Zeolite atomic species

Figure 1. Relative coherent and incoherent scattering cross-sections for some zeolite atomic species.

Irradiation was therefore carried out on silver doped A-zeo-lites at 77 K and a new silver atom species was detected with a different hyperfine splitting than that of the one observed at 4K. This species is designated as Ag°(A) with an isotropic hyperfine splitting of about 1985 MHz which is very close to that of the free atom value. Species Ag°(A) is the dominant species formed by irradiation at 77 K in all of the A-zeolites studied. In addition, species Ag°(B) is also visible at 77 K. However, upon warming above 77 K species Ag°(B) decays to apparently yield Ag°(A). 

Supported metal clusters play an important role in nanoscience and nanotechnology for a variety of reasons [1-6]. Yet, the most immediate applications are related to catalysis. The heterogeneous catalyst, installed in automobiles to reduce the amount of harmful car exhaust, is quite typical it consists of a monolithic backbone covered internally with a porous ceramic material like alumina. Small particles of noble metals such as palladium, platinum, and rhodium are deposited on the surface of the ceramic. Other pertinent examples are transition metal clusters and atomic species in zeolites which may react even with such inert compounds as saturated hydrocarbons activating their catalytic transformations [7-9]. Dehydrogenation of alkanes to the alkenes is an important initial step in the transformation of ethane or propane to aromatics [8-11]. This conversion via nonoxidative routes augments the type of feedstocks available for the synthesis of these valuable products. 

When the sample contains several atom species, r, the expression (4) has many different terms and therefore it is quite difficult in complex systems such as zeolite and sorbate to interpret the spectra in terms of dynamics. However for periodic systems (such as crystals) the integration over energy of equation (4) leads to ... 

Synthetic zeolites are usually microcrystalline and furthermore typically contain four 10-electron atomic species (Si, AP, O and Na ) which makes them difficult to study by conventional techniques of structural elucidation. The development of high-resolution solid-state NMR techniques, such as magic-angle spinning (MAS), gave zeolite chemistry a powerful structural tool to monitor all elemental components of such frameworks. 

Gas transport through nonporous inorganic membranes falls into two categories. It is known that the conventional solution-diffusion permeation mechanism is valid for nonporous membranes of silica, zeolite and inorganic salts. It is no longer so when the membrane is metallic in nature (Hwang and Kammermeyer, 1975). Diatomic gases such as O2, H2 and N2 dissolve atomically in the metallic membrane (see (3.3.67)). While a conventional flux expression is valid for atomic species i dissolved in the membrane, Le. 

The variation in the lattice vibration of the solid products was examined by utilizing the FT-IR technique at successive DGC process times and the results are presented in Fig. 5. The absorption bands at 550 cm and 450 cm" are assigned to the vibration of the MFI-type zeolite and the internal vibration of tetrahedral inorganic atoms. The band 960 cm" has been assigned to the 0-Si stretching vibration associated with the incorporation of titanium species into silica lattice [4], This indicates that the amorphous wall of Ti-MCM-41 was transformed into the TS-1 structure. 

Taking into consideration the preparation procedures, the Mo content of 2.1Mo/SC, and the Co/Mo atomic ratio of ca. unity at the maximum HDS activity, highly dispersed Co-Mo binary sulfide clusters, possibly COjMOjSx, in the supercage of the NaY zeolite are suggested for catalytically active species. The HDS activity of the CoSx-MoSx/NaY was not changed even after a 20-h treatment at 673 K in a stream of HjS/H (Fig.3), demonstrating a high thermal stability of the active species. 

An in situ infrared investigation has been conducted of the reduction of NO by CH4 over Co-ZSM-5. In the presence of O2, NO2 is formed via the oxidation of NO. Adsorbed NO2 then reacts with CH4. Nitrile species are observed and found to react very rapidly with NO2, and at a somewhat slower rate with NO and O2. The dynamics of the disappearance of CN species suggests that they are reactive intermediates, and that N2 and CO2 are produced by the reaction of CN species with NO2. While isocyanate species are also observed, these species are associated with A1 atoms in the zeolite lattice and do not act as reaction intermediates. A mechanism for NO reduction is proposed that explains why O2 facilitates the reduction of NO by CH4, and why NO facilitates the oxidation of CH4 by O2. 

In Ag-SAPO-ll/C2H4 zeolite the EPR at 77 K shows the spectra of Ag° atoms and C2H5 radicals. After annealing at 230 K those species disappeared and then an anisotropic EPR sextet was recorded. Based on DFT calculation the structure of complex was proposed in which two C2H4 ligands adopted eclipsed confirmation on either side of the Ag atom. As a result the overwhelming spin density is localised on ethylene orbitals. 

The high activity of Cr-Cl catalyst may result probably from the formation of more active species such as Cr02+ complex cation during the ammoxidation reaction. Some authors [7] assumed that Cr(V) species occurred inside the zeolite structure as complex cation such as Cr02+ coordinated to two framework oxygen atoms and suggest the following two step process as the most probable pathway of chromate formation ... 

Catalytic oxidative dehydrogenation of propane by N20 (ODHP) over Fe-zeolite catalysts represents a potential process for simultaneous functionalization of propane and utilization of N20 waste as an environmentally harmful gas. The assumed structure of highly active Fe-species is presented by iron ions balanced by negative framework charge, mostly populated at low Fe loadings. These isolated Fe sites are able to stabilize the atomic oxygen and prevent its recombination to a molecular form, and facilitate its transfer to a paraffin molecule [1], A major drawback of iron zeolites in ODHP with N20 is their deactivation by accumulated coke, leading to a rapid decrease of the propylene yield. 

The unusually high stability of DAY zeolites prepared from USY-B and having SiO /Al O ratios over 100 indicates that the non-framework aluminum species present in USY-B play no role in enhancing the stability of this zeolite. It is the highly silicious framework, in which most of the aluminum has been replaced by silicon atoms, that is responsible for the high stability of USY-B zeolites and of corresponding DAY zeolites. In zeolites with a lesser degree of framework dealumination (i.e. in USY-A), the non-framework aluminum species appear to play a role in the stabilization of the zeolites, since their removal results in materials of lesser stability (28). 

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