CO Desorption from Oxygen Species on Carbonaceous Surface: 1. Effects of the Local Structure of the Active Site and the Surface Coverage

A systematic theoretical study using the density functional theory is performed to provide molecular-level understanding on the desorption of carbon monoxide from surface oxygen complexes that are formed in the gasification and combustion of coal. Particularly, a CO molecule release from carbonyl oxygen complexes in the presence of different oxygen environments was analyzed. Molecular carbonyl models of different sizes in the zigzag, armchair, and tip shapes of the active sites were selected. It was found that the shape of the local active site has a strong effect on the CO desorption energy, and they are correlated with the broaden feature of the CO molecule desorption in the temperature-programmed desorption (TPD) experiments of oxidized carbonaceous material. The calculated desorption activation energy range is in good agreement with experimental data. Molecular size convergence analyses on the carbonyl models suggest that the smallest graphene molecular system for accurate desorption structure on char is a three-ring molecule. The activation energy and normal-mode analyses for selected carbonyl complexes suggest that carbonyl surface oxygen complexes are stable structures and that they can be considered as labile surface oxygen complexes. The CO molecule desorption energy is affected by the influence of different neighboring surface oxygen groups on the carbon surface as well as the aromatic character of the molecular models. The desorption energy analysis suggests that the CO molecule can be adsorbed on clean carbonized surface to form stable structures. The preadsorbed oxygen group decreases the CO adsorption sticking probability on oxidized materials, but it remains an energetically favorable process.