Kinetics and Mechanism of the Low-Temperature Water-Gas-Shift Reaction on Au/CeO2 Catalysts

Gold 2003
Vojtech Plzak, Rolf Juergen Behm, Rainer Leppelt,
Abstract Kinetics and Mechanism of the Low-Temperature Water-Gas-Shift Reaction on Au/CeO2 Catalysts
M. Kinne, R. Leppelt, V. Plzak*, and R.J. Behm
Dept. Surface Chemistry and Catalysis, University Ulm, 89081 Ulm, Germany
*Centre of Solar Energy and Hydrogen Research, Helmholtzstr. 8, D-89081 Ulm, Germany

The low temperature water-gas shift reaction (LTS) (CO + H2O  CO2 +H2) has gained new interest during the last few years since it can be used to improve the efficiency of reformer/ fuel-cell systems. Currently Cu/ZnO/Al2O3 is used as standard LTS catalyst [1] but very recently supported gold catalysts that were already known to exhibit very high activity for the CO oxidation as Au/TiO2, Au/Fe2O3 and Au/CeO2 have proven to be also active WGS catalysts [2-5]. In these studies the authors did mainly focus on the activity of there catalysts but did not present systematic kinetic data. Although much work has already been invested into the investigation of the LTS reaction the kinetics and the mechanism are still under discussion.
In this study we focus on a new Au/CeO2 system, prepared by a modified deposition precipitation technique. CeO2 alone is already active for the WGS reaction at high temperature. Likewise, CO oxidation catalysts based on Au/CeO2 show a higher activity than Au/TiO2 and Au/Fe2O3 systems. Simple activity measurements proof that the activity of the Au/CeO2 catalyst is superior to conventional LTS catalysts even at temperatures as low as 180°C [6]. The measurements which were carried out in a micro reactor with on-line GC analytics reveal reaction orders of 0.5 for both, CO (partial pressure: 0.2-2kPa) and H2O (partial pressures: 0.7-4kPa), at 180°C. The apparent activation energy in the temperature range of 80-140°C, at 2% H2O, 1% CO and N2 as balance, was determined to 40 kJ/mol.
DRIFTS measurements carried out during CO adsorption and reaction are presented which help to identify the nature of the adsorption sites of educts, products, and possible intermediates or byproducts. These experiments clearly show that CO is nearly exclusively bonded on the Au particles. In addition, they reveal that large amounts of formate (identified by bands at, e.g., 1586cm-1 and 2833cm-1) and possibly carbonates are formed on the surface during the WGS reaction. In order to correlate the concentration of surface species with the measured reaction rates we combined the in-situ DRIFTS set-up with an on-line GC. Figure 1 shows the correlation between the reaction rate and the concentration of the formate (CO2 and OH are also shown). After a short deactivation period that is observed for all Au/MxOy systems, the catalyst is quite stable for more than 10h and both lines show an absolutely parallel behaviour, indicating a correlation between the formed formate and the reaction rate. The formate seems to be an intermediate and not just a by-product of the WGS on the Au/CeO2 catalyst. To support this thesis additional experiments were carried out.

Figure 1: Reaction rate and peak area of formate, CO2 and OH vs reaction time.

In TPD experiments performed after reaction significant amounts of CO2 460 µmol/g CO2 could be detected although we already knew from the DRIFT experiments (Figure 1) that large amounts of formates and possibly some carbonates but very little CO2 was adsorbed on the catalyst surface. Most probably, the CO2 is stored in form of formates. A closer look at the dissociation rate of the formates at 180°C reveals that they are very similar to the measured WGS reaction rate at this temperature.
Based on the obtained results we conclude that formates are indeed one of the possible intermediate in the WGS reaction on Au/CeO2 catalysts.

References
1. K. Kochlöffel, Handbook of Heterogeneous Catalysis, G. Ertl, H. Knözinger, J. Weitkamp, VCH, Weinheim, 1997, 1831.
2. H. Sakurai, A. Ueda, T. Kobayashi, and M. Haruta, Chem. Commun., (1997) 271.
3. F. Boccuzzi, A. Chiorino, M. Manzoli, D. Andreea, and T. Tabakova, J. Catal., 188 (1999) 176.
4. Q. Fu, A. Weber, and M. Flytzani-Stephanopoulos, Catal. Lett., 77 (2001) 87.
5. D. Andreeva, V. Idakiev, T. Tabakova, L. Ilieva, P. Falaras; A. Bourlinos, and A. Travlos, Catal. Today, 72 (2002) 51.
6. R. Leppelt, V. Plzak, M. Kinne, and R.J. Behm, in prep. 2003
Keywords: WGS, AuCeO2, Kinetics, Mechansim
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