Partial oxidation of ethanol to acetaldehyde over surface-modified single-walled carbon nanotubes

Applied Catalysis A: General 469 (2014) 8–17

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Partial oxidation of ethanol to acetaldehyde over surface-modified single-walled carbon nanotubes Inusa Abdullahi a , Taylor J. Davis a , Dong M. Yun b , Jose E. Herrera a,∗ a b

Department of Chemical and Biochemical Engineering, Western University, London, Ontario, Canada N6A 5B9 Department of Civil and Environmental Engineering, Western University, London, Ontario, Canada N6A 5B9

a r t i c l e

i n f o

Article history: Received 15 August 2013 Received in revised form 10 September 2013 Accepted 17 September 2013 Available online 27 September 2013 Keywords: Ethanol Partial oxidation SWCNT Optical absorption FTIR Raman TPO TPD XPS.

a b s t r a c t The catalytic activity of surface-modified single-walled carbon nanotubes (SWCNTs) for partial oxidation processes was tested. A battery of characterization techniques including temperature programmed desorption (TPD) Fourier transform infrared (FTIR), and X-ray photoelectron spectroscopy (XPS) were used to probe the degree and type of oxygen bearing functional groups introduced on the nanotube surface and responsible for catalytic activity. Raman and optical absorption spectroscopy and temperature programmed oxidation (TPO), were used to probe the SWCNTs structure before and after reaction to monitor structural changes that take place during partial oxidation of ethanol to acetaldehyde. The results indicate a strong link between ketonic surface groups (C O) on the SWCNTs surface and its catalytic activity for partial oxidation processes. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Carbon nanotubes (CNTs) are finding an increasing number of applications in catalysis, either as supports or catalysts on their own [1–3]. Carbon itself as a catalytic material has significant advantages over the traditional metal-supported systems, owing to the unique controllability of both its surface chemistry and electron density through surface functionalization [4]. Carbon based catalysis is in fact regaining lots of attention due to sustainability issues and recent push for the development of metal free technologies. It is recognized that carbon materials performance in catalysis can be influenced both by their texture and surface chemistry [1,4,5]. In contrast to amorphous carbon materials, CNT materials offer the opportunity of a better control on types and number of active sites. Multi-walled carbon nanotubes (MWCNTs) have been used as active catalysts in oxidative dehydrogenation (ODH) of butane, propane and ethylbenzene [6–9] while graphite nanofibers have also shown a significant activity and selectivity in the oxidation of ethanol to acetaldehyde and ethyl acetate [10]. The nature and concentration of surface functionalities on these carbon materials can be modified by appropriate thermal or chemical treatments.

For instance, gas or liquid phase oxidation can be used to increase the concentration of surface oxygen groups while heating under an inert atmosphere may remove some of these functionalities [8,11,12]. Herein, we report single-walled carbon nanotubes (SWCNTs) as an alternative to these carbon-based systems. Contrasting to MWCNTs and carbon nanofibers, SWCNTs offer the opportunity of a careful tailoring of active catalytic sites through well-established surface functionalization protocols [13–15] which can be used to obtain very specific types of catalytic chemical functionalities on the carbon surface. The main objectives of our work are to investigate the effect of SWCNT-based catalyst pretreatment temperatures in the attachment of oxygen functionalities on the carbon surface and its concomitant effect on ethanol partial oxidation activity and reaction selectivity as well as to identify the nature and concentration of the surface functionalities responsible for catalytic activity.

2. Experimental 2.1. Synthesis and purification of SWCNTs

∗ Corresponding author. Tel.: +1 519 661 2111x81262; fax: +1 519 661 3498. E-mail address: [email protected] (J.E. Herrera). 0926-860X/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apcata.2013.09.027

The SWCNTs used for partial oxidation of ethanol were obtained by catalytic decomposition of methane at 800 ◦ C on 2 wt.% Co–MgO

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catalyst. Full details of the synthesis and purification protocols have been described previously [16]. 2.2. SWCNTs characterization Raman spectra of SWCNTs were obtained to investigate their structure and structural changes during the introduction of surface functionalities and partial oxidation of ethanol. A ReniShaw microRaman 2000 system was used. The spectra were obtained using the 632.8 nm excitation laser wavelength (He–Ne laser) in the range of 120–2000 cm−1 in micro mode, and the acquisition time was 20 s at 2 mW for each spectra. Six scans were taken at different points of every sample and the average used as the representative spectra. TPO was used to identify the different carbonaceous species present in the SWCNT material before and after the introduction of surface functionalities and after partial oxidation of ethanol. The TPO experimental procedure has been described elsewhere [16]. TPD experiments were performed using the same experimental setup in a similar fashion, except that, in TPD the heating of the SWCNTs material was conducted in pure helium (Praxair, UHP) instead of O2 /He. The amounts of CO and CO2 desorbed from the SWCNTs samples under thermal helium treatment were quantitatively converted to methane in a methanator, and the FID used to monitor methane formed. Optical absorption spectroscopy was used to investigate the diameter distribution and carry chirality assessments of the SWCNT materials before and after the introduction of surface functionalities and after partial oxidation of ethanol. Optical absorption experimental procedure has been described previously [16]. FTIR spectra of as-purified and pretreated samples at different temperatures were recorded on a Vertex 70 FTIR Spectrometer (Bruker, Germany) in the range of 400–4000 cm−1 wavenumber range. The samples were analyzed as pellets mixed with ZnSe at a mass ratio of 1:100. A hydraulic press was used to pelletize the sample at 10 MPa for 3 min. XPS analysis was run on a Thermo Scientific K-Alpha XPS spectrometer (Thermo Fisher, East Grinstead, UK). The samples were run at a take-off angle (relative to the surface) of 90◦ . A monochromatic Al K␣ X-ray source was used, with a spot area (on a 90◦ sample) of 400 ␮m. Charge compensation was not needed. Position of the energy scale was adjusted to place the main C 1s feature (C C) at 284.6 eV. Spectrum parameters were; survey – 200 eV pass energy, low resolution – 150 eV pass energy, 0.2 eV step. Quantification for the high resolution – 25 eV pass energy. The instrument and all data processing was performed using the software (Avantage) provided with the instrument. Modified Scofield parameters were used which were provided with the instrument. The purified SWCNTs material was further characterized by chemical analysis using an ICPMS to determine the amount of impurities remaining after purification. 2.3. Ethanol partial oxidation tests SWCNTs powder was pelletized to 425 ␮m size before introduction in the reactor catalyst bed. Steady-state partial oxidation reactions of ethanol were conducted using 17 mg catalyst in a continuous flow fixed bed quartz glass reactor oriented vertically inside an electrically heated furnace at atmospheric pressure. The height of the catalyst in the reactor was approximately 0.2–0.4 cm while the heated length was 32 cm. The reactor was 50 cm long with an inner diameter of ∼5 mm. Temperature was measured with a Ktype thermocouple inserted into the furnace and positioned within the catalyst bed. To introduce oxygen functionalities before reaction, all catalysts were pretreated in 5% O2 /He gas mixture (Airgas, 88 mL/min) at 200, 250, 300, 350 and 400 ◦ C for 3 h. After pretreatment absolute ethanol (Brampton, Ontario) was introduced

9

Table 1 Experimental conditions used for partial oxidation of ethanol. He/O2 (mL/min) O2 (mL/min) C2 H5 OH(l) (mL/h) C2 H5 OH/O2 molar ratio

WHSV ((mol C2 H5 OH/mol C) h−1 )

88 64 48 24

1.33 0.97 0.73 0.36

4.4 3.2 2.4 1.2

0.11 0.08 0.06 0.03

0.16 0.16 0.16 0.16

into the reactor by vaporizing it into the flowing O2 /He mixture at 150 ◦ C using a microsyringe pump. Reactant conversions were varied by changing the O2 /He flow rate (1.47–0.40 cm3 s−1 ) at constant ethanol partial pressures (0.23 kPa) and temperature (200 ◦ C). The different weight hourly space velocity (WHSV) values used are shown in Table 1. The gas flow was regulated through precalibrated mass flow controllers with digital read-out units (MKS Instruments). These reaction conditions were selected to ensure ethanol conversions below 30% so that mainly differential conditions can be assumed; similar conditions have been previously reported in the literature [17–19]. Reactant and product concentrations were measured using an online gas chromatograph (Shimadzu gas chromatograph, GC-2014) containing a capillary column (BP-5, 30 m × 0.53 mm, 1.0 ␮m thickness) connected to a flame ionization detector. Catalytic tests were monitored for at least 8 h time on stream, varying the space velocity. Combustion products (CO and CO2 ) were not observed. The total carbon balance on the system was always above 98.0%. The catalyst behavior was evaluated in terms of the following parameters: the conversion of ethanol is defined as the ratio between reacted and fed ethanol. Conversion =

ethanol(in) − ethanol(out) × 100% ethanol(in)

(1)

Product selectivity is defined as ethanol transformed to each product with respect to the ethanol reacted, thus the percentage selectivity to acetaldehyde is; Selectivity =

acetaldehyde(out) × 100% ethanol(in) − ethanol(out)

(2)

A set of preliminary experiments were conducted in order to determine that the partial oxidation process is entirely catalytic (i.e. the product, acetaldehyde is not formed by oxidation of ethanol by oxygen functionalities on the nanotube). For this purpose, first a blank experiment (no SWCNT catalyst) was carried out in O2 /He (after exposing the system to the same pretreatment protocols in O2 /He described in the experimental section), we observed zero ethanol conversion at 200 ◦ C at all residence times, which proved that the contribution of homogenous gas-phase reactions was negligible. Another set of experiments were also conducted with the catalyst in O2 /He at 200 ◦ C, but without ethanol feed (after exposing the system to the same pretreatment protocols in O2 /He we described in the experimental section). We did not observed evolution of gas phase carbon bearing molecules under these reaction conditions. Further experiments were conducted with the catalyst in helium alone (without oxygen), in the presence of ethanol feed. Again, we observed zero ethanol conversion at 200 ◦ C at all space velocities. We repeated this experiment at higher temperatures (260 ◦ C) for the case of the catalyst pretreated in O2 /He at 200 and 300 ◦ C. No conversion was observed at all spaces velocities except for 0.36 h−1 WHSV which showed (∼4%) ethanol conversion with a high selectivity to ethylene (98%) as compared to ethanol conversions around 88% (acetaldehyde selectivity 96%) obtained in the presence of oxygen under the same conditions. This experiment again suggested that direct reaction between oxygen functionalities present in the nanotube surface and the ethanol

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Fig. 1. Ethanol conversion (left vertical axis) and acetaldehyde selectivity (right vertical axis) for catalyst pretreated at 300 ◦ C and partial oxidation conducted at 260 ◦ C, at 0.36 h−1 WHSV.

feed does account for the observed ethanol conversion. To confirm this, we carried a long term experiment at high ethanol conversions (O2 /He pretreatment at 300 ◦ C, 0.36 h−1 WHSV, reaction temperature 260 ◦ C). The results are shown in Fig. 1. By carrying a mass balance on the amount of oxygen necessary to convert the ethanol into acetaldehyde at the observed selectivity and conversion levels and comparing this value with the amount of oxygen functionalities present on the nanotube catalyst (as quantified by TPD, see Section 3.2.1), our results indicate that the process is catalytic. The time required for the oxygen functional groups present on the nanotube surface (0.714 mg of atomic oxygen as obtained by TPD) to be depleted if they alone were responsible for catalytic activity is around seven minutes, after which time the nanotube surface would have run out of oxygen functionalities to maintain the observed levels of ethanol conversion to acetaldehyde. This clearly is not the case, as we kept running the experiment for more 24 h. The results clearly show that ethanol conversion and acetaldehyde selectivity remain constant at 88% and 96% respectively. In order to verify quantitatively that the SWCNTs are not depleted of oxygen-bearing functionalities during reaction we conducted TPD experiments for pretreated catalyst before and after partial oxidation of ethanol as well, the obtained results are discussed in detail in Section 3.2.1. They indicate that the SCWNT materials are not depleted of oxygen during ethanol partial oxidation. 3. Results and discussion 3.1. Effect of catalyst pretreatment on catalytic activity and reaction selectivity Fig. 2 shows typical steady-state ethanol conversion and the selectivity to acetaldehyde at 200 ◦ C as a function of time on stream for three different weight hourly space velocities (WHSV), in the case of the SWCNTs catalyst pretreated at 200 ◦ C. Results for runs at 200 ◦ C obtained on catalysts pretreated at other temperatures are shown in Table 2. In all cases the selectivity to acetaldehyde was observed to be above 95%, only ethylacetate was observed as reaction byproduct besides acetaldehyde. These results indicate that with decreasing WHSV the conversion of ethanol increases and that ethanol conversion increased with pretreatment temperature up to 300 ◦ C. Above 300 ◦ C pretreatment temperature, ethanol conversions decreased. These results point to a link between catalyst pretreatment temperatures and catalytic activity, indicating

Fig. 2. Catalytic activity data of pretreated catalyst at 200 ◦ C for ethanol partial oxidation at 200 ◦ C. Left vertical axis: conversion of ethanol (N). Right vertical axis selectivity to acetaldehyde (䊉) as a function of time on stream at different weight hourly space velocities; WHSV 1 = 1.33 (mol ethanol/mol C h−1 ); WHSV 2 = 0.73 (mol ethanol/mol C h−1 ); WHSV 3 = 0.36 (mol ethanol/mol C h−1 ).

that catalytic activity reaches optimal value only at a certain pretreatment temperature. It was also observed that acetaldehyde selectivity did not change with catalyst pretreatment temperature. We further tested the activity of the catalyst and product selectivity at higher reaction temperatures, by increasing the reaction temperature at 20 ◦ C interval until 260 ◦ C. The results obtained for ethanol partial oxidation conducted at 260 ◦ C at 0.36 h−1 WHSV over the catalyst pretreated in O2 /He 300 ◦ C is shown in Fig. 1. A key parameter in carbon based catalytic materials is the surface chemistry of the carbon species active for catalysis. For the case of previously tested activated carbon and CNT-based materials in oxidative dehydrogenation reactions, the literature indicates that the active sites are most likely to be the carbonyl and quinone surface groups [6,7,11,20,21], although carboxylic functional groups have been also identified as active sites for dehydrogenation reactions [22,23]. To get a first insight on the role of oxygen heteroatoms present on the SWCNTs surface in partial oxidation activity, the SWCNT materials were subjected to removal of surface oxygen functionalities by thermal treatment in Ar/H2 for 3 h. To this end the SWCNT sample was first pretreated in O2 /He at 300 ◦ C followed by a helium stream for 30 min and finally exposed to a 25 mL/min, Ar/H2 (5% H2 and 95% Ar) flow for 3 h at two different temperatures (450 and 600 ◦ C) and used for ethanol partial oxidation at 200 ◦ C. As shown in Fig. 3, the hydrogen pretreatment significantly affected ethanol conversion, although selectivity was not affected. More experiments were performed at 200 ◦ C and at different space velocities using the catalyst treated in Ar/H2 at 600 ◦ C for 3 h, ethanol conversion dropped to 6, 14 and 17% compared to 19, 27 and 38% obtained on O2 /He pretreated SWCNTs at WHSV of 1.33, 0.73 and 0.36 h−1 respectively. Further experiments were conducted using Table 2 Observed steady state ethanol conversion and acetaldehyde selectivity data obtained at 200 ◦ C for different SWCNTs pretreatment temperatures. (WHSV h−1 )

1.33 0.97 0.73 0.36

Pretreatment temperature (◦ C) 400 350 300 250 Average ethanol conversion (%) at 200 ◦ C

200

12 15 19 30

9 11 13 17

16 19 23 34

19 22 27 38

14 17 21 31

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Fig. 3. Ethanol conversions on 300 ◦ C pretreated catalyst, then treated in hydrogen at 450 and 600 ◦ C and partial oxidation reactions conducted at 200 ◦ C for WHSV of 1.33 h−1 .

SWCNTs annealed at 800 ◦ C in pure helium for 2 h, with the aim to fully remove oxygen functionalities, at WHSV of 1.33, 0.73 and 0.36 h−1 , respective ethanol conversions obtained were 6, 9 and 12% compared again to 19, 27 and 38% observed obtained on catalyst pretreated in O2 /He at 300 ◦ C. These experiments suggest that oxygen functional groups on the catalyst surface are responsible for the catalytic activity for partial oxidation of ethanol on the nanotube materials. SWCNTs used for partial oxidation of ethanol were grown by catalytic decomposition of methane on 2 wt.% Co–MgO catalyst. To evaluate the potential impact of any remaining cobalt metal in the SWCNTs after purification, we carried a blank experiment using the catalyst used for SWCNT growth. For this purpose, the 2 wt.% Co–MgO catalyst was reduced in hydrogen and heated in helium under the same conditions used to grow the SWCNTs [16], pelletized and used as a catalyst for partial oxidation. For the selected run, 17 mg of the reduced Co–MgO catalyst was pretreated at 300 ◦ C in He/O2 and partial oxidation of ethanol conducted at 200 ◦ C at 1.33 and 0.36 h−1 WHSVs. The amount of cobalt in this catalyst is 2 wt.%, while in the SWCNTs after purification it was below 0.9 wt.% as confirmed by (ICPMS) chemical analysis. These blank experiment yielded an ethanol conversion 4 and 8% which was very low when compared to the one displayed by the purified SWCNT material (18 and 38%) under the same conditions (Fig. 4). Moreover, previous characterization results on this material [16] indicated that most of the surface cobalt species interact strongly with the MgO support. We are certain that these cobalt species are not present after purification since the acid treatment removed most of the MgO support (less than 0.002 wt.% remaining in the purified SWCNT materials as indicated by ICPMS analysis). Rather, any remaining cobalt species identified in the purified sample are most likely encapsulated inside graphitic layers in the SWCNTs tips and not exposed to the gaseous reactant, hence, unable to participate in ethanol partial oxidation reactions [6,24–26]. Therefore, it can be asserted that remaining cobalt species in the SWCNT purified material have a very minimal influence on the reaction; rather, the origin of catalytic action in partial oxidation is clearly traceable to the SWCNTs. 3.2. Catalyst characterization In order to investigate changes in structure/morphology of the catalyst due to pretreatment and partial oxidation, the pretreated and spent catalysts after reaction were characterized by Raman,

Fig. 4. (a) Ethanol conversion at 200 ◦ C on reduced 2 wt.% Co–MgO catalyst and 300 ◦ C pretreated SWCNTs catalyst for partial oxidation conducted at (a) WHSV 1.33 h−1 and (b) WHSV 0.36 h−1 .

optical absorption spectroscopy and temperature programmed oxidation. The results obtained were compared with those obtained on the purified material before and after O2 /He pretreatment at different temperatures and after ethanol partial oxidation. Fig. 5 shows the Raman spectra obtained on these materials for the samples before (a) and after (b) ethanol partial oxidation. For the samples before partial oxidation pretreated in O2 /He at 200, 250 and 300 ◦ C the Raman spectra is not significantly different. The radial breathing mode frequencies remained consistent and are similar to those observed in the material before O2 /He pretreatment, an indication that oxidation in oxygen did not lead to significant changes in their diameter distribution. On the other hand, higher D band intensities were observed in the Raman spectra of pretreated samples at 350 and 400 ◦ C. This is attributed to an increase in structural defects as a result of a larger oxygen content introduced at higher pretreatment temperatures [27,28]. It can also be observed in Fig. 5(a) that the SWCNT pretreated at 300 ◦ C in O2 /He shows a slight decrease in the intensity of the D band (compared to the samples pretreated at temperatures below 300 ◦ C), probably due to the removal of amorphous carbon during oxygen pretreatment at 300 ◦ C. Shown in Fig. 5(b) is the Raman spectra of SWCNTs after ethanol partial oxidation at 200 ◦ C. Again, for the samples pretreated below 300 ◦ C, the radial breathing mode frequencies and the D band remain essentially the same compared to the samples before partial oxidation, For sample pretreated at 350 and 400 ◦ C, the intensity of the D band in the Raman spectra showed

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(a)

400 °C pretreat

Intensity (a.u.)

350 °C pretreat

300 °C pretreat 250 °C pretreat 200 °C pretreat Before pretreat 0

500

1000

1500

2000

-1

Raman shift (cm )

(b)

400 °C pretreat Fig. 6. Optical absorption spectra of the SWCNT catalyst before and after pretreatment in O2 /He at 300 ◦ C and of the spent catalyst after ethanol partial oxidation at 200 ◦ C.

Intensity (a.u.)

350 °C pretreat 300 °C pretreat

100 ◦ C in oxidation temperature in their TPO profiles. This phenomenon has been previously observed and is attributed to the removal of amorphous carbon from the nanotube walls as a result of pretreatment in oxygen [16].

250 °C pretreat 200 °C pretreat Before pretreat 0

500

1000

1500

2000

-1

Raman shift (cm ) Fig. 5. Raman spectra of pretreated catalysts samples; (a) before and (b) after ethanol partial oxidation at 200 ◦ C.

a decrease after partial oxidation, an indication that some amorphous carbonaceous impurities might have been removed during ethanol conversion. The fact that the D band of the SWCNT catalysts pretreated in O2 /He at 200, 250 and 300 ◦ C remains similar after partial oxidation reaction temperatures suggest not only that the structure of the catalyst remained the same, but also that significant amorphous carbon deposition did not occur during partial oxidation tests. Further analysis of the morphology of the catalyst for ethanol partial oxidation was carried through optical absorption experiments on the nanotube sample before and after pretreatment in O2 /He at 300 ◦ C and after partial oxidation at 200 ◦ C. As shown in Fig. 6, the spectral features are almost identical with no measurable changes in diameter and chirality distributions. This further demonstrates the stability of the nanotube morphology under the reaction conditions tested. TPO was used to examine the stability of the catalyst during ethanol partial oxidation. In order to make a meaningful comparison, samples before and after pretreatment in O2 /He at 300 ◦ C and after partial oxidation at 200 ◦ C were tested. Fig. 7 shows the TPO profiles obtained. There was no shift in the oxidation profile to lower temperatures in the spent SWCNT catalysts compared to the material after pretreatment. This clearly indicates that there was no significant amount of amorphous or poorly graphitized carbon deposited on the catalyst during ethanol partial oxidation. This further confirms the stability of these catalysts during ethanol partial oxidation. Compared to the sample before pretreatment, the pretreated and spent catalyst samples showed an upshift of almost

3.2.1. TPD analysis To quantitatively confirm that the SWCNTs are not depleted of oxygen functional groups during reaction we conducted TPD experiments for pretreated catalyst before and after partial oxidation of ethanol. Fig. 8 shows the TPD profiles obtained for the case of a catalyst pretreated at 400 ◦ C (highly oxidized sample) before and after ethanol partial oxidation at WHSV between (1.33 and 0.36 h−1 ) for a total of 10 h time on stream. The TPD results indeed showed a slight gain in oxygen content on the nanotube surface after ethanol partial oxidation. The surface oxygen content for the pretreated catalyst before reaction was 7.2 wt.% and increased slightly to 7.5 wt.% in the spent sample after partial oxidation. We also probed the oxygen functionalities present on the SWCNTs-catalyst after pretreatment in O2 /He, using TPD, FTIR and XPS. Fig. 8 also shows typical TPD profiles obtained, in this case for as-purified and SWCNT samples pretreated in O2 /He at 200 and

Fig. 7. TPO profiles of SWCNTs before and after pretreatment in O2 /He at 300 ◦ C and spent catalyst after ethanol partial oxidation at 200 ◦ C.

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13

1064 spent 400 °C pretreat catalyst

2856

2927 FID signal (a.u.)

400 °C pretreat

3442

1467 1562

1428

951

1641 400 °C pretreat 350 °C pretreat

300 °C pretreat

As-purified

0

200

300 °C pretreat

Transmittance (%)

200 °C pretreat

400

600

800

250 °C pretreat 200 °C pretreat Annealed sample

Temperature (°C)

OH

Fig. 8. TPD profiles of as-purified, pretreated SWCNT samples in (5% O2 /He) gas mixture at different temperatures and spent catalyst after ethanol partial oxidation at 200 ◦ C.

C-O

C=O

1000

C-H C-C

C-H C-O C=C

4000 3500 3000 2500 2000 1500 1000

500

-1

Wavenumber (cm ) 300 ◦ C

before ethanol partial oxidation. Surface oxygen groups on carbon materials are removed during annealing in helium in the form of CO and CO2 . There is no universal agreement in the literature with respect to the assignment of carbonaceous materials TPD peaks to specific surface oxygen groups as the peak temperatures may be affected by the texture of the carbon material, the heating rate and the experimental setup used [8]. We quantified the number of oxygen functionalities removed during the TPD process by directing the stream containing CO2 and CO generated during the TPD process to a methanator, and the methane thus generated was quantified using an FID detector. The system was calibrated in such a way that the integrated area values can be converted to the corresponding number of moles of CO2 formed during desorption, from which the amount of functionalized carbon and oxygen on the SWCNT surface can be calculated. Table 3 reports the results obtained in terms of number of moles of CO2 evolved during TPD and the corresponding oxygen content (wt.%) on the SWCNT samples, assuming a 1:1 C/O molar ratio. The amount of oxygen functionalized carbon shows a general correspondence to catalytic activity of the pretreated catalyst for ethanol conversion as depicted in Table 2; except for the samples pretreated above 300 ◦ C which displayed lower catalytic activity for ethanol partial oxidation but clearly show a larger degree of oxygen functionalization as the Raman (Fig. 5a) and TPD data (Table 3) clearly indicates. To try to explain this observation we initially attempted the assignation of the peaks observed on the TPD profiles to different types of oxygen containing groups present on the catalyst surface such as carbonyl, quinone, lactone and ether chemical functionalities. However, the literature does not offer a clear assignment between peak position and type of oxygen functionality [8,11,29]. Table 3 Amount of O2 in SWCNT samples, obtained by TPD of pretreated samples at different temperatures. Catalyst pretreatment temperature (◦ C) Moles of CO2 evolved (␮mol)

Oxygen in sample (wt.%) by TPD

He-annealed sample (800 ◦ C) Pretreated in O2 /He at 200 ◦ C Pretreated in O2 /He at 250 ◦ C Pretreated in O2 /He at 300 ◦ C Pretreated in O2 /He at 350 ◦ C Pretreated in O2 /He at 400 ◦ C

1.1 2.7 3.7 4.2 5.1 7.2

2.4 3.9 9.2 10.9 12.7 26.8

Fig. 9. FTIR spectra of SWCNTs samples pretreated under different conditions.

3.2.2. FTIR spectroscopy analysis FTIR spectroscopy was used for the qualitative analysis of the functional groups present on the SWCNTs surface after pretreatment at different temperatures. This method is reported to be particularly effective for highly oxidized carbon samples as the intensity of the absorption bands is very small for samples with a low amount of oxygen functional groups [8]. The interpretation of the spectra is also complicated by the fact that each functional group originates several bands at different wavenumbers [8]. The FTIR spectra of SWCNTs pretreated at different temperatures and the SWCNTs sample annealed in helium at 800 ◦ C are shown in Fig. 9. The spectra show a broad band centered around 3445 cm−1 which indicates the presence of various hydroxyl moieties on the SWCNTs surface [30]. However the relative intensity of this band is lower for the case of the annealed SWCNTs sample. Bands observed at 2927 cm−1 and 2856 cm−1 can be assigned to aliphatic C H/C C stretching. These bands are usually located at defect sites on the sidewall surface of CNTs [31]. The C O band characteristic of quinone groups is observed at 1641 cm−1 [32]. The band at 1562 cm−1 is attributed to the C C aromatic stretching of CNT backbone [33] and bands in the range of 1467–1428 cm−1 can be attributed to -CH3 asymmetric bending bands, strongly adsorbed water (OH in-plane deformation vibration) and overlapping bands in region characteristic of C O moieties [33,34]. The band at 1064 and 951 cm−1 can be attributed to the stretching vibration of C O in ether and alcohol functional groups, respectively [8,31]. Peaks below 900 cm−1 are difficult to assign because they represent too complex a structural signature [35]. As shown in Fig. 9 there is not considerable shift in the peak positions of the SWCNTs pretreated at different temperatures, but the relative intensity of the peaks linked to carbon–oxygen functional groups increased with the O2 /He pretreatment temperature. In agreement with the Raman and TPD observations, the FTIR data suggests that more oxygen functionalities were attached to the SWCNTs at higher pretreatment temperatures. 3.2.3. XPS analysis XPS analyses were performed on selected pretreated samples before ethanol partial oxidation test. Table 4 shows the results of

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Table 4 Fitting results for the XPS deconvoluted C 1s spectra and proportion of oxygen functionalities on the surface of SWCNT samples (right column) pretreated at different temperatures before ethanol partial oxidation (values are in atomic %). Pretreated sample

Annealed sample in He (800 ◦ C) 200 ◦ C 300 ◦ C 400 ◦ C

XPS binding energy (eV)

Total surface oxygen content (over total atomic content in sample)

285.5 (sp3 ) C 287.6 C O 290.2 O C O (functionality % over total carbon content on sample)

292.4 ␲–␲*

292.4 ␲–␲*

51.6 54.6 53.0 50.5

8.8 8.1 8.5 10.1

4.9 4.6 4.6 2.9

43.5 40.9 42.5 46.6

6.9 7.5 8.4 7.5

the surface chemical analysis in terms of surface oxygen content while Figs. 10 and 11 show the high resolution spectra for the O1s and C1s peaks respectively. Data for the helium annealed sample was also obtained. Comparison of the information shown in Tables 3 and 4 clearly indicates that the amount of surface oxygen present in the samples is proportional to the amount of oxygen bearing functional groups probed by TPD. Compared to the O2 /He pretreated samples the concentration of surface oxygen in the sample annealed in helium was 1.1 wt.%, clearly indicating that the pretreatment in O2 /He introduces a large amount of oxygen bearing chemical functionalities. A fitting routine was carried for O1s high resolution data, and the spectra was deconvoluted into 4 peaks, designated as (1, 2, 3, 4) (Fig. 10) each assigned to different functional groups based on previously reported results [36–39]. While for the case of the

1.4 2.2 3.7 9.5

oxygen line the data was noisy for some samples due to the relative low concentration of oxygen, we attempted to do a fitting to identify the different types of oxygen species present in the sample surface. For consistency we used the same set of peaks and their positions when fitting all spectra. Peak 1 (B.E. ∼ 529.9 eV) can be attributed to residual inorganic oxygen. Peak 2 (B.E. ∼ 531.3 eV) can be assigned to oxygen doubly bonded to carbon (C O) groups in ketone and quinone. Peak 3 (B.E. ∼ 532.9 eV) is assigned to oxygen singly bonded to carbon (C O) in hydroxyl and ether groups, and peak 4 (B.E. ∼ 534.4 eV) can be attributed to (O C O) oxygen in esters and carboxyl groups. A similar analysis was carried on the high resolution XPS spectra of C1s region. In this case, the data were resolved into at least five peaks designated (1, 2, 3, 4, 5) (Fig. 11) that were assigned according to the literature [36,38–42] to sp2 carbon in polyaromatic structures (C C ∼ 284.6 eV), aliphatic sp3 carbon

Fig. 10. High resolution XPS O1s deconvoluted spectra of pretreated SWCNTs in (O2 /He) at different temperatures (a) 400 ◦ C, (b) 300 ◦ C, (c) 200 ◦ C and (d) annealed sample before ethanol partial oxidation.

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Fig. 11. High resolution XPS C1s deconvoluted spectra of pretreated SWCNTs in (O2 /He) at different temperatures (a) 400 ◦ C, (b) 300 ◦ C, (c) 200 ◦ C and (d) annealed sample before ethanol partial oxidation.

(C C ∼ 285.5 eV), carbonyl or quinone groups (C O ∼ 287.6 eV), carboxyl or ester groups (O C O, ∼290.2 eV) and a satellite peaks originated from ␲–␲* transitions (B.E. ∼ 292.4 eV) typical of highly ordered graphitic materials. For the case of this peak, the data is of high quality, and once again to ensure consistency on the data analysis we used the same set of peaks and positions when fitting all C1s spectra. A semi-quantification of the relative percentages of the corresponding functional groups is summarized in Table 4. The effect of pretreatment temperature on concentration and type of oxygen functionalities on SWCNTs can then be assessed based on the information presented in Table 4. It is evident that O2 /He oxidation pretreatment results an increase of the O/C ratio for all samples, confirming the effective incorporation of surface oxygen groups. This oxidation treatment is also reflected on both a decrease in the number of surface C C groups (sp2 ) with increasing pretreatment temperature indicating a less graphitic character of the surface, and an increase in C C groups (sp3 ) due to the formation of surface defects [43]. This shows that the degree of SWCNT functionalization is highly dependent on temperature of SWCNT pretreatment. As also seen in Table 4, the total concentration of oxygen-functionalized carbon, i.e. carbonyl (C O) plus carboxylic (O C O) surface groups, showed an increase at higher pretreatment temperatures. However, while the amount of carboxylic surface groups increases at higher temperatures, for the case of the carbonyl surface group (C O), this increment peaks at 300 ◦ C pretreatment temperature and then drops to a lower value when temperature of 400 ◦ C was used for O2 /He pretreatment. These results also suggest that even though more amount of oxygen can be attached to the SWCNT surface at a higher

temperature, there is a saturation state for surface oxidation in terms of the attachment of carbonyl (C O) surface functionalities, after which the surface concentration of (C O) decreases above the optimum pretreatment temperature, which in this study was found to be around 300 ◦ C. 3.3. Relationship between catalytic activity and SWCNT surface morphology To carry meaningful comparisons with similar systems in the literature used for ethanol partial oxidation the intrinsic catalytic activity of the SWCNTs samples (initial partial oxidation rates at zero conversion normalized by the total amount of carbon atoms present in the sample) was calculated. The rate of partial oxidation of ethanol was calculated according to the following equation: rC2 H5 OH =

Ft C 3600Mwt

(ethanol)

(3)

where Ft is the total volumetric flow (mL/s) of ethanol,  refers to the density of ethanol (g/mL), Mwt is the molecular weight of ethanol (g/mol), and C (ethanol) is the ethanol conversion. This calculation allows us to get an initial pseudo turn over frequency numbers (TOF); thus the values calculated indicate the number of ethanol molecules that get converted per time per amount of carbon atoms in the catalyst. Shown in Fig. 12 is the graph resulting from this extrapolation routine obtained for the case of partial oxidation carried at 200 ◦ C for the catalyst pretreated at 200 ◦ C. The same procedure was used to calculate the pseudo TOF for the reaction at 200 ◦ C for catalysts pretreated at other temperatures. In this

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I. Abdullahi et al. / Applied Catalysis A: General 469 (2014) 8–17

Table 5 Initial ethanol partial oxidation rates observed at 200 ◦ C (reported as turn over frequencies). Values were obtained by extrapolation to zero conversion. 200 ◦ C

O2 /He pretreatment temperature −1

◦

Turnover frequency (s ) at 200 C TOF (s−1 ) based on oxygen functionalized carbon atoms (TPD)

250 ◦ C −5

5.2 × 10 1.9 × 10−3

Fig. 12. Ethanol partial oxidation rates as function of fractional conversion for catalyst pretreated at 200 ◦ C and partial oxidation conducted at 200 ◦ C.

calculation, we are assuming that all carbon in the SWCNT sample acts as catalytic site. While this is certainly not accurate, the values obtained offer a low end limit for genuine TOF values. Table 5 show the results of these calculations as a function of pretreatment temperature in O2 /He for partial oxidation of ethanol carried at 200 ◦ C. These values indicate that the intrinsic activity of SWCNTs used for this reaction is at least two orders of magnitude lower that those observed on typical metal oxide catalytic systems [17,44,45]. While this seems to indicate that the SWCNT materials show a poor performance, it must be stated once again that this calculation was based on the assumption that all carbon atoms act as catalytic sites. Results from activity test, TPD and XPS analysis discussed previously, clearly indicate that the catalytic activity is linked to the number of oxygen bearing carbon functionalities in the nanotube and that indeed not all the carbon atoms act as catalytic sites. Hence, a better approximation on intrinsic catalytic activities can be obtained if these values are recalculated using the number of functionalized carbon atoms (as obtained from TPD experiments) instead of the total carbon content in the catalyst. The results of the calculations are also shown in Table 5. A comparison of these results to some values reported in the literature for ethanol partial oxidation obtained on typical metal oxide supported catalytic systems [17,44,45] is shown in Table 6. Our results show that the

300 ◦ C −5

7.2 × 10 2.0 × 10−3

350 ◦ C −5

10.0 × 10 2.4 × 10−3

400 ◦ C −5

7.9 × 10 1.6 × 10−3

5.0 × 10−5 6.9 × 10−4

intrinsic activity for ethanol partial oxidation of SWCNTs used for this reaction is very similar to those observed on typical catalytic systems (particularly those supported on alumina and silica), as shown in Table 6 demonstrating the suitability of SWCNT-based materials for partial oxidation catalytic processes. Previous studies on oxidative dehydrogenation of butane over MWCNTs suggested that the active site for this process consisted two adjacent ketonic groups (C O) [6,7]. Another recent study indicates that for the case of supported vanadium oxide catalyst there is a correlation between the apparent activation energies for ethanol oxidation to acetaldehyde and for ethane and propane oxidative dehydrogenation [45]. Fig. 13 shows the values obtained for the rates obtained at extrapolated zero conversions (per mole of carbon in the SWCNT) depicted in Table 5 as a function of SWCNT pretreatment temperature in O2 /He. Obtained partial oxidation rates first increase with the pretreatment temperature until, a temperature of 300 ◦ C, at higher temperatures observed initial rates decrease. Based on the above mentioned studies [6,7] it is plausible then to assume that, for the case of ethanol oxidation to acetaldehyde over SWCNTs, a ketonic functional group is responsible for the activity. Fig. 13 also shows the relative amount of kenotic (% of C O groups over the total surface carbon) and carboxylic (% of O C O groups over the total surface carbon) functional groups present on these samples as a function of pretreatment temperature. This analysis together with the values reported in Table 5 suggest that higher pretreatment temperature results in increased amounts of total oxygen on the SWCNTs; however, the relative amount of ketonic (C O) groups do not show an increase with higher pretreatment temperatures, but an optimum near 300 ◦ C. This finding strongly suggest that the ketonic surface groups (C O) are a critical component of the active sites responsible for ethanol partial oxidation in the functionalized SWCNT materials. As mentioned above, this assertion is supported by previous reports on oxidative dehydrogenation reactions in which carbonaceous materials were used as the active catalysts [6,7,11,20–23].

Table 6 Initial ethanol partial oxidation rates observed at 200 ◦ C on different catalytic systems. Values were obtained by extrapolation to zero conversion. Catalytic system

Partial oxidation rates(s−1 )

This study (using total carbon content) VOx /TiO2 [17] VOx /MCM-41 [17] VOx /TiO2 -Al [44]

5.0 × 10−5 –1.0 × 10−4 8.0 × 10−1 4.0 × 10−3 ∼7.0 × 10−3 –1.0 × 10−1 (extrapolated at ∼50% conversion) ∼5.0 × 10−3 1.6 × 10−3 –2.4 × 10−3

VOx /Al2 O3 [45] This study (using total number of oxygen-functionalized carbon atoms as obtained by TPD)

Fig. 13. Ethanol partial oxidation rate and oxygen functional group content as a function of SWCNTs pretreatment temperatures.

I. Abdullahi et al. / Applied Catalysis A: General 469 (2014) 8–17

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