CO oxidation on metal-free nitrogen-doped carbon nanotubes and the related structure–reactivity relationships

Abstract

The reduction or elimination of noble metal used in carbon monoxide (CO) oxidation remains a challenge. Based on systematic research with methods using density functional theory, metal-free nitrogen-doped carbon nanotubes (NCNTs) are found to be effective catalysts for CO oxidation. The reaction route involves the direct oxidization of CO to CO₂ by O₂ adsorbed on the surface of NCNTs, as in CO + O₂ → CO₂ + O. The remaining O atom can further oxidize CO to CO₂. The barrier heights of the rate-determining step range from 0.477 to 0.619 eV for different catalysts investigated in the present study. These values are quite close to those processes using noble metal catalysts. The catalytic ability of NCNTs is influenced by the tube length, diameter and number of doped N atoms. The barrier height for the rate-determining step slightly oscillates with increased tube length, and progressively increases with expanded tube diameter. The structure–reactivity relationships of NCNTs in CO oxidation are revealed. A lower negative charge of the doped N atom corresponds to more reactive NCNTs. NCNT–O₂ with large spin densities on the O atom also show more powerful reactivity for CO oxidation. The present paper provides a way for the development of metal-free catalysts for CO oxidation.