Role of Bi promotion and solvent in platinum-catalyzed alcohol oxidation probed by in situ X-ray absorption and ATR-IR spectroscopy

Modification of 5 wt% Pt/Al2O3 by Bi (0.9 wt%) affords a drastic improvement of catalytic activity in the liquid phase aerobic oxidation of benzyl alcohol. The nature of the solvent employed, cyclohexane or toluene, seems to influence the catalytic activity as well. We have investigated the catalysts under working conditions using in situ X-ray absorption spectroscopy (XAS) and attenuated total reflection infrared spectroscopy (ATR-IR), aiming at uncovering the roles of the metal promoter and the reaction medium. XAS confirms that Bi is oxidized more easily than Pt, maintaining the catalytic activity of the metallic Pt sites for a longer period of time. Interestingly, toluene contrary to cyclohexane reduced Pt to a large extent. The freshly reduced noble metal sites seem to directly interact with the solvent, inducing an immediate poisoning of the material and limiting its performance. This behaviour is not observed in the presence of Bi, whose geometric effect (site blocking) is interpreted as additionally limiting the adsorption of toluene and the premature deactivation of Pt. ATR-IR spectroscopy during CO adsorption on Pt and during reaction indicates that Bi is located rather on extended surfaces than on step or kink sites. Side products, CO and benzoate species, appearing during the reaction reveal that the geometric suppression of undesired reactions does not occur to the same extent on Pt-based catalysts as on Pd, suggesting that decarbonylation of the produced aldehyde on Pt may occur also on sites other than the (111) terraces.