No abstract

No abstract

Platinum is a key catalyst that is invaluable in many important industrial processes such as CO oxidation in catalytic converters, oxidation and reduction reactions in fuel cells, nitric acid production, and petroleum cracking.[1] Many of these applications utilize Pt nanoparticles supported on oxides or porous carbon.[2] However, in practical applications that involve high temperatures (typically higher than 300 8C), the Pt nanoparticles tend to lose their specific surface area and thus catalytic activity during operation because of sintering. Recent studies have shown that a porous oxide shell can act as a physical barrier to prevent sintering of unsupported metal nanoparticles and, at the same time, provide channels for chemical species to reach the surface of the nanoparticles, thus allowing the catalytic reaction to occur. This concept has been demonstrated in several systems, including Pt@SiO 2 , [3] Pt@CoO, [4] Pt/CeO 2 @SiO 2 , [5] Pd@SiO 2 , [6] Au@SiO 2 , [7] Au@SnO 2 [8] and Au@ZrO 2 [9] coreshell nanostructures. Despite these results, a sinter-resistant system has not been realized in supported Pt nanoparticle catalysts.Improved catalytic or photocatalytic properties are often achieved when metal nanoparticles are supported on oxides such as TiO 2 and CeO 2 that interact strongly with late transition metals. [2f, 5] Herein, we demonstrate a thermally stable catalytic system consisting of Pt nanoparticles that are supported on a TiO 2 nanofiber and coated with a porous SiO 2 sheath. In this system, the porous SiO 2 coating offers an energy barrier to prevent the migration of individual Pt atoms or nanoparticles because of its weak interaction with late transition metals, including Pt. The porous-SiO 2 /Pt/TiO 2 catalytic system was prepared in three steps (Figure 1): 1) deposition of Pt nanoparticles onto the surface of TiO 2 nanofibers; 2) coating of SiO 2 with cetyltrimethylammonium bromide (CTAB) as a pore-generating agent; and 3) calcination in air to generate a porous sheath of SiO 2 . By using this approach, we were able to produce a platinum-based catalytic system that can resist sintering up to 750 8C in air, while retaining the catalytic activity of the Pt nanoparticles.The TiO 2 nanofibers were prepared by electrospinning and subsequent calcination in air at 750 8C for 2 hours.[10] The as-prepared nanofibers had a rough surface and a polycrystalline structure that contained both anatase and rutile phases (69 % anatase and 31 % rutile; Figure S1 in the Supporting Information). Poly(vinyl pyrrolidone) (PVP) stabilized Pt nanoparticles were prepared by using the polyol method.[11]The as-synthesized Pt nanoparticles were uniform in size, with an average size of (3.1 AE 0.5) nm (Figure 2 a, b). These Pt nanoparticles were deposited onto the TiO 2 nanofibers by immersing the sample in a suspension of the Pt nanoparticles, which was prepared by a 10-fold dilution of the as-prepared Pt sample with ethanol. As shown in Figure 2 c, the Pt nanoparticles were well dispersed on the surface of each TiO ...

An improved, exact analysis of surface Ostwald ripening of a collection of nanoparticles is presented in an effort to redefine the critical radius involved in the kinetic models of ripening. In a collection of supported particles of different sizes, the critical radius is the size of the particle that is in equilibrium with the surrounding adatom concentration. Such a particle neither grows nor shrinks due to Ostwald ripening, whereas larger particles grow and smaller particles shrink. We show that previous definitions of critical radius are applicable only for limiting regimes where the Kelvin equation has been linearized. We propose a more universally applicable definition of critical radius that satisfies the constraints of mass balance.

The decomposition and removal of poly(amidoamine) (PAMAM) dendrimers from inorganic metal oxide surfaces frequently used as catalyst supports was investigated by the use of FT-IR spectroscopy. Spectra of fourth-generation hydroxyl-terminated PAMAM dendrimers (G4OH) on γ-Al 2 O 3 were collected first at room temperature and were subsequently analyzed with all bands assigned to the vibrational frequencies of dendrimer functional groups. Bands corresponding to amide and ethylenic groups decrease in intensity upon heating at 150°C, while new bands corresponding to surface carboxylate species appear in their stead. Thus, the process of dendrimer removal occurs in two stages: dendrimer decomposition to form adsorbed carboxylates followed by the removal of these carboxylates from the surface. The dendrimer generation (i.e., G3OH vs G4OH) does not affect the rate of this process. However, the temperature required for completion of the first stage rises with increasing G4OH weight loading. Other factors that influence the rate of overall dendrimer removal were found to include the type of gas-phase environment used and the presence or absence of metal species within the dendrimer. Specifically, an oxidizing environment, or the presence of either platinum or rhodium, facilitates complete dendrimer removal at lower temperatures. Finally, although the rate of dendrimer removal is very similar on both alumina and zirconia, the conformations of the adsorbed dendrimers on these supports are different.