Oxygen, chemisorption selective

Oxygen chemisorption methods were used to titrate surface vanadium sites in these studies. Raman, X-ray diffraction and isotopic labeling were done to support the dispersion results from chemisorption. A further conclusion was that as the % V increased for ethane oxidation reactions that the catalytic activity and selectivity was similar to that of unsupported vanadia. 

One of the best ways of characterizing a supported catalyst is determination of dispersion and effective surface area of the catalyticaUy active component. The dispersion of metal oxide catalysts can be determined by selective chemisorption of oxygen at appropriate temperatures [14-16]. The dispersions obtained from oxygen chemisorption measurements on various catalysts are presented in table 1. The N2 BET surface areas of various samples are also shown in this table. As can be noted, dispersion for 20 wt% catalyst is similar, within experimental limitations, irrespective of their origin. The BET surface area measurements also reveal that both the preparation methods yield similar type of catalysts in terms of physico-chemical characteristics. These catalysts were further evaluated for selective oxidation of / -methox doluene to p-... 

The active site on the surface of selective propylene ammoxidation catalyst contains three critical functionalities associated with the specific metal components of the catalyst (37—39) an a-H abstraction component such as Sb ", or Te" " an olefin chemisorption and oxygen or nitrogen insertion component such as Mo " or and a redox couple such as Fe " /Fe " or Ce " /Ce" " to enhance transfer of lattice oxygen between the bulk and surface... 

The mode of chemisorption of CO is a key-factor concerning selectivity to various products. Hydrocarbons can only be produced if the carbon-oxygen bond is broken, whereas this bond must stay intact for the formation of oxygenates. It is obvious that catalysts favoring the production of hydrocarbons must chemisorb carbon monoxide dissociatively (e.g. Fe) while those favoring the formation of oxygenates must be able to chemisorb carbon monoxide molecularly (e.g. Rh). 

It is tempting to associate directly the absence of ethylene oxide over catalysts with more than 40% Pd with the appearance of holes in the d-band. It could be assumed that ethylene is chemisorbed directly on Pd-rich alloys and rapidly decomposed, whereas on Ag-rich alloys ethylene is only adsorbed on top of an oxygen-covered surface leading to selective oxidation. However, the general conclusion from earlier kinetic studies (143) is that the rate-determining step over pure palladium also involves the latter mode of ethylene chemisorption. 

Hirschler and Hudson (36/6), however, favor the opinion that Bronsted sites are exclusively responsible for the activity of silica-alumina. In studying the adsorption of perylene and of triphenylmethane, they concluded that carbonium ions are not formed by a hydride abstraction mechanism as claimed by Leftin (362). Instead, triphenylmethane is oxidized by chemisorbed oxygen to triphenylcarbinol in a photo-catalyzed reaction, followed by reaction with a Bronsted acid giving water and a triphenylmethyl carbonium ion. After treatment with anhydrous ammonia, the organic compound was recovered by extraction as triphenylcarbinol. About thirteen molecules of ammonia per assumed Lewis site were required to poison the chemisorption of trityl ions. The authors explain the selective inhibition of certain catalyzed reactions by alkali by assuming that only certain of the acidic protons will ion-exchange with alkali ions. 

To exhibit such an active and selective catalytic effect, the catalyst must be a fairly good hydrogenation catalyst that is able to activate molecular hydrogen. It must also activate carbon monoxide without dissociating it. A nondissociative chemisorption permits the hydrogenation of carbon monoxide to occur on both oxygen and carbon. Considering the formation of surface methoxide in the second mechanism [Eq. (3.43)], a further requirement is that the catalyst not form a too stable metal methoxide. 

The activity and selectivity of a Cs unsaturated aldehyde (prenal) has been investigated in gaseous phase on a platinum polycrystalline sample. The aim of this work was the study of the respective effects of the pretreatment of the sample by oxygen and of the presence of a small partial pressure of H2S in the gas phase. The results gave us a better understanding of the surface mechanism and proved the importance of the chemisorption mode of the molecule. 

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