J. L. García Fierro

The nature of active sites for the oxidation of methane on La-based perovskites

Highly crystalline, monophasic LaFeO3 and LaCoO3 perovskites, prepared by the explosion method, are shown to be heterogeneous at surface level. The outmost atomic layers of these perovskites contain high concentrations of carbonate-type species. Their specific activities for methane combustion are in fact identical to La2O2CO3 and air-exposed La2O3. These results compared with pertinent data from the literature hint that surface heterogeneity may be often present in mixed oxides catalysts.

Acidity and catalytic behavior of vanadium-phosphorus-oxygen catalysts

Abstract The surface acidity of equilibrated and non-equilibrated vanadium-phosphorus oxide (VPO) catalysts was studied using basic probe molecules (ammonia, pyridine and acetonitrile) via Fourier transform-IR spectroscopy. The stronger bases detected the presence, in varying proportions, of both Lewis and Bronsted acid centers on all the solids assayed. All the catalysts used exhibited the X-ray diffraction pattern of vanadyl pyrophosphate and much higher Lewis/Bronsted ratios than the β-VOPO 4 compound. This indicates that the Lewis acidity observed on the used catalysts is not due to the presence of small amounts of V v species. The Lewis/Bronsted ratio almost invariably decreased with temperature, suggesting that the Bronsted sites are stronger than the Lewis centers. Acetonitrile served to differentiate Lewis centers of varying strength. The equilibrated catalysts which exhibited a better crystallized pyrophosphate phase showed an increase in very strong Lewis acidity whereas the stacking fold disordered non-equilibrated catalyst showed the presence of both medium strong and very strong Lewis centers. The oxidation of n-butane to maleic anhydride was studied using a plug flow reactor. Both the yield of maleic anhydride and the proportion of very strong Lewis sites increased with time-on-stream. These findings together with previous IR studies reported in the literature allow a better understanding of the relationship among structural features, acidity and catalytic behavior of VPO formulations.

Dispersion of molybdena and hydrodesulphurization activity of Mo/γ-Al2O3, AND Co (or Ni) Mo/γ-Al2O3 catalysts

Abstract Oxygen chemisorption and catalytic activity for hydrodesulphurization of gas-oil on a series of Mo/γ-Al2O3 and Co (or Ni) Mo/γ-Al2O3, oxide catalysts were studied. A linear relationship between molybdena loading, up to about 15 wt % MoO3, and equivalent molybdena area suggests a high molybdenum dispersion on the alumina surface. Similar behaviour for hydrodesulphurization activity versus equivalent molybdena area was observed, except for the highest molybdena content where a slightly higher activity was found. The equivalent molybdena area in Co (or Ni) promoted molybdena catalysts was slightly smaller than that found in non-promoted molybdena catalysts. However, catalytic activity for hydrodesulphurization was much higher in Co (or Ni) promoted catalysts. These results indicate clearly that Co (or Ni) in Mo containing catalysts does not increase molybdenum dispersion.

CO, O2, and CO2 adsorption on scandium oxide

Abstract The adsorption of O 2 , CO, and CO 2 on Sc 2 O 3 was studied in the temperature range −195 to 550 °C. Isobars and isosteric heats were calculated, and ir spectra were measured for the identification of surface species. Oxygen is physically adsorbed at low temperatures. A slow chemisorption of oxygen begins about 100 °C, while above 350–400 °C there appears to be incorporation of oxygen into the lattice. Carbon monoxide is physically and reversibly adsorbed at temperatures up to 25 °C; however, chemisorption sets in above 100 °C and the ir spectrum shows the presence of surface carbonate at 210 °C. The adsorption of carbon dioxide is fast, giving high surface coverage; the spectra show the presence of both symmetric and bidentate surface carbonate. The slow oxygen chemisorption obeys a pseudo-Langmuirian isotherm. The large CO 2 chemisorption and the CO physical adsorption are better fit by a Freundlich model.

Characterization of polymer systems based on sulfonated poly(2,6-dimethyl-1,4-phenylene oxide)

The sulfonation reaction of poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) has been carried out, incorporating the resulting product into pure PPO to study, in forthcoming research, the electrical and mechanical features of the composites with regard to their performance in fuel cells. Pure sulfonated polymers and their blends have been characterized from a microstructural and electrical point of view, by means of X-ray photoelectron spectroscopy (XPS), differential scanning calorimetry (DSC) and complex impedance spectroscopy. Membranes have been manufactured with excellent ionic conductivity at room temperature. © 2000 Society of Chemical Industry

A gravimetric, esr and oxygen chemisorption study of the effect of nickel on the reduction of molybdenum in NiMo/γ-Al2O3 catalysts

Abstract The reduction by hydrogen of a NiMo/γ-Al2O3 catalyst series with a constant molybdenum content (14 wt %) and varying amounts of NiO (0 – 8.1 wt %) has been studied by ESR and gravimetric measurements. The results from both techniques clearly showed that nickel markedly decreases the reducibility of molybdenum, especially at low nickel content. It is suggested that this effect of nickel may be associated with a strong interaction between the nickel and molybdenum phases and to the formation of polymeric molybdates induced by the simultaneous impregnation of nickel. Parallel determination of equivalent molybdena surface area (EMA) of catalysts by oxygen chemisorption on reduced samples showed that those containing nickel had smaller EMA values than the non-promoted molybdena catalyst. When more severe reduction conditions were used, the EMA values of the NiMo/γ-Al2O3 samples approached that of the Mo/γ-Al2O3 catalyst. This seems to indicate an important role of the extent of molybdenum reduction on the oxygen chemisorption.

Kinetics of activated chemisorption: Hydrogen on scandium oxide

The integral chemisorption kinetics of hydrogen on scandium oxide fits well the integrated Elovich equation. t0 values from Z(t) curves (Aharoni and Ungarishs method) agree with those obtained from q vs log(t + t0) plots. The effects of pressure, temperature and state of the surface have been studied. The adsorption is activated, with Ea = 83 ± 6 kJ mol−, Q = 67 ± 5 kj mol− for φ = 0.37. The remaining OH groups strongly influence both the initial rate and the adsorbed amount. Equilibrium adsorption follows the model of Freundlich.

Gas dynamics at low pressures in a vacuum microbalance

Determination of micromass changes of solid samples in a vacuum and controlled gaseous environments at different temperatures is a powerful method for studying surfaces. The weighing at low gas pressure is highly susceptible to temperature inhomogeneities which cause spurious mass changes, the phenomenon being called the thermomolecular flow effect. This effect was explained for some time on the basis of transverse and longitudinal Knudsen forces, both related to gradient pressure, which at low pressure accompany a thermal gradient. The problem is analyzed by the use of a BGK model and diffuse scattering at the surface of the inner and outer cylinders. Experimental results for helium, krypton, hydrogen oxygen and carbon dioxide are reported. In all cases the Knudsen forces are seen to exhibit a maximum ( volcano plots) in the transition regime. For several values of the Knudsen number ( Kn) the BGK model theory is found to be in good qualitative, as well as quantitative, agreement with the experimental data.

Catalytic and physico-chemical properties of a doped MoO3/SiO2 catalyst used in propylene oxidation

Oxygen chamisorbed at low temperature on a doped MoO3/SiO2 catalyst has been used to evaluate the dispersity of molybdena. The time on stream seems to increase considerably the dispersity of active phase, also observed by Scanning Electron Microscopt (SEM). IR spectroscopy and X-Ray Diffraction (XRD) analysis have shown interaction between Te and Mo oxides. A weak π complex of propylene on the surface, and carboxylate species on the oxidized surface have been detected.AbstractКислород, хемосорбированный на дотированном каталтзаторе MoO3 SiO2 при низкой температуре, был использован для определения дисперсноети окиси молибдена. Время на потоке значительно увеличиваэт дисперсность активной фазы, наблюдаемой также с помощью сканирующей электронной микроскопии. ИК спектроскопия и рентнего-диффракционный анализ указывают на взаимодействие между окисяами Те и Мо. Были детектированы слабый π-комлекс и карбоксилатные частицы на окисленной поверхности.

THERMAL TRANSPIRATION OF METHANE AND ETHANE AT HIGH TEMPERATURES

The thermal transpiration of methane and ethane, in the temperature range 150—550°C and pressures between 20 and 2000 μ , was studied by using the relative method. The results, together with data previously obtained for helium, neon and nitrogen were analyzed according to several theoretical expressions. Millers equation is the one that best describes the experimental results at high temperatures for methane, ethane, helium and neon. Liangs and Takaishi and Sensuis equations—the last one being the best suited for noble gases— also follow fairly well the experimental results. Kavtaradzes equation has a more limited applicability; it fits very well experimental data of gases such as nitrogen and carbon dioxide showing a low thermal transpiration effect.