PHOTOINDUCED ACTIVATION OF CO2 ON TiO2 SURFACES: QUANTUM CHEMICAL MODELING OF GROUND AND EXCITED STATES, AND IN SITU EPR STUDIES
Open Access
- Author:
- Indrakanti, Venkata Pradeep
- Graduate Program:
- Energy and Geo-Environmental Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- February 13, 2009
- Committee Members:
- Harold Harris Schobert, Dissertation Advisor/Co-Advisor
Harold Harris Schobert, Committee Chair/Co-Chair
James David Kubicki, Committee Chair/Co-Chair
Chunshan Song, Committee Member
Kwadwo Osseo Asare, Committee Member
Adrianus C Van Duin, Committee Member - Keywords:
- quantum calculations
DFT
CO2 activation
photocatalytic reduction of CO2
anatase
TiO2
CO2 utilization - Abstract:
- The conversion of CO2 using light energy (CO2 photoreduction) has the potential to produce useful fuels or valuable chemicals while decreasing CO2 emissions from the use of fossil fuels. This thesis describes computational and experimental studies on the initial steps of photoinduced CO2 activation on TiO2 surfaces necessary to develop a mechanistic understanding of CO2 photoreduction. Initially, the state of the art in the field is reviewed, and the performance of CO2 photoreduction catalysts is compared to that of solar hydrogen production catalysts. To design efficient CO2 photoreduction catalysts, we need to understand the intermediates and energetics of various reactions involved in the photoreduction of CO2 in greater detail. As a first step in this process, the ground states of CO2 chemisorbed on small clusters from various anatase surface planes were modeled. We find favorable agreement for the existence of three different CO2-surface complexes on small TiO2 clusters (Ti2O9H10) extracted from the (010), (001) and (101) anatase surface planes. Secondly, identifying the mechanism and the active sites involved in the formation of negatively charged CO2 species on TiO2 surfaces represents a significant advance in our understanding of CO2 photoreduction. Both post-Hartree-Fock calculations on small model surface clusters as well as density-functional theory (DFT) calculations on larger clusters indicate that conduction band electrons in irradiated, stoichiometric TiO2 surfaces may not be transferred to CO2. On the other hand, oxygen vacancies may act as the active sites for CO2 photoreduction. The role of oxygen vacancies in promoting the light-induced conversion of CO2 (CO2 photoreduction) on TiO2 surfaces is discussed Chapter 4 of this thesis. Two different side-on bonded bent-CO2 (bridging Ti-CO2δ•--Ti species) were formed on the reduced rutile (110) and anatase (010), (001) surfaces. Consistent with CO2 adsorption on other n-type metal oxides such as ZrO2, the bent-CO2 species do not gain further charge from the TiO2 surface under illumination and are likely photodesorbed as neutral species. Additionally, although the formation of species such as CO and HCHO is thermodynamically possible, we find that the energy needed to regenerate the oxygen vacancy on TiO2 surfaces is greater than that available through band-gap illumination. Therefore, CO2 reactions with water on irradiated anatase TiO2 surfaces are likely to be stoichiometric. The role of lanthanide doping of TiO2 in influencing electron-hole recombination was investigated using EPR (electron paramagnetic resonance) spectroscopy. In situ pulsed-EPR experiments on calcined TiO2 samples provide evidence for surface electron and hole-centers on TiO2. However, contrary to expectations, no additional paramagnetic centers created due to lanthanide doping of TiO2 could be detected in the EPR experiments. Additionally, irradiating TiO2 in the presence of CO2 did not create paramagnetic signals corresponding to the CO2•δ- radical anion.