Sigmund Schuldiner

Investigations of the Kinetics of Hydrogen and Oxygen Reactions on a Platinum Electrode in Acid Solution Using Pulse and Decay Techniques

Abstract : The influence of hydrogen partial pressure, current density, and temperature on the anodic oxidation of sorbed H and the anodic generation of sorbed O were determined. Electrode processes were evaluated and separated to give the following quantities: (a) the amount of H associated with a Pt surface and its immediate vicinity, (b) the extent to reaction of H atoms, absorbed in the metal interior, that migrate to the surface of the metal and are either ionized or react with O atoms, (c) the extent of reaction of H2 from the solution phase, (d) the amount of O adsorbed on the Pt surface, and (e) the amount of O absorbed in the Pt surface layers (skin). The kinetics of the open-circuit reaction of galvanostatically determined amounts of sorbed O with H2 were also determined.

Hydrogen Overvoltage on Bright Platinum III . Effect of Hydrogen Pressure

The effect of hydrogen pressure on the hydrogen overvoltage of bright platinum was determined in acid. From the anodic and cathodic overvoltages, kinetic parameters were determined. A mechanism controlled by slow combination of hydrogen atoms adsorbed on the platinum surface fits the experimental data. From the Langmuir adsorption isotherm it is shown that the surface of the active platinum electrode at equilibrium is sparsely covered with hydrogen atoms.

Electrochemical Behavior of the Palladium‐Hydrogen System. I. Potential‐Determining Mechanisms

The potential of saturated α‐palladium in hydrogen‐stirred solution compared to a Pt/H2 electrode in the same solution is 0.0495±0.0005 v. The potential‐determining reaction on α‐palladium is independent of hydrogen pressure. The potential‐determining equilibrium is postulated to be, H++e= lim Pd(Pd−H)α. Pure palladium spontaneously absorbs hydrogen in hydrogen‐stirred solution until the saturation limit of the α phase is reached. This limiting atomic ratio of H/Pd=0.025±0.005. Between a H/Pd atomic ratio of 0.03 to 0.36 both the α and β phases coexist and the mixed potential is determined by that of the α domains. In the H/Pd region 0.36 to 0.6, the potential is a function of the hydrogen content of the palladium.

Potential of a Platinum Electrode at Low Partial Pressures of Hydrogen or Oxygen

Abstract : The open-circuit potential on birght platinum in 1M H2SO4 was studied as a function of oxygen or hydrogen partial pressure from .10 to 10 to the minus 7th power atm. The very low rates of H2 or O2 flow which were required were produced by a special gas generator. Potentials predicted by the Nernst equation for the H+/H2 couple were not observed below .0010 atm H2 partial pressure. A Nernst relation with a slope of 0.06 v per decade of O2 partial pressure was found. The O2 leak into the closed system used was found to be about 10 to the minus 7th power atm. Based on the assumption that dissolved O2 decreases the effective H2 partial pressure at the electrode surface, an equation was developed for obtaining the effective H2 partial pressure. (Author)

Comparative Activity of (111), (100), (110), and Polycrystalline Platinum Electrodes in H2‐Saturated 1 M H 2 SO 4 under Potentiostatic Control

Potentiostatic measurements of the three principle faces of oriented single Pt crystals and polycrystalline Pt showed some interesting comparisons. True areas are based on Pt atom densities, and under the electrochemical treatment used, no changes in these areas were found once the initial surface cleaning was completed. The hydrogen overvoltage was independent of crystal orientation or other metallurgical factors; however, the passivation of the hydrogen oxidation reaction and the oxygen generation reactions were different for each single crystal orientation with a large difference between these faces and a polycrystalline bead electrode. Steady‐state relations between potential and amounts of associated H atoms and number of sites available for coverage with O atoms were determined also. These steady‐state values are generally less than the comparative amounts found under transient conditions. Oxygen evolution on the Pt faces was not observed below a potential of 1.8V. It appears that grain boundaries, stress, and impurity inclusions may have more effect in determining the catalytic activity for some reactions than does the actual atomic density or geometry of Pt atoms.

Interaction of Carbon Dioxide with Hydrogen Chemisorbed on a Platinum Electrode

Abstract : The effect of CO2 on the oxidation of H atoms chemisorbed on a Pt electrode was examined with potentiostatic and transient galvanostatic techniques. The results indicate that CO2 is not reduced, but that Had oxidation is inhibited by CO2, or possibly HCO3(-). Under these conditions the inhibited or blocked Had is stable at potentials negative to 0.300 V (vs N.H.E.). Dissociation of HCOOH to Had and CO2 may inhibit the further oxidation of HCOOH by a similar blocking mechanism. (Author)

POTENTIAL OF A PLATINUM ELECTRODE AT LOW PARTIAL PRESSURES OF HYDROGEN AND OXYGEN. PART II. AN IMPROVED GAS-TIGHT SYSTEM WITH A NEGLIGIBLE OXYGEN LEAK.

Abstract : A tight electrochemical system was constructed in which the O2 pressure (PO2) above the cell solution was less than 10 to the 9th power atm. A comparison of the potentials obtained with this system with data from AD-624 024 shows that very small amounts of O2 leaking into a closed system can have marked effects on potential behavior at low H2 pressures (PH2). Reduction of the O2 leak to negligible proportions showed that: (a) the Nernst equilibrium relation for the H(+)/H2 couple holds only for PH2 in excess of 0.000001 atm; (b) at PH2 less than 0.000001 atm, trace amounts of O2 even in the presence of several orders of magnitude more H2, acted as an electrode poison, causing a positive deviation from the theoretical Nernst behavior; (c) in O2-free solution, at PH2 below 0.000001 atm, the potential remained at 0.18 v positive to the normal hydrogen electrode (N.H.E.) and was independent of PH2. The potential-determining reaction in this region may be an exchange of H(+) in solution with H atoms dermasorbed in the Pt. The potential vs O2 partial pressure relation was essentially the same as found in previous work at this Laboratory. Residual hydrogen associated with Pt at potentials from 0.18 to 0.20 v did not react with oxygen. (Author)

A Study of Adsorption and Oxidation of Carbon Monoxide on Platinum Using Constant Current Pulses

Abstract : Using single anodic constant-current pulses, the steadystate concentration of both the carbon monoxide already adsorbed on the surface of a bright platinum electrode immersed in 1M H2SO4 and the additional amount of CO adsorbed during the application of the pulse was determined. At potentials of 0.3 to 0.4 v relative to the normal hydrogen electrode, a surface coverage of one CO molecule per surface Pt atom was observed at very low CO partial pressures; for CO partial pressures up to 1 atm, the coverage increased to about 1.06 CO molecules per Pt atom, indicating that additional CO physically adsorbs on the base chemisorbed layer. At 1.0 to 1.3 v the rate of adsorption of CO was slower than the rate of diffusion (at 25C), and indirect evidence suggests that CO is not adsorbed at potentials above 1.3 v. The mechanism of oxidation of CO appears to be an electrochemical oxidation of water to form chemisorbed oxygen atoms, which then rapidly react with chemisorbed CO to form CO2. (Author)

Oxidation of Hydrogen on a Passive Platinum Electrode

Abstract : Under potentiostatic, steady-state conditions and at anodic potentials above 0.7 V (NHE), the rate of oxidation of molecular hydrogen decreases at a high-activity Pt electrode in 1M H2SO4. It is shown that this decrease is not owing to the formation of oxygen species on the electrode surface. It is believed that this passive behavior of Pt is due to anion adsorption - at least between 0.7 and 1.2 V. Depending on potential and previous potential sequence, passivity in this region is evidently sensitive to the amount of sulfate ion adsorbed, its heat of adsorption, and the presence of dermasorbed oxygen. At higher potentials both sorbed oxygen species and sulfate ion may be present and may contribute to the passivity. In the 0.7 to 1.2 V passive region, hydrogen oxidation is electrochemical. There is no significant chemical oxidation via an oxygen intermediate. (Author)

Mechanisms of Hydrogen Producing Reactions on Palladium II . Diffusion of Electrolytic Hydrogen through Palladium

The rate of diffusion of hydrogen through several thicknesses of palladium for varying current densities was measured. The relationship between the diffusion current and the total polarizing current was established.