Intrinsic Selectivity and Structure Sensitivity of Rhodium Catalysts for C₂₊ Oxygenate Production

Abstract

Synthesis gas (CO + H2) conversion is a promising route to converting coal, natural gas, or biomass into synthetic liquid fuels. Rhodium has long been studied as it is the only elemental catalyst that has demonstrated selectivity to ethanol and other C2+ oxygenates. However, the fundamentals of syngas conversion over rhodium are still debated. In this work a microkinetic model is developed for conversion of CO and H2 into methane, ethanol, and acetaldehyde on the Rh (211) and (111) surfaces, chosen to describe steps and close-packed facets on catalyst particles. The model is based on DFT calculations using the BEEF-vdW functional. The mean-field kinetic model includes lateral adsorbate-adsorbate interactions, and the BEEF-vdW error estimation ensemble is used to propagate error from the DFT calculations to the predicted rates. The model shows the Rh(211) surface to be ∼6 orders of magnitude more active than the Rh(111) surface, but highly selective toward methane, while the Rh(111) surface is intrinsically selective toward acetaldehyde. A variety of Rh/SiO2 catalysts are synthesized, tested for catalytic oxygenate production, and characterized using TEM. The experimental results indicate that the Rh(111) surface is intrinsically selective toward acetaldehyde, and a strong inverse correlation between catalytic activity and oxygenate selectivity is observed. Furthermore, iron impurities are shown to play a key role in modulating the selectivity of Rh/SiO2 catalysts toward ethanol. The experimental observations are consistent with the structure-sensitivity predicted from theory. This work provides an improved atomic-scale understanding and new insight into the mechanism, active site, and intrinsic selectivity of syngas conversion over rhodium catalysts and may also guide rational design of alloy catalysts made from more abundant elements.

Insights into structure and dynamics of (Mn,Fe)Ox-promoted Rh nanoparticles

“…The promising performance of Mn-Fe-promoted silica-supported Rh catalysts in the synthesis of higher alcohols initiated a number of studies on the promoter effects of Fe [8][9][10][12][13][14][15][16][17][18] and Mn 9,11,17,[19][20][21][22][23][24][25] additives. However, it appears that these investigations do not converge in consistent conclusions.…”

“…The lack of clarity might be ascribed to different procedures and materials that have been used for catalyst synthesis and activation, and the varying promoter content applied resulting in different catalytic properties. 8 Beyond that, density functional calculations based on single crystals indicate substantial structure sensitivity in CO hydrogenation over pure Rh.…”

“…22,29,30 The complex reaction mechanism for CO hydrogenation over Rh-based catalysts basically involves CO dissociation, hydrogenation of adsorbed carbon species into CH x (x ¼ 0-3), CO insertion into adsorbed CH x to form acyl species (CH x CO) and their subsequent hydrogenation to ethanol. Defect sites are responsible for the dissociation of CO 28,29 with a particular low barrier on stepped Rh(211) surfaces, 8,28 leading to the selective formation of methane on this facet due to fast CO dissociation, while the rates for ethanol and acetaldehyde are comparable on both stepped Rh(211) and terrace Rh(111) sites. 8 However, the barrier of CO dissociation increases with decreasing particle size, 28 accounting for an optimum particle size in view of activity and C 2 oxygenate formation.…”

“…Defect sites are responsible for the dissociation of CO 28,29 with a particular low barrier on stepped Rh(211) surfaces, 8,28 leading to the selective formation of methane on this facet due to fast CO dissociation, while the rates for ethanol and acetaldehyde are comparable on both stepped Rh(211) and terrace Rh(111) sites. 8 However, the barrier of CO dissociation increases with decreasing particle size, 28 accounting for an optimum particle size in view of activity and C 2 oxygenate formation. The Rh particles in a real catalyst are characterized by a certain particle size distribution and the particles contain both terrace (111)-like and stepped (211)-like sites.…”

The interaction of CO molecules on Au–Rh bimetallic nanoclusters supported on a thin film of Al2O3/NiAl(100)

“…The adsorption of CO on solid surfaces and its chemical bonds with those surfaces have been extensively studied because CO is involved in many important reactions, such as dehydrogenation of methanol, 1-8 water-gas-shi reaction, 9-11 CO oxidation [12][13][14][15] and the Fischer-Tropsch process; [16][17][18][19][20][21][22] CO also commonly serves as a probe molecule to investigate surface structures. [23][24][25][26][27][28][29][30][31][32][33][34][35][36] A comparison of infrared (IR) stretching wavenumbers between metal carbonyls and chemisorbed CO molecules on solid surfaces indicates the highly localized nature of the surface chemical bonds.…”