CO2 photoreduction kinetics : an experimental and numerical approach

Abstract

An industrial scale response is required to mitigate the impact of anthropogenic CO2. Developing technologies that utilize CO2 offer an opportunity for developing economies as this fairly abundant resource can be used as a starting material towards fuels and/or fuel syngases. CO2 photoreduction is one possible avenue that uses light energy to drive the photocatalytic transformation of CO2 and a proton source such as H2O towards H2, CO and CH4. To advance this technology, the aim of this PhD was to develop CO2 photoreduction kinetic models that could be used for understanding the process, decision making and for potentially scaling photoreactors. To achieve this goal, novel experimental and numerical methodology were developed and are presented in this thesis as four submitted papers that include: 1. A CO2 photoreduction kinetic model perspective paper that reviewed the limited literature examples of CO2 photoreduction kinetics, identified their limitations and highlighted factors for collecting kinetic data that included: • Impact of included organic impurities as adventitious carbon that may undergo photocatalytic transformations leading to false positive kinetic data for the expected CO2 photoreduction products • Photoreactor design and its impact on producing extrinsic kinetic models that are geometry dependent • Photocatalyst coating methods impact on false positive production of products • Light transport challenges that lead to extrinsic kinetic models • Impact of temperature and pressure on mass transfer and kinetic data • Numerical methods used for evaluating kinetic model coefficients. A novel mean median multi-start trust-region was developed to find a global solution to the nonconvex nonlinear kinetic models and compared to particleswarm and genetic algorithms. 2. A sol-gel paper that investigated the impact of seven process parameters, usingan efficient and systematic Design of Experiments approach, on four properties of a TiO2 sol-gel coating for CO2 photoreduction. This approach demonstrated how Design of Experiments could be used for understanding key process parameters towards the reproducible synthesis of photocatalysts and coatings for CO2 photoreduction. 3. A CO2 photoreduction kinetic paper that applied the factors identified in theperspective for collecting kinetic data. High quality kinetic data was recorded by limiting the impact of adventitious carbon, a gas chromatography method that included calibration of the expected ppm production amounts and the use of a photodifferential photoreactor to encourage equal probability of active site participation towards intrinsic kinetic models. A novel probability LangmuirHinshelwood based kinetic model was developed to describe the kinetics of CO, H2 and CH4 production from CO2 photoreduction. These models included the impact of deactivation and considered the fractional coverage of the reagents from the assumed elementary surface reactions towards CH4, CO and H2. The coefficients of the nonconvex nonlinear kinetic model were estimated using the numerical method presented in the perspective article. 4. A mixed metal-oxide paper that described the impact of different ratios of TiO2 and ZnO on a SBA-15 support for CO2 photoreduction. This paper demonstrated how the experimental and numerical methodology presented in the first three papers could be generalised and applied for the development of new photocatalyst mixtures. Using a combination of a systematic mixture design and photocatalysis theory, the impact of sulfates as electron-hole trapsand/or radical intermediate scavengers was presented.