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

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.

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► O2 addition enhances water gas shift (WGS) more on Pd-Cu/CeO2 than Pd-Cu/Al2O3.
► Kinetics shows chemistry in H2O activation differs between Pd-Cu on CeO2 and Al2O3.
► CeO2 surface area appears in kinetics as the number of H2O activation sites.
► High surface area CeO2 and O2 addition increase the capacity for H2O activation.
► Infrared spectra of adsorbed COx and TPR evidence strong metal–ceria interaction.