Water promoted structural evolution of Ag nanocatalysts supported on alumina

Water is often involved in many catalytic processes,which can strongly affect structural evolution of catalysts during pretreatments and catalytic reactions.In this work,we demonstrate a promotional effect of H_(2)O on both oxidative dispersion and spontaneous aggregation of Ag nanocatalysts supported on alumina.Ag nanoparticles supported onγ-Al_(2)O_(3) and Ag nanowires on Al_(2)O_(3)(0001)can be dispersed into nanoclusters via annealing in O_(2)above 300℃,which is accelerated by introduction of H_(2)O into the oxidative atmosphere.Furthermore,the formed highly dispersed Ag nanoclusters are subject to spontaneous aggregation in humid atmosphere at room temperature.Ex situ and in situ characterizations in both powder and model catalysts suggest that formation of abundant surface hydroxyls and/or water adlayer on the Al_(2)O_(3) surface in the H_(2)O-containing atmosphere facilitates the surface migration of Ag species,thus promoting both dispersion and aggregation processes.The aggregation of the supported Ag nanostructures induced by the humid oxidative atmosphere enhances CO oxidation but inhibits selective catalytic reduction of NO with C_(3)H_(6).This work illustrates the critical role of H_(2)O in structure and catalytic performance of metal nanocatalysts,which can be widely present in heterogeneous catalysis.

Self-assembly of a photoluminescent metal-organic cage and its spontaneous aggregation in dilute solutions enabling timedependent emission enhancement

Charged metal-organic cages generally produce aggregates with various morphologies and different properties through the multiple supramolecular interactions in solution. Herein, a luminescent hexahedral metal-organic cage containing pyrene chromophores is successfully constructed through coordination-driven subcomponent self-assembly. The cage exhibits novel spontaneous aggregation in a dilute solution and time-dependent luminescence enhancement behavior during the subsequent incubation process. Dynamic light scatter(DLS) and transmission electron microscopy(TEM) results prove that the metalorganic cages can form blackberry-like aggregates in methanol dilute solution. Unexpectedly, the luminescent intensity of this system shows a linear increase with the extension of the incubation time in methanol, and this process is also reflected in the change in the quantum yield of the system(2% to over 80% after 5 days incubation time). Ultraviolet-visible(UV-vis),1 H nuclear magnetic resonance(1 H NMR) and mass spectra show that metal-organic cages can stably exist in dilute solution. Timedepended DLS and TEM data reveal that the aggregates of metal-organic cages are gradually changed from the dense state to the loose one, which may involve the transition of the system from an energy unstable state to a stable one, probably leading to the unusual time-dependent luminescent property. This unique time-dependent luminescent cage aggregate can be potentially applied as a “supramolecular time meter”.