Alumina Washcoat Research Articles

Washcoating of catalytic particulate filters studied by time-resolved X–ray tomography

In this paper we introduce a new application of time-resolved X-ray tomography (XRT) to imaging of the washcoating process in catalytic monolith filters with a honeycomb structure (DPF, GPF). The method is applied to several alumina washcoats with different particle size distributions deposited on a cordierite substrate. The results bring new insights into the drying dynamics of the porous filter wall as well as the on-wall catalyst layer, including the process of layer shrinking and cracking. The XRT results are supplemented by cross-section scanning electron microscopy images and mercury intrusion porosimetry of the final structures. In addition to the statistical analysis of the washcoat distribution, the impact of the washcoat structure on filter pressure loss is determined by experiments.

Characterization and Evaluation of Methane Oxidation Catalysts for Dual-Fuel Diesel and Natural Gas Engines

The UK has incentivized the use of natural gas in heavy goods vehicles (HGVs) by converting to dual-fuel (DF) diesel-natural gas systems to reduce noxious and greenhouse gas emissions. Laboratory and on-road measurements of DF vehicles have demonstrated a decrease in CO2 emissions relative to diesel, but there is an increase in greenhouse gas (CO2e) emissions because of unburned methane. Decreasing tailpipe emissions of methane via after-treatment devices in lean-burn compression ignition engines is a challenge because of low exhaust temperatures (∼400 °C) and the presence of water vapor. In this study, six commercially available methane oxidation catalysts (MOCs) were tested for their application in DF HGV vehicles. Each MOC was characterized in terms of the catalyst platinum group metal (PGM) loading (both Pd and Pt), particle size, catalytic surface area, and Pd:Pt ratio. In addition, the washcoat surface area, pore volume, and pore size were evaluated. The MOC conversion efficiency was evaluated in controlled methane-oxidation experiments with varying temperatures, flow rates, and gas compositions. Characteristic-conversion efficiency correlations demonstrate that the influential MOC characteristics were PGM loading (both Pd and Pt), Pd:Pt ratio, washcoat surface area, and washcoat pore volume. With 90 % methane oxidation at less than 400 °C in DF HGV exhaust conditions, sample 1 had the highest conversion efficiency because of a high PGM loading (330 g/ft3, 12,000 g/m3), a 5.9 Pd:Pt ratio, a high alumina washcoat surface area of 20 m2/cm3, and 74-mm3/cm3 pore volume. Additional studies showed increased MOC conversion efficiency with decreasing gas hourly space velocities (GHSVs) and increasing methane concentrations.

Development of a syngas-fired catalytic combustion system for hybrid solar-thermal applications

This paper describes the development and operation of a catalytic combustion system for use with syngas as an important component of a hybrid heating source for solar-thermal power generation. The reactor consists of a cylindrical ceramic monolith with porous alumina washcoat in which platinum is distributed as the catalyst. Two fuel-rich equivalence ratios were studied over a range of flow rates. The fuel-rich conditions permit low temperature combustion without the problem of hotspots likely to occur under fuel-lean conditions with hydrogen-containing fuels. Experimental data of temperature and species concentration at the exit of the reactor have been reported for a maximum fuel thermal input of 34kW. The system exhibited quick start-up with a light-off time of around 60s and a steady-state time of around 200s as determined from the transient temperature profiles. The experimental results have also been complemented with detailed two-dimensional numerical simulations for improved understanding of the combustion characteristics in the reactor. The simulations suggest that the combustion system can be operated at a turn-down ratios far in excess of 1.67, which is the maximum value that has been investigated in the present setup. Stable operation, quick startup, and high turn-down ratio are some of the key features that enable the proposed combustion system to accommodate the transients in solar-thermal applications.

Preparation and characterization of Pd/Al2O3 catalysts on aluminum foam supports for multiphase hydrogenation reactions in rotating foam reactors

Stirred tank reactors with catalytically active foam elements as stirrers are a promising reactor concept for intermediate and fine chemical industries. The fixation of the active component is the key benefit because of the improved catalyst handling. Catalytically active open-celled foam stirrers were prepared from 30PPI aluminum foams, creating a highly porous open-celled catalyst support. The foam samples were anodized after preliminary studies at a current density of 30mAcm−2 in sulfuric acid to attain a better adhesion to the coating material. The alumina washcoat was deposited by dipcoating in a 40wt.% alumina slurry. The samples were calcined at 600°C. The mean washcoat thickness was 17.8μm. The washcoat thickness was determined by applying X-ray tomography as a non-invasive characterization method. The catalyst layer thickness distribution is also investigated. A mean palladium loading of 1.7wt.% was obtainedby applying wet impregnation. The foam catalysts were tested as stirring elements in a stirred tank reactor with a volume of 600mL for the hydrogenation of a 20wt.% mixture of α-methylstyrene (AMS), while the rotation speeds are varied from 600 to 1500rpm and total pressures were varied from 5 to 15bar. The adherence of the catalyst layer was tested during more than 50 experiments. The catalytic performance of the catalytically active foam stirrers were compared with a commercial slurrycatalyst while the slurry catalyst was stirred by non-coated foam stirrers.

Preparation and characterisation of CuO/Al2O3 films deposited onto stainless steel microgrids for CO oxidation

Homogeneous and well-adhered alumina washcoat was deposited onto stainless steel microgrids. Alumina-washcoated stainless steel microgrids loaded with different amounts of copper were synthesised and characterised by several techniques including SEM-EDS, N2 adsorption at −196°C, XRD, temperature programmed reduction (H2-TPR), UV–visible–NIR DRS and XPS spectroscopy. Three types of copper species were detected in the series of the prepared microreactors: (i) CuO deposited on the surface of the substrate; (ii) highly dispersed CuO on the alumina surface and (iii) bulk CuO. As proven by H2-TPR experiments, the distribution of CuO between these three sites depended on copper loading.The prepared microreactors were successfully tested in CO oxidation reaction and compared to powder catalysts loaded with the same amount of copper. The results showed that the microstructured catalysts were more active than the powder catalysts when activated under reaction gas mixture. Indeed, a second catalytic run, performed on the same catalyst, revealed a decrease of the light-off temperature in the case of the prepared microreactors as result of their activation. The Cu species distribution seemed to play a decisive role in the activity of the microreactors. The CuO interacting with the substrate seemed to enhance the thermal conductivity of the microreactor and, thus, might better dissipate the heat generated, in the Cu/Al2O3 film, by the exothermic CO oxidation reaction. These catalytic features evidenced the potential of these microreactors as a promising alternative catalyst in the CO oxidation reaction.

Co-Ni Bimetallic Catalysts Coated on Cordierite Monoliths for Hydrazine Decomposition

The preparation conditions to obtain cobalt-nickel monolithic catalyst with high loading and homogeneous distribution are studied. Washcoating of raw monolith with gamma alumina solution was examined and characterized by SEM. A uniform layer of alumina washcoat as a support for active phase was achieved in this process. Also the effect of the molarity of solution and the number of impregnation were studied to obtain different loading of active phase on monolith walls. Catalysts with 38.81, 20.44 and 3.22 wt.% of loading were obtained by four steps of impregnation in active phase solution with 3.33, 1.42, 0.3 of molarity, respectively. It was found that increase in concentration of active phase loading leads to increase in active phase loading but reduces the uniformity of active phase distribution.

Characterization of Pt and Ba over alumina washcoated monolith for NOx storage and reduction (NSR) by FIB-SEM

The application of NOx storage and reduction (NSR) catalysts for the cleanup of diesel engine exhaust gases requires unequivocally a structured support. In this paper, a cordierite monolithic substrate on which Al2O3 was washcoated and then the active phases, namely Pt and Ba, were incorporated by adsorption from solution and impregnation, respectively, is innovatively characterized. Alternatively to the standard characterization techniques, which often are not applicable for structured catalysts, we used a FIB-SEM with the aim of determining Pt and Ba dispersion and distribution in the alumina washcoat. First, using the imaging ability of the FIB-SEM, it was observed that barium was evenly distributed along the washcoat surface. Secondly, with the cutting ability of the equipment, a thin washcoat film was lifted out from the washcoat in order to be observed by TEM. Pt and Ba were found to be well distributed all over the washcoat depth with an excellent platinum dispersion with particles around 1–2nm. The NOx storage and reduction experiments showed an excellent storage capacity (74%) and nitrogen selectivity (87%) which was related to the adequate contact between the active phases, Pt and Ba.

Effect of Bimetallic Pt-Rh and Trimetallic Pt-Pd-Rh Catalysts for Low Temperature Catalytic Combustion of Methane

‡Monometallic, bimetallic and trimetallic particles consisting of different weight compositions of Pt-Pd-Rh over pure alumina wash coats have been synthesized and their catalytic performance on methane conversion was studied from 150 to 600 o C. Different catalyst formulations with variable Pt, Pd and Rh contents for bimetallic and trimetallic systems were tried and Pt(1.5)Rh(0.3)/Al2O3 and Pt(1.0)Pd(1.0)Rh(0.3)/Al2O3 shows low T50 and T90 temperatures. Bimetallic and trimetallic particle synergism acts as three way catalysts and therefore, all the catalysts show 100% methane conversion. The effect of supports such as ZrO2 and TiO2 on methane combustion was investigated; from T50 and T90 results both Al2O3 and ZrO2 are suitable supports for low temperature methane combustion.

The activity of Pt/Al 2O 3 diesel oxidation catalyst after sulphur and calcium treatments

A fresh powder-like Pt/Al 2O 3 diesel oxidation catalyst was treated with sulphur (S–), sulphur–water (SW–), calcium–water (Ca–), sulphur–calcium–water (SCa–), and water (W–). BET, TEM–EDS, XPS, ICP-OES, AAS analyses and catalytic activity tests were used to find the effect of sulphur, calcium and water on the Pt/Al 2O 3 catalyst and its performance. Based on the ICP-OES and EDS results, sulphur was found to accumulate on the surface, whenever it was used in the pretreatments. Calcium was found in a very small amount on the Ca- and SCa-treated Pt/Al 2O 3 catalysts according to AAS and EDS results. For all the cases, the specific surface area was decreased after pretreatment. As expected, for the S- and SW-treated Pt/Al 2O 3 catalysts the light-off temperatures for CO and propene were higher compared to the corresponding fresh catalyst. Ca-, SCa-, and W-treated Pt/Al 2O 3 catalysts had almost the same light-off temperatures of CO and C 3H 6 than the fresh catalyst, instead. Based on the TEM–EDS results, the particle size of Pt and alumina washcoat microstructure was found to be unchanged after all the pretreatments. The pronounced deactivation is explained with sulphates formation on the active sites and with a decrease in the specific surface area.

Influence of the preparation procedure of NSR monolithic catalysts on the Pt-Ba dispersion and distribution

The preparation procedure of a structured NO x storage/reduction catalyst is investigated by optimizing washcoating of ceramic monoliths and subsequent incorporation of Pt and Ba as the active phases. Endurance and firm adherence of the alumina washcoat to the substrate has been achieved. Incorporation of platinum from Pt(NH 3) 4(NO 3) 2 has been studied by impregnation (filling the monolith channels) and adsorption from solution (ion exchange). Highly dispersed and well distributed platinum were obtained by the adsorption procedure at pH 11–12 and calcination temperature no higher than 450–500 °C. Barium was incorporated by dry or wet impregnation. The dry impregnation procedure led to a more homogeneous barium distribution on the surface of washcoat. The sample prepared by adsorption from solution and dry impregnation, for incorporation of Pt and Ba respectively, showed the best storage capacity, ca. around 2.1 μmol NO x per mg of BaO for a feedstream with 380 ppm NO and 6% O 2 in nitrogen, and a space velocity of 32,100 h −1, and also complete release and reduction of the stored NO x when oxygen was shifted to 2.3% H 2. Thus, a relation between good NO x storage behavior and Pt dispersion and Pt-Ba distribution, and the preparation procedure has been established.

Development of functionalized Fe–Al–Cr alloy fibers as innovative catalytic oxidation devices

Catalytic devices play a very important role to drastically reduce the noxious contaminants released by the combustion engines. Common devices consist of functionalized ceramic monoliths; an interesting alternative can be realized with a support material made of a high temperature compatible metal alloy, like FeCrAl. In the present study the FeAlCr fibers used to make a device similar to the ceramic monolith were thermally treated to obtain alumina whiskers, which were homogenously covered with alumina or ceria–zirconia–alumina washcoats. Pt or Pt and Pd have been loaded over the functionalized fibers obtaining an innovative oxidation catalyst. The systems were characterized by BET surface area measurements, hydrogen chemisorption and tested under high space velocity and fast heating run-up cycles, for hydrocarbon and CO conversion and particulate filtration ability. The good catalytic activity after severe aging treatments confirms the possibility of use of these innovative functionalized metallic stacks in air pollution control applications.

The influence of preparative parameters on the adhesion of alumina washcoats deposited on metallic supports

Well-adhered alumina washcoats on FeCrAl metallic supports were prepared using boehmite sols and alumina slurries. The microstructure and the surface performance of the washcoat/support were investigated by SEM, XRD, and ultrasonic vibration. The effects of the main preparative parameters on the coating adherence were studied. The optimal coating conditions are presented as follows: pre-oxidation of the metallic supports was performed at 900 °C for 10 h, the sol layer loadings were 2.0–6.6 wt.%, and the slurry layer loadings were less than 25.3 wt.%. The sol layer drying was performed at 30 °C for 1 h and that for the slurry layer the drying was performed at 120 °C for 2 h, and the coating calcining was performed at 900 °C for 2 h. The SEM photographs of coated samples show that alumina washcoats were well deposited on the metallic supports.

Basic Study of Adhesion of Several Alumina‐based Washcoats Deposited on Stainless Steel Microchannels

AbstractThe adhesion of several washcoats deposited on stainless steel microchannels was investigated by performing a mechanical test (drop test) after application‐oriented tests, temperature cycling, and water exposure. For this study alumina washcoats (γ‐Al2O3) and washcoats of commercially available alumina‐based catalyst powders (Pt/Al2O3, Rh/Al2O3) were used. The deposited washcoats showed very good adhesion not only for fresh samples but also after the application‐oriented tests.

Short-channel structured reactor for catalytic combustion: Design and evaluation

Emission control of hazardous air pollutants, including volatile organic compounds (VOC), is one of the priorities for environmental catalysis. Metallic structured short-channel reactors of various geometries are regarded as an alternative to ceramic monoliths. The paper presents the coupled results on reactor structure modelling and the catalyst testing. It is expected, that mass transfer coefficients for laminar flow partially developing in short-channels can be increased up to 10 times comparing to standard monoliths, but the increased transfer properties involve elevated pressure drop. Therefore, performance evaluation criteria (PEC) have been developed to properly estimate the structure performance. The criteria derived cover both the general case for the comparable mass transport and kinetics of the process and the restricted case for the diffusional process limitation. In comparison with monoliths, the best structures (sinusoidal and triangular) seem to be from 1.5 times upto 4 times more efficient. Chromium–aluminium (CrAl) steel was surveyed in terms of its applicability for carrier manufacturing and catalyst depositing. Alumina washcoat and cobalt catalyst were deposited as organic precursors using Langmuir–Blodgett method (LB). Additional noble metal promoters were deposited by chemisorption. Catalytic tests have proved that the small amounts of highly dispersed Co 3O 4 are sufficiently active in combustion of diluted n-hexane with 100% selectivity to CO 2.

Application of supported perovskite-type catalysts for vehicular emission control

Catalytic control of auto-exhaust emissions is one of the most successful applications of heterogeneous catalysis, both in commercial and environmental point of views. Although noble metal-based catalysts have dominated this area, efforts were always put in towards development of low cost non-noble metal-based catalysts. With the recent need of closed-coupled catalytic converter, thermal stability requirements have also become more severe, leading to the search for stable catalytic materials. Mixed oxides, including those perovskite type compounds with ABO3 structure have been extensively studied, mainly for their catalytic and electrical properties. Low surface area of these catalysts has so far been the most important limitation for their catalytic applications involving high space velocities, e.g. auto-exhaust catalysis. Various synthesis routes have been earlier attempted to improve their surface area, yet this was much inferior than the noble metal catalysts, dispersed on high surface area alumina. The in situ synthesis of these oxides on alumina is often associated with the formation of undesired phases, due to the reactive nature of perovskite precursors. However, alumina washcoat, commonly used for improving the surface area of ceramic and metallic catalyst supports, can be modified for perovskite applications. In situ synthesis of stabilized perovskites on modified alumina-washcoated supports offer high surface area and excellent catalyst adhesion. Although, it is difficult to ascertain the presence of pure perovskite type materials on support, such improved synthesis has resulted in remarkable improvement in their catalytic activity for their applications in auto-exhaust catalytic converters. This review presents our work on synthesis of various improved perovskite-type mixed oxides supported on modified alumina-washcoated cordierite honeycomb, their characterization, and detailed catalytic evaluations for possible application in automobile pollution control.

Pd/Pt promoted Co 3O 4 catalysts for VOCs combustion : Preparation of active catalyst on metallic carrier

Emission control of volatile organic compounds (VOCs) is one of the priorities for environmental catalysis. Metallic microstructural short-channel reactors of various geometries are regarded as an alternative to ceramic monoliths. The paper presents the results on the catalyst preparation and optimisation. Chromium-aluminium (CrAl) steel was surveyed in terms of its applicability for carrier manufacturing and catalyst depositing. Alumina washcoat and cobalt catalyst were deposited as organic precursors using Langmuir–Blodgett method (LB) onto precalcined CrAl sheets. Additional noble metal promoters (Pd and Pt) were deposited by chemisorption. The LB method occurred useful for the preparation of nanocomposite catalyst since it enabled controlling the quantity and even distribution of the deposited material. The catalyst surface at various stages of preparation was examined using SEM, XPS and AFM methods. Catalytic tests showed that small amount of Co 3O 4 spinel, highly dispersed on Al 2O 3 layer, is active in combustion of diluted n-hexane. The apparent activation energy for the obtained cobalt catalyst (about 52 kJ/mol) was twice as low as for a standard Pt/Al 2O 3 catalyst. The promoting effect observed for Pd-containing cobalt catalysts by the decrease in the activation energy to around 15 kJ/mol, was correlated to the high surface concentration and dispersion of the cobalt catalyst containing PdO.

Fischer–Tropsch synthesis using monolithic catalysts

Square channel cordierite monoliths have been loaded with alumina washcoat layers of various thicknesses (20–110 μm) and loaded with rhenium and cobalt resulting in a 0.1 wt.% Re/17 wt.% Co/Al 2O 3 catalyst. These monolithic catalysts have been tested in the Fischer–Tropsch synthesis in a temperature window (180–225 °C) under synthesis gas compositions ranging from stoichiometrically excess carbon monoxide to excess hydrogen (H 2/CO = 1–3). The results include data on the activity and selectivity of CoRe/Al 2O 3 monolithic catalysts for FTS under these process conditions. Washcoat layers thicker than about 50 μm appear to lead to internal diffusion limitations. Thinner washcoat layers yield, depending on the conditions, to larger amounts of α-olefins than alkanes for chain lengths below 10 carbon atoms. ASF and non-ASF chain length distributions are obtained for thin washcoats, whereby the chain growth probability increases from 0.83 to 0.93. Under certain conditions the amounts of alkanes even increase with chain length. These experimental results with different diffusion lengths have been used to analyze the effects of secondary reactions on FTS selectivity.

Development of methods of growing carbon nanofibers on silica glass fiber supports

Samples of carbon nanofibers (CNF) with different carbon contents (6.5–55 wt.%) are prepared by decomposition of CH 4 on Ni catalysts supported on silica glass fibers (SGF). The incipient wetness impregnation of support, introduction of the active component into alumina washcoat and ion-exchange methods were used to prepare Ni catalysts on SGF. On the optimal catalyst prepared on washcoated SGF, carbon forms as CNF of diameter 20–50 nm, and the carbon capacity is rather high—55 g C/g Ni. Diameter of the CNF corresponds to the size of catalyst particles of Ni metal, which have the cuboctahedral form typical for the formation of CNF with conical embedding of graphite layers with respect to the filament axis. The SGF with supported CNF is a mesoporous material with polydisperse pore distribution similar by structural and textural parameters to the granulated bulk CNF obtained in methane decomposition over high-loaded nickel-alumina catalysts. The XRD, ESR and DTA methods were used for phase and structural analysis and demonstrated that CNF has a uniform graphite-like structure and does not include the amorphous phase.

Engineering and chemical aspects of the preparation of microstructured cobalt catalyst for VOC combustion

VOC combustion is a demanding process for both reactor and catalyst design. The microstructural reactor made of wire gauze was studied by us as an alternative to ceramic monoliths. The reactor evaluation was based on modeling heat and mass transfer through the selected microstructures. Chromium–aluminum and chromium–nickel steel was surveyed in terms of their applicability for carrier manufacturing and catalyst depositing. The alumina washcoat and cobalt catalyst were deposited as organic precursors using Langmuir–Blodgett (LB) method. The amount of deposited materials was controlled by the LB parameters and verified by TG measurements. The carriers were characterized at various stages of the preparation by SEM/EDX, XPS and RM methods and tested in n-hexane combustion. Oxidation of the Al washcoat precursor led to the formation of γ-Al 2O 3 improving the properties of the Al 2O 3 layer (α phase) developed on the precalcined CrAl carrier. Oxidation of cobalt stearate deposited on the CrAl gave dispersed Co 3O 4 spinel. Although the obtained cobalt catalyst in an oxidized state initiated n-hexane combustion at higher temperature (370 °C) than an industrial Pt/Al 2O 3 catalyst used as a reference, it had twice as low activation energy (50 kJ/mol).

Effect of metal foam supports on the selective oxidation of CO on Fe-promoted Pt/γ-Al 2O 3

Fe promotion is known to significantly increase the activity of Pt catalysts for the selective oxidation of CO in H 2 for fuel cell applications. Use of metal foams as catalyst structural supports in catalytic reactors can improve reactor characteristics by lowering the pressure drop and increasing heat transfer. In a previous study on metal foam-supported Pt catalysts, it was shown that Cr and Fe impurities from the metal foam were present in significant quantities in the washcoat, improving the catalytic properties of the catalysts. This study addressed the effect that these Cr and Fe impurities may have on metal foam supported PtFe (5 wt.% Pt, 0.5 wt.% Fe based on the loading of alumina washcoat) catalysts. Foam-based PtFe catalysts with different numbers of pores-per-inch (30–50) and percentage relative metal densities were characterized, investigated for their activities and surface kinetic parameters for the selective oxidation of CO in H 2, and compared with a powdered γ-Al 2O 3-supported PtFe catalyst (also 5 wt.% Pt and 0.5 wt.% Fe) and foam-based Pt catalysts. The foam-based PtFe catalysts exhibited reaction properties similar to those of the powdered PtFe/γ-Al 2O 3 catalyst. The intrinsic site activities of all foam-based PtFe catalysts determined using isotopic transient kinetic analysis (ITKA) were approximately the same and seemed to be characteristic of Fe-promoted Pt/γ-Al 2O 3. Thus, the presence of Cr and Fe impurities in the washcoat from the metal foam does not seem to have a major impact on reaction on Fe-promoted Pt even though it appears to significantly affect both H 2 and CO chemisorption at 37 °C. This was not the case for the comparison previously of foam-based and powdered Pt/γ-Al 2O 3 catalysts. These foam-based PtFe catalysts exhibited only a slight change with time-on-stream of CO oxidation rate and % CO 2 selectivity and did not show the rapid initial deactivation to steady state, typically seen for both γ-Al 2O 3-supported Pt and PtFe catalysts. This initial stable activity and selectivity is preferable for commercial use.