Towards a Molecular Level Understanding of the Multi-Electron Catalysis of Water Oxidation on Metal Oxide Surfaces

Abstract

Earth abundant metal oxides play a central role as catalysts in the essential chemical transformations of sunlight to fuel conversion, which are the oxidation of water and the reduction of carbon dioxide. The rapidly growing interest in renewable fuel generation by using the energy of the sun has recently led to substantial breakthroughs in the use of first row transition metal oxides as catalysts for oxygen evolution from water. Substantive improvements of rates and lowering of overpotentials have been achieved by exploiting materials properties on the nanoscale, or taking advantage of the synergy of multiple metals. Moreover, knowledge derived from mechanistic investigations with structure specific spectroscopy is accelerating efficiency improvements. Monitoring by timeresolved FT-infrared spectroscopy reveals the molecular nature of active sites, while in situ X-ray and optical spectroscopy under reaction conditions provides insights into the electronic structure of the surface metal centers participating in the catalysis. By combining the bond specificity of vibrational spectroscopy with the metal electronic structure specificity of optical, X-ray absorption or photoelectron spectroscopy, a complete understanding of active surface sites on metal oxides begins to emerge. Charge flow driving the chemical transformations probed by optical spectroscopy across time scales from ultrafast to very slow reveals the processes that control the productive use of charges delivered to the catalyst. Coupling of the water oxidation catalysis at a metal oxide catalyst with carbon dioxide reduction at a heterobinuclear chromophore, which is the goal of the artificial photosystem approach, is demonstrated by a well-defined all-inorganic polynuclear unit.

In situ characterization of cofacial Co(IV) centers in Co 4 O 4 cubane: Modeling the high-valent active site in oxygen-evolving catalysts

“…In this regard, insights into the details of the OER mechanism in Co-OECs have been greatly aided by molecular model complexes. Complementary electrochemical studies of binuclear cobalt complexes and Co-OECs are consistent with the contention that OER occurs at a dicobalt edge site (23,(25)(26)(27). As illustrated in Fig.…”

Probing zeolites by vibrational spectroscopies

“…928 In an experiment at 10 ms time resolution, following 300 ms laser pulses in an ATR cell, surface superoxide species were shown to be kinetically competent intermediates in the photooxidation of water on 5 nm nanoparticles of Co 3 O 4 : the measured lifetime of superoxides was in the same range as the duration of the illumination pulse needed to produce O 2 . 928 In an experiment at 10 ms time resolution, following 300 ms laser pulses in an ATR cell, surface superoxide species were shown to be kinetically competent intermediates in the photooxidation of water on 5 nm nanoparticles of Co 3 O 4 : the measured lifetime of superoxides was in the same range as the duration of the illumination pulse needed to produce O 2 .…”

In situ X-ray absorption spectroscopy of transition metal based water oxidation catalysts

“…The over-potential is defined as the difference between the applied potential and the thermodynamic potential for water oxidation (1.23 V vs. SHE, pH = 0). As an example, the O 2 evolution rate per site is in the range of 0.01-0.1 O 2 /Co site for Co 3 O 4 with a moderate over-potential of 0.3-0.4 V. 5 The over-potential can be tuned either by a voltage in an electrochemical cell or by photoexcitation. Both an applied voltage and photo-excitation provide positive charges, or holes, that then accept electrons from water; while a voltage can continuously tune the potential of holes, photo-excitation (either by charge transfer from a lightabsorbing dye molecule or by exciting the catalyst itself) leads to holes at the potential defined by the lowest occupied band in the material, i.e.…”