Dynamic Oxygen on Surface: Catalytic Intermediate and Coking Barrier in the Modeled CO2 Reforming of CH4 on Ni (111)

We identify Ni–O phases as important intermediates in a modeled dry (CO2) reforming of methane catalyzed by Ni (111), based on results from in operando near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) experiments, corroborated by low energy electron diffraction (LEED) and scanning tunneling microscopy (STM) measurements. We find that, under a CO2 or CO2–CH4 atmosphere, the Ni–O phases exist in the forms of p(2 × 2)-structured chemisorbed oxygen (Chem-O), epitaxial NiO (111), or oxygen-rich NixOy (x < y, typically Ni2O3), depending on the chemical potential. The growth rates of the Ni–O phases have a negative correlation with temperature from 600 to 900 K, proving that their dynamic concentrations in the reaction are not limited by CO2 activation, but by their thermal stability. Between 300 and 800 K (1:1 CH4 and CO2 mixture), oxidation by CO2 is dominant, resulting in a fully Ni–O covered surface. Between 800 and 900 K, a partially oxidized Ni (111) exists which could greatly facilitate the effective conversion of CH4. As CH4 is activation-limited and dissociates mainly on metallic nickel, the released carbon species can quickly react with the adjacent oxygen (Ni–O phases) to form CO. After combining with carbon and releasing CO molecules, the Ni–O phases can be further regenerated through oxidation by CO2. In this way, the Ni–O phases participate in the catalytic process, acting as an intermediate in addition to the previously reported Ni–C phases. We also reveal the carbon phobic property of the Ni–O phases, which links to the intrinsic coking resistance of the catalysts. The low dynamic coverage of surface oxygen at higher temperatures (>900 K) is inferred to be an underlying factor causing carbon aggregation. Therefore, solutions based on Ni–O stabilization are proposed in developing coking resisting catalysts.