Fundamental Investigations of Water Splitting on Model TiO2 (Kimmel's BES WBS04 of Henderson's BES-Hydrogen, PNNL Scope #48526)
EMSL Project ID
18397
Abstract
Problem Statement - One of the major needs for the 21st century is the development of alternative energy sources to fossil fuels that do not contribute to greenhouse gas emissions. Hydrogen has considerable potential as an alternative fuel because it is carbon-free and because it facilitates use of more efficient power generation systems (e.g., fuel cells). However, the efficient production of hydrogen, without concomitant generation of carbon dioxide as a by-product, is a major technological challenge. For this reason, Fujishima and Honda's discovery in 1972 that water could be photocatalytically split on titanium dioxide (TiO2) electrodes continues to motivate photochemical research toward hydrogen as an alternative fuel, and in turn has revolutionized the field of heterogeneous photocatalysis. Yet, despite over thirty years of research, TiO2-based photocatalysts have failed to show viability for promoting chemical conversions using visible light. While materials limitation have proved to be a major obstacle toward a viable water splitting photocatalyst, advancements have come, but predominately from iterative improvements in catalyst and/or catalytic system design and not from deepening of the fundamental understanding of the molecular-level issues involved.Objectives - The objective of this proposal is provide a fundamental understanding of heterogeneous photocatalytic water splitting over model TiO2-based photocatalysts doped for absorption in the visible. We will address key issues such as:
- How does anion and cation doping modify the structural, chemical and/or electronic properties of TiO2 in such a way as to promote water splitting?
- How do specific dopants in specific structural environments in the bulk and/or surface influence e-/h+ pair lifetimes, charge separation and diffusion, and reaction mechanisms?
- What are the structural, mechanistic and kinetic details associated with both half-reactions?
- How does the polymorphic form of TiO2 (anatase or rutile), and the crystallographic orientation of that form, influence water splitting?
- Is the use of noble metal co-catalysts, traditionally employed for facilitating H atom combination to molecular hydrogen, necessary? If so, how do the properties of the metal particle (coverage, size, shape, location on the oxide surface, etc.) influence charge separation, charge transport, mass transport (across the oxide-metal interface) and overall reaction mechanism of hydrogen production?
Approach - We propose examining the fundamental properties of heterogeneous photocatalytic water splitting on visible-light active ion-doped TiO2 single crystal surfaces in a concerted approach that employsUHV-based surface science methods to probe the photochemistry and photodynamics of the reaction mechanism, kinetics, wavelength dependence and surface specificity of the visible-light promoted splitting of water. In addition, modulated atomic, molecular, electron and photon beams will be used to study the dynamics and kinetics of photocatalytic water splitting and hydrogen production on ion-doped TiO2 surfaces with and without decoration by Pt nanoclusters. These studies will employ state-of-the-art surface science equipment and molecular beam scattering instrumentation at PNNL to elucidate the excitation mechanisms, energy transfer pathways and reaction intermediates in this complex process. For example, radical beams of H, O, and OH will be used to prepare model systems that mimic possible intermediate states and kinetic bottlenecks along the reaction pathway(s). The effects of irradiation and/or thermal processing on hydrogen production from these intermediate states will be explored as a function of variables to include: dopant nature, concentration and location depth within the surface, wavelength of light, TiO2 surface structure and polymorph type, and supported Pt particle size and shape.
Project Details
Project type
Exploratory Research
Start Date
2006-03-31
End Date
2007-06-28
Status
Closed
Released Data Link
Team
Principal Investigator
Team Members
Related Publications
Chistopher Lane, Nikolay Petrik, Thomas Orlando, Greg Kimmel; J. Phys. Chem. C, (accepted).
Christopher Lane: "Low-Energy Electron Induced Processes in Molecular Thin Films Condensed on Silicon and Titanium Dioxide Surfraces", 2007. (Doctoral Dissertation)
Christopher Lane, Nikolay Petrik, Thomas Orlando, Greg Kimmel; J. Phys. Chem. C, (submitted)
Kimmel GA, NG Petrik, Z Dohnalek, and BD Kay. "Crystalline Ice Growth on Pt(111) and Pd(111): Nonwetting Growth on a Hydrophobic Water Monolayer." Journal of Chemical Physics 126(11):Art. No. 114702.
Lane, C.D., Petrik, N.G., Orlando, T.M., and Kimmel, G.A. “Electron-stimulated oxidation of thin water films adsorbed on TiO2(110)” Journal of Physical Chemistry C, 111, 16319-16329 (2007)
Lane, C.D., Petrik, N.G., Orlando, T.M., and Kimmel, G.A. “Site-dependent electron-stimulated reactions in water films on TiO2(110)” Journal of Chemical Physics 127, 224706 (1-9) (2007)
N. G. Petrik and G. A. Kimmel, Electron stimulated reactions in thin water films adsorbed on TiO2(110), 232nd ACS National Meeting, San Francisco, CA, September 10-14, 2006
N. G. Petrik ,C.D. Lane, T. Orlando, and G. A. Kimmel, Electron stimulated desorption and dissociation of thin water films adsorbed on TiO2 (110), 234th ACS National Meeting, Boston, MA, August 19-23, 2007
N. G. Petrik ,C.D. Lane, T. Orlando, and G. A. Kimmel, Electron stimulated reaction in thin water films adsorbed on TiO2 (110), AVS 54th International Symposium October 14-19, 2007, Seattle, WA