The Structure of Molecular and Surface Platinum Sites Determined by DNP-SENS and Fast MAS¹⁹⁵Pt Solid-State NMR Spectroscopy

Abstract

The molecular level characterization of heterogeneous catalysts is challenging due to the low concentration of surface sites and the lack of techniques that can selectively probe the surface of a heterogeneous material. Here, we report the joint application of room temperature proton-detected NMR spectroscopy under fast magic angle spinning (MAS) and dynamic nuclear polarization surface enhanced NMR spectroscopy (DNP-SENS), to obtain the ¹⁹⁵Pt solid-state NMR spectra of a prototypical example of highly dispersed Pt sites (single site or single atom), here prepared via surface organometallic chemistry, by grafting [(COD)Pt(OSi(O<i>t</i>Bu)₃)₂] (<b>1</b>, COD = 1,5-cyclooctadiene) on partially dehydroxylated silica (<b>1@SiO</b>_<b>2</b>). Compound <b>1@SiO</b>_<b>2</b> has a Pt loading of 3.7 wt %, a surface area of 200 m²/g, and a surface Pt density of around 0.6 Pt site/nm². Fast MAS ¹H{¹⁹⁵Pt} dipolar-HMQC and S-REDOR experiments were implemented on both the molecular precursor <b>1</b> and on the surface complex <b>1@SiO</b>_<b>2</b>, providing access to ¹⁹⁵Pt isotropic shifts and Pt-H distances, respectively. For <b>1@SiO</b>_<b>2</b>, the measured isotropic shift and width of the shift distribution constrain fits of the static wide-line DNP-enhanced ¹⁹⁵Pt spectrum, allowing the ¹⁹⁵Pt chemical shift tensor parameters to be determined. Overall the NMR data provide evidence for a well-defined, single-site structure of the isolated Pt sites.