Chemical deactivation of Fe-BEA as NH3-SCR catalyst—Effect of phosphorous

Abstract

Fe-BEA as catalyst for selective catalytic reduction (SCR) of NOX with NH3 was experimentally studied with focus on chemical deactivation caused by phosphorous exposure. Cordierite supported Fe-BEA samples were exposed to 10 or 50ppm H3PO4 for 14, 24 and 48h in a continuous gas flow reactor at 350°C. The catalytic activity of the samples was studied by NH3- and NO-oxidation, NH3 inhibition and NH3-SCR experiments. The phosphorous exposed samples were further characterized by NH3- and NO-TPD, XPS and XRD. The results from the activity studies show that the degree of deactivation due to phosphorous exposure is strongly dependent on the exposure time, while the rate of deactivation is the same for exposure with 10 and 50ppm H3PO4. The XPS results show that primarily phosphorous pentoxides (P2O5) are formed after short time of phosphorous exposure while longer time of exposure results in formation of metaphosphates (PO3−). The relative amount of metaphosphates after 48h of H3PO4 exposure was about 45% compared to phosphorous pentoxides. The storage capacity of NO was shown to decrease with increasing relative amount of metaphosphates.The activity studies show that longer time of phosphorous exposure results in significantly decreased activity indicating that the active iron species are very sensitive to phosphorous forming metaphosphates. We suggest that metaphosphates replace the hydroxyl groups on the active iron species in Fe-BEA as the main mechanism for the decreased activity for NH3-SCR in connection with phosphorous exposure. Furthermore, the NH3 inhibition experiments show that the increased amount of strongly bound ammonia due to phosphorous exposure does not contribute to buffer the active iron sites with ammonia during transient SCR conditions.