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Although the global demand for energy is steadily increasing, most of the current sources of energy are either nonrenewable, nonsustainable, or contribute toward greenhouse gases in the environment. In addition to wind power and biofuels, solar energy is a renewable and clean alternative energy source, but current methods used to convert it into transportable fuels are costly and inefficient. However, solar energy is used efficiently in Nature via the process of photosynthesis in plants, cyanobacteria, and algae.
PHOTOSYSTEM II and O2 EVOLUTION
Photosynthesis is a biochemical process by which green plants convert solar energy to carbohydrates. In this process, water is oxidized and dioxygen is formed via the photo-induced reaction:
The photosynthetic splitting of water (i.e. oxygen evolution or water oxidation) is the source of nearly all of the O2 in the atmosphere, and takes place in the oxygen evolving complex (OEC), which is located in the multisubunit membrane protein complex Photosystem II (PS II). The OEC is a cluster of four Mn atoms and one Ca atom (Mn4CaO5), and has been shown via X-ray spectroscopy and crystallography to be linked by oxo or hydroxo bridges. The OEC cycles through five different oxidation states, known as Si states (i = 0 - 4), coupling the one-electron photochemistry of the photosynthetic reaction center with the four-electron chemistry of water oxidation. Although the structure and mechanism of Mn4CaO5 has been extensively studied by various methods, the precise molecular details remain elusive.
SPECTROSCOPY and X-RAY CRYSTALLOGRAPHY of PS II
our ongoing investigations of the electronic and geometric
structure and changes of the Mn4CaO5
cluster, we have made extensive use of X-ray and EPR
spectroscopy and X-ray crystallography. We are using
high-resolution absorption and emission X-ray spectroscopy,
including Resonant Inelastic X-ray Scattering spectroscopy,
to study crystals and solutions of PS II and inorganic
models. We are using the femtosecond X-ray Free Electron
Laser Facilities at Stanford and elsewhere to collect X-ray
diffraction and emission spectra simultaneously, and X-ray
absorption spectra at the K-and L-edges of the Mn cluster in
its native and intermediates states at room temperature in a
time-resolved manner, to capture short-lived intermediates
and the step that includes the O-O bond formation. These
techniques that we are developing, using both XFELs and
synchrotron sources, will elucidate not only the structure
of the Mn4CaO5 catalytic complex, but
also the changes in the structure as it cycles through the
intermediate states, leading to an understanding of the
mechanism. The insights gained from natural photosynthesis
can then be applied in the design of inorganic-based energy
conversion systems that split water, and will contribute to
our search for cleaner, renewable carbon-neutral energy
Simultaneous X-ray Diffraction and X-ray Emission Spectroscopy of PS II in the Intermediate States at Room Temperature Using an XFEL
Droplet on Tape Method with an Acoustic Droplet Dispenser for XFEL Studies
Zone Plate Spectrometer for Mn L-edge Studies of PS II
SYSTEMS FOR ENERGY CONVERSION
Energy dispersive emission spectrometer configured for two energies. (bottom) Energy diagrams of transition metals, Mn, and Ti.
The decades-long search for efficient, long-lived photo- and electrocatalysts for water oxidation, using earth-abundant and cost effective metals such as Mn, Fe, Co, Ni or others, has begun to produce important results. However, crucial breakthroughs are still required for the development of highly active, stable catalysts that promote rapid water splitting at a negligible overpotential. Significant improvements in catalyst performance have been hindered by a lack of structural and mechanistic understanding that will assist in the design of new catalytic structures.
Promising catalytic materials include first row transition metal oxides. Related systems of importance are heterodinuclear units that exhibit tunability of redox properties. The metal-to-metal charge-transfer (MMCT) units can absorb light across the visible spectrum, resulting in a transient charge-transfer excited state that can drive chemical reactions. With their wide range of oxidation states and redox potentials, MMCT units can serve as tunable single photon, single electron pumps for driving catalysts, some of them showing remarkable efficiency.
All these systems are amorphous and often embedded in matrices, and cannot be studied using X-ray crystallography. XAS/EXAFS/XES are ideal methods to determine the electronic and geometric structure of such materials. We have gained experience in developing and using such methods in our work with the natural photosynthetic system and we have begun to specifically tailor the methods for the study of artificial systems.
Although, steady state X-ray K- and L-absorption and emission techniques have been an enormously important tool for understanding the mechanism of these catalysts, it is critically important to be able to follow the reactions, in real time and in situ, to understand the dynamics of the systems. We are using in situ X-ray spectroscopy to study water-oxidation/CO2 reduction catalysts that are initiated chemically, photochemically or electrochemically, to understand the fundamental processes by which the catalysts operate and the phenomena that limit their effectiveness, so one can improve the efficiencies. We also developed methods for detecting the emission spectra of two elements from dilute systems simultaneously using an energy-dispersive emission spectrometer that can collect the entire emission spectra. This detection method allows us to follow the sequential chemistry at two metal sites following ultrafast laser pump and X-ray excitation.
These proposed development of X-ray spectroscopy techniques for studying inorganic systems is in line with DOE interest in generating efficient catalysts for artificial photosynthetic systems.