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.

The increasing demand for short chain olefins like propene for plastics production and the availability of shale gas make the development of highly performing propane dehydrogenation (PDH) catalysts, robust toward industrially applied harsh regeneration conditions, a highly important field of research. A combination of surface organometallic chemistry and thermolytic molecular precursor approach was used to prepare a nanometric, bimetallic Pt−Mn material (3 wt % Pt, 1.3 wt % Mn) supported on silica via consecutive grafting of a Mn and Pt precursor on surface OH groups present on the support surface, followed by a treatment under a H 2 flow at high temperature. The material exhibits a 70% fraction of the overall Mn as Mn II single sites on the support surface; the remaining Mn is incorporated in segregated Pt 2 Mn nanoparticles. The material shows great performance in PDH reaction with a low deactivation rate. In particular, it shows outstanding robustness during repeated regeneration cycles, with conversion and selectivity stabilizing at ca. 37 and 98%, respectively. Notably, a material with a lower Pt loading of only 0.05 wt % shows an outstanding catalytic performance�initial productivity of 4523 g Cd 3 Hd 6 /g Pt h and an extremely low k d of 0.003 h −1 under a partial pressure of H 2 , which are among the highest reported productivities. A combined in situ X-ray absorption spectroscopy, scanning transmission electron microscopy, electron paramagnetic resonance, and metadynamics at the density functional theory level study could show that the strong interaction between the Mn II -decorated support and the unexpectedly segregated Pt 2 Mn particles is most likely responsible for the outstanding performance of the investigated materials.

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. 195 Pt solid-state NMR spectroscopy could be an ideal tool for this task because 195 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 195 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 ( 1 H or 31 P) to enable the rapid acquisition of 195 Pt MAS NMR spectra. We demonstrate that high-resolution wideline 195 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, sidebandselective excitation, and saturation pulses are incorporated into t 1 -noise eliminated dipolar heteronuclear multiple quantum coherence, perfect echo resonance echo saturation pulse double resonance, or J-resolved pulse sequences. The complete 195 Pt MAS NMR spectrum is then reconstructed by recording a series of 1D NMR spectra where the offset of the 195 Pt pulses is varied in increments of the MAS frequency. Analysis of the 195 Pt MAS NMR spectra yields the 195 Pt chemical shift tensor parameters. Zeroth order approximation density functional theory calculations accurately predict 195 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.

Despite being widely used in numerous catalytic applications, our understanding of reactive surface sites of highsurface-area γ-Al 2 O 3 remains limited to date. Recent contributions have pointed toward the potential role of highly reactive edge sites contained in the high-field signal (−0.5 to 0 ppm) of the 1 H NMR spectrum of γ-Al 2 O 3 materials. This work combines the development of well-defined, needle-shaped γ-Al 2 O 3 nanocrystals having a high relative fraction of edge sites with the use of state-of-the-art solid-state NMR to significantly deepen our understanding of this specific signal. We are able to resolve two hydroxyl sites with distinct isotropic chemical shifts of −0.2 and −0.4 ppm and different positions within the dipole−dipole network from 1 H− 1 H single-quantum double-quantum NMR. Moreover, the use of recoupling-time-encoded arbitrary-indirect-dwell dipolar heteronuclear multiple quantum coherence allows us to partially revise previous assignments for surface-aluminum sites in the proximity of these hydroxyl sites. Although previous work has ascribed the high-field signal to be correlated with a single four-coordinate Al-site with a substantial quadrupolar broadening of >10 MHz, we can identify the presence of two four-coordinate Al-sites with similar isotropic chemical shifts but different quadrupolar coupling constants of approximately 7 and >10 MHz, respectively. Recoupling-time-encoded data are thus able to differentiate sites that would otherwise only be achievable with access to multiple fields or usage of highly advanced NMR techniques.

Surface organometallic chemistry (SOMC) represents a unique synthetic platform for the preparation of model heterogeneous catalysts resembling those broadly applied in industry. SOMC techniques usually rely on the grafting of tailored molecular precursors onto the surface OH groups of oxide supports. The development of such precursors and the understanding of their reactivity with the supports are therefore crucial for the development of well-defined surface species. While a large number of organometallic precursors of early transition metals are known, only few examples of group-10 metal complexes are reported, in spite of the great interest for heterogeneous catalysts based on the Pt-group elements. Herein, we report the reactivity of a family of group-10 (Ni, Pd and Pt) alkyl complexes, towards partially dehydroxylated SiO 2 yielding well-defined supported species. We studied the effect of the metal, ligand, and support on the grafting mechanism of such precursors through a combined experimental and computational approach. Ultimately, we showed that at least two grafting pathways are possible for these compounds, namely the protonolysis of the M-alkyl bond by surface OH groups and the opening of strained siloxane bridges: the proportion of the two depending on the nature of the metal and its ancillary ligand.