Concentration, component, diffusion porous catalysts

The computer-reconstructed catalyst is represented by a discrete volume phase function in the form of 3D matrix containing information about the phase in each volume element. Another 3D matrix defines the distribution of active catalytic sites. Macroporosity, sizes of supporting articles and the correlation function describing the macropore size distribution are evaluated from the SEM images of porous catalyst (Koci et al., 2006 Kosek et al., 2005). Spatially 3D reaction-diffusion system with low concentrations of reactants and products can be described by mass balances in the form of the following partial differential equations (Koci et al., 2006, 2007a). For gaseous components ... 

Equation (74) were solved within the section of the reconstructed porous catalyst, represented by the 3D matrix. Here x, y, and z are the spatial coordinates in the porous catalyst, cy denotes the molar concentration of the kth component, Dff is the effective diffusivity of the /cth component, vkj is the stoichiometric coefficient of the component k in the /th reaction step, and r, is the reaction rate of the y th reaction step. 

Hore importantly, the response curves are noticeably affected where one or both of the components is adsorbable, even at low tracer concentrations. The interpretation of data is then much more complex and requires analysis using the non-isobaric model. Figs 7 and 8 show how adsorption of influences the fluxes observed for He (the tracer), despite the fact that it is the non-adsorbable component. The role played by the induced pressure gradient, in association with the concentration profiles, can be clearly seen. It is notable that the greatest sensitivity is exhibited for small values of the adsorption coefficient, which is often the case with many common porous solids used as catalyst supports. This suggests that routine determination of effective diffusion coefficients will require considerable checks for consistency and emphasizes the need for using the Wicke-Kallenbach cell in conjunction with permeability measurements. 

During recent years, studies of a number of hydrocarbon transformations catalyzed by porous solid oxides containing a transition metal, notably platinum, have evolved some concrete examples and demonstrations of truly polystep catalytic reactions. Specifically, these reactions have been shown to be performed by catalysts which contain geometrically separate and different catalyst components, each of which catalyzes separate steps. The chemical intermediates exist as true compounds, although often at undetected concentrations. The term true is used in this context to characterize the intermediate as a normal chemical species, existing independently of, and desorbed from, the catalyst phase, and subject to ordinary physical laws of diffusion. 

The catalyst structure is assumed to be macro-porous, so that transport mechanisms like viscous transport or Knudsen diffusion can be neglected. It is assumed that the component mass transport inside the particle is described by Pick s law of diffusion (due to the relatively low concentrations of the relevant components). 

Where 4 rr is the inner superficial area of the spherical shell, D, effis the effective overall diffusion coefficient of i component through the porous structure, dQ/dr is the concentration gradient of i component at the spherical surface, p is the particle density of catalyst, and R, is the rate of i component. 

Esquivel et al. (2010) present an all-polymer micro-DMFC fabricated with a SU-8 photoresistor. This development exploits the capability of SU-8 components to bond to each other by a hot-pressing process and obtain a compact device. The device is formed by a MEA sandwiched between two current collectors. The MEA consists of a porous SU-8 membrane filled with a proton-exchange polymer and covered by a thin layer of carbon-based electrodes with a low catalyst loading (1.0 mg/cm ). The current collectors consist of two metalhzed SU-8 plates provided with a grid of through-holes that make it possible to deliver the reactants to the MEA by diffusion. The components were then bonded to obtain a compact micro-DMFC. With this assembly, using a 4 M methanol concentration at a temperature of 40°C, a maximum power density of 4.15 mW/cm was obtained. 

---