Operando Attenuated Total Reflectance FTIR Spectroscopy: Studies on the Different Selectivity Observed in Benzyl Alcohol Oxidation

Abstract

Au, Pd, and AuPd supported on TiO2 were prepared by a sol immobilization route. Operando attenuated total reflectance (ATR) IR spectroscopy and catalytic batch reactor experiments were performed in parallel to elucidate the different catalytic performance of the catalysts in the liquid‐phase oxidation of benzyl alcohol. Pd/TiO2 exhibited a higher activity than AuPd/TiO2 and Au/TiO2, but the modification of Pd with Au demonstrated a significant stability enhancement. ATR‐IR spectroscopy evidenced that the presence of Au facilitates the desorption of byproducts, which thus reduces the extent of deactivation of the active sites caused by the irreversible adsorption of benzoate species. Although benzaldehyde was the main product in both catalysts, the nature of the byproducts differs. Pd/TiO2 favored the deoxygenation of benzyl alcohol to produce toluene as the main byproduct. Conversely, AuPd/TiO2 promoted the transformation of benzaldehyde to benzoic acid.

Molecular Origin of the Selectivity Differences between Palladium and Gold–Palladium in Benzyl Alcohol Oxidation: Different Oxygen Adsorption Properties

“…Under these conditions, AuPd/C has lower activity than Pd/C and shows a different product distribution with less formation of products from the “carbonyloxyl” pathway (e.g., benzene, benzoic acid, benzoate) (Table ) . Different selectivity is obtained over Pd/TiO 2 and AuPd/TiO 2 [60 °C, p O 2 =2 bar (1 bar=0.1 MPa), toluene as solvent] . It is clear that the support and the reaction conditions can affect the selectivity and mechanism of alcohol oxidation over Pd versus AuPd.…”

Selective Benzyl Alcohol Oxidation over Pd Catalysts

“…Further studies have shown that the selectivity toward each product is related to the presence of specific active sites [36] but also to the adsorption geometry and orientation of reactants and products [37]. In situ attenuated total reflection infrared (ATR-IR) spectroscopy combined with site selective blocking by CO allow to discriminate the different sites involved in the liquid-phase oxidation of BA on Pd/Al 2 O 3 [36,38,39]. Ferri et al [36] showed that the oxidative dehydrogenation of the alcohol to benzaldehyde, showed only little dependence on structure and occurred on all exposed Pd faces, whereas benzene, an undesired product deriving from decarboxylation occurred preferentially on hollow sites on Pd (111) planes.…”

Characterisation of gold catalysts

“…Villa and co-workers 251 used operando attenuated total reflectance (ATR) IR spectroscopy to elucidate the different catalytic activity supported of Au and Pd and bimetallic Au-Pd nanoparticles in the liquid-phase oxidation of benzyl alcohol. The evolution of surface species was monitored during the oxidation of benzyl alcohol using a batch-reactor cell and ATR-IR spectroscopy measurements were performed.…”