C O M M U N I C A T I O N S
involves the lattice oxygen (O2- or O- as a part of the lattice defect
site). Once the lattice oxygen is transferred to the substrate, there
are two processes that occur: (a) the migration of sub-lattice oxygen
to the surface and (b) the replenishment of the oxygen ion vacancy
from the gas phase to the bulk. These latter two steps are known
to be faster than the catalytic step (dissociation of R and â-hydrogen
of ethylbenzene).8 In the present study, the ODH turnover rates
increased as the concentration of surface-defect sites increased,
suggesting that the activation of R and â-hydrogen of ethylbenzene
substrate by the Ce3+-O--Ce4+-type surface-defect sites is the
rate-determining step.9 Hence, under steady-state conditions, the
rate of vacancy migration or oxygen ion mobility from the bulk to
the surface will not be limiting on the catalytic rate.
From the above investigations, we can propose a catalytic path-
way for the oxidative dehydrogenation of ethylbenzene on ceria
using N2O as the oxidant, considering that this ODH reaction fol-
lows a Mars-van Krevelen mechanism (Scheme S1). We can con-
Figure 2. Dependence of the concentration of defect sites and the activation
temperature required for 50 mol % conversion of ethylbenzene (T50) for
the different ceria samples prepared by different routes.
clude that a ceria sample enriched with surface Ce3+-O--Ce4+
-
type defect sites promotes the subsurface oxygen migration and
thus facilitates surface reorganization. This favors a lower temper-
ature of activation of the substrate. We find a direct relationship
between the concentration of the defect sites and the energy required
for the activation of the substrate on the one hand and a correlation
between the rate of EB conversion and the rate of consumption of
the defect sites on the other.
Acknowledgment. We are grateful to Dr. D. Srinivas, Dr. C.
V. Satyanarayana, and Dr. Veda Ramaswamy of National Chemical
Laboratory, Pune, for their contributions to the experimental data
and fruitful discussions. We thank CSIR, New Delhi, for the
financial support (Grant ES 21(0561)/03/EMR-II).
Supporting Information Available: Experimental part, Figures S1
and S2, and Scheme S1. This material is available free of charge via
Figure 3. (a) Representative EPR spectra of a ceria sample prepared by
alcoholysis method (Ceria-A) after in situ adsorption of ethylbenzene at
various temperatures and recorded at 298 K. Adsorption of ethylbenzene
vapors was carried out for a period of 1 h with an EB flow rate of 2.3 mL
h-1 at respective temperatures. (b) Variation in the concentration of defect
sites, as recorded from the EPR signal intensity, with time on adsorption
of EB on a ceria sample prepared by alcoholysis method (Ceria-A) at
598 K.
References
(1) (a) Yan, Z.; Chinta, S.; Mohamed, A. A.; Fackler, J. P., Jr.; Goodman, D.
W. J. Am. Chem. Soc. 2005, 127, 1604-1605. (b) Sterrer, M.; Diwald,
O.; Knozinger, E. J. Phys. Chem. B 2000, 104, 3601-3607.
(2) (a) Sanchez, A.; Abbet, S.; Heiz, U.; Schneider, W.-D.; Ha1kkinen, H.;
Barnett, R. N.; Landman, U. J. Phys. Chem. A 1999, 103, 9573-9578.
(b) Costuas, K.; Parrinello, M. J. Phys. Chem. B 2002, 106, 4477-4481.
(c) Orlando, R.; Millini, R.; Perego, G.; Dovesi, R. J. Mol. Catal. A 1997,
119, 253-262. (d) Kolmel, C.; Ewig, C. S. J. Phys. Chem. B 2001, 105,
8538-8543. (e) Ferrari, A. M.; Giordano, L.; Rosch, N.; Heiz, U.; Abbet,
S.; Sanchez, A.; Pacchioni, G. J. Phys. Chem. B 2000, 104, 10612-10617.
(f) Valentin, C. D.; Pacchioni, G.; Abbet, S.; Heiz, U. J. Phys. Chem. B
2002, 106, 7666-7673.
(3) (a) Trovarelli, A.; Leitenburg, C. d.; Boaro, M.; Dolcetti, G. Catal. Today
1999, 50, 353-367. (b) O’Hara, F. J. U.S. Patent 3 904 552, 1975. (c)
Sherrod, F. A.; Smith, A. R. U.S. Patent 4 758 543, 1988. (d) Murakamy,
A.; Unei, H.; Teranishi, M.; Ohta, M. U.S. Patent 5 190 906, 1993.
further indicates that these sites are responsible for the activation
of R and â-hydrogen of ethylbenzene substrate.
To understand whether the concentration of Ce3+ defect sites
can limit the rate of the reaction, a time on stream study was
conducted by in situ adsorption of ethylbenzene at 598 K on a ceria
sample prepared by the alcoholysis method (Figure 3b). Interest-
ingly, the rate of decrease in the EPR signal intensity, a measure
of Ce3+ defect sites, was about 1.26 count % per min which was
found to be in the same order of magnitude with that of the rate of
EB conversion (1.11 mol % per min). The time required for the
Ceria-A sample to achieve 50% conversion of EB at 598 K is
approximately 45 min, which is comparable to the time required
for the 50% decrease in the defect site concentration (Figure 3b).
Thus, we can suggest that the presence of a Ce3+-O--Ce4+-type
defect site is kinetically significant in determining the rate of ODH
of EB.
(4) Murugan, B.; Ramaswamy, A. V.; Srinivas, D.; Gopinath, C. S.;
Ramaswamy, V. Chem. Mater. 2005, 17, 3983-3993.
(5) Binet, C.; Badri, A.; Lavalley, J. C. J. Phys. Chem. 1994, 98, 6392-
6398.
(6) (a) Abi-Aad, E.; Bechara, R.; Grimblot, J.; Aboukais, A. Chem. Mater.
193, 5, 793-797. (b) Abi-Aad, E.; Bennani, A.; Bonnelle, J.-P. J. Chem.
Soc., Faraday Trans. 1995, 91, 99-104.
(7) Damyanova, S.; Perez, C. A.; Schmal, M.; Bueno, J. M. C. Appl. Catal.
A 2002, 234, 271.
(8) Shin, M. Y.; Chung, K. S.; Hwang, D. W.; Chung, J. S.; Kim, Y. G.;
Lee, J. S. Langmuir 2000, 16, 1109-1113.
(9) Chen, K.; Bell, A. T.; Iglesia, E. J. Catal. 2002, 209, 35-42.
In most of the oxidation reactions involving metal oxides as
catalysts, the rate-determining step is the surface reaction that
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