Distinguishing surface adsorbed hydrogen from bulk-dissolved hydrogen in supported Pd nanoparticles using in situ Pd L₃-edge XANES

Abstract

Abstract. In situ X-ray absorption spectroscopy at the Pd L 3 edge was applied to determine the particle size effect on the formation of palladium hydride and on surface hydrogen adsorption at room temperature. Pd L 3 edge XANES spectra allow direct detection of hydride formation via a characteristic spectral feature caused by the formation of a Pd-H anti-bonding state. This feature showed strong particle size dependence. The L 3 edge spectra were reproduced using full multiple scattering analysis and density of states calculations and the contributions of bulkdissolved and surface hydrogen to the XANES spectra could be distinguished. The ratio of hydrogen on the surface versus that in the bulk increased with decreasing particle size, and smaller particles dissolved less hydrogen. IntroductionThe particle size and support effects on catalytic performance are two well-known factors that affect the activity and selectivity of a reaction. For hydrogenation reaction catalyzed by palladium catalysts, the particle size effect can be critical because the formation of palladium hydride is particle size dependent. The amount of hydride that forms decreases with increasing dispersion of palladium in supported catalysts [1]. In contrast to the bulk metal, palladium particles smaller than 2.6 nm have been suggested to not form a hydride phase, even at high hydrogen pressures (P H2 =1 atm and T =298 K) [2]. The hydride has higher mobility than the surface hydrogen and can hydrogenate surface adsorbates upon emerging to the surface [3]. Thus, the amount of bulk-dissolved hydrogen seriously affects the activity and selectivity of structure-sensitive hydrogenation reactions [4].Geometrical and electronic information of palladium nano-particles and of palladium hydride can be obtained from X-ray absorption spectroscopy (XAS) at the K and L edges [5,6]. At the Pd K edge XAS, the formation of palladium hydrides was generally deduced from an increased interatomic distance in EXAFS analysis [5]. In contrast, Pd L 3 edge XANES allows the direct detection of palladium hydride via a signature peak at ~6 eV above the Fermi level, caused by the formation of a Pd-H anti-bonding state [6]. This ~6 eV peak can be theoretically simulated [7]. Distinction of bulkdissolved hydrogen from surface hydrogen in a palladium-catalyzed process remains challenging. It was recently reported that nuclear reaction analysis can distinguish surface adsorbed and bulkdissolved hydrogen on or in palladium nanocrystals on Al 2 O 3 NiAl (110) [8]. We performed XAS at the Pd L 3 edge to determine the electronic structural changes that occur in supported nano-sized

Comparative in Operando Studies in Heterogeneous Catalysis: Atomic and Electronic Structural Features in the Hydrogenation of Ethylene over Supported Pd and Pt Catalysts

“…The r -factors are typically smaller than 0.01, indicative of the overall good fitting quality. The structural parameters obtained for the initial hydrogen-rich atmospheres are in good agreement with the results of an earlier EXAFS study (at the L 3 -edge) of Pd/SiO 2 particles under similar conditions …”

“…Palladium incorporates hydrogen at random interstitial sites, causing lattice expansion and smaller distortions, but no transitions in the crystalline structure . This has been confirmed in several EXAFS studies of supported Pd particles. ,,, …”

“…The structural parameters obtained for the initial hydrogen-rich atmospheres are in good agreement with the results of an earlier EXAFS study (at the L 3 -edge) of Pd/SiO 2 particles under similar conditions. 83 The increased bond distances found in the hydrogen-rich atmospheres as compared to bulk palladium, the constant coordination numbers, and the increased Debye−Waller factors as compared to the ethylene-rich atmospheres are indicative of the formation of a palladium hydride phase. Palladium incorporates hydrogen at random interstitial sites, causing lattice expansion and smaller distortions, but no transitions in the crystalline structure.…”

“…58 This has been confirmed in several EXAFS studies of supported Pd particles. 63,64,69,83 The deintercalation and intercalation of hydrogen in the hydrogen-rich atmospheres during the forward and backward sequence, respectively, can be directly followed by the changes occurring in the bond distance. With the lattice expansion Δa and the bulk metal lattice constant a 0 the hydrogen content n of the palladium hydride phase PdH n can be determined via the subsequent equation, which has been shown to be valid over large pressure and temperature ranges: 84,85…”