Identifying the Active Phase on Atomically Dispersed Catalysts for Propane Dehydrogenation: Positively Charged vs Metallic Transition Metals

Atomically dispersed transition-metal catalysts have received increasing research interest in heterogeneous catalysis. However, the nature of the real active phase, specifically how the oxidation state of active species may affect the catalytic performance, remains elusive. In this work, ab initio molecular dynamics and large-scale molecular dynamics simulations based on neural network potentials have been employed to assess the structural stability of 52 single- and dual-atom catalysts with transition metals including Mn–Cu, Ru–Ag, and Os–Au embedded in the metal or oxygen vacancies on the defective TiO2 surface. On the thermodynamically stable surfaces, microkinetic analysis combined with results from DFT calculations indicates the metal atoms stabilized in the Ti vacancies with a positive oxidation state generally promote propane dehydrogenation (PDH) with the assistance of adjacent O sites, whereas those in the O vacancies exhibiting metallic properties act as a sole active site for C–H bond activation. The scaling relations established show that the adsorption energies of H and H&H can be used as two simple but effective PDH activity descriptors across both positively charged and metallic metal-doped surfaces. The calculated TOF under the realistic experimental conditions reaches a maximum at a slightly negative oxidation state, implying the Pt and Ir in the metallic state would dominate the kinetics of PDH. Moreover, a high selectivity toward propylene may be attained because the scaling relation between the activation energies for the C–H bond breaking in propane and propylene is broken in the absence of multiple metallic metal–metal sites on the atomically dispersed catalysts. An understanding of this structure–activity relationship is of vital importance for the rational design and optimization of heterogeneous catalysts for light alkane dehydrogenation.