Highly dispersed Au, Ag and Cu nanoparticles in mesoporous SBA-15 for highly selective catalytic reduction of nitroaromatics

This paper demonstrates a homogeneous dispersion of 4 wt% coinage metal nanoparticles (Au, Ag and Cu) of different morphologies in the pores (similar to 8 nm) of 3-aminopropyltriethoxysilane (APTES) modified mesoporous SBA-15 for selective catalytic reduction of m-dinitrobenzene to phenylenediamine. EDX, elemental mapping and HR-TEM analysis confirmed the uniform dispersal of metal nanoparticles within the mesoporous matrix having lattice fringes with a d-spacing of 0.232 nm for Au (111), 0.23 nm and 0.20 nm for Ag (111) and (200) and 0.25 nm for CuO (111) planes. XPS results illustrated the presence of Au and Ag in their metallic states whereas Cu was oxidized to CuO. XRD, TEM and surface area analysis revealed that formation of metal nanoparticles within the sieves led to a significant change in the surface structural and physicochemical properties. Metal nanospheres with increasing size i.e., similar to 5 nm (Au) < similar to 11 nm (Ag) < similar to 13 nm (Cu) were formed within the channels of SBA-15, while small nanorods (aspect ratio similar to 2-4 nm) were also formed in the case of Ag and Cu impregnation. The catalytic activity was found to depend on the nature, size and dispersion of metal nanoparticles relative to negligible reactivity of bare SBA-15. Au nanosphere (similar to 5 nm) impregnated SBA-15, having the lowest surface area (292 m(2) g(-1)), exhibited the best catalytic activity for m-dinitrobenzene reduction (k = 1.765 x 10(-1) min(-1)) with 89% selectivity to m-phenylenediamine.