The role of composition on the ammonia synthesis activity of nitrides, carbonitrides, carbides and osmium based compounds

Daisley, Angela (2020) The role of composition on the ammonia synthesis activity of nitrides, carbonitrides, carbides and osmium based compounds. PhD thesis, University of Glasgow.

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Abstract

The industrial route for producing ammonia is via the Haber-Bosch Process and requires high temperatures and pressures. When considered in its entirety, including the production of the necessary feedstreams, the process is stated to account for 2% of the world’s energy demand and 1.6% of global anthropogenic CO2 emissions. If a more active ammonia synthesis catalyst was developed, it may prove possible to operate under more moderate conditions, facilitating localised and sustainable production. One potential approach to the development of catalysts with enhanced activity by-passing the limiting scaling relationship invoked for metal catalysts is ammonia production via the Mars-van Krevelen mechanism, as possibly exhibited by some metal nitrides, such as Co3Mo3N.
The role composition and crystal structure type have on the ammonia synthesis activity of mixed metal nitrides, carbonitrides and carbides was investigated within this thesis. Fe3Mo3C, Ni6Mo6C, Ni3Mo3C and Ni2Mo3CxNy have been tested for their ammonia synthesis capabilities, to discover if the presence of lattice nitrogen was required. Fe3Mo3C was found to be inactive for ammonia synthesis at 400oC. However, when the temperature was increased to 500oC, the material became active, which may be related to the substitution of lattice carbon with nitrogen. Ni6Mo6C and Ni3Mo3C were inactive for ammonia synthesis at 400oC and 500oC, respectively. When Ni6Mo6C and Ni3Mo3C were investigated at 700oC, an induction period occurred before the two materials developed activity for ammonia synthesis; during this period, nitridation of the lattice occurred eventually leading to the formation of Ni2Mo3N. Ni2Mo3C could not be synthesised via a topotactic route from Ni2Mo3N and the carbonitride phase which was formed was active for ammonia synthesis at 400oC.
The lattice nitrogen in the filled b-Mn structured Ni2Mo3N and h-carbide structured Ni2GaMo3N was observed to exhibit different behaviour to that in the filled b-Mn structured Co2Mo3N and h-carbide structured Co3Mo3N and Fe3Mo3N. For Ni2Mo3N and Ni2GaMo3N, the bulk lattice nitrogen appeared to be relatively unreactive at 900oC, whereas Co2Mo3N, Co3Mo3N and Fe3Mo3N decomposed under these conditions. This suggests that both composition and crystal structure type may have an impact on the lattice nitrogen reactivity.
The potential structure sensitivity of osmium for ammonia synthesis was examined and it was found that the metal was more active when it was more highly dispersed. Supported mixed metal carbonyl clusters were also tested and were observed to have different activities to the supported monometallic equivalents.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Ammonia synthesis, nitrides, carbides, osmium.
Subjects: Q Science > QD Chemistry
Colleges/Schools: College of Science and Engineering > School of Chemistry
Funder's Name: Engineering and Physical Sciences Research Council (EPSRC)
Supervisor's Name: Hargreaves, Professor Justin
Date of Award: 2020
Depositing User: Angela Daisley
Unique ID: glathesis:2020-81701
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 07 Oct 2020 08:49
Last Modified: 07 Oct 2020 08:49
URI: http://theses.gla.ac.uk/id/eprint/81701
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