Reaction Pathways to CO and Methanol in CO2 Hydrogenation over Zn x ZrO y Catalysts

Abstract ZnxZrOy paired with zeolites/zeotypes enables tandem CO2 hydrogenation to light olefins via methanol, but CO formation remains a major selectivity barrier at the high temperatures required for olefin production. Combined kinetic studies using methanol conversion, in situ DRIFTS, isotope labels, and co-feeding experiments challenge the relevance of the reverse water−gas shift reaction for CO production in the hydrogenation pathway over Zn-doped ZrO2. For Zn0.19ZrOy, methanol selectivity extrapolates to 100% at zero CO2 conversion. Together with the irreversible decomposition of methanol, this identifies methanol as an intermediate on the way to CO, which is predominantly a terminal product that does not re-hydrogenate under CO2 hydrogenation conditions. For pure ZrO2, CO selectivity extrapolates to 91%, indicating the dominance of reverse water−gas shift. On Zn0.44ZrOy and Zn0.56ZrOy, ZnO phase separation enables the reverse water−gas shift reaction. At higher CO2 conversion, and thus with methanol formation and decomposition, this shifts CO formation from parallel reverse water−gas shift to consecutive decomposition of methanol precursors. Methanol conversion studies confirm that ZrO2 favors methanol dehydration to dimethyl ether. Adding Zn, however, promotes methanol conversion to CO, CO2, and methyl formate due to different methoxy adsorption configurations. The product distribution is strongly influenced by the presence of H2, H2O, and CO2. Co-feeding methanol with CO2 highlights competitive adsorption, enhancing dimethyl ether formation on ZrO2 but suppressing CO2 activation on ZnxZrOy. Zn affects intermediate adsorption through the distribution and reactivity of hydroxyl groups, destabilizing strongly bound (bi)carbonates, enabling their hydrogenation to formate and methoxy in CO2 hydrogenation. Water co-feeding is ambivalent, enabling recycling of methanol to CO2 instead of CO but suppressing overall CO2 conversion on ZnxZrOy through site competition. Notably, these kinetic conclusions do not generally extrapolate. CO formation exhibits a non-monotonic dependence on H2 pressure, reflecting a shift in the dominant reaction pathway.