Juan R. González-Velasco

Enhanced activity of zeolites by chemical dealumination for chlorinated VOC abatement

Abstract The objective of this work is to evaluate the dealumination via ammonium hexafluorosilicate treatment as an effective method for enhancing the catalytic performance of H-Y zeolite in chlorinated VOC oxidative destruction. A series of Y zeolites with various Si/Al ratios was prepared from a commercial sample and then tested for the catalytic decomposition of 1,2-dichloroethane as a model reaction. In general, these modified Y zeolites exhibited a higher activity than that of the parent material, the zeolite subjected to 50% dealumination resulting the most active catalyst. This increase in activity was associated with the development of strong acidity due to dealumination. Likewise, 50% dealuminated sample showed an improved catalytic behaviour for the destruction of other typical chlorinated pollutants, namely, dichloromethane (DCM) and trichloroethylene (TCE). The ease of destruction was found to follow this trend: 1,2-dichloroethane > dichloromethane > trichloroethylene.

Contribution of cerium/zirconium mixed oxides to the activity of a new generation of TWC

Abstract The catalytic activity of several cerium/zirconium mixed oxides for TWC applications has been investigated with the aim of optimising the Ce/Zr composition. Their catalytic activity was surprisingly high, considering that no noble metal was present in the composition, particularly for oxidation reactions (CO and C3H6), while their reduction activity was very small (NO). The specific activity of the mixed oxides for CO and C3H6 was found to be the contribution of the individual specific activity of the single oxides. The best activity in practice (lowest T0 and T50 and highest X773) was found for the mixed oxide with a Ce/Zr ratio of 80/20, as a combination of its high specific activity and the highest specific surface area.

Pervaporation of ethanol—water mixtures through poly(1-trimethylsilyl-1-propyne) (PTMSP) membranes☆

The separation of alcohol—water mixtures by pervaporation has been preferentially carried out through hydrophobic membranes because it is more practical to permeate alcohol through the membrane. Nevertheless, most polymeric membranes are water permselective as the molecular size of water is smaller than that of ethanol. Ethanol-permselective membranes have been limited to silicon- and fluorine-containing polymers, for example, polydimethylsiloxane (PDMS), poly(1-trimethylsilyl-1-propyne) (PTMSP) and poly(tetrafluoroethylene) derivatives. PTMSP is known to show preferential permeation of ethanol in the pervaporation of ethanol—water mixtures. The permselectivity of this material is related to the existence of free volume and to the hydrophobicity of the membrane surface. The separation factor (αH2OEtOH) reached a value of 10.7 at 10 wt% of ethanol in the feed, and after 450 h it decreased to an average value of 8. The initial value is similar to that of PDMS, a well-known etanol-permselective membrane. Although PTMSP presented good characteristics for the separation of ethanol—water mixtures, the separation factor and the specific permeation rate (R) decrease with operation time due to a swelling process; however, the latter reached a value similar to the initial one after this period.

Influence of water and hydrocarbon processed in feedstream on the three-way behaviour of platinum-alumina catalysts

Abstract The influence of water and the nature of the hydrocarbon -methane, ethylene, propene and 1-butene- processed in the feedstream on the three-way behaviour of a ceria-modified alumina supported platinum catalyst is investigated. Experiments were carried out with static feedstreams with air-to-fuel ratios between 14.13 and 15.17 as well as with oxidizing-reducing cycled feedstreams simulating the real automobile exhaust mixture. Conclusions are drawn on the importance of water-gas shift and steam reforming reactions and their relationship with inhibition self-poisoning effects of carbon monoxide and/or hydrocarbons. Comparison of data of the prepared catalyst and those obtained at the same operational conditions with two commercial catalysts, with only platinum/alumina and platinum-rhodium/ceria-alumina respectively, allows confirmation of the effects of ceria and establishes the role of rhodium when added to the catalyst.

Kinetic considerations of three-way catalysis in automobile exhaust converters

Abstract The activity of three-way catalysts is highly dependent on the reactants present in the automobile exhaust gases (CO, NO x , HC, O 2 , H 2 O, CO 2 , N 2 ) as well as their relative concentration. Thus, the influence of each reactant on the kinetic behavior of the whole mixture makes difficult to establish the accurate kinetics of the system. Activity experiments carried out close to the real operation conditions (GHSV, concentration, etc.) with a Pt/CeO 2 /Al 2 O 3 catalyst supplied data on the CO and HC oxidation and NO reduction reactions in environments formed by different reactant combinations (from binary mixtures to the whole mixture simulating the real conditions at the automobile converter). The obtained results have shown notable variations in the oxidation/reduction mechanisms depending on the presence (or absence) of components in the environment. The presence of water always promoted the three-way activity of the catalyst. The compensation effect applied to the CO, NO and HC conversions confirmed that kinetic expressions obtained with partial mixtures (not very close to the real converter environment) have only limited application for determining the whole kinetic scheme occurring in the automobile converters.

Synthesis of cordierite monolithic honeycomb by solid state reaction of precursor oxides

The synthesis of cordierite monolithic honeycombs by solid state reaction at high temperature of alumina, kaolin, talc and silica has been studied. The synthesized honeycombs have been characterized by DTA and XRD. The crystal composition of the samples have been observed to be very dependent on temperature and time of reaction. Thermal treatment of the monolithic precursors at 1400°C for 30 minutes allows formation of high purity cordierite with high crystallinity, maintaining the desired monolithic structure of the honeycombs.

Gas-phase catalytic combustion of chlorinated VOC binary mixtures

Abstract The complete oxidation of 1,2-dichloroethane (DCE), dichloromethane (DCM) and trichloroethylene (TCE) as individual chlorohydrocarbons and of their binary mixtures over protonic zeolites (H-ZSM-5, H-MOR and chemically dealuminated H-Y zeolite) has been studied using a conventional fixed bed flow reactor. Its catalytic performance was associated with the presence of strong Bronsted acidity. Both H-MOR and chemically dealuminated H-Y zeolites exhibited the highest destruction activity for the abatement of single and binary mixtures. The destruction of chlorinated mixtures induced an inhibition of the reactivity of each compound leading to a significant increase in the ignition temperature, which varied form one compound to other. On the other hand, a moderate decrease in chlorinated intermediates/by-products formation was noted. Likewise, the presence of hydrogen, either as a part of the additional chlorinated molecule or as a part of water generated as a reaction product, was determined to be important for efficient combustion of the chlororganics to HCl.

Nickel on silica systems. Surface features and their relationship with support, preparation procedure and nickel content

Abstract The characteristics and structure of several silica-supported nickel systems, prepared by impregnation and precipitation-deposition and in a wide range of nickel contents (from 0.58 up to 69 wt% Ni), have been studied. Using mainly conventional sorption techniques the textural properties - BET surface area, pore volume, average pore radius, pore size distribution - and surface chemical distribution - nickel surface area, reduction degree, dispersion, crystallite particle diameter - of the samples have been determined. All these characteristics have been related to each other and globally analyzed to advance in the understanding of the final structure of the catalysts and its relationship with support structure, preparation procedure and nickel content.

Effects of redox thermal treatments and feedstream composition on the activity of Ce/Zr mixed oxides for TWC applications

Abstract The activity of cerium/zirconium mixed oxides for TWC applications submitted to different thermal treatments under oxidising, reducing and redox-cycled atmospheres has been investigated. The effect of each individual reactant in the reaction system has been studied using feedstreams in which some components had been removed and substituted by nitrogen. The conclusions extracted from the individual reactions have been used to describe the role and effect of each reactant when all the components are present in the feedstream, and to explain the differences observed in the catalytic performance after different treatments. Finally, a Ce/Zr mixed oxide composition has been proposed as the best support for TWC applications.

State of the art in catalytic oxidation of chlorinated volatile organic compounds

Chlorine-containing organic compounds (Cl-VOC) require special attention due to their distinct toxicity, high stability and persistence in the environment. Removal of Cl-VOC by catalytic oxidation over a wide variety of catalysts has been presented in literature. This paper reviews the state of the art in this subject, including different model compounds, nature of catalysts, and oxidation activity. Catalyst selectivity (CO2 vs. CO and HCl vs. Cl2), by-products formation and the causes of deactivation are also analyzed as the most important factors in the catalyst selection for practical applications.