Zeolites conduction mechanism

In 1997, Anderson et al. reported that when more and more extra electrons are introduced into the zeolite hosts,[19] some zeolites indeed show an electrical conductivity increase. The loading of potassium into zeolite L increases the room-temperature conductivity of the latter by 10 000 times. Nevertheless, the measured temperature dependence of conductivity on temperature indicates that the conduction mechanism is characteristic of thermal activation, and therefore that the electrical conduction may involve the redox jump of the K32+/K3+ process (see Figure 9.7). The K/K-A host-guest compound exhibits interesting ferromagnetic behavior,1221 and this magnetism may be related with the formation of a superlattice.[231... 

The HT Mechanism. Figures 2, 3, and 4 resume the main characteristics of the HT conduction mechanism. The conduction can be ascribed to the mobility of the exchangeable cations. This is in agreement with conclusions by other authors (3, 6, 7, 19). Thus, in Figure 2, the conductivity at 19 kHz and 700 °K is represented as a function of the radius of the exchangeable ions for the X and Y zeolites. The specific conductivity of X is considerably higher than that of Y for samples... 

The LT Mechanism. The dielectric behavior of the zeolites in the low-temperature region is more complicated than at high temperatures. A frequency-dependent process is superposed on the ionic conductivity, and the relative intensity of the 2 phenomena varies from sample to sample. The understanding of the LT mechanism requires the interpretation of the dissipation factor tg 8, which is obtained experimentally as the ratio between the imaginary and the real part of the complex dielectric constant e"/e. For a purely ionic conduction mechanism, the plot of tg 8 as a function of the temperature at a fixed frequency would show a... 

The question about the conduction mechanism was also studied by Krogh Andersen et al They made long term d.c. experiments on four ammonium zeolites. The cell they used is shown in Fig. 14.2. The electrodes were platinum sputtered on to the pellets. In one type of experiment, the measuring cell was flushed with hydrogen gas saturated with water. The outlet from the cell passed through a vessel with an... 

From the conductivity measurements made until now, it is to be expected that all hydrated zeolites conduct protons (or hydroxyl ions). The conduction mechanism is most likely to involve a vehicle, either water or ammonia. In some experiments, small molecule amines and alcohols may have acted as vehicles. 

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. 

A 0.5-gram mass of either the organo-treated or inorganic cation exchanged zeolite and 50 mL of 10 mM/L arsenate or chromate aqueous solutions were placed into Erlenmeyer flasks and mechanically shaken in reciprocating mode to attain equilibrium. Different equilibrium periods for individual zeolite modifications and both aqueous oxyanions species have been established. The adsorption isotherm experiments were conducted using above mass/ volume ratio of samples with an initial metal concentrations ranged from 0.5 to 100 mM/L at laboratory temperature. The... 

Two methods for including explicit electrostatic interactions are proposed. In the first, and more difficult approach, one would need to conduct extensive quantum mechanical calculations of the potential energy variation between a model surface and one adjacent water molecule using thousands of different geometrical orientations. This approach has been used in a limited fashion to study the interaction potential between water and surface Si-OH groups on aluminosilicates, silicates and zeolites (37-39). 

An extensive study was conducted on the effect of chemical and structural aspects of zeolites on the fire performance of the intumescent system, ammonium polyphosphate-pentaerythritol (APP-PER), where a marked improvement of the fire-retardant properties within different polymeric matrices has been observed.56 58 The synergistic mechanism of zeolite 4A with the intumescent materials was investigated using solid-state NMR. Chemical analysis combined with cross-polarization dipolar-decoupled magic-angle spinning NMR revealed that the materials resulting from the thermal treatment of the APP-PER and APP-PER/4A systems were formed by carbonaceous and phosphocarbonaceous species, and that the zeolite enhances the stability of the phosphocarbonaceous species. 

This instrument has allowed several studies that provide information not obtainable by other means to be conducted. Four examples are presented as follows The first example concerns the question of the mechanism of emission of potassium ions from potassium zeolite [7]. Earlier studies had made the assumption that this was an S-L type of ion formation mechanism [8], implying that there was a neutral potassium atom flux accompanying the flux of atomic potassium cations. Experiments performed on this instrument clearly showed that this is not the case there was no detectable neutral atomic potassium flux accompanying the cation flux. Thus this instrument was used to answer a long-standing question with an experiment conducted in one afternoon and allowed the conclusion to be reached that the mechanism is potassium ions in the solid state subliming into the gas phase. 

Among the chemical reactions of interest catalyzed by zeolites, those involving alkanes are specially important from the technological point of view. Thus, some alkane molecules were selected and a systematic study was conducted, on the various steps of the process (diffusion, adsorption and chemical reaction), in order to develop adequate methodologies to investigate such catalytic reactions. Linear alkanes, from methane to n-butane, as well as isobutane and neopentane, chosen as prototypes for branched alkanes, were considered in the diffusion and adsorption studies. Since the chemical step requires the use of the more time demanding quantum-mechanical techniques, only methane, ethane, propane and isobutane were considered. 

Because of its unique heating mechanism, microwave irradiation has a special effect on the ion-exchange behavior of zeolite molecular sieves. Compared with traditional ion-exchange of rare-earth ions, microwave-irradiation-conducted ion-exchange has many... 

---