M.A. Centeno

Catalytic combustion of volatile organic compounds on Au/CeO2/Al2O3 and Au/Al2O3 catalysts

Catalytic oxidation of n-hexane, benzene and 2-propanol was investigated on Au/CeO2/Al2O3 and Au/Al2O3 catalysts prepared from the deposition–precipitation method and characterised by XRD, scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV–VIS and X-ray photoelectron spectroscopy (XPS) techniques. It is shown that ceria enhances the fixation and final dispersion of gold particles, leading to stabilise them in lower crystallite sizes. Catalytic results show that ceria improves the activity of gold particles in the oxidation of the tested volatile organic compounds (VOCs), probably by increasing the mobility of the lattice oxygen and controlling and maintaining the adequate oxidation state of the active gold particles.

Characterization of molybdenum hydrodesulfurization catalysts supported on ZrO2-Al2O3 and ZrO2-SiO2 carriers

Two series of Mo-containing catalysts supported on alumina and silica, respectively, modified with different amounts of zirconium, were prepared by wet impregnation method. X-ray diffraction, temperature-programmed reduction (TPR), X-ray photoelectron and Fourier transform infrared spectroscopic techniques were used for physicochemical characterization of the samples in oxidic, reduced and sulfided state. The effect of zirconium content on the state and dispersion of molybdenum is reported: introduction of a small amount of zirconium into the support leads to a significant increase of the dispersion of supported molybdenum oxide species, especially, on silica. ZrO2-Al2O3 supported Mo catalysts show higher Lewis acidity compared to those supported on ZrO2-SiO2, Zirconium influences the reducibility of molybdenum. The reduction of molybdenum oxide species on ZrO2-Al2O3 supports proceeds easily compared to that supported on ZrO2-SiO2, however increasing the Zr02 content leads to a decreasing of the hydrogen consumption during the reduction for both series catalysts due to a strong interaction between molybdenum species and modified supports. The maximum in HDS activity is observed for Mo/ZrO2-SiO2 catalysts with the lowest Zr content (6.6 wt.%) related to a higher dispersion of molybdenum sulfide species

NH3 interaction with chromium-doped WO3 nanocrystalline powders for gas sensing applications

Ammonia interaction with chromium-doped WO3 nanocrystalline powders was investigated. Chromium centres were characterised by EELS, Raman, XPS, EPR and UV-visible spectroscopy. Essentially, these studies revealed that chromium was well distributed on WO3, forming Cr(III) species and chromyl groups [CrO]3+. Test of thick-film gas sensors based on chromium-doped WO3 revealed that chromium addition boosted sensor response to NH3. Besides, it also avoided unsatisfactory dynamic behaviour previously found in gas sensors based on pure WO3. DRIFT spectroscopy and TPD were used to investigate the surface chemistry of ammonia over pure and chromium-doped WO3 nanocrystalline powders. It was concluded that chromium favours the combustion of ammonia into molecular nitrogen and nitrous oxide on chromium centres, which contributes to the improvement in the sensing properties of the material.

NO–NH3 coadsorption on vanadia/titania catalysts: determination of the reduction degree of vanadium

Abstract Diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) has been used to study NH3 and NO adsorption over a 15% w/w vanadia/titania catalyst. NH3 is adsorbed as coordinate NH3 and NH4+ species over the oxidised catalyst, leading to the reduction of the vanadia surface. At 300°C, adsorbed nitrosyls species are detected, suggesting that the oxidation of gaseous or adsorbed ammonia species takes place over the VO sites. Coadsorption experiments show that NO is able to reoxidise about the 57% of the reduced VO groups, resulting in N2, according to a NO+V□→1/2N2+VO reaction. On the other hand, NO is only adsorbed over vanadia reduced surfaces. The measure of the area of the 2ν(VO) bands results in an estimate of the oxidation state of vanadium. From this estimate it can be concluded that nitrosyls species are adsorbed on the catalyst surface for vanadium atoms having an oxidation state ranging from +4 to +3.1.

Thermal evolution of sol–gel-obtained phosphosilicate solids (SiPO)

Abstract Amorphous phosphosilicate solid with P–O–Si linkages was obtained by a sol–gel process starting from tetraethoxysilane (TEOS) and H 3 PO 4 . The evolution of the structure of as-prepared gel was investigated as a function of the heat-treatment temperatures, by scanning electron microscopy (SEM), X-ray powder diffraction (XRD), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and X-ray photoelectron spectra (XPS) studies. XPS reveals that the surface of the particles is poorer in Si than the bulk at low temperatures. Heating the gel allows its crystallisation through the formation of Si–O–P linkages at the expends of the Si–O–Si and P–O–P networks, while an enrichment of silicium at the surface is detected.

In situ DRIFTS study of the SCR reaction of NO with NH3 in the presence of O2 over lanthanide doped V2O5/Al2O3 catalysts

Abstract An in situ diffuse reflectance FTIR spectroscopy (DRIFTS) study of the selective catalytic reduction (SCR) of NO with NH3 in the presence of O2 has been carried out over vanadia/alumina and vanadia/lanthanide-doped alumina catalysts. The SCR reaction data may be interpreted on the basis of an Eley–Rideal type mechanism between gaseous NO and adsorbed ammonia species (NH+4 and/or coordinated NH3). Although both adsorbed ammonia species seems to be reactive, NH+4 groups are more effective in the SCR reaction. Thus, at the temperature of the highest NO conversion (Tmax), ammonia is adsorbed preferentially as NH+4 over the undoped catalyst, meanwhile over lanthanide-doped ones, the adsorption of coordinated NH3 becomes predominant, thus explaining the lower SCR activity of such type of catalysts. At lower temperatures, NO is adsorbed as nitrates and the NH3 adsorption as coordinated NH3 species is enhanced. These two factors drive the activity decrease.

NH 3 Interaction with Catalytically Modified Nano- WO 3 Powders for Gas Sensing Applications

Nanocrystalline powders of WO 3 , pure and with catalytic additives such as copper and vanadium, for ammonia gas detection are analyzed in detail. Material was annealed at two different temperatures (400 and 700 ° C) and catalytic additives were introduced in two different concentrations (0.2 and 2%)in order to study the gas sensor performances of these WO 3 -based materials. Crystalline structure characterization shows that a mixture of triclinic and monoclinic structure was present in the materials analyzed. Additive characterization reveals that catalytic metals were located as cations in the matrix lattice. Thick-film gas sensors based on pure WO 3 show an abnormal sensor response, which is attributed to a complex process originated by the oxidation of ammonia to NO. On the other hand, catalyzed WO 3 -based gas sensors show a more direct and simple sensor response. Interaction of ammonia with WO 3 was studied by diffuse reflectance infrared spectroscopy. Only pure WO 3 presented a W=O overtone band decrease and some nitrosil bands. In this case, NH 3 would react with the surface oxygen of terminal W=O bonds and would lead to the formation of NO. Catalyzed WO 3 avoided this reaction and so the unselective catalytic oxidation of NH 3 , improving sensor response. Influence of introduced additives on ammonia oxidation and thus on sensor response is discussed.

Washcoating of metallic monoliths and microchannel reactors

Abstract The most important parameters controlling the washcoating of metallic structures from catalytic slurries are reviewed. The slurry must be stable with adequate rheological properties controlled by the solid content, particle size and additives. The metallic substrate must be pre-treated to obtain an adherent surface scale compatible with the coating and presenting appropriate surface roughness. The quality of the produced coating (homogeneity, specific load and adherence) depends essentially on slurry properties (viscosity and solid content) and on the technique used to remove slurry excess.

Surface models for γ-Al2O3 from molecular dynamics simulations

Molecular dynamics simulations for γ-Al2O3 and La3+-doped γ-Al2O3 crystals have been performed. From bulk simulations a description of the crystal-terminating layers in both doped and pure alumina solids is reported. The molecular dynamics simulations allow us to describe systematically for the first time the surface of the γ-Al2O3 crystal, taking into account the actual stoichiometry of the solid. This description results in a model in which the number of different surface sites is increased with respect to the previously reported models. The number of different surface sites may account for the IR spectra of hydroxy groups adsorbed on γ-Al2O3.

An attempt to explain the role of CO2 and N2O as gas dopes in the feed in the oxidative dehydrogenation of propane

Dynamic effects brought about by the introduction of small amounts of CO2 and N2O in the reaction feed during the oxidative dehydrogenation of propane to propene on alpha-NiMoO4 catalysts are reported. CO2 promotes oxidation probably via the formation of adsorbed oxygen species, O(a), formed by the dissociation of CO2 on NiMoO4. Catalysts in presence of CO? work in a high oxidation state increasing conversion. CO2 is not an inert non-selective product. It can play an active role during the reaction. It could, via O(a): (i) oxidise propane to propene, (ii) increase the oxidation state on the surface of the catalyst and (iii) in high concentration, promote the non-selectivity of the catalyst. In presence of N2O, catalytic sites work in a more reduced state. N2O improves selectivity, probably by: (i) inhibiting the adsorption of O-2 and/or (ii) increasing reduction rate of the catalysts. The O* produced by dissociation of N2O can oxidise propane to propene. Adjusting the concentration of promoters could be an useful tool to modulate selectivity during the catalytic reaction