<i>Operando</i>X-ray Absorption Spectroscopy Identifies a Monoclinic ZrO<sub>2</sub>:In Solid Solution as the Active Phase for the Hydrogenation of CO<sub>2</sub>to Methanol
Athanasia Tsoukalou, Paula M. Abdala, Andaç Armutlulu, Elena Willinger, Alexey Fedorov, Christoph R. Müller
Abstract
Operando X-ray absorption spectroscopy (XAS) associates the superior activity and stability of the In2O3/m-ZrO2 catalyst for the direct hydrogenation of CO2 to methanol (300 °C, 20 bar) to indium sites with an average oxidation state of +2.3 atomically dispersed in the lattice of monoclinic ZrO2. The active sites in this solid solution m-ZrO2:In catalyst are In–Vo–Zr sites (Vo is an oxygen vacancy) that are stabilized in the lattice of poorly reducible m-ZrO2 against deactivation by over-reduction to In0. In contrast, the amorphous ZrO2 support does not form a (crystalline) solid solution with In2O3 and, as a result, In2O3/am-ZrO2 reduces to metallic In within minutes under the reaction conditions. Furthermore, a tetragonal ZrO2 support stabilizes dispersed india nanocrystals (In2O3/t-ZrO2) against over-reduction only partially, yielding a catalyst with an average oxidation state of the In sites below +2: i.e., In2O3/t-ZrO2 also suffers deactivation by over-reduction. Our results demonstrate that the phase of the ZrO2 support determines whether an active solid solution with india forms, which has major implications for the reducibility of In3+ sites and their local structure. Comparing the stability and activity of india-based catalysts, we identified the monoclinic solid solution m-ZrO2:In as a superior catalyst for the direct conversion of CO2 to methanol, which contains active In–Vo–Zr surface species that are significantly more stable toward reduction than In–Vo–In sites in bixbyite-type In2O3.