Alkali metal-modified C-FDU-15: Highly efficient adsorbents for adsorption of NO and O at low temperatures.

The alkali metal (M = Na, K, Rb, and Cs)-modified C-FDU-15 (M-C-FDU-15(x); x was the M/C-FDU-15 M ratio, and equal to 0.01-0.03) samples were prepared through an in situ process, and characterized by means of the TG, XRD, TEM, EDS, N adsorption-desorption, O-TPD, and CO-TPD techniques. The (NO + O) adsorption mechanism was investigated using the (NO + O)-TPD and DRIFTS techniques. The results show that the sequence of (NO + O) adsorption performance was Na-C-FDU-15(0.01) (104.1 mg/g) > K-C-FDU-15(0.01) (92.4 mg/g) > C-FDU-15 (76.2 mg/g) > Rb-C-FDU-15(0.01) (65.1 mg/g) > Cs-C-FDU-15(0.01) (62.3 mg/g). The alkali metal was uniformly distributed in C-FDU-15 and its doping enhanced the amount of the basic sites in the sample. Moreover, the optimal Na/C-FDU-15 M ratio was 0.02. (NO + O) were chemically adsorbed mainly in the forms of nitrite (NO) and nitrate (NO) on M-C-FDU-15(x). A more amount of NO was converted to nitrate than to nitrite. There were three key factors of enhancing the (NO + O) adsorption capacity of C-FDU-15 due to alkali metal doping: the first factor was the increasing of surface area and pore volume of the sample, the second one was the enhancement in amount of the active sites in the sample, and the third one was the smaller alkali metal ionic radius in the sample.