Showing papers on "Platinum published in 1988"

Methods to Advance Technology of Proton Exchange Membrane Fuel Cells

Abstract: Proton exchange membrane (PEM) fuel cells showing promise of a high level of performance have, up to the present time, used electrodes containing a high platinum loading (4 mg/cm2). We report improvements in performance of PEM fuel cells utilizing electrodes with only one‐tenth of this platinum loading by (i) extension of the three‐dimensional reaction zone by incorporation of a proton conductor (Nafion) into the electrode structure; (ii) optimization of the amount of Nafion impregnated into the electrode structure; (iii) hot‐pressing the impregnated electrodes to the Nafion membrane at 120°C and 50 atm; (iv) optimal humidification of reactant gases at a temperature above that of the cell (5°C for or air and 10°–15°C for ); and (v) operation at elevated temperatures and pressures. The performance of the cells was analyzed from measurements of cell potential vs. current density and of cell potential at constant current density vs. time. Cyclic voltammetry proved to be a useful tool to ascertain the electrochemically active area of the electrodes.

Electrodeposition of platinum microparticles into polyaniline films with electrocatalytic applications

Abstract: The electrodeposition of platinum microparticles into polyaniline (PA) films on glassy carbon (gc) electrodes and their catalytic activity for the reduction of hydrogen and the oxidation of methanol are described. Electrodeposited platinum microparticles are dispersed in a three-dimensional array in fibril-type polyaniline film electrodes as evidenced by scanning electron microscope photomicrographs. These Pt/PA/gc electrodes exhibit good activity with respect to the catalytic reduction of hydrogen and the catalytic oxidation of methanol. Since polyaniline is a conducting polymer at potentials positive of 0.2 V vs Ag/AgCl, the PA films contribute a substantial amount of charge during the oxidation of methanol at 0.6 V. In addition, they also offer a protecting matrix for the Pt microparticles against particle loss and contamination from the bulk solution. The electrodes exhibited excellent long-term stability in the acidic methanol solutions.

Modified Heterogeneous Platinum Catalysts for the Enantioselective Hydroge-Nation of α-Ketoesters

Abstract: A detailed investigation of supported platinum catalysts modified with cinchona alkaloids for the enantioselective hydrogenation of pyruvic acid esters has been carried out. It was confirmed that pretreatment of the catalysts has a strong effect on their performance, which is dependent both upon the catalyst type and the solvent used. Modified catalysts are up to 100 times more active than without cinchona alkaloids. Many commercially available catalysts are active with enantioselectivities of 20-90%. There is no simple correlation between performance and either the total or metal surface area. The most important process variables are pressure, temperature and modifier concentration. Any change in the combination Pt catalysts / cinchona alkaloid / α-ketoester leads to almost complete loss of enantioselectivity. Some mechanistic aspects of this interesting catalytic system are discussed.

Extended X-ray absorption fine structure studies of platinum-tin catalysts

Abstract: Extended X-ray absorption fine structure (EXAFS) studies at 140 K, along with studies of X-ray absorption edges themselves, were conducted on a Pt-Sn/Al/sub 2/O/sub 3/ catalysts that had previously been exposed to hydrogen at high temperatures (775-795 K). For comparison, similar studies were conducted on a Pt-Sn/SiO/sub 2/ sample. For the Pt-Sn/Al/sub 2/O/sub 3/ catalyst the extent of coordination of tin to platinum atoms or to other atoms of tin is small, but the extent of coordination to oxygen is substantial. An analysis of the energy shift of the K absorption edge of the tin indicates that the tin is not present as Sn/sup 4 +/. The EXAFS data, coupled with the absorption edge data, suggest that the tin is largely present as Sn/sup 2 +/. There appears to be a significant interaction between the platinum and tin, since the platinum is more highly dispersed on alumina when tin is present. After consideration of all the data, it seems reasonable to regard the catalysts as if it consisted essentially of platinum clusters dispersed on an alumina that has Sn/sup 2 +/ present at the surface. The EXAFS data on the Pt-Sn/SiO/sub 2/ sample lead to very different conclusions. In thismore » case, most of the tin has an oxidation state of zero, and the material consists predominantly of bimetallic entities of platinum and tin on silica.« less

Coadsorption of ethylene and potassium on platinum (111). 1. Formation of a .pi.-bonded state of ethylene

Abstract: High resolution electron energy spectroscopy (HREELS), temperature-programmed desorption (TPD), and ultraviolet photoelectron spectroscopy (UPS) have been combined to study the effect of preadsorbed potassium on the adsorption of ethylene on the platinum(111) single-crystal surface. Addition of potassium increases the relative amount of reversible ethylene adsorption at 100 K/. Upon coadsorption with potassium, the authors also observe a new low-temperature ethylene desorption state at 150 K, with E/sub d/ = 5-9 kcal mol/sup -1/, which is about one-half the binding energy of ethylene adsorbed on clean Pt(111). The origin of this new desorption state is the formation of a weakly interacting ethylene species, which is only slightly distorted from the gas-phase hybridization, as determined primarily by vibrational spectroscopy. An explanation is given for these effects in which the increased charge density at the platinum surface due to coadsorbed potassium inhibits the strong sigma-donation of adsorbed ethylene to platinum and greatly reduces the chemisorption bond strength.

Agglomeration of Platinum Particles Supported on Carbon in Phosphoric Acid

Abstract: The growth of platinum particle supported on an acetylene black was investigated fundamentally using a potential perturbation method in phosphoric acid at 205°C and atmospheric pressure. Platinum particles in a colony were agglomerated to form one particle depending on the potential of the electrode and number of particles in the colony. Platinum dissolution and redeposition were considered to be the main reasons for the agglomeration.

A new conducting polymer-coated glucose sensor

Abstract: A sensor for glucose formed in a one-step process is described based on a new doped polyindole film coated onto a platinum surface; a polymer-entrapped glucose oxidase electrode can be operated as an amperometric glucose sensor. The new glucose sensor has fast response time (25–40 s) with high storage and operational stability ( > 35 days). Electrochemical rate constants for the new enzyme electrode have been calculated using the expressions derived by Albery et al.(J. Chem. Soc., Faraday Trans. 1, 1986, 82, 1033). It has been found that unsaturated enzyme kinetics are rate-limiting for conducting polymer-coated enzyme electrodes formed in a one-step process.

Accumulation of cis-Diamminedichloroplatinum(II) and Platinum Analogues by Platinum-resistant Murine Leukemia Cells in Vitro

Abstract: Three murine leukemia lines resistant to cis -diamminedichloroplatinum(II) and one line resistant to diaminocyclohexane (DACH) platinum(II) complexes were compared to their platinum-sensitive parent lines to determine whether differences in net platinum accumulation were related to the resistant phenotype. The cis -diamminedichloroplatinum(II)-resistant lines (L1210PtR4, L1210DDP5, P388PtR4) and the DACH-resistant line (L1210DACH) were incubated in vitro with cis -diamminedichloroplatinum(II), [ sp -4-2-( 1R,2R )]-(1,2-cyclohexanediamine- N-N ′)dichloroplatinum(II), [ sp -4-2-( 1R,2R )]-(1,2-cyclohexanediamine- N-N′ )[ethanedioato(2-)- O,O ′]platinum(II), or dianinocyclobutanedicarboxylatoplatinum(II) and the time-dependent cellular platinum levels determined by flameless atomic absorption spectrophotometry. Cell lines resistant to a given platinum complex showed a reduction in the rate of platinum accumulation when compared to the sensitive line at 37°C. Intracellular levels of diaminocyclobutanedicarboxylatoplatinum(II) were too low to confidently measure under the conditions of this study. Our data suggest that the mechanism of platinum resistance in these cell lines may be related to a reduced accumulation of the platinum-containing drug, although patterns of cross-resistance suggest other mechanisms may be operative as well.

Macrocyclic complexes of the platinum metals

Abstract: The synthesis and redox properties of a range of platinum metal complexes incorporating poly-aza and poly-thia macrocyclic ligands are described. The insertion of palladium(II), rhodium(III), iridium(II1) and ruthenium(I1) into tetra-aza macrocycles (L) has been achieved, and the structures of octahedral complexes incorporating mutually cis chlorohydride ([IrCl(H)(L)]+) and chloro-carbonyl ([RuCl(CO)(L)]+) moieties are discussed in relation to the conformational characteristics of the coordinated macrocyclic ligand. Square planar complexes of palladlum(II), [Pd(L)]2t, can be reduced electrochemically to afford highly reactive palladium(1) species. The potential of the palladium(II)/(I) redox couple is found to be dependent upon the degree of N-alkylation of the macrocycle. The synthesis, structures and redox properties of the homoleptic hexa-thia complexes [M(L)2IX+ (L = 1,4,7-trithiacyclononane; M = Pd, Pt, Rh, Ir, Ru, 0 s ; x = 2 or 3) are described. The stereochemical and electronic properties of these products are related to the ability of these poly-thia crown ligands to stabilise mononuclear platinum(III), palladium(II1) and rhodiurn(I1) species. These results are compared to the structural and redox chemistry of the homoleptic complexes of 1,4,8,11-tetrathiacyclotetradecane and 1,4,7,10,13,16-hexathiacyclooctadecane. A range of mixed sandwich carbocyclic-macrocyclic complexes [M(CgMes) (L) 12' (M = Rh, Ir) and [M(arene)(L)I2+ (M = Ru, 0 s ) (L = 1,4,7-triazacyclononane, N,N',N''-trimethyl-1,4,7-triazacyclononane, 1,4,7-trithiacyclononane) have been synthesised, and the redox properties of these organometallic derivatives assessed. With larger tetraand hexa-thia macrocycles binuclear platinum metal complexes can be prepared.

Hydrogen and ammonia response of metal‐silicon dioxide‐silicon structures with thin platinum gates

Abstract: The hydrogen and ammonia sensitivity of metal‐oxide‐semiconductor (MOS) structures with platinum gates between 3 and 70 nm thick was investigated. The response to these gases was measured as a shift in the capacitance‐voltage (C‐V) curve of the MOS structure along the voltage axis. The measurements were made at an elevated temperature, mainly at 150 °C, where chemical reactions take place on the surface of the catalytic metal. The main purpose of the investigation was to determine if hydrogen and ammonia are detected by similar mechanisms. It is concluded that hydrogen molecules are dissociated and hydrogen atoms give rise to a dipole layer at the metal‐oxide interface, similar to the behavior of hydrogen in hydrogen sensitive MOS structures with thick catalytic metal, normally Pd, gates. Ammonia, on the other hand, appears to be detected through surface potential changes of the Pt film, which are capacitively coupled to the semiconductor surface through voids in the thin metal film.