Nature of the N-Pd interaction in nitrogen-doped carbon nanotube catalysts

In this work, the geometric and electronic structure of N species in N-doped carbon nanotubes (NCNTs) is derived by X-ray photoemission (XPS) and absorption spectroscopy (NEXAFS) of the N 1s core excitation. Substitutional N species in pyridine-like configuration and another form of N with higher thermal stability are found in NCNTs. The structural configuration of the high thermally stable N species, in the literature often referred to as graphitic N, is assessed in this work by a combined theoretical and experimental study as a 3-fold substitutional N species in an NCNT basic structural unit (BSU). Furthermore, the nature of the interaction of those N species with a Pd metal center immobilized onto NCNTs is of σ-type donation from the filled π-orbital of the N atom to the empty d-orbital of the Pd atom and a π back donation from the filled Pd atomic d-orbital to the π* antibonding orbital of the N atom. We have found that the interaction of pyridine N with Pd is characterized by a charge transfer typical of a covalent chemical bond with partial ionic character, consistent with the chemical shift observed in the Pd 3d core level of divalent Pd. Graphitic N sites interact with Pd by a covalent bond without any charge redistribution. In this case, the electronic state of the Pd corresponds to metallic Pd nanoparticles electronically modified by the interaction with the support. The catalytic reactivity of these samples in hydrogenation, CO oxidation, and oxygen reduction reaction (ORR) allowed clarifying some aspects of the metal carbon support interaction in catalysis

Die Oberflächendynamik des Pd2Ga und seine Reaktivität in der Flüssigphasenhydrierung von Phenylacetylen

Intermetallische Verbindungen des Pd-Ga Systems zeigen eine hochselektive und stabile katalytische Aktivität in der Gasphasenhydrierung von Acetylen. Dies wurde mit einer, von Pd verschiedenen, wohlgeordneten Kristallstruktur und einer starken Modifikation der elektronischen Struktur begründet sowie einer hohen Stabilität unter in situ Bedingungen. In dieser Arbeit wurde die Stabilität der intermetallischen Verbindung Pd2Ga und ihre Eignung als Katalysator für die partielle Hydrierung von Phenylacetylen in flüssiger Phase untersucht. Pd2Ga ist inert gegenüber Hydridbildung und das Volumen wird auch in einer Atmosphäre aus 20% O2 in He nicht unter 300 °C angegriffen, wie durch in situ XRD und in situ DTA/TG/MS Messungen gezeigt werden kann. Allerdings wurde, abweichend von bisherigen Annahmen, eine deutliche Diskrepanz zwischen der Struktur des Volumens und der Oberfläche von Pd2Ga festgestellt. Mit Hilfe von HR-TEM sowie XPS konnte nachgewiesen werden, dass selbst Spuren von H2O oder O2 intermetallisches Ga unter partieller Zersetzung von Pd2Ga zu oxidieren vermögen. Mechanische Belastung induziert Segregation von Ga des Volumens an die Oberfläche, welches anschließend oxidiert. Die Anwesenheit von Pd bewirkt eine hohe Reversibilität dieser Prozesse, wodurch die jeweilige elektronische Struktur der Oberfläche von Pd2Ga stark von der Vorbehandlung und der umgebenden Atmosphäre abhängt. Eine Rekonstruktion der intermetallischen Verbindung auf der Oberfläche findet unter den Bedingungen der Flüssigphasenhydrierung nicht statt, wird jedoch durch eine Hochtemperaturvorbehandlung in einer 5% H2/Ar-Atmosphäre erreicht unter Ausprägung einer nur sehr dünnen Passivierungsschicht. Gegenüber Pd zeigt Pd2Ga eine erwartete, deutlich verminderte Hydrieraktivität, die Oberfläche oxidiert allerdings auch unter diesen Bedingungen. Unter weitestgehendem Ausschluss von Spuren von H2O und O2 wird die Oxidation verhindert, es findet in diesem Fall jedoch ein Angriff auf das Volumen von Pd2Ga durch Phenylacetylen statt. Die Ergebnisse zeigen beispielhaft, dass eine gezielte starke elektronische Modifizierung eines Übergangsmetalles mit einem unedlen Metall eine verringerte Stabilität der Oberfläche bewirkt. Eine Terminierung der Volumenstruktur an der Oberfläche kann, wenn überhaupt, nur unter stark reduzierenden Bedingungen erhalten werden.Intermetallic compounds of the Pd-Ga system provide a highly selective and stable catalytic activity in the gas phase hydrogenation of acetylene, which was explained by the modified, well-ordered crystal structures and electronic structures as well as their high in situ stability, as compared to Pd. In this work the stability of the bulk and the surface of Pd2Ga and its reactivity towards the partial hydrogenation of phenylacetylene in the liquid phase were investigated. Pd2Ga is resistant against hydride formation and the bulk of the material is not affected in a 20% O2/He atmosphere at temperatures below 300 °C. However, in contrast to former suggestions, a considerable discrepancy between the bulk and the surface structure of Pd2Ga was found. By means of HR-TEM and XPS evidence was found that traces of O2 or H2O are sufficient for the oxidation of intermetallic Ga at the surface leading to partial decomposition of Pd2Ga. Mechanical load induces Ga segregation from the bulk to the surface with subsequent oxidation. The presence of Pd explains the relatively high reversibility of these processes, which ultimately leads to strong dependence of the surface structure on the pre-treatment and surrounding atmosphere. A re-formation of the intermetallic surface does not take place under the conditions of the liquid phase hydrogenation but is only achieved by a high temperature reduction in a 5% H2/Ar atmosphere, resulting in the formation of only a very thin passivation layer. Compared to Pd, Pd2Ga shows an expected, significantly lowered hydrogenation activity. Though, the surface oxidizes even under these conditions. Under the exclusion of traces of O2 and H2O to the greatest possible extent, the oxidation is prevented, however, in this case even the bulk material is attacked by phenylacetylene. The results exemplify, that a well-defined strong modification of a transition metal with a less noble metal causes a considerably lowered stability. A direct termination of the bulk structure at the surface can be - if at all - achieved only under strongly reducing conditions

Synthesis and Catalytic Properties of Nanoparticulate Intermetallic Ga-Pd Compounds

A two-step synthesis for the preparation of single-phase and nanoparticulate GaPd and GaPd(2) by coreduction of ionic metal-precursors with LiHBEt(3) in THF without additional stabilizers is described. The coreduction leads initially to the formation of Pd nanoparticles followed by a Pd-mediated reduction of Ga(3+) on their surfaces, requiring an additional annealing step. The majority of the intermetallic particles have diameters of 3 and 7 nm for GaPd and GaPd(2), respectively, and unexpected narrow size distributions as determined by disk centrifuge measurements. The nanoparticles have been characterized by XRD, TEM, and chemical analysis to ensure the formation of the intermetallic compounds. Unsupported nanoparticles possess high catalytic activity while maintaining the excellent selectivity of the ground bulk materials in the semihydrogenation of acetylene. The activity could be further increased by depositing the particles on α-Al(2)O(3)

Quantitative High-Angle Annular Dark-Field Scanning Transmission Electron Microscope (HAAF-STEM)Tomography and High Resolution Electron Microscopy of Unsupported Intermetallic GaPd₂ Catalysts

Apart from describing the occurrence and detailed crystallographic nature of novel five-fold twinned nanoparticles (<5 nm) of the selective hydrogenation catalyst GaPd2, the main thrust of this work is to demonstrate a method of characterizing, by electron tomography, the structural morphologies of large agglomerates, consisting of ca. 1800 nanoparticles, the individual sizes of which fall in the range of 1–30 nm in equivalent diameter. The so-called segmentation of electron tomograms, usually evaluated manually and vulnerable to subjective bias (as well as being laborious) is carried out by utilizing sophisticated, yet readily implementable, image processing techniques that facilitate versatile 3D nanometrological analysis. Such procedures will play an important role in the move toward quantitative 3D characterization at the nanoscale and are applicable to numerous other systems of technological and catalytic interest (such as fuel-cell electrodes) where there are agglomerates of nanoparticles and nanoclusters of various compositions, including those that are supported on high-surface-area solids

Surface dynamics of the intermetallic catalyst Pd2Ga, Part i Structural stability in UHV and different gas atmospheres

The structural and electronic properties of unsupported Pd2Ga were investigated after different pre-treatments. Pd2Ga provides with respect to elemental Pd a significantly modified electronic structure with its d-band center being shifted away from the Fermi level. It was found that the electronic structure of the surface depends strongly on its pre-treatment and on the chemical environment. We report a detailed bulk and surface characterization of the intermetallic compound by means of XRD, DTA/TG/MS, SEM, XPS, and HR-TEM. At moderate temperatures, the bulk of Pd2Ga is chemically resistant against H-2 or O-2 atmosphere and against mechanical load. Contrariwise its surface is highly sensitive against even traces of oxidizing agents, leading quickly to a disparity between bulk and surface structure and composition. The reversibility of this dynamic effect depends on the degree of decomposition and on the sample history. An almost pure intermetallic surface can only be achieved in highly reducing atmospheres. (C) 2013 Elsevier Inc. All rights reserved