(377f) Characterizing Rh Single-Atom Catalysts on ?-Al2O3 Using Density Functional Theory and CO Probe Molecule IR Spectroscopy

Despite the commercial use of three-way catalysts (TWC) for NO_x reduction for over 40 years, the mechanism by which the Rh component of TWC reduces NO to N₂, N₂O, and NH₃ remains a mystery [1]. Rh used in small weight loadings (<2%), characteristic of TWC, can reversibly restructure during catalysis between single-atoms and small nanoparticles (Fig. 1a) [2]; as such, well-defined materials are required to study the behaviors of these distinct forms of Rh catalysts. We use a combination of density functional theory (DFT) and CO probe-molecule IR to study the structure of atomically dispersed Rh on γ-Al₂O₃. When exposed to CO, an IR spectrum of a 0.1 wt. % Rh/γ-Al₂O₃ catalyst shows two predominant peaks at ~2090 and ~2020 cm^−1 at 293 K, corresponding to the symmetric and asymmetric stretches of a Rh(CO)₂(Fig. 1b). These features contrast with our previous finding on a 10 wt. % sample, which exhibited one large peak at 2067 cm^−1 at 293 K, corresponding to atop-bound CO* on a Rh nanoparticle (Fig. 1c) [3]. Our DFT calculations indicate that these features correspond to a Rh(CO)₂ coordinated to OH on the γ-Al₂O₃ support (Fig. 1d), with the Rh in a +1 formal oxidation state—in agreement with earlier characterization work [4]. Critically, water can dissociatively adsorb on the surface of the γ-Al₂O₃ support, which alters the local environment around the Rh(CO)₂, changing its stretching frequencies by up to ~10 cm^−1. Our work suggests that the first CO desorbs during temperature-programmed desorption from a partly hydrated γ-Al₂O₃, indicating that support hydration may be crucial in the operating behavior of TWC.

References

[1] Catalysis Reviews, 1994, 36, 433–457

[2] J. Am. Chem. Soc., 2013, 135, 1760–1771

[3] J. Phys. Chem. C, 2021, 125, 19733-19755

[4] J. Am. Chem. Soc., 1985, 107, 3139-3147