(571b) Gas-Induced Stability of Ceria-Based Wgs Catalysts
catalysts are among the most active materials for the low temperature water-gas
shift reaction. Different techniques
can be used to prepare highly dispersed gold in cerium oxide with or without
dopants. We have recently reported that a strong interaction between
gold and ceria is responsible for the WGS reaction activity [1,2]. Gold is
stabilized in oxidized form in ceria; the amount of stabilized gold correlates
with the number of surface oxygen defects of nanoscale ceria.
In highly reducing
environments, the gold-ceria interaction can be lost, zerovalent gold is
formed, ceria defects are annealed, and deactivation follows. This happens
readily above 300 oC, but much less at lower temperatures. The most
serious stability issue with ceria-based catalysts is formation of cerium(III)
hydroxycarbonate in shutdown to room temperature . However, we have found
that addition of a small amount of oxygen (0.5 mole%) in the feed gas can fully
stabilize the WGS activity of Au-nano ceria in realistic reformate gas streams
at temperature up to 300oC. It can also prevent the formation of
cerium(III)hydroxycarbonate at low temperatures. Shutdown to room temperature
with water condensation is now possible without any activity loss for the gold-ceria
catalysts. The key is the CO oxidation reaction by oxygen that is very fast and
not quenched at room temperature. On the other hand, gaseous oxygen addition
can not stabilize platinum-ceria catalysts after shutdown to room temperature
in the full WGS gas mixture since the light off temperature of CO oxidation
over platinum-ceria is much higher than that for gold-ceria.
In this presentation, we will
show evidence of the dynamic response of gold-ceria catalysts to different gas
environments accompanied by structural characterization by various techniques,
including in situ XANES and EXAFS.
1. Q. Fu, H. Saltsburg and M.
Flytzani-Stephanopoulos, Science 301 (2003) 935.
2. Q.Fu, W.Deng, H. Saltsburg, M. Flytzani-Stephanopoulos, Appl. Catal. B,
56 (2005) 57-68.
3. W. Deng, M.
Flytzani-Stephanopoulos, 19th NAM paper # O-305, Philadelphia, PA, May 22-27,
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