HCN adsorption on silica and titania supported Rh catalysts studied by FTIR

Abstract

The adsorption of HCN on SiO₂, TiO₂, Rh/SiO₂ and Rh/TiO₂ was investigated by FTIR. HCN adsorbs either in intact molecular form with participation of surface OH groups, or through the lone pair electrons of its nitrogen on oxides; the latter leads to the formation of isocyanide (Si–NC and Ti–NC) surface species. Some of the adsorbed isocyanides undergoes isomerisation forming cyanides (Si–CN and Ti–CN). Hydrogenation of the isocyanides causes the appearance of adsorbed aminomethylidine (CNH_2(a)) on oxides. At higher temperatures the isocyanide is oxidized to isocyanate (Si–NCO and Ti–NCO) with the help of OH groups of the oxides. The presence of Rh on silica promotes the formation of the above surface species due to the activity of Rh in HCN dissociation. No such promoter effects of Rh on titania were experienced. The dissociation of HCN on Rh produces CN_(a) on the metal.