J. Blanco

Honeycomb monoliths of activated carbons for effluent gas purification

Abstract During the incineration of urban waste incomplete mineralisation of some organic species can give rise to effluents contaminated with VOCs, furans and dioxins. These units produce large volumes of gas to be treated, but with low contents of these toxic species, for which the pressure drop caused by passing through a conventional pellet adsorption bed can be considerable. Another solution is to spray activated carbon powder into the contaminated effluent gas, although this is an expensive method. Thus, to avoid the problems associated with pressure drop or high operation costs, commercial activated carbons have been conformed as open-channel honeycomb monoliths. Their adsorption capacities towards an aromatic probe molecule, o-dichlorobenzene, chosen to simulate dioxins have been tested in a static regime. The results were analysed together with the textural and mechanical properties of the monolith composites in order to establish criteria by which the most suitable honeycomb monolith composite material for industrial use could be prepared.

Influence of NH3 and NO oxidation on the SCR reaction mechanism on copper/nickel and vanadium oxide catalysts supported on alumina and titania

The influence of ammonia and nitric oxide oxidation on the selective catalytic reduction (SCR) of NO by ammonia with copper/nickel and vanadium oxide catalysts, supported on titania or alumina have been investigated, paying special attention to N2O formation. In the SCR reaction, the VTi catalyst had a higher activity than VAl at low temperatures, while the CuNiAl catalyst had a higher activity than CuNiTi. A linear relationship between the reaction rate of ammonia oxidation and the initial reduction temperature of the catalysts obtained by H-2-TPR showed that the formation rate of NH species in copper/nickel catalysts would be higher than in vanadia catalysts. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) showed that copper/nickel catalysts presented ammonia coordinated on Lewis acid sites, whereas ammonium ion adsorbed on Bronsted acid sites dominated on vanadia catalysts. The NO oxidation experiments revealed that copper/nickel catalysts had an increase of the NO2 and N2O concentrations with the temperature. NO could be adsorbed on copper/nickel catalysts and the NO2 intermediate species could play an important role in the reaction mechanism. It was suggested that the presence of adsorbed NO2 species could be related to the N2O formation

Photocatalytic destruction of toluene and xylene at gas phase on a titania based monolithic catalyst

Abstract Toluene and xylene were subjected to gas-solid heterogeneous photocatalytic oxidation on a titania based monolithic catalyst, in order to investigate the potential of solar-driven detoxification as a clean and safe method for air purification and gas phase waste destruction. Thus, gaseous streams with toluene or xylene were conducted through a monolithic catalysts based on titania dispersed on a fibrous silicate and irradiated with a Xenon lamp in the presence of air, at process temperatures from 150 to 450°C. Destructions levels higher than 96% were achieved for toluene and 99% for xylene at Area Velocity values in the range of 5–8 m h-1. A non-negligible number of undesirable sub-products (furans and benzofurans) were identified in partial oxidation conditions. The concentration of these sub-products was higher in the thermal process than with the photocatalytic system. Textural properties of the catalyst, the nature of phases and their distribution on the surface and light absorption properties were studied using techniques such as mercury intrusion porosimetry, scanning electron microscopy (SEM-EDX), X-ray diffraction and diffuse reflectance UV-Vis. spectroscopy.

Alumina- and titania-based monolithic catalysts for low temperature selective catalytic reduction of nitrogen oxides

Abstract The selective catalytic reduction of NO+NO2 (NOx) at low temperature (180–230°C) with ammonia has been investigated with copper-nickel and vanadium oxides supported on titania and alumina monoliths. The influence of the operating temperature, as well as NH3/NOx and NO/NO2 inlet ratios has been studied. High NOx conversions were obtained at operating conditions similar to those used in industrial scale units with all the catalysts. Reaction temperature, ammonia and nitrogen dioxide inlet concentration increased the N2O formation with the copper-nickel catalysts, while no increase was observed with the vanadium catalysts. The vanadium-titania catalyst exhibited the highest DeNOx activity, with no detectable ammonia slip and a low N2O formation when NH3/NOx inlet ratio was kept below 0.8. TPR results of this catalyst with NO/NH3/O2, NO2/NH3/O2 and NO/NO2/NH3/O2 feed mixtures indicated that the presence of NO2 as the only nitrogen oxide increases the quantity of adsorbed species, which seem to be responsible for N2O formation. When NO was also present, N2O formation was not observed.

Kinetic study of the selective reduction of nitric oxide over vanadia-tungsta-titania/sepiolite catalyst

A kinetic study of the reduction of nitric oxide by ammonia in the presence of oxygen on a novel catalyst prepared as a physical mixture of V2O5-WO3/TiO2 catalyst and a natural silicate (sepiolite) was carried out. The steady-state differential reactor data were fitted to several equations using a discriminating method based on physical and statistic criteria. The best fit was obtained from a model involving adsorption of ammonia and oxygen which react with nitric oxide from the gas phase following an Eley-Rideal mechanism. The equation considered for this model reproduces well the experimental results obtained in a differential regime. Important limitations by internal diffusion were determined when the reaction took place in an integral regime. (Less)

Influence of the binder on the properties of catalysts based on titanium-vanadium oxides

The influence of the nature of the binder and its concentration in V2O5/TiO2 catalysts on their mechanical and catalytic properties has been studied. The characterization analysis showed that the agglomeration mechanism is different when an inorganic acid, such as H3PO4, or a natural silicate, such as sepiolite, were used. Two different patterns are proposed, which explain the effect of these binders on the performance of this type of catalyst in the selective catalytic reduction of NOx with NH3.

Titania based platinum monolithic catalysts for lean-burn DeNOx process

Monolithic catalyst supports were prepared with mixtures of titanium dioxide and a natural magnesium silicate. Some properties of relevance to their scale-up for industrial applications using monoliths manufactured with various commercial titanias and treated at different temperatures were compared. Thus, their textural properties, anatase phase stability and axial strength were evaluated. Taking into account these results, a support was selected for the preparation of platinum catalysts, which were prepared by varying the impregnation time and the platinum solution concentration. Equations were obtained describing the dependence of the catalyst platinum content on each of these parameters. Catalyst activities were tested in the catalytic reduction of nitrogen oxide with propylene in lean-burn conditions.

Phase distribution in titania-sepiolite catalyst supports prepared by different methods

In this study it has been shown that in samples prepared by mechanical mixing of titania and sepiolite, greater titania coverages could be obtained when the mixtures were kneaded in a concentrated acid medium. The distribution of sepiolite and titania at the surface of the mechanical mixtures was studied by the electrophoretic migration technique. The samples were further characterised by nitrogen adsorption, XRD and thermogravimetric techniques. The electrophoretic migration results showed that the addition of sepiolite to titania, kneaded in water, had a dramatic effect on the quantity of titanium oxide present at the support’s surface, both strongly decreasing the molar fraction of titania at the surface, and altering the electrophoretic properties of the mixtures. When kneaded in concentrated acid (HCl, 37%, m/m), the effect of sepiolite addition was greatly reduced. The XRD patterns and BET surface areas of the water-kneaded samples agreed with previously reported results. However, kneading in acid medium induced a slight increase in the BET surface areas of sepiolite and mixtures, but a small decrease for titania. This acid treatment did not alter the XRD patterns of titania and mixtures, compared to the water-kneaded samples. Although the XRD pattern of sepiolite was partially altered, a large part (71%) of sepiolite remained unchanged.

Characterization of alumina:sepiolite monoliths for use as industrial catalyst supports

A series of honeycomb monolithic catalyst supports based on alumina, sepiolite and mixtures of the two were prepared. The textural characterization of these was carried out after heat treatments up to 1473 K, in order to assess the relative merits or drawbacks in the use of sepiolite as a possible admixture to improve the mechanical strengths of the monoliths, whilst preserving high specific surface areas and porosities.

The use of sepiolite in the preparation of titania monoliths for the manufacture of industrial catalysts

Abstract In this work the merits of the use of a natural fibrous mineral, sepiolite, as a binder to produce titania based monoliths of high mechanical strength and abrasion resistance is discussed. The monoliths of square channels were conformed with an initial 7.5 channels cm −2 and 1 mm wall thickness. The textural characterization was made by mercury intrusion porosimetry (MIP), nitrogen adsorption/desorption (BET), and X-ray diffraction (XRD). The mechanical resistance, dimensional changes and weight losses at each stage of heat treatment were also determined. The thermal expansion coefficients (TEC) of the monoliths were determined between 200° and 400°C, since in practice the usual working temperature of DENOX catalysts lies between 250°–350°C.