The Impact of Microstructure Geometry on the Mass Transport in Artificial Pores: A Numerical Approach

Abstract

The microstructure of porous materials used in heterogeneous catalysis determines the mass transport inside networks, which may vary over many length scales. The theoretical prediction of mass transport phenomena in porous materials, however, is incomplete and is still not completely understood. Therefore, experimental data for every specific porous system is needed. One possible experimental technique for characterizing the mass transport in such pore networks is pulse experiments. The general evaluation of experimental outcomes of these techniques follows the solution of Fick's second law where an integral and effective diffusion coefficient is recognized. However, a detailed local understanding of diffusion and sorption processes remains a challenge. As there is lack of proved models covering different length scales, existing classical concepts need to be evaluated with respect to their ability to reflect local geometries on the nanometer level. In this study, DSMC (Direct Simulation Monte Carlo) models were used to investigate the impact of pore microstructures on the diffusion behaviour of gases. It can be understood as a virtual pulse experiment within a single pore or a combination of different pore geometries.

Advanced In Situ I-V Measurements Used in the Study of Porous Structures Growth on Silicon

“…The pores intersect with each other (mainly at = 0.5 mA/cm 2 ) due to the fact that they increase their diameter as they grow into the substrate. The mass-transfer-controlled electrochemical etching is determined by diffusion of the oxide forming species and the diffusion depends on the geometry of pores [11,17]. However, the geometry of pores changes in time during etching.…”

Effect of pore diameter and length on electrochemical CO2 reduction reaction at nanoporous gold catalysts

“…Given that local concentration gradients play a signicant role in tuning the competition between CO 2 RR and the parasitic hydrogen evolution reaction (HER) on at polycrystalline electrodes, 8,16,17 it can be expected that the mass transport limitations introduced by connement effects in and around the nanoporous channels will also affect the competition between these two reactions. [18][19][20] In this respect, some recent studies have indeed emphasized the importance of local diffusional gradients in tuning the CO 2 RR activity on nanoporous electrodes. [21][22][23][24][25] In general, it has been shown that with the increasing roughness/thickness of the nanoporous catalysts, the local pH at the surface also increases, resulting in the suppression of bicarbonate-mediated HER reaction (HCO 3 À + 2e À / H 2 + 2CO 3…”

Computational optimization of catalyst distributions at the nano-scale

“…Furthermore, the DSMC method has the additional advantages of allowing treatment of inverse collisions and ternary chemical reactions, which becomes especially problematic in attempts at solving the Boltzmann equation directly [19]. The DSMC method is therefore well suited to describe reactive nanoscale systems [12,22]. It is, in fact, the most widely used numerical algorithm in kinetic theory [23,24] and has been experimentally validated for a great number of applications, including nonequilibrium gas flows (e.g.…”