Local reaction conditions

There is no doubt that this changing morphology influences the intra-particle heat-and mass-transfer [9]. How strong this effect can be, for a given particle, is primarily a question of the local reaction conditions within a given reactor. A small amount of the particle volume, for... 

The challenge is to construct reduced chemistry models which are fast to evaluate, yet which still satisfy the error tolerance Eq. (13). One effective approach to this is the Adaptive Chemistry method (Schwer et al., 2003a, b), where different reduced chemistry models are used under different local reaction conditions. For example, in the 1—d steady premixed flame studied by Oluwole et al. (Oluwole et al., 2006), six different reduced chemistry models were used, and the full chemistry model only had to be used at about 20% of the grid points, Fig. 14. 

Hong F T 1987 Effect of local reaction conditions on heterogeneous reactions in bacteriorhodopsin membrane an electrochemical view J. Electrochem. Soc. 134 3044-52... 

This chapter provides a systematic account of the pertinent challenges and approaches in catalyst layer design. The hierarchy of structural effects and physical phenomena discussed includes materials design for high surface area and accessibility, statistical utilization of Pt evaluated on a per-atom basis, transport properties of charged species and neutral reactants in composite media with nano- to meso-porosity, local reaction conditions at internal interfaces in partially electrolyte-filled porous media, and global performance evaluated in terms of response functions for electrochemical performance and water handling. 

In this chapter, connections will be established between electrocatalytic surface phenomena and porous media concepts. The underlying logics appear simple, at least at first sight. Externally provided thermodynamic conditions, operating parameters, and transport processes in porous composite electrodes determine spatial distributions of reaction conditions in the medium, specifically, reactant and potential distributions. Local reaction conditions in turn determine the rates of surface processes at the catalyst. This results in an effective reactant conversion rate of the catalytic medium for a given electrode potential. 

Combination of the macrohomogeneous approach for porous electrodes with a statistical description of effective properties of random composite media rests upon concepts of percolation theory (Broadbent and Hammersley, 1957 Isichenko, 1992 Stauffer and Aharony, 1994). Involving these concepts significantly enhanced capabilities of CL models in view of a systematic optimization of thickness, composition, and porous structure (Eikerling and Komyshev, 1998 Eikerling et al., 2004). The resulting stmcture-based model correlates the performance of the CCL with volumetric amounts of Pt, C, ionomer, and pores. The basis for the percolation approach is that a catalyst particle can take part in reaction only if it is connected simultaneously to percolating clusters of carbon/Pt, electrolyte phase, and pore space. Initially, the electrolyte phase was assumed to consist of ionomer only. However, in order to properly describe local reaction conditions and reaction rate distributions, it is necessary to account for water-filled pores and ionomer-phase domains as media for proton transport. 

FIG U RE 3.17 The self-consistency problem in Pt electrocatalysis. The metal phase potential determines oxidation state and charging properties at the catalyst surface. These properties in turn determine the local reaction conditions at the Hehnholtz or reaction plane. At this point, structural design and transport properties of the catalyst layer come into play (as illustrated for conventional and ultrathin catalyst layers). Newly developed methods in the emerging field of first-principles electrochemistry attempt to find self-consistent solutions for this conpled problem. 

This section emphasizes the importance of local reaction conditions in nanopores of CLs. In this case, spatially varying charge distributions ions exert a major impact on electrochemical processes at internal pore surfaces. Such charge distributions, occurring in the region of the electrochemical double layer, invalidate the assumption of electroneutrality. In fact, the double layer concept itself becomes meaningless when the nominal thickness of the double layer, that is, the Debye length, is of the same order as the pore radius. 

Numerical solutions for (r, z), Cg+ (r, z), and coj (c z) can be readily obtained using standard mathematical software packages like MATLAB or Simulink . These solutions could be utilized in further analysis to calculate the effectiveness factor of the pore and study the response function between catalyst layer overpotential r]c and fuel cell current density jo. Thereby, the dependence of these effective performance quantifiers on local reaction conditions in pores could be rationalized. 

Understanding of the mesoscopic structure is needed in order to rationalize local reaction conditions in nanopores. The conclusive step is to relate the hierarchy of structural effects to the performance of a catalyst layer. Owing to the importance of catalyst layer performance modeling. Chapter 4 will be devoted to this topic. 

The transport properties of water-filled nanopores inside of agglomerates and the properties of the ionomer film at the agglomerate surface define local reaction conditions at the mesoscopic scale. These local conditions, which involve distributions of electrolyte phase potential, proton density (or pH), and oxygen concentration, determine the kinetic regime, under which interfacial electrocatalytic processes must be considered. Combining this information, a local reaction current can be found, which represents the source term to be used in performance modeling of the cathode catalyst layer. 

Laakonen, M., Moilanen, P., Alopaeous, V., and Aittamaa, J. (2006) Dynamic modelling of local reaction conditions in an agitated aerobic fermenter. AIChE J., 52, 1673-1689. 

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