Aliphatic Nitrile Adsorption on Al2O3 and ZrO2 As Studied by Total Internal Reflection Sum-Frequency Spectroscopy

Total internal reflection sum-frequency spectroscopy has been used as an in situ vibrational spectroscopic probe of the interface between neat liquid aliphatic nitriles (acetonitrile, butyronitrile) and flat oxide surfaces (ZrO2, Al2O3). Such oxide surfaces serve as models for high surface area supports used for supported metal catalysts that are effective for nitrile hydrogenation reactions. Two types of surface nitrile species were observed to form on the oxide surfaces when exposed to neat nitriles. A weakly bound species coordinates to the surface through the nitrogen of the CN group and exhibits a νCN stretching frequency very similar to that of the bulk liquid nitriles. The lower νCN frequencies of this species on Al2O3 as compared to ZrO2 suggest a stronger interaction with the former surface. It is proposed that this stronger interaction arises from increased hydrogen bonding that is caused by a larger amount of native OH on the Al2O3. A second type of species exhibits a further downshifted νCN stretch and is assigned to intermolecular polar interactions and nitrile clusters at the interface. Cluster formation is diminished in the case of butyronitrile, likely due to increased steric hindrance. Such steric hindrance is suggested by gauche defects detected by sum-frequency measurements in the C−H stretching region. Finally, using polarization-dependent measurements of the CH3 stretching mode, the average orientation of acetonitrile on these surfaces was found to be around 20−25° from the surface normal. The prospects for using this approach for examining model-supported metal catalysts are discussed in light of these findings.