Shape and Size Controlled Pt Nanocrystals as Novel Model Catalysts

Abstract

A tremendous development of nanotechnology enabled us to prepare precisely structure-controlled nanocrystals (NCs). Recently, such structure controlled NCs have been applied as model catalysts with controlled active sites to evaluate reaction mechanisms. Here, we review our recent works on preparation of shape controlled Pt NCs, such as cube and wire, and on application as model catalysts for olefin hydrogenation to assess active Pt sites. A hydrogen reduction method of Pt(II) ion was adopted in an aqueous phase to prepare Pt cube in the presence of shape forming agent (NaI) and organic protective agent (sodium polyacrylate or sodium succinate). We succeed to prepare Pt cube with high size and shape selectivity, followed by very small Pt cube of less than 3 nm. Such simultaneous control of both size and shape of Pt NCs has been successfully achieved by the sophisticated tuning of multiple conditions for the growth kinetics of Pt nuclei during hydrogen reduction of PtCl 4 2-in aq. N-, N-dimethylformamide (DMF) solution. The key strategy is to produce small Pt nuclei and to avoid the excessive growth of Pt nuclei, in conjunction with face selective adsorption of anionic protective and shape-forming agents by the control of solvent system. For the preparation of Pt nanowires with high anisotropy, template-assisted methods have been usually used. We have developed facile liquid phase preparation method at room temperature in air without any template. The key factors of the method are: firstly, to use aqueous organic solvent system to control solvent polarity; secondly, to use co-solvent DMF as shape forming agent; thirdly, to use excess NaBH 4 as a stabilizer of Pt nuclei to prevent formation of conventional particles. Single-crystalline Pt nanowires of 2 nm diameter and more than micron length were easily obtained by the reduction of Pt(IV) with excess NaBH 4 in water-DMF-toluene solution (1:8:5 volume ratio) in a short reaction period of 3 h. Then, we have applied shape-controlled Pt NCs protected with PAA prepared by our original methods, such as cube, tetrahedron and wire, as model catalysts for olefin (cis-and trans-stilbenes) hydrogenation in ethanol to evaluate the active facet of Pt catalysts; e.g., Pt(111) or Pt(100). The estimated TOF values for the hydrogenation of cis-and trans-stilbene decreased in the order: cube [ cuboctahedron [ tetrahedron * nanowire. This tendency indicates that Pt(100) show high activity compared with Pt(111). The result is compatible with measurement of XPS and Raman spectra, suggesting strong adsorption of reactant on Pt(100). Hydrogenation of olefins (1-hexene and cyclohexene) has been carried out over Pt cubes with different sizes (8.2-10.1 nm) as catalysts to get information about active sites of Pt catalysts, e.g., flat facet or edge/corner atoms. The results of similar TOF values among Pt cubes with different sizes imply that the active sites are flat facets.

Shape‐directed platinum nanoparticle synthesis: nanoscale design of novel catalysts

“…These are often added to the system as salts with halides or polyatomic anions. However, shape direction via hydrogen chloride, bromide, iodide and nitrate anions has also been reported, so the effects of both cation and anion must be taken into account when adding inorganic salts to tune shape.…”

“…Some common polymers used for this directing method include polyvinylpyrrolidone (PVP), block copolymers and even short peptide sequences . Other directing agents which also act as stabilizers include sodium citrate, sodium polyacrylate and sodium succinate …”

“…Shape control of metal nanoparticles has had a great deal of attention in the past decade. The enhanced catalytic properties and large scope of catalytic activities that platinum possesses are based on the tunable electronic structure of surface faceting that is exhibited with size and shape control . Exposure of different crystal facets can significantly alter catalytic selectivity by changing chemical and electronic interactions, thereby governing the nanoparticle's reactivity .…”

Shape‐Controlled Nanostructures in Heterogeneous Catalysis

“…For instance, the rate of hydrogenation of 2-methyl-3-butyn-2-ol with monodispersed surfactant-stabilized Pd nanospheres with sizes between 6 and 13 nm, prepared by a reverse microemulsion procedure, was reported to scale with the number of atoms on their (111) terraces; this suggests that those are the active sites for that reaction (Figure 1). [45,46] This type of correlation between nanoparticle size and activity that leads to the identification of the active catalytic sites is, however, often not valid because other factors may also contribute to the catalytic performance. [44] Shown in the main panel are the turnover frequencies (TOFs) for the reaction, calculated both per total number of surface atoms (*, left axis) and per number of atoms in (111) facets (~, right axis).…”

“…[45,46] Also, Pd catalysts are particularly useful for the selective hydrogenation of alkynes to alkenes, and that behavior was shown to be surface sensitive as well. A group of reports has focused on the study of these materials for the activity and selectivity of hydrocarbon hydrogenation reactions, mostly by using Pt or Pd metals with cubic, octahedral, or cuboctahedral shapes, which allows comparison of the performances of the square (100) and hexagonal (111) facets of these fcc metals.…”