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

Abstract

Atomically dispersed Rh active sites on oxide supports have gained significant traction in heterogeneous catalysis due to their unique reactivity. In many reactions of interest, carbon monoxide (CO) is involved as a reactant or intermediate, leading to the formation of highly stable gem-dicarbonyl species, Rh(CO)2, that kinetically limit reaction rates by requiring CO desorption to produce reactive monocarbonyl intermediates, Rh(CO). Here we report on the use of ultraviolet (UV) photon illumination to induce CO desorption from Rh(CO)2 species supported on Al2O3 and produce Rh(CO). In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was used to track the conversion of Rh(CO)2 to Rh(CO) under UV illumination and various environmental conditions. The kinetics of Rh(CO)2 photolysis and the resulting maximum yield of Rh(CO) produced via photolysis were influenced by temperature (143–423 K) and interactions between Rh(CO) x species and hydrated regions of the Al2O3 support. The formation kinetics and maximum yield of Rh(CO) were significantly promoted at elevated temperature and in water cofeed. DRIFTS studies coupled with in situ X-ray absorption spectroscopy measurements during photolysis suggested that UV photolysis of Rh(CO)2 to produce Rh(CO) is promoted by occurrence in hydroxylated regions of the support where coordination of Rh(CO) to support-derived OH and H2O stabilize the reactive intermediate. Kinetic and in situ DRIFTS studies of the water gas shift reaction (CO + H2O → CO2 + H2) on atomically dispersed Rh/Al2O3 under thermal conditions and during irradiation of varying wavelengths and photon flux support the hypothesis that UV photons can promote the rate of Rh(CO) formation from Rh(CO)2 to promote the water gas shift rate. These results suggest that UV illumination can be an effective strategy to promote chemistries on atomically dispersed Rh/Al2O3, where the rate is limited by desorption of CO from the coordinatively saturated Rh(CO)2 active site.

Bond Selective Photochemistry at Metal Nanoparticle Surfaces: CO Desorption from Pt and Pd

“…The direct observations here of the influence of photons on CO desorption kinetics from Pt nanoparticles, and the consistent wavelength dependence, substantiates the previous hypotheses. 48 The maxima observed at 440 nm for photon influence on the kinetics of chemical processes involving CO desorption from Pt is also consistent with the resonance observed via 2-dimensional Vis-IR sum frequency generation (2D SFG) for CO on Pt(111) at 490 nm and with UV−vis measurements (Figure S10). 23 This suggests that visible photoexcitation of an electronic transition at the Pt−CO interface is responsible for transferring vibrational energy into the potential energy surface associated with CO desorption, which can promote CO desorption rates and catalytic kinetics that are limited by CO desorption.…”

“…This is akin to photolysis that is well-known for organometallic complexes. 48,53 It is worth noting that the absorbance measured at ∼240 nm in the UV−vis measurements for Pt and Pd following H 2 reduction corresponds to an interband transition within the metal. While the interband transition extends into the visible region for both metals, the lack of photon-induced CO desorption from Pd nanoparticles suggests that the interband excitation of Pd is not effective for promoting this chemistry.…”

Active Site Entropy of Atomically Dispersed Rh/Al2O3 Catalysts Dictates Activity for Ethylene Hydroformylation

“…We note that this proposed mechanism of CO desorption from Rh­(CO) 2 is somewhat different from recent reports from our group, where initially oxidized RhO x /γ-Al 2 O 3 was reduced in H 2 prior to CO adsorption (in the current work, oxidized RhO x /γ-Al 2 O 3 is reduced by a 1000 PPM CO stream). ,, When using H 2 as the initial reductant prior to CO adsorption, the CO desorption process was proposed to occur in two distinct steps where first, mobile Rh­(CO) 2 species find “wet regions” of the support (produced from the H 2 O generated during RhO x reduction by H 2 ) and then CO desorbs to produce Rh monocarbonyl, Rh­(CO), species that are stabilized through coordination to OH species native to the support. In this case, Rh­(CO) 2 is still mobile and still must find a stationary state prior to CO desorption, but following desorption of the first CO, an additional ligand (OH or H 2 O) coordinates to Rh­(CO) and stabilizes it as an observable intermediate.…”

“…Efforts to improve supported metal catalyst activity and selectivity are typically motivated by relationships between the active site structure or composition and the activation or reaction enthalpies of kinetically relevent elementary steps. − While it is known that activation and reaction enthalpies are often correlated to activation and reaction entropies, it is less common for catalyst modifications to be motivated by expected changes in entropic barriers. − Recently, it has been proposed that certain atomically dispersed metal active sites (e.g., Cu/Chabazite zeolites and Rh/Al 2 O 3 ) are mobile on support surfaces and that kinetically relevant elementary steps occur only when the active site reaches an immobile state, requiring a change in active site entropy. − In these circumstances, changes in the entropy of active sites during their transition from mobile to stationary species can be kinetically relevant and thus it is hypothesized that controlling active site entropy may be an effective design strategy for promoting catalytic rates.…”

Dry reforming of methane over single-atom Rh/Al2O3 catalysts prepared by exsolution

“…The CO-DRIFTS experiments confirmed the dissolution of the Rh atoms into the alumina framework (Fig. 2c; the peaks are in agreement with those reported by Christopher and coworkers 19 ). Noble metal catalysts often suffer from CO poisoning, [20][21][22] and one would expect to see in the DRIFTS the linear Rh-CO bond vibration.…”