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.
Keywords: Ullmann coupling; active site; scanning tunneling microscopy; self-assembly strategy; steering surface reaction.