Mesoporous metal oxides with uniform porosity are of considerable interest. Their economical production on a large scale in an efficient manner, however, remains a challenging task for commercialization. In this work, we demonstrate for the first time a scalable, economic, energy and time efficient method for the synthesis of a crystalline mesoporous CeO 2 catalyst with tailored porosity, by utilizing colloidal SiO 2 as a template. The size and amount of colloidal particles can tune the porosity of the CeO 2 nanostructure as well as alter the heat transfer and heat balance of combustion. As-prepared CeO 2 possesses uniform 22 nm pores and a 0.6 mL/g pore volume, which is the largest pore volume for CeO 2 reported. The obtained mesoporous CeO 2 catalyst exhibited excellent activity for soot and carbon monoxide oxidation. In principle, this method can be applied to synthesize different high-porosity crystalline oxides, and mesoporous CuO was also successfully prepared, thus demonstrating the generality of the method. at 00:50:55 (UTC).See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. ■ REFERENCES(1) Schuẗh, F. Endo-and Exotemplating to Create High-Surface-Area Inorganic Materials. Angew. Chem., Int. Ed. 2003, 42, 3604−3622. (2) Ren, Y.; Ma, Z.; Bruce, P. G. Ordered mesoporous metal oxides: synthesis and applications. Chem. Soc. Rev. 2012, 41, 4909−4927. Figure 8. BJH pore size distribution curve and TEM image (inset, SAED) of CuO synthesized via CSCS.
Wadsley−Roth phases accommodate variable cation charge by crystallographic shear planes delineating blocks of the parent ReO 3 structure. The homologous series TiNb x O 2+2.5x provides possible new anode materials for lithium-ion batteries. The thermodynamic stability of three of these shear phases was determined by high-temperature oxide melt solution calorimetry. TiNb 2 O 7 , TiNb 24 O 62 , and TiNb 5 O 14.5 (often called Ti 2 Nb 10 O 29 ) all have positive enthalpies of formation from binary oxides (TiO 2 and Nb 2 O 5 ), implying that they are entropy-stabilized and only stable above some minimum temperature. Hence, shear phases may represent a new and extensive class of "entropy-stabilized oxides". Their thermodynamic stability decreases with the increasing Nb content. Entropies of formation were calculated using the measured enthalpy of formation and assuming that their synthesis temperature is their lowest temperature of stability and using calculated configurational entropies arising from cation disorder. TiNb 24 O 62 has a high entropy consistent with extensive disorder, whereas TiNb 2 O 7 and TiNb 5 O 14.5 appear to be substantially more ordered. These entropy values are further constrained by considering the stability of the Wadsley−Roth phases with respect to each other. TiNb 2 O 7 and TiNb 5 O 14.5 can be relatively stable intercalating anode materials, while TiNb 24 O 62 is likely to decompose near room temperature during extended battery cycling. This work accentuates the underlying role of thermodynamic studies in engineering electrochemically active materials with enhanced stability for next-generation lithium-ion batteries and beyond.
The stability of functional materials in watercontaining environments is critical for their industrial applications. A wide variety of metal−organic frameworks (MOFs) synthesized in the past decade have strikingly different apparent stabilities in contact with liquid or gaseous H 2 O, ranging from rapid hydrolysis to persistence over days to months. Here, we show using newly determined thermochemical data obtained by high-temperature drop combustion calorimetry that these differences are thermodynamically driven rather than primarily kinetically controlled. The formation reaction of a MOF from metal oxide (MO) and a linker generally liberates water by the reaction MO + linker = MOF + H 2 O. Newly measured enthalpies of formation of Mg-MOF-74 (s) + H 2 O (l) and Ni-MOF-74 (s) + H 2 O (l) from their crystalline dense components, namely, the divalent MO (MgO or NiO) and 2,5dihydroxyterephthalic acid, are 303.9 ± 17.2 kJ/mol of Mg for Mg-MOF-74 and 264.4 ± 19.4 kJ/mol of Ni for Ni-MOF-74. These strongly endothermic enthalpies of formation indicate that the reverse reaction, namely, the hydrolysis of these MOFs, is highly exothermic, strongly suggesting that this large thermodynamic driving force for hydrolysis is the reason why the MOF-74 family cannot be synthesized via hydrothermal routes and why these MOFs decompose on contact with moist air or water even at room temperature. In contrast, other MOFs studied previously, namely, zeolitic imidazolate frameworks (ZIF-zni, ZIF-1, ZIF-4, Zn(CF 3 Im) 2 , and ZIF-8), show enthalpies of formation in the range 20−40 kJ per mole of metal atom. These modest endothermic enthalpies of formation can be partially compensated by positive entropy terms arising from water release, and these materials do not react appreciably with H 2 O under ambient conditions. Thus, these differences in reactivity with water are thermodynamically controlled and energetics of formation, either measured or predicted, can be used to assess the extent of water sensitivity for different possible MOFs.
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