Infrared spectroscopy at high pressure: Interaction of H sub 2 and D sub 2 with Rh/Al sub 2 O sub 3

A novel high-pressure-infrared-cell reactor has been employed to study the interaction of ultrapure, high-pressure H{sub 2} and D{sub 2} with Rh/Al{sub 2}O{sub 3} films. The frequencies for the Rh-H and Rh-D stretching modes were measured to be 2,013 and 1,441 cm{sup {minus}1}, respectively. In addition, a band at 1,618 cm{sup {minus}1} for water on the Al{sub 2}O{sub 3} support was observed to develop concomitantly with the 2,013-cm{sup {minus}1} band. It was postulated that H{sub 2} dissociates on the Rh sites to produce a weakly bound Rh-H surface species, with the remaining H spilling over to the support where it reacted with surface hydroxyl groups to produce H{sub 2}O. The new Rh-H species was easily removed by reduction of H{sub 2} pressure above the catalyst. This weakly bound species is probably relevant to the catalytic chemistry for reactions involving H{sub 2} as a reactant over supported Rh. It was further suggested that the heretofore elusive Rh-H stretching mode for the previously postulated Rh carbonyl hydride surface species, observed spectroscopically during catalytic methanation, is present as a weak low-frequency shoulder on the band due to the C-O stretch for that species.