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Characterization of silica–carbon mesoporous matrix with embedded nickel nanoparticles synthesized by the polymeric precursor method
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Materials Chemistry and Physics 106 (2007) 286–291
Characterization of silica–carbon mesoporous matrix with embedded nickel nanoparticles synthesized by the polymeric precursor method A.A. Cavalheiro a,∗ , J.C. Bruno a , E.R. Leite b , J.A. Varela c a
Depto de Qu´ımica, Instituto de Biociˆencias, UNESP, Distrito de Rubi˜ao Junior, s/n, P.O. Box 510, Botucatu 18.618-000, SP, Brazil b CMDMC, Universidade Federal de S˜ ao Carlos, Rod Washington Luiz Km 25, P.O. Box 676, S˜ao Carlos 13.565-905, SP, Brazil c LIEC, Instituto de Qu´ımica, UNESP, R. Francisco Degni, s/n, Quitandinha, P.O. Box 355, Araraquara 14.801-970, SP, Brazil Received 7 October 2006; received in revised form 18 May 2007; accepted 2 June 2007
Abstract Nickel nanoparticles into silica–carbon matrix composites were prepared by using the polymeric precursor method. The effects of the polyester type and the time of pyrolysis on the mesoporosity and nickel particle dispersion into non-aqueous amorphous silica–carbon matrix were investigated by thermogravimetric analysis, adsorption/desorption isotherms and TEM. A well-dispersed metallic phase could be only obtained by using ethylene glycol. Weightier polyesters affected the pyrolysis process due to a combination of more amounts of carbonaceous residues and delaying of pyrolysis process. The post-pyrolyzed composites were successfully cleaned at 200 ◦ C for 1 h in oxygen atmosphere leading to an increase in the surface area and without the occurrence of carbon combustion or nickel nanoparticles oxidation. The matrix compos˚ mainly when tetraethylene glycol was used as polymerizing ites presented predominantly mesoporous with pore size well defined in 38 A, agent. © 2007 Elsevier B.V. All rights reserved. Keywords: Composite materials; Chemical synthesis; Thermogravimetric analysis
1. Introduction In metal–ceramic composites the morphology and particle size are important aspects for tuning in nanometric scale. In fact, a large variety of mesoporous inorganic materials can be prepared using several organic templates including surfactant self-assemblies and block copolymers [1–3], but the development of metal particles in the nanosize range remains as an important problem. The interesting properties of nanosized metal particles as catalysts of chemical reactions involving inorganic and organic compounds have impelled more and more the researches in this field. Indirect methods, in which metal nanosized particles are previously prepared, cannot avoid the particle agglomeration originated from the decrease in the particle size, which is a required characteristic to enhance the effectiveness of catalysis [4,5]. An alternative method found for the insertion of a metallic precursor into the support matrix has also been conduced. In
∗
Corresponding author. Tel.: +55 14 3811 6255. E-mail address: [email protected] (A.A. Cavalheiro).
0254-0584/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2007.06.004
this procedure, the impregnation or precipitation stage is followed by reduction using “in situ” nanoparticles nucleation into a commercial or pre-prepared mesoporous matrix. Therefore, the heterogeneity in the dispersion of the metal and its particle size are strongly dependent on the previous matrix structure and metal impregnation techniques [6,7]. Recently, some researchers have experimented a direct method to synthesize this type of nanocomposites [8,9]. In this procedure, the synthesis of mesoporous matrix and the metal nanoparticle embedding are carried out simultaneously, leading to metal–ceramic composites. The polymeric precursor method [10] can be present as a tool for this purpose, due to the involving of a complex agent for several metallic and semi metallic ions. The pyrolysis process of the cation-containing the polymeric resins in inert atmosphere, such as nitrogen gas flux, results in metallic nanoparticles already embedded into the mesoporous matrix ceramic [11,12]. This work aims at showing the influence of some parameters in the pyrolysis process and its consequences on mesoporous silica–carbon embedded nickel nanoparticles. For this purpose, the polymer chain size was adjusted by changing the diol part of the polymeric chain.
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2. Experimental procedure
3. Results and discussion
Ni-silica samples were synthesized using tetraethoxysilane (TEOS: >99%, Aldrich), citric acid (C6 H8 O7 : 99.9%, Merck) and nickel (II) nitrate hexahydrate (Ni(NO3 )2 ·6H2 O: 99.9%, Aldrich). Ethylene glycol (EtG), tetraethylene glycol (TeG) and a blend of both in a molar ratio of 2:1 (BdG) were used as polymerizing agent. The Ni content for the samples was 10 wt%, relative to the silica weight present in the composite, and the molar ratio of citric acid/metal (Si + Ni) was 2:1 whereas the diol/citric acid molar ratio was 1:2. The polymeric precursor was obtained by condensation at 80 ◦ C under stirring until solvent evaporation, followed by polymerization at 250 ◦ C for 2 h in a mufle type oven. In sequence, the resulting precursor was ground in a mortar and pyrolysed at 600 ◦ C for three different times (1, 3 and 7 h) in a tubular furnace under nitrogen gas flux of 30 cm3 min−1 . The samples will be hereafter referred as RaD60X, where RaD means the designation of the polymeric resins and X, the time of pyrolysis (1, 3 and 7 h). The pyrolysis process was investigated by thermogravimetric analysis (Netszch–Thermiche Analyse) using a 10 ◦ C min−1 in heating rate with a nitrogen flux of 30 cm3 min−1 . Measurements of the surface area, hysteresis curve and pore size distribution of the composite samples were obtained by nitrogen adsorption/desorption analysis (Micromeritics, model ASAP 2020). The samples were suspended in ethanol under ultrasonic vibration and brought onto a holey polymer film on a copper grid to be observed by transmission electron microscopy (Philips CM-200 FEG).
The influence of the polyester type was verified by the weight loss (TG) and the weight loss rate (dTG) in accordance with the synthesis conditions. In Fig. 1a, it is possible to observe that the weight loss for the polyester starts effectively around 300 ◦ C for all samples, and the final residue varies as a function of the molecular weight of the polyester. The molecular weight of BdG sample can be assumed as being similar to the diethylene glycol taking in account the pondered weight formula (2EtG:1TeG). The residual weight at 1000 ◦ C (final temperature of analysis) is close to 22% of the initial weight for TeG polyester, while these values are close to 32 and 26% for EtG and BdG polyesters, respectively. By calculation, it was determined that the weight ratio between the organic and inorganic components is a constant for all samples. Probably, the residual carbon amount is dependent on the citric acid/metal molar ratio. In the temperature range between 250 and 500 ◦ C the highest weight loss occurs, and the differential weight loss increases with the molecular weight of the polyester (Fig. 1b). In this temperature range the polyester cracking is started, but the difference among dTG peaks indicates that the polyester breaking is dependent on the diol type used as polymerizing agent. The diol type affects the thermal stability of the polyester, as observed for TeG sample, showing two dTG peaks. The first peak is local-
Fig. 1. Thermogravimetric analysis in nitrogen atmosphere for the different polyester precursors: (a) TG and (b) dTG for the range where high weight loss occurs.
Fig. 2. Thermogravimetric analysis in oxidizing atmosphere (a) and the comparative results between oxygen and nitrogen atmosphere (b) for the EtG607 composite sample.
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Table 1 Data of porosity and specific surface area obtained for EtG607 composite sample under oxidizing treatment for 1 h Sample
Specific surface area (m2 g−1 )
Cumulative pore area (m2 g−1 )
Cumulative pore volume (cm3 g−1 )
Average pore ˚ diameter (A)
As pyrolyzed Oxidized at 200 ◦ C Oxidized at 400 ◦ C
260 396 414
118 179 219
0.14 0.20 0.25
46 46 45
ized at 365 ◦ C and the second one at 385 ◦ C, slightly above the dTG peak of EtG sample, localized at 380 ◦ C. BdG sample also presents a peak at this temperature; it is possible to observe a shoulder at 365 ◦ C, similar to the first dTG peak found for TeG sample. The organic component originated from the pyrolysis process is also an important factor to be considered in order to evaluate the nature of these composite samples. When recorded under air atmosphere (or in general, oxygen-containing), TG techniques can also be used to study the homogeneity and the phase purity of carbons, like as nanotubes [13,14] and microporous carbons [15]. So, it is interesting to verify whether this simple and highly useful methodology can be extended to investigate the characteristics of the organic component present in the Ni/SiO2 composites. A typical curve for EtG607 is shown in Fig. 2a, where two significant weight losses are observed in two narrow ranges of temperature: weight loss 1 (50–150 ◦ C) and weight loss 2 (300–400 ◦ C). The first one is related to evaporation of water and adsorbed gases. The second one is related to carbon combustion due to the low stability of the carbon framework, reported elsewhere [16], indicating that the carbon residue has a non-graphitized nature. The comparative results between oxygen and nitrogen atmosphere for representative EtG607 sample are shown in Fig. 2b. For nitrogen flux, the first weight loss occurring during the heating ramping step is associated to the same event verified in oxygen flux, and the second weight loss is related to completion of the pyrolysis process of carbonaceous residue present in the composite matrix. For nitrogen flux the first weight loss occurs faster than the observed for oxygen flux, due to the oxidation of alkoxy groups to carboxyl groups when oxygen atmosphere is used. The presence of these more oxidized groups in the adsorbed molecules increases the thermal stability when compared to the correspondent non-oxidized molecules. However, the second weight loss occurs faster for oxygen flux due to the combustion of carbon from 400 ◦ C. These results can help to propose an effective way for cleaning the adsorbed carbonaceous residues originated from the partial pyrolysis of the polymeric precursor. We have successfully cleaned the post-pyrolyzed composites at 200 ◦ C for 1 h in oxygen atmosphere. It is probable that the success for the cleaning process is related to the presence of nickel nanoparticles in the composite acting as catalyst for the destruction of the carbonaceous residue, as reported in other works [17–20]. This condition for cleaning is a safe mode because it is carried out in temperatures below the ones of carbon combustion and nickel particle oxidation. The X-ray diffraction patterns for the EtG607 composites before and after cleaning at 200 and 400 ◦ C for 1 h
are shown in Fig. 3. Also, it is exhibited the patterns for the synthetic SiO2 cristobalite (PDF number 39-1425), cubic SiO2 similar to the one of the several MCM-48 silica material types (PDF number 51-1592) and the synthetic NiO bunsenite (PDF number 47-1039). It is possible to observe that the EtG607 composite sample presents low amounts of amorphous cristobalite and one type of MCM-48 phase, observed at lower 2θ degree. After cleaning the EtG607 sample at 200 ◦ C for 1 h, no oxidation reaction was verified, and the XRD pattern is similar to the non-oxidized sample, presenting only an increase in the amount of cristobalite and rising of a second peak related to a second type of MCM-48 with smaller pore diameter contribution. But when the carbon combustion occurs at 400 ◦ C, as demonstrated in Fig. 2a, the nickel nanoparticles are consequently oxidized, visualized by the rising of NiO peak. It occurs because high temperature and partial pressure of oxygen eliminate the amorphous carbon, which can be acting as protective for nickel nanoparticles. In addition, the peaks at low angle related to the two existing types of MCM-48 phases for the sample oxidized at 400 ◦ C are convoluted at only one, which is similar to the MCM-48 pattern. After cleaning at 200 ◦ C for 1 h, it was verified that the surface area increased over 50% if compared to sample as pyrolyzed. The variation of the specific surface area and porosity for these samples is shown in Table 1. It is possible to observe that the ratio between pore and surface area values remains constant after cleaning, denoting that the removing of the carbonaceous residues not only unblocked the pore, verified by the increase over 40% in the cumulative pore volume, but also cleaned the carbonaceous residues chemisorbed on the external surface. For
Fig. 3. X-ray diffraction patterns for EtG607 composite sample: (a) as pyrolyzed, (b) oxidized at 200 ◦ C for 1 h and (c) oxidized at 400 ◦ C for 1 h.
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nal surface did not change, observed by the slightly decrease in the average pore diameter. Fig. 4 shows the nitrogen adsorption/desorption isotherms for no cleaned composites obtained by pyrolysis at 600 ◦ C for 7 h. The isotherms represent a combination between types I and II (IUPAC classification). The uptake at low pressure corresponds to the adsorption filling of smaller pores, and is more prominent for TeG607 sample. On the other hand, the
Fig. 4. Effect of the polyester type on the N2 adsorption/desorption isotherms for the composite samples obtained by pyrolysis at 600 ◦ C for 7 h in N2 atmosphere.
oxidized at 400 ◦ C, the ratio between pore and surface area values increases, showing that only the pore opening is occurring, once the external surface area almost did not change, what agrees with the absence of carbon phase inside the mesoporous silica matrix. The cumulative pore volume increases 25% for this sample compared to the sample oxidized at 200 ◦ C, and although the unblocking process occurs in a wide range of pore diameter, the clean process for smaller pores is more effective, once the exter-
Fig. 5. Specific surface area and pore volume curves for the composite samples obtained by pyrolysis at 600 ◦ C in N2 atmosphere.
Fig. 6. Pore size distribution (BJH model) for the composite samples obtained by pyrolysis at 600 ◦ C in N2 atmosphere for: (a) 1, (b) 3 and (c) 7 h.
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composites prepared with ethylene glycol (EtG and BdG samples) present higher adsorbed volume at intermediary relative pressure, indicating the existence of more mesopores. At high relative pressure EtG and BdG samples present type II isotherm, being characteristic for an opened structure different from TeG sample. The hysteresis presented at low relative pressure for all samples can be explained by the model of interconnected mesopores similar to the one found in carbon adsorbents [21]. When the adsorbate size is bigger than 1/3 of the network cavity size, the pore blocking effects can occur, leading to a hysteresis loop in the adsorption/desorption isotherms at low relative pressure. As a consequence, the pore interconnection volume decreases and also the correspondent hysteresis loop. BdG sample presents a high hysteresis loop at low relative pressure, which was associated to less opened structure.
The effect of the time of pyrolysis and polyester type on the specific surface area and pore volume in the composites pyrolyzed at 600 ◦ C can be seen in Fig. 5. The surface area is more dependent on the polyester type than the time of pyrolysis. The variation of the surface area seems to be more influenced by the organic component present in the matrix composite, which is repeatedly removed along the pyrolysis process. This event can be observed by the correspondent pore volume augment. Analyzing the profile of the pore size distribution as a function of different composites for each time of pyrolysis (Fig. 6), it is possible to observe that for 1 h (Fig. 6a) the EtG sample possesses more adsorbed volume in low average diameter ˚ while TeG exhibits lower adsorbed volume. BdG sam(∼38 A), ple presents an intermediate behavior at this initial stage of pyrolysis. With the increase in the time of pyrolysis (Fig. 6b ˚ increases continually for and c), the adsorbed volume at ∼38 A
Fig. 7. TEM image for the composite samples obtained by pyrolysis at 600 ◦ C for 7 h: (a) EtG, (b) TeG, and (c) BdG. Representative electron diffraction pattern obtained for EtG sample (d).
A.A. Cavalheiro et al. / Materials Chemistry and Physics 106 (2007) 286–291
all of the samples and the sharp shape indicates a well-defined pore size. A significant increase in the pore volume close to ˚ was not verified for TeG sample, indicating that, when 100 A tetraethylene glycol is used as polymerizing agent, there is the predominance of mesopores in the matrix composite. Fig. 7 shows the TEM images for the composite samples obtained by pyrolysis at 600 ◦ C for 7 h. It is possible to observe the well-dispersed Ni nanoparticles in the matrix composite obtained from EtG polyester (Fig. 7a). When tetraethylene glycol is present in the polyester, TeG and BdG samples (Fig. 7b and c, respectively) the dispersion of nickel nanoparticles is strongly affected, increasing the particle size. A representative electron diffraction pattern (Fig. 7d) obtained for EtG sample, and similar to that verified for all of the composite samples investigated in this work, shows that the crystallization process does not occur even after 7 h of pyrolysis, allowing to affirm that the composite samples are a dispersion of Ni nanoparticles in a hybrid phase composed by non-aqueous amorphous silica and amorphous carbon. 4. Conclusions Mesoporous silica carbon matrix nickel embedded composites could be successfully synthesized by the polymeric precursor method. The polyester type and the time of pyrolysis affected the characteristics of the matrix, including size and dispersion of nickel nanoparticles and mesoporosity of the matrix. Well-dispersed metallic phase could be obtained from EtG polyester; however, TeG and BdG affected the dispersion of nickel nanoparticles, increasing the particle size. Weightier polyesters changed the pyrolysis process due to a combination of more amounts of carbonaceous residues and delaying of the pyrolysis process. The increase in the surface area could be conduced by a cleaning process performed at 200 ◦ C for 1 h in oxygen atmosphere, without affecting the nature of nickel particles. The surface area showed to be more dependent on the polyester type than the time of pyrolysis. The matrix composites presented predominantly mesoporous with pore size well ˚ mainly for TeG607 sample. defined in ∼38 A,
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Acknowledgment This work was supported by FAPESP. References [1] K.J.C. Van Bommel, A. Friggeri, S. Shinkai, Angew. Chem. Int. Ed. 42 (9) (2003) 980. [2] S.T. Hyde, G.E. Schroder, Curr. Opin. Colloid Interface Sci. 8 (1) (2003) 5. [3] S. Mecking, R. Thomann, Adv. Mater. 12 (13) (2000) 953. [4] S. Han, T. Hyeon, Chem. Commun. (1999) 1955. [5] M. Iwamoto, T. Abe, Y.J. Tachibana, J. Mol. Catal. A: Chem. 155 (1–2) (2000) 143. [6] R. Chen, J.-L. Shi, Y.-S. Li, J.-N. Yan, Z.-L. Hua, H.-G. Chen, D.-S. Yan, Adv. Mater. 15 (13) (2003) 1078. [7] F. Sch¨uth, Angew. Chem. Int. Ed. 42 (31) (2003) 3604. [8] F.C. Fonseca, G.F. Goya, R.F. Jardim, N.L.V. Carre˜no, E. Longo, E.R. Leite, R. Muccillo, Appl. Phys. A: Mater. Sci. Proc. 76 (4) (2003) 621. [9] G.F. Goya, F.C. Fonseca, R.F. Jardim, R. Muccillo, N.L.V. Carre˜no, E. Longo, E.R. Leite, J. Appl. Phys. 93 (10) (2003) 6531. [10] M.P. Pechini, US Patent No. 3330697, July 11, 1967. [11] E.R. Leite, N.L.V. Carre˜no, E. Longo, A. Valentini, L.F.D. Probst, J. Nanosci. Nanotech. 2 (1) (2002) 89. [12] N.L.V. Carre˜no, E.R. Leite, E. Longo, P.N. Lisboa-Filho, A. Valentini, L.F.D. Probst, W.H. Schreiner, J. Nanosci. Nanotech. 2 (5) (2004) 491. [13] A.G. Rinzler, J. Liu, H. Dai, P. Nikolaev, C.B. Huffman, F.J. RodriguezMacias, P.J. Boul, A.H. Lu, D. Heymann, D.T. Colbert, R.S. Lee, J.E. Fischer, A.M. Rao, P.C. Eklund, R.E. Smalley, Appl. Phys. A: Mater. Sci. Process. 67 (1) (1998) 29. [14] A.C. Dillon, T. Gennett, K.M. Jones, J.L. Alleman, P.A. Parilla, M.J. Heben, Adv. Mater. 11 (13) (1999) 1354. [15] J. Rodriguez-Mirasol, T. Cordero, L.R. Radovic, J.J. Rodriguez, Chem. Mater. 10 (2) (1998) 550. [16] R. Ryoo, S.H. Joo, S. Jun, J. Phys. Chem. B 103 (37) (1999) 7743. [17] Y. Zhang, D.J. Draelants, K. Engelen, G.V. Baron, J. Chem. Technol. Biotechnol. 78 (2–3) (2003) 265. [18] H. Depner, A. Jess, Fuel 78 (1999) 1369. [19] H. Zhao, D.J. Draelants, G.V. Baron, Ind. Eng. Chem. Res. 39 (9) (2000) 3195. [20] J. Hepola, P. Simell, Appl. Catal. B: Environ. 14 (3–4) (1997) 287. [21] R.J.M. Pellenq, B. Rousseau, P.E. Levitz, Phys. Chem. Chem. Phys. 3 (7) (2001) 1207.
Materials Chemistry and Physics 106 (2007) 286–291
Characterization of silica–carbon mesoporous matrix with embedded nickel nanoparticles synthesized by the polymeric precursor method A.A. Cavalheiro a,∗ , J.C. Bruno a , E.R. Leite b , J.A. Varela c a
Depto de Qu´ımica, Instituto de Biociˆencias, UNESP, Distrito de Rubi˜ao Junior, s/n, P.O. Box 510, Botucatu 18.618-000, SP, Brazil b CMDMC, Universidade Federal de S˜ ao Carlos, Rod Washington Luiz Km 25, P.O. Box 676, S˜ao Carlos 13.565-905, SP, Brazil c LIEC, Instituto de Qu´ımica, UNESP, R. Francisco Degni, s/n, Quitandinha, P.O. Box 355, Araraquara 14.801-970, SP, Brazil Received 7 October 2006; received in revised form 18 May 2007; accepted 2 June 2007
Abstract Nickel nanoparticles into silica–carbon matrix composites were prepared by using the polymeric precursor method. The effects of the polyester type and the time of pyrolysis on the mesoporosity and nickel particle dispersion into non-aqueous amorphous silica–carbon matrix were investigated by thermogravimetric analysis, adsorption/desorption isotherms and TEM. A well-dispersed metallic phase could be only obtained by using ethylene glycol. Weightier polyesters affected the pyrolysis process due to a combination of more amounts of carbonaceous residues and delaying of pyrolysis process. The post-pyrolyzed composites were successfully cleaned at 200 ◦ C for 1 h in oxygen atmosphere leading to an increase in the surface area and without the occurrence of carbon combustion or nickel nanoparticles oxidation. The matrix compos˚ mainly when tetraethylene glycol was used as polymerizing ites presented predominantly mesoporous with pore size well defined in 38 A, agent. © 2007 Elsevier B.V. All rights reserved. Keywords: Composite materials; Chemical synthesis; Thermogravimetric analysis
1. Introduction In metal–ceramic composites the morphology and particle size are important aspects for tuning in nanometric scale. In fact, a large variety of mesoporous inorganic materials can be prepared using several organic templates including surfactant self-assemblies and block copolymers [1–3], but the development of metal particles in the nanosize range remains as an important problem. The interesting properties of nanosized metal particles as catalysts of chemical reactions involving inorganic and organic compounds have impelled more and more the researches in this field. Indirect methods, in which metal nanosized particles are previously prepared, cannot avoid the particle agglomeration originated from the decrease in the particle size, which is a required characteristic to enhance the effectiveness of catalysis [4,5]. An alternative method found for the insertion of a metallic precursor into the support matrix has also been conduced. In
∗
Corresponding author. Tel.: +55 14 3811 6255. E-mail address: [email protected] (A.A. Cavalheiro).
0254-0584/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2007.06.004
this procedure, the impregnation or precipitation stage is followed by reduction using “in situ” nanoparticles nucleation into a commercial or pre-prepared mesoporous matrix. Therefore, the heterogeneity in the dispersion of the metal and its particle size are strongly dependent on the previous matrix structure and metal impregnation techniques [6,7]. Recently, some researchers have experimented a direct method to synthesize this type of nanocomposites [8,9]. In this procedure, the synthesis of mesoporous matrix and the metal nanoparticle embedding are carried out simultaneously, leading to metal–ceramic composites. The polymeric precursor method [10] can be present as a tool for this purpose, due to the involving of a complex agent for several metallic and semi metallic ions. The pyrolysis process of the cation-containing the polymeric resins in inert atmosphere, such as nitrogen gas flux, results in metallic nanoparticles already embedded into the mesoporous matrix ceramic [11,12]. This work aims at showing the influence of some parameters in the pyrolysis process and its consequences on mesoporous silica–carbon embedded nickel nanoparticles. For this purpose, the polymer chain size was adjusted by changing the diol part of the polymeric chain.
A.A. Cavalheiro et al. / Materials Chemistry and Physics 106 (2007) 286–291
287
2. Experimental procedure
3. Results and discussion
Ni-silica samples were synthesized using tetraethoxysilane (TEOS: >99%, Aldrich), citric acid (C6 H8 O7 : 99.9%, Merck) and nickel (II) nitrate hexahydrate (Ni(NO3 )2 ·6H2 O: 99.9%, Aldrich). Ethylene glycol (EtG), tetraethylene glycol (TeG) and a blend of both in a molar ratio of 2:1 (BdG) were used as polymerizing agent. The Ni content for the samples was 10 wt%, relative to the silica weight present in the composite, and the molar ratio of citric acid/metal (Si + Ni) was 2:1 whereas the diol/citric acid molar ratio was 1:2. The polymeric precursor was obtained by condensation at 80 ◦ C under stirring until solvent evaporation, followed by polymerization at 250 ◦ C for 2 h in a mufle type oven. In sequence, the resulting precursor was ground in a mortar and pyrolysed at 600 ◦ C for three different times (1, 3 and 7 h) in a tubular furnace under nitrogen gas flux of 30 cm3 min−1 . The samples will be hereafter referred as RaD60X, where RaD means the designation of the polymeric resins and X, the time of pyrolysis (1, 3 and 7 h). The pyrolysis process was investigated by thermogravimetric analysis (Netszch–Thermiche Analyse) using a 10 ◦ C min−1 in heating rate with a nitrogen flux of 30 cm3 min−1 . Measurements of the surface area, hysteresis curve and pore size distribution of the composite samples were obtained by nitrogen adsorption/desorption analysis (Micromeritics, model ASAP 2020). The samples were suspended in ethanol under ultrasonic vibration and brought onto a holey polymer film on a copper grid to be observed by transmission electron microscopy (Philips CM-200 FEG).
The influence of the polyester type was verified by the weight loss (TG) and the weight loss rate (dTG) in accordance with the synthesis conditions. In Fig. 1a, it is possible to observe that the weight loss for the polyester starts effectively around 300 ◦ C for all samples, and the final residue varies as a function of the molecular weight of the polyester. The molecular weight of BdG sample can be assumed as being similar to the diethylene glycol taking in account the pondered weight formula (2EtG:1TeG). The residual weight at 1000 ◦ C (final temperature of analysis) is close to 22% of the initial weight for TeG polyester, while these values are close to 32 and 26% for EtG and BdG polyesters, respectively. By calculation, it was determined that the weight ratio between the organic and inorganic components is a constant for all samples. Probably, the residual carbon amount is dependent on the citric acid/metal molar ratio. In the temperature range between 250 and 500 ◦ C the highest weight loss occurs, and the differential weight loss increases with the molecular weight of the polyester (Fig. 1b). In this temperature range the polyester cracking is started, but the difference among dTG peaks indicates that the polyester breaking is dependent on the diol type used as polymerizing agent. The diol type affects the thermal stability of the polyester, as observed for TeG sample, showing two dTG peaks. The first peak is local-
Fig. 1. Thermogravimetric analysis in nitrogen atmosphere for the different polyester precursors: (a) TG and (b) dTG for the range where high weight loss occurs.
Fig. 2. Thermogravimetric analysis in oxidizing atmosphere (a) and the comparative results between oxygen and nitrogen atmosphere (b) for the EtG607 composite sample.
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Table 1 Data of porosity and specific surface area obtained for EtG607 composite sample under oxidizing treatment for 1 h Sample
Specific surface area (m2 g−1 )
Cumulative pore area (m2 g−1 )
Cumulative pore volume (cm3 g−1 )
Average pore ˚ diameter (A)
As pyrolyzed Oxidized at 200 ◦ C Oxidized at 400 ◦ C
260 396 414
118 179 219
0.14 0.20 0.25
46 46 45
ized at 365 ◦ C and the second one at 385 ◦ C, slightly above the dTG peak of EtG sample, localized at 380 ◦ C. BdG sample also presents a peak at this temperature; it is possible to observe a shoulder at 365 ◦ C, similar to the first dTG peak found for TeG sample. The organic component originated from the pyrolysis process is also an important factor to be considered in order to evaluate the nature of these composite samples. When recorded under air atmosphere (or in general, oxygen-containing), TG techniques can also be used to study the homogeneity and the phase purity of carbons, like as nanotubes [13,14] and microporous carbons [15]. So, it is interesting to verify whether this simple and highly useful methodology can be extended to investigate the characteristics of the organic component present in the Ni/SiO2 composites. A typical curve for EtG607 is shown in Fig. 2a, where two significant weight losses are observed in two narrow ranges of temperature: weight loss 1 (50–150 ◦ C) and weight loss 2 (300–400 ◦ C). The first one is related to evaporation of water and adsorbed gases. The second one is related to carbon combustion due to the low stability of the carbon framework, reported elsewhere [16], indicating that the carbon residue has a non-graphitized nature. The comparative results between oxygen and nitrogen atmosphere for representative EtG607 sample are shown in Fig. 2b. For nitrogen flux, the first weight loss occurring during the heating ramping step is associated to the same event verified in oxygen flux, and the second weight loss is related to completion of the pyrolysis process of carbonaceous residue present in the composite matrix. For nitrogen flux the first weight loss occurs faster than the observed for oxygen flux, due to the oxidation of alkoxy groups to carboxyl groups when oxygen atmosphere is used. The presence of these more oxidized groups in the adsorbed molecules increases the thermal stability when compared to the correspondent non-oxidized molecules. However, the second weight loss occurs faster for oxygen flux due to the combustion of carbon from 400 ◦ C. These results can help to propose an effective way for cleaning the adsorbed carbonaceous residues originated from the partial pyrolysis of the polymeric precursor. We have successfully cleaned the post-pyrolyzed composites at 200 ◦ C for 1 h in oxygen atmosphere. It is probable that the success for the cleaning process is related to the presence of nickel nanoparticles in the composite acting as catalyst for the destruction of the carbonaceous residue, as reported in other works [17–20]. This condition for cleaning is a safe mode because it is carried out in temperatures below the ones of carbon combustion and nickel particle oxidation. The X-ray diffraction patterns for the EtG607 composites before and after cleaning at 200 and 400 ◦ C for 1 h
are shown in Fig. 3. Also, it is exhibited the patterns for the synthetic SiO2 cristobalite (PDF number 39-1425), cubic SiO2 similar to the one of the several MCM-48 silica material types (PDF number 51-1592) and the synthetic NiO bunsenite (PDF number 47-1039). It is possible to observe that the EtG607 composite sample presents low amounts of amorphous cristobalite and one type of MCM-48 phase, observed at lower 2θ degree. After cleaning the EtG607 sample at 200 ◦ C for 1 h, no oxidation reaction was verified, and the XRD pattern is similar to the non-oxidized sample, presenting only an increase in the amount of cristobalite and rising of a second peak related to a second type of MCM-48 with smaller pore diameter contribution. But when the carbon combustion occurs at 400 ◦ C, as demonstrated in Fig. 2a, the nickel nanoparticles are consequently oxidized, visualized by the rising of NiO peak. It occurs because high temperature and partial pressure of oxygen eliminate the amorphous carbon, which can be acting as protective for nickel nanoparticles. In addition, the peaks at low angle related to the two existing types of MCM-48 phases for the sample oxidized at 400 ◦ C are convoluted at only one, which is similar to the MCM-48 pattern. After cleaning at 200 ◦ C for 1 h, it was verified that the surface area increased over 50% if compared to sample as pyrolyzed. The variation of the specific surface area and porosity for these samples is shown in Table 1. It is possible to observe that the ratio between pore and surface area values remains constant after cleaning, denoting that the removing of the carbonaceous residues not only unblocked the pore, verified by the increase over 40% in the cumulative pore volume, but also cleaned the carbonaceous residues chemisorbed on the external surface. For
Fig. 3. X-ray diffraction patterns for EtG607 composite sample: (a) as pyrolyzed, (b) oxidized at 200 ◦ C for 1 h and (c) oxidized at 400 ◦ C for 1 h.
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nal surface did not change, observed by the slightly decrease in the average pore diameter. Fig. 4 shows the nitrogen adsorption/desorption isotherms for no cleaned composites obtained by pyrolysis at 600 ◦ C for 7 h. The isotherms represent a combination between types I and II (IUPAC classification). The uptake at low pressure corresponds to the adsorption filling of smaller pores, and is more prominent for TeG607 sample. On the other hand, the
Fig. 4. Effect of the polyester type on the N2 adsorption/desorption isotherms for the composite samples obtained by pyrolysis at 600 ◦ C for 7 h in N2 atmosphere.
oxidized at 400 ◦ C, the ratio between pore and surface area values increases, showing that only the pore opening is occurring, once the external surface area almost did not change, what agrees with the absence of carbon phase inside the mesoporous silica matrix. The cumulative pore volume increases 25% for this sample compared to the sample oxidized at 200 ◦ C, and although the unblocking process occurs in a wide range of pore diameter, the clean process for smaller pores is more effective, once the exter-
Fig. 5. Specific surface area and pore volume curves for the composite samples obtained by pyrolysis at 600 ◦ C in N2 atmosphere.
Fig. 6. Pore size distribution (BJH model) for the composite samples obtained by pyrolysis at 600 ◦ C in N2 atmosphere for: (a) 1, (b) 3 and (c) 7 h.
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composites prepared with ethylene glycol (EtG and BdG samples) present higher adsorbed volume at intermediary relative pressure, indicating the existence of more mesopores. At high relative pressure EtG and BdG samples present type II isotherm, being characteristic for an opened structure different from TeG sample. The hysteresis presented at low relative pressure for all samples can be explained by the model of interconnected mesopores similar to the one found in carbon adsorbents [21]. When the adsorbate size is bigger than 1/3 of the network cavity size, the pore blocking effects can occur, leading to a hysteresis loop in the adsorption/desorption isotherms at low relative pressure. As a consequence, the pore interconnection volume decreases and also the correspondent hysteresis loop. BdG sample presents a high hysteresis loop at low relative pressure, which was associated to less opened structure.
The effect of the time of pyrolysis and polyester type on the specific surface area and pore volume in the composites pyrolyzed at 600 ◦ C can be seen in Fig. 5. The surface area is more dependent on the polyester type than the time of pyrolysis. The variation of the surface area seems to be more influenced by the organic component present in the matrix composite, which is repeatedly removed along the pyrolysis process. This event can be observed by the correspondent pore volume augment. Analyzing the profile of the pore size distribution as a function of different composites for each time of pyrolysis (Fig. 6), it is possible to observe that for 1 h (Fig. 6a) the EtG sample possesses more adsorbed volume in low average diameter ˚ while TeG exhibits lower adsorbed volume. BdG sam(∼38 A), ple presents an intermediate behavior at this initial stage of pyrolysis. With the increase in the time of pyrolysis (Fig. 6b ˚ increases continually for and c), the adsorbed volume at ∼38 A
Fig. 7. TEM image for the composite samples obtained by pyrolysis at 600 ◦ C for 7 h: (a) EtG, (b) TeG, and (c) BdG. Representative electron diffraction pattern obtained for EtG sample (d).
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all of the samples and the sharp shape indicates a well-defined pore size. A significant increase in the pore volume close to ˚ was not verified for TeG sample, indicating that, when 100 A tetraethylene glycol is used as polymerizing agent, there is the predominance of mesopores in the matrix composite. Fig. 7 shows the TEM images for the composite samples obtained by pyrolysis at 600 ◦ C for 7 h. It is possible to observe the well-dispersed Ni nanoparticles in the matrix composite obtained from EtG polyester (Fig. 7a). When tetraethylene glycol is present in the polyester, TeG and BdG samples (Fig. 7b and c, respectively) the dispersion of nickel nanoparticles is strongly affected, increasing the particle size. A representative electron diffraction pattern (Fig. 7d) obtained for EtG sample, and similar to that verified for all of the composite samples investigated in this work, shows that the crystallization process does not occur even after 7 h of pyrolysis, allowing to affirm that the composite samples are a dispersion of Ni nanoparticles in a hybrid phase composed by non-aqueous amorphous silica and amorphous carbon. 4. Conclusions Mesoporous silica carbon matrix nickel embedded composites could be successfully synthesized by the polymeric precursor method. The polyester type and the time of pyrolysis affected the characteristics of the matrix, including size and dispersion of nickel nanoparticles and mesoporosity of the matrix. Well-dispersed metallic phase could be obtained from EtG polyester; however, TeG and BdG affected the dispersion of nickel nanoparticles, increasing the particle size. Weightier polyesters changed the pyrolysis process due to a combination of more amounts of carbonaceous residues and delaying of the pyrolysis process. The increase in the surface area could be conduced by a cleaning process performed at 200 ◦ C for 1 h in oxygen atmosphere, without affecting the nature of nickel particles. The surface area showed to be more dependent on the polyester type than the time of pyrolysis. The matrix composites presented predominantly mesoporous with pore size well ˚ mainly for TeG607 sample. defined in ∼38 A,
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