Chemisorption of SO2 on graphite surface: A theoretical ab initio and ideal lattice gas model study

The first step of the reaction of SO2 with the graphite surface, corresponding to the chemisorption process, was investigated at ab initio MP2/6-31G(d)//HF/6-31G(d) level of theory using pyrene and its dehydrogenated derivatives as models. We have considered three possible adsorption sites: the insaturation of the fused aromatic rings (basal plane), the benzyne structure of the armchair edge and the triplet biradical species of the zigzag edge. Our results show that the adsorption on the fused aromatic rings is very unfavorable, with adsorption energies positive by 80-90 kcal mol−1. On the other side, the adsorption on the armchair edge have energies in the range of −5 to −51 kcal mol−1, while the adsorption on the zigzag edge is the most favorable, with energies of −61 to −100 kcal mol−1. Adsorption thermodynamics data were obtained from ab initio calculations combined with an ideal lattice gas model, which leads to a Langmuir like adsorption isotherm. At 900 °C, corresponding to the experimental conditions, only the zigzag edge will be fully covered with SO2 molecules, which bound to the surface through two C-O bonds, forming a five member ring, or one C-O and one C-S bond through a four member ring. The present study can be relevant for SO2 adsorption on carbon nanotubes.