AcidâBase Properties of N-Doped Carbon Nanotubes: A Combined

Article pubs.acs.org/cm

Acid−Base Properties of N‑Doped Carbon Nanotubes: A Combined Temperature-Programmed Desorption, X‑ray Photoelectron Spectroscopy, and 2‑Propanol Reaction Investigation Klaus Friedel Ortega,† Rosa Arrigo,‡ Benjamin Frank,§ Robert Schlögl, and Annette Trunschke* Department of Inorganic Chemistry, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14195 Berlin, Germany S Supporting Information *

ABSTRACT: Chemical and electronic properties of N-doped multiwalled carbon nanotubes (NCNTs) synthesized by NH3 treatment of preoxidized CNTs at 300, 500, and 700 °C have been investigated by a set of surface sensitive techniques. Temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS) were applied to characterize the nature, thermal stability, and binding state of N and/or O containing surface functional groups. Acid− base properties in aqueous phase were analyzed by potentiometric pH titration, while the catalytic reaction of 2-propanol probed the acid−base behavior of the materials in the gas phase. NH3 treatment at 300 °C leads to an acid−base bifunctional surface, predominantly decorated with imide species. Contrarily, pyrrolic N is the most abundant moiety present on the sample modified at 500 °C. Here, only small fractions of lactam groups and pyridinic species are present. The incorporation of N at 700 °C leads to a carbon nanotube (CNT) surface with a well-defined basicity due to pyridinic N, which serves as a Lewis basic site converting 2-propanol into acetone. Furthermore, the oxidative stability of NCNTs strongly depends on the nature of N-containing species. Regarding the oxidative stability, NCNTs obtained at 700 °C behave similar to the pristine CNTs, whereas the lower NH3 treatment temperatures are detrimental for this property. The combination of dedicated techniques reveals links between structural and functional properties of surface species that change dynamically with temperature.

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INTRODUCTION N-doped multiwalled carbon nanotubes (NCNTs) have gained considerable attention due to their tunable acid−base properties along with their specific electronic structure induced by the incorporation of electron-rich N surface species into the carbon nanotube (CNT) framework. As a metal-free catalyst, NCNTs have been employed as solid base in the Knoevenagel condensation of benzaldehyde with ethyl cyanoacetate.1 Its initial activity correlates with the amount of pyridinic N species embedded into the nanocarbon. Beneficial effects of N-doping have also been observed in red-ox catalysis or electrochemistry.2,3 When used as a catalyst support, N-containing surface species can stabilize highly dispersed metal nanoparticles by strong metal-heteroatom interaction. We recently showed that NCNTs generated by post-treatment of preoxidized CNTs (O-CNTs) with NH3 at 600 °C stabilize Pd nanoparticles with an average diameter of 2.7 nm, being smaller in size than those formed on CNTs functionalized at lower temperatures.4 The former catalyst led to the highest turnover frequency in the liquid phase oxidation of benzyl alcohol to benzaldehyde with an activity twice as high as for Pd supported on O-CNTs. Similar results were obtained over Au− Pd nanoparticles supported on either O- or N-doped CNTs, respectively,5 and others.6 Accordingly, the functional properties of CNTs are critically influenced and tunable by surface modification, which has been previously shown by Tessonnier et al. for a selection of commercial nanocarbons.7 Here, fundamental correlations © XXXX American Chemical Society

between surface and bulk defect chemistry with material properties such as oxidation stability or catalytic activity in redox reactions have been established. Consequently, a significant part of the present manuscript is dedicated to the transfer of these principles to the N-doped systems with a focus on temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). Compared to the “common” O functionalization of nanocarbons, the complexity of N incorporation into a graphitic framework becomes evident when attempts are made to understand the underlying reaction mechanisms. For instance, the amination of a preoxidized nanocarbon surface via NH3 treatment induces several reactions between the surface O species and the gaseous N precursor, followed by secondary transformations. The nature and abundance of functional groups characterizing the final state of a given graphitic surface are strongly affected by the amination temperature, which gives rise to dynamic rearrangements as well as decomposition of surface species. Below 300 °C, the N-containing functional groups are mainly created by a dehydration mechanism.8 Ammonium salts formed by neutralization of carboxylic acids with NH3 thermally decompose to amides, which can be further converted into nitrile groups.9 The reaction of carboxylic anhydrides with NH3 results in imides, which subsequently decarboxylate at 300−500 °C Received: April 26, 2016 Revised: September 7, 2016

A

DOI: 10.1021/acs.chemmater.6b01594 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials Table 1. Textural Properties and Elemental Composition of Pristine and Functionalized CNTs elemental analysis [wt %]

a

IDa

sample

SBET [m2 g−1]

Sμp [m2 g−1]

Vtot [mm3 g−1]

Vμp [mm3 g−1]

C

H

N

Ob

N/Oc

12832 16603 16646 16645 16644

HP-CNT O-CNT 300-NCNT 500-NCNT 700-NCNT

304 338 323 324 350

0 34 15 14 39

1370 1340 1310 1350 1510

0 20 10 10 30

97.2 92.0 94.8 96.4 97.4

0.00 0.03 0.02 0.03 0.02

0.0 0.0 1.3 1.2 1.0

2.8 8.0 3.9 2.4 1.6

0.38 0.57 0.71

Internal sample number to distinguish reproductions of sample preparation. bDifference to 100%. cMolar basis. measurement. The titrants used were 0.01 M NaOH and 0.01 M HCl solutions, respectively, depending on the samples to be investigated. The titrants were prepared by dilution of 1 M Titrisol standards (Merck Millipore). Temperature-Programmed Desorption. Temperature-programmed desorption (TPD) was carried out in a home-built setup equipped with a gas chromatograph (Varian CP-4900 Micro-GC) and a mass spectrometer (Pfeiffer Omnistar) for online gas analysis. Weighted amounts of sample were loaded into a fixed-bed quartz reactor (inner diameter: 9 mm), which was placed in a self-constructed furnace with an isothermal zone of 4 cm at the upper temperature limit. Prior to ramping, weakly adsorbed H2O was removed in a stream of He (25 mLn min−1) at 100 °C for 1 h. Thereafter, experiments were started by linearly heating the reactor by 5 °C min−1 to 1035 °C. This temperature was maintained for 30 min before cooling down to ambient temperature. Temperature-Programmed Oxidation. Temperature-programmed oxidation (TPO) experiments were performed in a TGDSC Netzsch STA 449C Jupiter thermobalance coupled to a Pfeiffer Omnistar mass spectrometer for online gas analysis. Differential scanning calorimetry (DSC) curves recorded during TPO provided the enthalpies associated with the combustion of the carbon samples. In a typical run, 10 mg of sample was loaded in an Al2O3 crucible, which was heated at 5 °C min−1 from ambient temperature to 950 °C in synthetic air (21% O2 in Ar) at a flow rate of 100 mLn min−1. The maximum temperature was maintained for 30 min before cooling to ambient temperature. Elemental Analysis. Elemental analyses (CHNS) were done with a Thermo FlashEA 1112 NC Analyzer. X-ray Photoelectron Spectroscopy. X-ray photoelectron spectroscopy (XPS) was performed at ambient temperature under UHV (base pressure 99 wt %, bulk density of 140−230 kg m−3) with an outer diameter of 13 nm and an inner diameter of 4 nm on average, respectively, were obtained from Bayer MaterialScience11 (HP-CNT). 20 g of HP-CNT was treated at 122 °C for 4 h in 1 L of concentrated HNO3 (Carl Roth, 65 wt %, p.a.) under reflux. After cooling to ambient temperature, the oxidized sample was thoroughly washed and filtrated with Millipore water until neutral pH. Finally, the resulting nanocarbon was dried at 110 °C in air for 3 days (O-CNT). N-Doping was performed by flowing a gas stream of 150 mLn min−1 comprising 50% NH3 in Ar through a tubular quartz reactor (inner diameter: 40 mm; length: 740 mm) loaded with 4 g of O-CNT. In order to minimize concentration and temperature gradients, the reactor was placed in a swinging furnace. The functionalization was initiated by heating the sample at 5 °C min−1 to 300, 500, and 700 °C, respectively, maintaining the target temperature for 6 h. Thereafter, the samples were cooled to ambient temperature in NH3/Ar. The name of N-doped nanocarbons is composed of the functionalization temperature followed by NCNT (Table 1). N2 Adsorption. N2 adsorption at −196 °C was performed using a Quantachrome Autosorb-6B KR instrument. 30 mg of powdered substance was degassed for 2 h at 200 °C before the measurements. The specific surface areas were calculated using the BET model.12 Pore size distributions were derived from the desorption branches of the isotherms according to the BJH method13 assuming a cross-sectional area of the N2 molecule of 0.162 nm2. Total pore volumes were determined at a relative pressure of p/p0 = 0.97. Micropore analyses were performed by applying the MP-method with a statistical thickness interval of 0.05.14 Potentiometric pH Titrations. Potentiometric pH titrations were carried out with a Mettler DL 77 autotitrator and monitored by a Mettler Toledo DGi114-SC electrode. In a typical experiment, 100 mg of powdered sample was suspended in 50 mL of a 0.1 M NaCl solution. Equilibration was achieved by vigorously mixing the suspension overnight. In order to minimize side effects from dissolved CO2, the mixture was degassed under Ar for 30 min prior to starting a B


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Chemistry of Materials

this value is more than doubled to 8.0%. For the NH3 treated samples, the N concentration slightly decreases from 1.3% to 1.0% with increasing treatment temperature. Inversely, the amount of O decreases from 3.9% to 1.6%. Consequently, the rise of the N/O molar ratio from 0.4 to 0.7 may indicate that N containing functional groups, which persist at higher temperatures, do not contain O. Temperature-Programmed Desorption. Differences in type and amount of surface species become evident by comparing the volatile compounds detected during TPD experiments. The temperature profiles of released gases (CO2, CO, HCN, N2, H2, H2O, HNCO, and C2N2) are shown in Figure 1. CO2, CO, and N2 were quantified by gas chromatography, whereas for HCN only a semiquantitative analysis was done. Notably, the concentrations of the individual components presented in Figure 1 are very different. However, the data are shown in one figure to directly compare the evolution temperatures of N- and O-containing compounds. The complex decomposition processes of surface species frequently include both, N and O. Some mechanistic aspects of the thermal decomposition of surface functional groups will be discussed by referring to studies performed on the corresponding pristine organic substances. The assignments are summarized in Table 2, quantification is provided in Table 3. O-CNTs. The oxidized CNTs essentially release CO2, CO, and H2O (black lines in Figure 1a,b,f). The wet oxidation confers the system acidic properties by introducing an abundance of carboxylic acid entities. Their degradation to CO2 is evidenced in Figure 1a by the major peak at 275 °C. Further steps in CO2 evolution indicates the decomposition of carboxylic anhydrides and lactones at 435 and 650 °C, respectively.25 In general, O functional groups on nanocarbons, which release CO by decomposition, have a higher thermal stability. However, a small amount of CO is already released at 300 °C (Figure 1b). H2O is detected in this temperature regime as well (Figure 1f), which is most likely due to a condensation between adjacent carboxylic acid groups at around 275 °C. Carboxylic anhydrides generated this way subsequently decompose into equimolar amounts of CO2 and CO at 275 and 300 °C, respectively. The condensation of neighboring phenolic species may contribute to the evolution of H2O at 275 °C.26 The second step in CO evolution observed at 460 °C stems from carboxylic anhydrides, which are more stable than the species intermediary formed by the condensation of carboxylic acid entities during TPD (see above). Compared to the corresponding peak of CO2 evolution (435 °C, Figure 1a), the peak of CO evolution is shifted by 25 °C toward higher temperatures, which may indicate a sequential decomposition.27 The most dominant feature in the CO-TPD profile is a broad peak at 600−850 °C due to the degradation of phenols and ethers.25 Two shoulders are indicated above 850 °C, which can be ascribed to ketonic and quinoidic carbonyl groups.25 Beyond 900 °C, the dehydrogenation of the H-terminated carbonaceous backbone begins. The homolytic C−H bond cleavage leads to H• radicals, which subsequently recombine to H2 (Figure 1e) or initiate the decomposition of highly stable O functional groups to form H2O (Figure 1f). Summarizing the TPD analysis of O-CNT (Figure 1, Table 3), its surface is characterized by a mixture of O-containing species. Phenolic groups and ethers, as well as carboxylic acid groups and anhydrides, are the dominating species, whereas

Scheme 1. Reaction Pathways Associated with the Catalytic Reaction of 2-Propanol20

4.2% 2-propanol was provided by passing 50 mLn min−1 N2 through a saturator at 20 °C. A U-shaped quartz reactor with an inner diameter of 9 mm was filled with 150 mg of catalyst (250 and 355 μm). Prior to the measurements, the samples were heated in N2 to 350 °C for 2 h (heating ramp: 5 °C min−1). Reaction temperatures were varied in 25 K steps from 200 to 300 °C at constant inlet flow rate, with each set of parameters remaining unchanged for 12 h.

Xi‐PrOH =

C‐balance =

χpropylene + χacetone + 2χdiisopropyl ether χi‐PrOH,in

× 100%

χi‐PrOH,out + χpropylene + χacetone + 2χdiisopropyl ether χi‐PrOH,in

(1) × 100%

(2)

Si =

νχ i i ∑ νχ j j

× 100% (3)

The conversion X of 2-propanol (eq 1) was calculated on the basis of the ratio of product concentrations χ (mol %) to the amount of 2propanol fed into the reactor. Propylene, acetone, and diisopropyl ether (DIPE) were the only C containing products detected. Cbalances (eq 2) near 98% were typically obtained. Selectivity S is defined as the molar concentration of a product with respect to the sum of all products detected (eq 3).

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RESULTS AND DISCUSSION Textural Properties and Elemental Composition. Structural parameters derived from N2 adsorption−desorption experiments are summarized in Table 1. The isotherms identified as type III are indicative for weak adsorbent− adsorbate interactions (Figure S1a). CNTs form entangled agglomerates due to strong noncovalent interactions arising from their high aspect ratio. The observed H3 hysteresis has often been encountered in carbonaceous systems with slitshaped pores.21 BET surface areas SBET increase after O and subsequent N functionalization (Table 1). In contrast, the total pore volume Vtot decreases, except for 700-NCNT providing the highest Vtot of all samples. The micropore surface areas Sμp exhibit a different trend. Harsh oxidation opens the CNT caps making the inner channels accessible. Thus, introduction of O species by HNO3 treatment of HP-CNT increases Sμp from initially zero to 34 m2 g−1, which corresponds to the increase in SBET. Reversely, NH3 treatment at 300 and 500 °C reduces SBET by the same extent as Sμp, lowering Sμp by more than 50%. The decomposition of O functional groups and thermal effects might be the origin of this observation. The treatment temperature of 700-NCNT enables the production of H2 and highly reactive species like H, NH2, and NH radicals, respectively, via thermal decomposition of NH3.22 However, the thermal stability of functional groups is significantly decreased in the presence of H2.23 Thus, radical species may serve as etching agents to increase microporosity.24 The elemental analysis (Table 1) shows that HP-CNT already contains approximately 2.8% O. After HNO3 oxidation, C


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Figure 1. Evolution of (a) CO2, (b) CO, (c) HCN, (d) N2, (e) H2, (f) H2O, (g) HNCO, and (h) C2N2 during TPD analyses of O- and N-modified CNTs. D


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Table 2. Decomposition Products, TPD Temperature Peak Maxima, and XP O 1s/N 1s Contributions Characteristic of O- and N-Containing Functional Groups Present on Oxidized and N-Doped CNTs

Table 3. Amounts of CO2, CO, and N2 as well as Relative Abundance of HCN As Determined by TPD sample

CO2 [μmol gCNT−1]

CO [μmol gCNT−1]

N2 [μmol gCNT−1]

HCN [V mol gCNT−1]

O-CNT 300-NCNT 500-NCNT 700-NCNT

919 263 87 13

1836 847 410 137

294 271 239

9.4 9.5 8.4

E


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mechanism could be considered. Here, a four-centered cyclic transition state is suggested to generate CO2 followed by the scission of a C−C bond resulting in the formation of HCN at 400−600 °C. Furthermore, the thermal decomposition of succinimide generates isocyanic acid (HNCO). The major contributions to the mass spectrum of HNCO arise from m/z = 42 and 43.31 These traces, which were recorded during TPD experiments, show a maximum at 510 °C. According to Figure 1c,g, HNCO evolution is located in the same regime as the broad low temperature peak of HCN. This further indicates the presence of imide species on the surface of 300-NCNT. Likewise, the decomposition of lactams yields HCN and HNCO.32 Lactams can be formed by the reaction of NH3 with lactones (Scheme 3). Since both cyclic functionalities

lactone and thermally more stable carbonyl groups are also present. 300-NCNT. Major changes on the CNT surface become evident after treating O-CNT in NH3 at 300 °C (orange lines in Figure 1). The CO2 profile evidences the complete disappearance of carboxylic acid groups (275 °C) and a substantial loss of anhydrides (435 °C) but an increase in CO2 evolution above 650 °C due to either lactones or imides (Figure 1a). The loss in the intensity of the CO peak at 460 °C (Figure 1b) further confirms the reduced abundance of carboxylic anhydrides. Furthermore, lower amounts of phenols, ethers, and carbonyl species are present on the surface of 300NCNT as evidenced by the reduced CO release at 600−950 °C. The total loss of surface O species after NH3 treatment at 300 °C is documented in Table 3. 300-NCNT possesses additional surface features that lead to the formation of HCN and N2 upon heating. Carboxylic acids react with NH3, forming amides via dehydration of ammonium salts,9 whereas imides are generated through ring opening accompanied by subsequent elimination of H2O (Scheme 2).

Scheme 3. Formation of Lactams via Ring-Opening of Lactones

Scheme 2. Formation Pathway of Amides (top) and Imides (bottom) through Neutralization of Carboxylic Acids or Ring-Opening of Cyclic Anhydrides, Respectivelya

a

decompose to the same volatile compounds, TPD cannot distinguish between imides and lactams. However, the low temperature peak in the HCN profile (Figure 1c) is too broad to be explained by a single functional group. This is supported by the profile of HNCO centered at 510 °C (Figure 1g), which shows a shoulder at 700 °C. These observations suggest that mainly imides, but also lactams, are present on 300-NCNT. In analogy to the O-containing functional groups, an assignment can be done for the N-containing species: Imides decompose at lower temperatures and structurally correspond to anhydrides, whereas lactams are thermally more stable and show a similarity to lactones. At above 700 °C, the HCN profile of 300-NCNT exhibits two additional features, which could be attributed to N in aromatic groups, i.e., pyrrolic and pyridinic structures. In the pyrolysis of pyrrole and pyridine, several major decomposition products have been identified, including HCN and H2.33,34 Although the mechanisms were established for the model compounds, similar decomposition pathways might govern the depletion of analogous functionalities on the surface of carbonaceous materials. As depicted in Figure 1e, the evolution of H2 already begins at 200 °C. The presence of two peaks centered in a similar position as compared to the high temperature features in the HCN profile (805 and 920 °C) suggest that two additional species with different thermal stability exist. The first feature is tentatively assigned to pyrrolic N and the second to pyridine-like species.35 As observed in Figure 1d, N-containing functional groups may also decompose under formation of N2. In contrast to other volatile compounds, the pathways explaining the formation of molecular N2 are not straightforward. This product cannot be directly linked to a certain kind of species but rather implies that surface diffusion plays an important role (eq 4).36

Amides can subsequently dehydrate to nitrile species.

The interconversion of amides into nitrile groups (Scheme 2) is reported to take place at around 200 °C,8 which is 100 °C below the synthesis temperature of 300-NCNT. Consequently, the presence of amides on this sample is unlikely. As shown in Figure 1f, a small peak of H2O evolves at 145−260 °C. Its low intensity compared to the dominating feature centered at 485 °C suggests only a residual fraction of surface N species being in amide configuration. The condensation of neighboring phenol groups may also contribute to the low-temperature peak of H2O as discussed for O-CNT. Nitrile groups formed according to Scheme 2 may be responsible for the small shoulder in the HCN evolution curve (Figure 1c) at 200−300 °C. Again, the concentration of nitriles, if present at all, is small, because the decomposition temperature of 250 °C is below the synthesis temperature of 300-NCNT. As evidenced by HCN, N2, and HNCO evolution depicted in Figure 1c,d,g, respectively, the decomposition of N-containing functional groups on 300-NCNT significantly rises above 300 °C. Theoretical calculations on the decomposition of phthalimide28 suggest a decarboxylation route involving four transition states leading to the formation of benzonitrile, which further degrades to HCN via a free radical mechanism.29 Thus, the evolution of CO2 at 350−600 °C may correspond to the superimposed decomposition of residual anhydrides and imides, and the formation of HCN at 400−600 °C is related to the thermal degradation of imide species. According to a report on the pyrolysis of succinimide,30 a decarboxylation

−C(N) + −C(N) → N2 + −2Cfas

(4)

This process requires a mobile surface species, which yields N2 upon recombination with another N-containing functionality, leaving two free active sites (fas) −Cfas on the surface. The evolution of cyanogen (C2N2) during TPD of N-containing F


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is still present after modification at 500 °C. This agrees with our finding that lactams possess a higher thermal stability than imides. Changes are also observed in the HCN trace (Figure 1c). Not only is the low temperature peak absent, but also in particular the intensity of the feature assigned to pyrrolic N strongly increases. The maximum at 600−700 °C might be indicative for the presence of lactams. Pyridinic N was likewise generated as evidenced by the feature centered at 920 °C. As discussed above, the low temperature evolution of H2 is due to the thermal depletion of pyrrolic and pyridinic surface species (Figure 1e). Although the N2 profile is similar to sample 300NCNT, the low temperature contribution in 500-NCNT is completely absent (Figure 1d). This may suggest that the generation of N2 at low temperatures is mainly linked to imides. In this context, C2N2 is only observed at higher temperatures, probably due to the decomposition of lactams. In summary, according to TPD, lactams are the dominating N-containing surface species on 500-NCNT. Residual lactones cannot be excluded. The TPD traces point at the decomposition of pyrrolic as well as pyridinic species. 700-NCNT. The amination at 700 °C eliminates all CO2 contributions (green line in Figure 1a). Hence, carboxylic acids, anhydrides, and lactones were quantitatively converted by NH3. Similarly, only a small amount of CO evolves at 980 °C (Figure 1b), indicative for traces of carbonyl species (Table 3). The corresponding HCN profile displays two features that fulfill the above statements on surface processes (Figure 1c). While the amount of pyrrolic species decreases (785 °C), the concentration of pyridinic N substantially increases (905 °C). The peaks of H2 release (Figure 1e) are centered at the same temperatures as the features identified in the HCN profile. In contrast to the samples functionalized at 300 and 500 °C, no HNCO evolves in the course of TPD from 700-NCNT (Figure 1g), which is indicative for the absence of imides and lactams. Interestingly, similar to the sharp CO peak identified in 300NCNT and 500-NCNT (Figure 1b), the N2 profile of both samples exhibit a sharp feature located exactly at the same temperature (Figure 1d). This peak completely vanishes after chemical modification at 700 °C, which may imply that the decomposition of lactams also gives rise to N2 formation. Finally, 700-NCNT does not show any contribution of C2N2 (Figure 1h). In summary, pyridine-like species are the dominant surface features on 700-NCNT, while pyrrolic N is also present. On the basis of TPD, practically no imides and lactams occur on the surface of 700-NCNT. To conclude, the TPD analyses clearly show that the amination temperature governs the nature of N-containing surface species. Generally, a mixture of various species is present and the following species dominate at the surface of the respective N-containing materials:

nanocarbons evidences the mobility of surface species.36 Although it has been stated that C2N2 cannot be detected during TPD as a consequence of its thermal decomposition,35 our study shows that a compound with m/z = 52 is released. This value corresponds to the most intense contribution in the mass spectrum of C2N2.37 For 300-NCNT, the evolution of C2N2 takes place already at low temperatures (Figure 1h). Probably, a larger fraction decomposes into N2 over the active surface and only a small amount desorbs into the gas phase, which would explain the poor signal-to-noise ratio. Besides the discussed reaction pathways between N- and Ocontaining functional groups, the dynamic character of the CNT surface has to be taken into consideration. The thermally induced rearrangements of surface entities change the state of the starting material by altering the distribution of functionalities. A comparison of the profiles of CO released from 300NCNT and O-CNT (Figure 1b) clearly shows that the overall intensity decreases after NH3 treatment. Furthermore, the feature arising from the decomposition of anhydrides is strongly reduced. However, the TPD profile of 300-NCNT has a sharp peak of CO release at around 800 °C, which is not detected for O-CNT. The narrow peak width suggests that the underlying process occurs fast. This feature probably may be linked to the decarbonylation of lactam groups into pyrrolic species as depicted in Scheme 4. Scheme 4. Thermally Induced Decarbonylation of Lactams Forming Pyrrolic Structures

In summary, according to TPD, some residual O-containing functional groups like anhydrides and lactones may remain on the surface of O-CNT after NH3 treatment at 300 °C. The most abundant surface species on 300-NCNT are imides, while lactams are present to a smaller extent. The TPD traces indicate the presence of pyrrole-like and pyridine-like surface species, but the latter may also be formed via rearrangements of surface lactams during TPD at temperatures higher than the synthesis temperature of 300-NCNT. The XPS analysis will show that actually the latter reaction does not occur and notable amounts of pyridinic surface species are already present on 300-NCNT. 500-NCNT. In comparison to 300-NCNT, the CO2 profile of 500-NCNT is characterized by the complete absence of the anhydride peak and the presence of a minor contribution at 650 °C (gray line in Figure 1a), which is due to the decomposition of lactones. Since the functionalization was performed at 500 °C, anhydrides were converted to imides in the presence of NH3. However, a large fraction of imide groups may already be decomposed during the amination since decomposition of imides on 300-NCNT was observed at around the synthesis temperature of 500-NCNT (500 °C, Table 2). This assumption is supported by the absence of the HCN and HNCO peaks at 400−600 °C (Figure 1c,g). CO2 evolution at 600−750 °C may partly arise due to the decomposition of residual imide species (Figure 1a). The absence of anhydrides is confirmed by the complete absence of CO desorbing at 350−600 °C (Figure 1b). However, the sharp CO feature at 800 °C, which was ascribed to the conversion of lactams into pyrrolic species (Scheme 4),

(i) imides on 300-NCNT; (ii) lactams and pyrrolic N on 500-NCNT; (iii) pyridinic and pyrrolic N on 700-NCNT. However, the TPD data are not necessarily unambiguous. Thermally induced rearrangements of functional groups must be taken into account;38 i.e., surface functional groups generated at a given temperature may undergo consecutive surface transformations in the course of the TPD experiment at up to 1000 °C. In the temperature range higher than the synthesis temperature and lower than the decomposition of G


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lactam and imide species both located at around 399.7 eV.39 Peaks in the high BE region at 401.1−401.6 eV (N3) originate from quaternary N. This type of species substitutes a C atom in the graphene layer forming three σ-bonds with vicinal C atoms. Alternatively, N3 could represent a protonated pyridine-like species. The presence of N−O bonds can be confirmed by spectral features at 402−406 eV. Surface functional groups like pyridine-oxide8 and NO240 have been identified in this energy range. Figure 2 displays the N 1s ranges of XP spectra of the modified CNTs. N-containing functional groups are absent in the backbone of HP-CNT and O-CNT, respectively. Instead, N1, N2, and N3 species can be distinguished by deconvolution of the spectra of the materials obtained by NH3 treatment of OCNT. Pyridinic N (N1) gives rise to the lowest BE at 398.3 eV. The relative intensities between N1, N2, and N3 indicate that the former is the most abundant species of 700-NCNT in agreement with TPD. As the treatment temperature is lowered, the relative contribution of N1 progressively decreases (Figure 3).

respective surface species, a dynamic mixture of thermally more stable surface species may exist. Accordingly, gas phase decomposition products observed during TPD may represent both surface species present after synthesis and surface species generated in situ during TPD. Therefore, XPS has been applied as a complementary method to analyze the bonding state of the heteroatoms after synthesis. Reversely, the high-quality TPD profiles can contribute to an improved assignment of single features of the XP spectra. Surface Analysis by XPS. The elemental composition of the modified CNTs as determined by XPS is summarized in Table 4. Whereas the liquid phase oxidation in refluxing HNO3 Table 4. Elemental Composition Determined by XPS sample O-CNT 300-NCNT 500-NCNT 700-NCNT

N [at %]

O [at %]

C [at %]

N/O

3.4 3.3 2.6

13.0 7.1 3.0 2.0

87.0 89.5 93.8 95.4

0.00 0.48 1.10 1.30

yields an O concentration of around 13 at %, this value is drastically reduced to about 2.0 at % after NH3 treatment at 700 °C. On the contrary, the amount of N ranging between 2.6 and 3.4 at % decreases only slightly with the temperature of the functionalization. These trends are in line with those obtained from elemental analysis (Table 1). Similarly, the increasing N/ O ratio as a function of treatment temperature indicates a progressive increase of the fraction of N species that is directly bound to the carbon backbone without the presence of O in the functional group. N 1s Core-Level Spectra. The N 1s core level spectra of carbonaceous materials (Figure 2) identify different regions in the binding energy (BE).8,38 Peaks at 398.4−398.8 eV (N1) are generally ascribed to pyridinic N. The corresponding N atoms are bound to two C atoms at the edge of graphene layers. Signals at 399.5−400.2 eV (N2) are usually attributed to N atoms at edge sites, which are bound to two C atoms and one H atom (pyrrolic N). Similar configurations are also found in

Figure 3. Relative concentrations of N1, N2, and N3 based on XPS considering the total N content as a function of NH3 treatment temperature.

The interpretation of the N2 signal is more complex. As opposed to N1, which is ascribed to a single configuration of N, this feature comprises a set of surface species all containing N− H bonds, i.e., imides, lactams, and pyrrolic N. According to TPD, imides decompose at 400−600 °C and they are the dominant species in 300-NCNT. Consequently, a component at 399.8 eV detected on the basis of the N 1s difference spectrum of 500-NCNT and 300-NCNT (Figure S2a) is attributed to imide species. Neither imides nor lactams are present in 700-NCNT, but lactams are highly abundant in 500NCNT. Thus, the component at 399.7 eV in the N 1s difference spectrum of 700-NCNT and 500-NCNT (Figure S2b) is assigned to lactams. The N2 species in the N 1s spectrum of 700-NCNT is basically pyrrolic N characterized by a BE of 399.6 eV. A peak at 401.0 eV in all NCNT spectra in Figure 2 appears close to the value reported for quaternary N species (N3).38 A combined TP-XPS study revealed a maximum concentration of N3 species at 300 °C,38 which corresponds to the NH3 treatment temperature applied for the preparation of 300NCNT. As evidenced in Figure 3, the N3 signal progressively decreases with increasing NH3 treatment temperature, while

Figure 2. N 1s ranges of XP spectra of the modified CNTs. H


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Chemistry of Materials the N1 signal increases. However, N3 can also be interpreted in terms of protonated pyridinic species. The concurrent presence of O-containing functionalities like carboxylic acids or phenolic groups, which are capable of forming H bridges with pyridinic N to form pyridinium species, may induce a quaternary type of arrangement as illustrated in Scheme 5.41 Our TPD data Scheme 5. Pyridinic N (1) Exhibiting the Configuration of Quaternary N (2), which Is Induced by H-Bonding with an Adjacent Phenolic Group, Schematically Illustrating the Configuration of N3

confirm a gradual removal of phenolic groups from 300-NCNT to 700-NCNT (Figure 1). Consequently, the amount of potential sites capable of creating H bonds is decreased. Thus, the amount of pyridinic N increases as the concentration of pyridinium species decreases. Therefore, we postulate that the N3 signal corresponds to pyridinic N that undergoes H bonding to phenol groups, which are located in striking distance to the N atom according to Scheme 5. As the decomposition of phenolic groups proceeds with increasing temperature, the N3 peak disappears in favor of the N1 peak. This is also reflected by TPD, which is in agreement with the dominating presence of pyridinic N species in 700-NCNT in contrast to the samples modified at lower temperatures. As evidenced by the absence of spectral features above 402 eV in Figure 2, no species containing N−O bonds are present on any of the NCNT samples studied. As mentioned above, in contrast to observations made in other investigations,10,40 no N functional groups have been detected on O-CNT. O 1s Core-Level Spectra. The assignments of the chemical bonds contributing to the O 1s core-level spectrum are less straightforward.10 So far, the peaks have been assigned as follows:

Figure 4. O 1s ranges of XP spectra of modified CNTs and difference spectra (hatched areas including the result of the fit) of the spectra shown in the corresponding sections.

between two treatment temperatures help one to understand changes observed in the CO2 and CO profile of the samples in the course of TPD analyses. The difference spectrum of O-CNT and 300-NCNT can be mathematically fitted with four peaks (Figure 4, bottom). One type of CO double bond (O2 at 531.4 eV) and two types of C−O single bonds (O3 at 532.6 eV and O4 at 533.6 eV) are detected. Physically adsorbed H2O (O5 at 535 eV) is also present. According to TPD, carboxylic acids completely disappear after functionalization at 300 °C, whereas anhydrides and lactones are only partly removed. Likewise, the amount of ketone/quinone species is reduced. These changes are due to the NH3 treatment, which leads to the formation of N species like lactams and imides by reaction of NH3 with O-containing species according to Schemes 2 and 3, respectively. Chemical shifts in conjugated carbonyl compounds range between 531.0 and 531.9 eV.46,47 Therefore, the peak at 531.4 eV is assigned to ketone moieties (O2). Anhydrides, lactones, lactams, and imides contain ketone moieties and may, therefore, contribute to the O2 signal after treatment at 300 °C. On the basis of TPD, ether-like O (O3) and phenolic species (O4) are present on O-CNT to a significantly higher extent than on 300-NCNT. These considerations agree with a balanced distribution of O2, O3, and O4 species in the difference spectrum of O-CNT and 300-NCNT. Interestingly, no quinoidic carbonyl groups (O1) appear in the fit of the difference spectrum of O-CNT and 300NCNT indicating that these groups are absent in 300-NCNT. In O-CNT, quinoidic carbonyl groups might be present, since the spectrum of O-CNT shows some intensity at the corresponding BE of O1 (530.7 eV). This observation is in agreement with the characteristics of the corresponding profiles of CO in the TPD experiments. In addition to the features discussed above, O1 species are clearly present in the difference spectrum between 300-NCNT and 500-NCNT (Figure 4, middle). On the basis of TPD and the N2 signal in the N 1s spectrum, imide species are the

The signal at 535 eV (O5) has been attributed to adsorbed water (and/or O2) based on the thermal behavior of the peak.10 It should be noted that a broad O 1s peak centered at 533 eV has also been interpreted in terms of H2O adsorbed on oxidized carbonaceous materials.43 The BE range at 532−533 eV (O3) has been likewise attributed to carboxylate species owing to the resonance structure of the latter, which hinders the distinction between a C−O single (O3) and CO double (O2) bond.42 Notably, functionalized samples without sufficient metallic conductivity can provide differential charging, which may explain shifts of all peaks toward higher binding energies.44,45 Figure 4 displays the O 1s ranges of XP spectra of the modified CNTs. Difference spectra that illustrate the difference I


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Acid−Base Properties in Aqueous Phase. Titration curves were recorded for the pristine, oxidized, and ammonia treated samples in order to investigate the impact of chemical modification on the acid−base properties. Figure S3a shows the state of the pristine CNTs and the effect arising from oxidation in concentrated HNO3. The starting pH of HP-CNT is around 7. Furthermore, the lack of an inflection point evidences the negligible concentration of acidic protons. In agreement with the steps observed in the TPD profile of CO2, a large amount of carboxylic acid entities and cyclic anhydrides is present on the surface of O-CNT. This is in line with the increase in acidity after HNO3 treatment, which is reflected in a low initial pH of about 3.5. According to the first derivative of the titration curve of O-CNT (Figure S3a) at least two different types of Brønsted acid sites are neutralized. Apparent acid dissociation constants (pKa) were derived from the titration curves by determining the pH at the corresponding half-equivalent points. Both species exhibit a middle-strong acidity with pKa values of 4.0 and 6.2, respectively (Table 5). The total amount

dominant functional groups in 300-NCNT; however, the latter are practically absent in 500-NCNT. Instead, 500-NCNT mainly contains lactams and pyrrolic species. Therefore, it is assumed that the component at 530.7 eV (O1) in the difference spectrum corresponds to a carbonyl group CO vicinal to a N atom in imides and lactams, since the local configuration of the carbonyl group in imides is similar to that in lactams (and pyridone-like species). Accordingly, imides and lactams cannot be distinguished on the basis of O 1s core level spectra. Due to the complete absence of carboxylic acid groups, the contribution to the O2 signal in the spectrum of 300-NCNT arises from anhydrides and lactones. As expected from the different intensities in the CO TPD profiles, 300-NCNT contains more ether-like (O3) and phenolic (O4) species compared to 500-NCNT. The residual O content in 700-NCNT is rather low, which complicates the analysis of the spectrum (Figure 4, top). As confirmed by the presence of O1, lactams are present on 500NCNT but absent on 700-NCNT, which is in agreement with TPD. The two other weak signals in the difference spectrum at 532.0 and 533.3 eV do not agree with any of the common assignments. The difference spectrum basically reflects the Ocontaining functional groups in 500-NCNT. According to TPD, in addition to lactams, only some residual lactones should be present on 500-NCNT. The signal at 532.0 eV is, therefore, assigned to the carbonyl groups in isolated lactone groups (O2b). Accordingly, the signal at 533.3 eV is attributed to the corresponding ether-like O3 in these lactones (O3b). The slight shift of the BEs of the two species to higher energies compared to the tabulated values of O2 and O3, respectively, might be explained by differential charging due to the high treatment temperature44,45 or to the missing interaction of these groups with other heteroatom-containing species on the surface. Hence, the O 1s core level spectra of nitrogen-functionalized CNTs studied in the present work can be specified as follows:

Table 5. Quantitative Analysis of the Potentiometric pH Titration initial pH

proton donor sites [μmol g−1]

O-CNT

3.5

630

300-NCNT 500-NCNT 700-NCNT

7.3 7.4 7.7

sample

a

proton acceptor sites [μmol g−1]

pKa

110 120 160

4.0, 6.2 7.2 7.3 7.5

Apparent pKa in the case of O-CNT and pKb for NCNT.

of sites calculated at the equivalent point is in reasonable agreement with the amount of CO2 released during TPD experiments, indicating that the acidic character is predominantly determined by carboxylic acids and cyclic anhydrides. The acid−base properties are reverted after N incorporation via NH3 treatment (Figure S3b). In spite of the significant differences observed between the N-doped samples in the TPD experiments, their titration curves are rather similar. This especially applies to 300-NCNT and 500-NCNT. In both cases, the initial pH is around 7.3, which is similar to the value obtained for HP-CNT. Contrary to the pristine CNTs, the titration curves belonging to these samples exhibit an inflection point, from which the amount of neutralizable sites can be derived. The values thus obtained are in the range between 110 and 120 μmol g−1. A slightly increased basicity can be appreciated in the case of 700-NCNT as evidenced by an initial pH close to 8. This observation is in line with the results obtained by TPD. Most of the N present is in the form of pyridinic species. Compared to 300-NCNT and 500-NCNT, the inflection point of this sample has the largest shift toward higher volumes. The apparent pKb values derived from the halfequivalent points slightly differ, lying between 7.2 and 7.5. The experimentally determined value corresponds to an integral pKb value derived from the superposition of the acid−base properties of the various surface species present on the CNTs. Imides and lactones prevail on 300-NCNT, whereas pyrrolic and pyridinic N dominates to a different extent on the samples treated in NH3 at higher temperatures. The acid−base character of these functional groups strongly differs. Consequently, the overall behavior of the nanocarbons in aqueous phase is determined by the extent to which N in different

In summary, the combination of TPD and XPS generated a comprehensive picture of the surface chemistry of N-functionalized CNTs. The details of the assignments are summarized in Table 2. Figure 3 quantitatively illustrates the general trends in surface composition with respect to the N-containing surface species. With increasing synthesis temperature, O is progressively substituted by N. The NH3 treatment of O-CNTs at 300 °C results in a surface, which is basically covered by imide groups. Some pyridinic and protonated pyridinic species are also present. An equal distribution of pyridinic species and lactams accounts for the surface composition of NCNTs synthesized at 500 °C. NH3 treatment at 700 °C predominantly results in pyridinic surface species. Generally, various species are simultaneously present, implying multifunctionality of the corresponding materials. J


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Table 6. Product Selectivity at Conversion below 1%, Basicity (Acetone-to-Propene Ratio), and Acidity (Formation Rate of Dehydration Products) of NCNT Samples Determined in 2-Propanol Transformation

a

sample

Ta [°C]

Xi‑PrOH [%]

SPb [%]

SAc [%]

SDIPEd [%]

rP+DIPE [μmol m−2 h−1]

rA [μmol m−2 h−1]

rA/rP+DIPE

300-NCNT 500-NCNT 700-NCNT

225 300 225

0.73 0.63 0.62

69.1 14.9 1.8

21.9 83.6 98.2

9.0 0 0

0.62 0.13 0.01

0.19 0.71 0.77

0.3 6 54

Temperature of isoconversion. bP: propylene. cA: acetone. dDIPE: diisopropyl ether.

their intrinsic activities. Certainly, 300-NCNT contains more O functional groups than 500-NCNT. Cyclic anhydrides, which are hydrolyzed in the presence of H2O, can further contribute to the acidic character of 300 NCNT. Thus, in case of 500NCNT, the dehydration activity yielding propylene might be ascribed to the presence of lactams, while the dehydrogenation of 2-propanol most likely takes place over pyridinic species. As derived from TPD and XPS investigations, 700-NCNT exhibits the highest abundance of pyridinic species, which is reflected in the almost exclusive formation of acetone. Similar to 500-NCNT, the only dehydration product formed was propylene. Notably, 300-NCNT being the only sample containing imide species produces DIPE. Correlations between the acid−base characteristics of Srbased oxidic catalysts derived from adsorption experiments employing basic probe molecules and the catalytic transformation of 2-propanol suggest that the dehydrogenation-todehydration-ratio can be used as a measure for the basicity of a catalytic system.50 In contrast, acidity correlates with the activity of 2-propanol toward dehydration products.51 Formation rates of propylene and DIPE decrease by a factor of 50 with increasing NH3 treatment temperature, indicating a strong decrease in surface acidity (Table 6). Consistently, the ratio of formation rates rA/rP+DIPE used as a measure for the basicity significantly increases from 0.3 to 54. These trends are evidently in line with the nature of species dominating the surfaces of the investigated samples. To address the deactivation phenomena (Figure S4), the catalysts were investigated by elemental analysis after reaction (Table S1). The N content of the catalysts is practically not changed (Table 1 and Table S1). The slight reduction observed for 300-NCNT is most likely related to the pretreatment in N2 at 350 °C, which is slightly higher than the temperature of functionalization. In contrast, the O content increases. On the basis of this finding, it is suggested that the deactivation is caused by irreversible adsorption of reactants and products on the CNT surface. Besides the adsorption of 2-propanol in the form of isopropoxide species, acetone may be likewise irreversibly bound to N species. Consistently, between three to five times more H is found on the samples after reaction. Either C−H bonds as constituent fractions of adsorbed species or irreversibly bound H2 resulting from the 2-propanol dehydrogenation might be reasons for this observation. Oxidative Stability of N-Doped CNTs. The impact of surface functional groups on the degree of graphitization and the availability of defect sites serving as centers at which combustion of CNTs is preferably initiated has been studied by TPO. Thermogravimetry (TG) is a suitable technique for understanding the combustion stages, whereas differentialscanning calorimetry (DSC) provides integral heats released in the course of the oxidation. Coupling these methods with online gas analysis permits one to monitor the combustion products evolving during the experiments. The parameters of TPO were carefully adjusted to avoid a significant (>5%)

configurations is attached to or incorporated into the carbonaceous framework. Notably, solvation effects have a strong influence on the behavior of surface species, which is one reason for differences usually observed in the acid−base properties between liquid phase and gas phase. Catalytic Reaction of 2-Propanol. The conversion of 2propanol was performed over the N-modified CNTs in order to elucidate their differences in terms of acid−base properties in gas-phase reactions. According to Scheme 1, 2-propanol is converted to propylene (dehydration) and DIPE over Brønsted acid centers, whereas its dehydrogenation to acetone is favored on Lewis acid−base pairs.20 The samples were pretreated at 350 °C. According to TPD, this implicates only slight changes of the surface of 300-NCNT (Figure 1). Conversion, selectivities, and C balance at different reactions temperatures are presented in Figure S4. The conversion over pristine HP-CNT was practically zero (Figure S4d). Obviously, all catalysts deactivate at the higher reaction temperatures. Moreover, the product selectivity changes, indicating that the probe reaction induces changes of the catalyst surface even at low temperature and low conversion. The experiments demonstrate that a single-point measurement may provide a misleading description of the actual state of the surface when the selected reaction conditions lead to severe reconstruction of active sites.48 In the present case, however, the general character of the surface reflected by the product mixture is consistent under all reaction conditions. The selectivity at comparable conversion and formation rates of the products are summarized in Table 6. 300-NCNT exhibits an acid−base bifunctional surface as evidenced by selectivity toward dehydration and dehydrogenation products. The ratio of propylene to DIPE changes with reaction temperature due to changes in the acid−base character of the surface.48 On the basis of the results obtained by TPD and XPS, imides are the dominating species in 300-NCNT, whereas pyridinic N is present to a lower extent. Cationic sites have been considered as active sites for dehydrogenation of the secondary alcohol to acetone.49 Since the amount of O is near 7 at % according to XPS, the Lewis basicity of surface oxo (O−) and superoxo (O2−) ions present on the CNT surface should be considered.20 Imide species possess an enhanced N−H acidity associated with the negative inductive effect of the carbonyl groups next to the N atom. Thus, the dehydration activity of 300-NCNT most likely arises from the acidic character of these species, but oxygen in functional groups cannot be excluded to contribute to the reaction as well. The major product over 500-NCNT is acetone (Figure S4). No DIPE was detected indicating a change in the distribution of Brønsted acid centers. The nature of the surface differs from that of 300-NCNT since lactam/pyridone species as well as pyrrolic N are attached to the carbonaceous backbone. 300NCNT is about five to six times more active than 500-NCNT in spite of a comparable N concentration. This could be related to differences in the number of active centers and/or changes of K


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Figure 5. Thermogravimetric changes (a) and DSC signals (b) during TPO of pristine, oxidized, and NH3 treated CNTs.

conversion of O2; thus, the fine structure of profiles recorded over the CNT samples can be attributed to differences in the oxidation kinetics and not to O2 gas transport limitation. Instead, heat transfer effects cannot be excluded due to the extremely high exothermicity of carbon combustion.52 Figure 5 displays the TG curves and DSC signals of the pristine, oxidized, and N-doped CNTs. Information about the (extrapolated) temperature of oxidation onset, the T50 value representing the temperature at which 50% of the initial mass has undergone combustion, and the overall heat of oxidation can be derived (Table S2). HP-CNT and 700-NCNT possess the highest stability against oxidation. T50 is lowered by 26 °C after the pristine material has been oxidized in HNO3. XPS, TPD, and elemental analysis showed that the amount of O significantly increases after the wet chemical treatment introducing various types of surface species strongly differing from each other in terms of thermal stability. In fact, the TG curve of O-CNT begins to drop already at around 200 °C, which is far below oxidation. In the case of 300-NCNT and 500-NCNT, the T50 value is further lowered. However, a significant improvement is observed for 700-NCNT, which exhibits the highest onset temperature of the series reaching almost 570 °C. A good agreement is found between T50 and the position of the peak maximum in the DSC curves (Tmax.DSC). The integral heats of combustion of the CNTs calculated from DSC range from −24 to −28 kJ·g−1, which is in good agreement with those obtained for charcoals53,54 and soots.55 For comparison, a heat of combustion of −32.8 kJ·g−1 has been determined for the oxidation of graphite.56 An interesting value is the fwhm of the combustion peaks in the DSC curves, which becomes narrower as the mass loss becomes steeper in the TG curves. On the basis of this observation, it can be stated that the combustion of HP-CNT, 300-NCNT, and 700-NCNT occurs faster. While 500-NCNT oxidizes slower, the combustion of O-CNT requires the longest time. This observation suggests that lower oxidative stability is related to the introduction of defects, which is also reflected in the increase in micropore area (Table 1). Instead, a slow combustion may be related to the presence of O species serving as a passivation layer. The profiles of volatile combustion products CO2 (m/z = 44) and NOx (m/z = 30) are shown in Figure S5. Carbon dioxide is the major product. No NOx evolution was observed for HP-CNT and O-CNT. The small contributions seen in the

corresponding profiles are rather attributed to an overlap arising from m/z = 29, which is a fraction in the MS pattern of CO2 that may partially overlap with m/z = 30. With respect to the N-doped CNTs, the mass traces ascribed to NO x significantly differ from the CO2 profiles. Therefore, an overlap can be discarded. The values determined for T50 and Tmax.DSC are in excellent correspondence with the temperatures of the maxima determined in the CO2 and NOx curves. An interesting finding is the shoulder in the NOx traces observed in 300NCNT and 500-NCNT located at 490 °C. A corresponding feature is neither found in the CO2 profiles nor in the NOx evolution of 700-NCNT. 700-NCNT only contains N atoms incorporated into the carbon lattice in form of substitutional species or at the edges as pyrrolic and pyridinic N, whereas 300NCNT and 500-NCNT also contain N atoms that are part of functional groups attached to the carbonaceous backbone, like imides and lactams, both exhibiting a thermal stability below 750 °C. Hence, the shoulder characterized by an onset near 450 °C is associated with the decomposition of these species. This leads to an increase in the defect density lowering the (extrapolated) onset temperature. In addition, less O is present on NCNTs compared to O-CNT. The absence of passivating O accelerates the nanocarbon combustion. In line with this argument, the exclusive presence of structurally more stable N species in 700-NCNT lowers the amount of defect sites, consequently increasing the onset temperature to a value comparable to that of the pristine material.

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CONCLUSION N-functionalized CNTs synthesized by post-treatment of OCNTs in NH3 at 300, 500, and 700 °C have been investigated with a set of complementary surface sensitive characterization techniques. Both abundance and type of O and N functional groups were assessed by TPD and XPS. Profiles of volatile compounds like CO2 and CO clearly confirm the presence of carboxylic acids, acid anhydrides, and lactones in addition to phenols, esters, and carbonyl species on oxidized CNTs. Their availability on a carbon surface is a key factor since they serve as reaction centers for NH3 leading to successful incorporation of N. Imides, which are formed by ring-opening of cyclic anhydrides in the presence of NH3, have been identified as the major N-containing species at a treatment temperature of 300 °C. This is indicated by the evolution of HCN and HNCO at low temperatures. The assignment is additionally supported L


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by the strong intensity of the N2 signal at 399.8 eV and the O1 feature at 530.7 eV assigned to the CO of pyridone-like species. A further increase in the functionalization temperature to 500 °C incorporates lactams and pyrrolic N to a larger extent. The HCN profile of the material is characterized by the disappearance of the imide contribution and the appearance of an asymmetric peak at higher temperatures. A modification at 700 °C yields a surface dominated by pyrrolic and pyridinic N, which was confirmed by the characteristic peaks in the HCN profile as well as by the respective N2 (399.7 eV) and N1 (398.4 eV) signals in the N 1s XP core level spectrum. Quaternary N atoms already formed at low functionalization temperatures give rise to the N 1s spectral feature at 401 eV (N3). Substitutional N and protonated pyridinic N-species formed via H bonding with nearby located OH groups contribute to the N3 signal. While the acid−base properties of NCNTs seemed to be similar in the aqueous phase, significant differences were observed in the gas phase as evidenced by the catalytic transformation of 2-propanol. The ratio between the rates of formation of dehydrogenation and dehydration products was used as a quantity to assess the basicity, which increased from 0.3 to 54 with increasing NH3 treatment temperature. Furthermore, the parallel formation of dehydration and dehydrogenation products reflects the acid−base bifunctional surface of 300-NCNT. On the contrary, an acetone selectivity of almost 100% regardless of the reaction temperature for the sample obtained at 700 °C is characteristic for a nanocarbon with a defined surface basicity. Pyridinic species serving as Lewis basic sites were identified as part of the active acid−base pair necessary for the turnover of 2-propanol toward dehydrogenation products. The resistance of NCNT samples toward oxidation also strongly depends on the nature of Ncontaining surface species. The analytical approach presented in this work is suitable for linking the structural information on N species with the acid− base and oxidation properties of the modified surface resulting from the superimposed properties of the individual functional groups. Furthermore, this integral concept can be applied in order to investigate the impact of N incorporation on different types of nanocarbons including active carbon, carbon nanofibers, graphite, or graphene. Undoubtedly, the results allow a better understanding of the complex surface characteristics, which are important in terms of the application of CNTs as functional materials.

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R.A.: Diamond Light Source Ltd., Diamond House, Harwell Science and Innovation Campus, Didcot, Oxfordshire OX11 0DE, Oxon, England, United Kingdom. § B.F.: BasCat - UniCat BASF Joint Lab, Technische Universität Berlin, Sekr. EW K 01, Hardenbergstraße 36, D-10623 Berlin, Germany. Funding

Financial support by the Federal Ministry of Education and Research (BMBF) within the CarboKat project (FKZ03X0204C) of the Inno.CNT alliance is gratefully acknowledged. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank M. Hashagen, Dr. A. Tarasov, and Dr. B. Johnson for assistance on experiments and scientific discussions.

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(1) van Dommele, S.; de Jong, K. P.; Bitter, J. H. NitrogenContaining Carbon Nanotubes as Solid Base Catalysts. Chem. Commun. 2006, 4859−4861. (2) Chen, C.; Zhang, J.; Zhang, B.; Yu, C.; Peng, F.; Su, D. Revealing the Enhanced Catalytic Activity of Nitrogen-Doped Carbon Nanotubes for Oxidative Dehydrogenation of Propane. Chem. Commun. 2013, 49, 8151−8153. (3) Kundu, S.; Nagaiah, T. C.; Xia, W.; Wang, Y.; Dommele, S. V.; Bitter, J. H.; Santa, M.; Grundmeier, G.; Bron, M.; Schuhmann, W.; Muhler, M. Electrocatalytic Activity and Stability of NitrogenContaining Carbon Nanotubes in the Oxygen Reduction Reaction. J. Phys. Chem. C 2009, 113, 14302−14310. (4) Arrigo, R.; Wrabetz, S.; Schuster, M. E.; Wang, D.; Villa, A.; Rosenthal, D.; Girsgdies, F.; Weinberg, G.; Prati, L.; Schlögl, R.; Su, D. S. Tailoring the Morphology of Pd Nanoparticles on CNTs by Nitrogen and Oxygen Functionalization. Phys. Chem. Chem. Phys. 2012, 14, 10523−10532. (5) Villa, A.; Wang, D.; Spontoni, P.; Arrigo, R.; Su, D.; Prati, L. Nitrogen Functionalized Carbon Nanostructures Supported Pd and Au−Pd NPs as Catalyst for Alcohols Oxidation. Catal. Today 2010, 157, 89−93. (6) Frank, B. Nanocarbon Materials for Heterogeneous Catalysis. In Nanocarbon-Inorganic Hybrids; Eder, D., Schlögl, R., Eds.; Walter de Gruyter: Berlin, Boston, 2014. (7) Tessonnier, J.-P.; Rosenthal, D.; Hansen, T. W.; Hess, C.; Schuster, M. E.; Blume, R.; Girgsdies, F.; Pfänder, N.; Timpe, O.; Su, D. S.; Schlögl, R. Analysis of the Structure and Chemical Properties of Some Commercial Carbon Nanostructures. Carbon 2009, 47, 1779− 1798. (8) Kundu, S.; Xia, W.; Busser, W.; Becker, M.; Schmidt, D. A.; Havenith, M.; Muhler, M. The Formation of Nitrogen-Containing Functional Groups on Carbon Nanotube Surfaces: A Quantitative XPS and TPD Study. Phys. Chem. Chem. Phys. 2010, 12, 4351−4359. (9) Stöhr, B.; Boehm, H. P.; Schlögl, R. Enhancement of the Catalytic Activity of Activated Carbons in Oxidation Reactions by Thermal Treatment with Ammonia or Hydrogen Cyanide and Observation of a Superoxide Species as a Possible Intermediate. Carbon 1991, 29, 707− 720. (10) Arrigo, R.; Hävecker, M.; Wrabetz, S.; Blume, R.; Lerch, M.; McGregor, J.; Parrott, E. P. J.; Zeitler, J. A.; Gladden, L. F.; KnopGericke, A.; Schlögl, R.; Su, D. S. Tuning the Acid/Base Properties of Nanocarbons by Functionalization via Amination. J. Am. Chem. Soc. 2010, 132, 9616−9630. (11) From September 1, 2015 Bayer MaterialScience is operating under the name Covestro AG.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b01594.

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REFERENCES

Results of BET, pH-titration, TPO, and catalysis (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Addresses †

K.F.O.: Universität Duisburg-Essen, Fakultät für Chemie, Anorganische Chemie and Center for Nanointegration Duisburg-Essen (CENIDE), Campus Essen, Universitätsstr. 7, D-45141 Essen, Germany. M


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