Real Active Species Research Articles

Deciphering the Structural and Chemical Transformations of Oxide Catalysts during Oxygen Evolution Reaction Using Quick X-ray Absorption Spectroscopy and Machine Learning.

Bimetallic transition-metal oxides, such as spinel-like CoxFe3-xO4 materials, are known as attractive catalysts for the oxygen evolution reaction (OER) in alkaline electrolytes. Nonetheless, unveiling the real active species and active states in these catalysts remains a challenge. The coexistence of metal ions in different chemical states and in different chemical environments, including disordered X-ray amorphous phases that all evolve under reaction conditions, hinders the application of common operando techniques. Here, we address this issue by relying on operando quick X-ray absorption fine structure spectroscopy, coupled with unsupervised and supervised machine learning methods. We use principal component analysis to understand the subtle changes in the X-ray absorption near-edge structure spectra and develop an artificial neural network to decipher the extended X-ray absorption fine structure spectra. This allows us to separately track the evolution of tetrahedrally and octahedrally coordinated species and to disentangle the chemical changes and several phase transitions taking place in CoxFe3-xO4 catalysts and on their active surface, related to the conversion of disordered oxides into spinel-like structures, transformation of spinels into active oxyhydroxides, and changes in the degree of spinel inversion in the course of the activation treatment and under OER conditions. By correlating the revealed structural changes with the distinct catalytic activity for a series of CoxFe3-xO4 samples, we elucidate the active species and OER mechanism.

Zhang-Rice singlets state formed by two-step oxidation for triggering water oxidation under operando conditions

The production of ecologically compatible fuels by electrochemical water splitting is highly desirable for modern industry. The Zhang-Rice singlet is well known for the superconductivity of high-temperature superconductors cuprate, but is rarely known for an electrochemical catalyst. Herein, we observe two steps of surface reconstruction from initial catalytic inactive Cu1+ in hydrogen treated Cu2O to Cu2+ state and further to catalytic active Zhang-Rice singlet state during the oxygen evolution reaction for water splitting. The hydrogen treated Cu2O catalyst exhibits a superior catalytic activity and stability for water splitting and is an efficient rival of other 3d-transition-metal catalysts. Multiple operando spectroscopies indicate that Zhang-Rice singlet is real active species, since it appears only under oxygen evolution reaction condition. This work provides an insight in developing an electrochemical catalyst from catalytically inactive materials and improves understanding of the mechanism of a Cu-based catalyst for water oxidation.

Unveiling coordination transformation for dynamically enhanced hydrogen evolution catalysis

The in situ conversion of Co-based catalysts to Co(OH)2 with oxyanion-coordination induced by MXene dynamically enhances the HER performance. The non-precious metal catalyst demonstrates catalytic activity and stability that exceed benchmark Pt/C.

Auto-programmed construction of an iron-incorporated cobalt-molybdenum complex towards enhanced electrocatalytic water oxidation

Development of metal complexes which can be used directly as electrocatalysts for oxygen evolution reaction (OER) is arousing great interest although still in its infancy. Here, an amorphous iron-incorporated cobalt-molybdenum-dithiooxamide (Dto) complex, Fe-CoMo(Dto), which can be applied directly for water oxidation, has been developed using an one-step room-temperature coordination polymerization and self-assembly process. In-situ electrochemical activation of Fe-CoMo(Dto) during electrocatalytic operation promotes structural evolution to form electrocatalytically active species for OER. Detailed investigation of the intermediates obtained after different cyclic voltammetry (CV) cycles of OER measurement showed that amorphous spherical Fe-CoMo(Dto) nanoparticles evolve gradually into corresponding metal (oxyhydr)oxides with hexagonal nanoplate shapes, which have been identified to be real active species for enhanced OER activity. Consequently, the so-activated Fe-CoMo(Dto) complex exhibits remarkable electrocatalytic activity for OER, with a low overpotential of 294 mV at a current density of 10 mA cm−2 and a reduced Tafel slope of 66 mV dec−1.

Inorganic Salts and Dehydrating Agents Cooperatively Promoted Ru-Catalyzed Ethylene Methoxycarbonylation Using CO2 as a CO Surrogate

The use of CO2 as a CO surrogate for the carbonylation of olefin has attracted considerable attention due to its abundance, readily availability, nontoxicity, and recyclability. In this work, we describe the synthesis of methyl propionate (MPA), a key intermediate for methyl methacrylate in the commercial Lucite Alpha process, by the ruthenium-catalyzed methoxycarbonylation of ethylene with CO2 as a carbonyl source. An efficient approach to producing MPA has been developed by adding metal halide promoters and dehydrating agents. Control experiments suggest that the NHC-Ru-hydride may be the real active species formed in situ by the reaction of Ru3(CO)12 with ionic liquid (IL). NMR data demonstrate that inorganic salts favor the formation of active species, which is an important issue for their promotion effect. In terms of the strategy to overcome chemical equilibrium by the addition of dehydrating agents and IL participation in the formation of NHC-Ru-hydride active species, a tasked IL containing a siloxyl group was employed to Ru-catalyze the methoxycarbonylation of ethylene, which showed higher catalytic efficiency in comparison to IL without a siloxyl group.

Synthesis of glucopyranoside benzimidazolium-based ionic liquids for Pd-catalyzed aqueous Suzuki reaction

Several novel benzimidazolium-based ionic liquids (BzIm ILs) with free and acetyl protected glucopyranoside unit were synthesized and used as ligand for Pd-catalyzed environment-friendly Suzuki reaction. The BzIm ILs with the acetyl protection shows the highest activity, probably because the sugar has a large steric hindrance and acetyl groups can coordinate with Pd. The nuclear magnetic resonance spectra of Pd(OAc)2/Glc-BzIm ILs free glucopyranoside indicate that Pd-C is formed. The Pd nanoparticles protected by ILs with a size of 5.3 (0.6) nm are considered to be real active species. A series of fluorene core arylfluorene derivatives were prepared with isolated yields above 90%.

In-situ generate robust Fe–Ni derived nano-catalyst featuring surface reconstruction for enhanced oxygen evolution reaction

Oxygen evolution reaction (OER) catalysts with highly efficient and cost-effective are cardinal for hydrogen production through water electrolysis. Herein, a novel strategy based on the theory of molecular crystallization and atomic diffusion is described to construct the FeOOH@Ni3(NO3)2(OH)4/NF. It requires an overpotential of 248 mV at the current density of 100 mA cm−2 for OER. The in-situ Raman spectroscopy test exploring the catalytic actives unravels that NiOOH is one of the real active species and a small amount of NiFe2O4 is generated during OER process. The analysis of the mechanism shows that NiOOH converted from the intermediate product of Ni(OH)2 derived from Ni3(NO3)2(OH)4 in the process of OER. NiOOH and FeOOH mainly work together contributing to boosting intrinsic catalytic activity. This work may provide a new insight into fabricating strategy for other nano-catalysts. The in-situ Raman measurement provides a valid and reliable means to probe into the catalytic active site and catalytic mechanism in the catalytic process.

Swapping catalytic active sites from cationic Ni to anionic F in Fe–F–Ni3S2 enables more efficient alkaline oxygen evolution reaction and urea oxidation reaction

Electrolysis of water for producing hydrogen instead of traditional fossil fuels is one of the most promising methods to alleviate environmental pollution and energy crisis. In this work, Fe and F ion co-doped Ni3S2 nanoarrays grown on Ni foam substrate were prepared by typical hydrothermal and sulfuration processes for the first time. Density functional theory (DFT) calculation demonstrate that the adsorption energy of the material to water is greatly enhanced due to the doping of F and Fe, which is conducive to the formation of intermediate species and the improvement of electrochemical performance of the electrode. The adsorption energy of anions (F and S) and cations (Fe and Ni) to water in each material was also calculated, and the results showed that F ion showed the most optimal adsorption energy of water, which proved that the doping of F and Fe was beneficial to improve the electrochemical performance of the electrode. It is worth noting that the surface of Fe–F–Ni3S2 material will undergo reconstruction during the process of water oxidation reaction and urea oxidation reaction, and amorphous oxides or hydroxides in situ would be formed on the surface of electrode, which are the real active species.

The CO2 cycloaddition to epoxides and aziridines promoted by porphyrin-based catalysts

Thanks to its intrinsic eco–compatibility and high atom economy, the direct insertion of CO2 into three–membered rings, such as epoxides and aziridines, has become one of the most investigated strategies for the synthesis of cyclic carbonates and oxazolidin-2-ones. In this view, the following short account summarizes our recent results regarding the use of ruthenium porphyrin complexes and metal-free porphyrins, in combination with TBACl (tetrabutyl ammonium chloride), to promote the CO2 conversion into cyclic carbonates as well as N–alkyl and N-aryl oxazolidin-2-ones. Experimental data are described in combination with performed DFT studies that allowed proposing a reaction mechanism for the CO2 cycloaddition to aziridines promoted by metal-free porphyrins in which the adduct formed by TPPH2 and TBACl represents the real catalytic active species.

Dynamic anion regulation to construct S-doped FeOOH realizing 1000 mA cm−2‑level‑current‑density oxygen evolution over 1000 h

Iron oxyhydroxide (FeOOH) as the real active species of Fe-based electrocatalysts holds great promise in industrial water electrolysis. However, the poor conductivity and OER kinetics hinder its catalytic performance. Herein, we have developed a facile dynamic anion regulation strategy to enhance OER activity of FeOOH. Of the seven common anions (VO33-, MoO42-, WO42-, S2-, H2PO4-, H2PO2- and F-), S2- was found to have the best regulatory effect. The obtained 0.01 S-FeOOH+1000/IF exhibits industrial-level OER current output of 1000 mA cm−2 with low overpotential of 358 mV. Importantly, this catalyst can operate stably at 1000 mA cm−2 for at least 1000 h. Theoretical calculation reveals that the doping of sulfur can drastically lower the energy barrier of rate-determining step during OER. Meanwhile, 0.01 S-FeOOH+1000/IF in alkaline anion exchange membrane (AEM) cells demonstrates a stable cell voltage of 2.12 V to reach 1000 mA cm−2 over 24 h.

Construction of hierarchical P/Ni-Ag@AgCoPBA hollowstructures for boosting water oxidation activity

Prussian blue analogue (PBA) is considered as one of the promising candidate materials as OER catalysts owing to its surface reconstruction to form M−OH/M−O species, serving as real active species. Herein, a novel P/Ni co-doped hierarchical Ag3[Co(CN)6] hollow microcuboids (P/Ni-Ag@AgCoPBA HHMCs) assembled from two-dimensional nanosheets are constructed for the first time. Benefitting from the unique hierarchical hollowstructures, controllable compositions and the abundant exposed active sites resulting from deep reconstruction, the P/Ni-Ag@AgCoPBA HHMCs show excellent activity with an overpotential (0.245 V) at 10 mA cm−2, and it can work smoothly for 24 h in alkaline electrolyte.

1,2,3-Triazine formation mechanism of the fairy chemical 2-azahypoxanthine in the fairy ring-forming fungus Lepista sordida.

2-Azahypoxanthine (AHX) was first isolated from the culture broth of the fungus Lepista sordida as a fairy ring-inducing compound. It has since been found that a large number of plants and mushrooms produce AHX endogenously and that AHX has beneficial effects on plant growth. The AHX molecule has an unusual, nitrogen-rich 1,2,3-triazine moiety of unknown biosynthetic origin. Here, we establish the biosynthetic pathway for AHX formation in L. sordida. Our results reveal that the key nitrogen sources that are responsible for the 1,2,3-triazine formation are reactive nitrogen species (RNS), which are derived from nitric oxide (NO) produced by NO synthase (NOS). Furthermore, RNS are also involved in the biochemical conversion of 5-aminoimidazole-4-carboxamide-1-β-D-ribofuranosyl 5'-monophosphate (AICAR) to AHX-ribotide (AHXR), suggesting that a novel biosynthetic route that produces AHX exists in the fungus. These findings demonstrate a physiological role for NOS in AHX biosynthesis as well as in biosynthesis of other natural products containing a nitrogen-nitrogen bond.

Understanding the active sites of Fe-N-C materials and their properties in the ORR catalysis system.

Metal–N–C-based catalysts prepared by pyrolysis are frequently used in the oxygen reduction reaction (ORR). Zeolitic imidazolate frameworks (ZIFs), a type of metal organic framework (MOF), are selected as precursors due to their special structure and proper pore sizes. A series of Fe–N–C catalysts with different concentrations of 2-methylimidazole were prepared with a simple solvothermal-pyrolysis method, and the transformation productivity, morphology and ORR activity were investigated. It was found that the Fe–N–C catalyst with a 2-methylimidazole concentration of 0.53 mol L−1 had the best performance. In 0.1 M KOH solution, the half-wave potential was 0.852 V (vs. RHE), with the highest electrochemically active surface area (ECSA) of 94.1 cm2, and the ORR reaction was dominated by a 4-electron process. The current only decreased by 10.5% after 50 000 s of chronoamperometry (CA), while the half-wave potential only decreased 20 mV in 3 M methanol. Additionally, this catalyst cannot be poisoned by Cl− and SO32− ions in the ORR process. Finally, some typical ions including SCN−, Fe(CN)63− and Fe(CN)64− were used to inhibit the active sites, and it was determined that Fe(ii) is the real active species. The series of synthesis and testing experiments has significance in guiding optimization of the synthesis conditions and analysis of the mechanism of active sites in Fe–N–C materials.

Regulating metal active sites of atomically-thin nickel-doped spinel cobalt oxide toward enhanced oxygen electrocatalysis

Two-dimensional (2D) transition metal oxides are promising bifunctional electrocatalysts for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). Herein, atomically-thin nickel-doped spinel cobalt oxide (NCO) was prepared as OER/ORR catalyst through a simple hydrothermal method. Electrochemical test results show that the NCO nanosheets exhibit bifunctional catalytic performances with the overpotential of 275 mV for OER and the onset-potential of 0.92 V for ORR. Moreover, the assembled Zn−air battery based on the material exhibit excellent performance among transition metal oxides based oxygen electrocatalysts. Characterization results demonstrate that the Ni dopants contribute to creating the oxygen vacancies and promoting the electron transfer from nickel to cobalt. More importantly, the ratio of Co2+ and Ni3+ in the octahedral oxygen gap are significantly increased, which serve as the active sites for ORR and OER, respectively. In addition, the 2D nanostructure and Ni dopants help to facilitate the charge transfer during the reaction and the real active species of OER process is more biased towards Ni ions which were proven by in-situ Raman. This work offers a strategy for preparing highly active and durable spinel transition metal oxides materials for bifunctional oxygen catalysts.

Facile fabrication of O vacancy rich CuVOx nanobelt@NiO nanosheet array for hydrogen evolution reaction

An integrated strategy combining nanoengineering and defect engineering is put forward to design efficient electrocatalyst for hydrogen evolution reaction (HER), by which an oxygen (O) vacancy rich CuVOx nanobelt@NiO nanosheet composite is obtained via a facile hydrothermal process combined with a calcination step. During the HER, CuVOx is reduced to VOx-Cu2O/Cu as the real active species with increased O vacancies. The lamellar NiO shell provides a large number of active sites as well as super-hydrophilic surface, which effectively facilitate the H2O adsorption and decomposition to Hads intermediates. It is found that the intrinsic reaction activity for the conversion of Hads to H2 could be improved by the VOx-Cu2O core with rich O vacancy defects. The obtained CuVOx@NiO composite electrode exhibits remarkable HER activity in alkaline solution with an overpotential as low as 74 mV at 10 mAcm−2, which also shows excellent stability and a fast kinetic reaction with a Tafel slope of 60.8 mVdec−1. This pioneer work provides a novel catalyst structure and superb properties for the HER in the alkaline electrolyzer.

In-situ probing the rapid reconstruction of FeOOH-decorated NiMoO4 nanowires with boosted oxygen evolution activity

Understanding the dynamic surface reconstruction is of vital importance for the development of oxygen evolution reaction (OER) electrocatalysts. Herein, we report FeOOH-decorated NiMoO4 nanowires as OER pre-catalysts. In-situ Raman spectroscopy elucidates that modified NiMoO4 undergoes fast reconstruction, including significant Mo leaching and simultaneous formation of Ni(Fe)OOH intermediates, which are determined as the real active species toward the OER. Rapid and deep reconstruction results in amorphous nanowires with uniform Fe distribution and great size expansion. Impressively, the reconstructed pre-catalyst exhibits outstanding catalytic activity, ultra-low overpotential, and extraordinary long-term stability under high current densities. Our work provides a facile and effective surface engineering approach of pre-catalysts and highlights the evolution of self-reconstruction chemistry for accelerating the kinetics of the OER.

Operando capturing of surface self-reconstruction of Ni3S2/FeNi2S4 hybrid nanosheet array for overall water splitting

Earth-abundant catalysts for both Oxygen evolution reaction (OER) and Hydrogen evolution reaction (HER) are highly required for electrochemical water splitting. Transition metal sulfides have been considered as efficient catalysts for both OER and HER, but their real active species in operation condition are still controversial. Herein, self-supported Ni3S2/FeNi2S4 nanosheets (Ni-Fe-S) are synthesized and in-situ Raman measurement is conducted to monitor the phase transformation under realistic conditions. Combined with a series of ex-situ technologies, it can be concluded that, Ni-Fe-S nanosheets function as pre-catalysts which undergo dynamically structural reconstruction during electrolysis. The active species are determined to be γ-NiOOH for OER and metallic Ni0 for HER in 1 M KOH media, respectively. As-converted products only need overpotentials of 50 mV for HER and 201 mV for OER to achieve the current density of 10 mA cm-2, respectively. A two-electrode electrolyzer can afford a current density of 10 mA cm-2 at a low cell voltage of 1.55 V. This work provides new perspectives to recognize the catalytic evolution process of nickel-based sulfides for water splitting and sheds light on the rational design of bifunctional materials in energy devices.

In‐Situ Activated NiFePBA‐FeOOH Electrocatalyst for Oxygen Evolution Reaction and Zinc‐Air Battery

AbstractDeveloping low‐price, highly efficient and durable electrocatalysts for sluggish oxygen evolution reaction (OER) is still a significant challenge for metal‐air batteries and electrochemical water splitting technology. Here, cost‐effective OER catalyst composing of nickel‐iron Prussian blue analogue and iron oxyhydroxide (NiFePBA‐FeOOH) was successfully prepared by facile hydrothermal method. Designed NiFePBA‐FeOOH catalyst exhibits superior OER performance with low overpotential, small Tafel slope and stable durability. Systematic investigations with in‐situ Raman and Transmission electron microscope reveal that the as‐prepared catalyst has undergone in‐situ electrochemical transformation to generate real catalytic active species of nickel and iron oxyhydroxides. Particularly, the assembled zinc‐air battery with NiFePBA‐FeOOH catalyst also displays a long‐term lifetime of 165 h only presenting a slight increase in voltage gap. Additionally, the successful exploration of cobalt‐iron PBAs with iron oxyhydroxides further demonstrates the universality of PBAs‐oxyhydroxides composites for energy storage and conversion applications.

From the grafting of NHC-based Pd(II) complexes onto TiO2 to the in situ generation of Mott-Schottky heterojunctions: The boosting effect in the Suzuki-Miyaura reaction. Do the evolved Pd NPs act as reservoirs?

The assumption that the real active species involved in the Suzuki-Miyaura reaction are homogeneous, heterogeneous or both is often proposed. However a lack of characterization of the true catalytic entities and their monitoring makes assumptions somewhat elusive. Here, with the aim of getting new insights into the formation of active species in the Suzuki-Miyaura reaction, a family of palladium(II) complexes bearing bis(NHC) ligands was synthesized for immobilization at the surface of TiO2. The studies reveal that once the complexes are anchored onto TiO2, the mechanism governing the catalytic reaction is different from that observed for the non-anchored complexes. All complexes evolved to Pd NPs at the surface of TiO2 under reaction conditions and released Pd species in the liquid phase. Also, this reactivity was boosted by the in situ generation of Mott-Schottky heterojunctions, opening new routes towards the design of heterogenized catalysts for their further implementation in reverse-flow reactors.

Self-template synthesis of hollow Fe-doped CoP prisms with enhanced oxygen evolution reaction activity

The development of efficient, durable and low cost electrocatalysts is crucial but extremely challenging for the oxygen evolution reaction (OER). Herein, we develop a self-template strategy to synthesize hollow Fe-doped CoP prisms (Fe-CoP) via ion exchange of cobalt acetate hydroxide with [Fe(CN)6]3- and phosphorization-induced transformation of CoFe-PBA (Co/Fe-containing prussian blue analogue) prisms in N2 atmosphere. The obtained Fe-CoP not only inherits the hollow prism-like morphology of CoFe-PBA, but also forms rich mesoporous channel. The Fe-CoP prisms exhibit extraordinary OER performances in 1.0 M KOH, with a low overpotential of 236 mV to deliver a current density of 10 mA cm−2 and a low Tafel slope of 32.9 mV dec–1. Moreover, the presented electrocatalyst shows good long-term operating durability and activity. The XPS and TEM analysis confirm that Fe-CoP has undergone surface reconstruction in the process of electrocatalytic OER, and the in situ formed oxides and oxyhydroxides are the real active species to boost OER. This work provides a promising pathway to the design and synthesis of efficient and robust electrocatalysts with hierarchical hollow structure for boosting OER.