FTIR TDS STUDIES OF REACTION PATHS AND SURFACE INTERMEDIATES FOLLOWING MULTILAYER ADSORPTION OF FORMAMIDE ON NI(111)

The adsorption and reaction of HCONH2 and HCOND2 molecules on Ni(111) were studied following initial multilayer exposure with thermal desorption spectroscopy (TDS) and reflection absorption infrared spectroscopy (RAIR). At 90 K, formamide molecules (HCONH2) adsorb on Ni(111) forming multilayers. Three molecular desorption peaks were observed at 160, 170, and 190 K, respectively. Heating this system to above 220 K leads to the decomposition of formamide molecules. Two parallel decomposition paths were observed. One path leads to the production of NH3, H-2, and CO. The other path, which is first reported here, gives HNCO and H-2. Possible surface precursors for the two decomposition paths are discussed. The temperature-dependant RAIR spectra indicated that ammonia (NH3(a)) was formed at about 230 K, isocyanic acid (HNCO(a)) at about 285 K, and CO at about 285 K. The observed desorption temperature from TDS are 260 K for ammonia (NH3), 325 K for isocyanic acid (HNCO), 340 K for hydrogen (H-2), and 410 K for carbon monoxide (CO). The competing route for formamide decomposition that leads to HNCO production on Ni(111) has not been reported on other transition-metal substrates. Strong hydrogen bonding between adsorbed formamide molecules was indicated by peak broadening of nu-a(NH2), nu-s(NH2), and nu-C = O modes in the RAIR spectra. Hydrogen bonding is suggested between the O atom of the carbonyl (C=O) group of one formamide molecule and H atom of amine group (NH2) of another neighbor formamide molecule. From the RAIR spectra of the isotope-labeled formamide molecule HCOND2, we have identified the mass 43 amu decomposition product as HNCO rather than HOCN.