The catalytic reaction of CO and alcohols over supported gold catalysts

Nuhu, Abdullahi 2008. The catalytic reaction of CO and alcohols over supported gold catalysts. PhD Thesis, Cardiff University.

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Abstract

Studies of CO, methanol and some higher alcohol oxidations over gold supported on TiO₂ (Degussa), Y-AI₂O₃ and SiCh were investigated. The methods of preparation of catalysts used are deposition precipitation and incipient wetness impregnation. Several parameters have been investigated for CO oxidation over Au/TiO₂ prepared by deposition precipitation, such as temperature programmed pulse flow reaction, isothermal and continuous flow CO oxidation, anaerobic CO reaction, calcination temperature, effect of moisture (in the presence of water, and methanol), kinetics and CO oxidation in the presence of hydrogen etc. The catalysts are demonstrated to have high activity even at room temperature. Gold supported on TiO₂ (Degussa) was characterized by BET surface area method, powder X-ray diffraction, SEM, EDAX, XPS, and Raman spectroscopy. The CO oxidation reaction studied on A11/Y-AI₂O₃ and Au/SiC catalysts prepared by deposition precipitation (DP) and incipient wetness impregnation (IW) methods showed that the A11/Y-AI₂O₃ catalyst prepared by DP method is much more active than A11/Y-AI₂O₃ and Au/SiC₂ prepared by the incipient wetness impregnation method. Presumably, the low performances of the IW catalysts are ascribed due to presence of chloride which leads to gold sintering in the catalyst. However, the performance of these catalysts with respect to CO oxidation was less than the Au/TiO₂ catalyst prepared by DP method. The characterization of the catalyst shows the BET surface area of A11/Y-AI₂O₃ and Au/SiCh catalysts to be 128 and 320m<super>2</super>/g respectively. The XRD of the A11/Y-AI₂O₃ shows mainly the support, which indicated high dispersion of gold on the catalyst. Nevertheless, XPS, SEM, Raman Spectroscopy and EDAX were used for characterisation of the A11/Y-AI₂O₃ and A11/S1O2 catalysts The CO oxidation reaction studied show that the activity of the catalyst decreases in the following order: Au/ TiCb > A11/Y-AI₂O₃ > Au/SiO₂. The adsorption and reaction of methanol with Au/TiO₂, A11/Y-AI₂O₃ and Au/SiO₂ catalyts using temperature pulse programmed reaction over the reactor, TPD, SEM, EDAX, XPS and DRIFTS were investigated. Several factors were investigated such as pH, calcination, comparison with the reference WGC catalyst, kinetic isotope effect etc. The TiO₂ (P25), Y-AI₂O₃ and SiO₂ surface adsorbed about a half monolayer of methanol, much of it in a dissociative manner forming methoxy groups associated with the cation sites and hydroxy! groups at anions. For, TK2, the methoxy is relatively stable until about 250&deg;C, at which point decomposition occurs, producing mainly dimethyl ether by bimolecular surface reaction. As the concentration of methoxy on the surface diminishes, so the mechanism reverts to a de-oxygenation pathway, producing mainly methane and water (at 330 C in TPD), but also with some coincident CO and hydrogen. In contrast, in the case of Y-AI₂O₃ and SiO₂, dimethyl ether (DME) was observed as the main product. The effect of gold catalysts prepared by DP and IW on the reactivity is marked. The pathway which gives methane, which is characteristic of TiCh (P25) for Au/TiO₂ catalysts, remains the same but a new feature of the reaction is the evolution of CO₂ and H₂ at lower temperature, and the elimination of the DME-producing state. Clearly, this is associated with the presence of gold and appears to be due to the high amount of formate species on the catalyst surface. The formate species involved in the reaction of methanol over TiO₂ and Au/TiO₂ catalysts results in a combustion pathway being followed, with complete conversion occurring by 130&deg;C. Similarly, the main methanol oxidation reaction observed on A11/Y-AI₂O₃ and Au/SiO₂ are dehydration products, mainly, DME, with CO and hydrogen as the main products. The oxidation reaction of higher alcohol (ethanol, propan-l-ol, and propan-2-ol) has been studied over Au/TiC₂ catalyst prepared by deposition precipitation (DP) method using Temperature Programmed Pulsed Flow Reaction, TPD, and DRIFTS. TiO₂ (P25) adsorbed about half monolayer of ethanol, propan-lol and propan-2ol. The presence of the gold (as in the case of Au/TiO₂ catalysts) eliminates most of the dehydration products and increases the production of formate species, which results in a combustion pathway being followed, with complete conversion of ethanol, propan-1-ol and propan-2-ol. Trie results are mainly dehydrogenation, dehydration, de-oxygenation and decomposition products but, in each case, the reaction is a complete oxidation reaction.

Item Type:	Thesis (PhD)
Status:	Unpublished
Schools:	Chemistry
Subjects:	Q Science > QD Chemistry
ISBN:	9781303213328
Date of First Compliant Deposit:	30 March 2016
Last Modified:	12 Feb 2016 23:12
URI:	https://orca.cardiff.ac.uk/id/eprint/54729

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