(653a) Rhodium Single-Atoms Vs. Nanoclusters on Titania for Reverse Water Gas Shift Reaction: A First-Principles Study

Heterogeneous catalyst research offers promising strategies for converting CO₂ emissions into useful chemical feedstocks, providing an economic incentive for CO₂ conversion. To advance the effort for cost-effective CO₂ reduction, catalysts must be highly active and selective while minimizing the amount of precious metal required. Here, single-atom catalysis offers multiple advantages over larger nanoparticles. Single-atom catalysts are often highly active and selective due to their single-site nature and their unique electronic properties. Additionally, single-atom catalysts can achieve nearly 100% metal atom dispersion.

The thermocatalytic reduction of CO₂ by H₂ proceeds via two competing reaction mechanisms—the reverse water gas shift reaction (rWGSR, CO₂ + H₂ ⇌ CO + H₂O) and methanation (CO₂ + 4H₂ ⇌ CH₄ + 2H₂O). Single-atom Rh₁ catalysts on TiO₂ show high selectivity toward the rWGSR compared with larger Rh nanoclusters,¹ but the origin of this size-dependent selectivity has not been fully explained at an atomistic level. In this talk, we report density functional theory (DFT) calculations and mean-field microkinetic simulations that explain the high rWGSR selectivity of Rh₁ on anatase TiO₂(101) compared with Rh nanoclusters.

The plausible active sites of Rh₁/TiO₂(101) were found based on DFT-computed formation energies, CO-Rh₁ vibrational frequency analysis, and microkinetic modeling. Predicted turnover frequencies and activation barriers for Rh₁/TiO₂(101) at a variety of sites indicate a preferred mechanism through oxygen vacancies. Rh₁/TiO₂(101) is selective toward CO rather than CH₄ because of the weaker adsorption of CO, larger barrier for C-O bond dissociation, and lack of nearby metal sites for H₂ dissociation relative to nanoclusters. Ultimately, this study clarifies the origin of the higher rWGSR selectivity of Rh₁ catalysts compared with Rh nanoclusters on anatase TiO₂, which has strong parallels to other single-atom and nanocluster systems for CO₂ reduction.

1. J. C. Matsubu, V. N. Yang and P. Christopher, J. Am. Chem. Soc., 2015, 137, 3076–3084.