- Title
- Selective oxidation of methane with nitrous oxide over cobalt catalyst
- Creator
- Khan, Naseer Ahmed
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2016
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- In spite of the highly fluctuating oil prices seen over the past number of years, the availability of relatively inexpensive sources of liquid hydrocarbons is diminishing, and the development of economically and environmentally viable alternative technologies to supply these hydrocarbons is vital. Projecting into the short and mid-term futures, the world economy is expected to continue to depend on hydrocarbon resources for stability, and thus technologies to produce these resources are important. Synthesis gas (CO and H₂), can be produced from a number of sources and feedstocks, such as biomass, waste and natural gas, and synthesis gas can be converted into liquid fuels such as hydrocarbons and alcohols, which are currently produced in large quantities in the conventional petrochemical industry. The selective oxidation of hydrocarbons, and in particular natural gas of which CH₄ the primary constituent, is notoriously difficult to control, mainly due to propensity of the reaction to generate to products of complete combustion. N₂O is a potent greenhouse gas (CO₂(e) = 300) and its concentration in the atmosphere is continuously increasing [Lee, Ryu et al, International Journal of Greenhouse Gas Control, January 2011, Pages 167–176]. Despite its greenhouse gas notoriety, N₂O is also recognised as a more selective oxidant than O₂ and has been used to selectively oxidise CH₄ to produce valuable products including synthesis gas. The production of synthesis gas, via its reaction with N₂O could be beneficial environmentally, as it could be used to provide valuable products when used to reaction with natural gas. The main aim of this research work is to investigate the scope of selective oxidation of natural gas (mainly CH₄) with N₂O over cobalt catalysts. The catalytic work was carried out in a continuous flow fixed bed micro reactor. Catalysts were prepared according to literature methods, as well as a new novel deposition method. The techniques used for catalyst characterization were powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), In situ Fourier transform infrared spectroscopy (FTIR), nitrogen adsorption/desorption isotherms, temperature program desorption (TPD), chemisorption, transmission electron microscopy (TEM). The elemental composition of catalyst samples was determined by using inductively coupled plasma optical emission spectrometer (ICP-OES). Based upon the experimental results obtained, the oxidation of CH₄ with N₂O over cobalt exchanged ZSM-5 (Co-ZSM-5) catalyst produced synthesis gas at a relatively high yield (19 %). When O₂ (instead of N₂O) was used as an oxidant over Co-ZSM-5 catalyst, synthesis gas was not formed. Similarly, when CH₄ and N₂O reacted over a cobalt on alumina catalyst, synthesis gas was not produced. Interestingly, H₂ was not detected as a reaction product when CH₄ with N₂O reacted over a Fe-ZSM-5 catalyst and only COx and H₂O were formed. The effect of the molar feed ratio (N₂O/CH₄ = 1, 3, and 5) was examined, in order to establish conditions which resulted in highest H₂ yield. It was found that maximum attainable H₂ yield over Co-ZSM-5 was about 19 % by using a feed ratio of 3 (i.e. N₂O/CH₄ = 3) at a temperature of 550 °C. In general, N₂O used as a limiting reactant in the reactant feed oxidized CH₄ to synthesis gas, otherwise excess ratio of N₂O (N₂O/CH₄ =5:1), produced H₂O and CO₂. This research work proposes a new, direct mechanism for synthesis gas production, while the majority of the published literature focuses on either a direct or indirect reaction mechanism for synthesis gas formation. The results suggest that H₂ is produced from the decomposition of surface intermediates, namely CH₃OH and CH₂O. Under favourable reaction conditions over the Co-ZSM-5 catalyst, it is proposed that N₂O oxidizes CH₄ to CH₃OH, which is subsequently converted into CH₂O and H₂. Furthermore, CH₂O is also highly reactive and decomposes into CO and H₂. The XPS analysis suggested that cobalt cations which are situated at exchange sites of ZSM-5 are active for synthesis gas formation. The dissociation of neat N₂O was studied and Co2+ and Co3+ are proposed as active sites.
- Subject
- CH₄; N₂O; Co-ZSM-5
- Identifier
- http://hdl.handle.net/1959.13/1312052
- Identifier
- uon:22339
- Rights
- Copyright 2016 Naseer Ahmed Khan
- Language
- eng
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