The Division of Chemistry supports William Eckenhoff of Rochester University as an American Competitiveness in Chemistry Fellow. Dr. Eckenhoff will work on studying the photocatalytic activity of molybdenum and tungsten dithiolate complexes for the reduction of water to hydrogen. The PI will collaborate with scientists at the 'Powering the Planet' Center for Chemical Innovation. The ultimate goal of this research is to develop inexpensive, efficient photocatalysts for the generation of hydrogen fuel. For his plan for broadening participation, Dr. Eckenhoff will participate in a number of activities (including the American Chemical Society's Project SEED) which are specifically aimed at mentoring high-school age students from underrepresented groups.
Research like that of Dr. Eckenhoff is aimed at developing improved photocatalysts for water reduction - to enable the generation of chemical fuel from water and sunlight. The results of research like this can help society come up with energy alternatives to fossil fuels. The efforts at broadening participation being pursued by Dr. Eckenhoff are aimed at encouraging young people from underrepresented groups to pursue careers in the sciences.
The objective of this proposal was to identify promising molybdenum and tungsten complexes to act as proton reduction catalysts in light-driven hydrogen production. Starting with the known activity of the heterogeneous MoS2 for this process, we synthesized and investigated over 20 molybdenum- and tungsten-sulfur complexes. Our original hypothesis was that molybdenum dithiolene complexes might be particularly active due to their well-known trigonal prismatic geometry, mimicking that of MoS2. These complexes were in fact to be the most active of those we surveyed. Of particular interest were complexes of the type [Mo(bis-dithiolene)2(isonitrile)2]. With each complex, we examined many different photosensitizer/electron donor pairing, including [Ru(bpy)3]2+/ascorbic acid, fluorescein/triethylamine, Eyosin Y/triethanolamine, and many others. In all cases, [Ru(bpy)3]2+/ascorbic acid systems were found to be the most active, achieving up to 500 TON. To better understand how these complexes functioned as catalysts, we investigated their mechanism, specifically that of [Mo(bdt)2(t-BuNC)2] (bdt=bis-benzenedithiolate, t-BuNC=t-butylisocyanide), which was one of the more active complexes. We found that each of these complexes was also active as electrocatalysts and that catalysis occurred after a 2 electron reduction. However, the nature of the catalyst after a 2 electron reduction is not clear, especially since these complexes are best described as coordinatively saturated. Thus, in a separate experiment, we reduced [Mo(bdt)2(t-BuNC)2] by 2 electrons using Na/Hg, which was removed after the reduction. The NMR spectra of this reduced complex showed that the t-BuNC ligands dissociated, opening the Mo center for catalysis. Interestingly, addition of acid restored the original complex and produced detectable quantities of hydrogen gas. This same experiment was also monitored by UV-vis spectroscopy and an intermediate was detectable shortly after the addition of acid, leading us to conclude that a molybdenum hydride was formed, which eventually lead to the evolution of hydrogen gas. A mechanism of hydrogen production was postulated based on this evidence and supported by computational results as well. This work was submitted o the Journal of the American Chemical Society and is currently under revisions for submission to Inorganic Chemistry. In the process of conducting this research, two undergraduate students worked with the PI on a daily basis and were trained in a wide variety of topics ranging from synthesis and purification, X-ray crystallography, spectroscopy and more. Their senior research projects were related to this work. The PI also participated in another project under the umbrella of light-driven hydrogen production. In this scheme, strongly absorbing rhodamine dyes were attached to platinized titania (TiO2) so that light absorption by the dyes would allow for ultrafast injection into the conduction band of TiO2 and then eventually to the platinum islands, which serve as microelectrodes for proton reduction. Similar dyes had been shown to be highly active in hydrogen production in conjunction with cobaloxime catalysts. We began by optimizing every parameter (pH, [Pt], [dye], etc). Unexpectedly, we discovered that the chalcogen atom present within the rhodamine structure (O, S, or Se) drastically affected hydrogen production, similar to what was seen in homogeneous systems. This indicated that the long-lived triplet excited state was responsible for electron injection into TiO2, counter to our original hypothesis that this would be ultra-fast. Nanosecond spectroscopy revealed that under the conditions of lightdriven hydrogen production, this injection is actually quite slow, making the triplet state an important consideration. This work was also submitted to the Journal of the American Chemical Society and is currently under revision for submission to the Journal of Physical Chemistry. The PI worked closely with a graduate student to collect this data, assisting and training as needed. In addition to training a number of students, the PI endeavored to broaden the outreach of his fellowship. The PI organized efforts to have a University of Rochester team participate the in the National Chemistry Week demonstrations at the Rochester Museum and Science centers both years of his fellowship. He also joined a nearly non-existent Younger Chemistry Council and helped in the efforts to rebuild the organization. Several events were held including a workshop that the PI personally organized for career tips. An ACS career speaker was brought in to perform the workshop where nearly 40 undergraduate students, graduate students, and post-docs engaged in this event.
