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

Ellie K Dann,Alexandre Goguet,Peter P Wells,Emma K Gibson,Richard A Catlow,Diego Gianolio,Anna Kroner,Paul Collier,Christopher Hardacre,Agnes Raj,Tugce Eralp Erden

doi:10.1021/acs.chemmater.7b02552

Abstract

The rational design of catalyst materials 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 overlooked. A spectroscopic method for advanced in situ characterization is employed to capture the formation of PdO nanoparticles supported on γ-Al2O3 from two alternative molecular precursors - Pd(NO3)2 and Pd(NH3)4(OH)2. Time-resolved diffuse reflectance infrared Fourier transform spectroscopy is able to identify the temperature assisted pathway for ligand decomposition, showing that NH3 ligands are oxidized to N2O and NO– species, whereas NO3– ligands assist in joining Pd centers via bidentate bridging coordination. Combining with...

Highlights

Supported metal nanoparticles are a cornerstone of heterogeneous catalysis, and by extension, the chemical industry
Optimizing the preparation method for smaller nanoparticle size and improved dispersion is a common theme in heterogeneous catalyst design: to improve specific surface area, lower precious metal content, and increase metal−support interfacial regions while maintaining thermostability toward sintering
There are various methods for the preparation of supported metal catalysts such as controlled colloidal routes,[1,2] deposition precipitation,[3] grafting techniques,[4] and atomic deposition, all of which result in nanoparticle materials with differing properties

Summary

■ INTRODUCTION

Supported metal nanoparticles are a cornerstone of heterogeneous catalysis, and by extension, the chemical industry. While diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) is used to follow the vibrational modes of adsorbed and coordinated inorganic molecular species at the surface, X-ray absorption fine structure (XAFS) is used to follow the local coordination environment and oxidation state of Pd throughout the bulk of the sample Using both techniques at the same time with online mass spectrometry of the effluent gas gives the advantage of confidently assigning features that change with response to sample environment and allow interpretation of the mechanisms involved in ligand decomposition and metal nanoparticle formation. The value of σ2, the mean square relative displacement of absorber and backscatter atoms, is known to increase with temperature.[24] the linear dependence of σ2 with temperature was fitted using nonphase corrected Fourier transformed Pd K-edge data collected of the calcined PdO/γ-Al2O3 between 500 to 20 °C, where all other parameters (delr and S02) are assumed to be fixed This variance in σ2 with temperature, reported in Figure S1 of the Supporting. Effluent gases were analyzed by a MKS 2000 multigas FTIR analyzer

■ RESULTS AND DISCUSSION

■ ACKNOWLEDGMENTS

■ REFERENCES