Combined surface science and catalytic studies of hydrocarbon reactions over platinum single crystal surfaces

Technical Report · Wed Aug 01 04:00:00 EDT 1984

OSTI ID:6216951

Zaera, F

Ethylidyne, which forms during room temperature chemisorption of ethylene over Pt (lll), was shown to be stable towards rehydrogenation or deuterium exchange under atmospheric conditions. Ethylene hydrogenation over Pt (111) surfaces at near room temperature displayed kinetic parameters that agreed with those for supported catalysts, LEED, thermal desorption spectroscopy, and HREELS indicate the presence of ethylidyne on the surface after reaction. Exchange of ethane with D/sub 2/ over Pt (111) between 470/sup 0/ and 620/sup 0/K gave an activation energy of 19 kcal/mole. The product distribution peaked at one (d/sub 1/) and six (d/sub 6/) deuterium atoms per ethane molecule, indicating two competitive mechanisms. A diffuse 2 x 2 LEED pattern suggested the presence of ethylidyne on the Pt surface, which can be easily hydrogenated to give d/sub 6/ ethane. Above 550/sup 0/K, some hydrogenolysis was also detected, with turnover frequencies about three orders of magnitude smaller than for the exchange. Isobutane, neopentane, n-hexane and methylcyclopentane (MCP) conversion were also investigated over Pt surfaces of different orientations. Surface defects were important for the light alkanes but not for n-hexane or MCP reactions. Higher activities on stepped and kinked surfaces for the light alkanes correlated with patches of bare low coordination Pt atoms. The opposite was true for the heavier alkanes. A correspondence between this behavior and the different bond-shift and cyclic mechanisms was established. Potassium poisoned all n-hexane reactions due to an electronic interaction of the potassium with the surface. Rhenium, alloyed with Pt, increased the selectivity for hydrogenolysis. Sulfur over the latter system reduces all activity, but particularly methane formation.

• Research Organization: Lawrence Berkeley Lab., CA (USA)
• DOE Contract Number: AC03-76SF00098
• OSTI ID: 6216951
• Report Number(s): LBL-18311; ON: DE85005162
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
360104* -- Metals & Alloys-- Physical Properties
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
400300 -- Organic Chemistry-- (-1987)
ADDITIVES
ALKALI METALS
ALKANES
ALKENES
CATALYSIS
CATALYTIC EFFECTS
CHEMICAL REACTION KINETICS
CHEMICAL REACTIONS
CHEMISORPTION
CRYSTALS
DEUTERIUM
ELEMENTS
ETHANE
ETHYLENE
HEXANE
HYDROCARBONS
HYDROGEN ISOTOPES
HYDROGENATION
ISOTOPES
ISOTOPIC EXCHANGE
KINETICS
LIGHT NUCLEI
METALS
MONOCRYSTALS
NUCLEI
ODD-ODD NUCLEI
ORGANIC COMPOUNDS
PLATINUM
PLATINUM METALS
POTASSIUM
REACTION KINETICS
REFORMER PROCESSES
RHENIUM
SEPARATION PROCESSES
SORPTION
STABLE ISOTOPES
SURFACES
TRANSITION ELEMENTS