The Importance of Nano‐Confinement in Nanoporous Catalysts: Atom Probe Tomography and FIB / SEM study 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. 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 as well as the importance of the residual Ag in the catalytic process. Ozone is used 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 of different compositions. To get better insight of the Ag distribution within the Au backbone, we first analysed nanoporous Au catalysts (composition: Au 97 ‐Ag 3 ) by atom probe tomography (APT). APT is a powerful technique to characterize the composition and 3D structure of materials at the atomic‐scale, but the presence of pores make the analysis and reconstruction difficult. New developments in sample preparation are required, and we were able to successfully image npAu samples by atom probe tomography and analyse the segregation of Ag atoms in the npAu sample ( Fig. 1 ). Complimentary experiments were performed on a bulk sample of the same composition, and XPS and APT experiments confirm the surface segregation of Ag (as silver oxide species) after ozone treatment, which is then reduced after exposing the catalyst to reaction conditions. Further experiments were performed on bulk Ag 70 ‐Au 30 samples which were exposed to ozone and reaction conditions. Ozone induces the segregation of Ag at the surface, which forms a distinct black layer of silver oxides. Below this oxide, a homogeneous Ag‐depleted region (Ag 56 ‐Au 44 ) can be observed, and extends over a depth of a few μm (the depth depends on the duration of the ozone treatment). As it can be seen on Fig. 2 , this region undergoes severe morphological changes, and the bulk sample becomes porous. FIB cross‐section analysis proves the segregation behaviour and long‐range diffusion of Ag in bulk samples, as compared to the nanoscale‐segregation observed by APT, correlating previous observations by E‐TEM. The nanoconfinement induced by the specific architecture of the nanoporous sample is then responsible of the long term stability and efficiency of the catalyst. This study highlights the importance of ozone treatment in the segregation of Ag at the surface, which can dramatically influence the local chemistry and morphology of a catalyst. The combination of APT, FIB/SEM and XPS allows studying the surface and subsurface compositional and morphological changes of the sample after various physicochemical treatments, and also allows the segregation behaviour of Ag in different Au‐based catalysts to be correlated.