Holli

Metal carbide species have been proposed as a new type of chemical entity to activate methane in both gas-phase and condensed-phase studies. Herein, methane activation by the diatomic cation MoC⁺ is presented. MoC⁺ ions have been prepared and mass-selected by a quadrupole mass filter and then allowed to interact with methane in a hexapole reaction cell. The reactant and product ions have been detected by a reflectron time-of-flight mass spectrometer. Bare metal Mo⁺ and MoC₂H₂⁺ ions have been observed as products, suggesting the occurrence of ethylene elimination and dehydrogenation reactions. The branching ratio of the C₂H₄ elimination channel is much larger than that of the dehydrogenation channel. Density functional theory calculations have been performed to explore in detail the mechanism of the reaction of MoC⁺ with CH₄. The computed results indicate that the ethylene elimination process involves the occurrence of spin conversions in the CC coupling (doublet→quartet) and hydrogen atom transfer (quartet→sextet) steps. The carbon atom in MoC⁺ plays a key role in methane activation because it becomes sp³ hybridized in the initial stages of the ethylene elimination reaction, which leads to much lower energy barriers and more stable intermediates. This study provides insights into the CH bond activation and CC coupling involved in methane transformation over molybdenum carbide-based catalysts.

Methane activation and CC coupling: Methane conversion to ethylene has been observed in its reaction with MoC⁺ (see scheme). Hydrogen atom transfer (HAT) from CH₄ to C rather than to Mo initiates the reaction. Further CC coupling and hydrogen transfers lead to the formation of C₂H₄. The bond strength preservation of MoC is important in methane activation.

The photoredox activation of organic substrates with visible light is a powerful methodology that generates reactive radical species under very mild conditions. When combined with another catalytic process in a dual catalytic system, novel, visible-light-promoted transformations have been realized that do not proceed using either catalyst in isolation. In this minireview, the state of the art in organic reactions mediated by dual catalytic systems merging photoredox activation with organo-, acid or metal catalysis is discussed.

De(light)ful catalysis! The merger of photoredox catalysis with another catalytic mode can result in novel, visible-light-promoted reactions that do not proceed by using either catalyst independently. Herein, the different ways that two catalytic modes can operate in tandem are highlighted, focusing on dual-catalyzed organic processes that merge photoredox with organo-, acid, and transition-metal catalysis.

The dramatic effect of substrate impurities on the performance of a specific ruthenium catalyst system is demonstrated in the benchmark metathesis reaction of diethyl diallylmalonate. Based on detailed two-dimensional GC–time-of-flight MS measurements, the significant influence of small amounts of different contaminations, especially various organic halides, is shown. This work serves as an incisive example of the importance of impurities to catalyst performance, also in homogeneous catalysis, which is often ignored in academic research.

Pure or not pure, that is the question: The presence of small amounts of impurities has a dramatic influence on the reactivity and selectivity of a novel ruthenium-based ring-closing metathesis (RCM) catalyst.

Ordered mesoporous NiAl and NiCeAl catalysts with different Ce/Al molar ratios were facilely synthesized by using the improved evaporation-induced self-assembly method. The characterization results confirmed that the ordered mesoporous structure was well sustained in the Ce-incorporated NiAl materials (Ce/Al molar ratio<4 %). Compared with NiAl mesoporous materials, Ce-incorporated mesoporous materials demonstrated higher specific surface areas, larger pore volumes, and more uniform pore sizes. The catalytic test conducted by using methane dry reforming revealed that compared with NiAl catalysts, all the Ce-promoted catalysts demonstrated improved initial catalytic activity, which was due to the high dispersion and the high reduction degree of active Ni species in NiCeAl catalysts. The stable alumina framework, confinement effect of ordered mesopores, and high oxygen mobility contributed to the improved catalytic stability of NiCeAl catalysts. For comparison, the mesoporous NiCeAl catalyst (denoted as NiCeAl-IMP) was also prepared by using the conventional impregnation method. The agglomeration of Ni particles was observed during the stability test for the Ni-impregnated catalyst, which accelerated the rate of carbon deposition. The NiCeAl catalyst (Ce/Al molar ratio=1 %) demonstrated excellent resistance to the formation of graphitic carbon species owing to the redox property, while a large amount of graphitic carbon species was deposited over the NiAl sample, which was responsible for deactivation.

Everything begins with an order: Ordered mesoporous NiAl and NiCeAl (R) catalysts with various Ce contents were prepared by using the improved evaporation-induced self-assembly method. Compared with NiAl catalysts, Ce-incorporated catalysts demonstrated higher specific surface areas, larger pore volumes, and more uniform pore sizes. The incorporation of Ce promoted the high dispersion and high reducibility of Ni species, which led to an improved catalytic activity for methane dry reforming.

Au/TiO₂ catalysts prepared by a deposition–precipitation process and used for CO oxidation without previous calcination exhibited high, largely temperature-independent conversions at low temperatures, with apparent activation energies of about zero. Thermal treatments, such as He at 623 K, changed the conversion–temperature characteristics to the well-known S-shape, with activation energies slightly below 30 kJ mol⁻¹. Sample characterization by XAFS and electron microscopy and a low-temperature IR study of CO adsorption and oxidation showed that CO can be oxidized by gas-phase O₂ at 90 K already over the freeze-dried catalyst in the initial state that contained Au exclusively in the +3 oxidation state. CO conversion after activation in the feed at 303 K is due to Au^III-containing sites at low temperatures, while Au⁰ dominates conversion at higher temperatures. After thermal treatments, CO conversion in the whole investigated temperature range results from sites containing exclusively Au⁰.

Ionic or metallic: Au³⁺ ions on TiO₂ (see HAADF-STEM image of a freshly prepared sample) can catalyze the oxidation of CO at low temperatures. The reaction rates at Au³⁺-containing centers are similar to those found at metallic gold clusters. However, the apparent activation energies are very low, which is probably due to the opposing influence of the true activation energy and the adsorption enthalpy of CO on Au³⁺ centers.

John Wiley and Sons, Hoboken, 2013. 380 pp., hardcover, € 132.00.—ISBN 978-0470915349

TiO₂ photocatalysts loaded with isolated Pt atoms were successfully synthesized by a simple and convenient technique as described by H. F. Wang and H. G. Yang et al. in their Communication on page 2138 ff. Isolated Pt atoms can stably anchor on TiO₂ and exhibit a high photocatalytic hydrogen evolution performance compared with Pt nanoparticles or clusters. Moreover, the configurations of the isolated Pt atoms and their catalytic hydrogen evolution activity were calculated by large-scale periodic DFT analysis. The results obtained for such model catalysts not only opens a door for synthesizing high-efficiency catalytic materials, but also has a great potential to reduce the high cost of commercial noble metal catalysts in industry.

The reaction of [Cp*Ir(bzpy)NO₃] (1; bzpy=2-benzoylpyridine, Cp*=pentamethylcyclopentadienyl anion), a competent water-oxidation catalyst, with several oxidants (H₂O₂, NaIO₄, cerium ammonium nitrate (CAN)) was studied to intercept and characterize possible intermediates of the oxidative transformation. NMR spectroscopy and ESI-MS techniques provided evidence for the formation of many species that all had the intact Ir–bzpy moiety and a gradually more oxidized Cp* ligand. Initially, an oxygen atom is trapped in between two carbon atoms of Cp* and iridium, which gives an oxygen–Ir coordinated epoxide, whereas the remaining three carbon atoms of Cp* are involved in a η³ interaction with iridium (2 a). Formal addition of H₂O to 2 a or H₂O₂ to 1 leads to 2 b, in which a double MeCOH functionalization of Cp* is present with one MeCOH engaged in an interaction with iridium. The structure of 2 b was unambiguously determined in the solid state and in solution by X-ray single-crystal diffractometry and advanced NMR spectroscopic techniques, respectively. Further oxidation led to the opening of Cp* and transformation of the diol into a diketone with one carbonyl coordinated at the metal (2 c). A η³ interaction between the three non-oxygenated carbons of “ex-Cp*” and iridium is also present in both 2 b and 2 c. Isolated 2 b and mixtures of 2 a–c species were tested in water-oxidation catalysis by using CAN as sacrificial oxidant. They showed substantially the same activity than 1 (turnover frequency values ranged from 9 to 14 min⁻¹).

Signs of three: Three intermediates from the oxidative transformation of a Cp*–iridium water-oxidation catalyst have been intercepted and characterized by using NMR spectroscopy, ESI-MS and, in one case, X-ray crystallography. Progressive oxidation of Cp* has been observed, whereas the benzoylpyridine ancillary ligand remains intact. Isolated intermediates and their mixture are still active in water-oxidation catalysis (see scheme).

Metal-free systems, including frustrated Lewis pairs (FLPs) have been shown to bind CO₂. By reducing the Lewis acidity and basicity of the ambiphilic system, it is possible to generate active catalysts for the deoxygenative hydroboration of carbon dioxide to methanol derivatives with conversion rates comparable to those of transition-metal-based catalysts.

Less is more: Metal-free systems, including frustrated Lewis pairs (FLPs), have been shown to bind CO₂. By reducing the Lewis acidity and basicity of the ambiphilic system, it is possible to generate active catalysts for the deoxygenative hydroboration of carbon dioxide to methanol derivatives with conversion rates comparable to those of transition-metal-based catalysts (see scheme).

Supported nanoparticles (NPs) of nonplasmonic transition metals (Pd, Pt, Rh, and Ir) are widely used as thermally activated catalysts for the synthesis of important organic compounds, but little is known about their photocatalytic capabilities. We discovered that irradiation with light can significantly enhance the intrinsic catalytic performance of these metal NPs at ambient temperatures for several types of reactions. These metal NPs strongly absorb the light mainly through interband electronic transitions. The excited electrons interact with the reactant molecules on the particles to accelerate these reactions. The rate of the catalyzed reaction depends on the concentration and energy of the excited electrons, which can be increased by increasing the light intensity or by reducing the irradiation wavelength. The metal NPs can also effectively couple thermal and light energy sources to more efficiently drive chemical transformations.

An effective energy boost: Electrons in nonplasmonic transition-metal nanoparticles absorb light energy by interband absorption (see picture) and drive a wide range of well-established organic reactions with high efficiency at ambient temperatures.