Eduardo E. Miró

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.

The nature of cobalt species in Co and PtCoZSM5 used for the SCR of NOx with CH4

Abstract A thorough characterization of CoZSM5 and PtCoZSM5 before and after catalytic use was carried out using a battery of techniques. The bimetallic solid was more selective for N2 production. The TPR profiles showed significant differences. No solid, either fresh or used, exhibited any of the characteristic cobalt oxide X-ray reflections. The XPS data provided information concerning cobalt dispersion. The Raman spectroscopy clearly indicated that Co3O4 species were present only in the monometallic zeolites while a form of highly dispersed CoxOy moieties became dominant in the PtCoZSM5. The diffuse reflectance spectroscopy showed that Co2+ species in the monometallic solids were preferentially located at the main channels while in PtCoZSM5 these cations moved to higher coordination lattice sites. Through the combination of these tools, a much better understanding of the synergetic effect of Pt incorporated to CoZSM5 has been achieved. In view of these findings, related work previously published is revisited.

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.

Effects of NO and solids on the oxidation of methane to formaldehyde

The selective oxidation of methane has been studied both in the presence and absence of solids (inert or catalysts) with and without NO added, at 1 bar of total pressure. NO enhances the yield to formaldehyde, while the solids favor its decomposition. These results, together with abundant literature data, show a maximum for formaldehyde yield of about 4.0%.

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.

Kinetics and mechanism of CO oxidation over Cu mordenite

Cu mordenite (CuM) has proved to be highly active for the oxidation of CO with oxygen. The effect of pretreatment, the kinetics, and the mechanism of the CO + O2 reaction have been studied using a continuous-stirred tank reactor (CSTR), Bennett-type unit, and standard BET system. Redox cycles performed using COO2 showed that the sample was stable and could be reversibly reduced and oxidized many times at temperatures up to 500 °C. The extent of reduction was 0.8 e/Cu. Pretreatment in CO at 750 °C did not affect the reversibility of the redox cycles but produced a larger valence change, 1.8 e/Cu, even at reduction temperatures as low as 300 °C. XRD patterns show the appearance of finely dispersed CuO on the partially destroyed mordenite lattice. This solid, CuM∗, shows different catalytic behavior compared to CuM. The kinetic studies on the latter were performed in the range 200–340 °C. Between 200 and 250 °C the rate function was zero order in CO and close to first order in O2. In the upper temperature range this pressure dependency became first order in CO and zero order in O2. The Arrhenius plot shows a break at 250 °C. At temperatures higher than 250 °C the oxidation reaction on CuM is severely limited by mass transport. On CuM∗ the reaction rate was first order in CO and zero order in O2 over the entire temperature range, 200–325 °C. The reduction and catalytic behavior of CuOSiO2 and CuOγ-Al2O3 were also studied to confirm the important role played by copper oxide produced by the CO pretreatment on CuM∗. The results obtained are analyzed in terms of the reaction mechanisms, and the predominance of individual steps, due to either different pretreatments and/or operating conditions, is assessed.

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.

Continuous-stirred tank reactor (CSTR) transient studies in heterogeneous catalysis: CO oxidation over CuY zeolite

Abstract CuY zeolites develop a high activity to catalyze CO oxidation upon reduction in CO at 750 °C. A continuous-stirred tank reactor (CSTR), Bennett-type unit, operated both in the transient and steady-state regimes was used to determine the rate constant, the adsorption equilibrium constant, and the effective diffusivity under reaction conditions. The reactor, loaded with powdered catalyst, was operated at reaction temperatures between 150 and 300 °C, atmospheric pressure, and usually in an oxydizing atmosphere ( CO O 2 ≤ 2 ). Under these conditions the rate function was found to be first order in CO pressure and zero order in oxygen pressure. The weighted, rather than ordinary moments method, was used for it was shown to give a better estimation of the reaction parameters. The use of step functions was useful to detect an activation process which occurs when the solid is contacted with the reacting mixture. The reaction was shown to be diffusion limited in the temperature range studied. The diffusivity obtained was D = 2.5 × 10 −7 exp(−4.0/ RT ), the adsorption constant K = 8.2 exp(3.9/ RT ) and the surface rate constant k = 1.6 × 10 5 exp(−13.4/ RT ). Energy values are given in kilocalories per mol. These data are analyzed in terms of relevant literature information on related systems.