New insights into the role of the electronic properties of oxide promoters in Rh-catalyzed selective synthesis of oxygenates from synthesis gas

A series of 2.5% Rh/M@Al2O3 model catalysts were prepared by supporting Rh on high-area gamma-Al2O3, resulting in a surface covered by a monolayer (4.5-7 atoms/nm(2)) of MO promoter oxides (M = Fe, V. Nb, Ta, Ti, Y, Pr, Nd, Sm). The catalysts were extensively characterized and evaluated for the conversion of synthesis gas to oxygenates at 553 K, 5.0 MPa, H-2/CO = 1, and space velocity adjusted to attain CO conversion around 15%. The broad range of products formed depending on the specific promoter were, for the first time, quantitatively described using the selectivity parameter (Phi) defined here, which indicates, for a given reaction product, the contribution of carbon atoms derived from dissociative (C-dis) and nondissociative (C-ins) activation of CO. Both the catalytic activity and, more interestingly, the selectivity pattern given by the Phi parameter were correlated with the electronic properties of the MOx promoters (i.e., electron-donating/electron-withdrawing capacity) for an extensive series of catalysts. Low-temperature and at-work CO-FTIR experiments suggested that the high activity and hydrocarbon selectivity displayed by catalysts promoted by more electron-withdrawing (acidic) oxide promoters (e.g., TaOx) were related to a higher proportion of bridged Rh-2(CO)(B) adsorption sites and to a higher electron density (i.e., a higher electron back-donation ability) of the Rh-0 surface sites, both factors promoting CO dissociation events. In contrast, linear CO adsorption on Rh-0 sites displaying decreased electron back-donation in catalysts promoted by electron-donating (basic) oxides (e.g., PrOx, SmOx) was likely related to nondissociative CO activation and thus to the selective formation of oxygenates. TEM. XPS, and CO-FTIR results pointed to differences in morphology, rather than size or partial electronic charge, of the nano-sized Rh-0 crystallites as the likely cause for the different proportions of CO adsorption sites. The Rh NP morphology, both as-reduced and at-work, is a function of the electronic properties of the underlying promoter oxide. (C) 2011 Elsevier Inc. All rights reserved.