Influence of morphology of hollow silica-alumina composite spheres on their activity for hydrolytic dehydrogenation of ammonia borane

Hollow silica-alumina composite spheres were prepared by a polystyrene(PS)template method using various amounts of PS suspension.Homogeneous hollow spheres prepared using 40 g were found to be with a diameter of about 300 nm in scanning electron microscopy,and transmission electron microscopy demonstrated their hollow sphere morphology.From the nitrogen adsorption isotherm results,the homogeneous hollow spheres prepared using 40 g of the PS suspension were found to be an ordered pore structure.The activities of the hollow spheres prepared using various amounts of the PS suspension for hydrolytic dehydrogenation of ammonia borane were compared.The results showed that 10,7,and 6 mL of hydrogen were evolved from the aqueous ammonia borane solution in about 40 min in the presence of the hollow spheres prepared using 40,80,and 120 g of PS suspension,respectively.The homogeneous hollow spheres with an ordered pore structure showed the highest activity among all the hollow spheres.The amount of acid sites and the coordination number of aluminum active species were characterized using neutralization titration and solid-state ^(27)Al magic angle spinning nuclear magnetic resonance spectroscopy.The homogeneous hollow spheres with an ordered pore structure had high amount of acid sites and 4-coordinated aluminum species.The relative proportion of 4-coordinated aluminum species was related to the dispersion of aluminum species.These results indicate that the homogeneous hollow spheres with an ordered pore structure showed the high activity because of high amount of acid sites induced by the highly dispersed aluminum species.

Synthesis of Micro-Mesoporous Ti-MOR/Silica Composite Spheres in Oil-in-Water Microemulsion System

The hierarchically structured micro-mesoporous spheres(MMS) composed of mesoporous silica and Ti-containing mordenite (Ti-MOR) zeolite were self-assembled in an oil-in-water microemulsion system containing tetrabutyl orthosilicate as silica source, cetyltrimethylammonium bromide as template and aluminum sulfate as additive. The composite materials possessed the connatural microporosity of zeolite together with the disordered mesopores(5.41 nm) in silica part. With a special focus on the importance of aluminum sulfate additive, a possible formation mechanism has been proposed, in which double electrostatic interactions played the crucial role of mediating the mesosilica species and zeolite crystals. The obtained MMS materials, with a tunable particle dimension(250-720 μm) and a changeable content of active component Ti-MOR(44%-70%), showed enhanced catalytic activity and lifetime in the liquid-phase ammoximation of cyclohexanone in comparison to the parent Ti-MOR powder.

Multi-scale Modeling of the Effective Chloride Ion Diffusion Coefficient in Cement-based Composite Materials

N-layered spherical inclusions model was used to calculate the effective diffusion coefficient of chloride ion in cement-based materials by using multi-scale method and then to investigate the relationship between the diffusivity and the microstructure of cement-basted materials where the microstructure included the interfacial transition zone （ITZ） between the aggregates and the bulk cement pastes as well as the microstructure of the bulk cement paste itself. For the convenience of applications, the mortar and concrete were considered as a four-phase spherical model, consisting of cement continuous phase, dispersed aggregates phase, interface transition zone and their homogenized effective medium phase. A general effective medium equation was established to calculate the diffusion coefficient of the hardened cement paste by considering the microstructure. During calculation, the tortuosity （n） and constrictivity factors （Ds/Do） of pore in the hardened pastes are n^3.2, Ds/Do=l.Ox 10-4 respectively from the test data. The calculated results using the n-layered spherical inclusions model are in good agreement with the experimental results; The effective diffusion coefficient of ITZ is 12 times that of the bulk cement for mortar and 17 times for concrete due to the difference between particle size distribution and the volume fraction of aggregates in mortar and concrete.

INTERFACE EFFECT ON THE EFFECTIVE BULK MODULUS OF A PARTICLE-REINFORCED COMPOSITE

Classical micromechanical methods for calculating the effective moduli of a heteroge- neous material are generalized to include the interface(surface)effect.By using Hashin's Composite Sphere Assemblage(CSA)model,a new expression of the bulk modulus for a particle-reinforced com- posite is derived.It is emphasized that the present study is within the finite-deformation framework such that the effective properties are not influenced by the interface stress itself solely,but influenced by the change of the interface stress due to changes of the shape and size of the interface.Hence some inadequacies in previous papers are pointed out.

A composite sphere assemblage model for porous oolitic rocks:Application to thermal conductivity

The present work is devoted to the determination of linear effective thermal conductivity of porous rocks characterized by an assemblage of grains(oolites) coated by a matrix. Two distinct classes of pores, i.e.micropores or intra oolitic pores(oolite porosity) and mesopores or inter oolitic pores(inter oolite porosity), are taken into account. The overall porosity is supposed to be connected and decomposed into oolite porosity and matrix porosity. Within the framework of Hashin composite sphere assemblage(CSA)models, a two-step homogenization method is developed. At the first homogenization step, pores are assembled into two layers by using self-consistent scheme(SCS). At the second step, the two porous layers constituting the oolites and the matrix are assembled by using generalized self-consistent scheme(GSCS) and referred to as three-phase model. Numerical results are presented for data representative of a porous oolitic limestone. It is shown that the influence of porosity on the overall thermal conductivity of such materials may be significant.