Conduction electron paramagnetic resonance of metal nanoparticles in AlMCM-41 aluminosilica mesoporous molecular sieves

Mesoporous aluminosilica molecular sieves AlMCM-41 are loaded with different amounts of Ag+, Pd2+ and Pt2+ ions by varying the Si/Al ratio, which changes the ion exchange capacity. These metal ions are reduced to metal particles by hydrogen and characterized by conduction electron paramagnetic resonance (CEPR). Metal particle sizes were calculated from the Kawabata theory that relates CEPR linewidths to metal particle size. The CEPR linewidths are independent of metal ion loading, which suggests that the metal particles are located inside the AlMCM-41 channels and size constrained by the channel diameter. The calculated metal particle diameters are about 1 nm, which is smaller than the 3.5 nm channel diameter of AlMCM-41. The validity of Kawabata's theory to such systems may need reexamination.