Small concentrations of chemical promoters significantly influence reaction outcomes in industrial metal catalysts, affecting product selectivity and catalytic activity. However, the mechanisms underlying these effects remain limited. To investigate promotion mechanisms in heterogeneous catalysis, we studied ethylene epoxidation over Ag/α-Al₂O₃ catalysts, focusing on Cs and Cl promoters. Industrial ethylene epoxidation catalysts, typically promoted with alkali (Cs, K), chlorine, and rhenium, can achieve approximately 90% ethylene oxide (EO) selectivity compared to around 50% for unpromoted catalysts. To explore the catalyst’s phase space, we utilized first-principles density functional theory augmented with machine-learned artificial neural network potential coupled with grand canonical Monte-Carlo simulations (DFT-ANNP-GCMC). Additionally, we employed operando surface-enhanced Raman spectroscopy (SERS), kinetic measurements, and experiment-guided microkinetic modeling to examine how Cs and Cl promoters affect oxygen intermediates and the properties of Ag/α-Al₂O₃, arriving at a picture that depicts an intricate interplay between different mechanisms depending on reaction conditions.
Our results indicate that the unpromoted Ag/α-Al₂O₃ surface hosts a diverse array of intermediates under reaction conditions, including atomic oxygen (Raman peaks: 300–600 cm⁻¹), surface oxides (600–800 cm⁻¹), and molecular oxygen (800–1200 cm⁻¹). We established a correlation between surface oxygen species and EO selectivity, showing that enhancing EO selectivity requires controlling the distribution of specific oxygen motifs while minimizing non-selective oxygen species. Further experiment-guided microkinetic modeling revealed that Cs promotion increases oxygen coverage and reduces the O₂ dissociation barrier compared to unpromoted surfaces. On the other hand, Cl promotion significantly decreases the reaction rate while greatly enhancing EO selectivity. Our findings demonstrate that the influence of Cs and Cl on EO selectivity is highly dependent on reaction conditions. These insights advance the understanding of promoter mechanisms in silver-based catalysis and can guide the design of more efficient catalysts for ethylene epoxidation.
