M.G. Musolino et al. / Applied Catalysis A: General 379 (2010) 77–86
85
pentanal as substrate with respect to the analogous co-precipitated
sample.
In other words, the main difference between the behaviour of Pd
and that of Pt, Ru, Rh and other metals rests on the assumption that,
on the majority of platinum metals series the strength of common
Lewis acids is sufficient to induce an “electrophilic C O activation”
whereas, when supported palladium is used, the peculiarity of its
electronic properties [12] implies the necessity of a stronger redox
couple presence.
5. Conclusions
Scheme 2. Induced carbonyl interaction of aliphatic aldehydes or ketones with
PdMe catalysts.
The results reported indicate that the behaviour of supported
palladium catalysts, in the hydrogenation of carbonyl compounds,
can be modified using an appropriate preparation method (co-
precipitation technique) and a more fitting metal oxide as support.
So far literature reports indicated that H2 reduction of aliphatic
aldehydes and ketones is a very difficult reaction to occur with Pd
systems. Our data suggest that it can be easily performed mainly
by changing the electronic properties of palladium, allowing forma-
tion, duringthe activation procedureat473 K, ofbimetallic particles
with PdCo or PdZn in less extent, or of an alloy in the case of a PdNi
system.
not fit completely the aromatic series data. Therefore, electronic
effects play an important role in driving the observed activity.
For clearness sake we prefer to discuss separately results refer-
ring to PdCo, PdNi and PdZn from those concerning PdFe and PdCe
for their different electronic peculiarities.
4.1. Hydrogenation promoted by PdCo, PdNi and PdZn catalysts
A comparison between the aromatic and aliphatic carbonyl reac-
tivity suggests that the first is easily activated on a palladium
surface since the ꢀ value –* is appreciably shorter than that of
the aliphatic one (reduction of aromatic carbonyls is easy to be car-
ried out with common palladium catalysts). However, if the energy
value of the d band at the Fermi level of palladium is increased,
population of * orbitals and then activation of the C O bond can
occur. This is indeed what happens when bimetallic ensembles or
an alloy are formed, during the activation procedure, in PdCo, PdZn
(in less extent) and PdNi catalysts. Similarly PdFe and PdCe cat-
alysts form suitable redox couples, Fe3+/Fe2+ or Ce4+/Ce3+ on the
palladium surface; interaction of Fe3+ or Ce4+ with the oxygen moi-
ety of the carbonyl lowers the * energy level and favours aliphatic
aldehydes activation. A comparison with the reactivity of Pd/CoO
and Pd/Fe2O3, prepared by impregnation, supports this conclusion.
Aliphatic aldehydes or ketones are extensively reduced by PdCo
or PdNi, whereas only pentanal and propanal (very slowly) and, in
a few extent, cyclohexanone, react with H2 in presence of PdZn.
The observed carbonyl reduction appears to be mainly related to
the amount of bimetallic PdMe “ensembles” formation (as con-
firmed by TPR, XRD and XPS measurements). Accordingly, when
we use the impregnated Pd/CoO only a very slow reaction occurs
with pentanal. The TPR profile (Fig. 3) clearly indicates, in the last
case, the absence of any PdCo species formation and therefore a
scarce metal–support interaction. On the other hand, it is perfectly
understandable the easy reactivity of the aromatic aldehyde with
the PdCoI catalyst.
tivity of aliphatic carbonyl compounds should be mainly found on
the change of the electronic properties of palladium promoted by
the second metal. XANES and EXAFS studies have so far demon-
strated that on PdCo [21] and PtCo [59] species a shift of electrons
from Co to Pd (Pt) occurs. On this basis it may be assumed that, on
the surface, clusters formed by both Pd(0) and Co(Ni)(0) atoms, in
intimate contact, may favour an electronic transfer from Co(Ni) to
palladium, leading to a negative charge on Pd and a positive one on
Co(Ni).
References
[1] D. Gallezot, D. Richard, Catal. Rev. -Sci. Eng. 40 (1998) 81–126.
[2] U.K. Singh, M.A. Vannice, J. Catal. 199 (2001) 73–84.
[3] P. Reyes, H. Rojas, G. Pecchi, J.L.G. Fierro, J. Mol. Catal. A: Chem. 179 (2002)
293–299.
[4] P. Mäki-Arvela, J. Hájek, T. Salmi, D.Yu. Murzin, Appl. Catal. A: Gen. 292 (2005)
1–49.
[5] Z. Poltarzewski, S. Galvagno, R. Pietropaolo, P. Staiti, J. Catal. 102 (1986)
190–198.
As a consequence, the d orbitals of Pd at the Fermi level should
be shifted upward, thus favouring the back donation of electrons
to the *CO. The hydrogenation process could be also helped by the
positively charged Co or Ni neighbours acting as electron-attractors
towards the carbonyl oxygen cloud (Scheme 2).
[6] S. Galvagno, A. Donato, G. Neri, R. Pietropaolo, Catal. Lett. 8 (1991) 9–14.
[7] V. Ponec, Appl. Catal. A: Gen. 149 (1997) 27–48.
[8] T.B.L.W. Marinelli, V. Ponec, J. Catal. 156 (1995) 51–59.
[9] M.A. Aramendía, V. Borau, C. Jiménez, J.M. Marinas, A. Porras, F.J. Urbano, J.
Catal. 172 (1997) 46–54.
[10] L. Sordelli, R. Psaro, G. Vlaic, A. Cepparo, S. Recchia, C. Dossi, A. Fusi, R. Zanoni,
J. Catal. 182 (1999) 186–198.
[11] S. Sato, R. Takahashi, T. Sodesawa, M. Koubata, Appl. Catal. A: Gen. 284 (2005)
247–251.
4.2. Hydrogenation promoted by PdFe and PdCe catalysts
XPS spectra of PdCe and PdFe catalysts indicate the presence
of both Fe3+/Fe2+ and Ce4+/Ce3+ couples on the surface. Besides,
TPR profiles are similar and both propose an easy reduction of
Fe3+ and surface Ce4+ to Fe2+ and Ce3+, respectively, in presence
of palladium particles. On the light of our structural evidences, in
absence of any alloy on the surface, the most convincing picture,
that we can draw for reduction of aliphatic aldehydes, in our opin-
ion, implies an oxidative approach by Fe3+ or Ce4+ on the oxygen
[12] F. Delbecq, P. Sautet, J. Catal. 152 (1995) 217–236.
[13] V. Ponec, G.C. Bond, Catalysis by Metals and Alloys, Series Surf. Sci. and Catal.,
96, Elsevier, Amsterdam, 1995.
[14] F. Pinna, F. Menegazzo, M. Signoretto, P. Canton, G. Fagherazzi, N. Pernicone,
Appl. Catal. A: Gen. 219 (2001) 195–200.
[15] H.-U. Blaser, A. Indolese, A. Schnyder, H. Steiner, M. Studer, J. Mol. Catal. A:
Chem. 173 (2001) 3–18.
[16] J.-Q. Yu, H.C. Wu, C. Ramarao, J.B. Spencer, S.V. Ley, Chem. Commun. (2003)
678–679.
[17] M.J. Gracia, J.M. Campelo, E. Losada, R. Luque, J.M. Marinas, A.A. Romero, Org.
Biomol. Chem. 7 (2009) 4821–4824.
[18] G. Neri, G. Rizzo, L. De Luca, A. Donato, M.G. Musolino, R. Pietropaolo, React.
Kinet. Catal. Lett. 93 (2008) 193–202.
[19] G. Neri, G. Rizzo, L. De Luca, A. Donato, M.G. Musolino, R. Pietropaolo, Appl.
Catal. A: Gen. 356 (2009) 113–120.
moiety of a carbonyl molecule, their reduction to Fe2+ and Ce3+
,
respectively and the subsequent lowering of the *CO favouring
the activation of the carbonyl bond on the Pd surface. Accordingly
a very slow reaction occurs on the impregnated Pd/Fe2O3 using