Solid-state NMR and TGA studies of silver reduction in chabazite

Silver-exchanged molecular sieves have shown great promise in applications ranging from antimicrobial materials to the adsorption of xenon and iodide, two key contaminants emitted from nuclear reactors. In this work, solid-state 27Al and 29Si MAS NMR and TGA were used to study silver reduction in silver-exchanged chabazite under various thermal conditions. The solid-state NMR results for both 27Al and 29Si show that there are no major changes in the chabazite during silver reduction in an argon stream; however a progressive structural change does take place in the hydrogen stream. The structural change likely involves breaking the silicon oxygen bond of the Si-O-AI fragment of chabazite, leading to the formation of extra-framework aluminum oxide. The TGA results at temperatures up to 600 degrees C indicate that silver reduction is less complete in an argon stream than in a hydrogen stream. In this paper we propose that silver reduction occurs via the following reactions: 2(Ag + ZO-)+H2O --> 1/2O2+2Ag0 + 2ZOH and nAg + mAg = Ag(m+n)n+ (in an argon stream); and Ag(+) + ZO(-) + 1/2H2 = Ag0 + ZOH and 2ZOH = ZO(-) + Z(+) + H2O (in a hydrogen stream).