Stable Water Oxidation in Acid Using Manganese-Modified TiO2 Protective Coatings.

Accomplishing acid-stable water oxidation is a critical matter for achieving both long-lasting water-splitting devices and other fuel-forming electro- and photocatalytic processes. Because water oxidation releases protons into the local electrolytic environment, it becomes increasingly acidic during device operation, which leads to corrosion of the photoactive component and hence loss in device performance and lifetime. In this work, we show that thin films of manganese-modified titania, (Ti,Mn)O x, topped with an iridium catalyst, can be used in a coating stabilization scheme for acid-stable water oxidation. We achieved a device lifetime of more than 100 h in pH = 0 acid. We successfully grew (Ti,Mn)O x coatings with uniform elemental distributions over a wide range of manganese compositions using atomic layer deposition (ALD), and using X-ray photoelectron spectroscopy, we show that (Ti,Mn)O x films grown in this manner give rise to closer-to-valence-band Fermi levels, which can be further tuned with annealing. In contrast to the normally n-type or intrinsic TiO2 coatings, annealed (Ti,Mn)O x films can make direct charge transfer to a Fe(CN)63-/4- redox couple dissolved in aqueous electrolytes. Using the Fe(CN)63-/4- redox, we further demonstrated anodic charge transfer through the (Ti,Mn)O x films to high work function metals, such as iridium and gold, which is not previously possible with ALD-grown TiO2. We correlated changes in the crystallinity (amorphous to rutile TiO2) and oxidation state (2+ to 3+) of the annealed (Ti,Mn)O x films to their hole conductivity and electrochemical stability in acid. Finally, by combining (Ti,Mn)O x coatings with iridium, an acid-stable water-oxidation anode, using acid-sensitive conductive fluorine-doped tin oxides, was achieved.

Silica-supported isolated gallium sites as highly active, selective and stable propane dehydrogenation catalysts.

Single-site gallium centers on the surface of silica are prepared via grafting of [Ga(OSi(OtBu)3)3(THF)] on SiO2-700 followed by a thermolysis step. The resulting surface species corresponds to well-defined tetra-coordinate gallium single-sites, [([triple bond, length as m-dash]SiO)3Ga(XOSi[triple bond, length as m-dash])] (X = -H or [triple bond, length as m-dash]Si) according to IR, X-ray absorption near-edge structure and extended X-ray absorption fine structure analysis. These gallium sites show high activity, selectivity and stability for propane dehydrogenation with an initial turnover frequency of 20 per h per gallium center, propylene selectivity of ≥93% and remarkable stability over 20 h. The stability of the catalyst probably results from site-isolation of the active site on a non-reducible support such as silica, diminishing facile reduction typical of Ga2O3-based catalysts.

Magnetic Memory from Site Isolated Dy(III) on Silica Materials.

Achieving magnetic remanence at single isolated metal sites dispersed at the surface of a solid matrix has been envisioned as a key step toward information storage and processing in the smallest unit of matter. Here, we show that isolated Dy(III) sites distributed at the surface of silica nanoparticles, prepared with a simple and scalable two-step process, show magnetic remanence and display a hysteresis loop open at liquid 4He temperature, in contrast to the molecular precursor which does not display any magnetic memory. This singular behavior is achieved through the controlled grafting of a tailored Dy(III) siloxide complex on partially dehydroxylated silica nanoparticles followed by thermal annealing. This approach allows control of the density and the structure of isolated, "bare" Dy(III) sites bound to the silica surface. During the process, all organic fragments are removed, leaving the surface as the sole ligand, promoting magnetic remanence.

Three-Dimensional Structure Determination of Surface Sites.

The spatial arrangement of atoms is directly linked to chemical function. A fundamental challenge in surface chemistry and catalysis relates to the determination of three-dimensional structures with atomic-level precision. Here we determine the three-dimensional structure of an organometallic complex on an amorphous silica surface using solid-state NMR measurements, enabled through a dynamic nuclear polarization surface enhanced NMR spectroscopy approach that induces a 200-fold increase in the NMR sensitivity for the surface species. The result, in combination with EXAFS, is a detailed structure for the surface complex determined with a precision of 0.7 Å. We observe a single well-defined conformation that is folded toward the surface in such a way as to include an interaction between the platinum metal center and the surface oxygen atoms.

C-H Activation on Co,O Sites: Isolated Surface Sites versus Molecular Analogs.

The activation and conversion of hydrocarbons is one of the most important challenges in chemistry. Transition-metal ions (V, Cr, Fe, Co, etc.) isolated on silica surfaces are known to catalyze such processes. The mechanisms of these processes are currently unknown but are thought to involve C-H activation as the rate-determining step. Here, we synthesize well-defined Co(II) ions on a silica surface using a metal siloxide precursor followed by thermal treatment under vacuum at 500 °C. We show that these isolated Co(II) sites are catalysts for a number of hydrocarbon conversion reactions, such as the dehydrogenation of propane, the hydrogenation of propene, and the trimerization of terminal alkynes. We then investigate the mechanisms of these processes using kinetics, kinetic isotope effects, isotopic labeling experiments, parahydrogen induced polarization (PHIP) NMR, and comparison with a molecular analog. The data are consistent with all of these reactions occurring by a common mechanism, involving heterolytic C-H or H-H activation via a 1,2 addition across a Co-O bond.

Low Temperature Activation of Supported Metathesis Catalysts by Organosilicon Reducing Agents.

Alkene metathesis is a widely and increasingly used reaction in academia and industry because of its efficiency in terms of atom economy and its wide applicability. This reaction is notably responsible for the production of several million tons of propene annually. Such industrial processes rely on inexpensive silica-supported tungsten oxide catalysts, which operate at high temperatures (>350 °C), in contrast with the mild room temperature reaction conditions typically used with the corresponding molecular alkene metathesis homogeneous catalysts. This large difference in the temperature requirements is generally thought to arise from the difficulty in generating active sites (carbenes or metallacyclobutanes) in the classical metal oxide catalysts and prevents broader applicability, notably with functionalized substrates. We report here a low temperature activation process of well-defined metal oxo surface species using organosilicon reductants, which generate a large amount of active species at only 70 °C (0.6 active sites/W). This high activity at low temperature broadens the scope of these catalysts to functionalized substrates. This activation process can also be applied to classical industrial catalysts. We provide evidence for the formation of a metallacyclopentane intermediate and propose how the active species are formed.

Highly Active Subnanometer Au Particles Supported on TiO2 for Photocatalytic Hydrogen Evolution from a Well-Defined Organogold Precursor, [Au5(mesityl)5].

A highly efficient H2 evolution photocatalyst based on TiO2 supported subnanometer Au particles was developed on the basis of the reaction of a gold(I) molecular precursor [Au5Mes5] (Mes = 2,4,6-trimethylphenyl), with titanium dioxide partially dehydroxylated at 120 °C. IR, UV-vis, elemental analysis, XANES, and STEM-EDX show that the deposition of [Au5Mes5] onto TiO2 leads to the formation of both subnanometer Au particles and chemisorbed [Au5Mes5]. The remaining organic ligands are removed via a mild treatment under H2, yielding subnanometer gold(0) particles. A range of Au loadings (0.3, 0.9, 2.4 wt %) with similar particle sizes (<1 nm) on TiO2 are obtained and tested in methanol-assisted photocatalytic hydrogen production under UV light. These catalysts display significantly higher activity than a commercial reference Au-TiO2 catalyst. The presence of chemisorbed [Au5Mes5] in the as-synthesized catalyst further improved activity, albeit at the expense of stability. This work demonstrates a simple synthetic route to obtain subnanometer Au particles on TiO2 that display exceptional activity in photocatalysis.

Proton transfers are key elementary steps in ethylene polymerization on isolated chromium(III) silicates.

Mononuclear Cr(III) surface sites were synthesized from grafting [Cr(OSi(O(t)Bu)3)3(tetrahydrofurano)2] on silica partially dehydroxylated at 700 °C, followed by a thermal treatment under vacuum, and characterized by infrared, ultraviolet-visible, electron paramagnetic resonance (EPR), and X-ray absorption spectroscopy (XAS). These sites are highly active in ethylene polymerization to yield polyethylene with a broad molecular weight distribution, similar to that typically obtained from the Phillips catalyst. CO binding, EPR spectroscopy, and poisoning studies indicate that two different types of Cr(III) sites are present on the surface, one of which is active in polymerization. Density functional theory (DFT) calculations using cluster models show that active sites are tricoordinated Cr(III) centers and that the presence of an additional siloxane bridge coordinated to Cr leads to inactive species. From IR spectroscopy and DFT calculations, these tricoordinated Cr(III) sites initiate and regulate the polymer chain length via unique proton transfer steps in polymerization catalysis.

Polymerization of ethylene by silica-supported dinuclear Cr(III) sites through an initiation step involving C-H bond activation.

The insertion of an olefin into a preformed metal-carbon bond is a common mechanism for transition-metal-catalyzed olefin polymerization. However, in one important industrial catalyst, the Phillips catalyst, a metal-carbon bond is not present in the precatalyst. The Phillips catalyst, CrO3 dispersed on silica, polymerizes ethylene without an activator. Despite 60 years of intensive research, the active sites and the way the first CrC bond is formed remain unknown. We synthesized well-defined dinuclear Cr(II) and Cr(III) sites on silica. Whereas the Cr(II) material was a poor polymerization catalyst, the Cr(III) material was active. Poisoning studies showed that about 65 % of the Cr(III) sites were active, a far higher proportion than typically observed for the Phillips catalyst. Examination of the spent catalyst and isotope labeling experiments showed the formation of a Si-(µ-OH)-Cr(III) species, consistent with an initiation mechanism involving the heterolytic activation of ethylene at Cr(III) O bonds.