Sites For Catalytic Reaction Research Articles

Activated Carbons from Fast Pyrolysis Biochar as Novel Catalysts for the Post-Treatment of Pyrolysis Vapors, Studied by Analytical Pyrolysis

Biochars are attractive materials for carbon catalysts since their carbon content and surface area are relatively high and minerals present in biochar can act as active sites for catalytic reactions. In this study, biochars from the fast pyrolysis of birch, pine, and unbarked willow were activated and acid washed. These materials were tested as catalysts for a post-treatment of pine wood pyrolysis vapors, aiming at stabilizing the vapors before their condensation. All the unmodified biochars had high content of minerals, those being highest in willow due to the bark. After the activation treatments, the surface areas and pore volumes of all biochars significantly increased. All studied biochars and activated carbon catalysts reduced the oxygen content of the pyrolysis degradation products. This effect was more pronounced for compounds derived from polysaccharides vs. lignin. The most promising catalyst for vapor upgrading was unwashed activated carbon from willow, having high surface areas and pore volumes together with high mineral contents. These properties together promoted the high conversion of polysaccharide-derived products (anhydrosugars, acids, and pyrans) into CO2. Release of highly oxidized degradation products may indicate that reductive stabilization takes place via hydrogen migration from the polysaccharide-derivatives to lignin derivatives, mediated by the carbon catalyst.

Study of water transport mechanism based on the single straight channel of proton exchange membrane fuel cell

At normal operating temperatures (below 100 °C), the water in the proton exchange membrane fuel cell (PEMFC) is in the form of a mixture of gas and liquid phases. The content of water guarantees that the membrane has good proton conductivity, but it may also block the transmission of the reaction gas to the catalytic reaction sites. Based on the single straight channel of the PEMFC, the water content of the model at various voltages and the water distribution of the different parts are studied. The results indicate that the amount of water in the model generally increases with the reduction in the voltage. When the voltage is set from 1.1 V to 0.7 V, the increase in the amplitude of the water content is higher than 0.7 V–0.4 V, and the water content is highest at 0.5 V. Under each voltage, the largest water content occurs in the gas diffusion layer, the second one occurs in the channel, the third one occurs in the proton exchange membrane, and the water content in the catalyst layer is lowest.

Converting coals into carbon-based pH-universal oxygen reduction catalysts for fuel cells

Tapping into the potentialities of coals as valuable resources that can be transformed into state-of-the-art energy-related materials is now a burgeoning and fascinating research area, which accords with the clean utilization of coals. In the present work, coal-based porous nitrogen-doped carbon (CPNC) electrocatalysts toward the oxygen reduction reaction (ORR) at fuel cell cathodes have been easily prepared by the mechanochemistry-assisted strategy. Catalyst preparation combines the low-temperature pyrolysis with the high-temperature annealing, which guarantees nitrogen-containing substances sealed and confined in the carbon layers and skeletons and effectively converts them into nitrogen-relevant bonds, simultaneously regulating architectural features (e.g. specific surface areas, pore width distributions, defects and edges). The optimal metal-free CPNC electrocatalyst owns onset potentials of 0.97, 0.80 and 0.87 V, half-wave potentials of 0.84, 0.64 and 0.65 V, limiting current densities of 4.96, 4.60 and 4.90 mA cm−2, electron-transfer numbers of 3.93, 3.92 and 3.91, together with fine current stability and methanol tolerance, in 0.1 M KOH, 0.5 M H2SO4 and 0.1 M phosphate buffer solution (pH = 7.0), respectively. This is the first report on high-performance coal-based pH-universal oxygen reduction electrocatalysts, attributed to the effective trade-off between textural properties of catalysts (related to surface areas and pore structures) and the number of catalytic reaction sites (associated with nitrogen configuration distributions). Accordingly, this work has established the basis by which earth-abundant coals are readily converted into high-performance oxygen reduction carbonaceous electrocatalysts that are able to be integrated into various fuel cell test beds.

Oxygen assisted butanol conversion on bifunctional carbon nanotube catalysts: Activity of oxygen functionalities

Catalytic conversion of n-butanol to high-value-added product is considered as a highly important route towards biomass derived fine chemicals. The present work reports the applications of non-metallic multi-walled carbon nanotube (CNT) as bifunctional catalysts for oxygen assisted n-butanol conversion reactions. Oxidative dehydrogenation (ODH) and dehydration of n-butanol on CNT yielded aldehyde and alkene compounds via redox and acid catalytic routes, respectively. CNT exhibited high activity (over 50% n-butanol conversion) and stability (over 120 h) under gentle reaction conditions (<330 °C), which are competitive to conventional metal-based catalysts. Carboxylic acid groups were identified as catalytic active sites for dehydration reactions via linking the reactivity (kinetic parameters) with the surface composition and structural information of CNT catalysts. The redox activity could be related to ketonic carbonyl groups, and interestingly the intrinsic ODH catalytic activity is also influenced by degree of graphitization for CNT materials.

MOF-derived Co3O4-C@FeOOH as an efficient catalyst for catalytic ozonation of norfloxacin

Metal-organic frameworks (MOFs) ZIF-67-derived Co3O4-C@FeOOH composite was prepared, characterized and used as an efficinet catalyst for ozonation of norfloxacin (NOF). Results showed that ZIF-67-derived Co3O4-C composite maintained the polyhedral structure of ZIF-67. After modification, abundant amorphous FeOOH nanowire attached on the surface of Co3O4-C composite, resulting in Co3O4-C@FeOOH interwoven polyhedrons. Furthermore, the specific surface area of the formed composite was about 2.5 times that of Co3O4-C composite, which might provide more active sites for catalytic reaction. Compared with single ozonation system, the catalytic ozonation process (Co3O4-C@FeOOH/O3) had better performance in NOF mineralization under the same operating conditions. Moreover, in the presence of Co3O4-C@FeOOH, faster O3 decomposition and higher •OH concentration were observed, which could explain the significant enhancement of TOC removal. The co-existence of Fe and Co in various valence states in catalyst might improve the conversion of Co(III)/Co(II) and Fe(III)/Fe(II), which would increase the catalytic activity in catalytic ozonation process. Besides, several main intermediate products were detected and possible NOF degradation pathway was proposed.

Bifunctional N-Doped Tungsten Trioxide Microspheres as Electrode Materials for Lithium-Ion Batteries and Direct Methanol Fuel Cells

Oxygen vacancies (OVs) have emerged as an efficient strategy to modulate the electronic structures, conductivity, and electrochemical properties of WO3. However, OVs are easily absorbed by water molecules or oxygen atoms, resulting in the poor stability of its electrocatalytic reaction. In the present work, we have demonstrated that nitrogen doping provides an alternative to improve the electrochemical properties of WO3, which plays a series of roles in reducing the band gap, improving the electron mobility, accelerating the Li+ ion diffusion, and providing more active sites for catalytic reactions. Consequently, N-doped WO3 microspheres exhibit low discharge–charge voltage gaps and high capacity retention as well as exceptional rate performance when used as anode materials for lithium-ion batteries (LIBs). Furthermore, the as-prepared Pt/N-WO3 catalyst also exhibits excellent electrocatalytic properties toward methanol electro-oxidation (MOR) including high activity, strong poison tolerance, and outstanding long-term stability owing to the synergetic effects between Pt and N-WO3.

Identification of catalytic sites for cerium redox reactions in a metal-organic framework derived powerful electrocatalyst

The development of N-doped carbon materials with highly catalytic activity for catalyzing electrode reactions as alternatives to noble metal is of great importance for the practical implementation of redox flow batteries (RFBs). However, in addition to C-Nx sites, the high structural heterogeneity of N-doped carbon materials often contains metal-Nx sites and graphene encapsulated metal nanoparticle simultaneously, as a result of their metal-assistant synthesis method. Thus it is challenging to identify the predominant active structure within N-doped carbon catalysts for RFBs, which normally receives far less attention in the field of RFB research. Herein, we report a general metal-organic frameworks assisted approach to high-yield growth of nitrogen-doped carbon nanotubes (NCNTs) onto 3D graphite felt (GF) substrates without the requirement of any additional C and N sources or costly and complex CVD systems. The obtained NCNTs-modified GF (NCNT-GF) exhibits high electrocatalytic activity and stability toward cerium redox reactions. Based on the moderate and selective NH4Cl-purification process and the electrochemical poison experiments, we definitely elucidate the spectator role of atomically dispersed metal-Nx sites and graphene encapsulated metal nanoparticles, and the pivotal role of C-Nx sites in the NCNT-GF materials for cerium redox reactions.

Graphene oxide induced assembly and crumpling of Co3O4 nanoplates

Cobalt (II, III) oxide (Co3O4) has been widely studied and applied in various fields, however, it suffers from slow mass and electron transfer during applications. Herein, crumpled Co3O4 and Co3O4/reduced graphene oxide (rGO) with tunable 2D-in-3D structures were prepared by combining spray pyrolysis with a graphene oxide (GO) template. The 2D Co3O4 nanoplates were interconnected with each other to form a 3D ball with many wrinkles, resulting in defect enrichment on the abundant boundaries of the nanosheets, which provided more active sites for catalytic reactions. In addition, the unique 2D-in-3D structure allowed fast mass transfer and structural stability. Furthermore, the assembled structure could be understood as being composed of uniformly distributed oxygen-containing functional groups pinning metal cations on the GO surface through electrostatic interaction, and the 2D structure of the GO enabled the in situ converted Co3O4 to grow along the GO surface with excellent dispersion. Taking advantage of the above, the Co3O4/rGO balls demonstrated an excellent oxygen evolution reaction performance, an overpotential of 298 mV at a current density of 10.0 mA cm−2 and a current density of 115.9 mA cm−2 at the overpotential of η = 500 mV.

Hierarchical Architectured Ternary Nanostructures Photocatalysts with In(OH)3 Nanocube on ZnIn2S4/NiS Nanosheets for Photocatalytic Hydrogen Evolution

Charge carriers separation and surface catalytic reactions are two crucial steps in photocatalytic processes; the rational design of photocatalysts by taking these two factors into consideration is thus of great importance to advance the associated performance. Herein, a self‐sacrificial strategy is developed to fabricate a ternary nanostructure photocatalyst, forming a hierarchical architecture of In(OH)3 nanocube decorated NiS‐ZnIn2S4 (ZIS) hybrid nanosheets (ZIS/In(OH)3‐NiS). Such a unique structure provides the hybrid photocatalyst with a facilitated path for efficiently separating the charge carrier and abundant sites for catalytic reactions. Systematic characterizations that reveal the strong electronic interactions in the ternary ZIS/In(OH)3‐NiS leads to fast electron transfer from excited ZnIn2S4 to NiS nanosheets, which provide catalytic sites for hydrogen evolution reaction. The comprehensive photocatalysis studies demonstrate that ZnIn2S4/In(OH)3‐NiS exhibits ultrahigh photocatalytic activity toward hydrogen generation with a high rate of 7010 μmol g−1 h−1, which ranks as one of the highest among ZnIn2S4‐based photocatalysts reported so far. This work provides an attractive and effective way to develop high‐activity photocatalysts without using precious metal cocatalysts. The investigation brings us one step closer to understanding the structure‐determining properties of nanohybrid architecture, and provides a valuable reference to develop cost‐effective and practical photocatalysts for a variety of applications.

Size Dependent Dielectric Properties of TiO&lt;sub&gt;2&lt;/sub&gt; Nanoparticles

In this study, influence of nanoparticles size on optical and dielectric properties of TiO2 nanoparticles is investigated through thermal treatment of hydrous amorphous titania synthesized by chemical precipitation method at temperatures 300 °C and 600 °C. The average sizes of nanoparticles estimated respectively are ~ 8 nm and ~ 22 nm. Although the optical bandgap energy of both samples remains the same the Eg Raman mode observed at 144 cm-1 for bulk TiO2 is shifted to 150 cm-1 only for nanoparticles calcined at 300 °C. The shift is ascribed to the size as well as higher density of surface defects. Moreover, the presence of surface defects like oxygen vacancies which provide effective sites for catalytic reaction are confirmed by EPR and photoluminescence studies. The oxygen vacancies enhances space charge polarization and consequently results in higher dielectric constant. In addition, the peak shift of loss tangent which determines the mobility of charge carriers is found to be size dependent. Hence calcination temperature has significant influence on defect levels which in turn determine the optical and dielectric properties of TiO2 nanoparticles.

Ozone synthesis from oxygen in narrow‐gap hybrid discharge integrated with oxide coating: The role of surface catalytic reactions

AbstractIntegrating oxide coatings with narrow‐gap hybrid discharge (HD) can modulate ozone (O3) synthesis. O3 concentration () and energy yield (η) for HD can be enhanced by single‐valent oxides, whereas they are reduced by multivalent oxides. The discharge and oxide characterizations suggest that surface catalytic reactions play a crucial role in the synthesis of O3: single‐valent oxide coatings facilitate O3 production by offering abundant catalytic sites for surface catalytic reactions; surface catalytic reactions on multivalent oxide coatings suppress O3 production because of the dominating O3 decomposition over highly active sites of oxygen vacancies. This study is inspirational for designing and fabricating high‐efficiency O3 generators.

Defect-Rich Multishelled Fe-Doped Co3O4 Hollow Microspheres with Multiple Spatial Confinements to Facilitate Catalytic Conversion of Polysulfides for High-Performance Li-S Batteries.

Over the past decade, lithium-sulfur (Li-S) batteries have been thought of as promising alternatives for the new generation of battery systems. Although the Li-S batteries possess high-theoretical energy density (2600 Wh kg-1) and capacity (1675 mAh g-1), the problems of poor electron and ion conduction, volumetric expansion, and sulfur immobilization greatly impede the wide applicability of Li-S batteries. Herein, a defect-rich multishelled Co3O4 microsphere structure doped with Fe was synthesized via a one-step hydrothermal method and subsequent thermal treatment. The unique multishelled structure provides multiple spatial confinements for lithium polysulfides trapping and buffering the volume variation during cycling. Moreover, the rich oxygen defect designed by controlled Fe doping can provide numerous catalytic sites for polysulfide redox reactions. Attributed to the synergistic effect of structural design and oxygen-defect fabrication, the sulfur composite electrode delivers a notable cycle performance, presenting a much lower capacity fading of 0.017% per cycle over 1000 cycles at 1 C and an excellent rate capability of 571.3 mAh g-1 at 5 C. This work proposes a potential approach for designing a transition metal oxide-based multishelled hollow structure combined with oxygen defect, which also offers a new perspective on high-performance Li-S batteries.

New Versatile Synthetic Route for the Preparation of Metal Phosphate Decorated Hydrogen Evolution Photocatalysts.

Photocatalytic hydrogen generation will benefit from the realization of more active but less expensive cocatalysts compared with noble metal counterparts. Herein we developed a universal vapor deposition method that selectively uses the thermal decomposition products of sodium hypophosphite as a phosphorus source for the fabrication of inexpensive and highly efficient metal phosphate (MPi) modified CdS nanorods. We find that the modification with a bimetal phosphate (i.e., 5 wt % NiCoPi) leads to an activity enhancement by a factor of approximately 52 in boosting visible-light-driven hydrogen evolution relative to the pristine CdS nanorods. The photocatalyst exhibits a high hydrogen generation rate of 13.44 mmol·g-1·h-1, which is much higher than that of its single metal counterparts (NiPi, 8.70 mmol·g-1·h-1; CoPi, 5.79 mmol·g-1·h-1) and 1 wt % Pt modified CdS (1.33 mmol·g-1·h-1). Its apparent quantum efficiency reaches 23.5% at 420 nm. Furthermore, it also shows remarkable photostability for eight consecutive cycles of photocatalytic activity tests with total reaction time of 24 h. The excellent photocatalytic performance of the photocatalyst is believed to be associated with the in situ formed NiICoP and NiCoIIIPi cocatalysts, which not only play an important role in photogenerated charge separation but also provide highly active catalytic reaction sites for the corresponding hydrogen evolution reaction and the sacrificial agent oxidation reaction.

Well-Defined Materials for Heterogeneous Catalysis: From Nanoparticles to Isolated Single-Atom Sites.

The use of well-defined materials in heterogeneous catalysis will open up numerous new opportunities for the development of advanced catalysts to address the global challenges in energy and the environment. This review surveys the roles of nanoparticles and isolated single atom sites in catalytic reactions. In the second section, the effects of size, shape, and metal-support interactions are discussed for nanostructured catalysts. Case studies are summarized to illustrate the dynamics of structure evolution of well-defined nanoparticles under certain reaction conditions. In the third section, we review the syntheses and catalytic applications of isolated single atomic sites anchored on different types of supports. In the final part, we conclude by highlighting the challenges and opportunities of well-defined materials for catalyst development and gaining a fundamental understanding of their active sites.

In situ visualization of loading-dependent water effects in a stable metal-organic framework.

Competitive water adsorption can have a significant impact on metal-organic framework performance properties, ranging from occupying active sites in catalytic reactions to co-adsorbing at the most favourable adsorption sites in gas separation and storage applications. In this study, we investigate, for a metal-organic framework that is stable after moisture exposure, what are the reversible, loading-dependent structural changes that occur during water adsorption. Herein, a combination of in situ synchrotron powder and single-crystal diffraction, infrared spectroscopy and molecular modelling analysis was used to understand the important role of loading-dependent water effects in a water stable metal-organic framework. Through this analysis, insights into changes in crystallographic lattice parameters, water siting information and water-induced defect structure as a response to water loading were obtained. This work shows that, even in stable metal-organic frameworks that maintain their porosity and crystallinity after moisture exposure, important molecular-level structural changes can still occur during water adsorption due to guest-host interactions such as water-induced bond rearrangements.

CuO/Cu2O nanoflake/nanowire heterostructure photocathode with enhanced surface area for photoelectrochemical solar energy conversion

Branched heterostructures play an important role in solar energy harvesting through an efficient separation of photogenerated charges as well as enhancement in catalytic reaction sites. Here we report the CuO/Cu2O nanoflake/nanowire heterostructure with the enhanced surface area directly grown on Cu foil through scalable process steps ensuring easy large-scale fabrication. A hydrothermal treatment on Cu2O nanowires, obtained through anodization of Cu foil and argon annealing, leads to a partial conversion of Cu2O into CuO nanoflakes resulting in a heterostructure with a highly enhanced surface area facilitating a higher semiconductor-electrolyte interface. Brief annealing at 250 °C for 5 min on the sample results in a photocurrent density of −1.9 mA/cm2 (at −0.3 V vs. Ag/AgCl under AM1.5G illumination), which is about an order of magnitude higher than the Cu2O nanowires. Further, CuO/Cu2O shell/core heterostructure is fabricated through direct annealing of Cu2O nanowires to understand the effect of highly enhanced surface area over the charge separation due to heterostructure. A comparative study on both heterostructures by photocurrent, electrochemical impedance, and Mott Schottky measurements reveals that although CuO/Cu2O heterojunction helps in charge separation, however, the enhanced surface area obtained through nanoflake morphology has a dominating effect in deciding high photocurrent density.

Highly Efficient Metal-Free Nitrogen-Doped Nanocarbons with Unexpected Active Sites for Aerobic Catalytic Reactions.

Nitrogen (N)-doped nanocarbons (NDN) as metal-free catalysts have elicited considerable attention toward selective oxidation of alcohols with easily oxidizable groups to aldehydes in the past few years. However, finding a new NDN catalytic material that can meet the requirement of the feasibility on the aerobic catalytics for other complicated alcohols is a big challenge. The real active sites and the corresponding mechanisms on NDN are still unambiguous because of inevitable coexistence of diverse edge sites and N species based on recently reported doping methods. Here, four NDN catalysts with enriched pyridinic N species and without any graphitic N species are simply fabricated via a chemical-vapor-deposition-like method. The results of X-ray photoelectron spectroscopy and X-ray absorption near-edge structure spectra suggest that the dominating N species on NDN are pyridinic N. It is demonstrated that NDN catalysts perform impressive reactivity for aerobic oxidation of complicated alcohols at an atmospheric pressure. Eleven kinds of aromatic molecules with single N species and tunable π conjugation systems are used as model catalysts to experimentally identify the actual role of each N species at a real molecular level. It is suggested that pyridinic N species play an unexpected role in catalytic reactions. Neighboring carbon atoms in pyridinic N species are responsible for facilitating the rate-determining step process clarified by kinetic isotope effects, in situ nuclear magnetic resonance, in situ attenuated total reflectance infrared, and theoretical calculation. Moreover, NDN catalysts exhibit a good catalytic feasibility on the synthesis of important natural products (e.g., intermediates of vitamin E and K3) from phenol oxidation.

Enhanced photocatalytic activity for tetracyclines degradation with Ag modified g-C3N4 composite under visible light

Tetracycline antibiotics (TCs) have high residual concentrations in the environment, which can cause serious problems with antibiotic resistance genes as well as bacterial resistance, and thus pose a threat to human health and ecosystem. The application of photocatalytic technology to degrade pollutants has received more and more attention due to its energy-saving and high-efficiency characteristics. In this work, special emphasis was placed on the degradation and mechanisms of three kinds of tetracycline antibiotics (including tetracycline (TC), chlortetracycline (CTC) and oxytetracycline (OTC)) by using novel Ag/g-C3N4 nano-sheets photocatalysts. The results showed that the heterojunction formed by the combination of g-C3N4 and Ag, being not only beneficial to supply more reaction active sites for catalytic reaction but also suppressing the recombination of photogenerated electron-holes pairs, can achieve 2–3 times the degradation efficiency of pure g-C3N4. The Ag(8%)/g-C3N4 sample showed the highest photocatalytic efficiency among these composites and the photocatalytic activities of TCs were improved with increasing pH. In addition, this work assessed the effects of TCs degradation intermediates on Escherichia coli (E. coli) activities and used TC as an example to infer the degradation pathway. The degradation mechanisms were also explored. It suggested that photogenerated holes were the main active species and the inhibition effect of degradation products on E. coli was significantly reduced compared with TCs. This proved again that the degradation of TCs by Ag/g-C3N4 was effective and reliable.

Emerging WS2/montmorillonite composite nanosheets as an efficient hydrophilic photocatalyst for aqueous phase reactions

Tungsten disulfide (WS2) as one of transition metal dichalcogenides exhibits excellent catalytic activity. However, its catalytic performances in aqueous phase reactions are limited by its hydrophobicity. Here, the natural hydrophilic two-dimensional clay was used to enhance the dispersibility of WS2 in aqueous phase. WS2/montmorillonite (WS2/MMT) composite nanosheets were prepared via hydrothermal synthesis of WS2 on the surface of montmorillonite from WCl6 and CH3CSNH2. The microstructure and morphology show that WS2 nanosheets are assembled parallelly on the montmorillonite with the interface interaction. Through the support of montmorillonite, WS2/MMT possesses higher photocatalytic ability for aqueous phase reactions than WS2, which could be due to the synergistic effect of higher adsorption property, higher hydrophilicity, dispersibility and more catalytic reaction site. The strategy could provide new ideas for obtaining novel hydrophilic photocatalyst with excellent performance.

Biodegradable CoS2 nanoclusters for photothermal-enhanced chemodynamic therapy

Retaining in tumors for cancer diagnosis/treatment with sequential elimination from body is crucial to the clinical translation of inorganic medicaments. Herein, monodispersed CoS2 nanoclusters (CoS2 NCs) assembled from ultrasmall nanocrystals oligomerization via the La Mer scheme were successfully developed for chemodynamic therapy (CDT) of cancer cells due to the Fenton-like catalytic activity. Owing to their excellent photothermal conversion ability, both in vitro and in vivo cancer cell-killing efficacy of CoS2 NCs could be greatly enhanced upon near infrared (NIR) light irradiation, suggesting that generated heat would accelerate the Fenton-like catalytic reaction rate. Moreover, the structure of loosely stacking of CoS2 nanocrystals not only endowed CoS2 NCs more catalytic reaction sites to convert endogenous H2O2 into cytotoxic OH in tumor region, but also facilitated them degrading into ions in vivo and rapidly excreting from body by feces and urine through the reticuloendothelial systems, without causing any appreciable toxicity confirmed by systematic toxicological evaluation. Thus, our study demonstrated that these CoS2 NCs with biodegradable behaviors and highly integrated properties might have broad applications for cancer treatment.