Adsorption of olefins on zinc oxide. Observations by carbon-13 nuclear magnetic resonance

Ali, Ishmail T.

doi:10.1021/j150609a032

Cited by 11 publications

(6 citation statements)

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“…This is corroborated by the 13 C NMR spectra of 2,5-DHF and other substrates adsorbed onto ZnS-B 1 from the gas phase, as described for ZnO [31] (although comparison of the latter results with the adsorption studies on aqueous suspensions is reliable only in some basic aspects).…”

supporting

confidence: 65%

“…In agreement with the more covalent character of ZnS, the adsorption shifts are smaller than reported for ZnO (Dd 2 ± 4). [31] For THF and 2,5-DHF the two a-carbon atoms appear as broad singlets shifted to lower field (Dd 0.7 and 1.5, respectively), and indicate adsorption through the oxygen atom. In the case of 2,5-DHF the small downfield shift (Dd 0.4) observed for the olefinic signals suggests that the C ± C double bond is also involved, resulting in adsorption parallel to the zinc sulfide surface.…”

mentioning

confidence: 98%

“…[31] A susceptibility correction (Dd 1.2) was applied to the chemical shifts, based on the upfield shift (Dd À 1.2) observed for TMS physisorbed onto ZnS-B 1 .…”

mentioning

confidence: 99%

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Semiconductor Type A Photocatalysis: Role of Substrate Adsorption and the Nature of Photoreactive Surface Sites in Zinc Sulfide Catalyzed C-C Coupling Reactions

et al. 1999

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The mechanism of the photodehydrodimerization of 2,5-dihydrofuran (2,5-DHF) by suspended zinc sulfide powders was investigated through emission, inhibition, adsorption, and quantum yield studies. Zinc and cadmium ions influenced the emission intensity only marginally but strongly inhibited the reaction, so the photoreactive surface sites were not identical with the emitting states, which had lifetimes in the 0.1 ± 24 ns range. Adsorption isotherms for these metal ions and 2,5-DHF in aqueous solution indicated the presence of mono-and multilayer adsorption. Zn 2 and Cd 2 were both adsorbed physically and by metal sulfide precipitatation, but in the case of Cd 2 a lattice exchange mechanism with ZnS afforded CdS and dissolved zinc ions also. 13C NMR spectra and the good agreement between the calculated number of zinc sites and the measured amount of 2,5-DHF in the saturated solvent ± solute surface monolayer indicated that 2,5-DHF was adsorbed perpendicular to the surface at all the available zinc sites. The true adsorption constant (170 AE 30 L mol À1 ) was consistent with the pseudo-constant (260 AE 50 L mol À1) obtained from the concentration dependence of the reaction rate. 13 C NMR signals of the a-carbon atoms of 2,5-DHF and THF adsorbed onto ZnS from the gas phase were downfield shifted by 1.5 and 0.7 ppm as compared to those of the free substrates, respectively. The downfield shift of the olefinic signals was about 0.4 ppm. It is postulated that a dissociative electron transfer from adsorbed 2,5-DHF to the reactive hole afforded a proton and the dihydrofuryl radical. The corresponding C ± H bond dissociation energies were calculated by ab initio methods for various substrates. As expected, the apparent quantum yield of various substrates increased linearly with decreasing C ± H bond dissociation energy. The intermediate dihydrofuryl radical dimerized to the products in the adsorbed state, as indicated by the linear increase of the square root of the reaction rate with increasing 2,5-DHF surface concentration and by competition experiments with THF/2,5-DHF mixtures. The reaction inhibition by Zn 2 and Cd 2 could be analyzed in terms of the Stern ± Volmer model only when the surface concentration was considered. Inhibition by Cd 2 is about three times faster than inhibition by Zn 2 , in which case no zinc metal was observed.

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confidence: 65%

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confidence: 98%

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Semiconductor Type A Photocatalysis: Role of Substrate Adsorption and the Nature of Photoreactive Surface Sites in Zinc Sulfide Catalyzed C-C Coupling Reactions

et al. 1999

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“…Adsorption Studies by 13 C NMR Spectroscopy. By analogy with NMR investigations of the adsorption of olefins onto ZnO, 2,5-DHF was adsorbed from the gas phase onto the various powders and the 13 C NMR spectrum was measured. Because of very long recording times, magnetic field instabilities arose, as indicated by a small highfield shift (−0.16 ppm) of TMS, and all the following values were therefore corrected for this.…”

Section: Resultsmentioning

confidence: 99%

“…Substrate Adsorption. Adsorption of gaseous cyclopentene onto SiO 2 is indicated by shifts of + 0.7 and −0.8 ppm for the olefinic and α-carbon 13 C NMR signals, respectively . In the case of 2,5-DHF, corresponding shifts of +0.6 and +1.3 ppm were observed.…”

Section: Discussionmentioning

confidence: 95%

On the Mechanism of Radical C−N Coupling in Type B Semiconductor Photocatalysis: A High-Pressure Study^,

2000

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The CdS and CdS/SiO2 photocatalyzed addition of 2,5-dihydrofuran (2,5-DHF) to azobenzene in methanol solution affords 3-(2,3-dihydrofuryl)hydrazobenzene (1) as the major and hydrazobenzene (2) as the side product. According to 13C NMR and adsorption data, 2,5-DHF is adsorbed parallel to the surface through hydrogen bonding with [OH/SH] groups; adsorption constants of 18 and 30 L mol-1 are obtained for CdS and CdS/SiO2, respectively. This enhanced effect of the silica support favors formation of the addition product 1. Different from that, azobenzene exhibits adsorption constants identical within experimental error (11126 and 1059 L mol-1) for both photocatalysts and adsorbs most likely at Brønsted acid centers. These results are corroborated by corresponding adsorption studies with SiO2. The much better adsorption of 2,5-DHF onto CdS when performed in aqueous suspension is rationalized by comparison of adsorbent and adsorptive polarities, as determined by Reichardt's dye. The absence of any addition products of methanol to a mutual dihydrofuryl radical cation suggests that oxidation by the reactive valence band hole occurs via dissociative electron transfer affording a proton and the dihydrofuryl radical. The latter undergoes C−N coupling with PhN−N(H)Ph, formed in a proton-coupled reduction by the reactive electron, to afford 1. Formations of 1 and 2 exhibit the same Arrhenius activation energies of 11 kJ mol-1. The rates of both reactions are decreased upon increasing the solvent viscosity by increasing pressure or by using various alcohols. From the average activation volume of 17 cm3mol-1, obtained for 1 and 2 at pressures up to 120 MPa, it is postulated that radical diffusion in the solvent−solute surface multilayer rather than C−N bond formation is the rate-determining step.

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Adsorption and ¹³C N.m.r. studies of ethene, ethane and carbon monoxide on zn‐ and cd‐exchanged A‐zeolites

Emesh

1985

J. Chem. Tech. Biotechnol.

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The adsorption of C2H4, C2Hb and CO has been studied on Zn'+-exchanged A-zeolites. Partial exchange with Zn2+ or Cd2+ increases the adsorption capacity for C2H4 and CO. Complete Zn2+ exchange leads to a decrease, indicating a complicated variation of cation positions with exchange level. N .m.r. measurements are consistent with adsorption of C2H4 as weak n-complexes on the divalent cations. Weak specific adsorption complexes are also demonstrated for CO.

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Adsorption of olefins on zinc oxide. Observations by carbon-13 nuclear magnetic resonance

Cited by 11 publications

References 4 publications

Semiconductor Type A Photocatalysis: Role of Substrate Adsorption and the Nature of Photoreactive Surface Sites in Zinc Sulfide Catalyzed C-C Coupling Reactions

Semiconductor Type A Photocatalysis: Role of Substrate Adsorption and the Nature of Photoreactive Surface Sites in Zinc Sulfide Catalyzed C-C Coupling Reactions

On the Mechanism of Radical C−N Coupling in Type B Semiconductor Photocatalysis: A High-Pressure Study^,

Adsorption and ¹³C N.m.r. studies of ethene, ethane and carbon monoxide on zn‐ and cd‐exchanged A‐zeolites

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Adsorption of olefins on zinc oxide. Observations by carbon-13 nuclear magnetic resonance

Cited by 11 publications

References 4 publications

Semiconductor Type A Photocatalysis: Role of Substrate Adsorption and the Nature of Photoreactive Surface Sites in Zinc Sulfide Catalyzed C-C Coupling Reactions

Semiconductor Type A Photocatalysis: Role of Substrate Adsorption and the Nature of Photoreactive Surface Sites in Zinc Sulfide Catalyzed C-C Coupling Reactions

On the Mechanism of Radical C−N Coupling in Type B Semiconductor Photocatalysis: A High-Pressure Study,

Adsorption and 13C N.m.r. studies of ethene, ethane and carbon monoxide on zn‐ and cd‐exchanged A‐zeolites

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On the Mechanism of Radical C−N Coupling in Type B Semiconductor Photocatalysis: A High-Pressure Study^,

Adsorption and ¹³C N.m.r. studies of ethene, ethane and carbon monoxide on zn‐ and cd‐exchanged A‐zeolites