a geometric effect of tin revealed for the poisoning of cobalt most active sites for co hydrogenation
Résumé
The effect of tin on cobalt-based catalysts used for CO hydrogenation was studied by operando diffuse reflectance FT-IR spectroscopy (DRIFTS). Tin was selected as a modifier as to obtain a poisoning effect that could help discriminating active sites and also because tin does not adsorb CO at the temperatures investigated (210-230°C). Hence, the carbonyl bands observed by FT-IR could only be attributed to CO adsorbed on cobalt atoms. Four catalysts were studied: a parent 15 wt.% Co/Al2O3 and three Co-Sn catalysts with Co/Sn molar ratios equal to 30, 60 and 120.
The setup and analytical methods are identical to those described elsewhere [1,2]. The rate of formation of methane, propene and methanol were monitored for the four catalysts. Increasing the tin content led to a significant activity loss. The operando DRIFTS spectrum measured over the tin-free Co/Al2O3 exhibited both linear CO(ads), characterized by bands above 2000 cm-1, and bridged and/or multibonded CO(ads), associated with bands below 2000 cm-1.
The Sn-modified samples exhibited a markedly lower fraction of bands below 2000 cm-1, indicating fewer corresponding sites when Sn was present. A decomposition of the carbonyl signal was carried out to quantify the changes in the DRIFTS band signals. The band most affected by tin was that located at around 1860 cm-1, referred to as “Bridged CO(ads)”, the intensity of which decreased with increasing Sn content. Interestingly, a plot of the normalized rates of product formation against the fraction of bridged carbonyls present at the surface of the catalyst revealed an essentially linear relationship.
These data strongly suggests that the sites associated with bridged CO(ads) (possibly edges or steps of cobalt nanoparticles) are the most active sites for CO hydrogenation under these conditions and, potentially, for the Fischer-Tropsch synthesis. Note that step sites have also been proposed as being the most active sites for CO dissociation [3,4].
In addition, the heat of adsorption of the CO species at full coverage was estimated using an IR-based technique [5] on the four catalysts after 16 hours of stabilization under reaction conditions. These values were almost the same for all samples, ca. 80 kJ/mol, stressing the absence of any electronic effects of tin on the adsorption of CO on cobalt atoms.