8
4
L. Yang et al. / Journal of Catalysis 276 (2010) 76–84
from in situ XRD measurements (Fig. 8). This observation together
with the XPS characterization for the sample after reaction sug-
indicated that the presence of vanadium species suppressed the
reactivity of lattice oxygen in the working catalyst.
0
I
gests that it is not the Cu but the Cu that functions for the epox-
idation of C by O . Our in situ XRD studies further suggest that
the presence of VO
species accelerates the transformation of Cu0
to Cu in reactant gas flow at reaction temperature (Figs. 7 and
). We speculate that the V species in the H -reduced catalyst
may easily activate O and the activated oxygen species may then
be incorporated into Cu to form Cu O.
3
H
6
2
Acknowledgments
x
I
This work was supported by the National Natural Science Foun-
dation of China (Nos. 20773099, 20625310 and 20923004), the
National Basic Research Program of China (No. 2010CB732303),
the Research Fund for the Doctoral Program of High Education
(No. 20090121110007), and the Key Scientific Project of Fujian
Province (No. 2009HZ0002-1).
8
2
O
3
2
2
2
It is expected that the nature of the active oxygen species deter-
mines the reaction route. Generally, the lattice oxygen is nucleo-
philic and mainly catalyzes the allylic oxidation of C
acrolein. Our previous studies showed that the addition of K into
3
H
6
to
+
Appendix A. Supplementary data
II
SBA-15 or SiO
2
-supported CuO
Cu to higher temperatures [31–33]. In the present paper, through
–TPR and C –TPR for the catalysts after reactions (Figs. 11B
and 13), we have demonstrated that the reactivity of lattice oxygen
x 2
shifted the H –TPR peak for Cu to
0
H
2
3 6
H
in the working catalyst was suppressed by the presence of VO
cies. Moreover, we have confirmed that the reaction of C H
6
x
spe-
with
References
3
the lattice oxygen in the working catalyst provided acrolein, CO
and CO , and no formation of PO was observed. This further sup-
ports the idea that the lattice oxygen is not responsible for C
epoxidation. This may partly explain the experimental fact that
the modification of Cu by VO shifts the main partial oxidation
product from acrolein to PO. Actually, Cu O is a well-known cata-
lyst for the allyllic oxidation of C to acrolein [40–42]. Consider-
ing that the TOFs for both C conversion and PO formation were
remarkably enhanced by VO modification, we speculate that ac-
tive oxygen species with an electrophilic character may be gener-
ated over the VO
ꢀCu catalyst. Vanadium species at lower valence
[1] J.R. Monnier, Appl. Catal. A 221 (2001) 73.
[
2] T.A. Nijhuis, M. Makkee, J.A. Moulijn, B.M. Weckhuysen, Ind. Eng. Chem. Res. 45
2006) 3447.
3] M.G. Clerici, G. Bellussi, U. Romano, J. Catal. 129 (1991) 159.
2
(
3
H
6
[
[4] R. Meiers, U. Dingerdissen, W.F. Hölderich, J. Catal. 176 (1998) 376.
[
[
[
5] T. Hayashi, K. Tanaka, M. Haruta, J. Catal. 178 (1998) 566.
6] E.E. Stangland, K.B. Stavens, R.P. Andres, W.N. Delgass, J. Catal. 191 (2000) 332.
7] A.K. Sinha, S. Seelan, S. Tsubota, M. Haruta, Angew. Chem. Int. Ed. 43 (2004)
1546.
x
2
3 6
H
[
[
8] T.A. Nijhuis, T. Visser, B.M. Weckhuysen, Angew. Chem. Int. Ed. 44 (2005) 1115.
9] B. Chowdhury, J.J. Bravo-Sárez, M. Daté, S. Tsubota, M. Haruta, Angew. Chem.
Int. Ed. 45 (2006) 412.
3 6
H
x
[10] E. Sacaliu, A.M. Beale, B.M. Weckhuysen, T.A. Nijhuis, J. Catal. 248 (2007) 235.
11] G.Z. Lu, X.B. Zuo, Catal. Lett. 58 (1999) 67.
[12] J. Lu, M. Luo, H. Lei, C. Li, Appl. Catal. A 237 (2002) 11.
13] A. Palermo, A. Husain, M. Tikhov, R.M. Lambert, J. Catal. 207 (2002) 331.
[14] F. Zemicheael, A. Palermo, M. Tikhov, R.M. Lambert, Catal. Lett. 80 (2002) 93.
[
x
IV
III
states (V and V ) may play a role in the activation of O
electrophilic oxygen species. Future studies are needed to elucidate
the nature of the active oxygen species over our VO
ꢀCu catalysts.
2
to form
[
[
[
15] G.J. Jin, G.Z. Lu, Y.L. Guo, Y. Guo, J.S. Wang, X.H. Liu, Catal. Lett. 87 (2003) 249.
16] A. Takahashi, N. Hamakawa, I. Nakamura, T. Fujitani, Appl. Catal. A 294 (2005)
x
3
4.
17] J. Lu, J.J. Bravo-Suárez, A. Takahashi, M. Haruta, S.T. Oyama, J. Catal. 232 (2005)
5.
18] J. Lu, J.J. Bravo-Suárez, M. Haruta, S.T. Oyama, Appl. Catal. A 302 (2006) 283.
19] W. Yao, Y. Guo, X. Liu, Y. Guo, Y. Wang, Y. Wang, Z. Zhang, G. Lu, Catal. Lett. 119
(2007) 185.
20] Y. Lei, F. Mehmood, S. Lee, J. Greeley, B. Lee, S. Seifert, R.E. Winans, J.W. Elam,
R.J. Meyer, P.C. Redfern, D. Teschner, R. Schlögl, M.J. Pellin, L.A. Curtiss, S. Vajda,
Science 328 (2010) 224.
[
8
5
. Conclusions
[
[
The unsupported copper powder was less effective for C
epoxidation by O . The modification of unsupported copper by
vanadium significantly enhanced both C conversion and PO
3 6
H
[
2
3 6
H
selectivity. The catalyst with a V/Cu atomic ratio of 0.11–0.20
was the most efficient for PO formation. Over the catalyst with a
V/Cu ratio of 0.11, PO selectivities of 35% and 16% could be ob-
[21] M. Ojeda, E. Iglesia, Chem. Commun. (2009) 352.
[
22] S. Lee, L.M. Molina, M.J. López, J.A. Alonso, B. Hammer, B. Lee, S. Seifert, R.E.
Winans, J.W. Elanm, M.J. Pellin, S. Vajda, Angew. Chem. Int. Ed. 48 (2009) 1467.
23] J. Huang, T. Akita, J. Faye, T. Fujitani, T. Takei, M. Haruta, Angew. Chem. Int. Ed.
48 (2009) 7862.
[
3 6
tained at C H conversions of 0.78% and 2.7%, respectively. On
the other hand, over the Cu catalyst without modification, higher
temperatures (573–593 K) were required for obtaining similar
[24] R.L. Cropley, F.J. Williams, O.P.H. Vaughan, A.J. Urquhart, M.S. Tikhov, R.M.
Lambert, Surf. Sci. 578 (2005) L85.
[
25] R.L. Cropley, F.J. Williams, A.J. Urquhart, O.P.H. Vaughan, M.S. Tikhov, R.M.
Lambert, J. Am. Chem. Soc. 127 (2005) 6069.
C H
3 6
conversions, and acrolein was the main partial oxidation
product. The pretreatment of the VO
ꢀCu catalyst was also crucial.
The pre-reduction of catalyst by H could lead to significantly bet-
[26] D. Torres, N. Lopez, F. Illas, R.M. Lambert, Angew. Chem. Int. Ed. 46 (2007)
055.
x
2
2
[
[
27] J.R. Monnier, G.W. Hartley, J. Catal. 203 (2001) 253.
28] J. Lu, M. Luo, H. Lei, X. Bao, C. Li, J. Catal. 211 (2002) 552.
ter catalytic performances for PO formation than the oxidative pre-
treatment. We observed an induction period for PO formation over
the reduced catalyst. Our characterizations showed that the pres-
ence of vanadium caused an increase in the dispersion of copper,
[29] J. Lu, M. Luo, C. Li, Chin. J. Catal. 25 (2004) 5.
[
30] O.P.H. Vaughan, G. Kyriakou, N. Macleod, M. Tikhov, R.M. Lambert, J. Catal. 236
2005) 401.
31] H. Chu, L. Yang, Q. Zhang, Y. Wang, J. Catal. 241 (2006) 225.
(
[
which might contribute to the rise in catalytic activity. In situ
[32] Y. Wang, H. Chu, W. Zhu, Q. Zhang, Catal. Today 131 (2007) 495.
[33] W. Zhu, Q. Zhang, Y. Wang, J. Phys. Chem. C 112 (2008) 7734.
XRD studies demonstrated that Cu0 in the reduced catalyst was
[
[
34] W. Su, S. Wang, P. Ying, Z. Feng, C. Li, J. Catal. 268 (2009) 165.
35] I. Onal, D. Düzenli, A. Seubsai, M. Kahn, E. Seker, S. Senkan, Top. Catal. 53
(2010) 92.
2
partially transformed into Cu O in propylene oxidation and the
existence of VO promoted this transformation. XPS studies con-
x
I
[
[
[
36] S. Sato, R. Takahashi, T. Sodesawa, K. Yuma, Y. Obata, J. Catal. 196 (2000) 195.
37] A. Gervasini, S. Bennici, Appl. Catal. A 281 (2005) 199.
38] J. Morales, J.P. Espinos, A. Caballero, A.R. Gonzalez-Elipe, J.A. Mejias, J. Phy.
Chem. B 109 (2005) 7758.
firmed that Cu mainly existed on the surface of the catalyst after
reaction. XPS studies also informed us that some vanadium species
III
IV
were changed from V to V during the reaction. The combination
of the characterization results with the catalytic reaction results
[39] G. Silversmit, D. Depla, H. Poelman, G.B. Marin, R.D. Gryse, J. Electron
Spectrosc. Relat. Phenom. 135 (2004) 167.
I
suggests that Cu is the active site for propylene epoxidation and
[
[
40] J.L. Callahan, R.K. Grasselli, AIChE J. 9 (1963) 755.
41] B.J. Wood, H. Wise, R.S. Yolles, J. Catal. 15 (1969) 355.
III
IV
vanadium species at lower valence states (V and V ) may play
a role in the activation of O . Our H –TPR and C –TPR studies
3
H
6
[42] T. Inui, T. Ueda, M. Suehiro, J. Catal. 65 (1980) 166.
2
2