Manganese oxides occur naturally as minerals in at least 30 different crystal structures, providing a rigorous test system to explore the significance of atomic positions on the catalytic efficiency of water oxidation. In this study, we chose to systematically compare eight synthetic oxide structures containing Mn(III) and Mn(IV) only, with particular emphasis on the five known structural polymorphs of MnO2. We have adapted literature synthesis methods to obtain pure polymorphs and validated their homogeneity and crystallinity by powder X-ray diffraction and both transmission and scanning electron microscopies. Measurement of water oxidation rate by oxygen evolution in aqueous solution was conducted with dispersed nanoparticulate manganese oxides and a standard ruthenium dye photo-oxidant system. No Ru was absorbed on the catalyst surface as observed by XPS and EDX. The post reaction atomic structure was completely preserved with no amorphization, as observed by HRTEM. Catalytic activities, normalized to surface area (BET), decrease in the series Mn2O3 > Mn3O4 ≫ λ-MnO2, where the latter is derived from spinel LiMn2O4 following partial Li(+) removal. No catalytic activity is observed from LiMn2O4 and four of the MnO2 polymorphs, in contrast to some literature reports with polydispersed manganese oxides and electro-deposited films. Catalytic activity within the eight examined Mn oxides was found exclusively for (distorted) cubic phases, Mn2O3 (bixbyite), Mn3O4 (hausmannite), and λ-MnO2 (spinel), all containing Mn(III) possessing longer Mn-O bonds between edge-sharing MnO6 octahedra. Electronically degenerate Mn(III) has antibonding electronic configuration e(g)(1) which imparts lattice distortions due to the Jahn-Teller effect that are hypothesized to contribute to structural flexibility important for catalytic turnover in water oxidation at the surface.

Microporous metal-organic frameworks (MOFs) have attracted tremendous attention because of their versatile structures and tunable porosity that allow almost unlimited ways to improve their properties and optimize their functionality, making them very promising for a variety of important applications, especially in the adsorption and separation of small gases and hydrocarbons. Numerous studies have demonstrated that MOFs with multifunctional groups, such as open metal sites (OMSs) and Lewis basic sites (LBSs), interact strongly with carbon dioxide and are particularly effective in its capture and separation from binary mixtures of CO(2) and N(2). In this feature article, we briefly review the current state of MOF development in this area, with an emphasis on the effect of multifunctional groups on the selectivity and capacity of MOFs for the CO(2) capture from flue gas mixtures.

As a new family of adsorbent materials, porous metal-organic frameworks (MOFs) have attracted enormous attention over the past decade.[1] Having a large surface area, [2] tunable pore size and shape, [3] adjustable composition and functionalizable pore surface, [4] MOFs show unique advantages and promises for potential applications in adsorption-based storage and separation technologies for small gas molecules such as H 2 , CO 2 , and CH 4 . [1b,d, 5] CO 2 capture from flue gases is of particular importance in reducing greenhouse gas emissions and in preserving environmental health. A flue gas mixture is composed of nitrogen, carbon dioxide, water vapor, oxygen, and other minor components such as carbon monoxide, nitrogen oxides, and sulfur oxides.[1b, 6] Separation of low-concentration CO 2 (about 10-15 %) from nitrogen-rich streams remains a challenging task at the present time. Adsorption-based CO 2 capture and separation is considered an effective way and may have a real potential if adsorbents with both high CO 2 selectivity and capacity near room temperature (up to 50 8C) and in the lowpressure range can be developed. [7] Recent studies have revealed a number of MOFs that show a high performance in capturing and separating CO 2 from N 2 and other small gases under conditions mimicking power plant flue gas mixtures. [8]

No abstract