Hydrogen oxidation catalysis

Hydrogen oxidation catalysis happens to be more difficult to obtain than hydrogen production, if noble metals are excluded. In particular, several nanoparticulate catalysts such as transition metal oxides/sulfides-based nanoparticles catalyze H2 evolution [29-34], while only tungsten carbide has been demonstrated to be active for H2 oxidation [35]. Even in the case of organometallic catalysts, only few complexes have proved to be able to catalyze H2 oxidation rather than evolution (see below). 

A brief summary of current and potential processes is given in Table 8.1. As shown in the table, most of the reactions are hydrolysis, hydrogenolysis, hydration, hydrogenation, oxidation, and isomerization reactions, where catalysis plays a key role. Particularly, the role of heterogeneous catalysts has increased in this connection in recent years therefore, this chapter concerns mostly the application of heterogeneous solid catalysts in the transformation of biomass. An extensive review of various chemicals originating from nature is provided by Maki-Arvela et al. [33]. 

In contact with fluorine, when it is cold, nickel oxide glows. It reacts violently with hydrogen peroxide (catalysis of its decomposition ). Finally, in contact with a mixture of hydrogen sulphide and air, it glows and causes this gaseous mixture to detonate. 

Poisoning of platinum fuel cell catalysts by CO is undoubtedly one of the most severe problems in fuel cell anode catalysis. As shown in Fig. 6.1, CO is a strongly bonded intermediate in methanol (and ethanol) oxidation. It is also a side product in the reformation of hydrocarbons to hydrogen and carbon dioxide, and as such blocks platinum sites for hydrogen oxidation. Not surprisingly, CO electrooxidation is one of the most intensively smdied electrocatalytic reactions, and there is a continued search for CO-tolerant anode materials that are able to either bind CO weakly but still oxidize hydrogen, or that oxidize CO at significantly reduced overpotential. 

In this chapter, we have discussed the application of metal oxides as catalysts. Metal oxides display a wide range of properties, from metallic to semiconductor to insulator. Because of the compositional variability and more localized electronic structures than metals, the presence of defects (such as comers, kinks, steps, and coordinatively unsaturated sites) play a very important role in oxide surface chemistry and hence in catalysis. As described, the catalytic reactions also depend on the surface crystallographic structure. The catalytic properties of the oxide surfaces can be explained in terms of Lewis acidity and basicity. The electronegative oxygen atoms accumulate electrons and act as Lewis bases while the metal cations act as Lewis acids. The important applications of metal oxides as catalysts are in processes such as selective oxidation, hydrogenation, oxidative dehydrogenation, and dehydrochlorination and destructive adsorption of chlorocarbons. 

The discovery of titanium substituted ZSM-5 (TS-1) and ZSM-11 (TS-2) have led to remarkable progress in oxidation catalysis (1,2). These materials catalyze the oxidation of various organic substrates using aqueous hydrogen peroxide as oxidant. For example, TS-1 is now used commercially for the hydroxylation of phenol to hydroquinone and catechol (/). Additionally, TS-1 has also shown activity for the oxidation of alkanes at temperatures below 1()0 °C (3,4). 

The chemistry of electrochemical reaction mechanisms is the most hampered and therefore most in need of catalytic acceleration. Therefore, we understand that electrochemical catalysis does not, in principle, differ much fundamentally and mechanistically from chemical catalysis. In addition, apart from the fact that charge-transfer rates and electrosorption equilibria do depend exponentially on electrode potential—a fact that has no comparable counterpart in chemical heterogeneous catalysis—in many cases electrocatalysis and catalysis of electrochemical and chemical oxidation or reduction processes follow very similar if not the same pathways. For instance as electrochemical hydrogen oxidation and generation is coupled to the chemical splitting of the H2 molecule or its formation from adsorbed hydrogen atoms, respectively, electrocatalysts for cathodic hydrogen evolution—... 

Hydrogenation of a, p- unsaturated aldehydes Oxidation Catalysis Pt/Al203 (unmodified)... 

For certain metal systems, the chemical properties of bimetallic DENs include selective extraction from the dendrimer interior into organic solvents. Catalytic properties include homogeneous hydrogenation catalysis heterogeneous hydrogenation and oxidation catalysis have also been examined. Homogeneous hydrogenation studies indicate that... 

This section is concerned with the activation of hydrocarbon molecules by coordination to noble metals, particularly palladium.504-513 An important landmark in the development of homogeneous oxidative catalysis by noble metal complexes was the discovery in 1959 of the Wacker process for the conversion of ethylene to acetaldehyde (see below). The success of the Wacker process provided a great stimulus for further studies of the reactions of noble metal complexes, which were found to be extremely versatile in their ability to catalyze homogeneous liquid phase reaction. The following reactions of olefins, for example, are catalyzed by noble metals hydrogenation, hydroformylation, oligomerization and polymerization, hydration, and oxidation. 

Attempts to support models of the catalytic activity and the operative mechanism with results of theoretical considerations have been reported for the oxygen reduction [iii] and hydrogen oxidation [iv]. Electrocatalytic electrodes are indispensable parts of fuel cells [v]. A great variety of electrocatalytic electrodes has been developed for analytical applications [vi]. See also electro catalysis, catalytic current, -> catalytic hydrogen evolution, catalymetry. 

The discovery of titanium silicalite-1 (TS1) by Enichem scientists (20-22) and its commercial use as a catalyst for a variety of selective oxidations with aqueous hydrogen peroxide under mild conditions (Figure 1.3) constituted a major breakthrough in oxidation catalysis. 

The pre-edge intensity very often increases only upon removal of any adsorbed water vapor [64]. The water vapor acts as a ligand, which changes the intensity of the pre-edge peak. This is crucial for the understanding of how the oxidation catalysis works, since TS-1 is mostly used in aqueous solutions with hydrogen peroxide. It has been suggested that EXAFS can show the presence of titanium peroxo species on TS-1 [65]. 

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