16
R. Rinaldi et al. / Journal of Molecular Catalysis A: Chemical 301 (2009) 11–17
show that the Ni sites are not highly accessible, not providing suit-
able catalytic sites for the cleavage of acetaldehyde to yield CO and
CH4 as indicated in Eq. (2). The Ni/SiO2 sample is active only above
4. Conclusions
We have explored the potential use of colloidal Ni NPs sup-
ported in silica and activated carbon as catalyst for hydrogenation
of cyclohexene and for steam reforming of ethanol. Despite the
homogenous size of the colloidal Ni NPs, this synthetic route was
not successful to provide active Ni catalysts. The impregnation of
Ni NPs on silica resulted in high metal dispersion, which would
provide an efficient participation of the supported NPs in the cat-
alytic processes. However, the application in the hydrogenation of
cyclohexene and in the steam reforming of ethanol revealed a lack
of catalytic activity. The detailed surface analysis of the Ni/SiO2
sample showed that the capping-ligands are not easily and fully
removed from the Ni surface in the posterior steps of catalyst prepa-
ration, inhibiting the surface to activation of reactants. This has
been shown particularly important for the case of Ni catalysts as
studied here, because other catalysts produced by impregnation of
Ni salts resulted in active materials for steam reforming of ethanol
[33]. The interaction between silica Aerosil surface and Ni NPs is
strong enough to partially displace the capping ligands, efficiently
grafting the Ni NPs on the silica surface, which avoids the Ni NPs
sintering even at 623 K. However, this interaction could not pro-
vide a suitable Ni surface for catalysis. Further studies in a broader
set of colloidal systems are desirable to draw a better picture of the
potential use of this methodology in the rational design of catalysts.
7
50 K. In this case, however, the molar ratio of CO:CH4 formed is
higher than 1, which indicates that the decomposition of acetalde-
hyde is not the only pathway of CO formation. The higher amount of
CO than CH is likely to be generated by the partial oxidation of coke
4
formed on the catalyst surface [34] or oxidation of residues of NPs
capping ligands. Interestingly, we observed formation of ethylene
above 600 K, indicating that Ni/SiO2 possesses acidity, probably
due to formation of nickel(II) silicates[9] under the reaction con-
ditions. It is important to remark that sintering occurs when the
temperature is above 700 K, completely changing the material as
the temperature is further increased (Fig. 7b).
Putting all these pieces of information together, we can get some
insights on the factors responsible for the low catalytic activity of
Ni NPs supported on silica. In principle, the impregnation of the Ni
NPs on silica Aerosil strips mostly of the P-ligands (TOP and TOPO)
as revealed by XPS (Fig. 4). However, other ligands remain strongly
bonded on the Ni/SiO2 surface as detected by DRIFTS experiments
(
Fig. 5). Ni NPs seem to be strongly anchored on the silica surface
through siloxane groups, and no sintering of Ni NPs takes place
below 623 K under a reducing atmosphere (Fig. 7).
Interestingly, while XPS revealed the presence of metallic Ni
sites in the Ni/SiO2 sample, no adsorption of CO is detected by
DRIFTS and no catalytic activity is found for the untreated samples.
One reasonable explanation is, despite being an outmost surface
sensitive technique, XPS probes few atomic sublayers from the sur-
face. However, the catalytic reaction and CO adsorption takes place
only on the directly exposed Ni sites on the surface. Hence, this spe-
cific Ni(0) sites, detected by XPS, could not be accessible to interact
with CO.
Acknowledgments
LME-LNLS is acknowledged for the use of TEM. R.R. is grateful
to CNPq for the post-doc fellowship at the LNLS facilities and to Mr.
Fabio Zambello for his assistance in part of this work.
The treatment of Ni/SiO2 under reducing atmosphere partially
cleans the Ni NPs surface from the capping ligands (Fig. 5). However,
it is noteworthy that only linear CO bonded is detected in the CO
adsorption experiments (Fig. 6). This contrasts strongly with sup-
ported Ni catalysts obtained from impregnation of Ni salts [9,33],
which also show CO bands assigned to several modes of bridged
bonded CO. The interaction of the remaining capping ligands on
the Ni NPs surface can change the electronic behavior of the Ni
sites, restricting the coordination of CO and other small molecules.
CO adsorption experiments revealed that the Ni sites on a sam-
ple reduced at 623 K are more electrophilic than the Ni sites on
a sample reduced at 423 K, since the IR-band assigned to linear
References
[1] Y.H. Cui, H.Y. Xu, Q.J. Ge, Y.Z. Wang, S.F. Hou, W.Z. Li, J. Mol. Catal. A: Chem. 249
(2006) 53.
[
2] H. Wilmer, M. Kurtz, K.V. Klementiev, O.P. Tkachenko, W. Grunert, O. Hinrichsen,
A. Birkner, S. Rabe, K. Merz, M. Driess, C. Woll, M. Muhler, Phys. Chem. Chem.
Phys. 5 (2003) 4736.
[3] A.J. Nagy, G. Mestl, R. Schlögl, J. Catal. 188 (1999) 58.
[4] G. Pina, C. Louis, M.A. Keane, Phys. Chem. Chem. Phys. 5 (2003) 1924.
[
[
5] R. Schlögl, S.B.H. Abd, Angew. Chem. Int. Ed. 43 (2004) 1628.
6] J.A. Schwarz, C. Contescu, A. Contescu, Chem. Rev. 95 (1995) 477.
[7] J.R.A. Sietsma, J.D. Meeldijk, M. Versluijs-Helder, A. Broersma, A.J. van Dillen,
P.E. de Jongh, K.P. de Jong, Chem. Mater. 20 (2008) 2921.
[
8] J.R.A. Sietsma, J.D. Meeldijk, J.P. den Breejen, M. Versluijs-Helder, A.J. van Dillen,
P.E. de Jongh, K.P. de Jong, Angew. Chem. Int. Ed. 46 (2007) 4547.
9] K. Hadjiivanov, M. Mihaylov, D. Klissurski, P. Stefanov, N. Abadjieva, E. Vassileva,
L. Mintchev, J. Catal. 185 (1999) 314.
−1
bonded CO shifted from 2064 to 2084 cm . The higher CO stretch-
[
∗
ing frequency shows that the back-bonding Ni(3d) → is less
CO
[
10] A. Infantes-Molina, J. Merida-Robles, P. Braos-Garcia, E. Rodriguez-Castellon, E.
Finocchio, G. Busca, P. Maireles-Torres, A. Jimenez-Lopez, J. Catal. 225 (2004)
effective, which indicates a lower electron density on Ni in Ni/SiO2
reduced at 623 K. These changes clearly show that new interac-
tions between the Ni NPs and the support are established during
the removal of the capping ligands. The interactions involving the
Ni NPs and the silica surface maintain the Ni NPs stable against
sintering up to 623 K.
479.
[11] G.A. Somorjai, F. Tao, J.Y. Park, Top. Catal. 47 (2008) 1.
[12] R. Narayanan, M.A. El-Sayed, Top. Catal. 47 (2008) 15.
[
13] J. Park, E. Kang, S.U. Son, H.M. Park, M.K. Lee, J. Kim, K.W. Kim, H.J. Noh, J.H. Park,
C.J. Bae, J.G. Park, T. Hyeon, Adv. Mater. 17 (2005) 429.
[14] H. Winnischofer, T.C. Rocha, W.C. Nunes, L.M. Socolovsky, M. Knobel, D. Zanchet,
ACS Nano. 2 (2008) 1313.
The construction of catalyst from supports and colloidal
nanoparticles is a much more complex process than the conven-
tional impregnation of metal salts followed by reducing steps. The
removal of the capping ligands, which are essential for tailoring
the properties of nanoparticles in the colloidal synthesis, is the
central challenge that remains to be circumvented in the case of
late 3d metal NPs. The main problem to be solved is to remove
the capping ligands without oxidizing the NPs surface. This is very
required, since the oxides of late 3d metals usually are reduced at
high temperatures (e.g. NiOx-species reduces to Ni(0) above 700 K
[15] Y. Zhu, C.N. Lee, R.A. Kemp, N.S. Hosmane, J.A. Maguire, Chem. Asian J. 3 (2008)
650.
[16] B.J. Liaw, S.J. Chiang, C.H. Tsai, Y.Z. Chen, Appl. Catal. A: Gen. 284 (2005) 239.
[17] S.J. Chiang, B.J. Liaw, Y.Z. Chen, Appl. Catal. A: Gen. 319 (2007) 144.
[
[
18] R. Xu, T. Xie, Y. Zhao, Y. Li, Nanotechnology 18 (2007) 055602.
19] J. Park, E. Kang, S.U. Son, H.M. Park, M.K. Lee, J. Kim, K.W. Kim, H.-J. Noh, J.-H.
Park, C.J. Bae, J.-G. Park, T. Hyeon, Adv. Mater. 17 (2005) 429.
[20] J. Yang, T.C. Deivaraj, H.P. Too, J.Y. Lee, Langmuir 20 (2004) 4241.
21] D. Li, S. Komarneni, J. Am. Soc. 89 (2006) 1510.
[
[
[
22] C.A. Stowell, B.A. Korgel, Nano Lett. 5 (2005) 1203.
23] G.G. Couto, J.J. Klein, W.H. Schreiner, D.H. Mosca, A.J.A. Oliveira, A.J.G. Zarbin, J.
Colloid Interf. Sci. 311 (2007) 461.
[24] A. Opitza, S.I.-U. Ahmedb, J.A. Schaefera, M. Scherge, Wear 254 (2003) 924.
[25] B. Hornetz, H.-J. Michel, J. Halbritter, J. Mater. Res. 9 (1994) 3088.
[26] M.C. Oliveira, A.M.B. Rego, J. Alloys Compd. 425 (2006) 64.
[
9]). At these temperatures, the sintering processes are rather prone
to occur, destroying all the properties tailored by the colloidal syn-
thesis of NPs.
[27] J. Pinkas, Z. Bastl, M. Slouf, J. Podlaha, P. Stepnicka, New J. Chem. 25 (2001) 1215.