In this research, silicon carbide (SiC) has been confirmed to be catalytically active for the formation of polycyclic aromatic hydrocarbons (PAHs) from acetylene (C2H2). Aromatization reactions of C2H2wasexamined by gas-phase experiments in vacuo using α-SiC and β-SiC respectively, characterized with in situ time-of-flight mass spectrometry (TOF-MS)in which abundant information of intermediates was record-ed. The significance of this catalytic reaction lies in the fact that the reagents –acety-lene, the catalyst –SiC, and the PAH products are all coexisting in interstellar environment and this reaction indicates one of the possible channels for the formation of PAHs. Reaction products obtained were further confirmed by gas chromatography mass spectrometry (GCMS). The SiC catalysts were analyzed before and after the re-action using high-resolution transmission electron microscopy (HRTEM), X-ray dif-fraction(XRD) and derivative thermogravimetry (DTG).Results of experiments indicate that SiC catalyzes aromatization of C2H2starting from 400℃with activities dependent on temperature and notable difference has been observed between α and β crystal forms. For a long-time reaction at a particular temperature, the changes in productions of the four major PAH products, i.e. naphthalene, anthracene, phenan-threne and pyrene showed the transition from small PAHs to larger PAHs. Carbon deposition on the catalyst depends on temperature, which explains the deactivation of SiC catalyst at high temperatures.
Based on experimental results, two new mechanisms for activation of acetylene on SiC that are closely connected with SiC surface dangling bonds have been proposed: 1)breakage of C-H bond by interaction of H with dangling bonds; 2) radical addition of hydrocarbon to form a surface ring structure which is then desorbed from the surface. Subsequent ring-growth process is suggested to follow the H abstraction / C2H2addition (HACA) mechanism. The activation effect of SiC dangling bonds was then extended to aromatization of propylene (C3H6) and propane (C3H8) starting from 650℃ and 750 ℃respectively in similar conditions. Comparison among reactions of C2H2, C3H6and C3H8provides further evidence for the proposed mechanism. / published_or_final_version / Chemistry / Master / Master of Philosophy
| Identifer | oai:union.ndltd.org:HKU/oai:hub.hku.hk:10722/208605 |
| Date | January 2014 |
| Creators | Zhao, Tianqi, 趙天騏 |
| Publisher | The University of Hong Kong (Pokfulam, Hong Kong) |
| Source Sets | Hong Kong University Theses |
| Language | English |
| Detected Language | English |
| Type | PG_Thesis |
| Rights | The author retains all proprietary rights, (such as patent rights) and the right to use in future works., Creative Commons: Attribution 3.0 Hong Kong License |
| Relation | HKU Theses Online (HKUTO) |
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