Carbon-Involved Near-Surface Evolution of Cobalt Nanocatalysts: An in Situ Study

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLE1 Nov 2021Carbon-Involved Near-Surface Evolution of Cobalt Nanocatalysts: An in Situ Study Feng Yang†, Haofei Zhao†, Wu Wang, Qidong Liu, Xu Liu, Yuecong Hu, Xinrui Zhang, Sheng Zhu, Dongsheng He, Yingying Xu, Jiaqing He, Rongming Wang and Yan Li Feng Yang† Beijing National Laboratory for Molecular Science, Key Laboratory for the Physics and Chemistry of Nanodevices, State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871 Department of Chemistry, Guangdong Provincial Key Laboratory of Catalytic Chemistry, Southern University of Science and Technology, Shenzhen, Guangdong 518055 , Haofei Zhao† Beijing Advanced Innovation Center for Materials Genome Engineering, Center for Green Innovation, Beijing Key Laboratory for Magneto-Photoelectrical Composite and Interface Science, School of Mathematics and Physics, University of Science and Technology Beijing, Beijing 100083 , Wu Wang Department of Physics, Southern University of Science and Technology, Shenzhen, Guangdong 518055 , Qidong Liu Beijing National Laboratory for Molecular Science, Key Laboratory for the Physics and Chemistry of Nanodevices, State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871 , Xu Liu Beijing National Laboratory for Molecular Science, Key Laboratory for the Physics and Chemistry of Nanodevices, State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871 , Yuecong Hu Beijing National Laboratory for Molecular Science, Key Laboratory for the Physics and Chemistry of Nanodevices, State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871 , Xinrui Zhang Beijing National Laboratory for Molecular Science, Key Laboratory for the Physics and Chemistry of Nanodevices, State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871 , Sheng Zhu Beijing National Laboratory for Molecular Science, Key Laboratory for the Physics and Chemistry of Nanodevices, State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871 , Dongsheng He Core Research Facilities, Southern University of Science and Technology, Shenzhen, Guangdong 518055 , Yingying Xu Beijing Advanced Innovation Center for Materials Genome Engineering, Center for Green Innovation, Beijing Key Laboratory for Magneto-Photoelectrical Composite and Interface Science, School of Mathematics and Physics, University of Science and Technology Beijing, Beijing 100083 , Jiaqing He Department of Physics, Southern University of Science and Technology, Shenzhen, Guangdong 518055 , Rongming Wang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Beijing Advanced Innovation Center for Materials Genome Engineering, Center for Green Innovation, Beijing Key Laboratory for Magneto-Photoelectrical Composite and Interface Science, School of Mathematics and Physics, University of Science and Technology Beijing, Beijing 100083 and Yan Li *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Beijing National Laboratory for Molecular Science, Key Laboratory for the Physics and Chemistry of Nanodevices, State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871 https://doi.org/10.31635/ccschem.020.202000595 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail When carbon-containing species are involved in reactions catalyzed by transition metals at high temperature, the diffusion of carbon on or in catalysts dramatically influences the catalytic performance. Acquiring information on the carbon-diffusion-involved evolution of catalysts at the atomic level is crucial for understanding the reaction mechanism yet also challenging. For the chemical vapor deposition process of single-walled carbon nanotubes (SWCNTs), we recorded in situ the catalyst state (solid and molten) composition as well as near-surface structural and chemical evolution at the cobalt catalyst–tube interface with carbon permeation using aberration-corrected environmental transmission electron microscopy and synchrotron X-ray absorption spectroscopy. The nucleation of SWCNTs was linked with an alternating dissolving and precipitating cycle of carbon in catalysts close to the nucleation site. Understanding the dynamics of carbon atoms in catalysts brings deeper insight into the growth mechanism of SWCNTs and facilitates inferring mechanisms of other reactions. The methodologies developed here will find broad applications in studying catalytic and other processes. Download figure Download PowerPoint Introduction Transition metals (Fe, Co, Ni, Cu, etc.) are in widespread use as heterogeneous catalysts for many reactions of both scientific and industrial interest. For example, such catalysts play a leading role in petrochemical engineering processes, including Fisher–Tropsch synthesis,1,2 selective hydrogenation of alkyne,3,4 and reforming and pyrolysis of methane.5 Carbon atoms in transitional-metal catalysts may act as either promotors or poisoners. At low carbon content, carbon atoms can induce the formation of metal carbide intermediates, which then serve as catalytically active sites.4,6–10 However, at high carbon content, graphite or coke can emerge on top of the metal surface, and even surround and encapsulate metal nanoparticles, fully blocking the active sites.1 Transition metals are also generally applied as highly efficient catalysts for the synthesis of carbon nanomaterials, including SWCNTs11–21 and graphene.22,23 The structure and composition of catalysts are crucially important and determinative for achieving high selectivity in diameter and chirality for SWCNT growth.12,13,24–32 During the chemical vapor deposition (CVD) process, carbon atoms may diffuse on the surface or into the body of transitional-metal nanoparticles, resulting in a diverse and heterogeneous composition and structure of the catalysts.27,30,33–35 Hence, the identity of the active catalytic species remains controversial. The carbon enrichment in metal nanoparticles also modifies their electronic and wetting properties,34 which in turn leads to different growth modes of SWCNTs on catalysts.36 In the controlled growth of graphene, the selection of metal substrate with suitable affinity to carbon is also a key issue. A precise tuning of carbon solubility in catalysts by combining different transition metals precisely controlled the production of mono- or bilayer graphene.23,37 These reports highlight the importance of the interaction between metal and carbon, which needs further investigation. The key challenge is to determine the behavior of carbon atoms in metal catalysts, especially to obtain atomic scale and dynamic information under a reactive environment. In situ and dynamical study are effective methods to monitor the structural evolution of the catalyst in the reaction process and provides quantitative information about the catalyst size, shape, and coordinations.38–40 Techniques based on environmental transmission electron microscopy (ETEM)41–47 are powerful tools for the investigation of such a process at high spatial and temporal resolution. The in situ imaging by ETEM have revealed the nucleation process of multiwalled carbon nanotubes (MWCNTs) associated with the morphological evolution of highly deformable catalysts.41,48–58 However, less is known about the structural and compositional evolution of catalysts at atomic scale. SWCNTs normally grow at higher temperature, and, with lower carbon feeding rate than MWCNTs, SWCNTs are more brittle to the electron-beam irradiation. Therefore, it is more challenging to study in situ the evolution of catalysts at atomic scale under carbon feeding during the growth of SWCNTs. There have been many controversial hypotheses and theoretical simulations on the growth process of SWCNTs.59 However, the atomistic mechanisms of SWCNT growth via in situ observation are still uncertain. For example, the state and composition of active catalyst (solid, liquid or metastable, and metallic state or carbide), the pathway of carbon atom diffusion (on the surface or through the catalyst bulk), and the nucleation site of SWCNTs (on carbon-rich or poor region of catalyst, on flattened surface or step of catalyst) are still debated.33,60–63 These uncertainties bring great difficulties for in situ studies of the growth kinetics of SWCNTs.64 Three groups reported the nucleation of SWCNTs on metal carbides Co2C, Co3C, and Fe3C, respectively,33,61,63 but Picher et al.61 found a different approach of SWCNT formation where the single-layer graphene converts to a curved cap on the atomic step of the Co2C nanocrystal when MgO supported Co–Mo catalyst was used. They also reported that Co was converted to Co–Co2C mixture with lattice strain using the same catalyst at a slightly higher temperature. The carbon-rich phase (Co2C) was generally in contact with the support (MgO), and the carbon-poor phase (metallic Co) was in contact with the SWCNT rim,62 which was different from other reports.61,63 The contradiction of the real nucleation sites is most probably due to the lack of understanding of carbon atom diffusion and interaction in the catalyst as well as near-surface composition. One of the challenges for probing the carbon atoms in catalyst is the experimental difficulty of atomically and chemically resolving the catalyst structure and composition. Recently, with the dramatic progress of aberration-corrected ETEM techniques, scientists can acquire more in situ information on the gas–solid reaction-induced catalyst surface and subsurface evolution.65–69 Many factors, such as the overlapped lattice of the catalyst and the support60 and electron-beam irradiation,70 bring great complexity to the in situ observation. Moreover, the near-surface lattice strain was frequently observed in bimetallic nanoparticles owing to the metal elemental diffusion and outward segregation.69,71–74 Although TEM imaging is useful to investigate the atomic structure, it is neither chemically sensitive nor capable of differentiating the elements. The feasible way to differentiate elements is to simultaneously combine atomic imaging with electron energy-loss spectroscopy (EELS), thereby allowing for unambiguous identification of the elemental composition. Such an ability to capture the dynamics of carbon involved structural evolution under reaction conditions is particularly important for understanding the origins of SWCNT growth. Moreover, ETEM is useful for investigating individual catalyst nanoparticles one-by-one, and X-ray-based spectroscopic techniques can be the complementary means to offer overall information about the chemical and structural evolution of working catalysts. Herein, taking the catalytic synthesis of SWCNTs as a model process, we investigate the diffusion behavior of carbon atoms in the near-surface of cobalt catalysts under carbon feeding conditions by using a state-of-the-art aberration-corrected ETEM. The insufficient carbon supply is a general problem in ETEM study due to the low pressure in the ETEM chamber and the tiny heating area of the in situ chip-heating system. Here, we used cobalt acetate [Co(OAc)2] as both the catalyst precursor and the solid carbon source. The in situ decomposition of acetate groups upon heating brings about a localized carbon feeding high enough to facilitate the growth of SWCNTs. Through atomic-scale visualization by ETEM and elemental identification by in situ EELS, we found that the cobalt catalyst (composed of Co–Co3C) was activated through carbon dissolving in near-surface. The dynamic structural evolution of catalyst revealed that the carbon dissolution and precipitation in cobalt facilitated the growth of SWCNTs. These findings were also proved by in situ ambient-pressure synchrotron X-ray absorption spectroscopy (XAS). Experimental Methods ETEM experiments The in situ study was conducted by using an aberration-corrected Titan G2 80-300 (Hillsboro, Oregon, USA) ETEM (operated at 80 kV) with a resolution of 1.0 Å. The ETEM chip with amorphous SiOx/SiNx membrane was mounted onto a micro-electro-mechanical system-based micro heater (ThermoFisher Scientific, Waltham, MA). The Co(OAc)2 catalyst precursor was dissolved in ethanol (0.1 mmol·L−1) and dropped onto the ETEM chips. We used two methods to prepare Co catalyst and perform in situ SWCNT growth. (1) The as-prepared specimen was first heated to 600 °C for 30 s and stabilized in high vacuum (∼10−5 Pa) at 600 °C for 10 min with the electron-beam off. Then, TEM images and videos were captured with the electron-beam on. During annealing at 600 °C, the Co(OAc)2 decomposed to metallic Co nanoparticles, carbon, and other reductive species (CO, C2H6, and H2). The carbon containing species acted as a carbon source to feed the growth of carbon nanotubes. (2) The as-prepared specimen was first heated to 500 °C in pure O2 (50 Pa) for 30 min to form Co3O4 and reduced in H2 (50 Pa) at 700 °C for 30 min to form metallic Co catalysts. Then, CH4 (99.99%), CH4/H2 (v/v=7/3), or CO (99.999%) was introduced to the TEM chamber to grow SWCNTs at 600–750 °C at a pressure of 28–503 Pa. The pure metallic Co catalyst prepared from (2), which contained no carbon, was also used to compare the near-surface lattice spacings with catalyst containing carbon. The length of carbon cap and SWCNT were determined from the top of the cap or tube to the end of the graphitic wall in contact with the catalyst nanoparticle. In situ scanning TEM-EELS experiments In situ scanning TEM (STEM)-EELS data were collected in another, but otherwise the same, aberration-corrected Titan G2 80–300 ETEM. It was equipped with a Gatan image filter (Quantum 936) with an energy dispersion of 0.25 eV operated at an acceleration voltage of 300 kV. The zero-loss EELS spectrum was acquired immediately after obtaining the core-loss EELS spectra on individual nanoparticles. The obtained EELS data were further analyzed by Digital Micrograph software (Pleasanton, USA). The position of core-loss EELS was corrected with the corresponding zero-loss peak and the extrapolated background was then subtracted from the edge of interest. A Fourier-ratio deconvolution was performed to remove the effect of plural scattering. Two-dimensional Gaussian fitting of TEM image The two-dimensional (2D) Gaussian fit to the intensity distribution of the atomic columns from high-resolution TEM (HRTEM) images was conducted using the StatSTEM V3.0 code run on MATLAB (R2015b; Natick, Massachusetts, USA).74 More details are shown in Supporting Information Figure S6 and corresponding discussion. Ex situ X-ray photoelectron spectroscopy The X-ray photoelectron spectroscopy (XPS) experiments were performed using an AXIS Supra/Ultra Imaging X-ray Photoelectron Spectrometer (Manchester, United Kingdom). The binding energy of the XPS spectra was calibrated to that of C 1s (284.6 eV). It was performed and exposed to a monochromated Al Kα (1486.6 eV). We prepared Co catalysts and performed low-pressure (1.79 kPa) ethanol-CVD growth of SWCNTs at 900 °C. Scanning election microscopy (SEM) and TEM images of as-grown SWCNTs are shown in Supporting Information Figure S5. In situ synchrotron XAS The X-ray absorption fine structure (XAFS) spectra at Co K-edge were obtained at BL14W-1 beamline of Shanghai Synchrotron Radiation Facility operated at 3.5 GeV. For in situ X-ray absorption near-edge spectroscopy (XANES) measurements, a homemade CVD apparatus was equipped in BL14W-1. The quartz tube was strictly parallel to the beam direction. The MgO supported Co catalyst (Co 5 wt %) thin flake was inserted into the quartz tube, facing the beam direction at 90°. The in situ Co K-edge XANES spectra were collected in a transmission mode at 700 °C with the energy range of 7513–8509 eV. First, we introduced H2/He (10% H2) (200 cm3·min−1) into the furnace and acquired the XANES spectra. The gas was switched to CH4/He (10% CH4) (20 cm3·min−1) and was introduced into the furnace to react for 15 min, then the XANES spectra were obtained. The data were interrupted using Athena and Artemis codes to remove background, normalize edge-step, and fit the extended XAFS (EXAFS) data using standard procedures. The absorption edge of Co foil (E0 = 7709 eV) was used to calibrate the energy edge of all samples. Results and Discussion In situ observation of the growth of SWCNTs from cobalt catalysts The Co(OAc)2 dispersed on the SiO2/SiNx membrane of the ETEM chip was annealed under vacuum (∼10–5 Pa) at 600–750 °C for 10 min (Experimental Methods). The Co(OAc)2 decomposed to CoO and further transformed to metallic Co as revealed by the in situ selected-area electron-diffraction patterns shown in Supporting Information Figure S1. In this process, the carbon species or other carbon-hydrogen species generated from the decomposition of the acetate groups became reductants, which was confirmed by mass spectrometry equipped in ETEM ( Supporting Information Figure S2). Meanwhile, the carbon species also acted as the carbon source to feed the growth of SWCNTs. Figures 1a–1d show a sequenced frame extracted from a video of a single-shell graphitic cap nucleated from an ∼4 nm Co nanoparticle. At a moment designated as 0 s, a graphitic shell formed on the Co nanoparticle surface, with a pair of diffraction spots indicated by boxes in fast Fourier transformation (FFT)75 corresponding to lattice spacing of face-centered-cubic (fcc) Co (Figure 1e). At 41 s, the Co nanoparticle became structurally inhomogeneous. Two new pairs of diffraction spots indicated by circles, 2.5 and 2.0 Å in spacing with 90° interplanar angle, appeared and accorded well with the diffraction pattern of Co3C carbide (ICSD 617389, space group: Pnma, a = 5.07 Å, b = 6.70 Å, c = 4.53 Å, α = β = γ = 90°) (Figure 1f, Supporting Information Table S1). Indeed, they neither corresponded to fcc/hexagonal-close-packing (hcp) Co nor Co2C. Then the graphite shell lifted off at 59 s to form a cap with a size of ∼1 nm (Figures 1c, 1d, 1g, and 1h). The active catalyst nanoparticle remained structurally inhomogeneous with the co-existence of Co and Co3C phases. To avoid the effect of electron-beam irradiation, we also performed the nanotube growth with the electron-beam off and then captured the ETEM images with the electron-beam on. Figures 1i and 1j show typical single-shot ETEM images of individual SWCNTs nucleating from cobalt catalysts at 600 °C. The FFT derived from the interface of catalyst-nanotube also shows the diffraction spots of metallic Co and Co3C (Figure 1k). More active catalysts containing Co and Co3C are shown in Supporting Information Figures S3 and S4. In previous reports, metallic Co,60,76 Co–Co2C,62 and Co3C33 had been assigned as active species in growing SWCNTs. The carbon feeding conditions might cause the contradiction of the identity of the active species from different reports.20 Our observations in ETEM demonstrated generally that inhomogeneous fcc/hcp Co–Co3C were active species for nucleating SWCNTs. Figure 1 | (a–h) Time-sequenced ETEM images of a cap growing from cobalt catalyst acquired at 750 °C (a–d) and corresponding FFT of the particle region (e–h). (i–k) ETEM images of SWCNTs grown on catalysts (i and j) and FFT (k) of the particle region marked in (j). (l) Schemes showing each stage of nucleation and corresponding catalyst species. (m) Length of the cap as a function of growth time measured from sequential ETEM images as shown in Supporting Information Figure S3. Catalyst composition was determined from the FFT in each snap point. Download figure Download PowerPoint To gain deeper insight into the growth dynamics of SWCNTs, we linked the catalyst evolution to the nucleation rate of SWCNTs (Figures 1l and 1m). The nucleation rate was plotted as a function of the SWCNT length versus growth time (Figure 1m). The catalyst structure of each snap point was determined from FFT of time-sequenced ETEM images ( Supporting Information Figure S3). It is obvious that the rate-limiting step was the incubation of graphitic shell on metallic Co nanoparticle, which lasted approximately 120 s. The catalyst maintained the metallic state without Co3C species during the incubation process. Then in the follow-up stages, the formation of structurally inhomogeneous Co–Co3C at ∼120 s in of the carbon the active catalyst nanoparticle remained inhomogeneous containing Co and The time 120 s was as the incubation time for the nucleation of which was under low pressure of carbon The of incubation time been by the of carbon atoms dissolving into catalyst nanoparticles and These observations that carbon dissolving and the of Co3C might be in the nucleation of cap and To further the general role of catalysts, we performed the XPS to the composition of catalysts after growth of SWCNTs ( Supporting Information Figure The spectra were recorded at approximately Co where the background was subtracted by the The two with low intensity at and eV were to Co and Co of metallic The Co peak at eV and the peak at eV were to Co3C (Figure between Co and was which further indicated the of a It is obvious that both Co3C and Co in the catalysts. XAS was also used to investigate the structure of cobalt catalysts. In situ XANES of MgO supported Co catalysts recorded at Co K-edge was collected under H2 and CH4 at 700 °C, The of Co under H2 a first peak of Å observed from the of cobalt The first of and Å appeared when to CH4 which were to and based on fitting (Figure in situ and situ X-ray were with ETEM observations that the Co catalysts were to form Co–Co3C during the growth of SWCNTs. Figure | Ex situ Co XPS of catalysts after the growth of SWCNTs on The and Co3C chemical were The the peak of the Co3C In situ Co K-edge Fourier of of Co catalysts under H2 and CH4 at 700 °C. Co3O4 nanoparticles were used as and performed at temperature. Download figure Download PowerPoint of Co by carbon spatial permeation To determine the distribution of Co3C species in active catalyst nanoparticles during the growth of SWCNTs, we investigation on the interface of Figure shows a single-shot ETEM image of a nanotube growing from a cobalt nanocrystal at 600 °C. FFT of this nanoparticle was as inhomogeneous Co–Co3C (Figure which with previous to the of Co spots marked with circles, the resulting FFT the region of which was the catalyst (Figure However, derived from the of Co3C spots marked with boxes an distribution of Co3C in the catalyst (Figure In we found from that the Co3C region was close to the nucleation site of the nanotube (Figure marked by in situ data that the Co3C species to the nucleation site of nanotubes ( Supporting Information Figure S3). Figure | high-resolution ETEM image showing a SWCNT nucleating on cobalt catalyst acquired at 600 °C. FFT of the particle region marked by in and formed using spots of Co and Co3C in corresponding of Co3C is marked by in 1 Download figure Download PowerPoint To the carbon dissolved in the catalysts, we a of the lattice spacings of the of catalysts. Figure shows a high-resolution ETEM image of a graphitic cap on Co catalyst at 600 °C. The corresponding FFT a resolution information of Å. We assigned atomic columns to and Co3C (Figure which the co-existence of Co and A fit to the intensity of distribution of the atomic columns was used to acquire the precise which the resolution to been used in previous The spacings were determined from and then for each shell ( Supporting Information Figure Figure the of the lattice of Co catalysts. The ( 1 1 1 Co lattice spacings of the active catalyst nanoparticle with a cap from Å to Å from the region to the Such an lattice of the surface be by the carbon atoms dissolving into Co We the lattice to the of pure metallic Co nanoparticles of size without to carbon The pure metallic Co nanoparticles under H2 show obvious lattice the surface at 600 °C ( Supporting Information Figure These indicated that the lattice of Co nanoparticles during carbon feeding were by the carbon dissolving in nanoparticles. We also analyzed the surface lattice spacings of other active catalysts of either or A general lattice of from the surface the region was observed in all the active catalysts (Figure The of the active catalyst nanoparticles were in the range of The lattice show obvious on the catalyst size ( Supporting Information Figure For we also measured the near-surface spacings of catalyst without It was found that the surface = a spacing and the lattice spacings were close to theoretical ( ( 1 1 1 Co = Å (Figure Supporting Information Figure that carbon atoms diffuse into the of Co nanoparticles. Figure | The ETEM image and of the position of individual atomic columns by Gaussian The of spacings is as from surface to FFT of near-surface of this nanoparticle. The ( 1 1 1 Co lattice spacings of active and catalysts as a function of sequential atomic from the surface to region of lattice measured from both and Co active catalysts. The of lattice was by lattice measured from different of the same catalyst as shown in Figure The the measured from surface to region to were from the standard