Dry Reforming of Methane on Single-Site Ni/MgO Catalysts: Importance of Site Confinement

Single-site catalysts (SSCs) have drawn considerable attention, because of their superior behaviors in catalysis. However, the origin of promoting the effect of a single site is not well understood. Here, we take the single-atom Ni1/Mg(100) and single-site Ni4/Mg(100) catalysts as a case study to elucidate their behaviors under the complex dry reforming of methane (DRM, CO2 + CH4→ 2CO + 2H2) reaction by combining theoretical modeling (density functional theory and kinetic Monte Carlo simulation) and experimental studies. The synergy between single Ni atom and MgO is found to improve the binding property of MgO; yet, it is not enough to dissociate CO2 and CH4. It can be achieved by the single-site Ni4/MgO(100) catalyst, enabling the formations of CO, H2, and H2O under the DRM conditions. During this process, coking, as observed for bulklike Ni particles, is eliminated. By confining the reaction to occur at the isolated Ni sites in the SSC, the Ni4/MgO(100) catalyst is able to balance the CO2 and CH4 activations, which is identified as the key for tuning the DRM activity and selectivity of Ni/MgO catalysts. The theory-identified promotion introduced by increasing the size of MgO-supported Ni clusters from Ni1 to Ni4 and the MgO-introduced site confinement of single-site catalysts are verified by corresponding experimental studies, highlighting the essential roles of confined sites in tuning the performance of SSCs during complex catalytic processes.