EPR of silver–ammonia adducts in γ-irradiated AgNa-A zeolite with low silver content

Abstract

An EPR study has been performed on a low Ag⁺-exchanged Na-A zeolite (Ag_0.1Na_11.9-A) on which ammonia had been adsorbed followed by exposure to γ-radiation at 77 K. The formation of Ag atoms, normally observed for zeolites with a high Ag content, is abated by the presence of ammonia. New EPR spectra with the silver hyperfine coupling constant slightly smaller than that of isolated silver atoms were observed. The new spectra were assigned to two silver–ammonia complexes, [Ag–NH₃]⁰ and [Ag–(NH₃)₂]⁰, the former being more thermally stable than the latter; the latter decayed at 150 K. The EPR parameters of [Ag–NH₃]⁰ were determined as follows: g_iso=2.005 and a_iso(¹⁰⁷Ag)=38.0, A_⊥(¹⁴N)=1.4, A_‖(¹⁴N)=1.7 and a_iso(¹H)≈0 mT. The formation reactions and possible locations of the adduct radical are discussed in terms of annealing temperatures and the amount of ammonia introduced.