Influence of Sn content on the hydrogenation of crotonaldehyde catalysed by colloidally prepared PtSn nanoparticles.

Bimetallic PtSn nanoparticles (NPs) of well-defined size and metal composition were prepared by means of colloidal methods. The mean particle diameter was about 2 nm for all samples irrespective of the Pt/Sn-ratio, which enables a systematic study of the influence of the composition on the catalytic properties while excluding particle size effects. The hydrogenation of crotonaldehyde was investigated as a reaction for which chemoselectivity is known to be a challenging task. Already very low atomic Sn contents (≈10%) were found to lead to a significantly improved activity which may be attributed to an electronic effect of Sn on Pt. For further increasing tin contents the activity decreased gradually. This trend was accompanied by a steady increase in selectivity towards the desired product (crotylalcohol). The results show that the highest crotylalcohol time yields can be obtained by using catalysts with an atomic Sn content of approximately 23%. In contrast, maximum crotylalcohol selectivities are achieved by using catalysts with a high tin content (>50%).

Pt based PEMFC catalysts prepared from colloidal particle suspensions--a toolbox for model studies.

A colloidal synthesis approach is presented that allows systematic studies of the properties of supported proton exchange membrane fuel cell (PEMFC) catalysts. The applied synthesis route is based on the preparation of monodisperse nanoparticles in the absence of strong binding organic stabilizing agents. No temperature post-treatment of the catalyst is required rendering the synthesis route ideally suitable for comparative studies. We report work concerning a series of catalysts based on the same colloidal Pt nanoparticle (NP) suspension, but with different high surface area (HSA) carbon supports. It is shown that for the prepared catalysts the carbon support has no catalytic co-function, but carbon pre-treatment leads to enhanced sticking of the Pt NPs on the support. An unwanted side effect, however, is NP agglomeration during synthesis. By contrast, enhanced NP sticking without agglomeration can be accomplished by the addition of an ionomer to the NP suspension. The catalytic activity of the prepared catalysts for the oxygen reduction reaction is comparable to industrial catalysts and no influence of the particle size is found in the range of 2-5 nm.

Colloidal synthesis and structural control of PtSn bimetallic nanoparticles.

PtSn bimetallic nanoparticles with different particle sizes (1-9 nm), metal compositions (Sn content of 10-80 mol %), and organic capping agents (e.g., amine, thiol, carboxylic acid and polymer) were synthesized by colloidal chemistry methods. Transmission electron microscopy (TEM) measurements show that, depending on the particle size, the as-prepared bimetallic nanocrystals have quasi-spherical or faceted shapes. Energy-dispersive X-ray (EDX) analyses indicate that for all samples the signals of both Pt and Sn can be detected from single nanoparticles, confirming that the products are actually bimetallic but not only a physical mixture of pure Pt and Sn metal nanoparticles. X-ray diffraction (XRD) measurements were also conducted on the bimetallic particle systems. When compared with the diffraction patterns of monometallic Pt nanoparticles, the bimetallic samples show distinct shifts of the Bragg reflections to lower degrees, which gives clear proof of the alloying of Pt with Sn. However, a quantitative analysis of the lattice parameter shifts indicates that only part of the Sn atoms are incorporated into the alloy nanocrystals. This is consistent with X-ray photoelectron spectroscopy (XPS) measurements that reveal the segregation of Sn at the surfaces of the nanocrystals. Moreover, short PtSn bimetallic nanowires were synthesized by a seed-mediated growth method with amine-capped bimetallic particles as precursors. The resulting nanowires have an average width of 2.3 nm and lengths ranging from 5 to 20 nm.