Synthesis-Structure-Performance Relationships for Supported Metal Catalysts

Abstract

Heterogeneous catalysts, which consist of many metal nanoparticles supported on highly porous, mechanically strong and chemically inert supports, are at the center of many existing as well as new and more sustainable processes, such as energy conversion and storage, nanoelectronics and the catalytic production of fuels and chemicals. For rational development of these materials, a comprehensive understanding of the processes that play a role during their synthesis is of utmost importance. Moreover, degradation of such materials by growth of nanoparticles under process conditions is detrimental for their functionality. Therefore, understanding the method of degradation and devising strategies to prevent these is paramount. The synthesis of heterogeneous catalysts can be done in many different ways. However, the more complicated the technique, the more difficult and expensive they become to perform on a large scale, which is necessary if the catalyst is ever to be used on an industrial scale. The method of impregnation and drying is often preferred, whereby a metal precursor dissolved in water is impregnated onto a premade porous support, which is subsequently dried to remove the solvent and calcined to decompose the precursor into the desired metal (oxide). Metal nitrate precursors, which generally have a high solubility in water, are used to obtain high loadings in a single step. Moreover, by impregnating only the pores of the support, no filtration or waste water is produced. Unfortunately, the resulting size and distribution of the metal nanoparticles are often difficult to control, with large particles or small particles clumped together into large aggregates often the result. Catalysts that suffer from aggregation are those used in the conversion of syngas, a mixture of H2 and CO, to more useful products. In the Fischer-Tropsch synthesis, syngas is converted into long hydrocarbon chains for the production of clean and more sustainable transportation fuels and lubricants from feedstocks other than crude oil. Cobalt is the desired metal for Fischer-Tropsch synthesis, and is commonly prepared on an industrial scale via impregnation of a porous support with cobalt nitrate solution. However, aggregates of cobalt nanoparticles are often reported, although their effect on the activity, selectivity and stability of the catalyst performance is not well understood. Alternatively, syngas can be converted into methane known as the methanation reaction, used to clean up the hydrogen feed for industrial ammonia plants and for the production of synthetic natural gas. This reaction is performed over nickel catalysts, which exhibit a high activity but suffer from Ostwald ripening via nickel carbonyl species, leading to rapid deactivation. The present work focuses on the synthesis of cobalt and nickel catalysts from their respective metal nitrate precursors via impregnation. Of main interest was to achieve control over the degree of aggregation and the particle size by studying the synthesis, in particular the drying step, and to elucidate the effects of particle size and nanoscale distribution on the performance of the Fischer-Tropsch and methanation reactions.