DOE OSTI2015
Part 1. Design of Biomimetic Catalysts Based on Amphiphilic Systems The overall objective of our research is to create biomimetic catalysts from amphiphilic molecules. More specifically, we aim to create supramolecular systems that can be used to control the microenvironment around a catalytic center in a biomimetic fashion and apply the learning to construct supramolecular catalysts with novel functions found in enzymatic catalysts. We have prepared synthetic molecules (i.e., foldamers) that could fold into helical structures with nanometer-sized internal hydrophilic cavities. Cavities of this size are typically observed only in the tertiary and quaternary structures of proteins but were formed in our foldamer prepared in just a few steps from the monomer. Similar to many proteins, our foldamers displayed cooperativity in the folding/unfolding equilibrium and followed a two-state conformational transition. In addition, their conformational change could be triggered by solvent polarity, pH, or presence of metal ions and certain organic molecules. We studied their environmentally dependent conformational changes in solutions, surfactant micelles, and lipid bilayer membranes. Unlike conventional rigid supramolecular host, a foldamer undergoes conformational change during guest binding. Our study in the molecular recognition of an oligocholate host yielded some extremely exciting results. Cooperativity between host conformation and host–guest interactions was found to “magnify” weak binding interactions. In other words, since binding affinity is determined by the overall change of free energy during the binding, guest-induced conformational change of the host, whether near or far from the binding site, affects the binding. This study has strong implications in catalysis because enzymes have been hypothesized to harvest similar intramolecular forces to strengthen their binding with the transition state of an enzyme-catalyzed reaction. The supramolecular and amphiphilic principles used in the foldamer catalysts were extended to a few other systems, particularly to interfacially cross-linked reverse micelles and micelles. These features enabled unusual catalytic features such as basic/nucleophilic catalysis under acidic conditions. We were able to create highly active metal nanoclusters catalysts whose local environment could be tuned by the organic framework. We were even able to create a “catalytic nanomachine” that grabs the substrate to the encapsulated Au clusters, which efficiently convert the substrate to the product that is rapidly ejected due to its different binding properties. Our research has important impacts on fundamental and applied energy-related sciences. On the fundamental level, it tests important biocatalytic principles on relatively simple synthetic systems and is expected to afford deeper understanding of biological catalysis. On the practical level, the research is anticipated to lead to “smart” catalysts and open up exiting applications in chemical analysis, reaction control, and materials synthesis. Part 2. Electrochemical Reduction of CO₂ The primary objective of our research involving the electrochemical reduction of carbon dioxide is to apply a multidisciplinary approach toward developing a greater understanding of the problem of efficiently converting CO₂ to hydrocarbons through electrochemical routes. Our goal is to provide a better understanding of the principles that underlie the electrocatalytic reduction of CO₂ at electrode surfaces and the molecular pathways that lead to desired compounds. This understanding is essential for the design and development of new catalytic materials for the selective production of renewable feedstocks. The electrochemical reduction of CO₂ involves the formation of various reaction products and adsorbed intermediates whose distribution depends upon the nature of the electrode material and the electrochemical conditions, including applied potential, solvent, and electrolyte, used during reduction. Our efforts are focused on developing a detailed picture of how these various parameters impact the reaction pathway for CO₂ reduction and to tune this reaction to favor the formation of specific products by careful selection of electrode materials and reaction conditions. Our experimental plan includes a detailed examination of the electrode material as well as a quantitative evaluation of the reaction conditions using several in-situ analysis tools that give a direct measure of adsorbed and solution species that appear during CO₂ reduction. During the first year of funding for this aspect of the project, we have developed an on-line electrochemical measurement tool utilizing membrane-introduction mass spectrometry to detect reactions products directly during the electrochemical reduction of CO₂on various sputtered electrode materials. In addition, we have examined the role of a selection of different electrode compositions (including single component and binary electrodes) on product distributions.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗