NH3-TPD-MS study of Ce effect on the surface of Mn- or Fe-exchanged zeolites for selective catalytic reduction of NOx by ammonia

Abstract

The selective catalytic reduction (SCR) of NOx with NH3 is considered to be a promising technique for the efficient reduction of highly detrimental NOx emitted from diesel engine vehicles to N2. This study was focused on a series of catalysts with ZSM-5 as support, prepared by Mn- or Fe-exchange followed by wet impregnation of Ce, or Fe or Mn. These catalysts were characterized by temperature-programmed desorption coupled with a mass spectrometer using ammonia (NH3-TPD-MS). Specifically, NH3-TPD-MS was used as a means of identifying the various strengths of acid sites and their relative abundance in an attempt to explain the effect of the catalyst surface acid sites on DeNOx activity. Acid sites with adsorption energies ranging from 47.0 to 75.6kJ/mol were detected for all of the catalysts. For the same concentration of exchanged metal it was found that the DeNOx activity depends strongly on the type of metal. Furthermore, the acid site strength and distribution depend on the active metal and correlate with the observed DeNOx catalyst activity. Additionally, SEM metal mapping images confirmed the presence of well dispersed active metal on the surface of all catalysts. The catalysts with bimetallic active phase were stable and demonstrated high NOx conversion over a broad temperature range. Impregnation of metal-exchanged zeolites with Ce enhanced the low temperature NOx conversion. Observed differences of activity between the various catalysts of this study may be due to the formation of new ammonia activation sites. The ammonia desorption profile during the elevation of temperature was correlated to the DeNOx activity.