Temperature-Programmed Reduction of NiO/Al2O3 by Biochar In Situ Generated from Citric Acid

Abstract

The reduction of metal oxides by biochar is an important reaction for many biomass utilization technologies. This work investigated the temperature–programmed reduction (TPR) of NiO/Al2O3 by in situ generated biochar from citric acid pyrolysis. Firstly, NiO/Al2O3 was loaded with citric acid by impregnation and then heated from ambient temperature to 900 °C in a N2 flow. The process was on–line analyzed by the TGA–FTIR technique. Secondly, a series of intermediates was obtained and characterized by XRD, CHNO elemental analysis, and temperature programmed oxidation (TPO). Lastly, a control experiment of unsupported NiO was conducted to show the influence of Al2O3 support on the NiO reduction. Results showed that the whole heating process could be resolved into two parts that is citric acid pyrolysis and NiO reduction at a heating rate of 5 °C/min. The NiO reduction occurred above 400 °C with the biochar from citric acid pyrolysis as reductant. In the temperature–programmed reduction process, the Al2O3–supported NiO exhibited three reduction phases in contrast with only one reduction phase for the unsupported NiO. A hypothesis was proposed to explain this. The presence of Al2O3 support may result in different deposition sites of biochar (on NiO or on Al2O3), and consequently different reduction mechanisms.