Irregularly Shaped NiO Nanostructures for Catalytic Lean Methane Combustion

NiO nanomaterials prepared using a solid–liquid NH3·H2O precipitation method (NiO-NSL) were tested in the catalytic combustion of methane. The NiO-NSL presented a characteristic rod-like nanostructure with a length of about a few hundred nanometers except for a part of the nanoparticles. For comparison, the NiO nanomaterials prepared by the traditional liquid-phase NH3·H2O precipitation method (NiO-NLL) were tested in the same reaction conditions. NiO-NSL exhibited significantly higher methane combustion activity than NiO-NLL and achieved the complete combustion of methane at 390 °C, which was outstanding in non-noble metal-based catalyst. X-ray photoelectron spectroscopy (XPS) and hydrogen-temperature-programmed reduction (H2-TPR) results indicate that the surface Ni2+ content of NiO-NSL was higher than that of NiO-NLL, and the presence of more Ni2+ might be responsible for the enhanced activity. DFT calculations prove that the energy barrier for C–H bond activation on Ni2+ was lower than that on Ni3+, which was consistent with the higher methane catalytic combustion activity of NiO-NSL. In addition, when the precipitating agent was replaced with NaOH and (NH4)2CO3, the generalization of the solid–liquid precipitation method in the preparation of the NiO catalysts was also tested. The results show that the solid–liquid precipitation method proposed in this work was still applicable when NaOH was used as a precipitant. However, with the use of (NH4)2CO3 as a precipitant, the methane catalytic activity of the NiO nanoparticles prepared by the solid–liquid precipitation method was reduced to a certain extent compared with the traditional liquid-phase precipitation method. This research can open up a highly efficient and environmentally friendly method for the synthesis of methane combustion catalysts.