Reduced formation of peroxide and radical species stabilises iron-based hybrid catalysts in polymer electrolyte membrane fuel cells

The incorporation of Pt into an iron-nitrogen-carbon(Fe NC)catalyst for the oxygen reduction reaction(ORR)was recently shown to enhance catalyst stability without Pt directly contributing to the ORR activity.However,the mechanistic origin of this stabilisation remained obscure.It is established herein with rotating ring disc experiments that the side product,H_(2)O_(2),which is known to damage FeNC catalysts,is suppressed by the presence of Pt.The formation of reactive oxygen species is additionally inhibited,independent of intrinsic H_(2)O_(2) formation,as determined by electron paramagnetic resonance.Transmission electron microscopy identifies an oxidised Fe-rich layer covering the Pt particles,thus explaining the inactivity of the latter towards the ORR.These insights develop understanding of Fe NC degradation mechanisms during ORR catalysis,and crucially establish the required properties of a precious metal free protective catalyst to improve Fe NC stability in acidic media.

Preparation and Microstructural Characterization of Activated Carbon-Metal Oxide Hybrid Catalysts:New Insights into Reaction Paths

In catalysis processes, activated carbon （AC） and metal oxides （MOs） are widely used either as catalysts or as catalyst supports because of their unique properties. A combination of AC and MO nanoparticles in a single hybrid material usually entails both chemical and microstructural changes, which may largely influence the potential catalytic suitability and performance of the resulting product. Here, the prepa- ration of a wide series of AC-MO hybrid catalysts is studied. Three series of such catalysts are prepared by support first of MO （Al2O3, Fe2O3, SnO2, TiO2, WO3, and ZnO） precursors on a granular AC by wet impregnation and oven-drying at 120 ℃, and by subsequent heat treatment at 200 or 850℃ in inert atmosphere. Both the chemical composition and microstructure are mainly investigated by powder X-ray diffraction. Yield and ash content are often strongly dependent on the MO precursor and heat treatment temperature, in particular for the Sn catalysts. With the temperature rise, trends are towards the transformation of metal hydroxides into metal oxides, crystallinity improvement, and occurrence of drastic composition changes, ultimately leading to the formation of metals in elemental state and even metal carbides. Reaction paths during the preparation are explored for various hybrid catalysts and new insights into them are provided.

Phthalocyanine-derived catalysts decorated by metallic nanoclusters for enhanced CO_(2)electroreduction

Electrochemical CO_(2)reduction(CO_(2)RR)over molecular catalysts is a paramount approach for CO_(2)conversion to CO.Herein,we report a novel phthalocyanine-derived catalyst synthesized by a two-step method with a much improved electroconductivity.Furthermore,the catalyst contains both Ni-N4sites and highly dispersed metallic Ni nanoclusters,leading to an increased CO_(2)RR currents by two folds.Isotope labelling study and in situ spectroscopic analysis demonstrate that the existence of metallic Ni nanoclusters is the key factor for the activity enhancement and can shift the CO_(2)RR mechanism from being electron transfer(ET)-limited(forming*COO^(-))to concerted proton-electron transfer(CPET)-limited(forming CO).

Diffusion process in enzyme-metal hybrid catalysts

Enzyme-metal hybrid catalysts bridge the gap between enzymatic and heterogeneous catalysis,which is significant for expanding biocatalysis to a broader scope.Previous studies have demonstrated that the enzyme-metal hybrid catalysts exhibited considerably higher catalytic efficiency in cascade reactions,compared with that of the combination of separated enzyme and metal catalysts.However,the precise mechanism of this phenomenon remains unclear.Here,we investigated the diffusion process in enzyme-metal hybrid catalysts using Pd/lipase-Pluronic conjugates and the combination of immobilized lipase(Novozyme 435)and Pd/C as models.With reference to experimental data in previous studies,the Weisz-Prater parameter and efficiency factor of internal diffusion were calculated to evaluate the internal diffusion limitations in these catalysts.Thereafter,a kinetic model was developed and fitted to describe the proximity effect in hybrid catalysts.Results indicated that the enhanced catalytic efficiency of hybrid catalysts may arise from the decreased internal diffusion limitation,size effect of Pd clusters and proximity of the enzyme and metal active sites,which provides a theoretical foundation for the rational design of enzyme-metal hybrid catalysts.

Conversion of CO_(2)to added value products via rWGS using Fe-promoted catalysts:Carbide,metallic Fe or a mixture?

Fe-based catalysts are efficient systems for CO_(2)conversion via reverse water-gas shift(rWGS)reaction.Nevertheless,the nature of the active phase,namely metallic iron,iron oxide or iron carbide remains a subject of debate which our paper is meant to close.Fe0 is a much better catalyst for the rWGS than Fe_(3)C.The activity of Fe0 can be promoted by the addition of Cs and Cu whose presence hinders iron carburisation while favouring both higher conversion and enhanced selectivity.When the samples are aged in the rWGS reaction mixture during stability test a new phase appear:Fe_(5)C_(2),resulting in a more active but less selective catalysts than Fe0 for the rWGS reaction.Hence our results indicate that we could potentially achieve an optimal activity/selective balance upon finely tuning the proportion Fe/Fe_(5)C_(2).Beyond the fundamental information concerning active phase we have observed the presence of advanced Fischer-Tropsch-like products at ambient pressure opening new opportunities for the design of hybrid rWGS/Fischer-Tropsch systems.

Heterostructured Electrocatalysts for Hydrogen Evolution Reaction Under Alkaline Conditions

The hydrogen evolution reaction(HER) is a half-cell reaction in water electrolysis for producing hydrogen gas. In industrial water electrolysis, the HER is often conducted in alkaline media to achieve higher stability of the electrode materials. However, the kinetics of the HER in alkaline medium is slow relative to that in acid because of the low concentration of protons in the former.Under the latter conditions, the entire HER process will require additional effort to obtain protons by water dissociation near or on the catalyst surface. Heterostructured catalysts, with fascinating synergistic effects derived from their heterogeneous interfaces, can provide multiple functional sites for the overall reaction process. At present, the activity of the most active known heterostructured catalysts surpasses(platinum-based heterostructures) or approaches (noble-metal-free heterostructures) that of the commercial Pt/C catalyst under alkaline conditions, demonstrating an infusive potential to break through the bottlenecks. This review summarizes the most representative and recent heterostructured HER catalysts for alkaline medium. The basics and principles of the HER under alkaline conditions are first introduced, followed by a discussion of the latest advances in heterostructured catalysts with/without noblemetal-based heterostructures. Special focus is placed on approaches for enhancing the reaction rate by accelerating the Volmer step. This review aims to provide an overview of the current developments in alkaline HER catalysts, as well as the design principles for the future development of heterostructured nano-or micro-sized electrocatalysts.

Comparison of dry reforming of methane in low temperature hybrid plasma-catalytic corona with thermal catalytic reactor over Ni/γ-Al_2O_3

In the current study, the hybrid effect of a corona discharge and γ-alumina supported Ni catalysts in CO2 reforming of methane is investigated. The study includes both purely catalytic operation in the temperature range of 923-1023 K, and hybrid catalytic-plasma operation of DC corona discharge reactor at room temperature and ambient pressure. The effect of feed flow rate, discharge power and Ni/γ-Al2O3 catalysts are studied. When CH4/CO2 ratio in the feed is 1/2, the syngas of low Ha/CO ratio at about 0.56 is obtained, which is a potential feedstock for synthesis of liquid hydrocarbons. Although Ni catalyst is only active above 573 K, presence of Ni catalysts in the cold corona plasma reactor （T≤523 K） shows promising increase in the conversions of methane and carbon dioxide. When Ni catalysts are used in the plasma reaction, H2/CO ratios in the products are slightly modified, selectivity to CO increases whereas fewer by-products such as hydrocarbons and oxygenates are formed.

Enhanced catalytic performance for direct synthesis of dimethyl ether from syngas over a La_2O_3 modified Cu-ZrO_2/γ-Al_2O_3 hybrid catalyst

A series of hybrid catalysts were made by physically mixing Cu-ZrO2 and γ-A12O3, for former it was modified with different loadings of La2O3 prepared by co-precipitation method. The catalysts were characterized by BET, XRD, N2O-adsorption, EXAFS, H2-TPR, NH3-TPD techniques and evaluated in the synthesis of dimethyl ether from syngas. The results show that La2O3 promoted catalysts displayed a significantly better catalytic performance compared with Cu-ZrO2#y-A12O3 catalyst in CO conversion and DME selectivity, and the optimum catalytic activity was obtained when the content of La2O3 was 12 wt%. The characterizations reveal that high copper dispersion, facile reducibility of copper particles and appropriate amount of acidic sites are responsible for the superior catalytic performance.

Building highly active hybrid double-atom sites in C_(2)N for enhanced electrocatalytic hydrogen peroxide synthesis

Two-electron(2 e^(-))oxygen reduction reaction(ORR)shows great promise for on-site electrochemical synthesis of hydrogen peroxide(H_(2)O_(2)).However,it is still a great challenge to design efficient electrocatalysts for H_(2)O_(2)synthesis.To address this issue,the logical design of the active site by controlling the geometric and electronic structures is urgently desired.Therefore,using density functional theory(DFT)computations,two kinds of hybrid double-atom supported on C_(2)N nanosheet(RuCu@C_(2)N and PdCu@C_(2)N)are screened out and their H_(2)O_(2)performances are predicted.PdCu@C_(2)N exhibits higher activity for H_(2)O_(2)synthesis with a lower overpotential of 0.12 V than RuCu@C_(2)N(0.59 V),Ru_(3)Cu(110)facet(0.60 V),and PdCu(110)facet(0.54 V).In aqueous phase,the adsorbed O_(2)is further stabilized with bulk H_(2)0 and the thermodynamic rate-determining step of 2 e^(-) ORR change.The activation barrier on PdCu@C_(2)N is 0.43 eV lower than the one on RuCu@C_(2)N with 0.68 eV.PdCu@C_(2)N is near the top of 2 e^(-) ORR volcano plot,and exhibits high selectivity of H_(2)O_(2.)This work provides guidelines for designing highly effective hybrid double-atom electrocatalysts(HDACs)for H_(2)O_(2)synthesis.

Dimethyl ether as circular hydrogen carrier:Catalytic aspects of hydrogenation/dehydrogenation steps

The intermittent nature of renewable resources requires for most applications the development of efficient and cost-effective technologies for steady supply of electrical energy.The storage of energy in the form of hydrogen chemically bound within organic molecules(rather than physically as compressed gas or cooled liquid)represents an alternative approach that is attracting great research interest.Compared to other liquid organic hydrogen carriers(LOHCs),dimethyl ether(DME)appears to have the largest potential impact on society,especially if inserted in technological chains of CO_(2) sequestration and utilization,so to determine an effective mitigation of environmental issues,without any net effect on the carbon footprint.Specifically,the steps of H2 storage and H2 release can take place in two coupled chemical processes,constituted by the exothermic synthesis of DME via CO_(2) hydrogenation and the endothermic steam reforming of DME,respectively.Herein,the latest advances in the development of heterogeneous bifunctional and hybrid catalysts for the direct hydrogenation of CO_(2) to DME are thoroughly reviewed,with special emphasis on thermodynamics,catalyst design and process feasibility.Despite many aspects behind the mechanism of DME synthesis from H2-CO_(2) streams are still to be uncovered,the recent progress in the research on H2 release by DME steam reforming is increasing the interest for effectively closing this binary H2 loop,in view of future green deals and sustainable research developments.

Ferric acetylacetonate/covalent organic framework composite for high performance photocatalytic oxidation

Ferric acetylacetonate/covalent organic framework(Fe(acac)_(3)/COF)composite was synthesized by interfacial polymerization method at room temperature.The crystal structure,morphology and porosity property of the composite were characterized by X-ray diffraction,scanning electron microscope,transmission electron microscope and nitrogen adsorption.The interaction between Fe(acac)_(3) and COF was investigated by Fourier transform infrared spectra and X-ray photoelectron spectroscopy.The Fe(acac)_(3)/COF composite was used as a photocatalyst for the oxidation of benzyl alcohol under mild conditions.It exhibits high activity and selectivity for the reaction,of which the mechanism was investigated by determining its photoelectric properties.The Fe(acac)_(3)/COF catalyst developed in this work has application potential in other photocatalytic reactions.

Design and synthesis of hybrid solids based on the tetravanadate core toward improved catalytic properties

Five inorganic-organic hybrid vanadates based on tetravanadate cores, transition metals and N-donor ligands have been designed and synthesized under hydrothermal conditions, namely, [Zn（elM）312V4O12 （1）, [Zn（plM）3]2V4O12.H2O （2）, [Zn（iplM）3]2V4O12 （3）, [Co（eIM）3]2V4O12.H2o （4）, [Cu（eIM）2（H20）]2V4O12 （5） （eIM = 1-ethylimidazole, plM = 1-propylimidazole, ipIM = isopropylimidazole）. All compounds were fully characterized by single-crystal XRD, powder XRD, elemental analysis, TGA, and FT-IR spectroscopy. The hybrid zinc vanadates （1-3） and cobalt vanadate （4） exhibit interesting 2D folded structures and the hybrid copper vanadate （5） presents a 1D chain configuration. All compounds can catalyze olefin epoxidation reactions when using TBHP （TBHP = tert-butyl hydroperoxide） as an oxidant in acetonitrile. The introduction of transition metal ions into tetravanadate cores not only improved the catalytic activity but also fulfilled the heterogeneous catalytic behavior. 1-5 all exhibit extraordinary efficiency in converting olefins to the corresponding epoxides with high conversion and selectivity （particularly, cony. up to 97.1%, sele. up to 100% for 1 ）. Leaching test was also carried out to prove the heterogeneous behavior.

Bi/Bi2O3 nanoparticles supported on N-doped reduced graphene oxide for highly efficient CO2 electroreduction to formate

Electrocatalytic CO2 reduction(CO2 ER)into formate is a desirable route to achieve efficient transformation of CO2 to value-added chemicals,however,it still suffers from limited catalytic activity and poor selectivity.Herein,we develop a hybrid electrocatalyst composed of bismuth and bismuth oxide nanoparticles(NPs)supported on nitrogen-doped reduced graphene oxide(Bi/Bi2 O3/NrGO)nanosheets prepared by a combined hydrothermal with calcination treatment.Thanks to the combination of undercoordinated sites and strong synergistic effect between Bi and Bi2 O3,Bi/Bi2 O3/NrGO-700 hybrid displays a promoted CO2 ER catalytic performance and selectivity for formate production,as featured by a small onset potential of-0.5 V,a high current density of-18 mA/cm2,the maximum Faradaic efficiency of85%at-0.9 V,and a low Tafel slope of 166 mV/dec.Experimental results reveal that the higher CO2 ER performance of Bi/Bi2 O3/NrGO-700 than that of Bi NPs supported on NrGO(Bi/NrGO)can be due to the partial reduction of Bi2 O3 NPs into Bi,which significantly increases undercoordinated active sites on Bi NPs surface,thus boosting its CO2 ER performance.Furthermore,a two-electrode device with Ir/C anode and Bi/Bi2 O3/NrGO-700 cathode could be integrated with two alkaline batteries or a planar solar cell to achieve highly active water splitting and CO2 ER.