Ionization and electron transfer reactions in Linde type Y zeolites

Kasai, Paul H.; Bishop, R.J.

doi:10.1021/j100638a010

Cited by 105 publications

(29 citation statements)

References 2 publications

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“…Important concepts related to this area, for instance, have been advanced by studying the interaction of low ionization energy metals (essentially alkali metals) with highly ionic polycrystalline oxides such as zeolites. [8][9][10] Also in the case of nonporous ionic oxide like MgO the study of the early stages of the interaction between low ionization energy metals and the surface provides interesting evidence about the defects present at the surface and their reactivity. Me-MgO systems (Me ) Li, Na, K, Rb, Mg) have been investigated by our group over the years using EPR spectroscopy, and a number of different paramagnetic centers were identified on the oxide surface depending on the amount and type of metal vapor used.…”

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confidence: 99%

Partial Ionization of Cesium Atoms at Point Defects over Polycrystalline Magnesium Oxide

et al. 2001

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Evaporation of Cs atoms onto dehydrated polycrystalline MgO leads to the formation of surface color centers in correspondence of surface point defects. EPR spectroscopy has revealed that the adsorbed Cs atoms are partially ionized, and a fraction of the electron spin density is delocalized onto a surface oxygen vacancy or trap. The observed defect can thus be written as Cs δ+ (trap) δ-. These results give evidence of the preferential interaction of the metal atoms with specific surface defect sites in the early stages of the metal-support interaction. The reaction of these centers with molecular oxygen leads to bleaching of the surface with formation of the O 2 -superoxide radical anion. A fraction of the adsorbed superoxide ions are adsorbed on "regular" Mg 2+ sites while the remaining ones are adsorbed on top of Cs + ions.The nature of the metal-support interaction in heterogeneous catalysis has both intrigued and stimulated researchers for many years. 1 To unravel the complexities of these interactions, elaborate experimental and theoretical methodologies have developed, in particular by studying the deposition of single metal atoms or small metal clusters on metal oxide thin films. [2][3][4][5] These investigations have revealed that, far from being an inert host, the support can influence the electronic properties of the deposited atoms through specific interactions with surface sites, and in some cases enhanced catalytic reactivity has been observed. 3,4 These studies, carried out on well-defined oxide surfaces, are extremely important because they provide clear evidence for the size-dependent catalytic activity of small clusters and the relationship with the intrinsic electronic and geometric structure of the cluster. [3][4][5][6] The changes in catalytic activity have been interpreted by assuming a size-dependent interaction of the small clusters with the substrate. 3,7 In other words, the morphological defects such as low coordination ions and point defects clearly play a crucial role in stabilizing metal atoms at the metaloxide interface and ultimately influencing the catalysis of the supported metal particles.Investigations into the metal-oxide interface, and the changes to the electronic structure of the supported clusters, has been explored for a number of years also over polycrystalline metal oxide surfaces. Important concepts related to this area, for instance, have been advanced by studying the interaction of low ionization energy metals (essentially alkali metals) with highly ionic polycrystalline oxides such as zeolites. 8-10 Also in the case of nonporous ionic oxide like MgO the study of the early stages of the interaction between low ionization energy metals and the surface provides interesting evidence about the defects present at the surface and their reactivity. Me-MgO systems (Me ) Li, Na, K, Rb, Mg) have been investigated by our group over the years using EPR spectroscopy, and a number of different paramagnetic centers were identified on the oxide surface depending on the amount and type ...

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confidence: 99%

Partial Ionization of Cesium Atoms at Point Defects over Polycrystalline Magnesium Oxide

et al. 2001

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“…Consistent with this mechanism is the observation of short-lived Na 3 2+ clusters at wavelengths greater than 510 nm (Fig. 1a) produced when electrons are trapped by the counterbalancing sodium cations (10,13,(31)(32)(33).…”

Section: Dianisylmethane (An 2 Ch 2 ) In Nay Ky Csy and Hymentioning

confidence: 54%

Dynamics of the transient species generated upon photolysis of diarylmethanes within zeolites Deprotonation and oxidation reactions

Shea¹,

Schepp²,

Keirstead³

et al. 2005

Can. J. Chem.

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The oxidation of diarylmethanes is a multistep process involving initial formation of a radical cation, deprotonation of the radical cation to the radical, and oxidation of the radical to the carbocation. The dynamics and efficiency of the last two steps in this process, namely deprotonation and oxidation, in acidic zeolites and non-acid zeolites are examined in the present work as a function of the acidity of the diarylmethane radical cations and the oxidation potential of the diarylmethyl radicals. Our results indicate that rate constants for deprotonation strongly depend on the acidity of the radical cations, but not on the composition of the zeolites. In addition, oxidation of the radicals to the diarylmethyl cations is strongly dependent on both the oxidation potential of the radicals and the oxidizing ability of the zeolite. This dependence allows oxidation potentials of the zeolites to be estimated.Résumé : L'oxydation de diarylméthanes est un processus en plusieurs étapes qui implique la formation initiale d'un cation radical, une déprotonation du cation radical et une oxydation du radical en carbocation. On a étudié la dynamique et l'efficacité des deux dernières étapes de ce processus, soit la déprotonation et l'oxydation, dans des zéolites acides et des zéolites non acides, en fonction de l'acidité des cations radicaux diarylméthanes et du potentiel d'oxydation des radicaux diarylméthyles. Les résultats obtenus indiquent que les constantes de vitesse pour la déproto-nation dépendent fortement de l'acidité des cations radicaux, mais pas sur la composition des zéolites. De plus, l'oxydation des radicaux en cations diarylméthyles est fortement dépendante à la fois sur le potentiel d'oxydation des radicaux et sur l'habilité des zéolites à oxyder. Cette dépendance permet d'évaluer les potentiels d'oxydation des zéo-lites.

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“…This is consistent with previous report^.^^,^^ Irradiated zeolites exposed to air at room temperature also show three g tensors in the EPR spectrum taken at 77 K.34 These g values, gxx = 2.0026, g, , = 2.0085, and g,, = 2.0575, have been observed in N&Y zeolites.34 In NaX zeolites, g,, = 2.078 has been assigned to a 0 2 -superoxide species adsorbed on Na+. 35 The absence of hyperfine structure in this sample suggests that there is no interaction between 0 2 -and electron holes associated with aluminum.34…”

Section: Discussionmentioning

confidence: 87%

“…23Na spectra were acquired at 79. 35 MHz with a 5-7 kHz MAS speed, a 10" pulse tip angle, and a 0.5-1 s recycle delay. 23Na spectra were referenced to solid NaCl (0 ppm).…”

Section: Magnetic Angle Spinning Nuclear Magnetic Resonance (Mas Nmr)mentioning

confidence: 99%

Characterization and Catalytic Studies on Defect Sites Formed upon the Thermal Decomposition of Sodium Ionic Clusters in NaX Zeolite

Simon¹,

Edwards²,

Suib³

1995

J. Phys. Chem.

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Sodium ionic clusters have been prepared by chemical vapor deposition of sodium metal at 225 "C and 1 x Torr in NaX zeolites. A maximum loading of approximately 1.2 wt % sodium was vaporized onto dehydrated NaX zeolite. Ionic clusters prepared in this study have been determined to have the formula Na65+. In this complex the sodium ions are arranged in octahedral geometries in the sodalite cages of NaX zeolite. Identification of these species was accomplished by electron paramagnetic resonance, magic angle spinning nuclear magnetic resonance. Fourier transform infrared spectroscopy, luminescence spectroscopy, and X-ray diffraction experiments. These ionic clusters have a localized electron trapped in the center of the octahedra, giving rise to a color center. Infrared analysis of pyridine adsorption onto NaX and NaX exposed to sodium vapor (Na(v)/NaX) revealed that initial Bransted acidities of untreated NaX zeolites are low and that a complete loss of Bronsted acidity is observed upon treatment with Na vapor. The catalytic properties of materials exposed to sodium vapor have been studied in isomerization reactions. Defect sites or holes on framework oxygen sites are proposed as the source of catalytic activity at elevated temperatures ('340 "C).A catalytic mechanism for isomerization of cyclopropane to propylene over positively charged defect sites associated with Na65f clusters in Na(v)/NaX zeolites is discussed. Activation energies, based on Arrhenius plots for first-order reactions, and rates of propylene formation are determined for cyclopropane isomerization reactions over defect sites in Na(v)/NaX catalysts. Two reaction pathways are proposed for the treatment of sodium vapor on these materials. The dominating reaction produces Nk5+ clusters in the sodalite cages of NaX zeolites. In a second reaction, Na vapor reduces Bransted acid sites on terminal hydroxyl groups. The latter reaction leads to the formation of hydridic species on framework silicon atoms at high temperatures and the generation of electron holes on framework oxygen. Activation of defect sites has been observed only at high temperatures. Electron paramagnetic resonance studies show that naphthalene, sublimed on defect sites, results in the formation of naphthalene radicals.

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Ionization and electron transfer reactions in Linde type Y zeolites

Cited by 105 publications

References 2 publications

Partial Ionization of Cesium Atoms at Point Defects over Polycrystalline Magnesium Oxide

Partial Ionization of Cesium Atoms at Point Defects over Polycrystalline Magnesium Oxide

Dynamics of the transient species generated upon photolysis of diarylmethanes within zeolites Deprotonation and oxidation reactions

Characterization and Catalytic Studies on Defect Sites Formed upon the Thermal Decomposition of Sodium Ionic Clusters in NaX Zeolite

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Ionization and electron transfer reactions in Linde type Y zeolites

Cited by 105 publications

References 2 publications

Partial Ionization of Cesium Atoms at Point Defects over Polycrystalline Magnesium Oxide

Partial Ionization of Cesium Atoms at Point Defects over Polycrystalline Magnesium Oxide

Dynamics of the transient species generated upon photolysis of diarylmethanes within zeolites  Deprotonation and oxidation reactions

Characterization and Catalytic Studies on Defect Sites Formed upon the Thermal Decomposition of Sodium Ionic Clusters in NaX Zeolite

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Dynamics of the transient species generated upon photolysis of diarylmethanes within zeolites Deprotonation and oxidation reactions