Model Studies with Gold: A Versatile Oxidation and Hydrogenation Catalyst

Abstract

Historically, scientists have considered gold an inert catalyst constituent. However, in recent decades, chemists have discovered that nanoscale gold shows exceptional activity for many chemical reactions. They have investigated model gold surfaces in order to obtain fundamental understanding of catalytic properties. In this Account, we present our current understanding of oxidation and hydrogenation reactions on the Au(111) single crystal as a planar representative of gold catalysts, revealing the interesting surface chemistry of gold. We begin by comparing two inverse reactions, alcohol oxidation and aldehyde hydrogenation, on a Au(111) surface. Beyond the expected different chemistry, we observe intriguing similarities since the same surface is employed. First, both molecular oxygen and hydrogen have high barriers to dissociation on Au(111), and frequently chemists study reactions here by using atomic O and H to populate the surfaces. Recombinative desorption features of oxygen and hydrogen are apparent at ∼500 and ∼110 K, lower than other transition metals. These results indicate that oxygen and hydrogen have low desorption activation energies and weakly chemisorb on the surface, likely leading to selective reactions. On the oxygen-precovered Au(111) surface, alcohols are selectively oxidized to aldehydes. Similarly, weakly bound hydrogen atoms on Au(111) also show chemoselective reactivity for hydrogenation of propionaldehyde and acetone. The second similarity is that the gold surface activates self-coupling of alcohol or aldehyde with oxygen or hydrogen, resulting in the formation of esters and ethers, respectively, in alcohol oxidation and aldehyde hydrogenation. During these two reactions, both alkoxy groups and alcohol-like species show up as intermediates, which likely play a key role in the formation of coupling products. In addition, the cross coupling reaction between alcohol and aldehyde occurs on both O- and H-modified surfaces, yielding the production of esters and ethers, respectively. Thus, we can tune the molecular structure of both esters and ethers by selecting the corresponding aldehyde and alcohol for the coupling reaction. These studies indicate that gold is a versatile active catalyst for various reactions, including oxidation and hydrogenation transformations. Despite the very different chemistry for these two reactions, we can establish an intrinsic relationship due to the distinct catalytic properties of gold. It can show activity for selective reactions on both O- and H-covered Au(111) and further induce the coupling reaction between surface reactants and adsorbed O/H to produce esters and ethers. This comparison demonstrates the unique surface chemistry of gold and enhances understanding of its catalytic properties.