Elucidating the Rate-Determining Step of CO Hydrogenation to Branched Hydrocarbons over m-ZrO 2

The isosynthesis reaction, involving CO hydrogenation over ZrO2 catalysts, represents a promising route for the selective production of branched hydrocarbons, particularly isobutene, and is relevant as part of broader CO2 valorization strategies. Isosynthesis over m-ZrO2 primarily yielded isobutene and other C1−C4 hydrocarbons, with considerable selectivity toward undesired CO2 formation. The reactivity of formate and methoxy surface species was studied by quantitative operando diffuse reflectance FTIR spectroscopy (DRIFTS) under chemical transients carried out by removing CO or H2 following steady-state catalytic conversion. Two distinct formate species were identified: a rapidly decomposing fraction and a slowly decaying one. The reactivity of fast formates is dominated by fast adsorption−desorption kinetics, but a minor fraction reacts further, forming methoxy groups. In contrast, the slow formates behave essentially as spectators, slowly desorbing without significant conversion to products of interest. In the absence of hydrogen, the DRIFTS signal of methoxy species decayed at rates that closely matched those of hydrocarbon formation. This suggests that methoxy surface dynamics governs the rate-determining steps of hydrocarbon formation in the isosynthesis. These results identify surface methoxy groups as a central-branching, rate-determining intermediate from which multiple hydrocarbon pathways originate.