A STUDY OF CATALYTIC PROPERTIES OF PEROVSKITE-TYPE OXIDES LAMNYCO1-YO3 .1. THE RELATIONSHIP BETWEEN THE TYPE OF OXYGEN SPECIES AND THE CATALYTIC PROPERTY IN AMMONIA OXIDATION

The type of oxygen species in perovskite-type oxides LaMnyCo1-yO3 (y = 0.0, 0.1, 0.3, 0.5, 0.7, 0.9, 1.0) has been studied by means of XRD, XPS and TPD. The catalytic activity in ammonia oxidation was also investigated. It was found that there were three desorption peaks in TPD curve corresponding to three types of oxygen species (alpha, beta, beta'). The desorption temperatures were 293 K less-than-or-equal-to T(alpha) less-than-or-equal-to 773 K, 773 K less-than-or-equal-to T(beta) less-than-or-equal-to K and T(beta') greater-than-or-equal-to 1073 K respectively. The relationship among the composition, structure and the catalytic property of.the catalyst was correlated and could be explainned with a model based on solid defect reaction and the interaction between Co and Mn ions. The adsorption strength and quantity of a oxygen are proportional to the catalytic activity. The, result indicates that the synergetic effect between B-site ions seems to the benefit of the ammonis oxidation reaction.

ELECTROCHEMICAL STUDY OF ISOPOLY-OXOMETALLATES AND HETEROPOLY-OXOMETALLATES FILM MODIFIED MICROELECTRODES .3. FURTHER STUDY OF ELECTRODE PROCESS OF ISOPOLYMOLYBDIC ANION DOPED IN POLYANILINE FILM

The isopolymolybdic anion-polyaniline film modified carbon fiber (CF) microelectrode with high stability and electroactivity in aqueous acid solution has been successfully prepared by cycling the potential between -0.15 V and +0.85 V vs. sce at 100 mV s-1 or applying constant potential (+0.85 V) for electropolymerization in a 0.5 M H2SO4 solution containing 5.0 x 10(-2) M aniline and 5.0 x 10(-3) M H4Mo8O26. The electrochemical behaviour of the isopolymolybdic anion entrapped in the polyaniline film is strongly influenced by the sweep-potential range besides the acidity of electrolyte solution. In some acidic electrolyte solution (eg 0.5 M H2SO4), the change of the sweep-potential range causes the structure alternation of the isopolymolybdic anion and resulting in a new electrode process. The cyclic voltammogram of Mo8O264- in 0.5 M H2SO4 solution exhibits three two-electron reversible waves between +0.70 and -0.20 V. However, when the potential sweeps to the lower-limit of -0.3 V, where the fourth four-electron cathodic wave appears, the redoxidation process of the reduction product of Mo8O264- becomes relatively complicated. The 10-electron reduction product seems to change into other isopolyanion (this unknown structure isopolyanions are simply called [Mo-O]), which can be reoxidized to Mo8O264- by five successive two-electron oxidation steps from -0.30 to +0.70 V. However, when the lower-limit of the cycling potential is maintained at -0.30 V and the upper-limit reduces to +0.40 V from +0.70 V, the [Mo-O] in the film exhibits four two-electron reversible waves. We have presented a novel explanation about its electrode reaction mechanism on the basis of our experimental results.

SYNTHESIS AND CHARACTERIZATION OF POLY-ORTHO-METHYLANILINE WITH DIFFERENT DEGREES OF OXIDATION

Poly-ortho-methylanilines (POT) in three states fully oxidized, fully reduced and oxidized in varying degrees were synthesized by the reaction of common POT (C-POT) having nearly equal amounts of benzenediamine and quinonediimine units with iodine or phenyl-hydrazine, and the resulting polymers were characterized by IR, C-13-NMR, SEM and elemental analysis. The results showed that the quinonediimine unit in C-POT could be reduced by phenylhydrazine to the benzenediamine unit, forming the polymer with low OD (oxidation degree) or in a fully reduced state and that iodine-oxidation resulted in the increase of quinonediimine unit and decrease of benzenediamine unit. The solubility and flexibility of the formed polymers depend strongly on the amount of quinonediimine unit in it. It is necessary to reduce the content of quinonediimine structure unit in order to improve the solubility of aniline-class polymers.

APPLICATIONS OF RAPID SCAN SPECTROELECTROCHEMISTRY - DETERMINATION OF HETEROGENEOUS ELECTRON-TRANSFER RATE-CONSTANT OF BILIVERDIN OXIDATION

The rapid scan spectrometer was used to determine the heterogeneous electron transfer rate parameters for the oxidation of Biliverdin in DMF by single potential step thin layer spectroelectrochemical techniques and yielded an average formal heterogeneous electron transfer rate constant K(s, h)0' = 2.45 (+/-0.12) x 10(-4) cm s-1, electrochemical transfer coefficient alpha = 0.694+/-0.008. The oxidation process of Biliverdin was also studied and the formal potential E0 = 0.637 V (vs. Ag/AgCl) was obtained.

THE ANODIC-OXIDATION OF HYDRAZINE ON GLASSY-CARBON ELECTRODE

The anodic oxidation kinetics of hydrazine (N2H4) on glassy carbon (GC) electrode was examined by cyclic voltammetry, rotating disk and ring-disc electrode techniques. The possible mechanisms of N2H4 oxidation in both aqueous and nonaqueous solutions are proposed.

MEASUREMENT OF CATALYTIC ACTIVITY VARIATION FOR H2O2 OXIDATION AT A COBALT PORPHYRIN COATED ELECTRODE

A rapid rotation-scan method was used for the electrocatalytic oxidation of H2O2 at a cobalt protoporphyrin modified pyrolytic graphite electrode (CoPP/PG). The rate constant of H2O2 oxidation at the CoPP/PG electrode at different potentials and in different pH solutions was measured. The variation of catalytic activity with reaction charges (Q) passed through the electrode was analyzed. This provided a convenient electrochemical method to study the passivation and poisoning of catalytic sites with time.

AN EFFECTIVE TRIPHASIC CATALYTIC-SYSTEM FOR OXIDATION OF BENZYL ALCOHOL UNDER PHASE-TRANSFER CONDITIONS

A tri-phasic catalytic system consisting of aqueous hydrogen peroxide, benzyl alcohol and a solid catalyst such as tungsten trioxide has been proved effective for the oxidation of benzyl alcohol in the presence of cetyl trimethyl aniline bromide (CTMAB). At first, the oxide reacts with CTMAB to form a complex, which can be oxidized by aqueous hydrogen peroxide to form a peroxide which effectively oxidizes benzyl alcohol.

ELECTROCATALYTIC OXIDATION AND FLOW DETECTION OF HYDRAZINE COMPOUNDS IN LIQUID-CHROMATOGRAPHY AT A VITAMIN-B-12 ADSORBED GLASSY-CARBON ELECTRODE

A vitamin B-12 chemically modified electrode (CME) was constructed by adsorption of vitamin B-12 onto a glassy carbon surface. The electrode catalyzes the electrooxidation of hydrazine compounds over a wide pH range. The electrocatalytic behavior of hydrazines is elucidated with respect to the CME preparation conditions, solution pH, operating potential, mobile phase flow rate, and other variables. When applied to liquid chromatographic detection of the analytes, the vitamin B-12 CME yielded a linear response range over 2 orders of magnitude, and detection limits at the picomole level. The vitamin B-12 CME offers acceptable catalytic stability in both batch and flow systems.

THERMAL-OXIDATION OF POLYPROPYLENE CONTAINING HINDERED PIPERIDINE COMPOUNDS

The thermal oxidation behaviour of polypropylene containing tetramethylpiperidine compounds and corresponding pentamethylpiperidine compounds are compared using air oven aging, oxygen uptake and thermogravimetry. Carbonyl formation, the induction period of oxygen absorption and weight loss have been selected to characterize the degree of oxidation. The results show that the stabilizing effectiveness of pentamethylpiperidines is always higher than that of tetramethyl types. Radical-trapping mechanisms cannot explain this, because large amounts of nitroxyl radicals are formed by the tetramethylpiperidine compounds. The quenching of singlet oxygen appears to be involved in thermal oxidation of polypropylene containing pentamethylpiperidine compounds. Specific hydrogen bonding between pentamethylpiperidines and hydroperoxide may account for their better thermal stabilizing action than tetramethylpiperidines.

FLOW-INJECTION AMPEROMETRIC DETECTION OF HYDRAZINE BY ELECTROCATALYTIC OXIDATION AT A PRUSSIAN BLUE FILM-MODIFIED ELECTRODE

A Prussian Blue-modified glassy carbon electrode prepared by simple adsorption exhibited excellent electrocatalytic activity in the oxidation of hydrazine in acidic media. A film of the perfluorosulphonic acid polymer Nafion coated on top of the Prussian Blue-modified glassy carbon electrode can improve the mechanical stability of the Prussian Blue layer in the flow stream. Hydrazine was detected by flow-injection analysis at the modified electrode with high sensitivity. The limit of detection was 0.6 ng.

AMPEROMETRIC FLOW-INJECTION ANALYSIS OF HYDRAZINE BY ELECTROCATALYTIC OXIDATION AT COBALT TETRAPHENYLPORPHYRIN MODIFIED ELECTRODE WITH HEAT-TREATMENT

Chemically modified electrodes prepared by treating the cobalt tetraphenylporphyrin modified glassy-carbon electrode at 750-degrees (HCME) are shown to catalyze the electrooxidation of hydrazine. The oxidation occurred at +0.63 V vs. Ag/AgCl (saturated potassium chloride) in pH 2.5 media. The catalytic response is evaluated with respect to solution pH, potential scan-rate, concentration dependence and flow-rate. The catalytic stability of the HCME is compared with that of the cobalt tetraphenylporphyrin adsorbed glassy-carbon electrode. The stability of the HCME was excellent in acidic solution and even in solutions containing organic solvent (50% CH3OH). When used as the sensing electrode in amperometric detection in flow-injection analysis, the HCME permitted sensitive detection of hydrazine at 0.5 V. The limit of detection was 0.1 ng. The linear range was from 50 ng to 2.4-mu-g. The method is very sensitive and selective.

ELECTROCATALYTIC OXIDATION OF ASCORBIC ACID AT POLYANILINE FILM MODIFIED GLASSY CARBON ELECTRODES

A conducting polyaniline (PAn) film modified glassy carbon (GC) electrode was prepared by electrochemical polymerization. The electrochemical behavior of ascorbic acid (AH(2)) in aqueous solution at this PAn modified electrode was studied in detail. The experimental results show that PAn film modified electrode has good electrocatalytic activity on the oxidation of ascorbic acid in aqueous solution over a wide range of pH value, among which pH 4 is the optimum condition. The oxidation process of ascorbic acid at PAn film electrode can be regarded as an EC catalytic mechanism. The kinetic process of the catalytic reaction was investigated by rotating disk electrode (RDE) coated with PAn films. The rate constant of the catalytic reaction was evaluated. The catalytic peak currents are proportional to the concentrations tions of ascorbic acid in the range of 5 x 10(-2)-1 x 10(-6) mol . L-1. The PAn film elec trodes give very stable responce for the oxidation of ascorbic acid. The present investigation shows the posibility of using PAn film modified electrode for the determination of ascorbic acid.

ELECTROCATALYTIC OXIDATION OF ASCORBIC-ACID AT A PRUSSIAN BLUE FILM MODIFIED MICRODISK ELECTRODE

The preparation and the behaviour of a Prussian Blue (PB) film on a platinum microdisk electrode has been described. Electrocatalytic oxidation of ascorbic acid has occurred at the PB film modified microelectrode. This shows a typical example of a modified microelectrode in electrocatalysis following our previous theoretical studies (J. Electroanal. Chem., 309 (1991) 103) and the related catalytic reaction rate constant was determined.