Surface Modifications during a Catalytic Reaction: a Combined APT and FIB/SEM Analysis of Surface Segregation

Abstract

To improve the understanding of catalytic processes, the surface structure and composition of the active materials need to be determined before and after reaction. Morphological changes may occur under reaction conditions and can dramatically influence the reactivity and/or selectivity of a catalyst. Gold-based catalysts with different architectures are currently being developed for selective oxidation reactions at low temperatures [1]. Specifically, nanoporous Au (npAu) with a composition of Au 97 -Ag 3 is obtained by dealloying a Ag 70 -Au 30 bulk alloy. Recent studies highlight the efficiency of npAu catalysts for methanol oxidation using ozone to activate the catalysts before methanol oxidation. In this work, we studied the morphological and compositional changes occurring at the surface of Au-based catalysts in certain conditions. To do so, we first analysed nanoporous Au catalysts, with a composition of Au 97 -Ag 3 , by atom probe tomography (APT) to get better insight of the Ag distribution within the Au backbone. APT is a powerful technique to characterize the composition and 3D structure of materials at the atomic-scale. However, the presence of pores make the APT analysis and reconstruction difficult, thus new developments in sample preparation are required. Using a combination of in-situ e-beam-directed chemical vapor deposition in the focused ion beam (FIB) and lift-out techniques, we were able to successfully image npAu samples by atom probe tomography and analyse the segregation of Ag atoms in the npAu sample. From left to right, Figure 1 shows a schematic representation of npAu, a typical sample prepared for APT analysis (the pores have been filled with W) and the three-dimensional reconstruction of the sample; the latter suggests a non-uniform enrichment of Ag at the surfaces of nanoporous Au. Complimentary experiments were also performed on a bulk sample of the same composition using APT, FIB/SEM and X-Ray photoemission spectroscopy (XPS). This bulk sample was exposed to different treatments in a flow reactor: 1) thermal treatment: 1 hour at 150°C under flowing He – 2) ozone treatment: 150°C under a flow of 3% O 3 in O 2 -He with different exposure durations – 3) reaction conditions: 1 hour at 150°C under a flow of 6.5% MeOH and 20% O 2 -He. XPS and APT confirm the surface segregation of Ag (as silver oxide species) after ozone treatment, which is then reduced after exposing the catalyst to reaction conditions.