Publisher Summary This chapter discusses the characterization and reactivity of mononuclear oxygen species on oxide surfaces. Measurement of the electron paramagnetic resonance (EPR) parameters gives information which at the simplest level indicates the presence of a paramagnetic species but which can be sufficiently detailed to identify and characterize the adsorbed species. Measurements of the variation of the intensity of the signal with time can give kinetic data on reactions occurring at the surface. Although most of the EPR measurements have been carried out at low temperatures, there is evidence from the examples for some of the species that are stable at much higher temperatures and may play an important role in catalytic reactions.

Characterization and Reactivity of Molecular Oxygen Species on Oxide Surfaces

Oxidative coupling of methane over sodium-promoted manganese-silica and manganese-magnesia was carried out catalytically with a methane-air cofeed. A catalyst of 12.5% sodium permanganate on magnesia gave a 15.7% yield of ethane, ethylene and higher hydrocarbons at 925°C, 9600 GHSV and 50% air in methane. Methane conversion and selectivity to C 2 + hydrocarbons was 22–25% and 65–70%, respectively. A Mars-van Krevelen type mechanism is proposed in which methyl radicals are formed at surface metal oxide sites. The methyl radicals subsequently dimerize or react with other hydrocarbons in the gas phase.

Catalytic oxidative coupling of methane over sodium-promoted Mn/SiO2 and Mn/MgO

In MOS VLSI device scaling, two major limiting mechanisms are the punchthrough and source-drain breakdown. The punchthrough mechanism is generally considered a bulk-dominated effect. Drain-source avalanche breakdown is generally attributed to bipolar transistor action between drain and source, dominated by injection through the neutral substrate region. The present work includes an experimental verification and a qualitative model demonstrating that both punchthrough and drain-source avalanche breakdown limitations are surface and surface-depletion-region dominated mechanisms, respectively. The two mechanisms are treated simultaneously since both involve enhanced injection from the source due to drain-induced source-potential barrier lowering. The experimental verification is done over a wide range of relevant device parameters, channel implant concentration between 5 × 1014-1 × 1016cm-3for punchthrough and 2 × 1015-5 × 1016cm-3for drain-source avalanche breakdown, effective channel length of 1.0-30.0 µm for both mechanisms.

Surface conduction in short-channel MOS devices as a limitation to VLSI scaling

Surface states in some alkaline earth oxides

At 600°C hydrogen and oxygen are adsorbed on magnesium oxide doped with manganese to an extent which is dependent on the manganese concentration. E.s.r. studies indicate that adsorption of hydrogen results in a reduction of higher valence states to Mn2+, whereas adsorption of oxygen converts Mn2+ into Mn3+ and Mn4+, which concentrate at the oxide surface. The stoichiometry and mechanism of these processes have been investigated in powders of small (∼100 A) and large (∼600 A) average particle sizes. At 20°C the adsorption of hydrogen is little affected by manganese impurity whereas the adsorptions of oxygen is directly proportional to the manganese concentration. These adsorptions are weak forms of chemisotorption and do not appear to involve charge transfer.
 The e.s.r. spectra of Mn2+ in the small particles of MgO are interpreted in terms of a D term in the Hamiltonian and the presence of this axial component in the normally octahedral environment of Mn2+ is thought to arise from the proximity of the surface.

A. J. Tench

Papers

Surface reactivity of magnesium oxide doped with manganese: an e.s.r. and chemisorption study