Hydrogen oxidation reaction , rate constant

Electrocatalytic Oxidation of Hydrogen The rate constant of the hydrogen oxidation reaction (HOR), as measured by the exchange current density jo (i.e. the current... 

In the present study, the mechanism of photolytic ozonation was investigated in the laboratory by measuring ozone, hydrogen peroxide, and total oxidant levels in purified water as a function of time in a two-liter stirred tank reactor in which concurrent ozone sparging and UV irradiation were taking place. Experiments were run in the presence and absence of various radical scavengers and the results Interpreted in terms of reaction rate constants taken from the literature. This paper will discuss the mechanism of photolytic ozonation, show its relationship to other ozonation processes, and... 

Chemical kinetics at tubular electrodes (systems of circular cross-section) have been considered recently in [77, 78]. Both for catalytic ErevCcat and for E vCirr (follow-up) mechanisms the theory of linear-sweep voltammetry and cyclic voltammetry was elaborated. The effects of the reaction rate constant, of the flow velocity, and of the potential scan rate on the shape of current curves are presented graphically. The deductions derived from both theories were tested on the reduction of Fe(III) in the presence of hydrogen peroxide (catalytic system) and on the oxidation of 1,4-phenylenediamine in alkaline medium (E vCin. mechanism). 

In this equation, k. Is a pseudo reaction rate constant and y Is the fraction of reouclble oxides that are reduced. Hydrogen could reduce only about 20-30% of the surfaces oxides on the 304 stainless steel surface and 65-90% on Incoloy. Carbon monoxide and hydrogen sulfide, however, could reduce essentially all the surface oxides. 

Hydrogen oxidation reaction occurs on the free sites liberated during the time between COads oxidative removal, Equation 16.4, and CO re-adsorption from solution. Equation 16.2. At the low potential (E < 0.6 V) the rate constant of COb re-adsorption is much higher than the rate constant for COads oxidation, and in practical terms only an infinitely small number of platinum sites could be liberated for H2 oxidation. 

Solvent Isotope Effects. The involvement of the abstraction of the hydrogen atom from the thiol by MB- as a rate limiting factor at the higher pH s is supported by comparison of the oxidation reaction rates in H2O and D2O at different pH s. Table VII lists the observed rate constants for the oxidations of both mercaptoethanol and dithioerythritol at different acidities. 

Scheme 6.1 General proposed auto-oxidation mechanism for polymers. R= polymer chain, H= most labile hydrogen, X = any radical, and k = reaction rate constant. Adapted from Ref [1].

FIGURE 6.7 Heterogeneous reaction rate constant, k, of hydrogen oxidation at a Pt substrate with different potentials in 0.01 M HCIO4 and 0.1 M NaC104. The k values were obtained by fitting the experimental approach curves with the theoretical ones. Approach speed, 3 pm/s. (Reprinted from Zhou, J., Zu, Y., Bard, A. J., J Electroanal. Chem. 491, 22, 2000, with permission from Elsevier.)... 

The first detailed investigation of the reaction kinetics was reported in 1984 (68). The reaction of bis(pentachlorophenyl) oxalate [1173-75-7] (PCPO) and hydrogen peroxide cataly2ed by sodium saUcylate in chlorobenzene produced chemiluminescence from diphenylamine (DPA) as a simple time—intensity profile from which a chemiluminescence decay rate constant could be determined. These studies demonstrated a first-order dependence for both PCPO and hydrogen peroxide and a zero-order dependence on the fluorescer in accord with an earher study (9). Furthermore, the chemiluminescence quantum efficiencies Qc) are dependent on the ease of oxidation of the fluorescer, an unstable, short-hved intermediate (r = 0.5 /is) serves as the chemical activator, and such a short-hved species "is not consistent with attempts to identify a relatively stable dioxetane as the intermediate" (68). 

Fig. 2. Steps in advanced oxidation process (AOPs) involving o2one, hydrogen peroxide, and uv light of 254 nm. ( D) represents the doublet state ( ) represents quantum yield, and the other numbers associated with the reaction arrows are rate constants in units of (Af-s). Dashed arrows indicate...

Another important catalytic reaction that has been most extensively studied is CO oxidation catalyzed by noble metals. In situ STM studies of CO oxidation have focused on measuring the kinetic parameters of this surface reaction. Similar to the above study of hydrogen oxidation, in situ STM studies of CO oxidation are often conducted as a titration experiment. Metal surfaces are precovered with oxygen atoms that are then removed by exposure to a constant CO pressure. In the titration experiment, the kinetics of surface reaction can be simplified and the reaction rate directly measured from STM images. 

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