Alkali metals have been widely used as promoters in heterogeneous metal catalysts to improve activity, product selectivity, and stability. Despite their prevalence, the precise role remains not fully uncovered due to the complex interplay between catalytic components and reactants involving dynamic structural changes. Herein, we elucidate the promotional effect of alkali metal in hydrogenation reactions from a unified perspective. Using Na-doped Rh/TiO2 as a representative system, we demonstrate a phenomenon, termed "Strong Metal-Base Interaction (SMBI)". The basic Na species (e.g. Na2O) intimate to the Rh particles facilitates heterolytic H2 splitting, capturing H+ while leaving H- on Rh sites. Consequently, there are suppressed hydrogen spillovers from Rh to the TiO2 surface and electron-rich Rh sites, fundamentally altering the catalytic behavior during hydrogenation. SMBI enables fine control of both activity and selectivity: Na doping shifts product selectivity in CO2 hydrogenation completely from CH4 to CO, enhances C=C hydrogenation via weakened H adsorption and electron-enriched Rh, and suppresses C=O and N=O hydrogenation due to restricted H spillover. These findings provide a unified framework for understanding alkali metal promotion and offer a rational approach for designing efficient hydrogenation catalysts.
Jung et al. (Fri,) studied this question.