Reaction mechanisms heterogeneous

The boundary-layer equations represent a coupled, nonlinear system of parabolic partial-differential equations. Boundary conditions are required at the channel inlet and at the extremeties of the y domain. (The inlet boundary conditions mathematically play the role of initial conditions, since in these parabolic equations x plays the role of the time-like independent variable.) At the inlet, profiles of the dependent variables (w(y), T(y), and Tt(y)) must be specified. The v(y) profile must also be specified, but as discussed in Section 7.6.1, v(y) cannot be specified independently. When heterogeneous chemistry occurs on a wall the initial species profile Yk (y) must be specified such that the gas-phase composition at the wall is consistent with the surface composition and temperature and the heterogeneous reaction mechanism. The inlet pressure must also be specified. 

Heterogeneous chemistry at a wall creates a further constraint on the initial conditions. At the inlet, the gas-phase composition, surface composition, and temperature must be specified such that they are consistent with the heterogeneous reaction mechanism. Specifically, the net surface production rates for each surface species must vanish i = 0 (Section 11.10). 

Heterogeneous reaction mechanisms range from the very simple to the complex. Many features of the formalism presented in this chapter are illustrated by the catalytic combustion reaction mechanism given in Table 11.1. The surface-reaction mechanism is due to Sidwell et al. [361], which in turn had its origins with the work of Schmidt [173,174] and Deutschmann [96,97,101],... 

The surface composition, usually represented by site fractions Z, must adjust itself to be consistent with the local gas-phase composition, temperature, and the heterogeneous reaction mechanism. When the surface composition is represented by site fractions, the definition requires that... 

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 ... 

A generalized heterogeneous reaction mechanism can be expressed in symbolic formas... 

Approximately 75% of all heterogeneous reaction mechanisms arc surface-reaction-limited. 

Besides the heavy chemical industry, where catalysis is a dominant feature of most conversion processes, enzyme catalysis is a critical component of bio-chemical processes. All that was said about mechanisms of catalytic reactions applies to enzyme catalysis. As can be expected, there are additional factors in enzyme catalysis that complicate matters. Many enzymatic reactions depend on factors such as pH, ionic strength, co-catalysts and so on that do not normally play a role in conventional heterogeneous catalysis. Despite this, the understanding of mechanisms in enzyme catalysis has outpaced that in heterogeneous catalysis and can now serve as a guide to the search for heterogeneous reaction mechanisms. 

This chapter presented the analysis of various porous media transport models and its validation under reaction conditions. The elementary heterogeneous reaction mechanism reported by Hecht et al. has been extended for wider temperature conditions [64]. All model predictions are compared with the experiment reported by Hecht et al. [64]. In general DGM and MFM give almost the same results in all cases. Similarly MTPM and MMS model gives the same results for all cases. For the cases as considered here it is quite obvious that DGM and MFM are able to reproduce the experimental observation to a great extend. MTPM as well... 

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