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Virtual mechanism approach

Figure 8.2 The virtual mechanism approach based on (a) CH4/D2 surface area normalized exchange rates at 773 K (CH4/D2 = 1 1, 700 h ). (b) Relative ranking order for the exchange of 02/ 02 with oxide surfaces normalized at 773 K. (c) Ranking order for methanol stability based on the temperature at which 30% of the methanol feed was converted to carbon oxides. The same rank order found if different methanol conversion levels were chosen (From Hutchings, G.J. and Scurrell, M.S. CATTECH 2003, 7, 90-103.)... Figure 8.2 The virtual mechanism approach based on (a) CH4/D2 surface area normalized exchange rates at 773 K (CH4/D2 = 1 1, 700 h ). (b) Relative ranking order for the exchange of 02/ 02 with oxide surfaces normalized at 773 K. (c) Ranking order for methanol stability based on the temperature at which 30% of the methanol feed was converted to carbon oxides. The same rank order found if different methanol conversion levels were chosen (From Hutchings, G.J. and Scurrell, M.S. CATTECH 2003, 7, 90-103.)...
Possibly, the earliest design approach in this area has been applied by Dowden and co-workers (435) through the derivation and application of a virtual mechanism. By thermodynamic analysis of the target reaction and side reactions, it was concluded that the functions of a suitable catalyst were dehydrogenation and oxygen insertion. Oxidation reactions all led preferentially to the production of carbon oxides. A hypothetical surface mechanism by which the preferred products were formed was proposed, and this is shown in Figure 52. [Pg.1524]

Shokhen et al. [56] analyzed possible mechanisms for the reversible formation of the complex between papain, a prototype enzyme of cysteine proteases, and pep-tidy 1 aldehyde inhibitors, using the quantum mechanical (DFT)/self consistent reaction field (virtual solvent) approach. [Pg.217]

Several examples of the application of quantum mechanics to relatively simple problems have been presented in earlier chapters. In these cases it was possible to find solutions to the Schrtidinger wave equation. Unfortunately, there are few others. In virtually all problems of interest in physics and chemistry, there is no hope of finding analytical solutions, so it is essential to develop approximate methods. The two most important of them are certainly perturbation theory and the variation method. The basic mathematics of these two approaches will be presented here, along with some simple applications. [Pg.151]

It is much less clear how the adsorption leads to such a dramatic change as a potential decay of several hundred volts, occurring within milliseconds. This short time is difficult to associate with film thinning, as assumed in the adsorption mechanism of pit initiation. It is not only that the mechanism of dissolution changes so much that the current efficiency falls from virtually 100% to virtually zero, but also that the resistance of the oxide decreases by orders of magnitude. The control of the process is, to a great extent, taken over by the events at the O/S interface, judging from the capacitance values measured,115 which approach those typical of the electrochemical double layer (cf. Fig. 22). [Pg.442]

To overcome thermal entry effects, the segments may be virtually stacked with the outlet conditions from one segment that becomes the inlet conditions for the next downstream section. In this approach, axial conduction cannot be included, as there is no mechanism for energy to transport from a downstream section back to an upstream section. Thus, this method is limited to reasonably high flow rates for which axial conduction is negligible compared to the convective flow of enthalpy. At the industrial flow rates simulated, it is a common practice to neglect axial conduction entirely. The objective, however, is not to simulate a longer section of bed, but to provide a developed inlet temperature profile to the test section. [Pg.341]

However, despite their proven explanatory and predictive capabilities, all well-known MO models for the mechanisms of pericyclic reactions, including the Woodward-Hoffmann rules [1,2], Fukui s frontier orbital theory [3] and the Dewar-Zimmerman treatment [4-6] share an inherent limitation They are based on nothing more than the simplest MO wavefunction, in the form of a single Slater determinant, often under the additional oversimplifying assumptions characteristic of the Hiickel molecular orbital (HMO) approach. It is now well established that the accurate description of the potential surface for a pericyclic reaction requires a much more complicated ab initio wavefunction, of a quality comparable to, or even better than, that of an appropriate complete-active-space self-consistent field (CASSCF) expansion. A wavefunction of this type typically involves a large number of configurations built from orthogonal orbitals, the most important of which i.e. those in the active space) have fractional occupation numbers. Its complexity renders the re-introduction of qualitative ideas similar to the Woodward-Hoffmann rules virtually impossible. [Pg.328]


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Mechanical approach

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