Low-temperature water-gas shift: Strategy to lower Pt loading by doping ceria with Ca~(2+) improves formate mobility/WGS rate by increasing surface O-mobility

摘要

In one view, the metal-oxide synergy (e.g., Pt metal and cerium oxide) has been explained in terms of the dehydrogenation of formate formed on the surface of the partially reducible oxide (PRO) by Pt across the interface, with H2O participating in the transition state of forward formate decomposition. In this work, Ca-doping of the ceria component in Pt/ceria catalysts was demonstrated by TPR and TPR-XANES measurements to facilitate the temperature of ceria surface shell and bulk reduction steps, and by TPR-XANES to increase the extents of surface shell and bulk reduction of ceria. The results thus confirm, experimentally, past theoretical models, which suggested that divalent elements (e.g., Ca) enhance both O-mobiiity and reducibility of ceria by weakening the Ce-O bond through lattice strain. This strain was also detected in our XRD measurements. A recent surface diffusion model postulated that increasing oxygen surface diffusion also improves the mobility of 0-bound intermediates (e.g., formates, carbonates, carboxylates). In this work, in situ DRIFTS measurements confirm that improved formate decomposition rates were realized over the Ca-doped Pt promoted ceria catalysts possessing higher O-mobility relative to undoped Pt/ceria. In turn, improved LT-WGS rates were observed over the Ca-doped Pt/ceria catalysts. While the precise mechanism is still under debate, the enhanced mobility of O-bound intermediates by Ca-dopihg is suggested to be responsible for the significant boosts in CO conversion levels and TOFs observed during LT-WGS. Thus, doping ceria with elements like Ca provides a path forward for lowering the precious metal content (e.g., Pt), as well as the rare earth content (e.g., Ce) - catalytic components that are becoming increasingly expensive.