Sensitivity and Selectivity of Porous Electrodes in Heterogeneous Liquid-Based Catalytic Reactions: 3D Simulation Study

Abstract

Efficiency, selectivity and sensitivity are important issues in catalytic applications, such as fuel cells and electrochemical sensors. This paper discusses the catalytic activity of porous layers in heterogeneous reactions based on the impact of pore morphology on pore accessibility in liquids. We present three-dimensional simulations to discuss some critical geometrical characteristics that influence the overall catalytic activity of porous catalyst. Sensitivity is proportional to the overall catalytic activity of the surface area. However, selectivity depends on pore accessibility. Simulation results demonstrate that at constant k 0 , porous layers with small pores and large numbers of pores are selective to the species with high diffusion coefficient because of high pore accessibility. In contrast, porous electrodes with low number of large pores and a large top surface area are selective to the species with low diffusion coefficient because of low pore accessibility. Additionally, pore accessibility influences the diffusional resistance, which has an impact on the local pH-value. High diffusional resistance in the porous layer leads to an accumulation of reaction products and a modification in the concentration of buffer molecules, which change local pH-value and therefore the catalytic behavior. Several porous surfaces in various forms were investigated as heterogeneous catalysts in order to increase the catalytic activity, especially the electrocatalytic activity of electrodes. Catalytic activity is proportional to the reaction rate, which is defined by the number of molecules catalyzed per second. 1-3 Thus, catalytic activity depends on the rate of turnover on the active sites and on the number of active sites. Many research groups tried to adjust the catalytic activity through variation of electrode material composition. The mono-metal, 4 bimetal 12 Current density is one of measurable quantities that is influenced by the catalytic surface area. 15 Enhancement of the current density by increasing the surface roughness factor was observed for the oxidation of methanol, 16 ethanol 9 and glucose. 18,23,24 Park et al. 23,24 The high surface Rf of mesoporous electrode influences the faradaic z E-mail: [email protected] current of kinetic controlled reactions, as in case of glucose oxidation, more than the faradaic current of diffusion-controlled reactions, as in case of L-ascorbic acid and 4-acetomidophenol oxidation. The high Rf enhances the faradaic current of reactions with sluggish electron transfer because of highly enlarged nanoscopic area. Roughness factor was often used in literature as the experimental characterizing parameter to optimize the electrode catalytic activity in liquid mediums. However, it is critical to use Rf as a characterizing parameter apart from other morphological characteristics and regardless the catalytic properties as well as the diffusion coefficient of active species. The investigations revealed that the electrode selectivity and activity have complex dependency on electrode morphology, i.e., the pore-size, distance among pores and the film thickness. For example, the Pt-nanotube arrays electrode with low roughness factor (Rf = 286) and the directly deposited Pt electrode with comparable rough surface (Rf = 183) exhibited different sensitivity toward glucose, ascorbic acid, uric acid and acetamidophenol. 18 Furthermore, the observation of the reaction current, based on the mesoporous surface area, proves that the surface morphology influences the surface catalytic activity. It was observed by Koehler et al. 25 that the fabrication process of mesoporous electrode influenced the oxidation efficiency at the electrode surface. They presented a fabrication process by which Rf was increased by 111%, while the current density of glucose oxidation increased by 250%. Catalytic activity is not only influenced by Rf and the number of catalytic sites, but also by pore accessibility. The mechanism of mass transport of active species and electrode film roughness are key aspects of determining the catalytic activity. One main limitation of mass transport in pores is the pore morphology, i.e., pore-size and the pore connectivity