The reactivity of several Pd-O species toward CO oxidation was compared experimentally, making use of chemically, structurally and morphologically different model systems such as single-crystalline Pd(111) covered by adsorbed oxygen or a Pd(5)O(4) surface oxide layer, an oriented Pd(111) thin film on NiAl oxidized toward PdO(x) suboxide and silica-supported uniform Pd nanoparticles oxidized to PdO. The oxygen reactivity decreased with increasing oxidation state: O(ad) on metallic Pd(111) exhibited the highest reactivity and could be reduced within a few minutes already at 223 K, using low CO beam fluxes around 0.02 ML s(-1). The Pd(5)O(4) surface oxide on Pd(111) could be reacted by CO at a comparable rate above 330 K using the same low CO beam flux. The more deeply oxidized Pd(111) thin film supported on NiAl was already much less reactive, and reduction in 10(-6) mbar CO at T > 500 K led only to partial reduction toward PdO(x) suboxide, and the metallic state of Pd could not be re-established under these conditions. The fully oxidized PdO nanoparticles required even rougher reaction conditions such as 10 mbar CO for 15 min at 523 K in order to re-establish the metallic state. As a general explanation for the observed activity trends we propose kinetic long-range transport limitations for the formation of an extended, crystalline metal phase. These mass-transport limitations are not involved in the reduction of O(ad), and less demanding in case of the 2-D Pd(5)O(4) surface oxide conversion back to metallic Pd(111). They presumably become rate-limiting in the complex separation process from an extended 3-D bulk oxide state toward a well ordered 3-D metallic phase.

The adsorption and thermal desorption of Zn and ZnO on Pd(111) was studied in the temperature range between 300 and 1300 K with TDS, LEED, and CO adsorption measurements. At temperatures below 400 K, multilayer growth of Zn metal on the Pd(111) surface takes place. At a coverage of 0.75 ML of Zn, a p(2 x 2)-3Zn LEED structure is observed. Increasing the coverage to 3 ML results in a (1 x 1) LEED pattern arising from an ordered Zn multilayer on Pd(111). Thermal desorption of the Zn multilayer state leads to two distinct Zn desorption peaks: a low-temperature desorption peak (400-650 K) arising from upper Zn layers and a second peak (800-1300 K) originating from the residual 1 ML Zn overlayer, which is more strongly bound to the Pd(111) surface and blocks CO adsorption completely. Above 650 K, this Zn adlayer diffuses into the subsurface region and the surface is depleted in Zn, as can be deduced from an increased amount of CO adsorption sites. Deposition of >3 ML of Zn at 750 K leads to the formation of a well-ordered Pd-Zn alloy exhibiting a (6 x 4 square root 3/3)rect. LEED structure. CO adsorption measurements on this surface alloy indicate a high Pd surface concentration and a strong reduction of the CO adsorption energy. Deposition of Zn at T > 373 K in 10(-6) mbar of O2 leads to the formation of an epitaxial (6 x 6) ZnO overlayer on Pd(111). Dissociative desorption of ZnO from this overlayer occurs quantitatively both with respect to Zn and O2 above 750 K, providing a reliable calibration for both ZnO, Zn, and oxygen coverage.

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...

The formation, thermal decomposition, and reduction of small PdO particles were studied by high-resolution transmission electron microscopy and selected area electron diffraction. Welldefined Pd particles (mean size of 5-7 nm) were grown epitaxially on NaCl (001) surfaces and subsequently covered by a layer of amorphous SiO 2 (25 nm), prepared by reactive deposition of SiO in 10 -2 Pa O 2 . The resulting films were exposed to molecular O 2 in the temperature range of 373-673 K, and the growth of PdO was studied. The formation of a PdO phase starts at 623 K and is almost completed at 673 K. The high-resolution experiments suggest a topotactic growth of PdO crystallites on top of the original Pd particles. Subsequent reaction of the PdO in 10 mbar CO for 15 min and thermal decomposition in 1 bar He for 1 h were also investigated in the temperature range from 373 to 573 K. Reductive treatments in CO up to 493 K do not cause a significant change in the PdO structure. The reduction of PdO starts at 503 K and is completed at 523 K. In contrast, PdO decomposes in 1 bar He at around 573 K. The mechanism of PdO growth and decay is discussed and compared to results of previous studies on other metals, e.g., on rhodium.