Person:

Montemore, Matthew

The Au(110) surface offers unique advantages for atomically-resolved model studies of catalytic oxidation processes on gold. We investigate the adsorption of oxygen on Au(110) using a combination of scanning tunneling microscopy (STM) and density functional theory (DFT) methods. We identify the typical (empty-states) STM contrast resulting from adsorbed oxygen as atomic-sized dark features of electronic origin. DFT-based image simulations confirm that chemisorbed oxygen is generally detected indirectly, from the binding-induced electronic structure modification of gold. STM images show that adsorption occurs without affecting the general structure of the pristine Au(110) missing-row reconstruction. The tendency to form one-dimensional structures is observed already at low coverage (< 0.05 ML), with oxygen adsorbing on alternate sides of the reconstruction ridges. Consistently, calculations yield preferred adsorption on the (111) facets of the reconstruction, on a 3-fold coordination site, with increased stability when adsorbed in chains. Gold atoms with two oxygen neighbors exhibit enhanced electronic hybridization with the O states. Finally, the species observed are reactive to CO oxidation at 200 K and desorption of CO2 leaves a clean and ordered gold surface.

In the one-pot reaction between nitroarenes, aldehydes, and hydrogen, the desired outcome is the selective hydrogenation of nitroarenes to form aminoarenes that condense with aldehydes to form pharmaceutically relevant imines and N-alkylamines. One approach to facilitate the selective hydrogenation of nitroarenes over aldehydes involves using bimetallic catalysts with near equimolar ratios. However, structural characterization of metallic ensembles on the nanoparticle surface is challenging at such high alloying ratios, which hinders the elucidation of clear structure–property relationships. Here, we prepared a well-controlled series of dilute Pd-in-Au alloy catalysts with a fixed nanoparticle size as a model system to investigate the effects of surface Pd ensemble size, from single atoms to dimers and trimers, in the one-pot hydrogenation reaction between nitrobenzene and benzaldehyde as our probe reaction. The highest (near unity) selectivity to condensation products was achieved using the catalyst with the lowest Pd content prepared (Pd2Au98/SiO2), which predominantly exposed surface Pd single atoms as verified by surface-sensitive spectroscopy. Theoretical calculations reveal that Pd single atoms were inactive for benzaldehyde adsorption and thus enabled selective nitrobenzene hydrogenation. On the contrary, the adsorption of benzaldehyde became stronger than nitrobenzene for Pd trimers and larger ensembles, explaining the enhanced competitive adsorption from benzaldehyde with increasing Pd content. Our results demonstrate that the commonly used (near equimolar) alloying ratio is rather arbitrary and may not necessarily produce the highest selectivity to condensation products. Instead, we illustrate how nanoscale Pd ensemble size control tunes competitive kinetics to steer selectivity towards forming the desired condensation products.