Birdcage-Type CoOx-Carbon Catalyst Derived ... - ACS Publications

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Birdcage-Type CoOx-Carbon Catalyst Derived from Metal Organic Frameworks for Enhanced Hydrogen Generation Huanhuan Zhang, Yanping Fan, Baozhong Liu, Yanyan Liu, Saima Ashraf, Xianli Wu, Guosheng Han, Jie Gao, and Baojun Li ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06660 • Publication Date (Web): 20 Mar 2019 Downloaded from http://pubs.acs.org on March 20, 2019

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ACS Sustainable Chemistry & Engineering

Birdcage-Type CoOx-Carbon Catalyst Derived from Metal Organic Frameworks for Enhanced Hydrogen Generation Huanhuan Zhang,† Yanping Fan,† Baozhong Liu,*,† Yanyan Liu,*,‡ Saima Ashraf,§ Xianli Wu,§ Guosheng Han,§ Jie Gao,¥ and Baojun Li*,§ †

College of Chemistry and Chemical Engineering, Henan Polytechnic University, 2001 Century Avenue, Jiaozuo 454000, P. R. China ‡ Institute of Chemical Industry of Forest Products, CAF, National Engineering Lab for Biomass Chemical Utilization, Key and Open Lab on Forest Chemical Engineering, SFA, Nanjing 210042, P. R. China § College of Chemistry and Molecular Engineering, Zhengzhou University, 100 Science Road, Zhengzhou 450001, P. R. China ¥ Integrated Analytical Laboratories, 273 Franklin Road, Randolph, New Jersey 07869, United States * Corresponding Author. E-mail: [email protected] (B.Z. Liu), [email protected] (Y.Y. Liu) and [email protected] (B.J. Li).

Abstract: Ammonia borane can release abundant hydrogen with an applicable catalyst. In this article, Co-N-doped carbon spherical catalysts (Co@NCS) were synthesized via the composition of Co-metal organic frameworks and resorcinol-formaldehyde resin. The Co@NCS microsphere consisted of carbon-coated Co nanoparticles, with a particle size of 7.2 nm. Co-CoOx@NCS-n was generated by controllable oxidation of Co@NCS-n, and it exhibited more optimistic catalytic activity than Co@NCS-n in ammonia borane hydrolysis. Co-CoOx@NCS-II demonstrated a superior specific hydrogen generation rate of 5562 mL·min−1·gCo−1 at 298 K due to the synergistic effect between Co and CoOx. The activity had no significant deterioration during the stability test. These designed catalysts acted as a birdcage. It restricted the growth of nanoparticles on catalysts and prevented the active ingredients moving away from the catalysts. The catalytic mechanism in this reaction system was also studied. This novel synthetic strategy may further the development and application of noble-metal-free catalysts in the sustainable catalysis field. Keywords: Ammonia borane; Birdcage; Co-MOF; Hydrogen generation; Spherical

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Introduction In the last several years, excessive consumption of fossil fuels has caused growing environmental concerns.1,2 As a clean source-independent energy vector, hydrogen has aroused a great frenzy of research to improve the utilization efficiency of fossil fuels, which has also benefited the study of practical applications of renewable energy sources.3–6 Hydrogen has been used in vehicular applications because it is clean and non-poisonous, with water as the only combustion product.7,8 However, its low density, difficult storage and propensity to explode under constant atmospheric pressure have become major obstacles for the advancement of hydrogen energy technology. A safe and efficient hydrogen storage technology plays an extremely important role in the utilization of hydrogen resources.9–11 Solid-state hydrogen storage is a vital component of a hydrogen transport system. Ammonia borane (NH3BH3), the simplest borane–nitrogen (B–N) compound, is a stable and environmentally sustainable material with high hydrogen content (19.6 wt%) and low molecular weight (30.86 g·mol−1)3, making it the most attractive storage medium among all chemical hydrides.12–15 The hydrolytic cleavage of NH3BH3 complex is expounded as follows: NH3BH3 + 2H2O→ NH4BO2 + 3H2

(1)

From Eq. 1, one mole of NH3BH3 can release a large stoichiometric amount of hydrogen in the presence of a suitable catalyst.16 Despite these advantages, there is still an urgent desire to develop a high-performance catalyst, which can improve the kinetic properties for facile production of hydrogen from NH3BH3 under moderate conditions. The ideal general catalysts, such as Pt,17,18 Pd,19,20 Rh,21,22 Ru,23,24 and other noble metals, have an intriguing impact on hydrogen generation during NH3BH3 hydrolysis. Due to their high cost and global reserve scarcities, they cannot be employed on an industrial scale. Therefore, the 2 - Environment ACS Paragon -Plus

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development of a cost-effective noble-metal-free catalyst is imperative for hydrogen evolution by NH3BH3, possibly Fe, Ni, Cu, or Co.25–29 Recently, Co-based catalysts have caused concern for hydrogen generation from NH3BH3 or NaBH4. However, the single-dispersed Co nanoparticles (NPs) easily agglomerate. To solve this problem, core-shell NPs have been explored. Metal organic frameworks (MOFs) can prevent agglomeration via a wrapping strategy to fabricate core-shell structures, and they are beneficial for the dispersion and stabilization of active metal NPs.30–33 As precursors, MOFs can be used to form metal/carbon nanomaterial, and there are many reports of MOFs being used in the hydrogenation of NH3BH3.34–36 Surprisingly, the core-shell structures obtained from Co-MOFs can form a composite with carbon materials to further improve their physicochemical properties.37 However, these reports are barely observed. Many carbon precursors, such as resorcinol-formaldehyde (RF) resin, saccharide, pitch, polydopamine and others are also used to fabricate carbon materials. RF is a three-dimensional network-structured polymer, and it stands out due to its low cost, high specific surface area and porosity.38 Liu et al. demonstrated an approach to produce a large-scale RF resin polymer and carbon spheres; their results have positively influenced the utilization of RF resin.39 Pei et al. reported N-HPCB via RF resin had an excellent cycling stability and rate capability in Li-S batteries.40 Fang et al. synthesized Au@HCS with RF resin, and this exhibited high catalytic activity and recyclability in the catalytic reduction of 4-nitrophenol.41 Based on these previous reports, RF resin composites with Co-MOFs will have great physical and chemical properties for catalysts. In this article, spherical catalysts Co-CoOx@NCS-n were synthesized through a direct, simple calcination process in a nitrogen atmosphere, and then they were treated in air to adjust the surface active composition. These catalysts exhibited remarkable catalytic activity during



NH3BH3 hydrolysis due to unprecedented synergistic activation. These designed catalysts acted as a “birdcage” by restricting the growth of NPs and limiting the mobility of active ingredients. We also investigated hydrogen generation in the slurry-bed reactor and the catalytic mechanism of this reaction. Clearly, the changed microstructure and improved catalytic performance will be considered as a milestone in future investigations. Experimental section Preparation of materials

Co-MOF. An ethanol solution (12 mL) of Co(NO3)2·6H2O (0.75 g) was poured into N,Ndimethylformamide solution (48 mL) of terephthalic acid (TPA) (0.428 g) and stirred for 1 h. The mixture was transferred into a Teflon-lined stainless steel autoclave (150 mL), heated to 100 °C and kept for 12 h. After the autoclave was cooled to room temperature naturally, the Co-MOF precursor was obtained as purple powders (0.4106 g) after drying at 60 °C for 12 h in air. CMRF. Resorcinol (0.17 g), formaldehyde (0.26 mL) and ethylene diamine (0.17 mL) were subsequently added into a 250 ml beaker containing water-ethanol mixture solvent (70 : 30 mL : mL), magnetically stirred at 30 °C for 30 min. Co-MOF (0.500 g) was put into the above mixture and then kept at 30°C for 24 h. After that, the mixture was kept at 50 °C and dried thoroughly. CMRF was obtained as a brown powder (0.7928 g). Co@NCS-n and Co-CoOx@NCS-n. Co@NCS-n were obtained from CMRF after being heated up to 550 °C at a rate of 3 °C·min−1 and kept for 1 h in N2 atmosphere in quartz tube. A black powder was achieved and recorded as Co@NCS-II when the material cooled down to room temperature spontaneously. (Caution! The materials were easy to combust spontaneously in air atmosphere and must be immersed into ethanol under nitrogen protection). In a similar procedure, Co@NCS-III was obtained after heated at 700 °C. The above materials were activate-treated at


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200 °C for 22 h in air and denoted as Co-CoOx@NCS-n (n=II, III), respectively. Aged-CMRF-I was obtained from CMRF-I after being aged under N2 atmosphere at 400 °C. Characterization

The crystalline phase of as-prepared samples were characterized through X-ray powder diffraction (XRD, Bruker/D8-Advance, Cu-Kα radiation, λ = 1.5418 Å) in the 2θ range from 5° to 80°. The average crystallite size of Co NPs was estimated according to the Scherrer formula: Dh = λ/(βh·cosθh). In general, Dh is the domain size of the diffraction line, λ is the wavelength of the Cu Kα source used, βh is the width in radians of the diffraction peak measured at halfmaximum intensity (fwhm, full-width at half-maximum) and corrected for instrumental broadening and θh is the angle of the particular (hkl) reflection. The morphology of materials was characterized through emission scanning electron microscope (SEM, Carl Zeiss NTS GmbH), transmission electron microscope (TEM, FEI Tecnai G2 F20 S-TWIN electron microscope, operating at 200 kV). Thermal gravimetric analysis (TGA) was carried out on the STA 409 PC/PG (NETZSCH Germany) with a heating rate of 10˚C min−1. X-ray photoelectron spectroscopy (XPS) was done on a PHI quantera SXM spectrometer with an Al Kα = 1486.60 eV excitation source and the binding energies were calibrated by referencing the C1s peak (284.8 eV) to reduce the sample charge effect. Raman spectra were obtained on Renishaw-invia Raman detection system using a 532 nm laser. Magnetic hysteresis (M-H) curves were used the PPMS9T-type physical magnetic test system to measure, where a temperature of 300 K and the magnetic field strength was −40 kOe-40 kOe. The nitrogen adsorption-desorption isotherms were conducted on ASAP2420-4MP surface area and pore size analyzer (Micromeritics, USA) at 77 K. The specific surface areas (SBET) were calculated via using the Brunauer-Emmett-Teller (BET) means. The pore size distributions were evaluated using the non-localized density function theory (NLDFT) model. 5 - Environment ACS Paragon -Plus


Hydrogen generation in the batch reactor. The hydrolysis of NH3BH3 was performed through a water displacement method. First, catalyst (20 mg) was preloaded into a round-bottom flask (50 mL). Then NaOH solution (1 M, 10 mL) containing NH3BH3 (84 mg) was rapidly injected into the flask using a gastight syringe. A gas burette (200 mL) filled with water connected to the other side of reaction flask was employed to collect the gas. Experiment was conducted by constant-temperature magnetic stirring apparatus and the stirring speed was kept at 500 rpm. The hydrogen generation specific rate was calculated using the information from the stabilizing stages (140 mL of hydrogen generated) according the following formula: rB 

80(mL) [t 140  t 60](min)  wc ( g )

(2)

Here, rB is the hydrogen generation specific rate, t140 represents the time for 140 mL of hydrogen generation, and t60 for 60 mL and Wc is the Co weight in catalyst. Hydrogen Generation in the slurry-bed reactor. The continuous hydrolysis of NH3BH3 was tested through a water displacement method. Catalyst (20 mg) and water (80 mL) were placed into a glass slurry-bed reactor (100 mL) fixed on a magnetic stirrer. The NaOH (1 M) aqueous solution of NH3BH3 (0.136 M or 0.272 M, 60 mL) was put into the reactor bottom through an entrance at a flow rate of 1.20 mL·min−1 by an injector pump. The mixture exited from the reactor top through an export tube and the stirring rate was fixed at 500 rpm. Hydrogen left the reactor through a top tube and its volume was measured in the same inverted and water-filled gas burette in a water-filled vessel. The value of turnover frequency (TOF) in this dehydrogenation reaction was calculated by the following formula:


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TOF 

nH 2 nCo  t

(3)

Here nH2 is the mole of generated H2 during the 60-140 mL, while nCo is the total mole of Co in the sample. t is the reaction time in unit of hours during the 60-140 mL. The unit of TOF is h−1. In the all hydrogen production experiments, the hydrogen production of NH3BH3 was carried out at a self-stirring mode (self-stirring mode means that on the reaction system was stirred by an external magnetic field through the ferromagnetic catalysts in the absence of magneton). The recycling test was conducted at 298 K. When the previous cycle of hydrogen generation was completed, the as-synthesized Co-CoOx@NCS-II was attracted to the bottom of the flask via a magnet from the mixture. The separated Co-CoOx@NCS-II catalyst was collected by centrifugation and dried under vacuum at room temperature and then activated in air at 200 °C for 22 h. The catalytic hydrogen generation process was repeated 4 times. Results and discussion The fabrication procedure for Co@NCS-n was carried out using a hydrothermal process and solution-phase composition process, as illustrated in Figure 1a. The synthesis routes were implemented in two main sections. The Co-MOF precursor was prepared using a hydrothermal reaction between Co ions and TPA. Next, Co-MOF precursor composites were combined with solution-phase RF resin to produce CMRF (a spherical structure precursor, namely, CMRF refers to the compound after the composites with RF resin). Finally, a calcination process converted Co-MOF and Co ions back and forth under a nitrogen atmosphere. Meanwhile, N-doped carbon spheres were manufactured from the RF resin, and then spherical catalysts were obtained. The active-treatment samples of Co-CoOx@NCS-n were harvested from Co@NCS-n through controlled oxidation at 200 °C for 22 h in air. The designed catalysts acted as a “birdcage” by



restricting the growth of NPs and limiting the mobility of the active ingredients. TGA of CMRF was performed in argon atmosphere, indicating that CMRF could not thoroughly decompose at 400 °C. CMRF was aged under this condition, which was not comparable to treatment at 200 °C for 22 h (Figure S1).

Figure 1. (a) Structural evolution of Co@NCS-n and Co-CoOx@NCS-n, (b) SEM image of Co@NCS-II, (c-e) TEM and HRTEM images of Co@NCS-II, (f) SEM image of Co-CoOx@NCS-II, (g-i) TEM and HRTEM images of Co-CoOx@NCS-II, and the illustration in Figure (i) related to HRTEM, (j) high-angle annular darkfield scanning transmission electron microscopy (HAADF-STEM) image and the corresponding EDX-STEM element mapping images of Co-CoOx@NCS-II.


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The morphologies were analyzed by SEM and TEM. Co-MOF demonstrated a flake-like morphology (Figure S2a). When being composited with the RF resin, CMRF was transformed into a sphere (Figure S2b). After the calcination process, Co@NCS-II still remained spherical: the size of the single sphere was approximately 500 nm, and the particle size was 7.2 nm (Figure 1c and 1d). We adopted some measures to certify the structure of NPs on spheres in the experiments. Based on the XRD pattern, the broad peak at 25° (2θ) of NCS-II after acid etching of Co-CoOx@NCS-II was graphitic carbon, and the core-shell structure was clear (Figures S3 and S4). The NPs of Co-CoOx@NCS-II were highly dispersed on spheres after controllable oxidation, with a particle size of 6.2 nm (Figure 1g and 1h). The small particle size positively influences the hydrolysis of NH3BH3. The configuration of Co@NCS-II and Co-CoOx@NCS-II were determined by HRTEM (Figure 1e and 1i). Small particles and carbon layers were clearly. This further demonstrated that the NPs on Co@NCS-n or Co-CoOx@NCS-n were arranged in a core-shell structure. And the lattice fringes with a spacing of 0.205 nm, were matched to the (002) plane of Co (JCPDS Card No. 05-0702). The lattice fringes had spacing of 0.212 nm that was coherent to the (200) plane of CoO (JCPDS Card No. 75-041), and 0.240 nm may be likely to match the (222) plane of Co3O4 (JCPDS Card No. 42-1467).34,37,42 Finally, the broad peak at approximately 25° (2θ) was denoted as C (002). Other materials were harvested under different pyrolysis temperatures and denoted as Co@NCS-III, Co-CoOx@NCS-III and Aged-CMRF-I. They had similar morphologies as Co@NCS-II (Figure S5). According to XRD pattern analysis, some CoO and Co3O4 were observed after treatment at 200 °C in air (Figures S6 and 2a). The coexistence of Co and CoOx had an optimistic effect on hydrogen generation. Furthermore, energy dispersive X-ray spectroscopy (EDX) confirmed the existence and dispersion uniformity



of Co, O, N, and C in the catalysts, confirming that CoO and Co3O4 were produced during oxidation (Figure 1j).

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Figure 2. (a) XRD patterns of various catalysts, (b) Raman spectra of Co@NCS-II, Co-CoOx@NCS-II, Co@NCS-III, Co-CoOx@NCS-III, (c, d) fine XPS spectra of Co2p and O1s of Co@NCS-II and CoCoOx@NCS-II, (e) nitrogen adsorption and desorption isotherms of Co-CoOx@NCS-II and Co-CoOx@NCS-II, (f) pore size distributions of Co-CoOx@NCS-II and Co-CoOx@NCS-II calculated by NLDFT method.

The XRD patterns of Co-MOF, CMRF, Co@NCS-II and Co-CoOx@NCS-II are shown in Figure 2a. The diffraction peaks of Co-MOF corresponded to the XRD patterns of MOF-71,43 suggesting the formation and presence of Co-MOF as a self-assembled hybrid. The peak at 44.7° corresponded to (002) lattice facets of Co0 (JCPDS Card No. 05-0727).37 The peaks at 42.5° and 74.2° corresponded to (200) and (311) lattice facets of CoO (JCPDS Card No. 75-0418).42 The peaks at 19.0°, 31.2°, 36.8°, 38.5°, 55.6°, 59.3°, 65.2°, 77.3° and 78.4° corresponded with (111), (220), (311), (222), (422), (511), (440), (533) and (622) lattice facets of Co3O4 (JCPDS Card No. 42-1467).34 These results indicated the coexistence of Co, CoO and Co3O4 in Co-CoOx@NCS-II (Figures 2a and S6). Further details were determined using Raman spectra. The D bands were caused by the symmetric stretching vibration (radial breathing mode) within aromatic ring sp2 carbon atoms. The D bands were required to activate a defect. Therefore, the strength of the D band was generally used to measure the disorder of a structure. G bands are caused by first-order scattering of the E2g vibration mode of sp2 carbon rings. The intensity ratio ID/IG is often considered to be an important parameter to characterize the state of carbon atoms.44 The two broad peaks at approximately 1360 and 1594 cm−1 were ascribed to the D and G bands of the carbon, respectively (Figure 2b).45 The calculated ID/IG intensity ratios of the catalysts were 0.80 and 0.83, suggesting that graphite carbon accounted for a large percentage in the samples. Moreover, diffraction peaks at approximately 192, 474, 517, 614 and 682 cm−1 were caused by Co3O4, and corresponded to the five Raman-active modes (F2g, Eg, F2g, F2g, A1g) of Co3O4.46,47



The surface compositions of Co@NCS-II and Co-CoOx@NCS-II were further studied by XPS spectra. C1s, N1s, O1s and Co2p exist in these catalysts (Figure S7). The primary peaks, located at 299.38 eV (296.78 eV), 414.38 eV (411.78 eV), 546.38 eV (543.78 eV) and 815.38 eV (812.78 eV), were attributed to C1s, N1s, O1s and Co2p in Co@NCS-II (Co-CoOx@NCS-II), respectively (Figure S7). The improved O element in Co@NCS-II was generated due to the surface oxidation of Co. The Co2p spectrum of Co@NCS-II could be divided into some peaks. The peaks at 778.5 eV (Co2p3/2) and 793.5 eV (Co2p1/2) were assigned to Co0. The peaks at 780.9 eV (Co2p3/2) and 796.4 eV (Co2p1/2) were linked to Co2+, and the formation of Co2+ was due to surface oxidation of the samples.48 The peaks at 784.4 eV and 802.5 eV were shake-up satellite peaks.49 When compared with Co@NCS-II, another peak of Co3+ (781.8 eV) was probed in Co-CoOx@NCS-II; therefore, the Co element transformed into cobalt oxide during activation, and the other peaks are somewhat shifted. The two O1s peaks of Co@NCS-II could be identified as −OH (531.6 eV) and absorbed water (533.0 eV).34 Co-CoOx@NCS-II had additional peaks at Co=O (529.4 eV) and Co−O (530.4 eV) when compared with Co@NCS-II. This demonstrated that Co became CoOx after treatment at 200 °C in air (Figure 2c and 2d).50 The N1s peaks in these samples were broken down into pyridinic nitrogen (398.1 eV), pyrrolic nitrogen (399.2 eV), graphitic nitrogen (400.3 eV; Figure S8).49,51 The C1s peaks in Co@NCS-II corresponded to C=C (284.0 eV), C−C (284.5 eV), C−O (285.2 eV) and C−N (286.2 eV) bonds.52,53 The peaks in Co-CoOx@NCS-II exhibited little change compared to the precursors, namely, C=C (284.0 eV), C−C (284.8 eV), C−O (285.5 eV) and C=O (287.5 eV) bonds (Figure S9).54,55 Some element information from XPS is exhibited in Table S1. Based on images of the magnetism of four composites in the presence or absence of water (Figure S10), the existence of Co NPs revealed


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that the catalysts exhibited typical ferromagnetic behavior. It facilitates an effective magnetic separation of catalysts from the mixture system and benefits the catalytic reaction. The nitrogen adsorption-desorption isotherms measured at 77 K were used to estimate the specific surface areas and porous structures. The SBET of Co-CoOx@NCS-II and CoCoOx@NCS-III values were 64 and 308 m2·g−1, respectively (Figure 2e and f). Co-CoOx@NCSn had both micropores (

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