Steering Surface Reaction at Specific Sites with Self-Assembly Strategy

To discern the catalytic activity of different active sites, a self-assembly strategy is applied to confine the involved species that are "attached" to specific surface sites. The employed probe reaction system is the Ullmann coupling of 4-bromobiphenyl, C₆H₅C₆H₄Br, on an atomically flat Ag(111) surface, which is explored by combined scanning tunneling microscopy, synchrotron X-ray photoelectron spectroscopy, and density functional theory calculations. The catalytic cycle involves the detachment of the Br atom from the initial reactant to form an organometallic intermediate, C₆H₅C₆H₄AgC₆H₄C₆H₅, which subsequently self-assembles with its central Ag atom residing either on 2-fold bridge or 3-fold hollow sites at full coverage. The hollow site turns out to be catalytically more active than the bridge one, allowing us to achieve site-steered reaction control from the intermediate to the final coupling product, p-quaterphenyl, at 390 and 410 K, respectively.