Study of the thermal behavior of surface species formed by direct reaction of cyanogen with silica

Abstract

The pyrolysis of ▪SiNCO (the main species formed during the interaction at 420°C of cyanogen with silica outgassed at 1000°C) has been investigated by a slow thermal programmed desorption (TPD) method followed by isothermal pyrolysis (IP) at 1000°C. CO, CO 2 and N 2 are evolved but, whereas almost all the carbon can be restored to the gas phase, a large part of the nitrogen remains bound onto the surface as a silicon nitride. Treatment with oxygen at 1000°C allows restoration of all the nitrogen into the gas phase and regeneration of silica. CO 2 has been revealed as an important intermediary species able, at high temperatures, to react with silicon nitride and SiNC (or ▪SiCN) species formed both during the pyrolysis of ▪SiNCO and the C 2N 2/SiO 2 interaction at 420°C. The slow restoration of the remaining small amount of carbon (as CO in the gas phase) during the final isothermal treatment is connected with the very difficult oxidation of the ▪SiCN (or SiNC) species which occurs only above 900°C through the attack of siloxane bridges. When C 2N 2 is allowed to react with a weakly dehydroxylated sample large quantities of CO 2 are formed during the TPD at rather low temperatures (600–700°C) whereas CO and N 2 are evolved at higher temperatures. In this case, CO 2 formation is thought to be the result of the decomposition of a surface species, namely, ▪Si 2NCOOH which is produced by the reaction of a proximate silanol group with ▪SiNCO. The moderate quantities of hydrogen evolved during the ultimate high temperature treatment accounts for the pyrolysis of the very stable remaining ▪SiNHSi ▪ group.