Activation of dihydrogen on supported and unsupported silver catalysts

H 2 activation on silver was investigated by temporal analysis of products (TAPs), IR spectroscopy, and calorimetry. H 2 adsorption on silver catalysts is found to be activated. Activated hydrogen becomes available also on the surrounding support. The activation of dihydrogen on silica, silver, and...

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Bibliographic Details
Published in:Journal of catalysis 2010, Vol.269 (1), p.5-14
Main Authors: Hohmeyer, J., Kondratenko, E.V., Bron, M., Kröhnert, J., Jentoft, F.C., Schlögl, R., Claus, P.
Format: Article
Language:English
Subjects:
Catalysis
Catalysts
Chemistry
Differential scanning calorimetry (DSC)
Dissociative adsorption
Exact sciences and technology
General and physical chemistry
Hydrogen
Infrared spectroscopy (FTIR)
Isotopic exchange
Silica
Silver
Silver catalysts
Surface physical chemistry
Temporal analysis of products (TAP)
Theory of reactions, general kinetics. Catalysis. Nomenclature, chemical documentation, computer chemistry
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Summary:H 2 activation on silver was investigated by temporal analysis of products (TAPs), IR spectroscopy, and calorimetry. H 2 adsorption on silver catalysts is found to be activated. Activated hydrogen becomes available also on the surrounding support. The activation of dihydrogen on silica, silver, and silica-supported silver (9 wt% Ag) was investigated. Both silica and silver are individually able to dissociate dihydrogen. Silanol groups on silica undergo H → D exchange at 393 K in D 2 as detected by IR spectroscopy. HD is observed in temporal analysis of products (TAP) experiments when H 2 and D 2 are sequentially pulsed on silver at 673 K; even when the time delay between the isotopes is 4 s, HD is formed, indicating that long-lived surface hydrogen species are present. Differential scanning calorimetry (DSC) shows that the activation of dihydrogen is an activated process: heat signals evoked through H 2 pulses on Ag/SiO 2 grow with increasing temperature (373–523 K). Nonetheless, the presence of silver on the silica surface accelerates the Si–OH → Si–OD exchange. Investigation of the exchange kinetics on Ag/SiO 2 shows that diffusion processes of the activated hydrogen species are rate determining at higher temperatures (⩾373 K), when the activation of D 2 on silver becomes facile. Indications of diffusion limitation are observed already at 313 K on Pt/SiO 2. TAP and DSC measurements show that H 2 is more readily activated on silver that has been treated in O 2 at 673 K followed by reduction in H 2 at 673 K. Morphological changes induced to the silver surfaces or (sub)surface oxygen species are presumed responsible for this effect.
ISSN:0021-9517
1090-2694
DOI:10.1016/j.jcat.2009.10.008