Mechanistic Study of a Cs-Enhanced Pd/γ-Al2O3 Catalyst for the Reverse Water–Gas Shift Reaction

Abstract

In the reverse water–gas shift (RWGS) reaction, Pd-based catalysts generally exhibit a high CO2 hydrogenation activity and a low CO selectivity due to the competitive methanation reaction. In this work, cesium (Cs), an alkali-metal promoter with strong basicity, is introduced to modify Pd/γ-Al2O3 catalysts for improving CO2 activation and suppressing CH4 formation. The optimized Pd(1)-Cs(16)/γ-Al2O3 catalyst achieves a CO2 conversion of 65.07% at 600 °C, approaching thermodynamic equilibrium, and 100% CO selectivity. The introduction of Cs significantly modifies both the support and the Pd sites. The Cs species on γ-Al2O3 increase the density of surface basic sites, enhancing the CO2 adsorption and activation. Meanwhile, the Cs influences the Pd dispersion and increases the electron density of Pd, suppressing the formation of CH4. Operando ME-DRIFTS/IR reveals that carbonate, bicarbonate, and formate species are the key surface intermediates. It is speculated that the RWGS reaction over Pd(1)-Cs(16)/γ-Al2O3 follows a formate-mediated associative interfacial pathway, in which carbonate/bicarbonate species are hydrogenated into formate by Pd-derived hydrogen and subsequently converted to CO. This work provides mechanistic guidance for designing highly selective Pd-based RWGS catalysts.