Why a good catalyst can turn out detrimental to good polymerization

Steps at metal surfaces are chemically active sites, and hence they have an enormous influence on catalytic reactions. However, their higher catalytic activity does not necessarily imply improving the efficiency in more complex reactions, such as Ullmann coupling. Here, by investigating the interplay of step and terrace sites in the polymerization of 4,4'-dibromo-para-terphenyl molecules on Au to form poly-p-phenylene (PPP), we demonstrate that the restriction in molecular diffusion imposed by steps makes polymerization difficult. Combining a battery of surface-sensitive techniques, we have monitored the entire Ullmann polymerization process across different crystal planes by using a crystal surface macroscopically curved around the (111) plane. With this approach, it is possible to smoothly tune the step type and density across the sample, while keeping all reaction parameters identical, to prove that despite the significant reduction of debromination temperature near atomic steps, there is a delay in the formation of long PPP chains at densely stepped planes. Thus, we disentangle the different reaction steps and determine how to speed up and optimize the entire process through a proper choice of the vicinal orientation of the substrate.