Single-atom catalysts on ceria substrates: Exploring cluster and surface effects on methane activation

Abstract

The activation of methane (CH4) is a critical step in its conversion to high value products, which require the use of catalysts due to the higher stability of the C-H bond. In this study, we combined density functional theory calculations and the unit bond index-quadratic exponential potential approximation to investigate the first CH4 dehydrogenation in the TM/(CeO2)10 system, where TM represents a single transition-metal specie (Fe, Co, Ni, Cu) supported in the compact (CeO2)10 cluster. In addition, substrate size effects will be addressed by comparing our results with previous surface science studies by our group. We found a direct correlation between the magnitude of the TM adsorption energy on the CeO2 substrates (hollow and bridge sites) and the magnitude of charge transfer from the TM adatoms to the substrates, while CH4 has a weaker physisorption binding energy to the ceria cluster and the surface substrates. Molecular fragments derived from CH4 (CH3 and H) bind through chemisorption interactions to the TM and O sites, respectively. The estimated energy barrier for the cleavage of the C-H bond is lower when the reaction occurs in the cluster rather than on the surface. The molecular fragments exhibited better stabilization on cluster substrates, resulting in an exergonic dehydrogenation reaction; whereas, for surfaces, the reaction was endergonic. Among the selected TM species, Fe and Co adatoms are promising candidates for the first CH4 dehydrogenation on CeO2 substrates, which can be explained by the magnitude of the metal-support interaction and stabilization of the CH3 specie, which results in the lowest barriers among the evaluated transition metals.