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

As ascertained using highly dispersed model Rh/M@Al 2O 3 catalysts, the selectivity pattern in the synthesis of oxygenates from synthesis gas is dictated by the electron-withdrawing/donating power (Lewis acidity/basicity) of the underlying metal oxide promoter. [Display omitted] ► Study of promotion effects without overlap of metal nanoparticle size effects. ► The broad selectivity pattern is modeled by a single parameter, pondering the presence of carbon atoms derived from CO dissociation or insertion. ► Differences in the CO adsorption modes and electron back-donation are related to the different CO activation pathways. A series of 2.5% Rh/M@Al 2O 3 model catalysts were prepared by supporting Rh on high-area γ-Al 2O 3, resulting in a surface covered by a monolayer (4.5–7 atoms/nm 2) of MO x 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 ( Φ) 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 Φ parameter were correlated with the electronic properties of the MO x 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., TaO x ) 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., PrO x , SmO x ) 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