Oxide-supported Rh catalysts are important components of commercial three-way catalysts for pollution abatement. Despite their universal application, many mysteries remain about the active structure of Rh on oxide supports as these materials often contain a mixture of nanoparticles and single-atom Rh species on the same support, even after aging. Probe molecule Fourier transform infrared (FTIR) spectroscopy in this work shows that atomically dispersed Rh on γ-Al2O3 prefer to strongly bind CO when exposed to NO and CO mixtures and that light-off of NO reduction occurs at temperatures similar to CO desorption, suggesting that the first and rate-determining step in NO–CO reactions may be the desorption of CO from single-atom Rh dicarbonyl complexes, Rh­(CO)2. Two sets of symmetric and asymmetric stretching frequencies associated with distinct Rh­(CO)2 species are observed in FTIR spectra at 2084/2010 and 2094/2020 cm–1. During temperature ramps, the latter pair of bands at 2094/2020 cm–1 converts to the 2084/2010 cm–1 bands at 463 K before all symmetric and asymmetric bands disappear at 573 K. Bands then appear in the range of 1975–1985 cm–1 associated with Rh monocarbonyl, Rh­(CO), species upon the disappearance of the 2084/2010 cm–1 bands, suggesting that CO desorbs sequentially from Rh­(CO)2 by forming Rh­(CO) intermediates. Combined DFT and FTIR experiments suggest that local OH coverage on the γ-Al2O3 surface distinguishes the two Rh­(CO)2 species: the higher frequency species resides on a less hydroxylated region and migrates to a more hydroxylated region at higher temperatures, causing the CO vibrational frequency to decrease by ∼10 cm–1. CO desorption occurs from this Rh­(CO)2 structure with high local OH coverage, consistent with the DFT predicted trend of CO binding energies. Because of the coincidence of CO desorption with the light-off of NO reduction, local support hydroxylation of atomically dispersed Rh1/γ-Al2O3 catalysts likely affects both the Rh structure after CO desorption and the kinetics of NO reduction, studies of which are enabled by the Rh­(CO)2 model developed here.

Rh active sites are critical for NO x reduction in automotive three-way catalysts. Low Rh loadings used in industrial catalysts lead to a mixture of small nanoparticles and single-atom Rh species. This active-site heterogeneity complicates the interpretation of characterization and reactivity, making the development of structure–function relationships challenging. Density functional theory (DFT) investigations of Rh catalysts often employ flat, periodic surfaces, which lack the curvature of oxide-supported Rh nanoparticle surfaces, raising questions about the validity of periodic surface model systems. Here, we combine DFT with probe molecule Fourier transform infrared (FTIR) spectroscopy and high-resolution scanning transmission electron microscopy of supported Rh catalysts synthesized to insure against the in situ formation of single-atom Rh species to compare periodic and nanoparticle DFT models for describing the interaction of CO and NO with supported Rh nanoparticles. We focus on comparing the behavior of model systemsRh(111) and a 201-atom cubo-octahedral Rh nanoparticle (Rh201; ∼1.7 nm diameter)to explain the behavior of CO and NO bound to Rh nanoparticles with an average particle diameter of ∼2.6 nm. Our DFT calculations indicate that CO* occupies a mixture of threefold and atop modes on Rh(111), saturating at 0.56 ML CO* (473 K, 1 bar), while CO* saturates Rh201 near 1 ML. Similarly, NO* binds to threefold sites and saturates the Rh(111) surface at 0.67 ML but saturates the Rh201 particle surface at 1.38 ML, indicating that more NO* binds than there are Rhsurf atoms. Moreover, the adlayers on the Rh201 particle contain predominantly atop-bound CO*, with bridge CO* possible on particle edges and predominantly threefold NO* with bridge- and atop-bound NO* bound to edges and corners. These binding modes and higher coverages are made possible by the curvature of these nanoparticles and by the expansion of surface metal–metal bondsneither of which can occur on Rh(111)which together permit the adlayer to laterally relax, reducing internal strain. FTIR data for CO* on 10 wt % Rh/γ-Al2O3 show predominantly atop binding modes (2067 cm–1) with small broad peaks near bridge (1955 cm–1) and threefold (1865 cm–1) regions. Meanwhile, NO* FTIR spectroscopy also shows a mixture of atop (1820 cm–1) and threefold (1685 cm–1) NO* features, with similar features observed at reaction conditions (5 mbar NO, 1 mbar CO, 478 K), indicating that NO* dominates Rh surfaces during catalysis. Frequency calculations on these adlayers of Rh201 particles yield dominant frequencies that more closely resemble those observed in FTIR spectra and demonstrate how coverage and dipole–dipole coupling affect vibrational frequencies with surface curvature. Taken together, these results indicate that the Rh surface curvature alters the structure and spectral characteristics of NO* and CO* for Rh nanoparticles of ∼2.6 nm diameter, which must be accurately reflected in DFT models.