Effects of treatment in different atmosphere on Pt3Sn/C electrocatalysts for ethanol electro-oxidation

Pt3Sn/C catalyst was prepared by a modified polyol process and treated in air, H-2/Ar, and Ar atmosphere, respectively. XRD analyses indicate that all of these catalysts have face-centered cubic (fcc) crystal structure. Temperature-programmed reduction (TPR) experiments show that more Sn exists in zero-valence in the Ar-treated PtSn catalyst than in the others. Cyclic voltammetry (CV), chronoamperometry (CA) experiments, and the performance tests of direct ethanol fuel cell (DEFC) indicate that the catalytic activity of PtSn/C for ethanol oxidation was affected significantly by the chemical state of Sn in catalyst particles. The as-prepared PtSn/C gives the higher power density, while Ar-treated PtSn/C shows the lower cell performance. It seems that the multivalence Sn rather than the zero-valence Sn in the PtSn catalyst is the favorable form for ethanol oxidation. Energy dispersion X-ray analysis (EDX) of the PtSn/C-as prepared and PtSn/C (after stability test) shows the active species (platinum, tin, and oxygen) composition changed to a different extent. Further attempt to improve the catalyst stability is needed.

Effects of reaction conditions on the selective oxidation of propane to acrylic acid on Mo-V-Te-Nb oxides

An effective Mo-1 V(0.3)Te(0.23)Nb(0.12)Ox catalysts for the selective oxidation of propane to acrylic acid was successfully prepared by using rotavap method. The catalyst was characterized by XRD and shown to contain (V0.07Mo0.93)(5)O-14, (Nb0.09Mo0.91)O-2.8,3MoO(2)(.)Nb(2)O(5), Mo5TeO16 and/or TeMo4O13, Te4Nb2O13 and a new TeMO (TeVMoO or TeVNbMoO; M = Mo, V and Nb) crystalline phase as the major phase. Regardless of the intrinsic catalytic characteristics of the catalyst, the external reaction conditions would have strong effects on the catalytic performance for propane oxidation. So in this paper, the effects of reaction conditions were investigated and discussed, including temperature, space velocity, V(air)/V(C3H8) ratio and V(steam)/V(C3H8) ratio. A stability test was also carried out on Mo1V0.3Te0.23Nb0.12Ox catalyst. The experimental run was performed during 100 h under the optimized reaction conditions. During the 100 h of operation, propane conversion and acrylic acid selectivity remained at about 59 and 64%, respectively. (C) 2004 Elsevier B.V. All rights reserved.

Nafion/PTFE composite membranes for fuel cell applications

Porous polytetrafluoroethylene (PTFE) membranes were used as support material for Nafion((R))/PTFE composite membranes. The composite membranes were synthesized by impregnating porous PTFE membranes with a self-made Nafion solution. The resulting composite membranes were mechanically durable and quite thin relative to traditional perfluorosulfonated ionomer membranes (PFSI); we expect the composite membranes to be of low resistance and cost. In this study, we used three kinds of porous PTFE films to prepare Nafion/PTFE composite membranes of different thickness. Scanning electron micrographs and oxygen permeabilities showed that Nafion resin is distributed uniformly in the composite membrane and completely plug the micropores, there is a continuous thin Nation film present on the PTFE surface. The variation in water content of the composite and Nafion 115 membranes with temperature was determined. At the same temperature, water content of the composite membranes was smaller than that of the Nafion 115. In both dry and wet conditions, maximum strength and break strength of C-325(#) and C-345(#) were larger than those of Nafion 112 due to the reinforcing effect of the porous PTFE films. And the PEMFC performances and the lifetime of the composite membranes were also tested on the self-made apparatus. Results showed that the bigger the porosity of the substrate PTFE films, the better the fuel cell performance; the fuel cell performances of the thin composite membranes were superior to that of Nation 115 membrane; and after 180 h stability test at 500 mA/cm(2), the cell voltage showed no obvious drop. (C) 2002 Published by Elsevier Science B.V.

Synthesis and properties of novel polyimides from sulfonated binaphthalene dianhydride for proton exchange membranes

A sulfonated dianhydride monomer, 6,6-disulfonic-4,4'-binaphthyl-1,1',8,8'-tetracarboxylic dianhydride (SBTDA), was successfully synthesized by direct sulfonation of the parent dianhydride, 4,4'-binaphthyl-1,1',8,8'-tetracarboxylic dianhydride (BTDA), using fuming sulfuric acid as the sulfonating reagent. A series of sulfonated homopolyimides were prepared from SBTDA and various common nonsulfonated diamines. The resulting polymer electrolytes, which contain ion conductivity sites on the deactivated positions of the aryl backbone rings, displayed high proton conductivities of 0.25-0.31 S cm(-1) at 80 degrees C. The oxidative stability test indicated that the attachment of the -SO3H groups onto the dianhydride units did not deteriorate the oxidative stability of the SPI membranes.

Formation, Thermal Stability and Deformation Behavior of Graphite-Flakes Reinforced Cu-based Bulk Metallic Glass Matrix Composites

Graphite-flake reinforced Cu47Ti34Zr11 Ni-8 bulk metallic glass matrix composite was fabricated by water-cooled copper mould cast. Most of the graphite flakes still keep unreacted and distribute uniformly in the amorphous matrix except that some reactive wetting occurs by the formation of TiC particles around the flakes. It reveals that the presence of graphite flakes does not affect the onset of the glass transition temperature, crystallization reaction and liquidus of the metallic glass. The resulting material shows obvious serrated flow and higher fracture strength under room temperature compressive load, comparing with the monolithic bulk metallic glass (BMG). Three types of interaction between the shear bands and graphite flakes, namely, shear band termination, shear bands branching and new shear bands formation near the graphite flakes can be observed by quasi-static uniaxial compression test and bonded interface technique through Vickers indentation.

Wave-soil-pipe coupling effect on submarine pipeline on-bottom stability

Wave-soil-pipe coupling effect on the untrenched pipeline stability on sands is for the first time investigated experimentally. Tests are conducted in the U-shaped water tunnel, which generates an oscillatory how, simulating the water particle movements with periodically changing direction under the wave action. Characteristic times and phases during the instability process are revealed. Linear relationship between Froude number and non-dimensional pipe weight is obtained. Effects of initial embedment and loading history are observed. Test results between the wavesoil-pipe interaction and pipe-soil interaction under cyclic mechanical loading are compared. The mechanism is briefly discussed. For applying in the practical design, more extensive and systematic investigations are needed.

Thermal stability of double-ceramic-layer thermal barrier coatings with various coating thickness

Double-ceramic-layer (DCL) coatings with various thickness ratios composed of YSZ (6-8 wt.% Y2O3 + ZrO2) and lanthanum zirconate (LZ, La2Zr2O7) were produced by the atmospheric plasma spraying. Chemical stability of LZ in contact with YSZ in DCL coatings was investigated by calcining powder blends at different temperatures. No obvious reaction was observed when the calcination temperature was lower than 1250 degrees C, implying that LZ and YSZ had good chemical applicability for producing DCL coating. The thermal cycling test indicate that the cycling lives of the DCL coatings are strongly dependent on the thickness ratio of LZ and YSZ, and the coatings with YSZ thickness between 150 and 200 mu m have even longer lives than the single-layer YSZ coating. When the YSZ layer is thinner than 100 mu m, the DCL coatings failed in the LZ layer close to the interface of YSZ layer and LZ layer. For the coatings with the YSZ thickness above 150 mu m, the failure mainly occurs at the interface of the YSZ layer and the bond coat.