Active deposit 941 mass

The properties of the surface layers have a strong effect on the deposition process. The driving force of the electrochemical reaction is the potential difference over the electrochemical double layer. Adsorption of species can change this potential. For example, the additives used in electrodeposition adsorb in the Helmholtz layer. They can change the local potential difference, block active deposition sites, and so on. The thickness of the diffusion layer affects the mass-transfer rate to the electrode. The diffusion layer becomes thinner with increasing flow rate. When the diffusion layer is thicker than the electrode surface profile, local mass-transfer rates are not equal along the electrode surface. This means that under mass-transfer control, metal deposition on electrode surface peaks is faster than in the valleys and a rough deposit will result. 

Platinum, palladium, and rhodium are the major materials used industrially for the catalytic oxidation of CO. This initiated a variety of studies on single crystals, on supported particles, as well as on real catalysts in order to obtain important details about this catalytic process. In this section, an overview over the reactive properties of free Pt and Pd clusters will be presented and the catalytic CO combustion reaction on clusters of these metals will be discussed. Subsequently, the activity of deposited mass-selected Pt and Pd clusters in this reaction will be analyzed in order gain insight into the details of the catalytic reaction mechanisms on these materials. 

The model of deactivation describes the transformations of two boundary forms of the carbonaceous deposits during the catalytic hydrogenation of CO2. These are the hydrocarbon-formed active deposit (CH) and the graphitic inactive one (C)n. Thus deactivation is based on dehydrogenation of the active deposit into the inactive one that blocks active centers for hydrogenation. The active deposit, a product of polymerization of surface methane precursors (CH ), is simultaneously their consumer and producer. The mass balance of the active intermediates derived from the model assumptions gave the kinetic equation which quantitatively describes the deactivation. 

Fig. 5.23. ICRP dust deposition model. The radioactive or mass fraction of an aerosol which is deposited in the N-P, T-B and P regions is given in relation to the activity or mass median aerodynamic diameter (AMAD or MMAD) of the aerosol size distribution. The model is intended for use with aerosol size distributions having an AMAD or MMAD between 0.2 and 10 pm and whose geometric standard deviations are less than 4.5. Provisional deposition estimates further extending the size range are given by the dashed lines. For the unusual aerosol size distributions having an AMAD or MMAD greater than 20 jm, complete N-P deposition can be assumed. The model does not apply to aerosols with AMAD or MMAD below 0.1 jm.

Specific cake resistance can be calculated as a function of flux, TMP and deposited mass [15, 32]. ft is generally used to characterize the fouling cake structures, either in dead-end [32] or crossflow filtration [33]. Table 14.1 provides a list of specific cake resistance values for activated sludge, which was studied with different MBR operating conditions. 

Equation (19.16) is valid only if the reaction occurs tmder purely activation control. When a single metal is being deposited, mass-transport limitation can be taken into account by replacing the bulk concentrations in Eq. (19.16) with the surface concentration. At steady state this is rather simple, since... 

Usually they are employed as porous pellets in a packed bed. Some exceptions are platinum for the oxidation of ammonia, which is in the form of several layers of fine-mesh wire gauze, and catalysts deposited on membranes. Pore surfaces can be several hundred mVg and pore diameters of the order of 100 A. The entire structure may be or catalytic material (silica or alumina, for instance, sometimes exert catalytic properties) or an active ingredient may be deposited on a porous refractory carrier as a thin film. In such cases the mass of expensive catalytic material, such as Pt or Pd, may be only a fraction of 1 percent. 

Both share more or less the same merits but also the same disadvantages. The beneficial properties are high OCV (2.12 and 1.85 V respectively) flexibility in design (because the active chemicals are mainly stored in tanks outside the (usually bipolar) cell stack) no problems with zinc deposition in the charging cycle because it works under nearly ideal conditions (perfect mass transport by electrolyte convection, carbon substrates [52]) self-discharge by chemical attack of the acid on the deposited zinc may be ignored because the stack runs dry in the standby mode and use of relatively cheap construction materials (polymers) and reactants. 

A major consequence of the activities associated with the exploitation of mineral deposits (i.e., exploration, the development of mines and processing facilities, the extraction and concentration, which is also called beneficiation, of ores containing the desired minerals, and the decommissioning or abandonment of mine facilities) is the production of extremely large volumes of unwanted materials. Waste volumes vary from ca. 30% of the mass of the ore in the case of gypsum and other non-metals, to about 50% for base metals to more than 80% for strip-mined... 

Cadmium is an extremely toxic metal that finds its way into the aqueous environment as a result of some human activities. A major cause of cadmium pollution is zinc mining and processing, because natural deposits of ZnS ores usually also contain CdS. During the processing of these ores, highly insoluble cadmium sulfide ( sp = 7.9 X 10 ) maybe converted into considerably less insoluble cadmium hydroxide (.E p — 7.2 X 10" ). What mass of Cd (OH)2 will dissolve in l.OOx lO L of an aqueous solution ... 

The platinum concentrations in the platinized carbon blacks are reported to be between 10 and 40% (by mass), sometimes even higher. At low concentrations the specific surface area of the platinum on carbon is as high as lOOm /g, whereas unsupported disperse platinum has surface areas not higher than 10 to 15m /g. However, at low platinum concentrations, thicker catalyst layers must be applied, which makes reactant transport to reaction sites more difficult. The degree of dispersion and catalytic activity of the platinum depend not only on its concentration on the carrier but also on the chemical or electrochemical method used to deposit it. 

The simplest case is represented by curve 1. The activity depends linearly on the number of unpoisoned active sites. The interpretation of curves 2 and 3 is less obvious. In the former case the interpretation might be that the least active sites are poisoned first, whereas in the latter case the most active sites are poisoned preferentially. Mass-transfer limitations also play a role in poisoning behaviour. If, for example, the poison is deposited in the outer shell of the catalyst particles, a decrease in catalytic activity can be expected as qualitatively described by curve 3 in Fig. 3.37. 

In order to relate material properties with plasma properties, several plasma diagnostic techniques are used. The main techniques for the characterization of silane-hydrogen deposition plasmas are optical spectroscopy, electrostatic probes, mass spectrometry, and ellipsometry [117, 286]. Optical emission spectroscopy (OES) is a noninvasive technique and has been developed for identification of Si, SiH, Si+, and species in the plasma. Active spectroscopy, such as laser induced fluorescence (LIF), also allows for the detection of radicals in the plasma. Mass spectrometry enables the study of ion and radical chemistry in the discharge, either ex situ or in situ. The Langmuir probe technique is simple and very suitable for measuring plasma characteristics in nonreactive plasmas. In case of silane plasma it can be used, but it is difficult. Ellipsometry is used to follow the deposition process in situ. 

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