EXAFS, TDPAC and TPR characterization of PtInFerrierite - The role of surface species in the SCR of NOx with CH4

Ferrierite exchanged with In and with Pt, In (InFerrierite and PtInFerrieri te) is characterized by extended X-ray absorption fine structure (EXAFS), t ime differential perturbed angular correlation (TDPAC) and temperature-prog rammed reduction (TPR). The presence of different In and Pt species is corr elated with activity and selectivity during the NO selective reduction (SCR ) with CH4 in the presence of excess oxygen. Both catalysts are active and selective for the said reaction. When a dry feed is used, both monometallic and bimetallic catalysts behave similarly. However, when water is present in the feed, while InFerrierite loses activity in the whole temperature ran ge, PtInFerrierite increases its activity at temperatures above 450 degrees C. EXAFS characterization indicates that in PtInFerrierite only a fraction of Pt is at exchange positions, the remaining fractions adopting the form o f PtO2 crystals located in the outer surface of the zeolitic structure. Whe n this bimetallic catalyst is treated under wet reaction conditions, the fo rmation of small metal clusters due to the reduction of PtO2 crystals occur s. Thus, the increase in activity could be due to a catalytic effect of Pt metal clusters on the NO oxidation reaction (NO+1/20(2)-->NO2). TDPAC chara cterization indicates the presence of two In species both in the monometall ic and in the bimetallic catalysts: In2O3 and (InO)(+)Z(-) (In at exchange positions), the latter being the active In species for the reaction under s tudy. TPR results also suggest the presence of highly dispersed non-crystal line In oxide species not bonded to the zeolite matrix. Neither EXAFS resul ts nor TDPAC characterization show the presence of intermetallic Pt-In spec ies. The results of this work prove that EXAFS and TDPAC techniques, combin ed with TPR and activity measurements, are powerful tools for the character ization of bimetallic catalysts. (C) 2001 Elsevier Science B.V. All rights reserved.