Ultrathin Two‐Dimensional Nanostructured Materials for Highly Efficient Water Oxidation

Water oxidation, also known as the oxygen evolution reaction (OER), is a crucial process in energy conversion and storage, especially in water electrolysis. The critical challenge of the electrochemical water splitting technology is to explore alternative precious‐metal‐free catalysts for the promotion of the kinetically sluggish OER. Recently, emerging two‐dimensional (2D) ultrathin materials with abundant accessible active sites and improved electrical conductivity provide an ideal platform for the synthesis of promising OER catalysts. This Review focuses on the most recent advances in ultrathin 2D nanostructured materials for enhanced electrochemical activity of the OER. The design, synthesis and performance of such ultrathin 2D nanomaterials‐based OER catalysts and their property‐structure relationships are discussed, providing valuable insights to the exploration of novel OER catalysts with high efficiency and low overpotential. The potential research directions are also proposed in the research field.     

Progress in Ni-based anode materials for direct hydrocarbon solid oxide fuel cells

Ni-based anode materials of solid oxide fuel cells (SOFCs) are susceptible to carbon deposition and deactivation in direct hydrocarbon fuels, greatly limiting the commercialization. Extensive studies on finding new alternative anode materials have been developed; however, new problems such as low electrochemical performance and complex cell preparation process destroyed the further research passion of Ni-free anode materials. Considering the superior catalytic activity and mature technology of Ni-based anode materials, a large number of recent research results proved that it is still important and promising to solve the carbon coking of Ni-based anode materials. In this review, progress in four typically promising Ni-based anode materials free from carbon coking has been summarized, including the noble metals, ceria, Ba-containing oxides and titanium oxide. Correspondingly, the mechanisms that improve the carbon tolerance of Ni-based modified SOFCs anodes are clearly concluded, providing the materials and theoretical basis for the use of direct hydrocarbon SOFCs as early as possible.     

Sulphur poisoning of palladium catalysts used for methane combustion: effect of the support

Four different supported palladium catalysts (using alumina, silica, zirconia and titania as supports), prepared by incipient wetness impregnation, were tested as catalysts for methane oxidation in presence of sulphur dioxide. The catalyst supported on zirconia showed the best performance, whereas the silica-supported one showed the fastest deactivation. Temperature-programmed desorption experiments of the poisoned catalysts suggest that SO(2) adsorption capacity of the support plays a key role in the catalyst poisoning. In order to study the effect of promoters, expected to improve the thermal stability and thioresistance of the catalyst, commercial zirconia modified by yttrium and lantane was tested as supports. It was found that the presence of these promoters does not improve the performance of the zirconia-supported catalyst. A deactivation model -- considering two different active sites (fresh and poisoning), pseudo-first order dependence on methane concentration and poisoning rate depending on sulphur concentration and fraction of non-poisoned palladium -- was used for modelling the deactivation behaviour.     

Melt-assisted phase transformations of A/W/Mn/SiO<Subscript>2</Subscript> (A = Li,Na, K,Rb, Cs) composite catalysts

This paper analyzes results obtained in studies of A/W/Mn/SiO₂ (A = Li, Na, K, Rb, Cs) composite catalysts for the oxidative coupling of methane (OCM). Particular attention is paid to phase transformations. It is pointed out that the SiO₂ matrix is an active catalytic component of the composites, rather than an inert carrier of additives, and that the heterogeneous OCM process involves melts based on alkali metal tungstates, along with polycrystalline manganese oxides. The effects of the cation ratio and synthesis method on the phase composition of the A/W/Mn/SiO₂ (A = Li, Na, K, Rb, Cs) materials is examined.     

Nanostructured Metal‐Free Electrochemical Catalysts for Highly Efficient Oxygen Reduction

Replacing precious and nondurable Pt catalysts with cheap and commercially available materials to facilitate sluggish cathodic oxygen reduction reaction (ORR) is a key issue in the development of fuel cell technology. The recently developed cost effective and highly stable metal‐free catalysts reveal comparable catalytic activity and significantly better fuel tolerance than that of current Pt‐based catalysts; therefore, they can serve as feasible Pt alternatives for the next generation of ORR electrocatalysts. Their promising electrocatalytic properties and acceptable costs greatly promote the R&D of fuel cell technology. This review provides an overview of recent advances in state‐of‐the‐art nanostructured metal‐free electrocatalysts including nitrogen‐doped carbons, graphitic‐carbon nitride (g‐C₃N₄)‐based hybrids, and 2D graphene‐based materials. A special emphasis is placed on the molecular design of these electrocatalysts, origin of their electrochemical reactivity, and ORR pathways. Finally, some perspectives are highlighted on the development of more efficient ORR electrocatalysts featuring high stability, low cost, and enhanced performance, which are the key factors to accelerate the commercialization of fuel cell technology.     

Single-atom catalysts boost nitrogen electroreduction reaction

Ammonia (NH₃) is mainly produced through the traditional Haber–Bosch process under harsh conditions with huge energy consumption and massive carbon dioxide (CO₂) emission. The nitrogen electroreduction reaction (NERR), as an energy-efficient and environment-friendly process of converting nitrogen (N₂) to NH₃ under ambient conditions, has been regarded as a promising alternative to the Haber–Bosch process and has received enormous interest in recent years. Although some exciting progress has been made, considerable scientific and technical challenges still exist in improving the NH₃ yield rate and Faradic efficiency, understanding the mechanism of the reaction and promoting the wide commercialization of NERR. Single-atom catalysts (SACs) have emerged as promising catalysts because of their atomically dispersed activity sites and maximized atom efficiency, unsaturated coordination environment, and unique electronic structure, which could significantly improve the rate of reaction and yield rate of NH₃. In this review, we briefly introduce the unique structural and electronic features of SACs, which contributes to comprehensively understand the reaction mechanism owing to their structural simplicity and diversity, and in turn, expedite the rational design of fantastic catalysts at the atomic scale. Then, we summarize the most recent experimental and computational efforts on developing novel SACs with excellent NERR performance, including precious metal-, nonprecious metal- and nonmetal-based SACs. Finally, we present challenges and perspectives of SACs on NERR, as well as some potential means for advanced NERR catalyst.     

Recent Advances and Perspectives of Core-Shell Nanostructured Materials for Photocatalytic CO2 Reduction

Photocatalytic CO₂ conversion into solar fuels is a promising technology to alleviate CO₂ emissions and energy crises. The development of core-shell structured photocatalysts brings many benefits to the photocatalytic CO₂ reduction process, such as high conversion efficiency, sufficient product selectivity, and endurable catalyst stability. Core-shell nanostructured materials with excellent physicochemical features take an irreplaceable position in the field of photocatalytic CO₂ reduction. In this review, the recent development of core-shell materials applied for photocatalytic reduction of CO_{2 is introduced}. First, the basic principle of photocatalytic CO₂ reduction is introduced. In detail, the classification and synthesis techniques of core-shell catalysts are discussed. Furthermore, it is also emphasized that the excellent properties of the core-shell structure can greatly improve the activity, selectivity, and stability in the process of photocatalytic CO₂ reduction. Hopefully, this paper can provide a favorable reference for the preparation of efficient photocatalysts for CO₂ reduction.     

Low-cost catalysts for the control of indoor CO and PM emissions from solid fuel combustion

Cu-Mn based mixed oxide type low-cost catalysts have been prepared in supported form using mesoporous Al(2)O(3), TiO(2) and ZrO(2) supports. These supports have been prepared by templating method using a natural biopolymer, chitosan. The synthesized catalysts have been characterized by XRD, BET-SA, SEM, O(2)-TPD and TG investigations. The catalytic activity for CO as well as PM oxidation was studied, in a view of their possible applications in the control of emissions from solid fuel combustion of rural cook-stoves. The trend observed for the catalytic activity of the synthesized catalysts for CO oxidation was ZrO(2)>TiO(2)>Al(2)O(3) while for PM oxidation it was observed to be TiO(2)>ZrO(2)>Al(2)O(3). The effect of CO(2), SO(2) and H(2)O on CO oxidation activity was also investigated, and despite partial deactivation, the catalysts show good CO oxidation activity. An effective regeneration treatment was attempted by heating the partially deactivated catalysts in presence of oxygen. Redox properties of TiO(2) and ZrO(2) and their structure appeared to be responsible for their promotional activity for CO and PM oxidation reactions. These unordered mesoporous materials could be useful for such reactions where mass transfer is more important than shape and size selectivity.     

The Flexibility of an Amorphous Cobalt Hydroxide Nanomaterial Promotes the Electrocatalysis of Oxygen Evolution Reaction

Structural flexibility can be a desirable trait of an operating catalyst because it adapts itself to a given catalytic process for enhanced activity. Here, amorphous cobalt hydroxide nanocages are demonstrated to be a promising electrocatalyst with an overpotential of 0.28 V at 10 mA cm^?2, far outperforming the crystalline counterparts and being in the top rank of the catalysts of their kind, under the condition of electrocatalytic oxygen evolution reaction. From the direct experimental in situ and ex situ results, this enhanced activity is attributed to its high structural flexibility in terms of 1) facile and holistic transformation into catalytic active phase; 2) hosting oxygen vacancies; and 3) structure self‐regulation in a real‐time process. Significantly, based on plausible catalytic mechanism and computational simulation results, it is disclosed how this structural flexibility facilitates the kinetics of oxygen evolution reaction. This work deepens the understanding of the structure–activity relationship of the Co‐based catalysts in electrochemical catalysis, and it inspires more applications that require flexible structures enabled by such amorphous nanomaterials.     

La1-xSrxMnAl11O19-δ催化剂的甲烷燃烧性能研究

采用反相微乳液-共沉淀法制备了一系列以La、Sr作为镜面阳离子、锰离子作为活性组分的六铝酸盐催化剂La1-xSrxMnAl11O19-δ(x=0.2、0.4、0.5、0.6、0.8)。利用X射线衍射、比表面积分析等分析方法及甲烷燃烧对催化剂的结构和性质进行了考察,主要考察了不同含量的La和Sr离子的掺杂量对催化剂结构及对甲烷催化燃烧活性的影响。结果表明,La和Sr同时作为镜面阳离子,不但可以形成完整的六铝酸盐,而且所制备的催化剂具有较高的催化活性。不同含量的La和Sr离子掺杂对于催化剂的特性有较大影响。当x=0.5时,所制备的催化剂La0.5-Sr0.5MnAl11O19-δ具有较高的催化活性,起燃温度T10%=502℃,至683℃甲烷完全转化。催化剂在低转化率下的宏观动力学实验结果表明,甲烷催化燃烧在La1-xSrxMnAl11O19-δ催化剂上为一级动力学反应,反应速率受催化剂固有性质控制。     

Recent Advances in Atomic Metal Doping of Carbon‐based Nanomaterials for Energy Conversion

Nanostructured metal‐contained catalysts are one of the most widely used types of catalysts applied to facilitate some of sluggish electrochemical reactions. However, the high activity of these catalysts cannot be sustained over a variety of pH ranges. In an effort to develop highly active and stable metal‐contained catalysts, various approaches have been pursued with an emphasis on metal particle size reduction and doping on carbon‐based supports. These techniques enhances the metal‐support interactions, originating from the chemical bonding effect between the metal dopants and carbon support and the associated interface, as well as the charge transfer between the atomic metal species and carbon framework. This provides an opportunity to tune the well‐defined metal active centers and optimize their activity, selectivity and stability of this type of (electro)catalyst. Herein, recent advances in synthesis strategies, characterization and catalytic performance of single atom metal dopants on carbon‐based nanomaterials are highlighted with attempts to understand the electronic structure and spatial arrangement of individual atoms as well as their interaction with the supports. Applications of these new materials in a wide range of potential electrocatalytic processes in renewable energy conversion systems are also discussed with emphasis on future directions in this active field of research.     

Nanoparticle-supported catalysts and catalytic reactions – a mini-review

This article reviews recent developments of using nanometer-sized particular matrices for supporting catalysts and catalytic organic reactions. Immobilization of a catalyst onto the surface of an Au or magnetic (e.g. Fe₂O₃) nanoparticle allowed facile separation and recycling of the catalyst out of the reaction mixture. The activity and selectivity of the nanoparticle-supported catalytic species were found to be comparable to those of their parent catalysts in solution or their counterparts immobilized on the solid-phase. In addition, due to the unique surface structure of the nano-host, cooperative catalysis was observed in a dyad of amino acid residues and peptide analogues on the nanoparticle surface. Thereby, immobilization of catalysts on the surface of a nanoparticular matrix will not only facilitate the isolation and recovery of expensive catalysts, but also open a new avenue to regulate their activity and selectivity.     

Application of Ni:SiO2 nanocomposite to control the carbon deposition on the carbon dioxide reforming of methane

Stable Ni nanoparticles embedded in a mesoporous silica material were used as catalysts for the conversion of methane into synthesis gas. This catalyst has the singular properties of controlling the carbon deposition and deactivation of active sites. A comparative study of our nanocomposites with conventional catalysts showed that impregnation material presented a preferential encapsulation and growth of carbon nanotubes on the metal surface. The impregnated catalyst showed a higher tendency for carbon nanotube and whiskers formation.     

Closest Packing Polymorphism Interfaced Metastable Transition Metal for Efficient Hydrogen Evolution

Metastable materials are promising because of their catalytic properties, high-energy structure, and unique electronic environment. However, the unstable nature inherited from the metastability hinders further performance improvement and practical applications of these materials. Herein, this limitation is successfully addressed by constructing an in situ polymorphism interface (inf) between the metastable hexagonal-close-packed (hcp) phase and its stable counterpart (face-centered cubic, fcc) in cobalt–nickel (CoNi) alloy. Calculations reveal that the interfacial synergism derived from the hcp and fcc phases lowers the formation energy and enhances stability. Consequently, the optimized CoNi-inf exhibits an exceptionally low potential of 72 mV at 10 mA cm⁻² and a Tafel slope of 57 mV dec⁻¹ for the hydrogen evolution reaction (HER) in 1.0 m KOH. Furthermore, it is superior to most state-of-the-art non-noble-metal-based HER catalysts. No noticeable activity decay or structural changes are observed even over 14 h of catalysis. The computational simulation further rationalizes that the interface of CoNi-inf with a suitable d-band center provides uniform sites for hydrogen adsorption, leading to a distinguished HER catalytic activity. This work, therefore, presents a new route for designing metastable catalysts for potential energy conversion.     

Diatomite as high performance and environmental friendly catalysts for phenol hydroxylation with H2O2

A series of diatomite catalysts were treated and characterized. For the first time, the resulting materials were used in catalysis for the hydroxylation of phenol with H₂O₂ and showed very high hydroxylation activity due to the Fe species in the diatomite. The effect of HCl treatment, contents of catalysts and H₂O₂ were investigated and the active components of diatomite were discussed. The results show that diatomite is the promising candidate for industrial output due to their high catalytic activity, easy physical separation and very low costs.     

Hydrogen production of nickel–scandia-stabilized zirconia and copper/nickel–scandia-stabilized zirconia catalysts through steam methane reforming for solid oxide fuel cell operation

In this study, the catalytic activities of the steam methane reforming (SMR) reactions with two catalysts, including nickel–scandia-stabilized zirconia (Ni–SSZ) and copper/nickel–scandia-stabilized zirconia (Cu/Ni–SSZ), were examined and compared. The microstructure and crystallinity of the as-prepared catalysts were characterized by scanning electron microscopy, Raman spectroscopy, and X-ray diffraction. Mass spectrometer was applied in the outlet streams, in order to simultaneously monitor the time-dependent kinetics in the reactor for an activity test and conversion examination. Finally, thermogravimetric analysis (TGA) and Raman spectrometer were implemented for further verification of carbon residuals on the catalysts. It was found that the incorporation of Cu on Ni–SSZ imposed significant constraints on the growth of nickel crystallites from NiO during the annealing process in reducing atmospheres. The methane conversion of Ni–SSZ and Cu/Ni–SSZ catalysts (annealed at 300 °C for 2 h) was 36.2 and 26.0%, respectively. However, the amount of carbon residuals on Cu/Ni–SSZ catalyst (300 °C for 2 h) was 18.6%, which is lower than that of the Ni–SSZ catalysts (33.2%) from TGA results. Further Raman experiments revealed that more graphite-like carbon residuals and less defects or amorphous carbons (I_G/I_D?=?2.0) were found in the case of Cu/Ni–SSZ catalysts (300 °C for 2 h). Among the catalysts in this study, the Cu/Ni–SSZ catalyst (300 °C for 2 h) is considered as a promising catalyst for SMR reaction, since it has a fair methane conversion, and characterized higher CO₂ selectivity and lower CO selectivity without compromising the hydrogen purity. More importantly, the least amount of carbon residuals was found in Cu/Ni–SSZ catalyst (300 °C for 2 h), which assured a better lifetime.     

Sustainable Carbonaceous Materials Derived from Biomass as Metal‐Free Electrocatalysts

Although carbon is the second most abundant element in the biosphere, a large proportion of the available carbon resources in biomass from agriculture, stock farming, ocean fisheries, and other human activities is currently wasted. The use of sustainable carbonaceous materials as an alternative to precious metals in electrocatalysis is a promising pathway for transforming sustainable biomass resources into sustainable energy‐conversion systems. The development of rational syntheses of metal‐free carbonaceous catalysts derived from sustainable biomass has therefore become a topic of significant interest in materials chemistry. However, great efforts are still required to develop methods that are low cost, scalable, and environmentally friendly and which afford carbonaceous materials having an electrocatalytic performance comparable to, or even better than, existing precious metal catalysts. Herein, recent achievements in developing metal‐free carbonaceous catalysts based on biomass are reviewed and discussed and the critical issues which still need to be addressed are highlighted. The focus is on representative synthesis and optimization strategies applicable to different kinds of biomass, as well as studies of the physicochemical structure and electrochemical performance of the resulting metal‐free carbonaceous catalysts. Finally, some guidelines for the future development of this important area are provided.     

Fabrication of Supported AuPt Alloy Nanocrystals with Enhanced Electrocatalytic Activity for Formic Acid Oxidation through Conversion Chemistry of Layer‐Deposited Pt2+ on Au Nanocrystals

The exploitation of nanoconfined conversion of Au‐ and Pt‐containing binary nanocrystals for developing a controllable synthesis of surfactant‐free AuPt nanocrystals with enhanced formic acid oxidation (FAO) activity is reported, which can be stably and evenly immobilized on various support materials to diversify and optimize their electrocatalytic performance. In this study, an atomic layer of Pt²⁺ species is discovered to be spontaneously deposited in situ on the Au nanocrystal generated from a reverse‐microemulsion solution. The resulting Au/Pt²⁺ nanocrystal thermally transforms into a reduced AuPt alloy nanocrystal during the subsequent solid‐state conversion process within the SiO₂ nanosphere. The alloy nanocrystals can be isolated from SiO₂ in a surfactant‐free form and then dispersedly loaded on the carbon sphere surface, allowing for the production of a supported electrocatalyst that exhibits much higher FAO activity than commercial Pt/C catalysts. Furthermore, by involving Fe₃O₄ nanocrystals in the conversion process, the AuPt alloy nanocrystals can be grown on the oxide surface, improving the durability of supported metal catalysts, and then uniformly loaded on a reduced graphene oxide (RGO) layer with high electroconductivity. This produces electrocatalytic AuPt/Fe₃O₄/RGO nanocomposites whose catalyst‐oxide‐graphene triple‐junction structure provides improved electrocatalytic properties in terms of both activity and durability in catalyzing FAO.     

Active Sites and Mechanism of Oxygen Reduction Reaction Electrocatalysis on Nitrogen‐Doped Carbon Materials

The oxygen reduction reaction (ORR) is a core reaction for electrochemical energy technologies such as fuel cells and metal–air batteries. ORR catalysts have been limited to platinum, which meets the requirements of high activity and durability. Over the last few decades, a variety of materials have been tested as non‐Pt catalysts, from metal–organic complex molecules to metal‐free catalysts. In particular, nitrogen‐doped graphitic carbon materials, including N‐doped graphene and N‐doped carbon nanotubes, have been extensively studied. However, due to the lack of understanding of the reaction mechanism and conflicting knowledge of the catalytic active sites, carbon‐based catalysts are still under the development stage of achieving a performance similar to Pt‐based catalysts. In addition to the catalytic viewpoint, designing mass transport pathways is required for O₂. Recently, the importance of pyridinic N for the creation of active sites for ORR and the requirement of hydrophobicity near the active sites have been reported. Based on the increased knowledge in controlling ORR performances, bottom‐up preparation of N‐doped carbon catalysts, using N‐containing conjugative molecules as the assemblies of the catalysts, is promising. Here, the recent understanding of the active sites and the mechanism of ORRs on N‐doped carbon catalysts are reviewed.     

Iron‐Based Metal–Organic Frameworks as Catalysts for Visible Light‐Driven Water Oxidation

The development of earth‐abundant, active, and stable catalysts is important for solar energy conversion. Metal‐organic frameworks (MOFs) have been viewed as a promising class of porous materials, which may have innovative application in photocatalysis. In this paper, three types of Fe‐based MOFs and their aminofunctionalized derivatives have been fabricated and systematically studied as water oxidation catalysts (WOCs) for oxygen evolution under visible light irradiation. MIL‐101(Fe) possesses a higher current density and earlier onset potential and exhibits excellent visible light‐driven oxygen evolution activity than the other Fe‐based catalysts. It speeds up the oxygen evolution reaction rate with the higher initial turnover frequencies value of 0.10 s^?1. Our study demonstrates that Fe‐based MOFs as efficient WOCs are promising candidates for photocatalytic water oxidation process.