Progress in Catalytic Roles of Hexagonal Boron Nitride in Oxidative Dehydrogenation, Water Splitting, and Pollutant Degradation

This paper presents an overview of heterogeneous partial (amm)oxidation and oxidative dehydrogenation (ODH) of hydrocarbons. The review has been voluntarily restricted to metal oxide-type catalysts, as the partial oxidation field is very broad and the number of catalysts is quite high. The main factors of solid catalysts for such reactions, designated by Grasselli as the “seven pillars”, and playing a determining role in catalytic properties, are considered to be, namely: isolation of active sites (known to be composed of ensembles of atoms), Me–O bond strength, crystalline structure, redox features, phase cooperation, multi-functionality and the nature of the surface oxygen species. Other important features and physical and chemical properties of solid catalysts, more or less related to the seven pillars, are also emphasized, including reaction sensitivity to metal oxide structure, epitaxial contact between an active phase and a second phase or its support, synergy effect between several phases, acid-base aspects, electron transfer ability, catalyst preparation and activation and reaction atmospheres, etc. Some examples are presented to illustrate the importance of these key factors. They include light alkanes (C1–C4) oxidation, ethane oxidation to ethylene and acetic acid on MoVTe(Sb)Nb-O and Nb doped NiO, propene oxidation to acrolein on BiMoCoFe-O systems, propane (amm)oxidation to (acrylonitrile) acrylic acid on MoVTe(Sb)Nb-O mixed oxides, butane oxidation to maleic anhydride on VPO: (VO)2P2O7-based catalyst, and isobutyric acid ODH to methacrylic acid on Fe hydroxyl phosphates. It is shown that active sites are composed of ensembles of atoms whose size and chemical composition depend on the reactants to be transformed (their chemical and size features) and the reaction mechanism, often of Mars and van Krevelen type. An important aspect is the fact that surface composition and surface crystalline structure vary with reaction on stream until reaching steady state, which makes characterisation of active and selective surface sites quite difficult. The use of oxidants other than O2, such as H2O2, N2O or CO2, is also briefly discussed. Based on such analysis and recent discoveries and process developments, our perspective is given.

Emerging Nanomaterials for Light‐Driven Reactions: Past, Present, and Future

This minireview is an attempt to summarize our current knowledge on oxidative water splitting in photosynthesis. Based on the extended Kok model (Kok, Forbush, McGloin (1970) Photochem Photobiol 11:457-476) as a framework, the energetics and kinetics of two different types of reactions comprising the overall process are discussed: (i) P680+* reduction by the redox active tyrosine YZ of polypeptide D1 and (ii) Yz (ox) induced oxidation of the four step sequence in the water oxidizing complex (WOC) leading to the formation of molecular oxygen. The mode of coupling between electron transport (ET) and proton transfer (PT) is of key mechanistic relevance for the redox turnover of YZ and the reactions within the WOC. The peculiar energetics of the oxidation steps in the WOC assure that redox state S1 is thermodynamically most stable. This is a general feature in all oxygen evolving photosynthetic organisms and assumed to be of physiological relevance. The reaction coordinate of oxidative water splitting is discussed on the basis of the available information about the Gibbs energy differences between the individual redox states Si+1 and Si and the data reported for the activation energies of the individual oxidation steps in the WOC. Finally, an attempt is made to cast our current state of knowledge into a mechanism of oxidative water splitting with special emphasis on the formation of the essential O-O bond and on the active role of the protein in tuning the local proton activity that depends on time and redox state Si. The O-O linkage is assumed to take place at the level of a complexed peroxide.

ConspectusHydrogen is an ideal energy carrier and plays a critical role in the future energy transition. Distinct from steam reforming, electrochemical water splitting, especially powered by renewables, has been considered as a promising technique for scalable production of high-purity hydrogen with no carbon emission. Its commercialization relies on the reduction of electricity consumption and thus hydrogen cost, calling for highly efficient and cost-effective electrocatalysts with the capability of steadily working at high hydrogen output. This requires the electrocatalysts to feature (1) highly active intrinsic sites, (2) abundant accessible active sites, (3) effective electron and mass transfer, (4) high chemical and structural durability, and (5) low-cost and scalable synthesis. It should be noted that all these requirements should be fulfilled together for a practicable electrocatalyst. Much effort has been devoted to addressing one or a few aspects, especially improving the electrocatalytic activity by electronic modulation of active sites, while few reviews have focused on the synergistic modulation of these aspects together although it is essential for advanced electrochemical water splitting.In this Account, we will present recent innovative strategies with an emphasis on our solutions for synergistically modulating intrinsic active sites, electron transportation, mass transfer, and gas evolution, as well as mechanical and chemical durability, of non-precious-metal electrocatalysts, aiming for cost-effective and highly efficient water splitting. The following approaches for coupling these aspects are summarized for both cathodic hydrogen evolution reaction (HER) and anodic oxygen evolution reaction (OER). (1) Synergistic electronic modulations. The electronic structure of a catalytic site determines the adsorption/desorption of reactive intermediates and thus intrinsic activity. It can be tuned by heterogeneous doping, strain effect, spin polarization, etc. Coupling these effects to optimize the reaction pathways or target simultaneously the activity and stability would advance electrocatalytic performance. (2) Synergistic electronic and crystalline modulation. The crystallinity, crystalline phase, crystalline facets, crystalline defects, etc. affect both activity and stability. Coupling these effects with electronic modulation would enhance the activity together with stability. (3) Synergistic electronic and morphological modulation. It will focus on concurrently modulating electronic structure for improving the intrinsic activity and morphology for increasing accessible active sites, especially through single action or processing. The mass transfer and gas evolution properties can also be enhanced by morphological modulation to enable water splitting at large output. (4) Synergistic modulation of elementary reactions. Electrocatalytic reaction generally consists of a couple of elementary reactions. Each one may need a specific active site. Designing and combining various components targeting every elementary step on a space-limited catalyst surface will balance the intermediates and these steps for accelerating the overall reaction. (5) Integrated electrocatalyst design. Taking all these strategies together into account is necessary to integrate all above essential features into one electrocatalyst for enabling high-output water electrolysis. Beyond the progress made to date, the remaining challenges and opportunities is also discussed. With these insights, hopefully, this Account will shed light on the rational design of practical water-splitting electrocatalysts for the cost-effective and scalable production of hydrogen.

Single-atom catalyst as an advanced co-catalyst shows promising potential in energy fields such as water splitting, degradation of pollutants, chemical conversion field, etc. It takes advantage of increased active sites and sub-nanometer size effects with electron confinement by reducing the size of supported noble metals; furthermore, a single-atom catalyst maximizes the atomic efficiency of metals and provides a low atomic coordination number for the active center. In this work, we discovered that the isolated single atoms (SAs) stabilized by the support of g-C3N4 as named by SA-Pt/g-C3N4. Single-atom catalyst is synthesized by anchoring Pt atoms in graphitic carbon nitride (g-C3N4) with different Pt-loading [0.01wt% to 1wt% H₂PtCl₆·(H₂O)₆]. Under flexocatalytic process, the k rate constant of the SA-Pt/g-C3N4 with 0.01wt% of Pt is 3×10-3 s-1 for decomposition of dye molecules, which is 2 times the pristine g-C3N4. In addition, the flexocatalytic hydrogen evolution of SA-Pt/g-C3N4 provides H2 production, reaching nearly 125 μmolgh-1, which is 312% of the pristine g-C3N4 (~40 μmolgh-1). The working mechanism of flexocatalytic activity suggests the mechanical strain-induced flexoelectric potential (flexopotential) and proceeding with redox's electrochemical reaction. In addition, the single-atom act as a critical role in attracting electrons to accelerate the reaction process. The single-atom catalyst of 2D materials is highly potential for applying in water splitting and degradation of pollutants through the flexocatalytic process.

SnO2/ZnO hierarchical heterostructures have been successfully synthesized by combining electrospinning technique and hydrothermal method. Various morphologies of the secondary ZnO nanostructures including nanorods (NRs) and nanosheets (NSs) can be tailored by adding surfactants. Photocatalytic performance of the heterostructures was investigated and obvious enhancement was demonstrated in degradation of the organic pollutant, compared to the primary SnO2-based nanofibers (NFs) and bare ZnO. Furthermore, it was found that the H2 evolution from water splitting was achieved by photocatalysis of heterostructured nanocomposites after sulfurization treatment. This synthetic methodology described herein promises to be an effective approach for fabricating variety of nanostructures for enhanced catalytic applications. The heterostructured nanomaterials have considerable potential to address the environmental and energy issues via degradation of pollutant and generation of clean H2 fuel.

Chapter 1 - Basics of Photocatalysis

Designing cost-effective and highly stable heterostructures with synergistic active sites could simultaneously catalyze the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) for (sea) water splitting. However, there are still challenges in maintaining the catalytic performance of individual materials and in constructing intimate interfaces. Herein, a novel corrosion engineering method is provided to prepare self-standing Fe2O3-Pt/NF nanoflowers where ultra-small amounts of Pt combined with Fe2O3 are in situ grown on nickel foam (NF) in the corrosion system of "H2PtCl6-NaCl-FeCl3". The synthesized Fe2O3-Pt/NF shows the presence of a Pt-O bond, which can regulate the electronic structure of the active sites and optimize the binding energy of the reaction intermediates, leading to an improvement in catalytic performance. Compared with Pt/NF and FeOOH/NF, the Fe2O3-Pt/NF heterostructure exhibits remarkable electrocatalytic activities with overpotentials reaching 94 mV and 265 mV for the HER and OER, respectively, at a high current density of 100 mA cm-2 in alkaline solution. Furthermore, the self-assembled electrolytic cell employing Fe2O3-Pt/NF as the bifunctional electrode only requires potentials of 1.60 V and 1.61 V to achieve a current density of 100 mA cm-2 in overall water and seawater splitting, respectively. This material remained stable for 10 hours without obvious attenuation, indicating its good environmental adaptability and stability. Specifically, the enhanced catalytic activity and stability can be ascribed to the abundant active sites of nanoflowers, fast electron transfer rate of intimate interfaces, and strong electronic interaction between Pt atoms and Fe2O3. This work provides a new insight into the construction of highly efficient co-catalysts with intimate interfaces based on corrosion engineering methods.

The humans are leading a good life due to industrialization by utilizing the available natural resources such as fossil fuels, water, etc. The development of many new chemicals such as fertilizers helped to increase the food production; pesticides and herbicides in reducing the diseases, pests, and weeds in agriculture; and antibiotics in improving the life span of humans. At the same time, the wastes that are generated due to industrialization have led to environmental pollution (air, water, and soil pollution). The degradation of the pollutants is considered as a major task in reducing the environmental pollution. Microbial remediation is an alternative, natural, and cost-effective method of bioremediation for mitigating the environmental pollution. Bioremediation is a process of utilizing the live microorganisms (microbial remediation) and plants (phytoremediation) which degrade/convert the toxic pollutants into non-toxic compounds. This chapter reviews the microbial remediation, i.e., degradation of environmental pollutants by microorganisms, and mechanism and advantages of microbial remediation.

Photocatalytic approaches in the visible region show promising potential in photocatalytic water splitting and water treatment to boost water purification efficiency. For this reason, developing cost‐effective and efficient photocatalysts for environmental remediation is a growing need, and semiconductor photocatalysts have now received more interest owing to their excellent activity and stability. Recently, several metal oxides, sulfides, and nitrides‐based semiconductors for water splitting and photodegradation of pollutants have been developed. However, the existing challenges, such as high over potential, wide band gap as well as fast recombination of charge carriers of most of the semiconductors limit their photocatalytic properties. This review summarizes the recent state‐of‐the‐art first‐principles research progress in the design of effective visible‐light‐response semiconductor photocatalysts through several modification processes with a focus on density functional theory (DFT) calculations. Recent developments to the exchange‐correlation effect, such as hybrid functionals, DFT + U as well as methods beyond DFT are also emphasized. Recent discoveries on the origin, fundamentals, and the underlying mechanisms of the interfacial electron transfer, band gap reduction, enhanced optical absorption, and electron–holes separation are presented. Highlights on the challenges and proposed strategies in developing advanced semiconductor photocatalysts for the application in water splitting and degradation of pollutants are proposed.

The rational design of highly efficient and stable bifunctional catalysts for overall water splitting is vitally important. In this study, to increase the active catalytic sites of CeO2 for electrochemical water splitting, a ternary CeO2-CuO-Mn3O4 heterostructure, synthesized by coprecipitation method, is loaded on reduced graphene oxide (rGO) nanosheets in different amounts to produce CeO2-CuO-Mn3O4@rGO nanocomposites. It is found that CeO2-CuO-Mn3O4@rGO nanocomposites show higher electrocatalytic activity than unsupported samples, and the best activity is observed when the wieght ratio of CeO2-CuO-Mn3O4 is three times that of rGO. The CeO2-CuO-Mn3O4@rGO(3:1) requires low overpotentials of 130 and 270 mV for hydrogen and oxygen evolution reactions (HER and OER) at a current density of 10 mA cm−2. Furthermore, this material demonstrates a large electrochemically active surface area, low charge transfer resistance, suitable kintics, and high long-term stability for both OER and HER. Additionally, when CeO2-CuO-Mn3O4@rGO(3:1) is used as self-supported electrodes for the overall water splitting reaction, a low cell voltage of 1.68 V is obtained. This superior performance is due to: (i) active multi-metal sites that produce strong synergistic effects; (ii) the high conductivity of rGO, which faciliate favorable electron transfer; and (iii) the homogenous anchoring of CeO2-CuO-Mn3O4 on rGO, which increases the number of active sites available on the catalyst surface.

Defect-Engineered 3D hierarchical NiMo3S4 nanoflowers as bifunctional electrocatalyst for overall water splitting

The development of cost-effective, highly active, and stable electrocatalysts for water splitting to produce green hydrogen is crucial for advancing clean and sustainable energy technologies. Herein, we present an innovative in-situ synthesis of FeOOH nanorods@NiOOH nanosheets on nickel foam (FeOOH@NiOOH/NF) at an unprecedentedly low temperature, resulting in a highly efficient electrocatalyst for overall water splitting. The optimized FeOOH@NiOOH/NF sample, evaluated through time-dependent studies, exhibits exceptional oxygen evolution reaction (OER) performance with a low overpotential of 261 mV at a current density of 20 mA cm−2, alongside outstanding hydrogen evolution reaction (HER) activity with an overpotential of 150 mV at a current density of 10 mA cm−2, demonstrating excellent stability in alkaline solution. The water-splitting device featuring FeOOH@NiOOH/NF-2 electrodes achieves a voltage of 1.59 V at a current density of 10 mA cm−2, rivalling the state-of-the-art RuO2/NF||PtC/NF electrode system. Density functional theory (DFT) calculations unveil the efficient functionality of the Fe sites within the FeOOH@NiOOH heterojunction as the active OER catalyst, while the Ni centres are identified as the active HER sites. The enhanced performance of OER and HER is attributed to the tailored electronic structure at the heterojunction, modified magnetic moments of active sites, and increased electron density in the dx2-y2 orbital of Fe. This work provides critical insights into the rational design of advanced electrocatalysts for efficient water splitting.

Owing to their remarkable success in photocatalytic applications, multiferroic BiFeO3 and its derivatives have gained a highly promising position as electrode materials for future developments of efficient catalysts. In addition to their appropriate band gaps, these materials exhibit inherent intrinsic polarizations enabling efficient charge carrier separation and their high mobility without the need for additional co-catalysts. Here, we review the existing strategies for enhancing the photocatalytic performances of BiFeO3-based materials and we describe the physico-chemical properties at the origin of their exceptional photocatalytic behavior. A special focus is paid to the degradation of organic pollutants and water splitting, both driven through photocatalysis to unveil the correlation between BiFeO3 size, substitution, and doping on the one hand and the photocatalytic performances on the other hand. Finally, we provide practical recommendations for future developments of high-performing BiFeO3-based electrodes.

Recyclable and dual-functional Ag3PO4/3D graphene aerogel photocatalysts for simultaneous water oxidation and pollutant degradation