(289k) Ceria Supported Catalysts for the Low Temperature Water Gas Shift Reaction

In
recent years, there has been a renewed interest in the low temperature
water-gas shift (LTWGS) reaction because of its potential use in conjunction
with fuel cell power generation.  Proton exchange membrane fuel cells require
hydrogen of sufficiently high purity, as parts per million of CO poison the
noble metal catalysts used in the fuel cell.  The LTWGS reaction can be used as
one of the steps to achieve this purity.  Studies have shown that ceria and
ceria-zirconia supported catalysts are promising catalysts for the LTWGS
reaction [1,2].  Two mechanisms have been proposed for the reaction over
ceria-supported catalysts.  The first mechanism is a ceria-mediated redox
process, where CO adsorbed on the metal is oxidized by ceria [2].  The second
mechanism involves the formation of a surface formate intermediates which
decomposes to H₂ and unidentate carbonate prior to liberation of CO₂
[3].  Significant debate still exists as to which mechanism, or combination of
mechanisms [4], occurs during the reaction.  What is known is that the
reducibility of the support is important. 

The
addition of zirconia to ceria has been shown to improve the reducibility of the
support [5].  Studies of the high temperature methane reforming reactions have
shown that the formation of Ce³⁺ under reaction conditions exhibits
a maximum, with the highest reducibility being observed on a Ce_0.5Zr_0.5O₂
support.  The results of this work show that the LTWGS activity correlates well
with the reducibility of the oxide support with a maximum being observed at a
50% Ce composition.  Specifics of the proposed mechanism of reaction and the
effect of the support composition, feed composition, and support surface area
are discussed.

1.     
G. Jacobs, L. Williams, U. Graham,
G. Thomas, D. Sparks, B. Davis, Appl. Catal. A: General  252, 107
(2003). 

2.     
S. Zhao, R.J. Gorte, Catal.
Lett., 92 (1-2), 75 (2004). 

3.     
G. Jacobs, E. Chenu, P. Patterson,
L. Williams, D. Sparks, G. Thomas, B. Davis, Appl. Catal. A: General 
258, 203 (2004). 

4.     
Y. She, F. Feng, Z. Dardas, R.R.
Willigan, C.A. Newman, S.M. Opalka, R.Radhakrishnan, H. Cordatos, T.H.
Vanderspurt, Proceedings of the 13th International Congress on Catalysis, 11-16
July, 2004, Paris, France, P4-126.

5.     
F. B. Noronha, E. C. Fendley, R.
R. Soares, W. E. Alvarez, and D. E. Resasco, Chem. Eng. Journal, 82,
21(2001).