Immobilized platinum atoms and compounds are widely used single-site heterogeneous catalysts. Due to the low concentration of surface platinum sites and the disordered nature of surfaces and interfaces, structural characterization of heterogeneous Pt-catalysts is often challenging. The development of techniques to characterize the surface structure of these single-site catalysts will open up their rational synthesis and development. ¹⁹⁵Pt solid-state NMR spectroscopy is uniquely able to probe the local chemical environment and electronic structure of Pt centers due to the dependence of the chemical shift tensor on the identity and symmetry of the surrounding ligands. However, ¹⁹⁵Pt solid-state NMR spectra are often broad, making them challenging to observe using conventional 1D ¹⁹⁵Pt solid-state NMR experiments, especially from catalyst materials that usually have Pt loadings below a few weight %.

Here we demonstrate state-of-the-art solid-state NMR techniques to determine the molecular and electronic structure of surface-supported Pt single-sites. Specifically, using recent technological advances such as fast magic angle spinning (MAS), proton detection and dynamic nuclear polarization surface-enhanced NMR spectroscopy (DNP-SENS), we demonstrate ¹H-¹⁹⁵Pt correlation experiments and determine the molecular structure of a 3.7 wt.% single-site Pt species grafted on silica. Recently, we reported an accelerated approach to precisely characterize ¹⁹⁵Pt chemical shift tensors using proton detection and selective pulses. The presence of nearby hydrogen atoms can be exploited to indirectly observe ¹⁹⁵Pt spectra using the highly sensitive proton spins. This allows ¹⁹⁵Pt chemical shift anisotropy to be measured in minutes for pure compounds, and in only a few hours for model single-site catalysts with low loading.

For compounds that do not contain a proton in the vicinity of the Pt center such as platinum N-heterocyclic carbenes, we used DNP-enhanced ¹³C-¹⁹⁵Pt double resonance NMR experiments to determine the ¹⁹⁵Pt chemical shift tensors. A combination of NMR and density functional theory revealed the structures of two types of surface Pt sites, pointing to parallel grafting mechanisms.

The results presented here offer new insights into the surface structure of Pt-based catalysts and reinforce solid-state NMR spectroscopy as a critical method in the toolbox of catalysis science.

Nicolas Kaeffer

Appearances