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 CO 2 by H 2 often proceeds via two competing reaction mechanisms – the reverse water gas shift reaction (rWGSR, CO 2 +H 2 ⇌CO+H 2 O) and methanation (CO 2 +4H 2 ⇌CH 4 +2H 2 O). Atomically dispersed Rh 1 catalysts on TiO 2 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 Rh 1 active sites and rWGSR pathway on anatase TiO 2 (101), as well as the high rWGSR selectivity of Rh 1 compared with supported Rh x ( x =2–8 atoms) nanoclusters. DFT‐computed formation energies, vibrational frequency analysis, and microkinetic modeling suggest three plausible active sites: Rh 1 on titania (Rh 1 /TiO 2 (101)), Rh 1 with a nearby hydroxyl group (Rh 1 OH/TiO 2 (101)), and Rh 1 near an oxygen vacancy at a three‐fold coordinated site (Rh 1 near O 3c vac). Predicted turnover frequencies and apparent activation barriers for Rh 1 indicate a faster reaction involving CO 2 dissociation assisted by a support oxygen vacancy via Rh 1 near O 3c vac, as well as slower reactions involving Rh 1 OH/TiO 2 (101) or Rh 1 /TiO 2 (101) through a COOH intermediate. These Rh 1 sites are selective toward CO rather than CH 4 because of the weak adsorption of CO, large barrier for C−O bond dissociation, and the lack of nearby metal sites for H 2 dissociation, in contrast to Rh x nanoclusters, including Rh 2 dimers.