Catalyst Shelf Life: Its Effect on Nitrogen-Doped Carbon Nanotubes

Article pubs.acs.org/JPCC

Catalyst Shelf Life: Its Effect on Nitrogen-Doped Carbon Nanotubes Stefania Marzorati,†,§ Roberto Bresciani,† Stefano Checchia,† Stefano Antenucci,†,∥ Benedetta Sacchi,† Vladimiro Dal Santo,‡ Marco Scavini,†,‡ and Mariangela Longhi*,† †

Dipartimento di Chimica, Università degli Studi di Milano, Via Golgi 19, 20133 Milano, Italy CNR−Istituto di Scienze e Tecnologie Molecolari, Via Golgi 19, 20133 Milano, Italy

‡

ABSTRACT: In this work is presented a study of the shelf life of an iron-impregnated MgO (Fe/MgO) sample and an explanation of the effect of its aging on morphology and electroactivity of nitrogen-doped carbon nanostructures prepared by using this catalyst at different ages. Characterizations of Fe/MgO by thermogravimetric analysis (TGA), temperature-programmed desorption mass spectroscopy (TPD-MS), high-resolution Xray powder diffraction (HR-XRPD), and X-ray photoelectron spectroscopy (XPS) have been performed at different aging times. The catalyst is formed by crystalline MgO, Mg(OH)2, and FeO phases. During aging, CO2 adsorption induces the formation of amorphous carbonate, resulting in a decrease of specific surface area, a redistribution of porosity, a reduction of the iron percentage at the surface, and an increase of iron oxide particle size. Different surface and bulk properties are used to explain different morphologies and electroactivities of the nitrogen-doped carbon nanotubes synthesized at different catalyst aging times. The importance of a careful characterization of the catalyst as a function of shelf life, to optimize the synthesis and the morphology of the nitrogen-doped carbon nanotubes, is pointed out.

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INTRODUCTION In the last few decades, nanotechnology has attracted great interest because it spans a very wide range of scientific and engineering fields, and, due to its applicability in many disciplines, it is undergoing revolutionary developments. The first scientist who talked about nanotechnology, although lacking in calling it the specific word “nanotechnology”, was Richard Feynman in 1959, in his famous lecture “There’s plenty of room at the bottom”. He was talking about the possibility of “arranging atoms the way we want”. Among the almost countless nanosystems produced thenceforth, in 2002 an innovative form of carbon, carbon nanotubes (CNTs), was obtained and immediately had great impact on nanosciences because of unparalleled strength and high thermal and electrical conductivity.1 Because of the outstanding properties of nanotubes, many studies aim to possibly change their physical and chemical features, by introducing heteroatoms such as nitrogen (yielding nitrogen-doped CNTs, N-CNTs). Nanotubes doping can be achieved post-synthesis by functionalization or, more interestingly, during the synthetic process: within the graphene network, carbon atoms are substituted by heteroatoms; as an expected consequence, not only the morphology but also the final electronic properties become deeply affected. This makes N-CNTs interesting materials as electrocatalysts for oxygen reduction reaction (ORR).2,3 Many papers show that the ORR activity of N-CNTs can be modified by varying preparation method, precursor nature, and composition of the catalyst used in the nanotube synthesis.2−4 In this contest, it has been demonstrated that many parameters chosen during the chemical vapor deposition (CVD) growth affect the morphology of the final sample, i.e., the material supporting the catalyst, the metallic catalyst itself (nature, © XXXX American Chemical Society

oxidation state, etc.), the decomposing N-CNTs precursor, and other physical parameters such as the decomposition temperature.5 Catalytic chemical vapor deposition (CCVD) is widely used for the synthesis of CNTs. This method involves the catalytic decomposition of hydrocarbons or carbon monoxide on transition-metal particles.6,7 However, relatively few articles report detailed studies on the starting catalytic material.5,8 In the literature, the synthesis and characterization of Fe/MgO catalysts have been often reported,5,8−10 due to the easy removal of the support from carbon products by lixiviation without any damage.11 Moreover, to the best of our knowledge, the time evolution of Fe/MgO catalyst structure and its properties have never been reported. Conversely, in a previous paper of some of us10 about the synthesis of N-CNTs, surprising effects that aging time of the used catalyst (Fe/MgO) has on the produced N-CNTs were reported. In fact, by using Fe/MgO catalyst from the same batch but at different aging times in N-CNTs synthesis, nanotubes with different morphologies and electrocatalytical activities were obtained.10 After 1 day, samples are characterized by the presence of bundles and a multiwalled flexible teardrops chain; after 6 days, a mixed structure between nanochains and bamboo-like nanotubes appears. The formation of bamboo-like nanotubes is completed after 10 days, and this sample is the most active toward ORR. Finally, after a prolonged aging (70 days), the well-defined structure is mostly lost and the electrocatalytic activity is depressed.10 Received: May 24, 2017 Revised: July 13, 2017 Published: July 17, 2017 A

DOI: 10.1021/acs.jpcc.7b05037 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. N2 adsorption/desorption isotherms for all samples.

were heat-treated at 150 °C for 4 h under a N2 flow to remove adsorbed and undesired species from the sample surface. Thermogravimetric analysis (TGA) was performed in on a PerkinElmer TGA4000 under nitrogen atmosphere on samples weighing ∼2 mg. After 25 min in N2 flow at T = 30 °C, samples were heated up to 900 °C (10 °C min−1). Temperature-programmed desorption mass spectroscopy analysis (TPD-MS) was performed using a system composed by a U-shaped quartz reactor (positioned inside a furnace) and connected upstream to a gas line and a downstream online mass spectrometer with a quadrupole mass analyzer; the whole apparatus is described elsewhere.12 TPD-MS analysis was performed under Ar flow. After 30 min of equilibration and making sure that the gas tubes were clean (recording H2O, CO2, and Ar profiles), 20 mg of the sample, diluted in a quartz powder, was heated (10 °C min−1) up to 600 °C and then kept for 10 min at this temperature. X-ray photoelectron spectroscopy (XPS) measurements were performed with a M-Probe Instrument (SSI) equipped with a monochromatic Al Kα source (1486.6 eV). For all samples, the C 1s peak was taken as the internal reference at 284.6 eV. Precision of reported binding energies (BEs) is approximately ±0.2 eV. Peaks were fitted to Gaussian function after Shirley background correction13 using XPSPEAK 4.1 software. Quantitative data were carefully checked and reproduced several times. Uncertainty in spectral decomposition is estimated to be ±1%. High-resolution X-ray powder diffraction (HR-XRPD) measurements were performed at the ESRF (Grenoble) and Alba-CELLS (Barcellona) synchrotron facilities at the beamlines ID2214 and MSPD, respectively. Both are high-resolution powder diffraction beamlines equipped with a multi-1D detector and a Mythen linear detector, respectively.15 Powders samples were packed into 1 mm diameter Kapton capillaries and measured at room temperature. Kapton was selected in spite of quartz in order to minimize the diffraction by the capillary that would overlap with MgO diffraction peaks. For HR measurements the capillaries containing samples were

As a consequence, we believe that a deep study of the nature, the surface state, and the composition of the catalyst at different aging times is necessary in order to comprehend these tricky results. In this Article we report surface and bulk characterization of this catalyst at different aging times, aiming to correlate the catalytic properties and the electroactivity toward ORR of the synthesized nanostructure with the structure, microstructure, and surface properties of the MgO/Fe catalyst while aging.

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EXPERIMENTAL SECTION All chemicals and solvents were purchased from Sigma-Aldrich and used as received without further purification. According to the same procedure already adopted in a previous work,10 a catalyst based on MgO modified with an iron salt solution was prepared to be used as catalyst for the synthesis of N-CNTs (Fe/MgO). Briefly, MgO was synthesized by thermal decomposition (T = 400 °C, N2, 4 h) of MgCO3·Mg(OH)2· 5H2O. Modification with iron was obtained by suspending MgO in a Fe(NO3)3·9H2O solution (molar ratio 1:0.015) and sonicating it for 1 h. After filtration and drying (T = 115 °C, N2 flow, 5 h), the powder was pyrolyzed (T = 300 °C, N2, flow rate = 100 cm3 min−1, 7.5 h). The final product was placed in a tube closed with a cap and stored in a paper box in air without other precautions. As previously reported,10 a single Fe/MgO batch was prepared. At 1, 6, 10, and 70 days from the preparation, sample aliquots underwent analyses described below. In this Article, they are labeled as Fe/MgO1, Fe/MgO2, Fe/MgO3, and Fe/MgO4, respectively. These samples were employed to synthesize N-CNT samples, labeled CNT1, CNT2, CNT3, and CNT4, respectively. The Brunauer−Emmett−Teller (BET) specific surface area was obtained from the N2 adsorption/desorption isotherms at 77 K using a Micromeritics Tristar II apparatus. Specific surface area and porosity distribution were evaluated by BET and Barrett−Joyner−Halenda (BJH) theories using the instrument software (version 1.03). Before measurements, sample powders B


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structure of the Fe/MgO catalysts is subject to a modification with time that involves a rearrangement of the porosity distribution. This phenomenon might be due to a partial filling of small mesopores with a corresponding shrinkage of pores in favor of micropores formation. Thermal gravimetric analyses (TGA) of Fe/MgO1, Fe/ MgO2, and Fe/MgO4 samples are displayed in Figure 3. TGA

mounted on the axis of the diffractometer and spun during measurements in order to improve powder randomization. Fe/ MgO3 sample was analyzed at beamline ID22 @ ESRF using an energy of 31.0 keV (λ = 0.40002 Å); scans were collected up to a maximum wavevector Qmax [=4πsin θλ−1] ≈ 11 Å−1. Fe/ MgO4 sample was analyzed at beamline MSPD @ Alba-CELLS using an energy of 30.0 keV (λ = 0.4131 Å); scans were collected up to a maximum wavevector Qmax ≈ 11 Å−1. Rietveld refinements on the diffraction patterns were carried out using the program GSAS16 through its graphical interface EXPGUI.17 Phase weight fractions were determined from the relative scale factors returned by the refinement. Fitting accuracy was given by the program according to the parameter Rp.16

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RESULTS AND DISCUSSION Specific surface area and porosity distribution were determined by BET method. In Figure 1 N2 adsorption−desorption data are shown and specific surface area are reported. It is noticeable that the isotherm shape is similar. On the basis of IUPAC classification, they belong to the reversible type IV isotherm, with a hysteresis loop that is typical of mesoporous materials.18−20 The desorption branches are characterized by a H3-type hysteresis loop, which is attributed to a nonfixed aggregation of platelike particles yielding slit-shaped pores.18−20 BET data show remarkable differences in their surface area, surprisingly decreasing by aging, as reported in Figure 1, from 149 ± 1 m2 g−1 for Fe/MgO1 to 62.8 ± 0.1 m2 g−1 for Fe/ MgO4, the eldest sample. In Figure 2 pore area distribution of all samples is presented by subdividing pore diameter in four ranges: d < 2 nm

Figure 3. TGA curves of samples (atmosphere = nitrogen). (Inset) TGA of MgCO3·Mg(OH)2·5H2O (atmosphere = nitrogen).

data relative to sample Fe/MgO3 are missing. For comparison, TGA analysis of the precursor MgCO3·Mg(OH)2·5H2O is also shown in Figure 3 inset. In MgCO3·Mg(OH)2·5H2O a first small weight loss between 25 and 200 °C, attributable to physisorbed water, is followed by an appreciable loss between 210 and 300 °C, due to a crystallization water release,21 and, finally, by the main steep weight loss, between 380 and 500 °C, assigned to CO2 and water release from carbonate and hydroxyl decomposition, respectively.21 Considering TGA curves of the samples (Figure 3), three mass losses can be observed; the first one (T < 150 °C) is easily attributable to physisorbed water, and the amount of released mass does not depend on the aging time (Table 1). The third one (T > 300 °C) is attributable to the decomposition of the precursor partially undecomposed during the synthesis. In fact, as observed in the inset of Figure 3, at T = 400 °C, the calcination temperature, carbonates and hydroxyl groups decomposition is not complete, and moreover, it seems that there is not a dependence on the aging time of the amount of the mass loss (Table 1). On the contrary, the second one (150 < T < 300 °C) is more difficult to assign. In fact, it cannot be attributable to crystallization water release, as during the preparation task these samples have been pyrolyzed to 400 °C and, as observed above, crystallization water release is obtained between 210 and 300 °C and is complete at 400 °C. Moreover, it is interesting that this mass loss increases with aging time (Table 1). This behavior could be justified considering that during the shelf life Fe/MgO might adsorb some species that form stable compounds on the surface that are released between 210 and 300 °C. It is well-known that MgO and Mg(OH)2 are CO2 adsorbers.22−24 The carbonation process is slow at room temperature,24 with Mg(OH)2 being more efficient than MgO,24 although accelerated by air humidity,25 but it is thermodynamically favored (T = 298 K, pCO2 in air = 3 × 10−4 bar, ΔrGMgO = −28 kJ mol−1, ΔrGMg(OH)2 = −50 kJ mol−1). In Figure 4 TPD data for the CO2 (m/z = 44) profile of Fe/ MgO4 is shown. In the signal a relevant peak between 200 and

Figure 2. Pore area distribution of catalyst at different aging times: Fe/ MgO1, 1 day; Fe/MgO2, 6 days; Fe/MgO3, 10 days; Fe/MgO4, 70 days.

(micropores), 2 < d < 5 nm, 5 < d < 20 nm, 20 < d < 40 nm, and d > 40 nm. Pores with a diameter between 5 and 20 nm are the most abundant in all samples; apart from Fe/MgO4, which is characterized by the largest amount of these pores, there are no evident differences among the other samples. On the other hand, some interesting aspects could be pointed out considering smaller pores. Moving from Fe/MgO1 to Fe/ MgO2 and, finally, to Fe/MgO4, the decrease of surface area is in fact accompanied by an evident increase of micropores percentage and, as presented in Figure 2, a decrease in the abundance of pores between 2 and 5 nm. This suggests that the C


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The Journal of Physical Chemistry C Table 1. Weight Loss Attributions and Percentages of Samples sample

% mass loss (25−200 °C)

% mass loss (210−300 °C)

% mass loss (380−500 °C)

Fe/MgO1 Fe/MgO2 Fe/MgO4

2.5 2.5 2.5

1.4 1.2 3.7

5 4 6

Figure 5. Diffraction patterns and corresponding multiphase fits of (A) Fe/MgO3 and (B) Fe/MgO4 samples measured by synchrotron XRPD. Peaks marked with asterisks belong to the Mg(OH)2 phase; peaks marked with circles belong to the FeO phase; all the other peaks belong to the MgO phase. Figure 4. TPD-MS analyses curve for CO2 (m/z = 44) (Fe/MgO4 sample).

Additional scattering is clearly visible in both samples. The broad bump in the 2.0 < Q < 2.8 Å−1 range, a smaller, broad peak at Q = 3.5 Å−1, and a shoulder at Q = 4.0 Å−1 are all compatible with the presence of cubic FeO and trigonal Mg(OH)2 (space groups Fm3m and P3m, respectively). It must be noted that these features are sharper in the more aged Fe/ MgO4 sample, indicating the ongoing crystallization of the Mg(OH)2 separated from the MgO periclase phase. The low crystallinity of FeO in both samples, on the contrary, gives rise to broad signals in both diffraction patterns. A multiphase Rietveld refinement was performed on both XRPD patterns using these three phases. Refinements were first carried out on the Fe/MgO4 sample because its higher crystallinity compared with Fe/MgO3 allowed a more accurate peak attribution and provided a reliable first guess of the phase parameters for Fe/ MgO3. To reduce the number of free parameters, and thus parameter correlation, a single Debye−Waller factor was used for Mg, Fe, and O in each phase. Therefore, only one Debye− Waller parameter, four cell parameters, and three phase fractions were refined. For Fe/MgO3 sample the free parameters were a single Debye−Waller parameter, the cell parameter of the MgO phase, and three phase fractions. Refining the cell parameters of Mg(OH)2 and FeO led to instable refinement, probably due to their poor crystallinity and consequent superposition of broad diffraction peaks. As a consequence, the cell parameters for these two phases were fixed to the results of Fe/MgO4. As can be seen from the results reported in Table 2, the cell parameter of MgO did not change with time; this rules out formation of solvate crystals following encapsulation of water and/or CO2 in MgO that would have resulted in cell expansion. The weight phase fractions for Fe/MgO3 and Fe/MgO4 samples are reported in Table 2. The results of the quantitative phase analysis show that FeO is the main crystalline product of iron nitrate calcination and its fraction increases with time, suggesting that aging promotes the aggregation of iron species into FeO nanoparticles. Moreover, aging time increases the crystallinity of Mg(OH)2 phase. The decrease of Mg(OH)2 weight fraction with aging suggests that it is partially transformed into amorphous

300 °C with a maximum at T = 273 °C is observed. With the temperature ramp in TPD and in TGA analyses being the same, the temperature range in TPD is directly correlated to the one in TGA; therefore, it is possible to attribute the second mass loss in TGA to a release of CO2 adsorbed onto the surface of partially undecomposed precursor. This is in accordance with literature data.26 In fact, Song et al. show that adsorbed CO2 onto MgO, depending on the binding modes of CO2 molecules on MgO, is lost between 100 and 350 °C. Typically, in the range 200−350 °C, CO2, bonded at medium and strong basic sites to form bidentate or unidentate carbonate, respectively, is released.26 The maximum position of CO2 profile in Fe/MgO4 (T = 273 °C) indicates that CO2 strongly adsorbs at strong basic sites, forming unidentate carbonate (in ref 26, the maximum is located at T = 280 °C). The presence of strong basic sites is justified considering that calcination at 400 °C preferably induces the formation on MgO surface sites of strong basicity, i.e., low-coordination O2− anions.27 Adsorption of CO2 and, therefore, the formation of carbonates might also explain the decrease of specific surface area and the modification of porosity distribution with aging. In fact, considering the transformation of 1 g of MgO (0.025 mol) or 1 g of Mg(OH)2 (0.017 mol) into the equivalent amount of MgCO3 (0.025 or 0.017 mol, respectively), there is an increase of volume, from 0.279 to 0.712 cm3 for MgO (ΔV = +155%) and from 0.417 to 0.484 cm3 for magnesium hydroxide (ΔV = +16%) (at T = 25 °C, ρMgO = 3.58 g cm−3;28 ρMg(OH)2 = 2.4 g cm−3;29 and ρMgCO3 = 2.96 g cm−330). This expansion causes shrinkage of pores and, consequently, a decrease of the specific surface area experimentally observed with aging. A decrease of specific surface area and an increase of the smallest pores could hinder the diffusion of reactants toward catalytic sites, and thus the yield of nanostructures during the synthesis. Bulk composition of Fe/MgO3 and Fe/MgO4 was studied by high-resolution synchrotron XRPD. As shown in Figure 5, the most intense Bragg peaks at Q = 3.0 Å−1 and Q = 4.2 Å−1 belong to the MgO cubic periclase phase. D


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corresponding observed increase of volume might sink iron species below the attenuation length of the Fe 2p photoelectrons. With iron being the real catalyst for N-CNTs growth, a very low concentration of it on the surface of Fe/MgO4 might explain why N-CNTs were not obtained by using this catalyst in the synthesis.10 The spectra of iron in the highresolution region are characterized by a high noise related to low percentages of iron present on the surface; to obtain some information from these data, the region was therefore fitted considering only one Gaussian function, normalizing for the maximum and subtracting the baseline of each spectrum. Calculated maxima for all the samples have been superimposed, as shown in Figure 6. Calculated peaks relative to samples Fe/

Table 2. Results of the Refinements on Fe/MgO3 and Fe/ MgO4 Samplesa MgO

Mg(OH)2

FeO

sample

Fe/MgO3

Fe/MgO4

space group a/Å WF/% space group a/Å c/Å WF/% space group a/Å WF/% Uave/Å2 Rp

Fm3m 4.2284(1) 49.2(6) P3m ≡Fe/MgO4 ≡Fe/MgO4 38(1) Fm3m ≡Fe/MgO4 13(1) 0.0006(2) 0.0477

Fm3m 4.2242(2) 49(1) P3m 3.1349(3) 4.902(5) 33(2) Fm3m 4.324(2) 18(1) 0.0062(2) 0.0327

a For MgO phase, Mg(0, 1/2, 0) and O(0, 0, 0). For Mg(OH)2 phase, Mg(0, 0, 0), O(1/3, 2/3, 0.773), and H(1/3, 2/3, 0.581). For FeO phase, Fe(0, 1/2, 0) and O (0, 0, 0).

magnesium carbonate and/or its hydrated form, which probably constitutes the large amorphous scattering found in Fe/MgO samples (see the irregular background at low Q in Figure 5). These findings are therefore consistent with previous data from TGA and TPD as well as with the kinetic preferential carbonation of Mg(OH)2 in respect to MgO, as claimed by Fricker and Park.24 Results of XPS atomic percent compositions are reported in Table 3.

Figure 6. Peak fitting in XPS high-resolution spectra of Fe 2p3/2. (Inset) TEM images reproduced with permission from ref 10. Copyright 2015 Elsevier.

Table 3. Atomic Mg, O, C, and Fe Percent Sample Compositions and Mg/O Ratios sample

Mg %

O%

C%

Fe %

Mg/O ratio

Fe/MgO1 Fe/MgO2 Fe/MgO3 Fe/MgO4

38.7 34.5 42.9 41.9

51.2 48.7 49.0 53.1

7.6 16.0 11.6 4.3

0.5 0.4 0.6 0.1

0.8 0.7 0.9 0.7

MgO1, Fe/MgO2, and Fe/MgO4 clearly overlap, and surprisingly, the only nonoverlapping peak corresponds to Fe/MgO3 sample, which shifts to lower binding energies. This shift suggests that on the surface of Fe/MgO3 a higher surface amount of low-valent iron species than Fe3+ are present compared to the other samples of Fe/MgO. This result does not contradict previous XRPD results because these are bulk data, while XPS analysis involves the surface. It is worth remarking that the most structured and ORR-active carbon nanotubes in ref 10 were obtained by using Fe/MgO3 as a catalyst (see TEM images in the inset of Figure 6). The high-resolution C 1s region has been convoluted as shown in Figure 7; peaks relative percentage areas are reported in Table 4. As internal reference, bulk oxygen O 1s at BE = 530.5 eV has been used.31 In the first three samples only two peaks are detectable. The first one, at lower binding energies, BE = 285−286 eV, is the most intense and is due to adventitious carbon;31 the second one at BE = 289−291 eV is attributable to the carbon of magnesium carbonate,32−35 and its percentage increases with the aging time, in accordance with previous thermal data. In Fe/MgO4 a third peak is found at BE = 288 eV. This is assignable to carbon in the carbonyl group.35 Considering carbonate percentage and comparing it only to the sum of the contribution to the adventitious carbon percentage, it is possible to compare this percentage of the Fe/MgO4 sample to those of the first three samples (Table 4, data in the brackets). It is interesting to notice that carbonate percentage in the last sample confirms the similar upward trend of the first samples.

As expected, oxygen, magnesium, and iron are present on the surface of all samples. Carbon, as seen below, is mostly adventitious carbon. The Mg/O ratio is smaller than the stoichiometric one and independent of the aging time. The excess of oxygen could be due to the presence of either magnesium compounds containing more than one oxygen on the surface, e.g., hydroxide or carbonate, or iron oxide, whose presence is revealed by XRPD. However, considering iron percentage, the contribution of iron oxide to oxygen excess is very low and the major contribution is due to magnesium compounds, thus enforcing the hypothesis that amorphous magnesium carbonate and hydroxide are present in the sample. The absence of a trend with the aging time might be justified considering that carbonation involves at first only surface layers and then, during aging, deeper ones, which could be located below the attenuation length of the O 1s photoelectrons and therefore not be detectable. Moreover, carbonation, as seen above, involves MgO and, especially, magnesium hydroxide. In the latter case, the variation of Mg/O ratio is smaller (from 1:2 to 1:3, Δ = −0.17) than the former one (from 1:1 to 1:3, Δ = −0.67). For the first three samples (Fe/MgO1, Fe/MgO2, and Fe/MgO3), iron percentage remains almost unchanged with aging, but it drastically decreases in the last sample; this could be explained considering that the carbonation and the E


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Figure 7. XPS high-resolution spectra C 1s: (a) adventitious carbon; (b) carbonyl group; and (c) carbonate carbon.31−35

Figure 8. XPS high-resolution spectra O 1s: (a) MgO; (b) carbonate or CO; (c) OH; (d) chemisorbed water.31,34,36−38

Table 4. Relative Peak Areas (RPA%) of C 1s Peaks

Table 5. Relative Peak Areas (RPA%) of O 1s Peaks

C 1s % peaks

sample Fe/ MgO1 Fe/ MgO2 Fe/ MgO3 Fe/ MgO4

adventitious carbon 285−286 eV31

O 1s % peaks

carbonyl group 288 eV35

carbonates 290−291 eV32−35

86

14

84

16

83

17

63(82)

23

sample Fe/ MgO1 Fe/ MgO2 Fe/ MgO3 Fe/ MgO4

14(18)

MgO carbonate or CO 530.5 eV31 532 eV34

OH 533 eV36,37

46

31

24

35

49

16

31

53

16

15(20)

48(64)

12(16)

chemisorbed water 534 eV38

26

All these data confirm that during shelf life Fe/MgO, stored in a closed tube placed in a box in air, undergoes compositional variations and, as a consequence, modification of textural properties, mainly pore size distribution. As reported above, the very low percentage of iron at the surface in Fe/MgO4 can explain why this catalyst is not useful to form well-structured carbon nanostructures. However, differences in morphology and electroactivity of the nanostructures synthesized by using other catalysts are more difficult to rationalize considering only the iron amount on the catalyst surface. In fact, the first three samples have the same surface percentage of iron, although a larger amount of low-valent iron is detectable on the surface of Fe/MgO3 than on the other catalysts. Moreover, bulk analyses evidence that during aging an aggregation process of crystalline iron oxide occurs. This process could justify different morphologies of nanostructures. Actually, Yao et al. have

Similar results have been obtained considering O 1s highresolution region (Figure 8 and Table 5). In addition to an expected contribution of Mg−O at BE = 530.5 eV, taken as internal reference,31 which decreases with time, a contribution at BE = 532 eV, attributable to carbonate or carboxyl group34,36 and increasing with aging time, in accordance with previous data, is present. Finally, a peak at BE = 533 eV due to the hydroxide group37 is detectable. Moreover, in Fe/MgO4, a peak of chemisorbed water is also present.38 Data in the brackets were calculated by excluding the contribution of chemisorbed water. The increase of carbonate or CO contributions coupled with the decrease of MgO surface concentration with increasing shelf-life time well fits with the presence of amorphous (because it is not revealed by X-ray powder (XRP) diffraction) magnesium−carbonate at the surface of well-aged catalysts. F


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The Journal of Physical Chemistry C shown that the size of the iron particles determines the final morphology of N-CNTs and, as a consequence, the electrocatalytic activity of them.39 Larger aggregates induce the formation of thick hollow N-CNTs instead of thick bamboolike (nanocups) N-CNTs, while slender bamboo-like N-CNTs are only formed in the presence of very small aggregates.39 Similar results have been obtained in this work: CNT1, prepared by using Fe/MgO1, which might have the smallest aggregates of iron-based active centers, is characterized by the presence of bundles and multiwalled flexible tear-drop nanochains; CNT2, from Fe/MgO2, is characterized by a nanostructure morphology that varies from nanochains to bamboo-like; and, finally, in CNT3, synthesized by using Fe/ MgO3, the catalyst with the largest aggregates considering these three catalysts, a well-developed nanotube morphology appears with a continuous external wall and an internal structure clearly composed by nanocups.10 Moreover, in agreement with the results reported by Yao et al.,39 the most reactive structure toward ORR is the thick bamboo-like series of stacked nanocups, CNT3, which has the best performance toward oxygen reduction reaction (ORR).

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by MIUR under Project NAMEDPEM (PRIN 2011). V.D.S. thanks financial support from EIT Raw Materials through project FREECATS (Project no. 15054). The authors gratefully acknowledge the European Synchrotron Radiation Facility ESRF (Grenoble, France) (beamline ID22) and Alba-CELLS (Barcelona, Spain) Synchrotron Facility (beamline MSPD) for provision of beam time. They are also greatly indebted to Dr. Christina Drathen and Dr. François Fauth for kind assistance in using the ID22 and MSPD beamlines, respectively.

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CONCLUSIONS In this work the shelf life of Fe/MgO samples was studied in terms of both the material bulk and surface chemical composition and structure. Provided is an explanation of phenomena concerning the differences in the morphology and electroactivity of synthesized nitrogen-doped carbon nanotubes starting from Fe/MgO samples, as reported in ref 10. The catalyst is composed of MgO, Mg(OH)2, and FeO phases. During aging, CO2 is adsorbed on the more basic site on the surface of Fe/MgO with the formation of amorphous carbonate. This results in a decrease of specific surface area, due to the increased volume of MgCO3 with respect to those of MgO and Mg(OH)2, with a consequent decrease of mesopority and an increase of microporosity. Moreover, surface iron percentage decreases for long aging (70 days), most probably due to the carbonation process, while iron species aggregate with time. As already pointed out in the literature,39 the aggregation process could explain different morphologies and electroactivities of synthesized nanostructures as revealed in ref 10. Moreover, we have shown that prolonged aging depresses electrocatalytic properties of the MgO/Fe catalyst by reducing the amount of surface iron consequent to amorphous magnesium carbonate formation. Finally, the most important result of this work is highlighting that a careful characterization of the catalyst structure, microstructure, and composition (especially at the surface) as a function of shelf life is of utmost importance to optimize the synthesis, morphology, and electrocatalytic properties of nitrogen-doped carbon nanotubes.

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*Tel.: +390250314226; e-mail: [email protected]. ORCID

Mariangela Longhi: 0000-0002-0014-3576 Present Addresses §

S.M.: Università degli Studi di Milano, Dipartimento di Scienze e Politiche Ambientali, via Celoria 2, 20133 Milan, Italy ∥ S.A.: Università degli Studi di Milano, Dipartimento di Chimica, CRC Materiali Polimerici (LaMPo), Via Golgi 19, 20133 Milan, Italy G


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