Activation of dihydrogen on supported and unsupported silver catalysts

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 (⩾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.

H2 activation on silver was investigated by temporal analysis of products (TAPs), IR spectroscopy, and calorimetry. H2 adsorption on silver catalysts is found to be activated. Activated hydrogen becomes available also on the surrounding support.Figure optionsDownload high-quality image (193 K)Download as PowerPoint slide