Surface Spectroscopy and Catalytic Properties of Model Platinum Catalysts Exposed to Hydrocarbons

Hydrocarbonaceous deposits are normally present on Pt during hydrocarbon reactions. Carbon deposition is dehydrogenated during evacuation and appear as “graphitic”,“polymeric” and “deactivating” carbon. The latter may correspond to “disordered” carbon. C atoms on Pt can also be present. Activity and selectivities of “skeletal” reactions of hexane (isomerization, C5-cyclization, aromatization, fragmentation) are influenced by the amount and chemical state of carbon. This depends of the temperature of treatment and the presence of H2

Carbon accumulation, deactivation and reactivation of Pt catalysts upon exposure to hydrocarbons

The formation and catalytic effect of carbonaceous deposits was studied on monofunctional Pt catalysts: Pt black (PtN, i.e., reduced with hydrazine), Pt/SiO2 (EUROPT-1), Pt on “herringbone” graphite nanofiber (Pt/GNF-H, GNF being able to store hydrogen) and Pt/CeO2 (ceria tending to consume spilt over hydrogen). They were exposed to hexane or t,t-hexa-2,4-diene between 483 and 663 K, with or without H2. Hydrocarbon transformations during these deactivating exposures as well as in subsequent standard test reaction with hexane in hydrogen excess were studied. Carbon accumulation on Pt black after analogous deactivating treatments was also examined by electron spectroscopy (XPS and UPS). The abundance of hydrogen on Pt sites controlled the activity and selectivity containing much PtC species. The amount of surface C could reach 45% causing almost total activity loss, but even 30% C on Pt blacks decreased markedly the catalytic activity, due to massive 3D deposits. “Disordered” carbon selectively poisoned the formation of saturated C6 products and fragmentation. The yield of dehydrogenation to hexenes was a good universal indicator of deactivation for each catalyst. Four regions weredistinguished: “beneficial”, “selective”, “non-selective” and “severe” deactivation

Quantum Hall Resistance Overshoot in 2-Dimensional Electron Gases - Theory and Experiment

We present a systematical experimental investigation of an unusual transport phenomenon observed in two dimensional electron gases in Si/SiGe heterostructures under integer quantum Hall effect (IQHE) conditions. This phenomenon emerges under specific experimental conditions and in different material systems. It is commonly referred to as Hall resistance overshoot, however, lacks a consistent explanation so far. Based on our experimental findings we are able to develop a model that accounts for all of our observations in the framework of a screening theory for the IQHE. Within this model the origin of the overshoot is attributed to a transport regime where current is confined to co-existing evanescent incompressible strips of different filling factors.Comment: 26 pages, 10 figure

Preparation, characterization and catalytic testing of GePt catalysts

Unsupported and SiO2 supported GePt bimetallic catalysts were prepared by depositing Ge on to Pt underpotentially. Surface-sensitive cyclic voltammetry of Pt black indicated that Ge covered ca. 40–45% of the Pt surface, whereas XPS showed just 96% Pt and 4% Ge (normalized to Pt+Ge=100%). High-resolution Ge map of GePt black obtained by Energy Filtered TEM (EFTEM) showed Ge scattered in the near-surface regions. Both catalysts were tested in hexane (nH) transformation reactions between 543 and 603 K and 60 to 480 Torr H2 pressure (with 10 Torr nH), and compared with the parent Pt catalysts. GePt/SiO2 catalyst was also tested with methylcyclopentane (MCP). Adding Ge to Pt/SiO2 lowered the activity; the opposite effect was observed with GePt black. The selectivities of saturated products on bimetallic catalysts decreased, while those of hydrogenolysis products, benzene and hexenes increased in nH transformations over supported catalyst. The reverse effects were observed over the black samples where addition of Ge prevented accumulation of adventitious carbon. Ring opening was the main reaction with MCP, together with some fragments, benzene and unsaturated hydrocarbons. Ring opening of MCP became more selective with decreasing temperature and increasing hydrogen pressure. Ge on GePt black blocked contiguous Pt sites favoring the formation of coke precursors. The different catalytic behavior of GePt/SiO2 indicated somewhat different Pt–Ge interaction(s)