Transition Metal Oxide Catalysts Research Articles

Conversion of 5‐hydroxymethylfurfural and carbohydrates to 2,5‐diformylfuran over supported Mo‐based catalysts

Here, a series of nitrogen‐doped carbon (NC) supported transition metal oxide catalysts were prepared by pyrolysis of chitosan‐metal salt mixtures for the efficient synthesis 2,5‐diformylfuran (DFF) from 5‐hydroxymethylfurfural (HMF) and fructose. Among them, supported molybdenum‐based species on nitrogen‐doped carbon (Mo/NC) exhibited excellent activity and selectivity for selective oxidation of HMF to DFF. The presence of nitrogen species reduces the electron cloud density around the Mo species, thereby improving the catalytic activity. Moreover, the interaction between active MoO2 species on the catalyst surface and NC ensured the stability of the catalyst, resulting in no significant loss of activity after four catalytic cycles. 99.5% and 55.1% yields of DFF with full conversion were obtained from HMF and fructose respectively over Mo/NC‐chitosan‐600 in DMSO under ambient air. In order to enhance DFF yield from fructose, sulfonate groups were introduced into Mo/NC‐chitosan‐600 catalyst, leading to the highest DFF yield of 70.2%. In addition, this catalyst also allowed 10.0%, 22.9% and 33.3% DFF yields from glucose, sucrose and inulin, respectively.

Tailoring oxygen vacancies by synergy of dual metal cations in LaCo0.8M0.2O3 (M = Cu, Ni, Fe, Mn) towards catalytic oxidation of toluene

Enhancing the catalytic performance of transition metal oxide catalysts is prerequisite for the efficient catalytic oxidation of VOCs. Here the B-site partially substituted LaCoO3 (LaCo0.8M0.2O3, M = Cu, Ni, Fe and Mn) catalysts was synthesized for the catalytic oxidation of toluene. Among these catalysts, LaCo0.8Ni0.2O3 catalyst exhibits remarkable catalytic performance for toluene with T50 = 226 °C and T90 = 239 °C at an initial toluene concentration of 1000 ppm, whose T50 (and T90) reduced by 13 °C (and 34 °C) compared with that of LaCoO3. Confirmed by BET, XPS, and EPR characterizations, Ni substitution improves the specific surface area, Co3+/Co2+ ratio, Oads/Olatt ratio, and oxygen vacancies concentration. This improvement indicates the improvement of adsorption and activation of reactant, mobility of oxygen species, thereby contributing to the superior catalytic performance in toluene oxidation. The B-site of LaCoO3 occupied by dual cations caused the lattice distortion and formation of oxygen vacancies due to the ionic radii difference between Co and Ni cations. Moreover, the apparent activation energy of LaCo0.8Ni0.2O3, as the determining factor for the catalytic oxidation rate, was remarkably reduced. Our findings confirmed the synergy effect of Co and Ni cations at B-site on the enhancement of catalytic activity, which provides a promising strategy for oxygen vacancy engineering.

Electrocatalytic Decomposition of Lithium Oxalate-Based Composite Microspheres as a Prelithiation Additive in Lithium-Ion Batteries.

In conventional lithium-ion batteries (LIBs), the active lithium from the lithium-containing cathode is consumed by the formation of a solid electrolyte interface (SEI) at the anode during the first charge, resulting in irreversible capacity loss. Prelithiation additives can provide additional active lithium to effectively compensate for lithium loss. Lithium oxalate is regarded as a promising ideal cathode prelithiation agent; however, the electrochemical decomposition of lithium oxalate is challenging. In this work, a hollow and porous composite microsphere was prepared using a mixture of lithium oxalate, Ketjen Black and transition metal oxide catalyst, and the formulation was optimized. Owing to the compositional and structural merits, the decomposition voltage of lithium oxalate in the microsphere was reduced to 3.93 V; when being used as an additive, there is no noticeable side effect on the performance of the cathode material. With 4.2% of such an additive, the first discharge capacity of the LiFePO4‖graphite full cell increases from 139.1 to 151.9 mAh g-1, and the coulombic efficiency increases from 88.1% to 96.3%; it also facilitates the formation of a superior SEI, leading to enhanced cycling stability. This work provides an optimized formula for developing an efficient prelithiation agent for LIBs.

Thermocatalytic oxidation potential of cobalt(II,III) oxide against gaseous formaldehyde in relation to hydrothermal synthesis temperature

The catalytic activity of various transition metal oxides has been reported to be tuned by controlling their synthesis conditions. In this perspective, the thermocatalytic oxidation performance of cobalt(II,III) oxides (Co3O4) against formaldehyde (FA) is studied in relation to the temperature selected for their hydrothermal synthesis (e.g., 30 (as room temperature (RT)), 60, 120, and 250℃) with the corresponding sample codes as Co3O4-RT, Co3O4-60, Co3O4-120, and Co3O4-250, respectively. The best performing Co3O4-RT records the lowest T90 (temperature required to obtain 90 % conversion) for 50 ppm FA at 72 °C when operated at 0 % relative humidity (RH), 30 mg (catalyst mass: mcat), and 100,000 mL g-1h−1 (weight hourly space velocity). The performance of Co3O4-RT, if assessed in terms of reaction kinetic rate (r) at XFA = 10 %, is 2.02E-02 mmol g-1h−1. According to X-ray photoelectron spectroscopy and temperature programmed characterization analysis, the catalytic activities of Co3O4-RT improve greatly through the synergistic combination of diverse factors (e.g., high Co3+ content, abundant oxygen vacancies (OVs) and surface adsorbed oxygen (Oβ) species, and good surface lattice oxygen (Oα) mobility). The analysis of in-situ diffuse reflectance infrared Fourier transform spectroscopy indicates that the FA should be oxidized into water (H2O) and carbon dioxide (CO2) via dioxymethylene (DOM) and formate (HCOO–) as reaction intermediates. The density functional theory simulations suggest the (111) surface of Co3O4 to be catalytically active against FA. The output of this research is expected to help establish effective strategies for designing high-performance transition metal oxide catalysts for the catalytic oxidation of FA.

Bifunctional Mn‐Fe Oxide Catalysts for Zn‐Air Battery Air Electrodes Fabricated Through Atomic Layer Deposition

Zinc‐air batteries (ZABs) are one of many energy storage technologies that can help integrate renewable energy into the power grid. A key developmental goal for ZABs is replacing the precious metal catalysts at the air electrode with more abundant and inexpensive materials. In this work, a MnFexOy bifunctional catalyst is directly deposited on a ZAB air electrode using atomic layer deposition (ALD). With ALD, the atomic composition of the air electrode coating can be tuned based on catalytic activity. Characterization through electron microscopy, photoelectron spectroscopy and diffraction techniques indicate that the novel ALD film deposits as a nanocrystalline (Mn,Fe)3O4 cubic spinel. The mixed oxide catalyst outperforms its individual binary MnOx or FeOx constituents, operating at 50.7% bifunctional efficiency at 20 mA cm‐2. Moreover, the long term stability of the ALD catalyst is showcased by 600 h (1565 cycles) of ZAB cycling at 10 mA cm‐2. The efficiency retention of the bifunctional transition metal oxide catalyst is superior to a precious metal benchmark of Pt‐Ru‐C, with 84.7% efficiency retention after more than 1500 cycles versus only 66.2% retention for the precious metal catalyst. The ALD technique enables deep penetration of catalyst material into the air electrode structure, improving the cycling behaviour.

Operando X-ray Absorption Spectroscopy as a Powerful Tool for Uncovering Property-Activity Relationships for Oxygen Evolution Transition Metal Oxide Catalysts.

The development of a sustainable and environmentally friendly energy economy encompasses efficient hydrogen production from renewable energy via electrolysis. In this context, great efforts have recently been dedicated to the development of more efficient and cost-effective electrocatalysts. Understanding the mechanism of the oxygen evolution reaction (OER) on transition metal oxide catalysts is of great interest, but the reaction and system complexity render the characterization of active sites and the understanding of reaction mechanisms challenging. Time resolved Quick X-ray Absorption Spectroscopy (XAS) can provide dynamic snapshots of the electronic and local structure of nanocatalysts, revealing the 'real active phase' of the catalyst, which can substantially differ from the as-prepared catalyst powder or the catalyst in form of an electrode under non-operating conditions. In this contribution, several examples will be presented showing how operando XAS can reveal catalyst-support interactions, changes in the reaction mechanism, and dynamic reversible/irreversible changes in the electronic and local structure of OER catalysts.

Enhancement of Electrochemical Nitrogen Reduction Activity and Suppression of Hydrogen Evolution Reaction for Transition Metal Oxide Catalysts: The Role of Proton Intercalation and Heteroatom Doping

Enhancement of Electrochemical Nitrogen Reduction Activity and Suppression of Hydrogen Evolution Reaction for Transition Metal Oxide Catalysts: The Role of Proton Intercalation and Heteroatom Doping

Multivariate Bayesian Optimization of CoO Nanoparticles for CO2 Hydrogenation Catalysis.

The hydrogenation of CO2 holds promise for transforming the production of renewable fuels and chemicals. However, the challenge lies in developing robust and selective catalysts for this process. Transition metal oxide catalysts, particularly cobalt oxide, have shown potential for CO2 hydrogenation, with performance heavily reliant on crystal phase and morphology. Achieving precise control over these catalyst attributes through colloidal nanoparticle synthesis could pave the way for catalyst and process advancement. Yet, navigating the complexities of colloidal nanoparticle syntheses, governed by numerous input variables, poses a significant challenge in systematically controlling resultant catalyst features. We present a multivariate Bayesian optimization, coupled with a data-driven classifier, to map the synthetic design space for colloidal CoO nanoparticles and simultaneously optimize them for multiple catalytically relevant features within a target crystalline phase. The optimized experimental conditions yielded small, phase-pure rock salt CoO nanoparticles of uniform size and shape. These optimized nanoparticles were then supported on SiO2 and assessed for thermocatalytic CO2 hydrogenation against larger, polydisperse CoO nanoparticles on SiO2 and a conventionally prepared catalyst. The optimized CoO/SiO2 catalyst consistently exhibited higher activity and CH4 selectivity (ca. 98%) across various pretreatment reduction temperatures as compared to the other catalysts. This remarkable performance was attributed to particle stability and consistent H* surface coverage, even after undergoing the highest temperature reduction, achieving a more stable catalytic species that resists sintering and carbon occlusion.

Highly Dispersed Mn-Doped Ceria Supported on N-Doped Carbon Nanotubes for Enhanced Oxygen Reduction Reaction.

The weak adsorption of oxygen on transition metal oxide catalysts limits the improvement of their electrocatalytic oxygen reduction reaction (ORR) performance. Herein, a dopamine-assisted method is developed to prepare Mn-doped ceria supported on nitrogen-doped carbon nanotubes (Mn-Ce-NCNTs). The morphology, dispersion of Mn-doped ceria, composition, and oxygen vacancies of the as-prepared catalysts were analyzed using various technologies. The results show that Mn-doped ceria was formed and highly dispersed on NCNTs, on which oxygen vacancies are abundant. The as-prepared Mn-Ce-NCNTs exhibit a high ORR performance, on which the average electron transfer number is 3.86 and the current density is 24.4% higher than that of commercial 20 wt % Pt/C. The peak power density of Mn-Ce-NCNTs is 68.1 mW cm-2 at the current density of 138.9 mA cm-2 for a Zn-air battery, which is close to that of 20 wt % Pt/C (69.4 mW cm-2 at 106.1 mA cm-2). Density functional theory (DFT) calculations show that the oxygen vacancy formation energies of Mn-doped CeO2(111) and pure CeO2(111) are -0.55 and 2.14 eV, respectively. Meanwhile, compared with undoped CeO2(111) (-0.02 eV), Mn-doped CeO2(111) easily adsorbs oxygen with the oxygen adsorption energy of only -0.68 eV. This work provides insights into the synergetic effect of Mn-doped ceria for facilitating oxygen adsorption and enhancing ORR performance.

MOF-derived CuO@CB@CF cathode for O2 activation to generate ROSs for efficient electro-Fenton-catalysis

The generation of reactive radicals by molecular oxygen activation has become an important advanced oxidation technique. The combination of highly active carbon materials and stable transition metal oxides can activate molecular oxygen to produce hydrogen peroxide and further produce active free radicals at the same time, which is an efficient way for the removal of organic pollutants by heterogeneous electro-Fenton (HEF) system. In this study, a novel metal organic framework (MOF) derived CuO-carbon black-carbon felt (CuO@CB@CF) composite catalytic electrode has been prepared, which can generate H2O2 efficiently by selective O2 reduction through two-electron pathway with high pollutant removal efficiency. By optimizing the calcination temperature of MOF and loading capacity, the MOF-derived CuO@CB@CF has demonstrated excellent performance as a HEF catalytic electrode. Meanwhile, the effects of applied voltage, pH value, electrode area and aeration conditions have been studied. The excellent performance of MOF-derived CuO@CB@CF provides new insights into the removal of organic pollutants through electro-Fenton processes of transition metal oxide and carbon-based catalysts.

Transition Metal Oxide (TiO2) Catalyst Promoted by the Synthesis of Imidazoles

Transition Metal Oxide (TiO2) Catalyst Promoted by the Synthesis of Imidazoles

Manipulation of Electronic Effect and Assembly Architecture to Invoke Oxidation of Ethylbenzene by Hierarchical Co3O4 Wreaths.

A high-performance transition-metal oxide catalyst can be designed by appropriately integrating the concepts of morphology regulation and electronic structure modulation. In this work, hierarchical Co3O4 wreaths (CCW) enriched with oxygen vacancies (Ov) were facilely constructed for the selective oxidation of ethylbenzene (EB) to acetophenone (AP). Under the screened optimal reaction conditions, the CCW catalyst can offer a 79.1% conversion of EB (ri = 0.244 mol gcat-1 h-1) accompanied by a selectivity of 92.3% to AP. The good reaction performance can be attributed to the cooperation of defect engineering and architecture design, which can synergistically facilitate the EB oxidation performance by augmenting the intrinsic reactivity and accessibility of active sites. This work presents a reliable route to construct a high-performance transitional metal oxide catalyst via manipulation of electronic effect and assembly architecture for the selective oxidation of EB and beyond.

Co3O4-Based Materials as Potential Catalysts for Methane Detection in Catalytic Gas Sensors.

The present work deals with the development of Co3O4-based catalysts for potential application in catalytic gas sensors for methane (CH4) detection. Among the transition-metal oxide catalysts, Co3O4 exhibits the highest activity in catalytic combustion. Doping Co3O4 with another metal can further improve its catalytic performance. Despite their promising properties, Co3O4 materials have rarely been tested for use in catalytic gas sensors. In our study, the influence of catalyst morphology and Ni doping on the catalytic activity and thermal stability of Co3O4-based catalysts was analyzed by differential calorimetry by measuring the thermal response to 1% CH4. The morphology of two Co3O4 catalysts and two NixCo3-xO4 with a Ni:Co molar ratio of 1:2 and 1:5 was studied using scanning transmission electron microscopy and energy dispersive X-ray analysis. The catalysts were synthesized by (co)precipitation with KOH solution. The investigations showed that Ni doping can improve the catalytic activity of Co3O4 catalysts. The thermal response of Ni-doped catalysts was increased by more than 20% at 400 °C and 450 °C compared to one of the studied Co3O4 oxides. However, the thermal response of the other Co3O4 was even higher than that of NixCo3-xO4 catalysts (8% at 400 °C). Furthermore, the modification of Co3O4 with Ni simultaneously brings stability problems at higher operating temperatures (≥400 °C) due to the observed inhomogeneous Ni distribution in the structure of NixCo3-xO4. In particular, the NixCo3-xO4 with high Ni content (Ni:Co ratio 1:2) showed apparent NiO separation and thus a strong decrease in thermal response of 8% after 24 h of heat treatment at 400 °C. The reaction of the Co3O4 catalysts remained quite stable. Therefore, controlling the structure and morphology of Co3O4 achieved more promising results, demonstrating its applicability as a catalyst for gas sensing.

Design and optimization of CuCo-LDO catalysts via ZIF-67 sacrificial templates for enhanced toluene oxidation: A comprehensive study on morphology, structure, and catalytic activity

The investigation into the development of an active transition-metal oxide catalyst with controllable morphology for the degradation of volatile organic compounds (VOCs) is a subject of considerable importance and warrants further exploration. In this study, ZIF-67 was prepared and employed as a sacrificial template, with varying quantities of Cu ions incorporated to synthesize layered copper-cobalt double oxide (CuCo-LDO) featuring a modifiable surface state. A comprehensive characterization of the CuCo-LDO catalysts was conducted, encompassing structural properties, morphology, surface chemical state, and redox properties, utilizing a range of analytical techniques. Subsequent evaluations included testing the catalytic activity for toluene oxidation and assessing stability performance. The results revealed that differing Cu contents played a pivotal role in inducing changes in the morphology and structure of CuCo-LDO, thereby significantly influencing its oxidation activity towards toluene. Notably, the flower-like structure of 3CuCo-LDO (Cu% = 20.14 wt.%) exhibited outstanding toluene oxidation activity attributable to its unique structure and composition. This research contributes novel insights to the design of highs-performance catalysts targeting volatile organic compounds.

H2 production based on a ternary mixture of commercial CuO-NiO-TiO2 in a solar pilot plant

Glycerol is a by-product in biodiesel production (in the range of g·L−1), so its photoreforming by photocatalysis is a way of valorising it. TiO2 in photocatalysis has been widely studied, although its efficiency is limited by the high energy band gap, and the electron-hole recombination. Its combination with different semiconductors should improve charge separation, extending also the absorption from UV to visible light. Cu and Ni oxides are two of the most efficient low-cost transition metal oxide catalysts. Experiments were carried out in a 25 L pilot plant connected to a compound parabolic solar collector. Different combinations of the three semiconductors, based on the concentration of each metal on TiO2 (Me, 5%, 7.2% and 10%) were evaluated. Evonik P25-TiO2, CuO and NiO were combined by mechanical mixing. Hydrogen was quantified by a micro gas chromatograph, and copper and nickel leaching by ICP-MS. The best hydrogen production (0.060 mMol kJ−1) was attained with a proportion of 10:1 of TiO2:MeO, that corresponds to a total metal concentration of 7.2 wt%, being Cu and Ni in the same proportion. Metal content in solution increased as the reaction progressed, but Ni lixiviation of <0.012 mg L−1 was not significant. Significant Cu leaching (>1 mg L−1) was observed. This article presents novel results, in a solar pilot plant, for determining which ternary mixture can give better results, as well as metal leaching into water. Handling relevant volume of water in anoxic conditions can help to understand the application of this technology for the production of hydrogen.

Low-Temperature controlled synthesis of nanocast mixed metal oxide spinels for enhanced OER activity

The controlled cation substitution is an effective strategy for optimizing the density of states and enhancing the electrocatalytic activity of transition metal oxide catalysts for water splitting. However, achieving tailored mesoporosity while maintaining elemental homogeneity and phase purity remains a significant challenge, especially when aiming for complex multi-metal oxides. In this study, we utilized a one-step impregnation nanocasting method for synthesizing mesoporous Mn-, Fe-, and Ni-substituted cobalt spinel oxide (Mn0.1Fe0.1Ni0.3Co2.5O4, MFNCO) and demonstrate the benefits of low-temperature calcination within a semi-sealed container at 150–200 °C. The comprehensive discussion of calcination temperature effects on porosity, particle size, surface chemistry and catalytic performance for the alkaline oxygen evolution reaction (OER) highlights the importance of humidity, which was modulated by a pre-drying step. The catalyst calcined at 170 °C exhibited the lowest overpotential (335 mV at 10 mA cm−2), highest current density (433 mA cm−2 at 1.7 V vs. RHE, reversible hydrogen electrode) and further displayed excellent stability over 22 h (at 10 mA cm−2). Furthermore, we successfully adapted this method to utilize cheap, commercially available silica gel as a hard template, yielding comparable OER performance. Our results represent a significant progress in the cost-efficient large-scale preparation of complex multi-metal oxides for catalytic applications.

Catalyst development for O2-assisted oxidative dehydrogenation of propane to propylene.

O2-Assisted oxidative dehydrogenation of propane (O2-ODHP) could convert abundant shale gas into propylene as an important chemical raw material, meaning O2-ODHP has practical significance. Thermodynamically, high temperature is beneficial for O2-ODHP; however, high reaction temperature always causes the overoxidation of propylene, leading to a decline in its selectivity. In this regard, it is crucial to achieve low temperatures while maintaining high efficiency and selectivity during O2-ODHP. The use of catalytic technology provides more opportunities for achieving high-efficiency O2-ODHP under mild conditions. Up to now, many kinds of catalytic systems have been elaborately designed, including transition metal oxide catalysts (such as vanadium-based catalysts, molybdenum-based catalysts, etc.), transition metal-based catalysts (such as Pt nanoclusters), rare earth metal oxide catalysts (especially CeO2 related catalysts), and non-metallic catalysts (BN, other B-containing catalysts, and C-based catalysts). In this review, we have summarized the development progress of mainstream catalysts in O2-ODHP, aiming at providing a clear picture to the catalysis community and advancing this research field further.

Unraveling the Crystal Facet and Support Effects on the Oxygen Evolution Activity of Cobalt Oxide Using Single Nanoparticle Electrocatalysis

Transition metal oxide catalysts have recently gained special interest in alkaline water electrolysis due to their relatively high abundancy and high catalytic activity. However, their catalytic performance has typically been evaluated based on measurements at composite electrodes, that is by immobilizing a large number of these nanoparticles together with inactive binders and additives on a current collector/support and measuring only their integral activity. In such studies, it is challenging to deconvolute the catalyst intrinsic properties from matrix effects. For example, the measured catalytic trends can be influenced by blocking of catalyst active sites by the additives or by non-optimal iR compensation or by different porosity within the composite, impeding a direct comparison of nanocatalysts.[1] Alternatively, single-particle electrocatalysis by the “nano impact” approach has been applied to assess the intrinsic activity of nanoparticles (NPs) at the individual particle level.[2] Exploiting this method, we demonstrate how the catalyst’s crystal facet/shape influences the OER activity by comparing the activity of cubes and spheres of Co3O4.[3] In contrast to the tentative differences in OER activity recorded on ensembles, the single particle studies unambiguously reveal that Co3O4 cubes are more active than spheres when both NPs have otherwise identical parameters. We also can link our experimental data to theory data obtained from DTU+U calculations. Furthermore, we investigate catalyst-support effects and show the higher activity of Pt support than carbon.[4]. This approach enables the identification of highly active facets to guide shape-selective syntheses of improved metal oxide nanocatalysts for water oxidation.

Operando Surface X-Ray Diffraction Studies of Co Oxide Catalyst Films for Electrochemical Water Splitting

The need for environmental friendly and sustainable energy conversion has triggered renewed interest into the electrochemical and photoelectrochemical splitting of water. A key challenge in this field is the development of economically viable electrocatalyst materials for the oxygen evolution reaction (OER). Transition-metal oxide catalysts, in particular the cobalt (hydr)oxide materials, have been found as promising candidates, since they are earth-abundant, efficient and scalable over a wide range of electrochemical conditions [1]. They present good catalytic properties and stability in alkaline solution under ambient conditions. Although a wide variety of different catalysts with rather diverse nanoscale morphologies have been synthesized and studied, the atomic scale surface structure usually is still unknown or poorly defined. This makes it difficult to compare their electrocatalytic activity and correlate it with ab initio theoretical studies, which hinders the development of clear structure-reactivity relationships and unambiguous determination of the OER reaction mechanism.We presentsystematic comparative studiesof structurally well-defined, 18-35nm thick Co3O4 films, prepared by electrodeposition or physical vapor deposition (PVD) on Au(111), Au(100) and Ir(100) single crystals. In all cases a well ordered epitaxial Co3O4(111) arrangement results, but the film morphology differs substantially, as shown by AFM and surface X-ray diffraction (SXRD). In order to elucidate the quantitative relationship between the catalyst surface structure and the OER reactivity, synchrotron-based operando SXRD and electrochemical measurements were performed simultaneously under reaction conditions, using a dedicated operando cell which allows massive gas evolution and current densities of up to 150 mA/cm2 [2,3].We find that the Co3O4(111) layers prepared by PVD on Ir(100) stay perfectly stable in the pre-OER and OER potential regime, whereas all electrodeposited films exhibit reversible changes in the oxide structure and oxidation state, in agreement with a previous study of Co3O4 catalysts [2]. Specifically, we observe the reverse formation of an ultrathin X-ray-amorphous CoOx(OH)y skin layer above 1 V vs. RHE and the gradual buildup of tensile lattice strain in the underneath, remaining Co3O4 layer with increasing potential. These structural changes occur for all electrodeposited samples, albeit to a different extent, depending on the film morphology [3]. Studying the relationship between the effective thickness of this skin layer and the OER reactivity, we found clear evidence that the entire skin layer is a three dimensional OER zone, which strongly exceeds one monolayer.Our results suggest that even subtle changes of the surface morphology have large influence on the OER activity of the Co3O4 catalyst, which may be expected also for other spinel-type transition metal oxides. On the one hand, this reveals that great care as to be taken in comparing the OER activity of differently prepared catalysts. On the other hand, these observations may provide a base for targeted preparation of highly reactive oxide catalysts.[1] C. C. L. McCrory et al., Journal of the American Chemical Society 2015, 137, 4347.[2] F. Reikowski, et al., ACS Catalysis, 2019, 9, 3811.[3] T. Wiegmann, et al., ACS Catalysis, 2022, 12, 256.

Selectivity of Control of Oxygen Reduction Reaction over Mesoporous Transition Metal Oxides Catalysts for Electrified Purification Technologies

Introduction: The use of abundant feedstocks, such as water and air, in manufacturing of value-added products is important in achieving the UN’s Sustainable Development Goals.1 In this context, electrified direct synthesis of chemicals from oxygen in the air is of special interest since it avoids greenhouse gas emissions if the electricity used to drive the synthesis comes from sustainable sources such as solar- and wind-power. The controlling of oxygen reduction path over catalysts incorporation with oxygen evolution reaction as auxiliary reaction enables the electrosynthesis of value-added chemicals, oxygen and hydroxyl radical. The development of cheap materials e.g., mesoporous transition metal oxides, is urged to replace the expensive and scarce of platinum group metal (PGM)-based catalysts, with the ability to provide competitive reaction rate in alkaline media.2 Experimental: A template-free hydrothermal method is developed for the synthesis of mesoporous NiO and NiCo2O4. The materials’ physicochemical properties are characterized by X-ray diffraction, physisorption, electron microscopy, selected electron area diffraction, and X-ray photoelectron spectroscopy. The electrochemical properties and electrocatalytic oxygen activity are evaluated via cyclic voltammetry and electrochemical impedance spectra in alkaline electrolyte using a standard three electrodes electrochemical working station equipped with (ring) rotating disk electrode (RDE/RRDE) techniques.The catalysts coated on carbon fiber paper (CFP) are used as electrode assembled with anion exchange membrane in two-electrodes-electrochemical cell for device test. NiO- and NiCo2O4-modified CFP are used as cathode and anode, respectively, for constructing electrochemical generator of hydroxyl radical. 50 mL min-1 O2 flow was maintained in catholyte to enable oxygen saturation. 20 mg L-1 rhodamine B (RhB)was fed into cathode. Off-line UV-vis measurements were used for RhB quantification. In Electrochemical oxygen purifier, NiCo2O4-modified CFP are used as electrodes. 0.1 - 3 M KOH aqueous solutions were used as anolyte and air-saturated catholyte. The volume of produced oxygen on the anode after 2 h was collected and measured by a water displacement method. Results and Discussion: A facile template-free hydrothermal synthesis is developed for bifunctional oxygen electrocatalysts (OER and ORR) of NiO and NiCo2O4. Physicochemical characterization shows that both NiO and NiCo2O4 are mesoporous with the pore size within a range of 2 - 8 nm and specific surface areas of 60 and 120 m2 g-1, respectively. In addition, NiO and NiCo2O4 possess cubic crystal structures with the corresponding crystal sizes of ~6.0 and 9.3 nm.Electrochemical analysis shows that mesoporous NiCo2O4 has about two times larger capacitance and eight times smaller charge transfer resistance compared to NiO. NiCo2O4 shows higher electrocatalytic activity in OER and selectivity to water as the terminal product of ORR. On the contrary, ORR over NiO yielded hydroxyl radicals, as a product of a Fenton-like reaction of H2O2. The product selectivity in ORR is used to construct two electrolyzers of electrified purification of oxygen and generation of hydroxyl radicals. The optimization of configuration of hydroxyl radical generator enables the degradation of RhB up to 90% after 1 hour, and the electrochemical oxygen purifier shows high efficiency of about 100%. Significance: It is demonstrated that the synthesized mesoporous transition metal oxides via a simple template-free hydrothermal route, and the materials can be used as alternatives to PGM-based electrocatalysts for driving electrochemical cell towards to various products. The work also clearly shows how the composition and structure of catalysts affect the oxygen reactions, and the selectivity of ORR used for producing value-added chemicals coupling with OER. It is shown that the established electrochemical oxygen purifier and hydroxyl radical generator have a high conversion efficiency. Schematic 1. (a) Scheme of operation of the hydroxyl radical generator, (b) the degradation efficiency and rate constants as function of current densities; (c) Scheme of the oxygen purifier, and (d) the generated oxygen volume as a function of working power of the electrochemical cell with different anolyte concentrations. Acknowledgements: This work was supported by the competence center FunMat-II funded by the Swedish Agency for Innovation Systems (Vinnova, grant no 2016-05156), the Swedish Energy Agency (project no 42022-1), and Swedish Research Council (VR 2019-05577, Flexible metal-air primary batteries). Reference: 1 United Nations. Transforming our world: The 2030 agenda for sustainable developement. (UN General Assembly, New York, 2015).2 Yang, Y. et al. Electrocatalysis in Alkaline Media and Alkaline Membrane-Based Energy Technologies. Chem. Rev. 6, 6117–6321 (2022). Figure 1