A FTIR search for linkage isomerism of CN− ions adsorbed on oxides and zeolites

Abstract

Adsorption of HCN and DCN was studied at 150–600 K on TiO₂, ZnO, MgO and a set of X zeolites by means of FTIR spectroscopy. At the lowest temperatures adsorption is non-dissociative and occurs on surface OH groups of oxides, cations and coordinately unsaturated oxygen atoms. Bands at 2243–2147 cm⁻¹ assignable to CN⁻ ions start to grow in the spectra of ZnO at 170 K, at 250 K for MgO and at 300 K for TiO₂, where bands of polymeric species appear simultaneously. The structure of bands near 2200 cm⁻¹ in the spectra of the studied oxides does not display a temperature dependence characteristic of isomeric forms. Reversible temperature changes of the band of CN⁻ ions in Na-, K-, Rb- and CsX zeolites exposed to HCN provide evidence for linkage isomerism, when the ions bound to alkali cations via C- or N- atoms co-exist in the thermodynamic equilibrium. C-Bonded species account for the stronger band at 2200–2140 cm⁻¹, while the low-frequency shoulder of this band, which appears and grows at elevated temperatures, is due to the energetically less favorable N-bonded form. The enthalpy of isomerization, estimated from the temperature dependence of band intensities, decreases from ca 20 kJ mol⁻¹ or more for Na⁺ and K⁺ to 7.6 kJ mol⁻¹ for Cs⁺, in accordance with the electrostatic model.