Growth and structure of Pd films on ZnO(0001)

The growth and structure of Pd films on ZnO(0001) were investigated using high resolution electron energy loss spectroscopy, x-ray photoelectron spectroscopy, and low energy electron diffraction. Vapor deposited Pd films at 300 K were found to follow a two-dimensional (2D) island growth mode, in which 2D metal islands are formed up to a critical coverage at which point growth occurs primarily in a layer-by-layer fashion on top of the islands. Heating to only 350 K was found to be sufficient to induce partial agglomeration of Pd films into three-dimensional particles. In addition to causing further agglomeration into particles, heating to 700 K resulted in partial reduction of the ZnO surface and the formation of a PdZn alloy

Site requirements for the reactions of CH\u3csub\u3e3\u3c/sub\u3eSH and (CH\u3csub\u3e3\u3c/sub\u3e)\u3csub\u3e2\u3c/sub\u3eS\u3csub\u3e2\u3c/sub\u3e on ZnO(\u3csub\u3e10(1)Overbar 0\u3c/sub\u3e)

Temperature programmed desorption (TPD) was used to investigate the adsorption and reaction of CH3SH and (CH3)2S2 on the nonpolar (10(1) Overbar 0) surface of ZnO. Methanethiol was found to dissociate on the (10(1)Overbar 0) surface to produce adsorbed methylthiolates. The primary reaction pathways for the methylthiolates were methyl group transfer between adjacent thiolates to produce (CH3)2S at 510 K, and transfer of methyl groups to surface lattice oxygen to produce adsorbed methoxides which were oxidized to CH2O at 525 K and adsorbed formate. Dimethyldisulfide was found to dissociate via cleavage of the S-S bond to form adsorbed methylthiolates. The reaction pathways for thiolates produced in this manner were similar to those produced from CH3SH except for an additional low-temperature pathway for the production of CH2O. Comparison of the results obtained in this study to our previous study of the reaction of CH3SH and (CH3)2S2 on ZnO(0001) and published STM studies of ZnO (10(1) Overbar 0) and ZnO(0001) indicates that step edges are the active sites for the reaction of thiols and disulfides on these surface

TPD study of the reaction of CH\u3csub\u3e3\u3c/sub\u3eCH\u3csub\u3e2\u3c/sub\u3eSH and (CH\u3csub\u3e3\u3c/sub\u3eCH\u3csub\u3e2\u3c/sub\u3e)\u3csub\u3e2\u3c/sub\u3eS\u3csub\u3e2\u3c/sub\u3e on ZnO(0001) and ZnO

Temperature programmed desorption (TPD) was used to study the reaction of CH3CH2SH and (CH3CH2)2S2 on the (0001) and surface of ZnO. The interaction of these molecules with ZnO was found to be structure-sensitive. Both the thiol and disulfide adsorbed dissociatively on ZnO(0001) forming adsorbed ethylthiolate intermediates, while only molecularly on ZnO. This result indicates that exposed cation-anion site pairs and exposed cations are the active sites for dissociative adsorption of CH3CH2SH and (CH3CH2)2S2, respectively. Decomposition to produce ethylene and adsorbed sulfur atoms was the only reaction pathway observed for adsorbed ethylthiolates on the (0001) surface. This is in contrast to adsorbed methylthiolates which undergo coupling to produce dimethylsulfide and oxydesulfurization to produce aldehydes and adsorbed carboxylates

Novel SOFC anodes for the direct electrochemical oxidation of hydrocarbons

Recent developments in solid-oxide fuel cells (SOFC) that electrochemically oxidize hydrocarbon fuels to produce electrical power without first reforming them to H2 are described. First, the operating principles of SOFCs are reviewed, along with a description of state-of-the-art SOFC designs. This is followed by a discussion of the concepts and procedures used in the synthesis of direct-oxidation fuel cells with anodes based on composites of Cu, ceria, and yttria-stabilized zirconia. The discussion focuses on how heterogeneous catalysis has an important role to play in the development of SOFCs that directly oxidize hydrocarbon fuels

Interaction of Platinum Films with the (0001#) and (0001) Surfaces of ZnO

In this investigation, the growth, structure, and electronic properties of Pt films on the polar surfaces of ZnO were examined using high-resolution electron energy-loss spectroscopy (HREELS) and low-energhy, electron diffraction (LEED). The growth mode of vapor-deposited Pt films on ZnO(0001#) and ZnO(0001) at 300 K was found to be nearly layer-by-layer. The surfaces of Pt films produced in this manner exhibited hexagonal symmetry and were stable up to 600 K. At higher temperatures, the Pt agglomerated into particles which remained oriented with respect to the ZnO substrate. HREELS results indicate that there are only weak interactions at the Pt/ZnO(0001#) interface, while charge transfer and Schottky barrier formation occures at the Pt/ZnO(0001) interface

Systematic Studies of the Cathode-Electrolyte Interface in SOFC Cathodes Prepared by Infiltration

In this study, the effect of the morphology and ionic conductivity of the electrolyte material in SOFC composite cathodes is systematically studied. The specific surface area of prous yttria-stabilized zirconia (YSZ) scaffolds was varied by almost two orders of magnitude using different pore formers and surface treatment with hydrofluoric acid (HF). The effect of ionic conductivity on the performance of SOFC cathodes was studied for electrodes prepared by infiltration of 35 wt % LSF into 65% porous scandia-stabilized zirconia (ScSZ), YSZ, or yttria-alumina co-stabilized zirconia (YAZ) scaffolds of identical microstructure cathodes

Novel SOFC Anodes for the Direct Electrochemical Oxidation of Hydrocarbon

This paper describes recent developments in solid-oxide fuel cells (SOFC) that use Cu-based cermets as the anode for direct oxidation of hydrocarbon fuels, including liquids such as gasoline, to generate electrical power without the need for first reforming that fuel to H2. Cu-YSZ cermets were found to be stable in hydrocarbon environments, but exhibited low performance for direct oxidation. Reasonable power densities could only be achieved with the addition of a catalytic oxide, like ceria, with the Cu cermet. Electrochemical oxidation studies demonstrated that the initial products for reaction depend on the catalytic oxide. Finally, the effect of sulfur impurities in the fuel is discussed

Analysis of the Performance of the Electrodes in a Natural Gas Assisted Steam Electrolysis Cell

The performance of solid oxide electrolysis (SOE) cells while operating in the natural gas assisted steam electrolysis (NGASE) mode was evaluated. The SOE cells used yttria-stabilized-zirconia (YSZ) as the oxygen ion conducting electrolyte, Co–CeO2–YSZ as the H2–H2O electrode, and Pd-doped CeO2 YSZ source as the CH4-oxidation electrode. The cell electrochemical performance was evaluated as a function of the H2O/H2 ratio and the extent of conversion of CH4. The results of this study provide insight into the factors that control electrode performance and further demonstrate the viability of an NGASE cell for the production of H2

Determining the Ce\u3csub\u3e2\u3c/sub\u3eO\u3csub\u3e2\u3c/sub\u3eS-CeO\u3csub\u3ex\u3c/sub\u3e Phase Boundary for Conditions Relevant to Adsorption and Catalysis

The interaction of sulfur with ceria under highly reducing conditions was investigated. The phase boundary between CeO1.83 and Ce2O2S was determined for temperatures between 873 and 1073 K. This data was used to derive an empirical equation for ΔGfº of Ce2O2S in this temperature range. This equation along with thermodynamic data for cerium oxides and sulfides obtained form the literature was used to predict Ce-O-S phase diagrams at 873 and 973 K. These phase diagrams provide insight into the mechanism of the deactivation of ceria-based catalysts by sulfur under reducing conditions

Preparation of SOFC Anodes by Electrodeposition

Anodes for solid oxide fuel cells (SOFCs) have been prepared by electrodeposition of either Co or Ni into a layer of porous yttria-stabilized zirconia (YSZ), 60 µm thick. The YSZ, having 65% porosity, was prepared by tape casting with graphite pore formers and was attached to the dense YSZ electrolyte. After adding 10 vol % CeO2 by impregnation of aqueous solutions of CeNO3)3, followed by calcination at 723 K, the porous YSZ was made conductive by exposing it to n-butane at 1123 K to form a coating of carbon. As much as 40 vol % metal could be added to the porous layers, while the carbon could then be removed by exposing the anode to humidified H2 at SOFC operating temperatures. The ohmic losses in cells containing 40 vol % Co or 30 vol % Ni were unaffected by heating to 1173 K. Finally, a cell with 15 vol % Cu and 15 vol % Co was prepared by electrodeposition of Cu onto electrodeposited Co. No carbon formation was observed on the Cu–Co anode following exposure to dry methane at 1073 K