Characterization of external surfaces

1 Application of Surface Complexation Models for External Surfaces The formation of surface charges in the surface complexation model is demonstrated on the example of aluminosilicates. Aluminosilicates have two types of surface sites, aluminol and silanol (van Olphen, 1977). These sites, depending on pH, may form both protonated and deprotonated surface complexes. From the thermodynamic equilibrium point of view, the protonated and deprotonated surface complexes can be characterized by the so-called intrinsic stability constants, considering the surface electric work. For aluminol sites, 

For silanol sites, due to the very lowlg at pH 2 the protonation of silanol sites can be neglected (Hiemstra et al. 1989), only deprotonation is taken into account  

As seen from Equations 1.54-1.56, the intrinsic stability constants of surface reactions are dependent on two factors a chemical and an electric contribution. The chemical contribution is taken into consideration by the mass balance the electric contribution is treated by the charge balance. There are several surface complexation models that mainly differ in the description of the electric double layer that is used to calculate the surface potential, which is done by different double-layer models. These models have been mentioned previously in this chapter. Since, however, the terminology usually used in electrochemistry, colloid chemistry and, especially, in the discussions of surface complexation models is different, they are repeated again  

At high ionic strength, the electric double layer is considered to be plane the so-called constant capacity model (Helmholtz model) is applied. 

At low ionic strength, a diffuse double-layer model (Gouy-Chapman model) is used. 

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Characterization of external surface properties of zeolite ZSM-5 modified by 12-tungstosilicic acid... 

The external surface Bronsted and Lewis acidity of modified zeolites were characterized by FTIR of OH groups and adsorbed CO molecules and ESR of adsorbed nitroxyi radicals. The catalytic properties of the samples were tested in reactions of aromatics conversion toluene, pseudocumene, and triisopropylbenzene. The correlation between acidity and catalytic activity has been found and discussed. The three methods of external surface characterization can lead to the entire view on zeolite acidity. 

These results clearly show that streaming potential measurements allows the characterization of membrane surface/solution interface (external and pore walls or internal surfaces) giving information not only on the fouling particles/macro-molecules but also on characteristic membrane material parameters. 

For characterization purposes the most useful form of external modulation is electromodulation, because it provides the sharpest structure (third derivative of R in bulk or thin films) and is sensitive to surface or interface electric fields. The most widely used contacdess mode of electromodulation is termed Photoreflectance (PR) 5.7.8... 

In the preparation of Mo/KL, the addition of 1 was stopped when pH of the solution was lowered to about 3. The resulting Mo/KL contained only 2.1 wt% (74% of added molybdenum clusters) of molybdenum. Chlorine was absent, which indicates that the cluster 1 acted as a tetravalent cation. The LTL structure is characterized by a monodimensional system of channels, whose diameter (0.70 nm) [14] is close to the size of the cluster 1. It is conceivable that, once the cluster 1 was incorporated into an LTL main channel and present at a site near the external surface, the sites deep in the channel are no more accessible to another cluster. 

The presence of methylenic bands shifted at higher frequency in the very early stages of the polymerization reaction has also been reported by Nishimura and Thomas [114]. A few years later, Spoto et al. [30,77] reported an ethylene polymerization study on a Cr/silicalite, the aluminum-free ZSM-5 molecular sieve. This system is characterized by localized nests of hydroxyls [26,27,115], which can act as grafting centers for chromium ions, thus showing a definite propensity for the formation of mononuclear chromium species. In these samples two types of chromium are present those located in the internal nests and those located on the external surface. Besides the doublet at 2920-2850 cm two additional broad bands at 2931 and 2860 cm are observed. Even in this favorable case no evidence of CH3 groups was obtained [30,77]. The first doublet is assigned to the CH2 stretching mode of the chains formed on the external surface of the zeolite. The bands at 2931 and... 

Without adequate silanization, an aqueous film can short-circuit the d-pipette by linking its two halves. In the absence of a surface aqueous layer the currents flow between each of two pipettes and the external reference electrode. In contrast, when the liquid film connects two orifices, the current flows directly from one barrel to the other. Two experiments allowing the distinction between these two cases were carried out using a well-characterized reaction of potassium transfer from water into DCE facilitated by dibenzo-18-crown-6 (DB18C6) [11]... 

The adsorption capacity of a surface with respect to molecules of a given species is characterized by the total number N of molecules of the particular species retained by unit surface area under the conditions of equilibrium with the gas phase under the given external conditions (i.e., at a given pressure P and temperature T). An expression for N as a function of rf, rr, and 7j+ will be derived in Section II. 

Acidic micro- and mesoporous materials, and in particular USY type zeolites, are widely used in petroleum refinery and petrochemical industry. Dealumination treatment of Y type zeolites referred to as ultrastabilisation is carried out to tune acidity, porosity and stability of these materials [1]. Dealumination by high temperature treatment in presence of steam creates a secondary mesoporous network inside individual zeolite crystals. In view of catalytic applications, it is essential to characterize those mesopores and to distinguish mesopores connected to the external surface of the zeolite crystal from mesopores present as cavities accessible via micropores only [2]. Externally accessible mesopores increase catalytic effectiveness by lifting diffusion limitation and facilitating desorption of reaction products [3], The aim of this paper is to characterize those mesopores by means of catalytic test reaction and liquid phase breakthrough experiments. 

Unfortunately, the literature is relatively sparse with examples showing the water uptake profile onto crystalline, nonhydrating substances below RHq. This is most likely due to the difficulty in accurately measuring the small amounts of water that are sorbed. Alkali halides are an exception, however, likely due to their well-characterized particle morphologies [34—37]. Figure 2 shows a water uptake isotherm onto recrystallized sodium chloride [37]. Note that the amount of water sorbed as a function of relative humidity is normalized to the specific surface area of the sample. Since water is sorbed only to the external surface of... 

To study the promotion mechanism of Pt wire/FSM-16 in the PROX reaction, the Pt nanowires were extracted by HF/EtOH treatment from FSM-16, and the wires were again deposited on the external surface of FSM-16 from the ethanol solution. We found that the resulting external Pt wire/FSM-16 catalyst gave low TOFs (>35) and lower CO selectivity (>30%) in the PROX reaction [32]. This implies that the encapsulation of Pt wires in the silica channels of FSM-16 is a key to promote the selective CO oxidation in the PROX reaction. Furthermore, from the structural characterization by XANES, XPS and IR in CO chemisorption... 

When the internal diffusion effects are considered explicitly, concentration variations in the catalytic washcoat layer are modeled both in the axial (z) and the transverse (radial, r) directions. Simple slab geometry is chosen for the washcoat layer, since the ratio of the washcoat thickness to the channel diameter is low. The layer is characterized by its external surface density a and the mean thickness <5. It can be assumed that there are no temperature gradients in the transverse direction within the washcoat layer and in the wall of the channel because of the sufficiently high heat conductivity, cf., e.g. Wanker et al. 

Generally, several protocols are used for the characterization of sohd-catalyzed reactions under batch reaction conditions by NMR spectroscopy. In ex situ experiments, the conversion of reactants adsorbed on the catalyst is carried out in an external oven and stopped after a given reaction time by quenching, for example, in liquid nitrogen. Subsequently, the reaction products formed on the catalyst surface are investigated at room temperature by use of a standard MAS NMR probe. This protocol is repeated with a stepwise increment of the reaction time at the same temperature or with a stepwise increment of the reaction temperature for the same duration. In an in situ experiment, the catalytic conversion of the reactants is measured inside the NMR spectrometer by use of a high-temperature MAS NMR probe. 

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