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Metal Nanoparticles on TiOx/Pt(111) Ultrathin Films with Different Structures and Stoichiometries: a Study of their Properties and Reactivity
Full text linkIn this thesis a three years study about model catalysts is presented. The model catalysts have been obtained depositing metals (Au and Fe respectively) nanoparticles (NPs) on TiOx/Pt(111) ultrathin (UT) films. The role of the substrate morphology on the metal NPs nucleation and growth modes have been deeply analyzed through the common surface science techniques and supported by theoretical (DFT) calculations. Also the reactivity of the metal NPs has been considered, together with a study of the evolution of such systems in "high pressure" conditions.In questa tesi viene presentato un lavoro durato tre anni e riguardante la tematica dei catalizzatori modello. Tali catalizzatori sono stati ottenuti depositando nanoparticelle (NPs) metalliche (di Au e Fe rispettivamente) su film ultrasottili di TiOx cresciuti su Pt(111). Il ruolo della struttura del substrato nella nucleazione e crescita delle NPs metalliche è stato ampiamente analizzato attraverso le tecniche della scienza dei materiali e col supporto di calcoli teorici DFT. Anche la reattività delle NPs metalliche è stata considerata, assieme allo studio dell'evoluzione dei sistemi in condizioni di "alta pressione"
Vanadium oxide nanostructures on another oxide: The viewpoint from model catalysts studies
No full textInteraction of iron with a wagon wheel-like ultrathin TiOx film grown on Pt(111)
No full textThe structure and thermal evolution of Fe nanoparticles deposited on a wetting TiOx ultrathin film epitaxially grown on Pt(111) has been characterized by various surface science techniques. Combining the results obtained it is shown that, at room temperature, metallic Fe nucleates randomly and oxidizes at the interface. A thermal treatment causes Fe migration through the TiOx layer, forming a mixed oxide and a new hexagonal ultrathin film phase. Finally, the pristine TiOx phase motif is restored, due to the complete diffusion of Fe into the Pt substrate
Interaction of iron with a wagon wheel-like ultrathin TiOxfilm grown on Pt(111)
No full textThe structure and thermal evolution of Fe nanoparticles deposited on a wetting TiOx ultrathin film epitaxially grown on Pt(111) has been characterized by various surface science techniques. Combining the results obtained it is shown that, at room temperature, metallic Fe nucleates randomly and oxidizes at the interface. A thermal treatment causes Fe migration through the TiOx layer, forming a mixed oxide and a new hexagonal ultrathin film phase. Finally, the pristine TiOx phase motif is restored, due to the complete diffusion of Fe into the Pt substrate
Atomic Structure and Special Reactivity Toward Methanol Oxidation of Vanadia Nanoclusters on TiO2(110)
No full textWe have grown highly controlled VOx nanoclusters on rutile TiO2(110). The combination of photoemission and photoelectron diffraction techniques based on synchrotron radiation with DFT calculations has allowed identifying these nanostructures as exotic V4O6 nanoclusters, which hold vanadyl groups, even if vanadium oxidation state is formally +3. Our theoretical investigation also indicates that on the surface of titania, vanadia mononuclear species, with oxidation states ranging from +2 to +4, can be strongly stabilized by aggregation into tetramers that are characterized by a charge transfer to the titania substrate and a consequent decrease of the electron density in the vanadium 3d levels. We then performed temperature programmed desorption experiments using methanol as probe molecule to understand the impact of these unusual electronic and structural properties on the chemical reactivity, obtaining that the V4O6 nanoclusters can selectively convert methanol to formaldehyde at an unprecedented low temperature (300 K
Mobility of Au on TiOx substrates with different stoichiometry and defectivity
No full textAu nanoparticles deposited on titania films, where two nanophases of different stoichiometry and defectivity are co-present, were imaged on the same spot by scanning tunneling microscopy. The observed sizes are rather dissimilar as a consequence of the different mobility of Au on the two surfaces. The role of the stoichiometry, which can influence the Au−substrate interaction, and of the defects, which can trap the metal atoms, is discussed on the basis of theoretical calculations of diffusion energy barriers on the two surfaces
From novel PtSn/Pt(110) surface alloys to SnOx/Pt(110) nano-oxides
No full textUHV evaporation of Sn on (1 x 2) Pt(110), followed by UHV annealing, provides three different PtSn/Pt(110) surface alloys, each characterized by a specific low energy electron diffraction (LEED) and scanning tunneling microscopy (STM) pattern. When annealed in a controlled oxygen background, the PtSn/Pt(110) surface alloys can be transformed into SnOx/Pt(110) nano-oxides. This paper reports the experimental phase diagram of the metallic and oxidized surface superstructures. (C) 2013 Elsevier B.V. All rights reserved
Chemisorption of CO on Au/TiOx/Pt(111) Model Catalysts with Different Stoichiometry and Defectivity
No full textAu/TiOx/Pt(111) model catalysts were prepared starting from well characterized TiOx/Pt(111) ultrathin films, according to an established procedure consisting in a reactive evaporation of Ti, subsequent thermal treatment in O2 or in UHV, and final deposition of submonolayer quantities of Au. Temperature Programmed Desorption measurements were performed to compare the interaction of CO in the case of two reduced TiOx/Pt(111) substrates (indicated as w-TiOx and w′-TiOx, being the former characterized by an ordered array of defects that can act as template for the deposition of a stable array of Au nanoparticles), with the case of a stoichiometric rect′-TiO2/Pt(111) substrate. It was found that in all cases CO is molecularly adsorbed and two different desorption peaks are detected: one at ≈140 K corresponding to CO desorption from less active adsorption sites (terraces) of the Au nanoparticles and one at ≈200 K corresponding to CO desorption from Au nanoparticles step sites. After annealing at 770 K, the high temperature CO desorption peak is still present in the case of the defective reduced w-TiOx phase, supporting the good templating and stabilizing effect of such phase. On the rect′-TiO2 stoichiometric phase, the CO uptake decreases after annealing but only to a minor extent
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