Comparison of Cu–Ce–Zr and Cu–Zn–Al mixed oxide catalysts for water-gas shift Florian Huber, Hilde Meland, Magnus Rønning, Hilde Venvik*, and Anders Holmen Department of Chemical Engineering, Norwegian University of Science and Technology (NTNU), Trondheim, N-7491 Norway Cu–Zn–Al and Cu–Ce–Zr mixed oxide catalysts were prepared by two different methods, co-precipitation and flame spray pyrolysis. The performance of the catalysts was evaluated using the water-gas shift reaction with and without CO 2 and H 2 added to the feed. Cu–Ce–Zr catalysts are found not to be superior to Cu–Zn–Al catalysts in terms of initial activity and short-term stability. Their apparent activation energy appears to be less affected by increased concentrations of CO 2 and H 2 . KEY WORDS: Cu–Zn–Al; Cu–Ce–Zr; mixed metal oxides; water-gas shift. 1. Introduction The water-gas shift (WGS) reaction is one of the oldest catalytic processes employed in the chemical industry. There is renewed interest in this reaction because of its relevance for producing hydrogen for use in fuel-cell systems as well as its key role in automotive exhaust processes, since the hydrogen produced is an efficient reductant for NO x removal [1]. Improved WGS catalysts with high activity at relatively low tempera- tures and better stability than commercial Cu–Zn–Al catalysts are needed. Ceria is at present one of the most investigated metal oxides, especially in connection with its oxygen storage capacity (OSC) applied in three-way catalyst systems [2]. CeO 2 is reported to improve the stability of classical Cu–Zn–Al formulations [3], as is ZrO 2 [4]. In addition, the application of Ce–Zr mixed oxides has become attractive [5], since CeO 2 doped with Zr shows improved OSC and better stability [2]. In the present study, Cu–Zn–Al and Cu–Ce–Zr mixed metal oxide (MMO) catalysts were prepared both by co-precip- itation and flame spray pyrolysis, and investigated under WGS conditions. The aim of the study was to compare classical Cu–Zn–Al formulations to novel Cu–Ce–Zr MMO catalysts from different preparation procedures at various reaction conditions. 2. Experimental Four MMO catalysts were prepared. CuZn-CP was prepared by co-precipitation from nitrate salts with (NH 4 ) 2 CO 3 in aqueous solution at ambient temperature according to a patent of Norsk Hydro, dried at 90 °C and calcined at 350 °C (3 °C/min, 1 h) [6]. CuCe-UCP was prepared by homogeneous co-precipitation from nitrate precursors with urea in a ethylene glycol–water mixture at 95 °C, dried at 100 °C and calcined at 250 °C (2 °C/min, 30 min) [7]. CuZn-FSP and CuCe-FSP were prepared by flame spray pyrolysis (FSP) of organo– metallic salts dissolved in toluene [8]. The MMO cata- lysts were characterised by Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES), X-ray dif- fraction (XRD) and N 2 adsorption–desorption (BET). For more details on preparation and characterization of the catalysts we refer to [7, 8] for the co-precipitated and flame spray pyrolysed catalysts, respectively. The Cu dispersion of Cu–Zn–Al catalysts has previously been determined by N 2 O titration (e.g. 8 % for CuZn-CP) [3, 9]. However, N 2 O titration was not applied in the present study for Cu–Ce based catalysts, since the con- tributions from ceria may obscure the results [10]. The WGS activity was studied at atmospheric pres- sure applying well-known empirical evaluation criteria for heat mass transfer limitations [11]. The catalysts were placed in an externally heated tubular fixed-bed reactor set up with on-line GC analysis and pre-reduced in H 2 /N 2 . More details on the reduction and WGS testing of CuZn-CP/CuCe-UCP and CuZn-FSP/CuCe- FSP are given in [7, 8], respectively. Three different feed compositions were applied; CuZn-CP and CuCe-UCP were investigated under a simple WGS reactant mixture (25/125/350 NmL/min CO/H 2 O/N 2 , catalyst amount: 0.10 g) and a simulated reformate product mixture (25/ 125/60/175/115 NmL/min CO/H 2 O/CO 2 /H 2 /N 2 , 0.20 g). CuZn-FSP and CuCe-FSP were studied under a simple WGS reactant mixture (50/100/50 NmL/min CO/H 2 O/ N 2 , 0.10 g). The selectivity was practically 100% in all measurements, with trace CH 4 detected (at random) in some analyses and deviations in the carbon balance within 1%. Activation energies (E a ) were determined by the integral method assuming irreversible reaction, first order in CO concentration, constant concentration of H 2 O and plug-flow conditions for the simple WGS * To whom correspondence should be addressed. E-mail: Hilde.Venvik@chemeng.ntnu.no Topics in Catalysis Vol. 45, Nos. 1–4, August 2007 (Ó 2007) 101 DOI: 10.1007/s11244-007-0247-2 1022-5528/07/0800-0101/0 Ó 2007 Springer Science+Business Media, LLC