Nano-analysis of surface reaction kinetics of automotive exhaust gas catalyst

Abstract

This study aims to advance our comprehension of the reaction kinetics occurring in nano-sized noble metal catalysts. Advanced microscopy techniques were employed to investigate the precise details of oxidation and diffusion behavior at different temperatures. In particular, atom probe tomography (APT), a highly sophisticated characterization technique, was utilized to obtain accurate and detailed information regarding these processes. By employing this microscopy technique, a comprehensive analysis of the reaction kinetics of nano-sized noble metal catalysts could be achieved, contributing to the development of more efficient catalysts for automotive applications. Noble metal nanoparticles such as Pt, Pd, and PtPd exhibit distinct oxidation behaviors during NO conversion. Understanding the alterations in the catalyst surface is crucial, as it has a direct effect on the performance of the catalytic converter. For this study, wires were utilized made of pure Pt, pure Pd, and PtPd alloy to create sharp tip samples through electrochemical polishing or FIB annular milling. The hemispherical shape of the tip apex serves as a model for the nanosized catalyst surface. The tips were oxidized in the reaction chamber and investigated by APT. The effective oxide thicknesses were determined and compared to the NO conversion rates measured using a Flat Bed Reactor (FBR). Additionally, experimental results of studying the interdiffusion behavior of the Pt-Pd binary system at various temperatures are presented and discussed. To achieve this purpose, the samples were prepared and examined with two different methods, depending on the heating temperatures. For the temperature range between 673 and 973 K, the samples made of nano-sized multiple layers were investigated using APT. For the temperature range between 1073 and 1243 K micro-sized Pt/Pd diffusion couples were annealed and analyzed using energy dispersive X-ray spectroscopy (EDX). The interdiffusion coefficients for both methods were determined by representing them as Fourier series and the Boltzmann-Matano method, respectively. The latter study was conducted in collaboration with the department of Materials design. Consequently, the interdiffusion coefficients were compared with the results of density functional theory (DFT) simulations.