Thermal Chemistry of Nitromethane on Cu(111)

• Main Authors: Cui-Fang Syu, 許翠芳
• Other Authors: Chao-Ming Chiang
• Format: Others
• Language: zh-TW
• Published: 2012
• Online Access: http://ndltd.ncl.edu.tw/handle/72099342009174894269

碩士 === 國立中山大學 === 化學系研究所 === 100 === Nitromethane is the simplest organic-nitro compound as well as the archetype of an important class of high explosive. Homogeneous nitromethane reactions have been the subject of extensive studies. Particularly the unimolecular isomerization of nitromethane to methyl nitrite is proven to be competitive with simple C-N bond (bond energy 60 kcal/mol) rupture. The activation energy for the rearrangement was measured to be 55.5 kcal/mol and methyl nitrite has a very weak CH3O-NO bond energy 42 kcal/mol lower than that for homolysis. The thermal chemistry of nitromethane on Cu(111) was studied by a combination of temperature-programmed desorption (TPD) and reflection absorption infrared spectroscopy (RAIRS) techniques. TPD spectra show that the desorption features include the physisorbed multilayer and monolayer of CH3NO2 at 150 and 190 K, respectively. The major decomposition pathway is via cleavage of O-N bond to yield a major product NO, which is characterized by m/z 30(NO+). A possible contribution from isomerization of nitromethane to methyl nitrite (CH3NO2 CH3ONO) on the surface cannot be ruled out at 278 K. In addition to isomerization, the dehydrogenation products CO and CO2 are also unveiled as part of the desorption features at 314 and 455 K, respectively. We can further prove the reactivity of nitromethane on Cu(111) at 367 K by using the deuterated form of nitromethane which reveals the corresponding desorption TPR/D signals of D2, D2O and CD4. However, we find that nitromethane also reacts by dissociating the C-H bond and the O-N bond, however, leaving the C-N bond intact. Along this reaction channel, HCN desorbs as a product above 360 K, as evidenced by a broad desorption feature of m/z 27. Dimerization of CN to C2N2 occurs at 815 K. The RAIR spectroscopy demonstrates that nitromethane is indeed adsorbed on Cu(111) at 100 K. The formation of methoxy and formyl are supported by the observation of desorption of NO at 278 K with the characteristic NO stretching mode found at 1535 cm-1. Moreover, we assign side-bonded CN and aminomethylene (HC-NH2) present on Cu(111). After the surface is annealed to 330 K, a signature band at 2173 cm-1 is assigned to terminal-bounded CN stretching mode. This band eventually fades out above 900 K consistent with the evolution of cyanogen at 815 K.