TEMPERATURE-PROGRAMMED DESORPTION STUDY OF NO AND CO2 OVER CEO2 AND ZRO2

The adsorptive properties of CeO2 and ZrO2 were studied with respect t o NO and CO2 probe molecules using temperature-programmed desorption ( TPD). Four species were detected during thermal desorption of NO adsor bed on CeO2 and ZrO2, namely, NO (m/e = 30), N-2 (m/e = 28), N2O (m/e = 44) and O-2 (m/e = 32). The TPD profile suggest that there are two t ypes of adsorbed states of NO on the CeO2 and ZrO2 surfaces, one is th e weakly adsorbed NO which desorbs at about 170 degrees C and the othe r is the more strongly adsorbed NO which desorbs at about 450 degrees C. The adsorbed NO undergoes extensive decomposition to form N-2, N2O and O-2 during thermal desorption. The TPD spectrum obtained after CO2 adsorption on CeO2 are composed of CO2 desorption at 140 degrees C an d 440 degrees C. These peaks are assigned to monodentate and bidentate carbonate species in the adsorbed states. After the successive adsorp tion of NO and CO2 on the CeO2 and ZrO2 surfaces, the intensity of CO2 desorption peak in TPD is weaker than that in the case of single of C O2. However, the intensity of NO desorption is almost the same as in t he case of single NO adsorption. This indicated that the preadsorption of NO on cation sites of oxide surfaces affected the surrounding surf ace oxygen sites and blocked the CO2 adsorption. Furthermore, this als o indicates that the interaction of the oxide surface with NO is much stronger than that with CO2.