Site coverage

The computational approach couples the two-phase LB model for the liquid water transport and the DNS model for the species and charge transport for the CL.25-27,68 The two-phase simulation using the LB model is designed based on the ex-situ, steady-state flow experiment for porous media, detailed earlier in the section 4.3, in order to obtain the liquid water distributions within the CL microstructure for different saturation levels resulting from the dynamic interactions between the two phases and the underlying pore morphology. The details of the simulation setup are provided in our work.27,61 62 Once steady state is achieved, 3-D liquid water distributions can be obtained within the CL, as shown in Fig. 13. From the liquid water distributions within the CL structure, the information about the catalytic site coverage effect can be extracted directly. 

Figure 23. Catalytic site coverage relation as function of liquid water saturation for the CL structure.

With the evaluated site coverage and pore blockage correlations for the effective ECA and oxygen diffusivity, respectively, and the intrinsic active area available from the reconstructed CL microstructure, the electrochemistry coupled species and charge transport equations can be solved with different liquid water saturation levels within the 1-D macrohomogeneous modeling framework,25,27 and the cathode overpotential, q can be estimated. 

The mode of deactivation depends on the zeolite pore structure. With monodimensional zeolites such as MOR and with zeolites with trap cavities (large cavities with narrow apertures) such as ERI deactivation occurs mainly through pore blockage while with tridimensional zeolites such as Y and ZSM5 it occurs mainly through site coverage. 

Various mechanisms of coke poisoning active site coverage, pore filling as well as pore blockage have been observed in FCC [18, 19, 43] and Percolation theory concepts have been proposed for the modelling here of [45, 46, 47, 48]. This approach provides a framework for describing diffusion and accessibility properties of randomly disordered structures. 

During induction, catalyst activity and selectivities to aromatics and propene increase steadily. Improvement of catalyst performance is due to increase in Ga dispersion and formation of dispersed Ga species (Gao) which are efficient for the heterolytic recombinative release of hydrogen [18,191. The Ga/H-MFI catalyst then reaches its optimal aromatisation performance (stabilisation). Ci to C3 hydrocarbons productions are at their lowest. The gallium dispersion and the chemical distribution of Ga are optimum and balance the acid function of the zeolite. Reversible deactivation during induction and stabilisation of the catalyst is due to site coverage and limited pore blockage by coke deposition. 

The deactivation of cracking catalysts by coking with vacuum gas oils (VGO) is studied in relation to the chemical deactivation due to site coverage, and with the increase of diffusional limitations. These two phenomena are taken into account by a simple deactivation function versus catalyst coke content. The parameters of this function arc discussed in relation to feedstock analysis and change of effective diffiisivity with catalyst coke content. 

At low coke content, pore plugging is still negligible and decay is mainly due to site coverage. Consequently, the variation of deactivation function with coke content is only due to chemical deactivation, and it is proportional to the remaining activity ... 

Some cases of catalyst deactivation by over-oxidation platinum leaching, platinum particle growth and site coverage during reductive pretreatment as well as during reaction were presented for the oxidation of ethanol and methyl-a-D glucopyranoside (MGP) in combination with the use of various catalyst characterization techniques. 

The reductive conditions during catalyst pretreatment may cause an irreversible deactivation as a result of particle growth and site coverage especially at high pH. 

Figure 7. Experimental data and model calculations of glutamic acid adsorption on amorphous iron oxyhydroxide as a function of pH and total glutamate added. For model calculations a surface site coverage of 18 sites/adsorbed glutamate molecule was assumed, Fe(OH)s(am), lO M O.IM NaNOs, 25°C. Glutamic (O) acid 1.1 X added (A) (----) Model calculation.

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