Hydrogen oxidation reaction kinetic activity

The capability of SECM to detect and to image regions with different catalytic activities is well known [120-122]. So far, this technique has been applied to studies of mainly two electrocatalytic reactions, the hydrogen oxidation reaction (HOR) and the oxygen reduction reaction (ORR), which have important implications for fuel cells. Unlike reversible redox mediators usually employed in SECM experiments, the kinetics of oxygen and hydrogen reactions are strongly dependent on the catalytic activity of the substrate surface. 

The kinetics of the hydrogen oxidation reaction in acidic PEM fuel cells above room temperature are often so fast that they contribute a negligible voltage to the overall activation overpotential, which is therefore often assumed to be wholly attributable to the ORR [41]. This allows catalyst loadings on the anode to be as low as 0.05mgptcm without affecting overall fuel cell performance significantly [42] and means that catalyst development is mainly focused on the cathode. However, in alkaline media, the HOR on polycrystalline Pt has been... 

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). 

Although Fenton (1894) studied the violet color in caustic alkali during oxidation of tartaric and racemic acids by ferrous salt and hydrogen peroxide, no reaction kinetic model was offered. Fenton reported that the color disappeared when acid was added. Also, it has been observed that fresh external air is more active than room air. Fenton performed different experiments using various amounts of ferrous and hydrogen peroxides and... 

Oxidation of benzoin to benzil with Fe(III), in the presence of 2,2/-bipyridine or ferrozine is of first order in Fe(III) and benzoin. An inverse second-order dependence was observed with respect to hydrogen ions. For oxidation of substituted benzoins the reaction constant is p 1.2, indicating an electron-rich transition state and an inner sphere mechanism has been proposed.66 The order with respect to iodide, in the reaction of iodide ions with a diiron(III)-l,10-phenanthroline complex, is 2. The hydrolytic derivatives of the complex are not kinetically active. Both inner and outer sphere pathways are operative.67... 

The ciystalJine structure of the catalyst and the oxidation state of surface nickel are specially relevant in this case due to the fact that coke deposition as well as acetylene hydrogenation occur on the metallic nickel sites [2]. Therefore, the pretreatments carried out on the catalyst with the aim of obtaining the active species have a great influence on the relationship between coke deposition and the main reaction kinetics. 

The key to the success of the oxidation examples cited above is the ability of the catalysts used to exert proper kinetic control on the possible side reactions. Without it, thermodynamically favorable but undesired products such as CO2 and H2O are made instead. Controlling oxidation kinetics to stop at the desired oxygenated products is quite difficult, and has yet to be solved for many other systems. For instance, although many attempts have been made to develop a commercial process for the oxidation of propylene to propylene oxide, both the activity and the selectivity of the systems proposed to date, mostly based on silver catalysts, are still too low to be of industrial interest " propylene oxide is presently manufactured by processes based on chlorohydrin or hydrogen peroxide instead. In spite of these difficulties, though, recent advances in selective liquid phase oxidation of fine chemicals on supported metal catalysts have shown some promise, offering high yields (close to 100%) under mild reaction conditions." ... 

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