Importance of Species Heterogeneity in Supported Metal Catalysts

The structural heterogeneity of surface metal species, which is represented by the distribution in size, morphology, and local coordination environment of the active metal component, is almost inevitable in practical supported metal catalysts. This is often regarded as a major hindrance to the full utilization of metal loading and the high mass-specific catalytic activity. In this work, by quantitative evaluation of the individual reaction steps of a probe reaction, cyclohexanol dehydrogenation (an important reaction for hydrogen storage and transportation as well as high valued chemical production), we demonstrate that the inherent heterogeneity of supported Rhodium catalysts prepared by conventional synthesis has unique advantages in a complex heterogeneous catalytic reaction. The isolated Rh species (Rh₁) is extremely active for the first step of dehydrogenation, the transformation of cyclohexanol to cyclohexanone, while the Rh ensemble sites (Rh_e, including Rh clusters, Rh_n, and Rh nanoparticles, Rh_p) are highly efficient for the successive reaction step, cyclohexanone to phenol, for which the Rh₁ sites are almost inactive. Only with the coexistence of both active structures could the optimal reaction performance be achieved, which ambiguously demonstrates the importance of species heterogeneity in some multistep catalytic reactions. Our study provides a new view of the benefits from structural heterogeneity in practical catalysts and sheds light on the catalyst design strategy for complex catalytic reactions.