Is the Desorption of Benzene Rate-Limiting

From the rate expression for. surface reaction. Equation (10-26), we set 

After substituting for the respective surface concentrations, we solve the site 

Replacing in Equation (10-48) by Equation (10-49) and multiplying the numerator and denominator by Pp. we obtain the rate expression for desorption control  

Cumene decomposition rate law if desorption were limiting 

If deso tion limits, the initial rate is independent of partial pressure of cumene. 

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Is the Surface Reaction Rate-Limiting 677 Is the Desorption of Benzene Rate-Limiting 678... 

Before checking to see if Figure 10-12 is consistent with experimental observation, we shall derive the corresponding rate laws and initial rate plots when the surface reaction is rate-limiting and then when the desorption of benzene is rate-limiting. 

Thus, the reaction on Pd/Fsc is rate limited in the last step, the conversion of (C4H4)(C2H2) into CeHe- This is different from the Pd(lll) surface where the rate determining step for the reaction is benzene desorption. The calculations are consistent with the experimental data. In fact, on Pdi/Fsc, the computed barrier of 0.98 eV corresponds to a desorption temperature of about 300K, as experimentally observed (Fig. 1.100). On Pd (111) surfaces, the bonding of benzene is estimated to be 1.9eV. This binding is consistent with a desorption temperature of 500 K as observed for a low coverage of CeHe on Pd(lll) [489],... 

Vapor-phase alkylation of benzene by ethene and propene over HY, LaY, and REHY has been studied in a tubular flow reactor. Transient data were obtained. The observed rate of reaction passes through a maximum with time, which results from build-up of product concentration in the zeolite pores coupled with catalyst deactivation. The rate decay is related to aromatic olefin ratio temperature, and olefin type. The observed rate fits a model involving desorption of product from the zeolite crystallites into the gas phase as a rate-limiting step. The activation energy for the desorption term is 16.5 heal/mole, approximately equivalent to the heat of adsorption of ethylbenzene. For low molecular weight alkylates intracrystalline diffusion limitations do not exist. 

We now propose a mechanism for the hydrodemethylation of toluene. We assume that toluene is adsorbed on the surface and then reacts with hydrogen in the gas phase to produce benzene adsorbed on the surface and methane in the gas phase. Benzene is then desorbed from the surface. Since approximately 75% of all heterogeneous reaction mechanisms are surface-reaction-limited rather than adsorption- or desorption-limited, we begin by assuming the reaction between adsorbed toluene and gaseous hydrogen to be reaction-rate-limited. Symbolically, this mechanism and associated rate laws for each elementary step are ... 

The cyclotrimerization of acetylene to benzene has been studied by Rucker et al. (8J) over Pd(lll), (100), and (110) at pressure near 1 atm. The (110) surface was four-fold less active than Pd(lll) or (100), which contrasts with their relative selectivities during TDS under UHV conditions. The activity at high pressures was correlated with the fraction of the various surfaces that exposed clean Pd atoms, as probed by postreaction CO adsorption-desorption. In all cases, most of the surface was covered with a carbonaceous residue. The authors stated that the reaction rate is first-order in acetylene pressure for all three surfaces. The extensive data on the Pd(lll) surface clearly indicate first-order kinetics in that case. However, the limited data presented for Pd(110) seem (to the present author) to be better fitted by an order of —2.5, which is closer to the value of three suggested by the overall stoichiometry. 

The only step in the overall oxidation reaction cycle which is endothermic is step 2, which involves the direct insertion of oxygen into the C-H bond of benzene. This is costly since it requires the loss of aromaticity in the benzene ring. All other steps in the cycle are exothermic. Furthermore, matrix effects are absent in this reaction. The main role of the lattice appears to be to stabilize Fe + and prevent over-oxidation of N2O decomposing Fe " " oxyhydroxy dicationic clusters. The overall result is that the rate-limiting step for phenol formation is the rate of desorption of phenol. The relative concentration of the different sites varies with Fe loading, as illustrated in Fig. 4.29b. Whereas the rate of phenol formation increases steeply with the Fe content, when the Fe concentration is low, at higher Fe content N2O decomposition increases, but phenol production is constant. 

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