Enhancing the activity of Pd ensembles on graphene by manipulating coordination environment

Atomic dispersion of metal catalysts on a substrate accounts for the increased atomic efficiency of single-atom catalysts (SACs) in various catalytic schemes compared to the nanoparticle counterparts. However, lacking neighboring metal sites has been shown to deteriorate the catalytic performance of SACs in a few industrially important reactions, such as dehalogenation, CO oxidation, and hydrogenation. Metal ensemble catalysts (M_n), an extended concept to SACs, have emerged as a promising alternative to overcome such limitation. Inspired by the fact that the performance of fully isolated SACs can be enhanced by tailoring their coordination environment (CE), we here evaluate whether the CE of M_n can also be manipulated in order to enhance their catalytic activity. We synthesized a set of Pd ensembles (Pd_n) on doped graphene supports (Pd_n/X-graphene where X = O, S, B, and N). We found that introducing S and N onto oxidized graphene modifies the first shell of Pd_n converting Pd-O to Pd-S and Pd-N, respectively. We further found that the B dopant significantly affected the electronic structure of Pd_n by serving as an electron donor in the second shell. We examined the performance of Pd_n/X-graphene toward selective reductive catalysis, such as bromate reduction, brominated organic hydrogenation, and aqueous-phase CO₂ reduction. We observed that Pd_n/N-graphene exhibited superior performance by lowering the activation energy of the rate-limiting step, i.e., H₂ dissociation into atomic hydrogen. The results collectively suggest controlling the CE of SACs in an ensemble configuration is a viable strategy to optimize and enhance their catalytic performance.