Experimental and Theoretical Characterization of Rh Single Atoms Supported on γ-Al₂O₃ with Varying Hydroxyl Contents during NO Reduction by CO

Abstract

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.

Photolysis of Atomically Dispersed Rh/Al2O3 Catalysts: Controlling CO Coverage in Situ and Promoting Reaction Rates

“…Furthermore, recent DFT calculations 18,44,45 support the assignment of the 1985 cm −1 CO stretch to the Rh(CO) complex, formed as an intermediate during the sequential desorption of CO molecules from Rh(CO) 2 . Specifically, DFT calculations of the Rh(CO) species on anatase TiO 2 45 and Al 2 O 3 18 suggest that this species is stabilized by coordinating to an additional OH species that is native to the support, as shown in the inset of Figure 1 and discussed further below. Therefore, we conclude that UV photolysis causes the desorption of a single CO from Rh(CO) 2 to produce an observable quantity of Rh(CO), which is likely coordinated to an additional support OH under the explored conditions.…”

“…This results in a shift of CO stretching frequencies by ∼10 cm −1 , with lower wavenumbers corresponding to Rh(CO) 2 in more hydrated/hydroxylated regions of the support. 18 Because the lower frequency Rh(CO) 2 complex is more reactive during UV photolysis and Rh(CO) formation, we hypothesize that the photolysis process benefits from the increased local hydroxyl density around this complex, which will be discussed later.…”

“…At 143 K, trace H 2 O freezes (crystallizes as ice) on the Al 2 O 3 surface, excluding Rh(CO) 2 complexes into dry regions of the support and severely restricting their mobility, Figure 5c. Without this hydroxyl coordination, the two CO molecules are bound to Rh with similar binding energies 18 and desorb simultaneously, forming no observable Rh(CO) intermediate and leading to agglomeration of Rh atoms into small clusters, Figure 5d. We have previously observed the simultaneous desorption pathway during TPD from Rh supported on anatase TiO 2 , when oxidized Rh species were reduced by CO. 45 During TPD measurements, Rh(CO) 2 /TiO 2 intensity decreased without change in peak position or FWHM and without growth of an observable Rh(CO) feature.…”

“…Furthermore, the existence of Rh(CO) 2 as the most abundant reactive intermediate makes the spectroscopic identification of reactive intermediates following CO desorption challenging. 18,20 Various approaches have been used to weaken Rh−CO interactions in attempts to promote reaction rates, including selecting different oxide supports and modifying the local coordination environment of Rh via inorganic and organic promoters. 4,9,21−26 demonstrated that ultraviolet (UV) photon illumination can induce the desorption of a single CO molecule from Al 2 O 3 supported Rh(CO) 2 , even at cryogenic temperatures, to form Rh(CO).…”

Synthesis of Atomically Dispersed Rh Catalysts on Oxide Supports via Strong Electrostatic Adsorption and Characterization by Cryogenic Infrared Spectroscopy

“…Fragmentation occurs under CO environments because of the thermodynamic preference of Rh to exist as atomically dispersed gem dicarbonyl species, Rh(CO) 2 , on the oxide support rather than CO-saturated Rh clusters. − Notably, gem dinitrosyl, Rh(NO) 2 , formation and identification by FTIR has also been reported as a characterization method for atomically dispersed Rh species in zeolites. However, unlike atomically dispersed Rh species in zeolites, NO probe molecule FTIR spectra for atomically dispersed Rh on Al 2 O 3 and CeO 2 show no observable Rh( NO ) x signatures at ambient and higher temperatures . The absence of bands associated with Rh( NO ) x on these oxide supports at ambient or higher temperatures indicates that once NO adsorbs to Rh, it rapidly reacts to form N 2 O, NO 2 , or N 2 .…”