KINETIC-STUDY OF ELECTROCATALYTIC REDUCTION OF HYDROGEN-PEROXIDE AT IRON(III) PORPHYRIN MODIFIED GLASSY-CARBON ELECTRODE IN ALKALINE-MEDIUM USING THE ROTATION-SCAN METHOD

Reduction of hydrogen peroxide at a glassy carbon (GC) electrode modified with sigma-bonded pyrrole iron(III) octaethylporphyrin complex, (OEP)Fe(Pyr), was studied by cyclic voltammetry and a rotating disk electrode. In 0.1N NaOH solution, it is shown that such an (OEP)Fe(Pyr)/GC electrode has a significant catalytic activity towards hydrogen peroxide reduction (E(D) = -0.80 V, k = 0.066 cm s(-1)); however, the electrode stability is low. The deactivation is observed when the reaction charge (Q) is passing through the (OEP)Fe(Pyr)/GC disk electrode. A linear rotation scan method is applied to study the kinetic process by determining the disk electrochemical response (i(D)) to rotation rate (omega) at a definite disk potential (E(D)). Considering that the number of adsorbed electroreduced catalyst molecules (Red) varies according to the disk potential, a factor theta(= Gamma(Red)/(Gamma(Red) + Gamma(Ox))) is introduced to describe the electrode surface area fraction for electroreduced species. The obtained Koutecky-Levich equation is applicable whatever the potential is.

STUDY OF THE ELECTROCATALYTIC REDUCTION OF NITRITE WITH SILICOTUNGSTIC HETEROPOLYANION

Three couples of reversible redox waves of the SiW12O404- anion which are composed of two one-electron and one two-electron processes occur in the potential range +0.1 to -0.7 V in aqueous solutions. The first (most positive) and third (most negative) redox waves exhibit good electrocatalytic activities for nitrite reduction in acid solutions with pH < 2 for the former and with pH similar or equal to 4 for the latter. The behavior of the third catalytic wave, which is quite unusual, was studied in detail. The rate constant governing the reduction of NO2- by the first wave of SiW12O404- was measured by an ultramicrodisk electrode as 3.73 x 10(3) M(-1) s(-1).

ELECTROCATALYTIC REDUCTION OF DIOXYGEN BY AN ELECTROCHEMICALLY POLYMERIZED FLAVIN ADENINE-DINUCLEOTIDE FILM

An electrochemically polymerized flavin adenine dinucleotide (FAD) film electrode is reported for the first time. The polymerized film was prepared by a two-step method. The electrocatalytic reduction of dioxygen at a glassy carbon electrode (GCE) modifie

RAPID-DETERMINATION OF KINETIC-PARAMETERS FOR THE ELECTROCATALYTIC REDUCTION OF DIOXYGEN BY VITAMIN-B12 ADSORBED ON GLASSY-CARBON ELECTRODE

In this paper, the electrochemical behavior of vitamin B-12, ie cyanocobalamin (abbr. VB12) in a weak acidic aqueous solution and adsorbed on glassy carbon (GC) surface (abbr. VB12(ad)/GC) in different pH buffer solutions have been described by using cyclic voltammetry (cv). It is found that VB12 and VB12(ad)/GC exhibit catalytic activity for the electroreduction of O2 according to two reduction peaks at -0.50 and -1.00 V vs. sce; but their electrocatalytic activity is very unstable. Based on the method of hydrodynamic amperometry [B. Miller and S. Bruckenstein, J. electrochem. Soc. 117, 1033 (1970)], some kinetic parameters for the electrocatalytic reduction of O2 by VB12(ad)/GC have been determined rapidly by using a linear rotation-scan method [Rongzhong Jiang and Shaojun Dong, Electrochim. Acta 35, 1451 (1990)]. These kinetic parameters indicate that the reduction of O2 on VB12(ad)/GC gives water predominantly in both potential ranges which correspond to those two reduction peaks. Possible reaction mechanisms have been suggested.

APPLICATION OF AN ULTRAMICROELECTRODE IN HOMOGENEOUS CATALYTIC REACTIONS .4. ELECTROCATALYTIC REDUCTION OF ORGANOHALIDES BY METALLOPORPHYRIN AT ULTRAMICROELECTRODE

The electrocatalytic reduction of 1,2-dibromoethane and tetrabromoethane with CoTPP in DMF solutions containing 0.1 M TBAP was investigated at a Pt ultramicroelectrode. The experimental results indicated that CoTPP obviously exhibited catalytic activity for 1,2-dibromoethane and tetrabromoethane. The rate constants of 1,2-dibromoethane and tetrabromoethane in this system were calculated to be 0.14 x 10(3) and 0.5 x 10(2) M-1 S-1, respectively. The reaction mechanism of 1,2-dibromoethane and tetrabromoethane reduction electrocatalysed by CoTPP in 0.1 M TBAP DMF solution is discussed.

CATALYTIC REDUCTION OF HEMOGLOBIN AT THIONINE CHEMICALLY MODIFIED ELECTRODE AND FIA APPLICATIONS

Thionine-containing chemically modified electrode (cme) was constructed with glassy carbon substrate by potential sweep oxidation, electrodeposition and adsorption procedures, and electrocatalytic reduction of hemoglobin was carried out and characterized at the cme under batch and flow conditions. Comparison of the catalytic response toward hemoglobir obtained at the cme was made mainly in terms of the potential dependence, the detectability and long-term stability. When used in flow injection analysis (FIA) experiments with the detector monitored at a constant potential applied at -0.35 V vs sce, detection limit of 0.15-1.5 pmol level of hemoglobin injected was achieved at the cme, with linear response range over 2 orders of magnitude. All the cme s retained more than 70% of their initial hemoglobin response level over 8 h of continuous service in the flow-through system.

ELECTRODEPOSITION OF PLATINUM PARTICLES ON GLASSY-CARBON MODIFIED WITH COBALT PORPHYRIN AND NAFION FILM AND THEIR ELECTROCATALYTIC REDUCTION OF DIOXYGEN

In this paper, five types of chemically modified electrode (CMEs) prepared with the deposition of platinum particles on various surfaces of glassy carbon (GC) modified with cobalt porphyrin and Nafion(R) solution are characterized using the electron scanning microscope (SEM). Their activities in the four-electron reduction of dioxygen to water on the basis of their electrochemical data from cyclic voltammetric and rotating ring-disk electrode (RRDE) experiments are examined and compared. Platinum particles dispersed on the GC surface adsorbed with the cobalt porphyrin exhibit the highest activity for the electrocatalytic reduction of dioxygen. However it is interesting that the cobalt ion is lost from the center of the porphyrin ring during the preparation of the cobalt porphyrin + Nafion mixture solution, while the porphyrin ring still remains in the Nafion film, as shown by EDX analysis. The incorporation of the porphyrin may change the structure of the Nafion film into a looser form, since the platinum particles dispersed in the film are more readily exposed, resulting in more favourable mass transfer and higher activity for the electrocatalytic reduction of dioxygen.

ROLE OF H2S IN THE REDUCTION OF MOO3/AL2O3

MoO3/Al2O3 is reduced at least partly by sulfur which is formed from H2S in sulfidation with H2S/N2 mixture. SO2 formation during TPD of MoO3/Al2O3 with presorbed H2S provides evidence for the explanation.

Microwave effects on the selective reduction of NO by CH4 over an In-Fe2O3/HZSM-5 catalyst

Microwave effects have been shown to promote the activation of NOx molecules in the process of selective reduction of NO by CH4 over an In-Fe2O3/HZSM-5 catalyst and to enhance the water tolerance of this catalyst for NO reduction.

Catalytic reduction of NO over in situ synthesized Ir/ZSM-5 monoliths

The catalytic performance of Ir-based catalysts was investigated for the reduction of NO under lean-burn conditions over binderless Ir/ZSM-5 monoliths, which were prepared by a vapor phase transport (VPT) technique. The catalytic activity was found to be dependent not only on the Ir content, but also on the ZSM-5 loading of the monolith. With the decreasing of the Ir content or the increasing of the ZSM-5 loading of the monolith, NO conversion increased. When the ZSM-5 loading on the cordierite monolith was raised up to ca. 11% and the metal Ir content was about 5 g/l, the NO conversion reached its maximum value of 73% at 533 K and SV of 20 000 h(-1). Furthermore, both the presence of 10% water vapor in the feed gas and the variation of space velocity of the reaction gases have little effect on the NO conversion. A comparative test between Ir/ZSM-5 and Cu/ZSM-5, as well as the variation of the feed gas compositions, revealed that Ir/ZSM-5 is very active for the reduction of NO by CO under lean conditions, although it is a poor catalyst for the C3H8-SCR process. This unique property of Ir/ZSM-5 makes it superior to the traditional three-way catalyst (TWC) for NO reduction under lean conditions. (C) 2001 Elsevier Science B.V. All rights reserved.