Date of Award

2016

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

First Advisor

Christopher C. Landry

Abstract

Tungsten oxide (WO3) is a semiconducting transition metal oxide with interesting electronic, structural, and chemical properties that have been exploited in applications including catalysis, gas sensing, electrochromic displays, and solar energy conversion. Nanocrystalline WO3 can absorb visible light to catalyze heterogeneous photooxidation reactions. Also, the acidity of the WO3 surface makes this oxide a good thermal catalyst in the dehydration of alcohols to various industrially relevant chemicals. This dissertation explores the photocatalytic and thermal catalytic reactivity of nanocrystalline porous WO3 microparticles. Furthermore, investigations into the changes in WO3 reactivity are carried out after modifying the porous WO3 particles with gold nanoparticles (Au NPs). On their own, Au NPs are an important class of materials that have had a large impact in many fields such as catalysis, biomedical imaging, and drug delivery. When combined with WO3, however, their influence as part of a composite Au/WO3 catalyst has not been widely studied.

Porous WO3 microparticles were first prepared using mesoporous silica (SiO2) spheres as hard templates and the physical properties of these materials were fully characterized. A facile sonochemical method was used to deposit Au NPs on the WO3 surface. Using methylene blue (MB) as a photocatalytic probe, the reaction products and the catalytic activity of WO3 and Au/WO3 catalysts were compared. Composite Au/WO3 photocatalysts exhibited significantly greater rates of MB degradation compared to pure WO3. Interestingly, the observed mechanism of MB degradation was not vastly different between the two types of catalysts.

The gas-phase photocatalytic oxidation of methanol (MeOH) was studied to further understand the role of WO3 and Au NPs in these photocatalysts. Porous WO3 showed greater photooxidation rates compared to bulk WO3 because of its increased active surface area. Pure WO3 and Au NPs on porous SiO2 (SiO2-Au) were both active MeOH photooxidation catalysts and were highly selective to formaldehyde (HCHO) and methyl formate (MF), respectively. Two different mechanisms, namely band gap excitation of WO3 and surface plasmon resonance (SPR) on Au NPs, were responsible for this result. Again, the Au/WO3 composite catalysts showed greater photocatalytic activity than WO3, which increased with Au loading. This high activity led to the complete photooxidation of MeOH to carbon dioxide (CO2) over Au/WO3 catalysts.

Finally, the thermal catalytic transformation of MeOH under aerobic conditions was carried out to further characterize the acid and redox active sites of WO3 and Au/WO3 catalysts. Pure WO3 was highly selective for MeOH dehydration to dimethyl ether (DME), whereas Au/WO3 showed increased oxidation selectivity to products such as HCHO, FM, and COx. The Au NPs increased the reducibility of the WO3 species, which made surface oxygen atoms more labile and reactive towards MeOH. Also, the WO3 facilitated the formation of cationic Au (Au δ+) species. This combination of effects created through a strong Au/WO3 interaction increased the activity of WO3 species, but it decreased the activity of the Au NPs.

Language

en

Number of Pages

206 p.

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