Langmuir 1999, 15, 2617-2619
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Exclusive Formation of C16O in the Prompt Oxidation of Carbon Films by 18O2 on Sm216O3. Implications for the Mechanism of Oxygen Activation V. T. Amorebieta and A. J. Colussi* National Research Council of Argentina, P.O. Box 422, Mar del Plata, 7600 Argentina Received October 27, 1998. In Final Form: February 17, 1999 The oxidation of carbon films on Sm216O3-γ exposed to ∼1 Langmuir/s 18O2 at 1100 K solely yields C16O while the substrate is quantitatively capturing oxygen. This is evidence that chemisorbed 18O scrambles its label with the catalyst before oxidizing carbon or diffusing into the bulk oxide. O-diffusion coefficients measured in samaria under present conditions (DO g 8 × 10-9 cm2 s-1 above 1000 K) imply that scrambling is complete in a few nanoseconds. We infer that dissociative 18O2 chemisorption is assisted by the concomitant restructuring of extended surface regions, on which carbon films are oxidized by the statistically preponderant unlabeled oxygen species.
Introduction The remarkable fact that dioxygen activation must invariably precede exothermic oxidation reactions provides for kinetic control of important natural and industrial processes.1,2 Activation is generally accomplished by weakening the O2 bond via charge transfer into its antibonding HOMO. The concertedness of the ensuing oxidation reaction tends to mitigate the uphill activation stage. Many metal oxides catalyze economically important oxidations, such as the conversion of hydrocarbons into more valuable feedstocks.2-7 The detailed mechanism of catalytic action remains, however, unknown. Since the rather inert alkanes require O-atoms as reactive intermediates, dioxygen activation must proceed all the way through dissociation.7-14 Potential catalysts should be able (1) (a) Haneda, M.; Mizushima, T.; Kakuta, N. J. Phys. Chem. B 1998, 102, 6579. (b) Proshlyakov, D. A.; Pressler, M. A.; Babcock, G. T. Proc. Natl. Acad. Sci. U.S.A. 1998, 95, 8020. (c) Francisco, W. A.; Tian, G. C.; Fitzpatrick, P. F.; Klinman, J. P. J. Am. Chem. Soc. 1998, 120, 4057. (d) Neumann, R.; Dahan, M. Nature 1997, 388, 353. (2) (a) Haber, J. Heterogeneous Hydrocarbon Oxidation; ACS Symposium Series 636; American Chemical Society, Washington, DC, 1996; Chapter 2. (b) Haber, J. In Catalysis of Organic Reactions; Kosak, J. R., Johnson, T. A., Eds.; Marcel Dekker: New York, 1994; p 151. (3) (a) Krylov, O. V. Catal. Today 1993, 18, 209. (b) Bielanski, A.; Haber, J. Oxygen in Catalysis; Marcel Dekker: New York, 1991; Chapters 3 and 4. (4) Olah, G. A.; Molnar, A. Hydrocarbon Chemistry; Wiley: New York, 1994. (5) Crabtree, R. H. Chem. Rev. 1995, 95, 987. (6) Shilov, A. E. In Activation and Functionalization of Alkanes; Hill, C. L., Ed.; Wiley: New York, 1989. (7) Lunsford, J. H.; Cisneros, M. D.; Hinson, P. G.; Tong, T.; Zhang, H. Faraday Discuss. Chem. Soc. 1989, 87, 13. (8) (a) Amorebieta, V. T.; Colussi, A. J. J. Phys. Chem. 1988, 92, 4576. (b) Amorebieta, V. T.; Colussi, A. J. J. Phys. Chem. 1989, 93, 5155. (9) Amorebieta, V. T.; Colussi, A. J. J. Am. Chem. Soc. 1996, 118, 10236. (10) Amorebieta, V. T.; Colussi, A. J. J. Am. Chem. Soc. 1995, 117, 3856. (11) Colussi, A. J.; Amorebieta, V. T. J. Catal. 1997, 169, 301. (12) Feng, Y.; Niiranen, J.; Gutman, D. J. Phys. Chem. 1991, 95, 6558, 6564. (13) (a) Peil, K. P.; Marcelin, G.; Goodwin, J. G. In Methane Conversion by Oxidative Processes: Fundamental and Engineering Aspects; Wolf, E. E., Ed.; Van Nostrand Reinhold: New York, 1992. (b) Peil, K. P.; Goodwin, J. G.; Marcelin, G. J. Catal. 1991, 131, 143. (c) Peil, K. P.; Goodwin, J. G.; Marcelin, G. J. Am. Chem. Soc. 1990, 112, 6129. (d) Shannon, S. L.; Goodwin, J. G. Chem. Rev. 1995, 95, 677. (e) Voskresenkaya, E. N.; Roguleva, V. G.; Anshits, A. G. Catal. Rev.-Sci. Eng. 1995, 37, 101.
to assist O2 dissociation by stabilizing the intermediate fragments, either energetically or entropically. Kinetic studies of hydrocarbon oxidations by dioxygen catalyzed by metal oxides, such as Sm2O3, Li/MgO, and Sr/La2O3, reveal that O2 dissociation actually precedes oxidation reactions.8-12 Remarkably, the dissociative chemisorption equilibria on all oxides prove to be endothermic and highly exentropic, in contrast to the behavior expected for localized gas-solid adsorption.8-10 We argued that large entropy gains in chemisorption could only accrue from simultaneous catalyst restructuring.9,10 We also conjectured that labeled oxygen would immediately lose its identity, since a vast excess of unlabeled atoms becomes available for reaction through this mechanism. In this paper we report experiments on the oxidation of carbon films by 18O2 catalyzed by unlabeled samaria that confirm this hypothesis. Present results provide direct chemical evidence that metal oxides activate dioxygen via entropic, rather than energetic, assistance. Experimental Section A heatable Knudsen flow reactor (fused silica, cylindrical, 5 cm diameter, VR ) 90 cm3) was connected to an on-line modulated beam mass spectrometer (Extrel).8-10 18O2 (Merck, Sharp & Dohme, 99.9% 18O), 16O2 (Air Liquide, Argentina, >99.99%), and CH4 (Matheson, >99%) circulated through the reactor at overall pressures < 20 mTorr. The fast mass spectrometric detection system (40 eV electron impact ionization) that allows for monitoring the composition of effusing reaction mixtures in real time has been described in detail previously.9,10 A thin sample of samarium(III) oxide (Aldrich, 240 mg) powder could be rapidly (e1 s) inserted into or removed from the reactor by means of a sliding feedthrough. The catalyst periodically underwent conditioning under 100 mTorr 16O2 at 1200 K. This treatment leads to partial sintering, which was found necessary to prevent blowoff of catalyst particles (3 µm average diameter, as determined by optical microscopy) in the evacuated reactor. Carbon films were formed by flowing CH4 on the catalyst under anoxic conditions for protracted periods. After several minutes, depending on the conditions, the oxide is partially reduced and becomes inert toward methane oxidation, which then decomposes into (C + 2H2), producing a grayish carbon layer.8,14 These films could be burned off under 16O2, a procedure that consistently regenerates pristine Sm2O3. (14) Colussi, A. J.; Amorebieta, V. T. In Methane and Alkane Conversion Chemistry; Bhasin, M. M., Slocum, D. W., Eds.; Plenum: New York, 1995; p 131.
10.1021/la981517f CCC: $18.00 © 1999 American Chemical Society Published on Web 03/19/1999
2618 Langmuir, Vol. 15, No. 8, 1999
Letters
is a direct indication that samaria surfaces remain unlabeled throughout because of fast diffusional equilibration with the entire solid. Actually, it is possible to estimate the diffusion coefficient of oxygen from the slow decay of I32 and I34 signals at longer times, Figure 2. We analyze the data assuming evaporation from a semiinfinite planar layer normal to the depth coordinate z e 0 with appropriate boundary conditions. The following equations apply:16
F16 ) 2F32 + F34 ) -ACATDO ∂[16OS]/∂z ) RACAT[16OS] (1) ∂[16OS]/∂z ) -∂[18OS]/∂z
(2)
[16OS](t) ) [16OS]0 exp(R2t/DO) erfc[(R2t/DO)1/2] (3) Figure 1. Normalized signal intensities IX of species exiting the reactor filled with 2.4 nmol s-1 of 18O2 at 1000 K. Circles: I36 in the empty reactor. Up triangles, diamonds, down triangles, squares: I36, I32, I34, ∑ ) (I36 + I32 + I34), respectively, in the presence of 240 mg of Sm216O3.
[18OS](t) ) [16OS]0{1 - exp(R2t/DO) erfc[(R2t/DO)1/2]} (4) F16 ) F16,0 exp(R2t/DO) erfc[(R2t/DO)1/2]
(5)
where the subscripts S and 0 designate surface and initial values, respectively, R ) F16,0/(ACAT[16OS]0), and [16OS]0 ) 0.072 mol/cm3. A fit of eq 5 to the data of Figure 2 leads to DO ) 3.1 × 10-13 cm2 s-1 at 700 K, which is within 30% of the value calculated from the following expression:17
log(DO/cm2 s-1) ) - 5.04-5140/T
Figure 2. Total 16O outflow: F16 ) 2F32 + F34, during exposure of 240 mg of Sm216O3 to 4.8 nmol s-1 of 18O2 at 1000 K (circles) and at 700 K (squares). Solid lines calculated from eq 5 with DO/cm2 s-1 ) 8.0 × 10-9 (A), 6.6 × 10-11 (B), 7.3 × 10-13 (C), and 5.0 × 10-14 (D) at 1000 K. With DO/cm2 s-1 ) 3.1 × 10-13 (E) at 700 K. Curve B corresponds to DO calculated from eq 6. Curves C and D correspond to DO’s estimated from ref 13a.
Results and Discussion Oxygen Diffusion Rates in Samaria. Exposure of samaria samplesspreviously treated under low (
