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.     

Effect of the Incorporation Order of Pt- and Ba-Precursors on the Structure and Catalytic Performance of NSR Catalysts

The structural properties and the performance of two NO _X storage-reduction (NSR) catalysts prepared as a function of the deposition order of Pt- and Ba-precursors was investigated. The catalyst obtained by incorporating first Ba turned out to be more active mainly due to the larger regeneration extent achieved at temperatures below 350 °C. This behaviour was related to the larger external Pt/Ba interface exhibited by this catalyst.     

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.     

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.     

Optimization by lumped control of reactors with langmuir-hinshelwood catalyst deactivation

A method is proposed for solving the problem of temperature optimal control in tubular fixed-bed reactors with reaction systems described by Langmuir-Hinshelwood-Hougen-Watson kinetic equations. The optimization problem is formulated by N state differential equations corresponding to the N differential fixed-bed reactors in which the integral reactor is divided. It is solved using the control vector parameterization computational technique. The proposed method when applied to a simple reaction system reported previously in the literature gives analogous results, and thus validates the theory. This theory is applied to the dehydrogenation of benzyl alcohol to benzaldehyde. An analysis of optimality problem shows a strong influence of the temperature dependence of the ratio of reaction rate to deactivation reaction rate on the optimal policy.     

Catalytic behaviour of thermally aged Ce/Zr mixed oxides for the purification of chlorinated VOC-containing gas streams

In the present paper the thermal deactivation of a series of Ce/Zr mixed oxides (CeO₂, Ce_0.8Zr_0.2O₂, Ce_0.68Zr_0.32O₂, Ce_0.5Zr_0.5O₂, Ce_0.15Zr_0.85O₂ and ZrO₂) was investigated. In order to simulate long-term operation, samples were calcined at three different temperatures, namely 550, 750 and 1000 °C in air for 4 h. Structural, morphological and physico-chemical changes caused by high-temperature treatment were analysed by X-ray diffraction, BET measurements, NH₃-temperature-programmed desorption and temperature-programmed reduction with hydrogen, and the behaviour in the oxidation of chlorinated volatile organic compounds (1,2-dichloroethane and trichloroethylene). The catalytic properties of Ce/Zr mixed oxides could be accounted for on the basis of their promoted redox, as characterised by the percentage of oxygen vacancies, and acidic properties due to the incorporation of zirconium in the ceria lattice. An increase in the calcination temperature led to a progressive decrease in the catalytic activity as a result of the modifications provoked by induced thermal aging (decrease in surface area, larger crystal sizes, reducibility at higher temperatures and loss of acid sites). Ce_0.15Zr_0.85O₂ and Ce_0.5Zr_0.5O₂ showed the best resistance to deactivation with combustion temperatures still notably lower in comparison with the homogeneous reaction even after calcination at 1000 °C. Also slight changes in selectivity were evident resulting in favoured yields of hydrogen chloride, which was environmentally beneficial, and incomplete combustion products such as carbon monoxide and chlorinated intermediates.     

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.     

Catalytic properties of cobalt-promoted Pd/HAP catalyst for CO-cleanup of H2-rich stream

The promoting effect of Co addition on the behaviour of the Pd/HAP system in WGS reaction has been investigated. Hydroxyapatite-supported bimetallic PdCo samples have been prepared by two consecutive impregnation steps with solutions of Co and Pd precursors, respectively. The resulting catalysts have been thoroughly characterised by BET, XRD, H₂-TPR, XPS, OSCC and OSC techniques.The progressive addition of cobalt drives to an abundance of the Pd species exhibiting an intimate contact with Co and an eminent enhancement of the redox properties of the Pd/Co(x)/HAP catalysts. This results in an improvement of their performance in WGS reaction. The Pd species presenting an intimate contact with cobalt seems to be the responsible of the improvement of the CO conversion. However, this PdCo interaction leads to an increase in the methanation activity at the expense of WGS activity. When small amounts of O₂ is added to the WGS mixture, the activity is exclusively enhanced in the presence of Co species exhibiting easier reducibility.     

Performance of zeolites and product selectivity in the gas-phase oxidation of 1,2-dichloroethane

The deep oxidation of 1,2-dichloroethane (DCE) over H-type zeolites (H-Y, H-ZSM-5 and H-MOR) was evaluated. Experiments were performed on conditions of lean chlorocarbon concentration (around 1000 ppmv) under dry and humid conditions, between 200 and 550°C in a conventional fixed-bed reactor. The high density of Brønsted acid sites, proved by temperature-programmed desorption (TPD) of ammonia and diffuse reflectance FT-IR of adsorbed pyridine measurements, make H-ZSM-5 zeolite an effective catalyst for DCE decomposition. Vinyl chloride was identified as an intermediate in 250–400°C range. When vinyl chloride was destroyed at higher temperatures, all the zeolites showed a great selectivity (>90%) to HCl. CO was promoted in quantity reflecting the difficulty of its oxidation over these zeolite catalysts. The activity of the zeolites was reduced in the presence of water vapour (15,000 ppmv). It was noticed that the addition of water to the feed stream did not alter the order of activity observed in the dry experiments. Moreover, the presence of water in the DCE decomposition changed significantly the reaction product distribution. Vinyl chloride formation was found to be significantly lowered over the three zeolites, and selectivity to CO₂ formation was largely enhanced. The X-ray powder diffraction (XRD) analysis of the deactivated samples indicated partial destruction of the zeolite crystal structure during reaction.     

Evaluation of H-type zeolites in the destructive oxidation of chlorinated volatile organic compounds

The deep oxidation of 1,2-dichloroethane (DCE) and trichloroethylene (TCE) over H-type zeolites (H-Y and H-ZSM-5) was evaluated. Experiments were performed at conditions of lean hydrocarbon concentration (around 1000 ppmv) in dry air, between 200 and 550°C in a conventional fixed-bed reactor. H-ZSM-5 zeolite resulted more active than H-Y zeolite in the decomposition of both chlorinated volatile organic compounds. It was established that Brønsted acidity plays an important role in controlling the catalytic behaviour of the H-type zeolites. DCE was completely decomposed at 400°C, whereas TCE required higher temperatures (550°C). Vinyl chloride was identified as an intermediate in the DCE oxidation in the range of 250–400°C. When vinyl chloride is destroyed at higher temperatures, both zeolite catalysts show a high selectivity (>90%) towards HCl formation. Trace amounts of tetrachloroethylene were detected in the TCE oxidation, which peaked at 500°C. CO was promoted in quantity in the destruction of both DCE and TCE reflecting the difficulty of carbon monoxide oxidation over H-type zeolites.