Oxidation oxygen activation

Oxygen has also been shown to insert into butadiene over a VPO catalyst, producing furan [110-00-9] (94). Under electrochemical conditions butadiene and oxygen react at 100°C and 0.3 amps and 0.43 volts producing tetrahydrofuran [109-99-9]. The selectivity to THF was 90% at 18% conversion (95). THF can also be made via direct catalytic oxidation of butadiene with oxygen. Active catalysts are based on Pd in conjunction with polyacids (96), Se, Te, and Sb compounds in the presence of CU2CI2, LiCl2 (97), or Bi—Mo (98). 

A fresh surface of siUcon carbide is thus constantiy being exposed to the oxidizing atmosphere. Active oxidation takes place at and below approximately 30 Pa (0.23 mm Hg) oxygen pressure at 1400°C (66). Passive oxidation is determined primarily by the nature and concentration of impurities (67). 

FIGURE S.47 The role of glutathione and metabolic pathways involved In the protection of tissues against Intoxication by electrophiles, oxidants and active oxygen species. (Used with permission.)... 

In the very early stages of oxidation the oxide layer is discontinuous both kinetic and electron microscope" studies have shown that oxidation commences by the lateral extension of discrete oxide nuclei. It is only once these interlace that the direction of mass transport becomes of importance. In the majority of cases the metal then diffuses across the oxide layer in the form of cations and electrons (cationic diffusion), or as with the heavy metal oxides, oxygen may diffuse as ions with a flow of electrons in the reverse direction (anionic diffusion). The number of metals oxidising by both cationic and anionic diffusion is believed to be small, since a favourable energy of activation for one ion generally means an unfavourable value for the other... 

The presence of red ferric oxide usually indicates an inactive pit but black oxide infers active pitting. Condensate receivers are common points for oxygen infiltration (especially in smaller boiler plants). 

The chemical properties of oxide surfaces have been studied by several methods, including oxygen exchange. This method has been used to investigate the mechanisms of heterogeneous reactions for which oxides are active catalysts [36]. The dimerization step does not necessarily precede desorption and Malinin and Tolmachev [634], in one of the few reviews of decomposition kinetics of solid metal oxides, use this criterion to distinguish two alternative reaction mechanisms, examples being... 

C.G. Vayenas, C.Georgakis, J. Michaels, and J. Tormo, The role of PtOx in the isothermal rate and oxygen activity oscillations of the Ethylene Oxidation on Pt, J. 

Under microwave irradiation and applying MCM-41-immobilized nano-iron oxide higher activity is observed [148]. In this case also, primary aliphatic alcohols could be oxidized. The TON for the selective oxidation of 1-octanol to 1-octanal reached to 46 with 99% selectivity. Hou and coworkers reported in 2006 an iron coordination polymer [Fe(fcz)2Cl2]-2CH30H with fez = l-(2,4-difluorophenyl)-l,l-bis[(l//-l,2,4-triazol-l-yl)methyl]ethanol which catalyzed the oxidation of benzyl alcohol to benzaldehyde with hydrogen peroxide as oxidant in 87% yield and up to 100% selectivity [149]. An alternative approach is based on the use of heteropoly acids, whereby the incorporation of vanadium and iron into a molybdo-phosphoric acid catalyst led to high yields for the oxidation of various alcohols (up to 94%) with molecular oxygen [150]. 

Oxidation is initiated by formation of radicals which may be the result of enzyme catalysed reactions like oxygen activation by xanthine oxidase in... 

Catalysts include oxides, mixed oxides (perovskites) and zeolites [3]. The latter, transition metal ion-exchanged systems, have been shown to exhibit high activities for the decomposition reaction [4-9]. Most studies deal with Fe-zeolites [5-8,10,11], but also Co- and Cu-systems exhibit high activities [4,5]. Especially ZSM-5 catalysts are quite active [3]. Detailed kinetic studies, and those accounting for the influence of other components that may be present, like O2, H2O, NO and SO2, have hardly been reported. For Fe-zeolites mainly a first order in N2O and a zero order in O2 is reported [7,8], although also a positive influence of O2 has been found [11]. Mechanistic studies mainly concern Fe-systems, too [5,7,8,10]. Generally, the reaction can be described by an oxidation of active sites, followed by a removal of the deposited oxygen, either by N2O itself or by recombination, eqs. (2)-(4). 

Oxygen activation of molecules at metal surfaces was first established in the 1970s by surface spectroscopies (XPS and UPS) over a wide temperature range (80-400 K). Furthermore, the distinction was made between the reactivity of partially covered surfaces and the relative inactivity of the oxide monolayer. 

There are a few examples of 02 oxidations catalyzed by zeolite-encapsulated complexes. Encapsulated CoPc was active in the oxidation of propene to aldehyde, whereas the free complex was inactive.76 A triple catalytic system, Pd(OAc)2, benzoquinone, and a metal macrocycle, was used to oxidize alk-enes with molecular oxygen at room temperature.77,78 Zeolite-encapsulated FePc79-81 and CoSalophen80,82 complexes were used as oxygen-activating catalysts. 

Toxoflavin reduced the respiratory oxygen consumption of mitochondria and suppressed the oxidative phosphorylation activity (91 Mil). 

When oxygen is pumped to the catalyst the activity of oxygen on the silver catalyst-electrode increases considerably because of the applied voltage. It thus becomes possible to at least partly oxidize the silver catalyst electrode. In a previous communication it has been shown that the phenomenon involves surface rather than bulk oxidation of the silver crystallites (17). The present results establish the direct dependence of the change in the rates of epoxidation and combustion Ari and Ar2 on the cell overvoltage (Equations 2,3, and 5) which is directly related to the surface oxygen activity. 

Endogenous NO is produced almost exclusively by L-arginine catabolism to L-citrul-line in a reaction catalyzed by a family of nitric oxide synthases (NOSs) [3]. In the first step, Arg is hydroxylated to an enzyme-bound intermediate "-hydroxy-1.-arginine (NHA), and 1 mol of NADPH (nicotinamide adenine dinucleotide phosphate, reduced form) and O2 are consumed. In the second step, N H A is oxidized to citrulline and NO, with consumption of 0.5 mol of NADPH and 1 mol of 02 (Scheme 1.1). Oxygen activation in both steps is carried out by the enzyme-bound heme, which derives electrons from NADPH. Mammalian NOS consists of an N-terminal oxy-... 

A recent study (1) has demonstrated that the electrochemical oxidation of hydroxide ion yields hydroxyl radical ( OH) and its anion (O"-). These species in turn are stabilized at glassy carbon electrodes by transition-metal ions via the formation of metal-oxygen covalent bonds (unpaired d electron with unpaired p electron of -OH and O- ). The coinage metals (Cu, Ag, and Au), which are used as oxygen activation catalysts for several industrial processes (e.g., Ag/02 for production of ethylene oxide) (2-10), have an unpaired electron (d10s1 or d9s2 valence-... 

Fiolitakis and Hofmann—wavefront analysis supports Eley-Rideal/redox mechanisms. In 1982 and 1983, Fiolitakis and Hofmann231,232 carried out wavefront analysis to analyze the dependence of the microkinetics of the water-gas shift reaction on the oxidation state of CuO/ZnO. They observed three important mechanisms after treatment of the catalyst surface with different H20/H2 ratios. These included two Eley-Rideal mechanisms which converted the reactants via adsorbed intermediates, and a redox mechanism that regulated the oxygen activity, as shown in Scheme 56. The authors indicated that different mechanisms could be dominating at different sections along the length of the fixed bed reactor. 

Wacker oxidation of l-alkenes. The Wacker oxygenation of 1-alkenes to methyl ketones involves air oxidation catalyzed by PdCl2 and CuCU, which is necessary for reoxidation of Pd(0) to Pd(II).1 This oxygenation is fairly sluggish and can result in chlorinated by-products. A new system is comprised of catalytic amounts of Pd(OAc)2, hydroquinone, and 1, used as the oxygen activator.2 The solvent is aqueous DMF, and a trace of HClOj is added to prevent precipitation of Pd(0). Oxygenation using this system of three catalysts effects Wacker oxidation of 1-alkenes in 2-8 hours and in 67-85% yield. 

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