Rhodium Single‐Atom Catalysts on Titania for Reverse Water Gas Shift Reaction Explored by First Principles Mechanistic Analysis and Compared to Nanoclusters

Abstract

The thermocatalytic reduction of CO2 by H2 often proceeds via two competing reaction mechanisms – the reverse water gas shift reaction (rWGSR, CO2+H2⇌CO+H2O) and methanation (CO2+4H2⇌CH4+2H2O). Atomically dispersed Rh1 catalysts on TiO2 show high selectivity toward the rWGSR compared with larger Rh nanoclusters, but the origin of this size‐dependent selectivity has not been fully explained. Here we report density functional theory (DFT) calculations and microkinetic simulations that clarify the Rh1 active sites and rWGSR pathway on anatase TiO2(101), as well as the high rWGSR selectivity of Rh1 compared with supported Rhx (x=2–8 atoms) nanoclusters. DFT‐computed formation energies, vibrational frequency analysis, and microkinetic modeling suggest three plausible active sites: Rh1 on titania (Rh1/TiO2(101)), Rh1 with a nearby hydroxyl group (Rh1OH/TiO2(101)), and Rh1 near an oxygen vacancy at a three‐fold coordinated site (Rh1 near O3cvac). Predicted turnover frequencies and apparent activation barriers for Rh1 indicate a faster reaction involving CO2 dissociation assisted by a support oxygen vacancy via Rh1 near O3cvac, as well as slower reactions involving Rh1OH/TiO2(101) or Rh1/TiO2(101) through a COOH intermediate. These Rh1 sites are selective toward CO rather than CH4 because of the weak adsorption of CO, large barrier for C−O bond dissociation, and the lack of nearby metal sites for H2 dissociation, in contrast to Rhx nanoclusters, including Rh2 dimers.The thermocatalytic reduction of CO2+H2 by Rh/TiO2 proceeds via two competing reaction mechanisms depending on whether single atoms or nanoclusters are used. DFT and microkinetic modeling suggest a preferred reaction involving CO2 dissociation assisted by a support oxygen vacancy. Rh1 sites are selective toward CO rather than CH4 because of the weak adsorption of CO, large barrier for C−O bond dissociation, and the lack of nearby metal sites for H2 dissociation, in contrast to Rhx nanoclusters, including Rh2 dimers.