Synthesis of Nanocrystalline Ni/CeO2 Powders by Solution Combustion Method: Influence of Ceria Precursors and Fuels on the Catalysts Performance for Syngas Production by the Biogas Trireforming Reaction

摘要

The increasing of energy requirements and the environmental issues due to the use of fossil fuel together with the development of new technologies and business movement have encouraged the transition to a recycling and environmentally friendly based society. Moreover, some specific legislative tools, in most of the word aiming to increase the integration of renewable resources into the preexisting energy system, have transformed the biogas in one of the most widespread renewable fuel and the biogas energy is begin watched with keenest interest as environmentally-friendly alternative energy source instead of petroleum. An alternative and sustainable use of biogas, respect the conventional uses (ICE, micro-turbine), is based on syngas production as raw material for the synthesis of synthetic fuels with low environmental impact (hydrogen, methanol, dimethylether and synthetic gasoline) or other valuable chemicals. In this study, a solution combustion synthesis (SCS) method from different nitrate precursors (Ce(NO3)_3·6H2O, (NH4)_2Ce(NO3)6) and different fuels (oxalyldiydrazide, urea, carbohydrazide, glycerol) has been used to prepare nanocrystalline Ni/CeO2 powders to syngas production over the biogas trireforming process. The physicochemical properties of the final catalysts were determined by using different techniques: N2 adsorption-desorption isotherms (BET), X-ray diffraction (XRD), Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), temperature programmed reduction (TPR) and hydrogen chemisorption. Moreover the flame temperature associated to the synthesis was registered using a thermocouple in order to obtain a temperature profile of combustion. According to the results, crystallites size, superficial area and metal dispersion are strongly dependent on the nature of precursor, of the fuel and of the fuelt/nitrate ratio. In general, the increase of ceria and nickel crystallite sizes is closely related to higher combustion temperatures (> 1000°C), inversely, the increase of surface area and the metal dispersion is principally connected with the use of a fuel with lower combustion temperature as Urea (=600°C). Particularly, the samples prepared with ODH have shown a lower surface area (≤ 8 m~2/g) and higher ceria crystallites size (26-30 nm) respect the samples prepared with Urea (≈ 40m~2/g) with crystallites size between 9-15 nm as reported in figure 1. Moreover, the chemical-physical characterization of the catalysts reveals that almost all the samples show strong interaction between support and dispersed nickel and that some part of nickel incorporates into the CeO2 support.