Supplementary Materials for - Science Advances · 2019-10-25 · Fig. S4. Studies of the rotation...
Transcript of Supplementary Materials for - Science Advances · 2019-10-25 · Fig. S4. Studies of the rotation...
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Supplementary Materials for
Dissociating stable nitrogen molecules under mild conditions
by cyclic strain engineering
Gao-Feng Han, Xiang-Mei Shi, Seok-Jin Kim, Jeonghun Kim, Jong-Pil Jeon, Hyuk-Jun Noh, Yoon-Kwang Im, Feng Li, Young Rang Uhm, Chul Sung Kim, Qing Jiang*, Jong-Beom Baek*
*Corresponding author. Email: [email protected] (J.-B.B.); [email protected] (Q.J.)
Published 1 November 2019, Sci. Adv. 5, eaax8275 (2019)
DOI: 10.1126/sciadv.aax8275
This PDF file includes:
Supplementary Materials and Methods Supplementary Text Fig. S1. Comparison of the as-prepared samples in N2 and Ar. Fig. S2. GNPs after different nitrogenation times. Fig. S3. GNPs after different nitrogenation times. Fig. S4. Studies of the rotation speed of ball milling, the loading amount of graphite, the critical reaction pressure, and temperature of the ball-mill container. Fig. S5. Ball milling by ZrO2 balls. Fig. S6. The poisoning experiments. Fig. S7. The self-nitrogenating protection phenomenon of Fe balls. Fig. S8. The transfer experiments. Fig. S9. Extension to other carbon materials. Table S1. Elemental analysis of the GNP.
Supplementary Materials
Supplementary Materials and methods
Checkout pressure tightness of the container
In order to prevent any gas leakage during ball-milling, the pressure tightness of the container was
carefully checked by the following combined methods:
1. The working condition simulation. We checked the pressure tightness under actual
working conditions, except filled with inert Ar gas (9 bar). Due to the increase in surface
area during ball-milling, the as-prepared sample will adsorb gas, especially in the
beginning. Thus, this test was divided into two stages. In the first stage, ball-milling for 10
h, the change in surface area was negligible. After an additional 10 h ball-milling, the
volume change of charged gas should be less than 10 mL. This test was conducted in the
beginning.
2. Immersing the whole container in water. After being charged with N2 gas (8 bar), the
container was immersed in water, and held for 20 min to make sure no bubbles appeared.
We conducted this test two times, before and after ball-milling. The test after ball-milling
was to detect any leakage caused by the vigorous agitation. Unlike the previous working
conditions simulation test, this test was conducted for every experiment.
Calculation of consumed gas
The consumed gas (Vconsumed) was calculated by determining the difference in gas volume before
(Vcharged) and after ball-milling (Vremant).
Vconsumed = Vcharged – Vremant (S1)
The charged gas before ball-milling (Vcharged) was first measured by the water displacement
method, and then the container was filled again at the same pressure. The amount of remnant gas
after ball-milling (Vremant) was measured by the water displacement method.
The consumed gas (Vconsumed) was composed of two parts: one is the chemically fixed (Vchem); the
other is physically adsorbed (Vphys).
Vconsumed = Vchem + Vphys (S2)
It was noted that a small amount of the consumed gas was associated with physical adsorption
(Vphys). This was caused by an increase in the surface area of graphitic nanoplatelets (GNP) during
unzipping and delamination by ball-milling. The physically adsorbed gas (Vphys) was about 13.7
mL g−1
, which is approximately 3 times higher than the values determined with BET
measurements (4.6 mL g−1
). The possible reason is that there were many free edge sites before
exposure to air. Oxidation of the free edge sites would deteriorate the capacity for physical
adsorption.
Although the volume of consumed gas (Vconsumed) cannot be directly translated into the N content
in GNP, it can be used as an indirect indicator of N content. Since the preparation method is the
same, it results in the same amount of physical adsorption.
Characterizations
The material microstructures were characterized by high-resolution transmission electron
microscope (HR-TEM, JEM-2100). The surface morphology of the Fe balls was observed with
optical polarization microscopy (Leica DM 2500P). X-ray absorption spectroscopy was measured
at 6D UNIST-PAL beamline in the Pohang Accelerator Laboratory (Pohang, South Korea). X-
Ray diffraction data (XRD) were collected on a D/max2500V (Rigaku, Japan) using Cu-Kα
radiation (λ = 1.5418 Å) at a scan rate of 4° min−1
. Physical adsorption was analyzed by the
Brunauer-Emmett-Teller (BET) method using nitrogen adsorption-desorption isotherms on a
Micromeritics ASAP 2504N.
Fourier transform infrared spectra (FTIR) were recorded on a Perkin-Elmer Spectrum 100. The
samples were first mixed with potassium bromide (KBr, Sigma-Aldrich) by grinding, then were
pressed into pellets. The Raman spectra were obtained on a WITec Alpha300R with laser
wavelength of 532 nm.
The elemental analysis (EA) was performed on an Element Analyzer (TruSpec Micro CHNS,
LECO Corp.). All data were the average value of at least three analyses. The chemical
composition and bond natures were characterized by X-ray photoelectron spectra (XPS) on a
Thermo Fisher K-alpha XPS spectrometer. The deconvolution of the N 1s spectra was performed
by XPSPEAK 41, programmed by Raymund W.M. Kwok, the Chinese University of Hong Kong.
Time-of-flight secondary ion mass spectrometry (TOF-SIMS, TOF.SIMS5, IONTOF GmbH,
Germany) was used to analyze the trace composition of the Fe balls, and the as-prepared GNP.
The primary ion species was Bi with a dose of 2.0 × 109, and the raster area was about 100*100
µm2 for the Fe balls and 400*400 µm
2 for the as-milled GNP.
The Fe loading was determined by thermogravimetric analysis (TGA, STA8000, PerkinElmer
Instruments Co. Ltd, USA) at a heating rate of 5 °C min−1
in air. The thermal stability was
measured at a heating rate of 20 °C min−1
in N2. The sample loading was typically 30 µg. The
TGA was conducted by compensating buoyancy effect, which is caused by the change in gas
density. The buoyancy effect was eliminated by subtracting the background curve, which was
measured under the same conditions except without the sample.
Mossbauer spectra were taken in transmission mode with a conventional constant acceleration
spectrometer at room temperature. A 50 mCi 57
Co(Rh) was used as the γ-ray source. The isomer
shifts were calibrated by α-Fe foil. The Mossbauer spectra were fitted by using a least-squares
computer program. The isomer shift (IS), quadrupole shift (QS), hyperfine field at the site of the
Fe nucleus (H), and relative spectral area of Fe species were derived from the fitted spectra.
Supplementary Text
The method of calculating the thermal-equilibrium bulk concentration of N in the bulk
The thermal-equilibrium bulk concentration of N (wt.) at 1 bar of N2 is given by (15):
N (wt.) = 0.098 exp(− ΔHs ∕ RT) (S3)
where, R is the gas constant (1.987 cal mol−1
K−1
), the ΔHs (= + 7.2 kcal mol−1
) is the enthalpy for
the reaction:
1∕2 N2, gas ↔ N* (S4)
When T is room temperature (298 K, 25 °C), the mass percentage of N is calculated as 5.14 ×
10−7
. After transferring to atom percentage, the theoretical thermal-equilibrium bulk concentration
of Fe powders is 2.05 × 10−6
.
The method of calculating N content according to XRD patterns
The lattice constant has a relation with the doped content of N as follows (28):
a0, N = a0 + D (at.% N) (S5)
where the a0, N is the lattice constant after N doping, the a0 is the lattice constant of pure Fe, a0 =
2.8664 Å. D is the conversion factor.
The a0, N can be calculated with the combination of Bragg's law (Equation S6) and interplanar
spacing dhkl function (Equation S7):
n λ = 2 dhkl sinθ (S6)
dhkl = a0, N ∕ (h2 + k
2 + l
2)1∕2
(S7)
where the n is a positive integer, λ is the diffracted wavelength of Cu Kα, λ = 1.5418 Å. dhkl is the
distance between two adjacent and parallel planes of atoms, and h, k, and l are the Miller indices.
Here, we selected the strongest (110) facets in the XRD pattern. Since the sum of h, k, and l is
odd, thus n = 1. θ is the scattering angle, and the XRD pattern shows that θ is 22.052 °.
The a0, N is 2.9015 Å after calculation.
The D is derived from our DFT using Equation S5.
D = (a0, N, DFT − a0, DFT) ∕ (at.% N)
In our DFT model, the percentage of N is 1.8182%. The difference in lattice constant (a0, N, DFT −
a0, DFT) is 0.0180 Å. The D is calculated as 0.0099 Å.
Finally, in terms of Equation S5, the calculated N content is 3.5 at.%. This means that the N
atoms in the Fe powders are at thermal nonequilibrium.
The method of calculating nitrogen fixation yield
The nitrogen fixation yield is defined by normalizing the total amount of fixed nitrogen (N in the
obtained carbon materials) to the surface of Fe balls and the milling time. The calculation method
is as follows:
(S8)
where, the φnorm is the N content normalized to C (at∕at%), the M is the mass loading of graphite
(M = 15 g), the mN is the standard atomic weight of nitrogen (14), the mC is the standard atomic
weight of carbon (12), the N is the total numbers of the steel ball (N = 4477 for Φ 3 Fe balls, N =
962 for Φ 5 Fe balls), the A is the surface area of Fe balls (A = 0.283 cm2 for Φ 3 Fe balls, A =
0.785 cm2 for Φ 5 Fe balls), the T is the ball-milling time (h).
When the ball milling time is 3 h, the calculated fixation yield is 272.3 and 459.7 μg cm−2
h−1
for
Φ 3 and Φ 5 Fe balls, respectively.
The method of calculating maximum nitrogen fixation efficiency
The maximum fixation efficiency could be given by the following equations:
Fixation efficiency = 𝑃−𝑃𝑐𝑟𝑖𝑡
𝑃 (S9)
where the P is the pressure of total N2 supplied, and the Pcrit is the critical reaction pressure in the
reaction (0.8 bar). If the initial pressure is 8 bar, the maximum fixation efficiency is 90 %.
Fig. S1. Comparison of the as-prepared samples in N2 and Ar. XPS survey spectra of the
samples, which were prepared using Fe balls with diameters of (A) 3 mm and (B) 5 mm. The
FTIR spectra of the samples, which were prepared using Fe balls with diameters of (C) 3 mm and
(D) 5 mm. The Raman spectra of the samples, which were prepared using Fe balls with diameters
of (E) 3 mm and (F) 5 mm. The nitrogenation method was ball-milling graphite (15 g) with Fe
balls (Φ = 3 mm or 5 mm) in N2 or Ar (8 bar) at a rotation speed of 500 rpm for 10 h.
800 700 600 500 400 300 200
O 1s N 1s
Inte
nsity (
a.u
.)
Binding energy (eV)
N2, 5 mm
Ar, 5 mmC 1s
800 700 600 500 400 300 200
O 1s N 1sIn
tensity (
a.u
.)
Binding energy (eV)
N2, 3 mm
Ar, 3 mm
C 1s
4000 3000 2000 1000
N2, 5 mm
Tra
nsm
itta
nce (
a.u
.)
Wavenumber (cm-1)
Ar, 5 mm
u C-O
u C=Cu OH
u OH u CºNu C=Cu C-N, C-O
500 1000 1500 2000 2500 3000 3500
N2, 3 mm
Inte
nsity (
a.u
.)
Raman shift (cm-1)
Ar, 3 mm
4000 3000 2000 1000
u OH
N2, 3 mm
u C-N, C-Ou C=C
u OH
Tra
nsm
itta
nce (
a.u
.)
Wavenumber (cm-1)
Ar, 3 mm
u CºN
u C=C
u C-O
500 1000 1500 2000 2500 3000 3500
N2, 5 mm
Inte
nsity (
a.u
.)
Raman shift (cm-1)
Ar, 5 mm
BA
DC
FE
Fig. S2. GNPs after different nitrogenation times. (A) The XPS survey spectra. (B) The XPS N
1s spectra. (C) The XPS C 1s spectra. (D) The FTIR spectra. (E) The Raman spectra. (F) The
XRD patterns. The nitrogenation method was ball-milling graphite (15 g) with Fe balls (Φ = 3
mm) in N2 (8 bar) at a rotation speed of 500 rpm.
292 290 288 286 284 282
C 1s
30 h10 h3 h2 h
Inte
nsity (
a.u
.)
Binding energy (eV)
1 h
C-N
800 700 600 500 400 300 200
30 h
10 h
3 h
2 hIn
tensity (
a.u
.)
Binding energy (eV)
1 h N 1sO 1s
C 1s
10 20 30 40 50
30 h
10 h
3 h
2 h
Inte
nsity (
a.u
.)
2q ()
1 h
500 1000 1500 2000 2500 3000 3500
0 h
1 h
2 h
3 h
30 h
10 h
Inte
nsity (
a.u
.)
Raman shift (cm-1)
4000 3000 2000 1000
1 h
2 h
3 h
30 h
10 hT
ransm
itta
nce (
a.u
.)
Wavenumber (cm-1)
u OHu CºN
u C=C
u C-N, C-O
406 404 402 400 398 396
30 h
10 h
3 h
2 h
Inte
nsity (
a.u
.)
Binding energy (eV)
1 hN 1sBA
DC
FE
Fig. S3. GNPs after different nitrogenation times. (A) The FTIR spectra. (B) The Raman
spectra. The nitrogenation method was ball-milling graphite (15 g) with Fe balls (Φ = 5 mm) in
N2 (8 bar) at a rotation speed of 500 rpm.
500 1000 1500 2000 2500 3000 3500
0 h
1 h
3 h
30 h
10 h
Inte
nsity (
a.u
.)
Raman shift (cm-1)
4000 3000 2000 1000
1 h
30 h
10 h
3 h
Tra
nsm
itta
nce (
a.u
.)
Wavenumber (cm-1)
u OH u CºN
u C=C
u C-N, C-O
B
A
Fig. S4. Studies of the rotation speed of ball milling, the loading amount of graphite, the
critical reaction pressure, and temperature of the ball-mill container. (A) The nitrogen
content in as-prepared nitrogenated GNP as a function of rotation speed. The rotation speed was
selected at 500, 400, 300, and 200 rpm, respectively. The nitrogenation method was ball-milling
graphite (15 g) with Fe balls (Φ = 3 mm) in N2 (8 bar) for 6 h. (B) The nitrogenation method was
ball-milling graphite (5, 10, 15, and 20 g) with Fe balls (Φ = 5 mm) in N2 (8 bar) at a rotation
speed of 500 rpm for 3 h. (C) The void in the container was calculated by the slope of the charged
gas volume (Vcharged) as a function of charged pressure. The N2 volume was measured by water
displacement. The obtained void was about 242.4 mL. (D) Results of the critical reaction pressure
study, after ruling out the physical adsorption. The graphite was first pretreated by bal-milling for
6 h in N2 (8 bar) to make the surface area stable. The further nitrogenation method was ball-
milling graphite (15 g) with Fe balls (Φ = 5 mm) in N2 (2.0 bar, true pressure) at a rotation speed
of 500 rpm for 3 h. The N2 volume and N2 pressure are the truth volume and pressure in the
container. (E) The normalized content of N to C, determined by EA. The container temperature
was adjusted by changing the ball-milling and waiting (cooling) time. The nitrogenation method
was ball-milling graphite (15 g) with Fe balls (Φ = 5 mm) in N2 (8 bar) at a rotation speed of 500
rpm for 3 h.
200 300 400 5000
3
6
9
Norm
. N
to C
(at
at%
)
Rotation speed (rpm)
R2 = 0.996
0
2
4
6
mill 10 min, wait 30 min
Norm
. N
to C
(at
at%
)
mill 60 min, wait 10 min
0
200
400
600
N2 V
olu
me (
mL)
Before nitrogenation
After nitrogenation
0.0
0.5
1.0
1.5
2.0
2.5
N2 p
ressure
(bar)
Independent experiments
2 4 6 8500
1000
1500
2000
Vcharg
ed (
mL)
Pressure (bar)
Slope = 242.4
R2 = 0.9992
0
30
60
90
120
150
Norm
. V
consum
ed (
mL g
-1)
5 10 15 20
0
2
4
6
8
10
Loading of graphite (g)
Fe c
onte
nt
in G
NP
(w
t%)
A C
B
D
E
Fig. S5. Ball milling by ZrO2 balls. (A) The nitrogen content in as-prepared nitrogenated GNP
for Fe and ZrO2 balls. The nitrogenation method was ball-milling graphite (15 g) with Fe or ZrO2
balls (Φ = 3 mm) in N2 (8 bar). For fair comparison, the input kinetic energy (MFe ω2
Fe = MZrO2 ω2
ZrO2) and total rotation number (TFe ωFe = TZrO2 ωZrO2, 180,000 cycles) are same. The rotation
speed are 300 rpm and 353 rpm for Fe and ZrO2 balls, respectively. (B) The Zr3d XPS spectrum
of the worn-off ZrO2 debris in GNP.
190 188 186 184 182 180 178
ZrNOx
Counts
(a.u
.)
Binding Energy (eV)
Zr3dB
0
1
2
3
4
ZrO2N
orm
. N
to C
(at
at%
)Fe
A
Fig. S6. The poisoning experiments. A small amount of triethanolamine (TEA), glass powders,
KCl, and Na2S were separately added with graphite before ball-milling. The nitrogenation method
was ball-milling graphite (15 g) with Fe balls (Φ = 5 mm) in N2 (8 bar) at a rotation speed of 500
rpm for 3 h.
0
300
600
900
1200
KClNa2
STEA Glass
Vconsum
ed (
mL)
N/A
Fig. S7. The self-nitrogenating protection phenomenon of Fe balls. (A) Digital photos of the
Fe balls. (B) Optical microscope image after ball-milling in N2 gas. (C) Optical microscope image
after ball-milling in Ar gas. The darkened zones are the remnant graphitic nanoplatelets, or
oxidized Fe particles. Even after careful rinsing, the surface cannot be completely cleaned. The
anti-abrasion ability was derived from the Fe content in nitrogenated GNP. (D) The Fe content in
GNP. (E) The enhanced factor. The Fe content was determined by TOF-SIMS for Φ 3 mm, and
TGA for Φ 5 mm. The nitrogenation method was ball-milling graphite (15 g) with Fe balls (Φ = 3
mm or 5 mm) in N2 or Ar (8 bar) at a rotation speed of 500 rpm for 10 h.
10-5
10-4
10-3
10-2
10-1
100
Fe c
onte
nt
in G
NP
(w
t/w
t)
N2 Ar
F 5*
F 3
F 5*
F 3
0
1
2
3
Enhanced f
acto
r (v
s in N
2)
F 3 F 5*
B
A
E
D
C
Fig. S8. The transfer experiments. (A) XPS survey spectra of the mixture Fe and GNP (Fe +
GNP) in well transferred GNP. (B) The high-resolution C 1s of the Fe + GNP mixture in well
transferred GNP. (C) The FTIR of the transferred GNP. (D) The TGA of the transferred GNP in
N2. The buoyancy effect was eliminated by subtracting the background during TGA
measurements.
200 400 600 80080
85
90
95
100
Weig
ht
(%)
Temperature (°C)
-0.05
-0.04
-0.03
-0.02
-0.01
0.00
Transferred GNP
Derivative w
eig
ht
(%/°
C)
4000 3000 2000 1000
u organic carbonate
u C-H
d C-H
u C-N
r C-H
Tra
nsm
itta
nce (
a.u
.)
Wavenumber (cm-1)
u C=C
u CºN
u OH
296 294 292 290 288 286 284 282
Fe + GNP
Inte
nsity (
a.u
.)
Binding energy (eV)
Transffered GNP
800 700 600 500 400 300 200
C 1sFe 2p O 1s
N 1s
Inte
nsity (
a.u
.)
Binding energy (eV)
Fe + GNP
Transferred GNP
BA
DC
Fig. S9. Extension to other carbon materials. The nitrogenation method was ball-milling
carbon black (10 g) or graphite (15 g) with Fe balls (Φ = 5 mm) in N2 (8 bar) at a rotation speed
of 500 rpm for 30 h. The N2 was recharged every 10 h.
0 5 10 15 20 25 300
50
100
150
200
Norm
. V
consum
ed (
mL g
-1)
Milling time (h)
Graphite Carbon black
Table S1. Elemental analysis of the GNP. The content of O and Fe are not provided here
because they could not be detected by our EA device. The O originates from the oxidation of free
edge sites after exposure to air. The Fe content was measured using TGA or TOF-SIMS.
No. Total
time
(h)
Ball
diameter
(mm)
Gas Elemental content (wt%) Norm. N
(at∕at%)
Comments
C H N
1 1∕3 3 N2 93.9 - - -
Φ 3 mm, different
time in N2
2 1 3 N2 85.5 - 2.7 2.7
3 2 3 N2 72.3 0.3 4.0 4.7
4 3 3 N2 80.0 0.3 5.5 5.9
5 6 3 N2 78.3 0.3 7.8 8.6
6 10 3 N2 74.6 0.5 9.7 11.1
7 30 3 N2 71.7 0.2 12.0 14.3
8 10 3 Ar 94.0 - 0.7 0.65
9 1∕3 5 N2 93.0 - 0.9 0.79
Φ 5 mm, different
time in N2
10 1 5 N2 86.9 0.7 2.6 2.5
11 2 5 N2 84.5 0.7 4.5 4.6
12 3 5 N2 83.3 0.7 5.8 6.0
13 6 5 N2 83.3 0.7 8.3 8.5
14 10 5 N2 78.4 0.3 10.4 11.4
15 30 5 N2 75.4 0.7 14.4 16.3
16 3 5 N2 84.7 0.8 5.6 5.7 Long cooling time
17 10 5 Ar 80.4 2.7 4.7 5.0 Transferred GNP
18 3 5 N2 85.5 0.5 5.7 5.7 TEA
19 3 5 N2 84.7 0.4 5.6 5.6 Glass
20 3 5 N2 74.3 0.4 2.4 2.8 KCl
21 3 5 N2 77.1 0.6 1.5 1.6 Na2S
22 6 3 N2 86.6 0.5 6.0 6.0 400 rpm
23 6 3 N2 89.9 0.3 3.5 3.3 300 rpm
24 6 3 N2 97.4 0.2 1.5 1.3 200 rpm
25 6 3 N2 92.4 0.3 1.8 1.7 ZrO2, 353 rpm