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Theoretical and Experimental Characterization of Adsorbed CO and NO on γ‑Al2O3‑Supported Rh Nanoparticles

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journal contribution
posted on 02.09.2021, 19:08 authored by Alexander J. Hoffman, Chithra Asokan, Nicholas Gadinas, Pavlo Kravchenko, Andrew “Bean” Getsoian, Phillip Christopher, David Hibbitts
Rh active sites are critical for NOx 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.