Zusammenfassung
Poisoning phenomena recently observed during the conversion of three-way catalysis model feed (CO, NO propene, O2, without or with water) over supported Au catalysts [V. Ulrich et al., Applied Catalysis B 203 (2017) 572] were investigated by reaction studies (replacing propene by propane in model feed, study of CO oxidation without and with relevant additional components), by temperature-programmed oxidation of deposits (TG, TPO), by studying self-regeneration phenomena and by DRIFTS of CO adsorption and co-adsorption of CO and propene under different conditions. During CO oxidation in the presence of other components, two different poisoning phenomena were observed. In the presence of propane, propene, and/or NO, CO oxidation was significantly inhibited below 370 K over Au on Al2O3 or on La-stabilized Al2O3, but not on Au/CeZrOx where activity was relatively low anywhere. DRIFTS data on (co-)adsorption of CO and propene suggest that this poisoning is not merely a result of site blocking by the coadsorbate, but that electron transfer from the co-adsorbates towards the active sites, which are assumed to be at the perimeter of positively charged Au clusters with the support, might have additionally quenched the reaction. At higher temperatures, a different poisoning mechanism operates in propene-containing mixtures. From studies by TPO, TG, and DRIFTS, we conclude the formation of carbonaceous residues, which do not block, however, CO adsorption sites but the support surface around Au clusters instead. While this inhibits CO oxidation involving active support oxygen, the same oxygen species might combust the deposits at higher temperatures. This results in self-regeneration phenomena depending on the redox activity of the support, the temperature and the propene content in the feed. In transient experiments, it was observed that NO is most favorably reduced by intermediates of coke formation from propene. Under stationary conditions, however, NO reduction, mostly by CO, remains insufficient.
