Catalyst layer thickness

Platelet material AlMg3 Sputtered catalyst layer thickness 50-1400 nm... 

Chip material Silicon, aluminum Catalyst layer thickness 0.1 pm... 

Reactor type Single-channel chip micro reactor Zeolite catalyst layer thickness 3 pm... 

Micro channel material Iron Catalyst layer thickness 5 pm... 

General Motors has assessed the required activity of a catalyst that costs less compared to the current state-of-the-art Pt activity based on these con-straints. i Assuming that the catalyst layer thickness could be increased to MOO pm from the currently used 10 pm, GM has calculated that the minimum volume activity (i.e., Acm ) for a cathode catalyst that costs less should be at least 10% of the current Pt activity. In reality, this seems rather generous, given the recent trend to reduce catalyst layer thicknesses to optimize high-current performances. The DoE has developed a series of volume activity targets for nonprecious metal catalysts, with the 10% of Pt activity target (300 Acm 3 at 0.8 V, H2/O2) necessary by 2015. 

The catalyst layer is composed of multiple components, primarily Nafion ion-omer and carbon-supported catalyst particles. The composition governs the macro- and mesostructures of the CL, which in turn have a significant influence on the effective properties of the CL and consequently the overall fuel cell performance. There is a trade-off between ionomer and catalyst loadings for optimum performance. For example, increased Nafion ionomer confenf can improve proton conduction, but the porous channels for reactanf gas fransfer and water removal are reduced. On the other hand, increased Pt loading can enhance the electrochemical reaction rate, and also increase the catalyst layer thickness. 

ORR rate constant as defined by eq 61, 1/s ORR rate constant in Figure 11, cm/s thermal conductivity of phase k, J/cm K relative hydraulic permeability saturated hydraulic permeability, cm electrokinetic permeability, cm catalyst layer thickness, cm parameter in the polarization equation (eq 20) loading of platinum, g/cm molecular weight of species i, g/mol symbol for the chemical formula of species i in phase k having charge Zi... 

The most important electrokinetic data pertinent to fuel cell models are the specific interfacial area in the catalyst layer, a, the exchange current density of the oxygen reduction reaction (ORR), io, and Tafel slope of ORR. The specific interfacial area is proportional to the catalyst loading and inversely proportional to the catalyst layer thickness. It is also a strong function of the catalyst layer fabrication methods and procedures. The exchange current density and Tafel slope of ORR have been well documented in refs 28—31. 

An important consideration for the electronics of semiconductor/metal supported catalysts is that the work function of metals as a rule is smaller than that of semiconductors. As a consequence, before contact the Fermi level in the metal is higher than that in the semiconductor. After contact electrons pass from the metal to the semiconductor, and the semiconductor s bands are bent downward in a thin boundary layer, the space charge region. In this region the conduction band approaches the Fermi level this situation tends to favor acceptor reactions and slow down donor reactions. This concept can be tested by two methods. One is the variation of the thickness of a catalyst layer. Since the bands are bent only within a boundary layer of perhaps 10-5 to 10 6 cm in width, a variation of the catalyst layer thickness or particle size should result in variations of the activation energy and the rate of the catalyzed reaction. A second test consists in a variation of the work function of the metallic support, which is easily possible by preparing homogeneous alloys with additive metals that are either electron-rich or electron-poor relative to the main support metal. 

In another case study, the thickness of the catalyst layer was increased at constant WHSV by increasing the inlet flow rate. Increasing the catalyst layer thickness from 10 to 60 pm led to a decrease in conversion from 100 to < 70% for both reactions. A faster increase in the axial temperature was achieved for the thin coatings. 

The results of experimental research on the product yield dependence on catalyst layer thickness h and gas rate value Q are shown in Figs. 2 and 3 correspondently. Obtained results allowed us to fix optimal charge parameters for pyrolysis in the interests of the efficiency. 

K - specific product yield, g/gcat, h - catalyst layer thickness, mm... 

Figure 2. Graph of dependence of specific product yield on catalyst layer thickness.

In comparison with fixed beds consisting of catalyst particles, coated monoliths have the advantage of a tunable catalyst layer thickness, decoupling the internal diffusion distance within the catalyst from the external surface area. However, mainly as a consequence of the square channel geometry in commercially available monoliths, the thickness of the... 

Figure 14 compares bed effectiveness for PPR modules with flat screens of different catalyst layer thickness. Module D, with 7-mm-thick catalyst slabs, has a lower bed... 

D. Chen, L. Fengmei, and A. K. Ray, Effect of mass transfer and catalyst layer thickness on photocatalytic reaction, AIChE J. 46,1034-1045 (2000). 

Fig. 11 Schematic picture of the cathode catalyst layer and its composition, exhibiting the different functional parts. The typical catalyst layer thickness is l 10-20 pm.

In the intermediate regime the dominating contribution to rjo is a 2Mn(jo/Vf ) term, and, thus, as demonstrated in general terms, the electrode potential is almost l independent. The catalyst layer thickness could be varied without having to pay a penalty in electrode potential. 

Advanced design Control of the catalyst layer thickness and composition provide the means to optimize catalyst utilization and performance issues. Taking together all factors listed above, only 10-20% of the catalyst is effectively... 

Here x is the location of the back-ing/catalyst layer interface, A is the characteristic width of the interface smearing, which must be much smaller than the catalyst layer thickness for this interpolation to... 

Figure 33 shows the profiles of reaction rates across the catalyst layers in the inlet element (along the white line in Fig. 32). In this example, the reaction rate on the cathode side is almost constant along x, whereas on the anode side it rapidly grows with x. Most of the anode catalyst layer thickness is not used for reaction, except for the thin sublayer near the membrane, with the thickness of the order of 2-3 pm.

Equations 47-49 describe variations of parameters along the y coordinate of the catalyst layer (y = z/lc 1), where z is the catalyst layer thickness coordinate, y = 0 specifies the catalyst layer/gas interface, and y = 1 specifies the catalyst layer/ionomeric membrane interface (see Fig. 44), in which Rc (= /Ci/cr) is the protonic resistance through a unit cross-sectional area of the catalyst layer and 7d (=nFDC /lc ) is a characteristic diffusion-controlled current density in the catalyst layer. The thickness of the catalyst layer disappears from the equations by introducing Rc, ax, and I ). The experimental variables considered include the overpotential 1], the current density /, and the oxygen concentration C, when pox = 1 atm at the catalyst layer/gas interface. The O2 partial pressure, pox, at the catalyst layer/backing layer interface is determined, in turn, by the cathode inlet gas stream composition and stoichiometric flow rate and by the backing layer (GDL) transport characteristics. 

In fixed-bed reactor technologies, an important factor is the pressure drop over the catalyst bed. In order to minimize the mass transfer effects, knitted silica fiber catalysts having small diffusion distance (< 10 pm) and low pressure drop, were applied in this study. 10 layers (0.4 g) of the catalyst was placed in the reactor between stainless steel nets and glass beads were used as inert packing material. The catalyst layer thickness was 0.15 cm resulting in catalyst bed length of... 

F. The experiments were to be carried out in a single stage without recycling of the gas. The converters consisted of water-cooled tubes, 4.5 m. long, with a catalyst layer thickness of 10 mm. Catalyst volumes of 4800 cc. were used for each experiment. 

Figure 8. Effect of catalyst layer thickness on CO conversion for platinum...

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