Radical-surface interactions species

Most mechanistic studies have focused on elucidation of the role of alkali promoters. The addition of Li+ to MgO has been shown to decrease the surface area and to increase both methane conversion and selective C2 production.338,339 As was mentioned, however, besides this surface-catalyzed process, a homogeneous route also exists to the formation of methyl radicals.340-342 The surface active species on lithium-doped catalysts is assumed to be the lithium cation stabilized by an anion vacancy. The methyl radicals are considered to be produced by the interaction of methane with O- of the [Li+0-] center330,343 [Eq. (3.32)]. This is supported by the direct correlations between the concentration of [Li+0 ] and the concentration of CH3 and the methane conversion, respectively. The active sites then are regenerated by dehydration [Eq. (3.33)] and subsequent oxidation with molecular oxygen [Eq. (3.34)] ... 

Experiments generating sulfate radicals, SO/", by UV photolysis of S2O82 in aqueous suspensions of silica nanoparticles showed a fast disappearance of the aqueous sulfate radicals yielding two transient species with absorption maxima around 320 and 600 nm, respectively [20]. The results indicated that at pH 3-9 S04 radicals build up an adduct on the surface with maximum absorption at 320 nm. This adduct shows similar reactivity to that observed for the sulfate radical in aqueous solution. The transients with absorption maximum at 600 nm were identified as SiO surface defects formed from the reaction between the adduct and deprotonated geminal and single silanols. Other less oxidative radicals lead to different radical-silica interactions. For example, thiocyanate radicals react with deprotonated silanols, not involving silanol oxidation. 

The main objective of this article is to summarize the work performed at the Max-Planck-Institute for Plasma Physics in Garching over the past few years relevant to plasma-surface interaction processes in the system hydrogen and carbon. This includes a short review of the properties of amorphous, hydrogenated carbon layers, further on abbreviated as a-C H, determination of reaction probabilities of reactive species such as atomic hydrogen and methyl radicals, and investigation of the simultaneous interaction of these species and low-energy ions with hydrocarbon surfaces. The reviewed ma-... 

We let a beam of atomic hydrogen and a beam of methyl radicals interact with the film surface simultaneously. This can be considered the simplest of all multi-species experiments first, H and CH3 are the simplest radicalic hydrocarbon species. Second, by restricting ourselves to radicals, the interaction of the beams with the film is purely chemical and expected to be limited to the very surface. Third, the effect of each species separately is already known the previous section described the temperature dependent interaction of CH3 radicals with the a-C H surface. The interaction of atomic hydrogen with carbonaceous materials has been studied extensively in the past by various groups [53,56,57]. A rate equation model describing chemical erosion by atomic hydrogen is well-established [53,58]. 

To accomplish CARE, a catalyst and reactive species appropriate for the material to be processed are selected. We used SiC as a sample material because there are few efficient machining methods because of its hardness and chemically stability, as previously noted. We selected F and OH radicals as reactive species for the removal of SiC because these species chemically remove SiC in plasma and aqueous solutions. We used platinum as a catalyst in a hydrogen fluoride (HF) solution, because Ft has catalytic properties, such as the abiHty to dissociate various molecules, and the interaction between Pt surfaces and hydrogen fluoride molecules in an aqueous solution can generate F and OH by self-dissociation. 

Microwave or radio frequencies above 1 MHz that are appHed to a gas under low pressure produce high energy electrons, which can interact with organic substrates in the vapor and soHd state to produce a wide variety of reactive intermediate species cations, anions, excited states, radicals, and ion radicals. These intermediates can combine or react with other substrates to form cross-linked polymer surfaces and cross-linked coatings or films (22,23,29). 

Recently, Muha (83) has found that the concentration of cation radicals is a rather complex function of the half-wave potential the concentration goes through a maximum at a half-wave potential of about 0.7 V. The results were obtained for an amorphous silica-alumina catalyst where the steric problem would not be significant. To explain the observed dependence, the presence of dipositive ions and carbonium ions along with a distribution in the oxidizing strengths of the surface electrophilic sites must be taken into account. The interaction between the different species present is explained by assuming that a chemical equilibrium exists on the surface. 

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