לוח שנה משולב

This colloquium will be divided into two applications parts, dealing with synthesis of supported molecular catalysts and solid catalysts for photoprotection. In the first of these areas, we describe a mechanical approach for stabilizing supported weakly interacting active sites (i.e. those that interact non-covalently with the support) against aggregation and coalescence. We use silica as a prototypical example of a support, and an iridium pair-site catalyst incorporating bridging calixarene ligands as an active site. Atomic-resolution imaging of the Ir centers before and after ethylene-hydrogenation catalysis show the metals resisted aggregation and deactivation, remaining atomically dispersed and accessible for catalysis. When active sites are located at unconfined environments, the rate constants for ethylene hydrogenation are markedly lower compared with confining external-surface pockets [1], in line with prior observations of similar effects in olefin epoxidation catalysis [2,3]. Altogether, these examples represent new opportunities for enhancing reactivity on surfaces by synthetically controlling mechanical features of active site catalyst environments.

In the second of these areas, reactive oxygen species (ROS) are associated with several human health pathologies and are invoked in the degradation of natural ecosystems as well as building materials that are used in modern infrastructure (e.g., paints and coatings, polymers, etc). Natural antioxidants such as vitamin E function as stoichiometric reductants (i.e. reaction with ROS synthesizes rancid oils). While enzymes such as superoxide dismutase working in tandem with catalase decompose decompose ROS to H2O and O2 through H2O2 as an intermediate, these enzymes are fragile and costly. Other non-stoichiometric commercial antioxidants that degrade ROS include hindered amine light stabilizers (HALS). Here, we demonstrate that cerium carbonate acts as a degradation catalyst for photogenerated ROS, and describe the performance and characterization of this new catalyst using X-ray photoelectron spectroscopy, and in comparison with HALS and stoichiometric reductants. Our results demonstrate catalytic antioxidant activity of cerium carbonate when dispersed in polymethylmethacrylate polymer. FTIR data demonstrate that a dispersion of 2 wt. % cerium carbonate within the polymer essentially stops degradation by photogenerated ROS, which otherwise cause oxidation of the polymer backbone, in the control polymer lacking cerium carbonate. Experiments with methylene blue dye in aqueous solution demonstrate that cerium carbonate decreases the rate of ROS degradation of dye, in the presence of UV irradiation and air by 16 fold. These effects become even more pronounced (over 600 fold decrease in rate of ROS dye degradation) when cerium carbonate is paired with a photoactive metal oxide. The mechanism involved in this latter case crudely mimics the enzyme tandem sequence referred to above.

[1] C. Schöttle, E. Guan, A. Okrut, N. A. Grosso-Giordano, A. Palermo, A. Solovyov, B. C. Gates, A. Katz*, Journal of the American Chemical Society, J. Am. Chem. Soc. 2019, 141, 4010-4015.
[2] N. A. Grosso-Giordano, C. Schroeder, A. Okrut, A. Solovyov, C. Schottle, W. Chasse, N. Marinkoyic, H. Koller, S. I. Zones, A. Katz, Journal of the American Chemical Society 2018, 140, 4956-4960.
[3] N. A. Grosso-Giordano, A. S. Hoffman, A. Boubnov, D. W. Small, S. R. Bare, S. I. Zones, A. Katz, Journal of the American Chemical Society 2019, 141, 7090-7106.
[4] M. K. Mishra, J. Callejas, M. Pacholski, J. Ciston, A. Okrut, A. Van Dyk, D. Barton, J. C. Bohling, A. Katz, ACS Applied Nano Materials 2021, 4, 11, 11590-11600.