P. Kočí

Pore-scale modeling of non-isothermal reaction phenomena in digitally reconstructed porous catalyst

Meso-scale mathematical model of local reaction and transport processes in a porous, supported heterogeneous catalyst with bimodal pore-size distribution is presented. The model takes into account individual reaction steps on active sites (microkinetics), diffusion of reactants in macro-pores and meso-/micro-pores (molecular and Knudsen-diffusion), and heat generation and transport. The processes are modelled within a three-dimensional domain ( ≈ 10 × 10 × 10 μm3) of computer-reconstructed porous catalyst. The methodology is demonstrated on CO oxidation on Pt/γ-Al2O3. Several 3D porous structures are digitally reconstructed by the methods of particle packing and Gaussian-correlated random fields from typical electron-microscopy images of the catalyst. Typical dependences of overall reaction rate and effectiveness factor on the temperature and properties of the porous catalyst structure are evaluated. Relative importance of diffusion in macro- and meso-pores under varying temperature is demonstrated. Local optimum of effectiveness factor is found for the mixing ratio of catalyst support particles with two different sizes. The resulting temperature gradients over the studied section of catalyst are very small (approximately 0.2 K for the CO concentrations in the order of 1% mol). The results represent the local situation on the meso-scale, which can be interpreted as one discretisation point in the full-scale model of the reactor.

Multifunctional Aspects of Three-Way Catalyst: Effects of Complex Washcoat Composition

In this paper multifunctional aspects in operation of Pt/Rh/Ce/γ-Al2O3 three-way catalyst (TWC) are discussed on the basis of computed spatiotemporal patterns of reacting and deposited species on specific active sites distributed in the catalytic washcoat layer. A spatially pseudo-one-dimensional, heterogeneous model of a TWC reactor has been developed, with well-mixed gas phase and description of diffusion in the porous catalytic washcoat. The model includes a detailed non-stationary kinetic scheme for CO and C2H2 oxidation, oxygen storage on CeO2, CO2 adsorption on the support and NOx transformation and reduction. Pt, Rh, Ce and γ-Al2O3 sites are considered with different kinetic parameters. A methodology is developed for the studies of the effects of internal diffusion and washcoat composition (i.e. concentration and distribution of active Pt, Rh and Ce catalytic sites) on the conversion of main pollutants in automobile exhaust gas under transient conditions. Several cases of variable distribution of the catalytic sites in the washcoat layer are discussed with respect to the reactor performance. Effects of oscillations in the inlet oxygen concentration are studied. Complex interactions, particularly between reaction intermediates and the stored oxygen within the washcoat, cause the increase of conversions in the periodic operation when compared with the stationary one.

Simulated preparation of supported porous catalyst and evaluation of its reaction-transport properties

In this contribution, mathematical model for the description of solvent evaporation and noble metal precursors crystallization in a porous medium on nano-scale is presented. The methodology is based on the volume-of-fluid method and the model is validated by comparing the numerical simulation results with analytical solutions for evaporation from a single pore and for particle growth and Ostwald ripening of two freely suspended particles in a saturated solution. Impact of process conditions (initial concentration of the metal precursors, temperature, transport properties, nucleation) on the resulting porous catalyst is studied.