Co-Nx Sites Research Articles

The marriage of crystalline/amorphous Co/Co3O4 heterostructures with N-doped hollow carbon spheres: efficient and durable catalysts for oxygen reduction

The design of non-precious metal-N-C (M-N-C) materials with efficient oxygen reduction activity are extremely necessary for renewable energy technologies. These M-N-C electrocatalysts are often composed of metal nanoparticles and M-Nx sites simultaneously due to their structural heterogeneity induced by the high-temperature strategy. However, these metal nanoparticles usually suffer from a partial electrochemical/chemical oxidation or dissolution in strong alkaline electrolytes, which may decrease the number of active sites for electrocatalytic oxygen reduction reaction (ORR). Herein, we demonstrated a facile strategy to get this obstacle that fabricate the core-shell crystalline/amorphous Co/Co3O4 heterostructures incorporated in N-doped hollow carbon spheres (Co/Co3O4@NHCS). Specifically, the amorphous Co3O4 shell could effectively prevent the electrochemical oxidation/dissolution of metal Co core in strong alkaline solution. It could also create abundant interface that provide sufficient electron supply and reaction places, and thus make it promising the high efficiency of electron transfer. Furthermore, the N-doped defective graphitic carbon could optimize and facilitate the charge/mass transport that could maximize the catalytic activity. Deliberately coupling the crystalline/amorphous Co/Co3O4 heterostructures with Co-Nx sites are advantageous to achieve the remarkable catalytic activity and superior durability for ORR in alkaline media. As expected, the resulted Co/Co3O4@NHCS catalysts possess the superior ORR activity with a half-wave potential of 0.82V (vs. RHE), similar to that of the commercial Pt/C. Most importantly, it shows excellent durability with almost negligible loss in catalytic performance after 3500 cycles. This work opens an avenue for rational design the efficient ORR electrocatalysts toward practical applications in fuel cells.

The synergistic catalysis on Co nanoparticles and CoNx sites of aniline-modified ZIF derived Co@NCs for oxidative esterification of HMF

An efficient, sustainable and scalable strategy for the synthesis of porous cobalt/nitrogen co-doped carbons (Co@NCs) via pyrolysis of aniline-modified ZIFs, has been demonstrated. Aniline can coordinate and absorb on the surface of ZIF (ZIF-CoZn3-PhA), accelerate the precipitation of ZIFs, thus resulting in smaller ZIF particle size. Meanwhile, the aniline on the surface of ZIF-CoZn3-PhA promotes the formation of the protective carbon shell and smaller Co nanoparticles, and increases nitrogen content of the catalyst. Because of these properties of Co@NC-PhA-3, the oxidative esterification of 5-hydroxymethylfurfural can be carried out under ambient conditions. According to our experimental and computational results, a synergistic catalytic effect between CoNx sites and Co nanoparticles has been established, in which both Co nanoparticles and CoNx can activate O2 while Co nanoparticles bind and oxidize HMF. Moreover, the formation and release of active oxygen species in CoNx sites are reinforced by the electronic interaction between Co nanoparticles and CoNx.

Cobalt in N-doped carbon matrix catalyst for chemoselective hydrogenation of nitroarenes

Anilines as important intermediates for both organic synthesis and industrial manufactory are densely substituted with a variety of functional moieties, and the transformation of nitroarenes into corresponding anilines requires catalytically selective hydrogenation catalyst. Herein, we describe a simple pyrolysis strategy to prepare cobalt catalysts in nitrogen-doped carbon matrix applied in the selective hydrogenation of nitroarenes with molecular hydrogen. The Co/NC catalysts are obtained through thermal treatment of mixed precursors of cobalt phthalocyanine and melamine. The surface-modified Co particles with Co3O4 and CoNx sites are surrounded by N-doped carbon layers according to a series of structural characterization results. These Co/NC catalysts are capable of efficiently selective hydrogenation of nitrobenzene and various substituted nitroarenes into corresponding anilines under relatively mild reaction conditions. The optimal catalytic hydrogenation performance is contributed to the fast rate of H2 dissociated activation on the CoNx active sites and the facile adsorption of the reactant substances, which is verified by the isotopic H2-D2 exchange experiments, reactant adsorption and the ORR reaction tests. Furthermore, the heterogeneous Co/NC catalyst is highly stable without the Co leaching and deactivation issues during the recycling reaction runs.

Switching Co/N/C Catalysts for Heterogeneous Catalysis and Electrocatalysis by Controllable Pyrolysis of Cobalt Porphyrin

SummaryIdentifying the optimal synthetic and structural parameters in preparing pyrolyzed metal/nitrogen/carbon (M/N/C) materials is crucial for developing effective catalysts for many important catalytic processes. Here we report a group of mesoporous Co/N/C catalysts ranging from polymerized cobalt porphyrin to Co/N-doped carbons, which are prepared by pyrolysis of cobalt porphyrin using silica nanoparticles as templates at different temperatures, for boosting both heterogeneous catalysis and electrocatalysis. It is revealed that the polymerized cobalt porphyrin prepared at low temperature (500°C) is a polymer-like network with exclusive single-atom Co-Nx sites, and that the high-temperature-pyrolysis (>600°C) produces an electrically conductive Co/N-doped carbon, accompanied by part degradation of Co-Nx centers. We identify that the polymerized cobalt porphyrin with undecomposed Co-Nx centers is optimal for heterogeneous catalytic oxidation of ethylbenzene, whereas the electrically conductive Co/N-doped carbon is ideal for eletrocatalytic oxygen reduction. Our results provide new insights for rationally optimizing M/N/C catalysts for different reactions.

Co based N, S co-doped carbon hybrids for catalytic hydrogenation: Role of cobalt salt and doped S

A series of Co-based N, S dual-doped carbon catalysts were prepared successfully via the pyrolysis of porous organic polymers (POPs) impregnated with cobalt salt and characterized by TEM, XRD, XPS, etc. The successful construction of cobalt-nitrogen (Co-Nx) sites is confirmed by XPS spectra. It was found that the content of Co-Nx sites is markedly affected by the type of cobalt salt, and the catalyst derived by the pyrolysis of the complex of POPs with Co(NO3)2·6H2O displays the best activity and outstanding selectivity for catalytic hydrogenation of various aromatic nitro compounds, it also demonstrates good recyclability. According to the results of control experiments, the activity for catalytic hydrogenation is mainly attributed to the Co-Nx sites. In addition, the doped S species in carbon shells can promote the formation of the active sites, which can effectively improve the performance of catalysts. Thus, this work can provide an effective and green approach for design highly effective transition metal-based multi-doped carbon catalysts with abundant metal-Nx sites.

Aerobic oxidative cleavage of 1,2-diols catalyzed by atomic-scale cobalt-based heterogeneous catalyst

The oxidative cleavage of the C–C bonds in 1,2-diols is an important transformation in synthetic organic chemistry. The challenge for this reaction is to develop a recyclable catalyst and an efficient catalytic system that operates under mild conditions. Here we report an atomically dispersed cobalt (3.8 wt% Co) on N-doped carbon catalyst, which exhibits improved catalytic activity toward the oxidative cleavage of a variety of 1,2-diols into esters, ketones or aldehydes using molecular oxygen under mild conditions. For example, the oxidative cleavage of internal diols is achieved at ambient temperature and air pressure. The robust catalyst can be reused at least seven times without regeneration treatment. The formation of highly dispersed active Co-Nx sites is demonstrated by catalyst characterization and potassium thiocyanate poisoning experiment. Mechanistic insights into monosubstituted diols indicate a sequence reaction including stepwise oxidation/nucleophilic addition/C–C bond cleavage, and reveal two reaction pathways.

Co/CoOx nanoparticles inlaid onto nitrogen-doped carbon-graphene as a trifunctional electrocatalyst

Development of earth-abundant, efficient and stable catalysts for hydrogen evolution reaction (HER), oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalysis is an important subject in replacing precious metals to solve energy and environmental issues. Herein, we report a highly active trifunctional electrocatalyst for hydrogen and oxygen electrodes comprising Co/CoOx nanoparticles embedded onto nitrogen-doped carbon-graphene obtained by the pyrolysis of graphene oxide supported cobalt metal–organic framework (ZIF-67) under a nitrogen atmosphere. The as-prepared Co@NC-G-700 catalyst shows robust and highly active HER in both acid and base electrolyte with low overpotential (140 and 136 mV at 10 mA cm−2, respectively). In addition, this catalyst exhibits excellent ORR performance, including a positive half-wave potential (∼0.83 V vs RHE), a low Tafel slope of 47.9 mV dec−1, outstanding stability, and strong tolerance to CH3OH crossover. Furthermore, Co@NC-G-700 displays a good oxygen evolution reaction activity in alkaline media with an overpotential of only 322 mV at a current density of 10 mA cm−2. The surface Co-Nx sites should be responsible for the efficient HER and ORR, while Co-O-Co, Co-N-C, and Co-O-C sites might work together in OER.

Exploring Active Sites in Multi-Heteroatom-Doped Co-Based Catalysts for Hydrogen Evolution Reactions.

Today, metal-N- as well as metal-S-doped carbon materials are known to catalyze the hydrogen evolution reaction (HER). However, especially N- and S-co-doped catalysts reach highest activity, but it remains unclear if the activity is related to MNx or MSy (M=metal) sites. In this work we apply a simple method for multi-heteroatom doping and investigate the effect of cobalt content on the HER in acidic medium. The CoNx and CoSy sites were evidenced on the basis of structural characterization by Raman, X-ray induced photoelectron spectroscopy, and TEM. The presence of sulfur enables the formation of a larger number of CoNx sites. Structure-performance relationship proves that the HER activity is dominated by CoNx rather than CoSy sites. The most active catalysts also exhibit an excellent stability under galvanostatic conditions making them of interest for electrolyser application.

Scalable solid-state synthesis of coralline-like nanostructured Co@CoNC electrocatalyst for Zn-air batteries.

A facile and scalable solid-state synthesis strategy is developed to produce hierarchical coralline-like nanostructured electrocatalysts with cobalt nanoparticles and Co-NX sites for efficient oxygen reduction reaction, opening up an avenue for the mass production of non-precious metal catalysts for metal-air batteries and fuel cells, etc.

Single cobalt sites in mesoporous N-doped carbon matrix for selective catalytic hydrogenation of nitroarenes

A supported cobalt catalyst with atomically dispersed Co-Nx sites (3.5 wt% Co) in a mesoporous N-doped carbon matrix (named Co@mesoNC) is synthesized by hydrolysis of tetramethyl orthosilicate (TMOS) in a Zn/Co bimetallic zeolitic imidazolate framework (BIMZIF(Co,Zn)), followed by high-temperature pyrolysis and SiO2 leaching. A combination of TEM, XRD XPS and X-ray absorption spectroscopy studies confirm the absence of cobalt nanoparticles and indicate that these highly dispersed cobalt species are present in the form of Co-Nx. The exclusive formation of Co-Nx sites in the carbon matrix is attributed to the presence of a large amount of Zn and N in the BIMZIF precursor together with the presence of SiO2 in the pore space of this framework, extending the initial spatial distance between cobalt atoms and thereby impeding their agglomeration. The presence of SiO2 during high-temperature pyrolysis is proven crucial to create mesoporosity and a high BET area and pore volume in the N-doped carbon support (1780 m2 g−1, 1.54 cm3 g−1). This heterogeneous Co@mesoNC catalyst displays high activity and selectivity (>99%) for the selective hydrogenation of nitrobenzene to aniline at mild conditions (0.5–3 MPa, 343–383 K). When more challenging substrates (functionalized nitroarenes) are hydrogenated, the catalyst Co@mesoNC displays an excellent chemoselectivity to the corresponding substituted anilines.The presence of mesoporosity improves mass transport of reactants and/or products and the accessibility of the active Co-Nx sites, and greatly reduces deactivation due to fouling.

Towards understanding ORR activity and electron-transfer pathway of M-Nx/C electro-catalyst in acidic media

M-Nx/C (M = Fe, Co) type electro-catalyst as a promising alternative to Pt-based electro-catalyst for oxygen reduction reaction (ORR) in fuel cells has been studied for years. However, the mechanism of this four-electron process involving several successive steps and the dynamic intermediate (hydrogen peroxide) participation still remains obscure. In this study, a series of Co-Nx/C with varied densities of Co-Nx sites have been obtained for probing the ORR activity and pathway in acid media via adopting zinc ions as a size-comparable template to disperse and regulate Co-Nx sites. Importantly, it has been found that the half-wave potential can be positively correlated with the weight percentage of ionic cobalt species, which suggests the decisive role of the density of Co-Nx sites on ORR activity. More importantly, our results suggest that both Co-Nx/C and 20 wt% Pt/C catalyze ORR via two successive steps: ORR begins at the onset potential accompanying the generation of H2O2 intermediate, which is more easily to be adsorbed on platinum than on Co-Nx/C surface as detected by ring electrode; Subsequently at the half-wave potential and more negative, H2O2 is further reduced immediately and efficiently on platinum, but unfortunately at much lower rate on Co-Nx/C. Thus in addition to improving the initial ORR activity by maximizing the M-Nx coordination, it will be of great significance to endow the non-platinum catalysts with surface active sites capable of adsorbing and efficiently reducing the H2O2 intermediate for the complete oxygen reduction to water.

Rational design of hollow N/Co-doped carbon spheres from bimetal-ZIFs for high-efficiency electrocatalysis

To explore efficient non-noble metal-based electrocatalysts for oxygen reduction reaction (ORR), herein we developed a facile bottom-up approach for the fabrication of a hollow porous carbon sphere codoped with ultra-small Co nanoparticles and uniform nitrogen distribution (Co-HNCS) via one-step pyrolysis of a core-shell type precursor composing of polystyrene (PS) core and bimetallic ZIF (zeolite imidazolate framework) shell. The bimetallic Co-Zn-ZIFs (BMZIFs) was selected as the sacrifice template due to not only its high nitrogen content and regular porosity but also the superiority that Zn species in BMZIFs can both spatially separate Co species to suppress the aggregation of ultra-small Co NPs and be evaporated to afford extra pores during high-temperature pyrolysis. As expected, by adjusting the starting molar ratio of Zn to Co, we were able to prepare Co-HNCS-x (x represent the molar ratio of Co to total starting metal feeding) that exhibited unique hollow structure with large surface areas, enhanced mass transport, high porosities, tunable particle sizes and graphitization degrees, abundant highly active CoNx sites, and thus significantly improved ORR performance. Particularly, the optimal Co-HNCS-0.2 exhibited the remarkable ORR activity (the onset and half-wave potentials were 0.94 and 0.82Vvs. RHE, respectively) via an efficient four-electron-dominant ORR process in alkaline medium, which outperformed that of commercial Pt/C (20wt%, the onset and half-wave potentials were 0.93 and 0.80Vvs. RHE, respectively) and most of previously reported Co-based catalysts. Moreover, it also displayed much superior stability and tolerance to methanol as compared to Pt/C, further highlighting the merit of this facile synthesis approach. Our findings might inspire new thoughts on the development of precious-metal-free, highly-efficient and cost-effective ORR electrocatalysts derived from MOF.

Significant Enhancement of Water Splitting Activity of N-Carbon Electrocatalyst by Trace Level Co Doping.

Replacement of precious metal electrocatalysts with highly active and cost efficient alternatives for complete water splitting at low voltage has attracted a growing attention in recent years. Here, this study reports a carbon-based composite co-doped with nitrogen and trace amount of metallic cobalt (1 at%) as a bifunctional electrocatalyst for water splitting at low overpotential and high current density. An excellent electrochemical activity of the newly developed electrocatalyst originates from its graphitic nanostructure and highly active Co-Nx sites. In the case of carefully optimized sample of this electrocatalyst, 10 mA cm(-2) current density can be achieved for two half reactions in alkaline solutions-hydrogen evolution reaction and oxygen evolution reaction-at low overpotentials of 220 and 350 mV, respectively, which are smaller than those previously reported for nonprecious metal and metal-free counterparts. Based on the spectroscopic and electrochemical investigations, the newly identified Co-Nx sites in the carbon framework are responsible for high electrocatalytic activity of the Co,N-doped carbon. This study indicates that a trace level of the introduced Co into N-doped carbon can significantly enhance its electrocatalytic activity toward water splitting.

Molecular metal-Nx centres in porous carbon for electrocatalytic hydrogen evolution.

Replacement of precious platinum with efficient and low-cost catalysts for electrocatalytic hydrogen evolution at low overpotentials holds tremendous promise for clean energy devices. Here we report a novel type of robust cobalt–nitrogen/carbon catalyst for the hydrogen evolution reaction (HER) that is prepared by the pyrolysis of cobalt–N4 macrocycles or cobalt/o-phenylenediamine composites and using silica colloids as a hard template. We identify the well-dispersed molecular CoNx sites on the carbon support as the active sites responsible for the HER. The CoNx/C catalyst exhibits extremely high turnover frequencies per cobalt site in acids, for example, 0.39 and 6.5 s−1 at an overpotential of 100 and 200 mV, respectively, which are higher than those reported for other scalable non-precious metal HER catalysts. Our results suggest the great promise of developing new families of non-precious metal HER catalysts based on the controlled conversion of homogeneous metal complexes into solid-state carbon catalysts via economically scalable protocols.

Performance of nitrogen-containing macroporous carbon supported cobalt catalyst synthesized through in-situ construction of catalytic sites for oxygen reduction reaction

A novel method of in-situ catalytic site (CoNx) construction in macroporous carbon (MPC) is developed. The nitrogen-containing MPC-supported cobalt (Co/N–MPC) catalysts are synthesized via the pyrolysis of a mixture of glucose-urea resin, nano-CaCO3, and cobalt nitrate. The nano-CaCO3 functions as a template to fabricate MPC that provides high electric conductivity and large specific surface area. The catalytic CoNx sites are simultaneously created during the formation of N–MPC. The use of glucose-urea resin as the carbon and nitrogen sources significantly increases the nitrogen content as high as 8.8 at% in the MPC. The synthesized Co/N–MPC demonstrates superb catalytic activity toward oxygen reduction reaction. The direct borohydride fuel cell using the Co/N–MPC shows a power density as high as 170 mW cm−2 which is much higher than the cell using 10 wt.% Pt/C but slightly lower than the cell using 20 wt.% Pt/C as the cathode catalyst at ambient conditions.