Bernard J. Piersma

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)

Potentiostatic Current‐Potential Measurements on a Platinum Electrode in a High‐Purity Closed System

Abstract : In a high-purity, closed, electrochemical system, voltage regions at which very slow reactions are rate-controlling, were accurately separated and measured. In the potential range from 0.06 to 0.13 volt, the rate-controlling step is Pt-H + H(+) + e = H2 under conditions of rapid stirring with helium. A second region between 0.30 and 0.46 volt indicated a slow hydrogen-ion discharge. In the anodic range from 0.46 to >2 volts, four distinct regions were found: a region in which oxidizable impurities reacted; a region at which water was probably oxidized to give chemisorbed OH; a region in which chemisorbed and dermasorbed O was formed; and a region in which O2 was generated. Maximum oxidizable and reducible impurity levels were quantitatively determined both in solution and as adsorbed species on the working electrode surface. It is shown that the impurity levels were very low. Very small additions of hydrogen or oxygen in the critical transition region (net cathodic to anodic reactions) did not appear to have catalytic effects. However, at oxygen partial pressures above 0.0000001 atm, a poisoning effect was apparent. (Author)

Short‐Pulse Techniques II . Perturbation Times in Alkali‐Halide Systems

Abstract : A new technique involving the perturbation of a system from equilibrium using very short galvanostatic pulses and miniature cells was applied to study of the electrical bouble layer at Pt electrodes. The perturbation time, the time required for the double layer to begin charging, was determined for hydrogen- and alkali metal-halide systems and for reversible hydrogen systems in several acids and NaOH. The perturbation time was essentially independent of charging-current density and of the anion in solution but was strongly dependent on the cation species and electrolyte concentration. Models to explain the absence of electrode polarization during the initial flow of charge into the double layer are discussed. A model which involves a very fast Faradaic process and which is consistent with the experimental observations is proposed. It is suggested that the perturbation time, which is less than 100 nanosec for 1 M solutions, represents the time required for movement of ions into or out of the compact double layer. (Author)

Mechanisms and kinetics of aqueous formic acid and formate ion dehydrogenation on platinum and subsequent reaction of adsorbed hydrogen

Abstract The opencircuit chemical behavior of dissolved HCOOH and HCOONa at Pt electrodes was studied. A doublepulse electrochemical anodic charging technique was used to first strip the surface and to then quantitatively determine the amount of atomic hydrogen formed on the clean electrode from the dissociation of formic acid and formates and the net rate at which this atomic hydrogen was removed from the electrode surface by reaction with formate species. The integrated rates of the reaction of PtO ads with hydrogen and/or formate species, of the dehydrogenation of formic acid and formates on clean Pt, and of the atomic hydrogen removal were measured. The kinetics of the net atomic hydrogen removal process were determined and shown to be first order in respect to hydrogen atom concentration at the surface. The formation of residues from the organic free radicals which were the products of dehydrogenation retarded the further dissociation of formic acid and formates. Sulfuric acid solutions of very low pH values and molecular hydrogen could strongly affect the reaction rates and retarded the rate of hydrogen atom removal from the electrode surface. Generalizations concerning fuel cell anodic reactions are made.

ON THE ACTIVITY OF PLATINUM CATALYSTS IN SOLUTION. PART I. EFFECTS OF THERMAL TREATMENT AND CHEMICAL ETCHING ON THE PT-O/HYDROGEN SPECIFIC REACTION RATE.

Abstract : The effects of thermal treatment and chemical etching of platinum on the specific rate of the chemical reaction of chemisorbed oxygen with hydrogen were determined. The hydrogen was present in electrochemically clean 1M H2SO4 and in the derma of the metal. On successive thermal treatments of bright Pt beads, which were heated to the melting point and then slowly recrystallized under high temperatures, the specific rate varied randomly from trial to trial. Where heating was more uniform and the cooling rate slower, reaction of hydrogen with Pt-O was usually faster. Repeated aqua regia etching of a given Pt bead caused monotonic improvement until a rate between 0.014 and 0.021 amp/sq cm was attained. Rates on Pt wire electrodes, which probably differed from the flame-formed Pt beads both in average crystallite size and number of defects (created by the drawing process and only partially removed by subsequent annealing), were highly variable but considerably lower than on beads. Surfaces whose activity for the Pt-O/hydrogen reaction differed manyfold showed no differences in anodic charging curves. Electrochemical rates of water oxidation at +0.617 v and +0.587 v (NHE) and reduction of hydrogen ions at +0.300 v also did not differ. It appears that many electrochemical reactions are insensitive to these differences in surface condition. (Author)