Surface Intermediates in In-Based ZrO 2 -Supported Catalysts for Hydrogenation of CO 2 to Methanol

High Resolution Image Download MS PowerPoint Slide The influence of the phase of the ZrO 2 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 ZrO 2 -supported, In-based catalysts for CO 2 hydrogenation has been investigated. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) at 300 °C and 20 bar (H 2:CO 2:N 2 = 3:1:1 volume ratio) on m-ZrO 2:In, In 2 O 3 /t-ZrO 2, and In 2 O 3 /am-ZrO 2 catalysts (m-ZrO 2:In is a solid solution) shows that formate species (HCOO*) appear prior to methoxy species (*OCH 3 ), and both intermediates form faster on the more active m-ZrO 2:In catalyst. Only formate bands are detected for the In 2 O 3 /t-ZrO 2 catalyst. For these materials, indium sites are essential for the formation of HCOO* and *OCH 3 species as only carbonate species are observed on m-, t-, and am-ZrO 2 supports under CO 2 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-ZrO 2:In and In 2 O 3 /am-ZrO 2, respectively, but only a weak formate peak is observed for In 2 O 3 /t-ZrO 2 . The presence of a major methoxy peak and only a very minor formate signal in unsupported In 2 O 3 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 CO 2 hydrogenation; the tests also show that the methanol selectivity and space time yield decrease in the following order: m-ZrO 2:In > In 2 O 3 /t-ZrO 2 > In 2 O 3 /am-ZrO 2 for all of the contact times tested.