Non-noble Metal Catalysts Research Articles

Metal-modified Ni phosphides: Highly efficient catalysts for the hydrogenation of ethyl levulinate

The hydrogenation of ethyl levulinate (EL) to γ-valerolactone (GVL) is an important process in biomass conversion to value-added chemicals. However, it remains challenging to develop efficient non-noble metal catalysts with high activity and selectivity for this reaction. Herein, we investigated the effect of second metal modification on Ni3P catalysts, where a series of Ni-P-M/Al2O3 and Ni-M/Al2O3 catalysts (M = Fe, Co, Cu, and Zn) were prepared for EL hydrogenation. The results show that Ni-P-Cu/Al2O3 exhibited the highest activity, being 2.3 and 1.7 times more active than Ni-Cu/Al2O3 and Ni-P/Al2O3, respectively. Reaction evaluations at different temperatures revealed that Ni-P/Al2O3 and Ni-P-Cu/Al2O3 have a similar activation energy of ∼65 kJ/mol, lower than that of Ni-Cu/Al2O3 (86 kJ/mol). The combined characterization and reaction results suggested that the introduction of Cu enriched surface P in the Ni-P-Cu/Al2O3 catalyst, providing more active sites to enhance CO hydrogenation. These findings provide insights for optimizing non-noble metal catalysts for EL hydrogenation.

Europium Oxide Evoked Multisite Synergism to Facilitate Water Dissociation for Alkaline Hydrogen Evolution

AbstractExploring an efficient nonnoble metal catalyst for hydrogen evolution reaction (HER) is critical for industrial alkaline water electrolysis. However, it remains a great challenge due to the additional energy required for H─OH bond cleavage and the lack of enough H2O adsorption sites for most catalysts. Herein, the integration of oxophilic Eu2O3 with NiCo alloy with evoked multisite synergism to facilitate water dissociation for alkaline HER is proposed. The optimized Eu2O3‐NiCo exhibits excellent HER activity with a low overpotential of only 60 mV at 10 mA cm−2 and good electrochemical stability, which is superior to that of Eu2O3‐free NiCo and comparable to benchmark Pt/C. The key roles of Eu2O3 on the enhanced HER performance of Eu2O3‐NiCo are identified by in situ Raman spectroscopy and theoretical calculations. It is discovered that the Eu2O3 with strong oxophilicity facilitates the adsorption of H2O and the breakage of H─OH bonding while evoking the electron redistribution at Eu2O3/NiCo interface and accelerating the Volmer step in alkaline HER. Furthermore, the obtained Eu2O3‐NiCo as both anode and cathode displays excellent overall water‐splitting activity and stability in 1.0 M KOH solution. It is believed that this study provides an important inspiration to design high‐performance electrocatalysts toward HER based on rare‐earth materials.

Electrodeposited Porous Nickel–Copper as a Non-Noble Metal Catalyst for Urea-Assisted Anion Exchange Membrane Electrolysis for Hydrogen Production

Electrodeposited Porous Nickel–Copper as a Non-Noble Metal Catalyst for Urea-Assisted Anion Exchange Membrane Electrolysis for Hydrogen Production

Regulating the electronic structure by P-doping cobalt-based catalyst for atomic hydrogen mediated electrocatalytic dechlorination

Electrocatalytic dechlorination by atomic hydrogen (H*) is efficient, but limited by the low efficiency of H* production. Herein, a phosphorus-doped cobalt nitrogen carbon catalyst (Co-NP/C) was prepared, which had high catalytic activity in a wide pH range (3−11). The turnover frequency of Co-NP/C (3.54 min−1) was 1.21–59000 times superior to that of current Pd-based and non-noble metal catalysts (0.00006–2.92 min−1). Co-NP/C significantly enhanced H* generation, which was 1.52, 2.44, and 3.77 times stronger than that of Co-N/C, NP/C, and N/C, respectively, since the introduction of phosphorus was found enhanced the electron density of cobalt and regulated the electron transfer. Co-NP/C showed outstanding catalytic performance after ten cycles and could achieve nearly complete chloramphenicol removal. This regulation method was verified to be effective for other non-noble metal (Fe, Mn, Cu, Ni) phosphorus doped catalysts, proposing a general class for efficient electrochemical dechlorination, which would be of great significance for the elimination of chlorinated organic pollutants.

A networked iron and nitrogen-doped ZIF-8/MWCNTs heterostructure for oxygen reduction reaction.

Zeolitic Imidazolate Frameworks-8 (ZIF-8) is commonly used as an ideal precursor for non-noble metal catalysts because of its high specific surface area, ultra-high porosity, and N-rich content. Upon pyrolyzing ZIF-8 at 900 °C in Ar, the resulting material, referred to as Z8, displayed good activity toward the oxygen reduction reaction (ORR). Then the ZIF-8 was mixed with various conductive carbon materials, such as multiwall carbon nanotubes (MWCNTs), Acetylene black (ACET), Vulcan XC-72R (XC-72R), and Ketjenblack EC-600JD (EC-600JD), to form Z8 composites. The Z8/MWCNTs composite exhibited enhanced ORR activity owing to its network structure, meso-/microporous hierarchical porous structure, improved electrical conductivity, and graphitization. Subsequently, iron and nitrogen co-doping is achieved through the pyrolysis of a mixture comprising Fe, N precursor, and ZIF-8/MWCNTs, which is denoted as FeN-Z8/MWCNTs. The intrinsically high electrical conductivity of MWCNTs facilitated efficient electron transfer during the ORR, while the meso-/microporous hierarchical porous structure and network structure of Fe, N co-doped ZIF-8/MWCNTs promoted oxygen transport. The presence of Fe-containing species in the catalyst acted as activity centers for ORR. This strategy of preparing Z8 composites and modifying them with Fe, N co-doping offers an insightful approach to designing cost-effective electrocatalysts.

Practice of Ecological Aesthetics in Green Production of Bimetallic Carbide Catalyst for Oxygen Reduction Reaction: Integrating Technological Development with Ecological Protection

This work summarizes the disciplinary connotation of ecological aesthetics, discusses the social and philosophical background of the origination of ecological aesthetics, and applies ecological aesthetics to the research on the production processes of catalytic materials. It is found that compared with conventional chemical processes, catalytic materials synthesized using green chemical processes that conform to ecological aesthetics have advantages in raw material cost, energy consumption, environmental protection, operational complexity, and product performance. Based on this, it is proposed that, as green chemical processes develop to a certain extent, they will unify anthropocentrism and ecocentrism, and meet both human needs and ecological protection requirements. The mentioned green chemical processes adopt biomass lotus leaf stems as a carbon source to produce non-noble metal bimetallic carbide (C19Cr7Mo24)-based catalysts for oxygen reduction reaction (ORR). Its initial half-wave potential (E1/2) for catalyzing ORR in an alkaline medium is 0.903 V, the E1/2 retention rate after 50,000 cycles is 98.9%, and its peak power density in H2/O2 fuel cell reaches 1.47 W cm−2, making it one of the most active non-noble metal catalysts for ORR reported so far; its stability is unparalleled.

Less Is More: Selective-Atom-Removal-Derived Defective MnOx Catalyst for Efficient Propane Oxidation.

Defect manipulation in metal oxide is of great importance in boosting catalytic performance for propane oxidation. Herein, a selective atom removal strategy was developed to construct a defective manganese oxide catalyst, which involved the partial etching of a Mg dopant in MnOx. The resulting MgMnOx-H catalysts exhibited superior low-temperature catalytic activity (T50 = 185 °C, T90 = 226 °C) with a propane conversion rate of 0.29 μmol·gcat.-1·h-1 for the propane oxidation reaction, which is 4.8 times that of pristine MnOx. Meanwhile, a robust hydrothermal stability was guaranteed at 250 °C for 30 h of reaction time. The comprehensive experimental characterizations revealed that the catalytic performance improvement was closely related to the defective structures including the abundant (metal and oxygen) vacancies, distorted crystals, valence imbalance, etc., which prominently weakened the Mn-O bond and stimulated the mobility of surface lattice oxygen, leading to the elevation in the intrinsic oxidation activity. This work exemplifies the significance of defect engineering for the promotion of the oxidation ability of metal oxide, which will be valuable for the further development of efficient non-noble metal catalysts for propane oxidation.

Highly efficient conversion of biomass-derived furanic compounds into alkyl diols by selective hydrogenolysis using non-noble metal catalysts with tunable surface oxygen vacancies

Production of alkyl diols from biomass-derived furanic compounds by hydrogenolysis is a promising route to the fabrication of bio-based polyesters and polyurethanes. However, it faces many challenges such as the use of noble metal catalysts, or low hydrogenolysis activity of the furan rings and poor product selectivity with non-noble metal catalysts. In this work, non-noble metal catalysts Cu-Co3O4 (CuCox) catalysts were modified by magnesium, obtaining CuCoxMgy catalysts with tunable surface oxygen vacancies (OV). The CuCoxMgy catalysts demonstrated greatly improved activity and selectivity (approx. 50 %), comparable to those of noble metal catalysts, in the hydrogenolysis of furfuryl alcohol into 1,5-pentanediol (1,5-PeD). The obtained CuCoxMgy catalysts are even superior to the noble metal catalysts in terms of production rate or productivity. Extensive characterizations including XAS, EPR and XPS revealed that magnesium significantly promotes the formation of Co2+-OV pair sites on the catalyst surface, which simultaneously activate furan ring and hydroxyl group in furfuryl alcohol, leading to the efficient and selective cleavage of the C2-O1 bond to form 1,5-PeD. The CuCoxMgy catalysts also exhibited high activity and selectivity in hydrogenolysis of various other biomass-derived furanic compounds towards ring-opening products.

Insights into the catalytic mechanism of V-Mo@CN for the oxidation of 5-hydroxymethylfurfural to 2,5-diformylfuran

The efficient synthesis of 2, 5-diformylfuran (DFF) over non-noble metal catalysts has been a significant challenge in biomass conversion strategies. Herein, a bimetallic V-Mo oxide supported by nitrogen-doped carbon (V-Mo@CN) was fabricated for the efficient oxygenation of 5-hydroxymethylfurfural (HMF) towards DFF. Remarkably, under the optimal conditions, a full conversion of HMF was achieved with an impressive DFF yield of 97.1 %. This oxidation reaction was determined to be a first-order reaction with a relatively lower apparent activation energy of 46.76 kJ/mol and proceeded by the synergistic redoxes of V5+/V3+ and Mo6+/Mo4+ with the assistance of the highly mobile lattice oxygen. The synergism of the V-species, Mo-species and @CN support contributed to the high-efficiency oxidation of HMF to generate DFF. The synergistic action of the V-species and O-species was responsible for the competitive adsorption advantage of the C-OH in HMF with a vertical configuration, ensuring the highly selective and efficiency in oxidizing HMF toward DFF.

Weakening Mn-O Bond Strength in Mn-Based Perovskite Catalysts to Enhance Propane Catalytic Combustion.

Exploring highly efficient and robust non-noble metal catalysts for VOC abatement is crucial but challenging. Mn-based perovskites are a class of redox catalysts with good thermal stability, but their activity in the catalytic combustion of light alkanes is insufficient. In this work, we modulated the Mn-O bond strength in a Mn-based perovskite via defect engineering, over which the catalytic activity of propane combustion was significantly enhanced. It demonstrates that the oxygen vacancy concentration and the Mn-O bond strength can be efficiently modulated by finely tuning the Ni content in SmNixMn1-xO3 perovskite catalysts (SNxM1-x), which in turn can enhance the redox ability and generate more active oxygen species. The SN0.10M0.90 catalyst with the lowest Mn-O bond strength exhibits the lowest apparent activation energy, over which the propane conversion rate increases by 3.6 times compared to that on the SmMnO3 perovskite catalyst (SM). In addition, a SN0.10M0.90/cordierite monolithic catalyst can also exhibit a remarkable catalytic performance and deliver excellent long-term durability (1000 h), indicating broad prospects in industrial applications. Moreover, the promotional effect of Ni substitution was further unveiled by density functional theory (DFT) calculations. This work brings a favorable guidance for the exploration of highly efficient perovskite catalysts for light alkane elimination.

Construction of a p-p heterojunction Co3O4/MoS2 electrocatalyst for boosting hydrogen evolution reaction

Building p-p heterojunctions with Fermi level difference, due to the motion of energy levels, band bending, and internal electric field generation, can effectively improve the electronic structure of the interface, enrich the active sites of heterojunctions, and promote the adsorption of reaction intermediates, which is an effective approach to constructing high-performance electrocatalysts. In this study, a Co3O4/MoS2 p-p heterojunction catalyst with nano-particle/nano-flower structure was designed, inducing charge transfer from Co3O4 to MoS2, thereby increasing the negative charge of MoS2, improving *H adsorption through electrostatic interaction, enhancing electrocatalytic kinetics, and reducing reaction overpotential. The catalytic performance of MoS2 for hydrogen evolution reaction was effectively enhanced. In 1 M KOH solution, the current density was 10 mA cm−2, and the overpotential was 156 mV, with good stability after 40 h of operation. This work provides a new direction for the design of p-p heterojunction electrocatalysts and offers more choices for promoting the development of high-performance non-noble metal catalysts.

Advancements in printed components for proton exchange membrane fuel cells: A comprehensive review

Proton Exchange Membranes (PEM) are a promising technology for fuel cells and electrolyzer cells, in line with societal goals for clean and sustainable fuel conversion and generation, respectively, in a vast field of applications. PEM fuel cell (PEMFC) technology has an important advantage for energy conversion applications, because it requires a lower operating temperature and is lighter and more compact than other technologies, which makes it ideal for portable and transportation applications. However, open issues remain to be addressed for PEMFC that hinder their widespread adoption, both on the technical side, such as too high operational temperature range, cell efficiency, and long-term stability, as well as on the industrial side with implementation issues, material and device component availability, and cost. PEM technology research usually focuses on the synthesis of novel materials such as non-noble metal catalysts and non-halogenated membranes. In parallel, developing high-throughput and cost-effective manufacturing methods for components, in particular Membrane Electrode Assembly (MEA), can also lift an important barrier to their mass production, thus contributing to the same goals and securing industrial provision. From this perspective, printing technologies such as spray coating, inkjet printing, and screen printing have emerged as promising approaches with the required precision and scalability. This review covers recent advancements and developments in advanced PEMFC technology manufacturing, with a focus on 2D printing methodologies to fabricate some or all of the MEA components. Leveraging these techniques as simplified large-area manufacturing methods for PEM technology can become a more viable and industrially attractive solution. This work presents a comparison study addressing the key progress indicators both in process and in characterization to build a working framework in which both scientists, engineers and manufacturers can collaborate toward the industrial implementation of this emergent field.

Defective Tungsten Oxides with Stacking Faults for Proton Exchange Membrane Green-Hydrogen Generation.

Defects can introduce atomic structural modulation and tailor performance of materials. Herein, it demonstrates that semiconductor WO3 with inert electrocatalytic behavior can be activated through defect-induced tensile strains. Structural characterizations reveal that when simply treated in Ar/H2 atmosphere, oxygen vacancies will generate in WO3 and cause defective structures. Stacking faults are found in defects, thus modulating electronic structure and transforming electrocatalytic-inert WO3 into highly active electrocatalysts. Density functional theory (DFT) calculations are performed to calculate *H adsorption energies on various WOx surfaces, revealing the oxygen vacancy composition and strain predicted to optimize the catalytic activity of hydrogen evolution reaction (HER). Such defective tungsten oxides can be integrated into commercial proton exchange membrane (PEM) electrolyser with comparable performance toward Pt-based PEM. This work demonstrates defective metal oxides as promising non-noble metal catalysts for commercial PEM green-hydrogen generation.

WOx boosted hollow Ni nanoreactors for the hydrodeoxygenation of lignin derivatives

Reasonable design of non-noble metal catalysts with hollow open structure for hydrodeoxygenation (HDO) of lignin derivatives to value-added chemicals is of great significance but challenging. Herein, a novel MOF-derived multilayer hollow sphere coated nickel‑tungsten bimetallic catalyst (Ni2-WOx@CN-700) was fabricated via by confined pyrolysis strategy using bimetallic MOFs as a self-sacrificial template, which exhibits robust activity for the typical model HDO of vanillin to 2-methoxy-4-methylphenol (Yield of 100 % at 140 °C for no less than 10 cycles). The characterizations revealed that WOx facilitated the dispersion of Ni nanoparticles and adjusted the acidic capacity of the catalyst through the formed Ni-WOx heterojunction. Density functional theory (DFT) calculations confirms that WOx species enhanced the electron-rich nature of the active sites, while the adsorption energies of H2 and vanillin on Ni-WOx decreased from −0.572 eV and − 0.622 eV on Ni to −3.969 eV and − 4.922 eV, respectively. These results further indicated that the high activity of Ni2-WOx@CN-700 was attributed to the Ni-WOx heterojunction. Based on the characterizations and the thermodynamic calculations, the reaction mechanism was proposed. In addition, the catalyst shows good substrate universality, which enables its good commercial application prospect.

Cu-BTC-derived Cu@C as nano-heater embedded in 3D g-C3N4 for photothermal-coupled photocatalytic CO2 reduction under full-spectrum solar irradiation

This study developed a novel Cu@C/NCN-T composite for efficient photothermal catalytic CO2 reduction. The composite, consisting of carbon-coated Cu (Cu@C) derived from Cu-BTC embedded in three-dimensional graphitic carbon nitride (NCN-T), was immobilized on the surface of carbonized wood (C-wood) floating on water, creating a triphase system (CO2, H2O, and catalyst). The optimized composite demonstrated remarkable CO2 photothermal reduction performance, achieving a rate of 31.3 μmol·g−1·h−1. This performance surpassed that of pure NCN-T and Cu@C by 1.6 and 3.8 times, respectively. The combination of Cu and carbon materials endowed Cu@C/NCN-T with excellent light-absorption properties, resulting in a photothermal effect that elevated the surface temperature and facilitated the catalytic reaction. Furthermore, the Schottky junction formed between Cu@C and NCN-T promoted the separation of electron-hole pairs, with Cu@C acting as charge channels. A charge transfer mechanism was proposed to explain these findings. This catalyst engineering approach holds promise for the development of efficient non-noble metal catalysts for rapid CO2 photothermal reduction.

Anion Structure Regulation of Cobalt Silicate Hydroxide Endowing Boosted Oxygen Evolution Reaction.

Transition metal silicates (TMSs) are attempted for the electrocatalyst of oxygen evolution reaction (OER) due to their special layered structure in recent years. However, defects such as low theoretical activity and conductivity limit their application. Researchers always prefer to composite TMSs with other functional materials to make up for their deficiency, but rarely focus on the effect of intrinsic structure adjustment on their catalytic activity, especially anion structure regulation. Herein, applying the method of interference hydrolysis and vacancy reserve, new silicate vacancies (anionic regulation) are introduced in cobalt silicate hydroxide (CoSi), named SV-CoSi, to enlarge the number and enhance the activity of catalytic sites. The overpotential of SV-CoSi declines to 301mV at 10mA cm-2 compared to 438mV of CoSi. Source of such improvement is verified to be not only the increase of active sites, but also the positive effect on the intrinsic activity due to the enhancement of cobalt-oxygen covalence with the variation of anion structure by density functional theory (DFT) method. This work demonstrates that the feasible intrinsic anion structure regulation can improve OER performance of TMSs and provides an effective idea for the development of non-noble metal catalyst for OER.

Unraveling selectivity in non-noble metal-catalyzed hydrogenation of 5-hydroxymethylfurfural (HMF) through mechanistic insights

The development of heterogeneous catalysts for converting abundant biomass feedstocks to higher value products is one of the most challenges these days. 2,5-dihydroxymethylfuran (DHMF) and 2,5-dihydroxymethyltetrahydrofuran (DHMTHF), synthesized from HMF hydrogenation, serve as crucial precursors in various applications. Non-noble metal catalysts are particularly attractive for this reaction, given their affordability and impressive catalytic efficiency. This work unveils the origin of the unique selectivity over Ni and Cu through mechanistic investigation using density functional theory (DFT), thermodynamic and kinetic analyses. The results emphasize that temperature and solvent play a crucial role in altering the energetic stabilities of intermediates, thereby influencing the energetic span (δG), turnover frequency (TOF), and selectivity of the reaction. The theoretical results align well with experimental observations. At 373.15 K, the highest TOF values over Ni and Cu are predicted in the HMF-to-DHMTHF path (1.79 × 103h−1) and the HMF-to-DHMF path (4.01 × 105h−1), respectively, in gas phase—under low dielectric constant ε condition. In contrast, the highest TOF value for Ni is observed in the HMF-to-DHMF path under implicit water condition (ε = 78.4) at 298.15 K. Competitive DHMF desorption and further hydrogenation influence the reaction’s selectivity. These insightful fundamental findings reveal key descriptors essential for designing new heterogeneous catalysts or enhancing existing ones, with the aim of potentially impacting biomass upgrading and other hydrogenation reactions.

Hydrodeoxygenation of Oxygen-Containing Aromatic Plastic Wastes into Cycloalkanes and Aromatics.

Chemical recycling and upcycling offer promising approaches for the management of plastic wastes. Hydrodeoxygenation (HDO) is one of the appealing ways for conversion of oxygen-containing plastic wastes, including polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polycarbonate (PC), polyphenyl ether (PPO), and polyether ether ketone (PEEK), into cyclic alkanes and aromatics in high yields under mild reaction conditions. The challenge lies in achieving C-O activation while preserving C-C bonds. In this review, we highlight the recent advancements in catalytic strategies and catalysts for the conversion of these oxygen-containing plastic wastes into cycloalkanes and aromatics. The reaction systems, including multi-step routes, direct HDO and transfer HDO methods, are exemplified. The design and performance of HDO catalysts are systematically summarized and compared. We comprehensively discuss the functions of the catalysts' components, reaction pathway and mechanism to gain insights into the HDO process for efficient valorization of oxygen-containing plastic wastes. Finally, we provide perspectives for this field, with specific emphasis on the non-noble metal catalyst design, selectivity control, reaction network and mechanism studies, mixed plastic wastes management and product functionalization. We anticipate that this review will inspire innovations on the catalytic process development and rational catalyst design for the HDO of oxygen-containing aromatic plastics to establish a low-emission circular economy.

Alkali-modified copper manganite spinel for room temperature catalytic oxidation of formaldehyde in air

Formaldehyde (FA) is present ubiquitously in indoor environment as a hazardous pollutant with carcinogenic risks. For the efficient mitigation of FA, catalytic oxidation is a recommendable option to simultaneously satisfy both material cost (e.g., avoiding noble metals) and low-energy requirement (under dark and at room temperature (RT)). From this perspective, a cost-effective alkali modified copper manganite spinel (CuMn2O4) catalyst has firstly been prepared and employed for FA oxidation. Specifically, alkali (1 mol L−1 potassium hydroxide)-modified CuMn2O4 (1-CuMn2O4) achieves 100% FA (50 ppm (gas hourly space velocity of 4777 h−1)) conversion (XFA) at RT. The steady-state reaction rate of 1-CuMn2O4 at 10% XFA is 8.18 × 10−2 mmol g−1 h−1. According to in situ diffuse reflectance infrared Fourier transform spectroscopy, FA molecules are oxidized into water and carbon dioxide through dioxymethylene and formate intermediates. Based on density functional theory simulation, the higher catalytic performance of 1-CuMn2O4 for FA oxidation is attributed to the combined effects of firmer attachment of FA molecules to 1-CuMn2O4 surface, lower energy cost of FA adsorption, and lower desorption energy for the final products from the substrate surface. The present work is expected to provide insights into high-performing non-noble metal catalysts for RT oxidative removal of FA from indoor air.

Morphology engineering of Co3O4 spinel for efficient oxidation of lean methane at low temperature

Nowadays, the development of high-performance non-noble metal catalysts is an important research direction for controlling methane emissions from liquefied natural gas (LNG)-powered vehicles and vessels, and Co3O4 spinel has attracted great attention. In this work, Co3O4 catalysts with different morphologies (flower, sheet, particle, and cube) were synthesized by hydrothermal method and their catalytic performance was evaluated. The results showed that the flower-like Co3O4 (Co3O4-F) catalyst exhibited the highest activity in the lean methane oxidation with the T90 (temperature at the methane conversion of 90%) of 365 °C. Compared to other catalysts, Co3O4-F contained an amount of Co3+ sites and the weakest Co3+-O bond, which facilitated the extraction of active lattice oxygen and the initial oxidation of methane to intermediate products (formate and -COO-) on the catalyst surface. Meanwhile, the abundant oxygen vacancies on the surface of the Co3O4 catalyst could efficient activate O2 to form active oxygen species, which promoted the oxidation of intermediate products to CO2 and H2O. The methane oxidation over the Co3O4-F was confirmed to conform with Mars-van Krevelen mechanism. This work provides a facile way to enhance the methane oxidation activity of Co3O4 catalysts by modulating the morphology.