P. Avila

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.

Pillared clay and zirconia-based monolithic catalysts for selective catalytic reduction of nitric oxide by methane

Abstract New monolithic catalysts based on zirconia and pillared clays (PILC) have been studied for NOx removal by CH4 in the presence of oxygen. A comparative study of the influence of ZrO2 from various commercial sources for the system Pd–ZrO2 and the effect of the noble metal chosen for the system NM–PILC was carried out, trying to correlate the catalytic activity with the physico-chemical properties of these catalysts. The obtained results indicate that structure and surface acidity of the support plays an important role on the selectivity to NOx reduction, although properties such as the surface area or pore volume could also determine the overall activity of the monolithic catalysts.

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.

Photocatalysis for Continuous Air Purification in Wastewater Treatment Plants: From Lab to Reality

The photocatalytic efficiency of TiO(2)-SiMgO(x) plates to oxidize H(2)S was first evaluated in a flat laboratory reactor with 50 mL min(-1) synthetic air containing 100 ppm H(2)S in the presence of humidity. The use of the photocatalyst-adsorbent hybrid material enhanced the photocatalytic activity in terms of pollutant conversion, selectivity, and catalyst lifetime compared to previous H(2)S tests with pure TiO(2) because total H(2)S elimination was maintained for more than 30 operating hours with SO(2) appearing in the outlet as reaction product only after 18 h. Subsequently, the hybrid material was successfully tested in a photoreactor prototype to treat real polluted air in a wastewater treatment plant. For this purpose, a new tubular photocatalytic reactor that may use solar radiation in combination with artificial radiation was designed; the lamp was turned on when solar UV-A irradiance was below 20 W m(-2), which was observed to be the minimum value to ensure 100% conversion. The efficient distribution of the opaque photocatalyst inside the tubular reactor was achieved by using especially designed star-shaped structures. These structures were employed for the arrangement of groups of eight TiO(2)-SiMgO(x) plates in easy-to-handle channelled units obtaining an adequate flow regime without shading. The prototype continuously removed during one month and under real conditions the H(2)S contained in a 1 L min(-1) air current with a variable inlet concentration in the range of tens of ppmv without release of SO(2).

Influence of the methods of TiO2 incorporation in monolithic catalysts for the photocatalytic destruction of chlorinated hydrocarbons in gas phase

Abstract Three different methods of fixing titanium dioxide on a monolithic, natural magnesium silicate matrix, onto-the-wall extrusion, wash-coating and sol–gel, are compared. Photo-assisted oxidation tests with chlorinated hydrocarbons, trichloroethylene (TCE) alone and in mixtures with perchloroethylene (PCE) were carried out with the monolithic photocatalysts. Results show that the use of extruded titania monoliths provides significant advantages for best stability of anatase, porosity and resistance to loss of active phase due to erosion. Catalysts obtained by the sol–gel method maybe a good option for this application, but the coating method must still be improved.

INFLUENCE OF NITROGEN DIOXIDE ON THE SELECTIVE REDUCTION OF NOX WITH A CATALYST OF COPPER AND NICKEL OXIDES

Abstract The selective reduction of NOx, with ammonia on alumina-supported copper catalysts is shown to be effective when O2 or NO2 are present in the feed. Under steady state conditions, the presence of NO2 in the feed stream increases the overall rate of reduction of NOx and simultaneously reduces its dependence on the oxygen concentration. A maximum in activity is found for a molar inlet ratio NO2/NO ≈ 1. It has also been observed that the stoichiometry of the process is a function of the reaction temperature, with a secondary NH3 oxidation reaction appearing at temperatures above 500 K. As revealed by X-ray photoelectron spectroscopy, the copper remains mainly as Cu+ species at steady state conditions. Such species are reoxidized by NO2 more easily than by O2 or NO. An analysis of the transitional states when the feed composition is changed and infrared spectra of the adsorbed species allows us to propose a mechanism which explains the observed results.