Fischer–Tropsch synthesis on ceramic monolith-structured catalysts

This paper reports recent research results about the impact of different catalyst bed configurations on Fischer–Tropsch (FT) synthesis product distributions. A powdered CoRe/γ-alumina catalyst with a particle size ranging from 60 to 100 mesh was prepared and tested in a packed bed reactor. The same catalyst was ball milled and coated on a ceramic monolith support structure of channel size about 1 mm. The monolith catalyst module was tested in two different ways, as a whole piece and as well-defined channels. Steady-state reaction conversion was measured at various temperatures under a constant H₂/CO feed ratio of 2 and a reactor pressure of 25 bar. Detailed product analysis was performed. Significant formation of wax was evident with the packed particle bed and with the monolith catalyst that was improperly packed. By contrast, wax formation was not detected in the liquid product by confining the reactions inside the monolith channel. This study presents an important finding about the structured catalyst/reactor system, in that the product distribution highly depends on how the structured reactor is set up. Even if a catalyst is tested under identical reaction conditions (T, P, H₂/CO ratio), hydrodynamics (or flow conditions) inside a structured channel may have a significant impact on the product distribution.     

Monolithic Ru-based catalyst for selective hydrogenation of benzene to cyclohexene

A novel Ru-based cordierite monolithic catalyst was prepared for the selective hydrogenation of benzene to cyclohexene. The catalyst was characterized by elemental analysis, XRD, SEM-EDX and physisorption measurements. The performance of the catalyst was tested in a continuous monolithic fixed bed reactor (MFBR). Compared with particulate catalyst, the monolithic catalyst gave much higher selectivity. Monolithic catalyst with ZrO₂–Al₂O₃ as washcoating was found to be more active than that with Al₂O₃ as washcoating and a high cyclohexene yield of about 30% was achieved at a relatively lower LHSV. The egg-shell distribution of the active component, the large pores in the walls of cordierite monolith and the Taylor flow pattern formed in the monolith channels were considered to be the crucial reasons.     

Active and Stable Ni‐MgO Catalyst Coated on a Metal Monolith for Methane Steam Reforming under Low Steam‐to‐Carbon Ratios

A structured reaction system in the form of an Ni‐MgO catalyst reduced to nanoscale particle size and coated on a metallic monolith proved to be an active and stable system for methane steam reforming under a steam‐to‐carbon ratio of 1.5 and a temperature of 700 °C. The catalyst‐coated monolith exhibited higher stability and much higher CH₄ conversion than the same catalyst in a catalyst particle bed reaction system. The high activity is attributed to the properties of the metal monolith and to the small size of the catalyst particles on the coating, while the stability is ascribed to the NiO‐MgO solid solution formed in the Ni‐MgO catalyst. These results are better than the corresponding ones obtained with a conventional Ni‐Al₂O₃ catalyst reported previously [1] and comparable to the ones presented in the literature, with the advantage of working under a low steam‐to‐carbon ratio.     

Testing of a Ni‐Al2O3 Catalyst for Methane Steam Reforming Using Different Reaction Systems

Ni‐Al₂O₃ catalyst activity was tested for methane steam reforming using two different reaction systems: a catalyst particle bed (0.42–0.5 mm catalyst particles diluted in SiC) with a surface area‐to‐volume ratio SA/V of 910 m^–1 and a porosity ? of 52 % and a catalyst‐coated metal monolith with an SA/V of 3300 m^–1 and an ? of 86 %. Under a steam‐to‐carbon ratio of 2.5 and at a temperature of 700 °C, the highest specific reaction rates were found for the catalyst‐coated monolith. The high SA/V and ?, together with the high rate of heat transfer of the metal monolith were found to be responsible of this optimum behavior. However, in both systems, the Ni‐Al₂O₃ catalyst suffered a catalyst deactivation during operation.     

A novel monolith catalyst of plate-type anodic alumina for the hydrolysis of dimethyl ether

A novel monolith catalyst of plate-type anodic alumina was applied in the dimethyl ether (DME) hydrolysis reaction system. The reactivity of the anodic alumina with hydration treatments in DME hydrolysis reaction was investigated. The preferred hydration-treated temperature was found to be 80 °C and the anodic Al₂O₃/Al monolith exhibited higher activity than the commercial Al₂O₃ in DME hydrolysis reaction. Meanwhile, the anodic Al₂O₃/Al monolith was proven to have higher MeOH effluent mole percentage with less unfavorable side reactions than the ZSM-5 catalyst. The anodic γ-Al₂O₃/Al monolith had just 0.85% coking while the ZSM-5 catalyst had 8.81% after 100 h of continuous experiments.     

Preparation of a carbon monolith with hierarchical porous structure by ultrasonic irradiation followed by carbonization, physical and chemical activation

A two-step direct and simple method for the preparation of a hierarchical porous carbon monolith with micropores, mesopores and macropores is described. The two stages give more flexibility in the preparation of a porous carbon monolith. In step I a macroporous interconnected carbon monolith is prepared by ultrasonic irradiation during sol-gel polycondensation. The effects of sol-gel temperature, catalyst concentration and ultrasonic power on the structure of the monolith are investigated. In step II, mesopores are induced in the monolith by Ca(NO₃)₂ impregnation followed by CO₂ activation. The effect of activation temperature is also studied. A hierarchical interconnected carbon monolith with mean pore size diameter of 1.2 μm, BET surface area of 624 m²/g, mesopore volume of 0.38 cm³/g and micropore volume of 0.22 cm³/g has been obtained from Ca(NO₃)₂ impregnation of the macroporous carbon monolith followed by CO₂ activation at 850 °C.     

The Sulfur and Water Resistance of Modified Washcoated Zeolite-Exchanged Monolith Catalysts for SCR of NOx with Propane

The method of preparation of modified Cu-substituted zeolite DeNOx catalysts washcoated on monolith ceramics has been developed. Non-modified and modified monolith catalysts were tested in DeNOx reaction with propane. It was shown that the catalyst modified by cerium and containing titania together with H-ZSM-5 and Al₂O₃ in the washcoating layer demonstrates high level of activity, and its resistance during multiple cycles of poisoning by sulfur compounds and water at 400 and 500°C is also high.     

Coupling of highly exothermic and endothermic reactions in a metallic monolith catalyst reactor: A preliminary experimental study

An LaFe_0.5Mg_0.5O₃/Al₂O₃/FeCrAl metallic monolith catalyst for the exothermic catalytic combustion of methane and an Ni/SBA-15/Al₂O₃/FeCrAl metallic monolith catalyst for the endothermic reforming of methane with CO₂ have been prepared. A laboratory-scale tubular jacket reactor with the Ni/SBA-15/Al₂O₃/FeCrAl catalyst packed into its outer jacket and the LaFe_0.5Mg_0.5O₃/Al₂O₃/FeCrAl catalyst packed into its inner tube was devised and constructed. The reactor allows a coupling of the exothermic and endothermic reactions by virtue of their thermal matching. An experimental study in which the temperature difference between the chamber of the external electric furnace and the metallic monolith catalyst bed in the jacket was kept very small, by adjusting the power supply to the furnace, confirmed that the heat absorbed in the reforming reaction does indeed partly come from that evolved in the catalytic combustion of methane, and that the direct thermal coupling of the two reactions in the reactor can be realized in practice. When the temperature of the electric furnace chamber was 1088 K, and the gas hourly space velocities (GHSVs) of the reactant mixtures passed through the inner tube and the jacket were 382 h⁻¹ and 40 h⁻¹, respectively, the conversions of methane and CO₂ in the reforming reaction were 93.6% and 91.7%, respectively, and the heat efficiency reached 81.9%. Stability tests showed that neither catalyst underwent deactivation during 150 h on stream.     

Performance features of Pt/BaO lean NOx trap with hydrogen as reductant

The performance of a model Pt/BaO/Al₂O₃ monolith catalyst was studied using H₂ as the reductant. The dependence of product selectivities on operating parameters is reported, including the durations of regeneration and storage times, feed composition and temperature, and monolith temperature. The data are explained in terms of a phenomenological model factoring in the transport, kinetic, and spatio‐temporal effects. The Pt/BaO catalyst exhibits high cycle‐averaged NOx conversion above 100°C, generating a mixture of N₂ and byproducts NH₃ and N₂O. The cycle‐averaged NOx conversion exhibits a maximum at about 300°C corresponding to the NOx storage maximum. The N₂ selectivity exhibits a maximum at a somewhat higher temperature, at which point the NH₃ selectivity exhibits a minimum. This trend conveys the intermediate role of NH₃ in reacting with stored NOx. Both N₂ and N₂O are also formed during the storage steps from the oxidation of NH_x species produced during the regeneration. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Parallel synthesis and fast screening of heterogeneous catalysts

We are presenting an effective method to prepare and test heterogeneous catalysts much faster than by the conventional way. A catalyst array was prepared via an incipient wetness method by combination of different amounts of Pt, Zr, and V on Al₂O₃ by means of an automatic liquid handler. For catalytic testing for methane oxidation a ceramic monolith reactor module, the channels of which contain the different catalyst compositions, was developed in which up to 250 catalyst compositions can be prepared and tested in parallel. Gas samples from each channel of the monolith were analysed sequentially by a mass spectrometer by moving the QMS inlet capillary into the channels using a three-dimensional positioning system which works at high temperatures. By comparison of the testing results with experiments carried out in flow reactors it is shown that the monolithic reactor is an efficient tool for fast screening of heterogeneous catalysts. This revised version was published online in June 2006 with corrections to the Cover Date.     

Deactivation of Diesel Oxidation Catalysts by Sulphur in Laboratory and Engine-Bench Scale Aging

The activity of sulphur–water- and water-treated PtPd/Al₂O₃- and Pt/Al₂O₃-based monolith catalysts was investigated. The catalysts were characterized by X-ray photoelectron fluorescence, X-ray photoelectron spectroscopy, transmission electron microscopy and BET–BJH. The sulphur poisoning had a diminishing effect on the catalyst activity. The correlation between the laboratory-poisoned and engine-bench-aged catalyst activity was detected and found to have a relatively good correspondence.     

Beneficial effect of adding γ-AlOOH to the γ-Al2O3 washcoat of a PdO catalyst for methane oxidation

The beneficial effect of adding γ-AlOOH to the γ-Al₂O₃ washcoat of a ceramic cordierite (2MgO · 2Al₂O₃ · 5SiO₂) monolith, used to support a PdO catalyst, is reported for methane oxidation in the presence of water at low temperature (<500°C). The mini-monolith (400 cells per square inch (CPI), 1 cm diameter × 2.54 cm length; ~52 cells) was washcoated using a suspension of γ-Al₂O₃ plus boehmite (γ-AlOOH), followed by calcination and then deposition of Pd by wet impregnation. An optimum solid content of 25 wt% in the washcoat suspension was used to obtain a ~25 wt% washcoat on the monolith. The presence of γ-AlOOH enhanced the thermal and mechanical stability of the washcoat, provided that the γ-AlOOH content was <8 wt%. Temperature-programmed methane oxidation (TPO) showed that the addition of γ-AlOOH to the γ-Al₂O₃ washcoat decreased the catalyst activity. However, when H₂O (2 vol% and 5 vol%) was present in the feed gas, the γ-AlOOH improved the catalyst activity and stability. A γ-AlOOH content of ~5 wt% in the washcoat was determined to provide the highest catalyst activity and stability for CH₄ oxidation in the presence of water.     

Porous washcoat structure in CeO2 modified Cu-SSZ-13 monolith catalyst for NH3-SCR with improved catalytic performance

A series of CeO₂ modified Cu-SSZ-13 monolith catalysts were prepared by embedding CeO₂ into the washcoat of Cu-SSZ-13 monolith catalyst through solvent combustion method. These CexCu-SSZ-13 catalysts were studied in the selective catalytic reduction (SCR) of NO with NH₃, among which the Ce2Cu-SSZ-13 catalyst exhibited the best low-temperature activity, hydrothermal stability, and sulfur resistance. The physicochemical properties of the catalysts were characterized using multiple methods. Results showed that the acidity, redox capacity, and ammonia adsorption capacity significantly enhanced after CeO₂ modification, thus leading to the high performance of Ce2Cu-SSZ-13 catalyst. Furthermore, the introduction of CeO₂ induced the fast SCR reaction by promoting the oxidation of NO to NO₂. Analog calculation suggested that the porous structure generated via solvent combustion in the washcoat effectively increased the diffusion rate of reaction. In situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFT) analysis showed that Brønsted acid sites were the main active center and the reaction followed Eley–Rideal mechanism.     

Metallic monolith supported LaMnO3 perovskite-based catalysts in methane combustion

Metallic monolith supported LaMnO₃ perovskite-based catalysts are characterized by a high activity in methane combustion (95.5% conversion at 745 °C) and by a high thermal resistance. The activity of the catalysts depends on the duration and temperature of LaMnO₃ calcination. The same relation holds for the chemical composition of the catalyst surfaces when they are determined by the XPS method. The shortening of the time of LaMnO₃ perovskite calcination from 12.5 h to 8 h (700 °C) reduces the conversion of methane over a fresh catalyst. This is attributable to the lower amount of manganese (Mn:La = 0.48) on the surface of this catalyst compared to the catalyst whose perovskite was calcined for 12.5 h (Mn:La = 1.8). The extension of calcination time from 8 h to 12.5 h (at 700 °C) brings about a decrease in the specific surface area (SSA) from about 13.7 m²/g to 9.4 m²/g. After approximately 6 h on stream, the activities of the two catalysts become comparable. Aging of the catalyst with an LaMnO₃ active layer at 920 °C for 24 h reduces methane combustion to 82.5% (at 745 °C). The aging process changes the catalyst surface, where Al and C content increases and the Mn:La ratio decreases. The activity of the monolithic LaMnO₃ catalyst rises with the increase in the amount of the active layer from 11.5% to 17.8%. Methane conversion is greater over catalysts with an LaMnO₃ than with an LaCoO₃ active layer, but the LaMnO₃ catalysts show a lower resistance to thermal shocks.     

NH3-Slip Catalysts: Experiments Versus Mechanistic Modelling

The oxidation of ammonia on a Pt/Al₂O₃ coated monolith has been studied under automotive NH₃-slip catalyst conditions. Ammonia conversion as well as the selectivities towards the products N₂, N₂O and NO are well described by a mechanistic model that is based on reaction mechanisms originally developed for NH₃ oxidation in nitric acid production plants.     

Low-Temperature CO Oxidation over a Pt/Al2O3 Monolith Catalyst Investigated by Step-Response Experiments and Simulations

The ignition–extinction processes for CO oxidation over a Pt/Al₂O₃ monolith catalyst have been studied by flow-reactor experiments and simulations. The study was performed by stepwise changes of the inlet O₂ concentration ranging 0–20 vol% while the CO concentration and the inlet gas temperature were kept constant at 1.0 vol% and 423 K, respectively. Several features observed experimentally are qualitatively simulated with our model: (i) the ignition of the CO oxidation demands 8.0vol% O₂ (ii) corresponding to a catalyst ignition temperature of 433 K (due to the exothermicity of the reaction) and (iii) occurs in the rear part of the monolith where (iv) a local reaction zone is formed which (v) moves towards the reactor inlet as a function of time on stream. Additionally, the simulations show first order kinetic phase transitions, i.e. rapid adsorbate concentration changes, where the catalyst surface is predominantly CO covered in the low reactive state and almost completely oxygen covered in the high reactive state. For the ignition process the kinetic phase transition occurs after the actual catalytic ignition. However, the extinction process is more difficult to simulate dynamically without changing the model parameters for O₂ adsorption in the low and high reactive state, respectively. The influence of diffusion limitations and the role of formation of a less reactive Pt state under oxidising conditions is discussed.     

Preparation of Pt/beta zeolite-Al2O3/cordierite monolith for automobile exhaust purification

A two-step route was developed to prepare zeolite beta coatings on structured monolith. Al₂O₃ layer was deposited on cordierite substrate by slurry dip-coating in the first step and beta zeolite layer was then coated on Al₂O₃/cordierite by direct dynamic hydrothermal synthesis in the second step. The as-prepared beta zeolite-Al₂O₃/cordierite was characterized by means of XRD and SEM techniques and the stability of beta coatings was studied. Based on these results, the advantages of the two-step method were discussed. After the introduction of Pt to beta zeolite-Al₂O₃/cordierite, the obtained Pt/beta zeolite-Al₂O₃/cordierite monolith was tested as promising catalyst for the purification of automobile exhaust from real lean-burn engine.     

Influence of reaction products of K-getter fuel additives on commercial vanadia-based SCR catalysts: Part II. Simultaneous addition of KCl, Ca(OH)2, H3PO4 and H2SO4 in a hot flue gas at a SCR pilot-scale setup

A commercial V₂O₅–WO₃–TiO₂ corrugated-type SCR monolith has been exposed for 1000 h in a pilot-scale setup to a flue gas doped with KCl, Ca(OH)₂, H₃PO₄ and H₂SO₄ by spraying a water solution of the components into the hot flue gas. The mixture composition has been adjusted in order to have P/K and P/Ca ratios equal to 2 and 0.8, respectively. At these conditions, it is suggested that all the K released during biomass combustion gets captured in P–K–Ca particles and the Cl is released in the gas phase as HCl, thus limiting deposition and corrosion problems at the superheater exchangers during biomass combustion. Aerosol measurements carried out by using a SMPS and a low pressure cascade impactor have shown two distinct particle populations with volume-based mean diameters equal to 12 and 300 nm, respectively. The small particles have been associated to polyphosphoric acids formed by condensation of H₃PO₄, whereas the larger particles are due to P–K–Ca salts formed during evaporation of the water solution. No Cl has been found in the collected particles. During the initial 240 h of exposure, the catalyst element lost about 20% of its original activity. The deactivation then proceeded at slower rates, and after 1000 h the relative activity loss had increased to 25%. Different samples of the spent catalyst have been characterized after 453 h and at the end of the experiment by bulk chemical analysis, Hg-porosimetry and SEM-EDX. NH₃-chemisorption tests on the spent elements and activity tests on catalyst powders obtained by crushing the monolith have also been carried out. From the characterization, it was found that neither K nor Ca were able to penetrate the catalyst walls, but only accumulated on the outer surface. Poisoning by K has then been limited to the most outer catalyst surface and did not proceed at the fast rates known for KCl. This fact indicates that binding K in P–K–Ca compounds is an effective way to reduce the negative influence of alkali metals on the lifetime of the vanadia-based SCR catalysts. On the other hand, P-deposition was favoured by the formation of the polyphosphoric acids, and up to 1.8 wt% P was accumulated in the catalyst walls. Deactivation by polyphosphoric acids proceeded at about 0.2% day⁻¹. About 6–7% of the initial activity was lost due to the accumulation of these species. However, the measured relative activity reached a steady-state level during the last 240 h of exposure indicating that the P-concentration in the bulk reached a steady-state level due to the simultaneous hydrolysis of the polyphosphoric acids.     

Novel Ce0.8Pr0.2O2 nanotube arrays synthesized by hydrothermal method with excellent catalytic performance

In this research, nanotube arrays of Ce_0.8Pr_0.2O₂ were synthesized on a monolith cordierite honeycomb by the low temperature hydrothermal method. Pre-synthesized ZnO nano-rods on cordierite act as a hard template for Ce_0.8Pr_0.2O₂ deposition. The formation of Ce_0.8Pr_0.2O₂ nanotube arrays was characterized by XRD, selective chemical leaching, BET, and FESEM. The calculated surface area of Ce_0.8Pr_0.2O₂ nanotube arrays was 80?m²/g. The external diameter and the wall thickness of Ce_0.8Pr_0.2O₂ nanotube arrays were 250 and 100?nm, respectively. In addition, Ce_0.8Pr_0.2O₂ nanotube had a length of about 1?μm. Pd decorated nanotube arrays catalyst present a lower light-off temperature and a higher thermal stability than a commercial catalyst in CO conversion catalyst test. Thermal stability of this catalyst is 10 times more than the commercial catalyst at 200?°C. The excellent catalyst performance of Ce_0.8Pr_0.2O₂ nanotube arrays can be attributed to the presence of praseodymium, a high surface area, and a thick wall of Ce_0.8Pr_0.2O₂ nanotubes arrays.     

Effect of the Presence of Ceria in the NSR Catalyst on the Hydrothermal Resistance and Global DeNOx Performance of Coupled LNT–SCR Systems

The global performance of coupled LNT–SCR systems, addressed to high NOx-to-N₂ conversion, minimal ammonia slip and null N₂O production, as well as the hydrothermal resistance of single NSR and SCR monolith catalysts and their coupling is discussed. Pt–Ba/Al₂O₃ and Pt–Ce–Ba/Al₂O₃ were washcoated on cordierite monoliths as NSR catalysts, and Cu/CHA was washcoated on similar monoliths as SCR catalysts. Both monoliths were coupled in two subsequent reactors to conform the LNT–SCR system. Previously to washcoating, the fresh powder catalysts and after severe hydrothermal aging were fully characterized by N₂ adsorption–desorption isotherms at 77 K, X-ray diffraction, NH₃ temperature-programmed desorption, and H₂ chemisorption to relate textural and chemical characteristics with the DeNO_x performance. The Cu/CHA catalyst shows an excellent hydrothermal resistance for the NH₃–SCR reaction. Incorporation of ceria to the model Pt–BaO/Al₂O₃is beneficial for the NO-to-NOx oxidation and NO₂ storage, improving NO conversion at low temperature and reducing the NH₃ slip. However, addition of ceria is detrimental for the hydrothermal resistance of the NSR catalyst. However, this detrimental effect is minimized when the NSR catalyst is coupled with the Cu/CHA monolith downstream of the NSR catalyst, achieving the coupled LNT–SCR device high NO conversion and minimal NH₃ slip with superior N₂ selectivity for an extended temperature windows, including as low as 220 °C, and maintaining performance even after severe hydrothermal aging.