Infrared spectra of hydrocarbons chemisorbed on silica-supported metals. II. Ethylene on nickel and platinum over a range of temperatures

Abstract

Infrared spectra have been obtained of ethylene chemisorbed on silica-supported nickel over the temperature range -78 to 150 $^\circ$C, and on platinum over the range -145 to +150 $^\circ$C. Relatively similar spectra resulted when ethylene was chemisorbed on platinum at any temperature in the range down to -78 $^\circ$C. The dominant spectral features at 2920, 2880 and 2795 cm$^{-1}$ are assigned to associatively chemisorbed ethylene $\mathbf{M}$CH$_2$-CH$_2\mathbf{M}$ ($\mathbf{M}$ = surface metal atom); a weaker band at 3010 cm$^{-1}$ is attributed to surface $\mathbf{M}$CH=CH$\mathbf{M}$ species. On hydrogenation, most of the chemisorbed surface species were converted to gaseous ethane, but at 95 $^\circ$C and increasingly at 150 $^\circ$C, a weak spectrum of chemisorbed n-butyl groups was observed. From the large spectral intensity increase on hydrogenation (about tenfold) it is concluded that considerable initial dissociative chemisorption must also have occurred, probably in the form of surface carbide. A different spectrum occurred on adsorption at -145 $^\circ$C with a single strong band at 2907 cm$^{-1}$; this is ascribed to $\mathbf{M}_2$CH-CH$\mathbf{M}_2$ surface species, which revert at higher temperatures to those described above. The spectrum attributed to associatively chemisorbed ethylene on nickel (bands at 2870 and 2790 cm$^{-1}$) was the dominant spectroscopic feature only after short initial periods of chemisorption at -78 $^\circ$C. After prolonged periods of chemisorption at -78 $^\circ$C, or shorter periods at 20 $^\circ$C, the spectra indicated that surface dimerization was occurring, probably via a self-hydrogenation reaction; at 20 $^\circ$C an appreciable number of n-butyl groups were gradually formed on the surface by this process. On hydrogenation at -78 $^\circ$C, ethane and a smaller amount of n-butane were formed, whereas at 20 $^\circ$C the gas phase consisted of only a low pressure of n-butane; at both temperatures many chemisorbed n-butyl groups remained on the surface. Less dissociative adsorption occurs on nickel at those temperatures than on platinum as the spectral intensity increase on hydrogenation was about threefold. Extensive dissociation, probably to a carbide, occurs when ethylene is chemisorbed on nickel at 150 $^\circ$C. A large spectral intensity increase occurs on hydrogenation at this temperature and methane is instantaneously detected in the gas phase. The latter grows in intensity over a period of hours at the expense of the chemisorbed species. The spectroscopically observable species appear to be attached to the metal surface by $\sigma$ rather than by $\pi$ bonds.