DFT study on the formation of HCO on ice and hydrogen atom diffusion on metals

Abstract

The focus of this work is on the dynamics of hydrogen atoms (H) on both metal and ice surfaces which is important for a wide range of natural processes. It is key both in the study of chemical reactions in space and also in H diffusion on close packed transition metal surfaces. The studies are low-temperature in nature and include investigation using density functional theory (DFT) on the formation of an HCO radical on ice, surface dynamics following HCO formation, H hops over energy barriers, and quantum tunnelling of H. HCO is the fundamental molecule from which more complex organic molecules are formed and it is revealed that HCO formation is always preferred over COH on the three ice surfaces investigated which include hexagonal ice, low-density amorphous ice, and high-density amorphous ice. Simulations are subsequently run to examine the changes in surface dynamics following formation, revealing absorption of reaction energy by the surfaces. Three dimensional potential energy surfaces for H adsorption on metals areconstructed using a novel approach involving DFT single point energy calculations, replication through symmetry operations, and interpolation. The potential energy surfaces give direct insight of H/metal adsorption as well as forming the basis from which information such as minimum energy paths and diffusion rates can be extracted. By applying a quantum tunnelling correction to the classical diffusion rates, total rates which contain both H tunnelling as well as thermally activated hops over potential barriers are obtained, revealing information regarding H diffusion on metals at low temperatures, and finally comparison to H tunnelling measurements and results from vibrational analysis is made.