Study of the state of nickel in ion-exchanged NiNaY zeolite by IR spectroscopy using molecular hydrogen adsorbed at low temperatures

Abstract

The state of nickel cations in NiNaY zeolite subjected to thermal vacuum and oxidative-reductive treatment was analyzed using diffuse reflectance IR spectra of molecular hydrogen, CO, and hydroxy groups. In a sample dehydrated at 570 K, nickel forms oxide particles, which are presumably located in big cavities of a zeolite. Heating of the sample at 670 K results in decomposition of the oxide with the participation of hydroxy groups followed by the formation of stabilized Ni2+ cations in large cavities of the zeolite. Calcination of NiNaY zeolite in a hydrogen atmosphere at 730 K leads to Ni2+ cation reduction to the metal state and to the formation of acidic hydroxy groups. Nevertheless, Ni2+ cations can be regenerated by reoxidation of the reduced sample with oxygen at 670 K and further evacuation at 870 K. Thermal destruction of NiO in the reoxidized sample also proceeds on acidic hydroxy groups, but the temperature of this process is higher than the temperature of oxide decomposition in the initial NiNaY zeolite. The IR spectra of molecular hydrogen complexes with Ni2+ cations were observed for the first time. A considerable decrease in the H-H stretching vibration frequency for the coordinated molecules (up to 210 cm’1) can be explained by electron density redistribution between a hydrogen molecule and the cation in the complex