The mechanism of B, N co-doping for enhancing graphene catalytic performance: CO oxidation and O2 dissociation as model reactions

Abstract

Carbon-based metal-free catalysts are becoming increasingly attractive for the low cost and environmental friendliness. With one more valence electron than C, N doping has been extensively studied for the preparation feasibility. While the main group element B is comparable to transition metals in many reactions for co-existence of lone pair electrons and vacant orbitals. The synergistic effect of co-doping B and N has been explored both experimentally and theoretically. However, the in-depth understanding of the structure–property relationships of B, N co-doping in carbon materials remains to be uncovered. Herein, through extensive density functional theory (DFT) calculations, the doping sites (meta vs ortho) and number of N atoms around B on the synergetic promotion for O2 activation was investigated. Our results reveal that ortho-N captured more electrons from B compared with meta-N. Moreover, O2 activation effect is directly related to the energy level of empty sp3 orbital of B. Subsequently, O2 dissociation and CO oxidation were used as probe reactions to evaluate the catalytic performance of all B, N co-doped graphene structures. It is found that the introduced N atoms, especially meta-N creates another electrophilic center to accommodate the dissociated O atom, thus O2 dissociation process was promoted kinetically. Besides, CO oxidation proceeds easily on meta- and meta, ortho-N co-doped structures, while the strong interaction between O2 and ortho-N doped substrates hampers the reaction, which perfectly obeys the Sabatier Principle. This work not only uncovers the synergetic effect of BN co-doping in carbon-based materials, but also shed new light on developing metal-free catalysts.