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 catalysts offer 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, 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 activity of Rh1 on anatase TiO2 and compare Rh1 binding characteristics to 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 apparent activation barriers for Rh1/TiO2(101) at a variety of sites indicate two likely mechanisms: a fast reaction involving support oxygen vacancies, and a hydrogen-assisted mechanism involving Rh1OH species. Rh1/TiO2(101) is selective toward CO rather than CH4 because of 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, and discusses strong parallels to other single-atom and nanocluster systems for rWGSR.

Rhodium single-atom catalysts on titania for reverse water gas shift reaction explored by first principles mechanistic analysis and compared to nanoclusters

ACS Spring Meeting 2021

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