Thermodynamic evaluation of steam gasification mechanisms of carbonaceous materials

Reactions of water with clean, oxidized, and hydrogenated carbon surfaces were investigated using density functional theory. It was confirmed that H2O can dissociatively chemisorb through highly exothermic reactions on the active sites of clean zigzag and armchair carbonaceous models to yield stable intermediates such as hydroxyl, semiquinone, and cyclic ether functional groups. Since the main products of the carbon-steam gasification process are molecular hydrogen and carbon monoxide, several pathways for their evolution are proposed, all of which are endothermic reactions. The interaction of H2O with an oxidized surface in some cases is even more exothermic than the interaction with the active sites of a clean surface. However, it becomes endothermic when H2O interacts with a hydrogen saturated surface. Thus, the presence of hydrogen hinders the gasification reactions by blocking of the active sites rather than by removing surface oxygen. (C) 2009 Elsevier Ltd. All rights reserved.