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Polarization-modulation infrared reflection−absorption spectroscopy (PM-IRRAS), facilitating simultaneous selective detection of liquid phase and surface bound species, was applied to study the liquid-phase oxidation of benzyl alcohol to benzaldehyde on a Pd film. Experiments under aerobic and anaerobic conditions revealed significantly higher activity under anaerobic conditions and higher selectivity under aerobic conditions. Under anaerobic conditions toluene was the major side product. CO 2 and benzoic acid, not yet detected by in situ IR during this reaction, were observed as degradation and side products in the liquid phase. Enhanced formation of CO 2 and benzoic acid was observed under aerobic conditions where no surface signals could be detected. In contrast, under anaerobic conditions CO formed by decarbonylation of benzaldehyde was observed on the Pd (111) surface, delayed with respect to the formation profile of benzaldehyde in the liquid phase. Both CO and oxygen were found to poison the surface for alcohol oxidation. The poisoning by oxygen was more effectual than that by CO, which was attributed to the low surface area of the Pd film as well as the non-site-selective nature of the alcohol oxidation. No benzoate species were present on the Pd surface. This confirmed the hypothesis that benzoate species only adsorb on the basic support sites and not on Pd.
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Meier et al. (2009) studied this question.
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