EXAFS,TDPAC and TPR characterization of PtInFerrierite The role of surface species in the SCR of NO_x with CH_4

摘要

Ferrierite exchanged with In and with Pt, In (InFerrierite and PtInFerrierite) is characterized by extended X-ray absorption fine structure (EXAFS), time differential perturbed angular correlation (TDPAC) and temperature-programmed reduction (TPR). The presence of different In and Pt species is correlated 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 range, PtInFerrierite increases its activity at temperatures above 450°C. EXAFS characterization indicates that in PtInFerrierite only a fraction of Pt is at exchange positions, the remaining fractions adopting the form of Pt02 crystals located in the outer-surface of the zeolitic structure. When this bimetallic catalyst is treated under wet reaction conditions, the formation of small metal clusters due to the reduction of Pt02 crystals occurs. Thus, the increase in activity could be due to a catalytic effect of Pt metal clusters on the NO oxidation reaction (NO+ 1/202-+ NO2). TDPAC characterization indicates the presence of two In species both in the monometallic and in the bimetallic catalysts: In203 and (InO)+Z- (In at exchange positions), the latter being the active In species for the reaction under study. TPR results also suggest the presence of highly dispersed non-crystalline In oxide species not bonded to the zeolite matrix. Neither EXAFS results norTDPAC characterization show the presence ofintermetallic Pt-In species. The results of this work prove that EXAFS and TDPAC techniques, combined with TPR and activity measurements, are powerful tools for the characterization of bimetallic catalysts.