Ageing Effects on Exhaust Gas Catalysts: Microscopic Changes Captured by X-Ray Tomography

Abstract

In this work we examine the different aspects of catalyst ageing with effects ranging from the nano to the macro scale. Underlining the general importance of combining different characterisation techniques, like transmission electron microscopy (TEM), X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) for the nanoscale, we focus on the application of X-ray absorption micro-computed tomography (micro-CT) to capture macroscopic changes in the um to mm scale. Two series of tomographic measurements were carried out: (i) investigation of three differently treated samples by collecting one channel from a fresh, a conditioned and an aged monolith and (ii) examination of one single coated honeycomb channel with 4 wt% Pt/γ-Al2O3 and for comparison one with pure γ-Al2O3 washcoat, which have been measured in a non-destructive ex situ manner at the same position after each ageing treatment. Main observations of the tomographic study are: (1) coating inhomogeneities between different channels taken from the same honeycomb and between different honeycombs, (2) formation of cracks in the washcoat material and (3) formation of macroscopic Pt particles in the case of 4 wt% Pt/γ-Al2O3 washcoat. Particularly valuable is the non-destructive ex situ investigation after different ageing steps on the same channel using X-ray tomography.