An inorganic capping strategy for the seeded growth of versatile bimetallic nanostructures

Mesoporous Silica Encapsulated Platinum-Tin Intermetallic Nanoparticles Catalyze Hydrogenation with an Unprecedented 20% Pairwise Selectivity for Parahydrogen Enhanced Nuclear Magnetic Resonance

Supported noble metals offer key advantages over homogeneous catalysts for in vivo applications of parahydrogen-based hyperpolarization. However, their performance is compromised by randomization of parahydrogen spin order resulting from rapid hydrogen adatom diffusion. The diffusion on Pt surfaces can be suppressed by introduction of Sn to form Pt-Sn intermetallic phases. Herein, an unprecedented pairwise selectivity of 19.7 ± 1.1% in the heterogeneous hydrogenation of propyne using silica encapsulated Pt-Sn intermetallic nanoparticles is reported. This high level of selectivity exceeds that of all supported metal catalysts by at least a factor of 3. Moreover, the pairwise selectivity for alkyne hydrogenation is about 2 times higher than for alkene hydrogenation, an observation attributed to the higher coverage of the former and its effect on diffusion. Lastly, PtSn@mSiO₂ nanoparticles exhibited improved coking resistance, and any loss of activity is shown to be fully reversible through high-temperature oxidation-reduction cycling.

Colloidal Nanocrystals as Precursors and Intermediates in Solid State Reactions for Multinary Oxide Nanomaterials

ConspectusPolyelemental compounds with dimensions in the nanosized regime are desirable in a large variety of applications, yet their synthesis remains a general challenge in chemistry. One of the major bottlenecks to obtaining multinary systems is the complexity of the synthesis itself. As the number of elements to include in one single nano-object increases, different chemical interactions arise during nucleation and growth, thus challenging the formation of the targeted product. Choosing the reaction conditions and identifying the parameters which ensure the desired reaction pathway are of the uttermost importance. When, in addition to composition, the simultaneous control of size and shape is sought after, the development of new synthetic strategies guided by the fundamental understanding of the formation mechanisms becomes crucial.In this Account we discuss the use of colloidal chemistry to target multinary oxide nanomaterials, with focus on light absorbers which can drive chemical reactions. We propose the combination of soft and solid-state chemistries as one successful strategy to target this family of polyelemental compounds with control on composition and morphological features. To start with, we highlight studies where in situ forming nanoparticles act as reaction intermediates, which we found in both oxide (i.e., Bi-V-O) and sulfide (Cu-M-S, with M = V, Cr, Mn) nanocrystals (NCs). Examples of ternary sulfides are mentioned only with the purpose of showing that similar mechanisms can apply to different families of multinary nanomaterials. Using this new knowledge, we demonstrate that reacting pre-synthesized NCs with well-defined composition and size with molecular precursors allows significant control of these same property-dictating features (i.e., composition and grain size) in the resulting ternary and quaternary compounds. For example, nanostructured BiV_1-xSb_xO₄ thin films with tunable composition and nanostructured β-Cu₂V₂O₇ with tunable grain size were accessed from colloidally synthesized Bi_1-xSb_x NCs (0 < x < 1) and size-controlled Cu NCs reacted with a vanadium molecular precursor, respectively. The analysis of reaction aliquots revealed that the formation of these materials occurs via a solid-state reaction between the NC precursors and V-containing amorphous nanoparticles, which form in situ from the molecular precursors. With the aim to achieve better control on the reaction product, we finally propose the use of colloidally synthesized NCs as reactants in solid state reactions. As the first proof of concept, ternary metal oxide NCs, including CuFe₂O₄, CuMn₂O₄, and CuGa₂O₄ with defined size and shape regulated by the NC precursors were obtained. Considering the huge library of single component and binary NCs accessible by colloidal chemistry, the extension of this synthetic concept, which combines soft and solid-state chemistries, to a larger variety of polyelemental nanomaterials is foreseen. Such an approach will contribute to facilitate a more rapid translation of design principles to materials with the desired composition and structural features.

Pairwise semi-hydrogenation of alkyne to cis-alkene on platinum-tin intermetallic compounds

The molecular basis for the high cis-alkene selectivity over intermetallic PtSn for alkyne semi-hydrogenation is demonstrated. Unlike the universal assumption that the bimetallic surface is saturated with atomic hydrogen, molecular hydrogen has a higher barrier for dissociative adsorption on intermetallic PtSn due to the deficiency of Pt three-fold sites. The resulting molecular behavior of adsorbed hydrogen on intermetallic PtSn nanoparticles leads to pairwise-hydrogenation of three alkynes to the corresponding cis-alkenes, satisfying both high stereoselectivity and high chemoselectivity.

Kinetics, energetics, and size dependence of the transformation from Pt to ordered PtSn intermetallic nanoparticles

The outstanding catalytic activity and chemical selectivity of intermetallic compounds make them excellent candidates for heterogeneous catalysis. However, the kinetics of their formation at the nanoscale is poorly understood or characterized, and precise control of their size, shape and composition during synthesis remains challenging. Here, using well-defined Pt nanoparticles (5 nm and 14 nm) encapsulated in mesoporous silica, we study the transformation kinetics from monometallic Pt to intermetallic PtSn at different temperatures by a series of time-evolution X-ray diffraction studies. Observations indicate an initial transformation stage mediated by Pt surface-controlled intermixing kinetics, followed by a second stage with distinct transformation kinetics corresponding to a Ginstling-Brounstein (G-B) type bulk diffusion mode. Moreover, the activation barrier for both surface intermixing and diffusion stages is obtained through the development of appropriate kinetic models for the analysis of experimental data. Our density-functional-theory (DFT) calculations provide further insights into the atomistic-level processes and associated energetics underlying surface-controlled intermixing.