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Understanding gold catalysis: the effect of water and the presence of low-coordinated sites

For centuries, mankind has been fascinated by gold and more specifically by its ability to refrain from interacting with other chemical elements. This changes drastically when moving from macroscopic to nanoscaled objects. Gold nanoparticles are highly reactive and catalyze the oxidation of CO to CO2 at significantly lower temperatures than platinum, the traditional catalyst.
Water vapor enhances the CO2 production by almost a factor twenty. The origin of this effect is difficult to pinpoint due to the lack of proper methods to fully characterize the working catalyst in a chemical environment. To circumvent these problems and to obtain detailed information on elementary steps, model catalysts are used. In our case, this is the surface of a macroscopic single crystal (Figure 1(a)). This model system allows to separate the different explanations regarding gold's catalytic activity. It has many steps (Figure 1(b)), which are composed of gold atoms with a low coordination number, which should also be abundantly present on small nanoparticles.
 

Figure 1. (a) photograph of a gold single crystal mounted inside a UHV system. (b) schematic view of the Au(310) surface that was used as model catalyst. The coordination numbers, i.e. total number of nearest neighboring atoms, are given in blue. 

Previous studies in the Catalysis and Surface Chemistry group at the Leiden University started with probing the adsorption and desorption of water and CO from a stepped surface. From these results, we could conclude that the low-coordinated Au atoms did not dissociate H2O and that H2O does not react directly with CO. However, when using electrons to fragment H2O prior to exposing them to CO, we observed the production of CO2.
To explore the nature of the H2O fragmentation products, two studies were conducted at the SuperESCA beamline using X-ray Photoelectron Spectroscopy (XPS). The first study was focused on the adsorption and desorption of intact water on the stepped model catalyst. We observed that water binds chemically differently to step atoms than the more coordinated atoms in the terraces.
In the second and recent study, water was fragmented using electron impingement. Assignments of various species adsorbed on the Au surface in our XP spectra are shown in Figure 2(a). With XPS, we showed: (1) the absence of significant amounts of hydroxides (i.e. OH groups) and, (2) that mixtures of water and atomic oxygen were formed. This mixture reacts vigorously with CO, even at temperatures as low as 100 K. Figure 2(b) shows an example of the intensity changes of various peaks in the XP spectra at a catalyst temperature where CO is oxidized. Simultaneously released water (here D2O) from the O(ads)-water complex remains frozen on the surface.
 

Figure 2. (a) XP spectrum of the oxygen species on the Au(310) surface after electron irradiation. Several species can be observed: adsorbed water bonded to 9-fold coordinated Au atoms (black), adsorbed water bonded to 6 and 8-fold coordinated Au atoms (red), water bonded to O(ads) (blue), and O(ads) (green). (b) coverage dependence of the surface species under CO oxidation reaction conditions at 135 K. CO reacted with O(ads) to form CO2 and the O(ads)-stabilized water was transformed to water (D2O) without the O(ads) stabilization [black and red peaks in (a)]. In addition to the reaction, CO(ads) fills up the surface (purple).


The positive effect of water to the CO oxidation rate is believed to be related to the activation of O2. Several different reaction mechanisms were proposed that are either involving atomic oxygen or hydroxides. In our work, we show that the latter are not stable on gold nanoparticles and will react via

2 OH(ads) → H2O(ads) + O(ads)

This adsorbed atomic oxygen, O(ads), is the main oxidant in the CO oxidation reaction.  To understand and control selectivity in heterogeneous catalysis, it is paramount to know exactly what the active oxidant in the reaction is. This information is fundamental to rational design of improved catalysts. Although gold is currently only used in niche markets, it may very well become an important catalytic material as it allows for operation at lower temperatures and with higher selectivity for many other chemical reactions, hence reducing energy costs and greenhouse gas emission.

 

This research was conducted by the following research team:

Matthijs A. van Spronsen1, Kees-Jan Weststrate2, Angela den Dunnen3, Christine Hahn3, and Ludo B.F. Juurlink3


1 Huygens-Kamerlingh Onnes Laboratory, Leiden University, The Netherlands
2 Syngaschem BV, Eindhoven University of Technology, The Netherlands
3 Institute of Chemistry, Leiden University, The Netherlands



Contact person:

Matthijs Andrè van Spronsen, email: mavanspronsen@fas.harvard.edu


 

Reference

Matthijs A. van Spronsen, Kees-Jan Weststrate, and Ludo B. F. Juurlink, "A Comparison of CO Oxidation by Hydroxyl and Atomic Oxygen from Water on Low-Coordinated Au Atoms", ACS Catalysis 6 , 7051 (2016), doi: 10.1021/acscatal.6b01720

 

 
Last Updated on Friday, 21 October 2016 10:08