Core Level Shifts of Hydrogenated Pyridinic and Pyrrolic Nitrogen in

Dec 7, 2016 - A combination of N 1s X-ray photoelectron spectroscopy (XPS) and first principles calculations of nitrogen-containing model electrocatal...
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Core Level Shifts of Hydrogenated Pyridinic and Pyrrolic Nitrogen in the Nitrogen-Containing Graphene-Based Electrocatalysts: In-Plane vs Edge Defects Ivana Matanovic,†,‡ Kateryna Artyushkova,† Matthew B. Strand,§ Michael J. Dzara,§ Svitlana Pylypenko,§ and Plamen Atanassov*,† †

The Department of Chemical and Biological Engineering, Center for Micro-Engineered Materials (CMEM), University of New Mexico, Albuquerque, New Mexico 87131, United States ‡ Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States § Department of Chemistry, Colorado School of Mines, Golden, Colorado 80401, United States ABSTRACT: A combination of N 1s X-ray photoelectron spectroscopy (XPS) and first principles calculations of nitrogen-containing model electrocatalysts was used to elucidate the nature of the nitrogen defects that contribute to the binding energy (BE) range of the N 1s XPS spectra of these materials above ∼400 eV. Experimental core level shifts were obtained for a set of model materials, namely N-doped carbon nanospheres, Fe−N−carbon nanospheres, polypyrrole, polypyridine, and pyridinium chloride, and were compared to the shifts calculated using density functional theory. The results confirm that the broad peak positioned at ∼400.7 eV in the N 1s XPS spectra of N-containing catalysts, which is typically assigned to pyrrolic nitrogen, contains contributions from other hydrogenated nitrogen species such as hydrogenated pyridinic functionalities. Namely, N 1s BEs of hydrogenated pyridinic-N and pyrrolic-N were calculated as 400.6 and 400.7 eV, respectively, using the Perdew−Burke−Ernzerhof exchange-correlation functional. A special emphasis was placed on the study of the differences in the XPS imprint of N-containing defects that are situated in the plane and on the edges of the graphene sheet. Density functional theory calculations for BEs of the N 1s of in-plane and edge defects show that hydrogenated N defects are more sensitive to the change in the chemical environment in the carbon matrix than the non-hydrogenated N defects. Calculations also show that edge-hydrogenated pyridinic-N and pyrrolic-N defects only contribute to the N 1s XPS peak located at ∼400.7 eV if the graphene edges are oxygenated or terminated with bare carbon atoms.

1. INTRODUCTION One of the most significant current technological challenges is to fill the growing gap between energy demand and supply with clean, reliable, renewable, and affordable energy. Electrochemical power generation by means of fuel cells should provide at least a partial solution to both the problem of decreasing energy resources relative to population growth and that of the urgently needed reduction of carbon emissions. While platinum and platinum group metals (PGMs) have been the most effective catalysts for electro-oxidation of different fuels or reduction of oxygen, their high cost and market instability hinders their use in large-scale application of fuel cell technologies. A successful development of platinum group metal free (PGM-free) catalysts is therefore critical to the widespread commercialization of modern fuel cells systems. Several groups have successfully synthesized PGM-free catalysts consisting of transition metals (TM), nitrogen, and carbon, often abbreviated as TM−N−C catalysts, that exhibit high oxygen reduction activity and good durability.1−7 It was shown that these catalysts demonstrate activity for oxygen reduction reaction (ORR) and possess durability similar to that of © 2016 American Chemical Society

other state-of-the-art PGM-free catalysts. We have recently shown that manganese-containing catalysts synthesized in-house have good performance for the oxidation of small organic compounds in acidic media,8 which can be used as “poly-oxo-fuels”, and their electro-oxidation can be a critical step for technologies aimed at the introduction of bioderived fuels into electrochemical power generation. In addition, there are a great number of works on metal-free carbon-based electrocatalysts that show improved ORR performance and long-term stability, particularly in alkaline medium.9−11 Although there have been a number of experimental and theoretical studies6,12−25 aimed at understanding the nature of the active sites in TM−N−C and metal-free graphene-based catalysts, the mechanistic aspects of reactions catalyzed by these catalysts including the active sites responsible for the catalytic activity and selectivity are still debated. This particularly applies to the role of nitrogen-containing defects. While some studies Received: September 27, 2016 Revised: November 29, 2016 Published: December 7, 2016 29225

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aqueous alcohol solution was prepared by mixing 320 mL of 18.2 MΩ deionized (DI) H2O and 128 mL of ethanol (Pharmco-Aaper, HPLC grade) in a 1 L high-density polyethylene bottle. Then, 3.2 g of resorcinol (Sigma-Aldrich, >99%) was dissolved while stirring at 300 rpm. Once fully dissolved, 6.0 mL of ethylenediamine (Sigma-Aldrich, >99.5%) was dissolved in solution, followed by 4.8 mL of 37 wt % formaldehyde (Alfa Aesar, 36.5−38%). The solution was stirred at room temperature for 24 h, and then the bottle was sealed and transferred to a convection oven at 100 °C for 24 h. After the vessel was cooled in an ice bath, the product was isolated via centrifugation at 16 600 G for 20 min and then transferred to a quartz boat and dried for 12 h. After drying, the product was pyrolyzed in a Lindberg Blue-M tube furnace under flowing N2 by heating at 2 °C/min up to 350 °C, dwelling for 4 h, then heating at 5 °C/min up to 600 °C and dwelling for 2 h. After cooling to room temperature, pyrolyzed C spheres were rinsed with ethanol and dried. Fe was incorporated into C spheres by a wet-impregnation procedure using iron(II) tris(ethylenediamine) (EN) chloride (Fe[EN]3Cl2) synthesized in house.35 A dispersion was prepared in a 500 mL Erlenmeyer flask by adding 200 mg of C spheres and 90 mg of Fe[EN]3Cl2 (5 wt % Fe) to a 125 mL emulsion of ethanol and DI water (80% ethanol). The dispersion was then stirred with a magnetic stir bar at 500 rpm for 1 h and sonicated for 1 h. The dispersion was poured into Petri dishes and allowed to evaporate overnight at ambient temperature. The resulting powder was pyrolyzed in a Lindberg Blue-M tube furnace by placing the powder in a quartz boat under flowing N2 at 700 °C for 4 h. The temperature was ramped to 700 °C at a rate of 3 °C/min and allowed to cool at 5 °C/min, resulting in a fine black powder. 2.2. X-ray Photoelectron Spectra. X-ray photoelectron spectra were acquired on a Kratos Axis DLD Ultra X-ray photoelectron spectrometer using an Al Kα monochromatic source operating at 150 W. Reference spectra from polypyridine, polypyrrole, and pyridinium chloride were acquired using standard charge neutralization conditions. Charge compensation was accomplished using low-energy electrons at standard operating conditions of −3.1 V bias voltage, −1.0 V filament voltage, and filament current of 2.1 A. 99.9% pure Au powder was deposited on each sample, and Au 4f spectra were acquired for calibration purposes. No charge compensation was used for N-doped and N-doped carbon spheres functionalized with Fe. High-resolution spectra were acquired at 20 eV pass energy at 0.1 eV step interval. Data analysis and quantification were performed using CasaXPS software. A 70% Gaussian/30% Lorentzian line shape was utilized in the curve-fit of spectra. 2.3. Calculation Details. All the calculation were performed using DFT with the Perdew−Burke−Ernzerhof (PBE) functional36,37 and projector augmented-wave pseudopotentials38,39 as implemented in Vienna ab initio software package (VASP).40−43 Extended surfaces were modeled using a 4 × 2 orthorhombic graphene supercell containing 32 carbon atoms with the dimensions of 9.84 Å × 8.52 Å and a vacuum region of 15 Å or a 4 × 4 rhombic graphene supercell containing 96 atoms with the dimensions 17.04 Å × 14.76 Å and a vacuum region of 15 Å. Edge defects were modeled using nanoribbons, which were constructed from 4 × 2 orthorhombic supercells with the size of 9.84 Å × 23.52 Å, with a vacuum region of 15 Å in z- and y-directions. The electronic energies were calculated using Gaussian smearing of Fermi level of σ = 0.2 eV and gamma-centered 8 × 8 × 1 k-point grid in the case of the 4 × 2 orthorhombic supercells, 5 × 5 × 1 in the case

suggest that N-containing defects partially reduce oxygen to hydrogen peroxide via a two-step, 2e− per step mechanism,19−21 others propose that these sites can fully reduce oxygen to water via a one-step, 4e− mechanism.22−25 The role of hydrogenation of N defects such as pyridinic nitrogen has not been addressed properly in the investigation of ORR on N-containing materials. It has recently been suggested that these defects might play an important role in the ORR activity of this class of materials in alkaline media.26,27 Understanding structure−property relationships is crucial for pushing the current limits in the performance of PGM-free electrocatalysts. Namely, developing model catalysts with a known structure and chemistry can allow for the isolation of certain material properties. By modeling a functionality that is present in a catalyst of interest in a less complex material, the role of that property can more readily be determined through characterization and performance testing. X-ray photoelectron spectroscopy (XPS) has proven to be one of the key surface characterization techniques for analyzing the structure of heterogeneous catalysts due to its capability to provide critical molecular level information necessary for establishing relationships between structure and reactivity/selectivity.1,5,6,28−30 However, an understanding of the active sites in PGM-free catalysts is difficult to achieve because of the large degree of heterogeneity of such materials. These challenges can be at least partially mitigated though computational efforts. Density functional theory (DFT) calculations of core level binding energy (BE) shifts for well-defined defects of the PGM-free catalysts have been recently identified as an extremely helpful tool that can facilitate the interpretation of XPS measurements.31 Previous calculations include the calculation of core level BEs of graphitic N as well as N coordinated to Fe and Co in different TM−Nx motifs.32,33 As such, the combination of first-principles calculations and XPS analysis can potentially allow for the detailed interpretation and identification of both N- and TM-containing moieties in the high-resolution XPS spectra of various PGM-free materials. In this work we use a combination of experimental and theoretical approaches to elucidate the nature of the N defects that contribute to one of the least understood BE ranges: the N 1s spectra of N-containing carbon-based electrocatalysts. The broad peak with a maximum at 400.7 eV is usually attributed to the pyrrolic-N defect; however, our results suggest a more complex interpretation of the conventional assignment. We calculated the N 1s BE of the hydrogenated pyridinic-N and found that it is very similar to the N 1s BE of the pyrrolic-N. This suggests the peak typically assigned to pyrrolic-N species may also contain contributions from other hydrogenated nitrogen species such as hydrogenated pyridinic functionalities. Pyridine can become hydrogenated and/or protonated, and it is important to understand which BE this state contributes to so that accurate structure−property relationships can be built. In addition, for the first time, special emphasis is placed on the study of the difference in the XPS imprint of N-containing defects that are situated in the plane or on the edges of the graphene sheet.

2. EXPERIMENTAL SECTION 2.1. Model Catalyst Materials. Polypyrrole, polypyridine, and pyridine hydrochloride were purchased from SigmaAldrich. The synthesis of nitrogen-doped carbon nanospheres (C spheres) is based on a hydrothermal method and was adapted from a previously published method.34 Briefly, an 29226

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Figure 1. High-resolution N 1s spectra for (a) N-doped carbon nanospheres, (b) Fe−N−carbon nanospheres, (c) polypyrrole and polypyridine, and (d) pyridinium chloride.

of the 4 × 4 rhombic super cells, and 8 × 1 × 1 k-points in the case of the supercells used to model edge defects. In all the cases, plane-wave basis cutoff was set to 800 eV. The edge effects were formed from in-plane supercells by introducing a vacuum region in the y-direction and replacing all dangling bonds with hydrogen or oxygen. All structures were relaxed allowing for the change in the cell shape, cell size, and atom position until the convergence in energy was 1 × 10−5 eV. BEs were calculated as the difference between the Kohn− Sham orbital energy of the 1s core state of nitrogen and the Fermi level

3. RESULTS AND DISCUSSION 3.1. N 1s XPS Spectra. Experimental CLS obtained for a set of synthesized model materials are shown in Figure 1a and b. Pyridinic-N is observed at 398.4 eV; the position of this type of nitrogen can be constrained to the reference positions in N 1s spectra acquired from reference polypyridine material shown in Figure 1c. DFT calculations along with experimental studies have shown that nitrogen coordinated to metal is positioned 1.1−1.5 eV higher in BE than pyridinic-N, placing it around 399.7 eV, as shown in Figure 1b.31,32 The identity of the peak at 402.1−402.5 eV has been investigated in detail before.33 DFT calculations have confirmed that graphitic nitrogen as a single-point defect in a graphene network contributes to this energy. However, if other nitrogen defects, graphitic or other types, occur in close proximity to this defect site, the BE may vary over the range between 401 and 403 eV. Small peaks at a higher BE between 405 and 407 eV come from oxidized nitrogen species. The peak positioned at 400.7 eV is typically assigned to pyrrolic-N and also can be constrained to the reference positions in N 1s spectra acquired from reference of polypyrrole shown in Figure 1c. Pyridine hydrochloride,44 a suitable reference material for protonated and hydrogenated pyridinic-N, was also studied, and N 1s spectra are shown in Figure 1d. The position of hydrogenated pyridine is in very close proximity to pyrrolic-N, which is a form of hydrogenated nitrogen in a five-ring structure. The position of protonated nitrogen is at 402.3 eV, which contributes to the same BE as graphitic nitrogen. The peak at 400.7 eV can thus be called by a generalized name “hydrogenated nitrogens”. For clarity, we will use the terms hydrogenated pyridinic-N and pyrrolic-N in the

BE N1s = E N1s−E Fermi

where BEN1s is the ab initio computed BE and EFermi is the corresponding Fermi energy. BEs of the core electron were calculated in the final state approximation as implemented in VASP. In order to avoid any errors associated with the description of the core electrons, BEs are always expressed relative to the BE of the 1s electron of the reference pyridinic-N defect. The reference defect was chosen as one of the nitrogen-containing defects that is available both experimentally and theoretically. Core level shifts (CLS) are then calculated as CLSN1s (defect) = BE N1s(defect) − BE N1s(pyridinic‐N)

Experimental N 1s BEs of a certain defect can, thus, be obtained by adding DFT-calculated CLS to the experimentally determined BE of the pyridinic-N (398.4 eV). The procedure described above has been applied previously to calculate N 1s BEs of nitrogen-containing defects in graphene-like structures and has been proven to result in good agreement with the experiment.31−33 29227

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likely to be undistinguishable in the N 1s XPS spectra with a resolution smaller than 0.1 eV. Thus, hydrogenated nitrogen that is a part of a six- or five-membered ring will contribute to the same broad peak in the N 1s XPS spectra located between 400 and 402 eV centered around 400.7 eV. Interestingly, the N 1s BE of dehydrogenated nitrogen atoms that are in the vicinity of the hydrogenated nitrogen also change indicating that the protonation of nearby nitrogen induces the change in the charge distribution of these nitrogen atoms as well. N 1s BEs of these nitrogen atoms are calculated as −0.3, −0.2, and +0.4 eV in the case of nitrogen atoms in the vicinity of hydrogenated pyridinic-N and as +0.4 and +0.4 eV in the case of nitrogen atoms in the vicinity of the pyrrolic-N defect, respectively. The shift of N 1s BE toward higher BE in the case of pyridinic-N3 in the vicinity of hydrogenated pyridinic-N and pyridinic-N1 and pyridinic-N2 atoms in the vicinity of a pyrrolic-N atom (Figure 2) indicates that these pyridinic nitrogen atoms have more attractive N nuclei than the corresponding atoms that are not in a vicinity of hydrogenated nitrogen atoms. As confirmed by the charge density of these systems, which are shown in Figure 3, some electron density

following discussion. The protonation state of nitrogen is beyond the scope of the current paper. 3.2. DFT Core Level Shifts of Hydrogenated Pyridinic-N and Pyrrolic-N Defects. Unit cells used to model the pyridinic-N, hydrogenated pyridinic-N, and pyrrolic-N defects in different chemical environments are shown in Figure 2, and the corresponding DFT-calculated N 1s CLS are given in Table1. As shown experimentally, hydrogenated pyridinic-N

Figure 2. DFT-optimized structures of pyridinic-N hydrogenated pyridinic-N and pyrrolic-N as in-plane and as edge defects. Only one unit cell is shown.

Table 1. DFT-Calculated CLS (in eV) of N 1s Electron of the Pyridinic-N, Hydrogenated Pyridinic-N, and Pyrrolic-N as in-Plane and Edge Defectsa CLS (pyridinicN) in-plane defect

edge defect a

N1 N2 N3 N4 N1

0.0 0.0 0.0 0.0 −0.3

CLS (hydrogenated pyridinic-N) N1 N2 N3 N4 N1

−0.3 −0.2 +0.4 +2.2 +4.6

Figure 3. Charge density of the pyridinic-N, hydrogenated pyridinic-N, and pyrrolic-N as a part of the extended graphene sheet as calculated using DFT with PBE functional and visualized using Visual Molecular Dynamics51 at isovalue of 0.35.

CLS (pyrrolicN) N1 N2 N3

+0.4 +2.3 +0.4

N1

+4.4

is indeed transferred from pyridinic-N to H−N group of hydrogenated pyridinic-N and pyrrolic-N. Therefore, it is expected that the electron density of pyridinic-N slightly decreases making its nuclei slightly more attractive, while the charge on the hydrogenated nitrogen slightly increases making its nuclei less attractive. The same shift toward more positive BE in the case of dehydrogenated N in the vicinity of hydrogenated N is observed in the case of tetrapyrrole rings in porphyrins on metal surfaces.49 Namely, the difference between the N 1s BE of pyrrolic-N and dehydrogenated pyrrolic-N, which constitutes the tetrapyrrole ring, was calculated as 1.98 eV, while in our case it is calculated as 1.9 eV. Moreover, it was shown previously that the formation of an intermolecular hydrogen bond between pyridinic-N and carboxylic group in isonicotinic acid can shift the N 1s BE of pyridinic-N for 1.24−1.65 eV toward higher binding energies.50 In our case the interaction of pyridinic-N and neighboring N−H group is not as strong as in the case of isonicotinic acid, and therefore the shift of N 1s BE of pyridinic-N toward higher BEs in not as pronounced. Previous experimental and theoretical works suggest that different hydrogenated nitrogen species might play a very different role in the oxygen reduction mechanism of the nitrogen-containing graphene-based electrocatalysts.26,27,29 Detailed analysis of the relationship between activity and the amount of nitrogen species present in these materials established through spectroscopic studies showed that in the acidic media pyrrolic-N catalyzes undesired oxygen reduction to hydrogen peroxide, while pyridinic-N is the active site for

The values are given relative to the pyridinic-N from Figure 1.

and pyrrolic-N have almost the same N 1s core level BEs. Namely, CLS of hydrogenated pyridinic-N and pyrrolic-N are calculated as +2.2 and +2.3 eV, respectively, which correspond to the BE of +400.6 and 400.7 eV. These values are in excellent agreement with the experimental N 1s peak positioned at 400.7 eV. The shift toward higher BE in the case of hydrogenated pyridinic-N relative to the BE of pyridinic-N is expected because the formation of the covalent N−H bond decreases the electron density on the nitrogen atom and makes the electrostatic potential around the nitrogen nuclei more attractive.27,44,45 The same positive shift of N 1s BE was observed in CN groups of some organic acid−base complexes. Namely, it was found that the protonation of CN peak to form CNH+ can shift the N 1s CN peak by +2.3 eV toward higher BEs.46 Our calculated values for hydrogenated pyridinic and pyrrolic-N are also in good agreement with previously reported values for C6H5N·HCl of 400.2 eV,44 experimental data for C6H5N·HCl presented herein, pyrrolic-N in 2H-tetraphenylporphyrines on metal surfaces,47,48 and the pyrrolic-N in protoporphyrin IX molecules adsorbed on Cu surface of ∼400 eV.49 Furthermore, the small difference between the N 1s BE of hydrogenated pyridinic-N and pyrrolic-N indicates that these two defects are 29228

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Figure 4. DFT-optimized structures of hydrogenated pyridinic-N in different chemical environments and the corresponding N 1s CLS for a hydrogenated pyridinic-N atom. Only one unit cell is shown.

oxygen peroxide reduction to water.29 However, as the hydrogenation process depends not only on the pH but also on the electrode potential, pyridinic-N can be hydrogenated even in alkaline media. In contrast to pyrrolic-N, the presence of hydrogenated pyridinic-N was suggested to play an important role in the activation of the oxygen reduction reaction by locally destabilizing adjacent carbon atoms.27 While there is a large difference in the N 1s BE of pyridinic-N and hydrogenated pyridinic-N or pyrrolic-N, the similarity in the N 1s BE of pyrrolic-N and hydrogenated pyridinic-N could complicate understanding of the role that pyridinic nitrogen hydrogenation plays in the oxygen reduction reaction mechanism. 3.3. DFT Core Level Shifts: In-Plane vs Edge Defects. If we compare N 1s CLS of the pyridinic-N, hydrogenated pyridinic-N, and pyrrolic-N in-plane and on the edges (Figure 2), we can see that the change in the chemical environment of these defects has a huge influence on the N 1s CLS of hydrogenated nitrogen atoms but has a small effect on the N 1s CLS of dehydrogenated nitrogen atoms. Moreover, the difference between the N 1s CLS of edge-hydrogenated pyridinic-N and edge pyrrolic-N and the N 1s CLS of equivalent in-plane defects is more than 2 eV. The DFT-calculated CLS place the N 1s BE of edge-hydrogenated pyridinic-N and edge pyrrolic N at 403.0 and 402.8 eV in the N 1s XPS spectra. These high N 1s BEs indicate highly attractive N nuclei of the edgehydrogenated pyridinic-N and edge pyrrolic-N defects. In order to understand the effects that the different chemical environments have on the N 1s CLS of hydrogenated nitrogen, we created a set of defects that range between in-plane hydrogenated pyridinic-N and the corresponding edge defect (Figure 4). The defects were obtained by increasing the number of carbon atom vacancies and increasing the number of edge carbon atoms terminated with hydrogen atoms, which ultimately leads to the formation of a pore in an otherwise extended graphene sheet. As shown in Figure 4, depending on the environment of hydrogenated pyridinic-N DFT-calculated N 1s CLS range from +2.3 eV, which is close to the value obtained for in-plane hydrogenated pyridinic-N, to +4.5 eV, which is close to the value obtained for the edge-hydrogenated pyridinic-N defect. In contrast, the variations in the N 1s CLS of the surrounding pyridinic-N atoms were in the range of a couple hundred millielectronvolts. These results confirm that the N 1s BE values of hydrogenated nitrogen atoms are highly dependent on their chemical environment, which contributes to the increasingly more complicated N 1s XPS spectra in the high-BE region as compared to the low-BE region. Higher N 1s BE of the hydrogenated nitrogen edge defects can be explained by the change in the electrostatic potential density distribution on the edges due to the termination of the extended graphene sheet with hydrogen atoms.52 In addition, hydrogenated

nitrogen atoms are more sensitive to the change in the chemical environment, which could be attributed to the decreased electron density of the hydrogenated nitrogen atoms. This makes their core electrons less shielded from the nuclear charge and therefore more responsive to additional fluctuations in the electron density distribution. Indeed as seen in Figure 4, N 1s BE of the hydrogenated pyridinic-N increases as the number of the nearby pyridinic-N atoms decreases (marked on Figure 4 with blue and red circles) and the number of nearby C−H terminations increases. In order to understand how different terminations of the graphene sheet influence the N 1s BE of a hydrogenated pyridinic-N and pyrrolic-N defects, we first calculated the N 1s CLS of the edge pyridinic-N and pyrrolic-N defects in the case in which the dangling bonds, which are created by the termination of the infinite graphene sheet, are replaced by oxygen atoms instead of hydrogen (Figure 5). Surface oxide termi-

Figure 5. Pyridinic-N and pyrrolic-N defects on the graphene edges that are terminated either with oxygen or with bare carbon and the corresponding N 1s CLS of hydrogenated pyridinic-N and pyrrolic-N atoms. Only one unit cell is shown.

nation is expected to be present when defects in the graphene network are being formed during synthesis of these catalytic materials. As can be seen from the N 1s CLS presented in Figure 5, termination with oxygen atoms decreases the N 1s CLS of hydrogenated pyridinic-N and pyrrolic-N edge defects for −1.9 and −1.2 eV relative to the case in which the edges are terminated with hydrogen. This result is not surprising, as it is expected that the oxygen atoms will increase the electron density on the edges including the electron density on nitrogen atoms, therefore making their nuclei less attractive. Based on the DFT-calculated CLS from Figure 5, the N 1s BE of hydrogenated pyridinic-N and pyrrolic-N atoms on oxidized edges are, thus, expected to be located in the N 1s XPS spectra around ∼401.0−401.6 eV contributing to the high intensity and broadness of the 400.7 eV peak. We also studied hydrogenated pyridinic-N and pyrrolic-N edge defects in nanoribbons in which the edge atoms are only bonded to two neighboring carbon atoms (Figure 5). Namely, it has been proven that the termination with bare carbon atoms can exist on monolayer graphene and as such provides an additional model case.53,54 29229

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Therefore, we believe there is a pressing need for DFT studies to take into account the presence of different functional groups or bare carbon atoms on the edges or pores of the PGM-free catalysts and investigate how these environments affect the reactivity and selectivity of different active sites.

Interestingly, N 1s CLS of hydrogenated pyridinic-N and pyrrolic-N on the bare edges also decrease relative to the corresponding defects in hydrogen-terminated nanoribbons for −2.1 and −1.3 eV corresponding to N 1s BE of 400.9 and 401.5 eV, respectively. This result implies that the BEs of these defects, if they exist in the real catalyst, would also contribute to the broad peak located at ∼400.7 eV. However, note that the edge motifs we used represent only model cases, and more complex terminations and structural motifs are expected to exist on the edges of real materials. These could, for example, include the presence of different carbon rings or different functional groups (−O, −OH, −COOH) on the same edge. Consequently, the presence of edge N-defects and the variations in the N 1s BE due to the different edge environments also contribute to the complexity of the high-BE part of the N 1s spectra of TM−N−C catalysts. In conclusion, our DFT calculations of N 1s CLS of model hydrogenated and dehydrogenated pyridinic-N and pyrrolic-N defects show that the hydrogenated pyridinic-N and pyrrolic-N, both as in-plane or edge defects, are much more sensitive to the change in the chemical environment in the carbon matrix than the equivalent dehydrogenated defects, which contributes to the more convoluted appearance of the high-BE part of the experimental N 1s XPS spectra of the TM−N−C composites. Edge-hydrogenated N-pyridinic and N-pyrrolic defects are also expected to contribute to the broad pyrrolic-N peak located at ∼400.7 eV, but only if the defects are in the vicinity of oxygenated or bare edges, which is the case for these types of catalytic materials. Our N 1s CLS results also show that the bare edges or edges terminated with oxygen might have a very different charge distribution than the edges terminated with hydrogen, which would also imply a very different reactivity of the nitrogen-containing defects in these chemical environments. Therefore, we emphasize the importance of studying the edge models that include bare carbon atoms or different functionalities, such as oxygen, hydroxyl, or carboxyl groups, present in the vicinity of the active sites in order to understand their effect on the reactivity/durability of the PGM-free catalysts.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Plamen Atanassov: 0000-0003-2996-472X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Center for Microengineered Materials and start-up funds from Colorado School of Mines. VASP license was provided by Theoretical Division, LANL, which is supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC5206NA25396. Computational work was performed using the computational resources of EMSL, a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. This paper has been designated LA-UR-16-27267.

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4. CONCLUSIONS Based on our DFT-calculated BEs of N 1s in different nitrogencontaining defects and their comparison with the experimental N 1s XPS spectra of model materials, we can conclude that the hydrogenated N atoms in a five- or six-membered ring (pyrrolic-N and hydrogenated pyridinic-N) contribute to the same broad peak positioned at ∼400.7 eV. Our findings also show that the N 1s BEs of the hydrogenated pyridinic-N and pyrrolic-N are more sensitive to the change in the chemical environment in the carbon matrix than the pyridinic-N defect. Namely, different chemical environments can change the BE of pyrrolic-N or hydrogenated pyridinic-N for more than 2 eV, while in the case of the pyridinic-N different environments induce a change in the N 1s BE on order of a couple hundred millielectronvolts. This response of 1s electrons of pyrrolic-N and hydrogenated pyridinic-N contributes to a more convoluted appearance of the high-BE part of the experimental N 1s XPS spectra of the N-containing PGM-free catalysts. Based on the measured and calculated N 1s BE, we can also conclude that a type of the chemical groups, which are present on the graphene edges and in the vicinity of N-defects, have a substantial impact on the charge distribution of nitrogencontaining sites, thus changing their reactivity and selectivity. 29230

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