Molecular and Electronic Structure of Isolated Platinum Sites Enabled by the Expedient Measurement of Pt Chemical Shift Anisotropy.

Techniques that can characterize the molecular structures of dilute surface species are required to facilitate the rational synthesis and improvement of Pt-based heterogeneous catalysts. Pt solid-state NMR spectroscopy could be an ideal tool for this task because Pt isotropic chemical shifts and chemical shift anisotropy (CSA) are highly sensitive probes of the local chemical environment and electronic structure. However, the characterization of Pt surface-sites is complicated by the typical low Pt loadings that are between 0.2 and 5 wt% and broadening of Pt solid-state NMR spectra by CSA. Here, we introduce a set of solid-state NMR methods that exploit fast MAS and indirect detection using a sensitive spy nucleus (H or P) to enable the rapid acquisition of Pt MAS NMR spectra. We demonstrate that high-resolution wideline Pt MAS NMR spectra can be acquired in minutes to a few hours for a series of molecular and single-site Pt species grafted on silica with Pt loading of only 3-5 wt%. Low-power, long-duration, sideband-selective excitation, and saturation pulses are incorporated into -noise eliminated dipolar heteronuclear multiple quantum coherence, perfect echo resonance echo saturation pulse double resonance, or -resolved pulse sequences. The complete Pt MAS NMR spectrum is then reconstructed by recording a series of 1D NMR spectra where the offset of the Pt pulses is varied in increments of the MAS frequency. Analysis of the Pt MAS NMR spectra yields the Pt chemical shift tensor parameters. Zeroth order approximation density functional theory calculations accurately predict Pt CS tensor parameters. Simple and predictive orbital models relate the CS tensor parameters to the Pt electronic structure and coordination environment. The methodology developed here paves the way for the detailed structural and electronic analysis of dilute platinum surface-sites.