Multitechnique analysis of supported Pd particles upon dynamic, cycling CO/NO conditions: Size-dependence of the structure–activity relationship

The behaviour of Pd/Al2O3 supported catalysts with metal loading in the 0.5–4 wt.% interval has been examined during cycling CO/NO conditions in an attempt to interpret the size-dependence of the metal chemical activity within a dynamic redox situation. To this end, a synchronous, multitechnique approach using X-ray absorption (XAS) or hard X-ray diffraction (HXRD) in tandem with diffuse reflectance infrared spectroscopy (DRIFTS) and mass spectrometry (MS) was used to establish the size-dependence of chemically significant steps in formation of N2 and CO2. During cycling, EXAFS and HXRD reveal the existence of a reversible Pd morphology and phase change (PdCx formation and removal) phenomena in response to the gas atmosphere reducing/oxidizing nature. This is observed for Pd particles having dispersion values going from ca. 1–0.4. Using EXAFS Pd–Pd coordination numbers, we dynamically normalized the DRIFTS and MS signals obtained during cycling conditions and find that metal size/phase-effects appear of importance in CO2 formation but can be essentially neglected in N2 formation. Such (size-dependent) metal behaviour appears far from expectations coming from studies using non-cycling gas mixtures. The chemical relevance of NO, CO activation, dissociation and coupling steps are discussed in relation to the product formation and their size-dependence or independence.