SiO2—Al2O3和MgO催化剂上吸附CH3NO2的TPD和IR研究

用TPD和IR方法研究了CH_3NO_2在典型固体酸SiO_2-Al_2O_3和固体碱MgO催化剂上的吸附分解。结果表明,在SiO_2-Al_2O_3表面CH_3NO_2吸附转化为表面甲酰胺物种,后者在高温下分解为CO_2和NH_3。在MgO表面CH_3NO_2吸附形成多种表面化学物种,它们在升温过程中脱附,并通过表面亚硝基甲烷物种分解为NO、C_2H_4、C_2H_6和N_2O.讨论了CH_3NO_2分解过程中表面酸、碱中心的作用。

TPD and IR Studies on the Decomposition of Nitromethane Adsorbed on SiO_2-Al_2O_3 and MgO Catalysts

The adsorption and decomposition of CH3NO2 on the typical acidic SiO2-Al2O3 and the typical basic MgO catalysts were studied by means of temperature- programmed- desorption (TPD) and infrared spectroscopy(IR). On the surface of SiO2-Al2O3the adsorption of CH3NO2 is partly reversible. On the TPD profile, desorption of the reversibly adsorbed CH3NO2 produces one peak at about 353 K; the irreversibly adsorbed molecules are decomposed into CO2 and NH3 at above 373 K. Two peaks of CO2 are observed at about 443 and 553 K. However, observable evolution of NH3occurs only at above 573 K. IR study shows that the reversible adsorption of CH3NO2 results from an interaction of the nature of hydrogen-bonding on the surface, while the irreversible adsorption from an acid-catalyzed chemical transformation to surface formamide species (HCONH2). On raising temperature up to higher than 373 K, the surface formamide species decomposes to CO2 and NH3. It is suggested that the evolution of CO2 is a reaction limited while that of NH3 is a desorption limited process. On the surface of MgO, desorption of reversibly absorbed CHsNO2 produces one peak at about 383 K. Decomposition of irreversibly adsorbed CHsNO2 occurs at 473-573 K. H2O, NO, C2H4, C2H6 and N2O, as well as trace of H2 are observed as the decomposition products. Surface species observed in the IR spectra are assigned to CH2NO2]-, bidentite H3C-N and monodentate CH3-N-O respectively.The former two species return to CHsNO2on desorption and give the peak at -383 K. Formation of surface nitrosomethane(CH3NO) and "N2O"species from the monodentate species are shown by IR at above 473 K. It is proposed that the surface CH,NO2 is an key intermediate of the decomposition reaction of CH3NO2 on MgO. The roles of acid and base sites in the adsorption and decomposition of CH3NO2 are discussed.