Photocatalysis and Electrocatalysis for Energy Production
Improved catalysts are needed to advance important energy producing reactions such as the oxygen reduction reaction in a fuel cell, the reverse oxygen evolution in the water splitting reaction, and in the reduction of carbon dioxide. These photo- and electrocatalytic reactions are deceptively simple involving multiple electron and atom (or ion) transfer processes. We are interested in understanding how these processes occur at the catalytic interfaces such that new, improved catalysts can be designed. This is a new research area for our group, and two recently initiated projects are described below.
Photocatalysts for CO2 Conversion. There is considerable interest in converting CO2, a greenhouse gas, into fuels (methanol, methane) or fuel/chemical feedstock precursors (CO) using solar light. Most research has focused on semiconductor photocatalysts based on band gap excitation, but there have been isolated recent reports that oxo-bridged heterobimetallics (M-O-M') tethered to mesoporous silicas can show photocatalytic activity through a metal to metal charge transfer (MMCT) process (an electron transfer from one metal to the other). The MMCT bands can extend out into the visible region of the solar spectrummaking these catalysts potentially attractive for solar energy conversion into fuels. We haverecently found that a Ti(IV)-O-Sn(II) / SBA-15 material exhibits an MMCT band that extends into the visible region, and that catalyzes CO2 reduction with a high selectivity to CO. We are currently examining this catalyst in detail as well as exploring other M-O-M' / SBA-15 materials.
This research involves synthesis of the mesoporous silica supports, surface reaction chemistry to introduce the metal centers, and catalyst characterization involving ICP elemental analysis, FTIR, solid state NMR, nitrogen physisorption (BET analysis), and UV-Visible diffuse reflectance absorption spectroscopy. Characterization of paramagnetic metal sites will be available with the delivery of a new pulsed EPR spectrometer that is on order. We also have access to fs-to-ns pump-probe spectroscopy to explore the excited state dynamics of these photocatalysts through collaboration with the Laser Spectroscopy Group in the Chemical Sciences Division. Our group has multiple photochemical reactors for performing reactions and collecting products for quantitative analysis by GC methods.
Catalysis of Multielectron Electrochemical Reactions. A molecular level understanding of multielectron electrochemical reactions that involve coupled electron transfer and atom transfer reactions will allow the design and preparation of new, efficient electrocatalysts for fuel cell electrodes, for hydrogen production by water electrolysis, or for energy storage. Catalysis of the 4e- reduction of oxygen to water is a problem of fundamental interest in electrochemistry because the overpotential is too high at a useful rate. This project is using a bioinspired approach in which effective catalysts in solution are attached to conducting supports. The reduction of oxygen requires simultaneous electron and atom transfer reactions, and the fundamental questions we are addressing are how to control the potential and how to stabilize the intermediates in oxygen reduction so that the oxygen-oxygen bond can be broken in a 4e- transfer process. These questions are being explored through the design, synthesis, and characterization of active catalysts based on metalloporphyrin binding sites and organometallic electron exchange catalysts. This work involves a determination of the reactivity of the catalysts for oxygen reduction and identification of intermediates by in situ characterization of active electrocatalysts using X-ray absorption spectroscopy (XANES and EXAFS). These studies are conducted at the National Synchrotron Light Source at Brookhaven National Laboratory in collaboration with members of the Catalysis Group in the Chemical Sciences Division. A stopped flow kinetic spectrometer is available to measure the rate of reactions in solution.
The cobalt porphyrin shown in the figure having appended ferrocene groups was prepared and investigated as an oxygen reduction catalyst. The Co is the site of O2 binding and the ferrocene groups function as electron transfer catalysts with the carbon support.
The oxidation state of the Fe and Co centers are determined by XANES. Preliminary in situ electrochemical XANES measurements suggest that the oxidized species are described as ferricenium (Fe(III)), but Co(II), indicating the porphyrin may be a cation radical.
The kinetically sluggish formation of Co(III) is avoided in this mechanism. Future work will add an appended source of protons such as that shown in the figure above. Charge transfer from the ferrocenes and formation of Co(IV) allows a four electron process to occur in an efficient pathway for the reduction of oxygen.
Physical Organic Chemistry R & D Projects