On the Effect of Reduction and Ageing on the TWC Activity of Pt/Ce0.68Zr0.32O2 under Simulated Automotive Exhausts

The three-way activity under complex feed-streams of a series of Pt/Ce_0.68Zr_0.32O₂ catalysts, either fresh or submitted to different ageing treatments, both before and after reduction, has been studied and related to the species present on the catalysts in view of their characteristics and TPR behaviour.     

Screening of Fe–Cu-Zeolites Prepared by Different Methodology for Application in NSR–SCR Combined DeNOx Systems

The NO_x NH₃-SCR performance of several Cu and Fe catalysts supported on BETA and ZSM-5 zeolites has been studied in single SCR and double NSR–SCR configuration, and the activity related to the nature and reducibility of metal species on the catalyst surface. Intermediate ammonia formed in NSR improved greatly NO_x conversion at the exit of the double NSR–SCR configuration, which was practically totally converted to N₂.     

Thermal aging of Pd/Pt/Rh automotive catalysts under a cycled oxidizing–reducing environment

Mono- and multi-metallic (bi- and tri-) Pt, Pd and Rh supported on cerium-promoted alumina (La Roche, SAS-1/16) catalysts were tested for activity as TWC, both fresh [G.C. Koltsakis, and A.M. Stamatelos, Progr. Energy Combust. Sci. 23 (1997) 1] and after accelerated aging. Aging consisted of a treatment at 900°C for 5 h during which an oxidizing (2.5% O₂, 10% H₂O, in N₂) and a reducing (5.0% CO, 10% H₂O, in N₂) feedstream were cycled at 0.017 Hz through the catalyst. Activity tests were carried out by increasing temperature from 100 to 600°C at 3°C min⁻¹, while two oxidizing and reducing (±0.5 A/F) feedstreams were alternately (1 Hz) fed through the reactor at 125 000 h⁻¹ (STP). Conversion was continuously analyzed. Light-off temperature, T₅₀, conversion at 500°C (normal running temperature), X₅₀₀, and the stoichiometric window (A/F from 14.13 to 15.13) for stationary feedstreams, were determined.     

Selectivity of high surface area Ce0.68Zr0.32O2 for the new generation of TWC under environments with different redox character

A number of experiments were carried out with fresh and aged high surface area Ce_0.68Zr_0.32O₂ mixed oxide samples with complex feed-streams in order to determine their performance as three way catalyst (TWC). The results confirmed the oxygen storage capacity (OSC) of these samples, which favour NO conversion in cycled versus stationary stoichiometric operation. Several experiments carried out with different feed-streams have shown that a pre-reducing treatment can significantly improve both NO reduction at low-temperature and selectivity to N₂, which can be very useful to reduce the emission of nitrogen oxides immediately after starting the automobile. Although a significant reduction of the specific surface area of the mixed oxide takes place during ageing at 1173 K, it has been shown that the performance of the sample remains similar or even better than when fresh, due to an increased OSC. Thus, the reduction of CO and hydrocarbon emissions during start could be achieved by situating the catalytic converter nearer the exhaust to the engine, where the catalyst will be heated faster.     

Restenosis and remodeling after percutaneous transluminal carotid angioplasty

INTRODUCTION: The characteristics of restenosis and remodeling after carotid percutaneous transluminal angioplasty (PTA) were badly known. OBJECTIVE: To describe these characteristics in our series of carotid PTA. PATIENTS AND METHODS: A total of 78 cases of PTA for symptomatic > 70% atherosclerotic stenosis of the extracranial internal carotid artery, were selected from our series of PTA if follow-up was > 12 months. All of them were followed with extracranial continuous-wave Doppler. RESULTS: Restenosis of any degree was found in 17 cases (21.79%) and always asymptomatic. A restenosis > or = 70% was found in 5 cases (6.4%). Restenosis was mainly found in cases without residual stenosis nor dissection after PTA (p = 0.002). Restenosis was found in 16 cases (94.11%) in the first 6-months, with no progression thereafter. Remodeling of residual stenosis was frequent (17 cases; 53.11%) and found mainly during the first month after PTA. Its incidence was highest in patients with dissection treated with heparin. In cases with restenosis, remodeling was infrequent, incomplete and occurred after 18-24 months. CONCLUSIONS: 1. Significant restenosis after PTA due to myointimal proliferation, was infrequent. All cases were asymptomatic, under antiplatelet treatment. A new interventional procedure might not be necessary. 2. Complete remodeling was frequently found after 1-month control, mainly in arteries with some residual stenosis and dissection after PTA.     

Selective CO oxidation in H2 streams on CuO/CexZr1−xO2 catalysts: Correlation between activity and low temperature reducibility

Catalysts synthesized by incorporating CuO (7 wt.% of Cu) on six commercial Ce_xZr_1−xO₂ mixed oxides (x = 1, 0.8, 0.68, 0.5, 0.15, 0) have been prepared by conventional wetness impregnation method. These catalysts have been screened for CO oxidation in hydrogen streams (CO-PROX) and characterized by means of XRD, BET, Raman, XPS and H₂-TPR experiments. Activity towards CO oxidation in hydrogen streams has been discussed and correlated with the properties of the catalysts. XRD and Raman analysis of the supports show an increase of oxygen defect as Zr content increase. Below 150 °C the catalysts reducibility measured by H₂-TPR correlates with ceria content in the support, although an increase of Zr content in the support increases considerably the reduction degree of ceria in the 0–600 °C interval. Activity towards CO oxidation in hydrogen streams also correlates with Ce/Cu molar ratio and low temperature reducibility of copper species. Most of the catalysts give complete CO conversion with high selectivity operating with λ = 2. The most active catalysts is CuO supported on pure ceria, which is able to oxidize completely CO in the interval 96–164 °C, with maximum selectivity of 90%. On the other hand, the operation window becomes narrower as Zr content in the supports increases.     

Kinetic considerations of three-way catalysis in automobile exhaust converters

The activity of three-way catalysts is highly dependent on the reactants present in the automobile exhaust gases (CO, NO_x, HC, O₂, H₂O, CO₂, N₂) 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/CeO₂/Al₂O₃ 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.     

Effect of copper loading on copper-ceria catalysts performance in CO selective oxidation for fuel cell applications

Copper–ceria catalysts with three different Cu loadings (1, 7 and 15 wt%) were prepared by incipient wet impregnation, dried at 120 °C and calcined in air at 500 °C. The as-prepared catalysts were characterized by XRD, BET, Diffuse Reflectance Spectroscopy (DRS–UV–visible), Raman spectroscopy, CO and H₂-TPR, CO-TPR, CO-TPD and Oxygen Storage Capacity (OSC) measurements (with CO and O₂ concentration step-changes). The results indicated a good dispersion of copper for catalysts with 1 and 7 wt% Cu; however, bulk CuO was present for catalyst with 15 wt% Cu loading. Catalyst with 7 wt% Cu was observed to have very high capacity to release lattice oxygen to oxidize CO at low temperature. Activity results for CO oxidation in the absence and in the presence of 60% H₂, demonstrated a very similar performance for catalysts with 7 and 15 wt% Cu (both with T₁₀₀ = 112 °C), and much better than that of catalyst loaded with 1 wt% Cu. Catalyst with 7 wt% of copper shows very high activity (100% in a wide temperature window) and selectivity (higher than 85%), which makes an attractive for its use in purification of hydrogen for fuel cell applications. The presence of a mixture of CO₂ and H₂O inhibited catalyst activity, with CuO/CeO₂ catalyst with 7 wt% Cu exhibiting the best performance in the overall reaction temperature range. This could be attributed to the presence of highly disperse copper, only part of it in deep interaction with ceria. The effect of O₂/CO ratio (λ) and the potential reversibility of the inhibitory effect of CO₂ and H₂O were also investigated.     

Effect of process variables on Pt/CeO2 catalyst behaviour for the PROX reaction

A Pt/CeO₂ catalyst has been evaluated for CO oxidation, both in the absence of H₂ and in H₂-rich feedstreams. The catalyst shows high activity and selectivity at temperatures as low as 80 °C, what makes it a viable catalyst for the selective depletion of CO in the temperature range at which PEMFC operate. The effect of: oxygen excess during operation (λ)$(\lambda )$, the presence of either CO₂ (5%), H₂O (5%) or both in the feedstream, and the spatial time, on catalyst activity and selectivity has been evaluated.