Surface Intermediates in In-Based ZrO2-Supported Catalysts for Hydrogenation of CO2 to Methanol

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.