Infrared Spectra of Acetylene and Acetylene Derivatives Adsorbed on Alumina and Silica

An infrared study of adsorbed acetylenes has revealed several features of the nature and orientation of the adsorbed species. Acetylene, deuteroacetylene, methyl acetylene, and dimethyl acetylene are strongly chemisorbed at room temperature on alumina. Weak chemisorption also occurs with acetylene, deuteroacetylene, and methyl acetylene. The strongly held acetylene is held normal to the surface, while the weakly held acetylene is held parallel to the surface. Similar effects occur with methyl acetylene, all of the strongly held molecules being attached to the surface by the acetylenic end. Both the strongly and weakly adsorbed dimethyl acetylene is adsorbed parallel to the surface. The sites responsible for the strong chemisorption of dimethyl acetylene are different from those active in the strong chemisorption of acetylene. With silica, no strong chemisorption occurred at room temperature for either acetylene or dimethyl acetylene. For both adsorbents, the interaction between the OD (and OH) groups of the surface and the adsorbates has been studied. Exchange takes place between the highest frequency OH groups on alumina and the strongly adsorbed C2D2. An OD group at the expected frequency was observed after the exchange had taken place. The remaining two types of OH groups on alumina did not appear to interact with the strongly held species, but only with the weakly held species. With dimethyl acetylene, the silica OD groups interacted to much the same degree as did the two lower frequency alumina OD groups.