The structure of vanadia ultrathin films grown on TiO2 (110) in an oxygen ambient

Sambi, Mauro; Negra, Michela Della; Granozzi, G.

doi:10.1016/s0039-6028(00)00856-6

Cited by 24 publications

(23 citation statements)

References 14 publications

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confidence: 88%

“…1(a)], which were observed also with scanning tunneling microscopy (STM) [10,11] and atomic force microscopy [12]. Surprisingly, the V O termination has been found also for the most stable, (0001) surface of V 2 O 3 [4,13,14], whereas the VO 2 110 surface appears to follow the respective bulk termination [1,4,15,16].Since the thermally induced transformations of vanadium oxides may be initiated at the surface and even be restricted to the surface layers, it is important to study their surface structures in the early stages of transitions when several structures may coexist. In this respect, STM combined with scanning tunneling spectroscopy (STS) can provide direct information on both geometrical and electronic structures of oxide surfaces (see, e.g., [17] ).…”

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confidence: 90%

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Surface Metal-Insulator Transition on a Vanadium Pentoxide (001) Single Crystal

et al. 2007

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In situ band gap mapping of the V 2 O 5 001 crystal surface revealed a reversible metal-to-insulator transition at 350-400 K, which occurs inhomogeneously across the surface and expands preferentially in the direction of the vanadyl (V O) double rows. Supported by density functional theory and Monte Carlo simulations, the results are rationalized on the basis of the anisotropic growth of vanadyloxygen vacancies and a concomitant oxygen loss driven metal-to-insulator transition at the surface. At elevated temperatures irreversible surface reduction proceeds sequentially as V 2 O 5 001 ! V 6 O 13 001 ! V 2 O 3 0001. DOI: 10.1103/PhysRevLett.99.226103 PACS numbers: 68.47.Gh, 68.35.ÿp, 73.20.ÿr Vanadium oxides represent an important class of materials with high potential in many technological applications based on their diverse temperature-dependent electronic, magnetic, and catalytic properties (e.g., [1][2][3][4] and references therein). There is a large variety of the vanadium oxide phases such as VO (V 2 , rocksalt structure),Depending on the ambient conditions and temperature, phase transformations between these oxides can occur. They may involve the formation of mixed valence phases. In addition, several vanadium oxides undergo metal-to-insulator transitions (MIT), e.g., V 2 O 3 at 150 K, VO 2 at 340 K. These complex structural and electronic transformations may play a crucial role in the behavior of vanadia-based systems.Among the surface structures of vanadium oxides [4], the V 2 O 5 001 surface seems to be the most studied [3,[5][6][7][8][9][10][11][12]. This surface exposes vanadyl (V O) double rows along the [010] direction [ Fig. 1(a)], which were observed also with scanning tunneling microscopy (STM) [10,11] and atomic force microscopy [12]. Surprisingly, the V O termination has been found also for the most stable, (0001) surface of V 2 O 3 [4,13,14], whereas the VO 2 110 surface appears to follow the respective bulk termination [1,4,15,16].Since the thermally induced transformations of vanadium oxides may be initiated at the surface and even be restricted to the surface layers, it is important to study their surface structures in the early stages of transitions when several structures may coexist. In this respect, STM combined with scanning tunneling spectroscopy (STS) can provide direct information on both geometrical and electronic structures of oxide surfaces (see, e.g., [17] ). We here report thermally induced reconstructions observed by STM/STS on a V 2 O 5 001 single crystal surface. Data show that V 2 O 5 , which is not known for the MIT in the bulk, exhibits, however, the MIT at the surface which proceeds through the formation and anisotropic growth of vanadyl-oxygen vacancies and is followed by an irreversible surface reduction. The experimental findings are supported by density functional theory (DFT) and

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Surface Metal-Insulator Transition on a Vanadium Pentoxide (001) Single Crystal

et al. 2007

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“…To elucidate the ODH reaction mechanism over vanadia on a molecular level, different model systems have been invoked that range from single crystals [22,23] and thin films prepared on TiO 2 [24][25][26], W(110) [27] , Au(111) [27][28][29][30][31] , Rh(111) [32][33][34] , and Cu 3 Au(100) [35][36][37] to vanadia particles deposited onto oxide substrates [21,38,39] including ceria [40][41][42]. In addition to experimental investigations, density functional theory h as been employed to explore the structural characteristics and reactivity towards methanol of model vanadia systems [43][44][45][46][47][48].…”

Section: Introductionmentioning

confidence: 99%

Relating methanol oxidation to the structure of ceria-supported vanadia monolayer catalysts

Abbott

Uhl

Baron

et al. 2010

Journal of Catalysis

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Abstract.Vanadia "monolayer"-type catalysts supported on reducible oxides such as ceria previously have shown high activity for the selective oxidation of alcohols. Here, a model system consisting of vanadia particles deposited on well-ordered CeO 2 (111) thin films has been employed. Scanning tunneling microscopy (STM), photoelectron spectroscopy (PES), and infrared reflection absorption spectroscopy (IRAS) were used to characterize the VO x /CeO 2 surface as a function of vanadia loading. The formation of isolated monomeric species as well as two-dimensional vanadia islands that wet the ceria support was directly observed by STM. The vanadia species exhibit V in a +5 oxidation state and expose vanadyl (V=O) groups with stretching vibrations that blue -shift from ~1005 cm -1 , to ~1040 cm -1 with increasing coverage.Temperature programmed desorption (TPD) of methanol revealed three peaks for formaldehyde production. One is correlated with reactivity on the ceria support (565-590 K). Another is correlated with reactivity on large vanadia particles (475-505 K) similar to that previously observed on vanadia/silica and vanadia/alumina model systems. A low temperature reaction pathway (~370 K) is observed at low coverage, which is assigned to the reactivity of isolated vanadia species surrounded by a reduced ceria surface. It is concluded that strong support effects reported in the literature for the real catalysts are likely related to the stabilization of small vanadia clusters by reducible oxide supports.

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“…Recently, in an effort to better understand the relationships between the structure and reactivity of supported vanadia catalysts several groups have studied model systems composed of vapor-deposited vanadia films supported on well-defined metal oxide single crystal surfaces including TiO 2 (110) and CeO 2 (111) [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31]. In the studies of vanadia films on TiO 2 (110) it has been found that vapor deposition of vanadium metal, either in the presence of a background of O 2 or followed by annealing in O 2 , produces vanadia films that predominantly contain V +3 or mixtures of V +3 and V +4 for O 2 pressures less than 10 -6 Torr [22,23,29].…”

Section: Introductionmentioning

confidence: 99%

Reactivity of monolayer V2O5 films on TiO2(110) produced via the oxidation of vapor-deposited vanadium

2003

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The growth, and reactivity of monolayer V 2 O 5 films supported on TiO 2 (110) produced via the oxidation of vapor-deposited vanadium were studied using X-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD). Oxidation of vapor-deposited vanadium in 10 -7 Torr of O 2 at 600 K produced vanadia films that contained primarily V +3 , while oxidation in 10 -3 Torr at 400 K produced films that contained primarily V +5 . The reactivity of the supported vanadia layers for the oxidation of methanol to formaldehyde was studied using TPD. The activity for this reaction was found to be a function of the oxidation state of the vanadium cations in the film. AbstractThe growth, and reactivity of monolayer V 2 O 5 films supported on TiO 2 (110) produced via the oxidation of vapor-deposited vanadium were studied using X-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD).Oxidation of vapor-deposited vanadium in 10 -7 Torr of O 2 at 600 K produced vanadia films that contained primarily V +3 , while oxidation in 10 -3 Torr at 400 K produced films that contained primarily V +5 . The reactivity of the supported vanadia layers for the oxidation of methanol to formaldehyde was studied using TPD. The activity for this reaction was found to be a function of the oxidation state of the vanadium cations in the film.

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The structure of vanadia ultrathin films grown on TiO2 (110) in an oxygen ambient

Cited by 24 publications

References 14 publications

Surface Metal-Insulator Transition on a Vanadium Pentoxide (001) Single Crystal

Surface Metal-Insulator Transition on a Vanadium Pentoxide (001) Single Crystal

Relating methanol oxidation to the structure of ceria-supported vanadia monolayer catalysts

Reactivity of monolayer V2O5 films on TiO2(110) produced via the oxidation of vapor-deposited vanadium

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