Methane cracking over cobalt molybdenum carbides

The catalytic behaviour of Co3Mo3C, Co6Mo6C, Co3Mo3N and Co6Mo6N for methane cracking has been studied to determine the relationship between the methane cracking activity and the chemical composition. The characterisation of post-reaction samples showed a complex phase composition with the presence of Co3Mo3C, α-Co and β-Mo2C as catalytic phases and the deposition of different forms of carbon during reaction

Synthesis and methane cracking activity of a silicon nitride supported vanadium nitride nanoparticle composite

The co-ammonolysis of V(NMe2)4 and Si(NHMe)4 with ammonia in THF and in the presence of ammonium triflate ([NH4][CF3SO3]) leads to the formation of monolithic gels. Pyrolysing these gels produces mesoporous composite materials containing nanocrystalline VN and amorphous silicon imidonitride. Elemental mapping indicated a thorough distribution of VN with no evidence of large cluster segregation. Whilst not active for ammonia synthesis, the silicon nitride based materials were found to possess activity for the COx-free production of H2 from methane, which makes them candidates for applications in which the presence of low levels of CO in H2 feedstreams is detrimental

Phase transformations of ternary carbides, nitrides and carbonitrides

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

A Comparison of the Reactivity of the Lattice Nitrogen in Tungsten Substituted Co3Mo3N and Ni2Mo3N

The effect of the partial substitution of Mo with W in Co3Mo3N and Ni2Mo3N on ammonia synthesis activity and lattice nitrogen reactivity has been investigated. This is of interest as the coordination environment of lattice N is changed by this process. When tungsten was introduced into the metal nitrides by substitution of Mo atoms, the catalytic performance was observed to have decreased. As expected, Co3Mo3N was reduced to Co6Mo6N under a 3:1 ratio of H2/Ar. Co3Mo2.6W0.4N was also shown to lose a large percentage of lattice nitrogen under these conditions. The bulk lattice nitrogen in Ni2Mo3N and Ni2Mo2.8W0.2N was unreactive, demonstrating that substitution with tungsten does not have a significant effect on lattice N reactivity. Computational calculations reveal that the vacancy formation energy for Ni2Mo3N is more endothermic than Co3Mo3N. Furthermore, calculations suggest that the inclusion of W does not have a substantial impact on the surface N vacancy formation energy or the N2 adsorption and activation at the vacancy site

A Comparison of the reactivity of the lattice nitrogen in tungsten substituted Co3Mo3N and Ni2Mo3N

The effect of the partial substitution of Mo with W in Co3Mo3N and Ni2Mo3N on ammonia synthesis activity and lattice nitrogen reactivity has been investigated. This is of interest as the coordination environment of lattice N is changed by this process. When tungsten was introduced into the metal nitrides by substitution of Mo atoms, the catalytic performance was observed to have decreased. As expected, Co3Mo3N was reduced to Co6Mo6N under a 3 : 1 ratio of H2/Ar. Co3Mo2.6W0.4N was also shown to lose a large percentage of lattice nitrogen under these conditions. The bulk lattice nitrogen in Ni2Mo3N and Ni2Mo2.8W0.2N was unreactive, demonstrating that substitution with tungsten does not have a significant effect on lattice N reactivity. Computational calculations reveal that the vacancy formation energy for Ni2Mo3N is more endothermic than Co3Mo3N. Furthermore, calculations suggest that the inclusion of W does not have a substantial impact on the surface N vacancy formation energy or the N2 adsorption and activation at the vacancy site

Liquid phase hydrogenation of CO2 to formate using palladium and ruthenium nanoparticles supported on molybdenum carbide

We report the development of palladium nanoparticles supported on Mo2C as an active catalyst for the liquid-phase hydrogenation of CO2 to formate under mild reaction conditions (100 °C and 2.0 MPa of a 1:1 CO2:H2 mixture). A series of Pd/Mo2C catalysts were synthesised via the modified wet-impregnation (MIm) and sol-immobilization (SIm) techniques and evaluated for CO2 hydrogenation, in an aqueous 1M NaOH solution. MIm catalysts synthesised using PdCl2 dissolved in a 2M HCl solution gave the highest formate yield with turnover numbers of up to 109 after 19 h. We further report the crucial role of base and the pH of the reaction medium for the hydrogenation of CO2 to formate. Based on stability studies, electron microscopic characterisation and density functional theory calculations we found that Ru has a stronger affinity than Pd to Mo2C resulting in the development of a stable bimetallic RuPd/Mo2C catalyst for the hydrogenation of CO2 to formate salt

The integration of experiment and computational modelling in heterogeneously catalysed ammonia synthesis over metal nitrides

In this perspective we present recent experimental and computational progress in catalytic ammonia synthesis research on metal nitrides involving a combined approach. On this basis, it suggested that the consideration of nitrogen vacancies in the synthesis of ammonia can offer new low energy pathways that were previously unknown. We have shown that metal nitrides that are also known to have high activity for ammonia synthesis can readily form nitrogen vacancies on their surfaces. These vacancies adsorb dinitrogen much more strongly than the defect-free surfaces and can efficiently activate the strong N–N triple bond. These fundamental studies suggest that heterogeneously catalysed ammonia synthesis over metal nitrides is strongly affected by bulk and surface defects and that further progress in the discovery of low temperature catalysts relies on more careful consideration of nitrogen vacancies. The potential occurrence of an associative pathway in the case of the Co3Mo3N catalytic system provides a possible link with enzymatic catalysis, which will be of importance in the design of heterogeneous catalytic systems operational under process conditions of reduced severity which are necessary for the development of localised facilities for the production of more sustainable “green” ammonia