The gas-phase loading of [Zn4O(bdc)3] (MOF-5; bdc = 1,4-benzenedicarboxylate) with the volatile compound [Ru(cod)(cot)] (cod = 1,5-cyclooctadiene, cot = 1,3,5-cyclooctatriene) was followed by solid-state (13)C magic angle spinning (MAS) NMR spectroscopy. Subsequent hydrogenolysis of the adsorbed complex inside the porous structure of MOF-5 at 3 bar and 150 degrees C was performed, yielding ruthenium nanoparticles in a typical size range of 1.5-1.7 nm, embedded in the intact MOF-5 matrix, as confirmed by transmission electron microscopy (TEM), selected area electron diffraction (SAED), powder X-ray diffraction (PXRD), and X-ray absorption spectroscopy (XAS). The adsorption of CO molecules on the obtained Ru@MOF-5 nanocomposite was followed by IR spectroscopy. Solid-state (2)H NMR measurements indicated that MOF-5 was a stabilizing support with only weak interactions with the embedded particles, as deduced from the surprisingly high mobility of the surface Ru-D species in comparison to surfactant-stabilized colloidal Ru nanoparticles of similar sizes. Surprisingly, hydrogenolysis of the [Ru(cod)(cot)]3.5@MOF-5 inclusion compound at the milder condition of 25 degrees C does not lead to the quantitative formation of Ru nanoparticles. Instead, formation of a ruthenium-cyclooctadiene complex with the arene moiety of the bdc linkers of the framework takes place, as revealed by (13)C MAS NMR, PXRD, and TEM.
The loading of [Zn4O(bdc)3] (MOF-5; bdc = 1,4-benzenedicarbocylate) with nanocrystalline Cu and ZnO species was achieved in a two-step process. First, the solvent-free gas-phase adsorption of the volatile precursors [CpCuL] (L = PMe3, CNtBu) and ZnEt2 leads to the isolable inclusion compounds precursor@MOF-5. These intermediates were then converted into Cu@MOF-5 and ZnO@MOF-5 by hydrogenolysis or photoassisted thermolysis at 200−220 °C in the case of Cu and hydrolysis or dry oxidation at 25 °C followed by annealing 250 °C in the case of ZnO. 17O labeling studies using H2 17O (30%) revealed that neither the bdc linkers nor the central oxide ion of the Zn4O unit exchange oxygen atoms/ions with the imbedded ZnO species. The obtained material Cu@MOF-5 (11 wt % Cu), exhibiting an equivalent Langmuir surface of 1100 m2·g−1, was further characterized by powder X-ray diffraction (PXRD), X-ray absorption spectroscopy (XAS), and transmission electron microscopy (TEM). The Cu nanoparticles are homogeneously distributed over the MOF-5 microcrystals, occupying only about 1% of the cavities. Their size distribution appears to be polydisperse with a majority around 1 nm in size (by EXAFS) together with a minority of larger particles up to 3 nm (PXRD). Cu@MOF-5 was reversibly surface oxidized/reduced by N2O/H2 treatment, resulting in a (Cu2O/Cu)@MOF-5 material as revealed by PXRD and XAS. Depending on the preparation conditions of the ZnO@MOF-5 materials a variation of the ZnO loading from 10 to 35 wt % was achieved. PXRD, TEM, UV−vis, and 17O-MAS NMR spectroscopy gave evidence for a largely intact MOF-5 matrix with imbedded ZnO nanoparticles <4 nm being in the quantum size regime. Doubly-loaded (Cu/ZnO)@MOF-5 samples were prepared by gas-phase loading of ZnO@MOF-5 with [CpCuL] followed by thermally activated hydrogenolysis. The initial catalytic productivity in methanol synthesis from a CO/CO2/H2 gas mixture at 1 atm and 220 °C peaked at about 60% of an industrial reference catalyst. This result is particular surprising because of the comparably low Cu loading (1.4 wt %) and small Cu specific surface area <1 m2·g−1, thus suggesting a superior interfacial contact between the Cu and ZnO nanophases. However, the materials (Cu/ZnO)@MOF-5 were unstable under catalytic conditions over several hoours, the metal organic framework collapsed, and the final catalytic activities were poor.
”Systematic errors”, namely, oxygen vacancy sites in ZnO, are the active centers for the hydrogenation of CO to give methanol. Nanocrystalline ZnO with a high density of oxygen vacancies was prepared from special organometallic precursors, and its catalytic properties for methanol synthesis were studied.
A systematic study was conducted to understand the influence of two different dopant cations (Zr4+ and Hf4+) incorporated into the ceria lattice. A modified coprecipitation technique was employed to make the investigated Ce x Zr1-x O2 (CZ) and Ce x Hf1-x O2 (CH) mixed oxides. The study was comprised of extensive characterization of the prepared catalysts using different techniques, namely, X-ray powder diffraction (XRD), Raman spectroscopy (RS), X-ray photoelectron spectroscopy (XPS), ion scattering spectroscopy (ISS), transmission electron microscopy (TEM), UV−vis diffuse reflectance spectroscopy (UV−vis DRS), and BET surface area method. To assess the usefulness of these catalysts, oxygen storage−release capacity (OSC) and CO oxidation activity measurements were performed. The XRD analyses reveal that the CZ sample bears Ce0.75Zr0.25O2 and Ce0.6Zr0.4O2 phases and the CH sample possesses only the Ce0.8Hf0.2O2 phase after calcination at different temperatures (773−1073 K). RS measurements suggest a defective structure of the mixed oxides resulting in the formation of oxygen vacancies. The TEM results indicate nanometer-sized crystallites and there is no appreciable increase in the particle size even after high temperature treatments. The XPS studies reveal the presence of cerium in both Ce3+ and Ce4+ oxidation states. The ISS results indicate surface enrichment of cerium in the case of the CH sample, while such surface enrichment of cerium is not observed for the CZ sample. The UV−vis DRS measurements provide information about Ce4+ ← O2− and Ce3+ ← O2− charge transfer transitions. The absence of free ZrO2 and HfO2 in the mixed oxides tenders the clue about the formation of respective solid solutions. The CH catalyst exhibited better OSC and CO oxidation activity compared to that of the CZ sample. The OSC and CO oxidation activity results correlate well with the structural characterization data. The influence of ionic radii of dopant cations on the overall performance of the ceria-based mixed oxides is contemplated.
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