Understanding the Reducibility of CeO 2 Surfaces by Proton-Electron Transfer from CpCr(CO) 3 H.

CeO 2 is a popular material in heterogeneous catalysis, molecular sensors, and electronics and owes many of its special properties to the redox activity of Ce, present as both Ce 3+ and Ce 4+ . However, the reduction of CeO 2 with H 2 (thought to occur through proton-electron transfer (PET) giving Ce 3+ and new OH bonds) is poorly understood due to the high reduction temperatures necessary and the ill-defined nature of the hydrogen atom sources typically used. We have previously shown that transition-metal hydrides with weak M-H bonds react with reducible metal oxides at room temperature by PET. Here, we show that CpCr(CO) 3 H ( 1 ) transfers protons and electrons to CeO 2 due to its weak Cr-H bond. We can titrate CeO 2 with 1 and measure not only the number of surface Ce 3+ sites formed (in agreement with X-ray absorption spectroscopy) but also the lower limit of the hydrogen atom adsorption free energy (HAFE). The results match the extent of reduction achieved from H 2 treatment and hydrogen spillover on CeO 2 in a wide range of applications.