Mesoporous and Graphitic Carbide-Derived Carbons as Selective and

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Chemistry of Materials

Mesoporous and Graphitic Carbide-Derived Carbons as Selective and Stable Catalysts for the Dehydrogenation Reaction Jan Gläsel,† Jiangyong Diao,‡ Zhenbao Feng,‡ Markus Hilgart,† Thomas Wolker,† Dang Sheng Su‡ and Bastian J.M. Etzold† †

Lehrstuhl für Chemische Reaktionstechnik, Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen, 91058, Germany

‡

Catalysis and Materials Division, Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang, 110016, China KEYWORDS: carbon catalyst, ethylbenzene, styrene, direct dehydrogenation, carbide-derived carbon

ABSTRACT: Dehydrogenation of ethylbenzene to styrene is one of the most important catalytic processes in chemical industry. While it was demonstrated that nanocarbons like nanotubes, nanodiamond or nanographite show high performance, especially selectivity, these powders give rise to handling problems, high pressure drop, hampered heat and mass transfer, and unclear health risks. More common macroscopic carbon materials like activated carbons show unsatisfying selectivity below 80%. In this study mesoporous, graphitic and easy to handle carbon powders were synthesized based on the reactive extraction of titanium carbide in a novel temperature regime. This resulted in extraordinary properties like a mean pore diameter of up to 8 nm, pore volumes of up to 0.90 ml g-1 and graphite crystallite sizes exceeding 25 nm. Exceptional styrene selectivities of up to 95% were observed for materials synthesized above 1300 °C and pretreated with nitric acid. Furthermore, the long-term stability of these non-nanocarbon catalysts could be demonstrated for the first time during 120 h time-on-stream.

INTRODUCTION Dehydrogenation of ethylbenzene to styrene is one of the most important catalytic processes in chemical industry.1,2 Nowadays, relatively inexpensive multi-promoted iron oxide catalysts with good resistance to feed impurities are used. Yet, these catalysts also possess some significant drawbacks like employing environmentally critical chromium compounds and being prone to quick deactivation due to potassium loss or coke deposition.3-5 Furthermore, excess steam is commonly utilized as cofeed to suppress catalyst coking, which dramatically increases the energy input of the entire process.6 In this sense, carbon based metal-free catalysts are of interest as they are ecologically benign. Due to their higher heat conductivity they can also limit hot spot formation, one reason for coke deposition.7 In particular, the oxidative dehydrogenation route has attracted broad scientific attention in carbon catalysis by demonstrating the excellent suitability of these versatile catalytic materials.2,8-11 Nevertheless, the oxidative dehydrogenation route involves the risk in handling flammable mixtures in combination with an exothermic reaction. Thus, a growing interest in the direct

dehydrogenation (DDH) of ethylbenzene under oxidantfree conditions arises. Here, carbon catalysts can allow for a steam-free synthesis, which is highly desired from the energy input point of view.12-15 It was demonstrated that nanocarbons like nanotubes, nanodiamond or nanographite show high performance in DDH of ethylbenzene. Styrene selectivities greater than 97% could be obtained.12-14 Unfortunately, more common carbon materials with high surface area like activated carbons didn’t show similar performance and exhibit low selectivity below 80%.5,12 Thus, for the technical application it would be necessary to employ nanocarbon powders as catalysts, which give rise to handling problems, high pressure drop, hampered heat and mass transfer, and unclear health risks.16,17 Hence, developing easy to handle carbon based catalysts with similar selectivity and stability as nanocarbons remains an ambitious but desirable task. A reason for the excellent performance of nanocarbons seems to be the combination of both high crystallinity and easy accessibility of the exohedral surface. Probably this favors the interaction of ethylbenzene with the carbon surface.12 Hence, mesoporous and highly graphitic carbons could be the material of choice for the DDH reaction.

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In this study carbide-derived carbons (CDCs) are employed and synthesized in a novel temperature regime to obtain mesoporous, graphitic and easy to handle DDH catalysts. CDC is commonly produced by the reactive extraction of metals or metalloids from carbides employing chlorine gas as extraction agent. Hereby, the initial shape of the precursor is typically preserved. The resulting CDC can either show the shape of a macroscopic powder or be prepared as monolith,18,19 foam,20 coating,21 or fibers,22,23 thus allowing the synthesis of easy to handle carbon materials. Furthermore, the applied extraction conditions and the choice of the carbide precursor allow to tune the textural properties and carbon microstructure with high accuracy.24 The temperature influence on the resulting structure has been intensively studied up to approx. 1200 °C.25-32 Exemplarily, for titanium carbide based CDC (henceforth denoted as TiC-CDC) the pore size enlarges from ultramicroporous to supermicroporous with higher extraction temperature. In parallel the degree of graphitization also increases to some extent.27 However, these materials still lack adequate mesoporosity and crystallinity for the application in DDH. Kravchik et al. presented the synthesis of TiC-CDC at 1800 °C where highly graphitic CDC was obtained, showing that the crystallinity can be further improved.33 Nevertheless, this material showed an extremely low pore volume of 0.29 ml g-1, hence it will show a very low number of active sites as carbon catalyst. Herein, the synthesis of easy to handle TiC-CDC powders (average particle size 85 µm) in the temperature regime from 1300 till 1585 °C is studied for the first time. It could be shown that the resulting mesoporous carbons combine a high pore volume and distinct crystallinity. Additionally, the influence of post-synthesis oxidative treatments to improve the catalytic activity of these novel materials was investigated. The study proves that combining CDC synthesis temperatures of 1400 and 1500 °C with a nitric acid treatment leads to high performance catalysts showing a selectivity towards styrene above 95%, negligible deactivation after 20 h time-on-stream and possibility for reactivation by oxidation in diluted air if necessary for industrial scale run times.

MATERIALS AND METHODS Experiments. The reactive metal extraction from titanium carbide (Goodfellow; >99.8%; dmean = 85 µm) was carried out at 500–1585 °C using diluted chlorine in helium as extraction agent. After the chlorination, the resulting CDC was post-treated with diluted hydrogen in helium and subsequently cooled down under helium purge. Further details of the chlorination procedure and experimental setup can be found in the Supporting Information. In order to introduce oxygen-containing surface groups, the CDC was dispersed either in nitric acid (AppliChem; 65 wt.-%) or sulfuric acid (AppliChem; 50 wt.-%) at 90 °C for 2 h according to a method described in literature.32 The gas-phase oxidation was

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carried out in a muffle oven under static air conditions at 350 °C for 0.5 h. The samples are denoted in the following manner, e.g. TiC-CDC-800-HNO3 for CDC produced from the precursor TiC at 800 °C and post-treated with nitric acid. Dehydrogenation of ethylbenzene (EB) was carried out at 550 °C in a fixed-bed quartz reactor with 0.1 g of catalyst at atmospheric pressure. The reactant (2.8% EB, total flow rate 10 ml min-1, He as balance) was fed to the reactor from a saturator kept at 39.8 °C. The inlet and outlet gas analysis was performed using an on-line gas chromatograph equipped with two columns: a HP-5 capillary column for the hydrocarbons and a CarboPlot capillary column for the permanent gases, coupled to FID and TCD detectors, respectively. Reactivation of the used catalyst was carried out at 400 °C for 2 h in 5% O2 diluted in He. The ethylbenzene conversion (XEB) and styrene selectivity (SST) were calculated according to the following equations: XEB = 1 – FCEB, outlet / F0CEB, inlet

(1)

SST = CST, outlet / (CST, outlet + CBZ, outlet + CTOL, outlet)

(2)

F and F0 are the flow rates of the outlet and inlet. CEB, CST, CBZ and CTOL denote the concentrations of ethylbenzene, styrene, benzene and toluene, respectively. The resulting carbon balance varied in the range of (100±4)% in all experiments. Characterization. The textural properties were studied using physisorption analysis with nitrogen at -196 °C (Quantachrome; Autosorb 1P). The speciﬁc surface area was calculated using Brunauer–Emmett– Teller (BET) multi-point method. Pore size distributions were determined from the adsorption branch by applying a QSDFT model for carbon with slit/cylindrical pores (QuadraWin version 5.02). A mean pore diameter (MPD) was calculated assuming slit pores: MPD = 2 Vp SBET-1. Water adsorption/desorption isotherms were determined gravimetrically at 40.2 °C (Surface Measurement Systems Limited; DVS Advantage ET 1). Raman spectra were recorded on a Horiba Jobin Yvon HR 800 spectrometer using a HeNe laser operating at 633 nm. Powder XRD analysis was performed with a PANalyticalX’Pert Pro MPD diffractometer using CuKα-radiation (40 kV, 40 mA) in the range from 2 to 80° 2Θ in steps of 0.03° and an acquisition time of 5 s per step. The composition (C, H, N, S, O) was determined using an elemental analyser (Euro Vector; EA3000). Temperature-programmed desorption was investigated using a thermogravimetric balance (Setaram Instrumentation Setsys 1750 CS). Hereby typically 30 mg of the sample were heated from 20 to 900 °C with a ramp of 5 K min-1 under nitrogen flow. Study of the carbon stability under oxidizing atmosphere was carried out on a Netzsch STA 409 PC. Briefly, 20– 30 mg of the sample was heated from 20 to 1000 °C with a ramp of 5 K min-1 under a flow of air. TEM and EELS was performed with a FEI TECNAI F20 equipped with a GIF Tridiem operating at 200 kV.


Page 3 of 10 RESULTS AND DISCUSSION

The textural properties of the novel TiC-CDC materials synthesized above 1300 °C were characterized using highresolution nitrogen physisorption analysis and compared to material produced at lower temperatures (typical isotherms can be seen in Figure 1A). The isotherm for TiC-CDC-1200 (nomenclature: carbide precursorsynthesis temperature in degree Celsius) shows a wider knee in the low-pressure range compared to a classical type I isotherm indicating that the pore structure consists of a broad range of micropores.24,27,29,34,35

Volume adsorbed / ml g-1 STP

A 600 500

1200 °C

400 300 200

1400 °C

100 0 0.0

1500 °C 0.2

0.4

0.6

0.8

1.0

Relative pressure p/p0 / -

The adsorption isotherm of TiC-CDC-1500 indicates a type IV shape associated with a pronounced H2 hysteresis loop, which can be attributed to desorption phenomena taking place in ink-bottle type micro-/mesoporous networks.36 Pore size distributions (PSDs) derived with the QSDFT model are compared in Figure 1B for materials synthesized at 1200 and 1500 °C. Despite a minor portion of 2.0–4.0 nm sized mesopores, TiC-CDC-1200 mainly exhibits micropores. In contrast in the novel temperature regime (1500 °C) mesopores ranging from 3.8 to about 30 nm are formed. While microporosity is still present the associated pore volume presents only 3% of the total pore volume. Thus, the bimodal pore structure suggested by the shape of the isotherm is also confirmed by the PSD. As it is known from literature mesopores in TiC-CDC are mostly created by voids between graphitic ribbons. The change in porosity can be explained by the more graphitic character with increasing synthesis temperatures, which will be shown later.35 Further textural features are given in Table 1. Employing the mean slit pore diameter as pore structure indicator, it gets obvious that the trend of increasing pore size known till 1200 °C is even more pronounced from 1300 to 1585 °C. Regarding the specific pore volume, needed for the aimed catalytic application, a volcano type dependence with the synthesis temperature is seen (Table 1). The highest pore volume with remarkable 0.90 ml g-1 was measured for TiC-CDC-1400. Table 1. Nitrogen sorption data synthesized from 800 to 1585 °C. TSyn. / °C

B Diff. pore volume / ml nm-1 g-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1200

1300

1400

1500

1585

1674

1204

771

464

283

170

Vp / ml g

0.76

0.77

0.84

0.90

0.71

0.69

MPD / nm

0.91

1.28

2.18

3.88

5.02

8.12

2

-1

-1

TiC-CDC-1500 0.05 0.00 2.00 TiC-CDC-1200 1.00 0.00 1

10 Pore size / nm

Figure 1. A) Nitrogen adsorption (closed symbols) – desorption (open symbols) isotherms of TiC-CDC synthesized at 1200, 1400 and 1500 °C; B) QSDFT pore size distributions for TiC-CDC-1200 and -1500.

TiC-CDC

800

SBET / m g

0.10

for

In order to elucidate the changes in the carbon microstructure wide-angle X-ray diffraction, Raman spectroscopy, EELS and HRTEM analysis were performed. Starting with 1200 °C reflexes for the graphitic (002) and (100/101) diffractions get obvious in the XRD patterns (Figure 2A). However, the broad and shallow manner implies a still predominantly amorphous carbon microstructure. In contrast, these reflexes are clearly developed for materials produced at 1400 °C and above. Besides, the higher-ordered (004) and (110) reflexes can be observed, indicating a strong increase in crystallinity. At 1585 °C an in-plane crystallite size of 32 nm and a stacking height of 20 nm are reached. Furthermore, the absence of additional reflexes in all patterns suggests that no residues of the precursor are left in the carbon.



fringes with increased dimensions in both length and stacking height indicate the formation of graphitic ribbons (Figure 3B). Evaluation of the Raman spectra (Figure S2) corroborates this finding.

20

40

1585 °C 1500 °C 1400 °C 1300 °C 1200 °C 800 °C

(110)

(004)

Intensity / a.u.

(100/101)

(002)

A

60

80

2 Theta / ° (CuKα)

B 100 Figure 3. A, B) TEM images of TiC-CDC-1200 and -1585.

sp2 fraction / %

The increasing graphitic character is supposed to strongly influence the gas/solid surface interaction, thus being relevant in the aimed catalytic application.38 To probe this, water ad- and desorption isotherms of the as produced materials were recorded (Figure 4).

95

90 50 85 500

1000

1500

Synthesis temperature / °C Figure 2. A) X-ray powder diffraction patterns of TiC-CDC synthesized at different temperatures; B) Influence of the 2 synthesis temperature on the sp -carbon fraction derived by EELS analysis.

In agreement with results reported in literature, the fraction of sp2-carbon determined from the C K-edge EEL spectra (Figure S3A) remains around 92% till 1000 °C (Figure 2B).27,37 Raising the synthesis temperature further, the share of sp2-hybridized carbon increases to extraordinary 99% for TiC-CDC-1585, which is close to the number of HOPG. The wide-angle XRD and EELS results are further supported by the HRTEM images. TiC-CDC synthesized at 1200 °C (Figure 3A) mainly exhibits an amorphous character. However, the local appearance of stacked but curved fringes implies the onset of graphitization. Upon raising the extraction temperature to 1585 °C parallel

Water uptake / wt.-%

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800 °C 1200 °C 1300 °C 1400 °C

40 30 20 10 0 0

20

40

60

80

Target p/p0 / % Figure 4. Water adsorption/desorption isotherms recorded at 40.2 °C for different TiC-CDC materials.

TiC-CDC-800 initially shows a low water uptake till 40% relative humidity (RH), which increases strongly with higher RH. The water uptake even at high RH


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remarkably lowers with increasing synthesis temperature. Above 1300 °C, nearly no water uptake (below 5 wt.-%) can be detected, which is advantageous for the DDH as the interaction of ethylbenzene with the carbon surface is strengthened in parallel due to the greater hydrophobicity. Another crucial advantage of the enhanced crystallinity may be attributed to the rise in oxidation stability. We probed it with thermogravimetric analysis in air (Figure 5A).

A

Mass / wt.-%

80 60 40 20

attributed to the desorption of adsorbed species.39 Figure 5B compares the onset temperature of oxidation, which increases monotonously from 473 to 520 °C for materials synthesized at 500 up to 1000 °C. Raising the synthesis temperature further to 1300 °C gives an over proportional rise till 631 °C. Later on the onset temperature changes in a more monotonous fashion again. Thus, especially the materials synthesized above 1200 °C exhibit an extremely high thermo-oxidative stability, which might be important for the reactivation of coked DDH catalysts by oxidation.12 Active sites for the carbon catalyzed DDH are oxygencontaining functional surface groups, in particular, ketonic carbonyl groups, which are rich in electrons.12 As produced CDC shows an extremely low amount of surface functional groups.32,40 In order to introduce oxygen functionalization, the materials were treated with nitric and sulfuric acid, as well as oxidized in air. Thermogravimetric analysis in inert gas was used to study the introduced surface functionalization by temperatureprogrammed desorption (TPD), employing the mass loss signal as desorption indicator.41,42 Figure 6 shows exemplary DTG curves for nitric acid treated TiC-CDC (further data is given in the Supporting Information Figure S4).

100

TiC-CDC-1585 Tonset = 677 °C

TiC-CDC-500 Tonset = 473 °C

0 200

400

600

0.8

800

TiC-CDC-800-HNO3 DTG curve / -103 K-1

Temperature / °C

B 700

600 Tonset / °C

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.6

TiC-CDC-1500-HNO3

0.4 0.2 0.0

500

200

400

600

800

Temperature / °C 400 500

1000

1500

Synthesis temperature / °C Figure 5. A) Thermogravimetric curves under air for TiCCDC-500, -800, -1200, -1400 and -1585; B) Onset temperatures for oxidation in air as a function of the synthesis temperature.

No measurable ash content remains after complete oxidation illustrating the high purity of CDC. The minor weight loss until 200 °C seen in Figure 5A can be

Figure 6. Derivative thermogravimetric curves (DTG) for TiC-CDC-800, -1200, -1300, -1400, -1500 after treatment with nitric acid.

In agreement with literature, despite the loss of some volatile adsorbed species till 200 °C no significant further mass loss is observable for the as produced CDC.39 Liquidphase treatment with sulfuric acid and oxidation in air mainly introduced low-temperature desorbing groups e.g. carboxylic or sulfonic ones (Figure S4). Whereas after the functionalization with nitric acid also high-temperature desorbing groups are visible in the DTG curves. For TiCCDC-800-HNO3 the total mass loss corrected for water desorption amounts 33.3 wt.-%. The analogues


Chemistry of Materials synthesized at higher temperatures show similar but flattened DTG profiles. The desorbed amount decreases continuously with increasing synthesis temperature to only 1.4 wt.-% for nitric acid treated TiC-CDC-1500. The decreasing oxygen functionalization is also corroborated by elemental analysis (data given in the Supporting Information Table S2). The findings can be attributed to the enhanced crystallinity and thus lower reactivity.32 The applicability of TiC-CDC synthesized at different temperatures and post-treated differently as catalyst for the DDH of ethylbenzene was studied in a continuous fixed-bed setup at 550 °C and 20 h time-on-stream (TOS).

A 100

40

90

30

TiC-CDC-800-HNO3

20

TiC-CDC-1500-HNO3

10 0 0

5

10

15

80

70

Styrene selectivity / %

Ethylbenzene conversion / %

50

60 20

Time-on-stream / h

conversion on the selectivity could be suppressed to be able to compare the selectivity obtained for the different catalysts.43 The degree of conversion and selectivity towards styrene is exemplarily given for nitric acid treated TiCCDC-800 and -1500 in Figure 7A. For all experiments within the first 2 to 3 h an induction period is observed, where the conversion decreases while the selectivity builds up. The catalytic results after 19 h TOS are summarized in Figure 7B. For the materials synthesized at 800 and 1200 °C the influence of the oxidative pretreatment is clearly visible. As produced materials always show a relatively low selectivity, which can be slightly increased by air oxidation or sulfuric acid treatment. Functionalization with nitric acid greatly boosts the selectivity without reducing the degree of conversion, e.g. for TiC-CDC-800 from 67 to 85%. The selectivities for CDC based catalysts produced till 1200 °C are in accordance with results reported for other activated carbons.5,9 Remarkably, the styrene selectivity strongly increases for synthesis temperatures above 1200 °C. For instance TiC-CDC-1500-HNO3 reaches a value of 95%, which is close to the ones obtained with nanocarbons.12-14 While the selectivity increases, the conversion exhibits an inverse dependence with respect to the CDC synthesis temperature. Most probably the number of active sites decreases, as the specific surface area and amount of oxygen functionalities (Figure 6) lowers. As it is obvious from Figure 7A, the catalysts suffer minor to strong deactivation indicated by the decrease in degree of conversion with time-on-stream. The change in activity is always accompanied by an increase in selectivity.

air

H2SO4

20

HNO3

90

80 10 70

Experiments were carried out at low degree of conversions. In this regime an influence of the degree of

air

0.6

H2SO4

0.4

HNO3

0.2 S X

0.0 -0.2 -0.4

°C 00 15 °C 00 14 C ° 00 13

°C 00 15 0°C 0 14 C ° 00 13

°C 00 12

Figure 7. Experimental data on the DDH (0.1 g carbon, -1 -1 550 °C, GHSV = 6000 ml g h ) A) Development of selectivity and conversion with TOS for TiC-CDC-800-HNO3 and -1500HNO3; B) Selectivity and conversion after 19 h TOS.

as produced

°C 00 12

60

0.8

C 0° 80

0

Change from 5 h to 19 h / ∆% ∆h

100 as produced



30

-1

B

C 0° 80

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Figure 8. Increase in selectivity and decrease of conversion from 5 to 19 h TOS.

For the sake of comparison the average activity loss and selectivity increase between 5 (after the induction period) and 19 h TOS were determined and are given in Figure 8. While for TiC-CDC-800 the degree of


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conversion lowers with a rate of 0.36 to 0.24% per hour, the loss in activity is less pronounced with increasing synthesis temperature. Finally for TiC-CDC-1500-HNO3 the drop of 0.02% per hour is close to the analytic accuracy. The superior stability of these novel carbidederived carbon based catalysts is believed to stem from the enhanced graphitic crystallinity as evidenced above. Independent of the type of CDC employed, nitric acid treated samples always show a higher selectivity already from the beginning and therefore less change later on compared to their analogues. The rise in selectivity may originate from dynamic changes of the carbon surface under DDH reaction conditions. Less-stable carboxylic acid, anhydride and phenol groups are likely to decompose to CO2 and CO.44 Probably more selective active sites are formed on the carbon surface or become accessible during this process, thusly depressing the formation of the by-products toluene and benzene.


50

100 Air exposure

40

90

30

TiC-CDC-1300-HNO3

80

20 70

10 0 0

20

40

60

80

60 100 120

Time-on-stream / h Figure 9. Development of selectivity and conversion with TOS for cycling DDH followed by exposure to diluted air at 400 °C on TiC-CDC-1300-HNO3.

In a long-term experiment, lasting 120 h in total, additionally the possibility to reactivate the catalyst by oxidation in diluted air at 400 °C was studied (Figure 9). TiC-CDC-1300-HNO3 was chosen for this experiment as it showed a slight deactivation of 0.12% per hour (Figure 8) but inherits also sufficient oxidation stability (Figure 5). The experiment demonstrated that reactivation is possible without losing selectivity during 5 consecutive cycles. This result promises a successful reactivation also under industrial operations conditions, especially when using CDC produced at temperatures above 1300 °C, as it exhibits an even higher oxidation stability.

CONCLUSION

In summary, reactive extraction temperatures for TiCCDC above 1200 °C allow to produce graphitic carbons with inner porosity and mesoporous character. The catalytic performance of these materials was studied in the DDH of ethylbenzene under steam-free conditions. The styrene selectivity tremendously improved with nitric acid treatment of the CDC and increasing synthesis temperature. CDC material produced at 1400 °C or higher synthesis temperature leads to catalysts that reach styrene selectivities of 95%. Furthermore, the high-temperature TiC-CDCs show low deactivation and adequate oxidation stability, hence reactivation by burning deposited coke is possible. Compared to nanocarbons, the powder catalysts employed were easy to handle in a fixed-bed reactor and the conformal synthesis process would also allow producing even bigger sized powders for the technical application

ASSOCIATED CONTENT Supporting Information Further experimental details, low-temperature nitrogen sorption data, TPD, Raman, EELS and elemental analysis. This information is available free of charge via the Internet at http://pubs.acs.org/.


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AUTHOR INFORMATION Corresponding Author *E-Mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT JG and BE gratefully acknowledge funding from the European Union’s Seventh Framework Programme for research, technological development and demonstration within the SusFuelCat project under grant agreement No 310490 (www.susfuelcat.eu). We thank T. Ariyanto, J. Landwehr and V. Kolb for comments on the manuscript. Dr. A. Inayat is acknowledged for high-resolution nitrogen sorption analysis.

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(44) Likodimos, V.; Steriotis, T. A.; Papageorgiou, S. K.; Romanos, G. E.; Marques, R. R. N.; Rocha, R. P.; Faria, J. L.; Pereira, M. F. R.; Figueiredo, J. L.; Silva, A. M. T.; Falaras, P., Controlled surface functionalization of multiwall carbon

nanotubes by HNO3 hydrothermal oxidation. Carbon 2014, 69, 311-326.



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Mesoporous and Graphitic Carbide-Derived Carbons as Selective and

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