Article pubs.acs.org/JPCC
Effect of O2 and CO Exposure on the Photoelectron Spectroscopy of Size-Selected Pdn Clusters Supported on TiO2(110) Arthur C. Reber,† Shiv N. Khanna,*,† F. Sloan Roberts,‡ and Scott L. Anderson*,‡ †
Department of Physics, Virginia Commonwealth University, 701 W. Grace St., Richmond, Virginia 23284, United States Department of Chemistry, University of Utah, 315 S. 1400 E., Salt Lake City, Utah 84112, United States
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S Supporting Information *
ABSTRACT: Ultraviolet photoelectron spectroscopy of size-selected Pdn clusters supported on TiO2 shows a decrease in density of states in the TiO2 gap after the absorption of CO, while O2 does not result in a decrease in density. Oxygen is more electron withdrawing than CO, so the Pd clusters should become more positively charged when exposed to O2 than CO. More positively charged clusters are expected to have larger electron binding energies; thus, the observed shifts in the UPS spectra are at odds with conventional wisdom. We have performed a combined experimental and theoretical investigation of the UPS spectra of Pdn clusters with adsorbed O2 and CO. Bonding of both CO molecules and O atoms to the Pd clusters results in a decrease in the density of states in the TiO2 gap; however, O atom binding on top of the clusters also results in significant final state stabilization, restoring the density of states in the TiO2 gap. We also find that 4d−5s hybridization plays a critical role in the initial state energies in X-ray photoelectron spectroscopy and evaluate two methods for determining the final state shift via periodic calculations.
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INTRODUCTION X-ray and ultraviolet photoelectron spectroscopy, XPS and UPS, are powerful tools for determining the oxidation state of atoms and the valence electronic structure of a sample.1−4 The sample of interest is exposed to ionizing radiation, and electron binding energies are determined from the kinetic energy distribution of the ejected photoelectrons. Experimental energies are the differences between the energies of the N electron initial state of the system and the N − 1 electron final state. If the goal is to relate experimental measurements such as adsorbate-induced shifts in binding energies to theoretical results for adsorbate effects on orbital energies, and thus learn about adsorbate bonding, it is necessary to differentiate between initial state and final state shifts in the binding energies.5−16 To this end we have performed a comprehensive experimental and theoretical study of the photoelectron spectroscopy (PES) of Pdn (n = 1−7, 10, 15, 20, 25) clusters supported on TiO2, before and after exposure to CO and O2. The characterization of the electronic structure of catalysts after they adsorb reactants is of special interest for identifying the oxidation state of the catalyst during reactions and for understanding catalyst poisoning. Size-selected clusters are valuable tools for studying catalysis because many studies have found that the electronic structure plays an important role in the reactivity, and understanding the microscopic mechanism that controls the activity may allow for the design of more effective catalysts.13,17−42 Conventional wisdom is that the more positive the oxidation state of an atom, the larger the electron binding energy, while the more negative the oxidation state, the lower the binding energy. Experimental results © XXXX American Chemical Society
detailed here reveal that adsorption of CO to the Pd clusters causes a dramatic attenuation of the Pd density of states near the Fermi level, while when O2 adsorption occurs, there is little to no detectable change in the Pd density of states. One might expect that binding oxygen would withdraw charge from the Pd clusters, resulting in increased electron binding energies and decreasing the density of states near the Fermi energy; however, this does not appear to be the case. To understand the microscopic mechanism responsible for the unexpected adsorbate effects in the Pdn/TiO2 system, we measured XPS and UPS for size selected Pdn clusters deposited on rutile TiO2(110), n = 1−7, 10, 16, 20, and 25, before and after exposure to CO and O2. Theoretical studies were performed for PdnX/TiO2, n = 1−7, X = CO, O, and O2, and for Pd4Xm/TiO2, m = 0−4, X = O and CO. The effect of the O binding site on the final state shift was also investigated. We examine the role of O binding site on the initial and final state stabilization and the effect of Pd−CO and Pd−O bonding on the density of states in the TiO2 band gap region. We also investigate the role of 5s−4d hybridization of the Pd cluster on the XPS shifts and critically examine two methods for determining the UPS shifts.
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EXPERIMENTAL METHODOLOGY The cluster deposition beamline, UHV chamber (base pressure
