Selectivity Trends and Role of Adsorbate–Adsorbate Interactions in CO Hydrogenation on Rhodium Catalysts

Abstract

Predictive-quality computational modeling of heterogeneously catalyzed reactions has emerged as an important tool for the analysis and assessment of activity and activity trends. In contrast, more subtle selectivities and selectivity trends still pose a significant challenge to prevalent microkinetic modeling approaches that typically employ a mean-field approximation (MFA). Here, we focus on CO hydrogenation on Rh catalysts with the possible products methane, acetaldehyde, ethanol, and water. This reaction has already been subjected to a number of experimental and theoretical studies with conflicting views on the factors controlling activity and selectivity toward the more valuable higher oxygenates. Using accelerated first-principles kinetic Monte Carlo simulations and explicitly and systematically accounting for adsorbate–adsorbate interactions through a cluster expansion approach, we model the reaction on the low-index Rh(111) and stepped Rh(211) surfaces. We find that the Rh(111) facet is selective toward methane, while the Rh(211) facet exhibits a similar selectivity toward methane and acetaldehyde. This is consistent with the experimental selectivity observed for larger, predominantly (111)-exposing Rh nanoparticles and resolves the discrepancy with earlier first-principles MFA microkinetic work that found the Rh(111) facet to be selective toward acetaldehyde. While the latter work tried to approximately account for lateral interactions through coverage-dependent rate expressions, our analysis demonstrates that this fails to sufficiently capture concomitant correlations among the adsorbed reaction intermediates that crucially determine the overall selectivity.

Computational Insights into Alloying and Confinement Effects on Promoted Activity and Selectivity of C2 Oxygenate over Rh-Based Catalysts

“…In these elementary steps, various C–O bond dissociation ways (including CO*, CHO*, COH*, CH 2 O*, CHOH*, CH 3 O*, and CH 2 OH* dissociation) and C–C coupling (C*–CO*, CH*–CO*, CH 2 *–CO*, and CH 3 *–CO* coupling) have been taken into account. The other C–C coupling possibilities, such as CH x * –CHO* and CH x * –COH * coupling, are not considered either because CO* coverage has been proved to be the most dominant species on the Rh-based catalyst. ,, In the sight of the reaction rate, the CH x intermediates are more likely to couple with adsorbed CO*. It is worth noting that the acetaldehyde (CH 3 CHO) is chosen as the target C 2 -oxy product because it was supposed to be the main C 2 -oxy on Rh-based catalysts. , As for the byproducts, only the methane (CH 4 ) production is considered in the MK model, while CH 3 OH is not involved.…”

“…We started by studying the mechanism of syngas conversion on the pure Rh(111) surface and clarifying the challenges of C 2oxy production. According to both theoretical and experimental evidences, the Rh terrace facet (111) should be mainly responsible for the C 2 -oxy production, although it is less active 10,18,20 In the sight of the reaction rate, the CH x intermediates are more likely to couple with adsorbed CO*. It is worth noting that the acetaldehyde (CH 3 CHO) is chosen as the target C 2oxy product because it was supposed to be the main C 2 -oxy on Rh-based catalysts.…”

“…Schumann et al also found this reverse correlation in Rh catalysts with different sizes . Recent theoretical investigation based on kinetic Monte Carlo supported this relation . Those studies mean that the C 2 -oxy selectivity will decrease with the increase of activity.…”

“…19 Recent theoretical investigation based on kinetic Monte Carlo supported this relation. 20 Those studies mean that the C 2 -oxy selectivity will decrease with the increase of activity. Breaking the existed reverse relationship between activity and selectivity over pure Rh catalysts is necessary and challenging.…”

Catlas: an automated framework for catalyst discovery demonstrated for direct syngas conversion

“…1,5,6 Experimental and theoretical studies had shown that under-coordinated surfaces have low selectivity towards C 2+ oxygenates 6 and that less active terrace sites have some selectivity towards the desired products. 7 A more recent work has shown that the previous treatment of adsorbate-adsorbate interactions on Rh(111) surfaces was inadequate and arrived at the opposite conclusion for Rh, 8 so the understanding of this chemistry is still evolving. To form multi-carbon oxygenates, the catalyst must be simultaneously good at disassociating *CO to form *CH x intermediates on the surface and able to maintain *CO such that it may be inserted into the *CH x intermediates.…”