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 PtCl₄ 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 NaBH₄ 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 NaBH₄ 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.