Surface‐Bonded Precursor Determines Particle Size Effects for Alkene Hydrogenation on Palladium

Abstract

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.

Recent Advances in the Liquid‐Phase Synthesis of Metal Nanostructures with Controlled Shape and Size for Catalysis

“…Thus, p-bonded pentene are not in direct contact with Pd and desorbs intact, and hydrogenation proceeds only via di-s-bonded species, which are favored on the terrace sites of large particles. Pentene hydrogenation is thus structuresensitive (121). PVP-stabilized Pt nanoparticles with the sizes between 1.7 and 3.6 nm also confirmed structure insensitivity of ethylene hydrogenation (47).…”

“…Using well-defined Pd/Al 2 O 3 model catalysts, where the particle size was varied in a controllable manner, the size effect in ethylene and trans-2-pentene hydrogenation was studied (121). Ethylene was adsorbed as a p-bonded species irrespectively of the surface composition explaining its hydrogenation structure insensitivity.…”

Acetylene and Ethylene Hydrogenation on Alumina Supported Pd-Ag Model Catalysts

“…Under both conditions, the ethylene hydrogenation reaction was found to be structure insensitive. In contrast, hydrogenation of 2-pentenes exhibited a significant particle size effect [19]. These studies have demonstrated that metal particles deposited on well-ordered oxide films are suitable model systems for studying mechanisms of the hydrogenation reactions o n supported metal particles.…”

“…The low temperature (LT) state(s) can be attributed to the recombination of surface H species, which are more weakly bonded to the surface in the presence of sub-surface hydrogen. The sub-surface hydrogen is available for the reaction on the particles due to their limited size in contrast to Pd crystals where hydrogen may migrate into the bulk far away from the surface [16,17,19,22,23]. Upon heating of the Pd particles, surface H species are re-populated by the H atoms from the sub-surface region, finally leading to the situation where sub-surface hydrogen is depleted and the desorption becomes similar to the case of adsorption at 300 K.…”

Effect of Pd Nanoparticle Size on the Catalytic Hydrogenation of Allyl Alcohol

“…to size effects in hydrogenation reactions focus almost exclusively on supported (heterogeneous) Pd catalysts. [14][15][16] In addition to catalyst size, the preparation and structure of nanoparticles are also important factors that must be taken into account when comparing catalytic activity. This is because preparation methods, stabilizing ligands, and polydispersity can lead to activity changes that may mask true particle size effects.…”