Understanding alkali metal promotion in hydrogenation catalysis through Strong Metal-Base Interaction.

Abstract

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.