Anders Bo Laursen

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

Using TiO2 as a Conductive Protective Layer for Photocathodic H2 Evolution

Surface passivation is a general issue for Si-based photoelectrodes because it progressively hinders electron conduction at the semiconductor/electrolyte interface. In this work, we show that a sputtered 100 nm TiO(2) layer on top of a thin Ti metal layer may be used to protect an n(+)p Si photocathode during photocatalytic H(2) evolution. Although TiO(2) is a semiconductor, we show that it behaves like a metallic conductor would under photocathodic H(2) evolution conditions. This behavior is due to the fortunate alignment of the TiO(2) conduction band with respect to the hydrogen evolution potential, which allows it to conduct electrons from the Si while simultaneously protecting the Si from surface passivation. By using a Pt catalyst the electrode achieves an H(2) evolution onset of 520 mV vs NHE and a Tafel slope of 30 mV when illuminated by the red part (λ > 635 nm) of the AM 1.5 spectrum. The saturation photocurrent (H(2) evolution) was also significantly enhanced by the antireflective properties of the TiO(2) layer. It was shown that with proper annealing conditions these electrodes could run 72 h without significant degradation. An Fe(2+)/Fe(3+) redox couple was used to help elucidate details of the band diagram.

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.

Substrate Size-Selective Catalysis with Zeolite-Encapsulated Gold Nanoparticles†

Over the years, many strategies have been developed to address the problem of sintering of nanoparticle catalysts, including encapsulating metal nanoparticles in protective shells, and trapping nanoparticles in the cavities of certain zeolites in post-synthesis steps. In general, materials that contain metal nanoparticles that are only accessible via zeolite micropores are intriguing, specifically, but not exclusively, for catalytic applications. The encapsulation of carbon nanoparticles during zeolite crystallization is a well-known approach for making carbon–zeolite composites that afford mesoporous zeolites after combustion. Herein, we show that metal nanoparticles can also be encapsulated during zeolite crystallization, as exemplified by silicalite-1 crystals that are embedded with circa 1–2 nm-sized gold nanoparticles that remain stable and catalytically active after calcination in air at 550 8C. Moreover, we show that the encapsulated gold nanoparticles are only are accessible through the micropores of the zeolite, which makes this material a substrate-size selective oxidation catalyst. Currently, more than 175 different zeolite structures have been reported, and these can be tuned according to the desired acidity and/or redox properties. Expanding the scope from pure zeolites to hybrid materials, by combining the properties of zeolites with other components, significantly widens the field of zeolite materials design. Aside from posttreatment methods, two types of approaches have been pursued for preparing hybrid zeolite–nanoparticle materials. The first type of approach involves crystallization of the zeolite from a gel that contains metal ions that are immobilized in the zeolite during crystallization. With this kind of approach, it is very difficult to control the properties of the non-zeolite component in terms of, for example, particle size. The other type of approach is to first synthesize the nonzeolite component and subsequently encapsulate this in the individual zeolite crystals during crystallization. Indeed, this strategy is also well-known and an entire family of materials, known as mesoporous or hierarchical zeolite crystals, are based on the embedding of carbon nanoparticles, nanofibers, nanotubes, or other nanostructures during zeolite crystallization (and subsequent combustion) in a process known as carbon templating. 15, 16] Concerning the embedding of metal nanoparticles in zeolites, Hashimoto et al. reported a top down approach that features downsizing gold flakes to approximately 40 nm particles by laser ablation, and subsequent encapsulation of these particles during crystallization. A reduction in particle size by one order of magnitude is necessary for an efficient use of costly noble metals in catalytic applications. However, a reduction of the particle size enhances the tendency for sintering, owing to the increase in surface free energy. To mitigate this problem, we report herein a bottom-up approach for the preparation of hybrid zeolite-nanoparticle materials that contain small metal nanoparticles, dispersed throughout the zeolite crystals. This synthetic approach comprises three steps (Figure 1): First, a metal nanoparticle colloid is prepared with suitable anchor points for the generation of a silica shell. Second, the particles are encapsulated in an amorphous silica matrix. Third, the silica nanoparticle precursor is subjected to hydrothermal conditions in order for zeolite crystallization to take place. Using this approach, we successfully prepared a material that consisted predominantly of circa 1–2 nm sized gold particles that were embedded in silicalite-1 crystals. X-ray diffraction revealed that the material contained exclusively gold as well as MFI-structured material (generalized silicalite-1 crystal structure type). Figure 2 shows scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of the hybrid material that consists of gold nanoparticles embedded in silicalite-1 crystals. The SEM image reveals that the material is mainly composed of circa 1–2 mm long coffinshaped crystals, with a minor fraction of intergrown coffin[*] A. B. Laursen, K. T. Højholt, L. F. Lundegaard, S. B. Simonsen, S. Helveg, Prof. C. H. Christensen, K. Egeblad Haldor Topsøe A/S Nymøllevej 55, 2800 Kgs. Lyngby (Denmark) E-mail: chc@topsoe.dk kreg@topsoe.dk

MoS2—an integrated protective and active layer on n+p-Si for solar H2 evolution

A new MoS2 protected n(+)p-junction Si photocathode for the renewable H2 evolution is presented here. MoS2 acts as both a protective and an electrocatalytic layer, allowing H2 evolution at 0 V vs. RHE for more than 5 days. Using a MoSx surface layer decreases the overpotential for H2 evolution by 200 mV.

In situ transmission electron microscopy of light-induced photocatalytic reactions

Transmission electron microscopy (TEM) makes it possible to obtain insight into the structure, composition and reactivity of photocatalysts, which are of fundamental interest for sustainable energy research. Such insight can be used for further material optimization. Here, we combine conventional TEM analysis of photocatalysts with environmental TEM (ETEM) and photoactivation using light. Two novel types of TEM specimen holder that enable in situ illumination are developed to study light-induced phenomena in photoactive materials, systems and photocatalysts at the nanoscale under working conditions. The technological development of the holders is described and two representative photo-induced phenomena are studied: the photodegradation of Cu₂O and the photodeposition of Pt onto a GaN:ZnO photocatalyst.

Light‐Induced Reduction of Cuprous Oxide in an Environmental Transmission Electron Microscope

Photocatalysts for solar fuel production are subject to intensive investigation as they constitute one viable route for solar energy harvesting. Cuprous oxide (Cu2O) is a working photocatalyst for hydrogen evolution but it photocorrodes upon light illumination in an aqueous environment. Environmental transmission electron microscopy (ETEM) makes it possible to obtain insight into the local structure, composition and reactivity of catalysts in their working environment, which is of fundamental interest for sustainable energy research and is essential for further material optimization. Herein, photoreduction of Cu2O is studied in situ using a dedicated TEM specimen holder for light illumination.

Availability of elements for heterogeneous catalysis: Predicting the industrial viability of novel catalysts

Abstract Growing concern regarding the sustainability of the chemical industry has driven the development of more efficient catalytic reactions. First-generation estimates of catalyst viability are based on crustal abundance, which has severe limitations. Herein, we propose a second-generation approach to predicting the viability of novel catalysts prior to industrial implementation to benefit the global chemical industry. Using this prediction, we found that a correlation exists between catalyst consumption and the annual production or price of the catalyst element for 11 representative industrial catalytic processes. Based on this correlation, we have introduced two new descriptors for catalyst viability, namely, catalyst consumption to availability ratio per annum (CCA) and consumed catalyst cost to product value ratio per annum (CCP). Based on evaluations of CCA and CCP for selected industrial reactions, we have grouped catalysts from the case studies according to viability, allowing the identification of general limits of viability based on CCA and CCP. Calculating the CCA and CCP and their comparing with the general limits of viability provides researchers with a novel framework for evaluating whether the cost or physical availability of a new catalyst could be limiting. We have extended this analysis to calculate the predicted limits of economically viable production and product cost for new catalysts.