Solid materials with a given functionality are of relevance for many fields in chemical research. Surface functionalities are of specific interest and determine the properties of materials for processes that proceed at the interface of a solid and a fluid. Typical processes are heterogeneously catalyzed reactions, adsorption processes, and electrochemical reactions. Crucial factor for efficient processes are high specific surface areas that can be realized either by generation of pores in a solid or by formation of nanoscopic particles.
Zeolites comprise a class of crystalline, microporous solids. They are mainly based on aluminosilicates, the structure of which are formed by corner-sharing SiO4 and AlO4 tetrahedra. Presence of three-valent aluminium in tetrahedral oxygen coordination results in negative framework charges that are compensated by extra-framework cations. The latter are generally exchangeable alkali or alkaline earth cations that are located in micropores (< 2nm) that are an inherent feature of zeolite structures. Replacement of the cations by protons results in acidic zeolites that are excellent catalysts for petrochemical processes and the production of fine chemicals. The pore size and geometry of a given zeolite limit the sizes of molecules that can enter the pore systems, making zeolites highly selective catalysts and efficient adsorbents.
Solids with ordered mesopore arrangement can be synthesized either via soft- or hard-templating methods. Typical materials that are obtained via soft-templating are MCM-41 or SBA-15 silicas. Amorphous silica walls are condensed around hexagonally ordered arrays of soft micelles consisting of amphiphilic molecules. Removal of the organics results in silica with ordered mesopores (2-50 nm). Carbon deposition in the pores of such silica and removal of the solid silica is hard-templating. As the results either carbon rods (complete pore filling) or carbon tubes (only deposition of carbon layer on silica wall) are obtained. The image shows a typical electron density distribution through a section of an array of hexagonally arranged carbon tubes (CMK-5). It has been calculated from a low-angle XRD pattern via inverse Fourier Transformation of reflection intensities.
Adsorption and diffusion processes are strongly affected by the pore systems and surface properties of nanoporous materials. Physical adsorption on solids is determined by interaction of molecules in the fluid phase with a given surface. Pore geometries and surface functionalities are factors that can be modified in order to alter diffusive and/or adsorptive properties of a given material. Generation of hierarchical pore systems has been shown to generate catalysts with significantly improved performance as the result of enhanced diffusivity. Investigation of adsorptive and diffusive properties of nanoporous materials allows for a better understanding of the processes involved, which in turn allows design of optimized materials for catalytic and adsorptive applications.
High surface area materials serve as catalysts or supports for active species. There are typically synthesized via precipitation, sol-gel processing, or flame processes. “Exotemplating” is a versatile alternative, e.g., using porous carbon matrices as hard templates. The pores of the carbon matrix are filled with concentrated salt solution. Subsequent combustion of the carbon matrix converts the metal salt into the oxide. The particle size of the oxide is determined by the pore size of the carbon. High surface area oxides synthesized from activated carbon are typically obtained as fine powders or particles that adopt the shape of the parent carbon. Using carbon aerogel monoliths as exotemplate, alumina with remarkable textural properties (X-ray amorphous, surface area 300 m2/g, volume > 1.5 mL/g) can be synthesized.
Irradiation of a semiconductor photocatalyst with light of appropriate wavelength results in the excitation of an electron from the valence band into the conduction band, leaving a positively charged hole in the valence band. This electron then can be transferred to an electron acceptor whereas an electron from a donor recombines with the hole in the valence band. In this manner, redox reaction can be catalyzed on the surfaces of semiconductors, such as for example TiO2 or ZnO. Combining semiconductor properties with specific steric arrangements within the pores of nanoporous solids appears a promising approach to tune the photocatalytic properties of such solids.
Dr. Castro, Maria
Dr. Schmidt, Wolfgang N.