Thin-film models

The thin him (or stagnant layer) model is based on the assumption that a dissolved chemical has a uniform concentration throughout a surface water body, due to turbulent diffusion, except in a very thin layer at the water s surface. A similar assumption is made concerning the chemical concentration in overlying air. Within a few micrometers or millimeters of the water-air interface, it is assumed that the eddies responsible for turbulent diffusion are suppressed therefore, chemical transport in this thin layer (or him) can only occur by molecular diffusion, which is considered to be the rate-limiting step of air-water exchange (Fig. 2-14) (Liss and Slater, 1974). 

If the dimensionless Henry s law constant for a chemical, H, is much greater than 0.01—as is the case for a large number of solvents, fuels, and gases— resistance to gas exchange in the stagnant air layer immediately above the water can be neglected. The thin him model then describes the flux of a chemical into or out of the water by 

Under this theory, for water-side control, the gas exchange coefficient, few, is equal to the quotient DW/5W. If the atmospheric concentration of a chemical is essentially zero, Eq. [2-29] simplifies to 

It should be noted that film thickness cannot be measured directly few, however, can be estimated, and 8 can then be estimated from kw and an independent knowledge of Dw. Typical values of are in the range of 20 to 

In the most general case, which must be invoked when the value of the dimensionless Henry s law constant is on the order of 0.01, both resistances contribute to limiting the gas exchange rate. In this case, the complete expression for flux density must be used  

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We have undertaken a series of experiments Involving thin film models of such powdered transition metal catalysts (13,14). In this paper we present a brief review of the results we have obtained to date Involving platinum and rhodium deposited on thin films of tltanla, the latter prepared by oxidation of a tltanliua single crystal. These systems are prepared and characterized under well-controlled conditions. We have used thermal desorption spectroscopy (TDS), Auger electron spectroscopy (AES) and static secondary Ion mass spectrometry (SSIMS). Our results Illustrate the power of SSIMS In understanding the processes that take place during thermal treatment of these thin films. Thermal desorption spectroscopy Is used to characterize the adsorption and desorption of small molecules, In particular, carbon monoxide. AES confirms the SSIMS results and was used to verify the surface cleanliness of the films as they were prepared. 

GL 16] ]R 12] ]P 15] Using a simple thin-film model for mass transfer, values for the overall mass transfer coefficient were determined for both micro-channel processing and laboratory trickle-bed reactors [11]. The value for micro-reactor processing (fCL = 5-15 s ) exceeds the performance of the laboratory tool Ki a = 0.01-0.08 s ) [11, 12], However, more energy has to be spent for that purpose (see the next section). 

Random gene insertion, 72 453 Randomization, 5 388-389 70 811-813 commercial experimental design software compared, 8 398t Randomly oriented thin film model,... 

The thin-film model is the simplest and, therefore, most commonly used approach to estimate air-sea fluxes of gases. In this model, molecular diffusion is assumed to present a barrier to gas exchange in each of two layers. As illustrated in Figure 6.5, one layer is composed of a shallow region of the atmosphere that lies in direct contact with the sea surface. The second is a shallow layer of seawater tliat lies at the sea surface. These layers have depths less than 100 (am and, hence, are referred to as thin films. 

The final simple macrohomogeneous porous-electrode models are the ones that are more akin to thin-film models. In these models, the same approach is taken, but instead of gas diffusion in the catalyst layer, the reactant gas dissolves in the electrolyte and moves by diffusion and reaction. The... 

The rest of the comparisons were done for the cathode. The results all showed that the agglomerate model fits the data better than the porous-electrode model. However, it should be noted that the porous-electrode model used was usually a thin-film model and so was not very robust. Furthermore, the agglomerate model has more parameters that can be used to fit experimental data. Finally, some of the agglomerate models compared were actually embedded models that account for both length scales, and therefore, they normally agree better with the experimental data. 

Andreae and Raemdonck Q4) applied the thin film parameterization described above in a slightly different way. The thin film model predicts that the exchange coefficient is linearly related to the diffusivity of the gas as follows,... 

Figure 12.3. Conceptual representation of the thin-film model (from Levenspiel and Walton, 1954).

The transfer of PCBs across the air-water interface is a combination of gas absorption and volatilization. The net effect of these two transfers is typically described by the two thin film model adapted by Liss and Slater,43 as neither of these processes can be measured directly. This model is a function of the mass... 

Note that the surface electric field, induced by the incident IR radiation characterizing the thin-film model catalysts, is mainly determined by the NiAl substrate. Consequently, because only the components of the dynamic dipole moment that are perpendicular to the metallic substrate contribute to the SFG signal, the effective dipole moment of tilted molecules is reduced. As a result, the intensity of the signal characterizing tilted molecules is smaller than that of CO molecules oriented perpendicular to the substrate (such as those on the particle top facet). 

The presence of wind-induced ripples was quickly noted as being coincident with an enhancement in (Kanwisher, 1963). Careful experiments with various pairs of gases were used to identify n at different U in order to identify which, if any, of the models discussed previously might best represent air-water gas transfer (Jahne et al., 1987). These experiments showed that the thin-film model is too simple and that varied with Sc at low U as predicted by boundary layer models (see Section 6.03.2.1.5). However, once wind-induced waves were observed on the surface of the water, k was found to vary with in agreement with surface renewal... 

Two of the key assumptions of the thin-film model (see Section 6.03.2.1.1) are that the main bodies of air and water are well mixed, i.e., that the concentration of gas at the interface between the thin film and the bulk fluid is the same as in the bulk fluid itself, and that any production or removal processes in the thin film are slow compared to transport across it. It is quite likely that there are near-surface gradients in concentrations of many photochemically active gases. Little research has been published, although the presence of near-surface gradients (10 cm to 2.5 m) in levels of CO during the summer in the Scheldt estuary has been reported (Law et al., 2002). Gradients may well exist for other compounds either produced or removed photochemically, e.g., di-iodomethane, nitric oxide, or carbonyl sulfide (COS). Hence, a key assumption made in most flux calculations that concentrations determined from a typical sampling depth of 4-8 m are the same as immediately below the microlayer may well often be incorrect. 

Fig. 15.3 Preparation of epitaxial thin film model catalysts, (a) Electron micrograph of a Pt-AljOj model catalyst with a mean particle size of 5 nm the insets show the corresponding electron diffraction pattern and the (200) weak-beam dark-field image of a pyramidal Pt nanocrystal (b) an atomically resolved TEM micrograph of a slightly rectangular Pt particle. A structural model of a pyramidal Pt particle is presented in (c). To illustrate the epitaxial growth the NaCl substrate is also included...

Nanoparticle characterization by high-resolution transmission electron microscopy (HRTEM) and diffraction (TED) requires model catalysts with thicknesses <100 mn. This led to the development of thin film model catalysts which could be... 

Preparation and Nanoparticle Structure of Epitaxial Thin Film Model Catalysts... 

Apparently, epitaxial thin-film model catalysts provide a well-defined initial state for a systematic study of microstructural changes and structure-activity correlations. Model catalysts were prepared for various noble metal-oxide combinations, including Pt, Rh, Ir, Pd, Re supported by Al Oj, SiO, TiO, CeO, VO, Ga Oj, etc. The number density of the metal particles (island density particles per cm ) and their size can be controlled via the NaCl(OOl) substrate temperature during evaporation and the amount of metal deposited (as measured by a quartz microbalance), respectively (Pig. 15.4). 

Rupprechter G, Hayek K, Hofmeister H (1998) Electron microscopy of thin film model catalysts Activation of alumina supported rhodium nanoparticles. J Catal 173 409... 

Rupprechter G, Seeber G, GoUer H, Hayek K (1999) Structme-activity correlations on Rh/ AljOj and Rh/TiO thin film model catalysts after oxidation and reduction. J Catal 186 201 Haerudin H, Bertel S, Kramer R (1998) Surface stoichiometry of titanium suboxide, part I volumetric and FTIR study. J Chem Soc Faraday Trans 94 1481... 

Structure and Catalytic Activity of Thin Film Model Oxide Catalysts... 

Penner S, Klotzer B, Jenewein B (2007). Structural and redox properties of VO and Pd/VO thin film model catalysts studied by TEM and SAED. Phys Chem Chem Phys, 9, 2428... 

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