Particle size effects in Rh/Al₂O₃ catalysts as viewed from a structural, functional, and reactive perspective: the case of the reactive adsorption of NO

The structural-dynamic behaviour of γ-Al₂O₃ supported Rh nanoparticles under He, H₂/He, and NO/He has been investigated using a newly developed methodology that permits dispersive EXAFS (EDE), diffuse reflectance infra red spectroscopy (DRIFTS), and mass spectrometry (MS) to be applied simultaneously to the study of gas-solid interactions. This reveals a considerably variability in nanoparticle habit (for 11 Å diameter nanoparticles as a function of temperature), and between 8 Å and 11 Å particles in their response to NO. The selectivity (N₂/(N₂ + N₂O)) of the reactive interaction between NO and the supported Rh shows essentially no particle size dependence above 473 K: it is apparent, however, that considerable differences in some aspects of the structural behaviour of the 8 Å and 11 Å Rh particles do nonetheless, exist. At 373 < T < 473 K a clear divergence in structural, functional, and reactive response of the different sized supported Rh nanoparticles toward NO is observed. These observations are discussed in terms of the ability of different sized Rh particles to change structure in response to the reactive environment, the subsequent effect this has on the nitrosyl functionality that different phases may support, and the reactive pathways for NO conversion that may therefore arise.