Single-Atom Catalyst Restructuring during Catalytic Reforming of CH₄ by CO₂

The supported single-atom catalyst has been a significant type of heterogeneous catalyst as it often offers a catalytic performance not achievable by nanoparticle catalysts of a metal or compound such as oxide. It is well acknowledged that knowing the surface structure of a catalyst under catalytic conditions is significant for uncovering the intrinsic correlation between an observed catalytic performance and its corresponding surface structure. How the surrounding gaseous environment could restructure the surface of a single-atom catalyst is crucial for establishing such a correlation. Here Rh1-based single-atom catalyst, Rh1/CeO2, was prepared as a model of single-atom catalyst and methane reforming by CO2 to produce H2 and CO was chosen as a probe reaction to explore potential restructuring of pristine single-atom sites under reaction and catalytic conditions. It is found that the singly dispersed Rh1 atoms in surface region of the support readily sinter to form Rh nanoparticles with a size of 0.8–1.0 nm under a reduction condition in H2 at 350 °C. The formed Rh nanoparticles supported on CeO2 are fragmented into Rh clusters containing 6–10 atoms at 400 °C by CO, a product formed from CH4 reforming by CO2. Then, the Rh clusters are further broken into single-atom Rh1 sites coordinating with five oxygen atoms, Rh1O5 on CeO2, driven by more CO formed under the CH4 dry reforming condition at 600–700 °C. The coordination environment of the Rh1 atoms formed under a catalytic condition at 600–700 °C is distinctly different from the Rh atoms of Rh nanoparticles or clusters formed at 350–400 °C in the same flowing mixture of reactants. After catalysis at 700 °C, these active Rh1-based single-atom sites formed under catalytic conditions sinter to form Rh2 and Rh3 clusters along with cooling in the mixture of reactants to relatively low temperatures. These observed product-driven restructurings suggest the significance of considering product molecules while establishing a correlation between the surface structure of a single-atom catalyst and its corresponding catalytic performance. Such restructurings suggest an avenue for designing catalysts by engineering different catalytic sites through tuning surface structure at an atomic scale under different catalytic conditions.