Rh is an important component of automotive three-way catalysts (TWCs) for NOx reduction, and Rh loadings on such TWCs are so low that single-atom catalysts (SAC) form. Samples with only Rh SAC (0.05 wt.% Rh/γ-Al2O3) more selectively form NH3 at low temperatures (&lt;400 K) when exposed to CO-NO-H2O mixtures, while catalysts synthesized with high Rh loadings to generate particles without SAC (5-10 wt.% Rh/γ-Al2O3) form N2 and N2O. Notably, the mechanisms for NOx reduction on both Rh clusters and SAC remain unknown, the study of which requires a suitable model for Rh SAC on γ-Al2O3. Here, we combine density functional theory (DFT) studies of different single-atom catalyst models with experimental characterization of 0.05 wt.% Rh/γ-Al2O3 using CO and NO probe-molecule IR spectroscopy to identify the structure of these Rh SAC on γ-Al2O3. We study three facets of a γ-Al2O3 model, different charge-balancing surface species that alter the oxidation state of the Rh, and different surface OH coverages (θOH) derived from dissociated H2O to determine which model most closely matches experimentally observed binding energies and frequencies for CO and NO on Rh SAC. When exposed to CO, Rh SAC form Rh(CO)2 with symmetric and asymmetric stretch frequencies of ~2090 and ~2020 cm−1, respectively, which closely matches a DFT-calculated structure with Rh in a +1 oxidation state bound to OH on the γ-Al2O3(110) surface, with stretch frequencies of 2082 and 2017 cm−1. OH derived from H2O on γ-Al2O3 can alter these frequencies; when exposed to 3% H2O at 298 K, the full-width half-maximum (FHWM) of Rh(CO)2 stretch bands decreases from 23 cm−1 to 15 cm−1 and the band centers redshift by ~2 cm−1, indicating that the local environment of the Rh species becomes more uniform and that higher θOH decrease CO frequencies. Denser OH adlayers on γ-Al2O3 in DFT calculations alter the orientation of the Rh(CO)2, suggesting that the preferred configuration of these species depends sensitively on the state of the support. When exposed to NO instead of CO, no clear bands are observed in IR at 298 K, despite DFT predicting that NO binds more strongly than CO to Rh SAC. Frequencies more closely matching DFT-predictions are only observed when these catalysts are exposed to NO below 200 K, indicating that Rh single atoms are so active for NO reduction that NO reacts at room temperature. This rigorous characterization permits subsequent studies of NO reduction on Rh single-atoms supported on γ-Al2O3.

Characterizing Rh single-atoms catalysts on γ-Al2O3 using CO and NO probe-molecule IR and density functional theory

ACS Spring 2022

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