Irreversible surface-reaction-limited

Table 10-4. Irreversible Surface-Reaction-Limited Rate Laws...

There are a number of examples of tube waU reactors, the most important being the automotive catalytic converter (ACC), which was described in the previous section. These reactors are made by coating an extruded ceramic monolith with noble metals supported on a thin wash coat of y-alumina. This reactor is used to oxidize hydrocarbons and CO to CO2 and H2O and also reduce NO to N2. The rates of these reactions are very fast after warmup, and the effectiveness factor within the porous wash coat is therefore very smaU. The reactions are also eternal mass transfer limited within the monohth after warmup. We wUl consider three limiting cases of this reactor, surface reaction limiting, external mass transfer limiting, and wash coat diffusion limiting. In each case we wiU assume a first-order irreversible reaction. 

Table 10-5 gives the forms of rate laws for different reaction mechanit that are irreversible and surface-reaction-limited. 

Figure 2.9. Simulation based on mechanism (2.113) for the catalytic reaction between A and B. with competitive equilibrium adsorption of A and B, a rate-limiting irreversible surface reaction between Aads and B ds and a kinetically insignificant irreversible desorption of the product AB. Dashed lines represent the high temperature limits. Input values A// =-94kJ/mol, A// = -lOOkJ/mol, /r [A] = 10" = 2 10 the reaction rates have been scaled to unity at maximum.

Let us find the rate and apparent activation energy of the stationary process in a particular situation of the kinetically irreversible stepwise reaction of the CO oxidation, step 1 being the rate-limiting stage and intermediate K2 being dominant on the surface. The stationary rate of the overall stepwise process (4.60) is... 

A measure of the absence of internal (pore diffusion) mass transfer limitations is provided by the internal effectiveness factor, t, which is defined as the ratio of the actual overall rate of reaction to the rate that would be observed if the entire interior surface were exposed to the reactant concentration and temperature existing at the exterior of the catalyst pellet. A value of 1 for rj implies that all of the sites are being utilized to their potential, while a value below, say, 0.5, signals that mass transfer is limiting performance. The value of rj can be related to that of the Thiele modulus, 4>, which is an important dimensionless parameter that roughly expresses a ratio of surface reaction rate to diffusion rate. For the specific case of an nth order irreversible reaction occurring in a porous sphere,... 

If surface reaction is assumed to be rate limiting and irreversible (and no adsorbed inerts are involved), the overall rate expression for consumption of A becomes -rA = A aCa/(1 + KaCa + KbCb), where k is the surface reaction rate constant and Ka and A b are adsorption equilibrium constants. If the surface is only sparsely covered, i.e., KaCa + KbCb 1, this can be approximated as simply va kKACA = k CA-This illustrates how a simple power law rate expression can apply, under some circumstances, for what is actually a relatively complex mechanism. 

In the original work on this reaction by Papp et al.. over 25 models wet tested against experimental data, and it was concluded that the precedin mechanism and rate-limiting step (i.e., the surface reaction between adsorbe toluene and gas) is the correct one. Assuming that the reaction is essential) irreversible, the rate law for the reaction on clinoptilolite is... 

In fact the EADA is the limiting case of the SSA when the rates of adsorption and desorption are much faster than the rate of surface reaction. The use of SSA for the simple case of unimolecular irreversible reaction does not cause significant added complexity, however for more complex reactions the SSA causes considerable complexity and most CSD kinetic models are based on the EADA. 

The mechanism for this reaction is shown below. A, B2 and C. are In adsorption-desorption equilibriimi with the surface. The bimolecular surface reaction is irreversible and rate limiting. ... 

At relatively low pressures, what dimensionless differential equations must be solved to generate basic information for the effectiveness factor vs. the intrapellet Damkohler number when an isothermal irreversible chemical reaction occurs within the internal pores of flat slab catalysts. Single-site adsorption is reasonable for each component, and dual-site reaction on the catalytic surface is the rate-limiting step for A -h B C -h D. Use the molar density of reactant A near the external surface of the catalytic particles as a characteristic quantity to make all of the molar densities dimensionless. Be sure to define the intrapellet Damkohler number. Include all the boundary conditions required to obtain a unique solution to these ordinary differential equations. 

Two expressions are given below to calculate the effectiveness factor E. The first one is exact for nth-order irreversible chemical reaction in catalytic pellets, where a is a geometric factor that accounts for shape via the surface-to-volume ratio. The second expression is an approximation at large values of the intrapellet Damkohler number A in the diffusion-limited regime. 

Effect of the Damkohler Number on Conversion in Square Ducts. More conversion is predicted at higher Damkohler numbers because the rate of surface-catalyzed chemical reaction is larger. At a given axial position z, reactant conversion reaches an asymptotic limit in the diffusion-controlled regime, where oo. Actual simulations of I Abuik vs. f at /i = 20 are almost indistinguishable from those when p = 1000. The effect of p on bulk reactant molar density is illustrated in Table 23-5 for viscous flow in a square duct at = 0.20, first-order irreversible chemical reaction, and uniform catalyst deposition. These results in Table 23-5 for the parameter A, as a function of the Damkohler number p can be predicted via equations (23-80) and (23-81) when C = A and... 

However, only a few organic compounds behave in a polarographically reversible manner although many may involve a reversible electron transfer step, this is often followed by irreversible chemical reactions. Irreversible processes are those for which the current is limited mainly by the kinetics of the process at the electrode surface and not by diffusion. The nature of such current-potential curves can be described by reference to Figure 6. If electrochemical equilibrium obtains at the electrode surface, then a reversible wave is obtained (curve a). The irreversible wave (curve b) is more drawn out, i.e., for a given current, say, /i or I2, a higher cathodic potential is required. 

It is possible to determine the effect on polymers of different substances or media and to differentiate them [3]. When a medium causes a chemical reaction with a polymer, it is called a chemically active medium [2]. These effects are irreversible and the deterioration caused to the polymeric material is very important. Such media are acids, bases, oxidants, and all other substances that can cause chemical reactions such as substitution, addition, and hydrolysis. In some cases the chemical reaction is limited to the polymer surface and produces a protective layer against a deeper deterioration by the corrosive chemical. Such surface reactions are those produced by nitric acid on vulcanized PI and SB rubbers or by sulfuric acid on vulcanized polychloroprene and NR [4-7]. 

The SIMS technique, as originally developed, was limited to relatively low molecular weights (m/z < 500 Da) due to limitations of the quadrupole mass analyzer. Latter, the development of TOF-SIMS extended the mass range to 10,000 Da. The problem with SIMS is that organic compounds often yield both intense structurally characteristic secondary ions and abundant nonspecific secondary ions. Quantitative analysis with SIMS is difficult mainly because of ion optical and matrix effects. With inorganic samples, uses of internal standards are required. For organic systems, factors such as sample/matrix evaporation, sputtering, and irreversible chemical reaction need to be determined. SIMS is always a surface technique [14]. 

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