Model studies of heterogeneous catalytic hydrogenation reactions with gold.

Department of Chemical Engineering, University of Texas at Austin, Austin, TX 78712-0231, USA. .
Chemical Society Reviews (Impact Factor: 30.43). 02/2013; DOI: 10.1039/c3cs35523c
Source: PubMed

ABSTRACT Supported gold nanoparticles have recently been shown to possess intriguing catalytic activity for hydrogenation reactions, particularly for selective hydrogenation reactions. However, fundamental studies that can provide insight into the reaction mechanisms responsible for this activity have been largely lacking. In this tutorial review, we highlight several recent model experiments and theoretical calculations on a well-structured gold surface that provide some insights. In addition to the behavior of hydrogen on a model gold surface, we review the reactivity of hydrogen on a model gold surface in regards to NO2 reduction, chemoselective C[double bond, length as m-dash]O bond hydrogenation, ether formation, and O-H bond dissociation in water and alcohols. Those studies indicate that atomic hydrogen has a weak interaction with gold surfaces which likely plays a key role in the unique hydrogenative chemistry of classical gold catalysts.

1 Bookmark
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Dissociation of molecular hydrogen is an important step in a wide variety of chemical, biological, and physical processes. Due to the light mass of hydrogen it is recognized that quantum effects are often important to its reactivity. However, understanding of how quantum effects impact the reactivity of hydrogen is still in its infancy. Here, we examine this issue using a well-defined Pd/Cu(111) alloy that allows the activation of hydrogen and deuterium molecules to be examined at individual Pd atom surface sites over a wide range of temperatures. Experiments comparing the uptake of hydrogen and deuterium as a function of temperature reveal completely different behavior of the two species. The rate of hydrogen activation increases at lower sample temperature, whereas deuterium activation slows as the temperature is lowered. Density functional theory simulations in which quantum nuclear effects are accounted for reveal that tunneling through the dissociation barrier is prevalent for H2 up to ~190 K and for D2 up to ~140 K. Kinetic Monte Carlo simulations indicate that the effective barrier to H2 dissociation is so low that hydrogen uptake on the surface is limited merely by thermodynamics whereas the D2 dissociation process is controlled by kinetics. These data illustrate the complexity and inherent quantum nature of this ubiquitous and seemingly simple chemical process. Examining these effects in other systems with a similar range of approaches may uncover temperature regimes where quantum effects can be harnessed, yielding greater control of bond-breaking processes at surfaces and uncovering useful chemistries such as selective bond activation or isotope separation.
    ACS Nano 03/2014; · 12.03 Impact Factor
  • [Show abstract] [Hide abstract]
    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.
    Accounts of Chemical Research 03/2014; 47(3):750-60. · 24.35 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Pd–Au bimetallic catalysts have shown promising performance in numerous reactions that involve hydrogen. Fundamental studies of hydrogen interactions with Pd–Au surfaces could provide useful insights into the reaction mechanisms over Pd–Au catalysts, which may, in turn, guide future catalyst design. In this study, the interactions of hydrogen (i.e., adsorption, absorption, diffusion, and desorption) with Pd/Au(111) model surfaces were studied using temperature-programmed desorption (TPD) under ultrahigh-vacuum conditions. Our experimental results reveal Pd–Au bimetallic surfaces readily dissociate H2 and yet also weakly bind H adatoms, properties that could be beneficial for catalytic reactions involving hydrogen. The presence of contiguous Pd sites, characterized by reflection–absorption infrared spectroscopy using CO as a probe molecule (CO-RAIRS), was found to be vital for the dissociative adsorption of H2 at 77 K. The H adatom binds to Pd–Au alloy sites more strongly than to Au(111) but more weakly than to Pd(111) as indicated by its desorption temperature (200 K). With hydrogen exposure at slightly higher temperatures (i.e., 100–150 K), extension of a low-temperature desorption feature was observed, suggesting the formation of subsurface H atoms (or H absorption). Experiments using deuterium indicate that H–D exchange over the Pd–Au bimetallic surface obeys Langmuir–Hinshelwood kinetics and that H/D adatoms are mobile on the surface at low temperatures.
    The Journal of Physical Chemistry C 09/2013; 117(38):19535–19543. · 4.84 Impact Factor


Available from
May 29, 2014