AbstractSolution Combustion Synthesis is a versatile method for the production of various materials directly at the nanoscale. The main source of heat comes for the exothermic combustion reactions that take place once the water in the solution has evaporated, so it is a self-sustaining thermal synthesis process. It is already widely used to prepare catalysts for laboratory and industrial purposes, due to the advantages it offers. This research project has been designed to study and enable the clarification of the mechanisms of reactions that take place during the production of Solution Combustion Synthesis catalysts based on nickel. Furthermore, the hydrogenation of maleic acid toward the as-synthesized catalysts was investigated to outline the catalysts properties and behaviour during catalysis.
However, SCS is a very sensitive synthesis approach and in this work an effort was made to investigate the main parameters that influence the final products’ composition and properties.
Nickel nitrate hexahydrate (Ni(NO3)2·6H2O) and aluminium nitrate nonahydrate (Al(NO3)3·9H2O) were used as oxidizers and glycine as the reducer. In most cases distilled water was added to the initial gel mixture to facilitate mixing prior to SCS. The parameters that were investigated during this work were:* water quality in the initial SCS mixture* pre-treatment of water used in the initial SCS mixture* duration of heated stirring as a pre-treatment for SCS solution* Fuel (reducer) to oxidizer ratio and total fuel concentration in the initial SCS mixture * preheating temperature in correlation to a computational model * heating mode during SCS and* time in furnace after SCS was completed.
This research work targeted in the investigation of the role of water in producing Ni-based nanopowders using SCS. More specifically, nanopowders were produced using Ni(NO3)2, glycine as a fuel and distilled water as a diluter. Different amounts of distilled water were added in that initial batch, in order to find the influence of the degree of dilution on the composition of the final products. In this work it was found for the first time that the initial concentration of nitrates in the aqueous solution affects the characteristics and properties of Ni-based catalysts including their final composition, crystallite size and parameters of crystal lattice, pores size distribution and surface area. In addition, the relative amount of water in the initial aqueous solution appears to have a substantial effect on their catalytic activity in maleic acid liquid phase hydrogenation. The underlying mechanism for this effect appears to be the prolonged persistence and delayed decomposition of hydrates that form during the early preheating stages of SCS. This is especially significant for structure-sensitive catalytic reactions, such as catalysis in the liquid-phase.
The structure, composition, surface area and catalytic activity of Solution Combustion Synthesis (SCS) catalysts are all influenced by the conditions of preparation and in particular, the glycine concentration in the initial SCS solution. Regular three-dimensional (3D) flowerlike Ni-NiO hierarchical architectures were synthesized by SCS. The results have revealed a three-dimensional percolation network with hierarchical structure on the basis of nano-structured metal oxides and metals synthesized during SCS. Such hierarchically nanoporous catalysts have versatile structural properties such as increased surface area and large overall pore volume that can alleviate diffusional limitations of conventional nanocatalysts with solely microporous frameworks. The three-dimensional percolation-like network and hierarchical structure of nano-composites on the basis of metal oxides and metals obtained by combustion in solutions provides a distinct possibility of increasing the selectivity and activity of such catalysts.
Some of the as-synthesized catalysts were tested for catalytic liquid-phase hydrogenation of unsaturated hydrocarbons in order to investigate the effect of the synthesis parameters on the catalysts’ catalytic behaviour. It was concluded that all the parameters above influence the SCS process in a complex way and subsequently the final products’ composition and activity in the hydrogenation process. The results revealed that the presence of Ni-Al alloys in the final SCS catalytic structure enhances significantly their catalytic performance. Moreover, in the Ni(NO3)2-glycine system the presence of NiO in the final product composition is crucial, as it acts as a carrier for the metallic nickel, and in its absence the catalysts is almost inactive.
The results of this extensive research of various parameters that were investigated enlightened the underlying mechanisms of SCS and exhibited its complex nature. In conclusion, SCS can be used to synthesize nano-catalysts with specific properties and offers notable advantages over other methods of producing catalysts.
|Date of Award||Jan 2020|
|Supervisor||Alexander Chroneos (Supervisor) & Galina Xanthopoulou (Supervisor)|