Real-time monitoring of CO2 reduction on supported catalysts using near-surface molecular beam mass spectrometry (ns-MBMS)

Near-surface molecular-beam mass spectrometry (ns-MBMS) is a highly sensitive and non-destructive method for temporal monitoring of catalytic conversions on solid surfaces, yet not widely applied in heterogeneous catalysis. Herein, we apply and validate the use of ns-MBMS to follow the thermo-catalytic reduction of CO2 on oxide supported catalysts. In our study, we focused on two structurally different Ru/TiO2 catalysts known to behave distinctly differently toward CO2 conversion, one promoting CO2 methanation (Ru/TiO2-1), and the promoting CO formation via reverse-water-gas shift reaction (Ru/TiO2-2). This disparity in product selectivity for CO2 reduction allows us to clearly examine the limits of ns-MBMS not only for differences in CO2 conversion but also for drastic differences in product selectivity with reference to well-established catalytic systems in our previous work. The recorded mass spectra showed apparently similar reaction behavior compared to results reported using standard flow reactors, particularly the temperature dependence of CO₂-reduction product selectivity. In contrast to conventional catalytic measurements, ns-MBMS enabled direct observation of the temporal evolution of transient near-surface products, revealing the formation of species such as CO and C₂H₄ that had not been previously detected. These species relate to specific surface intermediates, such as COad, or spectator fragments like -CH₂- formed on Ru/TiO₂. Reactivity results observed using ns-MBMS experiments are supported by insights on the surface adlayer during CO2 reduction obtained from diffuse-reflectance FTIR spectroscopy.