Carbonate Formation and Decomposition on Atomic Oxygen Precovered Au(111)

Abstract

The carbonate formation and decomposition (CO 3 T CO 2 + O a ) reaction on gold is important from the point of view of lowtemperature CO oxidation. Carbonate formation has been proposed as a possible reaction intermediate in CO oxidation in several investigations of supported and unsupported gold clusters. [1][2][3][4] Therefore, an understanding of this reaction on Au(111) may provide additional insight. Carbonate formation and decomposition went undetected in previous studies on Au (110) 5 and Au(111). 6 However, a surface carbonate was readily formed when oxygen precovered Ag(110) was exposed to CO 2 at 300 K. [7][8][9][10] This surface carbonate decomposes to produce CO 2 at 485 K and the remaining oxygen atoms recombinatively desorbed at 590 K. [7][8][9][10] Owing to its similarity with silver, we would anticipate equally facile carbonate formation and decomposition reactions on gold. Similar reactions have also been reported on other surfaces. [11][12][13] Here we present experimental evidence with supporting density functional theory (DFT) calculations of carbonate formation and decomposition from the adsorption of oxygen-labeled carbon dioxide (C 18 O 2 ) on an atomic oxygen ( 16 O) precovered Au(111) surface. We studied the effects of CO 2 exposure, surface temperature, and oxygen coverage on carbonate formation and decomposition and also estimated reaction probabilities (∼10 -3 -10 -4 ) and activation energies as a function of conditions.Our experiments were performed in a UHV chamber that has been described elsewhere, [14][15][16][17][18][19] but details specific to this study are briefly summarized here. The Au(111) single crystal sample is mounted to a tantalum plate that can be resistively heated and is in thermal contact with a liquid nitrogen bath. Oxygen ( 16 O) atoms were deposited using a radio frequency (RF) plasma-jet source. 16 O but with no exposure to C 18 O 2 (due to natural isotopic abundance of 18 O). As expected, no surface-bound oxygen was lost during carbonate formation and decomposition, in agreement with previous studies. [8][9][10] Figure 1b shows the amount of mass 34 produced (from the spectra in Figure 1a) as a function of C 18 O 2 exposure.To further examine the role of preadsorbed atomic oxygen on carbonate formation, we varied the oxygen precoverage (0.18-2.1 ML) while keeping both C 18 O 2 exposure (30 L) and surface temperature (167 K) constant (Figure 2). Mass 34 production increases with increasing 16 O a coverage, likely because more reactive oxygen is accessible to C 18 O 2 on the surface. Similar results were obtained employing surface temperatures of 220 and 300 K. We estimated the reaction probability of carbonate formation assuming a statistical distribution 7 in the decomposition of the surface-bound carbonate 16 OC 18 O 2 and obtained values of ∼10 -3 -10 -4 (uncertainties of (50%). These small values are likely part of the reason why an earlier study on Au(111) 6 reported undetectable surface carbonate formation. An Arrhenius plot of the reaction probabili...

A comparative theoretical study of the catalytic activities of Au 2 - and AuAg- dimers for CO oxidation

“…The overall reaction is calculated to be exothermic by 137.11 kcal mol -1 . For low-temperature CO oxidation, it is generally agreed that carbonate species are an important intermediate [58]. Several investigations have observed its formation on supported and unsupported gold clusters [57][58][59][60][61].…”

On the Impact of Solvation on a Au/TiO2Nanocatalyst in Contact with Water

“…In heterogeneous catalysis, it has been shown that the presence of water increases the observed rate of CO oxidation [9,10]. The degree of rate enhancement depends on the type of support used.…”

Reactions of small molecules on gold single crystal surfaces

“…Experimental results supported by DFT calculations show carbonate formation and reaction on atomic oxygen precovered Au(111) [39]. Oxygen mixing was observed when 16 O ads precovered Au(111) was exposed to isotopically labelled CO 2 (C 18 O 2 ) at surface temperatures ranging from 77-400 K and initial oxygen coverages ranging from 0.18-2.1 ML.…”

“…Mullins and co-workers studied carbonate formation on Au(111) [38][39]. Experimental results supported by DFT calculations show carbonate formation and reaction on atomic oxygen precovered Au(111) [39].…”