Metal nanoparticle-decorated titanium dioxide (M@TiO₂) materials are an increasingly popular class of heterogeneous catalyst, useful in both photochemical and thermal systems. Heterogenous catalysts offer the advantage of reusability and ease of catalyst separation, when compared to similar homogeneous systems. M@TiO₂ catalysts also have the benefit of water/air environment stability, strong photoactivity for oxidation and reduction reactions, as well as easy and low cost synthesis of the catalyst. Other heterogeneous catalysts can offer better activity for certain reactions; however, M@TiO₂ materials are extremely versatile in a variety of different reactions and applications, and often are cheaper than other alternatives. In this dissertation, M@TiO₂ catalysts will be evaluated in hydrogen generation, solvent radical chemistry, and organic synthesis utilizing calcium carbide.
Firstly, M@TiO₂ were evaluated for photocatalytic hydrogen generation from pure water splitting, and with the presence of sacrificial electron donors (SEDs) such as methanol. Efficient pure water splitting is of great interest for fuel production as it offers a perfect cycle with hydrogen gas burning to reform water as the only product. However, quite often SEDs are utilized to boost hydrogen gas generation due to poor conversion from pure water. It is often assumed that a photocatalyst effective with a SED will also be effective with water splitting. This assumption was tested, by comparing a variety of different M@TiO₂ photocatalysts for both water splitting, and SED-based hydrogen generation. Interestingly, it was found that the trends of hydrogen generation between photocatalysts are not the same in pure water splitting, as when SEDs are present. For example, Pd@TiO₂ shows great activity with a 1% methanol solution; however, no considerable H₂ generation for pure water splitting. This shows that the mechanisms of hydrogen generation with water splitting, and when SEDs are present, are very different and not directly comparable. It was also found that M@TiO₂ materials offer decent hydrogen generation rates, especially when considering the overall cost of the material.
M@TiO₂ materials were then tested for their ability to photocatalytically form usable free-radicals from ethers. This was evaluated with scavenging of generated radicals by TEMPO, as well as monitoring the resulting H₂ production during the reduction portion of the system. Overall, it was found that M@TiO₂ photocatalysts are exceptional at forming radicals from ethers. All the ethers tested are able to undergo proton-coupled electron transfer (PCET) with the hole of TiO₂, as seen by the H₂ generation observed. The main considerations are instead for the ether-radical, and if the radical will fragment or primarily undergo other reactions. This led to only some of the ethers being able to form TEMPO-ether adducts. The photogenerated hole of TiO₂ is also strong enough to form benzylic radicals from toluene, highlighting the further versatility of the catalyst.
To further explore TiO₂-generated radicals, heterogeneous laser flash photolysis techniques were then developed. Laser flash photolysis of TiO₂ suspensions is an uncommon, and underdeveloped technique in the research field. It was considered if low concentration suspensions of TiO₂ could allow for lowered impact from the absorbance and scattering from the TiO₂ particles. This allowed for monitoring the transient absorbance of a benzylic radical from the reaction between 1,1-diphenylethylene and 1,3-dioxolane solvent radicals formed by the photogenerated hole of TiO₂. The strength of this transient signal also showed dependence on the solvent, with 1,4-dioxane showing lower signal as expected from it's reactivity. This technique, with further development, should prove useful in expanding the kinetic evaluation of radicals generated by TiO₂ suspensions.
Finally, Pd@TiO₂ was evaluated as a thermal catalyst for a Sonogashira-like reaction between calcium carbide and bromobenzene in DMSO under low water conditions. This palladium catalyst was effective in catalyzing the reaction; however, the more interesting aspect was in the chemistry of the calcium carbide itself. Calcium carbide is typically used for the in-situ formation of acetylene gas through the addition of water. However, it was found that in DMSO with low amounts of water, the formation of a soluble ethynyl calcium hydroxide intermediate could be selected for. This allowed for a more controlled and effective coupling with bromobenzene in solution. Further expansion on the use of this intermediate may be invaluable in expanding calcium carbide chemistry beyond the formation of acetylene gas.
| Identifer | oai:union.ndltd.org:uottawa.ca/oai:ruor.uottawa.ca:10393/45233 |
| Date | 08 August 2023 |
| Creators | Hainer, Andrew |
| Contributors | Scaiano, Juan C. |
| Publisher | Université d'Ottawa / University of Ottawa |
| Source Sets | Université d’Ottawa |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf |
| Rights | Attribution-NonCommercial 4.0 International, http://creativecommons.org/licenses/by-nc/4.0/ |
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