Frances H. Yang

Ab initio molecular orbital study of adsorption of atomic hydrogen on graphite: Insight into hydrogen storage in carbon nanotubes

Ab initio molecular orbital (MO) calculations are performed to study the adsorption of H atoms on three faces of graphite: (0001) basal plane, (1010) zigzag edge and (1121) armchair edge. The relative energies of adsorption (or C-H bond energies) follow the order: zigzag edge.armchair edge.basal-plane edge, in agreement with previous semi-empirical MO results. However, it is found that adsorption on the basal plane sites is exothermic and stable, in contrast to previous semi-empirical results. On the edge sites, the C-H bond energy decreases by nearly 30 kcal / mol when two H atoms are adsorbed on the same site. On the basal plane, the C-H bond energy decreases from 46 kcal / mol when two H are adsorbed on alternating sites to 27 kcal / mol when they are adsorbed on two adjacent sites. Literature MO results of H adsorption on the exterior wall of SWNT are in fair agreement with that on the basal plane of graphite. The value 27 kcal / mol agrees well with experiment (23 kcal / mol) of TPD of hydrogen from MWNT. Three common features exist in the reported experiments on hydrogen storage in carbon nanotubes: slow uptake, irreversibly adsorbed species, and the presence of reduced transition metals (Fe, Co or Ni). Combined with the MO results, a mechanism that involves H dissociation (on metal catalyst) 2 followed by H spillover and adsorption (on nanotubes) is proposed for hydrogen storage in carbon nanotubes.

Selective Adsorption of Organosulfur Compounds from Transportation Fuels by π‐Complexation

Abstract Ab initio molecular orbital (MO) calculations were performed on the adsorption bond energies between the main sulfur compounds and the Cu+ on CuCl and CuY zeolite, for desulfurization of transportation fuels by π‐complexation sorbents. The relative adsorption bond energies of these compounds were measured by the elution order, based on the breakthrough curves of these compounds, from a column of CuY zeolite using commercial diesel and gasoline as the influents. The order from the elution followed: 4,6‐dimethyldibenzothiophene ≥ dibenzothiophene > benzothiophene ≥ 2‐methylthiophene > thiophene. The order is in agreement with that predicted from the calculations. The calculated values for benzene and thiophene were also in excellent agreement with the measured values that we reported earlier. The methyl and benzo groups have an electron‐donating effect on the aromatic rings that undergo π‐complexation. For the sulfur compounds, the thiophene ring was bonded to Cu+. For π‐complexation on both CuY and CuCl, the amount of electron forward donation was more than that of electron back donation for thiophenic adsorbates, but the reverse was true for benzene and toluene.

Ab initio molecular orbital study of the mechanism of SO2 oxidation catalyzed by carbon

Ab initio molecular orbital calculations were performed on the possible pathways of the carbon-catalyzed oxidation of SO2 by O2/H2O to form sulfuric acid. Both zigzag and armchair edge sites of graphite, with and without surface oxide, were considered as the possible active sites. For the sites with oxide, both isolated and twin oxides were included. MO calculations at the B3LYP/6-31G(d)//HF/3-21G(d) level were used for calculating the energies of SO2 adsorption, oxidation and hydration. Based on these calculations, three viable pathways emerged, and all three took place on the zigzag edge sites. Hence the armchair sites were not viable sites. On the bare surface, the only possible pathway involved the formation of a sulfurous acid intermediate. Thus, SO2 was first adsorbed on the bare zigzag sites, followed by reaction with H2O to form H2SO3, which was further oxidized by O2 to form the end product. On the zigzag edge site with isolated oxide, both pathways with either SO3 or H2SO3 as the intermediate are possible. Chemisorption on the edge sites containing twin oxides was not viable. This latter result explains the seemingly conflicting results in the literature regarding the dependence of SO2 adsorption (and oxidation) on the amount of surface oxygen.