Study and optimization of the morphological characteristics of nanostructures for heterogeneous catalysis applications

Abstract: The distinct organization of nanostructured materials influences several catalytic characteristics, like selectivity, sensitivity or catalytic efficiency, which are important issues in catalytic applications, such as fuel cells and electrochemical sensors. Indeed, pores on electrodes provide a three-dimensional active area, which results in a high surface area and an increased reaction rate at the electrode. This work draws the attention to the optimal morphology of hierarchical and non-hierarchical nanostructures and the morphological characteristics that lead to a rational design of heterogeneous nanocatalysts, especially that have sluggish kinetics. Such optimization is achieved through a deep understanding of the catalytic activity of porous layers based on the impact of pore morphology on pore accessibility. For that purpose, four types of nanostructures, i.e., hierarchical external, hierarchical internal, non-hierarchical external and non-hierarchical internal nanostructures, were utilized in simulations of the electrode kinetics of organic and bioorganic fuels oxidation.

Generally, external nanostructures reveal higher utilization of catalytic surface than internal nanostructures. Simulation results of non-hierarchical nanostructures demonstrate that 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 a 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. Nonetheless, the mass transport in hierarchical external nanostructures is the best among the four investigated nanostructures. Hierarchical external nanostructures show an outstanding accessibility of active sites and a kinetic-controlled regime for the species with low diffusion coefficient. In contrast, hierarchical internal nanostructures show slightly less specific activity than nonhierarchical internal nanostructures because of high total diffusional resistance and longer diffusional path.

For hierarchical nanostructures optimization, macropore-size exhibit the most comprehensive characteristic for evaluating specific activity and current density of hierarchical nanostructures. The optimal current densities for external and internal hierarchical nanostructures are achieved at macropore-size ranges of 3.2 - 4.5 μm and 1.9 - 3.2 μm, respectively. The optimal mass activity of internal nanostructures is achieved at the porosity range of 40 - 50% whereas the optimal mass activity of external hierarchical nanostructures is achieved at high porosity values. External hierarchical nanostructures in comparison to internal hierarchical nanostructures tend to be the cost-effective catalysts that have high catalytic activity