[Role of surface-generated gas-phase radicals in heterogeneous catalysis] Final Technical Report, January 1, 1994 - December 31, 2001
A major theme in this research has been the role of surface-generated gas-phase radicals in heterogeneous catalysis, with emphasis on methyl radicals formed from methane. The activation of methane is of particular interest because of its abundance as a hydrocarbon resource. Previous studies on methyl radicals showed that they are an intermediate in the oxidative coupling of methane to form ethane and subsequently ethylene. More recent research on methyl radicals has focused on their coupling with allyl radicals, derived from propylene, to produce 1-butene and the methylation of ethylene to form propylene. As an extension of the work on methyl radicals, it has been shown that their reaction with vanadium oxide yields surface methoxide ions which either decompose to formaldehyde or react with water to form methanol. The surface and gas phase chemistry of methyl radicals also provided a link to the removal of NOx. In an attempt to explore the validity of a mechanism put forth by Vannice and co-workers, it was shown under this DOE grant that over basic metal oxides methyl radicals indeed react with gas phase NO to form nitrosomethane which is believed to be an intermediate in the selective catalytic reduction (SCR). Over one of these basic oxide catalysts (BaO/MgO), the decomposition of N2O was studied to determine the role of peroxide ions as an intermediate. Hydroxyl radicals also may be formed on surfaces and enter the gas phase where they can promote chain branching reactions during hydrocarbon oxidation. The formation of these radicals during the reaction of water with molecular oxygen over basic oxide catalysts has been studied in detail. A kinetic model provided evidence that these surface generated radicals may play a role in catalytic combustion. Since the hydroxyl radicals may undergo secondary reactions with a surface, this aspect of their chemistry was also explored. In contrast to the low probability of methyl radicals reacting with metal oxide surfaces, hydroxyl radicals have a high probability of sticking and reacting. A gold surface was even more effective in the removal of hydroxyl radicals than the metal oxides that were considered. Presumably, hydrogen peroxide is an intermediate in the reaction. The formation of hydrogen peroxide directly from molecular hydrogen and oxygen is the most recent extension of this research. This reaction was thought to occur over supported palladium in an acidified aqueous slurry; however, it has been shown in this laboratory that the active palladium is actually in the form of a colloid. The reaction rate is believed to be limited by the transport of reagents through a film between the gas bubbles and the surface of the colloidal particles.
- Research Organization:
- Texas A and M University (US)
- Sponsoring Organization:
- USDOE Office of Science (SC); Office of Basic Energy Sciences (US)
- DOE Contract Number:
- FG03-94ER14417
- OSTI ID:
- 794289
- Resource Relation:
- Other Information: PBD: 20 Mar 2002
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
37 INORGANIC
ORGANIC
PHYSICAL AND ANALYTICAL CHEMISTRY
54 ENVIRONMENTAL SCIENCES
ALLYL RADICALS
HETEROGENEOUS CATALYSIS
HYDROGEN PEROXIDE
HYDROXYL RADICALS
METHYL RADICALS
CHEMICAL REACTION KINETICS
SELECTIVE CATALYTIC REDUCTION
VANADIUM OXIDES
METHANE
NITROGEN OXIDES
SURFACE-GENERATED
GAS-PHASE RADICALS
PALLADIUM