Hydrogenation of unsaturated hydrocarbons occurs efficiently on noble-metal catalysts, such as platinum, rhodium, and palladium.[1] The reaction mechanism first proposed by Horiuti and Polanyi [2] in 1934 proceeds by a) hydrogen dissociation on the metal surface, b) alkene adsorption, c) subsequent hydrogen addition to alkene and, finally, d) desorption of the product (alkane). Real hydrogenation catalysts represent very complex systems for studying reaction mechanisms at the molecular level. Therefore, model systems with a reduced complexity have been invoked ranging from single crystals to metal particles deposited on oxide films. [3][4][5][6][7][8] The conclusions regarding reaction mechanism and structural sensitivity are often based upon experiments on single metal crystals.[3] In particular, hydrogenation of alkenes has been shown to be structure insensitive.Herein, we report results showing that alkene hydrogenation reaction under low-pressure conditions, which does not occur on Pd(111) single crystal, proceeds efficiently on palladium nanoparticles. We show that the formation of weakly bound "subsurface" hydrogen is a key factor for hydrogenation to occur efficiently. The subsurface hydrogen exists in both Pd systems. However, the nanoparticle dimensions are such that this hydrogen is accessible to the adsorbed alkene, and hydrogenation occurs. In contrast, for crystals, the hydrogen atoms diffuse so deep into the bulk that they are not accessible to an adsorbed alkene, and therefore hydrogenation does not occur.We have studied the surface chemistry of ethene and different pentene isomers on both Pd(111) single crystal and Pd particles deposited on a thin alumina film (Figure 1). The particles studied are approximately 5 nm in diameter and consist primarily (% 90 %) of (111) facets [8] (% 10 % are (100) facets). The experiments were performed in ultrahigh vacuum on clean and well-defined systems. Using the temperatureprogrammed desorption (TPD) technique, we have observed that a number of hydrocarbon transformations, such as dehydrogenation and H-D exchange, occur on both palla-

Reactions of linear alkenes with metal surfaces have been previously studied on model catalyst systems with varying degrees of complexity, from single crystals to metal particles deposited on well-ordered oxide films. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] Based on these studies, the majority of which were conducted with ethene, it has been suggested that alkene hydrogenation is structure insensitive. This means that the reaction only depends on the number of metal atoms on the surface and is independent of the crystallographic orientation of the particle facets and particle size. These size effects are difficult to study in real catalytic systems because the mean particle size and distribution cannot be controlled accurately. Herein, we report the particle size effects on alkene reactivity (ethene and trans-2-pentene) using well-defined Pd/Al 2 O 3 model catalysts, where the particle size can be varied in a controllable manner. Temperature-programmed desorption (TPD) indicated that alkene adsorption exhibits site-specific behavior, which is assigned to the reactions occurring separately on facets and on low-coordination-number atoms such as edge and corner atoms. A strong particle size effect (within the 1-5-nm range) is observed for the hydrogenation of pentene over a hydrogen-precovered surface, whereas the reaction for ethene is independent of size. These effects are explained by the reactions proceeding via di-s-bonded pentene, which is favored on the terrace sites of large particles, and p-bonded ethene.The morphology of the Pd model system has been studied intensively in our laboratory by scanning tunneling microscopy. [9,16] Thin alumina films were grown on a clean NiAl(110) single crystal, and subsequently Pd was vapordeposited onto the films (see the Experimental Section for details). An average particle size was determined from the amount of Pd deposited and could be varied within the range 1-5 nm with a narrow particle-size distribution (20 %); [16] the corresponding relationship is presented in reference [9]. To exclude any effects of morphology changes on Pd particles during TPD measurements, the samples were preannealed at 500 K before the adsorption experiments.

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Zeolite Y, with a Si/Al ratio 3.1, was prepared using Iraqi kaolin and tested as a catalyst in the liquid-phase esterification of oleic acid (a simulated free fatty acid frequently used as a model reaction for biodiesel production). XRD confirmed the presence of the characteristic faujasite structure of zeolite Y, and further analysis was conducted using BET adsorption, FTIR spectroscopy, XRF, DLS particle size and SEM. A range of experimental conditions were employed to study the reaction; alcohol/oleic acid molar ratio, temperature, and catalyst mass loading. The optimum conditions for the reaction were observed at 70 °C, 5 wt% catalyst loading and 6:1 ethanol to oleic acid molar ratio. The oleic acid conversion using the zeolite prepared from kaolin was 85% after 60 min, while the corresponding value for a commercial sample of HY zeolite was 76%. Our findings show that low Si/Al ratio zeolite Y is a suitable catalyst for esterification, which is in contrast to the widespread view of the unsuitability of zeolites, in general, for such applications.

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Die Reaktionen linearer Alkene mit Metalloberflächen sind bereits anhand von unterschiedlich komplexen Modellkatalysatorsystemen studiert worden -von Einkristallen bis zu trägerfixierten Metallnanoteilchen auf geordneten Oxidfilmen.[

Trägerfixierte Pd‐Nanoteilchen katalysieren die Hydrierung von Alkenen unter Vakuumbedingungen, Pd‐Einkristalle sind dagegen inaktiv. Entscheidend für die unterschiedliche Aktivität ist die Verfügbarkeit des unterhalb der Oberfläche gebundenen Wasserstoffs (siehe Bild), der in den Pd‐Nanoteilchen schwächer gebunden und leichter zugänglich ist als in Einkristallen.