Ceria-based Catalysts for the Production of H₂ Through the Water-gas-shift Reaction: Time-resolved XRD and XAFS Studies

Hydrogen is a potential alternate energy source for satisfying many of our energy needs. In this work, we studied H₂ production from the water-gas-shift (WGS) reaction over Ce_1−x Cu _x O₂ catalysts, prepared with a novel microemulsion method, using two synchrotron-based techniques: time-resolved X-ray diffraction (XRD) and X-ray absorption fine structure (XAFS). The results are compared with those reported for conventional CuO _x /CeO₂ and AuO _x /CeO₂ catalysts obtained through impregnation of ceria. For the fresh Ce_1−x Cu _x O₂ catalysts, the results of XAFS measurements at the Cu K-edge indicate that Cu is in an oxidation state higher than in CuO. Nevertheless, under WGS reaction conditions the Ce_1−x Cu _x O₂ catalysts undergo reduction and the active phase contains very small particles of metallic Cu and CeO_2−x . Time-resolved XRD and XAFS results also indicate that Cu^δ+ and Au^δ+ species present in fresh CuO _x /CeO₂ and AuO _x /CeO₂ catalysts do not survive above 200 °C under the WGS conditions. In all these systems, the ceria lattice displayed a significant increase after exposure to CO and a decrease in H₂O, indicating that CO reduced ceria while H₂O oxidized it. Our data suggest that H₂O dissociation occurred on the O_vacancy sites or the Cu–O_vacancy and Au–O_vacancy interfaces. The rate of H₂ generation by a Ce_0.95Cu_0.05O₂ catalyst was comparable to that of a 5 wt% CuO _x /CeO₂ catalyst and much bigger than those of pure ceria or CuO.