High Temperature Photocatalysis over Semiconductors.

[摘要] Due in large part to in prevalence of solar energy, increasing demand of energy production(from all sources), and the uncertain future of petroleum energy feedstocks,solar energy harvesting and other photochemical systems will play a major role in thedeveloping energy market. This dissertation focuses on a novel photochemical reactionprocess: high temperature photocatalysis (i.e., photocatalysis conducted aboveambient temperatures, T > 100°C).The overarching hypothesis of this process is that photo-generated charge carriersare able to constructively participate in thermo-catalytic chemical reactions, therebyincreasing catalytic rates at one temperature, or maintaining catalytic rates at lowertemperatures. The photocatalytic oxidation of carbon deposits in an operationalhydrocarbon reformer is one envisioned application of high temperature photocatalysis.Carbon build-up during hydrocarbon reforming results in catalyst deactivation,in the worst cases, this was shown to happen in a period of minutes with a liquidhydrocarbon. In the presence of steam, oxygen, and above-ambient temperatures,carbonaceous deposits were photocatalytically oxidized over very long periods (t ≥24 hours).This initial experiment exemplified the necessity of a fundamental assessment ofhigh temperature photocatalytic activity. Fundamental understanding of the mechanismsthat affect photocatalytic activity as a function of temperatures was achievedusing an ethylene photocatalytic oxidation probe reaction. Maximum ethylene photocatalyticoxidation rates were observed between 100°C and 200°C; the maximum photocatalytic rates were approximately a factor of 2 larger than photocatalytic rates at ambient temperatures. The loss of photocatalytic activity at temperatures above 200°C is due to a non-radiative multi-phonon recombination mechanism. Further,it was shown that the fundamental rate of recombination (as a function of temperature)can be effectively modeled as a temperature-dependent quantum efficiency term,and is directly driven by bulk photocatalyst crystal parameters: maximum phonon energy and the number of phonons allowed per unit cell. This analysis extends to multiple photocatalysts and can explain experimental observations of photocatalytic oxidation rates with varied reactant concentrations. Lastly, this dissertation applies this knowledge to a thermo-catalytic reaction (CO-oxidation) using a Au/TiO2 catalyst. The combined photo/thermo-catalytic reaction showed a 10-25% increase in CO conversion during a temperature programmed reaction experiment.

[发布机构] University of Michigan

[效力级别] Non-radiative Multi-phonon Recombination

[关键词] Photocatalysis;Non-radiative Multi-phonon Recombination;Photo-thermal Catalysis;TiO2;Chemical Engineering;Engineering;Chemical Engineering