WO2005072871A1

WO2005072871A1 - Catalyst

Info

Publication number: WO2005072871A1
Application number: PCT/GB2004/004812
Authority: WO
Inventors: Mei Yu Yeung; Shik Chi Tsang
Original assignee: Johnson Matthey PLC
Current assignee: Johnson Matthey PLC
Priority date: 2004-02-02
Filing date: 2004-11-16
Publication date: 2005-08-11
Anticipated expiration: 2006-08-02
Also published as: GB0402104D0

Abstract

A water gas shift catalyst comprising metal particles and a metal oxide material is disclosed. The metal particles comprise at least one precious metal and the metal oxide material comprises at least one reducible metal oxide. Substantially all of the metal particles are encapsulated by the metal oxide material such that the catalyst has substantially no activity for methanation. The loading of the metal particles is between 0.5-25wt% based on the weight of the metal oxide material. A process for preparing the catalyst is also disclosed.

Description

CATALYST The present invention relates to catalysts comprising precious metal particles and reducible metaϊ oxide materials. The catalysts are active for the water gas shift reaction. The invention further relates to processes for preparing the catalysts and the use of the catalysts in the water gas shift reaction. The water gas shift (WGS) reaction converts water and carbon monoxide into hydrogen and carbon dioxide: H₂O + CO → H₂ + C0₂ The reaction is commonly used to remove carbon monoxide from gas streams, e.g. hydrogen-rich gas streams produced by the reforming of hydrocarbon fuels. The reaction is catalysed by heterogeneous catalysts, which are usually classified as high, temperature WGS catalysts (capable of catalysing the reaction at 400-450°C) and low temperature WGS catalysts (capable of catalysing the reaction at 200-400°C). State-of-the-art WGS catalysts include iron/chromium/copper oxide (for high temperature WGS) and copper oxide/zinc oxide (for low temperature WGS).

WGS catalysts comprising precious metals dispersed on ceria have recently been developed, see e.g. Fu et al (Science, 2003 (301), 935-938). One problem with known precious metal WGS catalysts is that the methanation reaction competes with the WGS reaction:

The methanation reaction consumes hydrogen, so is especially undesirable when the WGS process is being used to remove carbon monoxide in a fuel processing system for hydrogen generation. The present inventors have sought to provide catalysts that are highly active for the WGS reaction but have reduced activity (or totally eliminated activity) for methanation.

Accordingly, the present invention provides a water gas shift catalyst comprising metal particles comprising at least one precious metal, and a metal oxide material comprising at least one reducible metal oxide. wherein substantially all of the metal particles are encapsulated by the metal oxide material such that the catalyst has substantially no activity for methanation, and wherein the loading of the metal particles is between 0.5-25 wl% based on the weight of the metal oxide material.

By the term "encapsulated" we mean that the metal particles are not at the surface of the catalyst and are therefore not accessible to gas molecules during the catalytic process. By the term "substantially all of the metal particles" we mean at least 95% of the metal particles, preferably at least 99% of the metal particles. Most preferably all of the metal particles are encapsulated by the metal oxide material. Preferably the catalyst has no activity for methanation.

It is a well-established theory that the catalytic sites for WGS are located at the interface of metal particles and a metal oxide, so it is surprising that a catalyst wherein substantially all the metal particles are encapsulated has high activity for the WGS reaction, Hardacre et al, J. Phys. Chem, 1994, 98, 10901-10905 disclose a model catalyst consisting of a Pt (111) single crystal encapsulated with ceria and show that the encapsulated Pt is active for CO oxidation. Hardacre et al suggest that in practical catalysts, partial rather than complete encapsulation of the Pt may be preferable. The present inventors have prepared catalysts by practical methods and have found that substantially complete encapsulation of metal particles provides advantageous WGS catalysts, Catalysts wherein at least some of the precious metals particles are encapsulated in reducible metal oxides are known from EP 602 865. However, the present inventors have found that WGS catalysts produced by co-precipitation methods similar to those disclosed in EP 602 865 are active for methanation. Microemulsion processes used by the present inventors ensure that substantially all of the metal particles are encapsulated and that the catalyst is substantially not active for methanation. A microemulsion is a dispersion of two immiscible liquids, generally termed "water^5* and "oil^"', which is stabilised by surfactants. Figure 1 shows a "water in oil'^* microemulsion. Surfactants (1) hold water within the oil. Adjacent to the surfactants (1 ) is a region of rigidly held water (2). In the centre there is a region of mobile water (3).

The metal particles in the WGS catalyst suitably comprise one or more of Au or Pt and preferably contain only Au and or Pt.

The loading of the metal particles is between 0.5-25 t% based on the weight of the metal oxide material, preferably between 2~15 t%. WGS activity is reduced at low loadings, possibly because small metal particles are embedded deep down within the metal oxide overlayers. Conversely high metal loading not only reduces the contact interface between the metal particles and the metal oxide but may also disturb the embedment (encapsulation) process.

The average diameter of the metal particles is suitably in the range 2-50nm, preferably 2-3 Onm. Smaller particles are advantageous because they provide better interaction between the metal and the metal oxide material for the same amount of metal. However, very small metal particles (diameter less than 2nm) can exhibit strange non- metallic properties so are not preferred. The at least one reducible metal oxide is suitably chosen from ceria, titania, iron oxide, manganese oxide or tin oxide and is preferably ceria. The metal oxide material may further comprise metal oxides that are not reducible, such as zirconia, alumina or lanthana. The catalysts of the invention can be prepared by a microemulsion method using a

"water in oil" microemulsion. Accordingly the present invention further provides a process for preparing a water gas shift catalyst comprising steps of: a) preparing a microemulsion comprising an organic solvent, a surfactant and water, wherein a precious metal precursor and a reducible metal oxide precursor are dissolved in the water; and b) adding a hydrolysis agent to the microemulsion and aging the microemulsion, thereby encapsulating precious metal particles in a layer of metal oxide material. In prior art catalyst preparation methods such as those disclosed in EP 602 865, precious metal particles and reducible metal oxide are co-precipitated in an aqueous solution. In an alternative method, reducible metal oxide is precipitated in the presence of a precious metal colloid, again in aqueous solution. In the microemulsion method used in tire present invention, precipitation of the precious metal and the reducible metal oxide will take place within aqueous regions within the microemulsion. Precipitation within the microemulsion provides a different catalyst compared to precipitation within an aqueous solution as disclosed in EP 602 865, The present inventors have found that the microemulsion method can be used to provide a catalyst wherein substantially all of the precious metal is encapsulated by the metal oxide. These catalysts are active for the WGS reaction and exhibit substantially no methanation activity.

The organic solvent is suitably a non-polar søh ent such as toluene, cyclohexane or a C₅-C₁₅ alkane, and is preferably toluene. The surfactant may be an ionic surfactant or a non-Ionic surfactant, and may be fluorinated. Suitable surfactants include CTAB

(cetyltrimcthylammonium bromide). AOT (sodium bis (2-ethyIhexyl) suifb-succinate),

CPC (cetylpyridinram chloride) and Triton ® X-Ϊ00. A preferred surfactant is CTAB. The organic solvent, surfactant and water must be in a suitable ratio for forming a microemulsion wherein the water is held within the organic solvent. Suitable ratios for forming a microemulsion will be known to the skilled person. The molar ratio of water: surfactant is suitably between 2-100:1 and is preferably about 30: 1. This ratio dictates the size of the regions of water within the microemulsion, so will affect the formation of the catalyst particles.

Suitable precious metal precursors will be well known to the skilled person and include NHAP Clβ and HAuCl₄.3H₂0, The concentration of the precious metal precursor in the aqueous regions of the microemulsion can be varied and this will affect the size of the precious metal particles. Suitable reducible metal oxide precursors will also be well known to the skilled person and include Ce(N0₃)₃,6H₂θ. The concentration of the reducible metal oxide precursor in the aqueous regions of the microemulsion can be varied, and this will affect the ratio of reducible metal oxide: precious metal (usually expressed as the loading of the precious metal).

The microemulsion may be prepared by mixing the surfactant with the organic solvent, and then adding aqueous solutions of the precious metal precursor and reducible metal oxide precursor. The precious metal precursor and reducible metal oxide precursor can be present in a single aqueous solution that is added to the organic solvent and surfactant. However, the present inventors have found that it is preferable to add the reducible metal oxide precursor after the addition of the precious metal precursor. Precious metal encapsulation is improved and methanation activity of the catalysts is reduced.

The hydrolysis agent is suitably sodium hydroxide or ammonia. The microemulsion is suitably stirred during the aging process. The aging process may take several days, e.g. up to six days may be required to achieve encapsulation of the precious metal particles by the reducible metal oxide. The skilled person can sample the reaction mixture throughout the aging process and measure the WGS activity of the sample or take microscopy images to see if substantial encapsulation has occurred.

The microemulsion can be filtered and dried to provide the catalyst. Optionally, the catalyst may be calcined at a temperature of about 400°C before use.

The invention further provides the use of a catalyst according to the invention in a water gas shift reaction. Hydrogen and carbon monoxide are suitably supplied to the catalyst and thereby converted to water and carbon dioxide. The catalyst may be used to remove carbon monoxide from a hydrogen stream that v,ill be supplied to a fuel cell. The invention further provides a catalyst according to the invention deposited on a catalyst substrate such as a monolith, which may be metallic or ceramic, or a heat exchanger substrate. The invention will now be described by reference to examples that are not intended to be limiting of the invention:

Example 1: Encapsulated 5wt% Pt/ Ceria 8.0915 g of cetyltrimethylammonium bromide (CTAB) w^ras added into 300.0 ml of dry toluene with vigorous stirring overnight. A suspension of CTAB in toluene was formed. The Pt precursor salt solution was prepared by dissolving 0.1930 g of ammonium letraehloroplatinate (II). ( AhPtCU, into 4.0 ml of deionised water. The aqueous solution of Pt precursor salt was added dropwise to the suspension of CTAB in toluene and was stirred overnight. A solution of 1 ,9869 g of sodium hydroxide dissolved in 4.0 ml of deionised water was added dropwise into the reaction mixture and stirred for another 2 hours. A ceria precursor solution was then prepared by dissolving 4.8420 g of cerium (III) nitrate hexahydrate, Ce(NG₃)₃ 6H₂O, salt into 4.0 ml deionised water. The ceria precursor solution was added dropwise into the reaction mixture. Then, the reaction mixture was aged for 6 days with stirring. After the ageing step, the reaction mixture was centrifuged for 20 minutes (1000 revs/min) to collect the solid product. The solid product was then washed four times with hot ethanol to remove the excess surfactants in the solution, The solution was centrifuged after each washing. The final product obtained was then dried in air overnight.

Example 2t Encapsulated 5wt% Pt/ 5wt% An/ Ceria

The catalyst was prepared as for example 1 except that 12.9844 g of CTAB was added into 300.0 ml of dry toluene and instead of the Pt precursor solution, a Pt-Au precursor salt solution was prepared by dissolving 0.2117 g of ammonium tetrachloroplatinate (II), (NH )₂PtCl₄. and 0.2266 g of hydrogen tetrachloroaurate (HI), HAuCl₄ 3H₂O, into 5.4 ml of deionised water. The sodium hydroxide solution contained 1.S570 g of sodium hydroxide in 4.6 ml of deionised water and the ceria precursor solution contained 4.5230 g of cerium (III) nitrate hexahydrate salt, Ce(N0₃)₃ 6H₂0, into 2.8 ml deionised water.

Example 3: Encapsulated lwt% Pt Ceria The catalyst was prepared as for example 1 except that the amounts of Pt precursor salt solution and ceria precursor solution were varied to give a loading of 1 wt% Pt.

Example 4; Encapsulated 10wt% Pt Ceria The catalyst was prepared as for example 1 except that the amounts of Pt precursor salt solution and ceria precursor solution were varied to give a loading of 10wt% Pt

Comparative Example 1 : Co-precipitated 5wt% Pt/ Ceria

0.1541 g ammonium tetracliloroplatinate (II),

was dissolved in a 100.0 ml aqueous solution of 0.2 M cerium (III) nitrate hexahydrate. Ce(NOj)₃ 6H₂0, and sprayed into a 250.0 ml ammonia solution under constant stirring. The precipitate in ammonia solution was allowed to age at room temperature with stirring for another two hours. The precipitate was collected by centrifugation (1000 revs/min) and washed twice with water and once with hot ethanol to remove any remaining ammonia and reaction byproduct. The solid was dried in a vacuum oven at 333 for 2 hours. Then, it was dried in a flow of nitrogen at 100 ml/ in as the temperature increased at a rate of 2 K/min up to 623 K where it remained for 5 hours. After the drying procedure, the solid was pre- reduced with 50ml/mϊn hydrogen at a temperature ramp (2 K/min) up to 523 K where it remained for 3 hours.

Comparative Example 2; Impregnated 5wt% Pt Ceria Pure ceria was synthesised by a micro-emulsion method. 8.1056 g of cetyltrimethylammonium bromide (CTAB) was added into 300.0 ml of dry toluene with vigorous stirring. A suspension of CTAB in toluene was formed immediately. A ceria precursor solution was prepared by dissolving 5.3480 g of cerium (III) nitrate hexahydrate. Ce(N0₃)₃ 6H₂0, salt into 6.0 ml deionised water. The ceria precursor solution was added dropwise to the suspension of CTAB in toluene and was stirred overnight. A solution of 2.2082 g of sodium hydroxide dissolved in 6.0 ml of deionised water was added dropwise into the reaction mixture. The reaction mixture was allowed to age for 6 days with stirring. After the ageing step, the reaction mixture was centrifuged for 20 minutes (1000 revs/min) to collect the solid product. The product collected was then washed four times with hot ethanol to remove the excess surfactants in the solution. The solution was centrifuged after each washing. The product obtained was dried in air for overnight. The sample was calcined under air with temperature increasing (2 K/min) up to 523 where it remained for 2 hours.

Pt was impregnated onto the ceria. A Pt precursor salt solution was prepared by dissolving 0.0483 g of ammonium tetrachloroplatinate (II), (NH s tCLt, in 0,85 ml deionised water. The Pt precursor solution was added onto 0.4769 g calcined ceria. The reaction mixture was dried in air with temperature increasing (5 K/min) up to 373 K where It remained for 1 hours. Then, the product was calcined in air with temperature increasing (20 K/min) up to 773 K for 2 hours.

Comparative Example 3; Encapsulated 27.11wt% Pt Ceria The catalyst was prepared as for example 1 except that the amounts of Pt precursor salt solution and ceria precursor solution were varied to give a loading of 27.11wt% Pt.

Catalytic Activity The reactor was in the form of a quartz tube inside a temperature programmable furnace with an internal diameter of 4 mm. The catalyst powder (normally 50 mg used) was then packed and sandwiched between two glass wool plugs in this quartz reactor tube and was located in the middle of the furnace zone. The feed gases were passed directly into the quartz reactor. The gas composition used was 0.77 % CH₄, 6.15 % CO, 7.68 % COa, 24.99 % IHk 23.08 % H₂O balance N₂. The total gas flow rate was set at 90 ml/mϊn with the GHSV = 108,000 h^"1. The reactor temperatures were raised between 120 °C and 500 °C and catalytic performances were evaluated at 150, 200, 250, 300, 350, 400, 450, 500°C intervals. All the incoming and outgoing lines were pre-heated at 120 °C. The catalyst was first placed in a stream of 70 ml/min pure H₂ with temperature programming from room temperature to 400 °C (10 °C/min) and was held at this temperature for 30 minutes. Then, it was cooled to room temperature under the H₂, The catalyst was then pre-conditioned at 300 °C with tlie reactant gas mixture overnight before the proceeding to the catalytic activity evaluation. The catalyst was then allowed to cool to 150°C with the same feed gases and studied the catalytic results at each temperature (from 150 to 500°C) for 30 minutes before tlie next measurement. The product gases from the reactor were analysed by a GC equipped with a methanator and a flame ionizatlon detector (FID). Three samples of product gases were taken at each temperature point to ensure the results were reproducible.

The WGS activity and methanation activity of the catalysts of examples 1-2 and comparative examples 1-2 are shown in the following table:

Tl e catalysts of the invention show high WGS activity, but are not active for methanation. By contrast, catalysts wherein precious metal particles are present on tlie surface of metal oxide material are active for both WGS and methanation. Figure 2 shows the WGS activity of the catalysts of examples 1, 3 and 4 and comparative example 3. The result shows that the best WGS activity is observed for the catalysts with 5 t% or 10wt% of platinum metal loading. For the catalysts of example 1 , 3 and 4 (5wt%, lwt% and 10 t% platinum metal loaded ceria catalysts), no methane production was detected during the whole temperature range ( 120 °C to 500 °C). For the catalyst of comparative example 3 (27.11 wt% platinum metal loaded ceria catalyst), about 5% methane production compared with the marker methane was detected.

Claims

1. A water gas shift catalyst comprising metal particles comprising at least one precious metal, and a metal oxide material comprising at least one reducible metal oxide, wherein substantially all of the metal particles are encapsulated by the metal oxide material such thai the catalyst has substantially no activity for methanation, and wherein Hie loading of the metal particles is between 0.5-25wt% based on the weight of the metal oxide material, to

2. A water gas shift catalyst according to claim 1 , wherein the metal particles contain only Au and/or Pt,

3. A water gas shift catalyst according to claim 1 or claim 2, wherein the average ] 5 diameter of tlie metal particles is in tlie range 2-50nm.

4. A water gas shift catalyst according to any preceding claim, wherein tlie at least one reducible metal oxide is chosen from ceria, titania, iron oxide, manganese oxide or tin oxide.

20 5. A process for preparing a water gas shift catalyst comprising steps of: a) preparing a microemulsion comprising an organic solvent, a surfactant and water, wherein a precious metal precursor and a reducible metal oxide precursor are dissolved in the water; and 25 b) adding a hydrolysis agent to tlie microemulsion and aging the microemulsion. thereby encapsulating precious metal particles in a layer of metal oxide material.

6. The use of a catalyst according to any one of claims 1 to 4 in a water gas shift process.

_> 0 7. A catalyst according to any one of claims 1 to 4 deposited on a catalyst substrate.