In situ EPR spectroscopy proved to be a versatile tool to identify active sites for photocatalytic hydrogen generation in modified Y2Ti2O7 and CsTaWO6 catalysts of pyrochlore structure, in which the metal cations are located in two different positions A and B. It was found that the B-sites exclusively occupied by titanium (Y2Ti2O7) and tantalum/tungsten (CsTaWO6) act as electron traps on the surface. From these sites, electron transfer to the co-catalysts proceeds. Thus, the B-sites are responsible for photocatalytic water reduction.
In the context of homogeneous catalysis, open-shell systems are often quite challenging to characterize. Nuclear magnetic resonance (NMR) spectroscopy is the most frequently applied tool to characterize organometallic compounds, but NMR spectra are usually broad, difficult to interpret and often futile for the study of paramagnetic compounds. As such, electron paramagnetic resonance (EPR) has proven itself as a useful spectroscopic technique to characterize paramagnetic complexes and reactive intermediates. EPR spectroscopy is a particularly useful tool to investigate their electronic structures, which is fundamental to understand their reactivity. This paper describes some selected examples of studies where EPR spectroscopy has been useful for the characterization of open-shell organometallic complexes. The paper concentrates in particular on systems where EPR spectroscopy has proven useful to understand catalytic reaction mechanisms involving paramagnetic organometallic catalysts. The expediency of EPR spectroscopy in the study of organometallic chemistry and homogenous catalysis is contextualized in the introductory Sect. 1. Section 2 of the review focusses on examples of C–C and C–N bond formation reactions, with an emphasis on catalytic reactions where ligand/substrate non-innocence plays an important role. Both carbon and nitrogen centered radicals have been shown to play an important role in these reactions. A few selected examples of catalytic alcohol oxidation proceeding via related N-centered ligand radicals are included in this section as well. Section 3 covers examples of the use of EPR spectroscopy to study important commercial ethylene oligomerization and polymerization processes. In Sect. 4 the use of EPR spectroscopy to understand the mechanisms of Atom Transfer Radical Polymerization is discussed. While this review focusses predominantly on the application of EPR spectroscopy in mechanistic studies of C–C and C–N bond formation reactions mediated by organometallic catalysts, a few selected examples describing the application of EPR spectroscopy in other catalytic reactions such as water splitting, photo-catalysis, photo-redox-catalysis and related reactions in which metal initiated (free) radical formation plays a role are included as well. EPR spectroscopic investigation in this area of research are dominated by EPR spectroscopic studies in isotropic solution, including spin trapping experiments. These reactions are highlighted in Sect. 5. EPR spectroscopic studies have proven useful to discern the correct oxidation states of the active catalysts and also to determine the effective concentrations of the active species. EPR is definitely a spectroscopic technique that is indispensable in understanding the reactivity of paramagnetic complexes and in conjunction with other advanced techniques such as X-ray absorption spectroscopy and pulsed laser polymerization it will continue to be a very practical tool.
A novel and simple hydrogen storage system was developed, based on the dehydrogenative coupling of inexpensive ethylenediamine with ethanol to form diacetylethylenediamine. The system is rechargeable and utilizes the same ruthenium pincer catalyst for both hydrogen loading and unloading procedures. It is efficient and uses a low catalyst loading. Repetitive reversal reactions without addition of new catalyst result in excellent conversions in both the dehydrogenation and hydrogenation procedures in three cycles.
In support of the hydrogen economy: An efficient and simple homogeneous hydrogen carrier system was developed based on the dehydrogenative coupling of ethylenediamine with ethanol to form diacetylethylenediamine. The same ruthenium pincer catalyst is used for both hydrogen loading and unloading reactions.
The efficiency of a single-junction photovoltaic cell is constrained by the Shockley-Queisser limit. Here, the authors adopt a triple-junction configuration which relaxes material and current-matching constraints, providing a generic strategy for advancing the efficiency roadmap of photovoltaic technologies.
Nature Communications doi: 10.1038/ncomms8730
Authors: Fei Guo, Ning Li, Frank W. Fecher, Nicola Gasparini, Cesar Omar Ramirez Quiroz, Carina Bronnbauer, Yi Hou, Vuk V. Radmilović, Velimir R. Radmilović, Erdmann Spiecker, Karen Forberich, Christoph J. Brabec
Why we are teaching science wrong, and how to make it right
Nature 523, 7560 (2015). http://www.nature.com/doifinder/10.1038/523272a
Author: M. Mitchell Waldrop
Active problem-solving confers a deeper understanding of science than does a standard lecture. But some university lecturers are reluctant to change tack.
Lifelong learning: Science professors need leadership training
Nature 523, 7560 (2015). doi:10.1038/523279a
Authors: Charles E. Leiserson & Chuck McVinney
To drive discovery, scientists heading up research teams large and small need to learn how people operate, argue Charles E. Leiserson and Chuck McVinney.
The active site of many non-noble metal cathodic oxygen reduction catalysts consists of a nitrogen-corodinated transition metal. Here, the authors report an iron-based electrocatalyst devoid of iron–nitrogen coordination, and demonstrate its high activity in acid and alkaline media.
Nature Communications doi: 10.1038/ncomms8343
Authors: Kara Strickland, Elise Miner, Qingying Jia, Urszula Tylus, Nagappan Ramaswamy, Wentao Liang, Moulay-Tahar Sougrati, Frédéric Jaouen, Sanjeev Mukerjee
Artificial photosynthesis is a means of harnessing solar energy to generate fuels but has traditionally been exploited for the generation of hydrogen. Here, Schreier et al . instead employ a perovskite photovoltaic device to effect the solar conversion of CO 2 to CO with high efficiency.
Nature Communications doi: 10.1038/ncomms8326
Authors: Marcel Schreier, Laura Curvat, Fabrizio Giordano, Ludmilla Steier, Antonio Abate, Shaik M. Zakeeruddin, Jingshan Luo, Matthew T. Mayer, Michael Grätzel
Palladium is an effective but expensive catalyst used in catalytic converters. Here, the authors show that defective Co 3 O 4 nanocrystals, synthesized via oxidation of carbon-encapsulated cobalt nanoparticles, display similar or even comparable catalytic activity to palladium for hydrocarbon combustion.
Nature Communications doi: 10.1038/ncomms8181
Authors: Han Wang, Chunlin Chen, Yexin Zhang, Lixia Peng, Song Ma, Teng Yang, Huaihong Guo, Zhidong Zhang, Dang Sheng Su, Jian Zhang
Despite intensive research on photochemical activation of sol–gel metal oxide materials, the relatively long processing time and lack of deep understanding of the underlying chemical courses have limited their broader impact on diverse materials and applications such as thin-film electronics, photovoltaics, and catalysts. Here, in-depth studies on the rapid photochemical activation of diverse sol–gel oxide films using various spectroscopic and electrical investigations for the underlying physicochemical mechanism are reported. Based on the exhaustive chemical and physical analysis, it is noted that deep ultraviolet-promoted rapid film formation such as densification, polycondensation, and impurity decomposition is possible within 5 min via in situ radical-mediated reactions. Finally, the rapid fabrication of all-solution metal oxide thin-film-transistor circuitry, which exhibits stable and reliable electrical performance with a mobility of >12 cm2 V−1 s−1 and an oscillation frequency of >650 kHz in 7-stage ring oscillator even after bending at a radius of <1 mm is demonstrated.
The general physicochemical mechanisms underlying photoactivated sol–gel reactions are described, with comprehensive chemical and structural analysis inducing rapid (<5 min) fabrication of various metal oxide films at low temperatures (<150 °C), and all-solution processed high-performance electronic devices and circuitry on ultrathin polymeric substrates are demonstrated. This will open new possibilities to prepare future electronic materials in a fast, scalable, and economic manner.