Electron spin resonance study of radiation-induced centers of chemical activity in a silica-alumina gel

Abstract

The esr spectrum of a 7-irradiated silica-alumina gel has a line 44.5 ± 0.5 G wide at g = 2.0088 ± 0.0005 which is partially resolved into six components separated by 8.5 ± 0.2 G; superimposed on this broad line is a narrow line at g = 2.0010 ± 0.0005. The broad line is identified with a positive hole trapped at an aluminum atom present substitutionally in the silica matrix. The narrow, line is attributed to an electron trapped at one of several kinds of oxygen vacancies and seems to result from a reaction of H atoms. An appreciable number of electrons are trapped in such a manner as to be undetectable in the esr spectrum. At large doses, the positive-hole intensity reaches a limiting value that corresponds to 16 X 1017 spins/g; such a value is comparable with a previously determined value of 13 X 1017 centers/g as the limiting yield of radiation-induced centers of chemical activity. On exposure of irradiated solid to each of 18 reagents and on irradiation in the presence of each of 12 reagents, without exception, behavior of the visible coloration parallels that of the broad esr signal. With H2 and H20 as the only exceptions, a correlation exists between ionization potential of a reagent and its reactivity with the trapped positive holes; such a correlation permits an estimate of the effective electron affinity of the positive hole as lying between 9.8 and 12.1 eV. When present during irradiation, O,, N20, and C02 act as traps for free electrons and enhance color and positive-hole signal of the solid; CO and S02 apparently act as traps for free positive holes (unstabilized 0~) to give C02-and SOg-. A model is presented to describe the generality of results obtained in radiation studies with silica-alumina gel.

Formation of Methylviologen Radical Monopositive Cations and Ensuing Reactions with Polychloroalkanes on Silica Gel Surfaces

“…In the y-radiolysis studies EPR spectra of holes on silica-alumina gel surfaces were stated to be poorly resolved and resulted in a broad line. 35 A part of MV,+ species is stable for several months in the absence of O2. As soon as oxygen/air is introduced into the sample, the blue color, and the corresponding spectral absorption The reaction of photogenerated MV,+ with O2 on a silica gel surface has been studied in both steady-state and timeresolved mode, where the formation of superoxide anion (O2"-)…”

Interaction of tetrameric silver with ammonia in AgCs-rho zeolite

“…In fact, in silicaÈalumina gel a similar signal was interpreted as a positive hole trapped on a bridging oxygen atom that is bonded to an aluminium present substitutionally in the silica matrix. 18 The isotropic pentet Q with spin Hamiltonian parameters mT and represents a tetrameric silver A iso \ 13.9 g iso \ 1.973 cluster located inside an octagonal prism.11 Similar Ag 4 3s ignals were observed in AgCs-rho zeolite after H 2 reduction10 as well as in c-irradiated AgH-rho zeolite.11 The third and fourth lines of pentet Q show some additional features and their intensities are lower than one would expect from the binominal intensity ratio. We earlier proposed that these irregularities resulted from the overlapping of a low intensity quartet of the cluster which is the precursor of Ag 3 2t he silver tetramer.11…”

Quenching of Excited States of Pyrene by Tetranitromethane on SiO2 Surfaces: Effect of Reaction Temperature, Silica Pore Size, and Surface Silanol Content

“…High surface area silica is one of the most widely used catalyst supports. As such, and along with alumina and zeolites, it has been of interest as a support for high-energy radiation studies. − Extensive photochemical studies are also reported on oxide surfaces. − The photoinduced reactions of free radicals, radical ions, and excited states have all been studied on these surfaces. From the view of kinetics the fractal, rigid, and heterogeneous nature of these surfaces necessitate a careful consideration of the detailed nature of the reactions on these surfaces.…”