Effect of an Ammonia Treatment on Structure ... - ACS Publications

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Effect of an Ammonia Treatment on Structure, Composition, and Oxygen Reduction Reaction Activity of FeNC Catalysts Ulrike I. Kramm,* Iris Herrmann-Geppert, Peter Bogdanoff, and Sebastian Fiechter* Helmholtz-Center Berlin for Materials and Energy (former Hahn-Meitner-Institut), Lise-Meitner-Campus, Hahn-Meitner-Platz 1, D-14109 Berlin, Germany ABSTRACT: For the first time, the effect on structure and composition by a second heat treatment in ammonia was studied in detail for FeNC catalysts prepared by oxalatesupported pyrolysis of chloroirontetrametoxyphenylporphyrin (Fe(TMPP)Cl). The oxygen reduction reaction (ORR) activity was determined by rotating ring-disk electrode (RRDE) measurements in acidic solution. To evaluate the origin of the observed increase in ORR activity, bulk elemental analysis 57Fe M€ossbauer spectroscopy, N2-sorption measurements, and X-ray diffraction were performed. A second heat treatment in ammonia was found to affect the ORR activity of FeNC catalysts significantly; induced by NH3 treatment, all catalysts contained iron nitride; as more nitride was formed, the effect on ORR activity was more pronounced. A complete removal of the nitride by a subsequent acid leaching was possible but decreased the ORR activity slightly. On basis of these results, we conclude that the nitride itself cannot be the site responsible for the main catalytic increase. However, its formation positively affects either the carbon environment or the constitution of catalytically active centers, thus enabling an increase of the turnover frequency.

1. INTRODUCTION Currently, platinum and its alloys exhibit the highest catalytic activities in fuel cells for the hydrogen oxidation reaction (HOR) and the oxygen reduction reaction (ORR). However, the comparable high price of Pt prevents commercial, large-scale usage of such fuel cells in, for example, the automotive industry. Due to the high overpotential, even for platinum in ORR, large quantities need to be utilized to get sufficient current densities. Therefore, the replacement of the cathode Pt catalyst is required to reduce costs. In 1964, Jasinski discovered the catalytic activity of metallophthalocyanines (Me = Co, Pt, Ni, Cu) as ORR catalyst in alkaline media.1 In the following years this observation was extended to several other N4-metallomacrocycles and to acidic environment24 as well. Using these well-defined molecules, Zagal et al. demonstrated the ORR takes place at MeN4 centers,5 where high active surface area and electron conductivity was obtained when macrocycles were impregnated on carbon supports. However, at low pH values these macrocycles showed only a limited stability. In 1976, Jahnke et al. showed that a heat treatment of such carbon-supported complexes led to remarkably increased ORR activities and improved stabilities.2 Even after a heat treatment at 1000 °C, a fraction of MeN4 centers remained intact.6,7 Thus, van Veen et al. proposed that the enhanced ORR activity after heat treatment was correlated to an improved electronic structure of active MeN4 centers.7 The impregnation of macrocycles on carbon black was reported by Bogdanoff et al. to enhance the ORR activity only to a certain upper limit depending on a specific loading.8 Consequently, r 2011 American Chemical Society

scientists searched for alternative preparation methods to improve the ORR activity and site density. In 1989, Gupta et al. successfully prepared FeNC and CoNC catalysts with sufficient activity by heating a mixture of carbon black, polyacrylonitrile, and related metal acetates (FeAc, CoAc),9 provided a minimum temperature of 600 °C was applied. Currently, several alternative approaches for the preparation of FeNC catalysts are known, such as the pyrolysis (i) of a carbonsupported metal salt in a nitrogen-rich atmosphere (ammonia, acetonitrile) or (ii) of a carbon-supported nitrogen-containing polymer in the presence of a metal salt.1016 For catalysts prepared at T g 900 °C in ammonia, the catalytic activity strongly depends on the concentration of micropores available in the catalyst.17 On the basis of this observation and with use of time-of-flight secondary ion mass spectroscopy (ToF-SIMS), X-ray induced photoelectron spectroscopy (XPS), and Raman spectroscopy, Dodelet et al. developed a model for active sites formed during such treatment.1719 According to their findings, the metal ions were coordinated to four pyridinic nitrogen atoms, whereas the nitrogen atoms were bonded to two adjacent graphene layers.18,19 To distinguish these centers from those of porphyrinbased catalysts, they were labeled as FeN2+2 center. In this work, we will address such catalysts as μ-FeNC, in order to emphasize that the micropore content (next to Fe and N) is a crucial factor to achieve the desired current densities. Received: August 3, 2011 Revised: October 18, 2011 Published: October 20, 2011 23417

dx.doi.org/10.1021/jp207417y | J. Phys. Chem. C 2011, 115, 23417–23427

The Journal of Physical Chemistry C The preparation of MeNC catalysts by plasma or reactive sputtering was reported by several authors as feasible techniques.2024 In the case of plasma treatment, the main advantages are (i) smaller particle sizes due to the missing sintering step of the macrocycles during carbonization and (ii) no need for a carbon support.23 To create larger densities of active sites, MeNC precursors (e.g., macrocyles, nitrogen heterocycles with added metal salt) can be heated in the presence of a template such as metal oxalates or silica. However, the subsequent removal of such templates by an acid leaching is mandatory in order to obtain high surface areas and site densities.6,8,2531 In the case of oxalate-supported pyrolysis, it was reported that several metal oxalates and (Me)NC precursors could be used.8,26,28,32,33 As iron oxalate was utilized as template in the pyrolysis of cobalt porphyrin, a spongelike carbon modification with a homogeneous distribution of catalytic centers over the complete material was obtained after a subsequent removal of the template by acid leaching.26,32 Moreover, the addition of sulfur to the precursor mixture enhanced the catalytic activity. During the heat treatment iron sulfide was formed instead of iron carbide, which was easily removed with a subsequent acid leaching. Such catalysts exhibited (i) lower concentrations of inactive byproduct, (ii) less graphitized carbon morphology, and (iii) more active sites (increased site density) compared to the catalyst prepared without sulfur addition.27 To understand the improved site density, we performed a comparative study for pyrolysis processes of sulfur-free and sulfur-added precursors by high-temperature X-ray diffraction (HT-XRD) and thermogravimetry coupled with mass spectroscopy (TG-MS).29 In this study, data obtained for the sulfur-free precursor revealed the iron carbide formation (and/or the related graphitization) induced a decomposition of MeN4 centers. As sulfur was added, the formation of carbides was suppressed and the final catalysts contained larger fractions of MeN4 centers than the sulfur-free catalyst. As these results indicate, an iron carbide formation should be avoided either by sulfur addition or by other processes to obtain a high density of catalytic active centers. They might be important with respect to any further effort in the preparation of MeNC catalyst.29 The activity of catalysts prepared by the oxalate-supported pyrolysis was improved by Koslowski et al. using a second heat treatment in different gas atmospheres (N2, CO2, or Ar).28,32 Catalysts that underwent a subsequent wet-chemical treatment were characterized by 57Fe M€ossbauer spectroscopy and rotating ring-disk electrode (RRDE).32 For the first time, a direct correlation of the iron content related to one specific FeN4 center and the kinetic current density was found. It was concluded that, in these porphyrin-based catalysts, the overall content of FeN4 centers was not of importance for the ORR activity, but the concentration of one specific FeN4 center with ferrous iron in low-spin state.32 In the case of carbon-supported chloroirontetramethoxyphenylporphyrin (Fe(TMPP)Cl), the effect of the heat-treatment temperature on ORR activity was investigated using as prepared catalysts and additionally heattreated material after a subsequent acid leaching.6 Similar to previous observations the ORR activity increased to a certain temperature (800 °C in this work) and decreased again for higher pyrolysis temperatures, whereas acid-leached samples exhibited higher current densities. Structural results obtained from XPS and 57Fe M€ossbauer spectroscopy gave evidence to the conclusion that the turnover frequency (TOF) correlated with (i) M€ossbauer’s isomer shift (as measure for the electron density of

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the iron site) related to the active FeN4 center and (ii) the N1s binding energy of the peak related to the metalnitrogen bond strength. These observations suggested a higher 3d electron density at the iron ion of active FeN4 centers was responsible for increased turnover frequencies after heat treatment.6 This is consistent with van Veen’s theory and our previous results, where only specific FeN4 centers were found to be ORR active in these types of catalysts.7,32 With reference to the correlation of TOF and isomer shift, catalysts with iron nitride failed to correlate as they exhibited unexpected high turnover frequencies.6 The effect of ammonia at 950 °C on ORR activity of carbon-supported Fe(TMPP)Cl was investigated by Meng et al.34 Low concentrations of ammonia (the ratio of NH3 in a NH3/Ar gas mixture varied) were sufficient to yield much higher ORR activities than the reference catalyst which was heat-treated in Ar atmosphere. The authors suggested the formation of a liquid iron nitride phase during the heat treatment which might thus play the crucial role for the generation of catalytic sites.34 A super proportional increase of current density was also found for catalysts prepared by the oxalate-supported pyrolysis with a second heat treatment in ammonia.28 In ref 28, it was proposed that different active sites (possibly similar to those of μFeNC catalysts, i.e., FeN2+2) compared to those of porphyrin-based catalysts (FeN4) might have been formed. The aim of this work is to clarify how a second heat treatment in ammonia affects the structural composition and ORR activity of porphyrin-based catalysts. Therefore, a catalyst was prepared by oxalate-supported pyrolysis of Fe(TMPP)Cl under addition of sulfur followed by a subsequent acid leaching of the heattreated product. With the aim to change the degree of reaction with ammonia, the time of the heat treatment in ammonia was varied for small quantities of the original catalyst. To prevent any electronic changes of active sites related to pyrolysis temperature,6 all catalysts were prepared at 800 °C. Catalysts were characterized after the second heat treatment and after a subsequent acid leaching. On the basis of structural analyses and electrochemical results, we discuss possible reasons for the superproportional increase in ORR activity. Finally, the achieved volumetric current density, turnover frequency, and site density of these ammonia-treated catalysts are compared to the DOE target and values reported for other (μ-)FeNC catalysts.

2. EXPERIMENTAL DETAILS 2.1. Catalyst Preparation. Preparation of the Original Catalyst by the Oxalate-Supported Pyrolysis. The oxalate-supported

pyrolysis was already described in detail for several metal porphyrins and metal oxalates.8,26,27,29,33 Therefore, the catalyst preparation is briefly summarized in this work. To obtain the original catalyst that was used as starting material for several seconds of heat treatment, two standard catalysts were blended. The preparation of each was as follows: Chloroirontetrametoxyphenylporphyrin (3.0 g, Fe(TMPP)Cl, TriPorTec, 96% purity), iron oxalate dihydrate (15.0 g, FeC2O4 3 2H2O), and sulfur (0.9 g) were mixed in a mortar, filled in a quartz boat, and transferred to a heating procedure under constant flow of nitrogen. The precursor was heated to 800 °C with a heating rate of 7.5 K/min, whereupon the temperature was held at 450 °C for 20 min and at 800 °C for 45 min. After the sample was cooled to 725 °C.36

Figure 2. Kinetic current density at 0.75 V as a function of the burn off induced by the ammonia treatment for catalysts before (2) and after a subsequent acid leaching (Δ).

first step of ammonia decomposition: 2xFe þ 2NH3 T 2Fex N þ 3H2 v

ð3:1Þ

second step of ammonia decomposition: 2Fex N T 2xFe þ N2 v

ð3:2Þ

Depending on the amount and constitution of iron, a fraction of ammonia will be disintegrated, and a smaller fraction of NH3 might react with the carbon matrix or surface groups. Regarding the carbon matrix, disordered carbon phases react much faster with ammonia compared to ordered phases; the related gasification reaction is given in refs 37 and 38. reaction of carbon with ammonia: Csolid þ NH3 f HCN v þ H2 v

ð3:3Þ

Induced by this reaction, the surface area is increased and basic N functionalities are integrated into the carbon matrix.17,19,38,39 Because the iron content as well as the carbon composition might change with time during the NH3 treatment, the burn off was chosen as the characteristic parameter to estimate the reaction with ammonia. It was defined by eq 3.4 as the difference in catalyst mass before (mass of original catalyst used for the ammonia treatment, mbefore) and after these treatments (mass achieved after ammonia heat treatment ( acid leaching, mafter) related to the initial mass (mbefore). mbefore mafter burn off ¼ ð3:4Þ mbefore For all samples with varying burn off (as indicated in Figure 1), RDE measurements were performed to determine the kinetic current density. The values obtained for 0.75 V were plotted in Figure 2 as a function of burn off for a potential of 0.75 V. As expected from our previous work,28 all samples showed much higher kinetic current densities compared to the original catalyst (in this work, factors of 10 30). Up to 25% burn off, there was a continuous increase of the ORR activity; for higher burn offs, the activity remained nearly constant.

Figure 3. Specific BET surface area (a), mesopore surface area (b), and micropore surface area (c) as a function of burn off for the catalysts directly obtained after the second heat treatment in ammonia (a, b; b, 9; c, 2) and those after the second heat treatment in ammonia followed by an acid leaching (a, O; b, Δ; c, 0).

Similar developments in activity were found by Jaouen et al. who prepared μ-FeNC catalysts at different pyrolysis temperatures (900, 950, and 1000 °C) by reaction of carbonsupported FeAc with ammonia. For an initial iron loading of 0.2 wt %, they found a maximum in kinetic current density toward oxygen reduction for 35% burn off, which was similar for all investigated pyrolysis temperatures.17 For our catalysts the maximum was at 25% burn off. This shift to a smaller burn off might have been induced by the higher iron content in catalysts presented in this work (compare Figure 7). Furthermore, in Jaouen et al. the ammonia treatment was needed to create active sites (FeN2+2/C-centers) that are believed to be formed in micropores.18,19 In contrast to Jaouen et al., our catalysts already exhibited active FeN4 centers integrated in the carbon network. 23420

dx.doi.org/10.1021/jp207417y |J. Phys. Chem. C 2011, 115, 23417–23427

The Journal of Physical Chemistry C

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Figure 4. Correlation of the kinetic current density at 0.75 V with the mesopore surface area for all catalysts obtained by the second heat treatment in ammonia, before (9) and after (0) a subsequent acid leaching.

N 2 -sorption measurements were performed to evaluate the influence of porosity and especially micropores on ORR activity. 3.2. N2-Sorption Measurements. In Figure 3, the Brunauer EmmettTeller (BET), mesopore, and micropore surface areas (SA) are presented as a function of burn off. Even the shortest ammonia treatment resulted in an increase of the overall surface area by approximately three times compared to untreated samples. Similar to our previous observations,26 the best correlation was found between the mesopore surface area and the kinetic current density as shown in Figure 4. The results indicate that the main ORR activity of our catalysts was generated in mesopores, even after these NH3 treatments. As mentioned in the Introduction, for μ-FeNC catalysts (prepared by NH3 treatment, T g 900 °C) it was shown that the ORR activity rises with increasing micropore surface area (determined by the density functional theory (DFT) method).17 The correlation of micropore surface area and ORR activity could not be obtained for our catalysts (neither using the Vt plot nor using the DFT method for calculating the micropore surface area). Thus, we conclude, for catalysts discussed in this work, the micropores play no significant role in the generation of ORR activity. This discrepancy could be due to the initial presence of active centers, lower NH3-treatment temperatures, or the presence of a different kind of active sites (FeN4 vs FeN2+2). To investigate the origin of the much higher activity, chemical and structural composition were determined. 3.3. Structural and Chemical Composition. In Koslowski et al.32 we prepared catalysts by the oxalate-supported pyrolysis of Fe(TMPP)Cl that were chemically and thermally treated to modify the structural composition and ORR activity. A correlation between one specific FeN4 unit (named D1 as characterized by 57Fe M€ossbauer spectroscopy: in average δiso = 0.34 mm s1, ΔEQ = 0.90 mm s1) and the kinetic current density toward oxygen reduction was found for the first time.32 M€ossbauer spectroscopy was performed on the original catalyst and catalysts directly obtained after the ammonia treatment (i.e., #A, #C. #E. #G, #I) to evaluate to which extent this iron modification or another might be responsible for the increased ORR activity, The effects of a subsequent acid leaching, the pendants of catalysts #C (highest burn-off, catalyst #D), and #E (highest ORR activity, catalyst #F) were studied as well. Related M€ossbauer spectra are presented in Figure 5.

Figure 5. 57Fe M€ossbauer spectra of the original catalyst (OC) and catalysts as obtained after the ammonia treatments (#I, #G, #E, #A, and #C) and after subsequent acid leaching (#F, #D). The labeling of iron species is indicated in the spectrum of sample #C, and the positions of the sextet lines are indicated by arrows in the spectrum of sample #G. In Table 1 the M€ossbauer parameters of each iron site and the assigned iron species are summarized.

The original catalyst (OC) as well as the ammonia-treated samples with a subsequent acid leach (#D, #F) exhibited similar M€ossbauer spectra containing the same three doublets and one singlet. In accordance with our previous publications, all three doublets (D1D3) were assigned to FeN4 centers that differ in their electronic environments, thus, yielding in different M€ossbauer parameters.6,40 The singlet was related to superparamagnetic iron,41 which could be encapsulated in the carbon matrix and hence is not easily removed by the acid treatments. In Table 1, the M€ossbauer parameters and the assigned iron modifications are summarized. All ammonia-treated but not leached catalysts contained an additional contribution; iron nitride (D4, sextet). The doublet is characteristic for a nitrogen-rich iron nitride (FexN, x < 2.1), while the sextet suggests a larger concentration of iron within FexN.42,43 With increasing nitrogen amount in FexN, the Curie temperature decreases from 535 to 9 K. Because M€ossbauer spectroscopy was performed at RT, doublets will appear for iron nitride species FexN with x e 2.1.42,44 Equations 3.1 and 3.2 indicate an iron nitride formation as intermediate product in the decomposition process of ammonia.36 Thus, it is reasonable to assume that this disintegration was initiated by the superparamagnetic iron present in all studied catalysts.28,29,32 Initially, these iron particles were encapsulated in the carbon matrix but 23421



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Table 1. M€ ossbauer parameters (δiso, Isomer Shift Related to α-Fe; ΔEQ, Quadrupole Splitting; H0, Internal Magnetic Field; fwhm, Full Width at Half-Maximum) and Assignment to Iron Species for Different M€ ossbauer Sitesa

a

The color code is similar to those used in Figures 5 and 8.

Figure 7. Results of bulk elemental analysis for iron (a), nitrogen (b), and sulfur (c) for catalysts directly obtained after the ammonia treatment (b, 9, 2) and after a subsequent acid leaching (O, 0, Δ).

Figure 6. (a) X-ray diffraction of the original catalyst (OC), catalysts with the longest ammonia exposure (#C) and highest kinetic current density (#E), and their acid-leached equivalents (#D and #F). (b) Reference data of different iron nitride modifications (JCPDS database).

might have been released by the partial carbon burn off during the second heat treatment in ammonia. The presence of the nitrogen-rich iron nitride was confirmed by X-ray diffraction as shown in Figure 6. X-ray diffraction measurements of the original catalyst (OC) and the ammonia-treated catalysts with the highest burn off (#C), highest ORR activity (#E), and their acid-leached pendants #D and #F, respectively, are presented in Figure 6. All catalysts showed broad reflections (approximately 2θ = 26 and 44°) assigned to the carbon (100) orientation. The diffraction patterns were similar to those of catalysts prepared by the oxalatesupported pyrolysis of porphyrins in the presence of sulfur, where disordered carbon agglomerates are formed during the heat treatment of the precursors.27,29 For both nonleached samples (#C, #E), additional sharp reflections were assigned to nitrogen-rich iron nitride, whereas the reflections were more pronounced in the sample with the highest burn off (please note:

In Figure 6, #C shows an additional reflection related to the sample holder (silicon) at 2Θ = 33°). Therefore, in agreement with M€ossbauer measurements, XRD confirmed the removal of iron nitride by acid leaching subsequently performed to the ammonia treatment. Elemental analysis was utilized to study changes in the elemental bulk composition induced by the NH3 treatment. In Figure 7, iron, nitrogen, and sulfur contents are given as a function of burn off. The results for sulfur (Figure 7c) were identical for both types of ammonia-treated catalysts (i.e., ( subsequent acid leaching). In each case, a drastic decrease in the sulfur content was monitored. A similar observation was found when foaming gas (90/10 N2/H2, not shown) instead of ammonia was applied during the second heat treatment (not shown). Therefore, we relate the removal of sulfur to its reaction with hydrogen that was formed in the process of ammonia decomposition (eq 3.2).45 As Figure 7a) demonstrates, the amount of iron in the ammonia-treated but not further conditioned catalysts rose linearly with increasing burn off. In contrast to this, the iron amount remained constant at 0.62 at. % for all acid-leached samples and so did the amount of nitrogen (Figure 7b, N ≈ 4.0 at. %). The nitrogen content of the nonleached samples increased to a maximum at 4.6 at. % nitrogen related to a burn off of 2535% and decreased afterward. Because the additional nitrogen due to the ammonia treatment was removed by a subsequent acid leaching, we concluded that an additional amount of nitrogen cannot fully account for the increase in activity. However, it should be noticed that all samples subsequently acidleached exhibited smaller kinetic current densities compared to their only ammonia-treated sister samples. Hence, we assume the removal of any related iron- and/or nitrogen-containing compound is associated with this partial decrease of ORR activity. According to our results, these compounds might be iron nitride particles or physisorbed nitrogen atoms. 23422



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Table 2. Summary of the Kinetic Current Density and Iron Contents Related to Each M€ossbauer Species for All Catalysts Investigated by M€ ossbauer Spectroscopya

a

Jkin(0.75)/

Fetotal(NAA)/

Fe(Sing)/

Fe(D1)/

Fe(D2)/

Fe(D3)/

Fe (FexN)/

label

burn off/%

(A g1)

(wt %)

(wt %)

(wt %)

(wt %)

(wt %)

(wt %)

OC #F

0.0 17.0

1.0 17.6

3.5 3.3

0.11 (0.02) 0.09 (0.03)

1.51 (0.06) 1.52 (0.20)

1.10 (0.14) 1.00 (0.09)

0.78 (0.09) 0.69 (0.22)

0.00 0.00

#I

17.5

13.2

3.9

0.25 (0.03)

1.28 (0.04)

0.57 (0.05)

1.72 (0.39)

0.11 (0.09)

#D

19.2

20.6

2.9

0.09 (0.03)

1.94 (0.08)

0.59 (0.09)

0.28 (0.16)

0.00

#G

23.3

20.2

5.3

0.36 (0.04)

1.96 (0.58)

0.63 (0.08)

2.07 (0.58)

0.30 (0.07)

#E

26.1

27.4

4.9

0.33 (0.03)

2.75 (0.10)

0.53 (0.09)

0.42 (0.19)

0.87 (0.06)

#A

37

24.5

5.0

0.61 (0.12)

1.81 (0.39)

0.52 (0.11)

1.32 (0.43)

0.75 (0.22)

#C

46.3

22.2

6.5

0.57 (0.07)

2.81 (0.18)

1.02 (0.14)

1.72 (0.19)

0.39 (0.08)

Catalysts are listed in order of increasing burn off.

3.4. Correlation of Iron Amounts with Kinetic Current Density. The amount of iron bonded in specific modifications

(as identified by M€ossbauer spectroscopy) was calculated to investigate the reason for the increase in activity induced by the second heat treatment in ammonia. A further additional purpose of this correlation was the investigation of those iron species responsible for the high ORR activity. Therefore, the overall iron content Fecatalyst (in weight percent as determined by NAA) and the relative absorption areas Amodification X within the M€ossbauer spectrum of each catalyst was used.6,32 Femodification X ðwt %Þ ¼

ðAmodification X ð%ÞÞðFecatalyst ðwt %ÞÞ 100% ð3:5Þ

The overall amount of iron, the calculated content of iron related to each absorption site Femodification X, and the kinetic current densities related to the oxygen reduction are summarized in Table 2 for all catalysts investigated by M€ossbauer spectroscopy. In Table 2 data are given in the sequence of increasing burn off. As mentioned above, it was possible to remove iron nitride (D4, sextet) by a subsequent acid leaching, while the catalytic activity stayed much higher compared to the original catalyst. Consequently, iron nitride was excluded as the origin of the main improvement in ORR activity. On the other hand, results indicated some oxygen might be reduced on the surface of these nitride particles, as the ORR activity was always smaller after acid leaching. Indeed, there are other transition metal nitrides able to reduce oxygen in acidic environment.46,47 Thus, iron nitride might reduce oxygen but is instable under acidic conditions. In a previous work, we suggested that induced by the NH3 treatment different active sites might have been formed.28 In Figure 5, however, no additional M€ ossbauer site related to a new type of FeN4 center was found. All investigated catalysts contained only such FeN4 centers already identified in previous works, where no ammonia was utilized.6,32 Therefore, another possible reason for the increased activity induced by the second heat treatment in NH3 might be the modification of any kind of the existing FeN4 centers (D1, D2, D3), to either reduce oxygen directly (D2, D3) or be more active (D1). These possibilities were investigated by a correlation of the iron contents of different FeN4 centers to the kinetic current densities Jkin, as shown in Figure 8. If, in addition to the already known active site D1, there was a significant contribution of the D2 and/or D3 centers to the ORR activity, a correlation between the relative amount of iron

bonded in this center and the kinetic current density should be obtained. According to Figure 8b,c, there are no indications for a significant contribution of the D2 and/or D3 centers to the kinetic current density. Moreover, the overall content of iron assigned to FeN4 centers (Figure 8d) is randomly distributed. Similar to our previous work,32 the iron content assigned to the doublet D1 correlates with the kinetic current density. However, the slope is about three times smaller matching the increase in TOF of approximately three times. As mentioned above, the catalysts discussed in Koslowski et al. were similar to those evaluated here, whereas changes of ORR activity and structural composition were achieved by different types of second treatments (wet-chemical and thermal) but no treatments with NH3 were involved. Therefore, it has to be clarified why the same active sites display a higher TOF after being exposed to a second heat treatment in ammonia. Meng et al. published a study on batches of Fe(TMPP)Cl impregnated on carbon black and heat-treated at 950 °C in a mixed gas atmosphere (Ar, NH3) with several different NH3 contents.34 Here, the formation of iron nitride induced by the presence of NH3 during the heat treatment was found as well, and samples containing iron nitride exhibited much higher ORR activities than those without nitride. Consequently, Meng et al. assumed that iron nitride was the origin of active site formation. In our recent work, we correlated the TOF with the isomer shift (which is related to the electron density at the iron nucleus) for carbon-supported Fe(TMPP)Cl pyrolyzed at different temperatures in inert gas atmosphere.6 Samples that contained iron nitride exhibited much larger TOFs than expected by considering the isomer shift. As a consequence, the related data points failed to correlate. Moreover, the results discussed in this paper show no significant change in isomer shift although a 3-fold turnover frequency was obtained. Thus, it is reasonable to conclude that any positive effect on the TOF induced by iron nitride does apparently not affect the electron density at the iron nuclei of active FeN4 centers.6 In addition, the integration of graphitic nitrogen heteroatoms led to higher ORR activities, which was explained by electron donation toward the FeN4 centers. However, as can be seen in Figure 7, the overall content of nitrogen is not necessarily increased. Therefore, the higher TOF induced by the ammonia treatment was not related to (i) an increasing electron density on active sites or (ii) a larger number of donating nitrogen atoms. Nevertheless, in Figure 9 the positive effect of iron nitride on ORR activity becomes evident. 23423



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Figure 8. Iron contents related to M€ossbauer sites D1 (a, 2), D2 (b, 9), and D3 (c, 1) and to all FeN4 centers (d, (, D1 + D2 + D3) as a function of the kinetic current density (0.75 V). The color code is similar to Figure 5 and Table 1.

Figure 9. Correlation of the iron content related to all FeN4 centers with the overall content of iron (a) and correlation of the iron content related to iron nitride with the kinetic current density at 0.75 V of the catalysts as obtained just after the ammonia treatment (b, b). The effect of acid leaching on ORR activity is indicated by Δ. Please note: acid-leached catalysts do not contain iron nitride; the values are those of their nonleached sister samples (i.e., FexN of #A for sample #B, FexN of #C for sample #D, and so on).

In Figure 9b, the content of iron nitride is given as a function of the kinetic current density for catalysts directly obtained after the second heat treatment in ammonia. The data were fitted with a linear slope of 0.055. Obviously, as more iron nitride is formed, a higher ORR activity is gained. In the case of the acid-leached equivalents (accompanied by the removal of iron nitride), the curve is shifted toward smaller current values, but the slope of ∼0.055 was still obtained. Although iron nitride was excluded as active site playing the major role for the catalytic activity, this observation underlines the fact that catalysts are positively affected by an iron nitride formation. One possible explanation for the higher TOF might be a changed reduction mechanism; thus, the hydrogen peroxide formation of the catalysts is affected. In Figure 10, the relative hydrogen peroxide formation is given as a function of the applied potential for the original catalyst and the sample with the highest ORR activity (#E) and its acid-leached product (#F). From Figure 10, it becomes evident that the hydrogen peroxide formation is increased for the NH3-treated catalysts. Furthermore, the curve of hydrogen peroxide vs potential shows a maximum at approximately 0.4 V, whereas the hydrogen

Figure 10. Relative hydrogen peroxide formation in dependence of the potential for the original catalyst (OC) and samples #E (highest ORR activity) and #F (acid-leached product of #E).

peroxide formation continuously decreased with decreasing potential for the original catalyst. In the case of the NH3-treated catalysts, the hydrogen peroxide formation displays an initial increase with decreasing potential. This could arise from an affected reduction mechanism by the modification with iron nitride. Otherwise, 23424



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Table 3. Comparison of Site Density SD, Turnover Frequency TOF, and Volumetric Current Density of Different (μ-)FeNC catalysts SD (av)/

SD (max)/

TOF(0.8V)/

max. Jvol(0.8V)/

measuring conditions

(1019 sites cm 3)

(1019 sites cm 3)

(e site1 s1)

(A cm3)

ref

RT, 0.5 M H2SO4

8.7

12.1

0.14

300 2.79

48 this paper

other FeNC catalystsa

RT, 0.5 M H2SO4

4.9

0.04

0.31

32

μ-FeNC catalysts (00.25 wt % Fe)

RT, 0.1 M H2SO4

0.14

0.14b

37

μ-FeNC catalysts (0.3 wt % Fe)

FC 80 °C

1.3c

0.40

0.82

37

μ-FeNC catalysts (1.0 wt % Fe, PFM)

FC 80 °C

4.3d

14.3e

98.3

48

CMFeC (4.9 wt % Fe)

FC 80 °C

21.1d

1.48e

50

48

PANIFeC (0.7 wt % Fe in FeN4)

FC 80 °C

0.79e

3.8f

49

DOE target value for 2015 NH3-treated FeNC catalysts

f

3.0

a

Catalysts were prepared by oxalate-supported pyrolysis of Fe(TMPP)Cl. The obtained original catalyst was subsequently wet-chemically and thermally treated to induce structural changes and changes of ORR activity. b The volumetric current density was calculated from the kinetic current density given in Figure 3 (2.4 A/g for U = 0.5 V SCE) for an iron content of 0.25 wt % Fe of ref 37 considering a Tafel slope of 70 mV/dec and assuming a catalyst density of 0.4 g cm3 (these values of the Tafel slope and catalyst density were taken for the calculations in the citated reference, as well). c Maximum site density that can be yielded before a decrease of the turnover frequency is observed. Please note: the cited work assumes the overall content of iron is ORR active.37 d The site density was estimated assuming that all iron is catalyticly active and the catalyst density is 0.4 g cm3. e The TOF was calculated with the given value of volumetric current density and the estimated site density according to eq 3.7. f The site density and volumetric current density were estimated assuming that only iron phthalocyanine-like species (compare Figure 7 of ref 49) are catalytically active and taken the current value (at 0.8 V) of the polarization curve in Figure 1a of ref 49. A catalyst density of 0.4 g cm3 was considered.

ORR active surface groups formed during the NH3 treatment yielding in an increased peroxide formation. Additionally, it is well-known that the carbon morphology is modified by a NH3 treatment.17,19,38 Hence, such modifications might be responsible for the increased TOF and/or hydrogen peroxide formation. However, on the basis of these results the entire process responsible for the improved ORR activity by NH3-induced iron nitride formation cannot be clarified; thus, further investigations are necessary. 3.5. Determination of Jvol, TOF, and SD. Because of the low costs of non-noble metal catalysts, the amount of catalyst used in FC can be increased to enable higher current and power densities. However, above a certain loading of catalyst, transport limitations will hinder a further increase in current densities. Therefore, a minimum value for Jvol is required. This value is calculated as the product of the mass-related current density Jkin. with the mass density of the catalyst material Fcat. as given in eq 3.6.

In this equation MFe denotes the molar mass of iron (MFe = 55.845 g mol1) and NA Avogadro’s constant; the catalyst density was assumed to be similar to other FeNC catalysts (Fcat. = 0.4 g cm3).14 While the site density is independent from the applied potential, the turnover frequency will rise with increasing overpotential (η = E0 U), i.e., decreasing potential U. In Figure 8a, the iron content is given as a function of kinetic current density for a potential of 0.75 V. Therefore, taking into account the obtained slope (i.e., weight percent of iron FeD1 [gFe gcat.1 100%] per kinetic current density Jkin. [A gcat.1] f yielding a slope of [gFe A1 100%]), an average turnover frequency for 0.75 V can be calculated according to

volumetric current density:

where the parameter e denotes the charge of a single electron. In literature target values for the volumetric current density (and turnover frequency) are given for a potential of 0.8 V.37,48 For catalysts, both potential values (0.75 and 0.8 V) are within the Tafel region (defined by a linear increase of lg(i) vs U and no mass-transport limitation). Therefore, the Tafel slopes of these NH3-treated catalysts (in average 80.9 mV dec1) can be applied to extrapolate the turnover frequency for 0.8 V as demonstrated in

Jvol ðUÞ ¼ Jkin: Fcat: ¼ SD TOFðUÞe

ð3:6Þ

Within the catalysts optimization process there are two possible modes to improve Jvol by (i) the increase of SD, i.e., the number of active sites per volume, and/or (ii) an enhanced TOF, i.e., the number of electrons that are transferred per active site and second. In what follows, both values will be determined for our catalysts. Regarding the site density the intercept y0 of the correlation line in Figure 8a was zero. Hence, all D1-related FeN4 centers seem to participate in the ORR. Therefore the site density could be estimated from the overall iron content related to D1 ([FeD1]) according to site density: SD ¼

½FeD1 NA Fcat:: MFe 100%

ð3:7Þ

average turnover frequency at 0.75 V: TOFð0:75VÞ ¼

100% MFe slope NA e

ð3:8Þ

average turnover frequency at 0.8 V: TOFð0:8VÞ ¼ TOFð0:75VÞ 10ð 50 mVÞ=ð80:9 mV=decÞ ð3:9Þ The maximum value of site density, the average turnover frequency, and the volumetric current density are given in Table 3 for a potential of 0.8 V. Additionally, Table 3 contains a comparison to the DOE target value for 2015 and to other FeNC catalysts.32,37,48,49 23425


The Journal of Physical Chemistry C As can be drawn from Table 3, catalysts investigated in this study are among those exhibiting the highest site densities compared to other FeNC catalysts, although only the content of iron related to the active sites was considered. This emphasizes the benefit of the oxalate-supported preparation, where without additional carbon support very high site densities are achieved.8,32,33 The turnover frequency of the presented NH3-treated catalysts are in agreement with the values reported by Jaouen and Dodelet for μ-FeNC catalysts prepared by an ammonia treatment of carbon-supported iron acetate (FeAc/CB) measured under similar conditions.37 However, in ref 37, the TOF was evaluated by considering the overall iron content. This approach should result in an underestimation of the TOF as only a fraction of the iron content in μ-FeNC catalysts is associated with the FeN2+2 sites in its most active state of catalytic activity.12 A forthcoming paper will treat this topic in detail. Moreover, catalysts prepared by the pore filling method (PFM) or the polymer approach showed TOF values up to 2 orders of magnitude higher.14,37,48,49 A change in conditions (from RRDE to FC) might enable even higher TOF values and volumetric current density for our catalysts. However, increased kinetics will not enable as high values as reported by Jaouen et al.48 This emphasizes the need for a further improvement in the turnover frequency of our catalysts for FC application in order to meet the target value for the volumetric activity. According to Kramm et al.6 and as already stated by other authors,18 improved TOFs were achieved, e.g., by increasing the catalyst preparation temperature.

4. CONCLUSION AND OUTLOOK To investigate the effect of ammonia treatment on catalysts prepared by the oxalate-supported pyrolysis of Fe(TMPP)Cl, an original catalyst was produced and small quantities were processed with a second heat treatment in ammonia, whereas the heat-treatment duration times were varied. N2-sorption measurements and RRDE results confirmed that most of the ORR activity was generated in mesopores similar to catalysts prepared by oxalate-supported pyrolysis.26 In appling structural analysis the relative content of FeN4 species was found to be similar for all catalysts. Furthermore, all ammonia-treated catalysts contained iron nitride which was easily removed in a subsequent acid leaching. On the basis of these results, we cannot confirm our previous assumption; a second heat treatment in ammonia leads to the formation of new, more-active iron-based sites.28 The results rather imply an improvement in the turnover frequencies of the same, formerly reported FeN4 centers6,32 compared to the standard catalysts and those subsequently heat-treated in other gases (e.g., N2, CO2). NH3 treatment induced an increase of TOF to 3.5 times higher values compared to our standard catalysts. Moreover, ORR activity seems to be partly generated by iron nitride, which was found to be unstable under acidic conditions. The presented results suggest that the formation of iron nitride accounts for the improved TOF values, as proposed by Meng et al.34 The potential dependent increased formation of hydrogen peroxide indicates a change in the reduction mechanism resulting in an enhanced TOF. However, on the basis of the presented results, the question of how the iron nitride formation affects the TOF cannot be answered satisfactorily. Although the catalysts exhibited comparatively high site densities, for FC application the TOF should be increased further.

ARTICLE

This might be achieved by (i) increasing the preparation temperature of the original catalysts, (ii) changing the temperature applied during the second heat treatment in ammonia, or (iii) increasing the amount of iron nitride that is formed during catalyst preparation. Furthermore, the effect of ammonia treatment on long-term stability of the catalysts has to be examined.

’ AUTHOR INFORMATION Corresponding Author

*Tel.: +49-30-8062-42927 (S.F.). Fax. +49-30-8062-42434 (S.F.). E-mail: [email protected] (U.I.K.); Fiechter@ helmholtz-berlin.de (S.F.).

’ ACKNOWLEDGMENT We thank Susann Schmidt and Alejandra Ramirez Caro for their valuable comments to improve the language. Furthermore, Neutron Activation Analysis at the reactor BER II of Dorothea Alber and her group is acknowledged. ’ REFERENCES (1) Jasinski, R. Nature (London) 1964, 201, 1212–1213. (2) Jahnke, H.; Sch€onborn, M.; Zimmermann, G. Top. Curr. Chem. 1976, 61, 133–182. (3) Kobayashi, N.; Nishiyama, Y. J. Electroanal. Chem. 1984, 181, 107–117. (4) van Veen, J. A. R.; Visser, C. Electrochim. Acta 1979, 24, 921–928. (5) Zagal, J. H.; Paez, M.; Silva, J. F. In N4-Macrocyclic Metal Complexes; Zagal, J. H., Bedioui, F., Dodelet, J.-P., Eds.; Springer: New York, 2006; pp 4182. (6) Kramm, U. I.; Abs-Wurmbach, I.; Herrmann-Geppert, I.; Radnik, J.; Fiechter, S.; Bogdanoff, P. J. Electrochem. Soc. 2011, 158, B69–B78. (7) van Veen, J. A. R.; van Baar, J. F.; Kroese, K. J. J. Chem. Soc., Faraday Trans. 1981, 77, 2827–2843. (8) Bogdanoff, P.; Herrmann, I.; Hilgendorff, M.; Dorbandt, I.; Fiechter, S.; Tributsch, H. J. New Mater. Electrochem. Syst. 2004, 7, 85–92. (9) Gupta, S. L.; Tryk, D.; Bae, I.; Aldred, W.; Yeager, E. B. J. Appl. Electrochem. 1989, 19, 19–27. (10) Bashyam, R.; Zelenay, P. Nature (London) 2006, 443, 63–66. (11) Bron, M.; Fiechter, S.; Hilgendorf, M.; Bogdanoff, P. J. Appl. Electrochem. 2002, 32, 211–216. (12) Herranz, J.; Jaouen, F.; Lefevre, M.; Kramm, U. I.; Proietti, E.; Dodelet, J.-P.; Bogdanoff, P.; Fiechter, S.; Abs-Wurmbach, I.; Bertrand, P.; Arruda, T.; Mukerjee, S. J. Phys. Chem. C 2011, 115, 16087–16097. (13) Lalande, G.; Côte, R.; Guay, D.; Dodelet, J.-P.; Weng, L. T.; Bertrand, P. Electrochim. Acta 1997, 42, 1379–1388. (14) Lefevre, M.; Proietti, E.; Jaouen, F.; Dodelet, J.-P. Science 2009, 324, 71–74. (15) Proietti, E.; Ruggeri, S.; Dodelet, J.-P. J. Electrochem. Soc. 2008, 155, B340–B348. (16) Wu, G.; More, K. L.; Johnston, C. M.; Zelenay, P. Science 2011, 332, 443–447. (17) Jaouen, F.; Lefevre, M.; Dodelet, J.-P.; Cai, M. J. Phys. Chem. B 2006, 110, 5553–5558. (18) Lefevre, M.; Dodelet, J.-P.; Bertrand, P. J. Phys. Chem. B 2002, 106, 8705–8713. (19) Charreteur, F.; Jaouen, F.; Ruggeri, S.; Dodelet, J.-P. Electrochim. Acta 2008, 53, 2925–2938. (20) Bradley Easton, E.; Bonakdarpour, A.; Dahn, J. R. Electrochem. Solid-State Lett. 2006, 9, A463–A467. (21) Harnisch, F.; Savastenko, N. A.; Zhao, F.; Steffen, H.; Br€user, V.; Schr€oder, U. J. Power Sources 2009, 193, 86–92. 23426



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Effect of an Ammonia Treatment on Structure ... - ACS Publications

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