Ordered-mesochannels Confined Silver Nanoparticles: Controlled Construction, Characterization And The Catalytic Behavior

Througout the last decades, nanocatalysis has long been paid much attentions due to its considerable contributions to industry. To explore nanosize effect in catalysis, one has to be able to synthesize monodispersed metal nanoparticles (e.g., silver) with high thermal stabilities, which has been proved to be extremely difficult in practice. Although some feasible routes have been developed to synthesize protected metal nanoparticles, it is difficult to prevent those obtained metal nanoparticles from sintering during the high-temperature reactions. Recently, due to their higher surface areas, uniform mesostructure and tunable pore size etc., the ordered mesoporous materials have initiated the intensive research of inclusion chemistry, which is featured with the assemblage of metal, metal oxide, organometal, as well as semiconductors, inside the channels of mesoporous inorganic materials.Current work mainly focused on the fabrication of monodispersed metal (silver) nanoparticles within the mesoporous silica materials with tunable pore-size. These mesoporous silica trapped metal nanoparticles have unique physical/chemical and, most importantly, catalytic properties. The details are as follows: (1) By finely tuning the reaction compositions, a temperature-induced phase transformation, from a swollen inverse hexagonal phase (H2) to a swollen lamellar phase (Lα) and then to a swollen normal micelles phase (L1), is set up in the alkane (from heptane to dodecane)/P123/water/TEOS quadru-component system, directed by which, corresponding complex silica materials, such as mesoporous nano-fibres, multilamellar vesicles (MLVs) and mesocellular foams(MCFs), are obtained. (2) By controlling initial reaction temperatures, alkanes with different chain length (C5-C16) have been used as swelling agents to the continuous pore size expansion of highly ordered SBA-15. For the first time, the pore size of highly ordered SBA-15 was extended to ca. 15 nm. (3) Amino functional groups have been grafted on the surface of ordered mesoporous silicas and further used to anchor the reducing species (-NH-CH2-OH) and then to assemble highly dispersed Ag nanoparticles in mild conditions. Furthermore, when amino groups are selectively grafted on the internal surface of ordered mesoporous silicas, the highly dispersed silver nanoparticles with very narrow size distribution could be fabricated within the channels of mesoporous silicas. Significantly, in-situ heating TEM and XRD have also been used to investigate the thermal stabilities of silver nanoparticles trapped within the mesopores of SBA-15, and possible coarsening mechanisms of mesoporous channels confined sivler nanoparticles have also been proposed. (4) By tuning the pore size of mesoporous silicas, silver nanoparticles with different particle size have been synthesized. It was found that silver nanoparticles showed size-dependent behavior in the dissociation of oxygen into Oγspecies. The smaller the Ag particles are, the easier the formation of Oγ. Most importantly, the size dependent oxygen dissociation behavior has been further confirmed in the selective oxidation reactions of carbon monoxide, methanol and methane at different reaction temperatures. Namely, smaller silver particles, which are apt to the Oγformation, show much higher selectivity.