Hydrogen plays a key role in many industrial applications and is currently seen as one of the most promising energy vectors. Many efforts are being made to produce hydrogen with zero CO2 footprint ...via water electrolysis powered by renewable energies. Nevertheless, the use of fossil fuels is essential in the short term. The conventional coal gasification and steam methane reforming processes for hydrogen production are undesirable due to the huge CO2 emissions. A cleaner technology based on natural gas that has received special attention in recent years is methane pyrolysis. The thermal decomposition of methane gives rise to hydrogen and solid carbon, and thus, the release of greenhouse gases is prevented. Therefore, methane pyrolysis is a CO2-free technology that can serve as a bridge from fossil fuels to renewable energies.
Hydrogen production by electrocatalytic water splitting will play a key role in the realization of a sustainable energy supply. Owing to their relatively high stability and activity, iridium ...(hydr)oxides have been identified as the most promising catalysts for the oxidation of water. Comprehensive spectroscopic and theoretical studies on the basis of rutile IrO2 have provided insight about the electronic structure of the active X-ray amorphous phase. However, due to the absence of long-range order and missing information about the local arrangement of structural units, our present understanding of the active phase is very unsatisfying. In this work, using a combination of real-space atomic scale imaging with atomic pair distribution function analysis and local measurements of the electronic structure, we identify key structural motifs that are associated with high water splitting activity. Comparison of two X-ray amorphous phases with distinctively different electrocatalytic performance reveals that high activity is linked to the ratio between corner- and edge-sharing IrO6 octahedra. We show that the active and stable phase consists of single unit cell sized hollandite-like structural domains that are cross-linked through undercoordinated oxygen/iridium atoms. In the less active phase, the presence of the rutile phase structural motif results in a faster structural collapse and deactivation. The presented results provide insight into the structure–activity relationship and promote a rational synthesis of X-ray amorphous IrOx hydroxides that contain a favorable arrangement of structural units for improved performance in catalytic water splitting.
Ni Single Atom Catalysts for CO2 Activation Millet, Marie-Mathilde; Algara-Siller, Gerardo; Wrabetz, Sabine ...
Journal of the American Chemical Society,
02/2019, Letnik:
141, Številka:
6
Journal Article
Recenzirano
Odprti dostop
We report on the activation of CO2 on Ni single-atom catalysts. These catalysts were synthesized using a solid solution approach by controlled substitution of 1–10 atom % of Mg2+ by Ni2+ inside the ...MgO structure. The Ni atoms are preferentially located on the surface of the MgO and, as predicted by hybrid-functional calculations, favor low-coordinated sites. The isolated Ni atoms are active for CO2 conversion through the reverse water–gas shift (rWGS) but are unable to conduct its further hydrogenation to CH4 (or MeOH), for which Ni clusters are needed. The CO formation rates correlate linearly with the concentration of Ni on the surface evidenced by XPS and microcalorimetry. The calculations show that the substitution of Mg atoms by Ni atoms on the surface of the oxide structure reduces the strength of the CO2 binding at low-coordinated sites and also promotes H2 dissociation. Astonishingly, the single-atom catalysts stayed stable over 100 h on stream, after which no clusters or particle formation could be detected. Upon catalysis, a surface carbonate adsorbate-layer was formed, of which the decompositions appear to be directly linked to the aggregation of Ni. This study on atomically dispersed Ni species brings new fundamental understanding of Ni active sites for reactions involving CO2 and clearly evidence the limits of single-atom catalysis for complex reactions.
Chemistry with its key targets of providing materials and processes for conversion of matter is at the center stage of the energy challenge. Most energy conversion systems work on (bio)chemical ...energy carriers and require for their use suitable process and material solutions. The enormous scale of their application demands optimization beyond the incremental improvement of empirical discoveries. Knowledge‐based systematic approaches are mandatory to arrive at scalable and sustainable solutions. Chemistry for energy, “ENERCHEM” contributes in many ways already today to the use of fossil energy carriers. Optimization of these processes exemplified by catalysis for fuels and chemicals production or by solid‐state lightning can contribute in the near future substantially to the dual challenge of energy use and climate protection being in fact two sides of the same challenge. The paper focuses on the even greater role that ENERCHEM will have to play in the era of renewable energy systems where the storage of solar energy in chemical carries and batteries is a key requirement. A multidisciplinary and diversified approach is suggested to arrive at a stable and sustainable system of energy conversion processes. The timescales for transformation of the present energy scenario will be decades and the resources will be of global economic dimensions. ENERCHEM will have to provide the reliable basis for such technologies based on deep functional understanding.
Renewable energy supply systems require knowledge‐based systemic chemical processes and optimized materials. This paper focuses on the even greater role that “ENERCHEM” will have to play in the era of renewable energy systems where the storage of solar energy in chemical carries and batteries is a key requirement. Energy is chemistry.
Electrocatalytic water splitting is the key process for the formation of green fuels for energy transport and storage in a sustainable energy economy. Besides electricity, it requires water, an ...aspect that seldomly has been considered until recently. As freshwater is a limited resource (<1% of earth's water), lately, plentiful reports were published on direct seawater (around 96.5% of earth's water) splitting without or with additives (buffers or bases). Alternatively, the seawater can be split in two steps, where it is first purified by reverse osmosis and then split in a conventional water electrolyser. This quantitative analysis discusses the challenges of the direct usage of non-purified seawater. Further, herein, we compare the energy requirements and costs of seawater purification with those of conventional water splitting. We find that direct seawater splitting has substantial drawbacks compared to conventional water splitting and bears almost no advantage. In short, it is less promising than the two-step scenario, as the capital and operating costs of water purification are insignificant compared to those of electrolysis of pure water.
In this analysis, we show that direct seawater splitting with or without additives faces significant challenges and bears almost no advantage with respect to the costs and energy demands to purify water prior to water electrolysis.
The design of heterogeneous selective oxidation catalysts based upon complex metal oxides is governed at present by a set of empirical rules known as “pillars of oxidation catalysis”. They serve as ...practical guidelines for catalyst development and guide the reasoning about the catalyst role in the process. These rules are, however, not based upon atomistic concepts and thus preclude their immediate application in for example computer-aided search strategies. The present work extends the ideas of the pillar rules and develops the concept of considering a selective oxidation catalyst as enabler for the execution of a reaction network. The enabling function is controlled by mutual interactions between catalyst and reactants. The electronic structure of the catalyst is defined as a bulk semiconductor with a surface state arising form a terminating over layer being different from the structure of the bulk. These components that can be identified by in situ analytical methods form a chemical system with feedback loops, which is responsible for generating selectivity during execution of the reaction network. This concept is based upon physical observables and could allow for a design strategy based upon a kinetic description that combines the processes between reactants with the processes between catalyst and reactants. Such kinetics is not available at present. Few of the constants required are known but many of them are accessible to experimental determination with in situ techniques.
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.
Understanding the growth mechanism of carbon nanotubes (CNTs) has been long pursued since its discovery. With recent integration of in situ techniques into the study of CNT growth, important insights ...about the growth mechanism of CNT have been generated, which have improved our understanding significantly. However, previous in situ experiments were mainly conducted at low pressures which were far from the practical conditions. Direct information about the growth dynamics under relevant conditions is still absent and thus is highly desirable. In this work, we report atomic-scale observations of multiwalled CNT (MWCNT) growth and termination at near ambient pressure by in situ transmission electron microscopy. On the basis of the real-time imaging, we are able to reveal that the working catalyst is constantly reshaping at its apex during catalyzing CNT growth, whereas at the base the catalyst remains faceted and barely shows any morphological change. The active catalyst is identified as crystalline Fe3C, based on lattice fringes that can be imaged during growth. However, the oscillatory growth behavior of the CNT and the structural dynamics of the apex area strongly indicate that the carbon concentration in the catalyst particle is fluctuating during the course of CNT growth. Extended observations further reveal that the catalyst splitting can occur: whereas the majority of the catalyst stays at the base and continues catalyzing CNT growth, a small portion of it gets trapped inside of the growing nanotube. The catalyst splitting can take place multiple times, leading to shrinkage of both, catalyst size and diameter of CNT, and finally the growth termination of CNT due to the full coverage of the catalyst by carbon layers. Additionally, in situ observations show two more scenarios for the growth termination, that is, out-migration of the catalyst from the growing nanotube induced by (i) Oswald ripening and (ii) weakened adhesion strength between the catalyst and CNT.
Nitrogen atoms are essential for the function of biological molecules and thus are and important component of fertilizers and medicaments. Bonds to nitrogen also find nonbiological uses in dyes, ...explosives, and resins. The synthesis of all these materials requires ammonia as an activated nitrogen building block. This situation is true for natural processes and the chemical industry. Knowledge of the various techniques for the preparation of ammonia is thus of fundamental importance for chemistry. The Haber–Bosch synthesis was the first heterogeneous catalytic system employed in the chemical industry and is still in use today. Understanding the mechanism and the translation of the knowledge into technical perfection has become a fundamental criterion for scientific development in catalysis research.
A triumph over trial and error: The Haber–Bosch industrial synthesis of ammonia (the picture shows an industrial installation for this process) has reached such a level of refinement that, in the energy balance the only significant loss is the preparation of the highly pure starting materials. Iron, ruthenium, or molybdenum compounds serve as the catalyst. The development of new catalysts is becoming increasingly effective through a deeper understanding of the basic princi ples involved and less dependent on trial and error.
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