Graphitic carbon nitride, g-C 3 N 4 , can be made by polymerization of cyanamide, dicyandiamide or melamine. Depending on reaction conditions, different materials with different degrees of condensation, properties and reactivities are obtained. The firstly formed polymeric C 3 N 4 structure, melon, with pendant amino groups, is a highly ordered polymer. Further reaction leads to more condensed and less defective C 3 N 4 species, based on tri-s-triazine (C 6 N 7 ) units as elementary building blocks. High resolution transmission electron microscopy proves the extended two-dimensional character of the condensation motif. Due to the polymerization-type synthesis from a liquid precursor, a variety of material nanostructures such as nanoparticles or mesoporous powders can be accessed. Those nanostructures also allow fine tuning of properties, the ability for intercalation, as well as the possibility to give surface-rich materials for heterogeneous reactions. Due to the special semiconductor properties of carbon nitrides, they show unexpected catalytic activity for a variety of reactions, such as for the activation of benzene, trimerization reactions, and also the activation of carbon dioxide. Model calculations are presented to explain this unusual case of heterogeneous, metal-free catalysis. Carbon nitride can also act as a heterogeneous reactant, and a new family of metal nitride nanostructures can be accessed from the corresponding oxides.

One of the main stumbling blocks in developing rational design strategies for heterogeneous catalysis is that the complexity of the catalysts impairs efforts to characterize their active sites. We show how to identify the crucial atomic structure motif for the industrial Cu/ZnO/Al(2)O(3) methanol synthesis catalyst by using a combination of experimental evidence from bulk, surface-sensitive, and imaging methods collected on real high-performance catalytic systems in combination with density functional theory calculations. The active site consists of Cu steps decorated with Zn atoms, all stabilized by a series of well-defined bulk defects and surface species that need to be present jointly for the system to work.

Alkynes can be selectively hydrogenated into alkenes on solid palladium catalysts. This process requires a strong modification of the near-surface region of palladium, in which carbon (from fragmented feed molecules) occupies interstitial lattice sites. In situ x-ray photoelectron spectroscopic measurements under reaction conditions indicated that much less carbon was dissolved in palladium during unselective, total hydrogenation. Additional studies of hydrogen content using in situ prompt gamma activation analysis, which allowed us to follow the hydrogen content of palladium during catalysis, indicated that unselective hydrogenation proceeds on hydrogen-saturated beta-hydride, whereas selective hydrogenation was only possible after decoupling bulk properties from the surface events. Thus, the population of subsurface sites of palladium, by either hydrogen or carbon, governs the hydrogenation events on the surface.

The low efficiency of the electrocatalytic oxidation of water to O2 (oxygen evolution reaction‐OER) is considered as one of the major roadblocks for the storage of electricity from renewable sources in form of molecular fuels like H2 or hydrocarbons. Especially in acidic environments, compatible with the powerful proton exchange membrane (PEM), an earth‐abundant OER catalyst that combines high activity and high stability is still unknown. Current PEM‐compatible OER catalysts still rely mostly on Ir and/or Ru as active components, which are both very scarce elements of the platinum group. Hence, the Ir and/or Ru amount in OER catalysts has to be strictly minimized. Unfortunately, the OER mechanism, which is the most powerful tool for OER catalyst optimization, still remains unclear. In this review, we first summarize the current state of our understanding of the OER mechanism on PEM‐compatible heterogeneous electrocatalysts, before we compare and contrast that to the OER mechanism on homogenous catalysts. Thereafter, an overview over monometallic OER catalysts is provided to obtain insights into structure‐function relations followed by a review of current material optimization concepts and support materials. Moreover, missing links required to complete the mechanistic picture as well as the most promising material optimization concepts are pointed out.

Production of the industrial chemical propylene oxide is energy-intensive and environmentally unfriendly. Catalysts based on bulk silver surfaces with direct propylene epoxidation by molecular oxygen have not resolved these problems because of substantial formation of carbon dioxide. We found that unpromoted, size-selected Ag3 clusters and approximately 3.5-nanometer Ag nanoparticles on alumina supports can catalyze this reaction with only a negligible amount of carbon dioxide formation and with high activity at low temperatures. Density functional calculations show that, relative to extended silver surfaces, oxidized silver trimers are more active and selective for epoxidation because of the open-shell nature of their electronic structure. The results suggest that new architectures based on ultrasmall silver particles may provide highly efficient catalysts for propylene epoxidation.

Butenes and butadiene, which are useful intermediates for the synthesis of polymers and other compounds, are synthesized traditionally by oxidative dehydrogenation (ODH) of n-butane over complex metal oxides. Such catalysts require high O 2 /butane ratios to maintain the activity, which leads to unwanted product oxidation. We show that carbon nanotubes with modified surface functionality efficiently catalyze the oxidative dehydrogenation of n-butane to butenes, especially butadiene. For low O 2 /butane ratios, a high selectivity to alkenes was achieved for periods as long as 100 hours. This process is mildly catalyzed by ketonic C=O groups and occurs via a combination of parallel and sequential oxidation steps. A small amount of phosphorus greatly improved the selectivity by suppressing the combustion of hydrocarbons.Transition metal oxides have been widely used as catalysts for the conversion of butane to C 4 alkenes, important industrial precursors for producing synthetic rubbers, plastics, and a number of industrially important chemicals. Despite a great deal of research, alkene selectivity in the current butane-to-butadiene process is severely limited (1). One important reason is that the unsaturated products are much more readily oxidized to CO 2 than is the starting alkane. The chemical complexity of polyvalent metal oxides, although found to be necessary for catalytic activity, impedes satisfactory selectivity through isolation of active sites (2-6). For this reason, the origin of the catalytic activity is debated, and there is as yet no generally accepted picture of the reaction mechanism (7, 8).Carbon materials have been reported to catalyze the oxidative dehydrogenation (ODH) of an aromatic molecule, ethylbenzene. However, conventional carbons, in particular activated carbon, underwent unavoidable deactivations due to coking or combustion (9-12). Recently, it was shown that only wellnanostructured carbons are stable and coke-free catalysts for styrene synthesis (12, 13). Activation of C-H bonds in the ethyl group is considered to be coordinated by the ketonic carbonyl (C=O) group. Ethylbenzene has an aromatic moiety that enables relatively facile activation. Here, we report on surfacemodified carbon nanotubes (CNTs) as a high-performance catalyst for the ODH of the much less active butane. Relative to metal-based catalysts, CNTs displayed an enhanced selectivity to C 4 alkenes, especially butadiene.We conducted the reaction at 400° or 450°C with an O 2 /butane ratio of 2.0. The product mixture contained only 1-butene, 2-butene, butadiene, CO 2 , CO, and residual reactants; the resulting carbon balance was 100 ± 3% ( fig. S1A) (14). In a blank experiment without catalyst, the alkene yield was as low as 0.9%. Over pristine CNTs, 88.9% of the converted butane was burnt, yielding 1.6% alkenes (Fig. 1A). Considering the intensive stability of CNTs in O 2 ( fig. S1B) (14), we conclude that the CO 2 during the reaction mainly originated from the oxidation of the hydrocarbon feedstock and not from burning of t...

Rechargeable lithium-ion batteries are essential to portable electronic devices. Owing to the rapid development of such equipment there is an increasing demand for lithium-ion batteries with high energy density and long cycle life. For high energy density, the electrode materials in the lithium-ion batteries must possess high specific storage capacity and coulombic efficiency. Graphite and LiCoO 2 are normally used and have high coulombic efficiencies (typically >90%) but rather low capacities (372 and 145 mAhg -1 , respectively). [1][2][3][4][5] Various anode materials with improved storage capacity and thermal stability have been proposed for lithium-ion batteries in the last decade. Among these, silicon has attracted great interest as a candidate to replace commercial graphite materials owing to its numerous appealing features: it has the highest theoretical capacity (Li 4.4 Siº4200 mAhg ) of all known materials, and is abundant, inexpensive, and safer than graphite (it shows a slightly higher voltage plateau than that of graphite as shown in Figure S1, and lithiated silicon is more stable in typical electrolytes than lithiated graphite [6]).The practical use of Si powders as a negative electrode in lithium-ion batteries is, however, still hindered by two major problems: the low intrinsic electric conductivity and severe volume changes during Li insertion/extraction processes, leading to poor cycling performance. [7][8][9][10][11][12][13][14][15][16][17][18][19][20] Tremendous efforts have been made to overcome these problems by decreasing the particle size, [7, 8a,b] using silicon-based thin films and silicon-metal alloys, [9,10, 20] dispersing silicon into an inactive/ active matrix, [11][12][13][14][15][16][17][18][19] and coating with carbon as well as using different electrolyte systems. [15, 20] In these approaches a variety of composites of active and inactive materials have been widely exploited in which the inactive component plays a structural buffering role to minimize the mechanical stress induced by huge volume change of active silicon, thus preventing the deterioration of the electrode integrity. [11][12][13][14][15][16][17][18][19] Recent work has demonstrated that anodes made of silicon/ carbon composites can combine the advantageous properties of carbon (long cycle life) and silicon (high lithium-storage capacity) to improve the overall electrochemical performance of the anode for lithium-ion batteries. [8c,9b, 11-13,15-17] In contrast to these rather complicated high-temperature processes we report here a new, simple, and green methodology for the simultaneous coating of preformed silicon nanoparticles in a one-step procedure with a thin layer of SiO x and carbon by the hydrothermal carbonization of glucose. This Si@SiO x /C nanocomposite with a typical core/shell structure, which was further modified by electrochemical in situ generation of a passivated layer, shows remarkably improved lithium-storage performance in terms of high reversible lithium-storage capacity (º1100 mAhg -1 ), ex...

Mixed bimetallic oxides offer great opportunities for a systematic tuning of electrocatalytic activity and stability. Here, we demonstrate the power of this strategy using well-defined thermally prepared Ir-Ni mixed oxide thin film catalysts for the electrochemical oxygen evolution reaction (OER) under highly corrosive conditions such as in acidic proton exchange membrane (PEM) electrolyzers and photoelectrochemical cells (PEC). Variation of the Ir to Ni ratio resulted in a volcano type OER activity curve with an unprecedented 20-fold improvement in Ir mass-based activity over pure Ir oxide. In situ spectroscopic probing of metal dissolution indicated that, against common views, activity and stability are not directly anticorrelated. To uncover activity and stability controlling parameters, the Ir-Ni mixed thin oxide film catalysts were characterized by a wide array of spectroscopic, microscopic, scattering, and electrochemical techniques in conjunction with DFT theoretical computations. By means of an intuitive model for the formation of the catalytically active state of the bimetallic Ir-Ni oxide surface, we identify the coverage of reactive surface hydroxyl groups as a suitable descriptor for the OER activity and relate it to controllable synthetic parameters. Overall, our study highlights a novel, highly active oxygen evolution catalyst; moreover, it provides novel important insights into the structure and performance of bimetallic oxide OER electrocatalysts in corrosive acidic environments.