Heterogeneous catalysis is central for efficient energy and matter transformation in chemistry industry. Enormous efforts have been exerted to precise understanding of active sites, including identification of their atomic structures and establishment of structure-activity relationship. Meanwhile, the interaction between adjacent active sites has long been underexplored. The emerging single-atom catalysts (SACs) can serve as ideal model systems to study new physics and chemistry brought by such cooperative effect (Scheme 1). By using FeN3 embedded in graphene as a model system, we first showed that neighboring FeN3 sites can exhibit cooperative spin transition induced by the adsorption of CO. The cooperative communication between two FeN3 sites is mediated by the graphene network. Then we studied impacts of adjacent FeN3 sites on the oxygen reduction reaction (ORR). The midway region between two adjacent FeN3 sites of proper distances was found to act as the active center for two-electron reduction pathway of ORR producing hydrogen peroxide. This pathway relies on weakly physisorption of all species related. Further, we found that as the distance between two FeN3 sites decreases, the energetics of potential-limiting step of four-electron reduction pathway of ORR is worse, suggesting a negative impact of proximity of FeN3 sites on ORR. In addition, we investigated the inter-site distance in a monodispersed molybdenum SAC on nitrogen reduction reaction (NRR). We found that as two MoN3 sites are sufficiently close, the energy change in rate-limiting step could be appreciably affected. Moreover, two adjacent MoN3 sites can induce a bridge-like adsorption pattern for N2. It then undergoes a barrierless N-N dissociation induced by partial protonation of each N atom, which is rarely seen in NRR. Finally, we showed that this bridge-on adsorption pattern of N2 can help break the scaling relationship that constrains the production of ammonia. In summary, our work indicate that impact of active sites density in SACs play a critical role in their catalytic activities.

Theoretical study on cooperative single-atom active sites towards breaking scaling relations

ACS Spring Meeting 2021

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