Surface oxidation of Pt and PtPd alloys during NO conversion investigated by Atom Probe Tomography

TL;DR

This study used atom probe tomography to analyze how surface oxidation of Pt and PtPd alloys during NO conversion is reversible and correlates with catalytic efficiency changes across temperature cycles.

Contribution

It provides nanoscale insights into the reversible redox behavior of Pt and PtPd alloy catalysts during NO conversion, linking surface oxidation states to catalytic performance.

Findings

Oxide thickness decreases during cooling and heating cycles.

Redox behavior is reversible and driven by reaction kinetics.

Surface oxidation correlates with NO conversion efficiency.

Abstract

In this study, the dynamic oxidation state changes of pure Pt and 50at% PtPd alloy catalysts were investigated during a temperature ramp from 80 to 450 C under a reactive gas mixture of 500 ppm NO and 3 % Oxygen in Nitrogen atmosphere. These changes are closely correlated with variations in NO conversion efficiency. Sharp tip specimens were prepared from Pt and PtPd alloy wires by electrochemical polishing and Focused Ion Beam annular milling, with the hemispherical apex serving as a model for nanoscale catalyst surfaces. The samples were exposed to the reactive atmosphere in a dedicated reaction chamber and subsequently analyzed using atom probe tomography. Effective oxide thicknesses and three-dimensional surface morphology were quantitatively evaluated. A pronounced decrease in oxide thickness was observed during the first cooling and second heating cycles, particularly below 200 C, indicating reversible redox behavior on both Pt and PtPd alloy surfaces. This behavior correlates with the inverse hysteresis of NO conversion measured for both systems, suggesting that the redox reversibility is driven primarily by reaction kinetics rather than thermodynamic stability.