Reaction Pathways to CO and Methanol in CO 2 Hydrogenation over Zn x ZrO y Catalysts

Zn x ZrO y paired with zeolites/zeotypes enables tandem CO 2 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 ZrO 2 . For Zn 0.19 ZrO y, methanol selectivity extrapolates to 100% at zero CO 2 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 CO 2 hydrogenation conditions. For pure ZrO 2, CO selectivity extrapolates to 91%, indicating the dominance of reverse water−gas shift. On Zn 0.44 ZrO y and Zn 0.56 ZrO y, ZnO phase separation enables the reverse water−gas shift reaction. At higher CO 2 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 ZrO 2 favors methanol dehydration to dimethyl ether. Adding Zn, however, promotes methanol conversion to CO, CO 2, and methyl formate due to different methoxy adsorption configurations. The product distribution is strongly influenced by the presence of H 2, H 2 O, and CO 2 . Co-feeding methanol with CO 2 highlights competitive adsorption, enhancing dimethyl ether formation on ZrO 2 but suppressing CO 2 activation on Zn x ZrO y . 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 CO 2 hydrogenation. Water co-feeding is ambivalent, enabling recycling of methanol to CO 2 instead of CO but suppressing overall CO 2 conversion on Zn x ZrO y through site competition. Notably, these kinetic conclusions do not generally extrapolate. CO formation exhibits a non-monotonic dependence on H 2 pressure, reflecting a shift in the dominant reaction pathway.