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

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 NOx absorber. Experiments on adsorption and desorption of NOx 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 NOx 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 degrees 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 NOx were obviously different on Pd-Fe-Ti and Fe-Ti.

Automated summary

This study aimed to develop a passive NOx absorber using a series of transition metal catalysts loaded on TiO2 with Pd as a promoter. The Mn-containing catalysts showed excellent NOx adsorption ability and appropriate desorption temperature, while Ce- and Cu-containing samples were not suitable. The loading of Pd reduced the valence states of transition metals and surface oxygen percentage. The types of adsorbed substances were not influenced by Pd in Mn, Ce, and Cu series, but the storage forms of NOx were significantly different on Pd-Fe-Ti and Fe-Ti.