Direct solid state NMR observation of the 105Pd nucleus in inorganic compounds and palladium metal systems

Abstract

The ability to clearly relate local structure to function is desirable for many catalytically relevant Pd-containing systems. This report represents the first direct ¹⁰⁵Pd solid state NMR measurements of diamagnetic inorganic (K₂Pd(IV)Cl₆, (NH₄)₂Pd(IV)Cl₆ and K₂Pd(IV)Br₆) complexes, and micron- and nano-sized Pd metal particles at room temperature, thereby introducing effective ¹⁰⁵Pd chemical shift and Knight shift ranges in the solid state. The very large ¹⁰⁵Pd quadrupole moment (Q) makes the quadrupole parameters (C_Q, η_Q) extremely sensitive to small structural distortions. Despite the well-defined high symmetry octahedral positions describing the immediate Pd coordination environment, ¹⁰⁵Pd NMR measurements can detect longer range disorder and anisotropic motion in the interstitial positions. The approach adopted here combines high resolution X-ray pair distribution function (PDF) analyses with ¹⁰⁵Pd, ³⁹K and ³⁵Cl MAS NMR, and shows solid state NMR to be a very sensitive probe of short range structural perturbations. Solid state ¹⁰⁵Pd NMR observations of ∼44–149 μm Pd sponge, ∼20–150 nm Pd black nanoparticles, highly monodisperse 16 ± 3 nm PVP-stabilised Pd nanoparticles, and highly polydisperse ∼2–1100 nm biomineralized Pd nanoparticles (bio-Pd) on pyrolysed amorphous carbon detect physical differences between these systems based on relative bulk:surface ratios and monodispersity/size homogeneity. This introduces the possibility of utilizing solid state NMR to help elucidate the structure–function properties of commercial Pd-based catalyst systems.