Growth and structural stability of well-ordered PdZn alloy nanoparticles

Abstract

Despite many recent attempts to unravel the structure of novel PdZn alloys, promising catalysts in methanol steam reforming, a detailed study on the formation of Pd-Zn alloy particles and of their structural and thermal stability is still missing. We take advantage of the unique properties of epitaxially grown Pd particles embedded in layers of amorphous ZnO and mechanically stabilized by SiO 2 , and present an electron microscopy study of the preparation of well-ordered PdZn alloy nanoparticles at surprisingly low reduction temperatures. They are formed by topotactic growth on the surface of the Pd nanoparticles and are structurally and thermally stable in a broad temperature regime (473 -873 K). At and above 873K, partial decomposition of PdZn and beginning interaction with the SiO 2 support has been observed. Keywords:Thin film model catalyst, hydrogen reduction, alloy formation, electron microscopy, selected area electron diffraction ManuscriptMuch recent effort has been invested in the development of suitable catalysts for methanol steam reforming 1-15 as this is one of the most promising processes for hydrogen production with a high hydrogen-to-carbon ratio 16 . Cu/ZnO catalysts have been commercially used to produce hydrogen with high selectivity and activity 2-6 , but they suffer from deactivation at reaction temperatures above 573 K 17 . Recently, novel Pd/ZnO systems 3-15 (as well as Pd/Ga 2 O 3 and Pd/In 2 O 3 18,19 ) have attracted more attention because of their enhanced long-term and thermal stability 20,21 . As unsupported pure Pd exhibits only a poor selectivity 22 , the observed high activity and selectivity for CO 2 formation was ascribed to the formation of distinct PdZn, PdIn and PdGa alloys upon reductive activation at elevated temperatures 18 . Best characterized is the Pd/ZnO system, where alloy formation has been studied and confirmed by X-ray diffraction (XRD) 18,23 , temperature-programmed reduction (TPR) 18,23 and X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS) [24][25][26] . Iwasa et al. 23 observed PdZn alloy formation upon reduction at very low T (≥ 473 K). Density functional studies revealed the close relationship between the electronic structure of PdZn and Cu-based catalysts giving rise to a similar catalytic performance in methanol steam reforming 27,28 . Comparatively few studies have been carried out on the structural characterization of the PdZn alloys by electron microscopy, and these were limited to overview imaging of powder catalysts in the as-prepared state and after hydrogen reduction, thereby mainly supporting XRD measurements 29,30 . The Pd-Zn system is known to form several stable bulk alloy phases 31 . The most important and thermally most stable is the PdZn (β1) phase, which crystallizes in a tetragonal (AuCu-type) L1 0 structure 32 . A key point for understanding the catalytic pecularities of the above-mentioned Pd-Zn alloy particles is also to determine their surface composition. Recent experiments in our laboratory 33 show that a well-orde...

From Oxide‐Supported Palladium to Intermetallic Palladium Phases: Consequences for Methanol Steam Reforming

“…In practice, it has been shown that a transition from metal particles, which consist of one component that is in contact with an oxidic support, toward supported bimetallic phases can be induced by pre-treatment with a reductive catalyst. [1,[31][32][33][34][35][36][37][38][39][40][41][42] In specific cases (e.g. PdÀZn catalysts), even a highly ordered bimetallic compound can be formed by reduction in H 2 , which may be a necessary prerequisite for the most selective and active state of the respective catalyst.…”

Hydrogen Production by Methanol Steam Reforming on Copper Boosted by Zinc‐Assisted Water Activation

“…Apart from advanced copper-based catalysts, [1,2] special attention is presently paid to the highly MSR-selective reduced state of Pd/ZnO, [3] containing a particularily stable intermetallic PdZn (1:1) active phase. [3,4] Therefore, we recently studied related "inverse" near-surface PdZn intermetallic phases, showing that three-dimensional PdZn active site ensembles are equally important for selective dehydrogenation of methanol (thus avoiding CO) and for efficient water activation. [5] For the less costly Cu/ZnO catalysts, originally designed for methanol synthesis, improvements towards a technical MSR application regarding sintering stability, pyrophoricity, and selectivity are still required.…”

Steigerung der Wasserstoffproduktion in der Methanol‐Dampfreformierung auf Kupfer durch Zink‐unterstützte Wasseraktivierung

“…Um die technischen Probleme und Risiken zu vermeiden, die mit der Benutzung, Lagerung und dem Transport von Wasserstoff verbunden sind, bietet sich Methanol als praktischer und verfügbarer Energieträger für die Bereitstellung von H 2 an. [3,4] Aus diesem Grund haben wir kürzlich "inverse" oberflächennahe intermetallische PdZn-Phasen studiert und gezeigt, dass dreidimensionale aktive/selektive PdZn-Ensembles sowohl die selektive Dehydrierung von Methanol steuern (wodurch die Produktion von CO verhindert wird) als auch eine effektive Wasseraktivierung bewirken. Wasserstoff kann durch die katalytische Methanoldampfreformierung (MSR) aus Methanol erzeugt werden: CH 3 OH + H 2 O!CO 2 + 3 H 2 .…”

“…hat aktuell auch der hoch MSRaktive reduzierte Zustand von Pd/ZnO, [3] mit einer speziell stabilen intermetallischen 1:1-PdZn-Phase, Aufmerksamkeit erregt. [3,4] Aus diesem Grund haben wir kürzlich "inverse" oberflächennahe intermetallische PdZn-Phasen studiert und gezeigt, dass dreidimensionale aktive/selektive PdZn-Ensembles sowohl die selektive Dehydrierung von Methanol steuern (wodurch die Produktion von CO verhindert wird) als auch eine effektive Wasseraktivierung bewirken. [5] Die billigeren Cu/ZnO-Katalysatoren, ursprünglich für die Metha-nolsynthese entwickelt, bedürfen noch einiger Verbesserungen in Bezug auf Sinterstabilität, Selbstentzündung und Selektivität, um für die technische Reformieranwendung geeignet zu sein.…”