Ordered mesoporous materials in catalysis

Siliceous ordered mesoporous materials are well-investigated and -covered by several excellent reviews. Less work has been directed to non-siliceous materials, although these could have more wide-ranging applications. This review will address the synthesis and properties of non-siliceous ordered mesoporous materials, covering oxidic, non-oxidic, metallic, and pure carbon frameworks. Organometallic frameworks, which are typically not synthesized in the presence of surfactants, are beyond the scope of this paper.

Non-siliceous Mesostructured and Mesoporous Materials†

The photocatalytic reduction of N₂ to NH₃ is typically hampered by poor binding of N₂ to catalytic materials and by the very high energy of the intermediates involved in this reaction. Solvated electrons directly introduced into the reactant solution can provide an alternative pathway to overcome such limitations. Here we demonstrate that illuminated hydrogen-terminated diamond yields facile electron emission into water, thus inducing reduction of N₂ to NH₃ at ambient temperature and pressure. Transient absorption measurements at 632 nm reveal the presence of solvated electrons adjacent to the diamond after photoexcitation. Experiments using inexpensive synthetic diamond samples and diamond powder show that photocatalytic activity is strongly dependent on the surface termination and correlates with the production of solvated electrons. The use of diamond to eject electrons into a reactant liquid represents a new paradigm for photocatalytic reduction, bringing electrons directly to reactants without requiring molecular adsorption to the surface.

Photo-illuminated diamond as a solid-state source of solvated electrons in water for nitrogen reduction

Great progress has been made in the preparation and application of ordered mesoporous metal oxides during the past decade. However, the applications of these novel and interesting materials have not been reviewed comprehensively in the literature. In the current review we first describe different methods for the preparation of ordered mesoporous metal oxides; we then review their applications in energy conversion and storage, catalysis, sensing, adsorption and separation. The correlations between the textural properties of ordered mesoporous metal oxides and their specific performance are highlighted in different examples, including the rate of Li intercalation, sensing, and the magnetic properties. These results demonstrate that the mesoporosity has a direct impact on the properties and potential applications of such materials. Although the scope of the current review is limited to ordered mesoporous metal oxides, we believe that the information may be useful for those working in a number of fields.

Ordered mesoporous metal oxides: synthesis and applications.

We report a strategy for the production of materials with structural hierarchy. The approach employs polymer microgels as templates for the synthesis of semiconductor, metal, or magnetic nanoparticles (NPs). We show that NPs with predetermined dimensions and size-dependent properties can be synthesized by using a very delicate balance between the reaction conditions, the composition and the structure of microgel templates, and the concentration of NPs in the microgel. Postheat treatment of microgels doped with semiconductor nanoparticles reduces NP polydispersity and allows control of their photoluminescence. Microgel templates are particularly beneficial in the synthesis of polymer microspheres heavily loaded with monodisperse superparamagnetic Fe3O4 NPs. Hybrid submicrometer-size microgels have promising potential applications in photonics, catalysis, and separation technologies.

Polymer microgels: reactors for semiconductor, metal, and magnetic nanoparticles.

Mesoporous titanium oxide was treated with bis(toluene) titanium under nitrogen at room temperature in toluene, leading to a new blue black material possessing conductivity values of up to 10-2 Ω-1 cm-1. XRD and nitrogen adsorption showed that the mesostructure was fully retained. Elemental analysis indicated that the material absorbed Ti from the organometallic, without any incorporation of the toluene ligand. There was also an increase of nitrogen from below the detection limit to 1.16%. XPS studies showed that the Ti framework was reduced by the organometallic and that the material had reduced nitride on the surface. There was also an emission at the Fermi level, suggesting metallic behavior. This was confirmed by variable-temperature conductivity studies, which showed a gradual decrease of resistivity with temperature. SQUID magnetometer studies revealed spin glass behavior with a degree of temperature independent paramagnetism, consistent with metallic properties. Solid-state 15N NMR studies on mater...

Room-temperature ammonia formation from dinitrogen on a reduced mesoporous titanium oxide surface with metallic properties.

Synthesis and Electronic Properties of Reduced Mesoporous Sodium Niobium Oxides

Recently we reported that mesoporous niobium oxide can be chemically reduced by Na-naphthalene while fully retaining its mesostructure. This was the first report of a molecular sieve acting as a stoichiometric electron acceptor. Herein we expand on the initial work by presenting a detailed study on Li-, Na-, K-, Rb-, and Cs-reduced samples of mesoporous Nb oxide, as well as Li-reduced mesoporous Ta and Ti oxides. While the Nb- and Ta-based materials fully retained their structure on reduction as determined by X-ray diffraction (XRD) and nitrogen adsorption, the Li-reduced Ti material retained high surface area and narrow pore size distribution, but lost its diffraction pattern, indicating an increased level of disorder in this material. X-ray photoelectron spectroscopy (XPS) and UV-visible reflectance spectroscopy revealed that all reduced mesoporous oxides studied have a similar electronic structure, corresponding to the presence of a disordered impurity band in the material lying between the valence band and the conduction band. Electron paramagnetic resonance (EPR) studies suggest that the electron in this impurity level is unpaired and best described as a free electron, only loosely bound to the alkali or transition metal. SQUID magnetometry showed that all reduced materials are paramagnetic, further confirming the presence of unpaired electrons in the structure. All materials in this study were insulating with the exception of the Li-reduced mesoporous Ti material, which was highly conducting, possibly due to an Anderson transition. Electrochemical studies on the unreduced mesoporous oxides demonstrated that while the Ta and Nb materials are capacitors with only a small degree of reversible electrochemical behavior in the bulk sample, the Ti material was an electrical conductor with fully reversible redox behavior.

Synthesis and Characterization of a New Family of Electroactive Alkali Metal Doped Mesoporous Nb, Ta, and Ti Oxides and Evidence for an Anderson Transition in Reduced Mesoporous Titanium Oxide

Mesoporous niobium oxide was treated at room temperature with bis(toluene) niobium to make a new black material that exhibits metallic properties. The metallic behavior is attributed to low-valent NbII in the walls of the porous structure and is fully supported by strong emissions at the Fermi level, variable temperature resistivity measurements, and temperature-independent paramagnetism. These materials possess higher conductivities than reported for any molecular sieve and also represent the first example of a metallic oxide-based molecular sieve. For comparison, the conductivity is almost 10 000 times greater than the highest value measured for an open structured mesoporous material. Treatment at room temperature with dinitrogen leads to formation of a thin nitride coat on the surface. Cleavage of dinitrogen is an extremely rare and important reaction that typically requires very forcing conditions in the solid state (high temperatures in excess of several hundred degrees, argon plasmas, etc.) or low-valent coordinatively stressed metal centers in the homogeneous phase. This is the first example of a molecular sieve mediating this process and because of the controlled porosity and high surface areas of these materials they are ideal candidates for model studies for nitrogen reduction and catalytic nitrogen incorporation into organic compounds. Since porous materials are often the catalytic support of choice for industrial processes, these materials may lead to the development of commercial nitrogen incorporation processes.

Synthesis of a Stable Metallic Niobium Oxide Molecular Sieve and Subsequent Room Temperature Activation of Dinitrogen

Herein we report the synthesis and characterization of a new family of mesoporous niobium oxide bis(cyclopentadienyl)nickel composites which exhibit superparamagnetic or spin glass behavior depending on the different loading levels of the organometallic in the pores. XRD and nitrogen adsorption studies confirm the retention of the mesostructure in these new materials upon introduction of the guest species. The Nb 3d region of the XPS provides evidence for the slight reduction of the Nb(V) oxide walls while the Ni 2p region shows the presence of both neutral and cationic bis-cyclopentadienyl nickel species with peaks at 856.3 eV and 861.9 eV respectively, as well as two more highly oxidized species at 865.5 eV and 867.3 eV. The presence of neutral and cationic bis(cyclopentadienyl)nickel is further confirmed by EPR. SQUID magnetic measurements show superparamagnetic behavior for the material with a Ni∶Nb ratio of 0.07∶1,
while at a ratio of 0.1∶1 the material shows spin glass behavior, a phenomenon normally associated with amorphous metal alloys. The difference in magnetic behavior is rationalized by an increased density of unpaired spins in the pore structure.

M. Vettraino

Papers

Room-temperature ammonia formation from dinitrogen on a reduced mesoporous titanium oxide surface with metallic properties.

Synthesis and Electronic Properties of Reduced Mesoporous Sodium Niobium Oxides

Synthesis and Characterization of a New Family of Electroactive Alkali Metal Doped Mesoporous Nb, Ta, and Ti Oxides and Evidence for an Anderson Transition in Reduced Mesoporous Titanium Oxide

Synthesis of a Stable Metallic Niobium Oxide Molecular Sieve and Subsequent Room Temperature Activation of Dinitrogen

Superparamagnetic and spin glass behavior in mesoporous niobium oxide bis(cyclopentadienyl)nickel composites