A survey of different materials as heterogeneous catalysts for ammonia synthesis

Poya, Yalinu (2020) A survey of different materials as heterogeneous catalysts for ammonia synthesis. PhD thesis, University of Glasgow.

Abstract

The Haber–Bosch Process which was developed in the early 1900’s, was a landmark achievement of the 20th Century. Today the process produces over 174 million tonnes of ammonia annually, establishing an accessible route to produce over 450 million tonnes of synthetic fertilizer which maintains food production for 40 % of the global population. The Haber–Bosch Process involves combining pure H2 and N2 feed streams directly over a promoted iron catalyst at temperatures of around 400–500°C and reaction pressures of over 100 atmospheres. The reaction is exothermic and is equilibrium limited, thus it is thermodynamically favoured at lower reaction temperatures. Despite this, the temperature of operation is dictated by the requirement to achieve acceptable process kinetics. The Haber–Bosch Process is generally conducted at large scale and the process and the generation of feedstock for the reaction are energy intensive using 1–2 % of the world’s manmade energy and contributing 1.6 % of manmade CO2 emissions. The development of small-scale ammonia plants that function using renewable feed stock and sustainable electricity, can lessen such disadvantages towards the environment and can further offer financial benefits. As a result, the development of catalysts suitable for small-scale ammonia plants using less harsh reaction conditions are of interest.
Supported catalysts have been extensively studied and are well known for catalysing a wide variety of reactions. There are some low surface area catalysts that have been reported to be highly active in ammonia synthesis and using a suitable support, the catalytic phase can be dispersed, thus possibly further improving ammonia synthesis activity. In this work, a series of supported molybdenum, cobalt and cobalt-rhenium based catalysts were synthesised and tested for ammonia synthesis. Oxide supports with contrasting properties such as SiO2, α–Al2O3, ZrO2, CeO2, CaO and MgO were applied and the active phases were added to the supports via a wetness impregnation based procedure. A comparison of the ammonia synthesis activities of the catalysts with their respective supports are surveyed in this thesis. The different catalyst systems possessed different characteristics that influenced ammonia synthesis activities. Supported molybdenum catalysts were mainly influenced by the presence of different molybdenum species, while the supported cobalt and cobalt-rhenium catalysts were influenced by the support used. The CoRe/MgO catalyst was found to possess the highest activity in this work under the reaction conditions applied.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Additional Information: Due to copyright issues the electronic version of this thesis is not available for viewing.
• Keywords: Ammonia synthesis, supported catalysts, transition metals.
• Subjects: Q Science > QD Chemistry
• Colleges/Schools: College of Science and Engineering > School of Chemistry
• Supervisor's Name: Hargreaves, Professor Justin
• Date of Award: 2020
• Depositing User: Yalinu Poya
• Unique ID: glathesis:2020-81272
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 08 Apr 2020 15:59
• Last Modified: 08 Apr 2020 15:59
• URI: https://theses.gla.ac.uk/id/eprint/81272