Oxygen reduction reaction activity of nitrogen-doped titanium oxide in acid media

Abstract

For the first time, nitrogen atoms have been doped into partially oxidized titanium carbonitride powder to improve its oxygen reduction reaction (ORR) activity in acid media. The catalysts were synthesized using two heat-treatment steps, the first using a mixed gas containing H2, O2, and N2 under fixed conditions, and the second using NH3 gas at various temperatures and times. Field-emission transmission electron microscopy images, X-ray diffraction patterns, Raman spectra, X-ray photoelectron spectra, and cyclic voltammograms were analyzed. Carbon-coated tetragonal rutile-TiO2 plates were produced by the first heat-treatment. The ORR activity in 0.1moldm−3 H2SO4 increased with increasing temperature in the second heat-treatment at temperatures up to 1073K when the number of oxygen defects in the rutile-TiO2 lattice was increased by nitrogen doping. However, it decreased with further increases in temperature when the crystal structure was completely converted to cubic TiN. Heat-treatments were performed on four types of commercial TiO2 powders under NH3 gas at the optimum temperature, 1073K, but they all showed activity at potentials more than 0.4V lower than those with the titanium-carbonitride-derived catalysts, in which ORR currents were observed at above 0.8V versus a reversible hydrogen electrode. It was found that carbon species in the titanium carbonitride precursor powders, which produced graphitic layers during the first heat-treatment, were necessary for improving the ORR activity by the second heat-treatment under NH3 gas. A nitrogen-doped rutile-TiO2 lattice with oxygen defects was suggested to be responsible for the ORR.