C.A. Querini

Simultaneous removal of soot and nitrogen oxides from diesel engine exhausts

In this paper, previously reported findings and new results presented here are discussed with the main objective of establishing the reaction mechanism for soot oxidation on different supports and catalysts formulations. Catalysts containing Co, K and/or Ba supported on MgO, La2O3 and CeO2 have been studied for diesel soot catalytic combustion. Among them, K/La2O3 and K/CeO2 showed the best activity and stability for the combustion of soot with oxygen. A reaction mechanism involving the redox sites and the surface-carbonate species takes place on these catalysts. On the other hand, Co,K/La2O3 and Co,K/CeO2 catalysts display activity for the simultaneous removal of soot and nitric oxide. The soot–catalyst contacting phenomenon was also addressed. A synergic La–K effect was observed in which the mechanical mixtures of soot with K–La2O3 showed higher combustion rates than those observed when K and La were directly deposited on the soot surface. The effect of the addition of Ba was explored with the aim of promoting the interaction of the solid with NO2, thus combining the NOx catalytic trap concept with the soot combustion for filter regeneration. Ba/CeO2 and Ba,K/CeO2 were effective in NOx absorption as shown in the microbalance experiments. However, the formation of stable nitrate species inhibits the soot combustion reaction.

Catalytic combustion of diesel soot on Co, K supported catalysts

Catalysts containing 12% Co and 4.5% K, supported on MgO and CeO2 have been studied for diesel soot catalytic combustion. It has been found that this reaction occurs by a redox mechanism when Co and K are deposited on any of the above-mentioned supports. On MgO-supported catalysts, CoOx species are responsible for the supply of oxygen by a redox reaction. In this catalyst, K plays different roles, one of them being the stabilization of the CoOx particles. On CeO2-supported catalysts, Co does not significantly improve the activity of the K/CeO2 catalyst, since in this case the support itself displays redox properties. XPS analyses indicate that the oxygen availability on the surface is much higher on CeO2 than on MgO. On both CeO2 and MgO-supported catalysts, K might provide a route for CO2 release through a carbonate intermediate species. The presence of NO in the gas phase improves the catalytic activity for soot elimination. NO is oxidized to NO2 on the Co, K/CeO2 catalyst, and NO2 is a stronger oxidizing agent than O2, therefore decreasing the temperature needed to burn the soot.

Catalytic combustion of diesel soot particles. Activity and characterization of Co/MgO and Co,K/MgO catalysts

Abstract The catalytic combustion of diesel soot particles was studied on Co/MgO (12 wt% Co) and potassium-promoted Co/MgO (1.5 wt% K) that were calcined at different temperatures in the 300 to 700°C range. Catalyst samples were characterized by various techniques including nitrogen adsorption (BET), temperature programmed reduction (TPR),X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), electron spin resonance (ESR),X-ray photoelectron spectroscopy (XPS) and temperature programmed oxidation (TPO). As observed by TPO experiments, the catalyst activity depends strongly on the calcination temperature: calcination at 300 and 400°C produced samples that were much more active than those calcined at higher temperatures, on which an inactive Mg Co mixed oxide is formed, as suggested by TPR, ESR and XRD results. FTIR shows carbonate species on the surface. Unpromoted samples seem to correlate their activity with the amount of reducible Co species present. Potassium not only increased the sample activity, probably due to the improvement in surface mobility, but also enhanced stability at high temperatures. Experiments with different soot to catalyst ratios showed no significant variation in combustion temperature. TheK-promoted catalyst burns off soot at a temperature lower than the one needed for calcination, thus proving to be a promising catalyst.

A study of platinum supported on tungstated zirconia catalysts

Abstract The reaction of n-butane with hydrogen at 573 K was studied over platinum supported on tungstated zirconia (PtWZ). A higher Pt dispersion increases the selectivity toward hydrogenolysis products and overall activity. The % coke after 90 min on stream at a H 2 /n-C 4 ratio of 3 was found to be less than 0.1 wt.-% in all cases. The presence of Bronsted acid sites upon reduction is confirmed by diffuse reflectance infrared spectroscopy of adsorbed pyridine. The WO 3 /ZrO 2 (WZ) support consists of a mixture of tetragonal and monoclinic zirconia and tetragonal tungsten oxide regardless of the presence of platinum.

The role of Re and S in the PtReSAl2O3 catalyst

Abstract The modifications in catalytic activity, selectivity and stability when a Pt Al 2 O 3 catalyst is promoted by Re and/or S is studied following several C 6 hydrocarbons reforming. A bifunctional reaction scheme, showing modifications from those previously proposed, is introduced in order to interpret the results. Even the catalyst properties of PtRe are not the sum of the ones of Pt and Re, PtRe has properties brought by Pt and by Re. The addition of Re and S modifies the hydrogenolysis-dehydrogenation capacity of Pt. Re decreases the dehydrogenating capacity of Pt, reducing coke formation (desired effect) and increasing the formation of lower molecular weight paraffins by hydrogenolysis (undesired effect). The undesired contribution of Re is partially suppressed by the addition of S, that decreases the hydrogenolytic capacity of PtRe and increases the production of benzene and coke. This opposite action of Re and S on Pt can be related to the opposite electronic transfers, from Re to Pt and from Pt to S. Both geometrical and electronic effects influence the changes in selectivity.

Deactivation of solid acid catalysts during isobutane alkylation with C4 olefins

Abstract Coke formation on solid acid catalysts during isobutane alkylation with C4 olefins was studied. Y-zeolite, mordenite and L-zeolite were investigated, as well as sulfated zirconia catalysts. Zeolites were used in protonic form or after ion exchange with lanthanum nitrate. Studies were carried out in liquid phase in a fixed-bed reactor. It was found that Y-zeolite exchanged with lanthanum, being the catalyst with the best stability, is the catalyst that forms higher quantities of carbonaceous deposits. The amount of coke on this catalyst can be as high as 13–14%. This coke requires temperatures higher than 500°C to be completely eliminated. Temperature-programmed analyses indicated that the amount of coke eliminated from the catalyst during partial regeneration, strongly depends on the heating rate. This is due to a competition between coke gasification and its modification in structure leading to an aromatic type of coke, which then requires higher temperatures to be burnt. Pore and surface area measurements carried out on deactivated catalysts suggest that a pore plugging mechanism takes place during the reaction. Catalysts with too strong acidity (e.g. protonic form of Y-zeolite or sulfated zirconia) have no activity for trimethylpentane production at 80°C, most probably due to a very fast deactivation, even though the amount of coke is notably lower than in the lanthanum-exchanged zeolites.

Ethyl ester production by homogeneous alkaline transesterification: Influence of the catalyst

In this work, the process for ethyl ester production is studied using refined sunflower oil, and NaOH, KOH, CH(3)ONa, and CH(3)OK, as catalysts. In all cases, the reaction is carried out in a single reaction step. The best conversion is obtained when the catalyst is either sodium methoxide or potassium methoxide. We found that during the transesterification with ethanol, soap formation is more important than in the case of methanol. The saponification reaction consumes an important fraction of the catalyst. The amount of catalyst consumed by this reaction is 100% in the case of using hydroxides as catalyst (KOH or NaOH), and 25%, and 28% when using CH(3)ONa and CH(3)OK as catalysts, respectively. Ethanol increases the catalyst solubility in the oil-ethyl ester phase, thus accelerating the saponification reaction. It is possible to obtain high conversions in a one-step reaction, with a total glycerine concentration close to 0.25%.

Influence of PtRe interaction on activity and selectivity of reforming catalysts

The influence of the preparation procedure on the Pt and Re interaction in PtRe/Al2O3 reforming catalysts has been studied. Three different preparation procedures have been used: the classical coimpregnation and successive impregnation techniques, and the recently reported catalytic reduction method. Catalyst activation was done either by direct reduction after metal deposition, or by calcination and reduction. The degree of interaction of the metals was indirectly measured by the cyclopentane hydrogenolysis reaction. It has been found that interaction between Pt and Re increases according to the sequence: coimpregnation (calcined and reduced), catalytic reduction (calcined and reduced), successive impregnations (reduced catalysts) and catalytic reduction (reduced catalysts). Calcination greatly diminishes the PtRe metal-metal interaction, as measured by cyclopentane hydrogenolysis. After sulfiding, the catalysts prepared by catalytic reduction with a calcination and reduction treatment display the highest n-heptane dehydrocyclization activity. Catalysts only reduced also have good activity for this reaction, but with poor stability.

Isobutane/butene alkylation : regeneration of solid acid catalysts

Abstract The regeneration of Y-zeolite catalysts used during the isobutane alkylation reaction is studied. Coke is characterized by temperature-programmed techniques and measuring the H/C ratio. The coke deposited under supercritical conditions is very similar (TPO profile and amount) to the coke deposited in liquid phase. The regeneration was carried out in many ways. Air was used for heating with at a low rate, and holding the temperature at low values for long times. Platinum was incorporated in the zeolite to catalyze the coke combustion or to provide an additional route for coke gasification, such as hydrogenation. Ozone was used to remove most of the coke, followed by a second step with H 2 or He. Hydrogen peroxide was studied as an alternative low-temperature oxidation compound. It was found that the ozone treatment, followed by a hydrogen treatment, is an effective way to regenerate this microporous catalyst. The ozone not only eliminates a large fraction of the coke but also changes the characteristics of the small amount of coke left on the catalyst, making it easier to be burnt. The treatment with hydrogen peroxide at 90°C also removes a large fraction of coke, but without changing significantly the characteristics of the coke left after the treatment.

Biodiesel production by two-stage transesterification with ethanol.

A two-stage process consisting of two reactions steps with glycerin separation and ethanol/catalyst addition in each of them was optimized for ethyl esters production. The optimal reaction temperature was 55 °C. At an ethanol/oil molar ratio of 4.25:1 (25%v/v alcohol with respect to oil), a 99% conversion value was obtained with low ethanol consumption. In contrast to methoxide catalysts, sodium and potassium hydroxide catalysts severely complicate the purification since no phase separation took place under most conditions. With a total sodium methoxide concentration of 1.06 g catalyst/100 g oil, and adding 50% of the catalyst in each reaction step, biodiesel with a total glycerin content of 0.172% was obtained. The optimal conditions found in this study make it possible to use the same industrial facility to produce either methyl or ethyl esters.