Three-way catalysts: past, present and future

In this article, the past and the state-of-the-art in Three-Way Catalyst (TWC) technology are reviewed. The main chemical reactions occurring in a gasoline engine are discussed and also the main reactions taking place in a TWC placed in the tailpipe, namely CO and hydrocarbons oxidation and nitrogen oxides reduction to molecular nitrogen. The main components of a TWC (substrates, noble metals and cerium oxides) and their role in the different chemical reactions occurring in a TWC are described. Finally, the problem of diesel vehicles gas aftertratment is described, and the current state-of-the art in catalytic converters for these vehicles are commented.

Synthetic scope and DFT analysis of the chiral binap-gold (I) complex-catalyzed 1,3-dipolar cycloaddition of azlactones with alkenes

The 1,3-dipolar cycloaddition between glycine-derived azlactones with maleimides is efficiently catalyzed by the dimeric chiral complex [(Sa)-Binap·AuTFA]2. The alanine-derived oxazolone only reacts with tert-butyl acrylate giving anomalous regiochemistry, which is explained and supported by Natural Resonance Theory and Nucleus Independent Chemical Shifts calculations. The origin of the high enantiodiscrimination observed with maleimides and tert-butyl acrylate is analyzed using DFT computed at M06/Lanl2dz//ONIOM(b3lyp/Lanl2dz:UFF) level. Several applications of these cycloadducts in the synthesis of new proline derivatives with a 2,5-trans-arrangement and in the preparation of complex fused polycyclic molecules are described.

Relevance of porosity and surface chemistry of superactivated carbons in capacitors

We show, through some examples, that chemical activation by alkaline hydroxides permits the preparation of activated carbons with tailored pore volume, pore size distribution, pore structure and surface chemistry, which are useful for their application as electrodes in supercapacitors. Examples are presented discussing the importance of each of these properties on the double layer capacitance, on the kinetics of the electric double-layer charge-discharge process and on the pseudo-capacitative contribution from the surface functional groups or the addition of a conducting polymer.

Domino 1,3-Dipolar Cycloadditions of N-Alkyl-α-Amino Esters with Paraformaldehyde: A Direct Access to α-Hydroxymethyl α-Amino Acids

N-Alkyl-α-amino esters undergo a domino reaction, based on the iminium cation generation, with paraformaldehyde, followed by a 1,3-dipolar cycloaddition of the stabilized azometh­ine ylide with another equivalent of formaldehyde. The resulting products are oxazolidines, which can be transformed after hydrolysis into α-hydroxymethyl α-amino acid or its derivatives. The diastereoselective 1,3-dipolar cycloaddition was performed using sarcosine (–)-menthyl or (–)-8-phenylmenthyl esters affording the cyclic product with moderate enantiomeric ratio.

Valoración del estado nutricional y sus diferencias en mujeres mayores de 60 años con y sin práctica de ejercicio

El estado nutricional en los mayores es un factor que se asocia fuertemente a su morbilidad y calidad de vida. Profundizar en los factores que influyen en la malnutrición, esencialmente en los tratables o modificables, permitiría plantear medidas preventivas orientadas a mejorar el estado nutricional y su calidad de vida. La práctica de ejercicio en adultos mayores se ha generalizado como la llave para un envejecimiento activo y saludable, demostrando su influencia en la prevención de problemas de autonomía e independencia en dicho grupo poblacional, sin embargo, no se ha particularizado suficientemente su relación con la presencia de riesgo de malnutrición en mayores, ya sea por exceso o defecto, ni tampoco en el grupo de mujeres mayores de 60 años. Existen resultados de estudios serios que establecen la alta prevalencia de malnutrición en adultos mayores, sin embargo, son muy pocos los estudios enfocados a la situación nutricional de la mujer mayor de 60 años. El presente estudio tuvo como objetivos el valorar el estado nutricional en mujeres mayores de 60 años según diferencias con la práctica de ejercicio físico; comparar el estado nutricional entre grupos con y sin práctica de ejercicio físico; identificar factores relacionados con la presencia de riesgo de malnutrición e identificar factores de riesgo de malnutrición dicho grupo etario. Es un estudio transversal, comparativo realizado dos grupos; uno formado por mujeres mayores de 60 años que practican ejercicio sistematizado, integrantes del Club de “La edad de oro” del Gimnasio Multidisciplinario del Centro Universitario Tampico-Madero(CUTM) de la UAT y otro grupo control que no practica ejercicio sistematizado y que acuden a la Clínica de Medicina Familiar del Instituto de Seguridad y Servicio Social de los trabajadores del estado (ISSSTE), en una muestra total de 418 mujeres estudiadas. Se incluyeron como variables; la evaluación del estado nutricional según la Escala “Mini Nutritional Assessment” (MNA), la detección de factores asociados: psicosociales, sociodemográfico, económicos y fisiológicos, por medio de la Encuesta de antropometría y estado actual: (EAEA) y la Encuesta de aspectos demográficos, de salud y físicos (EFA). Resultó de 418 mujeres mayores de 60 años, 208 pertenecientes al grupo sin ejercicio y 210 al grupo con ejercicio. Con una media de edad en el total de mujeres de 67.0 (SD4.7) años, con un rango mínimo de 60 años y máximo de 81 años. El 58.4% tiene estudios de nivel Bachillerato (medio superior) a nivel superior, el 53.6% tiene de 4 hijos a más y el 67.9% tiene un ingreso económico mensual igual o superior a $4001.00 pesos mexicanos ($246.64 euros). Según grupo de estudio, se encuentran diferencias en la edad, educación e ingreso económico, siendo el grupo que no hace ejercicio las de mayor edad, mayor nivel de estudios y mayores ingresos económicos (p< 0.05). La enfermedad actual con mayor porcentaje es la hipertensión (33%), seguida por diabetes (21%) y por ambas enfermedades (14%). Según grupo de estudio, se encuentra diferencias en la presencia de la diabetes, que es de mayor porcentaje en el grupo de mujeres que hace ejercicio (29% vs 13%). En relación a los resultados del MNA el 57.4% se encuentra en riesgo de malnutrición, encontrando diferencias estadísticas entre grupos de estudio, siendo el grupo que no hace ejercicio el de mayor riesgo nutricional (71% vs 44%). De acuerdo a niveles establecidos por la OMS, para el Índice de masa corporal (IMC), el 81.1% del total está ubicado en niveles de sobrepeso y obesidad y el 18.7% en nivel normal. Se realizó el análisis de regresión logística por separado: grupo de mujeres que hacen ejercicio y las que no hacen ejercicio. Los factores que resultaron influyentes para problemas nutricionales fueron igual para ambos grupos: los problemas psicológicos y la pérdida de peso reciente; al igual que para el grupo total, la no glicemia se comporta como factor protector para ambos grupos. En el grupo que hace ejercicio se identificó como factor influyente la pérdida del apetito, a diferencia del grupo que no hace ejercicio donde se identificó la presencia de hipertensión como factor influyente en el deterioro nutricional. Se concluye que la presencia de cifras altas de glucosa en sangre y problemas psicológicos o depresión leve son factores de riesgo a problemas nutricionales, independientes a realizar ejercicio o no. La presencia de anorexia vs no tenerla y la pérdida de peso reciente son factores de riesgo de malnutrición independientemente de hacer ejercicio o no hacerlo. Presentar hipertensión en el grupo que no hace ejercicio es un factor de riesgo de malnutrición. La práctica de ejercicio es un factor protector de malnutrición, incluso con enfermedad crónica actual, para la población estudiada.

Asymmetric hybrid capacitors based on activated carbon and activated carbon fibre–PANI electrodes

Composites consisting of polyaniline (PANI) coatings inside the microporosity of an activated carbon fibre (ACF) were prepared by electrochemical and chemical methods. Electrochemical characterization of both composites points out that the electrodes with polyaniline show a higher capacitance than the pristine porous carbon electrode. These materials have been used to develop an asymmetric capacitor based on activated carbon (AC) as negative electrode and an ACF–PANI composite as positive electrode in H2SO4 solution as electrolyte. The presence of a thin layer of polyaniline inside the porosity of the activated carbon fibres avoids the oxidation of the carbon material and the oxygen evolution reaction is produced at more positive potentials. This capacitor was tested in a maximum cell voltage of 1.6 V and exhibited high energy densities, calculated for the unpackaged active materials, with values of 20 W h kg−1 and power densities of 2.1 kW kg−1 with excellent cycle lifetime (90% during the first 1000 cycles) and high coulombic efficiency.

Tailoring the porosity of chemically activated hydrothermal carbons: Influence of the precursor and hydrothermal carbonization temperature

Advanced porous materials with tailored porosity (extremely high development of microporosity together with a narrow micropore size distribution (MPSD)) are required in energy and environmental related applications. Lignocellulosic biomass derived HTC carbons are good precursors for the synthesis of activated carbons (ACs) via KOH chemical activation. However, more research is needed in order to tailor the microporosity for those specific applications. In the present work, the influence of the precursor and HTC temperature on the porous properties of the resulting ACs is analyzed, remarking that, regardless of the precursor, highly microporous ACs could be generated. The HTC temperature was found to be an extremely influential parameter affecting the porosity development and the MPSD of the ACs. Tuning of the MPSD of the ACs was achieved by modification of the HTC temperature. Promising preliminary results in gas storage (i.e. CO2 capture and high pressure CH4 storage) were obtained with these materials, showing the effectiveness of this synthesis strategy in converting a low value lignocellulosic biomass into a functional carbon material with high performance in gas storage applications.

Total oxidation of naphthalene using palladium nanoparticles supported on BETA, ZSM-5, SAPO-5 and alumina powders

A range of catalysts based on Pd nanoparticles supported on inorganic supports such as BETA and ZSM-5 zeolites, a silicoaluminophosphate molecular sieve (SAPO-5) and γ-alumina as a standard support have been tested for the total oxidation of naphthalene (100 ppm, total flow 50 ml/min) showing a conversion to carbon dioxide of 100% between 165 and 180 °C for all the analysed catalysts. From the combined use of zeolites with PVP polymer protected Pd based nanoparticles, enhanced properties have been found for the total abatement of naphthalene in contrast with other kinds of catalysts. A Pd/BETA catalyst has been demonstrated to have excellent activity, with a high degree of stability, as shown by time on line experiments maintaining 100% conversion to CO2 during the 48 h tested.

Abatement of hydrocarbons by acid ZSM-5 and BETA zeolites under cold-start conditions

Simulated cold-start tests have been carried out to evaluate the performance of H-ZSM-5 and H-BETA zeolites as hydrocarbon traps under simulated gasoline car exhaust gases, paying special attention to the effect of water on their behaviour. It is concluded that the hydrothermal treatment of the zeolites in the acidic form contributes to the better performance of these materials as hydrocarbon traps since the stabilization of the zeolites takes place. Moreover, the decrease of the surface acidity of the zeolites results in an increase of the Si/Al ratio, which contributes to the decrease of the water affinity for adsorption sites. Thus, the competition with hydrocarbon molecules in the exhaust for the adsorption sites is reduced which increases their trap efficiency. The stabilized H-ZSM-5 is the zeolite that showed the best performance with a propene offset temperature of 240 °C, which should be high enough for the three-way catalyst to carry out its role as catalytic converter.

Total oxidation of naphthalene at low temperatures using palladium nanoparticles supported on inorganic oxide-coated cordierite honeycomb monoliths

A study on the preparation of thin films of ZSM-5 and BETA zeolites, and a SAPO-5 silicoaluminophosphate, supported on cordierite honeycomb monoliths by in situ synthesis was carried out for their use as catalyst supports. Furthermore γ-Al2O3 was also coated onto a cordierite honeycomb monolith by a dip-coating method for use as a standard support. Structured monolithic catalysts were prepared by impregnation of the aforementioned coated monoliths with polymer-protected Pd nanoparticles. The monolithic catalysts have been tested for the total oxidation of naphthalene (100 ppm, GHSV 1220 h−1). From the combined use of the zeolite with polymer-protected nanoparticles, enhanced catalytic properties have been found for the total abatement of naphthalene. The Pd/MBETA and Pd/MZSM-5 catalytic monoliths have shown excellent activity with a high degree of stability, even after undergoing accelerated ageing experiments.

CuH-ZSM-5 as Hydrocarbon Trap under Cold Start Conditions

Cold start tests are carried out to evaluate the performance of copper-exchanged zeolites as hydrocarbon traps under simulated gasoline car exhaust gases, paying special attention to the role of copper in the performance of these zeolites. It is concluded that the partial substitution of the protons in the parent H-ZSM-5 zeolite is highly beneficial for hydrocarbon trapping due to the formation of selective adsorption sites with specific affinity for the different exhaust components. However, it is also observed that uncontrolled exchanging process conditions could lead to the presence of CuO nanoparticles in the zeolite surface, which seem to block the pore structure of the zeolite, decreasing the hydrocarbon trap efficiency. Among all the zeolites studied, the results point out that a CuH-ZSM-5 with a partial substitution of extra-framework protons by copper cations and without any detectable surface CuO nanoparticles is the zeolite that showed the best performance under simulated cold start conditions due to both the high stability and the hydrocarbon retaining capacity of this sample during the consecutive cycles.

El sistema de reserva indígena como organización social de tutela estatal. El caso de Canadá

Este trabajo tiene como objetivo examinar el impacto y las consecuencias sociales contemporáneas del mantenimiento del sistema de organización social denominado reserva indígena en Canadá. Mediante un estudio de caso, se explora cómo la política organizacional y la estructura institucional contemporánea en las reservas indígenas canadienses está alimentando problemas psicosociales y de salud mental como, por ejemplo, abuso de sustancias psicoactivas, depresión, negligencia parental, violencia doméstica... Tres características estructurales son identificadas como problemas inherentes en la estructura organizacional de las reservas contemporáneas: el desempleo estructural, la dependencia a programas de asistencia social y el faccionalismo comunitario.

Investigation of Pd nanoparticles supported on zeolites for hydrogen production from formic acid dehydrogenation

Catalysts based on palladium nanoparticles supported on different zeolites (BETA, ZSM-5 and Y) were prepared and their catalytic performance in formic acid dehydrogenation was studied. The effects of the zeolite structure and porous texture on the catalytic activity were investigated by comparing the behavior of these samples. The results revealed that the samples based on BETA zeolite are promising catalysts for this application.

BETA Zeolite Thin Films Supported on Honeycomb Monoliths with Tunable Properties as Hydrocarbon Traps under Cold-Start Conditions

A high percentage of hydrocarbon (HC) emissions from gasoline vehicles occur during the cold-start period. Among the alternatives proposed to reduce these HC emissions, the use of zeolites before the three-way catalyst (TWC) is thought to be very effective. Zeolites are the preferred adsorbents for this application; however, to avoid high pressure drops, supported zeolites are needed. In this work, two coating methods (dip-coating and in situ crystallization) are optimized to prepare BETA zeolite thin films supported on honeycomb monoliths with tunable properties. The important effect of the density of the thin film in the final performance as a HC trap is demonstrated. A highly effective HC trap is prepared showing 100 % toluene retention, accomplishing the desired performance as a HC trap, desorbing propene at temperatures close to 300 °C, and remaining stable after cycling. The use of this material before the TWC is very promising, and works towards achieving the sustainability and environmental protection goals.

Efficient Diastereo- and Enantioselective Synthesis of exo-Nitroprolinates by 1,3-Dipolar Cycloadditions Catalyzed by Chiral Phosphoramidite⋅Silver(I) Complexes

Chiral complexes formed by privileged phosphoramidites and silver triflate or silver benzoate are excellent catalysts for the general 1,3-dipolar cycloaddition between azomethine ylides generated from α-amino acid-derived imino esters and nitroalkenes affording with high dr the exo-cycloadducts 4,5-trans-2,5-cis-4-nitroprolinates in high ee at room temperature. In general, better results are obtained using silver rather than copper(II) complexes. In many cases the exo-cycloadducts can be obtained in enantiomerically pure form just after simple recrystallization. The mechanism and the justification of the experimentally observed stereodiscrimination of the process are supported by DFT calculations. These enantiomerically enriched exo-nitroprolinates can be used as reagents for the synthesis of nitropiperidines, by ester reduction and ring expansion, which are inhibitors of farnesyltransferase.