Adsorption cluster models

Methanol Adsorption Cluster Models and Plane Wave Methods Applying Periodic Boundary Conditions... 

Duarte H A and Salahub D R 1998 Embedded cluster model for chemisorption using density functional calculations oxygen adsorption on the AI(IOO) surface J. Chem. Phys. 108 743... 

Fukunishi Y and Nakatsu] H 1992 Modifications for ab initio calculations of the moderately large-embedded-cluster model. Hydrogen adsorption on a lithium surface J. Chem. Phys. 97 6535-43... 

From the theoretical standpoint the above issues are addressed by quantum chemistry. On the basis of calculations of various cluster models [191] the properties of surfaces of solid body are being studied as well as issues dealing with interaction of gas with the surface of adsorbent. However, fairly good results have been obtained in this area only to calculate adsorption on metals. The necessity to account for more complex structure of the adsorption value as well as availability of various functional groups on the surface of adsorbent in case of adsorption on semiconductors geometrically complicates such calculations. 

Zygmunt, S. A., Mueller, R. M., Curtiss, L. A., Iton, L. E., 1998, An Assessment of Density Functional Methods for Studying Molecular Adsorption in Cluster Models of Zeolites , J. Mol. Struct. (Theochem), 430, 9. 

In heterogeneous catalysis, the catalyst often exists in clusters spread over a porous carrier. Experimentally, it is well established that reactivity and selectivity of heterogeneous reactions change enormously with cluster size. Thus, theoretical studies on clusters are particularly important to establish a basis for the determination of their optimal size and geometry. Cluster models are also important for studying the chemistry and reactivity of perfect crystal faces and the associated adsorption and desorption processes in heterogeneous catalysis (Bauschlicher et al, 1987). 

Mo(CO)6 and Co(CO)3NO NaY zeolite Adsorption from vapor phase and H2S treatment Intrazeolite Co2Mo2S i clusters, model hydrodesulfuration catalyst [25]... 

M. Philpott, Electrochemical Contact Adsorption Site Changes Driven by Field and Charge Fact and Theory, in Cluster Models for Surface and Bulk Phenomena, G. Pacchioni ed., Plenum, New York (1992/... 

Of course, certain features of overall kinetics are inaccessible via a cluster model method, such as the influence of pore structure on reactivity. The cluster model method cannot integrate reaction rates with concepts such as shape selectivity, and an alternative method of probing overall kinetics is needed. This has recently been illustrated by a study of the kinetics of the hydroisomerization of hexane catalyzed by Pt-loaded acidic mordenite and ZSM-5 (211). The intrinsic acidities of the two catalysts were the same, and differences in catalyst performance were shown to be completely understood on the basis of differences in the heat of adsorption of hexene, an intermediate in the isomerization reaction. Heats of adsorption are strongly dependent on the zeolite pore diameter, as shown earlier in this review (Fig. 11). 

Fig. 7. Energy-minimized structures of acetone and mesityl oxide adsorption complexes on a cluster model of HZSM-5 using DFT calculation. Note that in the case of the acetone complex, the proton remains bonded to the bridging oxygen, while in the case of the mesityl oxide complex, the proton is more fully transferred to the ketone. (Reprinted with permission from Haw et al. (7). Copyright 1996 American Chemical Society.)...

In cluster models constructed to mimic adsorption at on top positions an all electron description must always used for the on top metal centre. The accuracy of this type of embedding model is very high compared with full all electron calculations [19-21]. Modelling the adsorption at bridge positions should ideally be done using two all electron centra. Such calculations do, however, become rather costly, and a simplified approach is to correct the ECP results close to the bridge site by comparing ECP and all electron results obtained for two metal atoms and the adsorbate. 

The theoretical studies applying cluster model approach [148, 149] and periodic approximation [150] devoted to the description of interaction of dickite and kaolinite with the FA, MFA and DMSO molecules have been performed. These works have studied the position and the orientation of the adsorbed and intercalated organic molecules with respect to the surface of mineral, interaction between the organic molecule and the mineral, interaction energy, the influence of the intercalation and adsorption on changes of geometry parameters, electron structures of organic molecules, and the surfaces of the minerals. 

Ab initio B3LYP cluster model calculations have been performed to describe the adsorptive behaviour of NO on MgO solid [108]. The most preferable configurations of the NO, NO22" and N2O32 surface complexes were determined. The calculated IR frequencies of these species accounted well for the temperature dependence of the experimental IR spectra. 

We emphasize two natural limitations of the finite cluster model. It does not allow to make a statement about the dependence of essential parameters such as adsorption and transition energies on the level of surface coverage, and it does not account adequately for charge delocalization or surface relaxation phenomena. Further, it excludes by definition any information about the modification of the surface band structure as a consequence of the organic molecule adsorption. The following case study of 1-propanol on Si(001) - (2 x 1) is intended to clarify how these elements can be consistently incorporated into the description of the Si surface interaction with organic species. 

Fig. 6. Cluster models for (a, b) two types of adsorption of ROH molecules and for (c) the intermediate state (C2 ) of the H/D isotopic exchange reaction with terminal hydroxyl groups.

Fig. 9. Cluster model of the coordinative adsorption of a H20 molecule on (a) the face and (b) the edge of a silicon-oxygen tetrahedron in Si02.

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