Luis A. Gambaro

Study of the Adsorption/Oxidation of Methanol over Vanadium Pentoxide

A study of the electric properties of sintered polycrystalline samples of V2O5 under a CH3OH atmosphere and without added extra oxygen has been performed as a function of time. The gas composition was determined by mass spectrometry in the range 373–473 K. It has been concluded from the results that initial adsorption and further oxidation of CH3OH to different products occurs. A possible mechanism for reaction of the intermediates is suggested. The presence of (CH3O)2CH2 (methylal), H2CO, CO, HCOOCH3, CO2, CH3OCH2OH (hemimethylal) and H2O has been detected by mass spectrometry, their relative amounts being dependent on both the temperature and time of contact.

Temperature-programmed reduction of V2O5 and coprecipitated V2O5−TiO2 by hydrogen

Temperature-programmed reductions (TPR) with H2 of both pure V2O5 and coprecipitated V2O5−TiO2 systems with different titanium concentrations was performed. The original and the reduced samples following each TPR step were characterized by X-ray diffraction, Fourier transform infrared analysis and scanning electron microscopy.Within the temperature range in which TPR analysis was carried out (100–600°C) the V2O5 phase was reduced in two or three steps, while no variation in the TiO2 phase (anatase or rutile) was observed.In the first reduction step only superficial reduction of the oxides was detected. In the following steps, the H2 reacted with oxygen atoms of the V=O and V−O−V bonds. This led to important changes in the structure and morphology of the system.The experimental evidence allowed the conclusion that titanium stabilizes certain phases of vanadium oxides in which vanadium appears as V(+4) or as a mixture of V(+4) and V(+5). Moreover, when moderate and high titanium concentrations were used, the reduction temperature of the bulk V2O5 decreased markedly.ZusammenfassungEs wurde eine temperaturprogrammierte Reduktion (TPR) sowohl von reinem V2O5 als auch von kopräzipitierten V2O5−TiO2 Systemen mit unterschiedlicher Titankonzentration in Wasserstoff durchgeführt. Die Ausgangsprobe und die reduzierten Proben nach jedem TRP-Schritt wurden mittels Röntgendiffraktion, Fourier-IR-Analyse und Scanning-Mikroskopie charakterisiert.Innerhalb des Temperaturbereiches, in dem die TPR-Analyse durchgeführt wurde (100–600°C), wird die V2O5-Phase in zwei oder drei Schritten reduziert, während bei der TiO2-Phase (Anatas order Rutil) keine Veränderung beobachtet wird.Im ersten Reduktionsschritt kann nur eine oberflächliche Reduktion der Oxide festgestellt werden. In den anschließenden Schritten reagiert H2 mit den Sauerstoffatomen der V=O- und der V−O−V-Bindungen. Dies führt zu beträchtlichen Veränderungen bei Struktur und Morphologie des Systemes.Experimentelle Beweise erlauben den Schluß, daß Titan gewisse Phasen von Vanadiumoxiden, in denen Vanadium als V(+4) oder als Gemisch aus V(+4) und V(+5) vorkommt, stabilisiert. Werden mäßige oder hohe Titankonzentrationen eingesetzt, wird die Temperatur für die Körperreduktion von V2O5 merklich herabgesetzt.

Studies on the V2O5-TiO2 system part 1. Thermoelectric power and electrical conductivity

SummaryThe thermoelectric power and the resistivity of V2O5- TiO2 mixtures over ranges of composition and temperature from 20° to 500° in air have been measured, The mixtures were obtained by coprecipitation of aqueous solutions of NH4VO3 and TiCl4. calcined during 90 h at 550° and then sintered. Resistances were measured by the four points method.At the VO5/2 35–100% M range, the activation energies for conductivity change from 0,36 to 0.62eV, while for the thermoelectric power they change from 0,18 to 0.24eV. It can be assumed from these values that the conduction mechanism over this concentration range is due to the “hopping” of small polarons, arising principally, from the presence of V4+ ions.In the samples with a high TiO2 content, the activation energies for conductivity were 0.82 and 0.36eV, for the different samples.From the variation of thermoelectric power with temperature, it can be assumed that the ionization energy of the donors centers lies at 0.83eV under the conducting band. A mechanism for band conduction is inferred from the results, being the V4+ donating centers and the V5+ receptive centers.

Adsorption/oxidation of CH3OH on V2O5. A theoretical and experimental study

The adsorption/oxidation of CH3OH on V2O5 was studied in previous work by mass spectrometric analysis of the reaction atmosphere, and by measuring the electrical properties (Hall effect) and total pressure of the system. Such data allowed a reaction pathway to be proposed. In the present work, the variations in the surface oxidation number have been calculated via balances in the C and H atoms in the reaction atmosphere and models of the (010) plane of V2O5 and of CH3OH have also been utilized to find energetically favourable sites for the adsorption of CH3OH and its subsequent oxidation to H2CO. For this purpose, the variation in the total energy of the system was computed by Extended Hückel-type calculations (ASED, Atom Superposition and Electron Delocalization). From such experimental results, it was deduced that the H atoms are removed by the network oxygens and that methanol is adsorbed as the methoxy group. Such removal of H atoms has two consequences: high surface hydroxylation and surface reduction [V(5+) → V(4+)]. In support of these suggestions, the theoretical results have confirmed both the removal of H atoms and methoxy group formation, and have also revealed two possible adsorption sites for CH3OH on V2O5: the terminal oxygens (V=O) and the network bridging oxygens (V-O-V).

Studies on the system V2O5−TiO2 II. Infrared spectroscopy

Shifts of the 1019 cm−1 peak (V=O) to higher frequencies, and of the 820 cm−1 peak (V−O−V) to lower frequencies with increasing TiO2 content were observed. These shifts are correlated with the formation of oxygen vacancies asociated with V4+ or Ti4+ ions. Moreover, a correlation has been found with the electrical properties of the system.AbstractВ системе V2O5−TiO2 наблюдается сдвиг полосы V=O 1019 cm−1 в сторону более высоких частот, а полосы 820 cm−1 (V−O−V) всторону низших частот. Это было отнесено к образованию кислородных вакансий, связанных с ионами V4+ и Ti4+. Ъолее того, была найдена корреляция с злектрическими свойствами системы.

Selective oxidation of methanol on a system of V2O5 O2 catalysts. Influence of operating variables

The partial oxidation of methanol on a system of van-dium-titanium catalysts has been studied. Results demonstrated that the conversio degree of reactant and the selectivity tointermediate oxidation products depend markedly on temperature and residence time.

Studies on the V2O5−TiO2 system. IV: Oxygen chemisorption

The chemical adsorption of oxygen on V2O5 and V2O5 -TiO2 over the temperature range 100–450°C and at pressures between 10−2 and 3 Torr has been studied. Assays have been performed using a volumetric method, the isotherms obtained being in accordance with Langmuirs adsorption model. Isobaric plots at 1.5 Torr of these isotherms exhibit two maxima, at 250 and 400°C, respectively. These maxima have been interpreted in terms of the surface chemisorption of O2− and its diffusion into the bulk solid. The amount adsorbed at the 250°C maximum depends on the sample composition in the following decreasing adsorption order: VT80, VT50, VT10, VT20 and VT0. At the 400°C maximum, the same amount of O2 is adsorbed experimentally by all the TiO2-containing samples, this value being quite different from that corresponding to pure V2O5 (VTO). Possible explanations for this observation are based on (i) the relative magnitudes of the V/Ti surface ratios and (ii) the presence of Ti4+ in V2O5 crystals.

Studies on the V2O5−TiO2 system, V. Equilibrium pressures

The oxygen equilibrium pressures from pure V2O5 and co-precipitated V2O5−TiO2 system were measured in the range of 200–450 °C. The behavior of the equilibrium pressure with changes of temperature of the samples with and without TiO2 is attributed to Ti4+ interaction with the V2O5 lattice.AbstractРавновесное давление кислорода из чистой V2O5 и системы со-высажденных V2O5−TiO2 измеряли в интервале температур 200–450°C. Поведение равновесного давления с изменением температуры образца с и без TiO2 объясняется на основе взаимодействия Ti+4 с решёткой V2O5.

Adsorption-Oxidation ofCH3OHonV2O5; Reactivity of the Structural Defects

In previous work the CH3OH adsorption–oxidation process on the (010) plane of V2O5 was studied both theoretically and experimentally. In the present work, we have studied this process on idealized structural defects (kinks, steps, corners) of the (010) plane of V2O5. The study was based on calculations of the variation of the total system energy by the extended Huckel method (ASED — atom superposition and electron delocalization). The results demonstrated that the oxidation of CH3OH to H2CO on the (010) plane is more favorable than on the structural defects and that the kink can be suitable for the oxidation of CH3OH to CO2.