Photoemission and RHEED study of the supported Pt and Au epitaxial alloy clusters

Formation of Subsurface W5+ Species in Gasochromic Pt/WO3 Thin Films Exposed to Hydrogen

“…Unlike this reversible behavior after exposure to hydrogen at low temperatures, the series of spectra in Figure 3 corresponding to H 2 treatments at 623 K shows the occurrence of irreversible changes in the shape of the W 4f spectra and the evolution of three well-defined features that can be attributed to W 6+ (BE 36.3 eV), W 4+ (BE 33.8 eV), and W m+ (m ≤ 2, BE 31.7 eV) species. [16][17][18]34 This result clearly sustains an irreversible and deep chemical reduction of the tungsten oxide, as previously reported for this material. 35 A rough estimation based on the fitting analysis of these spectra renders that at least more than 48% of the tungsten atoms at the surface of the Pt/WO 3 films has been transformed into W n+ species (n < 6) by this treatment.…”

“…This shift is similar to that reported for Pt clusters or atoms deeply interacting with an electron-rich WO x substrate, where the photohole in the final state of the Pt atom undergoing photoemission would be energetically more favorable and would produce the observed BE shift. 34 This hypothesis is sustained by the analysis of the density of occupied electronic states at the valence band by XPS and UPS of the Pt/WO 3 thin films heated in hydrogen, which revealed the development of a high electron density in occupied states at the Fermi level of the system, as expected for a metal-like state (see the Supporting Information, Figure S1). BE shifts to higher values are common for metal 37 and oxide 38 clusters and are due to a small value of the relaxation energy in the final state of the system after photoemission.…”

Enhanced dispersion and the reactivity of atomically thin Rh layers supported by molybdenum oxide films

“…5B show a slight enhancement of Mo/Rh ratios up to 650 K for the Rh clusters (0.4 ML) supported either by MoO 3 or MoO 2 films, which can be attributed to some surface diffusion of MoO X species onto the Rh particles. In a similar experiment, an SMSI effect was found for Pt particles supported by WO 3 on a titania substrate after a heat treatment at 570 K, manifested in the suppression of CO adsorption capability [6]. In the following, the carbon monoxide uptake of molybdenum oxide supported Rh layer formed at 300 K is determined as a function of annealing temperature by CO adsorption-desorption cycles.…”

“…The thermally assisted diffusion of Mo ions into the titania represents the reversal of the sign of surface dipole moments, leading to WF enhancement. On a similar blue-colored TiO 2 (110) single crystal, following the decomposition of Mo(CO) 6 and a subsequent heat treatment at 853 K in air, the formation of a substitutional near-surface alloy with a stoichiometry of Ti 1−X Mo X O 2 (x being close to 0.1) was found [24], corresponding to Mo 4+ ions embedded in the titania lattice. It is obvious from Fig.…”

“…There are several proposals to explain the promotional mechanism of molybdenum oxides [2,4], but model studies on wellcontrolled systems are rare. A recent work addressed the interaction of Pt deposit with WO 3 support, concluding that the so-called strong metal support interaction (SMSI) is responsible for the loss of CO adsorption capability on annealing the sample to 570 K [6]. An additive to a catalyst can be the molybdenum itself, which is a rather active metal towards different molecules, hence under catalytic conditions it can be oxidized, carbidized or may form other compounds of variable activities and promotional effect.…”