Søren Dahl

Molybdenum sulfides—efficient and viable materials for electro - and photoelectrocatalytic hydrogen evolution

This perspective covers the use of molybdenum disulfide and related compounds, generally termed MoSx, as electro- or photoelectrocatalysts for the hydrogen evolution reaction (HER). State of the art solutions as well as the most illustrative results from the extensive electro- and photoelectrocatalytic literature are given. The research strategies currently employed in the field are outlined and future challenges pointed out. We suggest that the key to optimising the HER activity of MoS2 is divided into (1) increasing the catalytic activity of the active site, (2) increasing the number of active sites of the catalyst, and (3) improving the electrical contact to these sites. These postulations are substantiated by examples from the existing literature and some new results. To demonstrate the electrocatalytic properties of a highly conductive MoS2 hybrid material, we present the HER activity data for multi-wall MoS2 nanotubes on multi-wall carbon nanotubes (MWMoS2@MWCNTs). This exemplifies the typical data collected for the electrochemical HER. In addition, it demonstrates that the origin of the activity is closely related to the amount of edges in the layered MoS2. The photoelectrocatalytic HER is also discussed, based on examples from literature, with an emphasis on the use of MoSx as either (1) the co-catalyst providing the HER activity for a semiconductor, e.g. Mo3S+4on Si or (2) MoS2 as the semiconductor with an intrinsic HER activity. Finally, suggestions for future catalyst designs are given.

Layered Nanojunctions for Hydrogen‐Evolution Catalysis

The production of chemical fuels by using sunlight is an attractive and sustainable solution to the global energy and environmental problems. Photocatalytic water splitting is a promising route to capture, convert, and store solar energy in the simplest chemical compound (H2). [1] Since the initial report of a photoelectrochemical cell using Pt-TiO2 electrodes for hydrogen evolution by Fujishima and Honda in 1972, considerable studies have been focused on the development of highly efficient and stable photocatalyst powder systems, and especially on using earth-abundant semiconductors and co-factors for water splitting. In practice, the achievement of the conversion of solar energy into hydrogen necessitates the spatial integration of semiconductors and co-catalysts to form surface junctions, so as to optimize the capture of light and to promote charge separation and surface catalytic kinetics. The construction of effective surface junctions is therefore of vital importance, and not only strongly depends on the properties, such as crystal structure, band structure, and electron affinity, of both semiconductors and catalysts but also on the interface between the two materials. In photocatalysis, an ohmic contact between photocatalysts and cocatalysts can allow the prompt migration of light-induced charge, thus resulting in an efficient photocatalytic reaction. Recently, we found that graphitic carbon nitride (g-CN), a polymeric melon semiconductor with a layered structure analogous to graphite, meets the essential requirements as a sustainable solar energy transducer for water redox catalysis; these requirements include being abundant, highly-stable, and responsive to visible light. g-CN is indeed a new type of visible-light photocatalyst that contains no metals, and has a suitable electronic structure (Eg = 2.7 eV, conduction band at 0.8 V and valence band at 1.9 V vs. RHE) covering the water-splitting potentials. An improvement in the efficiency of H2 production has been demonstrated by the introduction of nanohierarchical structures into g-CN. It is noted that, like many other photocatalysts, g-CN alone shows very poor electrocatalytic activities for water splitting and relies on surface co-catalysts to activate its photocatalytic functions. The co-catalyst cooperates with the light harvester to facilitate the charge separation and increases the lifetime of the photogenerated electron/hole pair, while lowering activation barriers for H2 or O2 evolution. Thus, the use of a co-catalyst leads to an increase in overall photocatalytic performance, including activity, selectivity, and stability. Generally, the efficiency of a given photocatalytic system is dependent on the ability of the co-catalysts to support reductive and/or oxidative catalysis. In particular, the structural characteristics and intrinsic catalytic properties of a co-catalyst are important. However, the study of the structural and electronic compatibility between g-CN and co-catalysts has been limited so far. The co-catalysts used are mainly platinum group metals or their oxides, which are scarce and expensive. Photocatalytic/catalytic systems based on abundantly available materials are certainly desirable for large-scale hydrogen production for future energy production based on water and sunlight. Among various hydrogen-evolution reaction (HER) catalysts, molybdenum sulfur complexes have received a lot of attention. MoS2 was found to be a good electrocatalyst for H2 evolution, and the HER activity stemmed from the sulfur edges of the MoS2 crystal layers. [10] When grown on graphene sheets, nanostructured MoS2 exhibited excellent HER activity owing to the high exposure of the edges and the strong electronic coupling to the underlying planar support. Incomplete cubane [Mo3S4] + clusters and amorphous MoS2 are also proven HER catalysts. Some of these HER catalysts have been used in photocatalytic H2 production and they exhibited a remarkable promoting effect. MoS2 has a similar structure to graphite; it has a layered crystal structure consisting of S Mo S “sandwiches” held together by van der Waals force. The fact that g-CN and MoS2 have analogous layered structures should minimize the lattice mismatch and facilitate the planar growth of MoS2 slabs over the g-CN surface, thus constructing an organic–inorganic hybrid with graphene-like thin layered heterojunctions (Scheme 1a). Such a distinct nanoscale structure has some advantages. It can increase the accessible area around the planar interface of the MoS2 and g-CN layers and diminish the barriers for electron transport through the co-catalyst, thus facilitating fast electron transfer across the interface by the electron tunneling effect. Also, thin layers can lessen the light blocking effect of the co-catalyst, thus improving the light utilization by g-CN. Importantly, the intrinsic band structures [*] Y. Hou, J. Zhang, G. Zhang, Y. Zhu, Prof. X. Wang Research Institute of Photocatalysis, College of Chemistry and Chemical Engineering, Fuzhou University Fuzhou 350002 (China) E-mail: xcwang@fzu.edu.cn

Bioinspired molecular co-catalysts bonded to a silicon photocathode for solar hydrogen evolution

The production of fuels from sunlight represents one of the main challenges in the development of a sustainable energy system. Hydrogen is the simplest fuel to produce and although platinum and other noble metals are efficient catalysts for photoelectrochemical hydrogen evolution, earth-abundant alternatives are needed for large-scale use. We show that bioinspired molecular clusters based on molybdenum and sulphur evolve hydrogen at rates comparable to that of platinum. The incomplete cubane-like clusters (Mo(3)S(4)) efficiently catalyse the evolution of hydrogen when coupled to a p-type Si semiconductor that harvests red photons in the solar spectrum. The current densities at the reversible potential match the requirement of a photoelectrochemical hydrogen production system with a solar-to-hydrogen efficiency in excess of 10%. The experimental observations are supported by density functional theory calculations of the Mo(3)S(4) clusters adsorbed on the hydrogen-terminated Si(100) surface, providing insights into the nature of the active site.

Discovery of a Ni-Ga catalyst for carbon dioxide reduction to methanol

The use of methanol as a fuel and chemical feedstock could become very important in the development of a more sustainable society if methanol could be efficiently obtained from the direct reduction of CO2 using solar-generated hydrogen. If hydrogen production is to be decentralized, small-scale CO2 reduction devices are required that operate at low pressures. Here, we report the discovery of a Ni-Ga catalyst that reduces CO2 to methanol at ambient pressure. The catalyst was identified through a descriptor-based analysis of the process and the use of computational methods to identify Ni-Ga intermetallic compounds as stable candidates with good activity. We synthesized and tested a series of catalysts and found that Ni5Ga3 is particularly active and selective. Comparison with conventional Cu/ZnO/Al2O3 catalysts revealed the same or better methanol synthesis activity, as well as considerably lower production of CO. We suggest that this is a first step towards the development of small-scale low-pressure devices for CO2 reduction to methanol.

Direct Observations of Oxygen-induced Platinum Nanoparticle Ripening Studied by In Situ TEM

This study addresses the sintering mechanism of Pt nanoparticles dispersed on a planar, amorphous Al(2)O(3) support as a model system for a catalyst for automotive exhaust abatement. By means of in situ transmission electron microscopy (TEM), the model catalyst was monitored during the exposure to 10 mbar air at 650 degrees C. Time-resolved image series unequivocally reveal that the sintering of Pt nanoparticles was mediated by an Ostwald ripening process. A statistical analysis of an ensemble of Pt nanoparticles shows that the particle size distributions change shape from an initial Gaussian distribution via a log-normal distribution to a Lifshitz-Slyozov-Wagner (LSW) distribution. Furthermore, the time-dependency of the ensemble-averaged particle size and particle density is determined. A mean field kinetic description captures the main trends in the observed behavior. However, at the individual nanoparticle level, deviations from the model are observed suggesting in part that the local environment influences the atom exchange process.

Hydrogen Production Using a Molybdenum Sulfide Catalyst on a Titanium-Protected n+p-Silicon Photocathode†

A low-cost substitute: A titanium protection layer on silicon made it possible to use silicon under highly oxidizing conditions without oxidation of the silicon. Molybdenum sulfide was electrodeposited on the Ti-protected n(+)p-silicon electrode. This electrode was applied as a photocathode for water splitting and showed a greatly enhanced efficiency.

Computational screening of perovskite metal oxides for optimal solar light capture

One of the possible solutions to the worlds rapidly increasing energy demand is the development of new photoelectrochemical cells with improved light absorption. This requires development of semiconductor materials which have appropriate bandgaps to absorb a large part of the solar spectrum at the same time as being stable in aqueous environments. Here we demonstrate an efficient, computational screening of relevant oxide and oxynitride materials based on electronic structure calculations resulting in the reduction of a vast space of 5400 different materials to only 15 promising candidates. The screening is based on an efficient and reliable way of calculating semiconductor band gaps. The outcome of the screening includes all already known successful materials of the types investigated plus some new ones which warrant further experimental investigation.

Electronic factors in catalysis: the volcano curve and the effect of promotion in catalytic ammonia synthesis

Abstract The activity and selectivity of heterogeneous catalysts are determined by their electronic and structural properties. In many cases, the electronic properties are determined by the choice of both the catalytically active transition metal and promoter elements. Density functional theory is used to calculate how these two factors affect the energies of the intermediates and transition states in the ammonia synthesis reaction. We show that a linear relationship exists between the activation energy for N 2 dissociation and the binding energy of adsorbed nitrogen. The ammonia synthesis activity under industrial conditions can be determined as a function of the nitrogen–surface interaction energy by combining the calculated adsorption energy–activation energy relation with a micro-kinetic model. The result is a volcano curve and we illustrate such relationships for both the non-promoted and alkali metal promoted transition metals. We conclude that promotion is most effective for the best non-promoted catalysts and that promotion will always be essential for obtaining an optimal ammonia synthesis catalyst. Analysis of the micro-kinetic model show that the best catalysts are those with the lowest apparent activation energies and they exhibit reaction orders between two asymptotic behaviors.

Solar-fuel generation: Towards practical implementation

Limiting reliance on non-renewable fossil fuels inevitably depends on a more efficient utilization of solar energy. Materials scientists discuss the most viable approaches to produce high-energy-density fuels from sunlight that can be implemented in existing infrastructures.

Support Effect and Active Sites on Promoted Ruthenium Catalysts for Ammonia Synthesis

The catalytic activities of three supported, barium-promoted ruthenium catalysts for ammonia synthesis are reported. The three supports are silicon nitride (Si3N4), magnesium aluminum spinel (MgAl2O4), and graphitized carbon (C). The effect of the promoter on the activity is strongly dependent on the choice of support material in accordance with several previous observations. Here, this dependence is ascribed to a difference in the affinity of the promoter for the different supports. It is shown how it is possible to image the barium promoter present on the surface of ruthenium crystals in passivated catalysts by conventional high-resolution transmission electron microscopy (HRTEM). By comparison with in situ HRTEM images obtained lately from similar catalysts, and with reference to recent density functional theory (DFT) calculations, we suggest that active B5-type sites on the surfaces of the ruthenium crystals are promoted by nearby promoter atoms via electrostatic interactions.