Probing the Formation and Evolution of Pd Nanoparticles on the Surface of γ-Al2O3 Using a Pd(II) Coordination Polymer as a Precursor: An In Situ X-ray Scattering and Spectroscopy Study

Due to the small size and very low metal loadings in heterogeneously supported nanoparticle catalysts, it is a characterization challenge to trace and control the formation and structural evolution of nanoparticles during their synthesis. In this report, we have probed the formation of alumina-supported Pd nanoparticles by thermal treatment of a 1:3 composite of a Pd(II) coordination polymer (Pd(II)-CP) and a gamma-Al2O3 powder using in situ X-ray total scattering and X-ray absorption spectroscopy. By comparing the pyrolysis process of Pd(II)-CP in the absence and presence of gamma-Al2O3, we found that the sintering of Pd nanoparticles by the coalescence of nucleation sites can be significantly reduced on gamma-Al2O3 surfaces. The introduction of gamma-Al2O3 provides far better control over the size, distribution, and speciation of the synthesized Pd nanoparticles. The results showed that the sintering of Pd nanoparticles does not occur on the surface of gamma-Al2O3 until the Pd(II)-CP structure is fully collapsed at 400 degrees C, most likely because the organic framework of Pd(II)-CP provides spatially distinct sites for Pd nucleation. Moreover, the interface of the gamma-Al2O3 surface and Pd(II)-CP protected the Pd nanoparticle nucleation sites from air oxidation up to 500 degrees C. Thereafter, a slow conversion of the Pd fcc phase to the PdO phase started taking place, which was facilitated by the formation of an amorphous PdOx phase as an intermediate.