Discovery of Photocatalysts for Hydrogen Production

This project for DOE was designed to address these materials-related issues through a combination of high-throughput screening of semiconductor candidates and theoretical modeling of nanostructures. High-throughput screening is an effective and economical way to examine a large number of candidates and identify those worthy of further study. Unfortunately, in the course of this project, we discovered no semiconductor candidates that can meet the DOE’s stringent requirements for an economically feasible photoelectrochemical process. However, some of our results indicated that several systems may have potential if further optimized. In particular, the published theoretical modeling work indicates that core-shell nanorod structures, if properly engineered, have the potential to overcome the shortfalls of current semiconductors. Although the synthesis of the designed core-shell nanorod structures proved to be beyond the current capabilities of our laboratories, recent advances in the synthesis of core-shell nanorod structures imply that the designed structures can be synthesized. SRI is confident that once these materials are made they will validate our models and lead to economical and environmentally friendly hydrogen from sunlight and water. The high-throughput photolysis analysis module developed at SRI will also have utility in applications such as identifying catalysts for photo-assisted chemical detoxification, as well as non-photolytic applications such as hydrogen storage, which can take advantage of the ability of the analysis module to monitor pressure over time

Reaction of Dioxygen with a Cross-Conjugated Carbon-Carbon Double Bond in a Bis-Macrocycle Diiron Compound

Dioxygen at atmospheric pressure attacks a cross-conjugated carbon-carbon double bond in a diiron complex to form two, like, keto macrocyclic iron(II) complexes. This reaction occurs with high yield in both solution and in the solid state. A dioxetane intermediate is, therefore, invoked. The rate of the reaction is very dependent on the nature of the axial ligands on the low-spin iron(II) ions in the bimetallic complex. The rate is at least a factor of 104 faster with DMF ligands than with CH3CN axial ligands. This rate dependence is explained by stabilization of a peroxo biradical transition state en route to a dioxetane intermediate. The keto-macrocyle product has the carbonyl group conjugated with a β-diimine in a six-membered chelate ring. The conformation of this keto macrocycle is fixed on the NMR time scale and the spectra of all ten non-equivalent protons in the complex can be unambiguously assigned. The keto β-diimine ligand is an excellent π-acceptor as indicated by the high Fe(II) to Fe(III) oxidation potential of the compound and by the Mössbauer spectrum, which shows a low value for the center shift and a high value for the quadrupole splitting parameter

Evolution of C-rich SiOC ceramics. Part 1: Characterization by integral spectroscopic techniques: solid-state NMR and Raman spectroscopy

Abstract Carbon-rich Si–O–C polymer-derived ceramics (PDCs) were investigated by various spectroscopic techniques, in order to characterize the evolution of their predominantly amorphous microstructure upon thermal treatment up to 1450°C. Particular attention was addressed to modifications of the excess free carbon phase present in these materials. Surprisingly, the carbon clusters exhibited high stability above the pyrolysis temperature. Despite the high volume fraction of carbon, only a very limited carbothermal reduction process was detected. This study is divided into two parts: PartI deals with characterization tools that reveal a rather low lateral resolution and are hence termed here as integral spectroscopic techniques, i.e., solid-state NMR and Raman spectroscopy. In contrast, PartII illustrates the experimental results obtained from the very same materials characterized by spectroscopic and imaging techniques with high lateral resolution, i.e., electron energy-loss spectroscopy (EELS), high-resolution transmission electron microscopy (HRTEM), and energy-filtered TEM. In addition to materials characterization, emphasize of both papers is also to compare the information gained by either integral or local spectroscopy techniques and to highlight the strengths and weaknesses of either approach.</jats:p