Metallic Monolith Research Articles

Deep oxidation of methane on granulated and monolith copper—manganese oxide catalysts

Deep oxidation of methane on the granulated Cu—Mn-mixed oxide catalyst and metallic monolith catalysts coated with the same oxide was studied. The experimental kinetic curves for both monolith and granulated catalysts are satisfactorily described by the first-order rate law. The values of activation energies, reaction rate constants, and feed flow rates for the specified conversion almost coincide for both types of the catalysts. The data obtained confirm the possibility of a quantitative comparison of the activities of the granulated and monolith catalysts. The activity of the monoliths is proportional to the concentration of the active component.

Mathematical model of a monolith catalytic incinerator

A set ofl-dimensional mathematical models were developed to simulate both the steady state and transient performance of monolithic catalytic incinerators for VOC abatement. In modelling transient performance, quasi-steady state gas phase was assumed since transient response time is determined primarily by the thermal inertia of the monolith. Higher inlet gas temperatures and lower gas velocities were predicted to give higher conversion and faster response times. VOC concentration had little influence on the performance within the concentration ranges used. A catalytic incinerator is shown to operate typically under mass transfer limited conditions, and monolith channel density and shape have significant influence on the conversion and monolith heating time. The metallic monolith was predicted to show superior steady state and transient responses due to its lower thermal inertia generated by higher cell density and thinner wall.

Thermal stability of metal-supported catalysts for reduction of cold-start emissions in a wood-fired domestic boiler

The aim of the present work is to develop a catalyst based on a mixture of manganese oxides and platinum supported on a metallic monolith for abatement of emissions from wood combustion, particularly during the cold-start phase. The activity and the thermal stability of the catalysts have been studied in the laboratory, before performing tests in a wood-stove. The effect of the hydrothermal treatment at 900°C on the adherence of the washcoat onto a metallic substrate was studied using scanning electronic microscope. It revealed well-adhering washcoat onto the metallic support due to the growth of the alumina whiskers during the treatment. The influence of the amount of washcoat, as well as the influence of the concentration of manganese oxides in it (Mn: 5 to 20 mol%/Al 2O 3) on the activity of fresh and hydrothermally-treated catalysts were studied. The activity tests were carried out using a mixture of carbon monoxide, naphthalene and methane in the presence of air, steam and carbon dioxide to resemble the flue gases from wood combustion. On the fresh catalysts, containing the same total amount of manganese, a high concentration of manganese oxides in the washcoat favoured the oxidation of carbon monoxide and naphthalene, whereas a lower concentration of manganese oxides in the washcoat gave higher activity for the oxidation of methane. An increased total amount of manganese oxides in the catalysts, which had the same amount of washcoat, resulted in an increase in activity for the oxidation of the three combustibles. After thermal treatment at 900°C for 270 h in steam, most of the manganese oxide catalysts were activated for the oxidation of carbon monoxide and naphthalene while only being slightly deactivated for the oxidation of methane. The addition of manganese oxides in the washcoat, however, lowers the temperature of the γ- to α-alumina phase transformation. Platinum (0.5 mol%) was added to the manganese oxide (10 mol%) catalyst to improve its activity. A platinum catalyst was also tested for comparison. The platinum and the mixed catalysts showed similar activity for the oxidation of carbon monoxide and naphthalene, while the mixed catalysts were more active for the oxidation of methane. A similarly mixed MnO x –Pt (10–0.5 mol%) catalyst supported on Al 2O 3 stabilised with 3% lanthanum, but at larger scale, was tested in a wood-stove. The possibility of pre-heating the catalyst during the start-up phase was studied. The tests revealed a strong decrease of the carbon monoxide and unburned hydrocarbons emissions during the start-up phase when the catalyst was pre-heated with hot air compared with no pre-heating or no catalyst.

Monoliths in catalytic oxidation

Catalytic combustion is useful to avoid emission of nitrogen oxides, to combust fuel gas of different calorific levels, and to combust low contents of badly smelling or hazardous gaseous compounds. After dealing with some characteristics of catalytic combustion it is argued that catalytic combustion to a final temperature lower than about 800°C calls for a rapid transport of thermal energy out of the reactor. A fluidized bed in which combustion has been successfully performed is dealt with as well as a reactor filled with metal bodies sintered to each other and to the wall of the reactor. To achieve a sufficiently high catalytically active surface area a thin layer of silicone rubber is applied to the surface of the metal bodies and subsequently pyrolyzed to a highly porous layer of silica. To raise the thermostability alumina can be added to the silica layer. To establish a final temperature above 900°C the homogeneous gas-phase combustion can be ignited by a solid catalyst or the reaction can be performed completely catalytically. Since the combustion reaction proceeds very rapidly at elevated temperatures, a large gas flow can be utilized, which calls for a reactor exhibiting a low-pressure drop. For catalytic combustion monoliths and gauzes are appropriate. The chemical composition of ceramic and metallic monoliths is dealt with as well as the cell densities and wall thicknesses of commercial monoliths. The application of active components to the surface of the walls of monoliths is subsequently discussed. Since monoliths do not allow radial mixing, a homogeneous gas mixture has to be fed to the monolith to prevent very high temperature levels moving randomly over the channels of the monolith and deactivating the catalyst. With monoliths in gas turbines often catalytic ignition is used. To limit the temperature a fraction of the fuel feed is injected into the homogeneous combustion chamber. A number of alternatives of transporting the fresh fuel to the homogeneous combustion zone is mentioned. The cause of the catalyst temperature being higher than that of the gas flow is dealt with as well as the low volatility at elevated temperatures required for the catalytic components. Selection of the catalytically active materials is discussed and the procedure to bring the gas flow at the light-off temperature of the catalyst. Monolithic combustors used in radiant heaters display often an oscillatory behavior. After dealing with the cause of the oscillations, prevention by means of a flame arrestor is mentioned.

Investigation of CO oxidation and NO reduction on three-way monolith catalysts with transient response techniques

The kinetics of CO oxidation and NO reduction reactions over alumina and alumina-ceria supported Pt, Rh and bimetallic Pt/Rh catalysts coated on metallic monoliths were investigated using the step response technique at atmospheric pressure and at temperatures 30–350°C. The feed step change experiments from an inert flow to a flow of a reagent (O 2, CO, NO and H 2) showed that the ceria promoted catalysts had higher adsorption capacities, higher reaction rates and promoting effects by preventing the inhibitory effects of reactants, than the alumina supported noble metal catalysts. The effect of ceria was explained with adsorbate spillover from the noble metal sites to ceria. The step change experiments CO/O 2 and O 2/CO also revealed the enhancing effect of ceria. The step change experiments NO/H 2 and H 2/NO gave nitrogen as a main reduction product and N 2O as a by-product. Preadsorption of NO on the catalyst surface decreased the catalyst activity in the reduction of NO with H 2. The CO oxidation transients were modeled with a mechanism which consistent of CO and O 2 adsorption and a surface reaction step. The NO reduction experiments with H 2 revealed the role of N 2O as a surface intermediate in the formation of N 2. The formation of N N bonding was assumed to take place prior to, partly prior to or totally following to the N O bond breakage. High NO coverage favors N 2O formation. Pt was shown to be more efficient than Rh for NO reduction by H 2.

A Catalytic Combustor for High-Temperature Gas Turbines

Catalytic combustion has been the subject of thorough research work for over two decades, mainly in the U.S. and Japan. However, severe material problems in the ceramic or metallic monolith prevented regular operation in most cases. Still, during these two decades, turbine inlet temperatures were raised remarkably, and lean premix combustors have become standard in stationary gas turbines. In view of these facts, a simple “monolith-in-tube” concept of a catalytic combustor was adapted for the use in high-temperature gas turbines. Its essential feature is the fact that a considerable portion of the homogeneous gas phase reaction is shifted to the thermal reactor, thus lowering the catalyst temperature. This is achieved by the employment of very short catalyst segments. The viability of this concept has been demonstrated for a variety of pure hydrocarbons, alcohols as well as common liquid fuels. Extensive experimental investigations of the atmospheric combustor led to the assessment of parameters such as reference velocity, fuel-to-air ratio, and fuel properties. The maximum combustor exit temperature was 1673 K with a corresponding catalyst temperature of less than 1300 K for diesel fuel. Boundary conditions were in all cases combustion efficiency (over 99.9 percent) and pressure loss (less than 6 percent). Additionally, a model has been developed to predict the characteristic values of the catalytic combustor such as necessary catalyst length, combustor volume, and emission characteristics. The homogeneous reaction in the thermal reactor can be calculated by a one-dimensional reacting flow model.

Cerium-exchanged zeolites grown on metal gauze: A new catalyst system applicable for NO x reduction in mobile engine exhaust

ZSM-5 and mordenite (MOR), directly grown on a stainless-steel metal gauze support and subsequently exchanged with cerium ions, were applied as a NO reduction catalyst. These supported cerium-zeolite catalysts were found to show turnover frequencies comparable to that of an unsupported cerium-exchanged mordenite. Furthermore, the thermostability of these materials was examined by a calcination-activity test cycle and Ce-ZSM-5 was found to be more stable than Ce-MOR. The present approach to grow zeolite directly on a support offers a new possibility to apply zeolite efficiently on a metal monolith applicable for NO x emission control of mobile sources.

Kinetics of nitrate reduction in monolith reactor

The kinetics of nitrate reduction with hydrogen was studied in a metallic monolith reactor, where the catalytically active material was Cu-doped Pd supported on Al 2O 3. The experiments were carried out at 60°C and at hydrogen pressures of 2 atm and 4 atm. The reactor was operated in semibatch with respect to hydrogen, whereas the liquid phase was in batch recirculating through a storage tank. The main reaction products were nitrogen and ammonia; nitrite was formed as an intermediate product. The reduction kinetics was described with a rate model based on a mechanism where absorbed NO is the key intermediate on the catalyst surface. The mass transfer parameters were estimated from correlations and the mass transfer model was combined to the kinetic model. The reaction kinetics and the mass transfer were included in a plug-flow type of model for the monolith and the kinetic parameters were estimated with non-linear regression analysis. Simulation of the experimental conditions showed that the rate model can be used in nitrate hydrogenation.

Catalytic honeycomb reactors with electrical heating

Experimental data on the ignition and time course of temperatures and conversion for CO oxidation both on an electrically heated ceramic monolith reactor and the system metallic monolith (preheater)—ceramic monolith (afterburner) and presented. Three types of nonstationary two-phase models are described, numerical algorithms for their solution are discussed and their applicability to description of experimental data is evaluated. The spatially one-dimensional AD model, including axial dispersion in the gas phase and axial heat conduction in the solid phase, is recommended particularly for its robust computational properties.

Modelling of heat transfer in metallic monoliths consisting of sinusoidal cells

A mathematical model is presented to describe heat transfer in a metallic monolith comprising channels of sinusoidal cross-section. Model equations were solved numerically and when the results were compared with literature experimental data for the Fecralloy R monolith, fairly good agreement was found. Radial temperature profiles were calculated using the model for constant wall temperature and a flat temperature profile at the inlet of the monolith. Effective thermal conductivities and wall heat transfer coefficients for the monolith were evaluated based on these profiles. The coefficients were compared with those for packed beds of electrocorundum pellets. Effective thermal conductivities for the monolith were found to be lower than those for packed beds, while wall heat transfer coefficients are much higher than those for packed beds. Overall heat transfer coefficients are greater for amonolith.

Grace Camet® metal monolith catalytic emission control technologies

The properties of Grace Camet® metal monolith catalysts are compared with washcoated commercially-available ceramic substrate-based catalysts. Camet® catalysts have a higher void fraction and geometric surface area per unit volume. Mass transfer and pressure drop correlations developed for Camet® catalysts show that Camet® catalysts have a higher mass transfer-limited activity at a given pressure drop. The implications of substrate design on catalytic performance are discussed. The paper provides information on the commercial performance of Grace Camet® three-way, carbon monoxide oxidation, and Low Temperature-Dual Function catalysts.

Camet oxidation catalyst for cogeneration applications

Environmental pollution control regulations have required the use of oxidation catalysts to reduce emissions of carbon monoxide and unburned hydrocarbons from cogeneration plants. Oxidation catalyst performance requirements include low pressure drop and high conversion efficiency. The conversion limit is controlled by external mass-transfer limitations determined by the properties of the monolith substrate. Conventional monoliths contain identical channels that pass straight through the monolith and are parallel to the flow direction. This paper discusses the conversion and pressure drop performance of a new metal monolith substrate based on a folded metal foil that is corrugated in a herringbone pattern. The external mass- transfer pressure drop performance of this type of metal monolith catalyst is compared with conventional ceramic substrate based catalysts.

Emission Control for Gas Turbines

Johnson Matthey are reporting successful commercialisation of oxidation catalysts for the clean-up of gas turbine exhaust. The reduction of carbon monoxide and hydrocarbons is achieved with a platinum-rhodium catalyst which minimises the formation of sulphur trioxide. The catalyst system, supported on an energy saving, low pressure drop metal monolith, has been used continuously for five years on large industrial gas turbines.

Modelling of heat transfer in non-adiabatic monolith reactors and experimental comparisons of metal monoliths with packed beds

Heat transfer in metal honeycomb monoliths, under non-adiabatic conditions, is considered in this work. Using a simple geometric simulation of the monolithic structure, steady-state heat balances are written separately for the solid and gas phases of the simulated monolith. In the absence of chemical reaction the model is solved analytically to give solid and gas temperature profiles radially and axially within the monolith. An experimental system is used to compare heat transfer in a honeycomb metal monolith to the model predictions. Also, heat transfer in a packed bed of ceramic pellets is studied experimentally and compared to the metal monolith under similar, non-adiabatic conditions. Results point out the different heat-transfer merits of each of these two bed configurations.

Transient performance of catalytic combustors for gas turbine applications

The transient response of a monolith catalytic combustor to changes in operating conditions and monolith design parameters has been examined on the basis of a mathematical model. The model considers both catalytic and gas phase reactions and allows for axial conduction of heat in the solid substrate. The transient response time is shown to be determined primarily by the thermal inertia of the monolith with metallic monoliths, having lower thermal inertia, offering superior performance to ceramics. Improved performance is also favoured by a square cell geometry rather than triangular or sinusoidal geometry because of the better heat and mass transfer characteristics of the square cell geometry. Higher inlet temperatures, operating pressures and lower gas velocities are shown to result in faster response times. A step decrease in inlet gas temperature may result in catalyst overheating when either the catalytic (solid phase) or the homogeneous (gas phase) reactions are dominant.

Noble Metal Catalysts on Metallic Substrates

The use of platinum group metal catalysts supported on ceramic monoliths for control of gaseous emissions from automobiles and chemical plant is now well established. Research in the Johnson Matthey Laboratories and at AERE Harwell, aimed at meeting the demand for more effective catalysts, has resulted in a new generation of platinum catalysts in which the ceramic substrate is replaced by a metal monolith. These metal substrate platinum catalysts are found to be more compact and more efficient than conventional ceramic structures. At the same time they overcome the structural limitations of ceramics with greater resistance to thermal shock and mechanical failure. The development of the metal substrate catalyst, and its use in hostile high temperature environments such as automobile emission control and catalytic combustion is described.