Elucidating the roles of metallic Ni and oxygen vacancies in CO2 hydrogenation over Ni/CeO2 using isotope exchange and in situ measurements

Abstract

Improvement in processes to convert CO2 to value-added chemicals requires elucidation of catalytic mechanisms. Previous investigations of CO2 reduction by hydrogen over CeO2-supported Ni catalysts suggested that the selectivity to CO and CH4 was related to the Ni loading. In this work we utilized near ambient pressure X-ray photoelectron spectroscopy to confirm that Ni remained metallic under reaction conditions at different Ni loadings. The presence of partially reduced CeO2 was detected, suggesting that oxygen from CO2 did not re-oxidize the CeOx. Isotope exchange studies were performed using C18O2 in a batch reactor equipped with Fourier transform infrared spectroscopy and mass spectrometry to determine the involvement of surface and lattice oxygen. Our results demonstrate that oxygen exchange with the CeO2 support occurs beyond the surface layer under CO2 hydrogenation conditions. Kinetic studies also revealed that the oxygen exchange rate is fast with respect to the CO2 hydrogenation reaction rate and that the CeO2 oxygen exchange mechanism is modified by Ni.