No abstract

The interaction of a sodium cation and six-rings of a zeolite framework with different numbers of aluminum atoms was studied with the aid of a density functional method and cluster models. The cation preferentially interacts with the oxygen atoms connected to aluminum at shortest distances of about 220 pm. For six-rings with two or three aluminum atoms, sodium positions on each side of the rings were identified. At both locations Na+ interacts mainly with the oxygen atoms directed toward the inside of the six-ring. From a comparison of calculated Na−O distances with crystallographic data and vibrational frequencies with the IR spectra of CO on NaY zeolite, it is deduced that the crystallographic SII site in faujasites may actually consist of two cation positions. The sodium location in a ring with one aluminum atom differs from that in the other clusters; it is almost in the plane of the T atoms. The sodium cation was calculated to experience a very low-energy barrier for crossing the six-ring (ca. 10 kJ/mol), which allows easy displacement perpendicular to the ring upon interaction with guest molecules. The vibrational frequency of adsorbed carbon monoxide was calculated as a measure for the polarizing power of the sodium cation. Both the calculated binding energies of CO and vibrational frequency shifts are in agreement with the experimental values. According to our results, the aluminum content of the six-rings can be the reason for the asymmetric broadening of the stretching mode region in the IR spectra of CO adsorbed on NaY zeolite at low pressure. The calculations indicate that a discrimination of six-rings of different aluminum content via IR spectroscopy is difficult.

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Very small transition metal particles can be stabilized inside zeolite cavities. Both electron-enriched and electron-deficient encapsulated metal species have been proposed on the basis of experimental data. In this work, structure and adsorption properties of the cluster Pt 4 , in both neutral and electronically modified forms, have been studied computationally with the help of a scalar-relativistic density functional method. The species Pt 4 + has been chosen to represent the case of a metal particle interacting with an electron attracting zeolite host; likewise, Pt 4 -has been taken to mimic the effect of an electron-donating host. Adsorption of CO probe molecules at on-top, bridge, and 3-fold hollow sites of the moieties Pt 4 , Pt 4 + , and Pt 4 -has been investigated to determine a relationship between the cluster charge and the C-O vibrational frequency shift ∆ω(CO). The chemical effect of electron-donor and electron-acceptor species on the electronic structure of the Pt 4 clusters and on the properties of adsorbed CO probes has been also explicitly taken into account by employing various models XPt 4 CO (X ) Na, Na + , NH 3 ). Properties of adsorbed CO probe molecules were calculated to be rather sensitive to the electronic state and the adsorption site of the Pt 4 particles, in line with experimental findings. A linear correlation between the effective charge of the metal cluster and the adsorption-induced vibrational frequency shift ∆ω(CO) has been found for CO adsorbed at on-top positions.

A density functional investigation of the x-ray emission spectrum of NO adsorbed on Ru(001) has been carried out using model cluster calculations. The dipole matrix elements governing the emission probability were evaluated in the frozen ground-state approximation. The resulting simulated spectra exhibit all characteristic features of the experimental data. A detailed analysis of the electronic structure of the model clusters permits a complete rationalization of all observed trends. Furthermore, a picture of the surface chemical bond results in which the classical Blyholder frontier orbital model is extended to a three-orbital description for both the π and σ interactions. Comparison of different adsorption sites reveals that threefold coordinated NO features a stronger orbital interaction with the substrate than NO adsorbed in an on-top position.