Reactivity of nitric oxide with nickel ions in A-type zeolite

Abstract

Nitric oxide adsorption over nickel-exchanged A-type zeolites has been investigated by gravimetry, e.s.r. and i.r. spectroscopy. Depending upon the nickel content of the zeolite and its dehydration process, four different nickel—nitrosyl complexes have been observed. They differ in structure as well as stability. Spin concentrations and gravimetric measurements agree well with an overall stoichiometry of one NO molecule per nickel ion in all cases.

In zeolites containing less than one nickel ion per unit cell, two isolated Ni⁺—NO⁺ complexes have been detected by e.s.r. The nickel is tetrahedrally coordinated. These species are stable as the gas-phase NO is removed and show an i.r. absorption at 1876 cm^–1.

As nickel loading reaches two ions per unit cell, binuclear nickel—nitrosyl complexes are formed in the sodalite units. A bent (Ni⁺—NO⁺)₂ complex with a central NO is detected on fully dehydrated zeolites (g_iso= 2.26; v_NO= 1876–1882 cm^–1). A different binuclear species (g_iso= 2.30) appears on zeolites which have been dehydrated in a gas flow; this unstable complex is though to have a (Ni⁺—NO⁺)₂O structure, involving a bridging extra-framework oxygen. E.s.r. intensities give the relative proportions of the various species.