(570a) Characterizing Rh Particles and Single-Atoms Supported on ?-Al2O3 for NO Reduction Using Probe-Molecule IR Spectroscopy and DFT

Rh in automotive three-way catalysts is selective to N₂ during NO reduction, but the foundations of this selectivity remain poorly understood [1]. Rh deposited on oxide supports reversibly disperses to single-atoms or aggregates to clusters in CO (Fig. 1a) [2], a component of automotive exhaust and a critical reductant for NO. We combine probe-molecule FTIR on Rh/γ-Al₂O₃ samples with density functional theory (DFT) calculations to characterize supported Rh particles and single-atoms prepared to avoid mixtures of these two motifs within a single sample. Because CO disperses Rh, particularly at weight loadings <2%, a 10 wt% Rh/γ-Al₂O₃ sample was heavily reduced in H₂. CO and NO FTIR on these samples showed frequencies near 2067 and 1685 cm^−1, respectively, at 293 K (Fig 1b), in good agreement with DFT calculations. These calculations also suggest that single-crystal surfaces saturate near 0.7 ML of CO* or NO*, while coverages on nanoparticle models (0.8–4 nm in diameter) saturate near 1 ML (Fig 1d). In contrast, a 0.1 wt% Rh/γ-Al₂O₃ sample shows two bands at ~2090 and ~2020 cm^−1 upon exposure to CO with no observable peak at 2067 cm^−1 (Fig 1c), indicating the presence of only single-atom Rh. DFT calculations with various possible ligands confirm that these peaks match the symmetric and asymmetric stretches of Rh⁺¹(CO)₂ coordinated to an OH^(−1) on γ-Al₂O₃. This rigorous characterization must precede mechanistic studies on single-atoms and particles, whose different behaviors remain unexplained—such as NH₃ formation on single-atoms during NO reduction by CO and H₂O [3].

References

[1] Shelef, M.; Graham, G. W. Catalysis Reviews, 1994, 36, 433–457

[2] Ouyang, R.; Liu, J.-X.; Li, W.-X. J. Am. Chem. Soc. 2013, 135, 1760–1771

[3] Asokan, C.; Yang, Y.; Dang, A.; Getsoian, A.; Christopher, P. ACS Catal. 2020, 10, 5217–5222