Mechanism of hydrogenation of nitric oxide on platinum catalysts in acidic solution

To explain the mechanism of hydrogenation of NO in 2 M H₂SO₄, the kinetics of the hydrogenation of hydroxylamine to ammonia were studied. The rate constant of this reaction was compared with the ratio of rates at which NH₃OH⁺ and NH ₄ ⁺ were generated by the hydrogenation of NO. Both of these products are formed from NO via parallel reactions of a common intermediate. A mechanism for the hydrogenation of NO and the shift of the catalyst of the potential is proposed.

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NO 2M H₂SO₄ . NH₃OH⁺ NH₄ ⁺ NO. NO . . ., NO .

     

Reduction of nitric oxide,nitrous acid and nitrogen dioxide at platinum electrodes in acidic solutions: Review and new voltammetric results

The literature concerning the chemical and electrochemical reactions of nitric oxide, nitrous acid and nitrogen dioxide in aqueous solutions is reviewed briefly, with emphasis on electrochemical reductions at platinum electrodes in acidic solutions. The voltammetric behavior of NO and NO₂ at a Pt electrode in perchloric acid is virtually identical to that for HNO₂ and this is explained on the basis of a common electroactive precursor concluded to be NO⁺. Three cathodic waves are obtained for acidic solutions of NO, HNO₂ and NO₂. The first two waves correspond to reduction of NO⁺ to NO and N₂O₃ to NO, respectively. The presence of N₂O₃ results from decomposition of the parent compounds. The presence of Br^- or Cl^- in acidic solutions of the title compounds promotes the voltammetric reductions at lower H⁺ concentrations. This probably results from formation of electroactive nitrosyl halides.     

Nitrite impurities are responsible for the reaction observed between vitamin B12 and nitric oxide in acidic aqueous solution

The reaction between aquacobalamin, Cbl(H2O), and NO was studied at low pH. As previously reported, the final product of the reaction is the same as that obtained in the reaction of NO and reduced Cbl(H2O), viz. Cbl(NO-). Nevertheless, this reductive nitrosylation is preceded by a faster reaction (accompanied by small absorbance changes) that depends on the HNO2 concentration but not on the NO concentration. Kinetic and UV-vis spectroscopic data show that Cbl(NO2-) is generated during this reaction. Spectroscopic data show that the dimethylbenzimidazole group trans to the NO2- ligand is protonated and partially dechelated at pH 1, by which a reaction with NO is induced. DFT calculations were performed to compare the ability of NO and NO2- to bind to cobalamin and their influence on the stability of the dimethylbenzimidazole group. The reductive nitrosylation reaction shows a quadratic dependence on the HNO2 concentration and an inverse dependence on the NO concentration. It also strongly depends on pH and is no longer observed at pH > 4. On the basis of earlier work performed on a series of Co(III) porphyrins, a mechanism is proposed that can quantitatively account for the HNO2 and NO dependencies. The reductive nitrosylation reaction is practically dominated by a back reaction, i.e., the reaction between Cbl(NO-) and HNO2, which accounts for the strange NO and HNO2 concentration dependencies observed.     

Coulometric studies of the reduction of nitric oxide,nitrous acid and nitrogen dioxide in acidic halide media with a platinum flow-through electrode

The apparent values of n for the reduction of NO, HNO₂ and NO₂ in acidic halide solutions of intermediate concentration have been determined coulometrically. The value of n_app for HNO₂ is 1.0 in HCl solutions for intermediate flow rates at potential values in the range of the first two cathodic waves observed in voltammetric data. The value of n_app exceeds 1.0 at low flow rate. The values of n_app for reductions of NO in 5 M HC1 and NO₂ in 3.0 M HClO₄ containing 0.1 M bromide are 0.5. These results are explained on the basis of chemical reactions coupled to the electrode processes.     

Potential of zinc based-graphene oxide composite coatings on mild steel in acidic solution

This study introduces varying concentrations of graphene oxide (GO) as a filler into zinc chromate in forming composite coatings to improve the corrosion protection of mild steel. The purity of synthesized GO was inferred through the application of complementary characterization techniques, including FT-IR, XRD, Raman, SEM-EDX, and TEM analyses. GO doped zinc chromate coatings were deposited on the surface of mild steel through the brushing method. Electrochemical studies, i.e., electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization (PD) were conducted to elucidate the anticorrosion properties of the coated mild steel exposed to 0.5 ​M HCl solution. It was revealed that the highest anticorrosion protection was attained at low doping amount of 0.5% of GO with a corrosion rate of 0.036 mpy. Surface analyses revealed that incorporating GO into zinc chromate coating can effectively improve the anticorrosion properties and adhesion strength.     

Hydrogenation of carbon monoxide on platinum metals

Together with methane, methanol is the main product of the hydrogenation of CO in the presence of platinum, palladium, and iridium, applied to Y-Al₂O₃, at atmospheric pressure and temperatures of 473–573 K. Dimethyl ether is also formed on platinum and palladium, while small amounts of ethanol and acetaldehyde are formed on iridium. The hydrogenation of CO in the presence of Rh and Ru leads to the formation of normal C₁-C₅ alcohols and C₂-C₅ aldehydes. Reduction of the energy of the metal-carbon bond in the platinum metals (Pd, Ir, Pt, Rh, Ru) increases their specific catalytic activity with respect to the formation of methane and oxygenated organic compounds, and increases the selectivity for higher alcohols and aldehydes.Translated from Teoreticheskaya i éksperimental'naya Khimiya, Vol. 24, No. 1, pp. 75–81, January–February, 1988.     

Hydrogenation of styrene oxide in the presence of supported platinum catalysts to produce 2-phenylethanol

Gas-phase hydrogenation of styrene oxide was investigated using platinum catalysts deposited on magnesia, γ-alumina and activated carbon (AC), at atmospheric pressure and within a wide range of temperature (348–398 K). In order to correlate the chemical and textural properties with the catalytic activity, all catalysts were characterized by several techniques such as X-ray diffraction (XRD), temperature-programmed reduction (TPR), H₂-temperature-programmed desorption (TPD) N₂ physisorption and H₂ chemisorption. Obtained results indicate that the catalytic activity and the selectivity were affected by the nature of the support. In the presence of MgO or activated carbon, as supports, the main product was 2-phenylethanol (2-PEA). However, when the support was γ-alumina, the main product was phenylacetaldehyde (PAD). The basic character of the support led to the formation of the less substituted alcohol (2-PEA). This was obtained at high conversion (85%) with practically total selectivity (around 99%). However, more acid support such as γ-alumina led to the formation of the more substituted alcohol 1-phenylethanol (1-PEA) and phenylacetaldehyde, mainly due to the isomerisation of the epoxide. Consequently, the acid–base character of the support plays an important role in the selectivity of this reaction.     

Mechanism of electroreduction of allyl alcohol at platinized platinum electrode in acidic aqueous solution.

The electroreduction of allyl alcohol to form propene at the platinized platinum electrode in acidic aqueous solution has been studied using CV plots, IR, ESR, and MS spectra, and a semiempricial MO method (MOPAC7/AM1, PM3). From the determinations of charge-transfer coefficients, reaction orders and apparent activation energy for the given reaction, the detection of the intermediates such as C(3)H(5)(+), C(3)H(5)(*), and C(3)H(5)(-) species, and PM3 calculations of charge distribution and frontier orbital energies of the reaction species C(3)H(5)OH and C(3)H(5)(+), the authors suggest that in acidic aqueous solution the production of propene via reductive splitting of the C-OH bond situated in the allyl position of allyl alcohol obeys a carbonium ion-carbanion mechanism.     

Overlap of the oxide and hydrogen regions of platinum electrodes in aqueous acid solution

Two unusual features of noble metal electrode surfaces, active states of the metal and their anodic oxidation products (hydrous oxides), are of increasing interest at the present time owing to the important role of such species in electrocatalysis. The extent to which the hydrous oxide reduction process overlaps with the hydrogen adsorption region was investigated for platinum in acid solution. At least three distinct hydrous oxide reduction peaks (or regions) were observed and in some cases one of these peaks commenced at ca. 0.0 V, i.e. it was almost totally within the hydrogen gas evolution region. Following repeated hydrous oxide growth and reduction, which disrupted and thus activated the metal surface, a sequence of four low-level premonolayer oxidation peaks (each of which has been noted earlier by other authors) appeared in the positive sweep. As discussed earlier for copper in base, the transitions giving rise to such peaks are assumed to be mediator generation reactions, which strongly influence electrocatalytic processes occurring on platinum at low potentials. Electronic Publication     

Hydrogenation of cyclopropylbenzene in presence of palladium and platinum blacks

Summary Substantial differences were found in the course of reaction in the hydrogenation of cyclopropylbenzene in the presence of platinum and palladium blacks.     

Adsorption of polyethylene glycol on platinum electrode from acidic solutions

The effect of the molecular mass of polyethylene glycol (PEG) on its adsorption on platinized platinum from aqueous solutions of 0.5 M H₂SO₄ and 1 M HCl is studied using the methods of open-circuit potential shifts and voltammetry. In sulfuric acid solutions, the PEG adsorption is accompanied by dehydrogenation and hydrogenation processes, which probably involve the terminal groups of polymers. For PEG with the molecular mass of 600–40000, the established stationary surface states turn out to be close to one another. Anions Cl^? inhibit the PEG adsorption and electrooxidation. The adsorption behavior of PEG samples studied substantially differs from that of ethylene glycol under comparable conditions.     

Mechanistic study of electrocatalytic oxidation of formic acid at platinum in acidic solution by time-resolved surface-enhanced infrared absorption spectroscopy

Surface-enhanced infrared absorption spectroscopy (SEIRAS) combined with cyclic voltammetry or chronoamperometry has been utilized to examine kinetic and mechanistic aspects of the electrocatalytic oxidation of formic acid on a polycrystalline Pt surface at the molecular scale. Formate is adsorbed on the electrode in a bridge configuration in parallel to the adsorption of linear and bridge CO produced by dehydration of formic acid. A solution-exchange experiment using isotope-labeled formic acids (H(12)COOH and H(13)COOH) reveals that formic acid is oxidized to CO(2) via adsorbed formate and the decomposition (oxidation) of formate to CO(2) is the rate-determining step of the reaction. The adsorption/oxidation of CO and the oxidation/reduction of the electrode surface strongly affect the formic acid oxidation by blocking active sites for formate adsorption and also by retarding the decomposition of adsorbed formate. The interplay of the involved processes also affects the kinetics and complicates the cyclic voltammograms of formic acid oxidation. The complex voltammetric behavior is comprehensively explained at the molecular scale by taking all these effects into account.     

CF2HCl在铂黑上的催化解离动力学

本文研究了CF2HCl在铂黑表面上的单分子反应,在压力较高时为零级,压力较低时为一级,表明此反应为典型的催化反应,测定了CF2HCl在铂黑表面上催化反应的活化能为36.2kcal/mol.     

Reactivity of hydrogen adsorbed on platinum black towards ethylene

TPD studies of the reactivity of hydrogen adsorbed on dispersed Pt-black towards ethylene at –90, 30 and 200°C indicate that molecular forms of hydrogen with E_des from 3 to 16 kcal/mol are reactive species.

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Pt- –90, 30, 200°C . , . 3 16 /.

     

The effect of platinum oxide films on reaction kinetics at platinum electrodes

It has been known for some time that the pre-treatment of platinum electrodes often effects subsequent electrochemical reactions. Part of the effect of pre-treatment is due to the fact that anodized or chemically oxidized platinum electrodes become coated to some degree with a film of platinum oxide. This work was concerned with quantitative measurement of kinetic parameters as a function of the extent of oxide film formation. Whenever possible, variations in reaction mechanisms are proposed.

Most of the experimental evidence has been taken from current-potential curves but the techniques of chronopotentiometry and microscopy were also used.

The reduction of vanadium^V chromium^VI, arsenic^V, iodate and oxygen were investigated as well as the oxidation of vanadium^IV, arsenic^III, oxalic acid, and formic acid. The presence of the platinum oxide film effected the reactions studied in a variety of ways but in every case some variation in the kinetic parameters of the reactions studied was recorded. For a number of cases, a modified oxygen bridge theory was found useful.