Size-Selective Reactivity of Subnanometer Ag4 and Ag16 Clusters on

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Size-Selective Reactivity of Subnanometer Ag and Ag Clusters on TiO Surface Po-Tuan Chen, Eric C. Tyo, Michitoshi Hayashi, Michael J Pellin, Olga V. Safonova, Maarten Nachtegaal, Jeroen Anton van Bokhoven, Stefan Vajda, and Peter Zapol J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b11375 • Publication Date (Web): 08 Mar 2017 Downloaded from http://pubs.acs.org on March 16, 2017

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The Journal of Physical Chemistry

Size-Selective Reactivity of Subnanometer Ag4 and Ag16 Clusters on TiO2 Surface Po-Tuan Chen,‡,§, Eric C. Tyo,§, Michitoshi Hayashi, Michael J. Pellin, Olga Safonovaǁ, Maarten Nachtegaalǁ, Jeroen A. van Bokhoven ǁ,§, Stefan Vajda*, and Peter Zapol,*

Materials Science Division, Argonne National Laboratory, Argonne, IL 60439, USA

Center for Condensed Matter Sciences, National Taiwan University, Taipei 10617, Taiwan

ǁ

Paul Scherrer Institut, Villigen PSI, 5232, Switzerland

§

Institute for Chemical and Bioengineering, ETH Zurich, Zurich, 8093, Switzerland

§

PT and EC contributed equally to this paper.

Corresponding Authors E-mail: [email protected] E-mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT Size-selected Ag4 and Ag16 clusters on a titania surface have been studied for their potential in CO oxidation using theoretical calculations and X-ray absorption near edge spectroscopy. The first peak at the measured Ag K-edge of Ag16@TiO2 is more prominent in air than in carbon monoxide environment, but no variation was found between the spectra of Ag4@TiO2 in air and in carbon monoxide environments. Density functional theory calculations show a preference for molecular oxygen adsorption for Ag4@TiO2 and that for a dissociative one on Ag16@TiO2, while carbon monoxide reactions with adsorbed oxygen reduced the Ag16@TiO2 cluster. The dissociated oxygen atoms increased the oxidation state of Ag16 cluster, and resulted in the prominent first peak in Ag K-edge spectrum in quasi-particle theory calculations, with the subsequent carbon monoxide oxidation reversing the character of Ag K-edge spectrum associated with the reduction of the cluster. The results provide insight into the size selectivity of supported subnanometer silver clusters in their interactions with oxygen and carbon monoxide, with implications on the cluster catalytic properties in oxidative reactions.


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I. Introduction Size-selected subnanometer clusters of precious metals on oxide supports show unique catalytic properties for many industrially important reactions1-5. Very often, the chemical activity of the clusters is very different than that of the corresponding nanoparticles and bulk materials. Control of cluster size down to single atom precision provides unique means to modify chemical properties since the reactivity of metal clusters varies widely when the cluster size changes even by a single atom6. In particular, subnanometer gold and silver clusters have caused interest for selective reactions, such as carbon monoxide oxidation7, acetylene oxidation8, and NOx reduction9-10. Activity may strongly depend on deposited cluster size. For example, CO oxidation on Aun@TiO2(110) has been particularly well studied, and considerable activities are found typically for Aun in the size range of few atoms7. Subnanometer cluster attachment to a support provides stability and accessibility. Additionally, strong cluster-support interaction often leads to the formation of new active sites at the interfaces between metal particles and the surface or in the overlayer of the support11. Titanium dioxide is one of the most frequently studied support materials12-16. Characterization of supported cluster properties such as geometry, reactivity and oxidation state is needed to reveal their potential for catalysis applications. Considerable effort has gone into probing the effects of particle size on its physical and chemical properties, in particular since the discovery that supported nanoparticles with different sizes show various chemical activities. Experimental microscopy imaged and characterized fabrication of Ag nanoparticles on TiO2 12-13. X-ray absorption near-edge spectroscopy (XANES) and extended Xray absorption fine structure spectroscopy (EXAFS) have been used for the characterization of silver species on Ag@Al2O3 catalysts with different silver loadings9-10. The interaction of silver


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species with Al2O3 is a key factor responsible for the activity of these catalysts. More specifically, aluminium oxide, as an electron acceptor, makes Ag adsorbate acquire partial positive charge which facilitates its catalytic activity. As a result of the changes in the electronic structure, the features of oxidation state of Ag nano clusters (Agnδ+) in Ag@Al2O3 catalysts and their catalytic activity for the selective catalytic reduction have been determined by XANES. Oxidized Ag clusters are believed to be active species for reduction of NOx by hydrocarbons. Such measurements allow detailed probing of physical and chemical properties of nanometer size particles; however, the catalytic properties of supported subnanometer Ag clusters as a function of number of atoms have not been sufficiently well understood so far. Because of the small cluster size, it is difficult to characterize these clusters under reaction conditions. Although XANES has been used successfully to study changes in the oxidation state of the catalyst during redox reactions14, interpretation based on reference materials lacks specific details on the clusters. On the other hand, density functional theory (DFT) studies of Agn@TiO2 materials provide the relation of geometries and electronic structures of Ag clusters15-17. Analysis of in-situ XANES spectra of supported Ag clusters on the basis of theoretical calculations can provide more detailed interpretation of size-specific properties. Here, we used DFT calculations combined with multiple-scattering simulations (FEFF code18) to determine the electronic structure of Ag clusters on titania, measured by XANES the during exposure of the samples to air and CO. Ag4 is the smallest cluster that can have a three-dimensional structure, which is a tetrahedron without a support, while Ag16 has a much larger size. We performed FEFF calculations using DFT-optimized structures to theoretically examine Ag K-edge XANES spectra related to molecular and dissociative O2 adsorbed on Ag4 and Ag16 clusters in gas phase and on TiO2 surface. In particular, we aim to identify how the features in the


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Ag K-edge spectra are linked to the changes in the electronic structure and geometry of the Ag clusters undergoing oxidation and reduction reactions. Therefore, we compare the results of our XANES measurements of Ag4 and Ag16 clusters on TiO2 surface in air and CO environments at room temperature to the calculation results. Understanding the differences in the electronic structure of Ag4 and Ag16 exposed to these environments will help us elucidate size dependence of reactivity of supported subnanometer silver clusters. This gives us opportunity to explore the size selectivity of TiO2-supported subnanometer silver clusters in their interactions with oxygen and carbon monoxide to evaluate potential of tailoring cluster catalytic properties for oxidative reactions.

II. Methods 1. Computational Methods Structural optimizations and density of states (DOS) calculations were performed by DFT within the PBE generalized gradient approximation19 using Vienna ab initio simulation package (VASP)20-21. The net charges were obtained from a Bader analysis of the VASP electron densities22. The (110) rutile surface was represented by a periodic three tri-layer (nine atomic layers) slab structure, where two upper tri-layers were allowed to relax while the bottom tri-layer was frozen at the bulk geometry. Ag4 or Ag16 clusters were adsorbed at the relaxed top surface. While using a thicker slab is a safer choice, the use of three tri-layer slab is justified by absence of surface defects that participate in charge transfer with the cluster in our calculations and very small changes in the average Bader charge on Ti atoms for all studied adsorption structures. The periodic cell length in z-direction (surface normal) was set at 30 Å. Therefore, the vacuum layer between the adsorbed cluster and a periodic image of the slab is at least 13 Å. The surface unit cell was 5x2


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with the lengths of 14.794 Å and 12.993 Å along (-100) and (001) directions, respectively. Calculations were spin-polarized with an energy cut-off of 400 eV and a 2×2×1 Monkhorst–Pack grid23. The convergence criterion for the electronic self-consistent loop was set to 10−5 eV. The structures were optimized until forces on each atom decreased below 0.01 eV/Å. The reaction barrier for CO oxidation is determined using the climbing-image nudged elastic band (cNEB) method. This method is designed to rigorously locate the highest saddle point on the minimumenergy path for a given elementary reaction step24. XANES and extended X-ray adsorption fine structure (EXAFS) calculations of Ag are performed by FEFF8 code18. We used the Dirac–Hara model based on the plasmon-pole model for an electron gas for the self-energy25-26 for XANES calculations, and Hedin–Lundquist selfenergy18 for EXAFS calculations. The optimized geometries of the Ag4 and Ag16 on TiO2 were adopted from the DFT results. We used a single representative optimized geometry for each of the supported clusters, which might not be a global energy minimum. The XANES spectra were averaged over individual spectra of all Ag atoms. We also considered three silver-containing crystals (Ag bulk, Ag2O, and AgNO3) as references to discuss Ag in various oxidation states. Ag bulk is in space group Fm-3m (#225) with cubic close-packed structure and 4.09 Å lattice constant. Ag2O has Pn-3m space symmetry and lattice constant of 4.72 Å. The crystal structure of AgNO3 is orthorhombic (space group Pbca) with lattice constants a = 7.00 Å, b = 7.33 Å, and c = 10.12 Å.27 These XANES calculations are performed for a Ag atom in the middle of their 2x2x2 cluster. The self-consistent potential and full multiple scattering was calculated at 6.0 Å radius. We also tested the calculations of using 8.0 Å radius, the peaks’ positions and the relative difference of intensity of the first and second peaks in XANES remain the same. To compare the experimental and calculated spectra, a rigid shift of 40 eV, to lower energy was applied to each calculated


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spectrum; the calculated spectra of Ag4 or Ag16 on TiO2 with different adsorbed species were roughly aligned to the first peak of the signal in the corresponding experimental data.

2. Experimental Methods The support titania film was prepared by atomic layer deposition (ALD) atop of a naturally oxidized n-type (P-doped) Si wafer, yielding a 3 nm thin TiO2 film. This process consisted of applying alternating exposures to titanium tetrachloride (TiCl4) and H2O, using 0.5 s reactant exposures and purge times of 60 s following the TiCl4 and 120 s following the H2O at a deposition temperature of 50 °C. A molecular beam of silver clusters was produced in a vacuum apparatus by magnetron sputtering of a silver target using a mixture of argon and helium as the sputtering gas and helium as carrier gas. 28 Next, after their exit from the source, the positively charged particles contained in the propagating molecular beam were focused using a conical octupole (Extrel), passed through a linear octupole ion guide (Extrel) and mass selected by a quadrupole mass filter (Extrel) before the cluster ions of single size (i.e. either Ag4+ or Ag16+) were soft landed on the support. The charge of gas–phase cluster cations was neutralized by the current from a picoammeter (Keithley) that was used to bias the support during landing to control the kinetic energy of the clusters and to monitor in real time the flux of clusters reaching the support (that is, the current of charged clusters measured at the surface).The amount of deposited metal was monitored on line by integrating the deposition flux over time and the number of atoms in the cluster. A surface coverage corresponding to 15% atomic monolayer Ag was applied to an 8 mm diameter area of the 20 x 22 mm support, to avoid aggregation of clusters after landing.


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For collecting the X-ray absorption data, we used the SuperXAS (XAS = X-ray absorption spectroscopy) beam line of the Swiss Light Source, which is a dedicated beam line for X-ray spectroscopic studies, in particular for in situ characterization of catalysts.29 XAS provides element-specific information on the electronic and geometric structure of a selected component of a catalyst. We have used an in situ reactor of own design which allows the characterization of the catalyst exposed to reactive gas mixtures and under atmospheric pressure.30 The cluster samples were characterized at room temperature exposed to ambient air and to carbon monoxide, using X-rays aligned close to the grazing incidence angle to the surface of the sample, at α=0.2º. The X-ray absorption data were collected in fluorescence mode.

III. Results and discussion 1. Gas phase Ag4 and Ag16 clusters 1.1. Geometries of O2 adsorption on Ag clusters The O-Ag4 geometries were optimized using initial DFT geometries of molecular and dissociated oxygen adsorbed on Ag4 in the gas phase are adopted from Ref. [31] . Figures 1(a) and 1(b) show the optimized structures and relative energy of O2 and dissociated 2O on the Ag4, respectively. An equilibrium bond length of the free oxygen molecule is 1.24 Å. The bond length of O2 on the Ag4 cluster is 1.32 Å, since the interaction of O2 with the Ag cluster weakens the O=O double bond. The average of the bond lengths between O and a nearby Ag atom in Ag4_O2 is 2.30 Å, which is larger than the same average in Ag4_2O, 2.09 Å. The relative energy indicates that O2 adsorbing on Ag4 cluster is more stable than dissociated 2O adsorbing on the Ag4 cluster by 0.16 eV. This indicates that molecular adsorption of an oxygen molecule on Ag4 cluster is more


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energetically favorable than dissociated O2 adsorption. The binding energy of an adsorbate with a silver cluster is defined as Eb = Etotal – Ecluster – Eadsorbate. We calculated the binding energy of O2 on Ag4 to be -1.02 eV and the binding energy of dissociated 2O on Ag4 to be -0.86 eV. For comparison, previous calculations31 indicated the difference in binding energies of 0.31 eV. To optimize geometries of molecular and dissociated O2 adsorbed on Ag16, an initial geometry of Ag16 cluster is adopted from one of the structures in the Ref. [32]. There, the authors used a genetic algorithm followed by local optimization with DFT to find the most stable structures of Agn clusters. We also performed calculations based on another geometry of Ag16 cluster obtained in Ref. [33]. Both initial geometries resulted in consistent calculated XANES. Therefore, we only present results for one kind of initial geometry of Ag16 cluster. We calculated binding energies for oxygen molecule or two dissociated oxygen atoms on different sites on Ag16. Our initial calculations suggested that the molecule adsorbs preferentially in a parallel configuration on twofold bridge site on Ag16 than on a top site, a three-fold hollow site, or any site with end-on adsorption. The binding energy of O2 in parallel configuration on two-fold site is -1.18 eV. All other adsorption sites have binding energies below -1.00 eV. On the other hand, an oxygen atom was found to adsorb on the three-fold hollow site on Ag16 more strongly than on any other site. The adsorption energy of two oxygen atoms which are both on the three-fold hollow sites on Ag16 cluster is -2.19 eV. This site preference is consistent with the literature, which reports the preference of O2 on the bridge site on Ag(111)34 and O atom on the hollow site on Ag(111)35. Therefore, we have performed all subsequent calculations for these relatively stable cases only and did not attempt a global optimization or address fluxionality of the cluster structures. [Zhai, H.; Alexandrova, A. N., ACS Catalysis 2017, ASAP article DOI: 10.1021/acscatal.6b03243]. Figure 1(c) and 1(d) show the optimized structures and their relative energies. The bond length of O2 on


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the Ag16 cluster is 1.34 Å. The average of the bond lengths between O and nearby Ag atoms in Ag16_O2 is 2.27 Å, which is somewhat larger than the same average in Ag16_2O, 2.16 Å. The relative energy of Ag16_2O is -1.01 eV lower than that of Ag16_O2. Adsorption energies of different O2 configurations on Ag16 indicate that the molecule tends to dissociate upon adsorption. Dissociative adsorption is more favored on the larger cluster since in the smaller cluster upon oxygen dissociation more charge per atom is transferred to oxygen, thus losing more energy from weakening intra-cluster bonds, and, at the same time, energy gain from image charge interactions is smaller compared to larger clusters. Our computational results for gas phase clusters indicate that O2 is likely to dissociate to two oxygen atoms on the Ag16 cluster, while it prefers molecular adsorption on the Ag4 cluster.

1.2. Oxidation states and charges We determined the atomic charges using Bader analysis22 of converged electron densities to establish silver and oxygen oxidation states and to help interpret the Ag K-edge XANES. The calculated charges on Ag and adsorbed O atoms are shown in Table 1. As a result of adsorption, the Ag clusters become positively charged. Molecular adsorption on Ag4 results in the total charge on O2 of -0.50e and on Ag4 of +0.50e. Upon dissociative adsorption on Ag4, the total charge of two oxygens is -1.72e and of Ag4 is +1.72e. For the larger cluster size, for molecular adsorption, the charge of O2 is -0.64e and that of Ag16 is +0.64e. After dissociative adsorption, the total charge of two oxygens is -1.83e and that of Ag16 is +1.83e. As shown in Table 1, the dissociation changes the average charge of Ag from +0.04e to +0.11e and the average charge of oxygen from -0.32e to -0.92e. Overall, dissociative adsorption of oxygen enhances positive charge of Ag clusters. The


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charge distribution for CO adsorption on all Ag clusters given in Table 1 indicates that CO adsorption results in barely any electron transfer from or to the silver cluster.

1.3. Theoretical Ag K-edge XANES and comparison to reference spectra We used the real space multiple scattering method (FEFF code) to determine XANES spectra of Ag K-edge from the optimized geometries. Figure 2 shows calculated XANES for the bare Ag4 and Ag16 clusters, as well as those with adsorbed oxygen or carbon monoxide. The spectrum of Ag4 cluster reveals two peaks and a pre-peak shoulder at the first peak in the region of 40 eV near edge. For the Ag4 cluster, the difference in energy of two calculated peaks is ca. 30 eV. The intensities of the two peaks are approximatively the same. The molecular oxygen adsorption only slightly changes the XANES. However, the dissociative oxygen adsorption brings out a prominent first peak and no second peak. It is known8, 10 that oxidation of Ag nanoparticles yields a prominent first peak in its K-edge spectrum. There are two peaks in the region of 40 eV near edge for the Ag16 cluster in Figure 2. Their energy difference is ca. 25 eV. The peaks in the spectrum of the Ag16 cluster are notably sharper than those of the Ag4 cluster. The intensity of the first peak is lower than the second peak. The XANES of Ag16, Ag16_O2, Ag16_2O, and Ag16_CO look similar, since the adsorption of a single molecule does not significantly alter the average electronic state of silver atoms in the Ag16 cluster. Figure 3 (a) shows calculated K-edge XANES of bulk materials of AgNO3, Ag2O, and Ag to examine the relation between spectra and oxidation state. The spectrum of Ag bulk shows two peaks in the near edge with the energy difference of ca. 23 eV, and the intensity of the first peak is lower than the second peak. These features are similar to those observed for the Ag16 cluster. Ag2O and AgNO3 represent progressively larger positive charge on silver: +0.53e and +0.74e in


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Ag2O and AgNO3, respectively (see Table 1). The spectrum of Ag2O reveals a double peak at the near edge with energy spread ca. 12 eV, and there is a pre-peak shoulder for the first peak. The spectrum of AgNO3 displays a prominent first peak, with a 40 eV difference between the two peak energies. These features of AgNO3 and Ag2O are qualitatively comparable with the experimental spectra shown in Figure 3(b) validating our calculation methods for studies of systems with varying geometry and Ag charge state. The dissociative oxygen adsorption brings out a prominent first peak for Ag4_2O, consistent with a significant change of oxidation state of Ag, like in AgNO3. The average charge of Ag atoms in Ag4_2O is +0.43e much higher than the ones found in Ag4_O2 or Ag4_CO (see Table 1). However, the average charge of the Ag atoms in Ag16_2O is +0.11e, thus the XANES calculated for Ag16_2O remains approximately the same as for Ag16.

2. Calculated properties of supported Ag4 and Ag16 clusters 2.1. Geometries and adsorption energies Next, we performed DFT optimization for the TiO2-supported Ag clusters with adsorbed molecular or dissociated oxygen. The initial inputs of the geometries of adsorbed oxygen and the Ag clusters are adapted from those presented in the last section. We arranged the geometries onto the rutile TiO2 (110) surface and optimized them. The optimized geometry of the Ag4 cluster in Ag4@TiO2_O2 is of pyramid type shown in Figure 4(a). Here, two Ag atoms of the cluster bind to bridging O atoms on TiO2 (110) with bond lengths of 2.21 Å and 2.23 Å respectively. The average distance of the atoms of the oxygen molecule to a nearby Ag atom is about 2.36 Å. Similarly, three-dimensional geometries of Ag4 clusters were obtained on anatase (101) surface.36 For the Ag4@TiO2_2O case, shown in Figure 4(b), Ag4 is moderately bent rather than flat or pyramidal.


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The average distance of adsorbed oxygen to a nearby Ag atom is 2.06 Å. The bond lengths between the Ag atoms and surface O atoms are 2.14, 2.29, and 2.12 Å, respectively. The relative energy indicates that molecular adsorption on the Ag4 cluster on TiO2 is more stable by 0.67 eV than dissociative adsorption. This is higher than the unsupported cluster binding energy difference of 0.16 eV for these two configurations. The optimized structure of Ag16@TiO2_O2 is shown in Figure 4(c). Two silver atoms interact with bridging oxygen atoms of TiO2 (110) forming bonds with lengths of 2.26 Å and 2.19 Å. The average distance of an atom in oxygen molecule to a nearby Ag atom is 2.33 Å. Figure 4(d) shows the optimized structure of Ag16@TiO2_2O. The average distance of oxygen to a nearby silver is 2.13 Å. The bond lengths of silver atoms to the bridging oxygen of the surface are 2.34 and 2.24 Å. Relative energies of different O2 configurations on Ag16@TiO2 indicate that the molecule tends to dissociate upon adsorption. The energy of Ag16@TiO2_2O is lower by 1.14 eV than that of Ag16@TiO2_O2. For comparison, O2 also prefers a dissociative adsorption on a gas-phase Ag16 cluster with a relative energy of -1.01 eV. Therefore, oxygen molecule is likely to dissociate to two atoms on a supported Ag16 cluster, while it prefers molecular adsorption on Ag4 cluster or on Ag4@TiO2. Binding energy of O2 in various configurations with Ag@TiO2 was defined as Eb = Etotal – EAg@TiO2 – EO2. We obtained a binding energy of of -3.67 eV for the molecule on Ag4@TiO2 and the binding energy of -3.00 eV for two dissociated atoms on Ag4@TiO2. The binding energy of the molecule on Ag16@TiO2 is -3.84 eV and the binding energy of two dissociated atoms on Ag16@TiO2 is -4.98 eV. These binding energies are larger than the energies of corresponding O2 configurations on unsupported silver clusters. Because of electron transfer from silver clusters to


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the titania support, the more positively charged silver clusters facilitate stronger oxygen adsorption.

2.2. Charge states Interactions of Ag clusters with TiO2 surface and adsorbed molecules result in changes of their electronic structures and charge states. The calculated Bader charges of the supported silver clusters and adsorbed oxygen atoms are shown in Table 2. For the bare silver clusters, the TiO2 surface acts as an electron acceptor which accepts ca -1.30e charge from Ag4 and ca -2.05e charge from Ag16. Thus, the silver clusters are positively charged, and their charge state is further influenced by the adsorption of oxygen molecule or atoms, or carbon monoxide. Molecular adsorption of O2 on Ag4@TiO2 results in the charge of O2 being -0.42e and of Ag4 being +1.38e. Upon dissociative 2O adsorption on Ag4@TiO2, the total charge of 2O is -1.63e and the total charge of Ag4 is +2.48e. The dissociation of O2 changes the average charge of Ag atoms from +0.35e to +0.62e, and the average charge of oxygen atoms changing from -0.21e to -0.81e. For the Ag16 cluster, O2 molecular adsorption on Ag16@TiO2, the charge of O2 is -0.50e, the charge on Ag16 is +2.50e, while in the case of dissociated 2O adsorbed on Ag16@TiO2, the charge of 2O is -1.82e, the charge on Ag16 is +3.71e. As shown in Table 2, the dissociation of O2 changes the average charge of Ag atoms from +0.16e to +0.23e and the average charge of O atoms changing from -0.25e to -0.91e. The variation of average charge of Ag atom in Ag16 cluster is more moderate than in Ag4 cluster for the same amount of adsorbed oxygen. Average charges on Ti atoms of the slab are +2.62e to +2.63e in all calculations and the variation of Ti charges is small. The cluster has an overall positive charge, while the TiO2 slab possesses negative charge. Average charge on Ag atoms in Ag16@TiO2 strongly increases with the number


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of dissociated oxygen molecules, up to +0.64e per Ag atom for Ag16@TiO2_10O. At the same time, with increase in the number of adsorbed oxygens, the total positive charge on the Ag cluster with adsorbed oxygen slightly decreases, from +2.05e for bare cluster to +1.71e for Ag16+10O, and the negative charge on the TiO2 support decreases accordingly. Note that while we have not considered reducing species on the oxide support, previous studies for Au particle on rutile TiO2 found the charge state of the Au particle to be negative in a reducing chemical environment while the cluster obtained a net positive charge with coadsorbed oxidizing species to the oxide surface.37

2.3. Calculated XANES of Ag4@TiO2 and Ag16@TiO2 Figure 5 shows calculated XANES at the Ag K-edge for O2 and for dissociative 2O adsorbed on Ag4@TiO2 and Ag16@TiO2, as well as for CO adsorbed on the clusters. Ag4@TiO2 reveals two peaks at the region of 40 eV near edge. The difference in the positions of two calculated peaks is ca. 28 eV, smaller than that of Ag4 in the gas phase, which is ca. 30 eV. The intensity of the first peak is higher than the second peak indicating that the Ag cluster is an electronic donor to TiO2 surface. The spectrum of Ag4@TiO2_2O displays only one peak which is attributed to the dissociated 2O strongly increasing the charge state of Ag. The energy difference in the positions of the two peaks for Ag16@TiO2 is 24 eV. The spectra of Ag16@TiO2, Ag16@TiO2_O2, and Ag16@TiO2_2O display similar features, because a single adsorbed oxygen molecule only moderately increases Ag positive charge. We also observe a moderate increase of the average charge on the Ag atoms for Ag16@TiO2_2O shown in Table 2. To the contrary, the adsorption of carbon monoxide molecule does not change the XANES of Ag4@TiO2 and Ag16@TiO2 much, since interaction of the molecule with the cluster does not result in considerable charge transfer.


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2.4. Characterization of XANES for increasing oxygen coverage of Ag16@TiO2 Furthermore, we show that the theoretically calculated XANES for one to five dissociated O2 molecules adsorbing on Ag16@TiO2 are correlated with progressively increasing oxidation state. According to the above discussion, we have found a preference for O2 to dissociate on Ag16@TiO2. The binding energy of O2 on Ag16@TiO2 is -3.84 eV, and the binding energy of dissociated 2O on Ag16@TiO2 is -4.98 eV. We then added up to 10 O atoms on Ag16@TiO2 in the DFT calculations. After the geometry optimizations, the binding energies for 4O, 6O, 8O, and 10O adsorbed on Ag16@TiO2 are -10.86, -13.10, -16.70, and -21.07 eV, respectively. The binding energies per atom of 2O, 4O, 6O, 8O, and 10O adsorbed on Ag16@TiO2 are -2.49, -2.52, -2.18, -2.09, and -2.11 eV, indicating a trend of weaker binding with increasing number of oxygen atoms. (See Table 2). We note that since we did not search for global minima for the structure of the oxidized Ag clusters, the analysis of the binding energies is not detailed enough for a systematic study of the effect of cluster geometry and adsorption sites for oxygen atoms. We report the changes in the average charge per Ag atom in the cluster in Table 2. The average charge of Ag in Ag16@TiO2_2O is +0.23e. As the number of adsorbed O atoms increases, the average charge of Ag for Ag16@TiO2_4O, Ag16@TiO2_6O, Ag16@TiO2_8O, and Ag16@TiO2_10O are +0.34e, +0.45e, +0.53e, and +0.64e, respectively. Consequently, XANES displays a reduction of the second peak intensity, as shown in Figure 6. As a result, the XANES of Ag16@TiO2_10O is characterized by the prominent first peak, similar to Ag4@TiO2_2O. Figure 7 shows a variation for difference of intensity in the first and second peak. As the number of dissociated O2 adsorbed on Ag16@TiO2 increases, the intensity of the second peak decreases and the difference of intensity in the first and second peak becomes larger. The variation of the difference of intensity is approximately linear from 2O to 10O. However, with the increase of O


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atoms adsorbing on Ag16 cluster, the more stable sites (i.e. three-fold hollow sites) become saturated. The deviation from the linear trend may be attributed to the observation that part of O atoms adsorb on less stable sites. Adsorption of CO or O2 molecule does not significantly change average atom charge in a Ag16 cluster, thus the differences of intensity for the Ag16@TiO2_CO and the Ag16@TiO2_O2 are lower than those for dissociated oxygen adsorbed on Ag16@TiO2. On Ag4@TiO2, we obtain a similar result for the charge state. However, Ag4@TiO2_CO displays no second peak at the near edge. Thus, variation of charge state of Ag16 cluster changes its K-edge manifested in the difference of intensity in 1st and 2nd peak.

3. X-ray absorption spectroscopy and comparison to the calculated spectra 3.1. Experimental XANES spectra Experimental Ag K-edge spectra collected on the ALD titania-supported Ag4 and Ag16 clusters in air and in CO atmospheres are shown in Figure 8. Signal to noise ratio is good considering that we measure spectra of silver clusters at a submonolayer coverage on a flat support. From comparison to previously reported XANES of Ag nanoparticles in air8-9, we can determine that there are two bands within the 40 eV region of the edge. The first absorption peak in the experimental spectra of Ag4@TiO2_air and Ag4@TiO2_CO appeared at 25526 eV, while the second peak is at 25551 eV; the energy difference of these two peaks is 25 eV. The first absorption peak in the experimental spectra of Ag16@TiO2_air and Ag16@TiO2_CO appeared at 25524 eV, while the second peak at 25546 eV; the difference of these two peaks is 22 eV. In both cases, the theoretical calculations are in agreement with the experimental results. Specifically, Ag16@TiO2_air reveals the prominent first peak, indicating dissociative adsorption of oxygen in air. Ag4@TiO2_air presents no such a prominent peak consistent with the fact that Ag4@TiO2_O2


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is energetically more stable than Ag4@TiO2_2O. Therefore, both the experimental and theoretical results indicate that oxygen molecule would not dissociate on Ag4@TiO2 clusters. Figure 8 shows experimental spectrum of Ag4@TiO2_CO with a shape nearly identical with the experimental spectrum of Ag4@TiO2_air. Thus, we conclude that Ag4@TiO2 does not considerably change its charge state either in air or carbon monoxide environments.

3.2. Comparison of the Experimental and Theoretical EXAFS spectra Figure 9 displays measured and calculated extended X-ray adsorption fine structure (EXAFS) spectra of Ag4 and Ag16 clusters supported on TiO2 in CO gas environment. We have also performed calculations for CO oxidized on Ag16@TiO2, i.e. Ag16@TiO2_CO2, which gives spectra that appear to be similar to the results obtained for Ag16@TiO2_CO (not shown). In the wavenumber region of 0 – 6 Å-1 the results from calculation and experiment are comparable to each other, exhibiting similar patterns (see Figure 9). In particular, the calculated spectrum of Ag4@TiO2_CO reveals a broad band in the region of 1 – 3 Å-1 (Figure 9a) The feature is consistent with the experimental results obtained for the Ag4 cluster under CO (Figure 9b). There are two distinguishable peaks respectively at 2 Å-1 and 3 Å-1 in the calculated spectra which are corresponding to the peaks at 1 Å-1 and 3 Å-1 in the experiment. Moreover, the calculated spectra of both Ag4 and Ag16 reveal peaks at about 4.5 Å-1 and 5.5 Å1

. The two calculated peaks of Ag16 are shifted to a slightly lower wavenumber with respect to the

peaks of Ag4. The overall features are similar to those from the experiment. However, the calculated and experimental features diverge beyond 6 Å-1, in which the experimental data are more noisy. While the theoretical calculations are performed for a single subnanometer cluster on


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a perfect rutile surface, experimental measurements are performed on an ensemble of single size but not necessarily structurally identical clusters on ALD titania support, which in part can explain some of the discrepancies.

3.3. Origin of the prominent first peak Ag K-edge XANES originates from the excitation of core electrons to unoccupied electronic states. The spectral data contain a general feature: above the edges, a series of wiggles or oscillatory structures appear that are related directly to the transition from the initial states to final states of the material. Therefore, XANES of Ag K-edge is related to the projected DOS (pDOS) of unoccupied states. Thus, the pDOS calculation of unoccupied orbitals in Figure 10 is a useful tool to explain the experimental peaks. The Ag16 cluster on TiO2 is metallic. The pDOS beyond Fermi level of Ag is mainly attributed to 3d orbitals mixing with 4s orbitals. Because the orbitals of the Ag atoms hybridize with O atoms (including with the bridging oxygens of TiO2 surface), the pDOS of Ag and O correlate with each other. In Figure 10(a), pDOS of Ag atoms in Ag16@TiO2_10O display large population near Fermi level, which leads to the prominent first peak in Ag K-edge spectra. In the Ag16@TiO2_CO, C and O of CO are covalently bonded to each other and, as a result, CO interacts much weaker with Ag cluster. The pDOS of carbon and oxygen displays orbital coupling presenting in the energy region of 2.5 eV to 4.5 eV, shown in Figure 10(b). In addition, the pDOS of Ag atoms which bind to bridging oxygens of titania display a moderate intensity near the Fermi level, shown in Figure 10(b). The interaction with the surface oxygens results in the electron transfer from the silver cluster.

4. Discussion of reaction mechanism for CO oxidation


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In previous literature, two well-known Langmuir-Hinshelwood (LH) and Eley-Rideal (ER) mechanisms were studied for CO oxidation on Ag catalysts38-41. In particular, Vattuone et al. investigated the interaction of CO with molecularly and dissociatively adsorbed oxygen on Ag(001) using high resolution electron energy loss spectroscopy38. In their study, pre-adsorbed oxygen molecules or atoms on Ag(001) surface showed reactivity towards CO. The reaction followed the LH mechanism. Tang et al. proposed that Ag55 nanoclusters are promising to catalyze CO oxidation39. The barriers for CO reaction with Ag55O+ to form CO2 were 0.42 eV via the LH mechanism and 0.44 eV via the ER mechanism. With CO and O2 co-adsorbing on the catalyst, the LH mechanism was calculated to be likely dominant39-42. In this study, our subnanometer clusters are supported on the TiO2 surface. Supported subnanometer clusters are different in both structure and charge state from clusters studied in the gas phase and from the extended Ag surfaces. Therefore, the mechanism for CO oxidation is not necessarily the same. Our theoretical calculations and in-situ experimental observations of XANES indicate that the adsorption of an O2 molecule is preferable on Ag4@TiO2, while a dissociative adsorption of multiple oxygen atoms is favorable on Ag16@TiO2 when the clusters are exposed to oxygen. The decay of the prominent first peak in the experimental spectra upon the exposure of the oxidized Ag16 clusters to CO (Ag16@TiO2_CO) suggests that CO oxidation is feasible on Ag16@TiO2, yielding a reduced cluster. We note that reduction of oxidized gas-phase size-selected Ag anion clusters was also proposed based on the depletion of the oxygen-containing clusters upon the introduction of CO.43

To investigate how CO oxidation proceeds on Ag4@TiO2_O2 or

Ag16@TiO2_nO when they were exposed in CO gas, we propose the following mechanism for CO oxidation on Ag4@TiO2 and Ag16@TiO2.


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Under air, an oxygen molecule adsorbs on Ag4@TiO2: O2(gas) + *  O2*,

(1a)

where * indicates an available adsorption site on the cluster. Upon exposure to CO, we suggest these chemical reactions for the LH mechanism : CO(gas) + O2*  CO* + O2*,

(2a)

CO*+O2*  CO2(gas) + O*.

(3a)

Figure 11(a) shows the relative energies along the calculated reaction pathway. Comparison of calculated adsorption energy of O2 on Ag4@TiO2 of -3.67 eV to the calculated adsorption energy of CO in step (2a) of -0.63 eV indicates possible CO and O2 co-adsorption on Ag4@TiO2. Our calculation results reveal that CO oxidation in step (3a) is exothermic (-0.75 eV). However, the calculated reaction barrier of CO oxidation is 1.05 eV, which indicates that this reaction is slow. The proposed reactions are not likely to proceed, which is supported by the experimental observation of XANES. As shown in Figure 8, Ag K-edge of Ag4@TiO2_air and one of Ag4@TiO2_CO do not reveal any in-situ change in oxidation state indicative of CO oxidation. Next, we propose a mechanism for the reaction on Ag16. In air environment, dissociative O2 adsorption on Ag16@TiO2 is occurred in air environment: nO2(gas) + *  (2n)O*.

(1b)

Next, the following reactions are proposed on oxygen covered Ag16@TiO2 exposed to CO: CO(gas) + (2n)O*  CO2* + (2n-1)O*,

(2b)

CO* + (2n)O*  CO2(gas) + (2n-1)O*,

(3b)

Figure 11(b) reveals the relative energies of the proposed reactions. The calculated adsorption energy of CO in step (3b) is -0.05 eV. Because CO reveals only a weak physisorption, we suggest that the reaction can proceed either through LH (3b) or ER (2b) mechanism. The CO oxidation in


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step (3b) is exothermic by 2.85 eV; the reaction barrier is 0.60 eV. For comparison, DFT calculations of CO oxidation by atomic and molecular oxygen on cluster models of Ag (100) surface found lower reaction barriers for atomic oxygen, in agreement with our results on supported clusters [Lei, X.; Mbamalu, G.; Qin, C., J. Phys. Chem. C 2017, ASAP article DOI:10.1021/acs.jpcc.6b09105]. Also note that the reaction energies and barriers are dependent on the number of oxygen molecules, n, in step (3b). In the experimental XANES observations, Figure 8 reveals a prominent first peak in Ag K-edge in Ag16@TiO2_air, which indicates binding of dissociated oxygen atoms to silver. Subsequently, we introduce CO into Ag16@TiO2_air system. There is no prominent first peak for Ag16@TiO2_CO as shown in Figure 8, suggesting that the Ag16 cluster was reduced, presumably via the oxidation of CO, in agreement with the calculated reaction mechanism. Both the experimental and theoretical results indicate that the behavior of Ag16@TiO2 for CO oxidation is very different from that of observed for Ag4@TiO2. Thus, there is a unique size dependence of reactivity of size selected silver clusters.

IV. Conclusions We have investigated size-selected Ag4 and Ag16 clusters deposited on TiO2 surface and established their reactivity towards oxygen and subsequent CO. In situ XAS and DFT calculations showed that Ag16@TiO2 in air has a prominent first peak in Ag K-edge, attributed to an oxidized Ag16 cluster formed after the dissociation of O2 on the surface of the clusters. Exposure of the oxidized cluster to CO leads to the reduction of the cluster, providing an indirect experimental evidence that CO oxidation occurred. In contrast, similar changes were not observed in the spectra of the four-atom silver clusters indicating poor oxygen dissociation and CO oxidation capabilities of the smaller four-atom clusters.


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DFT calculations and real space quasi-particle theory elucidated corresponding cluster geometries, electronic properties, and XANES spectra. Oxygen molecules favor dissociation on the Ag16@TiO2; however, O2 dissociation has energy penalty on the Ag4@TiO2. The dissociated oxygen adsorption resulted in more positive charge on Ag clusters, which in turn resulted in a prominent first peak at the Ag K-edge of Ag4@TiO2_2O and Ag16@TiO2_10O. According to the calculated energy preference and a comparison of experimental and theoretical XANES, we suggest that oxygen dissociation only occurred on the Ag16@TiO2. Subsequently, exposure of the Ag16@TiO2 to carbon monoxide reduced the cluster, greatly reducing the intensity of the first peak at Ag K-edge. Experimental and theoretical Ag K-edges of Ag16@TiO2_CO are in agreement with each other. Because of their size-dependent activity, Ag subnanometer clusters have a potential to be a next generation oxidation catalyst, with their oxidation potential to be controlled through their size.

ACKNOWLEDGMENT The work of ECT, MJP, SV and PZ was supported by U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Materials Science and Engineering under Contract No. DE-AC02-06CH11357. We gratefully acknowledge the beam time awarded at the Swiss Light Source and computing resources provided on the Blues high-performance computing cluster operated by the Laboratory Computing Resource Center at Argonne National Laboratory. PTC and MH acknowledge support by Ministry of Science and Technology of Taiwan (Dragon Gate Program 103-2911-I-002 -595).


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17. Mazheika, A. S.; Matulis, V. E.; Ivashkevich, O. A., Quantum Chemical Study of Adsorption of Ag-2, Ag-4 and Ag-8 on Stoichiometric TiO2 (110) Surface. J Mol StrucTheochem 2010, 942, 47-54. 18. Rehr, J. J.; Albers, R. C., Theoretical Approaches to X-Ray Absorption Fne Structure. Rev Mod Phys 2000, 72, 621-654. 19. Perdew, J. P.; Burke, K.; Ernzerhof, M., Generalized Gradient Approximation Made Simple. Phys Rev Lett 1996, 77, 3865-3868. 20. Kresse, G.; Hafner, J., Abinitio Molecular-Dynamics for Liquid-Metals. Phys Rev B 1993, 47, 558-561. 21. Kresse, G.; Furthmuller, J., Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys Rev B 1996, 54, 11169-11186. 22. Henkelman, G.; Arnaldsson, A.; Jonsson, H., A Fast and Robust Algorithm for Bader Decomposition of Charge Density. Comp Mater Sci 2006, 36, 354-360. 23. Monkhorst, H. J.; Pack, J. D., Special Points for Brillouin-Zone Integrations. Phys Rev B 1976, 13, 5188-5192. 24. Henkelman, G.; Uberuaga, B. P.; Jonsson, H., A Climbing Image Nudged Elastic Band Method for Finding Saddle Points and Minimum Energy Paths. J Chem Phys 2000, 113, 99019904. 25. Hara, S., The Scattering of Slow Electrons by Hydrogen Molecules. J. Phys. Soc. Jpn. 1967, 26, 710-718. 26. Chou, S. H.; Rehr, J. J.; Stern, E. A.; Davidson, E. R., Abinitio Calculation of Extended X-Ray-Absorption Fine-Structure in Br-2. Phys Rev B 1987, 35, 2604-2614. 27. Lindley, P. F.; Woodward, P., An X-Ray Investigation of Silver Nitrate: A Unique Metal Nitrate Structure. J. Chem. Soc. A 1966, 123-126. 28. Yin, C., et al., Size- and Support-Dependent Evolution of the Oxidation State and Structure by Oxidation of Subnanometer Cobalt Clusters. Journal of Physical Chemistry A 2014, 118, 8477-8484. 29. Abdala, P. M.; Safonova, O. V.; Wiker, G.; van Beek, W.; Emerich, H.; van Bokhoven, J. A.; Sá, J.; Szlachetko, J.; Nachtegaal, M., Scientific Opportunities for Heterogeneous Catalysis Research at the SuperXAS and Snbl Beam Lines. CHIMIA International Journal for Chemistry 2012, 66, 699-705. 30. Sungsik, L.; Byeongdu, L.; Seifert, S.; Vajda, S.; Winans, R. E., Simultaneous Measurement of X-Ray Small Angle Scattering, Absorption and Reactivity: A Continuous Flow Catalysis Reactor. Nucl. Instrum. Methods Phys. Res. A, Accel. Spectrom. Detect. Assoc. Equip. 2011, 649, 200-203. 31. Klacar, S.; Hellman, A.; Panas, I.; Gronbeck, H., Oxidation of Small Silver Clusters: A Density Functional Theory Study. J Phys Chem C 2010, 114, 12610-12617. 32. Tian, D. X.; Zhang, H. L.; Zhao, J. J., Structure and Structural Evolution of Ag-N (N=322) Clusters Using a Genetic Algorithm and Density Functional Theory Method. Solid State Commun 2007, 144, 174-179. 33. Dhillon, H.; Fournier, R., Geometric Structure of Silver Clusters with and without Adsorbed Cl and Hg. Comput Theor Chem 2013, 1021, 26-34. 34. Xu, Y.; Greeley, J.; Mavrikakis, M., Effect of Subsurface Oxygen on the Reactivity of the Ag(111) Surface. J Am Chem Soc 2005, 127, 12823-12827. 35. Li, W. X.; Stampfl, C.; Scheffler, M., Oxygen Adsorption on Ag(111): A DensityFunctional Theory Investigation. Phys Rev B 2002, 075407,1-19.


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36. Yang, C. T.; Balakrishnan, N.; Bhethanabotla, V. R.; Joseph, B., Interplay between Subnanometer Ag and Pt Clusters and Anatase TiO2 (101) Surface: Implications for Catalysis and Photocatalysis. J Phys Chem C 2014, 118, 4702-4714. 37. Wang, Y. G.; Cantu, D. C.; Lee, M. S.; Li, J.; Glezakou, V. A.; Rousseau, R., CO Oxidation on Au/TiO2: Condition-Dependent Active Sites and Mechanistic Pathways. J Am Chem Soc 2016, 138, 10467-10476. 38. Burghaus, U.; Vattuone, L.; Gambardella, P.; Rocca, M., HREELS Study of CO Oxidation on Ag(001) by O-2 or O. Surf Sci 1997, 374, 1-8. 39. Tang, D. Y.; Chen, Z. Z.; Hu, J. P.; Sun, G. F.; Lu, S. Z.; Hu, C. W., CO Oxidation Catalyzed by Silver Nanoclusters: Mechanism and Effects of Charge. Phys Chem Chem Phys 2012, 14, 12829-12837. 40. Chang, C. M.; Cheng, C.; Wei, C. M., CO Oxidation on Unsupported Au-55, Ag-55, and Au25Ag30 Nanoclusters. J Chem Phys 2008, 124710, 1-4. 41. Kim, D. H.; Shin, K.; Lee, H. M., CO Oxidation on Positively and Negatively Charged Ag-13 Nanoparticles. J Phys Chem C 2011, 115, 24771-24777. 42. Jia, C. Y.; Zhang, G. Z.; Zhong, W. H.; Jiang, J., A First-Principle Study of Synergized O-2 Activation and CO Oxidation by Ag Nanoparticles on TiO2(101) Support. Acs Appl Mater Inter 2016, 8, 10315-10323. 43. Socaciu, L. D.; Hagen, J.; Roux, J. P.; Popolan, D.; Bernhardt, T. M.; Wöste, L.; Vajda, S., Strongly Cluster Size Dependent Reaction Behavior of CO with O2 on Free Silver Cluster Anions. J Chem Phys 2004, 120, 2078-2081.


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Table 1. Calculated charges for unsupported Ag4 and Ag16 clusters and with adsorbed molecular O2, dissociative 2O, and CO as well as for reference bulk Ag species. Ag(total)

Ag(avg. per atom)

Ag4

+0e

+0e

Ag4 _O2

+0.50e

Ag4 _2O

O(total)

O(avg. per atom)

+0.13e

-0.50e

-0.25e

+1.72e

+0.43e

-1.72e

-0.86e

Ag4 _CO

+0.12e

+0.03e

-1.85e

-1.85e

Ag16

+0e

+0e

Ag16 _O2

+0.64e

+0.04e

-0.64e

-0.32e

Ag16 _2O

+1.83e

+0.11e

-1.83e

-0.92e

Ag16 _CO

+0.12e

+0.01e

-1.84e

-1.84e

Ag2O (bulk)

+0.53e

AgNO3 (bulk)

+0.74e

Charge-C

+1.73e

+1.72e


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Table 2. Calculated charges for Ag4 and Ag16 clusters supported on TiO2 (110) and with adsorbed molecular O2, nO atoms, and CO. Ag(total)

Ag(avg. per atom)

O(total)

O(avg. per atom)

Ag4@TiO2

+1.30e

+0.32e

Ag4@TiO2_O2

+1.38e

+0.35e

-0.42e

-0.21e

Ag4@TiO2_2O

+2.48e

+0.62e

-1.63e

-0.81e

Ag4@TiO2_CO

+1.33e

+0.33e

-1.79e

-1.79e

Ag16@TiO2

+2.05e

+0.13e

Ag16@TiO2_O2

+2.50e

+0.16e

-0.50e

-0.25e

Ag16@TiO2_2O

+3.71e

+0.23e

-1.82e

-0.91e

Ag16@TiO2_4O

+5.43e

+0.34e

-4.34e

-1.08e

Ag16@TiO2_6O

+7.18e

+0.45e

-6.53e

-1.09e

Ag16@TiO2_8O

+8.51e

+0.53e

-9.24e

-1. 15e

Ag16@TiO2_10O

+10.20e

+0.64e

-11.52e

-1.15e

Ag16@TiO2_CO

+2.12e

+0.13e

-1.80e

-1.80e

Charge-C

+1.81e

+1.74e


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Figure 1. Geometry and relative energy of (a) molecularly absorbed O2 and (b) dissociatively adsorbed 2O on Ag4 cluster, and (c) molecularly adsorbed O2 and (d) dissociatively adsorbed 2O on Ag16 cluster.


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Figure 2. Theoretical Ag K-edge XANES for gas phase bare Ag4 and Ag16 clusters, and with molecularly adsorbed O2, dissociatively adsorbed 2O, and adsorbed CO molecule


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Figure 3. Comparison of theoretical (a) and experimental (b) Ag K-edge XANES spectra for bulk AgNO3, Ag2O, and Ag standards.


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Figure 4. Geometry and relative energy of (a) molecular O2 and (b) dissociative 2O adsorbed on Ag4 cluster and (c) molecular O2 and (d) dissociative 2O adsorbed on Ag16 cluster. Relative energy with respect to molecular adsorption.


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Figure 5. Theoretical Ag K-edge XANES for bare Ag4 and Ag16 clusters supported on TiO2 (110) surface and with molecularly adsorbed O2, dissociatively adsorbed 2O, and adsorbed CO molecule.


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Figure 6. Oxygen coverage dependence: Theoretical Ag K-edge XANES for Ag16 cluster supported on TiO2 (110) surface with 2 to 10 O atoms (i.e., 1 to 5 dissociated O2 molecules) bound to the cluster.


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Figure 7. Difference of the intensities of the first and second peak of theoretical XANES spectra with an adsorbed CO molecule, a molecularly adsorbed O2, as wells as increasing number of dissociated O atoms.


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Figure 8. Experimental Ag K-edge XANES acquired for Ag4 and Ag16 clusters deposited on TiO2 surface in air and CO environments.


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Figure 9. (a) Theoretical and (b) experimental EXAFS obtained for supported Ag4 and Ag16 clusters in CO gas environment.


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Figure 10. Partial densities of states of titania-supported Ag16 cluster (a) with 10 adsorbed oxygen atoms and (b) and with adsorbed CO.


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Figure 11. Relative energies along the pathway of CO oxidation reaction on supported Ag4 (a) and Ag16 with 10 adsorbed O atoms(b).


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Table of Contents Graphic


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Size-Selective Reactivity of Subnanometer Ag4 and Ag16 Clusters on

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