Role of CeO2 support for Pd-Cu bimetallic catalysts for oxygen-enhanced water gas shift

Authors: Junichiro Kugai, Elise B. Fox, Chunshan Song

Abstract:
CeO₂-supported metal is one of the efficient CO shift catalysts for hydrogen production for compact polymer electrolyte membrane fuel cells (PEMFCs). The present study focuses on the role of support in oxygen-enhanced water gas shift (OWGS) in which a small amount of oxygen is added to a reformate gas to promote WGS. Pd-Cu bimetallic catalysts on various CeO₂ supports were prepared and their catalytic activities and physicochemical properties were investigated in comparison with Al₂O₃-supported catalyst. CeO₂-supported catalysts showed not only better performance, but also pronounced enhancement of WGS upon O₂ addition to the feed compared to Al₂O₃-supported catalyst. In kinetic study, the higher CO fractional order with CeO₂-supported catalysts than Al₂O₃-supported counterpart revealed H₂O participates in the reaction much more sensitively to CO partial pressure on the former, which further shows H₂O is readily activated with CeO₂. By the same CO fractional order for all the CeO2-supported catalysts, difference in CeO₂ properties was shown to appear in the OWGS rate as the site number for H₂O activation. The comparable apparent activation energy (Ea)for Al₂O₃-supported and CeO₂-supported catalysts regardless of the large difference in turnover rate also corroborates that the rate is expressed by the number of the H₂O activation sites. The upper limit of turnover rate above 90 m2/g of CeO₂ surface area was attributed to a change in the rate-controlling factor from H2O activation to CO activation (CO diffusion or CO oxidation ability inherent to metal species). The reducibility of PdO-CuO species measured by TPR and the surface carbonyl/carbonate ratio measured by FT-IR in CO atmosphere were contrastive between Al₂O₃-supported and CeO₂-supported catalysts, which suggested strong interaction at metal–ceria interface. XRD and field emission SEM image revealed that Pd-Cu bimetallic particles uniformly disperse on support by changing the particle size depending on ceria surface property and structure with minor Cu segregation. Ceria properties thus influence both dispersion of Pd-Cu species and reducibility (metal–support interaction) to control H₂O activation. Transient pulse studies on clean catalyst surface revealed WGS proceeds via association of the two surface adsorbates of short lifetime, but CO was never converted on pure metal, which corroborates metal–ceria interface is indispensable for high H2O activation.

Keywords:
Oxygen-enhanced water gas shift (OWGS)
Water gas shift (WGS)
Pd
Cu
Metal catalyst
CeO₂-supported Pd-Cu
Bimetallic catalysts

Published in: Applied catalysis A: General  (Volume 456, Pages 1-248, 10 April 2013)

Publisher: Elsevier

ISSN Information: 0926-860X

Role of CeO2 support for Pd-Cu bimetallic catalysts for oxygen-enhanced water gas shift

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