A Combined Density-Functional and IRAS Study on the Interaction of NO with Pd Nanoparticles: Identifying New Adsorption Sites with Novel Properties

Abstract

Nanocrystalline particles expose special adsorption sites close to edges and corners, giving rise to novel adsorption and reaction properties. The spectroscopic identification of these sites represents a great challenge, however. Here, we present results of a combined experimental and theoretical study on the adsorption of NO on Pd nanoparticles, using infrared reflection absorption spectroscopy (IRAS) and calculations based on densityfunctional theory (DFT). This approach facilitates identification of the adsorption sites available on the nanoparticles and reveals detailed information on their bonding properties, on the vibrational parameters of NO adsorbed on these sites, and on their sequence of occupation. With respect to all these aspects, the adsorption behavior of NO on the Pd nanoparticles notably differs from any single crystal reference data available. The IRAS studies are performed on well-defined Pd nanoparticles supported on an ordered Al 2 O 3 film on NiAl(110). The growth and structure of these particles has been characterized previously, predominately exposing (111) and a small fraction of (100) facets. Here, we systematically monitor the NO adsorption as a function of exposure in a temperature region between 100 and 300 K by means of time-resolved IRAS in combination with molecular beam (MB) dosing. We interpret these experimental data with the help of DFT calculations on the adsorption of NO on unsupported cuboctahedral Pd n clusters cut from Pd bulk and containing up to 140 atoms; for comparison, calculations of the reference adsorption complexes of NO on single-crystal Pd(111) surface have also been performed. NO molecules are shown to most favorably adsorb on hollow µ 3 -sites on (111) facets of Pd n clusters, closely followed by bridge µ 2 -sites at the edges between adjacent (111) facets. Both sites give rise to characteristic features in the vibrational spectrum and are populated sequentially. At higher coverage (and low temperature) on-top µ 1 -sites on the (111) facets begin to be occupied. At variance with the adsorption on Pd(111) surface, however, additional on-top-sites are available at the particle edges and corners, which reveal stronger NO adsorption. In spite of the strong adsorption in bridge (µ 2 ) coordination geometry at edges, our calculations predict that intermolecular repulsion between adjacent µ 2 -NO species gives rise to the formation of mixed bridge/on-top structures at high coverage. Similarly to the bridge NO at particle edges, the edge-and corner-related µ 1 -NO species reveal characteristic vibrational frequencies, allowing for direct verification of this prediction by IRAS. The present results make possible the identification and monitoring of the occupation of specific sites on Pd nanoparticles by NO during adsorption and reaction processes.

The Effect of Gamma-Al2O3Support on the NO Adsorption on Pd4Cluster

“…Adsorption in the bent arrangement favors overlap between the d z 2 orbital on the palladium atom and the polarized 2π* orbital of NO. 52 Reflecting the main type of interaction between the NO molecule and the supported palladium cluster, the total NBO charge density on the NO molecule is positive for all adsorption sites. Therefore, in general, there is charge transfer from the NO molecule to the palladium cluster, with higher values for charge transfer found for the adsorption sites Pd (45) and Pd (47).…”

Hydrogen Diffusion into Palladium Nanoparticles: Pivotal Promotion by Carbon

“…It has been proposed that carbon may decrease the activation barrier for the subsurface H(D) diffusion, thus promoting persistent hydrogenation. [4] To examine this hypothesis, we performed density-functional calculations on model cuboctahedral Pd 79 nanoparticles, which were shown to be representative for realistic description of surface interactions present on larger PdNPs experimentally studied in model catalysts, [9,10] especially for sites near particle edges. Comparison with Pd(111) slab models consisting of the surface unit cell (3 3) and six atomic layers of Pd, namely Pd(111)9 6 L, has been made to clarify importance of the mobility of surface palladium atoms for subsurface diffusion of adsorbed hydrogen.…”

Simulation of Near Edge X-ray Absorption Fine Structure (NEXAFS) Measurements of CO on Supported Pd Nanoparticles

“…The particle was constructed using parameters (M, m, s) = (12,8,4) and has an average diameter of 5.1 nm and a height of 0.81 nm. In the simulation, the incident photon beam (P) is linearly, p-polarized (hence b = 0°) with its direction always fixed at P = (1, h P , / P ) = (1, 180°, 180°).…”

“…For instance, one can employ scanning probes (scanning tunneling microscopy (STM) [1], atomic force microscopy [2]), electron microscopy (X-ray photoemission electron microscopy [3], transmission electron microscopy [4], diffraction (photoelectron diffraction [1]) and scattering techniques (grazing-incidence small angle X-ray scattering [5,6]) to monitor the morphology of the nanoparticles. To probe molecular adsorbates on nanoparticles, one can use STM to monitor their adsorption sites [7], infrared absorption spectroscopy to investigate their vibrational modes at different sites [8], and microcalorimetry to measure their sticking coefficients as well as adsorption energies on the nanoparticles [9]. To determine molecular orientations on nanoparticles, near-edge X-ray absorption fine structure (NEXAFS) is one of the few experimental tools available.…”