Adsorption and reaction complexes on the catalytic surface

In the foregoing discussion, the emphasis has been mainly on the properties of the catalyst, but it is evident that these must be regarded in close connection with the nature of the adsorbed hydrocarbons. Important information about this interaction can be gained from structure analysis of adsorption and reaction complexes, as well as adsorption measurements. 

Infrared spectra of propene and isobutene on different catalysts were measured by Gorokhovatskii [143]. Copper oxide, which converts olefins to butadiene and aldehydes, shows adsorption complexes different from structures on a V2Os—P2Os catalyst which produces maleic acid anhydride. Differences also exist between selective oxidation catalysts and total oxidation catalysts. The latter show carbonate and formate bands, in contrast to selective oxides for which 7r-allylic species are indicated. A difficulty in this type of work is that only a few data are available under catalytic conditions most of them refer to a pre-catalysis situation. Therefore it is not certain that complexes observed are relevant for the catalytic action. 

Sachtler [270] notes that the 7r-allyl complex can be attached to a metal ion or to an oxygen anion but doubts that a 7r-allyl metal complex can be stable at the high temperatures normally used. He draws attention to the fact that, in the case of aromatic oxidations, benzoates, maleinates, etc. are observed spectroscopically, indicating that a carbon—metal bond is not formed. 

Trifiro et al. [322], however, did not find (R—C=0) groups in an investigation of Sn02—V2Os catalyst by infrared spectroscopy. The spectra reveal the presence of Mo03 on the surface. If propene is adsorbed, the Mo=0 band of the oxide is influenced. The Mo=0 band disappears when acrolein is adsorbed (at room temperature). Desorption at 225°C restores this peak. 

Haber suggests that the relation between active sites of different types is associated with the nature of the active complex (ref. in Butt [67]). The final products can be classified in the following manner. 

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