Hira Lal

Two forms of chemisorbed sulfur on platinum and related studies

Abstract The effect of temperature on the potentiodynamic oxidation of adsorbed sulfur layers on platinum, obtained from H 2 S or SO 2 was studied. The broad oxidation peak at 1.2–1.3 V observed at room temperature is resolved at 80°C. Two distinct peaks are observed at 80°C, oxidation peak I (at 0.97 V) corresponding to the weakly bound sulfur and oxidation peak II (at 1.10 V) corresponding to the strongly bound sulfur. Evidence is adduced to show that these two forms of chemisorbed sulfur are distinguished by the number of platinum sites they occupy. At elevated temperatures an extension of the hydrogen region was observed during cathodic charging in the presence of adsorbed sulfur. This phenomenon was found to be reversible with respect to temperature and does not correspond to a desorption of sulfur.

The nature of species adsorbed on platinum from SO2 solutions

Abstract The present studies have shown that at θS 0.9 can be explained by the formation of a partial bilayer on top of the first layer wherein 70% of the sites are covered with S (2-site adsorption) and 30% with S (1-site adsorption). It has also been shown that the ‘difficultly reducible’ oxygen referred to by earlier workers is identical with the species formed by exposure of the electrode to SO2 at potentials above 0.2 V. Evidence for the presence of two forms of ‘difficultly reducible’ oxygen, corresponding to the two forms of chemisorbed sulfur, is presented.

A study of the oxidation of adsorbed films formed on platinized platinum in methanol, formic acid and carbon dioxide solutions

Summary The oxidation of films formed on platinized platinum in methanol, formic acid and carbon dioxide solutions has been studied. It is shown that approximately one site is bared for every electron involved in the oxidation of these films. It is concluded that the same species is absorbed in all the cases studied and that it has the composition HCO. It is probable that COOH radicals are also present to a certain extent. A mechanism for the formation of HCO during the adsorption of formic acid on platinum has been proposed.

Formic acid oxidation at platinized platinum electrodes: Part IV. Effect of preadsorbed sulfur and related studies

A study has been made of the effect of preadsorbed sulfur layer on the anodic oxidation of formic acid at a platinized platinum electrode. It is shown that, in the presence of the preadsorbed s sulfur layer, the oxidation process obeys the following rate expression, i=nFkcαg(θs) exp(αanFϕr/RT) where α≈0.75 and αan≈0.5. This is explained in terms of the following rate-determining step, (HCOOH)ads→C*OOH+H++e and involves the adsorption of formic acid on the sulfur-covered electrode surface. A strong catalytic effect of the sulfur layer is observed; the function g(θS) is greater than unity at all θS and exhibits two maxima at гS values of 0.3 and 0.6. It is suggested that these effects arise from a coverage-dependent variation of bond strength between adsorbed sulfur and platinum. Evidence is adduced that the adsorption of sulfur from H2S solutions involves 2-site adsorption at low coverages and 1-site adsorption at higher coverages. Possibility of multilayer adsorption under appropriate conditions is also discussed in the light of the results obtained.

Halide adsorption and the anodic oxidation of chemisorbed methanol on platinum

Summary The effect of bromide and irreversibly adsorbed iodide ions on the anodic oxidation of methanol residue chemisorbed on platinized platinum has been investigated. It is shown that irreversibly adsorbed iodide ions poison the surface by blocking a fraction of the available platinum sites and that this “poisoning” has no effect on the mechanism of the anodic oxidation process. This behaviour is in marked contrast to that of chloride and bromide ions, and is attributed to the strong chemisorption of iodide ions on platinum. It is also shown that the experimental rate equations for the anodic oxidation process in acid sulfate and acid chloride solutions include a function of the type, θ M (1-θ M ), indicating a virtual independence of the heat of adsorption of the methanol residue with coverage. The observed effect has been explained on the basis of the induced heterogeneity model of the platinum surface taken together with lateral interactions between the chemisorbed species.

A study of intermediates adsorbed on platinized platinum during the anodic oxidation of formaldehyde

Summary The chemisorption of carbonaceous species on platinized platinum from formaldehyde solutions has been studied. It is concluded from these studies that chemisorption of formaldehyde from acid sulfate solutions results in a species CO. In acid chloride solutions, however, the species consists of HCO (∼75%) and CO (∼25%). No effect of chloride ions on the mode of adsorption of methanol is observed. Possible causes for the effect of chloride ions on the mechanism of chemisorption of formaldehyde and methanol are discussed. Definite evidence has been found to show that the open-circuit adsorption of formaldehyde from moderately concentrated solution is accompanied by hydrogenation of the organic molecule. Kinetic parameters for the anodic oxidation of chemisorbed formaldehyde have been determined and are shown to be analogous to those for chemisorbed methanol, formic acid, and reduced carbon dioxide.

Kinetics of anodic oxidation of adsorbed films formed on platinized platinum in methanol, formic acid and carbon dioxide solutions

Summary The kinetics of anodic oxidation of the organic species adsorbed on platinized platinum from methanol, formic acid and CO 2 solutions has been investigated. The kinetic data are closely similar for the three films studied. It is shown that the mechanism of the reaction is greatly influenced by the presence of chloride ions. In acid sulfate solution, the rate-determining step involves a reaction between adsorbed organic and adsorbed hydroxyl radicals. In acid chloride solutions, however, water discharge is rate-determining. The change of mechanism has been attributed to chloride ion adsorption which increases the activation energy for the discharge of the water molecule.

Formic acid oxidation at platinized platinum electrodes: Part V. A further study of catalytic effect of preadsorbed sulfur

Abstract Oxidation of formic acid in the presence of a sulfur layer preadsorbed from SO2 solutions has been investigated. Two different behaviours are observed depending on the manner in which the desired coverage, θs, is obtained. When the desired coverage was obtained by adjusting the concentration of SO2, the catalytic effect observed was similar to that reported earlier [1] using H2S as the adsorbate. The log i−θs plot exhibited two maxima at θs=0.30 and 0.65. However, when the desired θs was obtained by a partial anodic stripping of an initial complete layer, the catalytic effect observed was very different. The log i−θs plot exhibited two linear regions with a cross-over at θs=0.35. The energy of activation for the oxidation of formic acid in the presence of adsorbed sulfur has been estimated. Investigations have also been made to study the effect of preadsorbed sulfur on the strongly chemisorbed organic species. Based on these results and reasonable coverage profiles for the growth and stripping of the sulfur layer, the following expression for the oxidation current is proposed i=(i)θT=0 exp ajθj where aj is a coefficient representing the heterogeneity arising due to the adsorption of species j, and θj is the fractional coverage with the species j. A rate expression which includes the function g(θs) in an explicit form has been formulated.

Pre-exponential factor and mechanism of anodic oxidation of chemisorbed methanol and formaldehyde on platinum

Summary The effect of temperature on the anodic oxidation of chemisorbed methanol and formaldehyde layers on platinized platinum has been studied in 0.5 M H2SO4 and in 0.1 M HCl+0.45 M H2SO4. The activation energy of the anodic oxidation process for the two films is the same and is attributed to a close structural similarity of the activated complexes formed in the two cases. The activation energy is the same in acid chloride as in the acid sulfate medium, and obeys the same potential dependence. The entropy of activation is virtually independent of electrode potential in 0.5 M H2SO4; in the acid chloride medium, however, it decreases markedly with increase in electrode potential. The results are interpreted in terms of the effect of chloride coverage on the free energy of activation. It is assumed, in conformity with literature data, that chloride coverage is also influenced by electrode potential and pH of the solution. On this basis, the kinetic data including the observed Tafel slopes and pH effects are explained in terms of a nonelectrochemical rate-determining step involving the adsorbed organic species and the adsorbed OH radical. Probable reaction paths are discussed. The relative effects of chloride ions and irreversibly adsorbed iodide ions on the kinetic parameters are examined in terms of the charge carried by the anion in the adsorbed state. It is suggested that the irreversibly adsorbed iodide anion is virtually a neutral particle and, as such, does not influence the reaction rate except through a blocking of the platinum sites.

Formic acid oxidation at plantinized platinum electrodes: Part III. Effect of preadsorbed iodide layer

Abstract A study has been made of the effect of an irreversibly adsorbed iodide layer on the anodic oxidation of formic acid at a platinized platinum electrode. It is shown that, in the presence of the preadsorbed iodide layer, the oxidation process obeys the following rate expression, i=nF k→ cα g(θI) exp(αanFφr/RT where α≈0.75 and αan≈0.5. This is explained in terms of the following rate-determining step, (HCOOH)ads→C*OOH+H++e and involves the adsorption of formic acid on the iodide covered surface. A strong catalytic effect of the iodide layer is observed; the function g (θI) passes through two maxima at θI values of 0.15 and 0.53. It is suggested that these effects arise from a coverage-dependent variation of bond strength between the adsorbed iodide and platinum.