Gas-phase catalytic oxidation of CO by Au(2-).

Chemical reactivity of transition metal clusters toward various molecules in gas phase has been a subject of numerous investigations,1,2 motivated by search of analogues to adsorption mechanisms and reactivity at bulk metal surfaces. However, the full catalytic cycle, involving detection of a product molecule XY, as a result of reaction between reactants X and Y in the presence of the metal cluster M (possibly through an intermediate compound MXY), has been demonstrated experimentally only in a handful of studies.2 While it is well-known that gold is chemically inert as bulk material, nanometer-sized dispersed gold particles often show remarkable catalytic activity.3 Most recently it was found that mass-selected deposited gold clusters AuN (N ) 2 20) on MgO(001) exhibit extremely sensitive size-effects in the catalytic activity to oxidize CO: only clusters with eight or more atoms catalyzed the reaction, and the CO2 yield had a nontrivial dependence on the cluster size for N g 8.4 A concurrent theoretical study revealed that charging effects (by electrons trapped in surface color centers of MgO) are important in activating the gold cluster and the adsorbed O2. It is then of interest to study also the reactivity and catalytic properties of small negatively charged gold clusters in the gas phase. Indeed, there are several experimental gas-phase studies5 on the reactivity of small anionic gold clusters with O2 and CO. We present here a detailed investigation, based on the density functional theory, on the interactions of oxygen and carbon monoxide with the gold dimer anion, which is the smallest gold cluster found to react with oxygen in the gas phase.5 We find that oxygen should bind molecularly to Au2. Furthermore, we studied a full catalytic cycle, producing two carbon dioxide molecules, and identified a key intermediate state, di-goldcarbonate, which should be detectable in the experiments.6 In this study the Kohn-Sham (KS) equations were solved using the Born-Oppenheimer local-spin-density molecular dynamics (BO-LSD-MD) method,7 including generalized gradient corrections (GGA),8 with nonlocal norm-conserving pseudopotentials9 for the 5d106s1, 2s22p2, and 2s22p4 valence electrons of Au, C, and O atoms, respectively. Bonding in gold clusters requires a relativistic treatment;10,11 a scalar-relativistic12 pseudopotential was used here. From comparison of several calculated properties of basic dimers Au2, Au2, AuO, AuO-, and molecules O2, CO, as well as CO2, to available experimental values in Table 1, we judge that calculations at the GGA level give a rather satisfactory description of the energetics and structure of these systems. The same methodology was used recently by us to study oxidation of CO at a single Pd atom adsorbed on a surface color center on a MgO(001) film,13 and the calculated reaction barriers agree favorably with the ones deduced from the experiment.13 We found two bonding configurations for Au2CO, shown in Figure 1. In the ground-state (GS) “end-bonded” configuration (Figure 1a) both the Au-Au and C-O bonds are significantly stretched. The calculated adsorption energy of CO is 0.96 eV. In the “side-bonded” configuration (Figure 1c), which is 0.41 eV less stable than the GS, the CO induces a break-up of the AuAu bond. This illustrates the ability of CO to bind to different sites of the metal cluster, reminiscent of various adsorption sites on transition metal surfaces (e.g., on-top, bridge, etc.). The bonding of CO to transition metal surfaces is generally characterized by charge transfer from the metal (donation) to the antibonding CO(π*) orbital, and back-donation to the metal mainly from the bonding CO(σ) orbital.19 Similar concepts apply * Corresponding author. E-mail: [email protected]. (1) Knickelbein, M. B. Annu. ReV. Phys. 1998, 50, 79. (2) Ervin, K. M. Int. ReV. Phys. Chem. 2001, 20, 127. (3) (a) Haruta, M. Catal. Today 1997, 36, 153. (b) Bond, G. C.; Thompson, D. T. Catal. ReV.-Sci. Eng. 1999, 41, 319. (4) Sanchez, A.; Abbet, S.; Heiz, U.; Schneider, W.-D.; Häkkinen, H.; Barnett, R. N.; Landman, U. J. Phys. Chem. A 1999, 103, 9573. (5) (a) Cox, D. M.; Brickman, R.; Creegan, K.; Kaldor, A. Z. Phys. D 1991, 19, 353. (b) Lee, T. H.; Ervin, K. M. J. Phys. Chem. 1994, 98, 10023. (c) Salisbury, B. E.; Wallace, W. T.; Whetten, R. L. Chem. Phys. 2000, 262, 131; (d) Bernhardt, Th.; Heiz, U. Private communication. (6) While at saturation two or more O2 and CO molecules may adsorb on Au2, we limit ourselves here to the low-coverage regime. (7) Barnett, R. N.; Landman, U. Phys. ReV. B 1993, 48, 2081. This method does not use periodic boundary conditions for the ionic system. (8) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. ReV. Lett. 1996, 77, 3865. (9) Troullier, N.; Martins, J. L. Phys. ReV. B 1991, 43, 1993. The core radii (in a0) are: Au: s(2.50), p(3.00), d(2.00); C: s(1.50), p(1.54); O: s(1.45), p(1.45), with Au(s), C(p), and O(p) as local components. KS orbitals are expanded in a plane-wave basis with 62 Ry energy cutoff. (10) Häkkinen, H.; Landman, U. Phys. ReV. B 2000, 62, R2287. (11) Grönbeck, H.; Andreoni, W. Chem. Phys. 2000, 262, 1. (12) (a) Kleinman, L. Phys. ReV. B 1980 21, 2630. (b) Bachelet, G. B.; Schluter, M. Phys. ReV. B 1982, 25, 2103. (13) Abbet, S.; Heiz, U.; Häkkinen, H.; Landman, U. Phys. ReV. Lett. 2001, 86, 5950. (14) Ho, J.; Ervin, K. M.; Lineberger, W. C. J. Chem. Phys. 1990, 93, 6987. (15) Cheeseman, M. A.; Eyler, J. R. J. Phys. Chem. 1992, 96, 1082. Table 1. GGA vs Experimental Data of Bonding