RESUMEN
The diminished surface-area-normalized catalytic activity of highly dispersed Pt nanoparticles compared with bulk Pt is particularly intricate, and not yet understood. Here we report on the oxygen reduction reaction (ORR) activity of well-defined, size-selected Pt nanoclusters; a unique approach that allows precise control of both the cluster size and coverage, independently. Our investigations reveal that size-selected Pt nanoclusters can reach extraordinarily high ORR activities, especially in terms of mass-normalized activity, if deposited at high coverage on a glassy carbon substrate. It is observed that the Pt cluster coverage, and hence the interparticle distance, decisively influence the observed catalytic activity and that closely packed assemblies of Pt clusters approach the surface activity of bulk Pt. Our results open up new strategies for the design of catalyst materials that circumvent the detrimental dispersion effect, and may eventually allow the full electrocatalytic potential of Pt nanoclusters to be realized.
RESUMEN
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.
RESUMEN
The GM1 ganglioside in the monolayer at the air/water interface shows a liquid expanded-liquid condensed phase transition. Due to a combination of the PM IRRAS results with quantum chemical calculations the structure and orientation of the GM1 molecule in the monolayer on the sub-molecular level is provided. The PM IRRAS studies of GM1 monolayers demonstrate that the phase transition is accompanied by a reorientation of sugar residues with simultaneous changes in the network of hydrogen bonds. The calculation of the IR spectrum of the GM1 molecule allowed us to describe individual nu(as)(COC) modes in sugar residues of the GM1 pentasaccharide. The visualization of the dipole moment vector of each analyzed nu(as)(COC) band allowed us to discuss the orientation of the polar head group of the GM1 in the monolayer. In the liquid expanded state the planes of two sugar residues: beta-Gal-(1-3)-betaGlc-(1-1)-Cer of the GM1 molecule are tilted by ca. 55 degrees with respect to the surface normal. The plane of the beta1,4-GalNAc sugar ring is inclined by 70 degrees towards the gold surface. The phase transition to the liquid condensed state causes simultaneous reduction (by approximately 10 degrees) of the tilt of planes of sugar residues. The plane of the alpha2,3-Nue5Ac residue (sialic acid) has a nearly perpendicular orientation to the gold surface. Upon the transition to the liquid condensed state the strength of hydrogen bonds formed by the carboxylate group in the alpha2,3-Nue5Ac residue decreases. In parallel, the strength of hydrogen bonds formed by N atoms of amide groups of the GM1 molecule increases. These events may be explained by loosening and/or breaking of hydrogen bonds between the carboxylate and hydroxyl groups in the alpha2,3-Nue5Ac residue in a densely packed monolayer, due to steric hindrances. The ceramide group coupling the polar head group with the hydrophobic hydrocarbon chains, forms strong hydrogen bonds. The C=O bond of the amide group is almost perpendicular to the gold surface orientation. The hydrocarbon chains of the GM1 exist in a liquid disordered state and their physical state and orientation are not affected by the phase transition.