Oxidation-reduction catalysis

This review has highlighted the key contributions of modern surface science to the understanding of the kinetics and mechanism of nitrogen oxide reduction catalysis. As discussed above, the conversion of NO has been taken as the standard to represent other NOx, and CO has typically been used as the reducing agent in these studies. The bulk of the work has been carried out on rhodium and palladium surfaces, the most common transition metals used in three-way catalytic converters. 

Steady state and transient experiments, the substantial though fragmented literature, and new interpretations are combined in an attempt to define and understand the catalytic kinetics for crrbon monoxide oxidation over cobalt oxide (C03O4) supported on alumina. The result is a rather coherent picture of oxidation-reduction catalysis by a metal oxide. It is shown that the dynamic methods yield vastly more information than steady state studies with significantly less experimental effort. 

Enzymes that perform oxidation-reduction catalysis generally require some type of cofactor to store electrons (and protons) during the catalytic cycle. The cofactors are usually low-molecular weight species that reversibly bind to the enzyme, but in some cases are intrinsic elements of the protein structure. For example, quinoenzymes contain covalently bound quinones derived from tyrosine or tryptophan residues in the protein. These quinocofactors represent a striking extension of protein side-chain reactivity. " ... 

Surface chemistry plays a vital role in many processes, including corrosion, oxidation, reduction, catalysis, and adsorption. As mentioned above, as a particle decreases in size, a greater percentage of its constituent atoms are featured on the surface compared with the atoms present in the bulk of the... 

Unfortunately, many enzymes which fall clearly within the scope of oxidation-reduction catalysis will have to be neglected because of limitations of space and the inadequate amount of information available about them. In this group are the hydrogenases, the sulfur oxidases, the nitrogen oxidases, and a host of related enzymes found in autotrophic bacteria, as well as in plant and animal organisms. 

JAROUSSE - MAKOSZA Phase transfer Phase transfer (PT) catalysis by quaternary ammonum salts of substitution, addition, carbonyl iormatlon, oxidation, reduction... 

COVALENT COMPOUNDS, METAL IONS OXIDATION-REDUCTION g) Base catalysis... 

Another important catalytic technology for removal of NOx from lean-burn engine exhausts involves NOx storage reduction catalysis, or the lean-NOx trap . In the lean-NOx trap, the formation of N02 by NO oxidation is followed by the formation of a nitrate when the N02 is adsorbed onto the catalyst surface. Thus, the N02 is stored on the catalyst surface in the nitrate form and subsequently decomposed to N2. Lean NOx trap catalysts have shown serious deactivation in the presence of SOx because, under oxygen-rich conditions, SO, adsorbs more strongly on N02 adsorption sites than N02, and the adsorbed SOx does not desorb altogether even under fuel-rich conditions. The presence of S03 leads to the formation of sulfuric acid and sulfates that increase the particulates in the exhaust and poison the active sites on the catalyst. Furthermore, catalytic oxidation of NO to N02 can be operated in a limited temperature range. Oxidation of NO to N02 by a conventional Pt-based catalyst has a maximum at about 250°C and loses its efficiency below about 100°C and above about 400°C. 

The third remarkable aspect of enzyme catalysis is the versatility of these species. They catalyze an extremely wide variety of reactions— oxidation, reduction, polymerization, dehydration, dehydrogenation, etc. Their versatility is a reflection of the range and complexity of the chemical reactions necessary to sustain life in plants and animals. 

Throughout this book a major stress is on catalysis in organisms. Catalysis is confined to non-metals and metal ions of attacking power, either as Lewis acids or in oxidation/reduction and this excludes the simplest ions such as Na+, K+ and Ca2+ (and Cl- among anions). The transition metal ions and zinc are the most available powerful catalysts. The metals in a transition series are known to have selective binding properties, exchange rates and oxidation/reduction states, which can be put to use in catalysis in quite different ways (Table 2.13). It is noticeable that especially the complexes of metal elements... 

In this mechanistic picture, the rhodium center goes through the catalysis with the unusual III —> V —> III oxidation/reduction cycle. 

Selectivity in catalytic oxidation/reduction and acid-base reactions has been a long-term challenge in the catalysis field. While it has been recognized that the control of molecular activation and reaction intermediates is critical in achieving high selectivity, this issue has not been adequately addressed and is a serious challenge to the field. 

The principal iron oxides used in catalysis of industrial reactions are magnetite and hematite. Both are semiconductors and can catalyse oxidation/reduction reactions. Owing to their amphoteric properties, they can also be used as acid/base catalysts. The catalysts are used as finely divided powders or as porous solids with a high ratio of surface area to volume. Such catalysts must be durable with a life expectancy of some years. To achieve these requirements, the iron oxide is most frequently dis-... 

Bipyridyl (continued) as ligand, 12 135-1% catalysis, 12 157-159 electron-transfer reactions, 12 153-157 formation, dissociation, and racemization of complexes, 12 149-152 kinetic studies, 12 149-159 metal complexes with, in normal oxidation states, 12 175-189 nonmetal complexes with, 12 173-175 oxidation-reduction potentials, 12 144-147... 

The other metal ion characteristics that are familiar to the inorganic chemist, such as their stereochemistry, their electronic configuration, and their oxidation reduction potential, are also very useful in biological phenomena. In fact, perhaps the major difference between the inorganic chemist and the biochemist in their view of metal ion catalysis is that the inorganic chemist produces the metal complexes that he studies, whereas the biochemist analyzes metal complexes that are naturally occurring. 

Metal Ion Catalysis Metals, whether tightly bound to the enzyme or taken up from solution along with the substrate, can participate in catalysis in several ways. Ionic interactions between an enzyme-bound metal and a substrate can help orient the substrate for reaction or stabilize charged reaction transition states. This use of weak bonding interactions between metal and substrate is similar to some of the uses of enzyme-substrate binding energy described earlier. Metals can also mediate oxidation-reduction reactions by reversible changes in the metal ion s oxidation state. Nearly a third of all known enzymes require one or more metal ions for catalytic activity. 

We believe that catalysis occurs by formation of a complex between acetaldehyde, peracetic acid, and the metal ion in the 3+ oxidation state. The metal ion could be acting as a superacid as for peracetic acid decomposition, although oxidation-reduction reactions within the complex cannot be ruled out. Here again, we have found a disturbing lack of catalytic activity of other trivalent metals (aluminum, iron, and chromium). Simple acid catalysis is not as effective as proved when using p-toluenesulfonic acid and acetyl borate. This indicates that at least more than one coordination position is needed to obtain a complex of the proper configuration. 

However, these experiments may not have established a mechanism for natural flavoprotein catalysis because the properties of 5-deazaflavins resemble those of NAD+ more than of flavins.239 Their oxidation-reduction potentials are low, they do not form stable free radicals, and their reduced forms don t react readily with 02. Nevertheless, for an acyl-CoA dehydrogenase the rate of reaction of the deazaflavin is almost as fast as that of natural FAD.238 For these enzymes a hydride ion transfer from the (3 CH (reaction type D of Table 15-1) is made easy by removal of the a-H of the acyl-CoA to form an enolate anion intermediate. 

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