Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Chemical and Biochemical Engineering

Supervisor

Dr. Hugo de Lasa

Abstract

Nowadays, the world experiences a high energy demand caused by the expansion of the industry sector as well as by increasing world population. There is, as a result, a steady depletion of non-renewable fossil fuels. This also leads to significant contaminant emissions such as CO2, contributing to green house gases and other noxious pollutants such as NOx and SOx. Thus, it is of high importance and interest to promote new alternative and environmental-friendly sources of energy. Heterogeneous photocatalysis as practiced in the present PhD dissertation is a promising alternative, producing hydrogen and simultaneously using a renewable organic scavenger (ethanol) at ambient conditions. In addition, heterogeneous photocatalysis can be, in principle, promoted by the interaction of a semiconductor material and photons in the solar light spectrum (UV-Visible-IR radiation).

The present PhD dissertation demonstrates that hydrogen can be produced photocatalytically using a modified Degussa P25 (TiO2)-Pt photocatalyst in a slurry medium under near-UV irradiation and having ethanol as a sacrificial reagent (scavenger). The modified DP25-Pt photocatalyst was prepared using the incipient wetness impregnation technique. The Pt modified photocatalyst exhibited a 2.73 eV reduced band gap.

Experiments were performed in a Photo-CREC Water II Reactor (PCW-II Reactor). This novel unit provides both radial and axial symmetrical irradiation profiles. Macroscopic energy balances developed in this unit, showed a 95% LVREA at 0.15 g of photocatalyst per liter of aqueous solution.

Runs in the PCW-II Reactor showed hydrogen formation via H radicals under oxygen free conditions. The use of 2 v/v% ethanol as sacrificial reagent enabled producing significant hydrogen amounts with the simultaneous formation of CH4 and C2H6 by products. It is proven that hydrogen formation in the presence of ethanol is a function of water solution pH and Pt loading on the TiO2 photocatalyst.

Regarding the consumption of an ethanol scavenger, experimental findings are supported by an “in series-parallel” reaction network and a kinetic model. Kinetic model parameters were estimated using numerical non-linear regression. These kinetic parameters were determined under rigorous statistical methods. These methods were adapted to give an adequate fit to the experimental data and to all the by product species resulting from the photocatalytic hydrogen production “in series-parallel” kinetic model. Furthermore, hydrogen production, in the context of the present research, was also described using an “in parallel” reaction network. In this case, once again, kinetic parameters were established using carefully determined statistical methods.

Concerning energy efficiencies, it was observed that the best obtained 7.9% quantum yield for hydrogen production indicates a good degree of photon utilization. This is particularly true in view of the fact that hydrogen production requires two simultaneous or quasi-simultaneous photons interacting with a semiconductor site. It was also proven via the Photochemical Thermodynamic Efficiency Factors (PTEFs) that observed PTEFs are in accordance with the thermodynamics remaining in all cases below 1.

One can thus, conclude, with the result of the present research, the value of a modified DP25-Pt photocatalyst operating in the Photo-CREC Water II Reactor for hydrogen production, via photocatalytic water splitting.


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