Hohmeyer, Jens ; Kondratenko, Evgenii ; Bron, Michael ; Kröhnert, Jutta ; Jentoft, Friederike C. ; Schlögl, Robert ; Claus, Peter (2010)
Activation of dihydrogen on supported and unsupported silver catalysts.
In: Journal of Catalysis, 269 (1)
doi: 10.1016/j.jcat.2009.10.008
Article, Bibliographie

Abstract

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 D₂ as detected by IR spectroscopy. HD is observed in temporal analysis of products (TAP) experiments when H₂ and D₂ 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 H2 pulses on Ag/SiO₂ 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/SiO₂ shows that diffusion processes of the activated hydrogen species are rate determining at higher temperatures (greater-or-equal, ≥ 373 K), when the activation of D₂ on silver becomes facile. Indications of diffusion limitation are observed already at 313 K on Pt/SiO₂. TAP and DSC measurements show that H₂ is more readily activated on silver that has been treated in O2 at 673 K followed by reduction in H2 at 673 K. Morphological changes induced to the silver surfaces or (sub)surface oxygen species are presumed responsible for this effect.

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