Unusual CO Adsorption Sites on Vanadium Oxide−Pd(111) “Inverse Model Catalyst” Surfaces

Mixing properties of Al2O3(0001)-supportedM2O3andMM'O3monolayers (M,M' = Ti, V, Cr, Fe)

Considering the importance of sub-monolayer transition metal oxides supported on another oxide in many industrial processes, with the help of a DFT +Uapproach, we provide information on the structural and electronic properties of pureM₂O₃and mixedMM'O₃oxide monolayers (M,M' = Ti, V, Cr, Fe) supported on anα-Al₂O₃(0001) support. With their structure in the prolongation of the alumina corundum lattice, the monolayers have non-equivalent surface and interface cations, which leads to two different cation configurations in the mixed oxides. In all cases, the interfacial charge transfer is weak, but strong cation-cation electron redistributions may take place as in TiVO₃, TiFeO₃, VFeO₃, and TiCrO₃in which actual redox processes lead to cation oxidation states different from the expected +3 value. We show that the tendency to mixing relies on the interplay between two very different driving forces. Cation-cation redox reactions, in most cases, strongly stabilise mixed configurations, but preference for a given cation position in the monolayer, because of surface energy reasons, may strengthen, weaken or even block the mixing tendency. By comparison with results obtained in bulk ilmenite, in free-standing monolayers and in MLs deposited on transition metal substrates, we evidence the flexibility of their electronic structure as a function of size, dimensionality and nature of support, as a lever to tune their properties for specific applications.

Model reaction studies on vanadium oxide nanostructures on Pd(111)

Deuterium desorption and reaction between deuterium and oxygen to water has been studied on ultrathin vanadium oxide structures prepared on Pd(111). The palladium sample was part of a permeation source, thus enabling the supply of atomic deuterium to the sample surface via the bulk. Different vanadium oxide films have been prepared by e-beam evaporation in UHV under oxygen atmosphere. The structure of these films was determined using low energy electron diffraction and scanning tunneling microscopy. The mean translational energy of the desorption and reaction products has been measured with a time-of-flight spectrometer. The most stable phases for monolayer and submonolayer VOx are particular surface-V2O3 and VO phases at 523 and 700 K, respectively. Thicker films grow in the form of bulk V2O3. The mean translational energy of the desorbing deuterium species corresponds in all cases to the thermalized value. Apparent deviations from this energy distribution could be attributed to different adsorption/desorption and/or accommodation behaviors of molecular deuterium from the gas phase on the individual vanadium oxide films. The water reaction product shows a slightly hyperthermal mean translational energy, suggesting that higher energetic permeating deuterium contributes with higher probability to the water formation.

Ultrathin Wagon-Wheel-like TiOx Phases on Pt(111): A Combined Low-Energy Electron Diffraction and Scanning Tunneling Microscopy Investigation

Ultrathin ordered titanium oxide films on a Pt(111) surface have been prepared by reactive deposition and characterized by low-energy electron diffraction and scanning tunneling microscopy (STM). According to the postdeposition annealing condition, three different phases have been prepared which show a wagon-wheel-like (hereafter ww) morphological pattern. Two of them can be prepared as single phases (w- and w'-TiO(x)) and one (w(int)-TiO(x)) as a mixed phase which always coexists with at least one of the other two phases. All of them are formed by a Ti-O bilayer, where the Ti atoms are located at the interface with the substrate, but they show a rather distinct STM ww pattern. The experimental STM contrast has been discussed on the basis of a Moiré-like model, i.e., as deriving from a modulation of the Ti occupancy of the different substrate sites (i.e., hollow, bridge and on-top sites). The major part of the STM data can be easily interpreted on the basis of this simplified model.

Ultrathin TiOx Films on Pt(111): A LEED, XPS, and STM Investigation

Ultrathin ordered titanium oxide films on Pt(111) surface are prepared by reactive evaporation of Ti in oxygen. By varying the Ti dose and the annealing conditions (i.e., temperature and oxygen pressure), six different long-range ordered phases are obtained. They are characterized by means of low-energy electron diffraction (LEED), X-ray photoemission spectroscopy (XPS), and scanning tunneling microscopy (STM). By careful optimization of the preparative parameters, we find conditions where predominantly single phases of TiO(x), revealing distinct LEED pattern and STM images, are produced. XPS binding energy and photoelectron diffraction (XPD) data indicate that all the phases, except one (the stoichiometric rect-TiO2), are one monolayer thick and composed of a Ti-O bilayer with interfacial Ti. Atomically resolved STM images confirm that these TiO(x) phases wet the Pt surface, in contrast to rect-TiO2. This indicates their interface stabilization. At a low Ti dose (0.4 monolayer equivalents, MLE), an incommensurate kagomé-like low-density phase (k-TiO(x) phase) is observed where hexagons are sharing their vertexes. At a higher Ti dose (0.8 MLE), two denser phases are found, both characterized by a zigzag motif (z- and z'-TiO(x) phases), but with distinct rectangular unit cells. Among them, z'-TiO(x), which is obtained by annealing in ultrahigh vacuum (UHV), shows a larger unit cell. When the postannealing of the 0.8 MLE deposit is carried out at high temperatures and high oxygen partial pressures, the incommensurate nonwetting, fully oxidized rect-TiO2 is found The symmetry and lattice dimensions are almost identical with rect-VO2, observed in the system VO(x)/Pd(111). At a higher coverage (1.2 MLE), two commensurate hexagonal phases are formed, namely the w- [(square root(43) x square root(43)) R 7.6 degrees] and w'-TiO(x) phase [(7 x 7) R 21.8 degrees]. They show wagon-wheel-like structures and have slightly different lattice dimensions. Larger Ti deposits produce TiO2 nanoclusters on top of the different monolayer films, as supported both by XPS and STM data. Besides the formation of TiO(x) surfaces phases, wormlike features are found on the bare parts of the substrate by STM. We suggest that these structures, probably multilayer disordered TiO2, represent growth precursors of the ordered phases. Our results on the different nanostructures are compared with literature data on similar systems, e.g., VO(x)/Pd(111), VO(x)/Rh(111), TiO(x)/Pd(111), TiO(x)/Pt(111), and TiO(x)/Ru(0001). Similar and distinct features are observed in the TiO(x)/Pt(111) case, which may be related to the different chemical natures of the overlayer and of the substrate.

Adsorption and reaction of methanol on clean and oxygen modified rhodium/vanadium surface alloys

The dehydrogenation of methanol on Rh(111), on a Rh(111)/V subsurface alloy and on Rh(111) with V islands has been studied with and without preadsorbed oxygen using a supersonic molecular beam and temperature programmed desorption. The reactivity is highest for the V islands surface without oxygen. But this surface is deactivated due to CO dissociation. The subsurface alloy is less reactive than the islands, but still more active than the Rh(111) surface. The reaction products are carbon monoxide and hydrogen only. With preadsorbed oxygen Rh(111) is the most active surface, but a strong dependence of the activity on the amount of preadsorbed oxygen is found for all three surfaces. The reaction products with preadsorbed oxygen are water, hydrogen, carbon dioxide and carbon monoxide. The reactions follow the same mechanism on all surfaces, but the activation energy of the individual reaction steps is different leading to significant changes in the thermal desorption spectra and in King and Wells-type experiments.