The formation of a PdZn alloy from a 4.3% Pd/ZnO catalyst was characterized by combined in situ high-resolution X-ray diffraction (HRXRD) and X-ray absorption spectroscopy (XAS). Alloy formation started already at around 100 °C, likely at the surface, and reached the bulk with increasing temperature. The structure of the catalyst was close to the bulk value of a 1:1 PdZn alloy with a L1 o structure (R PdÀPd = 2.9 Å, R PdÀZn = 2.6 Å, CN PdÀZn = 8, CN PdÀPd = 4) after reduction at 300 °C and above. The activity of the gas-phase hydrogenation of 1-pentyne decreased with the formation of the PdZn alloy. In contrast to Pd/SiO 2 , no full hydrogenation occurred over Pd/ZnO. Over time, only slight decomposition of the alloy occurred under reaction conditions.

The particle size effect on the formation of palladium hydride and on surface hydrogen adsorption was studied at room temperature using in situ X-ray absorption spectroscopy at the Pd K and L3 edges. Hydride formation was indirectly observed by lattice expansion in Pd K edge XANES spectra and by EXAFS analysis. Hydride formation was directly detected in the L3 edge spectra. A characteristic spectral feature caused by the formation of a Pd−H antibonding state showed strong particle size dependence. The L3 edge spectra were reproduced using full multiple scattering analysis and density of state calculations, and the contributions of bulk absorbed 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.

The catalytically active phase of silica-supported palladium catalysts in the selective and non-selective hydrogenation of 1-pentyne was determined using in situ X-ray absorption spectroscopy at the Pd K and L(3) edges. Upon exposure to alkyne, a palladium carbide-like phase rapidly forms, which prevents hydrogen to diffuse into the bulk of the nano-sized particles. Both selective and non-selective hydrogenation occur over carbided particles. The palladium carbide-like phase is stable under reaction conditions and only partially decomposes under high hydrogen partial pressure. Non-selective hydrogenation to pentane is not indicative of hydride formation. The palladium carbide phase was detected in the EXAFS analysis and the K edge XANES showed representative features.

No 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

No abstract

No abstract