(49c) Decoupled Scaling Relationships of Atomic and Molecular Adsorption Energies On Transition-Metal Carbide Catalysts
Currently in the United States, natural gas is the primary feedstock for the production of many major chemical commodities such hydrogen and ethylene. The sustainable synthesis of these chemical energy carriers via the catalytic reduction of abundant resources such as carbon dioxide and water with renewable energy and without fossil fuel requires the development of efficient and inexpensive catalysts for large-scale production. The design of transition metal or metal alloy catalysts is often limited by the scaling of adsorption energies of reactants and reaction products. This work uses electronic-structure calculations to present how transition-metal carbide surfaces can decouple the scaling of hydrogen, carbon and oxygen adsorption energies - thereby increasing the degrees of freedom for the rational design of novel catalytic materials. Throughout the 12 studied transition metal carbide surfaces, carbides show an oxophilic departure from the scaling relations of adsorption energies on transition metal surfaces and a carbophobic shift of carbon adsorption energies by 0.30 to 1.8 eV relative to the (211) surface of the parent metal. Given a highly hybridized and delocalized electronic structure, the metal-projected d-band centers correlate with the reported adsorption energies. However, the preference of carbides for oxygen adsorption, site preferences of adsorbates, and the tendency of a surface to form C-C bonds between the lattice carbon and adsorbed carbon may be understood due to shifts in the sp-band center and an increasing degree of electron localization with decreasing electronegativity of the metal. Correlation of atomic adsorption energies with relevant molecular adsorbates such as CO, HCO, COOH or HCO2 as well as the activating or passivating influence of carbon vacancies or oxygen adatoms on adsorption energies respectively will be demonstrated.