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Atomically-Dispersed Co and Cu on N-doped Carbon for Reactions involving C-H Activation Jiahan Xie, James D. Kammert, Nicholas Kaylor, Jonathan Zheng, Eunjin Choi, Hien N. Pham, Xiahan Sang, Eli Stavitski, Klaus Attenkofer, Raymond R. Unocic, Abhaya K. Datye, and Robert J Davis ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b00141 • Publication Date (Web): 22 Mar 2018 Downloaded from http://pubs.acs.org on March 22, 2018
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Atomically-Dispersed Co and Cu on N-doped Carbon for
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Reactions Involving C-H Activation
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Jiahan Xie,a James Kammert,a Nicholas Kaylor,a Jonathan Zheng,a Eunjin Choi,a,b
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Hien N. Pham,c Xiahan Sang,d Eli Stavitski,e
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Klaus Attenkofer,e Raymond R. Unocic,d Abhaya K. Datye,c Robert J. Davis a,*
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a: Department of Chemical Engineering, University of Virginia, 102 Engineers’ Way, PO Box
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400741, Charlottesville, VA 22904-4741, United States
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b: Department of Emerging Materials Science, Daegu Gyeongbuk Institute of Science
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&Technology, Daegu, 42988, Republic of Korea
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c: Department of Chemical and Biological Engineering and Center for Micro-Engineered
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Materials, University of New Mexico, Albuquerque, NM 87131, United States
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d: Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, TN
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37831, United States
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e: National Synchrotron Light Source II, Brookhaven National Laboratory, Upton, NY, 11976,
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United States
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*Author to whom correspondence is addressed
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Phone: 1 - 434 - 924 - 6284
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Email:
[email protected] 22 23 24 25 1 / 40
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Abstract:
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Atomically-dispersed Co(II) cations coordinated to nitrogen in a carbon matrix (Co-N-C)
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catalyze oxidative dehydrogenation of benzyl alcohol in water, with a specific activity
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approaching that of supported Pt nanoparticles. Whereas Cu(II) cations in N-doped carbon
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also catalyze the reaction, they are about an order of magnitude less active compared to Co(II)
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cations. Results from X-ray absorption spectroscopy suggest that oxygen is also bound to
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N-coordinated Co(II) sites, but that it can be partially removed by H2 treatments at 523 – 750
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K. The N-coordinated Co(II) sites remained cationic in H2 up to 750 K and these stable sites
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were demonstrated to be active for propane dehydrogenation. In-situ characterization of Cu(II)
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in N-doped carbon revealed that reduction of the metal in H2 started at about 473 K,
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indicating a much lower thermal stability of Cu(II) in H2 relative to Co(II). The demonstrated
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high catalytic activity and thermal stability of Co-N-C in reducing environments suggests this
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material may have broad utility in a variety of catalytic transformations.
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Key words:
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Heterogeneous catalyst; C-H activation; Alcohol oxidation; Propane dehydrogenation;
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Non-precious metal; N-doped carbon; in-situ XAS; Coordination environment.
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Introduction
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The catalytic transformation of carbon feedstocks to liquid fuels and useful chemicals
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usually involves the activation of C-H bonds, whether the feedstock source is petroleum,
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natural gas or biomass.1–3 Precious-metal catalysts have been recognized for many years as
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possessing the features needed to activate C-H bonds.4,5 However, high costs limit their
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application in large-scale and distributed systems.6 Therefore, a strong motivation exists to
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discover or engineer earth-abundant elements in chemical environments that catalyze desired
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chemical transformations similar to those typical of precious metals. Recently, non-precious
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metals confined in a nitrogen-containing carbon matrix (M-N-C) were developed for the
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electro-catalytic oxygen reduction (ORR) and the water splitting reaction, in which
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comparable activity and stability to the well-known Pt catalysts were observed.7–12
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Considering its demonstrated redox activity, M-N-C catalysts were further applied in organic
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reactions involving oxidative C-H activation at mild temperature (usually lower than 400 K),
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such as dehydrogenation, coupling, esterification and nitrification.13–24
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The M-N-C catalysts are commonly prepared by a high-temperature pyrolysis method,
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which usually produces a variety of metal species simultaneously, such as exposed or
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encapsulated metal nanoparticles along with the atomically-dispersed metals in a variety of
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coordination environments. The co-existence of these different species complicates
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investigations into the nature of the catalytically-active sites. Catalysts predominantly
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containing atomically-dispersed Fe and Co, which also exhibited catalytic activity in ORR or
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C-H activation, were utilized to study the coordination environment and chemical state
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around the metal.17,19,22,25–44 Atomic dispersion of the metals on these catalysts has been
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verified by aberration-corrected scanning transmission electron microscopy (AC-STEM) and
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X-ray absorption spectroscopy (XAS). The chemical state and coordination environment
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were also probed by XAS, along with X-ray photoelectron spectroscopy (XPS), Mössbauer 3 / 40
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spectroscopy, electron energy loss spectroscopy (EELS) and density-functional theory
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(DFT).17,19,30–32,34,38,39,41,43,45 Although the existence of nitrogen-coordinated metal sites (MNx)
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has been well documented, the coordination number of N atoms to the active metal center is
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still unresolved.19,23,40,41,43 Additionally, in-situ XAS and XPS have been used to study
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Fe-based catalysts for ORR,32,34,37,39 but in-situ characterization of materials for chemical
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catalysis is lacking.
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In a recently-published work,21 atomically-dispersed Co and Cu in N-containing carbon
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exhibited the highest activity for catalytic alcohol oxidation among a series of M-N-C
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catalysts. In the current work, Co and Cu catalysts were comprehensively characterized and
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correlated with reaction rates to identify and quantify the active sites. In-situ XAS was
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performed to investigate the evolution of Co and Cu sites under oxidizing and reducing
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conditions at different temperatures (up to 750 K). In addition to oxidative dehydrogenation
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of benzyl alcohol at mild temperature (353 K), the dehydrogenation of propane to propene
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was demonstrated over Co-N-C.
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Experimental Methods
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Preparation of catalysts
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Black Pearls 2000 from Cabot Corporation and Davisil 636 silica from Sigma-Aldrich
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were used as the carbon and silica supports in this study. The aqueous ammonia was
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purchased from Fisher Scientific. All other chemicals were purchased from Sigma-Aldrich
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Corporation or Alfa Aesar and gases were from Praxair Inc. distributed by GTS-Welco. The
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M-N-C catalysts were prepared by a modified pyrolysis method.13,21 First, an ethanol solution
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of 1,10-phenanthroline was mixed with an aqueous solution of Co(NO3)26H2O or
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Cu(NO3)23H2O with a 1:2 molar ratio of metal to 1,10-phenanthroline. The mixture was
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stirred for 20 min at 353 K and added dropwise to slurry of carbon black in a 0.1 M NaOH 4 / 40
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aqueous solution under vigorous stirring at 353 K. After 2 h, the slurry was cooled, filtered,
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washed thoroughly with distilled, deionized water (DI water) and dried overnight at 343 K.
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The filtered medium was colorless, indicating the efficient adsorption of metal complex onto
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the carbon support. The prepared materials were subsequently impregnated with an acetone
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solution of dicyandiamide and dried at 343 K overnight. The amount of impregnated
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dicyandiamide relative to carbon support was 40 wt% for samples with 1 wt% metal loading
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or 80 wt% for catalysts with a 3 or 5 wt% metal loading. The obtained solid was pyrolyzed at
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873, 973 or 1073 K for 2 h with a ramping rate of 10 K min-1 under ultra-high-purity N2 flow
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(100 cm3 min-1). To investigate the influence of ammonia treatment,10,44 Co and Cu catalysts
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pyrolyzed at 973 K with 1 wt% nominal metal loading were treated at 1223 K in NH3 with a
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ramping rate of 10 K min-1 for 2 h under NH3 flow (100 cm3 min-1). To remove the
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nanoparticles that were observed on the optimized Co-N-C, the catalyst was treated in 1 M
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HCl solution overnight at room temperature, filtered, washed thoroughly and dried at 393 K.
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The acid-treated catalyst was denoted as Co-N-C-HCl. To prepare a metal-free
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nitrogen-doped carbon (N-C), an acetone solution of dicyandiamide (0.4 g) was impregnated
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into carbon black (0.5 g) followed by a pyrolysis step at 973 K for 2 h under a dinitrogen
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flow of 100 cm3 min-1. As references, supported Co (5 wt%) and Cu (1.5 wt%) nanoparticles
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were prepared by incipient wetness impregnation using an aqueous solution of
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Co(NO3)26H2O or Cu(NO3)23H2O as the metal precursor and silica, carbon black or N-C as
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the support. After impregnation, the Co precursors on carbon or N-doped carbon were
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decomposed by thermal treatment in N2 flow (100 cm3 min-1) at 973 K. The C or
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N-C-supported Cu nanoparticles were prepared by reducing the impregnated Cu(NO3)2 under
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H2 flow at 623 K. The nanoparticles on Co/SiO2 were prepared by reducing the impregnated
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support under H2 flow at 923 K. Nitrogen-doped carbon layers were subsequently added to
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carbon-supported Co and Cu nanoparticle by a modified pyrolysis methods using 5 / 40
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4-aminoantipyrine as the N-C precursor.46,47 Additionally, a well-dispersed 2.5 nominal wt%
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Co/SiO2 catalyst was synthesized by an ion exchange method described elsewhere (denoted
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as CoOx-SiO2).48,49 Briefly, 4.875 g of acid-washed silica was dispersed in 98 cm3 of DI water
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under vigorous stirring and then heating to 373 K. A solution containing 0.573 g Co(NH3)6Cl3,
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2.765 cm3 aqueous ammonia, and 104 cm3 DI water was added dropwise over 10 min to the
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SiO2 slurry, followed by stirring at 373 K for 1 h. Afterwards, the mixture was cooled to room
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temperature, washed in DI water, dried, and calcined in 100 cm3 min-1 air at 573 K for 2 h
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after heating with a 1 K min-1 ramp rate. As references, the commercial carbon-supported 3
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wt% Pt nanoparticles (Pt/C) (from Sigma-Aldrich) with a surface Pt fraction of 0.5950 were
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evaluated in alcohol oxidation reaction. Silica-supported Pt nanoparticles (Pt/SiO2) were
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prepared by incipient wetness impregnation (details described elsewhere51) with a surface Pt
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fraction of 0.40 and evaluated in the propane dehydrogenation reaction
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Characterization of catalysts
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The X-ray diffraction (XRD) patterns were recorded using a PANalytical X'Pert Pro
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MPD (Multi-Purpose Diffractometer) instrument with Cu Kα radiation (45 kV, 40 mA) and
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scanning of 2θ from 10° to 70° with a step size of 0.0025° at a rate of 0.5° min−1.
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The scanning transmission electron microscopy (STEM) was performed on an FEI Titan
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80-300 operating at 300 kV or a JEOL 2010 F instrument operating at 200 kV. High angle
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annular dark filed (HAADF) STEM images were acquired using a Nion UltraSTEM 100
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operated at 60 kV and equipped with a probe aberration corrector, to achieve a spatial
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resolution of 1.1 Å. A convergence angle of 31 mrad was used, with HAADF detector inner
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and outer collection angles of 86 mrad and 200 mrad, respectively. The electron energy loss
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spectroscopy (EELS) was collected with a dispersion of 0.5 eV using Gatan Enfina
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spectrometer. 6 / 40
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The metal loading on Cu-N-C and Co-N-C-HCl was determined by inductively coupled
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plasma atomic emission spectroscopy (ICP-AES, performed by Galbraith Laboratories (2323
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Sycamore Drive, Knoxville, TN 37921).
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The XPS was performed on a Thermo Scientific ESCALAB 250 spectrometer equipped
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with a focused monochromatic Al Kα X-ray radiation source (1486.6 eV) and a
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hemispherical analyzer with a 6-element multichannel detector. The incident X-ray beam was
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45° off normal to the sample while the X-ray photoelectron detector was normal to the
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sample. A large area magnetic lens with a 500 µm spot size in constant analyzer energy mode
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was utilized with a pass energy of 20 eV for region scans. Charge compensation was
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employed during data collection with an internal electron flood gun (2 eV) and a low energy
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external Ar ion flood gun. The binding energy of the C 1s peak assigned at 284.5 eV, which is
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attributed to the support, was used to reference the peak positions.52
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X-ray absorption spectroscopy (XAS) at the Co K and Cu K edges was collected at
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beamline 8-ID at the National Synchrotron Light Source II, Brookhaven National Laboratory,
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operating at a ring energy of 3.0 GeV and beam current 325 mA. The Cu and Co metal foils
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(Goodfellow Corporation) were used as references at the Cu (8979 eV) and Co (7709 eV) K
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edges,
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(NH3)6CoCl3, Co(II) phthalocyanine, CuO, Cu(NO3)23H2O, and Cu(II) phthalocyanine
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powders were crushed and pelletized with boron nitride to provide an absorption edge jump
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of near unity. Spectra were collected by suspending the pellets in the beam path inside a
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temperature controlled transmission chamber with Kapton windows. A gas mixture of 5% H2
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in N2 or 10% O2 in N2 was flowed through the transmission chamber at 100 cm3 min-1 to
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provide the reducing and oxidizing conditions, respectively. The XAS data were processed
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using the Demeter software package created by Bruce Ravel.53 The Co-N, Co-O Cu-N/O and
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Cu-Cu amplitude reduction factors were obtained from fitting to Co phthalocyanine, Co3O4,
respectively.
Chemical standards including CoO,
Co(NO3)26H2O, Co3O4,
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Cu phthalocyanine and Cu foil standards.
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Oxidative dehydrogenation of benzyl alcohol
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The high-pressure, semi-batch alcohol oxidation reactions were performed in a
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50 cm3 Parr Instrument Company 4592 batch reactor with a 30 cm3 glass liner. An aqueous
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benzyl alcohol solution (10 cm3) and catalyst were added to the glass liner. The glass liner
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was inserted into the reactor, sealed, purged with Ar, heated to 353 K and initiated by
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pressurizing the reactor with O2. The conversion of alcohol during the initial heating stage
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was negligible. Liquid samples were periodically taken, and the catalyst was removed using a
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0.2 µm PTFE filter before product analysis with a Waters e2695 high performance liquid
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chromatograph (HPLC) equipped with refractive index detector. Product separation in the
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HPLC was accomplished on an Aminex HPX-87H column (Bio-Rad) operating at 318 K
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with an aqueous 5 mM H2SO4 solution as the mobile phase flowing at 5 cm3 min−1. The
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retention times and calibration curves were determined by injecting known concentrations of
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standards. The carbon balance was greater than 90 % unless otherwise stated.
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The amount of catalyst added to the reactor was chosen so that the alcohol oxidation rate
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would not be limited by O2 mass transfer from the gas to the liquid.50 Selectivity to a specific
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product is defined as moles of that product formed divided by moles of all products produced.
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The initial reaction rates were calculated from the initial conversions of the alcohol (573 K), the Co(II) cations remained atomically-dispersed and
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coordinated to nitrogen in the carbon at up to 750 K. The stable Co(II) cations in N-doped
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carbon catalyze the dehydrogenation of propane to propene at 773 K. Cu (II) cations confined
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in nitrogen-doped carbon also catalyze aerobic alcohol oxidation at 353 K but the turnover
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frequency of the highly active CuNx sites is an order of magnitude lower than that associated
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with CoNx. Although Cu(II) cations remained in the N-doped carbon matrix at 373 K under
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reducing conditions, partial reduction of the Cu was observed above 473 K. These findings
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provide new insights into the structural and chemical properties that may allow Co-N-C and
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Cu-N-C catalysts to become desirable replacements for a variety of precious metal catalysts.
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Associated Content
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Supporting Information
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The Supporting Information is available free of charge on the ACS Publications websites at
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DOI: (TO BE ADDED AFTER ACCEPTANCE)
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Additional analytical and characterization data including optimization of Co and Cu catalysts, 18 / 40
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alcohol oxidation rates over different material, X-ray photoelectron spectra, X-ray absorption
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spectra, XRD patterns, STEM images and EELS mapping.
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Acknowledgements
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This work is supported by the US NSF under grant numbers EEC-0813570 (Center for
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Biorenewable Chemicals, CBiRC) and CBET-1157829. A portion of the microscopy research
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was conducted at the Center for Nanophase Materials Sciences in Oak Ridge National Lab,
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which is a DOE Office of Science User Facility. This research used resources of the National
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Synchrotron Light Source II, a U.S. Department of Energy (DOE) Office of Science User
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Facility operated for the DOE Office of Science by Brookhaven National Laboratory under
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Contract No. DE-SC0012704. Helpful discussion with Zhongwen Luo, Prof. T. Brent Gunnoe
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and Prof. Matthew Neurock are acknowledged.
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MN4C4
Single-Atom
Catalysts
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2002, 106, 4714–4720.
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O.; Curtiss, L. A.; Stair, P. C.; Miller, J. T.; Hock, A. S. Selective Propane
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Dehydrogenation with Single-Site CoII on SiO2 by a Non-Redox Mechanism. J. Catal.
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650
Tables and Figures
651
Table 1. Kinetic isotope effect for benzyl alcohol oxidationa Catalyst
Co-N-C
Cu-N-C
Reaction
RateD -6
-1
RateH/ -1
(10 mol s gcat )
RateD
C6H5CD2OH+H2O
2.3
2.9
C6H5CD2OH+H2O+0.1 M NaOH
10
1.6
C6H5CD2OH+H2O
0.57
2.8
C6H5CD2OH+H2O+0.1 M NaOH
3.9
1.8
652
a. Reaction conditions: 10 cm3 0.05 M benzyl alcohol aqueous solution, 10 mg catalyst, 1
653
MPa O2, 353 K. Conversion was less than 20% when calculating the rate.
654
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655
Table 2. Weight loadings of metal sites associated with atomically-dispersed Co and Cu
656
catalysts Overall metal loading (wt%) Catalyst
ICP-AESa
XPSb
Co-N-C-HCl
1.5
1.3±0.2
Cu-N-C
1.2
0.5±0.1
657
a. The relative standard deviation of ICP-AES measurement is less than 10% (provided by
658
Galbraith Laboratories).
659
b. The metal loadings are derived from the quantitative XPS analysis shown in Table S3.
660
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661
ACS Catalysis
Table 3. Rates of benzyl alcohol oxidation over reference materialsa Entry
Metal loadingb
Catalyst
Rate (10-6 mol s-1 gcat-1)
(wt%) 1
CoPhen/C
1
0.013
2
CuPhen/C
1
0.010
3
N-C
−
0.041
4
Cu/C
1.5
0.035
5
Cu/N-C
1
0.022
6
N-C-Cu/C
1.5
0.024
7
Co/C
5
0.040
8
Co/N-C
5
0.085
9
N-C-Co/C
5
0.062
10
Co/SiO2
5
0.0026
11
CoOx-SiO2
2.5
0.0025
12
Pt/C
3
30
3
662
a. Reaction conditions: 10 cm 0.05 M benzyl alcohol aqueous solution, 1 MPa O2, 353 K,
663
rates were calculated based on the alcohol conversion after 15 min of reaction.
664
b. The nominal metal loadings are calculated based on the amounts of precursors for catalyst
665
preparation.
666
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667
Table 4. Results from the analyses of Co EXAFS from Co-N-C-HCl catalyst before and after
668
H2 treatmenta Conditions As-prepared After H2 treatmentb
Shell Coordination number
r (Å)
∆σ2 (Å2)
∆E0 (eV)
Co-N
2.4±0.9
1.91±0.03
0.005±0.003
-0.3±0.6
Co-O
2.1±0.9
2.09±0.03
0.005±0.005
-0.3±0.6
Co-N
2.4±0.9
1.89±0.03 0.0041±0.003 -0.3±0.6
Co-O
1.2±0.9
2.06±0.05 0.0037±0.005 -0.3±0.6
669
a: Fitting parameters: Fourier transform range, ∆k, 2-14 Å-1; fitting range ∆R, 1-2.2 Å;
670
k2-weighting, S02(Co-N/O) = 0.775 (Calibrated from Co phthalocyanine). The fitted Fourier
671
transforms are present in Figure 6.
672
b: Co-N-C-HCl catalyst was treated in H2 from room temperature to 750 K and then cooled to
673
313 K.
674
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675 676
Figure 1. Influence of adsorbed acids on metal sites on the reaction rate of benzyl alcohol
677
oxidation over a) Co-N-C and b) Cu-N-C catalysts. Reaction conditions: 10 cm3 0.1 M benzyl
678
alcohol aqueous solution, 10 mg catalyst, 353 K, 1 MPa O2, the reaction rates were calculated
679
from the conversion after 15 min of reaction. The results from linear fit of first four points
680
(dashed lines) are presented in Table S6.
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681 682
Figure 2. a) Normalized XANES at the Co K-edge and b) the k2-weighted Fourier transform
683
(not corrected for phase shift) of Co EXAFS associated with Co-N-C-HCl at room
684
temperature, at 373 K under 10% O2 and at 373 K under 5% H2; and c) normalized XANES
685
at the Cu K-edge and d) the k2-weighted Fourier transform (not corrected for phase shift) of
686
Cu EXAFS associated with Cu-N-C at room temperature, at 373 K under 10% O2 and at 373
687
K under 5% H2.
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Figure 3. AC-STEM images of a) Co-N-C-HCl and b) Cu-N-C.
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690 691
Figure 4. a) Co 2p3/2 X-ray photoelectron spectrum of Co-N-C-HCl; b) Cu 2p3/2 X-ray
692
photoelectron spectrum of Cu-N-C and N 1s X-ray photoelectron spectra of c) Co-N-C-HCl
693
and d) Cu-N-C.
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Figure 5. a) Normalized XANES at the Co K-edge and b) the k2-weighted Fourier transform
696
(not corrected for phase shift) of Co EXAFS associated with Co-N-C-HCl under 5% H2 flow
697
from 373 to 750 K; c) normalized XANES at the Cu K-edge and d) the k2-weighted Fourier
698
transform (not corrected for phase shift) of the Cu EXAFS associated with Cu-N-C under 5%
699
H2 flow from 373 to 573 K.
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700 701
Figure 6. a) The k2-weighted experimental and fitted Fourier transforms of Co EXAFS
702
associated with Co-N-C-HCl as-prepared and after H2 treatment at 750 K followed by
703
cooling in H2 to 313 K and b) the contributions of different paths including Co–N and Co–O
704
in k-space for the Co-N-C-HCl, as-prepared and after H2 treatment at 750 K followed by
705
cooling in H2 to 313 K. The fitting parameters are presented in Table 4.
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706 707
Figure 7: a) Conversion of propane and b) nominal TOF during propane dehydrogenation
708
over 0.05 g Co-N-C-HCl and 0.1 g CoOx-SiO2. TOF was based on total Co loading in the
709
sample. Reaction conditions: 773 K, 0.1 MPa, N2:propane = 3:1.
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