Oxygen adsorbed cobalt oxide

Associated with these surfaces, three distinct forms of oxygen can exist lattice oxygen, 0, oxygen adsorbed on [Co], Oco and oxygen adsorbed on [0l], 0q. Halpern and Germain (4) have also reported the presence of three different kinds of oxygen on a cobalt oxide surface. 

The studies of Garner and his co-workers in the years 1928-1939, which had established the existence of two types of carbon monoxide and hydrogen chemisorption on oxides and which identified irreversible chemisorption with incipient reduction, were followed in the immediate postwar period by an intensive study of the properties of copper oxide (12-15). The work was later extended to nickel oxide (16) and cobalt oxide (17,18). With each of these oxides it was established that carbon monoxide was capable of reacting not only with lattice oxygen, but also with adsorbed oxygen. The concept of irreversible chemisorption involving a carbonate ion and ulti-... 

We are investigating bifunctional catalysts in which one component of the catalyst adsorbs or oxidizes CO and the other component dissociates water. Our present research is focusing on metal-support combinations to promote this bifunctional mechanism. The metallic component is chosen to adsorb CO at intermediate adsorption strengths (platinum [Pt], Ru, palladium [Pd], PtRu, PtCu, cobalt [Co], ruthenium [Ru], silver [Ag], iron [Fe], copper [Cu], and molybdenum [Mo]). The support is chosen to adsorb and dissociate water, typically a mixed-valence oxide with redox properties or oxygen... 

The amount of oxygen chemisorbed decreased with decreasing temperature except at —78°. This anomaly was not found when similar measurements were made with nickel and copper. A tentative explanation may be that at this low temperature, oxygen may be somewhat strongly physically adsorbed in addition to that required for oxide formation. This physical adsorption could accoimt for the larger amounts of oxygen taken up at this temperature on cobalt oxide. However, more experimental work is definitely needed before any decision can be made. 

CoOx may affect the adsorption of CO or O2 on R. Since at low temperatures the reaction rate on R is determined by the slow adsorption of oxygen due to CO inhibition, it is most likely that CoO serves as 0-supplier for die reaction. No influence of cobalt oxide on the CO adsorption on R was detected by IR measurements. If we assume that R-Co alloy formation does not play an important role in the CO/O2 reaction over Pt/CoOx/SiQj, several models may account for flie observed effects. According to our first model, cobalt cations enhance the adsorption of O2 on R by an increased electron back-donation into the anti-bonding orbitds of O2, which facilitates O2 dissociation. The increased back donation may be induced by the electrical field of the cobalt cations. The second model is shown schematically in figure 4. CO is adsorbed on R. O2 dissociates on CoO and the dissociation may be assisted by the presence of O-vacancies present on cobalt oxide. COa on R will react with Oa on cobalt. This reaction will then take place at the interface between R and CoOx. It is also possible that Oa migrates fi"om tiie CoO to R, in which case the reaction proceeds on the R surface (third model). The authors are in favour of the last two models since R itself is already able to dissociate O2 around 100 K if fi ee R sites are available (no CO inhibition) [33]. 

Cobalt(Il) dicobalt(Ill) tetroxide [1308-06-17, Co O, is a black cubic crystalline material containing about 72% cobalt. It is prepared by oxidation of cobalt metal at temperatures below 900°C or by pyrolysis in air of cobalt salts, usually the nitrate or chloride. The mixed valence oxide is insoluble in water and organic solvents and only partially soluble in mineral acids. Complete solubiUty can be effected by dissolution in acids under reducing conditions. It is used in enamels, semiconductors, and grinding wheels. Both oxides adsorb molecular oxygen at room temperatures. 

Investigations by Trifiro et al. (54-57) using infrared spectroscopy have led to the identification of a covalent metal-oxygen double bond which is present in a great majority of the selective oxidation catalysts. On the other hand, this bond is systematically absent from total oxidation catalysts such as the oxides of iron, nickel, and cobalt (55, 56). Trifiro et al. (57) and Akimoto and Echigoya (58) have also reported that there is a direct interaction between adsorbed propylene or acrolein and this... 

In the previous sections of this review, it has been shown that most effective catalysts for the selective oxidation of propylene contain at least two types of metal oxides—an amphoteric or low-valence oxide, such as bismuth, tin, iron, or cobalt, and an oxide of a high valence metal, such as molybdenum or antimony. Moreover, it has been suggested several times that each of these metal oxide components may give rise to an active site for example, propylene may adsorb on an active site associated with one of the metal oxide components, and oxygen may adsorb on an active site associated with another metal oxide component. This problem has been studied using spectroscopic, adsorption, and kinetic techniques. It now seems appropriate to consider some of these studies in detail, attempting to relate the solid structure of the catalyst to the active sites wherever possible. 

For optimum activity, nickelic, cobaltic, and ferric oxides must be prepared in a finely divided state, free from adsorbed impurities. Cobaltic and nickelic oxides show a variable oxygen content, being mixtures of more than one oxide, and give up or take up oxygen continuously, according to conditions (22,49). 

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