Phase transformations of ternary carbides, nitrides and carbonitrides

QD Chemistry

Alshibane, Ihfaf ; Hargreaves, Justin

University:University of Glasgow

Department:School of Chemistry

关键词: Ternary carbides, nitrides and carbonitrides, ammonia synthesis, methane cracking.;   

Others  :  http://theses.gla.ac.uk/30702/1/2018Alshibanephd.pdf

来源: University of Glasgow

【 摘 要 】

The development of novel efficient catalytic materials to improve the efficiency of industrial processes has been the driving force for many academic and industrial studies. The general approach adopted to enhance the activity of a given catalytic formulation is usually based on enhancing the structural and structural properties (e.g. crystal size and surface area) by adopting new synthesis methods, by supporting the active phase or by modifying the reactivity of the parent materials by adding dopants. However, in a less studied approach, it has been shown that the presence of interstitial species such as carbon or nitrogen can modify the electronic structure of parent metals apparently conferring, in the case of systems such as molybdenum carbide, properties akin to precious metals. This approach allows not just improvement of the catalytic activity in an incremental manner but also the design entirely new catalytic formulations. In this context, the effect of the interstitial elements carbon and nitrogen upon the activity of a range binary and ternary molybdenum based materials for ammonia synthesis and methane cracking has been investigated within this thesis. The performance of Co3Mo3N, Co3Mo3C, and Co6Mo6C for ammonia synthesis has been compared. Depending on the chemical composition, significant difference in catalytic activity was apparent. In contrast to Co3Mo3N, which is active at 400 °C, Co3Mo3C was found to be only active at a reaction temperature of 500 °C. Furthermore, in-situ NPD revealed that the catalytic activity of ternary cobalt molybdenum systems is associated with the presence of N in the 16c Wyckoff crystallographic site. Co6Mo6C was found to be inactive under the conditions tested.The same comparison between the chemical composition and the catalytic activity has been made in the context of methane cracking. Although all the prepared materials (i.e.Co3Mo3N, Co6Mo6N, Co3Mo3C, and Co6Mo6C) displayed catalytic activity, differences as a function of chemical composition were observed. Among the evaluated catalysts, the Co6Mo6N sample showed the highest activity. However, in-situ and post-reaction analysis revealed a significant phase transformation during reaction which explains the differences in terms of catalytic reactivity. In summary, this thesis details a comparison between the catalytic performance of a range of binary and ternary molybdenum based materials presenting different chemical compositions. Particular attention has been directed towards the role of, and the interconversion between, lattice C and N species with the intention of elucidating their influence upon catalytic behaviour.