Catalyst
stability in the liquid phase under polar conditions,
typically required for the catalytic conversion of renewable platform
molecules, is a major concern but has been only sparsely studied.
Here, the activity, selectivity, and stability of Ru-based catalysts
supported on TiO2, ZrO2, and C in the conversion
of levulinic acid (LA) to γ-valerolactone (GVL) has been studied
at 30 bar of H2 and 423 K in dioxane as solvent. All catalysts
showed excellent yields of GVL when used fresh, but only the Ru/ZrO2 catalyst could maintain these high yields upon multiple recycling.
Surprisingly, the widely used Ru/TiO2 catalyst showed quick
signs of deactivation already after the first catalytic test. XPS,
CO/FT-IR, TGA, AC-STEM, and physisorption data showed that the partial
deactivation is not due to Ru sintering or coking but rather due to
reduction of the titania support in combination with partial coverage
of the Ru nanoparticles, i.e. due to a detrimental strong metal–support
interaction. In contrast, the zirconia support showed no signs of
reduction and displayed high morphological and structural stability
even after five recycling tests. Remarkably, in the fresh Ru/ZrO2 catalyst, Ru was found to be fully atomically dispersed on
the fresh catalyst even at 1 wt % Ru loading, with some genesis of
Ru nanoparticles being observed upon recycling. Further studies with
the Ru/ZrO2 catalyst showed that dioxane can be readily
replaced by more benign solvents, including GVL itself. The addition
of water to the reaction mixture was furthermore shown to promote
the selective hydrogenation reaction.
Ultrathin single-crystalline RuO 2 (110) films supported on Ru(0001) are employed as model electrodes to extract kinetic information about the industrially important chlorine evolution reaction (CER) in a 5M concentrated NaCl solution under well-defined electrochemical conditions and variable temperatures. A combination of chronoamperometry (CA) and online electrochemical mass spectrometry (OLEMS) experiments provides insight into the selectivity issue: At pH = 0.9, the CER dominates over oxygen evolution, whereas at pH = 3.5, oxygen evolution and other parasitic side reactions contribute mostly to the total current density. From temperaturedependent CA data for pH = 0.9, we determine the apparent free activation energy of the CER over RuO 2 (110) to be 0.91 eV, which compares reasonably well with the theoretical value of 0.79 eV derived from first-principles microkinetics. The experimentally determined apparent free activation energy of 0.91 eV is considered as a benchmark for assessing future improved theoretical modeling from first principles.
Dehydrogenation promoters greatly enhance the performance of SiO 2 −MgO catalysts in the Lebedev process. Here, the effect of preparation method and order of addition of Cu on the structure and performance of Cupromoted SiO 2 −MgO materials is detailed. Addition of Cu to MgO via incipient wetness impregnation (IWI) or coprecipitation (CP) prior to wet-kneading with SiO 2 gave similar butadiene yields (∼40%) as when Cu was added to the already wet-kneaded catalyst. In contrast, the catalyst prepared by impregnation of Cu on SiO 2 first proved to be the worst catalyst of the series. TEM, XRD, and XPS analyses suggested that, for all catalyst materials, Cu 2+ forms a solid solution with MgO. This was confirmed by UV−vis, XANES, and EXAFS data, with Cu being found in a distorted octahedral geometry. As a result, the acid−base properties, as determined by Pyridine-and CDCl 3 −IR as well as NH 3 -TPD, are modified, contributing to the improved performance. Operando XANES and EXAFS studies of the evolution of the copper species showed that Cu 2+ , the only species initially present, is extensively reduced to a mixture of Cu 0 and Cu + , leaving only a limited amount of unreduced Cu 2+ . This formation of Cu 0 is the result of the reducing environment of the Lebedev process and is thought to be mainly responsible for the improved performance of the Cu-promoted catalysts.
The one‐step synthesis and characterization of a new and robust titanium‐based metal–organic framework, ACM‐1, is reported. In this structure, which is based on infinite Ti−O chains and 4,4′,4′′,4′′′‐(pyrene‐1,3,6,8‐tetrayl) tetrabenzoic acid as a photosensitizer ligand, the combination of highly mobile photogenerated electrons and a strong hole localization at the organic linker results in large charge‐separation lifetimes. The suitable energies for band gap and conduction band minimum (CBM) offer great potential for a wide range of photocatalytic reactions, from hydrogen evolution to the selective oxidation of organic substrates.
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