Uses of Physical Vapor Deposition Processes in Photoelectrochemical Water Splitting Systems

Most of the hydrogen on planet earth is found bound to oxygen atoms in water, making H2O one of the most promising H2 storage molecules. Large availability, non-toxicity and low cost are among the advantages of using H2O as a H2 gas source. However, the decomposition of water into H2 and O2, called water splitting, needs a large amount of energy, increasing the final cost per kg of hydrogen produced. In this context, the energy provided by the sun may be used to power photoelectrochemical cells (PEC) for water splitting to produce cheap and high purity H2. This mini-review will show recent advances on the use of physical vapor deposition (PVD) methods to improve semiconducting electrode performance. PVD enables the preparation of thin layers of expensive materials over photoelectrodes, therefore decreasing PEC systems manufacture costs. Moreover, the interface of between the semiconductor and the evaporated materials can be optimized under high vacuum conditions used in PVD processes and more efficient PEC systems can be obtained.

Iron-catalyzed hydrogenation and dehydrogenation reactions with relevance to reversible hydrogen storage applications

Today’s energy concerns require the development of suitable solutions for the storage of energy from renewable resources. Although the chemical storage of energy using molecular hydrogen as energy carrier is one of the best options, this type of energy storage requires the conversion of hydrogen to liquid organic hydrogen careers (LOHCs) for practical reasons. This goal is challenging and highly desirable at the same time. In comparison to dihydrogen, hydrogen storage in LOHCs offers easier handling and minimum dangers involved in their production, storage, and reconversion. To achieve efficient processes based on LOHCs highly active catalyst systems are required which ideally are based on cheap and abundant metals such as iron. This review summarizes recent advances in ironcatalyzed hydrogenation and dehydrogenation reactions, with relevance to reversible hydrogen storage in small molecules. It entails the dehydrogenation reactions of formic acid and methanol water mixtures, the reverse reaction, the hydrogenation of CO2, dehydrogenation of alcohols, and the hydrogenation of different carbonyl compounds as the formal reverse reaction, as well as hydrogenation and dehydrogenation reactions of N-heterocyclic compounds and hydrogen release reactions from amino boranes.

Nanoparticle-Catalysts for Hydrogen Storage Based on Small Molecules

In this mini-review, selected contributions on the development of hydrogen storage systems based on small molecules using nanocatalysts for hydrogen generation will be described. The discussion is centered on the most applied compounds such as formic acid, metal hydrides, amine-boranes, alcohols, hydrocarbons, hydrazine and water. In addition, an overview of the most important aspects relating to the application of the metal nanoparticles in each reaction is also considered.

Acid-Catalyzed Dehydration of Fructose to 5-(Hydroxymethyl)furfural

Hydroxymethylfurfural (abbreviated as HMF), also 5-(hydroxymethyl)furfural, is an organic compound derived from dehydration of certain sugars. HMF is primarily considered as a starting material for liquid transportation fuels and polyester building block chemicals. The most convenient synthetic method of HMF is based on acidcatalyzed triple dehydration of fructose. Although there are many studies about fructose dehydration to 5-HMF since this field started to be investigated, it is necessary to provide a new review about fructose dehydration to 5-HMF. In the following, we will make a summary (in detail) of catalytic systems of fructose dehydration to HMF achieved by different acid catalysts, including mineral and organic acids, metal complexes, heteropoly acid-based materials, Ionic Liquids, ion-exchange resins, zeolites, functionalized carbonaceous materials and mesoporous silica materials. It has been demonstrated that nearly full conversion of fructose and 100% HMF selectivity could be obtained with some acidic catalytic systems up to now.

Lactic acid and hydrogen from glycerol via acceptorless dehydrogenation using homogeneous catalysts

Acceptorless dehydrogenation of alcohols has emerged as a powerful methodology for the valorization of biomass derived platform chemicals and building blocks. In this review we provide a short overview of the advantages and possible product outcomes of this method. The main focus will be devoted to the conversion of glycerol, which is the major waste product of biodiesel production, to lactic acid. While extensive research addresses the development of heterogeneous catalysts, recently new and highly active iridium and ruthenium complexes have also been reported. These novel homogeneous catalysts are even more active than the already reported heterogeneous systems and enable the direct conversion of glycerol into lactic acid and molecular hydrogen. While the product hydrogen might be used either as fuel or as reducing agent for other processes, lactic acid is a platform chemical widely employed by the polymer, pharmaceutical and food industries. The used catalytic methodology is atom-economic, waste-free and is uniquely suited for the efficient conversion of renewable resources.

Alumina modified by niobia supported nickel catalysts for hydrodechlorination of 1,2-dichloroethane

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In situ time-resolved HEROS study of Pt electronic structure during regenerative H2-O2 cycles

We report an in situ time-resolved high-energy resolution off-resonant spectroscopy (HEROS) study with unprecedented 100 ms time resolution revealing the unoccupied electronic states of platinum during regenerative oxidation and reduction cycles. The study depicted a slowed oxidation step in comparison with reduction. The oxidation cycle is composed of two characteristic stages, namely adsorption of dissociated oxygen followed by partial oxidation of Pt subsurface. Besides improved temporal resolution of the experiment, the detected reduction process of platinum showed no intermediate features and was completed in a single step within a few seconds.

Chemical hydrogen storage by methanol: Challenges for the catalytic methanol synthesis from CO2

Methanol is a very promising chemical hydrogen carrier molecule. The well-established industrial methanol synthesis process is a reference case for the desired sustainable synthesis from CO

Turbostratic carbon supported Ni-Pd alloys in aqueous-phase hydrodechlorination of 1,1,2-trichloroethene

Two step modification of Norit CNR115 active carbon led to formation of mesoporous turbostratic carbon structure. Ni-Pd catalysts were prepared by incipient wetness impregnation of turbostratic active carbon with an aqueous solution of nickel and palladium chloride salts and were investigated by the Temperature-Programmed Reduction (TPR), Scanning Electron Microscopy (SEM), High Resolution Transmission Electron Microscopy (HRTEM), X-Ray Diffraction (XRD) and Temperature- Programmed Hydrogenation (TPH). The aqueous-phase hydrodechlorination of 1,1,2-trichloroethene (TCE) were carried out in a batch reactor at room temperature. Addition of palladium to nickel catalysts resulted in increasing of TCE conversion from 85% for Ni100 to more than 90% for Ni95Pd05 after 150 minutes of reaction. Aqueous-phase hydrodechlorination of TCE led to the formation of hydrocarbons (ethane and ethene) as the main products. Temperature-programmed hydrogenation of the catalysts after kinetic run have shown that during reactions with TCE only a small amount of carbon species were deposited on the catalysts surface and that chloride species were not observed.

Effect of silver on the catalytic hydrodechlorination of tetrachloromethane over silica-supported palladium-silver catalysts

Two silica-supported palladium based catalysts characterized by overall metal loadings 10 wt.%, and atomic Pd : Ag ratios of 100 : 0 and 70 : 30, were prepared by incipient wetness impregnation, subjected to reduction in hydrogen and characterized by XRD. It was found that incipient wetness co-impregnation of silica resulted in the formation of a catalyst characterized by relatively small metal crystallites (~11 nm) and significant extent of Pd-Ag alloying. The catalytic performance was investigated in the hydrodechlorination of tetrachloromethane in the gas phase. Silver addition to palladium in the co-impregnated catalysts had very pronounced effect, vastly decreasing the overall activity (conversion) and also the selectivity towards hydrocarbons, propensity characteristic for the monometallic palladium catalysts working in CCl4 hydrodechlorination carried out in the gas phase. Instead, large amounts of C2HxCly dimeric products were formed. These dimers are considered as possible coke precursors. Post-reaction samples of both catalysts contained large amount of carbon which entered the Pd and Pd-Ag phases. This carbon can be easily removed by treatment with hydrogen at 450ºC.