Surface Intermediates in In-Based ZrO<sub>2</sub>-Supported Catalysts for Hydrogenation of CO<sub>2</sub> to Methanol
Athanasia Tsoukalou, Nikolai S. Bushkov, Scott R. Docherty, Deni Mance, Alexander I. Serykh, Paula M. Abdala, Christophe Copéret, Alexey Fedorov, Christoph R. Müller
Abstract
The influence of the phase of the ZrO2 support (monoclinic, tetragonal, and amorphous, referred to as m-, t-, and am-, respectively) on the nature of the surface species involved in methanol synthesis and the rates of their formation on ZrO2-supported, In-based catalysts for CO2 hydrogenation has been investigated. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) at 300 °C and 20 bar (H2:CO2:N2 = 3:1:1 volume ratio) on m-ZrO2:In, In2O3/t-ZrO2, and In2O3/am-ZrO2 catalysts (m-ZrO2:In is a solid solution) shows that formate species (HCOO*) appear prior to methoxy species (*OCH3), and both intermediates form faster on the more active m-ZrO2:In catalyst. Only formate bands are detected for the In2O3/t-ZrO2 catalyst. For these materials, indium sites are essential for the formation of HCOO* and *OCH3 species as only carbonate species are observed on m-, t-, and am-ZrO2 supports under CO2 hydrogenation conditions. The nature of the reaction intermediates is confirmed by ex situ solid-state nuclear magnetic resonance (NMR), where both methoxy and formate species are detected in m-ZrO2:In and In2O3/am-ZrO2, respectively, but only a weak formate peak is observed for In2O3/t-ZrO2. The presence of a major methoxy peak and only a very minor formate signal in unsupported In2O3 indicates that an india–zirconia interface is required for the effective stabilization of formate species. Catalytic tests in a fixed bed reactor are consistent with both CO and MeOH being primary products of CO2 hydrogenation; the tests also show that the methanol selectivity and space time yield decrease in the following order: m-ZrO2:In > In2O3/t-ZrO2 > In2O3/am-ZrO2 for all of the contact times tested.