Combined In Situ XAFS/DRIFTS Studies of the Evolution of Nanoparticle Structures from Molecular Precursors

Abstract

The rational design of catalysts is of great industrial significance, yet there is a fundamental lack of knowledge in some of the most well-established processes e.g. formation of supported nanoparticle structures through impregnation. Here, the choice of precursor has a significant influence on the resulting catalytic properties of the end material, yet the chemistry that governs the transformation from defined molecular systems to dispersed nanoparticles is often over-looked. A spectroscopic method for advanced in situ characterization is employed to capture the formation of PdO nanoparticles supported on γ-Al<sub>2</sub>O<sub>3</sub> from two alternative molecular precursors; Pd(NO<sub>3</sub>)<sub>2</sub> and Pd(NH<sub>3</sub>)<sub>4</sub>(OH)<sub>2</sub>. Time resolved DRIFTS is able to identify the temperature assisted pathway for ligand decomposition, showing that NH<sub>3</sub> ligands are oxidised to N<sub>2</sub>O and NO<sup>-</sup> species, whereas, NO<sub>3</sub><sup>-</sup> ligands assist in joining Pd centres via bidentate bridging co-ordination. Combining with simultaneous XAFS, the resulting nucleation and growth mechanism of the precious metal oxide nanoparticles are resolved. The bridging ability of palladium nitrate aids formation and growth of larger PdO nanoparticles at lower onset temperature (<250°C). Conversely, impregnation from [Pd(NH<sub>3</sub>)<sub>4</sub>]<sup>2+</sup> results in well isolated Pd centres, anchored to the support, which require higher temperature (>360°C) for migration to form observable Pd-Pd distances of PdO nanoparticles. These smaller nanoparticles have improved dispersion and an increased number of step and edge sites compared to those formed from the conventional Pd(NO<sub>3</sub>)<sub>2</sub> salt, favouring a lower light off temperature for complete methane oxidation.