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44179-AC5
Chemistry of Thin Gold Films on Titania

Charles Buddie Mullins, University of Texas (Austin)

Carbonate Formation on Au(111)

We conducted experiments of carbonate formation and decomposition from the adsorption of oxygen-labeled carbon dioxide (C¹⁸O₂) on an atomic oxygen (¹⁶O) pre-covered Au(111) surface. We studied the effects of CO₂ 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 in which a 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 (¹⁶O) atoms were deposited using a radio frequency (RF) plasma-jet source. The ¹⁶O_a/Au(111) surface was exposed to C¹⁸O₂ by backfilling the chamber and carbonate ¹⁶OC¹⁸O¹⁸O was formed. The surface carbonate decomposes to form either C¹⁸O₂ or ¹⁶O C¹⁸O leaving ¹⁸O_a or ¹⁶O_a adatoms on the surface. Upon heating, the oxygen atoms undergo recombinative desorption to produce ¹⁶O₂ (mass 32) and ¹⁶O¹⁸O (mass 34), as observed in TPD. Thus, carbonate formation and decomposition were detected via the increased presence of mass 34 ¹⁸O¹⁶O in a temperature programmed desorption (TPD) spectrum after the ¹⁶O_a covered Au(111) surface was exposed to C¹⁸O₂. We did not observe ¹⁸O₂ (mass 36) in TPD due to the very small surface concentration of ¹⁸O.  This method was employed after other strategies proved unsuccessful due to the low reaction probability.
In summary, we obtained evidence for carbonate formation and reaction on atomic oxygen pre-covered Au(111). Oxygen mixing was observed when ¹⁶O_a pre-covered Au(111) was exposed to isotopically labeled CO₂ (C¹⁸O₂) at surface temperatures ranging from 77 – 400 K and initial oxygen coverages ranging from 0.18 ML – 2.1 ML. Subsequent desorption of isotopically mixed oxygen (¹⁶O¹⁸O, mass 34) is observed as a by-product of carbonate formation and decomposition on the surface. Carbonate formation occurs with a very small reaction probability (~ 10^-3 - 10^-4) and is most favorable at low surface temperatures.
Effect of Adsorbed Water on CO Oxidation on Au(111)
Water-oxygen interactions and CO oxidation by water on the oxygen pre-covered Au(111) surface were studied using molecular beam scattering techniques, temperature programmed desorption (TPD), and density functional theory (DFT) calculations. Water thermally desorbs from the clean Au(111) surface with a peak temperature of ~ 155 K, however on a surface with pre-adsorbed atomic oxygen a second water desorption peak appears at ~ 175 K. DFT calculations suggest that hydroxyl formation and recombination are responsible for this higher temperature desorption feature. TPD spectra support this interpretation showing oxygen scrambling between water and adsorbed oxygen adatoms upon heating the surface. In further support of these experimental findings, DFT calculations indicate rapid diffusion of surface hydroxyl groups at temperatures as low as 75 K. Regarding the oxidation of carbon monoxide, if a C¹⁶O beam impinges on a Au(111) surface covered with both atomic oxygen, (¹⁶O) and isotopically labeled water (H₂¹⁸O), both C¹⁶O¹⁶O and C¹⁶O¹⁸O were produced, even at surface temperatures as low as 77 K. Similar experiments performed by impinging a C¹⁶O beam on a Au(111) surface covered with isotopic oxygen, (¹⁸O) and deuterated water (D₂¹⁶O) also produced both C¹⁶O¹⁶O and C¹⁶O¹⁸O but less than that produced using ¹⁶O and H₂¹⁸O.  These results unambiguously show the direct involvement and promoting role of water in CO oxidation on oxygen covered Au(111) at low temperatures.  On the basis of our experimental results and DFT calculations, we propose that water dissociates to form hydroxyls (OH and OD) and these hydroxyls react with CO to produce CO₂.  Differences in water-oxygen interactions and oxygen scrambling were observed between ¹⁸O/H₂¹⁶O and ¹⁸O/D₂¹⁶O with the latter producing less scrambling. Similar differences were also observed in water reactivity towards CO oxidation in which less CO₂ was produced with ¹⁶O/D₂¹⁶O than with ¹⁶O/H₂¹⁶O.  These differences are likely due to primary kinetic isotope effects due to the differences in O-H and O-D bond energies.

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