+PDE ; We describe here an original methodology related to the "build-the-bottle-around-the-ship" approach yielding a highly ordered silica matrix containing regularly distributed Pt nanoparticles (NPs) located inside the silica walls, Pt@{walls}SiO(2). The starting colloidal solution of crystalline Pt nanoparticles was obtained from Pt(dba)(2) (dba = dibenzylidene acetone) and 3-chloropropylsilane. The resulting nanoparticles (diameter: 2.0 +/- 0.4 nm determined by HRTEM) resulted hydrophilic. The NPs present in the THF colloidal solution were incorporated inside the walls of a highly ordered 2D hexagonal mesoporous silica matrix via sol-gel process using a templating route with tetraethylorthosilicate, TEOS, as the silica source, and block copolymer (EthyleneOxide)(20)(PropyleneOxide)(70)(EthyleneOxide)(20) (Pluronic P123) as the structure-directing agent. Low-temperature calcination of the crude material at 593 K led to the final solid Pt@{walls}SiO(2). Characterization by IR, HRTEM, BF-STEM and HAADF-STEM, SAXS, WAXS, XRD, XPS, H(2) chemisorption, etc. of Pt@{walls}SiO(2) confirmed the 2D hexagonal structuration and high mesoporosity (870 m(2)/g) of the material as well as the presence of stable 2-nm-sized crystalline Pt(0) NPs embedded inside the walls of the silica matrix. The material displayed no tendency to NPs sintering or leaching (Pt loading 0.3 wt.%) during its preparation. Pt@{walls}SiO(2) was found to be a stable, selective and highly active hydrogenation catalyst. The catalytic performances in propene hydrogenation were tested under chemical 'regime conditions in a tubular flow reactor (278 K, propene/H(2)/He = 20/16/1.09 cm(3)/min, P(tot) = 1 bar) and were found superior to those of an homologous solid containing Pt NPs along its pore channels Pt@{pores}SiO(2) and to those of a classical industrial catalysts Pt/Al(2)O(3), (TOF = 2.3 s(-1) vs. TOF = 0.90 and 0.92 s(-1), respectively, calculated per surface platinum atoms). Pt@{walls}SiO(2) also catalyzes fast and selective styrene hydrogenation. A material containing by design Pt NPs both in its walls and in its pores, Pt@{walls + pores}SiO(2), is also described. (C) 2011 Elsevier Inc. All rights reserved.