Catalyst layer reaction conditions

Relationship of stationary conversion with area loading of decalin and a correlation curve between reaction area needed for 50 kW power and stationary conversion, (a) Stationary conversion versus area loading of decalin and (b) reaction area needed for 50 kW versus one-pass conversion. Catalyst platinum nanoparticles supported on ACC (5 wt-metal%), 0.87 g (three layers). Reaction conditions boiling and refluxing by heating at 280°C and cooling at 25°C. 

Characterization techniques become surface sensitive if the particles or radiation to be detected come from the outer layers of the sample. Low energy electrons, ions and neutrals can only travel over distances between one and ten interatomic spacings in the solid state, implying that such particles coming off a catalyst reveal surface-specific information. The inherent disadvantage of the small mean free path is that measurements need to be carried out in vacuum, which conflicts with the wish to investigate catalysts under reaction conditions. 

Vibrational spectroscopies are particularly useful for the analysis of the adsorbed layers on metallic particles. Among them, infrared spectroscopy is of widespread use and provides a powerful tool in the study of metal-based catalysts under reaction conditions. Under the approximation of vibrational and rotational coordinate separation, the vibrational wavefunction by is a function of the internal coordinates (Qk) and is a solution of the vibrational hamiltonian. Assuming a quadratic approximation of the potential energy in terms of the internal coordinates, then ... 

Vibrational spectroscopies are particularly useful for the analysis of the adsorbed layers on metallic particles. Among them, infrared spectroscopy is of widespread use and provides a powerful tool in the study of metal-based catalysts under reaction conditions. Under the... 

Both reactions were carried out under two-phase conditions with the help of an additional organic solvent (such as iPrOH). The catalyst could be reused with the same activity and enantioselectivity after decantation of the hydrogenation products. A more recent example, again by de Souza and Dupont, has been reported. They made a detailed study of the asymmetric hydrogenation of a-acetamidocin-namic acid and the kinetic resolution of methyl ( )-3-hydroxy-2-methylenebu-tanoate with chiral Rh(I) and Ru(II) complexes in [BMIM][BF4] and [BMIM][PFg] [55]. The authors described the remarkable effects of the molecular hydrogen concentration in the ionic catalyst layer on the conversion and enantioselectivity of these reactions. The solubility of hydrogen in [BMIM][BF4] was found to be almost four times higher than in [BMIM][PFg]. 

The higher activity of the catalyst [(mall)Ni(dppmo)][SbFg] in [BMIM][PFg] (TOF = 25,425 h ) relative to the reaction under identical conditions in CFF2C12 (TOF = 7591 h ) can be explained by the fast extraction of products and side products out of the catalyst layer and into the organic phase. A high concentration of internal olefins (from oligomerization and consecutive isomerization) at the catalyst is known to reduce catalytic activity, due to the formation of fairly stable Ni-olefin complexes. 

Table 11.2 and assume A=100, which is rather conservative value, to compute J via Eq. (11.32) and O via Eq. (11.22). The results show t p 0.91 which implies that the O2 backspillover mechanism is fully operative under oxidation reaction conditions on nanoparticle metal crystallites supported on ionic or mixed ionic-electronic supports, such as YSZ, Ti02 and Ce02. This is quite reasonable in view of the fact that, as already mentioned an adsorbed O atom can migrate 1 pm per s on Pt at 400°C. So unless the oxidation reaction turnover frequency is higher than 103 s 1, which is practically never the case, the O8 backspillover double layer is present on the supported nanocrystalline catalyst particles. 

The reactions are still most often carried out in batch and semi-batch reactors, which implies that time-dependent, dynamic models are required to obtain a realistic description of the process. Diffusion and reaction in porous catalyst layers play a central role. The ultimate goal of the modehng based on the principles of chemical reaction engineering is the intensification of the process by maximizing the yields and selectivities of the desired products and optimizing the conditions for mass transfer. 

The transient response of DMFC is inherently slower and consequently the performance is worse than that of the hydrogen fuel cell, since the electrochemical oxidation kinetics of methanol are inherently slower due to intermediates formed during methanol oxidation [3]. Since the methanol solution should penetrate a diffusion layer toward the anode catalyst layer for oxidation, it is inevitable for the DMFC to experience the hi mass transport resistance. The carbon dioxide produced as the result of the oxidation reaction of methanol could also partly block the narrow flow path to be more difScult for the methanol to diflhise toward the catalyst. All these resistances and limitations can alter the cell characteristics and the power output when the cell is operated under variable load conditions. Especially when the DMFC stack is considered, the fluid dynamics inside the fuel cell stack is more complicated and so the transient stack performance could be more dependent of the variable load conditions. 

The interpretation is straightforward. At reaction conditions the concentration in the film is lowered by reaction, and, as a consequence, the driving force for mass transfer increases. In a homogeneous system this results in high values of Ha. In a slurry reactor this enhancement can occur if the catalyst particles are so small that they accumulate in the film layer. Table 5.4-4 summarizes expressions for the reaction rate or enhancement factor for various regimes. 

The reactivity of vanadyl pyrophosphate (VO)2P207, catalyst for n-butane oxidation to maleic anhydride, was investigated under steady and unsteady conditions, in order to obtain iirformation on the status of the active surface in reaction conditions. Specific treatments of hydrolysis and oxidation were applied in order to modify the characteristics of the surface layer of the catalyst, and then the unsteady catalytic performance was followed along with the reaction time, until the steady original behavior was restored. It was found that the transformations occurring on the vanadyl pyrophosphate surface depend on the catalyst characteristics (i.e., on the PfV atomic ratio) and on the reaction conditions. 

In the present work, the transient reaetivity and the ehanges of the snrface charaeteristies of an eqnihbrated VPP in response to modifications of the gas-phase composition have been investigated. As the VN atomic ratio is one of the most important factors affecting the catalytic performance of the VPP (6), two catalysts differing in VN ratio were stndied. Data obtained were used to draw a model about the nature of the surface active layer, and on how die latter is modified in function of the reaction conditions. 

The opposite behavior was observed after the treatment of the two catalysts in the steam-containing stream, at 380°C. The catalyst P/V 1.06 did not show any change of catalytic performance, whereas in the case of P/V 1.00 the treatment rendered the catalyst less active but more selective than the sample equilibrated in the reactive atmosphere at 380°C. This means that with P/V 1.00, the active layer is not fully hydrolyzed under reaction conditions, and that a hydrolyzed surface is more selective than the active surface of the equilibrated P/V 1.00 catalyst. On the contrary, the active surface of catalyst P/V 1.06 either was already hydrolyzed under... 

The results of the unsteady-state reactivity tests and of the catalysts characterization allow us to propose a model for the active layer of VPP under reaction conditions, illustrated in Figure 55.5. In this model, the surface is in dynamic equilibrium with the gas phase, and its nature is a function of both reaction... 

The oxidation is carried out over layers of platinum-rhodium catalyst and the reaction conditions are selected to favour reaction 1. Yields for the oxidation step are reported to... 

Relationship of charged amount of decalin with catalyst-layer temperature. Catalyst support granular activated carbon, 0.285 g. Charged amount of decalin 0,1.0, and 3.0 mL. Reaction conditions boiling and refluxing by heating at 210°C and cooling at 5°C. 

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