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Clusters as precursors of nanoparticles supported on carbon nanofibers
10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.
Clusters as precursors of nanoparticles supported on carbon nanofibers Deborah Vidick, Sophie Hermans and Michel Devillersa a
Université catholique de Louvain, Unité de chimie des matériaux inorganiques et organiques, Place L. Pasteur 1, 1348 Louvain-la-Neuve, Belgium [email protected]
Abstract Carbon nanofibers were functionalized following two distinct strategies, in order to introduce anchors for the grafting of clusters at their surface. In the first case, chelating phosphine groups were built using a multi-step synthesis, while in the second case, ammonium groups were introduced to create positive charges at the surface. To prove the success of the functionalization, a [Ru5PtC(CO)14(COD)] cluster was covalently grafted onto CNF-PPh2 while a negatively charged (NEt4)[FeCo3(CO)12] cluster was incorporated on CNF-NMe3+. Nanometer-sized RuPt and FeCo particles were evidenced by TEM. Keywords: carbon nanofibers, molecular clusters, functionalization, grafting
1. Introduction In recent years, the use of carbonyl clusters as sub-colloïdal defined metallic entities has received much interest [1]. These molecules may act as models for heterogeneous catalysts by virtue of the ‘cluster-surface analogy’ and make the connection between the molecular level and the bulk. Indeed, a cluster is defined as a multi-center transition metal complex including at least three metal atoms, linked by a minimum of two metalmetal bonds, and stabilized by a layer of organic ligands, usually CO. Clusters can be used as precursors for the preparation of heterogeneous nano-structured catalysts, by incorporation on a support and thermal activation [2]. To improve the control on the catalysts synthesis, the incorporation of the cluster on the support can be optimized through surface functionalization. The ligands sheath can then be removed selectively to yield supported nanoparticles of controlled size and composition.
2. Experimental section All manipulations were carried out under nitrogen by using Schlenk techniques. The solvents were distilled before use and stored under nitrogen on molecular sieves, and the obtained products were stored under Ar. [Ru5PtC(CO)14(COD)] (1) and (NEt4)[FeCo3(CO)12] (2) were prepared according to published procedures [3,4]. All other mentioned reactants were commercially available and used as received. The support was carbon nanofibers of the type PR24-XT-LHT-OX (noted CNFox) from Applied Sciences Inc.
2.1. Functionalization with chelating phosphines [5] The acidity of this support was estimated to be ~200 mmol/100g from XPS analysis. In a 100 mL Schlenk flask, 1g of CNFox was introduced with 5 ml SOCl2 and 40 ml of toluene. The mixture was refluxed (120°C) for five hours and filtrated. The obtained powder (CNF-Cl) was extensively washed with toluene and dried under vacuum. In
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the second step, 900 mg of CNF-Cl was refluxed (120°C) with 1.3 equivalents of ethylenediamine in 30 mL toluene for four hours. Then, the mixture was filtrated, and the obtained powder (CNF-NH2) was extensively washed with toluene and dried under vacuum. In the third step, 2.5 equivalents of CH2O and HPPh2 were introduced in a 100 mL Schlenk flask together with 7.5 mL of methanol. The mixture was stirred at 70°C for ten minutes and was then cooled to room temperature. 800 mg of CNF-NH2 were concurrently placed in a 100 mL Schlenk flask with 12.5 ml of methanol. When the first mixture reached room temperature, it was added to the CNF-NH2 suspension and stirred for fifteen minutes at room temperature. Then, 25 mL of toluene were added and the mixture was stirred at 70°C for 24h. Finally, the mixture was filtrated and the resulting powder (CNF-PPh2) was extensively washed with methanol and dried under vacuum.
2.2. Functionalization with ammonium groups The procedure was identical to the one described in section 2.1 for the first two steps except that N,N-dimethylethylenediamine (1.3 equivalents) was used instead of ethylenediamine. In the third step, 800 mg of CNF-NMe2 were placed in a 100 mL Schlenk flask with 30 ml of acetone and 5 equivalents of methyl trifluoromethanesulfonate (0.91 mL). The mixture was stirred at room temperature for 24 hours. Finally, the mixture was filtrated and the resulting powder (CNF-NMe3+) was extensively washed with acetone and dried under vacuum.
2.3. Grafting of metal complexes The amount of cluster engaged in each grafting experiment corresponds to a theoretical 5 wt.% metal loading on the support after ligands removal. In a typical experiment, 8.7 mg of cluster 1 was stirred with 95 mg of CNF-PPh2 in 10 mL of toluene and 10 mL of dichloromethane at room temperature, in the dark, for five days. The solid was filtrated, washed with dichloromethane and dried at room temperature under vaccum. The same procedure was used to graft cluster 2 on CNF-NMe3+ (using 15 mg of 2 and 95 mg of support with acetone/toluene 1:1 v/v).
2.4. Physico-chemical methods of characterization Atomic absorption measurements were carried out on a Perkin Elmer atomic absorption spectrometer 3110. XPS analyses were performed on a Kratos Axis Ultra spectrometer (Kratos Analytical – Manchester – UK) equipped with a monochromatized aluminium X-ray source (powered at 10 mA and 15 kV) and an eight channeltrons detector. The sample powders were compacted with a spatula into small stainless steel troughs of inner diameter 4 mm and 0.5 mm depth. Charge stabilisation was achieved by using the Kratos Axis device. Spectra were decomposed with the CasaXPS program (Casa Software Ltd., UK). TEM images were obtained with a LEO 922 OMEGA energy filter transmission electron microscope. The samples were suspended in hexane under ultrasonic treatment; a drop of the supernatant was then deposited on a holey carbon film supported on a copper grid, which was dried overnight under vacuum at room temperature before analysis.
3. Results and discussion The aim of support functionalization is to introduce anchors for the grafting of clusters at its surface. Two different functionalization strategies of carbon nanofibers were envisaged (Figure 1). In the first case, chelating phosphine groups were introduced at the surface of nanofibers in three steps. This strategy allows a covalent grafting of
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Clusters as precursors of nanoparticles supported on carbon nanofibers
clusters through ligand exchange. The other functionalization strategy was used in order to introduce positive charges at the surface of the support when dealing with negatively charged clusters. It is realised through formation of a pending arm ending with an ammonium group. Table 1 shows the XPS results for functionalization samples. At each step, an increase of surface concentration of the heteroatom of interest was observed. R
O
Cl
CNF
O H2N(CH2)2 N(R)2
N
F3 CSO2 O-
R
N
NH CNF
R = H, CH3
O F3CSO2OCH3
NH CNF
SOCl2
O
OH
PPh2 PPh2 N
CNF
HPPh2/CH2O O
NH
CNF
Figure 1. Functionalization of CNFox. Table 1. XPS results for functionalization samples. Surf. At. Ratios
CNFox
CNF-Cl
CNF-NH2
CNF-NMe2
CNF-PPh2
CNF-NMe3+
O/C Cl/C N/C P/C S/C F/C F/N
0.121 0.009 0.010 -
0.114 0.023 0.013 0.010 -
0.104 0.011 0.060 0.008 -
0.081 0.010 0.052 0.004 -
0.066 0.005 0.031 0.013 0.002 -
0.111 0.005 0.031 0.020 0.053 1.688
To prove the success of the functionalization, a [Ru5PtC(CO)14(COD)] cluster was covalently grafted onto CNF-PPh2 while a negatively charged (NEt4)[FeCo3(CO)12] cluster was incorporated on CNF-NMe3+. Table 2 shows the results of metal loading and XPS characterization of these samples. We can see that experimental values for Ru/C, Pt/C and Co/C ratios are higher than calculated values, which means that, a priori, clusters form small and well-dispersed particles on the support. The samples were characterized by TEM to visualize particles sizes (Figure 2). The size of particles observed was about two nanometers for the Ru5Pt cluster and ten nanometers for the FeCo3 cluster.
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Table 2. Loading and XPS results for the incorporation of clusters on functionalized nanofibers. Samples Cluste r
Support
Ru5Pt
CNF-PPh2
Atomic absorption Metal Grafting loading yield (%) (wt.%) 44
2.2
FeCo3 CNF-NMe3+ 40 2 calculated value corresponds to bulk molar ratios.
M/Ccalca (Ru) 0.002 (Pt) 0.0004 (Co) 0.003
XPS M/Cexp before activation 0.013 0.004 0.011
M/Cexp after activation 0.010 0.003 0.015
(a)
(a)
(b)
Figure 2. TEM image of (a) [Ru5PtC(CO)14(COD)] grafted on CNF-PPh2 and (b) (NEt4)[FeCo3(CO)12] incorporated on CNF-NMe3+.
4. Conclusion The goal of this study was to covalently graft potential coordination sites at the surface of carbon nanofibers to allow the incorporation of clusters. Two functionalization strategies were investigated : the incorporation of chelating phosphine groups and the incorporation of ammonium groups. To prove the success of the functionalization, two organometallic compounds were used - [Ru5PtC(CO)14(COD)] and (NEt4)[FeCo3(CO)12] as precursors of supported bimetallic nanoparticles. Atomic absorption allowed us to confirm the incorporation of the metals at the surface of the functionalized supports. Small metallic particles were observed by TEM. In the case of [Ru5PtC(CO)14(COD)], the particles were approximately 2 nm in size, while in the case of (NEt4)[FeCo3(CO)12] they were, on average, about 10 nm in diameter.
Acknowledgement The authors gratefully acknowledge the FNRS, FRIA and PAI Inanomat for funding.
References [1] W. Eberhardt, 2002, Surf. Sci., 500, 242-270. [2] P. Braunstein, L. A. Oro, P. R. Raithby, 1999, Metal Clusters in Chemistry, Wiley-VCH, Weinheim. [3] S. Hermans, T. Khimyak, B. F. G. Johnson, 2001, J. Chem. Soc., Dalton Trans., 3295-3302. [4] P. Chini, L. Colli, M. Peraldo, 1960, Gazz. Chim. Ital., 90, 1005-1020. [5] C. Willocq, S. Hermans, M. Devillers, 2008, J. Phys. Chem. C, 112, 5533-5541.
Clusters as precursors of nanoparticles supported on carbon nanofibers Deborah Vidick, Sophie Hermans and Michel Devillersa a
Université catholique de Louvain, Unité de chimie des matériaux inorganiques et organiques, Place L. Pasteur 1, 1348 Louvain-la-Neuve, Belgium [email protected]
Abstract Carbon nanofibers were functionalized following two distinct strategies, in order to introduce anchors for the grafting of clusters at their surface. In the first case, chelating phosphine groups were built using a multi-step synthesis, while in the second case, ammonium groups were introduced to create positive charges at the surface. To prove the success of the functionalization, a [Ru5PtC(CO)14(COD)] cluster was covalently grafted onto CNF-PPh2 while a negatively charged (NEt4)[FeCo3(CO)12] cluster was incorporated on CNF-NMe3+. Nanometer-sized RuPt and FeCo particles were evidenced by TEM. Keywords: carbon nanofibers, molecular clusters, functionalization, grafting
1. Introduction In recent years, the use of carbonyl clusters as sub-colloïdal defined metallic entities has received much interest [1]. These molecules may act as models for heterogeneous catalysts by virtue of the ‘cluster-surface analogy’ and make the connection between the molecular level and the bulk. Indeed, a cluster is defined as a multi-center transition metal complex including at least three metal atoms, linked by a minimum of two metalmetal bonds, and stabilized by a layer of organic ligands, usually CO. Clusters can be used as precursors for the preparation of heterogeneous nano-structured catalysts, by incorporation on a support and thermal activation [2]. To improve the control on the catalysts synthesis, the incorporation of the cluster on the support can be optimized through surface functionalization. The ligands sheath can then be removed selectively to yield supported nanoparticles of controlled size and composition.
2. Experimental section All manipulations were carried out under nitrogen by using Schlenk techniques. The solvents were distilled before use and stored under nitrogen on molecular sieves, and the obtained products were stored under Ar. [Ru5PtC(CO)14(COD)] (1) and (NEt4)[FeCo3(CO)12] (2) were prepared according to published procedures [3,4]. All other mentioned reactants were commercially available and used as received. The support was carbon nanofibers of the type PR24-XT-LHT-OX (noted CNFox) from Applied Sciences Inc.
2.1. Functionalization with chelating phosphines [5] The acidity of this support was estimated to be ~200 mmol/100g from XPS analysis. In a 100 mL Schlenk flask, 1g of CNFox was introduced with 5 ml SOCl2 and 40 ml of toluene. The mixture was refluxed (120°C) for five hours and filtrated. The obtained powder (CNF-Cl) was extensively washed with toluene and dried under vacuum. In
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D. Vidick et al.
the second step, 900 mg of CNF-Cl was refluxed (120°C) with 1.3 equivalents of ethylenediamine in 30 mL toluene for four hours. Then, the mixture was filtrated, and the obtained powder (CNF-NH2) was extensively washed with toluene and dried under vacuum. In the third step, 2.5 equivalents of CH2O and HPPh2 were introduced in a 100 mL Schlenk flask together with 7.5 mL of methanol. The mixture was stirred at 70°C for ten minutes and was then cooled to room temperature. 800 mg of CNF-NH2 were concurrently placed in a 100 mL Schlenk flask with 12.5 ml of methanol. When the first mixture reached room temperature, it was added to the CNF-NH2 suspension and stirred for fifteen minutes at room temperature. Then, 25 mL of toluene were added and the mixture was stirred at 70°C for 24h. Finally, the mixture was filtrated and the resulting powder (CNF-PPh2) was extensively washed with methanol and dried under vacuum.
2.2. Functionalization with ammonium groups The procedure was identical to the one described in section 2.1 for the first two steps except that N,N-dimethylethylenediamine (1.3 equivalents) was used instead of ethylenediamine. In the third step, 800 mg of CNF-NMe2 were placed in a 100 mL Schlenk flask with 30 ml of acetone and 5 equivalents of methyl trifluoromethanesulfonate (0.91 mL). The mixture was stirred at room temperature for 24 hours. Finally, the mixture was filtrated and the resulting powder (CNF-NMe3+) was extensively washed with acetone and dried under vacuum.
2.3. Grafting of metal complexes The amount of cluster engaged in each grafting experiment corresponds to a theoretical 5 wt.% metal loading on the support after ligands removal. In a typical experiment, 8.7 mg of cluster 1 was stirred with 95 mg of CNF-PPh2 in 10 mL of toluene and 10 mL of dichloromethane at room temperature, in the dark, for five days. The solid was filtrated, washed with dichloromethane and dried at room temperature under vaccum. The same procedure was used to graft cluster 2 on CNF-NMe3+ (using 15 mg of 2 and 95 mg of support with acetone/toluene 1:1 v/v).
2.4. Physico-chemical methods of characterization Atomic absorption measurements were carried out on a Perkin Elmer atomic absorption spectrometer 3110. XPS analyses were performed on a Kratos Axis Ultra spectrometer (Kratos Analytical – Manchester – UK) equipped with a monochromatized aluminium X-ray source (powered at 10 mA and 15 kV) and an eight channeltrons detector. The sample powders were compacted with a spatula into small stainless steel troughs of inner diameter 4 mm and 0.5 mm depth. Charge stabilisation was achieved by using the Kratos Axis device. Spectra were decomposed with the CasaXPS program (Casa Software Ltd., UK). TEM images were obtained with a LEO 922 OMEGA energy filter transmission electron microscope. The samples were suspended in hexane under ultrasonic treatment; a drop of the supernatant was then deposited on a holey carbon film supported on a copper grid, which was dried overnight under vacuum at room temperature before analysis.
3. Results and discussion The aim of support functionalization is to introduce anchors for the grafting of clusters at its surface. Two different functionalization strategies of carbon nanofibers were envisaged (Figure 1). In the first case, chelating phosphine groups were introduced at the surface of nanofibers in three steps. This strategy allows a covalent grafting of
829
Clusters as precursors of nanoparticles supported on carbon nanofibers
clusters through ligand exchange. The other functionalization strategy was used in order to introduce positive charges at the surface of the support when dealing with negatively charged clusters. It is realised through formation of a pending arm ending with an ammonium group. Table 1 shows the XPS results for functionalization samples. At each step, an increase of surface concentration of the heteroatom of interest was observed. R
O
Cl
CNF
O H2N(CH2)2 N(R)2
N
F3 CSO2 O-
R
N
NH CNF
R = H, CH3
O F3CSO2OCH3
NH CNF
SOCl2
O
OH
PPh2 PPh2 N
CNF
HPPh2/CH2O O
NH
CNF
Figure 1. Functionalization of CNFox. Table 1. XPS results for functionalization samples. Surf. At. Ratios
CNFox
CNF-Cl
CNF-NH2
CNF-NMe2
CNF-PPh2
CNF-NMe3+
O/C Cl/C N/C P/C S/C F/C F/N
0.121 0.009 0.010 -
0.114 0.023 0.013 0.010 -
0.104 0.011 0.060 0.008 -
0.081 0.010 0.052 0.004 -
0.066 0.005 0.031 0.013 0.002 -
0.111 0.005 0.031 0.020 0.053 1.688
To prove the success of the functionalization, a [Ru5PtC(CO)14(COD)] cluster was covalently grafted onto CNF-PPh2 while a negatively charged (NEt4)[FeCo3(CO)12] cluster was incorporated on CNF-NMe3+. Table 2 shows the results of metal loading and XPS characterization of these samples. We can see that experimental values for Ru/C, Pt/C and Co/C ratios are higher than calculated values, which means that, a priori, clusters form small and well-dispersed particles on the support. The samples were characterized by TEM to visualize particles sizes (Figure 2). The size of particles observed was about two nanometers for the Ru5Pt cluster and ten nanometers for the FeCo3 cluster.
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D. Vidick et al.
Table 2. Loading and XPS results for the incorporation of clusters on functionalized nanofibers. Samples Cluste r
Support
Ru5Pt
CNF-PPh2
Atomic absorption Metal Grafting loading yield (%) (wt.%) 44
2.2
FeCo3 CNF-NMe3+ 40 2 calculated value corresponds to bulk molar ratios.
M/Ccalca (Ru) 0.002 (Pt) 0.0004 (Co) 0.003
XPS M/Cexp before activation 0.013 0.004 0.011
M/Cexp after activation 0.010 0.003 0.015
(a)
(a)
(b)
Figure 2. TEM image of (a) [Ru5PtC(CO)14(COD)] grafted on CNF-PPh2 and (b) (NEt4)[FeCo3(CO)12] incorporated on CNF-NMe3+.
4. Conclusion The goal of this study was to covalently graft potential coordination sites at the surface of carbon nanofibers to allow the incorporation of clusters. Two functionalization strategies were investigated : the incorporation of chelating phosphine groups and the incorporation of ammonium groups. To prove the success of the functionalization, two organometallic compounds were used - [Ru5PtC(CO)14(COD)] and (NEt4)[FeCo3(CO)12] as precursors of supported bimetallic nanoparticles. Atomic absorption allowed us to confirm the incorporation of the metals at the surface of the functionalized supports. Small metallic particles were observed by TEM. In the case of [Ru5PtC(CO)14(COD)], the particles were approximately 2 nm in size, while in the case of (NEt4)[FeCo3(CO)12] they were, on average, about 10 nm in diameter.
Acknowledgement The authors gratefully acknowledge the FNRS, FRIA and PAI Inanomat for funding.
References [1] W. Eberhardt, 2002, Surf. Sci., 500, 242-270. [2] P. Braunstein, L. A. Oro, P. R. Raithby, 1999, Metal Clusters in Chemistry, Wiley-VCH, Weinheim. [3] S. Hermans, T. Khimyak, B. F. G. Johnson, 2001, J. Chem. Soc., Dalton Trans., 3295-3302. [4] P. Chini, L. Colli, M. Peraldo, 1960, Gazz. Chim. Ital., 90, 1005-1020. [5] C. Willocq, S. Hermans, M. Devillers, 2008, J. Phys. Chem. C, 112, 5533-5541.