In Situ DRIFTS and Modulation–Excitation Studies for CO2 Hydrogenation on Nanostructures of CeO2: Unraveling the Dynamic Nature of CO2↔Hydroxylated-Ceria Interfaces

Abstract

A combination of DRIFTS and transient studies with Modulation Excitation and Phase Sensitive Detection (ME-PSD) was used to study the hydrogenation of CO2 on ceria nanocubes and nanospheres. Up to five different types of OH groups were produced after the reaction of molecular H2 with the ceria nanostructures at a temperature of 250 °C typically used in catalytic processes. The formation of these OH groups induced morphological changes in the ceria nanostructures that affected the adsorption of CO2 and influenced reaction paths for the reverse water–gas shift (RWGS) reaction (CO2 + H2 → CO + H2O). The behavior of surface adsorbates and hence the associated reaction mechanisms were found to be highly morphology dependent, reflecting the dynamic nature of these CO2↔hydroxylated-ceria interfaces. In general, the reaction of CO2 with OH groups produced a rich set of formate and hydroxycarbonate species, and the existence of surface OH groups affected the stability of regular carbonates formed by the reaction of CO2 with O centers of the oxide substrates. In transient or ME-PSD studies, performed with the exposure of the ceria nanostructures to CO2/H2 mixtures at 250 °C, hydrogen carbonates and formates were identified as dynamic species in adsorption/desorption processes that have a limited contribution to the RWGS reaction. On both catalytic surfaces, hydroxyl groups and carbonate adsorbates initially experienced a rapid, kinetically controlled reduction (OHadsorbed + 0.5H2,gas → Ovacancy + H2Ogas) and oxidation (CO3,ads + Ovacancy→ COgas + 2Ooxide) process that was part of the RWGS reaction. The ceria nanocubes had an additional reaction pathway, where direct carbonate decomposition, involving the transformation of transient and surface tridentate carbonates, enhanced the RWGS rate. Overall, these results illustrate the tremendous impact that hydrogen can have on the adsorption and reaction of CO2 with oxide surfaces and strongly emphasize the need to consider different oxide nanostructures to successfully optimize the performance of CeO2-based catalysts for CO2 hydrogenation.