Single zeolite-based systems

The dual-layer ASC system herein studied consisted of commercial state-of-the-art PGM and SCR catalysts the PGM catalyst was a Pt/Al203-based system, while an Fe-zeolite was used as the NH3-SCR catalyst component. The original ASC monolith was a dual-layer system with an SCR layer coated on top of a PGM one. For the present study, in addition to the dual-layer monolith, the SCR and PGM catalysts were supplied both in the form of precursor powders of the monolith washcoat and in the form of single washcoated monoliths (400 CPSI, wall thickness = 5 mils). 

The positions of non-framework cations in aluminosilicate zeolites can control or fine-tune their sorptive and catalytic properties. Measurement, however, requires careful and usually protracted analyses of accurate single crystal or powder dif action data. In cases for which extensive experimental data are available, statistical mechanics analyses can yield insight into relative site energies [53-55] etirlier analyses have also attempted to quantify the relative importance of short and long-range interactions in controlling site occupancy patterns [56]. Earlier atomistic simulations in this area [57-62] had mixed results. Recent developments in methods and interatomic potentials have allowed non-framework cation positions to be simulated based solely on a knowledge of the framework structure in zeolite systems for which validatory experimental data are available [113]. 

The Coupled-clusters (CC) method[7] based on the cluster expansion of the wavefunction has been established as a highly reliable method for calculations of ground state properties of small molecules with the spectroscopic accuracy. When this method is used together with a flexible basis set it recovers the dominant part of the electron correlation. Typically, CC variant explicitly considering single and double excitations (CCSD) is used. In order to save computer time the contributions from triple excitations are often calculated at the perturbation theory level (notation CCSD(T) is used in this case). CCSD(T) method can be routinely used only for systems with about 10 atoms at present. Therefore, it cannot be used directly in zeolite modeling, however, results obtained at CCSD(T) level for small model systems can serve as an important benchmark when discussing the reliability of more approximate methods. 

The FTIR technique has proven to be a powerful method for investigating adsorption, desorption, and diffusion of single components or binary mixtures in microporous solids such as zeolites. In the latter case of mixtures, the phenomena of codiffusion and counter-diffusion became accessible to measurement, which was not possible with methods of investigation based on changes of weight, volume, or pressure. Even with the powerful and most important NMR techniques (see Chap. 3 of the present volume), the study of multicomponent (e.g., H2-D2) self-diffusion rather than co- and counterdiffusion experiments is possible (see Sect. 1 and [6]). The only prerequisite for the IR method is that the IR spectra, which are contributed by the components of the mixture, can be sufficiently decomposed. This, however, was easily achieved for all systems studied so far, owing to appropriate computer programs nowadays available. Certainly, the computational methods... 

FIGURE 4.19. Comparison of experimental equilibrium data for the system ri-heplane-cyclohexane on I3X zeolite with the theoretical predictions of the generalized statistical model [Eq. (4.18)] based on single-component data, (From ref. 17.)... 

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