N-doped reduced graphene oxide promoted nano TiO2 as a bifunctional adsorbent/photocatalyst for CO2 photoreduction: Effect of N species

A series of TiO2/nitrogen (N) doped reduced graphene oxide (TiO2/NrGO) nanocomposites with varying concentration and bonding configurations of nitrogen were synthesized by a one-step urea-assisted hydrothermal method, and applied to photoreduction of CO2 with H2O vapor in the gas-phase under the irradiation of a Xe lamp. The effect of the N dopant (doping quantity and bonding configuration) on the catalytic performance of TiO2/NrGO was examined. In particular, TiO2/NrGO-300, with a 300:1 mass ratio of urea/GO in precursor solution, had the highest CO production yield (356.5 μmol g−1), manifesting a significant 4.4 and 2.2-fold enhancements of CO yield over pure TiO2 and TiO2/rGO, respectively. More significantly, TiO2/NrGO showed excellent catalytic stability during the prolonged reaction, while catalytic deactivation was observed for both pristine TiO2 and TiO2/rGO after a few hours. The promoting effects of N dopants on the structure and activity of TiO2/NrGO were investigated. It was demonstrated that NrGO with an appropriate N quantity and N-bonding configuration acted as a dual-functional promoter, simultaneously enhancing CO2 adsorption on the catalyst surface and facilitating electron-hole separation, while eventually boosted the photocatalytic performance. Experimental results in this work provide a better understanding of the critical roles of N dopants in the synthesized composites and also inspire the ongoing interest in better design of other N-doped graphene based materials for photoreduction of CO2.