Mechanistic Insight into Formate Production via CO2 Reduction in C

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C: Energy Conversion and Storage; Energy and Charge Transport 2

Mechanistic Insight into the Formate Production via CO Reduction in C-C Coupled Carbon Nanotube Molecular Junctions Shubhadeep Pal, Sreekanth Narayanaru, Biswajit Kundu, Mihir Ranjan Sahoo, Sumit Bawari, D. Krishna Rao, Saroj Kumar Nayak, Amlan J. Pal, and Tharangattu N. Narayanan J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b08933 • Publication Date (Web): 23 Sep 2018 Downloaded from http://pubs.acs.org on September 24, 2018

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The Journal of Physical Chemistry

Mechanistic Insight into the Formate Production via CO2 Reduction in C-C Coupled Carbon Nanotube Molecular Junctions Shubhadeep Pal,[a] Sreekanth Narayanaru,[a] Biswajit Kundu,[b] Mihir Sahoo,[c] Sumit Bawari,[a] D. Krishna Rao,[a] Saroj K. Nayak,[c] Amlan J. Pal,[b] and Tharangattu N. Narayanan[a]* [a]

Tata Institute of Fundamental Research - Hyderabad, Sy. No. 36/P, Gopanapally Village,

Serilingampally Mandal, Hyderabad 500107, India. [b]

Department of Solid State Physics, Indian Association for the Cultivation of Science, Kolkata

700032, India. [c]

School of Basic Sciences, Indian Institute of Technology, Bhubaneswar 751013 India.

AUTHOR INFORMATION Corresponding Author [email protected] or [email protected]

ACS Paragon Plus Environment

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ABSTRACT

Electronic band structure modification of carbon nanotubes (CNTs) through Suzuki coupling has been predicted recently. Here, scanning tunneling microscopy/spectroscopy studies of a molecular junction developed through C-C coupled CNTs are conducted to probe the local density of states variation along the junction, and distorted band structure of CNT at the junction is unraveled, in tune with the predictions. The charge transfer, among CNTs and C6H4 in the junction, aided band structure modification helps an efficient *COOH adsorption in coupled CNTs (CCNT) than in pristine CNTs - via density functional theory based calculations. This indicates the possibilities of CCNT based electrochemical CO2 reduction. Formate production at low potential (-0.9V vs RHE) in neutral pH (6.8) is demonstrated with CCNT, while no formic acid production is observed in uncoupled CNTs. This study opens a fundamental insight into the development of novel catalysts based on carbon materials 'beyond doping' towards CO2 reduction via band engineering.


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TOC GRAPHICS

Carbon nanotubes based molecular junctions are developed and local density of states variations along the junction are mapped using scanning tunnelling spectroscopy. This molecular junction is demonstrated for selective formate production via electrochemical CO2 reduction at low overpotential, while no such liquid product is formed from uncoupled nanotubes. A mechanistic insight into this selective product formation is depicted in this article.


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1. Introduction Recently, authors studied the theoretical and experimental possibilities of covalently coupled junctions in carbon nanotube (CNTs) via Suzuki coupling, resulting in a porous 3D solid of CNTs.[1] An augmented hydrogen evolution reaction (HER) in acidic media is observed from these 3D solids in comparison to uncoupled CNTs, and charge transfer aided band structure modifications in CNTs are predicted as the possible reasons towards high performance.[1] Though the density functional theory (DFT) based calculations showed variation in the density of states (DOS) of CNTs when coupled through C6H4,[1] a direct experimental proof was lacking. Scanning tunneling microscopy/ spectroscopy (STM/S) can probe the local DOS (LDOS) of a molecular junction

[2,3,4]

and the modification in electronic band structure of CNTs can be

studied. Further, such a molecular junction will also be useful in developing molecular circuitries, those are highly important in molecular electronics and spintronic devices.[5 ,6, 7, 8, 9, 10, 11]

Electrochemical conversion of abundant molecules such as water, carbon dioxide, nitrogen etc. into high value products (hydrogen, hydrocarbons, ammonia etc.) is receiving tremendous scientific interest due to the importance of fossil free paths towards high energy density fuel production.[12,

13]

There is significant progress in the development of catalysts for

CO2 reduction, and metal-free systems are highly sought after.[14,15,16] Nitrogen is an efficient dopant in graphitic systems like carbon fibers, CNTs, graphene, and graphene QDs towards the development of an efficient CO2 reduction catalyst.[17,18] The role of nitrogen centers in carbon is to introduce charge density fluctuation in lattice to make the *COOH (radical) bonding effective


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for following the CO2 reduction reaction.[ 18,19] High yield C1 to C4 electrochemical products were reported from these nitrogen doped structures, though the roles of different nitrogen doping centers in graphitic lattice towards CO2 reduction are still contentious.[18]Meanwhile, selective separation of different products formed in a single reaction is also challenging. Selectivity of the catalysts towards a product varies drastically due to the competition among different products themselves (due to small energy landscape variations)[20,21] and also due to its competition with HER in aqueous medium.[22,23] Carbon monoxide (CO) is identified as the major product in many of the N doped systems, and formate production is identified in another N doped carbon system.[24] Recently, high yield formate production by electrocatalytic CO2 reduction reaction is reported from boron doped graphene system, where the role of boron doping is identified as the same as that of nitrogen doping.[25] Tuning the product selectivity by engineering the catalyst, and understanding the selective mechanism are highly important in designing a catalyst.[21,26] A recent theoretical work discusses the importance of nitrogen doping and curvature in deciding the limiting potential of CNT/graphene based system for producing practically important products such as CO and CH3OH.[27] CNTs are also pursued for CO2 reduction reaction catalysis, and Ajayan et al. reported a high yield production of CO (80%) in low over potential (η ~ -0.26 V) with aligned and N doped MWCNTs.[18] The superior activity of NCNTs is attributed to the low free energy barrier for the liming *COOH adsorption step in CO2 reduction reaction. Production of liquid fuels like formic acid/ethanol/methanol through CO2 reduction is desirable, but only metals like bismuth deliver formic acid selectively at very high over-potentials.[25] T. J Meyer et al. have reported NCNTs for 'selective and robust' electrocatalysis for CO2 reduction without metals to form formate.[24] They also report that polyethyleneimine (PEI) modification of NCNTs can further


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reduce the overpotential with high current density where PEI acts as a co-catalyst. They observed a high yield formate production (87%) at ~ -1.15V vs RHE in 0.1M KHCO3.[24] In this work, we report the formation of formate at low overpotential (thermodynamic potential + η: ~ -0.9 V vs RHE) in neutral medium (0.1M NaHCO3) from metal free, covalently coupled CNTs.[1] Coupled CNTs based electrocatalysis is found to result to selective formate production, as proven via electrochemical and NMR methods. DFT studies are conducted to understand the role of molecular junctions in *COOH adsorption and further reduction reaction, and a mechanistic insight into the phenomenon is given via band structure analyses and its experimental validation using STS studies. 2. Experimental Methods 2.1. Synthesis: Details of the molecular junction formation and porous 3D CNT developments are discussed in our previous report.[1]] MWCNTs are used in the present study, and an extensive washing and purification (to remove metal impurities in ppm level) is done before the electrochemical and STM/S studies. The purification of as-purchased MWCNTs resulted in their oxidation and the oxidized CNTs have been taken for comparison and are named as CNT_OH or pristine (bare) CNTs. CNT_OH is transformed to CNT-Cl through chlorination with thionylchloride refluxing and the halogenated CNTs are used for Suzuki coupling reaction forming large scale interconnected porous 3D CNTs,[28] and hereafter they are referred to as CCNT. The reaction in a nutshell is as follows: Multi wall Carbon nanotube (MWCNT) is purchased from Cheaptube Incorporation with inner diameter 5-10nm, 10-30 micron long tubes with purity 95% . It has electrical conductivity close to 100 S/cm which tells that MWCNT is metallic. To make interconnected CCNT, the following method is pursued: 100mg sample is refluxed under


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strong nitric acid for 1day at 1200C. This will remove loosely attached metals and bring hydroxy (-OH) and carboxy (-COOH) groups into the basal plane of MWCNT structure as well as into the edge or defective carbon. After that sample has been taken under refluxing with thionyl cloride (SOCl2 ) and 1,2-dichlorobenzene (C6H4Cl2) mixture at 800C. Then it is filtered with Tetrahydrofuran (THF) to remove excess SOCl2.

This will introduce Chlorine group by

eliminating oxigenated groups. Hence the final suzuki coupling reaction is performed with 40mg chlorinated sample, 60mg Palladium catalyst with 20mg 1,4-diboronic acid with 160mg cesium carbonate base medium in Tolune for 3days at 1100C. The final product is vaccuum filter several times with methanol and DI water (resistivity ~ 18Mohm) and at the final stage it is washed with 10X diluted aqua regia solution. 2.2. STM/S and Raman Studies: The STM/S measurements are carried out at ultra-high vacuum with a base pressure of the microscope chamber as 2.8×10-10 Torr. CCNT is dispersed in isopropyl alcohol (IPA) and a diluted solution is drop casted on a newly cleaved highly ordered pyrolytic graphite (HOPG) surface. STM measurement is carried out with Pt/Ir (80%:20%) tip with PAN style UHV-STM of M/s RHK Technology. During the approach of the tip, a current of 0.2 nA is set at 2.0 V. Differential tunnel conductance (dI/dV), which represents the LDOS of the material,[29] is recorded using a lock-in amplifier (16 mV rms, 935 Hz). Micro-Raman spectroscopy is carried out with Renishaw micro-Raman spectrometer with 532 nm (~2.33eV) laser excitation. 2.3. Electrochemistry: Three-electrode electrochemical studies are carried out by modifying a 3 mm glassy carbon electrodes with bare CNT or CCNT inks (working electrode) in 0.1M sodium bicarbonate (NaHCO3) solution with saturated Ag/AgCl as reference electrode and graphite paper as counter electrode. The electrochemical cell containing 20ml of 0.1M NaHCO3


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is continuously saturated with ultra-pure Argon (Ar) gas to pass out other unwanted gas interference (and control experiment) prior to the experiment, and then saturated with ultra-pure CO2 gas for reduction reactions. All cyclic voltammetry (CV) data reporting here are carried out at 50mV/s scan rate. 2.4. Product Analysis: Bulk electrolysis is carried out in 0.1 M NaHCO3 solution in presence and absence of CO2 using a homemade gas tight H-cell (figure S1). The counter electrode used here is a stainless steel (SS304) mesh, due to its high oxygen evolution capability.[30] Electrochemical surface area (ECSA) is calculated with Randles-Sevcik equation.[31] All the current densities reporting here are calculated using ECSA. The Faradaic efficiency (F.E., %) of foramate production is calculated using nuclear magnetic resonance (NMR) studies. NMR spectra are recorded on 300 MHz Bruker nano bay spectrometer with single pulse experiment using 300 flip angle and longer recycle delay to achieve complete relaxation of all the proton spin states to equilibrium, so that the product quantification will be accurate. Gas chromatography (Thermo Trace GC- 1110) is performed to study the formation of other gaseous products formed during the bulk electrolysis (figure S5). 2.5. Computational Details - Density Functional Theory (DFT):

Geometrical structure

optimization and electronic structure calculations of metallic SWCNT (5,5) (resembling metallic MWCNTs) with three unit cells and their C-C coupled structures have been studied using the abinitio DFT package SIESTA.[32] The band analyses and energy calculations are also verified with VASP, which is found to be consistent with SIESTA based analyses. Geometrical structure optimization and electronic structure calculations of single walled carbon nanotube (5,5) with three unit cells and coupled carbon nanotubes have been performed


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by using ' Spanish Initiative for Electronic Simulations with Thousands of Atoms ' (SIESTA) Package code

[32]

which is based on density functional theory (DFT).

[33,34]

Double zeta with

polarization (DZP) with split type basis set is used for the expansion of wave functions. FermiDirac occupation fuction is taken at electronic temperature 298k. The Perdew–Burke–Ernzerhof (PBE)

[35]

exchange-correlation functional of the general gradient approximation (GGA) was

approximated for the calculation. Kinetic energy cutoff of 500 Ry was used to describe the plane wave. Sufficient vacuum is provided in XY plane of the super cell to avoid spurious interactions between images produced due to periodic boundary condition. A k-mesh of 1x1x12 was used to sample the surface Brillouin zone in Monkhorst-Pack method

[36]

for geometry relaxation

calculation upto atomic force tolerance 0.004eV/Ang with every Molecular Dynamic (MD) step according to GC (Gradient Conjugate ) method in variable cell configuration whereas a higher k-grid of 1x1x24 was approached for density of states (DOS) calculation with fermi smearing energy 0.2eV. The self-consistent Kohn-Sham equation was solved iteratively until the energy difference is lower than 10-4 eV between two consecutive steps. Single walled carbon nanotube (5,5) with supercell 1x1x3 was taken into account for our study. Another system called dual-cnt-benzene which consists of two carbon nanotubes connected by a C6H4 linker where the plane of linker is perpendicular to axis of nanotube as shown in the Fig 4.C. COOH* functional is connected to both single nanotube and coupled nanotubes in various sites which are diagrammatically represented in the Fig4D.The systems were allowed to fully relax for structural optimization. The systems with ground sate energy are considered as stable configuration and proceeded for further electronic calculations. Band structure and density of states (DOS) of all above systems were calculated for comparison.


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3. Results and Discussion Figure 1A shows the STM image of a CCNT, where a Y-junction is identified. The longer tube arms are joined at the marked point (d) (figure 1B). This STM image resembles the molecular junction identified using high resolution transmission electron microscope (HR-TEM) images shown in our previous report (figure S8). The HOPG layers beneath are also visible in both pictures (figure 1A-B). This junction is used for LDOS studies and two far away locations indicated as (c) and (e) are also studied. From the locations (c) and (e), MWCNT like LDOS is expected while contrast is expected from the probable molecular junction at the point marked (d).


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Figure 1: STM images of CCNTs (A) and (B). LDOS measurements at the respective point (c), (d), and (e) are shown in (C), (D), and (E) respectively. (F) and (G) Histograms of valance band and conduction band populations at the respective points of CCNT. The dI/dVvsV characteristics are probed to map DOS.[29,37] Since the bias is applied to the sample with respect to the tip, electrons are injected to the conduction band at suitable positive voltages and the peaks in the LDOS spectra (at positive voltages) hence corresponded to the location of conduction band.[ 38,39] Similarly, the peak at the negative voltages imply withdrawal of electrons from the sample. Finite DOS is found at sample locations at (c) and (e) with valence band and conduction band positioned at -0.3 eV and +0.26 eV from Fermi level, respectively. This band gap of 0.5-0.6 eV shows MWCNT is metallic in nature (figure 1F). While the STM tip placed at position (d), shows a distorted band structure. The valence band and conduction bands positions are changed to -0.4 eV to +0.53 eV from Fermi level, where the Fermi level is shifted towards the valence band. And the band gap becomes ~0.9V (figure 1G). This is the result of the decrement in carriers from MWCNT and pushes band positions with no carriers at Fermi level.


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Figure 2: (A) DFT based energy relaxed structure of CCNT with the charge transfer from CNTs to the radical as inferred from the band structure alignment (work functions are mentioned). (B) Charge distribution on CNTs both coupled and uncoupled systems and their bond length (C-C) variations (C). (D) The micro-Raman spectra of CCNT and bare CNT (CNT_OH).

To understand the CNTs band structure modulation while coupling them through a molecule/radical, DFT based charge transfer studies are conducted. Figure 2A shows the energy relaxed structure of CCNT with the work function (W.F.) values of CNTs (5,5) and C6H4,


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calculated using DFT. The W.F. of CNT is calculated as 4.293 eV. The W.F. of C6H4 is calculated as 4.735 eV, indicating possible charge transfer from CNT to C6H4. Hirshfield charge population[40] studies show charge density distribution in carbon atoms of CCNTs, while the uncoupled CNT has no charge distribution (figure 2B). This indicates the charge transfer among CNTs and C6H4 (figure 2A). Further, the charge transfer from CNT(5,5) to C6H4 cause depletion of π electrons making it p-doped. This is reflected in the Hirshfield charge analyses, where the CNT surface acquires +0.04e positive charge while C6H4 molecule gets -0.08e negative charge. This causes the Fermi level to shift downward. The geometrical distortion happening at the interface is discussed in the later part. Suzuki coupling affects both geometry of the bonds and the charge density distribution, which can be useful in the adsorption of external molecules/radicals. The C-C bond distance distribution shows that all the 160 C-C bonds in 3 unit cells of CNT(5,5) are at 1.438 Å in relaxed structure, whereas 88 bonds out of 160 in the CCNT have been squeezed, hardening some of the covalent C-C bonds, which affects the phonon vibration modes (figure 2C).[41,42] This leads to an upshift of resonance Raman peak (G').[43] The representative Raman spectra of CNT and CCNT is shown in figure 2D. The G' resonance Raman mode changes more significantly if the electronic structure of CNT is distorted.[43] Collectively, an upshift in G' of ~9 cm-1 is observed in CCNT while other peaks are exactly match with CNT, indicating the band distortion in CCNT.


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Figure 3: (A) CVs at 50mV/s in 0.1M NaHCO3 with CNT and CCNT after saturating with CO2 gas. (B) Chronoamperometry study during bulk electrolysis set up. (C) NMR analyses of the electrolyte after bulk electrolysis (after 2 hours) using CNT and CCNT showing the formate formation in CCNT. (D) The F.E. of formate production calculated at different potentials using NMR. In order to verify the CO2 reduction efficacies of CCNT and CNT modified electrodes, the following experiments are conducted: 0.1M NaHCO3 has been taken as electrolyte having pH close to 8.3 which goes down to 6.8 when it gets saturated with CO2. For CV measurements, electrolyte is saturated with Argon (Ar) to avoid all unnecessary molecular interference for 1hrs


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with 20ml of electrolyte. This electrolyte is then saturated with CO2 and the CV is recorded using three electrode system (Ag/AgCl reference electrode) with RHE corrected scale as shown in figure 3A. An enhanced Faradaic current is observed with CCNT modified electrode (figure 3A, red), indicating that the CCNT electrode is kinetically more active than CNT with a low onset potential of -0.9V (where as that of CNT is -1.2V). Further, a bulk electrolysis at -0.9 V (RHE) is carried out in CO2 saturated 0.1M NaHCO3 solution as shown in figure 3B. It is clear from figure 3B that the current density is much higher (~7-8 times than CNT) for CCNT, indicating higher yield. The products formed (liquid) is analysed using NMR (figure 3C). The formation of formate (peak at 8.3 ppm) is evident in NMR spectrum of CCNT based electrolysis sample in CO2 saturated electrolyte, while no formate peak is observed in 0.1 M NaHCO3 alone based electrolysis [supporting information, figure S4], indicating that the formate production is due to the purged CO2 reduction. Further, the NMR data of the sample collected after the electrolysis (4hrs) in phosphate buffer (pH=6.8, which is devoid of carbonate or bicarbonate) is also shown in figure S4(B). The presence of formate is evident in the NMR analyses, indicating the formation of formate through the reduction of CO2 only. In both electrolysis conditions no formate peak is observed in CNT sample. A gas chromatography study is conducted to check other gas products (except H2) formed at this potential. The F.E. of formate production at -0.9 V using CCNT is found to be ~ 9%, while that of CNT is zero (figure 3D). The absence of any other product formation both in gas chromatography analyses and NMR indicate that the rest of the faradaic efficiency at -0.9 V (vs RHE) can be assigned to electrochemical hydrogen generation. Enhancement in coupling can be made by extensive oxidation and functionalization for cathodic material, and anodic electrode - SS304, can be modified or electrochemically


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deposited with non-precious metal-oxide having low OER potential[44] to enhance overall faradic efficiency, and these studies are in-progress. To achieve a theoretical insight into enhanced CO2 reduction at CCNT electrode at lower potential, change in the energy (∆E) of CNT-COOH and CCNT-COOH structures while forming from their individual structures are studied using DFT analyses (figure S9 Table S1 in supporting information. The geometrical distortion happening at the interface and the resultant bond length changes are shown in figure S10). The ∆E for *COOH attached to any carbon atom of CNT is found to be −0.88 eV (∆E=∆ECNT-COOH - ∆ECNT

–

∆ECOOH)[18]. In CCNT, *COOH placed at a

slightly positively charged carbon (+0.002e) has a ∆E of −1.01 eV. This indicates that CCNT has more facile *COOH adsorption and it is found that CNT-HCOOH has binding energy of ~-0.77 eV, whereas it lower for CCNT at ~-0.53, implying easier desorption.


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Figure 4. Relaxed geometry with electronic band structure along with DOS for (A) CNT (B) CNT-COOH (C ) CCNT (D) CCNT-COOH. Further theoretical insight to this facile process in CCNT is gained by band structure analyses using the same relaxed structures(figure 4). The symmetric band structure along high


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symmetry k points (X-G-X), the path along tube axis.[ 45, 46] Finite DOS at Fermi level, indicates the metallic nature of (5,5) armchair CNT. Coupling makes a distortion in the symmetric valence and conduction band alignments of CCNT (figure 4C). Introducing the COOH group, energy bands in CNT near the Fermi level becomes less dispersive (E-k curve flattens) and this indicates the higher effective mass of electrons making it less mobile for the reaction (figure 4B). In case of CCNT, the COOH functionalization makes the band more dispersive (E-k curves with higher slope, figure 4D) indicating mobile electrons at the Fermi level which can take part in the CO2 reduction reaction. 4. Conclusions Molecular junction of CNTs formed through the Suzuki coupling reaction is studied by STM/S and a detailed DFT based band structure analyses. The STS and DFT band structure analyses give similar result indicating the LDOS modification at the junction, opening up possibilities of such molecular species in molecular electronics. The electrochemical CO2 reduction in neutral pH at lower overpotential (-0.9 V vs RHE) is achieved with this coupled CNTs resulting to a liquid product - formate, while no such liquid product is formed with bare CNTs (CNT_OH). Molecular origin of this enhanced CO2 reduction reaction

from CCNT is unraveled and

identified as due to the charge transfer among CNT and C6H4 coupling radical, introducing charge densities in CNTs. This study uncovers the engineering possibilities of CNTs via covalent coupling and molecular junction formation towards the development of product selective catalysts for electrochemical reactions.


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SUPPORTING INFORMATION Electrode development and bulk electrolysis set up, FE% calculations, NMR (control experiments), GC method and data, micro-Raman analyses, TEM images of junctions (before and after purification –without Pd), and DFT details and data are available in supporting information.

ACKNOWLEDGMENT TNN, SP, SN, SB, and KR acknowledge Tata Institute of Fundamental Research for the financial support and the National Facility for High-Field NMR, TIFR Hyderabad for NMR analyses. SN acknowledges postdoctoral fellowship from SERB- NPDF ( PDF/2016/ 002763). MS and SKN acknowledge Ministry of Human Resources Development, India for support through Centre of Excellence on Novel Energy Materials.

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