DFT study of selective alpha-fluoride elimination of adsorbed CF3(ads) on both ag(111) and cu(111) surfaces

摘要

Total-energy calculations based on (1) density functional theory (DFT) in connection with ultrasoft pseudopotential (USP) and generalized gradient spin-polarized approximation (GGSA), (2) the partial structural constraint path minimization (PSCPM) method, and (3) an analysis tool of the partial density of states (PDOS) have been used to investigate the possible energetic profile for the selective activation of C-F bonds, that is, the single a-fluoride elimination of adsorbed CF3(ads) on both Cu(111) and Ag(111) surfaces leading to adsorbed CF2(ads) and F-(ads) on both surfaces. Following our proposed most possible reaction pathway, namely, the diffusion of the hcp-hollow site of CF3(ads) toward the top site accompanied by the single a-fluoride elimination to form the fcc-hollow site of F(ad,), our calculated. energy barrier on the Ag(111) surface is significantly larger than (similar to 0.462 eV) that on the Cu(111) surface. We attribute this unusual hi.-h-energy barrier for the single a-fluoride elimination of adsorbed CF3(ads) to forming a productlike distorted transition-state structure on the Ag(111) surface, that is, the larger stretching of a C-F bond and the larger distortion of bond lengths of Ag-Ag on the Ag(111) surface, in comparison with a less energy barrier to forming a reactant-like distorted transition-state structure on the Cu(111) surface, that is, smaller stretching of a C-F bond and smaller distortion of bond lengths of Cu-Cu on the Cu(111) surface. Consequently, the single a-fluoride elimination of adsorbed CF3(ads) to form adsorbed CF2(ads) and F-(ads) leading to the formation of CD2CF2(g), CD2=CD2(g), and CF2 = CF2(g) through coupling reactions with CD2(ads) coadsorbed on the Ag(111) surface will be suppressed by the methylene (CD2) insertion into the Ag-CF3(ads) bond with CD2(ads) coadsorbed on the Ag(111) surface to initially form adsorbed Ag-CD2CF3(ads) and to continually form CD2=CF2(g) through the beta-fluoride elimination on the same surface. Finally, our calculated surface electronic states, that is, PDOS, of both Cu(111) and Ag(111) surfaces and our calculated bonding nature, that is, PDOS, of both carbon and fluorine within adsorbed CF3(ads) on the same surfaces at their different transition-state structures, that is, reactant-like on Cu(111) versus productlike on Ag(111), are investigated to obtain further insight into the effect of both surface electronic states and C-F bond strength on their different reactivity for the single alpha-fluoride elimination of adsorbed CF3(ads) on both surfaces.