Vi Parvulescu

Selective catalytic reduction of NO with NH3 over mesoporous V2O5-TiO2-SiO2 catalysts

Mixed vanadia-titania-silica catalysts (3 or 6 wt% V2O5, and 16-34 wt% TiO2) were one-pot prepared by sol-gel and hydrothermal methods in the presence of surfactants. Sodium silicate (25.5-28.5% silica) or tetraethylorthosilicate was used as a precursor for silica; tetraisopropylorthotitanate or titanyl acetylacetonate, for titania; and vanadyl sulfate or vanadium acetylacetonate, for vanadia. Cetyltrimethylammonium bromide, octadecyltrimethylammonium bromide, or dodecylamine was used as a surfactant. The catalysts were characterized by adsorption and desorption curves of N-2 at 77 K, NH3-DRIFTS, H-2-TPR, XRD, in situ Raman spectroscopy, XPS, and TEM. The catalysts were tested in NO reduction with ammonia using a total flow rate of 100 ml/min and a feed composition of nitric oxide 0.1 vol%, ammonia 0.1 vol%, oxygen 3 vol%, in helium. Vanadia was found to be entrapped in these catalysts as V(V) species in which the population of V=O monomeric bonds strongly depended on the dispersion. Titanium also existed in a very oxidated state, and for high dispersions it adopted a tetrahedral coordination. These structures led to surfaces on which mainly Lewis acid sites are effective under reaction conditions. Under such conditions, the dominant route followed an Eley-Rideal mechanism, yielding in such a way very high activity and selectivity. A comparison with a conventional V2O5-TiO2 catalyst led to the conclusion that the intrinsic activity of one-pot prepared polymeric sol-gel catalysts is higher

NO decomposition over bicomponent Cu-Sm-ZSM-5 zeolites

Bicomponent Cu-Sm-ZSM-5 zeolites with different Cu and Sm contents have been investigated, and compared with monocomponent Cu-ZSM-5 and Sm-ZSM-5. Sm-ZSM-5 is almost inactive in NO decomposition, producing only N2O. Zeolites containing around 0.20 wt% Sm and between 2.2 and 2.7 wt% Cu are substantially more active than the pure Cu-ZSM-5, the increase in NO transformation per total Cu atoms reaching 100%. The catalysts were studied using XPS, O-2-TPD and FT-LR of adsorbed NO. In addition, the transient period between the first contact of the catalyst with NO and 3 h was monitored. The bicomponent Cu-Sm-ZSM-5 catalysts exhibit a very strong O-2-TPD band around 370 degrees C compared to the monocomponent catalysts. FT-IR shows a strong increase of NO adsorption in the (NO)(2) form, and a corresponding decrease of the bands of isolated adsorbed NO. The presence of Sm does not significantly prevent the loss of Cu dispersion during a heat treatment in helium, nor the Cu++/Cu+ ratio. Although it is impossible to conclude unequivocally, it seems that samarium has two roles. It modifies the exchange of copper with the zeolite and, consequently, the dispersion and aggregation of copper

Hydrocarbons oxidation with hydrogen peroxide over germanic faujasites catalysts

Highly crystalline near-faujasite Na-GeX, and its protonated and sulfated forms have been prepared and used as catalysts fur the oxidation of several hydrocarbons with different structures (benzene, toluene, cyclohexane) in the presence of H2O2. The catalysts have been characterized using surface area measurements, X-ray diffraction (XRD) and infrared (IR) techniques. The catalytic experiments have been performed both at normal and autogenic pressure. Autogenic pressures highly improved the oxidation degree of the hydrocarbons. It was found that the oxidation of hydrocarbons in the presence of H2O2 depended not only on the formation of Ge-peroxo species but also on the acidic properties of the catalyst, and also involved the activation of O-2 formed in situ

NO decomposition over physical mixtures of Cu-ZSM-5 with zeolites or oxides

The paper deals with the investigation of the activity of bicomponent Cu-M-ZSM-5 zeolites (M = Ce, Sn, TI) and of the physical mixtures of Cu-ZSM-5 with M-ZSM-5 or with the corresponding oxides of these metals in NO decomposition. Physical mixtures of M-ZSM-5 with those corresponding oxides were investigated as well. Copper exchange degree in the zeolites varied between 21.2 and 128% while the M content varied in inverse order, between 27.2 and 18.8%. The catalysts were characterized by NO-DRIFT, O-2-TPD and XPS. Catalytic tests were performed in a microreactor in the temperature range 673-873 K. It was found that the presence of a second component may indeed lead to a better positioning of Cu, resulting in an improvement of its catalytic properties. This effect is conditioned by the introduction of M together with Cu in the exchange solution. Physical mixtures of Cu-ZSM-5 with M-ZSM-5 or with M oxides are not effective, confirming the importance of the co-exchange. The chemical nature of M is a key factor in this process

Toluene oxidation in a plasma-catalytic system

Oxidative removal of toluene in a dielectric barrier discharge reactor combined with manganese catalysts downstream was investigated. Toluene input concentration was varied in the range of 415-2227 ppm. The discharge was operated in pulsed mode, with short pulses of 23-35 kV peak voltage. At 7 W average power, toluene conversion was 60%-70%, independent on the toluene input concentration and on the total gas flow rate in the range of 110-330 SCCM (SCCM denotes cubic centimeter per minute at STP). Toluene total oxidation was favored at high residence time of the gas in the discharge zone and low toluene concentration, when the main reaction product was CO2 with selectivities of 80%-85%. The addition of the catalysts led to a 15%-20% increase in toluene conversion with respect to the values obtained in the plasma, due to oxidation with ozone on the catalyst surface. (c) 2006 American Institute of Physics.

First in situ Raman study of vanadium oxide based SO2 oxidation supported molten salt catalysts

In situ Raman spectroscopy at temperatures up to 500 °C is used for the first time to identify vanadium species on the surface of a vanadium oxide based supported molten salt catalyst during SO2 oxidation. Vanadia/silica catalysts impregnated with Cs2SO4 were exposed to various SO2/O2/SO3 atmospheres and in situ Raman spectra were obtained and compared to Raman spectra of unsupported “model” V2O5–Cs2SO4 and V2O5–Cs2S2O7 molten salts. The data indicate that (1) the VV complex VVO2(SO4)23− (with characteristic bands at 1034 cm−1 due to ν(V=O) and 940 cm−1 due to sulfate) and Cs2SO4 dominate the catalyst surface after calcination; (2) upon admission of SO3/O2 the excess sulfate is converted to pyrosulfate and the VV dimer (VVO)2O(SO4)44− (with characteristic bands at 1046 cm−1 due to ν(V=O), 830 cm−1 due to bridging S–O along S–O–V and 770 cm−1 due to V–O–V) is formed and (3) admission of SO2 causes reduction of VV to VIV (with the ν(V=O) shifting to 1024 cm−1) and to VIV precipitation below 420 °C.

Preparation and characterization of mesoporous zirconium oxide. Part 2.

Mesoporous zirconium oxides were prepared by the sol-gel procedure under basic catalysis and in the presence of a series of surfactants: CnH2n+1N(CH3)(3)Br, with n = 8-18. Zirconium propoxide was used as a precursor. Basic conditions were created by NaOH (pH > 11) or ammonia (pH = 10). The influence of the preparation parameters was examined by modifying several parameters such as Zr:H2O and Zr:surfactant molar ratios. The sol-step and then the polymerization step were achieved at different temperatures. Drying of the samples was carried out by lyophilization, or in vacuum oven at 383 K for 48 h. Calcination of the materials was made on the basis of TG-DTA results. The oxides were characterized by adsorption-desorption isotherms of N-2 at 77 K, XRD, SAXS, solid-state C-13 CP/MAS NMR, FTIR, and SEM

Preparation, characterisation and catalytic behaviour of cobalt–niobia catalysts

Cobalt-niobia catalysts were prepared using the colloidal sol-gel technique. Niobium chloride or niobia oxide were used as precursor. The differences between the procedures used are due to the methods of preparation of the colloidal suspensions and gelification. The catalysts were characterised using adsorption and desorption curves of Kr and N-2 at 77 K, H-2-Chemisorption, XRD, FT-IR, XPS and electron microscopy investigations. Preparation of these catalysts without experimental precautions led to a very inhomogeneous structural and textural material. In contrast, the colloidal sol-gel technique controls both the structure of the niobia oxide and the tailoring of cobalt. A strong metal support interaction effect (SMSI) was present irrespective of the sample preparation variant. Although the rate of butane hydrogenolysis was low for all catalysts, a correlation between TOF and the catalyst crystallite size was found. Selectivity to methane, ethane, propane or to isomerization also depends on the catalyst crystallite size

Chemoselective oxidation of 2-thiomethyl-4,6-dimethyl-pyrimidine and 2-thiobenzyl-4,6-dimethyl-pyrimidine over titania-silica catalysts

Mixed oxides titania-silica catalysts (3 wt.% TiO2) were prepared using the sol-gel technique. The catalysts were characterized using adsorption-desorption isotherms of N-2 at 77 K, XRD, SAXS, UV-Vis, FTIR and XPS. They were tested in sulfoxidation of 2-thiomethyl-4,6-dimethyl-pyrimidine and 2-thiobenzyl-4,6-dimethyl-pyrimidine in presence of H2O2. The catalysts were used as such or after modification with L(+)-tartaric or R(-)-alpha-metoxyphenylacetic acid at 373 K. The chemoselectivity was found to be influenced by the solvent, properties of catalyst and nature of the substrate. The enantiomeric excess (e.e.) obtained using modified catalysts was lower than 30%

Preparation, characterization and catalytic properties of Co-Nb2O5-SiO2 catalysts

Co-Nb(2)Q(5)-SiO2 catalysts were prepared using three different sol-gel procedures: (i) the colloidal sol-gel method using NbCl5 and SiCl4 as precursors; (ii) the polymeric sol-gel method using niobium ethoxide and tetraethyl-orthosilicate (TEOS); (iii) an intermediate procedure between the colloidal and polymeric sol-gel method in which the precursors were those utilized in the CSG but dissolved in a mixture of anhydrous ethanol and CCl4. In all procedures, the elimination of the solvent carried out between 80 and 110 degrees C was followed by a reduction in hydrogen flow (30 ml min(-1)) at 773 K. Following these procedures, samples containing 10 wt.% Co and 15 wt.% niobium oxide (expressed as Nb2O5) were obtained. The characterization of the catalysts was performed using various techniques: N-2 adsorption and desorption curves at 77 K, NH3- and H-2-chemisorption, TPO, XPS, XRD, and solid state H-1 MAS-NMR. Hydrogenolysis of butane was evaluated. The low reaction rates are assigned to the effect of the metal size, whereas the isobutane selectivity as well as the relatively high stability is due to the acidity of the support. (C)2000 Elsevier Science B.V. All rights reserved.