Silica-supported subnanometric PtZn particles, prepared via surface organometallic chemistry, are highly productive and selective for propane dehydrogenation.
Alkane dehydrogenation over heterogeneous catalysts has attracted renewed attention in recent years. Here, well-defined catalysts based on isolated metal sites and supported Pt-alloys prepared via SOMC are discussed and compared to classical systems.
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 195 Pt 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(OtBu) 3 ) 2 ] (1, COD = 1,5-cyclooctadiene) on partially dehydroxylated silica (1@SiO 2 ). Compound 1@SiO 2 has a Pt loading of 3.7 wt %, a surface area of 200 m 2 /g, and a surface Pt density of around 0.6 Pt site/nm 2 . Fast MAS 1 H{ 195 Pt} dipolar-HMQC and S-REDOR experiments were implemented on both the molecular precursor 1 and on the surface complex 1@SiO 2 , providing access to 195 Pt isotropic shifts and Pt−H distances, respectively. For 1@SiO 2 , the measured isotropic shift and width of the shift distribution constrain fits of the static wide-line DNPenhanced 195 Pt spectrum, allowing the 195 Pt 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.
Nonoxidative dehydrogenation
of light alkanes has seen a renewed
interest in recent years. While PtGa systems appear among the most
efficient catalyst for this reaction and are now implemented in production
plants, the origin of the high catalytic performance in terms of activity,
selectivity, and stability in PtGa-based catalysts is largely unknown.
Here we use molecular modeling at the DFT level on three different
models: (i) periodic surfaces, (ii) clusters using static calculations,
and (iii) realistic size silica-supported nanoparticles (1 nm) using
molecular dynamics and metadynamics. The combination of the models
with experimental data (XAS, TEM) allowed the refinement of the structure
of silica-supported PtGa nanoparticles synthesized via surface organometallic
chemistry and provided a structure–activity relationship at
the molecular level. Using this approach, the key interaction between
Pt and Ga was evidenced and analyzed: the presence of Ga increases
(i) the interaction between the oxide surface and the nanoparticles,
which reduces sintering, (ii) the Pt site isolation, and (iii) the
mobility of surface atoms which promotes the high activity, selectivity,
and stability of this catalyst. Considering the complete system for
modeling that includes the silica support as well as the dynamics
of the PtGa nanoparticle is essential to understand the catalytic
performances.
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