Search

The adsorption equilibria of a series of n-alkanes and 2-methyl- and 3-methyl-branched alkanes in the C5−C9 range on ZSM-22 zeolite samples were determined using the tracer chromatographic technique at temperatures between 343 and 623 K. Adsorption experiments were performed with a calcined ZSM-22 sample referred to as “open” ZSM-22 and an as-synthesized ZSM-22 zeolite sample in which the 1,6-diaminohexane template was left inside the pores, denoted as “closed” ZSM-22. In the “open” ZSM-22, the zeolite micropores are available for adsorption. In the “closed” ZSM-22, adsorption can occur in the pore mouths and on the external surface. The closing of the channels of ZSM-22 results in a significant decrease in the Henry constant and adsorption enthalpy and entropy of linear alkane molecules. Contrarily, the adsorption enthalpy and entropy of branched alkanes on “open” and “closed” ZSM-22 are nearly identical, indicating that the preferred adsorption sites for monobranched alkanes are not located inside the pores, but specifically at the entrance of the pore. For the pore mouth adsorption mode, a characteristic compensation between adsorption enthalpy and entropy was observed. The entropy losses upon adsorption inside the narrow ZSM-22 channels are much larger than in channel openings. These data support the pore mouth catalysis model for alkane hydro-isomerization on ZSM-22 type catalysts.

The adsorption of benzene on Pt(111) was analyzed using first-principles density functional theoretical cluster and periodic slab calculations. The preferred adsorption site at low coverage is the bridge(30) site with an adsorption energy of 117 kJ/mol. At the bridge(30) site, two of the C pz orbitals are well aligned for overlap with the metal dz^{2 and dyz orbitals, leading to a strong C−Pt bond and a strong adsorption energy. The molecule's second important site is the hollow(0) site with an adsorption energy of 75 kJ/mol. Comparing calculated and experimental vibrational frequencies confirms the preference for the bridge site at low coverage and also indicates that adsorption at the hollow(0) site becomes preferred at higher coverage. Adsorption at the hollow(30), the bridge(0) and at the atop sites was found to be unfavorable.}

Experimental results for the adsorption frequencies of SCN^- on a silver electrode obtained with the surface-enhanced Raman spectrometry (SERS) method are presented and discussed. These results were confronted with a quantum chemical DFT-based study. Different clusters were used from the simple monoatomic case to a much more realistic representation of the silver electrode by means of a Ag23 cluster simulating the (100) surface. The calculations were performed at the B3PW91/LanL2DZ and the BP86/TZP level. The interaction energies indicate the importance of the cluster size and calculation level. The cluster size was increased until the results were converged. Calculated vibrational frequencies for the different possible adsorption geometries on a (100) surface are compared with experiment. Together with the interaction energy results, they indicate that the preferred interaction via the sulfur atom on a hollow site of the silver electrode is in agreement with the hard and soft acids and bases principle.

Ab initio density functional theory calculations have been carried out on a model reaction involved in coke formation during the thermal cracking of hydrocarbons, namely, the addition of the ethylbenzene radical to ethene. This study enables one to get more microscopic insight into the mechanistic and kinetic aspects of the reaction. A profound ab initio conformational analysis of the formed products, reactants, and transition states is made. The impact of internal rotations on the two kinetic parameters deduced from transition state theory (TST), the activation energy and the preexponential factor, has been studied in detail. Furthermore, we report on the various components that govern the kinetic parameters. Preexponential factors are very sensitive to the accuracy of constructing the microscopic partition functions. Internal rotations play a dominant role in the reaction mechanism, and their impact on the preexponential factor is large. Hence, a very accurate handling of internal rotations is of crucial importance. We present a new algorithm to extract exactly on a quantum mechanical basis the partition functions of the internal rotations. The calculations as presented here are especially important for complex reaction schemes, for which experimental data are not always available.