Huai-Yong
ZHU, et
al.
Nano-Titanate Absorbent for Radioactive Waste
Nano-Titanate Absorbent for Radioactive Waste
http://www.qut.edu.au/about/news/news?news-id=37568
31 October 2011
Technology
Makes Storing Radioactive Waste Safer
Queensland University of Technology (QUT) researchers have developed new technology capable of removing radioactive material from contaminated water and aiding clean-up efforts following nuclear disasters.
The innovation could also solve the problem of how to clean up millions of tonnes of water contaminated by dangerous radioactive material and safely store the concentrated waste.
Professor Huai-Yong Zhu from QUT Chemistry said the world-first intelligent absorbent, which uses titanate nanofibre and nanotube technology, differed from current clean-up methods, such as layered clays and zeolites, because it could efficiently lock in deadly radioactive material from contaminated water.
The used absorbents can then be safely disposed without the risk of leakage, even if the material became wet.
"One gram of the nanofibres can effectively purify at least one tonne of polluted water," Professor Zhu said.
"This saves large amounts of dangerous water needing to be stored somewhere and also prevents the risk of contaminated products leaking into the soil."
The technology, which was developed in collaboration with the Australian Nuclear Science and Technology Organisation (ANSTO) and Pennsylvania State University in America, works by running the contaminated water through the fine nanotubes and fibres, which trap the radioactive Cesium (Cs+) ions through a structural change.
"Every year we hear of at least one nuclear accident. Not only is there a risk of contamination where human error is concerned, but there is also a risk from natural disasters such as what we saw in Japan this year," he said.
Professor Zhu and his research team believed the technology would also benefit industries as diverse as mining and medicine.
By adding silver oxide nanocrystals to the outer surface, the nanostructures are able to capture and immobilise radioactive iodine (I-) ions used in treatments for thyroid cancer, in probes and markers for medical diagnosis, as well as found in leaks of nuclear accidents.
"It is our view that just taking the radioactive material in the adsorbents isn't good enough. We should make it safe before disposing it," he said.
"The same goes for Australian sites where we mine nuclear products. We need a solution before we have a problem, rather than looking for fixes when it could be too late."
With a growing need to find alternatives to meet global energy needs, Professor Zhu said now was the time to put safeguards in place.
"In France, 75 per cent of electricity is produced by nuclear power and in Belgium, which has a population of 10 million people there are six nuclear power stations," he said.
"Even if we decide that nuclear energy is not the way we want to go, we will still need to clean-up what's been produced so far and store it safely," he said.
"Australia is one of the largest producers of titania that are the raw materials used for fabricating the absorbents of titanate nanofibres and nanotubes. Now with the knowledge to produce the adsorbents, we have the technology to do the cleaning up for the world."
Media contact: Alita Pashley, QUT media officer, 07 3138 1841 or alita.pashley@qut.edu.au
http://staff.qut.edu.au/staff/zhuhy/
Prof. Huai-Yong Zhu
WO 2008034190
METAL OXIDE NANOFIBRE FILTER
METAL OXIDE NANOFIBRE FILTER
2008-03-27
Inventor(s): ZHU HUAI YONG [AU]; KE XUEBIN [AU] + (ZHU, HUAI YONG, ; KE, XUEBIN)
Applicant(s): UNIV QUEENSLAND [AU]; ZHU HUAI YONG [AU]; KE XUEBIN [AU] + (QUEENSLAND UNIVERSITY OF TECHNOLOGY, ; ZHU, HUAI YONG, ; KE, XUEBIN)
Classification: - international: B01D39/00; B01D61/14 - European: B01D39/20D4; B01D61/02F; B01D67/00M12; B01D69/12; B01D71/02P
Abstract -- A substantially ceramic nanofilter in the form of a hierarchical structure of layers of metal oxide nanofibre non-woven meshes with increasing filtration ability on top of a mechanically strong but relatively porous substrate allows for high flux with nanometre separation capability. The nanofilter has application in the water purification, dairy, pharmaceutical, petrochemical and radioactive material processing industries. Particularly important, is the application of the nanofilter to filtering out viral and bacterial pathogens from water and air.
FIELD OF THE INVENTION
The present invention relates to substantially ceramic filters, and more particularly to metal oxide nanofibre filters.
BACKGROUND OF THE INVENTION
Ceramics have found a wide range of uses in today's society. They have applications in the aerospace, medical, military and communications industries. This versatility is due to their unique properties. Ceramic materials are very stable chemically, thermally and mechanically, and in addition are frequently bio-inert. Ceramic materials are generally porous and this means they can be very useful as filters.
Simply, the medium to be filtered flows through the channels of the filter carrier and particles are retained if their size exceeds the diameter of the filter pores. The filtrate permeates through the pores and can then be subjected to subsequent process stages. Ceramic materials are used as filters in the water and air purification, pharmaceutical, dairy, radioactive materials processing and chemical industries. Particularly important applications of this technology are in wastewater processing, as air filters in breathing equipment and in filtration of drinking water.
Every day, an estimated 3,000 to 6,000 people worldwide die from diseases caused by contaminated water. Ceramic filters with nano-sized pores which are capable of removing species larger than 60 nm have great importance as a potential solution to this tragedy. Filters of this selectivity allow the removal of bacteria and many pathogenic viruses from our water supply, air supply and even from our blood. The recent Severe Acute Respiratory Syndrome (SARS) and 'bird flu' epidemics resulted in many deaths and in affected areas people sought protection by the use of respiratory masks which were capable of filtering out the virus. These viruses fall into the 80-200 nm range.
Already, ceramic microfilters and nanofilters are routinely used in most developed countries to clean wastewater before discharge. The biggest advantages of ceramic filters over others, such as polymer filters, are better corrosion resistance, ability to withstand leaching and higher mechanical stability, all of which result in long operation lifetimes. Normal filtration at low temperature using organic filters causes fouling that has to be removed periodically. Steam cleaning is one way to remove it but is not possible without damaging the polymer filters. The ability of ceramic filters to withstand temperatures as high as 500<0>C means they are much easier to clean and thereby regenerate. The use of chemicals to remove fouling is also problematic for organic polymer filters because, unlike ceramic filters, they are not chemically inert.
Ceramic filters are typically produced through the sol-gel method which is well known in the art. In this method, the inorganic precursors go through the formation of a colloidal suspension (sol) and gelation of the sol to form a network in a continuous liquid phase (gel). At the functional group level, three reactions are generally used to describe the sol-gel process: hydrolysis, alcohol condensation, and water condensation. The support layer is generally thin and has a pore size of 1 [mu]m while the uppermost layer has a pore size in the nanometre range. Typical materials used to create porous filters employing the sol gel process are alkoxysilanes, alumina (AI2O3), titania (TiO2), silica (SiO2), zirconia (ZrO2) and mixed oxides. One of the drawbacks of the sol gel method is that the control of pore size is often difficult due to formation of irregular shaped particles. Pinholes and cracks can also appear in the top layer during the drying phase. This means that many sol particle layers may have to be put down to achieve effective filtration. This increases the mean pore length through which the filtrate must pass and so, as mentioned above, results in a greatly decreased flux. Low porosity and the presence of dead end pores which cannot contribute to filtration are also common problems. It is currently extremely difficult to obtain porous ceramic filters with both good selectivity and a sufficiently large filtration flux. Tepper et al, US patent 6,838,005, teaches the use of aluminium hydroxide fibres to form a composite filter to remove nano size viruses and other particulates from drinking water and other fluids. The filter is formed from an alumina sol mulched with glass microfibers. The resulting filter can suffer from the weaknesses described above for sol gel produced filters. The use of the glass is an attempt to compensate for the mechanical weakness of the filter. They found that attempts to double the thickness of the alumina filter resulted in a halving of the flow rate which means that a trade off must be made between the mechanical strength of the filter and the flow rates which can be achieved. WO 1998/21164 employs a functionally gradient ceramic structure as a substrate in a ceramic filter and teaches a method of producing said filter. Two surfaces are provided with a decreasing pore size between the two. Again this filter is synthesised starting from a colloidal suspension of ceramic particles. The control over the pore size in the resulting filter provided by this method is poor and the layers produced are relatively thick. This results in a decreased rate of flux and poor separation.
WO 2007/054040 teaches the use of a number of polymeric nanofibres to produce a filter for removing biological and physical impurities. The disadvantages of polymeric nanofilters compared to ceramic nanofilters are well known and include shorter operational lifetimes, lower stability to varying temperatures, problems with swelling in various solvents and greater difficulty in introducing surface modifications.
OBJECT OF THE INVENTION
The object of the invention is to overcome or at least alleviate one or more of the above problems and to provide for substantially ceramic nanofilters which possess high selectivity and are also capable of good filtration rates.
SUMMARY OF THE INVENTION
In a first aspect, although it need not be the only or indeed the broadest form, the invention resides in a substantially ceramic nanofilter comprising:
(a) a porous substrate;
(b) one or more intermediate layers of nanofibre non-woven meshes coated onto the substrate; and (c) a top layer of nanofibre non-woven mesh coated onto the upper surface of the one or more intermediate layers, wherein the pore sizes of the nanofibre non-woven meshes decrease in each consecutive layer to a desired minimum size in the top layer.
In a second aspect the invention resides in a process for generating a metal oxide nanofibre non-woven mesh including the steps of:
(a) suspending nanofibres of metal oxides in an aqueous, alcohol or acetone solution to form a suspension; (b) treating the suspension to make it homogeneous;
(c) coating the suspension onto a substrate;
(d) drying the coating in air; and
(e) calcining the coating to produce the non-woven mesh. Advantageously, the metal oxides of the second aspect may be selected from a wide range of suitable metal oxides such as aluminium oxides, titanium oxides, zinc oxides, rare earth oxides, copper oxides and the like.
In a third aspect, the invention resides in a process for producing a substantially ceramic nanofilter including the steps of: (a) coating a porous substrate with a first layer of nanofibres wherein the length of the nanofibres is greater than the pore size of the substrate;
(b) coating the first layer of nanofibres with a second layer of nanofibres wherein the length of the nanofibres in the second layer is greater than the pore size of the first layer; and
(c) repeating the process of coating a new layer of nanofibres on top of the uppermost layer until the desired number of layers is achieved, wherein the length of the nanofibres in the new layer is greater than the pore size of the uppermost layer, to thereby produce a substantially ceramic nanofilter.
Further features of the present invention will become apparent from the following detailed description. The term nanofibres will be used in this specification when discussing the suspensions of metal oxides being used to generate the non-woven meshes of the nanofilter. It should be understood that this also includes nanorods, nanotubes, nanobelts and the like which are formed by certain of the metal oxides discussed herein.
Throughout this specification, unless the context requires otherwise, the words "comprise", "comprises" and "comprising" will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.
BRIEF DESCRIPTION OF THE FIGURES
In order that the invention may be readily understood and put into practical effect, preferred embodiments will now be described by way of example with reference to the accompanying figures wherein:
FIG 1 shows a Scanning Electron Micrograph (SEM) of (a) a cross- sectional view of a nanofilter according to one embodiment of the present invention, including SEM's of the intermediate, (b) and (c), and top layers (d);
FIG 2 shows (a) a schematic representation of the nanofilter of FIG 1 showing the substrate, intermediate and top layers and relating each of these layers to electron micrograph images (b)-(d);
FIG 3 shows a titanate fibre with anatase nanocrystals coated on its surface;
FIG 4 shows (a) a graphical representation of the changing flux (black squares) and selectivity (clear circles) of a nanofilter according to one embodiment of the present invention to a solution of 60 nm latex spheres, as it is built up from its various layers and (b) a latex sphere of 60 nm diameter filtered out by the top layer of the nanofilter;
FIG 5 (b) is an SEM of a solution of latex spheres of 108 nm diameter before and after (insert) filtration through a nanofilter of pore size less than 60 nm;
FIG 5 (c) is an SEM of a solution of latex spheres of 200 nm diameter before and after (insert) filtration through a nanofilter of pore size less than 60 nm;
The present inventors have developed a method of creating substantially ceramic nanofilters by generating metal oxide nanofibre non- woven meshes. These non-woven meshes can be created to give pore sizes of diminishing diameters simply by the choice of the ceramic nanofibres used. The mesh layers are continuously constructed one on top of the other, layer upon layer, starting on a porous substrate base, to give nanofilters which have a filtration ability based on the pore size of the top layer.
It should be appreciated that while the pore size generally diminishes in going from the porous substrate, through the one or more intermediate layers to the top layer, each nanofibre non-woven mesh which is put down does not necessarily have a smaller pore size than the one directly below it.
This is because each of the one or more intermediate layers may be made up of a number of sub-layers e.g. three sub-layers of titanate nanofibres laid down one on top of the other. Likewise the top layer may consist of a number of sub-layers e.g. three sub-layers of [gamma]-alumina nanofibres.
The one or more sub-layers within the one or more intermediate layers and top layer, will often be produced using the same suspension of nanofibres and so the pore sizes of each individual sub-layer being coated down are the same and all contribute to the filtration ability of the intermediate and/ortop layers. Each intermediate layer and the top layer will, therefore, often contain more than one sub-layer of nanofibre non-woven mesh. As stated previously however, the pore size will decrease in going from the substrate through to each of the one or more intermediate layers and, finally, to the top layer.
The term "layer" will, therefore, be used herein to describe a distinct section of the nanofilter i.e. individually, the one or more intermediate layers and the top layer. The term "sub-layer" will be used to describe each of the individual nanofibre non-woven meshes which are laid down to collectively form each layer.
In the embodiments described herein the nanofilter is constructed from a porous substrate onto which is coated one or more sub-layers of the same metal oxide nanofibre non-woven mesh, to form the intermediate layer. One or more sub-layers of a different metal oxide nanofibre non-woven mesh are then coated on top of this to form the top layer. However, it will be appreciated that it is possible, using the same process, to lay down a number of intermediate layers which may each comprise one or more sub-layers of the same or differing metal oxide nanofibre non-woven meshes. The use of a number of intermediate layers of different metal oxide nanofibres may be structurally or functionally useful to support the application of a top layer of metal oxide nanofibres of desired dimensions.
It should be appreciated that the term "intermediate layer(s)" as used herein refers to the one or more layers the first of which is laid down upon the porous substrate i.e. they are located between the porous substrate and the top layer. Each of these one or more intermediate layers may be made up of one or more sub-layers of nanofibre non-woven mesh.
The term "top layer" as used herein refers to the layer which is laid down on top of the uppermost intermediate layer. As referred to earlier the top layer may be made up of one or more sub-layers of nanofibre non-woven mesh.
The term "titanate nanofibres" as used herein refers to hydrogen titanate nanofibres (H2TJaO7). It would be understood by a person of skill in the art that these nanofibres can be converted to TiO2(B) and anatase TiO2 nanofibres upon heating above approximately 300 <0>C. These fibres are useful for catalytic degradation of organic compounds using UV light. Other titanates such as sodium titanate (Na2Ti3O7), which can be converted to Na2Ti6Oi3 upon heating above approximately 300 <0>C, are also considered suitable to form metal oxide nano-fibre non-woven meshes but were not used in the particular examples herein which recite the use of "titanate nanofibres".
FIG 1 is a composite Scanning Electron Micrograph (SEM) of a cross- sectional view of a ceramic nanofilter according to one embodiment of the present invention. Image (a) is a cross section through the nanofilter, exposing the layers used in its construction. The porous [alpha]-alumina substrate can be seen at the left of the image and coated onto its surface, forming the intermediate layer is the non-woven mesh of titanate nanofibres. The top layer, at the right hand side of the image is a non-woven mesh of [gamma]-alumina nanofibres. Image (b) is an SEM of the filter surface after the substrate has had one coating of a 0.05-1.0 wt% titanate nanofibre suspension applied to it. Hydrogen titanate is particularly useful in the embodiments described as it bonds well to both the substrate and other metal oxide nanofibres. This image clearly shows the structure of the non-woven meshes of the present invention. Image (c) is an SEM of the filter surface after the substrate has had three coatings of a 0.05-1.0 wt% titanate nanofibre suspension applied to it, thereby demonstrating how the non-woven mesh intermediate layer has been built up from individual sub-layers in comparison to the single sub-layer in image (b). Image (d) is an SEM of the filter surface after the titanate intermediate layer has been coated with a 0.05-1.0 wt% suspension of AIO(OH) nanofibres. The decrease in pore size compared to the images of the titanate layer is clearly visible.
FIG 2 shows a schematic representation, in part (a), of the substrate, intermediate and top layers of a ceramic nanofilter according to one embodiment of the present invention and relates each of these layers to actual electron micrograph images. Image (b) is an SEM of the surface of the substrate, in this case [alpha]-alumina, demonstrating a relatively loose organization of particles and indicating pore sizes in the micrometer range. Image (c) is a Transmission Electron Micrograph (TEM) of the titanate nanofibres. The non-woven mesh structure can be seen and the relative position of this layer in the construction of the nanofilter is indicated in the schematic (a). Finally, image (d) is a TEM of the AIO(OH) nanofibres which are converted into [gamma]-alumina nanofibres during subsequent calcinations, forming the top layer of the nanofilter. This image indicates how the non- woven mesh has formed pore sizes capable of nano-filtration.
A suitable porous substrate should provide mechanical strength to the nanofilter and should have pore sizes sufficiently large to allow high flux but not so large as to prohibit the forming of a layer of nanofibres on its surface.
Examples are porous glass, ZnO, [alpha]-alumina, Zr[theta]2, T[Iota]O2, aluminosilicate and other ceramics. Suitably, the substrate is [alpha]-alumina or porous glass. The substrate will have pore diameters of 1-20 [mu]m depending on the flux desired and the dimensions of the nanofibres chosen to be coated onto the surface of the substrate. In one form the substrate will have pore diameters of 5-18 [mu]m. Preferably, the substrate will have pore diameters of 10-16 [mu]m.
In an alternative embodiment the porous substrate is titanium micromesh. The pore size of the titanium micromesh will be between 75 to
150 [mu]m. Typically, the titanium micromesh pore size is between 80 to 125 [mu]m. In a preferred embodiment the titanium micromesh pore size is about
100 [mu]m. The thickness of the mesh will be in the order of 1 mm.
The use of a titanium micromesh as the porous substrate provides a number of advantages to the final nanofilter. It provides a framework which allows for a very high flow rate, has great mechanical strength and allows for excellent binding with nanofibres and nanotubes. The one or more intermediate layers and top layer of non-woven mesh metal oxide nanofibres can be laid down upon the micromesh substrate to generate a substantially ceramic nanofilter.
The ceramic porous substrates e.g. alumina may be chemically treated to enhance adhesion with the intermediate layer of non-woven mesh nanofibres. Non-limiting examples are the treatment of the substrate surface with acid or caustic soda to bring about activation. This involves the generation of hydroxyl groups on the surface which aid in bonding with the intermediate layer in contact with the substrate during calcination. Advantageously, the surface of the substrate does not have to be made smooth as is required in some processes. This allows the use of a wider range of materials to act as the substrate and also reduces cost and time spent on preparation. Advantageously, the metal oxides of the present invention may be selected from a wide range of suitable metal oxides such as aluminium oxides, titanium oxides (FIG 12), cerium oxides (FIG 8), zinc oxides (FIG 10), rare earth oxides, copper oxides (FIG 9), boehmite, alumina, cerium oxide, titanate, zirconium dioxide, niobate, rare earth oxides and the like to produce non-woven meshes with differing pore sizes, thereby providing filtration selectivity. Figures 7-12 are a series of SEM's of different metal oxides in the form of nanofibres or nanorods which can be used to generate the non- woven meshes in the construction of a nanofilter. These images demonstrate some of the diversity available when selecting the metal oxide based on dimensions and inherent properties.
The intermediate layer can be formed from a range of suitable metal oxide nanofibres. The material chosen will depend on the pore size of the substrate. Suitable nanofibres will have a length greater than the diameter of the substrates pores. The nanofibres selected for the intermediate layer will have good compatibility with biological systems i.e. non-toxic, photostable and no dissolution in water. In one embodiment of the present invention, the intermediate layer is constructed from titanate nanofibres. ZnO nanorods (FIG 10), niobate nanofibres (FIG 11) and rare earth nanorods (FIG 7, praseodymium oxide) with a length in the range 1-10 [mu]m are further examples of materials suitable for use in the construction of this intermediate layer.
In another embodiment, the intermediate layer nanofibres are coated with a substance which enables the ceramic filters to photocatalytically decompose organic or biological species such as viruses and bacteria. This may aid in clearing the pores and maintaining filtration functionality of the nanofilter. One non-limiting example of such a substance is the use of anatase CT[Iota]O2) nanocrystals. FIG 3 shows how these nanocrystals can be coated onto the surface of a titanate nanofibre. As was mentioned previously, the hydrogen titanate nanofibres themselves may also be converted by heating into TiOa(B) or anatase nanofibres which are both effective in photocatalytic decomposition or organic matter and biological species. Other suitable examples are the painting of a layer of In2O3ZTa2O5, anatase, rutile or TiO2(B) onto the chosen nanofibres to allow the decomposition of organic pollutants in water using visible light.
The selected nanofibres are dispersed in a solution which is aqueous, acetone or an alcohol to form a suspension greater than 0.001 wt%. Preferably, the suspension is 0.05-1.0 wt%. In a preferred embodiment, the suspension is 0.2 wt%. Suitably, the solution is ethanol, an ethanol/water mixture or acetone. The solution may already contain various additives such as are discussed below.
Substances such as polyelectrolytes and the like may be present in the suspension to assist electrostatic self-assembly of the nanofibres. Surfactants may also be present to provide control over the viscosity of the coating suspension.
The suspension may be treated to form a homogeneous suspension by a number of chemical and physical means. One such example is sonication. The suspension is then applied to the substrate. The suspension can be applied by a number of appropriate means such as are known in the art, for example, dip-coating and spin-coating. The coating process may be repeated a number of times resulting in a number of sub-layers of the same nanofibre being laid down and which together form the intermediate layer. This allows the desired filtration capacity to be achieved. The layer is then dried in air at 323-523 K followed by calcination at 523-973 K. A non-woven mesh of nanofibres is formed with pore sizes substantially smaller than those of the substrate.
The top layer of the filter can be formed from a number of suitable metal oxide nanofibres. The nanofibres are chosen so that their length is greater than the diameter of the pores of the intermediate layer. In one preferred embodiment, the nanofibres are boehmite (AIO(OH)) nanofibres. Small nanofibres of rare earth oxides, Zr[theta]2 and alumina nanofibres coated with other oxides are also suitable for forming the top layer. The metal oxide nanofibres used to generate the top layer will also be chosen based on the functionality required from the nanofilter, which is related to the pore sizes achieved. For example, if the purpose of the nanofilter is to filter out bacteria and viruses then metal oxides which achieve pore sizes in the range of about 30 nm to about 60 nm in the top layer would be suitable. If the application of the nanofilter is to separate biological substances such as DNA or chlorophyll then metal oxide nanofibres which result in pore sizes of approximately 10 nm would be chosen.
In one embodiment the nanofibres of the top layer will result in a non- woven mesh with pore sizes of 1-100 nm. Suitably, the top layer will have pore sizes of 5-80 nm. Normally the pore sizes of the top layer will be between 10-60nm.
A number of metal oxide nanofibres are considered suitable for use in the nanofilter of the present invention. Some are particularly suitable for use in either the intermediate layer(s) or the top layer. It should be appreciated, however, that any metal oxide nanofibre may be useful in forming these layers and the particular one chosen will depend, in part, on the pore size of either the substrate (if laying down the nanofibre non-woven mesh directly upon this) or the last layer generated. The pore size the particular nanofibres form, their mechanical strength and desired functionality e.g. photocatalytic decomposers will all be taken into consideration when choosing the metal oxide nanofibres to form any one layer.
The top layer is formed in a similar manner to the intermediate layer. The selected nanofibres are dispersed in acetone, an alcohol or an aqueous solution to form a suspension greater than 0.001 wt%. Preferably, the suspension formed is between 0.01-5.0 wt%. In a preferred embodiment, the suspension is 0.2 wt%. The suspension may be treated to form a homogeneous suspension by a number of chemical and physical means. One such example is sonication. In addition, substances such as polyelectrolytes and the like may be present in the suspension to assist electrostatic self-assembly of the nanofibres. Further, the suspension may contain non-ionic polymers such as poly(ethylene)oxide, poly(ethylene)glycol and other water soluble polymers, to aid in controlling the thickness of the top layer.
The suspension is then applied on top of the intermediate layer and the solvent evaporated to leave a non-woven mesh of nanofibres. The suspension can be applied to the upmost intermediate layer by a number of appropriate means such as are known in the art, for example, dip-coating and spin-coating. The coating may be repeated a number of times resulting in a number of sub-layers. The resulting layer is then dried in air at between 323-523 K followed by calcination at between 523-923 K. In one embodiment, when using boehmite nanofibres, a temperature of 393 K is used for drying in air and the calcination is carried out at 723 K for 2h. During calcination the boehmite nanofibres are converted to [gamma]-alumina (AI2O3) nanofibres. The [gamma]-alumina nanofibres retain a similar morphology to the parent boehmite nanofibres. This results in a non-woven mesh of nanofibres with smaller pore sizes than the intermediate layer on which it is generated.
Thus, a hierarchical structure of non-woven meshes with increasing filtration ability on top of a mechanically strong but relatively porous substrate is achieved.
Further, post-construction, modification of the surface of the nanofilter can confer additional, desirable properties on the nanofilter. For example, grafting various silanes onto the filter surface by impregnation or gaseous reaction alters surface properties of the filter such as the relative hydrophobicity. Other chemical reactions on the filter surface can achieve fine-tuning of the porous structure and alter the affinity of the nanofibres for various elements. These nanofibre non-woven meshes do not suffer from cracks or pinholes to the same degree as, for example, layers created by the sol gel process and, as a direct result of the mesh-like structure, have flow rates at 60 nm separation that are 10-100 times greater than those of conventional filters with similar separation ability. This is due to a number of factors inherent in the non-woven meshes, for example the absence of dead-end pores and the fact that all the pores are interconnected. This can result in high porosity levels of 70% or greater. This is demonstrated graphically in FIG 4 (a) which shows the changing flux and selectivity of the filter as it is built up from its various sub-layers and layers. The filter selectivity was determined based on the filtering of a solution containing latex spheres of 60 nm diameter. The different points represent the flux (black squares) and filtration (clear circles) of the layers as they are built, one upon the other. Therefore, S = substrate alone, T1 = one titanate coating (on the substrate), T2 = two titanate coatings, T3 = three titanate coatings and Al = 3 boehmite coatings on top. The relatively high flux rates demonstrated can be achieved at lower pressures and so, with less energy consumption.
FIG 4 (b) shows how a latex sphere of 60 nm diameter from the above filtration solution has been filtered out by the non-woven mesh of [gamma]-alumina nanofibres forming the upper layer of the nanofilter.
FIG 5 (a)-(c) is a series of SEM's from three different solutions of latex spheres before and after (insert) filtration through the nanofilter. In the image shown in (a) the spheres were of 60 nm diameter, in (b) 108 nm and in (c) 200 nm. A portion of the solutions before filtration were analysed by SEM and these form the main part of each image. After filtration through a nanofilter with a [gamma]-alumina top layer, and so, pore sizes of 60 nm or less, a portion of the filtrate was sampled and analysed by SEM. These images form the insert in the top right hand corner of the corresponding p re-filtration image (a)-(c). These images show that all species of diameter 200 nm or greater are filtered off. Spheres of 108 nm are almost completely removed and even 60 nm diameter spheres are filtered off to a very large degree.
So that the invention may be more readily understood and put into practical effect, the skilled person is referred to the following non-limiting examples. EXAMPLES
Example 1
Preparation of a ceramic nanofilter was performed using a porous [alpha]- alumina disk with a diameter of 30 mm and thickness of 2-3 mm as the substrate. The pore sizes of the [alpha]-alumina substrate are approximately 10 [mu]m. Titanate fibres (20-30 [mu]m long and 40-100 nm thick) were dispersed in ethanol to give a 0.2 wt% suspension and sonicated for 10 min using an ultrasonic finger to achieve a homogenous suspension. This suspension was used to coat the substrate using a spin-coat processor. The coating was applied at a spinning velocity of 1000 r/min for 2 min and used approximately 0.5 ml_ of the fibre suspension for the coating.
The spin-coating process was repeated twice followed by drying in air at 393 K and then calcination at 773 K for 4 h. The heating rate employed is 1 K/min starting from 393 K. These three sub-layers collectively form the intermediate layer.
A 0.2 wt% suspension of boehmite (AIO(OH)) nanofibres (60-100 nm long and 2-5 nm thick) in ethanol was then made up and applied on top of the calcined titanate nanofibre non-woven mesh in the same manner. After coating and drying in air at 393 K, calcining at 723 K for 2h then results in the boehmite fibres being converted to y -alumina nanofibres (AI2O3). The v- alumina nanofibres form the top layer of the nanofilter and retain a similar morphology to the parent boehmite nanofibres. This process results in a non- woven mesh of [gamma]-alumina nanofibres (top layer) with smaller pore sizes than the underlying titanate nanofibre non-woven mesh (intermediate layer) all formed upon the [alpha]-alumina substrate. The result is shown in cross section in FIG 1 (a).
Example 2
The filtration properties of the titanate non-woven mesh (intermediate layer) formed from three coating cycles (three sub-layers) on the substrate were tested before the boehmite top layer was applied by filtering an aqueous suspension of latex spheres. Suspensions of spheres of a variety of sizes were made up to be 0.1 wt% and were used in the following experiment. 30 ml_ of these suspensions were put through the titanate nanofilter layer using a vacuum system which maintains a pressure difference between the feeding fluid and permeated fluid of 20 KPa.
The concentrations of the latex spheres in the original suspension and the filtrate were determined using both scanning electron microscopy (SEM) and UV-Visible spectroscopy. The titanate layer was able to filter out 100% of latex spheres with a diameter of 200 nm or greater. It also retained a flux of about 800 L/m<2>/h. The alumina substrate alone has a flux of about 1200 L/m<2>/h. FIG 4 exemplifies these results for the 60 nm diameter latex sphere solution. For comparisons sake the filtration and flux were tested at each stage of filter construction i.e. on the substrate alone (S), one titanate coating on the substrate (T1 ), two titanate coatings (T2), three titanate coatings (T3) (these three titanate coatings or sub-layers together make up the intermediate layer) and with the final [gamma]-alumina layer on top (Al).
Example 3
The filtration capability of the completed filter (3 coatings of titanate nanofibres on the [alpha]-alumina substrate followed by 3 coatings of the y- alumina nanofibres on top) was tested.
A 10 wt% aqueous suspension of 60 nm latex spheres was diluted to 0.01 wt% with water. 30 ml_ of the dilution was used to test permeation of the filter. A pressure difference of 20 KPa between the feeding fluid and permeated fluid was stably maintained by a vacuum system. The time taken for every 5 ml_ of fluid to filter through the filter under test was recorded.
The fluids before and after filtration were sampled for analysis. SEM images were collected on an FEI Quanta 200 Environmental SEM. A JEOL JSM 6400F Field Emission SEM was also used to obtain images of high resolution. The samples are coated with gold using a BioRad SC500 sputter coater.
The specimens from the liquid samples were prepared by dropping 5 [mu]l of solution on a glass slide and drying under vacuum. The efficiency of filter separation could be estimated directly by comparing the numbers of latex spheres in images of the specimens taken before and after filtration.
These images are shown in FIG 5 which demonstrates that at pore sizes less than 60 nm all spheres of diameter greater than 200nm are filtered off.
Those of diameter 108nm are almost completely filtered off and spheres of 60 nm diameter are removed to a very large extent.
The morphology of the nanofibres was recorded on a Philips CM200 Transmission Electron Microscope. UV-visible spectroscopy on a Caryl 00 (Varian Inc.) spectrophotometer was also utilized to analyze the concentration change of the fluids before and after filtration. The intensity of the absorption band at 200 nm was adopted to determine the concentration using a standard plotting curve.
In this case 96.8% of spheres with a diameter of 60 nm or greater were filtered out. The flux passing through the nanofilter was still found to be relatively high at 600 L/m<2>/h. This is 70-80% that of the titanate non-woven mesh filter alone and half that of the alumina substrate alone. It also represents a flux of 10-100 times greater than that of ceramic filters prepared in a more conventional manner exhibiting similar separation ability.
Example 4 A glass filter substrate was placed into 50 ml of a HNO3 solution containing 0.5 g of HNO3 and 49.5 g H2O. It was sonicated for about 10 min in an ultrasonic bath and then washed with deionised water. The substrate was then dried in air at 393 K for 4 h.
A suspension of titanate nanofibres was prepared by dispersing 0.08 g of nanofibres into 50 ml ethanol with vigorous stirring to give a white suspension with a fibre content of 0.2 wt%. Stirring was continued for a further 30 min.
The pre-cleaned substrate was loaded on the chuck of a spin coat processor and the above suspension was dropped at a spin velocity of 1000 r/min. The coating process took 1 min. The coating process is repeated 2-4 times. The total consumption of one solution was about 0.4 g (using 0.5 ml of the suspension). The substrate surface was thereby covered with titanate nanofibres.
After spin-coating, the coated substrate is dried firstly at room temperature for 12h and then at 393 K for 4 h. The substrate is then calcined at 773 K for 4 h to attach the non-woven mesh layer. The heating rate is 1 K/min, starting from 393 K.
Similarly, a 0.2 wt% suspension of boehmite nanofibres was applied on top of the calcined non-woven mesh of titanate nanofibres. The coating process is repeated 2-4 times.
The nanofilter product was mounted on a filtration set-up to assess its separation efficiency. A 10 wt% aqueous dispersion of latex spheres of 60 nm diameter was diluted to 0.01 wt% with water. 30 ml_ of the dilution was used to test permeation passing through a prepared filter. A pressure difference of 20 kPa was maintained between the feeding fluid and the filtrate, by a vacuum system, to drive the filtration. The time taken for each 5 ml_ of fluid to pass through the filter was recorded. The fluids before and after filtration were sampled for analysis. UV-visible spectroscopy (UV-vis) on a Caryl 00 (Varian Inc.) spectrophotometer was utilized to analyse the concentration change of the latex spheres in the fluids before and after filtration. The intensity of the absorption band at 205 nm was adopted to determine the concentration using a standard plotting curve. FIG 6 demonstrates the filtration flux and selectivity of this nanofilter as it is built up from its various layers by filtering a 0.01 wt% solution of 60 nm latex spheres (S is for the glass substrate alone; T1 represents the filter after the first coating with titanate nanofibres; T2 after the second coating with titanate nanofibres; T3 after the third coating with titanate nanofibres and Al represents the filter after three coatings of [gamma]-alumina nanofibres on top of the coatings of titanate nanofibres).
The completed filter comprising a non-woven mesh of titanate nanofibres (three coatings) and a non-woven mesh of alumina nanofibres (three coatings) is able to filter out 93 % of latex spheres with a diameter of 60 nm, with a maximum flux of 994 Um<2>Zh. Such a flux is significantly greater than the flux of filters prepared under conventional approaches which exhibited similar separation. The glass substrate (pore size 10-16 [mu]m) alone has a flux of more than 4000 Um<2>IU. The nanofilter provided by the present invention will be useful for a range of applications in the water purification, dairy, pharmaceutical, petrochemical and radioactive material processing industries. Particularly important, is the application of the nanofilter to filtering out viral and bacterial pathogens from water and air. The hierarchical structure of non-woven mesh layers with increasing filtration ability on top of a mechanically strong but relatively porous substrate allows for high flux with nanometre separation capability.
The surface of the substrate used does not have to be made smooth as is required in some processes thereby making the filter cheaper and faster to produce.
Throughout the specification the aim has been to describe the preferred embodiments of the invention without limiting the invention to any one embodiment or specific collection of features. It will therefore be appreciated by those of skill in the art that, in light of the instant disclosure, various modifications and changes can be made in the particular embodiments exemplified without departing from the scope of the present invention.
US 7169348
Metal Oxide Nanoparticles in an Exfoliated Silicate Framework
Metal Oxide Nanoparticles in an Exfoliated Silicate Framework
2004-12-02
Inventor(s): ZHU HUAI [AU]; LU GAO QING [AU] + (ZHU HUAI, ; LU GAO QING, ; ZHU HUAI YONG)
Applicant(s): ZHU HUAI, ; LU GAO QING, ; UNIVERSITY OF QUEENSLAND
Classification: - international: B01J20/10; B01J21/16; B01J29/04; B01J35/00; B01J35/10; B01J37/03; B05D7/24; C01B33/40; C01B33/44; (IPC1-7): B05D7/00; B32B5/16 - European: B01J20/10; B01J21/16; B01J29/04P; B01J35/00D; B01J35/10; B01J37/03B2
Abstract - A method of producing metal oxide nanoparticles in an exfoliated silicate framework by forming an aqueous exfoliated silicate suspension by mixing a layered clay into water until clear. To which is added an acidic hydrated metal species precursor solution, formed by dissolving one or more transitional metal salts, and a non-ionic surfactant. This solution is then aged to precipitate a product precursor before the product precursor is separated and washed. The product precursor is calcined to form metal oxide nanoparticles in an exfoliated silicate framework.
FIELD OF THE INVENTION
[0001] THIS INVENTION relates to metal oxide nanoparticles in an exfoliated laponite framework, methods of producing and uses of same. More particularly but not exclusively, the invention relates to the method of producing transition metal oxide nanoparticles in an exfoliated laponite framework and their use as catalysts, catalyst supports, adsorbents and/or in photoelectronics and electromagnetics.
BACKGROUND OF THE INVENTION
[0002] Fine particles of transition metal oxides, in the several nanometer range, are of particular interest for their potential use in photoelectronics, electromagnetics and as catalyst, catalyst supports and adsorbents. Typically transition metal oxides in this several nanoparticle range are in the form of fine powders and whilst their powdery nature increases the particle surface area they are subject to agglomeration which affects their general performance. Furthermore these fine powders are very hard to recover when used in aqueous systems, thus leading to a potential difficulty in downstream separation.
[0003] Various new techniques have been adopted to develop solids of large metal oxide surface area. Antonelli, D. M. and Ying, J. Y., Angew. Chem., Int.Ed. Engl., 1995, 34(18), 2014-2017 and Yang, P. et al, Nature 1998, 396, 152-165 described the formation of TiO2 mesoporous molecular sieves have been synthesised by templated synthesis. Kresge, C. T., et al, Nature, 1992,359,710-712 and Inagaki, S, et al, J. Chem. Soc. Chem. Commun., 1993, 680-682 earlier described the synthesis of mesoporous silica or aluminosilicate. Some approaches, such as starting with metal alkoxides and conducting the synthesis in non-aqueous systems, were employed to overcome serious difficulties in the synthesis, such as those reported in Antonelli, D, M, and Ying, J. Y., Angew Chem Int Ed Engl, 1995, 34(8), 2014-2017. However these processes still did not provided products with suitable pore size and surface areas.
[0004] In response to the desire to develop materials with larger pore sizes than zeolites, a class of thermally stable porous materials, pillared layered clays (PILCs) were developed from swellable layered clays, such as smectite, in the late 1970s. Numerous references describe the process, mechanism and properties of PILC's, for example Brindley, G. W. and Semples, R. E., Clay Miner, 1977, 12, 229. It is well understood that when dispersed in water, the layered clays swell because of hydration of the interlamellae cations which act as counterions to balance the negative charges of clay layers, which in turn allows inorganic polycations, the so-called pillar precursors, to be intercalated into the interlayer gallery by cation exchange. During subsequent heating above 400[deg.] C., the intercalated polycations are converted to oxide pillars, which prop the clay layers apart. A permanent micropore system is thus formed. Whilst pillaring has become a well-established technique for the synthesis of porous materials the materials produced are limited to microporous solids of a moderate porosity (typical characteristics being, pore volume of 0.15-0.40 cm<3> /g and BET surface area of 150-450 m<2> /g), such as those described by Burch, R. Ed, "Pillared clays, Catalysis Today", Elsevler: New York, 1998, Vol 2 and Mitchell, I. V., Ed. "Pillared layered structures, current tends and applications", Elsevier Applied Science, London 1990. As the pore size is limited by the formation of pillars, which in turn are limited by the size of the cations being incorporated into the clay structure, it is extremely difficult to obtain large pillar precursors that are identical in size, and result in a catalyst having very high porosity.
[0005] Galameau, A., et al., Nature, 1995, 374, 529, reported a successful synthesis of mesoporous solids termed as porous clay heterostructures (PCHs) from layered clays using quaternary ammonium surfactants as template agents. Layered clays were first intercalated with surfactants, tetraethoxide orthosilicate (TEOS), were allowed to hydrolyze and condense, surrounding the intercalated surfactants in the galleries of the clay particles. An open framework of silica formed within the clay layers after removal of the surfactants by heating. Products of this method however have poor porosity characteristics. In the formation of the PCHs the water content present needs to be carefully controlled to ensure that the TEOS is allowed to hydrolyse rendering the product outcome somewhat uncertain.
[0006] Suzuki, K. and Mari. T., Appl. Catal., 1990, 63, 181; Suzuki, K. et al., J. Chem. Soc.,Chem. Commum., 1991, 873 and Michot, L. and Pinnavala, T. J., Chem. Mater. 1992, 4 (6), 1433, describe the use of poly(vinylalcohol) (PVA) or alkyl polyether surfactants in the synthesis of aluminium pillared layered clays (Al-PILCs), which resulted in significant changes in the pore structure of the products. The Al-PILC prepared in the presence of PVA however have poor long-range order, and a relatively large pore volume, which mainly arises from mesopores. The PILC catalyst structures of the prior art, the clay layer remains intact while the pillar precursors intercalate into the clay layers by means of ion exchange processes which result in layered clays with a typical diameter of 1-2 [mu]m.
[0007] Whilst surfactants have been used to form a templates within catalyst or nanoparticle structures, all the methods of the prior art have suffered one or more limitations, including uncontrollable pore size, limited pore size range, poor porosity characteristics, the clay layers exhibit short range order, and/or their catalytic act is adversely effected when subject to high temperatures. The metal oxide nanoparticles in an exfoliated laponite framework when produced by the method of the invention show surprisingly good porosity characteristics and/or resistance to the effects of high temperatures.
DISCLOSURE OF THE INVENTION
[0008] In one form of the invention, although it need not be the only or indeed the broadest form, the invention resides in a method of producing metal oxide nanoparticles in an exfoliated laponite framework comprising the steps of:
forming an aqueous exfoliated laponite suspension having high pH, by mixing a layered clay into water until clear;
forming an acidic hydrated metal species precursor solution by dissolving one or more transitional metal salts;
adding a non-ionic surfactant and the hydrated metal species precursor solution to the aqueous exfoliated laponite suspension to form a product precursor;
ageing to precipitate a product precursor;
separating and washing the product precursor; and
calcining the product precursor to form metal oxide nanoparticles in an exfoliated laponite framework.
[0015] Preferably, the non-ionic surfactant is selected from Tergitol 15S-5, Tergitol 15S-7, Tergitol 15S-9, Tergitol 15S-12 or Tergitol 15S-30.
[0016] The transition metal salts are preferably selected from one or more of aluminium chloride; aluminium nitrate; aluminium hydroxychloride; ferric chloride; ferric nitrate; cerium chloride, lanthanum chloride; zirconium chloride; zirconium oxychloride; titanium (IV) isopropoxide, titanium chloride; chromium chloride.
[0017] Preferably, the aqueous exfoliated laponite suspension is formed by adding approximately 1 g laponite, per 50 mls water, and stirring until clear.
[0018] Suitably, 2 to 20 g of the nonionic surfactant is added per 200 ml of aqueous exfoliated laponite suspension.
[0019] The ageing step, is preferably achieved by subjecting the solution temperatures between 100[deg.] C. and 700[deg.] C. over a period of between one to three days.
[0020] Suitably, the calcining step is carried out at temperatures between 500[deg.] C. and 1100[deg.] C., for periods of up to or about 20 hours.
[0021] In another form of the invention, there is provided metal oxide nanoparticles in an exfoliated laponite framework wherein the metal ion of the metal oxide is selected from one or more of titanium, zirconium, aluminum, cerium, lathanum, iron, nickel and chromium.
[0022] In another form the invention provides metal oxide nanoparticles in an exfoliated laponite framework having characteristics selected from two or more of the following:
a. pore size between 3 and 9 nm;
b. surface area between 500 and 900 m<2> /g;
c. thermal stability at temperatures equal to or greater than about 500[deg.] C.; or
d. metal oxide nanoparticles particle size between 3 and 9 nm.
[0027] Preferably the metal oxide nanoparticles in the exfoliated laponite framework has characteristics selected from a surface area between 500 and 900 m<2> /g and metal oxide nanoparticles particle size between 3 and 9 nm; or a pore size between 3 and 9 nm, a surface area between 500 and 900 m<2> g and a thermal stability at temperatures equal to or greater than about 500[deg.] C.
[0028] In yet another form, the invention provides for an adsorbent comprising metal oxide nanoparticles in an exfoliated laponite framework, comprising metal oxide nanoparticles in an exfoliated laponite framework,
having a surface area between 500 and 900 m<2> /g and metal oxide nanoparticles particle size between 3 and 9 nm.
[0030] In still another form, the invention provides an photocatalyst comprising metal oxide nanoparticles in an exfoliated laponite framework having a pore size between 3 and 9 nm, surface area between 500 and 900 m<2/> g and thermal stability at temperatures equal to or greater than about 500[deg.] C.
[0031] The inventors have found that the metal oxide nanoparticle in an exfoliated laponite framework, unlike the PCHs and PILCs of the prior art, have no structure order and the metal oxide nanoparticles are linked and separated by silica pieces. The pore size of the metal oxide particles in the exfoliated laponite framework are typically 3 nm as opposed to 1-3 nm for the pillared clays, such as that produced by Suzuki supra. Furthermore it has been found that the method of the invention can be used for a variety of metal oxides rather than being applicable to only one metal oxide, such as the work reported by Suzuki supra only being applicable to alumina pillared clays.
[0032] Without wanting to be bound to a particular theory the inventors believe that the advantage achieved by the method of the invention is due in part to the reaction of the high pH synthetic clay suspension and the low pH of the precursor solution of hydrated metal species resulting in additional hydrolysis of the metal species. Furthermore it is believed that the clay is subjected to acid leaching by the precursor solution which leads to the clay loosing its original composition and structure, resulting in an amorphous silicate. It has been found that the clay particles and metal hydrates species have a high affinity to the non-ionic surfactant, which can be used to tailor the structure of the product. It also appears that the non-ionic surfactants act by separating the hydrolyzed species of metal elements, preventing them from further agglomeration and sintering during the drying and heating steps. During the heating process the non-ionic surfactant is volatilized, leaving a rigid structure of metal oxide nanoparticles incorporated into an exfoliated laponite framework, having high porosity.
BRIEF DESCRIPTION OF THE DRAWINGS
[0033] FIGS. 1A and B. UV-V is spectra for TiO2-laponite
[0034] FIG. 2. Comparison of catalytic performance of various Ni/ZrO2-laponites
[0038] For convenience the subsequent examples will refer to the metal oxide nanoparticles in an exfoliated laponite framework as metal oxide-laponites, for example titanium oxide nanoparticles in an exfoliated laponite framework will be referred to as titanium oxide-laponite or TiO2-Laponite, or metal oxide nanocomposites.
Example 1
Alumina-Laponite
[0039] Materials: The clay was Laponite RD, supplied by Fernz Specialty Chemicals, Australia. The clay powder has a BET specific surface area of 370 m<2> /g and a cation exchange capacity (CEC) of 55 mequlv/100 g of clay.
[0040] A commercial solution of aluminum hydroxychloride (Locron L from Hoechst, Germany) was used as the alumina source. It contains polyoxycations of aluminum hydrate with an Al2O3 content of 23.5+0.5 wt &, a ratio of OH/Al of 2.5, and a pH of about 3.5-3.7.
[0041] Four nonionic PEO surfactants, Tergitol 15S-n (n=5, 7, 9, and 12) from Aldrich, were used. The PEO surfactants have general chemical formula of C12-14H25-28O(CH2CH2O)nH and an average molecular weight of 420 for Tergitol 15S-5 (n=5) and of 730 for Tergitol 15S-12 (n=12).
[0042] Preparation of Alumina-Laponite Samples.
[0043] A 4.09 sample of Laponite was dispersed in 200 ml of water. The suspension was stirred until it became clear. Polyethylene oxide surfactant Tergitol 15-S-n, was added to the Laponite suspension. The amount of surfactant was varied from 0 to 20 g, and the different surfactants (n=5, 7, 9 and 12) were used to obtain a range of samples.
[0044] The suspension was stirred for 2 h to allow sufficient mixing. To this mixture was added 20 mL of the Locron L solution dropwise with continuous stirring. The suspension was then transferred to an autoclave after being stirred for 2 h and maintained at 100[deg.] C. for 2 days. A white precipitate was recovered from the mixture by centrifuging and washed with deionized water until it was free of chlorine (Cl<-> ) ions. The wet cake was dried in air and calcined at 500[deg.] C. for 20 h. The temperature was raised at a rate of 2[deg.] C./min. The PEO surfactants were evaporated in the temperature range between 100[deg.] C. and 250[deg.] C. The surfactants can be collected in a cooling trap during this stage and reused.
[0045] N2 adsorption-desorption isotherms were measured at liquid nitrogen (-196[deg.] C.) after a degassing period of 16 h at 230[deg.] C. The BET specific surface area, surface area, pore volume and mean diameter of the framework pores (pores formed in the galleries between the clay layers) in the calcined alumina intercalated laponite samples were derived from the data of the isotherms and summarized in Table 1 and 2. In Table 1, m indicates the amount of surfactant, Tergitol TS-15-9, introduced in the synthesis of the alumina-laponite nanocomposite.
TABLE 1
Surface area, pore volume and mean diameter of the
framework pores in the calcined alumina-laponite
nanocomposite samples.
Samples SBET(m<2> /g) Vp(cm<3> /g) dp(nm)<a>
m =
20 531 0.925 7.4
12 495 0.652 5.5
8 542 0.709 5.4
6 499 0.641 5.3
4 437 0.530 4.9
2 417 0.405 4.3
0 278 0.233 3.5
10 239 0.631 10.6
(alumina prepared
without Laponite)
<a> dP the mean diameter of the framework pores is hydraulic diameter, derived from the ratio of pore volume to pore surface area.
[0046]
TABLE 2
Surface area, pore volume and mean diameter of the
framework pores in the calcined alumina-laponite
nanocomposite samples prepared with different PEO
surfactants. The amount of the surfactant used is in the
synthesis is 8.0 g
Samples SBET(m<2> /g) Vp(cm<3> /g) dp(nm)
Tergitol 15-S-5 557 1.162 9.2
Tergitol 15-S-7 545 0.752 5.7
Tergitol 15-S-9 542 0.709 5.4
Tergitol 15-S-12 514 0.559 4.5
[0047] Alumina intercalated laponites prepared by the process of the invention and example 1, and further characterized were published by the inventors in "Engineering the Structures of Nanoporous Clays with Micelles of Alkyl Polyether Surfactants", Langmuir 2001, 17, 588-594, published on Jan. 6, 2001 and herein wholly incorporated by reference.
[0048] Further characterization and comparison data for alumina nanoparticles in exfoliated laponite appears below at example 10.
Example 2
Titanium Dioxide-Laponite
[0049] A laponite suspension was prepared as in Example 1. PEO surfactants, Tergitol 15-S-n, were added into the suspension. The amount of the surfactant was varied from 0 to 20 g, and the different surfactants (n=5, 7, 9 and 12) were used to obtain a range of samples.
[0050] The titanium hydrate sol was used as the TiO2 source. The sol was prepared from 12.9 g of titanium (IV) isopropoxide, Ti[OCH(CH3)2]4 and 176 ml of 1.0M HCl. It was then added dropwise into the mixture of surfactant and laponite suspension with continuous stirring. After prolonged stirring of 3 hours, the mixture was transferred into an autoclave and kept at 100[deg.] C. for two days. White precipitate was recovered from the mixture by centrifuging and washing with deionised water until it was free of Cl<- > ions. The wet cake was dried in air and calcined at the same conditions as in Example 1.
[0051] N2 adsorption-desorption isotherms were measured in a similar manner to Example 1. The BET specific surface area, surface area, pore volume and mean diameter of the framework pores (pores formed in the galleries between the clay layers) are listed in Table 3. In Table 3, m indicates the amount of the surfactant, Tergitol 15-S9 used in preparing the samples.
TABLE 3
Surface area, pore volume and mean diameter of the
framework pores in the calcined TiO2-laponite
nanocomposite samples.
Samples SBET(m<2> /g) Vp(cm<3> /g) dp(nm)<a>
m =
0 343 0.406 5.9
2 479 0.441 4.4
4 534 0.497 4.8
8 635 0.776 5.9
12 550 0.525 4.8
20 464 0.450 5.3
<a> dp the mean diameter of the framework pores is hydraulic diameter, derived from the ratio of pore volume to pore surface area.
[0052] Photodegradation of Rhodamine BG
[0053] The UV source was a 100W Hg lamp, Toshiba SHLS-1002A. Aqueous suspensions of Rhodamine 6G (usually 25 ml) and TiO2 nanoparticles in an exfoliated silicate framework (25 mg) were placed in a Pyrex vessel. TiO2 nanoparticle of 3.9, 4.4, 5.5 and 6.2 nm where prepared using a method similar to that described above with the pH of the solution varied. Prior to irradation, the suspensions were magnetically stirred in the dark for ca. 30 min to establish an adsorption/desorption equilibrium between the dye and the TiO2 particle surface. At given intervals of illumination, a sample of the TiO2 particulates were collected, centrifuged and then filtered thorough a millipore filter. The filtrates were analysed by UV-VIS spectroscopy using lambda Bio 20 spectrophotometer.
[0054] Comparing the DR-UV-VIS spectra of the samples FIG. 1A and 1B, it was found that absorbance of UV light by the sample with smaller anatase particle sizes occurs at the shorter wavelength, so called blue shift. The blue shift is consistent with the variation in the anatase particle size of the samples.
[0055] Larger species of the precursors of TiO2 nanoparticles were formed and condense to the acid-leached silicate layers. Introducing PEO surfactants in the synthesis significantly increased the porosity and surface areas in the composite solids. The TiO2 exists in anatase nano-particles, separated by voids and silicates platelets, and are accessible to organic molecules. The composite solids exhibited superior properties as photo-catalysts for degradation of Rhodamine 6G in aqueous solution.
[0056] The products of this example may be used for odour elimination of drinking water, degradation of harmful organic contaminants, like herbicides, pesticides, refractive dyes, and oil spills in surface water systems.
[0057] Further characterisation and comparison data for titanium oxide nanoparticles in exfoliated laponite appears below at example 10.
EXAMPLE 3
Zirconium Dioxide-Laponite
[0058] A laponite suspension was prepared as in Example 1. PEO surfactants, Tergitol 15-S-n, were added into the suspension. The amount of the surfactant was varied from 0 to 20 g, and the different surfactants (n=5, 7, 9 and 12) were used to obtain a range of samples.
[0059] An aqueous solution of ZrOCl2, from 32.23 g of ZrOCl2.8H2O and 200 ml water, was refluxed for 1 hour and used as the ZrO2 source. It was then added drop wise into the mixture of surfactant and laponite suspension with continuous stirring. After prolonged stirring of 3 hours, the mixture was transferred into an autoclave and kept at 100[deg.] C. for two days. White precipitate was recovered from the mixture by centrifuging and washing with deionised water until it was free of Cl<- > ions. The wet cake was dried in air and calcined at the same conditions as in Example 1.
[0060] N2 adsorption-desorption isotherms were measured in a similar manner to Example 1. The BET specific surface area, surface area, pore volume and mean diameter of the framework pores (pores formed in the galleries between the clay layers) are listed in Table 4. In Table 4, m indicates the amount of the surfactant, Tergitol 15-S-9 used in preparing the samples
TABLE 4
Surface area, pore volume and mean diameter of the
framework pores in the calcined ZrO2-
laponite nanocomposite samples.
Samples SBET(m<2> /g) Vp(cm<3> /g) dp(nm)<a>
m =
0 211 0.174 3.3
2 401 0.401 4.0
4 391 0.360 3.6
8 459 0.430 3.9
12 465 0.406 3.6
20 304 0.377 4.4
<a> dp the mean diameter of the framework pores is hydraulic diameter, derived from the ratio of pore volume to pore surface area.
[0061] Further characterisation and comparison data for zirconium oxide nanoparticles in exfoliated laponite appear below at example 10.
[0062] With large metal oxide surface area and porosity, good thermal and chemical stability, these materials have been found to be superior support for advance catalysts. Nickel catalysts supported on Zr-laponite exhibit a great long-term stability for methane reforming with carbon dioxide with high conversion rate, compared with the conventional nickel catalyst on alumina support, discussed in further detail in Example 9.
Example 4
Iron Oxide-Laponite
[0063] The laponite suspension was prepared in a similar manner as in Example 1. 8.0 g of Terigol 15-S-9 was added into the suspension. A 0.2M Iron(III)nitrate solution [Fe(NO3)3], from 17.8 of iron nitrate, [Fe(NO3)3.9H2O] and 213 ml of deionised water was mixed with 2.33 g of sodium carbonate, Na2CO3 under vigorously stirring. The molar ratio of [Na2CO3]/[Fe<3+> ]was 1:1.
[0064] After prolonged stirring of 3 hours, this solution was added dropwise into the mixture of surfactant and laponite suspension with continuous stirring. The mixture was stirred for 3 hours. The precipitate was recovered from the mixture by centrifuging and washing with deionised water until it was free of Cl<- > ions. The wet cake was dried in air and calcined at 250[deg.] C. for 20 hours. Temperature was raised at a rate of 2[deg.] C./min.
[0065] N2 adsorption-desorption isoterms were measured in a similar manner to Example 1. The BET specific surface area, surface area, pore volume and mean diameter of the framework pores (pores formed in the galleries between the clay layers) are listed in Table 5. In Table 5, m indicates the amount of the surfactant, Tergitol 15-S-9 used in preparing the samples
TABLE 5
Surface area, pore volume and mean diameter of the
calcined Fe2O3-laponite samples.
Samples SBET(m<2> /g) Vp(cm<3> /g) dp(nm)<a>
m =
0 419 0.3066 2.9
8 472 0.5467 5.0
<a> dp mean diameter of the framework pores is hydraulic diameter, derived from the ratio of pore volume to pore surface area.
[0066] Further characterisation and comparison data for iron oxide nanoparticles in exfoliated laponite appears below at example 10.
Example 5
Chromium Oxide-Laponite
[0067] The laponite suspension was prepared in a similar manner as in Example 1. 8.0 g of Terigol 15-S-9 was added into the suspension. A 0.1M chromium(III)nitrate solution [Cr(NO3)3], from 17.6 g of chromium nitrate, [Cr(NO3)3.9H2O] and 213 ml of hot water (95[deg.] C.) was mixed with 4.66 g of sodium carbonate, Na2CO3 under vigorously stirring. The molar ratio of [Na2CO3]/[Cr<3+> ] was 2:1. The solution thus obtained was aged at 95[deg.] C. for 6 hours. This solution was added drop-wise into the mixture of surfactant and laponite suspension with continuous stirring. The mixture was stirred for 3 hours. The precipitate was recovered from the mixture by centrifuging and washed with deionised water 3 times. The wet cake was dried in air and calcined at 350[deg.] C. for 20 hours. Temperature was raised at a rate of 2[deg.] C./min.
[0068] N2 adsorption-desorption isotherms were measured in a similar manner to Example 1. The BET specific surface area, surface area, pore volume and mean diameter of the framework pores (pores formed in the galleries between the clay layers) are listed in Table 6. In Table 6, m indicates the amount of the surfactant, Tergitol 15-S-9 used in preparing the samples.
TABLE 6
Surface area, pore volume and mean diameter of the
calcined Cr2O3-laponite nanocomposite samples.
Samples SBET(m<2> /g) Vp(cm<3> /g) dp(nm)<a>
m =
0 240 0.2062 3.4
8 446 0.6262 5.6
<a> dp the mean diameter of the framework pores is hydraulic diameter, derived from the ratio of pore volume to pore surface area.
[0069] Further characterisation and comparison data for chromium oxide nanoparticles in exfoliated laponite appears below at example 10.
Example 6
Mixed Oxides of Cerium and Aluminium-Laponite
[0070] The laponite suspension was prepared in a similar manner as in Example 1. PEO surfactants, Terigol 15-S-9, was added into the suspension. The amount of the surfactant was varied from 0 to 20 g, to obtain a range of samples. A mixture solution of CeCl3 and Lacron L, from 3.5 g of CeCl3.7H2O, 20 ml of Locron L solution and 26 ml of water stirred for 2 hours and then transferred in to an autoclave and kept at 120[deg.] C. for 100 hours. This solution was added drop-wise into the mixture of surfactant and laponite suspension with continuous stirring. After prolonged stirring of 2 hours, the mixture is transferred into an autoclave and kept at 100[deg.] C. for two days. The precipitate was recovered from the mixture by centrifuging and washed with deionised water until it is free of Cl<- > ions. The wet cake was dried in air and calcined at 500[deg.] C. for 20 hours. Temperature was raised at a rate of 2[deg.] C./min.
[0071] N2 adsorption-desorption isotherms were measured in a similar manner to Example 1. The BET specific surface area, surface area, pore volume and mean diameter of the framework pores (pores formed in the galleries between the clay layers) are listed in Table 7. In Table 7, m indicates the amount of the surfactant, Tergitol 15-S-9 used in preparing the samples
TABLE 7
Surface area, pore volume and mean diameter of the
calcined CeO2/Al2O3-laponite samples.
Samples SBET(m<2> /g) Vp(cm<3> /g) dp(nm)<a>
m =
0 422 0.324 3.1
4 562 0.6767 4.8
8 599 0.755 5.3
12 589 0.7222 4.9
<a> dp the mean diameter of the framework pores is hydraulic diameter, derived from the ratio of pore volume to pore surface area.
[0072] Further characterisation and comparison data for cerium oxide/alumina nanoparticles in exfoliated laponite appears below at example 10.
Example 7
Mixed Oxides of Lanthanum and Aluminium-Laponite
[0073] The laponite suspension was prepared in a similar manner as in Example 1. PEO surfactants, Terigol 15-S-9, were added into the suspension. The amount of the surfactant was varied from 0 to 20 g, to obtain a range of samples. A mixture solution of LaCl3 and Locron L, from 3.5 g of LaCl3.7H2O, 20 ml of Locron L solution and 26 ml of water stirred for 2 hours and then transferred in to an autoclave and kept at 120[deg.] C. for 100 hours. This solution was added drop-wise into the mixture of surfactant and laponite suspension with continuous stirring. After prolonged stirring of 2 hours, the mixture is transferred into an autoclave and kept at 100[deg.] C. for two days. The precipitate was recovered from the mixture by centrifuging and washed with deionised water until it is free of Cl<- > ions. The wet cake was dried in air and calcined at 500[deg.] C. for 20 hours. Temperature was raised at a rate of 2[deg.] C./min.
[0074] N2 adsorption-desorption isoterms were measured in a similar manner to Example 1. The BET specific surface area, surface area, pore volume and mean diameter of the framework pores (pores formed in the galleries between the clay layers) are listed in Table 8. In Table 8, m indicates the amount of the surfactant. Tergitol 15S-9 used in preparing the samples
TABLE 8
Surface area, pore volume and mean diameter of the
calcined LaO2/Al2O3-laponite samples.
Samples SBET(m<2> /g) Vp(cm<3> /g) dp(nm)<a>
m =
0 438 0.321 2.9
8 587 0.636 4.3
<a> dp the mean diameter of the framework pores is hydraulic diameter, derived from the ratio of pore volume to pore surface area.
Example 8
Mixed Oxides of Cerium and Zirconium-Laponite
[0075] The laponite suspension was prepared in a similar manner as in Example 1. 8.0 g Terigol 15-S-9, was added into the suspension. A mixture solution of CeCl3 and ZrOCl2, from 1.8 g of CeCl3.7H2O, 16.1 g ZrOCl2.7H2O and 100 ml water, was stirred for half an hour and then transferred into an autoclave and kept at 100[deg.] C. for two days. The precipitate was recovered from the mixture by centrifuging and washed with deionised water until it is free of Cl<- > ions. The wet cake was dried in air and calcined at 500[deg.] C. for 20 hours. Temperature was raised at a rate of 2[deg.] C./min.
[0076] N2 adsorption-desorption isotherms were measured in a similar manner to Example 1. The BET specific surface area, surface area, pore volume and mean diameter of the framework pores (pores formed in the galleries between the clay layers) are listed in Table 9. In Table 9, m indicates the amount of the surfactant, Tergitol 15-S-9 used in preparing the samples.
TABLE 9
Surface area, pore volume and mean diameter of the
calcined CeO2/ZrO2-laponite samples.
Samples SBET(m<2> /g) Vp(cm<3> /g) dp(nm)<a>
m =
0 291 0.226 3.1
8 611 0.674 4.4
<a> dp the mean diameter of the framework pores is hydraulic diameter, derived from the ratio of pore volume to pore surface area.
Example 9
Nickel/Zirconium Oxide Nanoparticle in an Exfoliated Laponite
[0077] A zirconium oxide nanopartilce in an exfoliated laponite, subsequently referred to as a Zr-laponite was prepared in a similar manner to that described in Example 3 above.
[0078] The Ni/Zr-laponite was prepared by the incipient wetness impregnation method with aqueous solution of nitrates as metal precursors. The solids were dried overnight in air at 105[deg.] C. and then calcined at 500[deg.] C. in air for 4 hours in order to realize the complete decomposition of the nitrate. After this treatment, the catalyst was reduced at 500[deg.] C. in a stream of 50% H2/N2 for 2 hours. The nickel metal loading of catalyst was 9 wt %, except where specially stated.
[0079] Catalytic performance
[0080] Catalytic reaction experiments were conducted in a vertical fixed-bed reactor made of quartz tube under atmospheric pressure. 0.2 g catalyst was placed in the quartz tube. A thermocouple was placed in the tube with one end touching on the catalyst in order to measure the bed temperature. The mixture of reactants of methane and carbon dioxide with ratio of 1:1 was fed into the reactor at the flow rate of 60 ml/min (GHSV=18000 ml/gh). The analyses of reactant/product mixtures were carried out using an on-line gas chromatograph (Shimadzu-17A) equipped with a thermal conductivity detector. A carbosphere (80-100 mesh) column was used to separate H2, CO, CH4 and CO2. Prior to each reaction run, the catalyst was reduced in situ at 500[deg.] C. in 50% H2/N2 for 2 h. The activities of catalysts were investigated at temperatures between 500-800[deg.] C., and stability tests were conducted at a certain reaction temperature only.
[0081] Surface area and pore size of supports and catalysts
[0082] The surface areas and pore sizes of supports and catalysts were studied by nitrogen adsorption, and the results are listed in Table 10. The pillaring process greatly increases the surface area of these laponite materials. It was seen that when the amount of the introduced surfactant was increased, the surface area of Zr-Laponite increased, until the Zr-Laponite(12) with a 12 g amount of surfactant in the pillaring process, when its surface area decreased. This indicates that the surfactant amount of 12 g is more than enough in the pillaring process. All these supports are mesoporous and have higher surface areas than [gamma]-Al2O3, of the prior art. The surface area order of supports is Zr-Laponite(8)>Zr-Laponite(12)>Zr-Laponite(4)>Zr-Laponite(0). The order of catalysts is similar to that of supports. The surface area of catalyst treated at 500[deg.] C. is higher than at 600[deg.] C. It is also seen that the surface area of catalysts is generally reduced for those with high porous structure.
TABLE 10
Variation in pore structure of Zr supports and Ni/Zr catalysts
Surface Pore
Area Pore Volume Diameter Pore
Sample (m<2> /g) (cm<3> /g) (nm) Structure
Zr-Laponite (0) 211.4 0.174 3.3 Mesoporous
Zr-Laponite (4) 390.8 0.360 3.7 Mesoporous
Zr-Laponite (8) 458.6 0.41 4.1 Mesoporous
Zr-Laponite (12) 435.9 0.413 3.8 Mesoporous
Al2O3 112 0.286 10.3 Mesoporous
Ni/Zr-Laponite (0) 260.3 0.192 3.0 Mesoporous
(500[deg.] C.)
Ni/Zr-Laponite (0) 227 0.169 3.0 Mesoporous
(600[deg.] C.)
Ni/Zr-Laponite (8) 371.9 0.354 3.8 Mesoporous
(500[deg.] C.)
Ni/Zr-Laponite (8) 350 0.34 3.7 Mesoporous
(600[deg.] C.)
Ni/Zr-Laponite (4) 330 0.306 3.8 Mesoporous
(500[deg.] C.)
Ni/Zr-Laponite (12) 356 0.337 3.8 Mesoporous
(500[deg.] C.)
4.5% Ni/Zr-Laponite 371.8 0.351 3.8 Mesoporous
(8) (500[deg.] C.)
Ni/Al2O3 118 0.230 7.8 Mesoporous
[0083] Catalytic activity
[0084] These ZrO2-laponite composites were used as supports to prepare supported Ni catalysts for methane reforming with carbon dioxide. These catalysts exhibited high conversion and good stability, maintaining the high activity for over 170 hours at an operating temperature of 1023K. While the nickel catalyst on conventional support of activated alumina (Al2O3) showed substantial deactivation. The performances of the catalysts supported on a nanocomposite and on an activated alumina are shown in FIG. 2.
Example 10
Metal Oxide Nanoparticles in Exfoliated Laponite
[0085] A number of metal oxide nanoparticle in exfoliated laponites framework were formed using methods similar to those described above. It was found that the porosity of the nanocomposite was significantly increased by introducing polyethylene oxide (PEO) surfactants of small molecular weights, with a general chemical formula C12-14H25-29 O(CH2CH2O)nH (n=5-12). These surfactants have strong affinities to the surfaces of clay and metal hydrates. Therefore, they have a function of separating the hydrolyzed species of metal elements, preventing them from further agglomeration and sintering during the drying and heating processes.
[0086] Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartull, J. C.; Beck, J. S., Nature, 359, 710-712, 1992; Inagaki, S.; Fukushima, Y.; Kuroda, K. J. Chem. Soc. Chem. Commun., 680-682, (1993); and Huo, Q. et al.
[0087] Kresge, C. T.; Leonowicz, M. E; Roth, W. J.; Vartuli, J. C.; Beck, J. S., Nature, 359, 710-712, 1992; Inagaki, S.: Fukushima, Y.; Kuroda, K. J. Chem. Soc. Chem. Commun., 680-682, (1993); and Huo, Q. et al, Nature, 368, 317-321(1994) reported that in the templated synthesis the pore size of the product is proportional to the molecular size of the surfactant. But there is no such a trend observed in the composites made by the method of the current invention. The molecular size of the surfactant is not a sole determinant of the pore size of the product solids. During heating the surfactant volatilizes, leaving a rigid structure with high porosity. The BET specific surface area and porosity data of some nanocomposite samples are given in Table 11.
TABLE 11
BET specific surface area (BET S.A.) and pore
volume (Vp) of some metal oxide composite samples.
Samples prepared with Samples prepared
surfactant (PEO) without surfactant
BET
S.A. Vp BET S.A. Vp
Metal oxide (m<2> /g) (cm<3> /g) Metal oxide (m<2> /g) (cm<3> /g)
Al2O3- 542 0.709 Al2O3- 278 0.233
Al2O3/La2O3- 587 0.676 Al2O3/La2O3- 439 0.383
Al2O3/CeO2- 599 0.775 Al2O3/CeO2- 422 0.345
ZrQ2 459 0.430 ZrO2 248 0.146
ZrO2/CeO2 611 0.676 ZrO2/CeO2 291 0.185
TiO2- 635 0.776 TiO2- 343 0.405
Cr2O3- 670 0.461 Cr2O3- 894 1.124
Fe2O3- 434 0.547 Fe2O3- 419 0.307 are microporous solids (pore size below 2 nm) with a moderate porosity, while the nanocomposites are mesoporous solids. The structures of these two classes of solids are also profoundly different. The TEM images of the pristine laponite and three nanocomposites (as representatives) are given in FIG. 3.
[0089] Bundles of several clay platelets can be seen in the image of pristine laponite in FIG. 3a. They aggregate in a poor long-range order. For pillared clays, the clay layers were regarded as inert with respect to reaction and almost intact in term of chemical composition through pillaring process. The acid leaching may result in remarkable changes in composition and structure of the clay layers. The extent of the reaction varies substantially from clay to clay. FIG. 3b is the image of the sample prepared from a solution containing Keggin ions, [Al13O4(OH)24]<7+> , and the laponite dispersion, Al2O3-composite. In this solid, thin stringy structure of about 1 and 2 nm thickness are observed, which are singular and paired silicate platelets, respectively. It is noted that these platelets are entangled but with a separation in the nanometer range. This indicates that the platelets are intercalated with nanoparticles of alumina. The Keggin ion solution has a weak acidity (pH of 3.0-3.5), and there is no obvious acid leaching from the laponite platelets, according to the results of chemical composition of the sample (in Table 12). Substantial loss of magnesium that is in the clay layer is an indicative of the acid leaching. Thus, the laponite platelets remain intact, in terms of composition and framework structure. When a more acidic solution containing sol particles of titanium (IV) hydrate, was used, laponite platelets are obviously involved in reaction. Most of magnesium in the clay platelets was leached out (Table 2). The solids obtained after calcination at 500[deg.] C. contains mainly silica and titanium dioxide. X-ray powder diffraction pattern (not shown) indicates that TiO2 exists in anatase phase and we can see the aggregation of crystallites with random orientations in FIG. 3c. The size of the anatase crystalites can be estimated from domains of regular texture in the image, being about 3-9 nm. Both the x-ray pattern and TEM image do not indicate any crystal form of silica or silicate although silica accounts for over 50% of the sample mass. The silica in the samples is more likely amorphous as reaction product of the laponite clay. Similar behaviors were observed for other transition metal oxide nanocomposites. Energy dispersing x-ray spectroscopy (EDS) was used to analyze the chemical compositions at different regions over a sample. At least 5 regions were taken for one sample and the average region size was about 15 nm in diameter. We found no obvious difference from region to region and the overall composition of the sample was uniform, for all the samples. This means that in these samples metal oxide particles homogeneously disperse in exfoliated silicate media.
[0090] The image in FIG. 3d provides more information on the structure of the reaction product derived from laponite. This solid was obtained from reaction of laponite suspension with a solution containing chromium hydroxyl ions. In this particular case, only a small amount of Cr2O3 is left in the product solid (Cr2O3 content below 1%), which contains about 80 wt % of silica and 6 wt % of MgO, meanwhile most of the Mg content in the original laponite has been leached out during the synthesis. This solid provides a clear picture of the residue from the original laponite after the reaction. The structure of this solid is strikingly different from that of the original laponite. Various holes ranging from 3-20 nm can be seen in the image. These irregular pores reveal that the laponite platelets were seriously attacked, not only at edges but also on the basal surface of the platelets, leaving a porous framework of silica.
TABLE 12
Major chemical composition of the
samples shown In FIG. 3.
SiO2 Al2O3 MgO TiO2 Na2O Fe2O3 Cr2O3
Sample (%) (%) (%) (%) (%) (%) (%)
Laponite 51.10 0.07 23.20 -* 2.51 - -
Al2O3- 41.49 27.91 17.38 - - 0.04 -
composite
TiO2- 55.30 0.12 0.19 43.90 - 0.02 -
composite
Cr2O3- 79.87 - 6.13 - - 0.60 0.03
composite
*Not detectable.
[0091] <29> Si magic angle spinning nuclear magnetic resonance (<29> Si MASNMR) of the samples (FIG. 4) also indicate the different structure change in silicate platelets caused by the reaction.
[0092] <29> Si MASNMR spectrum of laponite displays two resonance peaks at -90 and -80 ppm (spectra a). Such chemical shifts are correlated to the SiO4 tetrahedras linked with 3 and 2 other SiO4 tetrahedras (Q<3 > and Q<2 > sites), respectively. This is expected for the structure of laponite clay layer. In the clay layer most SiO4 tetrahedras are linked to 3 other SiO4 tetrahedra, being in Q<3 > sites but the tetrahedra at the edges of the clay layers are linked to 2 other SiO4 tetrahedra and thus form the Q<2 > sites. The smaller amount of Q<2 > sites, compared with that of Q<3 > sites, is responsible for the low intensity of the peak at -80 ppm.
[0093] The chemical shifts for the Al2O3-composite sample is similar to that of laponite, with a major resonance at -91.7 ppm (spectra b). This means that the clay platelets remain almost intact in the reaction, being consistent with our observation on the TEM image. The TiO2-composite and Cr2O3-composite samples show substantially different MASNMR spectra. Broad resonance in the range from -110 to -80 ppm can be seen, reflecting poor short-range order. It also suggests a radical structure change of the silicate due to the reaction in the synthesis. The chemical shift at -104 ppm (a peak for Cr2O3-composite and a shoulder for TiO2-composite) should be assigned to Q<4 > sites where the SiO4 tetrahedra linked with 4 other SiO4 tetrahedra. In laponite clay structure there should be no Q<4 > sites, and this is confirmed by the spectrum of the clay. Thus the Q<4 > sites have resulted from the profound structure changes of the silicate in the synthesis.
[0094] These results suggest that the clay layers could be seriously attacked if the acidity of the precursor solution is strong. On the other hand, the laponite dispersion with a high pH inevitably induces further hydrolysis of the metal hydroxyl oligomers in the precursor solution, forming larger species, the precursors of metal oxide nanoparticles. These large species most likely condense to the surrounding silicate platelets, because they carry opposite electric charges. This leads to a composite structure in which metal oxide particles of several nanometers in size are dispersed among the exfoliated silicate media.
[0095] According to this mechanism, it is possible to alter the particle size of the metal oxides by manipulating the acidity of the precursor solution. Indeed, as we increased the H<+> /TI molar ratio of the precursor solution from 2.0 to 8.0, the mean size of anatase particle in the product TiO2-composites increases from 3.7 to 9.0 nm. This finding is of significant importance, which allows us to effectively tailor the structure of these solids for various applications.
[0096] In FIG. 5 catalytic performances for photo-degradation of 2, 4-dicholorophenol by a TiO2-composite and P25, a commercial ultra-fine titanium dioxide powder supplied by Degussa, are compared.
[0097] The overall photo-catalytic efficiency of the TiO2-composite is comparable to that of P25, which is known to be the best commercial TiO2 photo-catalyst and has a mean particles size of about 25 nm. The catalytic performance of the TiO2-composite proves that most of the surface of TiO2 crystals is accessible to the various molecules in solution. Furthermore, the TiO2-composite contains about 45% of TiO2. Therefore, the activity per mass of TiO2 for the TiO2-nanocomposite is superior. Besides, it is very difficult to recover P25 powder from water. This could leads to a potential difficulty in downstream separation. In contrast, the composite catalyst can be readily separated from aqueous solutions by filtration. The silicate layers in the samples not only act as media allowing TiO2 to disperse in nano-crystals but also link the distributed TiO2 nano-crystals to large granules which can be recovered easily.
[0098] Besides this, we also found that nickel catalysts supported on ZrO2-composite and Al2O3-composite exhibit high conversion rate for methane reforming with carbon dioxide. The catalyst maintains the high activity for over 170 hours, much longer than the catalyst supported activated Al2O3.
[0099] These findings highlight the potential of the metal oxide composites as advanced materials. These solids can be readily granulated to designed shapes and the grains have good mechanical strength because of the presence of a silicate framework structure.
[0100] In comparison to the prior art, the method of the current invention has a prominent advantage that it can be conducted in aqueous system at moderate conditions. Moreover, this synthesis utilises the reaction between the clay suspension and the oligomer solution to form composite nanostructure with assistance of PEO surfactant, being profoundly different from the synthesis of the well-known pillared clay materials. Actually, such a synthesis route is not limited to laponite, we have prepared composites from natural layered clays such as saponite and hectorite. This new synthesis technique allows us to design and engineer composite nanostructures with desirable pore and surface properties.
[0101] The metal oxide nanoparticles in exfoliated laponite produced by the process of the invention are highly porous with large surface areas, generally greater than 500-900 m<2> /g. The process of the invention also allows the pore framework to be tailored to meet use requirements.
[0102] The high thermal stability and porosity of the metal oxide nanoparticles in exfoliated laponite in addition to their cost effective production methods, make them more favourable than the currently available pillared clay catalyst.
[0103] Throughout the specification the aim has been to describe the preferred embodiments of the invention without limiting the invention to any one embodiment or a specific collection of features.
[0104] In addition, throughout this specification and claims which follow, unless the context requires otherwise, the word "comprise", and variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers or steps but not the exclusion of any other integer or group of integer.