6074
J. Phys. Chem. C 2009, 113, 6074–6087
Noble Metal Core-Ceria Shell Catalysts For Water-Gas Shift Reaction Connie Mei Yu Yeung† and Shik Chi Tsang* Wolfson Catalysis Centre, Inorganic Chemistry Laboratory, UniVersity of Oxford, Oxford, OX1 3QR U.K. ReceiVed: October 19, 2008; ReVised Manuscript ReceiVed: February 7, 2009
The metal-support interface is known to play a key role in many catalysis reactions. Using the microemulsion technique we report the preparation of a nanosized noble metal particle with an intimate contact with cerium oxide. The materials display better catalytic performance in the water-gas shift (WGS) reaction over the hydrogen-rich reformate-steam mixture without methane (side product) formation as compared with catalysts prepared by traditional methods. It is evident that the catalysts prepared by the microemulsion method show an unusual morphology and unique catalytic properties with the noble metal nanoparticle being embedded in a thin layer of cerium oxide, giving a maximum metal-support interface of three-dimensional interactions. In light of results obtained from material characterization as well as reaction rate orders and switching experiment studies over the catalysts, a discussion on possible reaction mechanism(s) taking place at the interface of this new type of catalyst is included in this paper. Introduction The below equation, known as the “water-gas shift” (WGS) reaction, is an important industrial process concerned with largescale hydrogen production for many chemical applications1,2
CO(g) + H2O(g) h CO2(g) + H2(g)
(1)
∆H°298 ) -41.2 kJ mol-1, ∆G°298 ) -28.6 kJ mol-1 The forecast demand for hydrogen is expected to increase in coming years with increases in its use as a fuel. This is because our energy systems will need to be renewable and sustainable, efficient and cost effective, convenient, and safe. Thus, hydrogen gas has been proposed as the perfect fuel for this future energy. Much of the increase in hydrogen use is expected in transportation and electricity generation applications. In particular, there is an increasing demand for a small-scale hydrogen production at low temperature, LT, for mobile and on-site stationary applications. Fuel cells are the primary technology that will advance such hydrogen uses. Typically, a proton-exchange membrane (PEM) fuel cell is one choice as it is capable of generating clean and efficient power for stationary and mobile source applications. It promises to deliver electricity with virtually no emission of harmful pollutants and with increased efficiency because it directly converts chemical energy to electricity, avoiding the thermodynamic mechanical cycle losses associated with combustion in conventional power generation. Ceria, CeO2, has been identified as one potentially important catalyst component for the LT hydrogen production from WGS.3 Pure ceria is well known to store and release oxygen and hydrogen, via forming surface and bulk vacancies or intermetallic M-Ce compounds. Ceria can also serve as a stabilizer to metal and alumina supports, maintaining a high dispersion of * To whom correspondence should be addressed. Phone: 44(0)1865282610. Fax: 44(0) 1865272659. E-mail:
[email protected]. † Present address: Johnson Matthey Catalyst, Belasis Avenue, Billingham, Cleveland TS23 1LB, U.K.
the catalytic metals.4-6 In addition, ceria can be used as a promoter in combination with other elements to give mixedoxide formulations. Cerium-oxide-containing WGS formulations have therefore attracted considerable interest from catalyst manufacturers.7 As a result, metal-ceria WGS catalysts have been developed as potentially better alternatives to the pyrophoric Cu-ZnO. For example, Li reported that copper or nickel on Ce(La)Ox catalysts prepared by urea precipitation-gelation displayed good LT shift activity at high space velocities when tested under low CO concentrations (2%).1 They attributed the high activity of their catalyst to the enhanced reducibility of ceria in the presence of the metal. Ruettinger developed a base metal nonpyrophoric particulate catalyst with a very promising catalytic behavior with respect to the requirements of fuel cell applications.8 This catalyst only showed a very slight temperature increase of 40 °C without any deactivation if it was (accidentally) exposed to air. Lost activity due to liquid water exposure was shown to be regenerated in situ or ex situ. Perhaps one of the most significant earlier results came from Swartz, who reported that Pt/CeO2 catalyst was nonpyrophoric but showed higher activity than Cu-based catalysts at a high space velocity.9 Recently, gold particles supported on CeO2 have been reported to be highly active for the WGS, with their improved activity at low temperature being explained as due to the synergism of the gold-metal oxide. 10,11 Mechanism elucidation of the WGS reaction may provide hints on how the noble metal-ceria interface works, which may lead to better catalyst formulation. However, there is still no agreement on the mechanism of the WGS reaction over these noble metal-ceria catalysts. There are at least two schools of thought, namely, a regenerative (redox) mechanism and an associative mechanism to explain the high activity of WGS using metal-promoted ceria catalysts.12-17 The regenerative mechanism involves successive oxidation and reduction of the surface (eqs 2 and 3). In this redox mechanism water dissociates completely to Oa and Ha and Oa is then titrated by carbon monoxide. The elementary reactions are shown as follows
10.1021/jp809247g CCC: $40.75 2009 American Chemical Society Published on Web 03/20/2009
Noble Metal Core-Ceria Shell Catalysts
O-cat + CO(a) f CO2 + []-cat
J. Phys. Chem. C, Vol. 113, No. 15, 2009 6075
(2)
H2O + []-cat f H2 + O-cat (rate-detemining step)
(3)
On the other hand, the associative mechanism involves reaction through adsorbed surface intermediates, such as a formate. Decomposition of formate then results in hydrogen and carbon dioxide products. In the formate mechanism, water dissociates to form OHa, which then reacts with carbon monoxide to produce HCOOa. This mechanism was summarized by Shido and Iwasawa who studied mechanism of the WGS reaction over CeO2 and Rh/CeO2.18 This postulation has been backed by the studies from Jacobs and co-workers.19-21 However, Meunier et al. have recently shown that the surface formate species observed by IR were essentially spectators during the catalysis.22 Surface carbonate has also been proposed as the main intermediate specie over reverse-WGS reaction which is thought to share the same mechanistic pathways as WGS under microscopic equilibrium.23,24 Very recently, Burch has pointed out that the WGS mechanism is strongly dependent on the choice of experimental conditions.25 According to his analysis, the redox mechanism has been proposed to be important at higher temperatures with the implication that the associative mechanism is important at low temperatures with further possibility that surface carbonates or carboxylates could be important intermediates involved in the rate determining step. Apart from the difficulties in mechanism elucidation there are also challenges in catalyst selectivity for stationary applications. In order to achieve a high carbon monoxide conversion in a commercial WGS reactor, a high steam to carbon monoxide ratio in the feed is usually established but the energy efficiency (energy is required to process surplus steam) is very low. If the WGS is primarily used for fuel processing at small scale, this is expected to be a major problem. Thus, a low steam to carbon monoxide ratio, R, would be more desirable in a fuel processor. However, reformate contains carbon monoxide, carbon dioxide, steam and hydrogen and many possible reactions (Reactions 4-10, see below) may take place as the side reactions other than the WGS reaction. These side reactions will become very significant especially when low R is used.2 For example, carbon and methane may be produced, lowering the overall yield of hydrogen production for the WGS reaction. The carbon deposition might also cause difficulties in subsequent steps in the process.
2CO h C + CO2
(4)
CO + H2 h C + H2O
(5)
CO2 + 2H2 h C + 2H2O
(6)
2CO + 2H2 h CO2 + CH4
(7)
CO + 3H2 h CH4 + H2O
(8)
CO2 + 4H2 h CH4 + 2H2O
(9)
C + 2H2 h CH4
(10)
To allow the WGS reaction to be operated at low R ratios without suffering the thermodynamic constraint, it has been
proposed to use H2-separation membrane technology for smallscale hydrogen production in some stationary sources. Thus, the hydrogen produced by the reaction is continuously removed from the reaction stream, which can also reduce the potential driving force for methane formation. The use of a H2-separation membrane would also make it possible for the WGS reaction to be carried out in one step, provided that a suitable catalyst is used.2 However, researchers in this area reported that carbon formation would block the catalyst bed inducing catalyst deactivation, increase the pressure drop across the catalyst bed, plugging and fouling equipment, and eventually leading to a run down of the process. If the H2-separation membrane is involved, the coke formed could also block the pores of the membrane, hence lowering its permeability. It would therefore be better to use WGS catalysts which show ultraselectivity to the WGS reaction with a high kinetic barrier to methane and carbon formation. However, all conventionally prepared metal/ ceria catalysts studied seem to be susceptible to methane and coke formation at low R ratios. Thus, an effective commercial catalyst based on metal/ceria has remained elusive. In order to provide a step change in this area of catalyst design (rather than screening more metal doped ceria), the intention of investigation in this work was to adopt the nanoscience/ nanotechnology approach to synthesize, characterize, and test new WGS nanocatalysts based on metal-ceria. We reported earlier that a metal nanoparticle of controlled size in a thin silica coating can be synthesized by the microemulsion technique.26,27 It was believed that this approach could lead to the custom designing of new metal-ceria-supported catalysts with desirable architecture by covering the nanometal with an ultrathin ceria oxide in order to maximize the local metal-support effect to gain the maximum activity/selectivity. Thus, the new approaches adopted in this research included the following. (a) Developing a new method for synthesizing well-defined nanosized metal/ alloy particles in solution. (b) Constructing a rigid inorganicbased molecular scaffold on the individual metal nanoparticle by creating a ceria overlayer in order to build up a required ‘local’ metal support interaction. It was anticipated that the oxide overlayer would greatly enhance the stability of the enclosed metal nanoparticle by reducing the propensity of their agglomeration. (c) Controlling the structure of the oxide overlayer (thickness and composition) in order to optimize the metal-support interactions. In this work, a microemulsion method for the synthesis of a metal nanoparticle of defined size in a ceria shell of controlled dimension with an intimate metal-ceria interface was employed. A preliminary account of our research on the synthesis and characterization has been published.28 This paper gives the comprehensive information on the detailed preparation, testing, and characterization of this new type of core-shell nanocatalyst with discussion of a possible mechanism(s) taking place at the interface. Experimental Section Catalyst Synthesis. Microemulsion-Prepared Pt/CeO2 Catalysts. The typical procedure for preparing 5 wt % Pt/ceria catalysts by the microemulsion (MEs) technique is as follows. CTAB was added into dry toluene with vigorous stirring. A water to surfactant ratio, W, of 30 was employed in this synthesis. A suspension of CTAB in toluene was formed immediately. Then, the Pt precursor salt solution with ceria was prepared by dissolving an appropriate amount of (NH4)2PtCl6 into the DI water. The aqueous solution of Pt precursor salt was then added dropwise to the suspension of CTAB in toluene
6076 J. Phys. Chem. C, Vol. 113, No. 15, 2009 and was stirred overnight. A solution of 0.22 g of sodium hydroxide predissolved in 1.630 mL of DI water was added into the reaction mixture and stirred for 2 h before adding a solution of 0.6060 g of cerium(III) nitrate hexahydrate. The reaction mixture was aged for 6 days with constant stirring. After the aging step, the reaction mixture was centrifuged for 20 min at 1000 rev min-1 in order to collect the product. The product was then washed with EtOH at least four times to remove surfactants. The solution was repeatedly centrifuged after each washing. The solid product obtained was dried overnight in air. The catalysts were then pretreated with the reactant gas mixture (8% CO, 10% CO2, 1% CH4, 32.5% H2, balancing with N2) at 400 °C before catalytic testing. Coprecipitation-Prepared Pt/CeO2 Catalyst. Two weight percent Pt/ceria catalysts synthesized by the coprecipitation method were prepared as follows. A 0.1541 g amout of ammonium tetrachloroplatinate(II), (NH4)2PtCl4, was dissolved in a 100 mL aqueous solution of 0.2 M cerium(III) nitrate hexahydrate, Ce(NO3)3 · 6H2O, and sprayed into a 250 mL ammonia solution under constant stirring. The precipitate was allowed to age at room temperature with stirring for another 2 h. Then, it was collected by centrifugation at 1000 rev min-1 and washed twice with water and once with ethanol to remove any remaining ammonia and reaction byproduct. The solid was dried at 60 °C in a vacuum oven for 2 h. Then, it was dried in a flowing stream of nitrogen at 100 mL min-1 at a temperature ramp of 2 °C min-1 from room temperature to 350 °C and then held for further 5 h. After the drying procedure, it was prereduced with 50 mL min-1 hydrogen at temperature programming of 2 °C min-1 up to 250 °C and held for further 3 h. Wet-Impregnation-Prepared Pt/CeO2 Catalyst. Five weight percent Pt/ceria catalysts synthesized by wet impregnation were prepared as follows. Cerium(III) nitrate hexahydrate was calcined in static air with temperature programming (25 °C min-1) from room temperature to 600 °C and held at 600 °C for a further 10 h. Ammonium tetrachloroplatinate(II) or hexachloroplatinate(IV) was dissolved in DI water. The Ptprecursor solution was then wet impregnated onto the calcined ceria. The product was dried under air with temperature programming (5 °C min-1) from room temperature to 100 °C and held at 100 °C for a further 10 h. The catalyst was then calcined in a flowing stream of nitrogen at 30 mL min-1 with temperature programming (20 °C min-1) from room temperature to 500 °C and held for a further 2 h. Catalytic Testing. The reaction conditions and testing procedures toward the WGS reaction are given as follows: The reactor used for this work was a 0.3 m long quartz tube with an internal diameter of 4 mm mounted vertically in a temperaturecontrollable furnace. The catalyst (normally 50 mg used) was packed and sandwiched between two glass wool plugs in the middle of the reactor. The feed gases were then allowed to pass directly through the catalyst bed of the reactor. The furnace temperature was controlled by a temperature controller, and the range of temperatures studied in this work ranged from 75 to 500 °C. Liquid water was delivered from a HPLC pump to a hot zone heated at above 120 °C in order to convert it to become a flow of steam before mixing with other incoming gases. All pipe lines connecting the HPLC pump to the reactor were wrapped with a heating tape in which the temperature was set at above 120 °C to avoid any condensation of the flow of steam. The final gas compositions used as reactant gases were 0.77% CH4, 6.15% CO, 7.68% CO2, 24.99% H2, 23.08% H2O, and balanced with N2. The total gas flow rate was set at 90 mL min-1 with GHSV of about 108 000 h-1. It is noted that two
Yeung and Tsang
Figure 1. Plot of the catalytic WGS activities (CO fraction conversion) versus reactor temperature over Pt/ceria samples prepared by different methods as compared to Cu/ZnO/Al2O3 and theoretical equilibrium data. The small offsets between the thermodynamic line and the experimental CO fraction conversion lines at high temperatures are attributed to experimental errors in temperature and product measurements. Reactant gas mixture (0.77% CH4, 6.15% CO, 7.68% CO2, 24.99% H2, 23.08% H2O, and balancing with N2): GHSV ) 108 000 h-1.
condensers kept in an ice/water bath were installed between the gas exit of the reactor and the inlet of a GC in order to trap any remaining water from the product mixture; otherwise, the separation efficiency of the GC column would have been impaired. Before catalytic testing all microemulsion (except goldcontaining catalysts) and wet impregnation prepared catalysts were pretreated as follows: The catalyst was prereduced with 70 mL min-1 pure hydrogen with a temperature ramping from room temperature to 400 at 10 °C min-1 and held at 400 °C for a further 30 min. After that, it was allowed to cool to room temperature under the same flow of hydrogen. The catalyst was then preconditioned at 300 °C under a flowing stream of reactant gases (reformate) overnight before commencing the catalyst testing. The catalyst was handled without air exposure between the reduction and preconditioning stages in order to avoid any chance of reoxidation of the noble metal. Coprecipitation catalysts and the commercial low-temperature WGS catalyst, Cu/ZnO/Al2O3, were preconditioned under the same feeds used for the catalytic testing. After the pretreatment, catalysts were tested over a temperature range of 120-500 °C. During testing, at least 30 min was given to the catalyst to reach steady state at a particular temperature and flow rate before the measurement of catalytic activity was taken. Three batches of product gas samples were analyzed for each temperature in order to ensure the data were reproducible. The methane content in the product gas was also recorded during catalytic testing in order to assess the degree of methane formation over a particular catalyst. The product gas mixtures were analyzed by an online GC using a PERKIN ELMER Auto System XL equipped with a methanator and flame ionization detectors. Catalyst Characterization. Transmission electron microscopy (TEM), X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction (TPR), DRIFT analysis, CO chemisorption, and UV-vis diffuse reflectance measurements were employed for the characterization of these core-shell catalysts.
Noble Metal Core-Ceria Shell Catalysts
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TABLE 1: Comparison of WGS and Methanation Activities of Various Catalysts catalyst
WGS activityc
CH4 formationd
a
55.4 53.2 58.6 62.5 70.6
0.03 1.51 13.60 0.00 0.00
Cu/ZnO/Al2O3 wet-impregnated 5% Pt/ceriaa coprecipitated 2% Pt/ceriaa MEs 5% Pt/ceriab MEs 5% Pt, 5% Au/ceriab
dispersione (d f) n.d. 20.3(4.4) 14.2(6.4) 0.8(58.4) 1.8(50.4)
a Catalysts prepared by traditional methods. b Using the microemulsion method, each metal particle is embedded in a thin overlayer of ceria. WGS activity expressed as a percent of CO conversion using 0.77% CH4, 6.15% CO, 7.68% CO2, 24.99% H2, and 23.08% H2O balanced with N2 at GHSV of 108 000 h-1 at 400 °C. d Percent of methane formation with respect to the input 0.77% CH4 at 500 °C. e Percent metal dispersion measured by CO chemisorptions. f Theoretical metal diameter, d, based on CO chemisorptions (notice that XRD shows no metal peaks, where small metal clusters
