Discrimination of Active Palladium Sites in Catalytic Liquid-Phase Oxidation of Benzyl Alcohol Davide Ferri, ² Cecilia Mondelli, ‡ Frank Krumeich, ² and Alfons Baiker* ,² Department of Chemistry and Applied Biosciences, ETH Zurich, HCI, CH-8093 Zurich, Switzerland, and Department of Inorganic, Metallorganic and Analytical Chemistry, UniVersity of Milan, Via Venezian 21, I-20133 Milan, Italy ReceiVed: September 5, 2006; In Final Form: October 13, 2006 Knowledge of the structure of active sites is a prerequisite for the rational design of solid catalysts. Using site-selective blocking by CO and isotope labeling combined with in situ attenuated total reflection infrared (ATR-IR) spectroscopy, we were able to discriminate the different sites involved in the liquid-phase oxidation of benzyl alcohol on Pd/Al 2 O 3 . The main reaction, that is, the oxidative dehydrogenation of the alcohol to the corresponding aldehyde, showed only little dependence on structure and occurred on all exposed Pd faces, whereas the undesired product decarbonylation occurred preferentially on hollow sites on (111) Pd faces. This explains why specific blocking of the latter sites, as realized in the industrially used Pd-Bi/Al 2 O 3 catalysts, leads to improved catalytic performance. Introduction The discrimination of catalytically active sites remains a great challenge when the solid catalyst operates in a liquid environ- ment because application of surface analytical methods is restricted to only few techniques capable of providing adequate information under reaction conditions. 1-4 Although UHV studies on single crystals 5,6 and model catalysts 7 represent an extremely valuable approach for gaining fundamental insight into the functioning of such catalytic systems, the interpretation of the data requires extrapolation to normal pressure and in particular to the liquid phase, which is often connected with great uncertainty. Well-defined supported metal nanoparticles have been applied successfully to bridge the “material gap” between single crystals and polycrystalline materials to understand the structure of active sites. 8 They allow mimicking catalytic particles 9 and metal-support interfaces 10 as found in real catalysts. The liquid-phase oxidation of alcohols is an example of a relevant industrial catalytic process where the use of single crystals combined with UHV techniques contributed greatly to unravel fundamental aspects of the reaction mechanism. 11 This operates via an oxidative dehydrogenation taking place on metallic Pd. 12 Recent in situ spectroscopic studies on commercial Pd/Al 2 O 3 catalysts including attenuated total reflection infrared (ATR-IR) spectroscopy 13,14 and X-ray absorption spectroscopy (XAS) 15 provided unique information on the state of the metal surface during reaction together with the observation of the complex reaction mechanism in which metal particles are involved. The molecular structure of the alcohol allows for a number of side reactions, which run parallel to the main alcohol dehydrogenation reaction, thus revealing that the catalyst surface is composed of different active sites enabling one (or more) specific reaction(s). Here, we report the discrimination of active sites exposed on a technical Pd/Al 2 O 3 catalyst during the liquid-phase oxidative dehydrogenation of benzyl alcohol. The sites active for the desired dehydrogenation reaction as well as those catalyzing the undesired decarbonylation of the product are identified using a combined approach including selective site blocking by CO adsorption, isotope labeling, and in situ attenuated total reflection infrared (ATR-IR) spectroscopy to probe simultaneously the catalyst surface during reaction and the reaction progress. Experimental Section Attenuated total reflection infrared (ATR-IR) measurements of benzyl alcohol oxidation were performed as described elsewhere. 14 Briefly, the reaction was monitored at 50 °C first during contact of the catalyst deposited on the ZnSe ATR-IR crystal with an Ar-saturated solution of the alcohol in cyclo- hexane (20 mM). Then Ar was replaced by air in the same solution without interrupting the liquid flow. Before reaction, the catalyst coating was contacted with H 2 -saturated solvent for about 20 min. This treatment affords reduced Pd. 15 In case of pre-equilibration of the Pd/Al 2 O 3 with CO, after catalyst reduction (H 2 -saturated solvent, 50 °C, 20 min) CO- saturated cyclohexane was admitted to the ATR-IR cell for about 1 h. Then the flow was switched to the Ar-saturated solution of benzyl alcohol (or 13 C-benzyl alcohol). Transmission electron microscopy (TEM) investigations were performed with a CM30ST microscope (Philips; LaB6 cathode, operated at 300 kV, point resolution ∼2 Å), while scanning transmission electron microscopy (STEM) investigations were * Corresponding author. E-mail: baiker@chem.ethz.ch. ² Department of Chemistry and Applied Biosciences, ETH Zurich. ‡ Department of Inorganic, Metallorganic and Analytical Chemistry, University of Milan. 22982 2006, 110, 22982-22986 Published on Web 10/31/2006 10.1021/jp065779z CCC: $33.50 © 2006 American Chemical Society