Ozone Decomposition on α-Fe2O3 Catalyst

The catalytic decomposition of ozone on an α-Fe₂O₃ catalyst has been investigated within the temperature range 23-65°C. A high initial decomposition degree followed by a decrease is observed. It is found that the water vapor in the air-ozone mixture exercises almost no effect on the process. IR-spectroscopy has shown that the nitrogen oxides formed in the ozone generator are the main reason for the deactivation of the catalyst. A scheme of the deactivation process is proposed.     

Effect of nickel incorporation on the acidity and stability of HZSM-5 zeolite in the MTO process

A study has been carried on the effect of Ni incorporation on the acidity (acid strength distribution and total acidity), on the hydrothermal stability of a HZSM-5 zeolite and on the kinetic performance of this catalyst in the MTO process at high temperature (up to 500 °C, in order to increase selectivity to olefins) and when there is a high water content in the feed (in order to minimise deactivation by coke). The incorporation of Ni in the HZSM-5 zeolite implies a decrease in the total acidity and in the acid strength of the zeolite and, consequently, the activity of the catalyst decreases. Nevertheless, the catalysts with Ni are hydrothermally more stable than the parent zeolite catalyst. A Ni content around 1 wt% is the optimum, as it allows for uninterrupted operation in reaction–regeneration cycles, with water contents higher than 50 wt% in the medium, without irreversible deactivation, whereas a higher Ni content produces an unnecessary loss of initial activity.     

Effect of the Porous Structure of a Catalyst on the Rate of Its Deactivation in the Claus Reaction

The effect produced by the content of different size pores and the diameter of a catalyst grain on the deactivation rate has been estimated based on the experimental data on the conversion of H₂S in the Claus reaction. Catalysts with different pore size distributions have been used for analysis. Deactivation has been shown to occur in different fashions for a fine catalyst fraction and catalyst grains. A mathematical model that describes the process of deactivation has been proposed.     

Effects of temperature and feed composition on catalytic dehydration of methanol to dimethyl ether over γ-alumina

Catalytic dehydration of methanol to dimethyl ether (DME) is performed in an adiabatic fixed bed heterogeneous reactor by using acidic γ-alumina. By changing the mean average temperature of the catalyst bed (or operating temperature of the reactor) from 233 up to 303 °C, changes in methanol conversion were monitored. The results showed that the conversion of methanol strongly depended on the reactor operating temperature. Also, conversion of pure methanol and mixture of methanol and water versus time were studied and the effect of water on deactivation of the catalyst was investigated. The results revealed that when pure methanol was used as the process feed, the catalyst deactivation occurred very slowly. But, by adding water to the feed methanol, the deactivation of the γ-alumina was increased very rapidly; so much that, by increasing water content to 20 weight percent by weight, the catalyst lost its activity by about 12.5 folds more than in the process with pure methanol. Finally, a temperature dependent model developed to predict pure methanol conversion to DME correlates reasonably well with experimental data.     

Alkali deactivation of high-dust SCR catalysts used for NOx reduction exposed to flue gas from 100 MW-scale biofuel and peat fired boilers: Influence of flue gas composition

Deactivation of vanadium–titanium deNO_x SCR (selective catalytic reduction) catalysts in high-dust position have been investigated in three 100 MW-scale boilers during biofuel and peat combustion. The deactivation of the catalyst samples has been correlated to the corresponding flue gas composition in the boilers. Too investigate the effect on catalyst deactivation a sulphate-containing additive was sprayed into one of the furnaces. Increased alkali content on the SCR catalyst samples decreased the catalytic deNO_x activity. The study has shown a linear correlation between exposure time in the boilers and alkali concentration (mainly potassium) on the samples. The results imply that mainly alkali in ultra fine particles (<100 nm) in the flue gas increased the alkali accumulation on the catalyst samples. Low correlation was found between particles larger than 100 nm and the catalyst deactivation. It was not possible to decrease the deactivation of the catalyst samples by the sulphate-containing additive. Although the additive had an effect in sulphating potassium chloride to potassium sulphate, it did not decrease the amount of potassium in ultra fine particles or the deactivation of the catalyst samples.     

Deactivation kinetics of a HZSM‐5 zeolite catalyst treated with alkali for the transformation of bio‐ethanol into hydrocarbons

The kinetics of deactivation by coke of a HZSM‐5 zeolite catalyst in the transformation of bioethanol into hydrocarbons has been studied. To attenuate deactivation, the following treatments have been carried out: (i) the zeolite has been subjected to a treatment with alkali to reduce the acid strength of the sites and (ii) it has subsequently been agglomerated into a macro and meso‐porous matrix of bentonite and alumina. The experimental study has been conducted in a fixed bed reactor under the following conditions: temperature, between 300 and 400°C; pressure, 1 atm; space‐time, up to 1.53 (g of catalyst) h (g of ethanol)^?1; particle size of the catalyst, between 0.3 and 0.6 mm; feed flowrate, 0.16 cm³ min^?1 of ethanol+water and 30 cm³ (NC) min^?1 of N₂; water content in the feed, up to 75 wt %; time on stream, up to 31 h. The expression for deactivation kinetics is dependent on the concentration of hydrocarbons and water in the reaction medium (which attenuates the deactivation) and, together with the kinetics at zero time on stream, allows the calculation of the evolution with time on stream of the yields and distribution of products (ethylene, propylene and butenes, C₁‐C₃ paraffins, and C₄‐C₁₂). By increasing the temperature in the 300–400°C range the role of ethylene on coke deposition is more significant than that of the other hydrocarbons (propylene, butenes and C₄‐C₁₂), which contribute to a greater extent to the formation of coke at 300°C. © 2011 American Institute of Chemical Engineers AIChE J, 58: 526–537, 2012.     

Effect of hydrogen sulphide on the deactivation of alumina-supported ruthenium methanation catalysts

The deactivation of a 0.5% ruthenium-alumina catalyst for the methanation reaction has been studied at 250 and 400 °C in the absence and presence of small concentrations of hydrogen sulphide (H₂S). The adequacy of a suggested mechanistic model for the methanation reaction has been confirmed and the model has been applied to the deactivation in the absence of H₂S, which is significantly improved with respect to an empirical methanation rate equation. The effect of the presence of 10 mg dm^?3 H₂S in the reactant stream on the catalyst deactivation rate has also been investigated.     

The study on the selective oxidation of H<Subscript>2</Subscript>S over the mixture zeolite NaX-WO<Subscript>3</Subscript> catalysts

At temperatures lower than 250 °C the deactivation of zeolite NaX catalyst occurred in the presence of water vapor. The gradual accumulation of water vapor on the surface of catalyst could cause deactivation of catalyst. The zeolite NaX-WO₃ catalysts were prepared to study a method preventing deactivation of catalysts from the adsorption of water vapor. The zeolite NaX-WO₃ (9 : 1) with a low content of WO₃ showed the highest conversion of H₂S. It is believed that the addition of WO₃ caused either a decrease of the strong adsorption of water vapor on the zeolite NaX or an increase of the reducibility of WO₃ by some interactions between zeolite NaX and WO₃. This paper is dedicated to Professor Hyun-Ku Rhee on the occasion of his retirement from Seoul National University.     

Water tolerance of DeNOx SCR catalysts using hydrocarbons: Findings, improvements and challenges

The recent developments on the effect of H₂O on deNO_x performance of a variety of SCR catalysts selectively removing NO_x by hydrocarbons in excess oxygen have been reviewed. In particular, the water tolerance of the catalyst is summarized to illustrate a common deactivation behavior of SCR catalyst for the reduction of NO by hydrocarbons under wet feed gas mixture. Earlier proposals elucidating the possible cause of the catalyst deactivation under wet conditions are discussed, focusing mainly on the catalyst characteristics. A promising way, which can improve the water tolerance and the hydrothermal stability of zeolite-based SCR catalyst, is also described.     

Effect of Temperature in Hydrocracking of Light Cycle Oil on a Noble Metal‐Supported Catalyst for Fuel Production

The effect of temperature has been studied in hydrocracking of light cycle oil (LCO), byproduct of fluidized catalytic cracking (FCC) units on a bifunctional catalyst (Pt‐Pd/HY zeolite). The increase in both temperature and H₂ partial pressure have an important attenuating effect on catalyst deactivation, given that they decrease sulfur equilibrium adsorption and enhance hydrocracking of coke precursors. Therefore, the catalyst maintains significant hydrodesulfurization and hydrocracking activity. As the temperature is increased, hydrocracking conversion and naphtha selectivity increase, although there is no significant dearomatization of the medium distillate fraction in the range of the studied experimental conditions. 400 °C is the more suitable temperature for obtaining a high yield of naphtha with a high content of i‐paraffins.     

Hydrothermal stability of HZSM-5 catalysts modified with Ni for the transformation of bioethanol into hydrocarbons

The hydrothermal stability of catalysts prepared from HZSM-5 zeolites doped with Ni (by impregnation) has been studied in the transformation of bioethanol into hydrocarbons, in order to remove the main barrier for the use of HZSM-5 zeolite catalysts in this process, which is the irreversible deactivation by dealumination of the zeolite above 400 °C with water in the reaction medium. The main effect of doping is the attenuation of the zeolite acid strength from 135 to 125 kJ (mol of NH₃)⁻¹ for a Ni content of 1 wt.%. The catalysts maintain a high level of activity and a high selectivity of propene and butenes, and Ni doping significantly attenuates irreversible deactivation of the catalyst by dealumination of the zeolite. The zeolite catalyst doped with 1 wt.% of Ni maintains its kinetic behaviour in reaction-regeneration cycles when the reaction step is carried out at 500 °C and with 5 wt.% of water in the feed. This catalyst allows operating at 400 °C without irreversible deactivation with bioethanol containing 75 wt.% of water.     

Elaboration of an Accelerated Oven CNG Heavy Duty Vehicles Catalyst Ageing for Road Ageing Simulation

The study of automotive catalyst deactivation is associated with technical, economical and environmental problems. The deactivation phenomenon can be divided into three main groups: thermal, chemical and mechanical. Both fresh and road aged commercial metallic honeycombs were used to determine an ageing impact on catalytic activity. Accelerated catalyst ageing procedures were performed to simulate thermal ageing process observed during the real conditions (30,000 km of mileage). Different water quantities and times ageing were used to simulate accelerated ageing process. SEM, DRX, N₂ adsorption and TPR analyses were carried out to explain catalyst deactivation.     

Developing a method for increasing the service life of a higher paraffin dehydrogenation catalyst,based on the nonstationary kinetic model of a reactor

The service life of an industrial catalyst can be prolonged by improving the technological conditions of its operation. This allows us to maximally eliminate the catalyst deactivation factors. A specific feature of the catalytic dehydrogenation of hydrocarbons is its nonstationarity produced by the deactivation of catalysts. The results of modeling the industrial catalytic process of C₉-C₁₄ paraffin dehydrogenation—the key stage in the production of linear alkylbenzenes—is discussed in this paper. We consider (1) thermodynamic analysis of reactions by means of quantum chemistry, (2) estimation of the kinetic model’s parameters by solving the inverse kinetic problem, (3) selection of an equation that describes the coke deactivation of a catalyst, and (4) development of a method for increasing the service life of a dehydrogenation catalyst using a nonstationary model based on the quantitative consideration of the water added to a reactor within a temperature range of 470–490°C. The higher alkane dehydrogenation flowsheet proposed on the basis of these models allows us to predict the operation of a reactor in different water supply regimes. It is shown that the service life of a catalyst grows by 20–30% on the average, if water is fed by increasing portions.     

气相有机污染物的光催化氧化

多相光催化氧化是日益引起重视的污染治理新技术。本文对近年来国外有关气相有机污染物的光催化氧化过程的研究结果进行了总结，在此基础上，讨论了该过程所涉及的反应机理、动力学、催化剂失活、水蒸气的作用、催化剂制备及光催化反应器设计等问题。     

Deactivation of a Pd/Al2O3 catalyst used in hydrodechlorination reactions: Influence of the nature of organochlorinated compound and hydrogen chloride

The deactivation of a 0.5 wt.% Pd on alumina catalyst used for the hydrodechlorination of tetrachloroethylene (TTCE), chlorobenzene (CBZ) and dichloromethane (DCM) in organic matrix in gas phase has been studied in this work. Experiments were carried out in a continuous fixed bed reactor, at a pressure range of 0.1–2 MPa. It was found that the reactivity and stability of the catalyst increases as pressure increases. Different compounds present quite different deactivation patterns: fast initial deactivation followed by a plateau for DCM and CBZ, continuous moderate deactivation for TTCE.The effect of hydrogen chloride was studied by adding to the reactor feed different amounts of hydrogen chloride, working at 225–300 °C.Fresh and used catalysts were characterised by nitrogen porosimetry, X-ray diffraction, transmission electronic microscopy, scanning electronic microscopy, thermogravimetry and temperature-programmed oxidation. Results indicate that carbonaceous deposits play a key role in the catalyst deactivation, being this phenomenon promoted by the presence of HCl.     

The Impact of Water on CO Oxidation with Au/TiO<Subscript>2</Subscript> Catalysts: Poison or Promotor? A Study with an Au–TiO<Subscript>2</Subscript>/MCM-48 Model Catalyst

Abstract  

CO oxidation was studied with a model catalyst containing Au and TiO _x nanoaggregates confined in a siliceous MCM-48 host. With this material, which has a particular small ratio between the TiO _x and Au components, activities well comparable to those of unconfined Au/TiO₂ catalysts were obtained in particular when a thermal activation in inert gas at temperatures between 523 and 673 K was applied. When the subsequent catalytic tests were performed in a feed containing ca. 20 ppm H₂O, strong deactivation phenomena were observed which could be reverted by repeated thermal treatment and are most likely caused by carbonate deposition. This deactivation was strongly attenuated when the water content of the feed was decreased to ca. 6 ppm, which suggests that water plays an important role in the formation of the poisoning species. With unconfined Au/TiO₂ catalysts, a promoting role of water on the formation of catalyst poison was observed as well, but to a much lower extent. The effect may therefore have escaped undetected so far as a contribution to the well-known catalyst deactivation by carbonate species.     

Deactivation and regeneration of hybrid catalysts in the single-step synthesis of dimethyl ether from syngas and CO2

Synthesis of dimethyl ether (DME) has been studied in a single reaction step, from H₂ + CO and H₂ + CO₂, in a fixed bed reactor on CuO-ZnO-Al₂O₃/γ-Al₂O₃ and CuO-ZnO-Al₂O₃/NaHZSM-5 hybrid catalysts. It has been proven that water content in the reaction medium (which is higher when CO₂ is fed) contributes to efficiently decreasing deactivation by coke in both catalysts and, consequently, when water is in the feed deactivation is insignificant for 30 h reaction. Nevertheless, water also decreases the activity of γ-Al₂O₃ acid function, due to its high adsorption capacity on the acid sites. Due to its importance in the viability of the industrial process, a study has been carried on the regeneration of both catalysts by coke combustion under controlled conditions (in order to avoid CuO sintering). For this study, the catalysts have been used under severe deactivation conditions. It has been proven that γ-Al₂O₃ does not have a suitable hydrothermal stability and that CuO-ZnO-Al₂O₃/NaHZSM-5 catalyst has an excellent performance and is suitable for using it in uninterrupted reaction–regeneration cycles.     

Slurry-Phase Fischer–Tropsch Synthesis Using Co/γ-Al2O3, Co/SiO2 and Co/TiO2: Effect of Support on Catalyst Aggregation

The catalytic performance of Co/γ-Al₂O₃, Co/SiO₂ and Co/TiO₂ catalysts has been investigated in a slurry-phase Fischer–Tropsch Synthesis (FTS). Although Co/SiO₂ catalyst shows higher CO conversion than the other catalysts, the intrinsic activity is much higher on Co/TiO₂ due to large pore size and low deactivation of large cobalt particles by reoxidation mechanism. Co/γ-Al₂O₃ catalyst confirms low formation rate of oxygenates and C₅₊ selectivity because of deactivation of catalyst due to catalyst aggregation and reoxidation by the in situ generated water during the FTS reaction. Long-chain hydrocarbons such as wax formed during FTS reaction generally contains water and trace amount of oxygenate which are conducive to the formation of a macro-emulsion of wax products. Formation of such macro-emulsion on the catalyst suggests that the presence of proper amount of alcohol content derived FTS reaction on large pore of catalyst inhibits the catalyst aggregation. The intrinsic activity (turn-over frequency; TOF) of cobalt-based catalysts, in a slurry-phase FTS reaction, is affected by the average pore size of catalyst, cobalt particle size, degree of reduction of cobalt species and possible reoxidation by in situ generated water.     

Selective kinetic deactivation model for methanol synthesis from simultaneous reaction of CO2 and CO with H2 on a commercial copper/zinc oxide catalyst

A kinetic model for the deactivation of copper/zinc oxide catalyst during the methanol synthesis has been developed. This model is of the Langmuir-Hinshelwood-Hougen-Watson type and considers two types of active sites for the deactivation of catalyst. One of the site types on copper is allocated for the deactivation of the catalyst due to carbon dioxide while another type is assigned for the deactivation of the catalyst due to carbon monoxide. The parameters of the deactivation rate equations based on the above concept have been determined using the experimental data of Hoffmann (1993). The validity of the deactivation model has been checked by comparing the results predicted by the model with experimental data different than of those used to evaluate the parameters of the model. The good agreement that noticed in this comparison confirmed the idea that CO and CO₂ are responsible at different extent for the deactivation of Cu/ZnO catalyst during methanol synthesis.     

Modeling of Pt-Sn/γ-Al2O3 deactivation in propane dehydrogenation with oxygenated additives

A reduction in catalyst activity with time-on-stream and formation of side products are the major problems associated with catalytic propane dehydrogenation. Coke formation on the catalyst surface is the most important cause for catalyst deactivation. Experiments have indicated that the presence of very small amounts of oxygenated additives such as water can reduce the amount of coke accumulated on the catalyst surface and enhance catalyst activity. Addition of water beyond an optimum level, however, would result in a loss of activity due to sintering of catalyst. Propane dehydrogenation over a Pt-Sn/γ-Al₂O₃ catalyst in the temperature range of 575 to 620 °C was investigated in the presence of small amounts of water added to the feed. A monolayer-multilayer mechanism was used to model the coke growth kinetics. Coke deposition and catalyst sintering were considered in a catalyst deactivation model to explain the observed optimum level in the amounts of water added to the feed. The model predictions for both propane conversion and coke formation with time-on-stream were in good agreement with experimental data.