Characterization of CeO2-supported Cu–Pd bimetallic catalyst for the oxygen-assisted water–gas shift reaction

Our recent work demonstrated that Cu–Pd bimetallic catalyst supported on a nano-crystalline CeO2 synthesized by urea gelation method is effective for oxygen-assisted water–gas shift (OWGS) reaction. The present study focuses on the roles of Cu and Pd in CuPd/CeO2 bimetallic catalysts containing 20–30 wt% Cu and 0.5–1 wt% Pd used in the OWGS reaction employing a combined bulk and surface characterization techniques such as EXAFS, XRD, TPR, CO chemisorption, and in situ XPS. The catalytic activity for CO conversion and the stability of catalyst during on-stream operation increased by the addition of Cu to Pd/CeO2 or Pd to Cu/CeO2 monometallic catalysts, especially when the OWGS reaction was performed at low temperatures, below 200 °C. The TPR of monometallic Cu/CeO2 showed reduction of CuO supported on CeO2 in two distinct regions, around 150 and 250 °C. The high temperature peak disappeared and reduction occurred in a single step around 150 °C upon Pd addition. In situ XPS studies showed a shift in Cu 2p peaks toward lower binding energy (BE) with concomitant shift in the Pd 3d peaks toward higher BE. An inward diffusion of Pd into the CeO2 support occurred upon reduction. On the other hand, inward diffusion of Cu occurred when Pd was present in the sample. These observations indicated the existence of synergistic interactions between Cu and Pd in these catalysts which could be responsible for the improved catalytic activity and stability of CuPd/CeO2 bimetallic catalyst. The EXAFS analysis of Cu showed no clear evidence of Cu–Pd alloy formation at the copper edge. However, evidence for Pd–Cu alloy was shown in the Pd edge, and Pd atoms are surrounded only by Cu atoms in the reduced CuPd/CeO2 bimetallic catalyst as revealed by EXAFS.