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Synthesis of nanocrystalline gold–carbon nanotube composites and evaluation of their sorption and catalytic properties
Microporous and Mesoporous Materials 120 (2009) 122–131
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Synthesis of nanocrystalline gold–carbon nanotube composites and evaluation of their sorption and catalytic properties E.C. Vermisoglou a,*, G.E. Romanos a,*, V. Tzitzios b, G.N. Karanikolos a, V. Akylas a, A. Delimitis c, G. Pilatos a, N.K. Kanellopoulos a a
Institute of Physical Chemistry, National Center for Scientific Research Demokritos, Agia Paraskevi, Athens 153 10, Greece Institute of Materials Science, National Center for Scientific Research Demokritos, Agia Paraskevi, Athens 153 10, Greece c Center for Research and Technology, Hellas/Chemical Process Engineering Research Institute, Thessaloniki 570 01, Greece b
a r t i c l e
i n f o
Article history: Received 30 March 2008 Received in revised form 4 September 2008 Accepted 17 October 2008 Available online 30 November 2008 Keywords: Carbon nanotubes Gold nanoparticles Adsorption CO conversion
a b s t r a c t Gold particles the size of nanometer scale supported on various substrates have gained enormous attention in the surface science and catalysis community. The more interesting application is the low temperature oxidation of CO. In the current work we proceed to the doping of commercial single carbon nanotubes with Au nanoparticles. The samples were characterized by X-ray diffraction and TEM. The characteristic XRD peaks denote the presence of the noble metal nanoparticles as well as a rough approximation of crystallite size and structure. TEM images gave us a clue about the size, the shape and dispersion of these particles on carbon nanotube surface. Sorption experiments of CO, CO2 and O2 at different temperatures ranging between 273 and 323 K were conducted to both carbon nanotubes and doped carbon nanotubes to elucidate the special interaction of gases with the noble metal nanoparticles and examine the role of the support in the conversion efficiency of CO. The activity of supported gold for CO oxidation in low temperatures was investigated by catalytic experiments in a specially developed continuous flow reactor, involving a packed bed of doped nanotubes. The reactor was interfaced with a gas chromatograph equipped with thermal conductivity detector and gas sampling valve. Ó 2008 Elsevier Inc. All rights reserved.
1. Introduction Supported gold nanoparticles have recently gained great attention due to their unique catalytic performance. Although gold in the bulk state is inert, it exhibits a remarkably high activity when finely dispersed on different supports [1–7]. The nature of the support plays a decisive role in stabilizing the nanoparticles, maintaining particle monodispersity, and enhancing catalytic activity of the resulting nanocomposites. A variety of materials have been proposed as gold supports that include metal oxides, such as TiO2, Al2O3, ZrO2, and MgO, and ordered porous substrates, such as SBA-15 and MCM-48 [8,9], while the resulting catalytic systems have been successfully used in several reactions including removal of NOx, methane combustion, hydrogenation of unsaturated species, and selective oxidation of alcohols [10–16]. The use of carbon nanotubes (CNTs) as catalyst support is inspired by the unique properties of these materials that include high surface area, resistance to strong acids and bases, and high mechanical flexibility and strength, as well as improved carbon– * Corresponding authors. Tel.: +30 2106503973; fax: +30 2106511766. E-mail addresses: [email protected] (E.C. Vermisoglou), groman@ chem.demokritos.gr (G.E. Romanos). 1387-1811/$ - see front matter Ó 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2008.10.040
metal interactions that have been found to enhance catalytic performance [17–23]. In addition, surface-modified CNTs have shown remarkable sorption characteristics [24], which are advantageous in catalytic processes, where, for instance, one or more of the reaction species are selectively adsorbed on the support and thus the reaction takes place at the particle–support interfaces. Moreover, flexibility in functionalizing nanotube surfaces can yield higher density of metallic particles, improved particle dispersion, and affinity of the resulting nanostructured catalysts in a variety of environments. In 1989, Haruta et al. reported that supported gold nanoparticles are extremely active in catalytic oxidation of CO [25]. Most of the results presented in the recent literature concerning the interaction of O2 and CO with gold clusters however, are the outcome of theoretical calculations, mainly based on the density functional theory principles [26–28]. Experimental investigations are infrequent and, in one of them [29], PES spectra revealed that the maximum number of chemisorbed CO molecules corresponds precisely to the available low-coordination apex sites on each Aum cluster, and that the CO chemisorption induces significant lowering of the electron binding energies to the Aum (CO)n chemisorbed clusters. In another work, the adsorption of CO on gold/TiO2 system was investigated by IRAS analysis [5]. By evaluating the DHads in
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2. Experimental section 2.1. Characterization of pristine carbon nanotubes and preparation of Au-doped SWNTs The SWNTs used in this work were purchased from Carbon Solutions, Inc. According to the manufacturer, the distribution of outer diameters is centred around 1.4 nm with a length distribution between 0.5 and 1.5 lm, while their end-cups have been removed by a purification procedure (purity 80–90%). Carboxylic groups with a concentration of about 6% per carbon were attached both to open ends and sidewalls of SWNTs by treatment in diluted HNO3. The oxygenated functional groups work as anchoring agents for the entrapment of Au3+, creating structures of gold complexes. The high carboxylic content explains the high loading of metal (6%), as determined by TGA analysis as well as by interpreting the adsorption results (see Section 3). In order to prepare the nanotube–gold composites, the acidtreated SWNTs were first dispersed in DMF. AuCl3 (99.9+%) was then added in the SWNT-in-DMF solution stirring under reflux at 200 °C for 1 h. The final solution was washed/centrifuged several times using ethanol in order to remove DMF and a clear solution is obtained, and then the sample was left to dry in air. The method is based on the reduction of Au3+ ions by DMF, which plays the role of solvent and reducing agent and have already reported in the lit-
80000
G-band
70000 60000 Intensity
a series of gold clusters on titania, the authors claim that the increase in CO heat of adsorption and the onset of catalytic oxidation occur concomitantly with respect to cluster size. Since the gold/CNTs system is scarcely investigated, we doped single wall carbon nanotubes (SWNTs) with gold nanoparticles with a main target to perform a thorough study on the adsorption properties and catalytic performance of the resulting hybrid nanocomposites. Sorption experiments of CO, O2, CO2 gases, which participate in the CO oxidation reaction, were carried out at different temperatures (ranging from 273 to 360 K) and partial pressures (ranging from vacuum to 100 kPa) in order to determine the heats of adsorption and evaluate the special interactions of CO and O2 with the gold nanoparticles. By comparing the results with those obtained from the non-decorated samples, useful information concerning the Au surface loading and the chemisorbed monolayer capacity was extracted. Subsequently, the activity of CNT-supported gold for CO oxidation at low temperatures was investigated by catalytic experiments under various reaction conditions (temperature, CO/O2 ratio, contact time) in a specially developed continuous flow reactor containing a packed bed of doped nanotubes. In recent investigations directed toward the elucidation of the catalytically active species of gold, extensive discussion and many hypotheses about the effects of gold cluster size related to the electronic structure are provided [30,31]. The role of low-coordinated gold atoms in clusters with non-bulk geometries [4], of cationic gold at the peripheries of supported clusters [20,32], and of anionic gold clusters [33] has also been investigated. However, the influence of gold oxidation states and cluster size on CO oxidation catalysis remain unresolved [33,34], although recent data [35] show that during CO oxidation both Au(I) and Au(0) are present in the working catalysts, and that since more catalytic sites incorporate Au(I), higher concentrations of cationic gold, which correspond to lower partial pressures of CO in the reacting mixture, lead to higher catalytic activity. To this point, we examined the performance characteristics of the CNT-supported gold nanoparticles after completely opposite activation procedures (oxidation/ reduction) in order to define the oxidation states of gold that are more active during the reaction, as well as to provide aspects on enhancing regeneration efficiency.
RBM
50000 40000
G'-band
30000 20000
D-band
10000 0 0
500
1000 1500 2000 2500 3000 -1 wavelength (cm )
Fig. 1. Raman spectrum of the gold-decorated SWNT’s.
erature [36], and the spontaneous deposition of the Au particles on the external surface of carbon nanotubes [37]. The Raman spectrum of the gold-decorated SWNTs is illustrated in Fig. 1, where the high quality of the involved SWNTs is confirmed by the high intensity ratio of the G over D band, and by the fact that the G’-band is completely free from defect contributions. 2.2. Instrumentation Raman spectra were obtained using an inVia Reflex (Renishaw) micro-Raman spectrometer using objective 50 with a focal point of 1.5 lm2 and a crystal laser excitation of 785 nm operating at 1.2 mW (1% of full power). Samples for transmission electron microscopy were prepared by extensive sonication of the CNTs in high-purity ethanol. A drop of the solution was deposited onto a Lacey C-film supported on a Cu grid and the solvent was allowed to evaporate in air. TEM analysis was then carried out in a JEOL 2011 HR-TEM, operating at 200 kV and fitted with an Oxford Instruments INCAx-sight EDS detector. TGA characterization was performed on a Pyris Diamond TG/DTA, Thermogravimetric/Differential, Thermal Analyser (Perkin–Elmer Instruments). Samples for X-ray diffraction were deposited in the form of powder on a glass. The instrument used was a Siemens D500 X-ray diffractometer. 2.3. Adsorption apparatus The adsorption isotherms were acquired using a homemade stainless steel gravimetric rig described elsewhere [30]. The balance head (CI), counterweight compartment, valves and tubing of the system were thermostated in an air-circulating bath (±0.1 K), whereas the sample compartment was separately thermostated in a silicon oil bath (±0.01 K). Measurements were performed with about 100 mg of sample to resemble the conditions of the reactor experiments. All results were corrected for buoyancy effects. Before the measurements the samples were outgassed at 460 K for 24 h under high vacuum conditions (105 mbar). 2.4. Plug flow reactor The catalytic activity for CO oxidation was examined in a plug flow reactor using 100 mg of catalyst in gas mixtures of CO-in-air and CO-in-O2, with total flow rates between 50 and 80 ml/min. The catalysts were packed in a quartz tube forming a bed with a length of 5 cm and a volume of 0.16 cm3. The gas flow of each stream was controlled by electronic mass flow controllers (Bronkhorst F-200CV) with a scale of 2–100 ml/min for CO and
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20–1000 ml/min for air and O2. The operation temperature was controlled using a thermocouple and was adjusted in the range of 303–573 K. Temperature data during the catalytic experiments corresponded to the value measured with a second thermocouple placed in the inlet immediately before the catalyst bed. The experiments were carried out both under steady state and gradient temperature conditions. During the steady state temperature experiments, kinetic data were acquired every 30 min after an initial exposure of at least 5 min on the reactants stream. The concentrations of CO2, CO, N2 and O2 were analyzed alternatively at the inlet and outlet of the reactor by a gas chromatograph (GC, SRI 8610C), equipped with a six port gas sampling valve and TCD detector. A Heysep D packed column (1/8” SS) with a length of 10 m ensured satisfactory resolution between N2, O2 and CO at a GC furnace temperature of 303 K. CO2 was eluted 10 min after injection at a furnace temperature of 373 K. The cool-down capacity of the GC furnace determined the frequency of gas sampling. The system was calibrated with gas mixtures of 1 %, 3 %, 5%, 10%, and 15% CO2 and CO-in-air. 2.5. Experimental conditions Table 1 shows the pre-treatment, reaction, and regeneration conditions involved for each of the examined catalysts. The sample code corresponding to the specific conditions is also included and will be further used as a reference. After the initial reductive activation of the sample 0_AuCNT, the catalytic performance was examined starting from the lowest temperature of 323 K and performing successive temperature increments of 50 K, each one starting after a cumulative CO feed of about 23 mmol. Thus, performance evaluation up to 573 K was accomplished. In the second catalytic test, the performance characteristics were also attained during successive temperature increments up to 573 K but with a higher ratio of O2/CO in the total gaseous stream, whereas regeneration had taken place under oxidative conditions. Kinetic tests (3, 4, 5, 6 in Table 1) were performed under stable temperature conditions after oxidative regeneration, and the performance evolution was monitored up to a total CO feed of 200 mmol. Activation of the second sample Au_CNT, took place under argon atmosphere at 373 K. The first test was performed at 303 K
by monitoring the evolution of the CO oxidation performance for a total CO dosage of 25 mmol. The subsequent tests following regeneration under oxidative conditions were performed at various temperatures for a total CO amount of about 70 mmol. The same procedure was followed for the third sample, III_AuCNT, which had been previously activated under oxidative conditions, using a smaller CO contact time and O2/CO ratio. 3. Results and discussion 3.1. Adsorption results The adsorption isotherms of CO at different temperatures for the Au-decorated and the as-received SWNTs are illustrated in Fig. 2a and b, respectively. The exothermic character of the CO physisorption on the SWNT’s is intensively proclaimed in Fig. 2b, where the adsorption capacity drops significantly with temperature. In contrary, an endothermic trend indicating the occurrence of chemisorption is obvious for the Au-decorated sample. Moreover, the CO adsorption capacity at 40 kPa for the Au-decorated sample, as compared with this of the bare nanotubes, is enhanced by a factor of 1.05, 1.5, 2.2 and 3.2 at the temperatures of 273, 290, 318 and 356 K, respectively. Additionally, above a pressure of 40 kPa, the CO isotherm curve for the Au/CNTs sample presents a shift upwards at 273 K and a shift downwards at 356 K. This is an indication that an alteration of the mechanism of adsorption to physisorption occurs as soon as the available for chemisorption Au clusters are occupied. However, the shift induced in the isotherm curves was observable only in the case of the extreme upper and lower temperatures and not for the intermediate ones, while a different trend of the curve shift between the extremely high and low temperatures is evident. This is because in the case of 273 K, the physisorption occurring to the bare part of nanotubes as well as to the CO-chemisorbed clusters is significantly enhanced, as compared to the one occurring at 356 K (exothermic character), whereas in the case of the intermediate temperatures, the differences in mass uptake caused by the different adsorption mechanisms are eliminated. Therefore, by subtracting the CO isotherm curve of the bare SWNTs at 356 K from the respective one of the Au-decorated SWNTs, it was possible in a simple way to deduce
Table 1 Catalytic performance experiments-regeneration conditions. Number of tests
Activation
Reaction conditions Temperature (K)
Total flow (ml/min)
CO contact time (min1)
O2/CO
323?573
80 (Air/CO)
54.8
1.75
323?573 573 523 473 573
57 (O2/CO) – – – –
54.8 – – – –
5.6 – – – –
Argon 373 K 24 h Regeneration Oxygen 573 K 24 h Oxygen 573 K 24 h Oxygen 573 K 24 h Oxygen 573 K 24 h
303
78 (O2/CO)
54.1
8.17
333 423 473 523
57 (O2/CO) – – –
54.8 – – –
5.6 – – –
Air 573 K 2 h Regeneration Air 573 K 2 h – –
306
78 (Air/CO)
20.82
4.82
473 523 573
– – –
– – –
– – –
0_AuCNT 1 2 3 4 5 6
Hydrogen 483 K 24 h Regeneration Oxygen 493 K 24 h Oxygen 493 K 24 h Oxygen 493 K 24 h & 573 4 h Oxygen 493 K 24 h Oxygen 493 K 24 h & 573 4 h
AuCNT 1 2 3 4 5 III_AuCNT 1 2 3 4
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a
0.25
356K 318K 290K 273K
0.18 0.16 0.14
mmols/gr
0.2 mmol/gr
b
273K 290K 318K 356K
0.3
0.15 0.1
0.12 0.1 0.08 0.06 0.04
0.05
0.02
0
0
0
20
40 60 P (kPa)
80
100
0
20
40 60 P (kPa)
80
100
80
100
Fig. 2. (a): CO isotherms of the Au-decorated sample. (b): CO isotherms of the undoped SWNTs.
a
0.6
b
Au+SWNTs
0.5
SWNTs_318K
1
SWNTs
Au+CNTs_290K
mmol/gr
0.4
mmol/gr
Au+SWNTs_318K
1.2
0.3 0.2
0.8
SWNTs_290K
0.6 0.4
0.1
0.2
0
0 0
20
40
60
80
100
P (kPa)
0
20
40
60
P (kPa)
Fig. 3. (a): CO2 isotherms at 318 K, (b): CO2 isotherms at different temperatures.
the % w/w ‘‘active” gold loading on the nanotubes. The following expression was involved:
mmolCG ¼ ðyÞ ðmmol=grÞG þ ðx yÞ f ðpÞ ðmmol=grÞC
ð1Þ
where, mmolCG is the absolute amount of CO adsorbed on the golddecorated sample, (mmol/gr)C is the normalized per gram amount of CO adsorbed on the SWNTs, (mmol/gr)G is the normalized per gram amount of CO adsorbed on the deposited gold clusters, x(gr) is the experimental mass of the gold-decorated sample, and y(gr) is the hypothetic mass of the Au nanoparticles in the Au-decorated sample. The correction factor f(p) was involved in order to exclude from the SWNT’s sample the nanotube active sites that were occupied by gold. This was calculated by the ratio of the CO2 isotherms of the doped and undoped samples at 318 K (Fig. 3a). As it is evident (Fig. 2a), the presence of Au clusters diminished the adsorption capacity for CO2, which was expected since there is not a special interaction of the Au clusters with the CO2 molecules. A further proof for this, constitutes the exothermic character of the CO2 isotherms of the gold-decorated sample (Fig. 3b). Moreover, since the amount of chemisorbed CO molecules per gram of gold (mmol/gr)G was unknown, we adopted the values referred in the recent literature [29] for the chemisorbed clusters Aum(CO)n. As it was shown by PES spectroscopy, gold clusters with m = 2, 3, 4, 5 chemisorb 2, 2, 3, and 4 molecules of CO, respectively. By assuming an equal in amount distribution of the deposited gold in Au2, Au3, Au4 and Au5 clusters we could calculate a chemisortion capacity of about 4 (mmol/gr)G. It should be noted
that although the application of this value of the gold chemisorption capacity for all the range of pressures examined is certainly under contestation, it was adopted as an indicator to reveal the CO pressure corresponding to the end of chemisorption. At this point, the adopted value is consistent and the y% content of gold calculated out of this must be correct. The plot derived out from Eq. (1), which expresses the% w/w gold (y) as a function of pressure, is illustrated in Fig. 4a. The shift in curve of Fig. 4a between 40 and 50 kPa denotes the aforementioned transition from chemisorption to physisorption, and a gold weight percentage of 2.75% was calculated at this point. However, the weight percentage of deposited gold calculated by this method was smaller than this estimated from the result of TGA analysis in oxygen atmosphere (Fig. 4b). After the complete oxidation of the decorated SWNTs (Fig. 4b rhombs) the remaining mass, which corresponds to the sum of the gold and nickel content of the decorated sample was about 11.6% of the initial. The nickel content defined by the TGA curve of pristine SWNTs (Fig. 4b-line) was about 6.2%. Thus a net gold content of about 5.4% was calculated. This discrepancy between the gold content values calculated by TGA and the gravimetric method was attributed to the fact that from the total deposited gold (5.4%w/w) only the 50% was active for catalysis. Further proof for this constitutes the particle size distribution (PSD) (Fig. 5a) derived from the examination of TEM images of the Au-decorated samples (Fig. 5b). A critical issue in order to derive the PSD of Au nanoparticles is to distinguish them from the Ni particles that had remained during the fabrication of
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a
b
3.5
Sample mass (%)
3 2.5
% w/w Au
Gold decorated SWNTs
100
2 1.5 1
Pristine SWCNTs
80 60 40 20
0.5 0
0 0
50
100
P (kPa)
0
500
1000
Temperature [oC]
Fig. 4. (a): %w/w amount of deposited gold where CO chemisorption occurs. (b): TGA curve of the Au-decorated SWNT’s.
SWNTs and which, as calculated by the TGA, amount up to 6.2%w/w in the pristine SWNTs. On this purpose the TEM image of pristine SWNTs is presented in Fig. 5c. As it can be observed there are curved distorted carbon structures on the external surface of the pristine nanotubes (dark cycles) that bear the Ni nanoparticles. The size of these nanoparticles is about 1–2 nm (inset Fig. 5c), which is much smaller than that of the Au particles, which tend to form larger aggregates during the reduction with DMF (Fig. 5b). The curved distorted structures bearing the Ni nanoparticles can also be distinguished on the Au-decorated sample (upper right part Fig. 5b). Additionally the difference in size between the Au and Ni nanoparticles is proclaimed by the XRD patterns of the pristine and Au-decorated SWNTs presented in Fig. 6. By involving the Sherrer equation a size of 6.8 and 21 nm was calculated for the Ni and Au nanoparticles respectively. The derived gold PSD implies that about 45% of the 5.4% w/w deposited gold particles have the size between 3 and 7 nm, which is established as the optimum size for the catalytic activity of gold, and this is in very good agreement with the result obtained by the isotherms subtracting method. As mentioned in the introduction, most of the literature dealing with oxygen chemisorption on Au nanoparticles presents theoretical calculations of the binding energy based on the use of density functional theory. The O2 interaction with charged and neutral gold clusters is found to exhibit a pronounced sensitivity to the cluster size and to its charge state [26–28]. In Fig. 7 we present experimental evidence of the specific interaction of O2 molecules with gold nanoparticles as the adsorption capacity is enhanced by a factor of 2–3 when compared with this of the undoped SWNTs. Moreover, a DH (kJ/mol) value of 4 for the case of bare SWNTs and 12 for the gold-decorated sample was calculated by applying the Clausius–Clapeyron equation. The observed exothermic character of the O2 chemisorption is consistent with the occurrence of dissociative adsorption [35], which, as has been predicted by DFT [26], is favored for larger gold clusters Aun (n > 3) and is accompanied by large changes in the structure of the metal host. Overall, the adsorption results revealed that the decoration of SWNTs with gold nanoparticles resulted in a significant increase in the adsorption capacity for CO and O2 and a decrease for CO2, when compared with the bared SWNTs. Thus, the developed materials constitute a good candidate to be applied as catalysts for low temperature CO oxidation reaction. The good dispersion properties of DMF in combination with the high content of SWNT’s in strongly acidic carboxylic groups resulted in an adequate loading with gold. The potential of carboxylic groups to reduce the anchored Au(III) led to a spontaneous formation of metallic gold that under the reductive conditions involved (DMF)
acts as a nucleation center for the formation of larger gold agglomerates with an almost equal number distribution (20–25%) in the size ranges of 3–5, 5–7 and 7–9 nm. 3.2. Performance results The low temperature CO oxidation efficiency of the as-prepared gold-decorated SWNTs was significantly degraded after their treatment in H2 atmosphere (483 K) for 24 h. During the temperature gradient tests that followed the H2 treatment (filled triangles in Fig. 8), the acquired conversion factors were limited to the moderate value of 1.3% in the temperature range between 323 and 473 K, whereas the highest efficiency was observed at 573 K and was no more than 24%. However after the regeneration of the same sample under extremely oxidative conditions (493 K, 24 h) an important enhancement of the performance was observed, with prominent CO conversions of 3.2% and 5% at 373 and 423 K, respectively (Fig. 8). The moderate conversion efficiency of the H2-activated sample once again denotes the significant reduction capacity of both the DMF solvent and the surface carboxylic groups. As a result of this reduction activity, most of the deposited gold (75%) on the starting material was present in the form of small (3–5 nm) and slightly bigger (5–13 nm) Au(0) particles (Fig. 5a), with a very small percentage remaining in the form of supported Au(III) complexes (probably those anchored to the surface phenolic groups). During the subsequent reductive activation in H2 at 483 K, the existing small particles serve as nucleation centers for the formation of bigger agglomerates, which are inactive during CO oxidation. These large agglomerates are clearly distinguished in Fig. 9a, which presents a TEM image of the hydrogen treated sample 0_AuCNT after the completion of the catalytic test runs. Thus the observed moderate activity of the material (1.3%) can be attributed to the presence of the remaining low amount of Au(III) complexes that under the H2 reductive atmosphere form the desired small Au particles. It is believed that the undesirable big agglomerates can be generated by two processes. The first is assumed to start from the reduction in H2 of the remaining Au(III) cations, forming metal clusters that migrate along the surface until collision with similar clusters occurs, resulting in coalescence. The second process involves Ostwald ripening when primary Au particles formed during ion exchange grow in the expense of smaller clusters [38] formed during reduction in H2. In our case, the second mechanism most likely occurs, since the remaining Au(III) complexes are not consumed in the formation of large metal agglomerates but they are rather participating to the formation of a slight portion of small gold clusters (<15 nm) that were responsible for
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a
Nanoparticle size distribution
[5-7]
20
[43-45]
[45-47]
[39-41]
[41-43]
[35-37]
[37-39]
[33-35]
[31-33]
[29-31]
[25-27]
[27-29]
[21-23]
[23-25]
[17-19]
[19-21]
0
[13-15]
5
[11-13]
10
[15-17]
15 [9-11]
% number of particles
[3-5]
25
[7-9]
30
Particle size [nm]
b
c
Fig. 5. (a): Au particle size distribution as-derived from TEM images. (b): TEM image of the Au-decorated SWNTs. (c): TEM image of the pristine SWNTs.
the existing, although moderate, catalytic activity of the final material. Independent of the mechanism, the deposited gold on the H2 reduced material (0_AuCNT) must have exhibited its highest percentage in the form of large Au clusters (>15 nm), which are
inactive for low temperature CO oxidation. An additional important remark coming out of this set of experiments (tests 1 and 2, sample 0_AuCNT) is that the formation of the bigger gold agglomerates is partially reversible, meaning that under the extremely oxidative conditions (O2 493 K, 24 h) applied before proceeding
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Intensity a.u.
Au (111)
Au (200) Ni (111)
pristine SWNTs
Ni (200)
Au (220)
Au (311)
Au SWNTs
28
38
48
58
68
78
2-theta degree Fig. 6. XRD spectrum of the as-derived and gold-decorated SWNTs.
0.3
mmol/gr
0.25 0.2 0.15 0.1 0.05 0 0
20
40 60 P (kPa)
80
100
Fig. 7. Oxygen isotherms at different temperatures.
with the second temperature gradient test (open cycles in Fig. 8), a fragmentation of the initial clusters to smaller ones occurs. This phenomenon has already been investigated in the recent literature by EXAFS data recorded at the end of an O2-TPO (temperature programmed oxidation at 490 K) treatment of gold clusters supported to MgO. The data showed a decrease in the Au–Au coordination number and an increase in the Au–O contributions of the gold clusters. The authors [35] noted that this oxidative fragmentation was
320
370 28 26 24 22 20 18 120
0
140
20
420
160 mmols CO
40
Temperature (K) 470
180
60 80 mmols (CO)
520
570
200
100
120
27.5 25 22.5 20 17.5 15 12.5 10 7.5 5 2.5 0 140
% conversion
AuCNTs 273K AuCNTs 290K AuCNTs 318K CNTs 290K CNTs 318K
0.35
not complete, and the resultant oxidized species were not the same as those presented in the initially prepared samples as Au(III) complexes. Instead, these oxidized species are not well defined structurally and consist of oxidized and aggregated species that might be regarded as gold oxide clusters. In this work it is shown that the species that are formed out of fragmentation are more active for CO oxidation at low temperatures (up to 473 K) than the large (>15 nm) Au clusters (Fig. 8). At higher temperatures (523 and 573 K) and under the reducing conditions involved during the catalytic test (where CO acts as an electron donor), the oxidized fragmented pieces are reorganized in larger clusters, and the catalytic conversion drops again to the values obtained with the H2-activated sample (Fig. 8, the two curves come into convergence). Additional evidence to this point constitutes the kinetic data acquired in both cases at a stable temperature of 573 K (inset in Fig. 8). The catalytic activity of the oxidized sample drops faster during the initial stages of the isothermal test due to re-agglomeration of the fragmented clusters, and after about 40 mmol of CO feed, the conversion efficiency comes into the same value as the one acquired at the same temperature for the H2-treated sample. This is an additional contribution to the conclusion that the existing slight portions of the small Au nanoparticles, which are responsible for this activity, do not take part in this oxidative fragmentation/ reductive re-agglomeration process. The kinetic tests 3 and 6 of the same sample (Table 1 and 0_AuCNT) are illustrated in Fig. 10a. Although performed at a high temperature (573 K), which was out of the purposes of this work,
Fig. 8. Evolution of the conversion efficiency of sample 0_AuCNT with temperature and cumulative CO feed. Triangles (Test 1): Sample activated with hydrogen 483 K, 24 h. Cycles (Test 2): Sample regenerated with oxygen at 493 K for 24 h. Inset: Evolution of the conversion efficiency with CO feed at 573 K.
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Fig. 9. TEM images of: (a) hydrogen treated sample and (b) air treated sample.
these tests provide additional information concerning the oxidative fragmentation of the gold agglomerates as well as on the retention of the catalytic activity of the small (<13 nm) gold nano-
a 100
b 14 regeneration conditions
90
12
80
493K O2_24h & 573K 4h
70
493K O2_24h
% conversion
% conversion
particles. The oxidative fragmentation occurring under more intense conditions (O2, at 573 K) must be thoroughly enhanced thus giving rise to an enhanced conversion efficiency of 90% for the first 5 mmol of CO feed. However the efficiency diminishes very fast and during the first 26 mmol of CO feed the conversion factor drops to about 50% which is also the value for the case when the sample was regenerated under softer oxidative conditions (O2 at 493 K). A conclusion educed out of this is that the excess amount of small clusters produced out of the fragmentation at higher temperatures is prone to a very fast agglomeration under the reducing conditions involved during the catalytic tests. On the contrary, the fragments generated out of the oxidative treatment at 493 K are far more stable and keep their significant catalytic activity (50% conversion) for more than 100 mmol of CO feed. Similar results at lower catalytic temperatures (tests 4 and 5) are presented in Fig. 10b where the faster deactivation rate of the sample regenerated under higher oxidative conditions is demonstrated. The up to now extended discussion on the CO oxidation performance of the 0_AuCNT sample mainly concerned the reducing activity of both the DMF that was used as a dispersion agent and solvent, and the carboxylic groups existing on the surface of the involved SWNTs. Due to the high reducing activity, the developed material contained particles of gold, which was already in its metallic form, as also shown by the XRD pattern (Fig. 6), where the characteristic peaks of metallic gold are clearly distinguished. Indeed, as it was already observed [26], Pt and Au nanoparticles (NPs) can be formed in the presence of CNTs, from metal precursors (salts) without the need to further involve a reducing procedure. This phenomenon was attributed to a direct redox reaction occurring between the metal ions and CNTs, the latter acting as the electron donors. In another aspect [18] phenolic groups interact with ([Au3+] A) precursors by anchoring the gold cations: C– OH + Au3+?C–OAu3+ + H+, while the interaction of ([Au3+] A) solutions with the less stable surface carboxylic groups results in surface decarboxylation and reduction from Au(III) to Au(0): C– COOH + Au3+? C + Au0 + H+ + CO2. During the succeeding treatment with H2 these particles served as nucleation centers for the formation of bigger agglomerates and the derived material was inactive for CO oxidation at low temperatures. In order to elucidate the origins of the catalytic activity of the developed materials we performed catalytic tests on two samples activated with argon at 373 K (AuCNT’s) and air at 573 K (III_AuCNT’s) for 24 and 2 h, respectively. Since no agglomeration of gold nanoparticles is expected to occur under the involved activation conditions, both samples must contain a 2.75% per weight loading of gold particles with a size between 3 and 7 nm, as already calculated by the interpretation of
60 50
10
regeneration conditions
8
493K O2_24h & 573K 4h 493K O2_24h
6 4
40
2
30
0
0
50
100 mmols CO
150
0
10
20
30
mmols
Fig. 10. (a): Kinetic experiments for sample 0_AuCNT. Evolution of the conversion efficiency with CO feed at 573 K. Squares (Test 3), Rhombs (Test 6). (b): Evolution of the conversion efficiency with CO feed at 523 K (Cycles, Test 4), 473 K (Triangles, Test 5).
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b
70 60 conversion%
8.0
423K
473K
III_AuCNT's
7.0
523K
473K
AuCNT's
6.0
523K
573K
80
conversion%
a
50 40 30
5.0 4.0 3.0
20
2.0
10
1.0 0.0
0 0
20
40
60
mmols CO
0
50
100 mmols CO
150
Fig. 11. (a): Kinetic experiments at 303 K. Open symbols: sample activated with Ar at 373 K for 24 h, Filled symbols: sample activated with Air at 573 K for 2 h. (b): Evolution of the conversion efficiency with CO feed at higher temperatures.
the adsorption isotherms (Section 3.1). This loading was adequate to produce significantly high conversion efficiency (70%) at 303 K, during the first 2.5 mmol of CO feed as illustrated in Fig. 11a. The initial activity of catalyst AuCNT’s was greater than that of catalyst III_AuCNT’s, but after about 20 mmol of CO, the activity of the two samples became nearly the same, reaching a plateau of 10% in CO conversion. The data suggest the presence of essentially the same supported gold species in both catalysts after steady state had been attained. The slightly higher activity of the AuCNT’s sample at the initial stages of testing may be attributed to the formation of an additional amount of small-size gold clusters during argon treatment. These were formed from the ionic gold Au(III) that has remained unreduced during the impregnation procedure (not reduced by DMF or carboxylic groups). The ability of argon at 373 K to reduce Au(III) complexes forming small clusters of metallic gold Au(0) can be compared with the reported activity of He at 373 K to form extremely small clusters of gold [4]. The He activity was educed out of EXAFS spectra showing first- and second-shell Au–Au coordination numbers of 4.0 and 1.0, respectively for a MgO sample impregnated with cationic gold and further treated under He atmosphere at 373 K for 2 h, whereas, a treatment at 573 K resulted in the formation of larger clusters [4]. On the other hand, as already mentioned, the oxidative treatment of the III_AuCNT’s sample must have provoked the oxidative fragmentation of larger gold agglomerates already existing on the raw sample, thus creating smaller clusters, which are also active for CO oxidation. This is also supported by the TEM image of the III_AuCNTs sample (Fig. 9b) where a large population of small in size (10–15 nm) Au clusters can be clearly distinguished. As it is also clear from the TEM image, the oxidative treatment in Air at 573 K has not distorted the structure of the nanotubes and thus the possible effect of exposed Ni nanoparticles on the catalytic performance must be excluded. Although not concluded by the TEM image, the number of these clusters must be significant since a substantial population of large agglomerates was observed on the starting material (Fig. 5a) that corresponded to a deposited gold w/w percentage of 0.7, 0.5 and 0.3% for sizes 15–17, 23–25 and 37– 39 nm, respectively. Therefore, we would expect a higher catalytic activity for sample III_AuCNT’s when compared to the AuCNT’s. However, although the sample III_AuCNT’s certainly contains a larger number of small gold nanoparticles (3–13 nm) than the AuCNT’s one, a significant population of these are in the form of oxidized species which are not well defined structurally [35] and must be less active than their metallic gold analogues of the same size. A general conclusion educed up to now is that the oxidized gold species coming out of the oxidative fragmentation during regeneration, have a CO oxidation catalytic activity that follows the order: CI < oxidized gold species < CII, where CI the catalytic
activity of Au particles larger than 15 nm and CII the catalytic activity of Au particles smaller than 13 nm. A common observation when comparing the kinetic data of all the performed catalytic experiments (Inset Figs. 8, 10a, 11a) is that, independently of the activation, regeneration, and catalytic test conditions, the catalyst deactivation proceeds with a fast rate at the very early stages of the test up to about 20 mmol of CO feed, which correspond to 200 mmol per gram of catalyst. The steady state catalytic efficiency remains almost stable for more than 400 mmol of CO feed per gram of catalyst and, as illustrated in Fig. 11b, in one of the most persistent tests the conversion was slightly diminishing for more than 1.1 mol/gr. Another observation is that for a sample exposed to sequential catalytic tests the conversion efficiency at steady state is independent of the regeneration conditions (moderately oxidative or extremely oxidative (Fig. 10a)) involved in between the tests. Finally, when involving different activation conditions (except from sample 0_AuCNT’s) the steady state conversion efficiency is identical during the first run (Fig. 11a), but differs significantly after regeneration (Fig. 11b). From the above results, it is clear that at the early stages of the catalytic test, deactivation proceeds via a reorganization process including sintering and oxidative fragmentation of the gold particles and clusters. As already discussed, two mechanisms describe the sintering process in supported metallic particles: (i) particle migration and coalescence, (ii) atom emission and recapture through diffusion from the smaller, high potential particles to the larger, low potential ones (Ostwald ripening). Recently, a new MC model of supported-catalyst sintering [39] was developed and showed that particles grow rapidly during the first stages of a catalytic test due to the first mechanism, whereas subsequent increases in average particle size are much slower as the growth mechanism shifts from coalescence of closely spaced particles to Ostwald ripening. Our data show that once the CO oxidation reaction attains steady state, meaning that the mechanism of deactivation proceeds through diffusion of gold atoms on the surface, a significant part of the total amount of deposited gold has already aggregated into clusters with large size as a result of the reductive activity of CO, and simultaneously, a part of these large clusters undergoes oxidative fragmentation to less active oxidized species due to the presence of oxygen, with the second mechanism most likely occurring at temperatures higher than 473 K. Thereinafter, the catalytic activity is attributed to the remaining small-size gold particles, as also suggested by Haruta and Date [40]. Concerning the regeneration of the samples (Fig. 11b), it is again confirmed that the more oxidative conditions lead to the production of fragmented clusters, which are more easily migrating and more prone to coalescence. From Fig. 11, it is clear that although the steady state CO conversion values acquired during the first run at 303 K
E.C. Vermisoglou et al. / Microporous and Mesoporous Materials 120 (2009) 122–131
were about 9 and 7% for the oxygen and argon-activated samples respectively, the succeeding regeneration of the argon-activated sample with pure oxygen instead of air (III_AuCNT’s) led to a faster rate of deactivation at the early stages of the subsequent catalytic tests and moreover to a lower steady state conversion efficiency. 4. Conclusions SWNTs constitute a good candidate to be used as support for gold nanoparticles due to their high content in carboxylic groups that act as anchoring agents for the formation of Au(III) complexes. The reducing activity of carboxylic groups eliminates the need for further treatment, as metallic nanoparticles are formed spontaneously after the impregnation procedure. In this work we showed that through the treatment of SWNTs with AuCl3 (99.9+%) in DMF, a 6% w/w gold particles loading was achieved from which 75% had a size below 13 nm. The developed material, after its treatment in argon atmosphere at 373 K, had a significant performance for low temperature CO oxidation and a conversion factor of 70% was evaluated at 303 K during the first 25 mmol of CO feed per gram of catalyst. The steady state conversion efficiency at the same temperature was about 10% and was kept almost constant for more than 500 mmol CO per gram of the catalyst. Deactivation during the catalytic tests (CO, O2 atmosphere) proceeds via particle migration at the very early stages accompanied by a tremendous drop in the catalytic activity. Deactivation during steady state proceeds via the Ostwald ripening mechanism. Regeneration of the catalyst under moderate oxidative conditions (air at 573 K) provoked the oxidative fragmentation of the large gold clusters. The resulted oxidized species presented quite satisfactory conversion efficiency and were more stable during the catalytic test than the ones produced after regeneration under more oxidative conditions (O2 at 573 K). Acknowledgments The authors would like to thank Dr. P. Falaras and Dr. V. Likodimos for the Raman spectroscopy analysis and G. Basina for X-ray diffraction analysis. Financial support by the European Network of Excellence INSIDE-PORES is gratefully acknowledged. References [1] M. Commoti, W.-C. Li, B. Spliethoff, Schüth, J. Am. Chem. Soc. 128 (2006) 917– 924.
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Contents lists available at ScienceDirect
Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Synthesis of nanocrystalline gold–carbon nanotube composites and evaluation of their sorption and catalytic properties E.C. Vermisoglou a,*, G.E. Romanos a,*, V. Tzitzios b, G.N. Karanikolos a, V. Akylas a, A. Delimitis c, G. Pilatos a, N.K. Kanellopoulos a a
Institute of Physical Chemistry, National Center for Scientific Research Demokritos, Agia Paraskevi, Athens 153 10, Greece Institute of Materials Science, National Center for Scientific Research Demokritos, Agia Paraskevi, Athens 153 10, Greece c Center for Research and Technology, Hellas/Chemical Process Engineering Research Institute, Thessaloniki 570 01, Greece b
a r t i c l e
i n f o
Article history: Received 30 March 2008 Received in revised form 4 September 2008 Accepted 17 October 2008 Available online 30 November 2008 Keywords: Carbon nanotubes Gold nanoparticles Adsorption CO conversion
a b s t r a c t Gold particles the size of nanometer scale supported on various substrates have gained enormous attention in the surface science and catalysis community. The more interesting application is the low temperature oxidation of CO. In the current work we proceed to the doping of commercial single carbon nanotubes with Au nanoparticles. The samples were characterized by X-ray diffraction and TEM. The characteristic XRD peaks denote the presence of the noble metal nanoparticles as well as a rough approximation of crystallite size and structure. TEM images gave us a clue about the size, the shape and dispersion of these particles on carbon nanotube surface. Sorption experiments of CO, CO2 and O2 at different temperatures ranging between 273 and 323 K were conducted to both carbon nanotubes and doped carbon nanotubes to elucidate the special interaction of gases with the noble metal nanoparticles and examine the role of the support in the conversion efficiency of CO. The activity of supported gold for CO oxidation in low temperatures was investigated by catalytic experiments in a specially developed continuous flow reactor, involving a packed bed of doped nanotubes. The reactor was interfaced with a gas chromatograph equipped with thermal conductivity detector and gas sampling valve. Ó 2008 Elsevier Inc. All rights reserved.
1. Introduction Supported gold nanoparticles have recently gained great attention due to their unique catalytic performance. Although gold in the bulk state is inert, it exhibits a remarkably high activity when finely dispersed on different supports [1–7]. The nature of the support plays a decisive role in stabilizing the nanoparticles, maintaining particle monodispersity, and enhancing catalytic activity of the resulting nanocomposites. A variety of materials have been proposed as gold supports that include metal oxides, such as TiO2, Al2O3, ZrO2, and MgO, and ordered porous substrates, such as SBA-15 and MCM-48 [8,9], while the resulting catalytic systems have been successfully used in several reactions including removal of NOx, methane combustion, hydrogenation of unsaturated species, and selective oxidation of alcohols [10–16]. The use of carbon nanotubes (CNTs) as catalyst support is inspired by the unique properties of these materials that include high surface area, resistance to strong acids and bases, and high mechanical flexibility and strength, as well as improved carbon– * Corresponding authors. Tel.: +30 2106503973; fax: +30 2106511766. E-mail addresses: [email protected] (E.C. Vermisoglou), groman@ chem.demokritos.gr (G.E. Romanos). 1387-1811/$ - see front matter Ó 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2008.10.040
metal interactions that have been found to enhance catalytic performance [17–23]. In addition, surface-modified CNTs have shown remarkable sorption characteristics [24], which are advantageous in catalytic processes, where, for instance, one or more of the reaction species are selectively adsorbed on the support and thus the reaction takes place at the particle–support interfaces. Moreover, flexibility in functionalizing nanotube surfaces can yield higher density of metallic particles, improved particle dispersion, and affinity of the resulting nanostructured catalysts in a variety of environments. In 1989, Haruta et al. reported that supported gold nanoparticles are extremely active in catalytic oxidation of CO [25]. Most of the results presented in the recent literature concerning the interaction of O2 and CO with gold clusters however, are the outcome of theoretical calculations, mainly based on the density functional theory principles [26–28]. Experimental investigations are infrequent and, in one of them [29], PES spectra revealed that the maximum number of chemisorbed CO molecules corresponds precisely to the available low-coordination apex sites on each Aum cluster, and that the CO chemisorption induces significant lowering of the electron binding energies to the Aum (CO)n chemisorbed clusters. In another work, the adsorption of CO on gold/TiO2 system was investigated by IRAS analysis [5]. By evaluating the DHads in
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2. Experimental section 2.1. Characterization of pristine carbon nanotubes and preparation of Au-doped SWNTs The SWNTs used in this work were purchased from Carbon Solutions, Inc. According to the manufacturer, the distribution of outer diameters is centred around 1.4 nm with a length distribution between 0.5 and 1.5 lm, while their end-cups have been removed by a purification procedure (purity 80–90%). Carboxylic groups with a concentration of about 6% per carbon were attached both to open ends and sidewalls of SWNTs by treatment in diluted HNO3. The oxygenated functional groups work as anchoring agents for the entrapment of Au3+, creating structures of gold complexes. The high carboxylic content explains the high loading of metal (6%), as determined by TGA analysis as well as by interpreting the adsorption results (see Section 3). In order to prepare the nanotube–gold composites, the acidtreated SWNTs were first dispersed in DMF. AuCl3 (99.9+%) was then added in the SWNT-in-DMF solution stirring under reflux at 200 °C for 1 h. The final solution was washed/centrifuged several times using ethanol in order to remove DMF and a clear solution is obtained, and then the sample was left to dry in air. The method is based on the reduction of Au3+ ions by DMF, which plays the role of solvent and reducing agent and have already reported in the lit-
80000
G-band
70000 60000 Intensity
a series of gold clusters on titania, the authors claim that the increase in CO heat of adsorption and the onset of catalytic oxidation occur concomitantly with respect to cluster size. Since the gold/CNTs system is scarcely investigated, we doped single wall carbon nanotubes (SWNTs) with gold nanoparticles with a main target to perform a thorough study on the adsorption properties and catalytic performance of the resulting hybrid nanocomposites. Sorption experiments of CO, O2, CO2 gases, which participate in the CO oxidation reaction, were carried out at different temperatures (ranging from 273 to 360 K) and partial pressures (ranging from vacuum to 100 kPa) in order to determine the heats of adsorption and evaluate the special interactions of CO and O2 with the gold nanoparticles. By comparing the results with those obtained from the non-decorated samples, useful information concerning the Au surface loading and the chemisorbed monolayer capacity was extracted. Subsequently, the activity of CNT-supported gold for CO oxidation at low temperatures was investigated by catalytic experiments under various reaction conditions (temperature, CO/O2 ratio, contact time) in a specially developed continuous flow reactor containing a packed bed of doped nanotubes. In recent investigations directed toward the elucidation of the catalytically active species of gold, extensive discussion and many hypotheses about the effects of gold cluster size related to the electronic structure are provided [30,31]. The role of low-coordinated gold atoms in clusters with non-bulk geometries [4], of cationic gold at the peripheries of supported clusters [20,32], and of anionic gold clusters [33] has also been investigated. However, the influence of gold oxidation states and cluster size on CO oxidation catalysis remain unresolved [33,34], although recent data [35] show that during CO oxidation both Au(I) and Au(0) are present in the working catalysts, and that since more catalytic sites incorporate Au(I), higher concentrations of cationic gold, which correspond to lower partial pressures of CO in the reacting mixture, lead to higher catalytic activity. To this point, we examined the performance characteristics of the CNT-supported gold nanoparticles after completely opposite activation procedures (oxidation/ reduction) in order to define the oxidation states of gold that are more active during the reaction, as well as to provide aspects on enhancing regeneration efficiency.
RBM
50000 40000
G'-band
30000 20000
D-band
10000 0 0
500
1000 1500 2000 2500 3000 -1 wavelength (cm )
Fig. 1. Raman spectrum of the gold-decorated SWNT’s.
erature [36], and the spontaneous deposition of the Au particles on the external surface of carbon nanotubes [37]. The Raman spectrum of the gold-decorated SWNTs is illustrated in Fig. 1, where the high quality of the involved SWNTs is confirmed by the high intensity ratio of the G over D band, and by the fact that the G’-band is completely free from defect contributions. 2.2. Instrumentation Raman spectra were obtained using an inVia Reflex (Renishaw) micro-Raman spectrometer using objective 50 with a focal point of 1.5 lm2 and a crystal laser excitation of 785 nm operating at 1.2 mW (1% of full power). Samples for transmission electron microscopy were prepared by extensive sonication of the CNTs in high-purity ethanol. A drop of the solution was deposited onto a Lacey C-film supported on a Cu grid and the solvent was allowed to evaporate in air. TEM analysis was then carried out in a JEOL 2011 HR-TEM, operating at 200 kV and fitted with an Oxford Instruments INCAx-sight EDS detector. TGA characterization was performed on a Pyris Diamond TG/DTA, Thermogravimetric/Differential, Thermal Analyser (Perkin–Elmer Instruments). Samples for X-ray diffraction were deposited in the form of powder on a glass. The instrument used was a Siemens D500 X-ray diffractometer. 2.3. Adsorption apparatus The adsorption isotherms were acquired using a homemade stainless steel gravimetric rig described elsewhere [30]. The balance head (CI), counterweight compartment, valves and tubing of the system were thermostated in an air-circulating bath (±0.1 K), whereas the sample compartment was separately thermostated in a silicon oil bath (±0.01 K). Measurements were performed with about 100 mg of sample to resemble the conditions of the reactor experiments. All results were corrected for buoyancy effects. Before the measurements the samples were outgassed at 460 K for 24 h under high vacuum conditions (105 mbar). 2.4. Plug flow reactor The catalytic activity for CO oxidation was examined in a plug flow reactor using 100 mg of catalyst in gas mixtures of CO-in-air and CO-in-O2, with total flow rates between 50 and 80 ml/min. The catalysts were packed in a quartz tube forming a bed with a length of 5 cm and a volume of 0.16 cm3. The gas flow of each stream was controlled by electronic mass flow controllers (Bronkhorst F-200CV) with a scale of 2–100 ml/min for CO and
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20–1000 ml/min for air and O2. The operation temperature was controlled using a thermocouple and was adjusted in the range of 303–573 K. Temperature data during the catalytic experiments corresponded to the value measured with a second thermocouple placed in the inlet immediately before the catalyst bed. The experiments were carried out both under steady state and gradient temperature conditions. During the steady state temperature experiments, kinetic data were acquired every 30 min after an initial exposure of at least 5 min on the reactants stream. The concentrations of CO2, CO, N2 and O2 were analyzed alternatively at the inlet and outlet of the reactor by a gas chromatograph (GC, SRI 8610C), equipped with a six port gas sampling valve and TCD detector. A Heysep D packed column (1/8” SS) with a length of 10 m ensured satisfactory resolution between N2, O2 and CO at a GC furnace temperature of 303 K. CO2 was eluted 10 min after injection at a furnace temperature of 373 K. The cool-down capacity of the GC furnace determined the frequency of gas sampling. The system was calibrated with gas mixtures of 1 %, 3 %, 5%, 10%, and 15% CO2 and CO-in-air. 2.5. Experimental conditions Table 1 shows the pre-treatment, reaction, and regeneration conditions involved for each of the examined catalysts. The sample code corresponding to the specific conditions is also included and will be further used as a reference. After the initial reductive activation of the sample 0_AuCNT, the catalytic performance was examined starting from the lowest temperature of 323 K and performing successive temperature increments of 50 K, each one starting after a cumulative CO feed of about 23 mmol. Thus, performance evaluation up to 573 K was accomplished. In the second catalytic test, the performance characteristics were also attained during successive temperature increments up to 573 K but with a higher ratio of O2/CO in the total gaseous stream, whereas regeneration had taken place under oxidative conditions. Kinetic tests (3, 4, 5, 6 in Table 1) were performed under stable temperature conditions after oxidative regeneration, and the performance evolution was monitored up to a total CO feed of 200 mmol. Activation of the second sample Au_CNT, took place under argon atmosphere at 373 K. The first test was performed at 303 K
by monitoring the evolution of the CO oxidation performance for a total CO dosage of 25 mmol. The subsequent tests following regeneration under oxidative conditions were performed at various temperatures for a total CO amount of about 70 mmol. The same procedure was followed for the third sample, III_AuCNT, which had been previously activated under oxidative conditions, using a smaller CO contact time and O2/CO ratio. 3. Results and discussion 3.1. Adsorption results The adsorption isotherms of CO at different temperatures for the Au-decorated and the as-received SWNTs are illustrated in Fig. 2a and b, respectively. The exothermic character of the CO physisorption on the SWNT’s is intensively proclaimed in Fig. 2b, where the adsorption capacity drops significantly with temperature. In contrary, an endothermic trend indicating the occurrence of chemisorption is obvious for the Au-decorated sample. Moreover, the CO adsorption capacity at 40 kPa for the Au-decorated sample, as compared with this of the bare nanotubes, is enhanced by a factor of 1.05, 1.5, 2.2 and 3.2 at the temperatures of 273, 290, 318 and 356 K, respectively. Additionally, above a pressure of 40 kPa, the CO isotherm curve for the Au/CNTs sample presents a shift upwards at 273 K and a shift downwards at 356 K. This is an indication that an alteration of the mechanism of adsorption to physisorption occurs as soon as the available for chemisorption Au clusters are occupied. However, the shift induced in the isotherm curves was observable only in the case of the extreme upper and lower temperatures and not for the intermediate ones, while a different trend of the curve shift between the extremely high and low temperatures is evident. This is because in the case of 273 K, the physisorption occurring to the bare part of nanotubes as well as to the CO-chemisorbed clusters is significantly enhanced, as compared to the one occurring at 356 K (exothermic character), whereas in the case of the intermediate temperatures, the differences in mass uptake caused by the different adsorption mechanisms are eliminated. Therefore, by subtracting the CO isotherm curve of the bare SWNTs at 356 K from the respective one of the Au-decorated SWNTs, it was possible in a simple way to deduce
Table 1 Catalytic performance experiments-regeneration conditions. Number of tests
Activation
Reaction conditions Temperature (K)
Total flow (ml/min)
CO contact time (min1)
O2/CO
323?573
80 (Air/CO)
54.8
1.75
323?573 573 523 473 573
57 (O2/CO) – – – –
54.8 – – – –
5.6 – – – –
Argon 373 K 24 h Regeneration Oxygen 573 K 24 h Oxygen 573 K 24 h Oxygen 573 K 24 h Oxygen 573 K 24 h
303
78 (O2/CO)
54.1
8.17
333 423 473 523
57 (O2/CO) – – –
54.8 – – –
5.6 – – –
Air 573 K 2 h Regeneration Air 573 K 2 h – –
306
78 (Air/CO)
20.82
4.82
473 523 573
– – –
– – –
– – –
0_AuCNT 1 2 3 4 5 6
Hydrogen 483 K 24 h Regeneration Oxygen 493 K 24 h Oxygen 493 K 24 h Oxygen 493 K 24 h & 573 4 h Oxygen 493 K 24 h Oxygen 493 K 24 h & 573 4 h
AuCNT 1 2 3 4 5 III_AuCNT 1 2 3 4
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a
0.25
356K 318K 290K 273K
0.18 0.16 0.14
mmols/gr
0.2 mmol/gr
b
273K 290K 318K 356K
0.3
0.15 0.1
0.12 0.1 0.08 0.06 0.04
0.05
0.02
0
0
0
20
40 60 P (kPa)
80
100
0
20
40 60 P (kPa)
80
100
80
100
Fig. 2. (a): CO isotherms of the Au-decorated sample. (b): CO isotherms of the undoped SWNTs.
a
0.6
b
Au+SWNTs
0.5
SWNTs_318K
1
SWNTs
Au+CNTs_290K
mmol/gr
0.4
mmol/gr
Au+SWNTs_318K
1.2
0.3 0.2
0.8
SWNTs_290K
0.6 0.4
0.1
0.2
0
0 0
20
40
60
80
100
P (kPa)
0
20
40
60
P (kPa)
Fig. 3. (a): CO2 isotherms at 318 K, (b): CO2 isotherms at different temperatures.
the % w/w ‘‘active” gold loading on the nanotubes. The following expression was involved:
mmolCG ¼ ðyÞ ðmmol=grÞG þ ðx yÞ f ðpÞ ðmmol=grÞC
ð1Þ
where, mmolCG is the absolute amount of CO adsorbed on the golddecorated sample, (mmol/gr)C is the normalized per gram amount of CO adsorbed on the SWNTs, (mmol/gr)G is the normalized per gram amount of CO adsorbed on the deposited gold clusters, x(gr) is the experimental mass of the gold-decorated sample, and y(gr) is the hypothetic mass of the Au nanoparticles in the Au-decorated sample. The correction factor f(p) was involved in order to exclude from the SWNT’s sample the nanotube active sites that were occupied by gold. This was calculated by the ratio of the CO2 isotherms of the doped and undoped samples at 318 K (Fig. 3a). As it is evident (Fig. 2a), the presence of Au clusters diminished the adsorption capacity for CO2, which was expected since there is not a special interaction of the Au clusters with the CO2 molecules. A further proof for this, constitutes the exothermic character of the CO2 isotherms of the gold-decorated sample (Fig. 3b). Moreover, since the amount of chemisorbed CO molecules per gram of gold (mmol/gr)G was unknown, we adopted the values referred in the recent literature [29] for the chemisorbed clusters Aum(CO)n. As it was shown by PES spectroscopy, gold clusters with m = 2, 3, 4, 5 chemisorb 2, 2, 3, and 4 molecules of CO, respectively. By assuming an equal in amount distribution of the deposited gold in Au2, Au3, Au4 and Au5 clusters we could calculate a chemisortion capacity of about 4 (mmol/gr)G. It should be noted
that although the application of this value of the gold chemisorption capacity for all the range of pressures examined is certainly under contestation, it was adopted as an indicator to reveal the CO pressure corresponding to the end of chemisorption. At this point, the adopted value is consistent and the y% content of gold calculated out of this must be correct. The plot derived out from Eq. (1), which expresses the% w/w gold (y) as a function of pressure, is illustrated in Fig. 4a. The shift in curve of Fig. 4a between 40 and 50 kPa denotes the aforementioned transition from chemisorption to physisorption, and a gold weight percentage of 2.75% was calculated at this point. However, the weight percentage of deposited gold calculated by this method was smaller than this estimated from the result of TGA analysis in oxygen atmosphere (Fig. 4b). After the complete oxidation of the decorated SWNTs (Fig. 4b rhombs) the remaining mass, which corresponds to the sum of the gold and nickel content of the decorated sample was about 11.6% of the initial. The nickel content defined by the TGA curve of pristine SWNTs (Fig. 4b-line) was about 6.2%. Thus a net gold content of about 5.4% was calculated. This discrepancy between the gold content values calculated by TGA and the gravimetric method was attributed to the fact that from the total deposited gold (5.4%w/w) only the 50% was active for catalysis. Further proof for this constitutes the particle size distribution (PSD) (Fig. 5a) derived from the examination of TEM images of the Au-decorated samples (Fig. 5b). A critical issue in order to derive the PSD of Au nanoparticles is to distinguish them from the Ni particles that had remained during the fabrication of
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a
b
3.5
Sample mass (%)
3 2.5
% w/w Au
Gold decorated SWNTs
100
2 1.5 1
Pristine SWCNTs
80 60 40 20
0.5 0
0 0
50
100
P (kPa)
0
500
1000
Temperature [oC]
Fig. 4. (a): %w/w amount of deposited gold where CO chemisorption occurs. (b): TGA curve of the Au-decorated SWNT’s.
SWNTs and which, as calculated by the TGA, amount up to 6.2%w/w in the pristine SWNTs. On this purpose the TEM image of pristine SWNTs is presented in Fig. 5c. As it can be observed there are curved distorted carbon structures on the external surface of the pristine nanotubes (dark cycles) that bear the Ni nanoparticles. The size of these nanoparticles is about 1–2 nm (inset Fig. 5c), which is much smaller than that of the Au particles, which tend to form larger aggregates during the reduction with DMF (Fig. 5b). The curved distorted structures bearing the Ni nanoparticles can also be distinguished on the Au-decorated sample (upper right part Fig. 5b). Additionally the difference in size between the Au and Ni nanoparticles is proclaimed by the XRD patterns of the pristine and Au-decorated SWNTs presented in Fig. 6. By involving the Sherrer equation a size of 6.8 and 21 nm was calculated for the Ni and Au nanoparticles respectively. The derived gold PSD implies that about 45% of the 5.4% w/w deposited gold particles have the size between 3 and 7 nm, which is established as the optimum size for the catalytic activity of gold, and this is in very good agreement with the result obtained by the isotherms subtracting method. As mentioned in the introduction, most of the literature dealing with oxygen chemisorption on Au nanoparticles presents theoretical calculations of the binding energy based on the use of density functional theory. The O2 interaction with charged and neutral gold clusters is found to exhibit a pronounced sensitivity to the cluster size and to its charge state [26–28]. In Fig. 7 we present experimental evidence of the specific interaction of O2 molecules with gold nanoparticles as the adsorption capacity is enhanced by a factor of 2–3 when compared with this of the undoped SWNTs. Moreover, a DH (kJ/mol) value of 4 for the case of bare SWNTs and 12 for the gold-decorated sample was calculated by applying the Clausius–Clapeyron equation. The observed exothermic character of the O2 chemisorption is consistent with the occurrence of dissociative adsorption [35], which, as has been predicted by DFT [26], is favored for larger gold clusters Aun (n > 3) and is accompanied by large changes in the structure of the metal host. Overall, the adsorption results revealed that the decoration of SWNTs with gold nanoparticles resulted in a significant increase in the adsorption capacity for CO and O2 and a decrease for CO2, when compared with the bared SWNTs. Thus, the developed materials constitute a good candidate to be applied as catalysts for low temperature CO oxidation reaction. The good dispersion properties of DMF in combination with the high content of SWNT’s in strongly acidic carboxylic groups resulted in an adequate loading with gold. The potential of carboxylic groups to reduce the anchored Au(III) led to a spontaneous formation of metallic gold that under the reductive conditions involved (DMF)
acts as a nucleation center for the formation of larger gold agglomerates with an almost equal number distribution (20–25%) in the size ranges of 3–5, 5–7 and 7–9 nm. 3.2. Performance results The low temperature CO oxidation efficiency of the as-prepared gold-decorated SWNTs was significantly degraded after their treatment in H2 atmosphere (483 K) for 24 h. During the temperature gradient tests that followed the H2 treatment (filled triangles in Fig. 8), the acquired conversion factors were limited to the moderate value of 1.3% in the temperature range between 323 and 473 K, whereas the highest efficiency was observed at 573 K and was no more than 24%. However after the regeneration of the same sample under extremely oxidative conditions (493 K, 24 h) an important enhancement of the performance was observed, with prominent CO conversions of 3.2% and 5% at 373 and 423 K, respectively (Fig. 8). The moderate conversion efficiency of the H2-activated sample once again denotes the significant reduction capacity of both the DMF solvent and the surface carboxylic groups. As a result of this reduction activity, most of the deposited gold (75%) on the starting material was present in the form of small (3–5 nm) and slightly bigger (5–13 nm) Au(0) particles (Fig. 5a), with a very small percentage remaining in the form of supported Au(III) complexes (probably those anchored to the surface phenolic groups). During the subsequent reductive activation in H2 at 483 K, the existing small particles serve as nucleation centers for the formation of bigger agglomerates, which are inactive during CO oxidation. These large agglomerates are clearly distinguished in Fig. 9a, which presents a TEM image of the hydrogen treated sample 0_AuCNT after the completion of the catalytic test runs. Thus the observed moderate activity of the material (1.3%) can be attributed to the presence of the remaining low amount of Au(III) complexes that under the H2 reductive atmosphere form the desired small Au particles. It is believed that the undesirable big agglomerates can be generated by two processes. The first is assumed to start from the reduction in H2 of the remaining Au(III) cations, forming metal clusters that migrate along the surface until collision with similar clusters occurs, resulting in coalescence. The second process involves Ostwald ripening when primary Au particles formed during ion exchange grow in the expense of smaller clusters [38] formed during reduction in H2. In our case, the second mechanism most likely occurs, since the remaining Au(III) complexes are not consumed in the formation of large metal agglomerates but they are rather participating to the formation of a slight portion of small gold clusters (<15 nm) that were responsible for
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a
Nanoparticle size distribution
[5-7]
20
[43-45]
[45-47]
[39-41]
[41-43]
[35-37]
[37-39]
[33-35]
[31-33]
[29-31]
[25-27]
[27-29]
[21-23]
[23-25]
[17-19]
[19-21]
0
[13-15]
5
[11-13]
10
[15-17]
15 [9-11]
% number of particles
[3-5]
25
[7-9]
30
Particle size [nm]
b
c
Fig. 5. (a): Au particle size distribution as-derived from TEM images. (b): TEM image of the Au-decorated SWNTs. (c): TEM image of the pristine SWNTs.
the existing, although moderate, catalytic activity of the final material. Independent of the mechanism, the deposited gold on the H2 reduced material (0_AuCNT) must have exhibited its highest percentage in the form of large Au clusters (>15 nm), which are
inactive for low temperature CO oxidation. An additional important remark coming out of this set of experiments (tests 1 and 2, sample 0_AuCNT) is that the formation of the bigger gold agglomerates is partially reversible, meaning that under the extremely oxidative conditions (O2 493 K, 24 h) applied before proceeding
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Intensity a.u.
Au (111)
Au (200) Ni (111)
pristine SWNTs
Ni (200)
Au (220)
Au (311)
Au SWNTs
28
38
48
58
68
78
2-theta degree Fig. 6. XRD spectrum of the as-derived and gold-decorated SWNTs.
0.3
mmol/gr
0.25 0.2 0.15 0.1 0.05 0 0
20
40 60 P (kPa)
80
100
Fig. 7. Oxygen isotherms at different temperatures.
with the second temperature gradient test (open cycles in Fig. 8), a fragmentation of the initial clusters to smaller ones occurs. This phenomenon has already been investigated in the recent literature by EXAFS data recorded at the end of an O2-TPO (temperature programmed oxidation at 490 K) treatment of gold clusters supported to MgO. The data showed a decrease in the Au–Au coordination number and an increase in the Au–O contributions of the gold clusters. The authors [35] noted that this oxidative fragmentation was
320
370 28 26 24 22 20 18 120
0
140
20
420
160 mmols CO
40
Temperature (K) 470
180
60 80 mmols (CO)
520
570
200
100
120
27.5 25 22.5 20 17.5 15 12.5 10 7.5 5 2.5 0 140
% conversion
AuCNTs 273K AuCNTs 290K AuCNTs 318K CNTs 290K CNTs 318K
0.35
not complete, and the resultant oxidized species were not the same as those presented in the initially prepared samples as Au(III) complexes. Instead, these oxidized species are not well defined structurally and consist of oxidized and aggregated species that might be regarded as gold oxide clusters. In this work it is shown that the species that are formed out of fragmentation are more active for CO oxidation at low temperatures (up to 473 K) than the large (>15 nm) Au clusters (Fig. 8). At higher temperatures (523 and 573 K) and under the reducing conditions involved during the catalytic test (where CO acts as an electron donor), the oxidized fragmented pieces are reorganized in larger clusters, and the catalytic conversion drops again to the values obtained with the H2-activated sample (Fig. 8, the two curves come into convergence). Additional evidence to this point constitutes the kinetic data acquired in both cases at a stable temperature of 573 K (inset in Fig. 8). The catalytic activity of the oxidized sample drops faster during the initial stages of the isothermal test due to re-agglomeration of the fragmented clusters, and after about 40 mmol of CO feed, the conversion efficiency comes into the same value as the one acquired at the same temperature for the H2-treated sample. This is an additional contribution to the conclusion that the existing slight portions of the small Au nanoparticles, which are responsible for this activity, do not take part in this oxidative fragmentation/ reductive re-agglomeration process. The kinetic tests 3 and 6 of the same sample (Table 1 and 0_AuCNT) are illustrated in Fig. 10a. Although performed at a high temperature (573 K), which was out of the purposes of this work,
Fig. 8. Evolution of the conversion efficiency of sample 0_AuCNT with temperature and cumulative CO feed. Triangles (Test 1): Sample activated with hydrogen 483 K, 24 h. Cycles (Test 2): Sample regenerated with oxygen at 493 K for 24 h. Inset: Evolution of the conversion efficiency with CO feed at 573 K.
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Fig. 9. TEM images of: (a) hydrogen treated sample and (b) air treated sample.
these tests provide additional information concerning the oxidative fragmentation of the gold agglomerates as well as on the retention of the catalytic activity of the small (<13 nm) gold nano-
a 100
b 14 regeneration conditions
90
12
80
493K O2_24h & 573K 4h
70
493K O2_24h
% conversion
% conversion
particles. The oxidative fragmentation occurring under more intense conditions (O2, at 573 K) must be thoroughly enhanced thus giving rise to an enhanced conversion efficiency of 90% for the first 5 mmol of CO feed. However the efficiency diminishes very fast and during the first 26 mmol of CO feed the conversion factor drops to about 50% which is also the value for the case when the sample was regenerated under softer oxidative conditions (O2 at 493 K). A conclusion educed out of this is that the excess amount of small clusters produced out of the fragmentation at higher temperatures is prone to a very fast agglomeration under the reducing conditions involved during the catalytic tests. On the contrary, the fragments generated out of the oxidative treatment at 493 K are far more stable and keep their significant catalytic activity (50% conversion) for more than 100 mmol of CO feed. Similar results at lower catalytic temperatures (tests 4 and 5) are presented in Fig. 10b where the faster deactivation rate of the sample regenerated under higher oxidative conditions is demonstrated. The up to now extended discussion on the CO oxidation performance of the 0_AuCNT sample mainly concerned the reducing activity of both the DMF that was used as a dispersion agent and solvent, and the carboxylic groups existing on the surface of the involved SWNTs. Due to the high reducing activity, the developed material contained particles of gold, which was already in its metallic form, as also shown by the XRD pattern (Fig. 6), where the characteristic peaks of metallic gold are clearly distinguished. Indeed, as it was already observed [26], Pt and Au nanoparticles (NPs) can be formed in the presence of CNTs, from metal precursors (salts) without the need to further involve a reducing procedure. This phenomenon was attributed to a direct redox reaction occurring between the metal ions and CNTs, the latter acting as the electron donors. In another aspect [18] phenolic groups interact with ([Au3+] A) precursors by anchoring the gold cations: C– OH + Au3+?C–OAu3+ + H+, while the interaction of ([Au3+] A) solutions with the less stable surface carboxylic groups results in surface decarboxylation and reduction from Au(III) to Au(0): C– COOH + Au3+? C + Au0 + H+ + CO2. During the succeeding treatment with H2 these particles served as nucleation centers for the formation of bigger agglomerates and the derived material was inactive for CO oxidation at low temperatures. In order to elucidate the origins of the catalytic activity of the developed materials we performed catalytic tests on two samples activated with argon at 373 K (AuCNT’s) and air at 573 K (III_AuCNT’s) for 24 and 2 h, respectively. Since no agglomeration of gold nanoparticles is expected to occur under the involved activation conditions, both samples must contain a 2.75% per weight loading of gold particles with a size between 3 and 7 nm, as already calculated by the interpretation of
60 50
10
regeneration conditions
8
493K O2_24h & 573K 4h 493K O2_24h
6 4
40
2
30
0
0
50
100 mmols CO
150
0
10
20
30
mmols
Fig. 10. (a): Kinetic experiments for sample 0_AuCNT. Evolution of the conversion efficiency with CO feed at 573 K. Squares (Test 3), Rhombs (Test 6). (b): Evolution of the conversion efficiency with CO feed at 523 K (Cycles, Test 4), 473 K (Triangles, Test 5).
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b
70 60 conversion%
8.0
423K
473K
III_AuCNT's
7.0
523K
473K
AuCNT's
6.0
523K
573K
80
conversion%
a
50 40 30
5.0 4.0 3.0
20
2.0
10
1.0 0.0
0 0
20
40
60
mmols CO
0
50
100 mmols CO
150
Fig. 11. (a): Kinetic experiments at 303 K. Open symbols: sample activated with Ar at 373 K for 24 h, Filled symbols: sample activated with Air at 573 K for 2 h. (b): Evolution of the conversion efficiency with CO feed at higher temperatures.
the adsorption isotherms (Section 3.1). This loading was adequate to produce significantly high conversion efficiency (70%) at 303 K, during the first 2.5 mmol of CO feed as illustrated in Fig. 11a. The initial activity of catalyst AuCNT’s was greater than that of catalyst III_AuCNT’s, but after about 20 mmol of CO, the activity of the two samples became nearly the same, reaching a plateau of 10% in CO conversion. The data suggest the presence of essentially the same supported gold species in both catalysts after steady state had been attained. The slightly higher activity of the AuCNT’s sample at the initial stages of testing may be attributed to the formation of an additional amount of small-size gold clusters during argon treatment. These were formed from the ionic gold Au(III) that has remained unreduced during the impregnation procedure (not reduced by DMF or carboxylic groups). The ability of argon at 373 K to reduce Au(III) complexes forming small clusters of metallic gold Au(0) can be compared with the reported activity of He at 373 K to form extremely small clusters of gold [4]. The He activity was educed out of EXAFS spectra showing first- and second-shell Au–Au coordination numbers of 4.0 and 1.0, respectively for a MgO sample impregnated with cationic gold and further treated under He atmosphere at 373 K for 2 h, whereas, a treatment at 573 K resulted in the formation of larger clusters [4]. On the other hand, as already mentioned, the oxidative treatment of the III_AuCNT’s sample must have provoked the oxidative fragmentation of larger gold agglomerates already existing on the raw sample, thus creating smaller clusters, which are also active for CO oxidation. This is also supported by the TEM image of the III_AuCNTs sample (Fig. 9b) where a large population of small in size (10–15 nm) Au clusters can be clearly distinguished. As it is also clear from the TEM image, the oxidative treatment in Air at 573 K has not distorted the structure of the nanotubes and thus the possible effect of exposed Ni nanoparticles on the catalytic performance must be excluded. Although not concluded by the TEM image, the number of these clusters must be significant since a substantial population of large agglomerates was observed on the starting material (Fig. 5a) that corresponded to a deposited gold w/w percentage of 0.7, 0.5 and 0.3% for sizes 15–17, 23–25 and 37– 39 nm, respectively. Therefore, we would expect a higher catalytic activity for sample III_AuCNT’s when compared to the AuCNT’s. However, although the sample III_AuCNT’s certainly contains a larger number of small gold nanoparticles (3–13 nm) than the AuCNT’s one, a significant population of these are in the form of oxidized species which are not well defined structurally [35] and must be less active than their metallic gold analogues of the same size. A general conclusion educed up to now is that the oxidized gold species coming out of the oxidative fragmentation during regeneration, have a CO oxidation catalytic activity that follows the order: CI < oxidized gold species < CII, where CI the catalytic
activity of Au particles larger than 15 nm and CII the catalytic activity of Au particles smaller than 13 nm. A common observation when comparing the kinetic data of all the performed catalytic experiments (Inset Figs. 8, 10a, 11a) is that, independently of the activation, regeneration, and catalytic test conditions, the catalyst deactivation proceeds with a fast rate at the very early stages of the test up to about 20 mmol of CO feed, which correspond to 200 mmol per gram of catalyst. The steady state catalytic efficiency remains almost stable for more than 400 mmol of CO feed per gram of catalyst and, as illustrated in Fig. 11b, in one of the most persistent tests the conversion was slightly diminishing for more than 1.1 mol/gr. Another observation is that for a sample exposed to sequential catalytic tests the conversion efficiency at steady state is independent of the regeneration conditions (moderately oxidative or extremely oxidative (Fig. 10a)) involved in between the tests. Finally, when involving different activation conditions (except from sample 0_AuCNT’s) the steady state conversion efficiency is identical during the first run (Fig. 11a), but differs significantly after regeneration (Fig. 11b). From the above results, it is clear that at the early stages of the catalytic test, deactivation proceeds via a reorganization process including sintering and oxidative fragmentation of the gold particles and clusters. As already discussed, two mechanisms describe the sintering process in supported metallic particles: (i) particle migration and coalescence, (ii) atom emission and recapture through diffusion from the smaller, high potential particles to the larger, low potential ones (Ostwald ripening). Recently, a new MC model of supported-catalyst sintering [39] was developed and showed that particles grow rapidly during the first stages of a catalytic test due to the first mechanism, whereas subsequent increases in average particle size are much slower as the growth mechanism shifts from coalescence of closely spaced particles to Ostwald ripening. Our data show that once the CO oxidation reaction attains steady state, meaning that the mechanism of deactivation proceeds through diffusion of gold atoms on the surface, a significant part of the total amount of deposited gold has already aggregated into clusters with large size as a result of the reductive activity of CO, and simultaneously, a part of these large clusters undergoes oxidative fragmentation to less active oxidized species due to the presence of oxygen, with the second mechanism most likely occurring at temperatures higher than 473 K. Thereinafter, the catalytic activity is attributed to the remaining small-size gold particles, as also suggested by Haruta and Date [40]. Concerning the regeneration of the samples (Fig. 11b), it is again confirmed that the more oxidative conditions lead to the production of fragmented clusters, which are more easily migrating and more prone to coalescence. From Fig. 11, it is clear that although the steady state CO conversion values acquired during the first run at 303 K
E.C. Vermisoglou et al. / Microporous and Mesoporous Materials 120 (2009) 122–131
were about 9 and 7% for the oxygen and argon-activated samples respectively, the succeeding regeneration of the argon-activated sample with pure oxygen instead of air (III_AuCNT’s) led to a faster rate of deactivation at the early stages of the subsequent catalytic tests and moreover to a lower steady state conversion efficiency. 4. Conclusions SWNTs constitute a good candidate to be used as support for gold nanoparticles due to their high content in carboxylic groups that act as anchoring agents for the formation of Au(III) complexes. The reducing activity of carboxylic groups eliminates the need for further treatment, as metallic nanoparticles are formed spontaneously after the impregnation procedure. In this work we showed that through the treatment of SWNTs with AuCl3 (99.9+%) in DMF, a 6% w/w gold particles loading was achieved from which 75% had a size below 13 nm. The developed material, after its treatment in argon atmosphere at 373 K, had a significant performance for low temperature CO oxidation and a conversion factor of 70% was evaluated at 303 K during the first 25 mmol of CO feed per gram of catalyst. The steady state conversion efficiency at the same temperature was about 10% and was kept almost constant for more than 500 mmol CO per gram of the catalyst. Deactivation during the catalytic tests (CO, O2 atmosphere) proceeds via particle migration at the very early stages accompanied by a tremendous drop in the catalytic activity. Deactivation during steady state proceeds via the Ostwald ripening mechanism. Regeneration of the catalyst under moderate oxidative conditions (air at 573 K) provoked the oxidative fragmentation of the large gold clusters. The resulted oxidized species presented quite satisfactory conversion efficiency and were more stable during the catalytic test than the ones produced after regeneration under more oxidative conditions (O2 at 573 K). Acknowledgments The authors would like to thank Dr. P. Falaras and Dr. V. Likodimos for the Raman spectroscopy analysis and G. Basina for X-ray diffraction analysis. Financial support by the European Network of Excellence INSIDE-PORES is gratefully acknowledged. References [1] M. Commoti, W.-C. Li, B. Spliethoff, Schüth, J. Am. Chem. Soc. 128 (2006) 917– 924.
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