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.