Oxide Supported Pd Nanoparticles Research Articles

Modulation of the superficial electronic structure via metal-support interaction for H2 evolution over Pd catalysts.

Electronic interactions can radically enhance the performance of supported metal catalysts and are critical for fundamentally understanding the nature of catalysts. However, at the microscopic level, the details of such interactions tuning the electronic properties of the sites on the metal particle's surface and metal–support interface remain obscure. Herein, we found polarized electronic metal–support interaction (pEMSI) in oxide-supported Pd nanoparticles (NPs) describing the enhanced accumulation of electrons at the surface of NPs (superficial Pdδ−) with positive Pd atoms distributed on the interface (interfacial Pdδ+). More superficial Pdδ− species mean stronger pEMSI resulting from the synergistic effect of moderate Pd–oxide interaction, high structural fluxionality and electron transport activity of Pd NPs. The surface Pdδ− species are responsible for improved catalytic performance for H2 evolution from metal hydrides and formates. These extensive insights into the nature of supported-metal NPs may open new avenues for regulating a metal particle's electronic structure precisely and exploiting high-performance catalysts.

Supports and modified nano-particles for designing model catalysts.

In order to design catalytic materials, we need to understand the essential causes for material properties resulting from its composite nature. In this paper we discuss two, at first sight, diverse aspects: (a) the effect of the oxide-metal interface on metal nanoparticle properties and (b) the consequences of metal particle modification after activation on the selectivity of hydrogenation reactions. However, these two aspects are intimately linked. The metal nanoparticle's electronic structure changes at the interface as a catalyst is brought to different reaction temperatures due to morphological modifications in the metal and, as we will discuss, these changes in the chemistry lead to changes in the reaction path. As the morphology of the particle varies, facets of different orientations and sizes are exposed, which may lead to a change in the surface chemistry as well. We use two specific reactions to address these issues in some detail. To the best of our knowledge, the present paper reports the first observations of this kind for well-defined model systems. The changes in the electronic structure of Au nanoparticles due to their size and interaction with a supporting oxide are revealed as a function of temperature using CO2 activation as a probe. The presence of spectator species (oxopropyl), formed during an activation step of acrolein hydrogenation, strongly controls the selectivity of the reaction towards hydrogenation of the unsaturated C[double bond, length as m-dash]O bond vs. the C[double bond, length as m-dash]C bond on Pd(111) when compared with oxide-supported Pd nanoparticles.

Density Functional Calculations and IR Reflection Absorption Spectroscopy on the Interaction of SO2 with Oxide-Supported Pd Nanoparticles

A systematic study on the interaction of sulfur dioxide (SO2) on BaO-supported Pd nanoparticles has been carried out using suitable models and state-of-the-art density functional (DF) calculations. Detailed information concerning the structure and energetics of the different conformations of adsorbed SO2 is provided as a function of coverage together with calculated infrared reflection absorption spectroscopy (IRAS) spectra. SO2 may adsorb on Pd(111) in several conformations, some active, η2-SbOa and η1-Sb, and others inactive in IRAS, η3-SaOaOa. SO2 is found to attach stronger to Pd nanoparticle edges and corners, a fact intimately related to catalyst poisoning by site blocking. On Pd nanoparticles, SO2 is found to preferably adopt adsorption conformations that depend on the specific region on the nanoparticle, thus adding site specificity to vibrational recognition. Molecular beam experiments and IRAS have been performed on a single-crystal-based Pd/BaAl2xO1+3x/NiAl(110) model NOx storage and reduction catalyst and its individual components. SOx formation on the oxide components, evolution of a SO2 multilayer, and adsorption of SO2 on BaO or Pd nanoparticles is linked to DF calculations. The effect of cation intermixing in the oxide support and overlap of absorption bands on the unequivocal discrimination of signals are discussed.

Adsorbate mobilities on catalyst nanoparticles studied via the angular distribution of desorbing products

Abstract Employing molecular beam methods, we have studied the angular distribution of products desorbing from oxide-supported Pd nanoparticles during catalytic oxidation of CO. A large range of particle sizes was covered by combining preparation techniques based on (1) metal evaporation and growth and (2) electron beam lithography. Whereas the CO 2 distribution is symmetric and independent of the reaction conditions on small Pd aggregates, a strong influence of the reactant incidence angles and fluxes was observed for large particles. For the first time, these observations experimentally verify the existence of local reaction rate variations on a catalyst nanoparticle as a consequence of limited adsorbate mobility. Furthermore, the results allow us to extract in situ information about surface diffusion rates.