Operando DRIFTS investigation of cobaltbased FisherTropsch catalysts exposed to trichloroethylene
Résumé
Biomass-derived syngas contain significant concentrations of chlorine that may contaminate downstream Fisher-Tropsch catalysts. We report herein an operando diffuse reflectance FT-IR spectroscopy (DRIFTS) study in which the hydrogenation of CO is monitored in the presence of 150 ppm of chlorine (introduced as 50 ppm of trichloroethylene, noted TCE) at ambient pressure.
The experimental setup was described elsewhere [1]. The rates of formation of the main products of interest sharply decreased upon the introduction of the chlorinated compound, typically showing a 10-fold drop. Interestingly, the drop in the carbonyl band area was far less marked and mostly due the removal of bridged/multi-bonded carbonyls. This suggests that each Cl atoms poisoned several surface sites and/or selectively poisoned the most active sites. The effect of Cl was partly and slowly reversible. Surprisingly, the Cl poisoning hardly affected the position of the on-top CO(ads) IR band, in contrast to the effect of surface coverage when CO was removed from the feed. In addition to site poisoning, our operando DRIFTS data highlights a strong electronic effect of Cl, which is able to counteract the effects of surface coverage of CO on the position of the IR band. Similar effects were observed on a silica-supported cobalt catalyst.