The Coulombic Nature of Active Nitrogen Sites in N-Doped

Apr 4, 2017 - The surface activity and charged nature of ionic surfactants (sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide (CTAB)) ar...
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The Coulombic Nature of Active Nitrogen Sites in Ndoped Nanodiamond Revealed In-situ by Ionic Surfactants Kuang-Hsu Wu, Yuefeng Liu, Jingjie Luo, Bolun Wang, Junyuan Xu, Cuong Pham-Huu, and Dangsheng Su ACS Catal., Just Accepted Manuscript • Publication Date (Web): 04 Apr 2017 Downloaded from http://pubs.acs.org on April 5, 2017

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ACS Catalysis

The Coulombic Nature of Active Nitrogen Sites in N-doped Nanodiamond Revealed In-situ by Ionic Surfactants Kuang-Hsu Wu,1 Yuefeng Liu,2,3 Jingjie Luo,1 Bolun Wang,1 Junyuan Xu,4 Cuong Pham-Huu,3 Dangsheng Su1,2* 1

Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang, Liaoning 110016, China; 2

Dalian National Laboratory for Clean Energy (DNL), Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, Liaoning 116023, China;

3

Institut de Chimie et Procédés pour l’Energie, l’Environnement et la Santé (ICPEES), ECPM, UMR 7515 of the CNRS-University of Strasbourg, 67087 Strasbourg Cedex 02, France; 4

International Iberian Nanotechnology Laboratory, Av, Mestre Jose Veiga, 4715-330 Braga, Portugal.

ABSTRACT: The surface activity and charged nature of ionic surfactants (SDS and CTAB) are used as in-situ probes for the Coulombic nature of active sites in N-doped nanocarbon. In this report, we demonstrate that, by selectively masking charge-carrying sites, the active nitrogen sites in N-doped nanodiamond for oxygen reduction reaction are positively charged moieties, rather than those uncharged or negatively charged groups. From the N group analysis by XPS and H2TPD, we put forward the concern that the active nitrogen groups in N-doped nanocarbon may contain oxygen and should not be simply classified by the four conventional nitrogen types by XPS binding energy.

KEYWORDS Coulombic interaction, in-situ characterization, ionic surfactant, nitrogen-doped carbon, nanodiamond, oxygen reduction reaction Nitrogen-doped nanocarbons, a highly interesting category of carbon materials, have received enormous attention due to their fascinating chemical properties for catalysis, energy storage and sensing.1 In particular, electrocatalysis of the oxygen reduction reaction (ORR) is among the highlight applications of N-doped nanocarbons, after the famous report on metal-free, active N-doped carbon nanotubes by Dai and the co-workers.2 Significant effort was made since then to understand the origin of the high activity and the reaction mechanism, although neither has been explicitly elucidated to-date. A general consensus about the unusually prominent ORR activity attributes to the local chemistry of incorporated nitrogen sites. For example, the Lewis acid-base properties and the local charge/spin densities in a highly conjugated π system are expected to determine the different adsorption mode of O2 molecule and thus its activation and reduction.3 Structureactivity relationship hence has become an important topic in the field and X-ray photoelectron spectroscopy (XPS) is at the heart of the characterization.1 However, the spectroscopy method only resolves the state of nitrogen by binding energy and cannot offer structural information and physical properties such as the local charged state of nitrogen sites in-situ when there involves resonance structures under biased potential. Indeed, there is a strong demand for alternative methods to reveal the chemical nature of active nitrogen groups.

Among the four types of nitrogen groups (pyridinic, pyrrolic, graphitic N and pyridinic N-oxide groups) proposed earlier in N-doped nanocarbons, both pyridinic and graphitic N are suspected to be the active species in many catalytic reactions,3 while only graphitic N carries a net positive charge. Inspired by the charged state of the N group, in this report we designed a protocol to reveal the Coulombic nature of the sites in-situ, in association with the electroactivity. The idea follows the charged property and the surface activity of ionic surfactants that would spontaneously act on the oppositely charged sites and specifically mask the corresponding groups with the hydrophobic tail, hampering the catalytic function of the target sites. Note that sites with identical charge with surfactant are exposed by electrostatic repulsion. The rationale of the protocol is depicted in Figure 1a. In this case, positively and negatively charged surfactants such as cetyltrimethylammonium bromide (CTAB) and sodium dodecyl sulfate (SDS) are particularly useful in determining the charged nature of functional nitrogen groups under in-situ reaction conditions. To prove the concept, we will require an N-doped nanocarbon with an electroactivity that can only be attained through N-doping. In our demonstrative experiment, we selected nanodiamond (ND) as the base material and employed a reported method for N-doping using ammonium carbonate to introduce nitrogen sites at 900 °C, namely N-doped

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nanodiamond (NND).4 No trace metals (Fe, Mn) were detected (< 0.01 wt%) by inductively coupled plasma optical emission spectrometry (ICP-OES). The choice of ND is addressed by the rich sp2 fragments at the surface and the ability to graphitize the outer layer at above 750 °C;5, 6 both features are beneficial for the incorporation of N species into the carbon lattice. We noted that there was ~10% weight gain after the pyrolysis, which suggests that a layer of Ndoped carbon derived from the precursors deposited on the surface of ND. Figure 1b displays a transmission electron microscopy (TEM) image of NND with an illustration of proposed surface structure. The nanoparticle (~8 nm) comprises of a shell of amorphous sheath with graphitic layers (d002 = 3.56 Å) and a diamond core (d111 = 2.04 Å). Since the NND was prepared by using nitrogen and oxygen rich precursors, the surface nitrogen structures may be more diverse than in the trivial classification; a list of possibly existing structures is also provided in Figure 1b. In addition to nitrogen-only groups, we also consider tertiary lactam/imide groups as possible N, O structures that may survive the heat treatment and induce charged states on the nitrogen as iminol tautomers through resonance under a biased potential.7, 8 This hypothesis is first endorsed by an XPS survey of the surface composition, as there are 2.2 at% and 3.6 at% of nitrogen and oxygen, respectively, given that most individual oxygen groups should be removed upon a thermal treatment at 900 °C.9 Nevertheless, the NND is a sound platform for proof-ofconcept experiment as long as the N-doping can effectively deliver a superior electroactivity than other nanocarbons.

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Electroactivity of the NND for ORR was first assessed in an O2-saturated 0.1 M KOH electrolyte by staircase voltammetry on a rotating ring-disk electrode (SCV-RRDE), with an addition of 10 wt% carbon black (CB) for conductivity; the morphology and the oxygen-free composition of CB are verified by TEM and XPS as given in Figure S1 (Supporting Information). The necessity of CB was addressed by a comparative study with the results provided in Figure S2. Without CB, NND by itself shows an onset potential (Eonset = –0.004 V) more positive than a representative oxygenated nanocarbon and pristine CB, but the current performance appears to be retarded (Figure S3, Supporting Information). At an optimal composition, the marked overall activity of NND(+CB) as described by an identical Eonset to NND and a one-step current behavior could be attained; these are the reaction features of an active N-doped nanocarbon.3 In addition, an average electron-transfer number above 3.6 could be calculated across the reaction potential range. On the basis of the results, we are confident to assert the possession of active nitrogen sites in the surface layer of the NND. For the Coulombic screening experiment for active nitrogen sites by ionic surfactants, we have used cationic CTAB and anionic SDS as our ionic surfactant probes. The concentration of surfactant was selected at or below the critical micelle concentration (cmc) to avoid complete coverage of the surface and unnecessary reorganization of surfactant packing. The study was conducted in the same electrochemical system and the ORR activity was examined by SCV-RRDE. A solution of 0.1 M KOH containing surfactant was injected in aliquots to the electrolyte for each measurement. The results are shown in Figure 2a and 2b.

Figure 1. (a) A schematic diagram depicting the idea of specific Coulombic interaction of ionic surfactants with charged N-sites on NND. (b) A high-resolution TEM micrograph of NND, with an illustration of the supposed structure and N, O functionalities: (i) pyridinic N, (ii) pyrrolic N, (iii) graphitic N, (iv) pyridinic N-oxide, (v) tertiary lactam N and (vi) imide N; only iminol forms are shown to illustrate the possible charged states. Figure 2. SCV-RRDE profiles (a,b), the first-derivative voltammograms (disk) (c,d) and the Tafel plots of NND(+CB) (e,f) in O2-saturated 0.1 M KOH containing at different surfactant

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(CTAB and SDS) concentrations, respectively. A step period of 2.0 s was used with an electrode rotation rate of 1600 rpm.

For cationic surfactant, the voltammograms exhibited a one-step current behavior with an unchanged Eonset, similar to that of pristine NND(+CB) composite, except for a lower limiting disk current density (jD) and a retarded current activation behavior. The non-shifting of the Eonset is also confirmed in the first derivative voltammogram (Figure 2c), which suggests that the active sites remained exposed, but the surface was generally covered by a layer of non-specifically adsorbed CTAB, hindering the diffusion of O2 molecules. This result is substantial as it implies that the active nitrogen sites on the NND, which efficiently activate O2 (justified by an Eonset close to 20% Pt/C) and express one-step current behavior, likely do not carry a negative charge. Besides the charge-specific binding, the non-specific adsorption by the surfactants can develop a soft, resistive layer on uncharged sites and hinder mass transport near the electrode surface. This process is reflected by the reduced limiting current density (Figure 2a and b) and the formation of a resistive layer can be tracked by electrochemical impedance spectroscopy (EIS) as shown in Figure S4 (Supporting Information). On the contrary, the voltammetry behavior in the presence of anionic surfactant is clearly distinguished from that with CTAB. Interestingly, the Eonset values in the presence of SDS are down-shifted to more negative potentials (as also evidenced in Figure 2d), the jD behavior becomes two-step and the ring current density (jR) profile resembles to that of typical oxygenated nanocarbons.10 Note that the Eonset of NND (+CB) at 1.6 mM SDS is downshifted to –0.080 V, which perfectly overlays on top of that for a representative oxygenated nanocarbon (Figure S5, Supporting Information). A control experiment on pristine CB was done (Figure S6) to clarify that the downshifting of the Eonset was not inherited from the conductive filler. The results hence suggest that the sites with negative and/or zero net charge carry a similar function to oxygenated nanocarbon, at least for the facilitated O2 activation. These results remarkably indicate that the sites responsible for the desired features in active Ndoped nanocarbon are blocked by SDS through specific adsorption. In other words, the active sites in NND are very likely in a positively charged state under reaction condition. The unblocked sites, on the other hand, appear to activate and reduce O2 in a similar manner to oxygenated nanocarbon. The selective binding by the anionic surfactant is further supported by an XPS investigation in Figure S7 (Supporting Information). The surfactant head group element only shows a shift in binding energy when there is a substantial change to the ionic binding, for example, when the dodecyl sulfate adsorbs and bound to the positively charged sites on the NND. In an opposite case, the quaternary N in CTAB does not show a binding energy shift when there is no selective binding.

Tafel analysis was also performed to evaluate the ORR behavior over NND(+CB) in the presence of the surfactants, as shown in Figure 2e and 2f. It is noticed that the Tafel slopes increased evenly upon the addition of surfactant, reflecting the retardation of ORR activation upon non-specific adsorption by the surfactants.11 Furthermore, the Tafel slopes of the CTAB system intersect at a value of –2.37, whereas the slopes with SDS do not intersect at all due to the shifting of the Eonset. This again suggests that cationic surfactant does not influence the intrinsic reaction activation of the system, a feature given rise by active nitrogen sites. To validate our approach effective, we also studied the system in presence of Brij-35 non-ionic surfactant and potassium sulfate salt. In the presence of non-ionic surfactant, the outcome (Figure S8, Supporting Information) was similar to that with CTAB (i.e., a non-shifting Eonset and one-step jD behavior). A gradual reduction of the limiting current density with increasing surfactant concentration is generally observed for all surfactants. This is due to the non-specific coverage of the electrode surface as discussed earlier. The similarity with CTAB is in agreement with the case without specific masking of the active sites through Coulombic binding. On the other side, excess K2SO4 was introduced to the system to check for any sulfate-specific binding (Figure S9, Supporting Information). The result obviously indicates that the SO42– anions without a hydrophobic tail do not penetrate through the electric double layer and merely act as a negatively charged electrolyte unit. To this end, the positively charged nature of active nitrogen sites in the NND is pretty clear. High-resolution XPS was then employed to establish a correlation with the functional structures. The N 1s spectrum in Figure 3a shows four nitrogen types by conventional assignment (pyridinic/nitrile N, 398.3 eV; pyrrolic/lactam N, 399.8 eV; graphitic N, 401.1 eV; pyridinic N-oxide/imide N, 403.2 eV), with pyrrolic/lactam N being dominating.3, 7, 12 If only oxygen-less N groups are concerned, the nature of the dominating N group (pyrrolic N) does not match with the observed Coulombic property of active sites. This led us to consider groups such as tertiary lactam and imides, as there is fair amount of oxygen in the NND. Their presence is partly supported by the O 1s spectrum in Figure 3b. There is trace C–OH group (533.8 eV) and the peak at 530.7 eV is assigned to carbonyl C=O groups. The peak at 532.0 eV includes a range of lactam/imide/lactone/carboxyl C=O groups (531.4–532.2 eV) and C–O–C groups (531.8–533.0 eV).13 Since C–O–C groups cannot survive the temperature and carboxyl groups must be accompanied by C–OH groups, this peak is likely dominated by lactam/lactone groups. Further evidence was given by H2 temperature programmed desorption (TPD) study (Figure 3c) with the aim to extract nitrogen species from the NND as NH3 (m/z = 17) and related signals. In general, there are four peaks of NH3 signal occur at about 200 °C, 510 °C, 750 °C and > 800 °C, relating to the dangling amines/lactams, imides, lactams and substituted N groups (pyrrolic and

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pyridinic N’s), respectively.8 Signals relating to nitriles and substituted N groups (HCN, m/z = 27) and lactam/imide groups (HNCO, m/z = 43) were also collected for clarification.14 The presence of lactam and imide is clearly confirmed by the pronounced signals for NH3 and HNCO at 510 °C and 750 °C, respectively. This means that some kinds of lactam and/or imide may be responsible for the activity enhancement in the ORR under reaction condition. As the Coulombic screening study suggested the active sites on the NND carry a positive charge, the results here point towards possible iminol forms of tertiary lactam and/or imide. As a final note, the finding of positively charged site in our post-doped NND does not rule out the possibility of other active sites with a different nature in N-doped carbons. For example, a type of model pyridinic N has been reported to involve carbanion intermediate in the ORR.15 While the paradox remains to be verified (which is currently under progress), the discovery is expected to be an incentive for full resolution of ORR-active structures.

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are currently under progress. Nevertheless, we hope this work can stimulate the method development for facilitating the understanding about the true chemical nature of active N-doped nanocarbons.

ASSOCIATED CONTENT Supporting Information. Experimental detail, TEM and XPS results of CB, SCV-RRDE profiles of NND(+CB), its components and references, and the profiles of NND(+CB) in electrolytes containing Brij-35 or K2SO4. SCV-RRDE profiles of CB and EIS result of NND(+CB) in the presence of surfactants. XPS results of the NND after reaction in the presence of surfactants. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * Prof. Dr. Dangsheng Su Tel: +86-24-23971577 Email: [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT The authors acknowledge the financial support from Institute of Metal Research, Chinese Academy of Sciences. KHW would like to acknowledge the support of International Postdoctoral Exchange Scholar Fellowship and PIFI Postdoctoral Research Fellowship (2017PM0002) from Chinese Academy of Sciences.

ABBREVIATIONS NND, N-doped nanodiamond; ND, nanodiamond; CB, carbon black; CTAB, cetyltrimethylammonium bromide; SDS, sodium dodecyl sulfate; RRDE, rotating ring-disk electrode; SCV, staircase voltammetry; EIS, electrochemical impedance spectroscopy; TPD, temperature programmed desorption; XPS, X-ray photoelectron spectroscopy; ORR, oxygen reduction reaction. Figure 3. XPS (a) N 1s and (b) O 1s spectra, and (c) H2-TPD profiles of NND under 5% H2/N2 atmosphere. Top: HNCO (m/z = 43), middle: HCN (m/z = 27) and bottom: NH3 (m/z = 17).

In conclusion, we have revealed the Coulombic nature of active nitrogen sites on the NND by using ionic surfactants as in-situ molecular probes and the results suggest that the active sites in our NND for ORR activity possess a positive charge. Supported by XPS and H2-TPD methods, we proposed iminol forms of tertiary lactams and imides as probable active site structures for the ORR in alkaline media. Even though the exact structure of the active site is not explicitly solved in the present work, many of the properties revealed hint towards an unconventional answer (i.e., non-pyridinic or graphitic N groups and the structure perhaps involves oxygen). Further studies to uncover the explicit surface chemistry of the NND and the active structure require additional investigations and

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9. Figueiredo, J. L.; Pereira, M. F. R.; Freitas, M. M. A.; Órfão, J. J. M. Carbon 1999, 37, 1379-1389. 10. Wu, K.-H.; Wang, D.-W.; Gentle, I. R. Carbon 2015, 81, 295304. 11. Fletcher, S. J. Solid State Electrochem. 2009, 13, 537-549. 12. Radovic, L. R.; Silva, I. F.; Ume, J. I.; Menéndez, J. A.; Leon, C. A. L. Y.; Scaroni, A. W. Carbon 1997, 35, 1339-1348. 13. Naumkin, A. V.; Kraut-Vass, A.; Gaarenstroom, S. W.; Powell, C. J. NIST X-ray Photoelectron Spectroscopy Database, Version 4.1 2012, National Institute of Standards and Technology Website, https://srdata.nist.gov/xps/Default.aspx (accessed: 02 Dec, 2016). 14. Hansson, K.-M.; Samuelsson, J.; Åmand, L.-E.; Tullin, C. Fuel 2003, 82, 2163-2172. 15. Li, Q.; Noffke, B. W.; Wang, Y.; Menezes, B.; Peters, D. G.; Raghavachari, K.; Li, L.-s. J. Am. Chem. Soc. 2014, 136, 33583361.

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Ionic surfactants can act as in-situ probes to reveal the Coulombic nature of active nitrogen sites in N-doped nanodiamond.

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Ionic surfactants can act as in-situ probes to reveal the Coulombic nature of active nitrogen sites in N-doped nanodiamond. 241x223mm (150 x 150 DPI)

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Figure 1. (a) A schematic diagram depicting the idea of spe-cific Coulombic interaction of ionic surfactants with charged N-sites on NND. (b) A high-resolution TEM micrograph of NND, with an illustration of the supposed structure and N, O functionalities: (i) pyridinic N, (ii) pyrrolic N, (iii) graphitic N, (iv) pyridinic Noxide, (v) tertiary lactam N and (vi) imide N; only iminol forms are shown to illustrate the possible charged states. 275x254mm (150 x 150 DPI)

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Figure 2. SCV-RRDE profiles (a,b), the first-derivative volt-ammograms (disk) (c,d) and the Tafel plots of NND(+CB) (e,f) in O2-saturated 0.1 M KOH containing at different surfactant (CTAB and SDS) concentrations, respectively. A step period of 2.0 s was used with an electrode rotation rate of 1600 rpm. 307x328mm (150 x 150 DPI)

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Figure 3. XPS (a) N 1s and (b) O 1s spectra, and (c) H2-TPD profiles of NND under 5% H2/N2 atmosphere. Top: HNCO (m/z = 43), middle: HCN (m/z = 27) and bottom: NH3 (m/z = 17). 241x234mm (150 x 150 DPI)

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