26
S. Hermans et al. / Applied Catalysis A: General 395 (2011) 19–27
tive correlation could be found between the Pd/C ratios determined
by XPS, the Pd dispersion measured by CO chemisorption and the
yields in glyoxalic or gluconic acids. These observations suggest that
high activities are related, on the one hand, to an important degree
of Pd reduction, and, on the other hand, to high Pd surface concen-
tration. The reactions being carried out in water, which is highly
demanding in terms of diffusion, especially with a hydrophobic
support such as carbon (knowing in the present case that it is highly
microporous), the trends observed are maybe not only linked to a
dispersion effect, but also to the accessibility of the active metal
that needs to be located on the external surface.
on maximizing the interactions between the metal precursors
and the carbonaceous support in aqueous phase are very active
in liquid phase glyoxal and glucose selective oxidations. The
obtained catalysts give performances superior to the corresponding
monometallic Pd/C and Au/C materials, and similar to the most per-
formant bimetallic Bi–Pd/C catalyst so far, but without any metal
leaching. The Au–Pd/C catalysts are characterized by high Pd/C sur-
face ratios and by a total degree of Pd reduction. Moreover, the high
activity is connected to the presence of small Pd particles. How-
ever, gold is present in the form of big particles, which makes the
monometallic Au/C materials inactive. The bimetallic cooperative
effect is explained by the presence of small amounts of gold in con-
tact with palladium. Indeed, the synergetic effect seems to require
an interface between the two metals to take place. In consequence,
the incorporation of Au on Pd rather than the opposite and the
concomitant activation of both metals influence positively the cat-
alytic performance. The use of NaBH4 as activating agent allowed
the best Au–Pd/C catalyst both in glucose and in glyoxal oxidation
to be obtained.
Regarding the bimetallic effect, SEM characterization coupled
with EDXS indicated that the least active catalysts in the two
reactions (catalysts 1 and 8 prepared as Pd(OAc)2-Au/C and HAuCl4-
Pd/C respectively) displayed both small Pd particles and big Au
particles, separated from each other. TEM showed that the Pd parti-
cles were homogeneous in terms of sizes and distribution and very
small (<5 nm). The preparation procedure explains this microstruc-
ture as one metal has been reduced before incorporating the second
one in both cases. The more active bimetallic materials display
either a homogeneous distribution of smaller Pd and Au particles
on the support (catalysts 3 and 5 prepared as Pd(OAc)2-HAuCl4/C
and HAuCl4-Pd(OAc)2/C respectively), or a precipitate covering the
carbon surface (catalyst 6 prepared as HAuCl4-Pd(OAc)2/C and acti-
vated with NaBH4). TEM examination of the finest particles in these
samples revealed a broader distribution of particles sizes centered
on 10 nm. In these cases, both metals have been reduced concomi-
tantly, and the occurrence of alloying/interfacing must be higher. It
appears thus that the bimetallic materials are not just an addition
of two independent monometallic systems but that the cooperative
effect observed must be due to bimetallic active sites. Indeed, when
carrying out a test with a physical mixture of two monometallic cat-
alysts, these best results could not be attained. Alloying in the best
bimetallic catalysts could not be unraveled by XRD or microscopy
probably a waste and only Au atoms in contact with Pd play a role.
Prati et al. have shown that a Pd–Au/C catalyst containing exclu-
sively bimetallic particles is more active that catalysts containing
tact with Au are suggested to be the active sites [56]. Hutchings
and co-workers argue that Au acts as an electronic promoter for Pd
[57]. This has to be related to Pd–Bi/C systems, where Mallat and
Baiker showed that Bi plays its promoter role when located on the
Pd surface [58]. Here, the incorporation order of the two metals also
plays a role, with incorporating Au in second position (thus giving
it a chance to cover Pd) giving better results.
Acknowledgements
The authors greatly acknowledge financial support from the Bel-
gian National Fund for Scientific Research (FNRS, Brussels) and the
Belgian State (Belgian Science Policy, IAP Project INANOMAT N◦
P6/17), as well as the Norit firm for supplying the carbon support.
Nathalie Meyer is warmly thanked for her assistance with the phys-
ical mixture test. We are also grateful to M. Genet, E. Ferain and J.-F.
Statsijns for useful discussions and technical assistance.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
References
[1] M. Haruta, Gold Bull. 37 (2004) 27–36.
[2] A.S.K. Hashmi, G.J. Hutchings, Angew. Chem. Int. Ed. 45 (2006) 7896–7936.
[3] C.W. Corti, R.J. Holliday, D.T. Thompson, Top. Catal. 44 (2007) 331–343.
[4] C. Della Pina, E. Falletta, L. Prati, M. Rossi, Chem. Soc. Rev. 37 (2008) 2077–2095.
[5] D.T. Thompson, Plat. Met. Rev. 48 (2004) 169–172.
[6] A.M. Venezia, V. La Parola, V. Nicoli, G. Deganello, J. Catal. 212 (2002) 56–62.
[7] M. Bonarowska, B. Burda, W. Juszczyka, J. Pielaszek, Z. Kowalczyk, Z. Karpinski,
Appl. Catal. B: Environ. 35 (2001) 13–20.
[8] M. Bonarowska, J. Pielaszek, V.A. Semikolenov, Z. Karpinski, J. Catal. 209 (2002)
528–538.
[9] J. Edwards, P. Landon, A.F. Carley, G.J. Hutchings, J. Mater. Res. 22 (2007)
831–837.
[10] G.C. Bond, A.F. Rawle, J. Mol. Catal. A: Chem. 109 (1996) 261–271.
[11] N. Dimitratos, C. Messi, F. Porta, L. Prati, A. Villa, J. Mol. Catal. A: Chem. 256
(2006) 21–28.
[12] J.A. Lopez-Sanchez, N. Dimitratos, P. Miedziak, E. Ntainjua, J.K. Edwards, D. Mor-
gan, A.F. Carley, R. Tiruvalam, C.J. Kiely, G.J. Hutchings, Phys. Chem. Chem. Phys.
[13] C.L. Bianchi, P. Canton, N. Dimitratos, F. Porta, L. Prati, Catal. Today 102 (2005)
203–212.
[14] N. Dimitratos, F. Porta, L. Prati, Appl. Catal. A 291 (2005) 210–214.
[15] N. Dimitratos, F. Porta, L. Prati, A. Villa, Catal. Lett. 99 (2005) 181–185.
[16] N. Dimitratos, L. Prati, Gold Bull. 38 (2005) 73–77.
Finally, we must point out that the bimetallic material activated
with NaBH4 displayed the highest XPS Pd/C and Au/C ratios, a pre-
cipitate covering the carbon surface, and was the most selective
catalyst in the transformation of glyoxal into glyoxalic acid, and the
most active in the oxidation of glucose. So, although this activating
agent decreased the selectivity in glyoxalic acid for monometallic
Pd/C catalysts, it seems to improve the catalytic performance in the
bimetallic formulations. TEM characterization showed that NaBH4
allowed the formation of smaller Pd particles than formalin, related
to higher Pd/C surface ratio, for monometallic Pd/C samples [23].
For Au also, the bimetallic AuPd/C sample activated with NaBH4 dis-
plays the highest Au surface ratio. Traces of residues from NaBH4,
such as boron, could not be ruled out and might also play a role in
improving the catalytic results obtained with this activation agent.
[17] N. Dimitratos, A. Villa, D. Wang, F. Porta, D.S. Su, L. Prati, J. Catal. 244 (2006)
113–121.
[18] L. Prati, A. Villa, F. Porta, D. Wang, D.S. Su, Catal. Today 122 (2007) 386–390.
[19] A. Villa, N. Janjic, P. Spontoni, D. Wang, D. Sheng Su, L. Prati, Appl. Catal. A: Gen.
364 (2009) 221–228.
[20] A. Villa, G.M. Veith, L. Prati, Angew. Chem. Int. Ed. 49 (2010) 4499–4502.
[21] W.C. Ketchie, M. Murayama, R.J. Davis, J. Catal. 250 (2007) 264–273.
[22] M. Haruta, Catal. Today 36 (1997) 153–166.
[23] A. Deffernez, S. Hermans, M. Devillers, J. Phys. Chem. C 111 (2007) 9448–9459.
[24] S. Hermans, A. Deffernez, M. Devillers, Catal. Today 157 (2010) 77–82.
[25] H. Hustede, H.J. Haberstroh, E. Schinzig, “Gluconic acid” in Ullmann’s Encyclo-
pedia of Industrial Chemistry, VCH, Weinheim, 1989, pp. 449–456.
[26] K. Buchholz, B. Godelmann, Biotechnol. Bioeng. 20 (1978) 1201–1220.
[27] K. Li, J.W. Frost, J. Am. Chem. Soc. 120 (1998) 10545–10546.
5. Conclusions
In this work, we have shown that Au–Pd/C catalysts pre-
pared on SX+ activated carbon by an adsorption method based