Product-promoted acetylene cyclotrimerization to benzene on Ag(111)

• Benzene promotes 100% selective acetylene cyclotrimerization on Ag(1 1 1). • Co-adsorption of benzene reduces the threshold surface coverage for trimerization. • STM shows attractive acetylene-benzene interactions. • DFT reveals stabilization of the C4 transition state by benzene. Acetylene cyclotrimerization to benzene on Ag(1 1 1) has recently been found to be 100% selective. However, it requires monolayer coverages, limiting its practical application due to the high pressure required to attain this surface coverage of acetylene near room temperature. Here, we show that co-adsorbed benzene, the reaction product, promotes cyclotrimerization at submonolayer acetylene coverages without compromising the 100% selectivity of Ag(1 1 1) via temperature-programmed desorption experiments. Scanning tunneling microscopy and density functional theory (DFT) calculations elucidate the origin of this effect whereby attractive acetylene–benzene interactions locally compress acetylene domains. DFT calculations further illustrate how co-adsorbed benzene reduces the transition-state energy for formation of the key C 4 intermediate, making cyclotrimerization competitive with desorption. Together, these findings reveal how product-reactant interactions can enhance C–C coupling.