AUTOMOTIVE EXHAUST AFTERTREATMENT
CATALYST FUNDAMENTLS Catalyst in its simplest term is a material that increase the rate (molecules converted by unit time) of a chemical reaction while itself not undergoing any permanent changre. Catalyst affects only the rate of the reaction, i.e.kinetics. It changes neither the thermodynamics of the reaction nor the equilibrium composition.
Rate of Reaction x y k0 ca cb exp( E RT ) o The activation energy for entire reaction represents the slowest of all steps involved in converting reactants to products.
TYPES OF CATALYSIS Heterogeneous catalysts Heterogeneous catalysts act in a different phase than the reactants. Most heterogeneous catalysts are solids that act on substrates in a liquid or gaseous reaction mixture. Homogeneous catalysts Homogeneous catalysts function in the same phase as the reactants, but the mechanistic principles invoked in heterogeneous catalysis are generally applicable.
MODELS OF REACTION KINETICS o Langmuir-Hinshelwood type with few global reaction steps. o Detailed catalytic surface reaction mechanism.
Absorption and Adsorption H H H H H H H H H H H H H H H H H H HH H H H H H H H H H H H H H H H H H 2 adsorption on palladium H 2 absorption palladium hydride Adsorption and Catalysis H H H H H H H H H adsorbate coverage q fraction of surface sites occupied H H H H H adsorbent
Adsorption Mechanisms Langmuir-Hinshelwood mechanisms: 1. Adsorption from the gas-phase 2. Desorption to the gas-phase 3. Dissociation of molecules at the surface 4. Reactions between adsorbed molecules The Reaction A 2 + 2B = 2AB may have the following mechanism A 2 + * = A 2 * A 2 * + * = 2A* B + * = B* A* + B* = AB* + * AB* = AB + *
Adsorption Mechanisms Eley-Rideal mechanism: 1. Adsorption from the gas-phase 2. Desorption to the gas-phase 3. Dissociation of molecules at the surface 4. Reactions between adsorbed molecules 5. Reactions between gas and adsorbed molecules The reaction A 2 + 2B = 2AB may have the following Eley-Rideal mechanism A 2 + * = A 2 * A 2 * + * = 2A* A* + B = AB + *
For the Eley-Rideal mechanism: the rate will increase with increasing coverage until the surface is completely covered by A*. For the Langmuir-Hinshelwood mechanism: the rate will go through a maximum and end up at zero, when the surface is completely covered by A*. This happens because the step B + * = B* cannot proceed when A* blocks all sites.
DISPERSED CATALYST MODEL The number of reactant molecules converted to products in a given time is directly related to the number of catalytic sites available to the reactants. It is common practice to disperse the catalytic component on a high-surface-area carrier such as Al 2O3.
REGIMES OF CATALYTIC SURFACE REACTIONS In the intrinsic surface reaction regime, the kinetics dominates over diffusion effects. In the significant pore diffusion regime, the concentration gradients in the pores become significant as a result of the diffusion through pore structures and surface reactions. The mass transfer regime, the reaction rate is limited by the mass transfer between the bulk gas and the outside surface of the washcoat. k Da V L
THREE-WAY CATALYST For homogeneous charged Spark Ignition engines, the aftertreatment device of choice is the Three-Way Catalyst (TWC). This is because TWC is extremely effective at simultaneously oxidizing HC and CO, as well as reducing NOx provided that the engine operates at a stoichiometric fuel air mixture. η cat m m m x,in x,in x,out
T.W.C METALS The three most commonly employed metals for the TWC are platinum (Pt), rhodium (Rh) and palladium (Pd). Platinum: is used to complete the oxidation of HC and CO. Rhodium: has attractive NOx reduction properties with a formation of ammonia in the process. low Palladium: can be used for oxidation and reduction in place of the Pt/Rh catalysts, but it has a smaller operating window for conversion.
SCHEMATIC DIAGRAM OF THE THREE-WAY CATALYTIC CONVERTER
GOVERNING PHENOMENA the following relevant rate processes are important: Bulk flow (gas) Interphase (gas surface) transfer Chemical reaction (surface) Heat generation (surface) Diffusion through washcoat (surface) Axial heat conduction (surface) Radial heat conduction (2D)
The efficiency of a catalyst is a function of three main factors: chemical species, temperature and residence time, so for catalyst modeling we must solve these equations: Bulk gas temperature (BGT) Bulk gas species (BGS) Surface temperature (ST) Surface species (SS) Surface intermediate species (SIS)
CLASSICAL APPROACHES Models of catalysts have existed in the literature since the late 1960s with the work by Vardi and Biller among the first reported.
CLASSICAL APPROACHES o Harned model
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