(253au) Computational Analysis of the Assembly and Catalytic Effects of Self-Assembled Monolayers

Authors: 
Kumar, G., Pennsylvania State University
Lien, C. H., University of Colorado Boulder
Medlin, J., University of Colorado
Janik, M. J., Pennsylvania State University
Self-assembled monolayers (SAMs) can be used to enhance selectivity in various nanoparticle catalyzed reactions by altering the availability of certain active sites or interacting with surface reaction species. We use density functional theory (DFT) to investigate the adsorption behavior of alkane-thiolates over Pd step and terrace surfaces. We also investigate reaction pathways for various hydrogenation reactions on SAM coated/uncoated surfaces, and compare results with experimental reactivity data. DFT calculations indicate that, at lower coverages, small chain alkane-thiolates prefer to adsorb on under-coordinated step sites. Due to stronger van der Waals interaction in longer chain thiolates, at higher coverage, long chain alkane-thiolates form highly ordered self-assembled structures on the terrace sites. This leaves the step sites open for catalytic reaction, selectively facilitating specific reaction paths. One example of such a selective path is the hydrogenation of furfural to furfuryl alcohol. On an uncoated Pd surface, the furan ring in furfural adsorbs lying flat on the terrace sites, facilitating decarbonylation to form furan. However, on SAM coated Pd catalysts, the furfural to furan pathways are hindered due to the SAM occupying the terrace sites. This forces the furan ring in furfural to attach upright on the step sites via the carbonyl oxygen, facilitating the conversion pathway to furfuryl alcohol. DFT results examining thiol assembly on different Pd facets, and its dependence on the chemical potential will also be discussed.