Lanthana-promoted [formula omitted]: II. Studies of CO hydrogenation

Abstract

A study of the influence of lanthana promotion on the hydrogenation of CO over Rh SiO 2 has been conducted. Lanthana-promoted Rh SiO 2 exhibits higher turnover frequencies for the synthesis of CH 4, C 2C 4 hydrocarbons, CH 3OH, and C 2 oxygenates than unpromoted Rh SiO 2 . The turnover frequency for each product goes through a maximum with increasing lanthana addition. Lanthana promotion is also found to increase the selectivity for the formation of CH 3OH, C 2 oxygenates, and C 2C 4 hydrocarbons and to decrease the selectivity for CH 4. The selectivity for C 2 oxygenates is optimized at low levels of lanthana addition. In situ infrared observations show that lanthana promotion blocks the chemisorption of CO onto surface Rh sites. Of particular interest is the observation of a band at 1725 cm −1. This feature is assigned to CO adsorbed on a Rh site immediately adjacent to an LaO x island. The low frequency of this vibration is attributed to a weakening of the CO bond caused by the interaction of lanthana cations with the oxygen end of the adsorbed CO. It is postulated that the interaction of the lanthana with the CO is responsible for the enhanced rates of CO dissociation and the formation CH 4 and C 2+ products. Infrared spectroscopy also provides evidence for acyl, formate, and acetate groups on the surface of lanthana-promoted Rh SiO 2 . The possible role of these species in the synthesis of products is discussed.