WO2011023953A1 - A catalyst and a method of making the same - Google Patents

A catalyst and a method of making the same Download PDF

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Publication number
WO2011023953A1
WO2011023953A1 PCT/GB2010/001614 GB2010001614W WO2011023953A1 WO 2011023953 A1 WO2011023953 A1 WO 2011023953A1 GB 2010001614 W GB2010001614 W GB 2010001614W WO 2011023953 A1 WO2011023953 A1 WO 2011023953A1
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metal
catalyst
cations
protecting agent
nanoparticles
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PCT/GB2010/001614
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French (fr)
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Graham John Hutchings
Jose Antonio Lopez-Sanchez
Nikolaos Dimitratos
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University College Cardiff Consultants Limited
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J37/00—Processes, in general, for preparing catalysts; Processes, in general, for activation of catalysts
    • B01J37/02—Impregnation, coating or precipitation
    • B01J37/0201—Impregnation
    • B01J37/0211—Impregnation using a colloidal suspension
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J23/00—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00
    • B01J23/38—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of noble metals
    • B01J23/40—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of noble metals of the platinum group metals
    • B01J23/42—Platinum
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J23/00—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00
    • B01J23/38—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of noble metals
    • B01J23/40—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of noble metals of the platinum group metals
    • B01J23/44—Palladium
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J23/00—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00
    • B01J23/38—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of noble metals
    • B01J23/48—Silver or gold
    • B01J23/52—Gold
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J35/00—Catalysts, in general, characterised by their form or physical properties
    • B01J35/20—Catalysts, in general, characterised by their form or physical properties characterised by their non-solid state
    • B01J35/23—Catalysts, in general, characterised by their form or physical properties characterised by their non-solid state in a colloidal state
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J35/00—Catalysts, in general, characterised by their form or physical properties
    • B01J35/30—Catalysts, in general, characterised by their form or physical properties characterised by their physical properties
    • B01J35/391—Physical properties of the active metal ingredient
    • B01J35/393—Metal or metal oxide crystallite size
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J37/00—Processes, in general, for preparing catalysts; Processes, in general, for activation of catalysts
    • B01J37/0009—Use of binding agents; Moulding; Pressing; Powdering; Granulating; Addition of materials ameliorating the mechanical properties of the product catalyst
    • B01J37/0018—Addition of a binding agent or of material, later completely removed among others as result of heat treatment, leaching or washing,(e.g. forming of pores; protective layer, desintegrating by heat)
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J37/00—Processes, in general, for preparing catalysts; Processes, in general, for activation of catalysts
    • B01J37/02—Impregnation, coating or precipitation
    • B01J37/0215—Coating
    • B01J37/0219—Coating the coating containing organic compounds
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J37/00—Processes, in general, for preparing catalysts; Processes, in general, for activation of catalysts
    • B01J37/02—Impregnation, coating or precipitation
    • B01J37/0215—Coating
    • B01J37/0221—Coating of particles
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J37/00—Processes, in general, for preparing catalysts; Processes, in general, for activation of catalysts
    • B01J37/06—Washing
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J37/00—Processes, in general, for preparing catalysts; Processes, in general, for activation of catalysts
    • B01J37/08—Heat treatment
    • B01J37/10—Heat treatment in the presence of water, e.g. steam
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J37/00—Processes, in general, for preparing catalysts; Processes, in general, for activation of catalysts
    • B01J37/16—Reducing
    • C—CHEMISTRY; METALLURGY
    • C07—ORGANIC CHEMISTRY
    • C07C—ACYCLIC OR CARBOCYCLIC COMPOUNDS
    • C07C51/00—Preparation of carboxylic acids or their salts, halides or anhydrides
    • C07C51/16—Preparation of carboxylic acids or their salts, halides or anhydrides by oxidation
    • C07C51/21—Preparation of carboxylic acids or their salts, halides or anhydrides by oxidation with molecular oxygen
    • C07C51/23—Preparation of carboxylic acids or their salts, halides or anhydrides by oxidation with molecular oxygen of oxygen-containing groups to carboxyl groups
    • C07C51/235—Preparation of carboxylic acids or their salts, halides or anhydrides by oxidation with molecular oxygen of oxygen-containing groups to carboxyl groups of —CHO groups or primary alcohol groups
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J21/00—Catalysts comprising the elements, oxides, or hydroxides of magnesium, boron, aluminium, carbon, silicon, titanium, zirconium, or hafnium
    • B01J21/06—Silicon, titanium, zirconium or hafnium; Oxides or hydroxides thereof
    • B01J21/063—Titanium; Oxides or hydroxides thereof

Definitions

  • the present invention relates to a catalyst and a method of making a catalyst.
  • the deposition-precipitation process may be complicated and difficult to optimise.
  • a further technique used to make catalysts is the impregnation method.
  • the metal particle sizes are large even at low metal loading and this may be disadvantageous.
  • catalysts provided by this method may comprise large amounts of chlorine, which are known to poison catalysis for many reactions (e.g. CO oxidation). Furthermore, the presence of chlorine can cause aggregation during calcination of the catalyst. This method may also lead to the formation of metal particles with a broad distribution of particle size, which may sometimes by disadvantageous.
  • a further method used to make metal-containing catalysts is the sol immobilisation technique.
  • Metal catalysts made using sol techniques are known to those skilled in the art ("Au-Pd supported nanocrystals prepared by a sol immobilisation technique as catalysts for selective chemical synthesis",
  • the method comprising the steps of: (i) Providing a solution comprising metal cations;
  • the method of the present invention provides a method of at least partially removing a protecting agent from a catalyst without one or more of the potential drawbacks of calcination.
  • the catalyst provided by the method of the present invention may be effective in catalysing gas and/or liquid phase
  • the solvent removes the protecting agent by dissolving the protecting agent.
  • the method of the present invention involves reduction in the presence of a protecting agent and then immobilisation on a support. In some instances there could be some protecting agent located between metal particles and the support (and/or between adjacent metal particles) which is not accessible to the solvent used in step (iv) .
  • the nanoparticles typically have a mean greatest dimension in a range of from 0.5nm to lOOnm, more typically in the range of from lnm to 50nm, optionally in the range of from lnm to 20nm and further optionally in the range of from 2nm to 7nm.
  • the nanoparticles may be approximately spherical and have a mean diameter of from 2nm to 7nm.
  • it is preferred for the nanoparticles to be relatively small, for example, having a mean diameter in a range of from lnm to 5nm.
  • the size of the nanoparticles may, for example, be measured using scanning transmission electron microscopy.
  • the protecting agent prevents uncontrolled coalescence of the metal, therefore helping to control and maintain particle size. In the absence of a protecting agent, coalescence will generally occur, with particles being much larger than desired.
  • protecting agent depends on the substrate to be used, since it is generally desirable to use a protecting agent which binds strongly to the substrate.
  • a suitable protecting agent will typically be soluble in a solvent which is one or more of inexpensive, readily-available and of low toxicity.
  • PVA polyvinyl alcohol
  • the protecting agent may typically comprise one or more- polymers, such as water soluble polymers.
  • the one or more polymers may be one or more dendrimers.
  • Polyols such as polyvinyl alcohol
  • polyvinyl pyrrolidone alfe examples " of suitable protecting agents which are advantageous in that they are soluble in water.
  • the protecting agent may comprise non-polymeric species.
  • non-polymeric species for example, monomeric surfactants, phosphonium salts (such as
  • the solvent may typically be aqueous, and may comprise at least 98% water by weight.
  • the solvent may comprise one or more alcohols (such as methanol, ethanol, propanol, butanol and pentanol) .
  • the solvent may comprise a mixture of water and one or more alcohols.
  • the solvent may be provided by a solution.
  • the solvent need not be aqueous.
  • the solvent may be an organic solvent.
  • the solvent may comprise one or more ether (such as diethyl ether), alkane (including cycloalkanes) , alkene (including cycloalkenes, such as cyclohexene and •benzene) and amine (such as triethyl amine) .
  • the choice of solvent will depend on the choice of protecting agent; the protecting agent has to be soluble in the solvent. Furthermore, the choice of solvent will typically determine the effective temperature at which step (iv) may be performed, since the boiling point of the solvent will typically be the highest temperature at which step (iv) is performed.
  • the metal ions may typically comprise ions of one or more transition metal, and more typically ions of one or more of gold, palladium, platinum, bismuth, copper, nickel, lead and cobalt.
  • the identity of the metal ions depends on the
  • the metal ions may preferably comprise ions of one or more of gold, palladium and platinum, more preferably one or both of gold and palladium. Metals derived from these ions are known to have excellent catalytic activity in relation to a large number of reactions.
  • the solution provided in step (i) may comprise a promoter or a pre-cursor to a promoter which forms a promoter in the
  • the promoter may comprise a metal.
  • the promoter may typically increase one or more of activity, selectivity or resistance to deactivation of a catalyst compared to a
  • the promoter may function by affecting the electronic properties of the catalyst metal (for example, by alloying to the catalyst metal, the alloy being electronically more favourable for catalysis than the catalyst metal without the promoter) and/or by affecting the morphology of the catalyst.
  • the promoter may comprise one or more of platinum, rhodium, ruthenium, bismuth, lead, caesium, copper, cobalt, indium, tin and antimony. If the catalyst metal comprises gold, then the promoter may typically comprise bismuth, tin, lead, indium or antimony. If the catalyst metal comprises palladium, the promoter may typically comprise lead or bismuth.
  • the substrate may comprise one or more of silica, carbon, titania (titanium dioxide), alumina and silicon.
  • the method of the present invention may be performed in a batch process (for example, by using some form of reflux apparatus) . Alternatively, the method may be performed in a continuous process.
  • the method of the present invention preferably comprises isolating the catalyst produced in step (iv) . This may
  • the catalyst produced in step (iv) may be removed from the suspension by filtration and then dried, for example, by heating to 100 0 C or so.
  • the drying process should not take place in conditions which are sufficiently aggressive to cause a noticeable change to the morphology and/or catalytic
  • Step (ii) typically comprises bringing a reducing agent into intimate admixture with the metal cations.
  • the choice of reducing agent will depend on the metal cation to be reduced, but the reducing agent may typically comprise a hydride reducing agent, H 2 , ferrous compounds or sodium amalgam.
  • Suitable hydride reducing agents include borohydrides, such as Group 1 metal borohydrides, e.g. NaBH 4 , and aluminium hydrides, such as Group 1 metal aluminium hydrides, e.g. LiAlH 4 , or alkylaluminium hydrides, e.g. diisobutylaluminium hydride, with NaBH 4 being a preferred reducing agent.
  • the temperature of the solvent in step (iv) may be in the range from 30 0 C to 130 0 C, optionally from 70 0 C to 130 0 C, typically from 85°C to 120 0 C and optionally from 9O 0 C to
  • the maximum temperature of the solvent used in step (iv) will typically be governed by the boiling point of the solvent. For example, if the solvent has a low boiling point (such as diethyl ether), the temperature of the solvent in step (iv) will be low. For diethyl ether, the temperature may typically range from 25 0 C to 35 0 C. If the solvent is a high boiling point solvent, then the temperature of the solvent in step (iv) is likely to be high. For example, the temperature of water used in step (iv) may typically be in the range from 70 0 C to 90 0 C.
  • the temperature of the solvent in step (iv) it is desirable for the temperature of the solvent in step (iv) not to exceed 150 0 C. It is anticipated that high solvent temperatures may have an adverse effect on the_morphology and/or catalytic activity of the metal
  • Steps (i) and (ii) may comprise providing a solution of cations of a first metal and cations of a second metal and reducing said cations to a first metal and a second metal in the presence of the protecting agent.
  • steps (i) and (ii) may comprise providing a solution of cations of a first metal and reducing said cations to a first metal in the presence of a protecting agent, and subsequently adding a solution of cations of a second metal and reducing said cations to a second metal in the presence of a protecting agent.
  • the concentration of the cations provided in step (i) may typically be in the range of from 0.005M to 0.1M.
  • step (iii) proceeds after steps (i) and (ii) .
  • the sol of metal nanoparticles is dispersed in the solvent following step (ii) .
  • the immobilising agent is added to the reaction mixture after the sol of metal nanoparticles has been formed in step (ii) .
  • the method may comprise, after step (iii) and before step (iv) , isolating the solid-supported metal nanoparticles.
  • Isolating the solid-supported metal nanoparticles may comprise one or more of filtration, drying and centrifugation.
  • Drying may typically be performed by one or more of: (a) Heating; (b) use of a dessicant; and (c) use of reduced pressure .
  • Centrifugation is typically followed by removal of liquid, for example, by decanting liquid from the solid-supported metal nanoparticles•
  • a catalyst producible by a method in
  • the catalyst of the second aspect of the present invention may have the features described above with reference to the method of the first aspect of the present invention.
  • a catalyst comprising metal nanoparticles immobilised on a substrate, the nanoparticles having a mean greatest dimension of from lnm to 20nm and the catalyst being capable of catalysing the gas phase oxidation of carbon- containing species and/or being capable of catalysing
  • the catalyst may be capable of catalysing the gas-phase oxidation of organic compounds (such as the oxidation of carbon monoxide, volatile organic compounds [VOCs] and the epoxidation of alkenes) .
  • organic compounds such as the oxidation of carbon monoxide, volatile organic compounds [VOCs] and the epoxidation of alkenes
  • the catalyst When performing the gas-phase oxidation of carbon monoxide, the catalyst preferably has an activity of at least 20Og of CO per gram of catalyst per hour, at 25°C, with 5000ppm CO in air, with a gas hourly space velocity of 3000 to 12000.
  • the nanoparticles have a greatest mean particle dimension of from Iran to 6nm and more preferably from 2nm to 5nm.
  • the catalyst is typically more active towards the oxidation of organic compounds than the Hopcalite compounds disclosed in "Copper manganese oxide catalysts for ambient temperature carbon monoxide oxidation: Effect of calcination on activity", -Hutch-ings- e-t al., Journal of Molecular Catalysis A; Chemical 305 (2009) 121-124.
  • the nanoparticles may be approximately spherical and have a mean diameter of from lnm to 6nm, preferably of from 2nm to 5nm.
  • the catalyst of the third aspect of the present invention may comprise those features described above with reference to the method of the first aspect of the present invention.
  • the nanoparticles comprise one or more transition metals, typically one or more of palladium, gold and platinum, for example, one or more of palladium and gold.
  • the catalyst of the third aspect of the present invention is typically isolated from a carrier fluid (such as a fluid which may have been used to make the catalyst of the third aspect of the present
  • the catalysts of the second and third aspects of the present invention may catalyse oxidation of glycerol.
  • a description of the oxidation of glycerol may be found in "Oxidation of glycerol using gold-palladium alloy-supported nanocrystals", Hutchings et al., Phys. Chem. Chem. Phys., 2009, vol. 11, pgs. 4952-4961.
  • Figure 1 shows the catalytic activity of various catalysts (including examples of catalysts made by a method in
  • the activity of the catalyst being defined by the mass of CO converted per g of catalyst used per hour;
  • Figure 2 shows the catalytic activity of various catalysts (including examples of catalysts made by a method in
  • Figure 3 shows the catalytic activity, as a function of calcination temperature, of a known catalyst towards the oxidation of carbon monoxide, the activity of the catalyst being defined by the % of CO converted;
  • Figures 4a and 4b show particle size distribution data for a gold-on-titania catalyst and a gold/palladium-on-titania catalyst respectively made by the method of the present invention .
  • aqueous solution of metal ions of the desired concentration was prepared.
  • PdCl 2 Johnson Matthey
  • HAuCl 4 -3H 2 O Johnson Matthey
  • a 0.1M solution of NaBH 4 (>9 ⁇ %, Aldrich) was freshly prepared.
  • the PVA solution was added to the solution of metal ions in the desired amount and the solution was stirred vigorously for 3 minutes.
  • the weight ratio of PVA to metal was typically about 1.2.
  • the PVA was used as a protecting agent to ensure that the metal did not coalesce to form large particles.
  • the desired amount of NaBH 4 solution was then added to form a metallic sol.
  • the molar ratio of NaBH 4 to total metal content was 5:1.
  • a change in colour was observed as the sol was formed; a dark-brown colour was observed whenever Au-Pd sols were formed and a red colour was observed when an Au sol formed.
  • the sol was immobilised by adding the chosen support (either activated carbon (G-60 carbon, Aldrich) or titania (Degussa P25) , acidified to pH 1 by sulfuric acid) under vigorous stirring conditions.
  • the amount of support material required was calculated so as to have a total final metal loading of lwt% .
  • the slurry so produced was stirred for two hours.
  • the flask was then submerged in an oil bath at 9O 0 C and kept under agitation using a magnetic stirrer for the desired time (typically 1 hour) .
  • the hot water at 90°C at least partially removed the PVA protecting agent from the surface of the gold by dissolving the PVA.
  • the suspension was then filtered in a Buchner flask and the catalyst washed with 2 L of distilled water before being covered with aluminium foil and placed in an oven at 110 0 C for 16 hours for removal of residual water. No leaching of metals from the catalysts was observed during this drying step.
  • Catalysts 1, 2 and 3 were based on Au only and were produced by subjecting the catalyst precursor (i.e. metal nanoparticles coated with PVA protecting agent) to 30, 60 and 120 minutes respectively in water at a temperature of approximately 90°C.
  • Catalysts 4 and 5 were based on a 1:1 molar ratio of Au and Pd, and were produced by subjecting the catalyst precursor (i.e. metal nanoparticles coated with PVA protecting agent) to 30 and 60 minutes respectively in water at a temperature of approximately 90 0 C.
  • Each of Catalysts 1, 2 and 3 were made using gold metal on a titania substrate, whereas each of catalysts 4 and 5 were made using gold and palladium metals on a titania substrate. Comparative Example 1
  • a catalyst was made using the General Method mentioned above, but omitting the step which at least partially removed the protecting agent.
  • the catalyst comprised lwt% gold on a titania substrate.
  • a catalyst was made using a sol-immobilisation method, but the protecting agent was partially removed by sintering at a temperature of 250°C in air for 3 hours, instead of being removed by contact with a heated solvent.
  • the catalyst comprised lwt% gold on a titania substrate.
  • A- catalyst was made using the General Method, mentioned above, but omitting the step which at least partially removed the protecting agent.
  • the catalyst comprised lwt% gold-palladium on a titania substrate.
  • the catalytic activity of Catalysts 1, 2 and 3 in relation to the oxidation of carbon monoxide was investigated.
  • the catalysts were tested for catalysis of CO oxidation using a fixed-bed microreactor .
  • An air flow rate of 19.1mL/min (with 5000 vppm CO) was passed over 35 mg of catalyst placed in the middle of the microreactor.
  • the glass reactor was submerged in a water bath maintained isothermally at 25 C C. Analysis of reactants and products was performed using on-line gas chromatography (VARIAN CP-3800) .
  • Figures 1 and 2 show the catalytic activity of the catalysts (Catalysts 1, 2 and 3) made using the method of the present invention in comparison to the catalysts of Comparative
  • Figure 1 shows, as a function of time, the mass of carbon monoxide oxidised per gram of catalyst per hour for each of Catalysts 1, 2 and 3, along with the catalysts of Comparative Examples 1 and 2 ( ⁇ - Comparative Example 1, ⁇
  • FIG. 1 shows, as a function of time, the % conversion of carbon monoxide for each of Catalysts 1, 2 and 3, along with the catalysts of Comparative Examples 1 (x - Comparative Example 1, ⁇ - Catalyst 1, ⁇ - Catalyst 2, A- Catalyst 3) .
  • the catalyst of Comparative Example 1 shows no (or very little) catalytic activity.
  • Catalyst 1 shows some catalytic activity, similar to that shown by the catalyst of Comparative Example 2, but a far lower catalytic activity than Catalysts 2 and 3, which appear to show approximately equivalent catalytic activity. This indicates that, in this case, more than 30 minutes (but less than 120 minutes)_ reflux in water at 90°C is required to remove the protecting agent.
  • Figure 3 shows the catalytic activity of various commercially- available hopcalite catalysts.
  • Figure 3 corresponds to Figure 1 of "Copper manganese oxide catalysts for ambient temperature carbon monoxide oxidation: Effect of calcination on activity", Hutchings et al., Journal of Molecular Catalysis A: Chemical 305 (2009) 121-124.
  • the data show the catalytic activity of hopcalite catalysts calcined at various temperatures as shown in the Figure, the activity being shown as % conversion of carbon monoxide as a function of time.
  • the data of Figure 2 for Catalysts 1 and 2 may be favourably compared with the data shown in Figure 3, indicating that the catalysts of the present invention (and made using the methods of the present invention) show some unexpected advantages.
  • Catalysts 4 and 5 comprising gold-palladium as catalytic metals were investigated in relation to the oxidation of benzyl alcohol.
  • the reaction was performed in a stirred reactor (10OmL, Autoclave Engineers Inline MagneDrive III). The vessel was charged with benzyl alcohol (4OmL) and catalyst (0.05g). The autoclave was then purged five times with oxygen, leaving the vessel at 10 bar gauge- (appr-ox. -150psi) . The stirrer -was set -to 1500 rpm and the reaction mixture was raised to the required temperature (in this case, 120 0 C) . Reaction timing started once the reaction temperature had been reached. Samples were taken periodically from the reactor via a sampling system. GC-MS and GC were used to analyse the products of the reaction. The products were identified by comparison with known standards and quantified by comparison with calibration samples.
  • Table 1 compares the catalytic efficiency of Catalysts 4 and 5 with the catalyst of Comparative Example 3. The data in Table 1 clearly indicate that Catalyst 5 gives a higher turnover frequency than the catalyst in which the protecting agent is not removed from the surface of the metal (Comparative Example 3) .
  • Catalyst 5 (comprising gold- palladium as catalytic metals) were investigated in relation to the oxidation of glycerol, firstly in glass reactors at 30 0 C and secondly in stainless steel reactors at 5 0 C. The method follows that of Hutchings et al., Physical Chemistry Chemical Physics, 2009, vol. 11, pages 4952-4961.
  • the catalytic reactions in the glass reactor were carried out using a 50 ml glass reactor.
  • the molar ratio of glycerol :metal was 1000:1).
  • the glass reactor was purged with oxygen five times and adjusted to the desired pressure of 3 bar. This pressure was maintained at a constant level throughout the experiment; hence as the oxygen was consumed in the reaction it was continuously replenished.
  • the reaction mixture was heated to the desired temperature (30 0C) and stirred for 4h. Samples were taken periodically from the reactor. In order to do this, the stirring was halted, the oxygen vented and a 0.5 ml sample was removed. After the sample was removed, the reactor was once again purged with oxygen 5 times and adjusted to the required pressure.
  • catalytic activity of Catalyst 5 towards glycerol can be compared to the catalytic activity of the analogous
  • the catalytic reactions in the stainless steel reactor were performed using a 50-mL Parr autoclave.
  • the autoclave was pressurised with oxygen (10 bar pressure) and the temperature adjusted to (and maintained at) 5°C.
  • the reaction mixture was stirred at 1500 rpm for 4h. Samples were extracted and analysed essentially as described above in relation to the experiments performed in the glass reactor. Table 4 shows the catalytic data for Catalyst 5 and for
  • Table 4 shows that Catalyst 5 demonstrates immediate catalytic activity not shown by the Comparative Example. The data above suggest that the presence of protecting agent may impede the activity of the catalyst.
  • nanoparticles was investigated using STEM-HAADF (scanning transmission electron microscopy - high angle annular dark field detector) . It was found that, prior to the removal of the protecting agent, average particle diameter was typically 4-6nm. Removing the protecting agent by using a solvent at elevated temperatures did not lead to a significant change in particle size. For example, the mean particle diameter of the gold nanoparticles on a titania support prior to the removal of the protecting agent was 4.6 nm. After the removal of the STEM-HAADF (scanning transmission electron microscopy - high angle annular dark field detector) . It was found that, prior to the removal of the protecting agent, average particle diameter was typically 4-6nm. Removing the protecting agent by using a solvent at elevated temperatures did not lead to a significant change in particle size. For example, the mean particle diameter of the gold nanoparticles on a titania support prior to the removal of the protecting agent was 4.6 nm. After the removal of the
  • the mean particle diameter was 5.2 nm, therefore showing a slight increase in particle size.
  • the mean particle diameter of the gold-palladium nanoparticles on a titania support prior to the removal of the protecting agent was 4.0 nm; after the removal of the protecting agent by treatment with a hot water solvent the mean particle diameter was 4.3 nm, therefore showing a slight increase in particle size .
  • Figures 4a and 4b shows particle size distribution data for the gold nanoparticles on a titania support and the gold- palladium nanoparticles on a titania support, respectively, after treatment with a hot water solvent.
  • the prior art method of removing the protecting agent by heating in air at 400 0 C caused a noticeable increase in size of the gold nanoparticles .
  • nanoparticles of gold-palladium on a titania support which were prepared by heating in air at 400 0 C had a mean particle size of 7. lnm, compared to a particle size preheating of 4nm.
  • Nanoparticles of gold-palladium on a carbon support which were prepared by heating in air at 400 0 C had a mean particle size of 36nm, compared to a particle size pre- heating of 5.4nm. It can therefore be demonstrated that the catalysts prepared using the method of the present invention provide small nanoparticles of metal which in certain
  • Treatment with the solvent provided an increase by a factor of about 2 of the surface ratio of gold: titania.

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Abstract

A method is provided for preparing a metal nanoparticle catalyst, the method comprising the steps of : (i) Providing a solution comprising metal cations; (ii) Reducing said metal cations in the presence of a protecting agent to form a sol of metal nanoparticles; (iii) Immobilising said metal nanoparticles on a solid support; and (iv) Treating the solid-supported metal nanoparticles with a solvent at a temperature above ambient temperature to at least partially remove the protecting agent. A catalyst made by this method is also disclosed.

Description

A catalyst and a method of making the same
The present invention relates to a catalyst and a method of making a catalyst.
There are many methods of making metal-containing catalysts. In deposition-precipitation the conditions used most often lead to incomplete transport of the metal to the support, with the degree of recovery (i.e. the actual metal loading
obtained) varying widely. The best specific activity of a catalyst is often observed when the recovery is far from complete. This is clearly not ideal for the scaling up of the procedure as required for industrial or environmental
applications due to the high cost of many of the metals used (for example, gold, palladium and platinum) . Furthermore, the deposition-precipitation process may be complicated and difficult to optimise.
A further technique used to make catalysts is the impregnation method. Typically the metal particle sizes are large even at low metal loading and this may be disadvantageous.
Furthermore, catalysts provided by this method may comprise large amounts of chlorine, which are known to poison catalysis for many reactions (e.g. CO oxidation). Furthermore, the presence of chlorine can cause aggregation during calcination of the catalyst. This method may also lead to the formation of metal particles with a broad distribution of particle size, which may sometimes by disadvantageous.
A further method used to make metal-containing catalysts is the sol immobilisation technique. Metal catalysts made using sol techniques are known to those skilled in the art ("Au-Pd supported nanocrystals prepared by a sol immobilisation technique as catalysts for selective chemical synthesis",
Hutchings et al . , Physical Chemistry Chemical Physics, 2008, vol. 10, pages 1921-1930, and Hutchings et al., Physical Chemistry Chemical Physics, 2009, vol. 11, pages 4952-4961.). Typically, metal ions are reduced to metal atoms in the presence of a protecting agent, which form nanoparticles of metal coated with a protecting agent. The coated particles are then brought into contact with a support and the protecting agent removed by heating in static air to approximately 200- 4000C for three hours or so (a process often referred to as "calcination") . The calcination process often causes a change in the morphology of the catalytic material and/or a change in the properties of the catalyst. Hagemeyer et al. (US6603038) propose an alternative method which involves immobilising a precursor of the metal on a support and then reducing the precursor to a metal. The _present invention s.eeks to mitigate againsjt one or more of the problems mentioned above.
There is, in accordance with a first aspect of the present invention, a method of preparing a metal nanoparticle
catalyst, the method comprising the steps of: (i) Providing a solution comprising metal cations;
(ii) .Reducing said metal cations in the presence of a
protecting agent to form a sol of metal
nanoparticles;
(iii) Immobilising said metal nanoparticles on a solid
support; and
(iv) Treating the solid-supported metal nanoparticles with a solvent at a temperature above ambient temperature to at least partially remove the
protecting agent. The method of the present invention provides a method of at least partially removing a protecting agent from a catalyst without one or more of the potential drawbacks of calcination. The catalyst provided by the method of the present invention may be effective in catalysing gas and/or liquid phase
reactions. It should be noted that the solvent removes the protecting agent by dissolving the protecting agent. Those skilled in the art will realise that not all of the protecting agent will necessarily be removed. The method of the present invention involves reduction in the presence of a protecting agent and then immobilisation on a support. In some instances there could be some protecting agent located between metal particles and the support (and/or between adjacent metal particles) which is not accessible to the solvent used in step (iv) .
The nanoparticles typically have a mean greatest dimension in a range of from 0.5nm to lOOnm, more typically in the range of from lnm to 50nm, optionally in the range of from lnm to 20nm and further optionally in the range of from 2nm to 7nm. The nanoparticles may be approximately spherical and have a mean diameter of from 2nm to 7nm. In certain circumstances (for example, in the production of catalysts for gas reactions), it is preferred for the nanoparticles to be relatively small, for example, having a mean diameter in a range of from lnm to 5nm. The size of the nanoparticles may, for example, be measured using scanning transmission electron microscopy.
Those skilled in the art will realise that a sol is a
colloidal suspension of solid particles.
The protecting agent prevents uncontrolled coalescence of the metal, therefore helping to control and maintain particle size. In the absence of a protecting agent, coalescence will generally occur, with particles being much larger than desired.
The choice of protecting agent depends on the substrate to be used, since it is generally desirable to use a protecting agent which binds strongly to the substrate. A suitable protecting agent will typically be soluble in a solvent which is one or more of inexpensive, readily-available and of low toxicity. For example, polyvinyl alcohol [PVA] has a strong affinity for titania and carbon which reduces the chance of the PVA being removed by subsequent processing steps and/or use of the catalyst.
The protecting agent may typically comprise one or more- polymers, such as water soluble polymers. The one or more polymers may be one or more dendrimers. Polyols (such as polyvinyl alcohol) and polyvinyl pyrrolidone alfe examples" of suitable protecting agents which are advantageous in that they are soluble in water.
Alternatively or additionally, the protecting agent may comprise non-polymeric species. For example, monomeric surfactants, phosphonium salts (such as
tetrakis (hydroxymethyl) phosphonium chloride), methoxyethanol, nitric acid and monoethanol amine are all examples of
materials which can stabilise sol formation.
The solvent may typically be aqueous, and may comprise at least 98% water by weight. The solvent may comprise one or more alcohols (such as methanol, ethanol, propanol, butanol and pentanol) . The solvent may comprise a mixture of water and one or more alcohols. The solvent may be provided by a solution. The solvent need not be aqueous. The solvent may be an organic solvent. The solvent may comprise one or more ether (such as diethyl ether), alkane (including cycloalkanes) , alkene (including cycloalkenes, such as cyclohexene and •benzene) and amine (such as triethyl amine) .
The choice of solvent will depend on the choice of protecting agent; the protecting agent has to be soluble in the solvent. Furthermore, the choice of solvent will typically determine the effective temperature at which step (iv) may be performed, since the boiling point of the solvent will typically be the highest temperature at which step (iv) is performed.
The metal ions may typically comprise ions of one or more transition metal, and more typically ions of one or more of gold, palladium, platinum, bismuth, copper, nickel, lead and cobalt. The identity of the metal ions depends on the
catalytic activity required. The metal ions may preferably comprise ions of one or more of gold, palladium and platinum, more preferably one or both of gold and palladium. Metals derived from these ions are known to have excellent catalytic activity in relation to a large number of reactions.
The solution provided in step (i) may comprise a promoter or a pre-cursor to a promoter which forms a promoter in the
catalyst. The promoter may comprise a metal. The promoter may typically increase one or more of activity, selectivity or resistance to deactivation of a catalyst compared to a
catalyst which does not comprise the promoter. The promoter may function by affecting the electronic properties of the catalyst metal (for example, by alloying to the catalyst metal, the alloy being electronically more favourable for catalysis than the catalyst metal without the promoter) and/or by affecting the morphology of the catalyst. The promoter may comprise one or more of platinum, rhodium, ruthenium, bismuth, lead, caesium, copper, cobalt, indium, tin and antimony. If the catalyst metal comprises gold, then the promoter may typically comprise bismuth, tin, lead, indium or antimony. If the catalyst metal comprises palladium, the promoter may typically comprise lead or bismuth.
The substrate may comprise one or more of silica, carbon, titania (titanium dioxide), alumina and silicon. The method of the present invention may be performed in a batch process (for example, by using some form of reflux apparatus) . Alternatively, the method may be performed in a continuous process.
The method of the present invention preferably comprises isolating the catalyst produced in step (iv) . This may
comprise one or both of filtration and drying. For example, the catalyst produced in step (iv) may be removed from the suspension by filtration and then dried, for example, by heating to 1000C or so. _ The drying process should not take place in conditions which are sufficiently aggressive to cause a noticeable change to the morphology and/or catalytic
activity of the metal nanoparticles. Drying may be performed by one or more of heating, use of a dessicant and reduction of pressure . Step (ii) typically comprises bringing a reducing agent into intimate admixture with the metal cations. The choice of reducing agent will depend on the metal cation to be reduced, but the reducing agent may typically comprise a hydride reducing agent, H2, ferrous compounds or sodium amalgam.
Suitable hydride reducing agents include borohydrides, such as Group 1 metal borohydrides, e.g. NaBH4, and aluminium hydrides, such as Group 1 metal aluminium hydrides, e.g. LiAlH4, or alkylaluminium hydrides, e.g. diisobutylaluminium hydride, with NaBH4 being a preferred reducing agent. The temperature of the solvent in step (iv) may be in the range from 300C to 1300C, optionally from 700C to 1300C, typically from 85°C to 1200C and optionally from 9O0C to
1050C. The maximum temperature of the solvent used in step (iv) will typically be governed by the boiling point of the solvent. For example, if the solvent has a low boiling point (such as diethyl ether), the temperature of the solvent in step (iv) will be low. For diethyl ether, the temperature may typically range from 250C to 350C. If the solvent is a high boiling point solvent, then the temperature of the solvent in step (iv) is likely to be high. For example, the temperature of water used in step (iv) may typically be in the range from 700C to 900C.
In any case, it is desirable for the temperature of the solvent in step (iv) not to exceed 1500C. It is anticipated that high solvent temperatures may have an adverse effect on the_morphology and/or catalytic activity of the metal
nanoparticles .
Steps (i) and (ii) may comprise providing a solution of cations of a first metal and cations of a second metal and reducing said cations to a first metal and a second metal in the presence of the protecting agent.
Alternatively, steps (i) and (ii) may comprise providing a solution of cations of a first metal and reducing said cations to a first metal in the presence of a protecting agent, and subsequently adding a solution of cations of a second metal and reducing said cations to a second metal in the presence of a protecting agent.
The concentration of the cations provided in step (i) may typically be in the range of from 0.005M to 0.1M.
For the avoidance of doubt, it should be noted that step (iii) proceeds after steps (i) and (ii) . Typically the sol of metal nanoparticles is dispersed in the solvent following step (ii) . Typically the immobilising agent is added to the reaction mixture after the sol of metal nanoparticles has been formed in step (ii) .
The method may comprise, after step (iii) and before step (iv) , isolating the solid-supported metal nanoparticles.
Isolating the solid-supported metal nanoparticles may comprise one or more of filtration, drying and centrifugation.
Drying (removal of liquid from the solid-supported metal nanoparticles) may typically be performed by one or more of: (a) Heating; (b) use of a dessicant; and (c) use of reduced pressure .
Centrifugation is typically followed by removal of liquid, for example, by decanting liquid from the solid-supported metal nanoparticles• In accordance with a second aspect of the present invention, there is provided a catalyst producible by a method in
accordance with a first aspect of the present invention.
The catalyst of the second aspect of the present invention may have the features described above with reference to the method of the first aspect of the present invention.
In accordance with a third aspect of the present invention, there is provided a catalyst comprising metal nanoparticles immobilised on a substrate, the nanoparticles having a mean greatest dimension of from lnm to 20nm and the catalyst being capable of catalysing the gas phase oxidation of carbon- containing species and/or being capable of catalysing
hydrogenation reactions.
The catalyst may be capable of catalysing the gas-phase oxidation of organic compounds (such as the oxidation of carbon monoxide, volatile organic compounds [VOCs] and the epoxidation of alkenes) .
When performing the gas-phase oxidation of carbon monoxide, the catalyst preferably has an activity of at least 20Og of CO per gram of catalyst per hour, at 25°C, with 5000ppm CO in air, with a gas hourly space velocity of 3000 to 12000.
It is preferred that the nanoparticles have a greatest mean particle dimension of from Iran to 6nm and more preferably from 2nm to 5nm. The catalyst is typically more active towards the oxidation of organic compounds than the Hopcalite compounds disclosed in "Copper manganese oxide catalysts for ambient temperature carbon monoxide oxidation: Effect of calcination on activity", -Hutch-ings- e-t al., Journal of Molecular Catalysis A; Chemical 305 (2009) 121-124.
The nanoparticles may be approximately spherical and have a mean diameter of from lnm to 6nm, preferably of from 2nm to 5nm.The catalyst of the third aspect of the present invention may comprise those features described above with reference to the method of the first aspect of the present invention. For example, it is preferred that the nanoparticles comprise one or more transition metals, typically one or more of palladium, gold and platinum, for example, one or more of palladium and gold. For the avoidance of doubt, the catalyst of the third aspect of the present invention is typically isolated from a carrier fluid (such as a fluid which may have been used to make the catalyst of the third aspect of the present
invention) .
The catalysts of the second and third aspects of the present invention may catalyse oxidation of glycerol. A description of the oxidation of glycerol may be found in "Oxidation of glycerol using gold-palladium alloy-supported nanocrystals", Hutchings et al., Phys. Chem. Chem. Phys., 2009, vol. 11, pgs. 4952-4961.
The present invention will now be described by way of example only with reference to the following Figures of which:
Figure 1 shows the catalytic activity of various catalysts (including examples of catalysts made by a method in
accordance with the present invention) towards the oxidation of carbon monoxide as a function of time, the activity of the catalyst being defined by the mass of CO converted per g of catalyst used per hour;
Figure 2 shows the catalytic activity of various catalysts (including examples of catalysts made by a method in
- accordance with the present invention) towards the oxidation of carbon monoxide as a function of time, the activity of the catalyst being defined by the % of CO converted;
Figure 3 shows the catalytic activity, as a function of calcination temperature, of a known catalyst towards the oxidation of carbon monoxide, the activity of the catalyst being defined by the % of CO converted; and
Figures 4a and 4b show particle size distribution data for a gold-on-titania catalyst and a gold/palladium-on-titania catalyst respectively made by the method of the present invention .
General Method
An aqueous solution of metal ions of the desired concentration was prepared. PdCl2 (Johnson Matthey) was used to make solutions of palladium ions and HAuCl4-3H2O (Johnson Matthey) was used to make solutions of gold ions. A solution of polyvinyl alcohol [PVA] (1 wt % solution, Aldrich, weight average molecular weight Mw = 9,000-10,000 g/mol, 80% hydrolysed) was prepared. A 0.1M solution of NaBH4 (>9β%, Aldrich) was freshly prepared.
The PVA solution was added to the solution of metal ions in the desired amount and the solution was stirred vigorously for 3 minutes. The weight ratio of PVA to metal was typically about 1.2. The PVA was used as a protecting agent to ensure that the metal did not coalesce to form large particles.
The desired amount of NaBH4 solution was then added to form a metallic sol. The molar ratio of NaBH4 to total metal content was 5:1. A change in colour was observed as the sol was formed; a dark-brown colour was observed whenever Au-Pd sols were formed and a red colour was observed when an Au sol formed. After 30 min of sol generation, the sol was immobilised by adding the chosen support (either activated carbon (G-60 carbon, Aldrich) or titania (Degussa P25) , acidified to pH 1 by sulfuric acid) under vigorous stirring conditions. The amount of support material required was calculated so as to have a total final metal loading of lwt% . The slurry so produced was stirred for two hours. It was found that two hours was sufficient for a great majority of the sol to be adsorbed onto the support, as was indicated by the decolouration of the liquid part of the slurry. After two hours of stirring, the slurry was filtered, the catalyst precursor (still with the PVA protecting agent) was washed thoroughly with distilled water and dried at 12O0C overnight. Washing with distilled water removes soluble contaminants such as sodium and chloride ions. Ig of the dried catalyst precursor (with the protecting agent still in place) was placed in a 10OmL round-bottom flask with 5OmL of distilled water and connected to a condenser. The flask was then submerged in an oil bath at 9O0C and kept under agitation using a magnetic stirrer for the desired time (typically 1 hour) . The hot water (at 90°C) at least partially removed the PVA protecting agent from the surface of the gold by dissolving the PVA. The suspension was then filtered in a Buchner flask and the catalyst washed with 2 L of distilled water before being covered with aluminium foil and placed in an oven at 1100C for 16 hours for removal of residual water. No leaching of metals from the catalysts was observed during this drying step.
Examples of catalysts
Various examples of catalysts in accordance with the present invention were made in accordance with the General Method mentioned above. Catalysts 1, 2 and 3 were based on Au only and were produced by subjecting the catalyst precursor (i.e. metal nanoparticles coated with PVA protecting agent) to 30, 60 and 120 minutes respectively in water at a temperature of approximately 90°C. Catalysts 4 and 5 were based on a 1:1 molar ratio of Au and Pd, and were produced by subjecting the catalyst precursor (i.e. metal nanoparticles coated with PVA protecting agent) to 30 and 60 minutes respectively in water at a temperature of approximately 90 0C. Each of Catalysts 1, 2 and 3 were made using gold metal on a titania substrate, whereas each of catalysts 4 and 5 were made using gold and palladium metals on a titania substrate. Comparative Example 1
A catalyst was made using the General Method mentioned above, but omitting the step which at least partially removed the protecting agent. The catalyst comprised lwt% gold on a titania substrate.
Comparative Example 2
A catalyst was made using a sol-immobilisation method, but the protecting agent was partially removed by sintering at a temperature of 250°C in air for 3 hours, instead of being removed by contact with a heated solvent. The catalyst comprised lwt% gold on a titania substrate.
Comparative Example 3
A- catalyst was made using the General Method, mentioned above, but omitting the step which at least partially removed the protecting agent. The catalyst comprised lwt% gold-palladium on a titania substrate.
The catalytic activity of Catalysts 1, 2 and 3 in relation to the oxidation of carbon monoxide was investigated. The catalysts were tested for catalysis of CO oxidation using a fixed-bed microreactor . An air flow rate of 19.1mL/min (with 5000 vppm CO) was passed over 35 mg of catalyst placed in the middle of the microreactor. The glass reactor was submerged in a water bath maintained isothermally at 25 CC. Analysis of reactants and products was performed using on-line gas chromatography (VARIAN CP-3800) .
Figures 1 and 2 show the catalytic activity of the catalysts (Catalysts 1, 2 and 3) made using the method of the present invention in comparison to the catalysts of Comparative
Examples 1 and 2. Figure 1 shows, as a function of time, the mass of carbon monoxide oxidised per gram of catalyst per hour for each of Catalysts 1, 2 and 3, along with the catalysts of Comparative Examples 1 and 2 (♦ - Comparative Example 1, ■
[large square] - Comparative Example 2, A- Catalyst 1, •
[small square] - Catalyst 2, • [circle] - Catalyst 3) . Figure 2 shows, as a function of time, the % conversion of carbon monoxide for each of Catalysts 1, 2 and 3, along with the catalysts of Comparative Examples 1 (x - Comparative Example 1, ♦ - Catalyst 1, ■ - Catalyst 2, A- Catalyst 3) . The catalyst of Comparative Example 1 shows no (or very little) catalytic activity. Catalyst 1 shows some catalytic activity, similar to that shown by the catalyst of Comparative Example 2, but a far lower catalytic activity than Catalysts 2 and 3, which appear to show approximately equivalent catalytic activity. This indicates that, in this case, more than 30 minutes (but less than 120 minutes)_ reflux in water at 90°C is required to remove the protecting agent. These data
demonstrate that removal of the protecting agent using a solvent produces a catalyst which is far more effective at catalysing the oxidation of carbon monoxide than a catalyst prepared using conventional sintering techniques to remove the protecting agents. It is expected that this beneficial effect will be observed in relation to the catalysis of other gaseous reactions, such as the oxidation of organic compounds (such as the oxidation of alkanes and alkenes, oxidation of volatile organic compounds [VOCs] and epoxidation of alkenes [such as propene] ) .
Figure 3 shows the catalytic activity of various commercially- available hopcalite catalysts. Figure 3 corresponds to Figure 1 of "Copper manganese oxide catalysts for ambient temperature carbon monoxide oxidation: Effect of calcination on activity", Hutchings et al., Journal of Molecular Catalysis A: Chemical 305 (2009) 121-124. The data show the catalytic activity of hopcalite catalysts calcined at various temperatures as shown in the Figure, the activity being shown as % conversion of carbon monoxide as a function of time. The data of Figure 2 for Catalysts 1 and 2 may be favourably compared with the data shown in Figure 3, indicating that the catalysts of the present invention (and made using the methods of the present invention) show some unexpected advantages.
The catalytic properties of Catalysts 4 and 5 (comprising gold-palladium as catalytic metals) were investigated in relation to the oxidation of benzyl alcohol. The reaction was performed in a stirred reactor (10OmL, Autoclave Engineers Inline MagneDrive III). The vessel was charged with benzyl alcohol (4OmL) and catalyst (0.05g). The autoclave was then purged five times with oxygen, leaving the vessel at 10 bar gauge- (appr-ox. -150psi) . The stirrer -was set -to 1500 rpm and the reaction mixture was raised to the required temperature (in this case, 1200C) . Reaction timing started once the reaction temperature had been reached. Samples were taken periodically from the reactor via a sampling system. GC-MS and GC were used to analyse the products of the reaction. The products were identified by comparison with known standards and quantified by comparison with calibration samples.
Table 1 compares the catalytic efficiency of Catalysts 4 and 5 with the catalyst of Comparative Example 3. The data in Table 1 clearly indicate that Catalyst 5 gives a higher turnover frequency than the catalyst in which the protecting agent is not removed from the surface of the metal (Comparative Example 3) .
Furthermore, whilst the turnover frequency for Catalyst 4 is comparable to the catalyst in which the protecting agent is not removed from the surface of the metal, the selectivity of Catalyst 4 is generally higher.
Figure imgf000017_0001
Table 1 The catalytic properties of Catalyst 5 (comprising gold- palladium as catalytic metals) were investigated in relation to the oxidation of glycerol, firstly in glass reactors at 300C and secondly in stainless steel reactors at 50C. The method follows that of Hutchings et al., Physical Chemistry Chemical Physics, 2009, vol. 11, pages 4952-4961.
The catalytic reactions in the glass reactor were carried out using a 50 ml glass reactor. The glycerol solution (0.3 M) (glycerol from Aldrich) and NaOH (molar ratio of NaOH: glycerol ratio = 2:1) were added into the reactor with the catalyst. The molar ratio of glycerol :metal was 1000:1). The glass reactor was purged with oxygen five times and adjusted to the desired pressure of 3 bar. This pressure was maintained at a constant level throughout the experiment; hence as the oxygen was consumed in the reaction it was continuously replenished. The reaction mixture was heated to the desired temperature (30 0C) and stirred for 4h. Samples were taken periodically from the reactor. In order to do this, the stirring was halted, the oxygen vented and a 0.5 ml sample was removed. After the sample was removed, the reactor was once again purged with oxygen 5 times and adjusted to the required pressure.
Each 0.5ml samples was diluted to 5 ml using the eluent and analysed using HPLC equipped with ultraviolet and refractive index detectors. Reactants and products were separated using a Metacarb 67H column eluted with 0.01M aqueous HsPO4 with a flow rate of 0.3 ml/min. Products were identified by comparison with standard samples. Quantification of the reactants consumed and products generated were performed by reference to an external calibration method.
The catalytic activity of Catalyst 5 towards glycerol can be compared to the catalytic activity of the analogous
comparative catalyst from which the protecting agent had not been removed (Comparative Example 3) . Table 2 shows the catalytic data for Catalyst 5 and Table 3 shows the catalytic data for Comparative Example 3.
Figure imgf000018_0001
Table 2
Figure imgf000019_0001
Table 3
It is clear from the data of Tables 2 and 3 that the catalyst of the present invention (Catalyst 5) performs better than the ■catai-yst—of the Comparative- Example (in wlrrch none of the protective agent has been removed from the catalyst prior to being used as a catalyst) . The data above suggest that the presence of protecting agent may impede the activity of the catalyst.
The catalytic reactions in the stainless steel reactor were performed using a 50-mL Parr autoclave. A glycerol solution (0.6 M) and NaOH (molar ratio of NaOH:glycerol ratio = 2:1) was added into the reactor and the desired amount of catalyst (molar ratio of glycerol :metal ratio = 500:1-2000:1) was suspended in the solution. The autoclave was pressurised with oxygen (10 bar pressure) and the temperature adjusted to (and maintained at) 5°C. The reaction mixture was stirred at 1500 rpm for 4h. Samples were extracted and analysed essentially as described above in relation to the experiments performed in the glass reactor. Table 4 shows the catalytic data for Catalyst 5 and for
Comparative Example 3 in the steel reactor. selectivity /%
time in Catalyst conv glyceric oxalic tartronic glycolic formic hours /% acid acid acid acid acid
Comparative
0.5 example 3 0
Comparative
4 example 3 38.4 73.6 0.6 9.0 12.1 4.7
0.5 Catalyst 5 • 9.0 n.d. n.d. n.d. n.d. n.d.
4 Catalyst 5 42.1 70.4 0.4 13.9 10.2 5.0
Table 4
In Table 4, "n.d." indicates that the value was not
determined.
Table 4 shows that Catalyst 5 demonstrates immediate catalytic activity not shown by the Comparative Example. The data above suggest that the presence of protecting agent may impede the activity of the catalyst.
Effect of processing conditions on size of nanoparticles
The effect of processing conditions on the size of
nanoparticles was investigated using STEM-HAADF (scanning transmission electron microscopy - high angle annular dark field detector) . It was found that, prior to the removal of the protecting agent, average particle diameter was typically 4-6nm. Removing the protecting agent by using a solvent at elevated temperatures did not lead to a significant change in particle size. For example, the mean particle diameter of the gold nanoparticles on a titania support prior to the removal of the protecting agent was 4.6 nm. After the removal of the
protecting agent by treatment with a hot water solvent, the mean particle diameter was 5.2 nm, therefore showing a slight increase in particle size.
The mean particle diameter of the gold-palladium nanoparticles on a titania support prior to the removal of the protecting agent was 4.0 nm; after the removal of the protecting agent by treatment with a hot water solvent the mean particle diameter was 4.3 nm, therefore showing a slight increase in particle size .
Figures 4a and 4b shows particle size distribution data for the gold nanoparticles on a titania support and the gold- palladium nanoparticles on a titania support, respectively, after treatment with a hot water solvent.
It is noticeable that there is not a significant increase in particle diameter after removal of the protecting agent using a hot water solvent. Furthermore, the catalytic activity of the catalyst thus produced is noticeably better than the catalytic activity of the supported gold nanoparticles covered in protecting agent. It is worth noting that the particular examples of the method of the present invention used in the manufacture of the catalysts analysed in Figures 4a and 4b use two drying steps (one prior to treatment with the hot water solvent and one post-treatment with solvent). It is expected that it would be more usual to omit the drying step prior to treatment with the solvent to remove the protecting agent and it is expected that the omission of one drying step may lead to less of an increase in particle size. The prior art method of removing the protecting agent by heating in air at 4000C (the process of calcination) caused a noticeable increase in size of the gold nanoparticles . For example, nanoparticles of gold-palladium on a titania support which were prepared by heating in air at 4000C had a mean particle size of 7. lnm, compared to a particle size preheating of 4nm. Nanoparticles of gold-palladium on a carbon support which were prepared by heating in air at 4000C had a mean particle size of 36nm, compared to a particle size pre- heating of 5.4nm. It can therefore be demonstrated that the catalysts prepared using the method of the present invention provide small nanoparticles of metal which in certain
circumstances are catalytically superior to catalysts prepared using conventional techniques. Gold _p.articl_es on a titania support (1 wt_% Au/TiO2) were examined using a variety of techniques to study the removal of the PVA from the particles. Total carbon analysis indicated that about 20-30% of the carbon present had been removed by the treatment with solvent, whereas gold analysis showed that no gold was lost from the sample as a result of the treatment with the solvent. Laser Raman spectroscopy demonstrated that PVA is removed by the solvent treatment. X-ray photoelectron spectroscopy was used to measure the surface ratio of
gold: titania before and after treatment with the solvent.
Treatment with the solvent provided an increase by a factor of about 2 of the surface ratio of gold: titania.
Where, in the foregoing description, integers or elements are mentioned which have known, obvious or foreseeable
equivalents, then such equivalents are herein incorporated as if individually set forth. Reference should be made to the claims for determining the true scope of the present
invention, which should be construed so as to encompass any such equivalents. It will also be appreciated by the reader that integers or features of the invention that are described as preferable, advantageous, convenient or the like are optional and do not limit the scope of the independent claims.

Claims

01614 23 Claims
1. A method of preparing a metal nanoparticle catalyst, the method comprising the steps of: (i) Providing a solution comprising metal cations;
(ii) Reducing said metal cations in the presence of a
protecting agent to form a sol of metal
nanoparticles;
(iii) Immobilising said metal nanoparticles on a solid
support; and
(iv) Treating the solid-supported metal nanoparticles
with a solvent at a temperature above ambient temperature to at least partially remove the protecting agent.
2. A method according to claim 1 wherein the protecting
agent comprises one or more polymers.
3. A method according to claim 2, wherein the protecting
agent comprises one or more water soluble polymers.
4. A method according to any one preceding claim, wherein the protecting agent comprises non-polymeric species.
5. A method according to claim 4, wherein the non-polymeric species comprises monomeric surfactants, phosphonium salts, methoxyethanol, nitric acid and monoethanol amine,
6. A method according to any one preceding claim, wherein the solvent is aqueous.
7. A method according to any one preceding claim, wherein the solvent comprises one or more alcohols, preferably one or more polyols.
8. A method according to any one preceding claim, wherein the solvent comprises one or more ether, alkane, alkene, and amine.
9. A method according to any one preceding claim, wherein the metal ions comprise ions of one or more transition metal.
10. A method according to claim 9 wherein the metal ions comprise ions of one or more of gold, palladium and platinum.
11. A method according to any one preceding claim
wherein the substrate comprises one or more of silica, carbon, titania, alumina and silicon.
12. A method according to any one preceding claim, the method comprising isolating the catalyst produced in step
(iv) .
13. A method according to claim 12 wherein isolating the catalyst produced in step (iv) comprises one or both of filtration and drying.
14. A method according to any one preceding claim,
wherein step (ii) comprises bringing a reducing agent into intimate admixture with the metal cations.
15. A method according to claim 14 wherein the reducing agent comprises a hydride reducing agent, H2, ferrous compounds or sodium amalgam.
16. A method according to any one preceding claim, wherein the temperature of the solvent in step (iv) is in the range from 30°C to 1300C.
17. A method according to claim 16, wherein the
temperature of the solvent is step (iv) is in the range of from 85°C to 120°C.
18. A method according to claim 17, wherein the
temperature of the solvent is step (iv) is in the range of from 900C to 1050C.
19. A method according to any one preceding claim,
wherein steps (i) and (ii) comprise providing a solution of cations of a first metal and cations of a second metal and reducing said cations to a first metal and a second metal in the presence_ of the. protecting agent.
20. A method according to any one of claims 1 to 18, wherein steps (i) and (ii) comprise providing a solution of cations of a first metal and reducing said cations to a first metal in the presence of a protecting agent, and subsequently adding a solution of cations of a second metal and reducing said cations to a second metal in the presence of a protecting agent.
21. A method according to any one preceding claim,
wherein the concentration of the cations is in the range of from 0.005M to 0.1M.
22. A method according to any one preceding claim
wherein the method comprises, after step (iii) and before step (iv) , isolating the solid-supported metal
nanoparticles .
23. A method according to claim 22 wherein isolating the solid-supported metal nanoparticles comprises one or more of filtration, drying and centrifligation .
24. A catalyst producible by the method of any one
preceding claim.
25. A catalyst comprising metal nanoparticles
immobilised on a substrate, the nanoparticles having a mean greatest dimension of from lnm to 20nm, and the catalyst being capable of catalysing gas phase oxidation of carbon-containing species and/or being capable of catalysing hydrogenation reactions.
26. A catalyst according to claim 25 wherein the
catalyst has an activity of at least 20Og of CO per gram
-of catalyst per hour, at 25°C, with 5000ppm CO in air, with a gas hourly space velocity of 3000-12000.
27. A catalyst according to claim 25 or claim 26,
wherein the nanoparticles having a mean diameter of from lnm to 6nm.
28. A catalyst according to claim 27, wherein the
nanoparticles having a mean diameter of from 2nm to 5nm.
29. A catalyst according to any one of claims 24 to 28, wherein the nanoparticles comprise one or more of palladium, gold and platinum.
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