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Synthesis of complex transition metal nitrides and their ammonia synthesis activity

Synthesis of complex transition metal nitrides and their ammonia synthesis activity
Synthesis of complex transition metal nitrides and their ammonia synthesis activity
In this study, some nanocrystalline quaternary interstitial nitrides adopting the filled b-Mn and the η-6 carbide type structures have been synthesised using a modified Pechini method. This method, which involves dissolving metal salts with citric acid followed by the evaporation and ammonolysis of the resulting oxide precursors, has resulted in the nanoparticle materials. It has been shown to be a useful tool for producing homogenous quaternary nitrides in a fast and simple way. The structural properties of the samples were examined by powder X-ray diffraction and the data was further analysed and quantitatively refined using Rietveld refinement methods. Other characterisation techniques such as SEM, TEM, TGA and elemental analysis of carbon, hydrogen and nitrogen content were also undertaken. The surface areas of the materials were measured by the Brunauer- EmmettTeller (BET) method, using nitrogen adsorption isotherms. The refinement of Ni2-xCuxMo3N and Ni2WxMo3-xN powder X-ray diffraction data confirms a linear dependence of lattice parameters versus Cu or W content where ! = 0.0, 0.1, 0.2, 0.3, and then the lattice parameters no longer change as the doping level is increased, indicating that copper or tungsten can be substituted at the nickel or molybdenum sites in Ni2Mo3N structure up to a solubility limit of ! = 0.3. Ni2-xFexMo3N was observed to have a linear increase in the lattice parameters up to ! = 1. These metal nitrides show good ammonia synthesis activity at 500 °C under ambient pressure. The activity fell with copper content and increased with iron and tungsten content, although these changes appear to be related to changing surface areas of the catalysts. The η-6 Co3No3M structure is a promising material for ammonia synthesis. The substitution of molybdenum with tungsten into this structure in the formula Co3WxMo3N resulted in some single phases where ! = 0.1, 0.2, 0.4, 0.7 and 0.8. However, the compositions with ! = 0.3, 0.5 and 0.6 were found to have a trace impurity of Co3Mo. These materials showed good activity for ammonia synthesis, but less than that of the parent compound Co3Mo3N. The oxide intermediates from the citrate gel process cleanly converted to the metal nitride under the ammonia synthesis gas stream (75% H2, 25% N2) providing more active catalysts than those pre-nitrided by ammonolysis.
University of Southampton
Alsobhi, Samia Ibrahim N
38a7cc19-8013-4947-98ac-f6d3b56982c9
Alsobhi, Samia Ibrahim N
38a7cc19-8013-4947-98ac-f6d3b56982c9
Hector, Andrew
f19a8f31-b37f-4474-b32a-b7cf05b9f0e5

Alsobhi, Samia Ibrahim N (2019) Synthesis of complex transition metal nitrides and their ammonia synthesis activity. Doctoral Thesis, 175pp.

Record type: Thesis (Doctoral)

Abstract

In this study, some nanocrystalline quaternary interstitial nitrides adopting the filled b-Mn and the η-6 carbide type structures have been synthesised using a modified Pechini method. This method, which involves dissolving metal salts with citric acid followed by the evaporation and ammonolysis of the resulting oxide precursors, has resulted in the nanoparticle materials. It has been shown to be a useful tool for producing homogenous quaternary nitrides in a fast and simple way. The structural properties of the samples were examined by powder X-ray diffraction and the data was further analysed and quantitatively refined using Rietveld refinement methods. Other characterisation techniques such as SEM, TEM, TGA and elemental analysis of carbon, hydrogen and nitrogen content were also undertaken. The surface areas of the materials were measured by the Brunauer- EmmettTeller (BET) method, using nitrogen adsorption isotherms. The refinement of Ni2-xCuxMo3N and Ni2WxMo3-xN powder X-ray diffraction data confirms a linear dependence of lattice parameters versus Cu or W content where ! = 0.0, 0.1, 0.2, 0.3, and then the lattice parameters no longer change as the doping level is increased, indicating that copper or tungsten can be substituted at the nickel or molybdenum sites in Ni2Mo3N structure up to a solubility limit of ! = 0.3. Ni2-xFexMo3N was observed to have a linear increase in the lattice parameters up to ! = 1. These metal nitrides show good ammonia synthesis activity at 500 °C under ambient pressure. The activity fell with copper content and increased with iron and tungsten content, although these changes appear to be related to changing surface areas of the catalysts. The η-6 Co3No3M structure is a promising material for ammonia synthesis. The substitution of molybdenum with tungsten into this structure in the formula Co3WxMo3N resulted in some single phases where ! = 0.1, 0.2, 0.4, 0.7 and 0.8. However, the compositions with ! = 0.3, 0.5 and 0.6 were found to have a trace impurity of Co3Mo. These materials showed good activity for ammonia synthesis, but less than that of the parent compound Co3Mo3N. The oxide intermediates from the citrate gel process cleanly converted to the metal nitride under the ammonia synthesis gas stream (75% H2, 25% N2) providing more active catalysts than those pre-nitrided by ammonolysis.

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Samia Al Sobhi thesis_ Jun2020 (1)
Restricted to Repository staff only until 30 June 2022.
Available under License University of Southampton Thesis Licence.
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Published date: November 2019

Identifiers

Local EPrints ID: 447593
URI: http://eprints.soton.ac.uk/id/eprint/447593
PURE UUID: 46b328b4-5d2c-4b37-9ce0-3cf86492894a
ORCID for Andrew Hector: ORCID iD orcid.org/0000-0002-9964-2163

Catalogue record

Date deposited: 16 Mar 2021 17:45
Last modified: 17 Mar 2021 02:35

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Contributors

Author: Samia Ibrahim N Alsobhi
Thesis advisor: Andrew Hector ORCID iD

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