Carbon monoxide chemisorbed, structure

Williams ED, Weinberg WH. 1979. The geometric structure of carbon monoxide chemisorbed on the ruthenium (001) surface at low temperatures. Surf Sci 82 93. 

Fig. 6. The structure of carbon monoxide chemisorbed on Ni(100) in a c(SxS) periodic arrangement. The carbon atoms are bonded to single metal atoms.

The vibrational spectra of carbon monoxide chemisorbed on various metal surfaces are a function of coverage and temperature [128]. Review the experimental techniques that were employed and discuss the surface structure sensitivity, the coverage and temperature dependencies of the adsorbed molecule. 

If the relationship is accepted, the question remains whether it shows that the bridged form of chemisorbed carbon monoxide is more easily hydrogenated or whether the structure. of the adsorbed carbon monoxide and the catalytic activity both reflect some third factor. 

It had been found that removal of hydrogen from nickel at 350°C, instead of at room temperature, produced a profound difference in the properties of the nickel with respect to ethylene chemisorption (4). Therefore the chemisorption of carbon monoxide was repeated using nickel which had been degassed at 350°C. This produced a startling difference compared to the results shown in Fig. I now most of the carbon monoxide was chemisorbed in the linear structure (5). Similar experiments have not been made with palladium but it is reasonable to predict that the ratio of linear to bridged carbon monoxide would be increased by a more thorough removal of hydrogen from this metal. 

The shifts can be explained on the basis that the first carbon monoxide added to the surface tends to chemisorb in the Pt=C= structure. As more carbon monoxide is added It becomes more difficult for platinum to contribute bonding electrons thus the proportion of the Pt-CJsO structure increases. 

The band at 2060 cm-i disappears after evacuation of carbon monoxide at room temperature (Table I, lb). A fraction of the reversibly adsorbed gas is therefore located on cationic sites. Since the band at 1960-1970 cm- is observed after the evacuation, an irreversible fraction of carbon monoxide is also chemisorbed on cationic sites. A subsequent adsorption of oxygen produces, however, the disappearance of this band (Table I, Ic), demonstrating that oxygen interacts with carbon monoxide irreversibly adsorbed on cationic sites. No gas is evolved from the surface during the adsorption of oxygen. The interaction product therefore remains in the adsorbed state and its structure must be similar to the structure of species formed previously during the adsorption of carbon monoxide since no new band appears in the spectrum after the adsorption of oxygen. 

Interpretation of the EXAFS of [Os3(CO)i2] absorbed on silica in terms of a structure [Os3(CO)ioH(OS=)] has been confirmed by comparison with [OS3-(CO)ioH(OSiPh3)], which was prepared from the reaction between [Os3(CO)i2] and Si(OH)Ph3. The same species is formed by physisorption of [Os3(CO)ioH3] and of [Os6(CO)i8l on silica. Carbon monoxide inhibits the transformation to [Rh8(CO)i8] which occurs when [Rh4(CO)i2] is chemisorbed on highly divided silica. Both [Rh4(CO)i2] and [Rh8(CO)x8] are converted to high nuclearity rhodium particles when the chemisorbed carbonyls are treated with water vapour, but surface-bound [Rh(CO)2(OSi=)] is formed when the chemisorbed polynuclear carbonyls are treated with oxygen gas. The molecular compounds [Rh4(CO)i2] and [Rh8(CO)i8] can be regenerated in that sequence when [Rh(CO)2(OSi=)] is treated with CO. In this context, the observation that Na2[M(CO)5] reduces CO2 to form [M(CO)8] (M=Mo, W) and carbonate ion may provide a model for the surface process. ... 

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