Influence of the Ce–Zr promoter on Pd behaviour under dynamic CO/NO cycling conditions: a structural and chemical approach

Abstract

The behaviour of the Pd phase in three way catalysts (TWCs) having a Ce–Zr oxide as a promoter component has been examined under CO/NO cycling conditions. In order to extract information on the role of the noble metal and the metal–promoter interface, two catalysts in, respectively, the presence or the absence of the promoter and having a similar Pd particle size were analyzed by a synchronous multitechnique approach using X-ray absorption (XAS) or high energy X-ray diffraction (HXRD) in combination with diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) and mass spectrometry (MS). From a structural point of view it is shown that the promoter oxide strongly influences Pd behaviour, limiting morphological (size/shape) and structural (formation of carbide/oxide phases) modifications occurring under dynamic conditions in the absence of the promoter. Activity measurements show that these phenomena have important chemical consequences in terms of CO/NO pollutant metal coverage(s), the occurrence of NO and/or CO dissociation steps as well as the subsequent transformation of the pollutants into CO₂ and N₂.