Chemical deactivation of Fe-BEA as NH3-SCR catalyst –Effect of Phosphorous

• 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 50 ppm H3PO4 for 14, 24 and 48 h 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 werefurther 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 50 ppm 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 relativeamount of metaphosphates after 48 h 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.

Ämnesord

NATURVETENSKAP  -- Kemi -- Fysikalisk kemi (hsv//swe)

NATURAL SCIENCES  -- Chemical Sciences -- Physical Chemistry (hsv//eng)

TEKNIK OCH TEKNOLOGIER  -- Kemiteknik -- Kemiska processer (hsv//swe)

ENGINEERING AND TECHNOLOGY  -- Chemical Engineering -- Chemical Process Engineering (hsv//eng)

Nyckelord

NO adsorption

Fe-BEA

XPS

Chemical deactivation

NH3-SCR

Poisoning

Phosphorous

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