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 N-2 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 N-2 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. (C) 2010 Elsevier Inc. All rights reserved.