Pd-M-TiO₂ (M=Mn, Cu, Ce and Fe) as passive NO_x adsorber (PNA) at low temperature

Abstract

A series of transition metal Mn, Cu, Ce and Fe were loaded on TiO₂ by sol-gel method with noble metal Pd as promotor for the application of passive NO_x absorber. Experiments on adsorption and desorption of NO_x were conducted and characterization methods such as X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and in situ Fourier transform infrared spectroscopy (in situ DRIFTS) were involved. The experimental results show that Mn-contained catalysts, Mn-Ti and Pd-Mn-Ti, performed excellent NO_x adsorbing ability and appropriate desorption temperature window. On the other hand, Ce- and Cu-contained samples were not suitable for the purpose of PNA. In addition to the low adsorption capacity, these two series of catalysts released massive amount of NO below 150 °C. Characterization results indicated that Pd was highly dispersed on all catalysts. The loading of Pd lowered not only the valence states of transition metals but surface oxygen percentage as well. From in situ DRIFTS tests, the Pd had little influence on the types of adsorbed substances for Mn, Ce and Cu series. However, the storage forms of NO_x were obviously different on Pd-Fe-Ti and Fe-Ti.

Keywords

passive NO_x adsorber / low temperature absorption / manganese / Pd-Fe interaction

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