M.A. Gutiérrez-Ortiz

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.

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.

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.

Preparation, activity and durability of promoted platinum catalysts for automotive exhaust control

Abstract The effects of the addition of calcia, ceria and lanthana to alumina-supported platinum catalysts on the simultaneous control of hydrocarbon, carbon monoxide and nitrogen oxide automobile emissions (three-way catalyst behaviour) were analyzed. The activity of the prepared samples was determined with steady-state, reducing and oxidizing, simulated feedstreams as well as with a cycled oxidizing-reducing feedstream averaged at the stoichiometric conditions which resembled the exhaust air/fuel fluctuations in a closed-loop emission control system. Activity of the catalysts was also analyzed after conducting accelerated thermal and chemical ageing in order to test their durability. Under normal operating conditions of the automobile engine, Pt/Al2O3 catalysts promoted by rare-earth oxides are able to achieve high HC, CO and NO conversions. The behaviour of the catalysts in the cold start period was determined by analysis of light-off temperatures and a comparison was made with those corresponding to some commercial samples and others reported in the literature. The catalysts prepared in this work showed lower light-off temperatures than those of commercial and reported Pt/Al2O3 catalysts but these temperatures were not so low as with PtRh/Al2O3. In all cases, the prepared catalysts resulted in a better resistance to accelerated ageing. Samples with ceria showed the best resistance to accelerated ageing.

Role of water and other H-rich additives in the catalytic combustion of 1,2-dichloroethane and trichloroethylene.

In several practical applications gas streams containing chlorinated volatile organic compounds with variable chemical nature (namely, 1,2-dichloroethane and trichloroethylene) and a significant moisture content (15000ppm) must be addressed. In this paper the control of such emissions by catalytic oxidation over Ce/Zr mixed oxides was analysed. Results in terms of activity and selectivity were compared with those obtained when other H-rich additives (1000ppm), such as hexane or toluene, were fed. High activity was found from mixed oxides featuring a suitable combination of a large population of acid sites, easily accessible oxygen species, and hydrophobic nature attributable to cerium content. The presence of additional H-rich compounds in the feed stream (water, toluene or hexane) tended to decrease the catalytic activity due to the blockage and/or competition for actives sites. However, the increased presence of hydrogen atoms in the stream notably promoted the selectivity to hydrogen chloride instead of molecular chlorine.

Optimal inlet temperature trajectories for adiabatic packed reactors with catalyst decay

Abstract An algorithm for simulation and optimization of adiabatic fixed-bed reactors subject to catalyst deactivation is developed. It is applied to the low-temperature CO converter, the kinetic model for which is deduced from some industrial data and considering chlorine as the main cause of catalyst poisoning. Sintering is not considered due to the short range of variation of temperature in the catalyst bed. Optimal inlet temperature trajectories are determined to maximize the obtained total amount of reaction over a fixed operation time. Approximate optimal trajectories in discrete steps of temperature are proposed to be followed in industrial practice.

Deep catalytic oxidation of chlorinated VOC mixtures from groundwater stripping emissions

The total oxidation chlorinated VOCs mixtures, with composition similar to those expected from groundwater air strippers, over Pd/Al2O3 and Pt/A2O3 catalyst at temperatures from 200 to 550°C was investigated. 1,2-Dichloroethane showed greater oxidability, followed by dichloromethane and 1,1- dichioroethylene and trichloroethylene. The addition of hydrocarbons to a chlorinated feed produced changes in the reactivity order of the chlorinated VOCs, indicating the existence of a complex reaction system due to the energetically non-uniform catalytic surface.