| The conversion of solar to chemical energy is the basis for the natural, life-sustaining process of photosynthesis, and is one of the most promising means of meeting human energy demands in an environmentally responsible manner. Photosynthesis relies upon the photochemical generation of a charge-separated state, followed by efficient electron and hole transfer to appropriate cathodic and anodic catalysts, respectively. In the present work, a porphyrin-sensitized titanium (IV) oxide photoanode functions as an array of artificial reaction centers. Absorption of a visible photon, by a porphyrin sensitizer, gives rise to an excited-state electron transfer, resulting in charge separation. The photochemically generated hole is localized on a surface-bound porphyrin dye, and is reduced via a mediated electron transfer from a suitable biofuel substrate. The photochemically generated reducing equivalent resides in the titanium (IV) oxide conduction band, and is competent to migrate through an external circuit to an appropriate cathodic catalyst, where hydrogen production occurs. The overall process is the photoelectrochemical oxidation of a biofuel, with the concomitant production of hydrogen gas. The photochemical step provides the activation energy required to drive the reaction at an appreciable rate. Depending upon the type of biomass used, some portion of the incident light energy is stored in the hydrogen gas product. The performance of this artificial photosynthetic construct has been tested with glucose or ethanol as the biofuel substrate, using either platinum or hydrogenase as the catalyst for hydrogen production. |
