M.A. Ulla

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.

Synthesis and characterization of ZSM-5 coatings onto cordierite honeycomb supports

Zeolite ZSM-5 layers (up to ca. 30% by weight) have been synthesized on cordierite substrates, following either a direct hydrothermal synthesis procedure or a secondary growth method, in this case after seeding of the support. The Si/Al ratio in the synthesis gel ranged from 14 to 100, but layers with a high Al content (i.e. a low Si/Al ratio) could not be prepared directly on the cordierite support. However, MFI layers with a low Si/Al ratio were readily grown after depositing an intermediate Si-rich layer. The results also show that the Si/Al ratio of the synthesis gel has a direct effect on the morphology, crystallinity and orientation of the MFI layer formed.

Cobalt-containing catalysts for the high-temperature combustion of methane

Cobalt was supported on ZrO2, La-doped ZrO2 and La2O3 through atomic layer epitaxy (ALE) and wet impregnation. The rate data obtained at 770 K is compared with literature information about cobalt inserted in other matrixes. The ALE technique using ZrO2 and La-doped ZrO2 yielded the best cobalt-containing catalysts. Bulk and surface characterization techniques provided key clues to understand the origin of the large difference in catalytic activity reported for cobalt-containing formulations.

Surface chemistry and catalytic activity of La1 − yMyCoO3 perovskite (M = Sr or Th): 2. Hydrogenation of CO2

Abstract The partial substitution of La III by either Sr II or Th IV in lanthanum cobaltate perovskite affects both the rate of hydrogenation of carbon dioxide and the distribution of products. The reaction of cyclopropane with hydrogen was used as a parallel test reaction in order to ascertain the nature of the active sites developed on these solids upon reduction. The active site density was calculated from the measured amount of chemisorbed hydrogen at 25 °C on the oxides reduced at temperatures between 250 and 500 °C. The catalytic activity was measured in a standard recirculation system using a H 2 : CO 2 = 4:1 ratio, total pressure of 160 Torr, and 280 °C reaction temperature. The total conversion of CO 2 into products (activity) on LaCoO 3 is little affected by the extent of reduction of the sample. More sensitive to this parameter are both the rate of methanation and the production of C + 2 compounds. Reduced La 0.8 Th 0.2 CoO 3 is very stable giving a constant activity, the highest of all the solids assayed and almost exclusively producing methane. La 0.6 Sr 0.4 CoO 3 shows a sharp maximum in activity and high selectivity to methane when prereduced at 300 °C. However, when reduced at increasing temperatures the overall activity sharply drops while the selectivity to higher hydrocarbons increases very rapidly. In all cases the unreduced solids present induction periods, which indicate that the oxide is being reduced in situ by the reacting mixture. In runs designed to measure the extent of deactivation due to coke deposition it was found that the degree of activity decay was inversely correlated with the methanation selectivity. The test reaction was conducted in the same system at H 2 : cyclopropane = 1:1 ratio, P = 170 Torr, and 250 °C. The overall activity and the product distribution toward isomerization, hydrogenation, and hydrogenolysis is very sensitive to both the nature of the solid involved and the extent of reduction. The initial rate of formation of hydrogenation plus hydrogenolysis products when plotted vs extent of reduction produces curves which are similar to those observed in methanation activity. To gain further insight into both the matrix and promoter effect a series of catalysts were prepared containing different combinations of Co, Sr, La, and Th supported on either celite or La 2 O 3 . The matrix effect is most important in the Sr-substituted oxide, less so in LaCoO 3 , and unimportant in the Th-containing perovskite. The promoter effect for C + 2 production follows the order Sr > La ⪢> Th. This and previous studies made on crystalline mixed oxides, together with data available in the literature, allowed us to propose a model to interpret the effect of lanthanum replacement upon the catalytic activity, selectivity, and stability of these solids.

Preparation of highly accessible mordenite coatings on ceramic monoliths at loadings exceeding 50% by weight

A mordenite layer with a high accessibility has been synthesised on cordierite monolith supports; substantial loadings of mordenite were achieved (above 50 wt%) under the synthesis conditions used.

Characterization of hydrogenation active sites on LaCoO3 perovskite

Abstract LaCoO 3 becomes active for hydrogenation of ethene upon reduction in hydrogen at temperatures between 300 and 490 °C. Several aspects of the reacting system were studied in order to ascertain the nature of the active sites generated in this manner. Catalyst deactivation was evaluated by comparing rates between two successive experiments. An upper limit was estimated for the amount of polymeric residues formed after a single run: 1.1 ± 0.5 × 10 14 molecules of C 2 per square centimeter. Reduced LaCoO 3 also catalyzed the self-hydrogenation of ethene. When a mixture of C 2 H 4 :D 2 = 1:1 was reacted over LaCoO 3 reduced to varying extents multiple-exchanged ethenes and ethanes were formed. The exchange patterns were almost unaffected by the extent of reduction. The effect of pretreatment temperatures was also evaluated. The solid in its reduced form was particularly sensitive to high-temperature treatments. The amounts of CO chemisorbed when plotted vs extent of reduction gave curves that were almost identical to the activity plots. The results reported here, discussed in terms of the current literature, are consistent with a model in which finely dispersed Co 0 , formed in the oxide matrix upon reduction, is the locus of hydrogenation activity.

Catalytic diesel soot elimination on Co-K/La2O3 catalysts: Reaction mechanism and the effect of NO addition

Catalysts containing Co and/or K supported on La2O3 have been studied for diesel soot catalytic combustion. While supported Co provides redox sites for the reaction, potassium and the support itself contribute to create additional sites for soot consumption by forming carbonates intermediates. The formation of a perovskite structure after high temperature treatment leads to the lost of activity. The presence of NO in the gas phase improves the catalytic activity for soot elimination. NO is oxidized to NO2 on the catalyst surface, and NO2 is a stronger oxidizing agent than O2, therefore decreasing the temperature needed to burn the soot.

Identification of the species formed in the Fe/MgO system: A Raman and Mössbauer study

A set of samples of Fe/MgO, in the concentration range between 1.5 and 9 wt% of Fe, was prepared by the impregnation method and then calcined at 1073 K. All of them were characterized by XRD and Raman and Mössbauer spectroscopies. The data obtained show that the solids are made up of MgFeO4 nanoclusters and Fe3+ ions dispersed in the MgO matrix. The size of the clusters and the amounts of each phase were calculated using the XRD and Mössbauer data.

Characterization of freeze-dried precursors of La1.85Sr0.15CuO4

Abstract The production of acetate precursors of the La 1.85 Sr 0.15 CuO 4 superconductor by freeze-drying was studied and compared with the solid obtained when the same starting solution was evaporated to dryness. The structural, morphological and thermal changes occurring during processing were followed using a battery of techniques such as X-ray diffraction (XRD), infrared spectroscopy (IR), scanning electron microscopy (SEM), and differential thermal analysis (DTA). It was shown that freeze-drying leads to the production of homogeneous precursors which reflect the molecular dispersion of the starting solution. Evaporated precursors showed the opposite characteristics for they are inhomogeneous, exhibiting severe phase segregation. Several key factors in the thermal decomposition of the precursor to obtain the superconducting oxide were detected, the most important being the following: (i) The mechanism of decomposition of copper acetate is significantly affected by the gas atmosphere. Under vacuum or N 2 , Cu(C 2 H 3 O 2 ) sublimes and copper is lost. (ii) The melting of Cu(C 2 H 3 O 2 ) 2 and its complex decomposition mechanism led to a transient liquid phase during calcination. This emphasizes the need for carefully designing and controlling the heating strategy and atmosphere composition used during the transformation of the precursor in the superconducting oxide.