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Zeolites single atoms

The extreme case of an ultrananoparticle consists of one single atom. Such mono-atomic Pt sites have been realized in Pt/H-zeolites that are prepared at low temperature. They are identified by their unique selectivity in catalyzing the U/D exchange between cyclopentane and D2. Numerous data in the literature show that over conventional Pt... [Pg.145]

Smaller single-component, single-atom adsorbates, such as Ar, in zeolite A have also been investigated (20, 28, 29, 130). Kono and Takasaka (130) calculated the sorption characteristics using classical statistical mechanics methods. The particular MC method that they used is applicable at sorbate concentrations higher than infinite dilution. They found that a London... [Pg.60]

In Faujasites. Bezus et al. (49) reported in 1978 statistical calculations on the low-coverage adsorption thermodynamics of methane in NaX zeolite (Si/Al = 1.48). As for single-atom adsorbates described earlier, the agreement between their calculated values and a range of experimental values was excellent. Allowing for different orientations of the molecule, they calculated a value of 17.9 kJ/mol for the isosteric heat of adsorption at 323 K. Experimental values available for comparison at that time (134-136) ranged from 17.6 to 18.8 kJ/mol. Treating the methane molecule as a hard-sphere particle, with a radius of 2 A, resulted in a far lower heat of adsorption (12.6 kJ/mol). Further calculations (99) yielded heats of adsorption of 19.8 and 18.1 kJ/mol for methane in NaX and NaY zeolites, respectively. [Pg.62]

The potential energy of a single atom has to account for all the interaction energies with the atoms of the molecule, with those of the zeolite wall, and also for the internal bond bending, vibration, etc. Dual interactions are calculated... [Pg.191]

It has already been mentioned that the formation of ultrastable Y zeolites has been related to the expulsion of A1 from the framework into the zeolite cages in the presence of steam (8,9), and the filling of framework vacancies by silicon atoms (11,12). This results in a smaller unit cell size and lower ion- exchange capacity (6). It also results in a shift of X-ray diffraction peaks to higher 20 values. Ultrastable Y zeolites prepared with two calcination steps (USY-B) have a more silicious framework than those prepared with a single calcination step (USY-A). Furthermore, since fewer aluminum atoms are left in the USY-B framework, its unit cell size and ion-exchange capacity are also lower and most of the nonframework aluminum is in neutral form (18). [Pg.167]

Catalysis by Metal and Oxide Nanoparticles, Single Metal Atoms and Di-Nuclear Oxo-Ions in Zeolites... [Pg.139]

In a recent work (207), in situ ll and Al MAS NMR spectroscopy was used to investigate the Bronsted acid sites of zeolite HZSM-5 during the conversion of methanol under CF conditions. As a consequence of the application of MAS instead of the spin-echo technique, only framework Al atoms involved in quadrupolar interactions according to Cqcc < 5 MHz could be observed. Upon injection of methanol into the MAS NMR rotor reactor filled with calcined zeolite HZSM-5, a single Al MAS NMR signal arose at 54 ppm at reaction temperatures up to... [Pg.189]

Bandyopadhyay and Yashonath (31), in an extension of their work on MD studies of noble gas diffusion, presented MD results for methane diffusion in NaY and NaCaA zeolites. The zeolite models were the same as those used in the noble gas simulations (13, 15, 17, 18, 20, 28, 29) and the zeolite lattice was held rigid. The methane molecule was approximated as a single interaction center and the guest-host potential parameters were calculated from data of Bezus et al. (49) (for the dispersive term) and by setting the force on a pair of atoms equal to zero at the sum of their van der Waals radii (for the repulsive term). Simulations were run for 600 ps with a time step of 10 fs. [Pg.24]


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See also in sourсe #XX -- [ Pg.42 , Pg.53 , Pg.54 , Pg.55 , Pg.56 , Pg.57 , Pg.58 , Pg.59 , Pg.60 , Pg.61 ]




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