Activation and Reactions of CO 2 on a K-Promoted Au(111) Surface Arnold Pe ´ter Farkas and Frigyes Solymosi* Reaction Kinetics Research Group, Chemical Research Centre of the Hungarian Academy of Sciences, Institute of Solid State and Radiochemistry, UniVersity of Szeged, P.O. Box 168, H-6701 Szeged, Hungary ReceiVed: July 1, 2009; ReVised Manuscript ReceiVed: October 8, 2009 The adsorption of potassium and CO 2 and their interaction on a Au(111) surface have been studied using high-resolution electron energy loss spectroscopy, thermal desorption spectroscopy, work function measure- ments, and Auger electron spectroscopy. Potassium adsorbs in cationic form on Au(111) at low coverage, depolarizing to a neutral metallic state at high coverage. CO 2 adsorbs only weakly on clean Au(111), desorbing with a T p ∼ 124 K. Its binding energy is greatly enhanced by the presence of potassium and led to the formation of a CO 2 - anion radical characterized by losses at 957, 1360, and 1620 cm -1 . An enhancement of a weakly held, unperturbed CO 2 also occurred on a K-dosed surface, which was attributed to the formation of an unstable cluster compound (CO 2 ) n · CO 2 - . The surface concentration and the reactivity of the CO 2 - radical sensitively depend on the K coverage and on the state of potassium. At low K coverages, CO 2 - dissociated into CO (a) and O - (a) . At higher coverages, it disproportionated into stable CO 3 - (a) and CO (a) - ; they decomposed and released only above 500 K. CO formed on a K-dosed surface gave vibration losses at 1940-1700 cm -1 and was also stabilized by potassium. 1. Introduction Following the pioneering works of Haruta et. al, 1,2 the application of gold as a catalyst receives gradually increasing attention. 3-5 A remarkable feature of the gold catalyst is its great sensitivity to the size of the crystallites. 2-8 Au represents an almost unique catalytic behavior in the selective oxidation of CO besides H 2 . Although the dissociation of NO on supported Au is very limited, its catalytic efficiency in the NO + CO reaction is commensurable with that of supported Rh. 9,10 The reaction also involves the formation of NCO species. 10,11 To explain the catalytic performance of supported Au in various reactions, more extensive studies are required on the interaction of reactants and products with well-defined Au surfaces under UHV conditions. In the present paper, an account is given on the adsorption and reactivity of CO 2 on pure and K-promoted Au(111) surfaces. This work is a part of our research program concerning the activation and catalytic transformation of CO 2 into more valuable compounds. 12,13 Our laboratory belongs to the pioneer ones, which considered CO 2 not only a harmful chemical for the climate but also a useful C-containing raw material for the future. 14,15 Nowadays, a very extensive research is carried out worldwide in a possible utilization of CO 2 . 16 The activation of very stable CO 2 is a great challenge, as CO 2 adsorbs weakly and nondissociatively even on Pt metals. 17,18 Potassium, however, is an efficient promotor, and due to its electron-donation character, it can transform the neutrally adsorbed CO 2 into a more reactive negatively charged CO 2 . This was first illustrated in the case of K-dosed Pd(100) surfaces, 19,20 followed by extensive studies on Rh(111), 21-23 Pt(111), 24,25 Pd(111), 26,27 Ru(0001), 28-30 Ag(111), 31 Co(1010), 32 Mo 2 C/ Mo(100), 33 Cu(110), Cu(115), 34 and Cs-dosed Cu(110) sur- faces. 35 2. Experimental Section The experiments were performed in a two-level UHV chamber with a routine base pressure of 5 × 10 -10 mbar produced by a turbomolecular pump. The chamber was equipped with facilities for Auger electron spectroscopy (AES), high- resolution electron energy loss spectroscopy (HREELS), and temperature-programmed desorption (TPD). The HREEL spec- trometer (LK, ELS 3000) is situated in the lower level of the chamber and has a resolution of 20-40 cm -1 (fwhm). The count rates in the elastic peak were typically in the range of 1 × 10 4 -1 × 10 5 counts/s (cps). All spectra reported were recorded with a primary beam energy of 6.5 eV and at an incident angle of 60° with respect to the surface normal in the specular direction. When spectra in off-specular direction were taken, the electron beam incidence angle was decreased by 15°, keeping all other parameters fixed. Work function changes, based on secondary electron energy threshold, were measured with the same electron gun and analyzer used in AES. The Au(111) crystal was secured to a Mo plate, which was connected via a copper block directly to a liquid nitrogen reservoir. The sample was heated with a tungsten spiral situated at the back of the sample from 100 to 900-1000 K. Its temperature was monitored by a chromel-alumel thermocouple pressed firmly into a hole drilled into the side of a crystal and was controlled with a feedback circuit to provide a linear heating rate of ca. 2 K/s. The dosing temperature was ∼95-100 K, unless otherwise noted. The heating rate for TPD measurements was 4.0 K s 1- from 95 to 100 K to the selected temperature. Gases were dosed through a capillary with a diameter of 0.1 mm, which terminated 2 cm from the sample. The pressure around the sample was about 10 -7 Pa during dosing. To establish the contribution of the desorption of CO 2 from the sample holder, we followed the previously applied method. 19-23 After exposition, the crystal was turned away from the direct line to the mass spectrometer and TPD measurement was performed. In this way, we measured only a negligible amount of CO and CO 2 compared to the gases desorbed from the Au(111). * To whom correspondence should be addressed. Fax: +36-62-420-678. E-mail: fsolym@chem.u-szeged.hu. J. Phys. Chem. C 2009, 113, 19930–19936 19930 10.1021/jp9061779 CCC: $40.75  2009 American Chemical Society Published on Web 10/21/2009