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Porous nanographene formation on γ-alumina nanoparticles via transition-metal-free methane activation

γ-Al2O3 nanoparticles promote pyrolytic carbon deposition of CH4 at temperatures higher than 800 °C to give single-walled nanoporous graphene (NPG) materials without the need for transition metals as reaction centers. To accelerate the development of efficient reactions for NPG synthesis, we have investigated early-stage CH4 activation for NPG formation on γ-Al2O3 nanoparticles via reaction kinetics and surface analysis. The formation of NPG was promoted at oxygen vacancies on (100) surfaces of γ-Al2O3 nanoparticles following surface activation by CH4. The kinetic analysis was well corroborated by a computational study using density functional theory. Surface defects generated as a result of surface activation by CH4 make it kinetically feasible to obtain single-layered NPG, demonstrating the importance of precise control of oxygen vacancies for carbon growth.


S1.2 Reaction Kinetics
Thermogravimetry was conducted on a thermogravimeter (Netzsch, STA 2500 Regulus) under a steady flow of He with various concentration of CH 4 at the total flow rate of 100 mL min −1 . The standard reactor volume was 50 mL. To suppress air contamination during the analysis, the thermogravimeter was surrounded by a chamber filled with Ar gas supplied at a flow rate of 1 L min -1 . Typically, 32 mg of γ-ANPs was used for the kinetic analysis of CH 4 -CVD. The samples were loaded on the reactor of the thermogravimeter, and first heated from room temperature to a specified temperature (1128 -1173 K) at 10 K min −1 under a steady flow of He. This was followed by the constant-temperature heating at the specified temperature for 30 min under a steady flow of He. After the pretreatment, CH 4 was introduced to the reactor with a specified partial pressure to initiate CH 4 -CVD. The rate of CH 4 -CVD for the kinetic analysis of the first-layer deposition was determined at an inflection point. Formation of nanoporous graphene was confirmed by x-ray diffraction and Raman spectroscopy ( Figure S1).
In situ infrared (IR) spectra of the Al 2 O 3 nanoparticles (ca. 18.0 mg) at the specified temperature were recorded on a Nicolet 6700 FT-IR spectrometer (ThermoScientific) with a diffuse reflectance infrared Fourier transform (DRIFT) method under a steady flow of Ar at 30 mL min −1 . The intensity was reported as the Kubelka-Munk function. S1 The temperature of the sample during the in-situ IR measurements was monitored by using an infrared thermometer (Keyence, FT-H40K) to ensure the temperature. We performed 120 scans.
Temperature programmed desorption (TPD) was measured on a gas chromatogram (GC, Varian 490-GC, GL Science). Approximately 1 g of γ-ANP was gently packed in a quartz reactor tube with stacked height of 3~4 cm using quartz wool (4-9 μm, Toso Company, Ltd.). The reactor tube was heated at the rate of 10 K min -1 under a steady flow of He (200 mL min -1 ), and the evolved H 2 O was analyzed by GC. The sampling interval was approximately 2.5 min. Figure S2. Kinetic analysis of CH 4 -CVD for porous nanographene with or without pretreatment of H 2 gas before CH 4 -CVD. For the control experiment, He was introduced instead of H 2 for the same period. (a) Weight changes during CH 4 -CVD at 900°C as monitored by TG. CH 4 was introduced to the reactor at 0 min. (b) The rate of reaction for CH 4 -CVD.

S2.4 Stability of Surface OH groups toward Surface Activation
In order to analyze the stability of the OH groups on the γ-ANP surface for the surface activation, we performed the in-situ IR experiments under a steady flow of CD 4 . We find that almost all protons are labile in the presence of CD 4 at temperatures higher than 600 ºC (Figures S4,S5) while the structure of bulk region remained almost unchanged during the CH 4 -CVD according to the 27 Al NMR spectra ( Figure S9). The D-H exchange between CD 4 and isolated OH groups occurred on the γ-ANP surface above 600 ºC, and the OH stretching band at ν OH = 3701 cm −1 depressed with time constants of 1.2 min and the OD stretching band at ν OD = 2730 cm −1 evolved as shown in Figure S4.
This isotope shift can be quantitively rationalized by the change of the reduced mass μ by the H-D exchange. The vibration frequency ν of the OH stretching mode is described in eq. (S1) under the harmonic approximation, where m O = 16 amu, and m H = 1 amu are the masses of oxygen and hydrogen atoms, respectively. The effect of isotope exchange on the vibrational force constant k f is negligible and the frequency of the deuterated system ν OD can be written as the rate between reduced mass of OD group μ OD and that of OH μ OH, , and the frequency of the original system ν OH = 3701 cm −1 , where m D = 2 amu is the mass of deuterium. Resultant is calculated to be 2693 cm −1 , which qualitatively agrees with the experimental value (2730 cm −1 ).
(a) (b) Figure S4. Temporal profiles of the OH stretching bands in IR spectra of ANPs at 900 ºC (a) in the presence of CH 4 (2 mL min −1 ) and (b) in the presence of CD 4 (2 mL min −1 ). Depletion at 2350 cm −1 is due to CO 2 . Figure S5. Time-course of IR spectra of -ANPs in the presence of CD 4 at the elevation rate of 16.7 K min −1 from 600 ºC to 850 ºC. Depletion at 2350 cm −1 is due to CO 2 . Table S1. Summary of H 2 O/NH 3 TPD a and CH 4 -CVD.

S2.5 Stability of Oxygen Vacancy Sites and Reactivity of CH 4 on Them
Evolved gas Conditions for pre-activation b H 2 O c NH 3 Rate of reactions d 700 ºC for 30 min 1.1 mmol g -1 33 μmol g -1 ---900 ºC for 30 min 1.4 mmol g -1 19 μmol g -1 4.8 × 10 -9 mol s −1 1000 ºC for 30 min 1.5 mmol g -1 21 μmol g -1 3.6 × 10 -9 mol s −1 a The details are shown in the section S1.3; b under a steady flow of He; c The amount of water desorbed at the temperatures higher than 300 ºC; d The rate for the first-layer deposition under the standard CH 4 -CVD condition at 900 ºC with a steady flow of CH 4 (20 mL min -1 ). Figure S6. Black line: Energy profile for the formation of a CH 4 σ complex and the subsequent C-H bond cleavage on a γ-Al 2 O 3 (100) surface. Red line: Conversion of CH 4 to CH 3 OH on a γ-Al 2 O 3 (100) surface. The reactive site is a 5-coordinated-Al. Figure S7. (a) Geometry of a γ-Al 2 O 3 (100) surface with an adsorbed CH 4 and (b) the same geometry without CH 4 for clarity. Red: oxygen, Blue; aluminum, gray: carbon, and white: hydrogen atoms. The reactive site is tetrahedrally-coordinated (4-coordinated) Al center, and the coordinate is the same for CH 4 * in Figure 4a of the manuscript. The coordinates are shown in Section S3.

S2.6 Density Functional Chemical Calculations
(a) (b)  Figure S8. Energy profile for the formation of a CH 4  complex and the subsequent C-H bond cleavage on a -Al 2 O 3 (100) surface with no oxygen defect. The reactive site is an octahedrallycoordinated (6-coordinated) Al center, and the clouded surface gives radical mechanism rather than the Lewis acid-base mechanism for the bond cleavage reaction. Figure S9. Normalized MAS 27 Al NMR of γ-ANPs before and after CH 4 -CVD. Relative intensity of the peak for octahedrally coordinated Al-center ( [6] Al) in the up-field ( = 9 ppm) S17 was enhanced as compared with that for tetrahedrally coordinated  Al) in the downfield ( = 68 ppm) S17 after CH 4 -CVD. Figure S10. XRD of γ-ANPs before (ANP) and after CH 4 -CVD (C/ANP).

S2.8 Structural Analysis of ANPs by XRD
S2.9 Crystal Structure Dependency on CH 4 -CVD Figure S11. Time-course of weight changes during CH 4 -CVD on various crystal structures of Al 2 O 3 at 900 ºC as monitored by TG.
Curvature reflecting the difference in the rates of CH 4 -CVD reactions was recognized at nearly single-layered deposition of carbon (Figs. 1a and 1b), but there was exception at higher partial pressure of CH 4 or at lower temperatures showing the curvature at the number of carbon layers > 1. We also noticed such exceptions for -ANP and -ANP (Fig. S11). This could indicate that a mixture growth on the Al 2 O 3 nanoparticles (first layer) and on carbon layer (second layer) was significant under these conditions. Thus, lower partial pressure of CH 4 and higher reaction temperatures as well as the use of -ANP would be important for single-layered carbon deposition.

S3. Appendix: Optimized Structures and Energies in Quantum Chemistry Calculations
S3.1 Optimized Structures and Energies for Figure 4 Coordinate for a γ-Al 2 O 3 (100) surface with CH 4 -complex with a surface proton density of 7.1 μmol m -2 at a 4-coordinated Al site.  Figure S8 Coordinate for a γ-Al 2 O 3 (100) surface with CH 4 -complex at a 6-coordinated Al site.