Matti Härkönen

Modelling of catalytic monolith converters with low- and high-temperature NOx storage compounds and differentiated washcoat

Two types of catalytic washcoats with different properties can be present simultaneously in a monolith channel, e.g. the flat foils and the corrugated ones in metallic monolith can be individually coated with specific type of the washcoat. The model of such multiphase, differentiated NOx storage and reduction (NSR) catalytic monolith converter has been developed. The important reactions as are the oxidation of carbon monoxide, hydrocarbons and hydrogen, the reduction of nitrogen oxides (NOx), the water gas shift and the steam reforming reactions, the NO/NO2 transformation, and the oxygen and NOx storage are considered. Unknown kinetic parameters of the NSR are evaluated from transient experiments with the samples of two different types of NSR catalysts. The first catalyst is of the Pt–γ-Al2O3–CeO2 type with alkali earth metals (e.g. barium) as NOx storage components (active at lower temperature). The second one is of the PtRh–γ-Al2O3–CeO2 type with both alkali earth metals and alkali metals (e.g. potassium) as NOx storage components (active at higher temperature). Simulations of periodic lean/rich operation of the low-temperature, the high-temperature and the differentiated (combined) NSR converters are performed. The results of the computations agree well with the experimental data. The dependences of integral NOx conversion on the lengths of the lean and rich phases and on the temperature of the inlet gas are discussed. The efficiencies of the low-temperature, the high-temperature and the differentiated NSR converters are compared at different operating conditions.

Stationary kinetics of essential reactions on automobile exhaust PtRh/Al2O3 catalyst

Abstract The essential oxidation and reduction reactions over ceria promoted Pt Rh/Al2O3 catalyst were studied in a laboratory testing system consisting of a metallic three-way catalyst. Oxidation of CO and propene as well as reduction of NO were examined under steady state conditions at 100–400°C and at atmospheric pressure. The concentrations of CO, CO2, total hydrocarbons, NO/NOx and O2 at the monolith outlet were measured for different gas feeds. The presence of water had a considerable effect on the kinetics. Water enhanced the oxidation reactions at low temperatures, while water-gas shift reaction and steam reforming took place at higher temperatures. Steady-state rate equations based on plausible surface reaction mechanisms for the oxidation of CO and propene as well as the reduction of NO with CO were derived for dry and wet conditions. The kinetic parameters included in the rate equations were estimated with nonlinear regression analysis. The experimental and the predicted concentrations were in good agreement, indicating that the kinetic models with their underlying mechanistic assumptions are able to predict the behaviour of the Pt Rh/Al2O3 catalyst.

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 (O2, CO, NO and H2) 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/O2 and O2/CO also revealed the enhancing effect of ceria. The step change experiments NO/H2 and H2/NO gave nitrogen as a main reduction product and N2O as a by-product. Preadsorption of NO on the catalyst surface decreased the catalyst activity in the reduction of NO with H2. The CO oxidation transients were modeled with a mechanism which consistent of CO and O2 adsorption and a surface reaction step. The NO reduction experiments with H2 revealed the role of N2O as a surface intermediate in the formation of N2. The formation of NN bonding was assumed to take place prior to, partly prior to or totally following to the NO bond breakage. High NO coverage favors N2O formation. Pt was shown to be more efficient than Rh for NO reduction by H2.

NOx reduction by urea in the presence of NO2 on metal substrated SCR catalysts for heavy-duty vehicles

The emission limitations for heavy-duty vehicles are coming stricter between 2005 - 2008 in Europe, Japan and United States. In addition to engine, fuel and control modifications, efficient exhaust gas after treatments like oxidation/deNO x catalysts and particulate filters are needed. In mobile truck applications the system should operate at low (<300°C) and stand high temperatures (500-650°C) in transient driving conditions. Coated V 2 O 5 /TiO 2 -WO3 based SCR catalysts on thin metal foil substrates have been studied here in laboratory and engine experiments. The open-coating method enables the high volumetric amount of SCR catalyst evenly coated on high cell density substrates (e.g. 600 cpsi). A new washcoat composition with platinum loading has been used in pre-oxidation catalyst to reach the NO 2 concentrations, which initiate the SCR reaction clearly below 300°C. Dip-coated new catalyst structures with lower cell densities and mixing channel shapes improved the efficiencies in mass transfer controlled region. The coated mixer structure confirmed uniform urea hydrolysis and ammonia generation particularly at low temperatures. The NH 3 slip was cut down by the optimization of urea injection and a small post-oxidation catalyst. The combinations of different metallic catalysts match well into modular truck silencer structures.

The optimization of light-duty diesel oxidation catalysts for preturbo, closed-coupled and underfloor positions

Diesel engines are very popular in European passenger cars and their technology has been developed to have cleaner raw emissions and lower fuel consumption. Therefore the exhaust temperatures are extremely low in urban driving conditions. The current diesel European driving cycle (EDC) and diesel catalyst ageing in different positions (Preturbo, CC and UF) were simulated successfully according to diesel light-duty exhaust gas conditions with laboratory equipment. A small mixer type EcoXcell structure was used in Preturbo position with high Pt loading to enhance in particular CO and hydrocarbon oxidations. The small metal substrated pre and larger main catalyst with active, zeolite containing washcoat were developed to decrease emissions. Both experimental and calculation simulations gave a prediction for grams per kilometer emissions for a single or combined catalyst system. The reaction and ageing rate based design can be used to optimize the diesel aftertreatment system. Engine experiments with a full-size Preturbo catalyst combined with a larger catalyst proved the efficiency of the developed catalysts.

Advanced Metallic Three-Way Catalysts with Optimized Washcoat Performance

The major challenge for future catalyst systems was to develop more thermally stable washcoats for close coupled operating conditions and for engines operating under high speed and load conditions. To design these future emission systems extensive research and development was undertaken to develop methods to disperse and stabilize the key catalytic materials for operation at much higher temperatures. The second priority was to design catalysts that are more effective under low exhaust temperature exhaust conditions and have improved oxygen storage properties in the washcoats. Incorporating new materials and modified preparation technology a new generation of metallic catalyst formulations emerged, those being trimetallic K6 (Pt:Pd:Rh) and bimetallic K7 (Pd+Pd:Rh). The target was to combine the best property of Pt:Rh (good NO{sub x} reduction) with that of the good HC oxidation activity of Pd and to ensure that precious metal/support interactions were positively maintained. Both K6 and K7 concepts contain special catalyst structures with optimized washcoat performance which can be employed either individually or combined in a single or double brick converter configuration. Improvement in light-off, thermal stability and transient performance with these new catalyst formulations has clearly been shown in both laboratory and vehicular testing.