Molecular orbital theory calculations of the H₂O-carbon reaction

Carbon gasification with steam to produce H₂ and CO is an important reaction widely used in industry for hydrogen generation. Although the literature is vast, the mechanism for the formation of H₂ is still unclear. In particular, little has been done to investigate the potential of molecular orbital theory to distinguish different mechanism possibilities. In this work, we used molecular orbital theory to demonstrate a favorable energetic pathway where H₂O is first physically adsorbed on the virgin graphite surface with negligible change in molecular structure. Chemisorption occurs via O approaching the carbon edge site with one H atom stretching away from the O in the transition state. This is followed by a local minimum state, in which the stretching H is further disconnected from the O atoms and the remaining OH group is still on the carbon edge site. The disconnected H then pivots around the OH group to bond with the H of the OH group and forms H₂. The O atom remaining on the carbon edge site is subsequently desorbed as CO. The reverse pathway occurs when H₂ reacts with the surface oxygen to produce H₂O.