J . Phys. Chem. 1988, 92, 4514-4516
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hydrogen evolution as a function of platinum loading on titanium dioxide particles. The platinum particle size distributions were in the range of 5-30 A. Heller et a1.I8 have prepared and characterized light-transmitting platinum films. One of their key conclusions is that, at the primary level, the monocrystalline stable platinum particles are approximately 5 0 A. In conclusion, a new interfacial photochemical reaction at the photosynthetic membrane interface is reported. This reaction has been used to flash photoprecipitate platinum clusters, and the number of flashes required to observe the onset of hydrogen evolution has been used to characterize the cluster size. While (17) Kiwi, J.; Grltzel, M. J . Phys. Chem. 1984, 88, 1302. (18) Heller, A.; Aspnes, D. E.; Porter, J. D.; Sheng, T. T.; Vadimsky, R. G . J . Phys. Chem. 1985, 89, 4444.
photoprecipitation of metal colloids at the photosynthetic membrane interface is obviously not a general technique for catalyst preparation, it is, however, probably true that other metals (or combinations of metals) can be used to metallize chloroplasts, thereby imparting a variety of catalytic properties to the composite material. Acknowledgment. The author thanks J. P. Eubanks for technical support; M. A. Neal and P. S. Mattie for secretarial support; W. A. Arnold, B. Z. Egan, A. Heller, and M. Andrews for helpful discussions, comments, and criticism on the manuscript; and C. S. MacDougall for performing the X-ray fluorescence analysis. This research was supported by the Office of Basic Energy Sciences, U S . Department of Energy, under Contract DEAC05-840R21400 with the Martin Marietta Energy Systems, Inc.
Reactlvlty of the Ca/HCI van der Waals Complex J. P. Visticot,* Service de Physique des Atomes et des Surfaces, CEN Saclay, 91 191 Gif sur Yvette cedex, France
B. Soep, and C. J. Whitham Laboratoire de Photophysique Moleculaire batiment 21 3, Universite Paris Sud, 91405 Orsay cedex, France (Received: May 4, 1988)
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We have observed the chemiluminescent reaction Ca('P) + HCl CaCl(A or B) + H starting from a Ca/HC1 van der Waals complex. The complex is produced by a Ca laser vaporization source coupled to a supersonic expansion. No effect of the p orbital orientation upon the A211/B2Z+branching ratio is observed. The analysis of the beam gas experiment by Rettner and Zare allows interpretation of the results.
The reactivity of aligned atoms or oriented molecules is expected to depend strongly upon the direction of the When there exists more than one exit channel for the reaction, changing this direction can also change the branching ratio to these various exit channels. Rettner and Zare, in a beam gas experiment, have shown an important effect of the direction of the laser polarization in the reaction of Ca(4s4p1PI) with HC1.334 In particular, they have shown that the alignment of the Ca p orbital perpendicular to the direction of approach enhances production of the CaCI(A211) state, while the CaC1(B22+) product is favored by parallel approach. The main disadvantage in preparing the polarization of the atomic Ca reagent is that the memory of this preparation can be lost in the collision when the impact parameter is different from 0. Such a problem does not exist if we start the reaction from a Ca/HC1 van der Waals complex because there is much less averaging of the impact parameter. A recent study of the Hg/H2 van der Waals complex has shown that, by tuning a laser near the resonance transition of Hg, it is possible to selectivity excite the 2: or II excited surface and a very different reactivity was found for each excitation.5 The purpose of this communication is to present the first observation of the Ca/HCI complex and of the chemiluminescent reaction induced by the absorption of a photon. ( 1 ) Bernstein, R. B.; Herschbach, D. R.; Levine, R. D. J. Phys. Chem. 1987, 91, 5365. (2) Simons, J. P. J . Phys. Chem. 1987, 91, 5378. (3) Rettner, C. T.; Zare, R. N. J . Chem. Phys. 1981, 75, 3636. (4) Rettner, C. T. Zare, R. N. J . Chem. Phys. 1982, 77, 2416. (5) Breckenridge, W. H.; Jouvet, C.; Soep, B. J . Chem. Phys. 1986, 84,
1443.
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The Ca + HCl system has been chosen because it does not react when the calcium atom is in its ground state (AHN 38 kJ/mol) and the complex should be easily formed in a supersonic expansion. Moreover, when the calcium atom is electronically excited, a strong chemiluminescent emission from the CaCl A211and B2E+ states has been o b s e r ~ e d . ~ * ~ . ~ - ~ The principle of the experiment is to excite the Ca/HCl complex in the vicinity of the Ca 4sz 'SO-4s4p1PI resonance line which prepares this complex on an excited potential energy surface. The dynamical evolution of the complex on the excited surface leads to the reaction and forms CaCl in an excited state A or B whose fluorescence is detected. The spectrum then obtained by sweeping the laser wavelength is called the action spectrum. The experiment consists of a supersonic pulsed beam coupled to a laser vaporization source of the type described by S m a l l e ~ . ~ When the valve opens, a mixture of argon and HCI starts to flow into a 1-mm-diameter channel. About 1 cm downstream, it reaches a rotating calcium rod where the beam of the second harmonic of a YAG laser is focused. The laser pulse evaporates the calcium about 1 ms after the opening of the valve when the Ar/HC1 flux is a maximum. Then, the mixture of Ca, Ar, and HCl is expanded into vacuum and, due to the cooling of this expansion, the Ca/HC1 complexes are formed. The beam of a second laser crosses the molecular beam about 15 mm downstream. This second pulse is delayed with respect to the first by aproxi(6)Brinkmann, U.; Telle, H. J . Phys. B 1977, 10, 133. (7) Brinkmann, U.; Schmidt, V. H.; Telle, H. Chem. Phys. Lett. 1980, 73, 530. (8) Telle, H.; Brinkmann, U. Mol. Phys. 1980, 39, 361. (9) Smalley, R. E. Laser Chem. 1983, 2, 167.
0 1988 American Chemical Society
The Journal of Physical Chemistry, Vol. 92, No. 16, 1988 4575
Letters
1 Co-HC!
CaCl EMISSION SPECTRUM
~
I
ACUW spEcrRuM MO#ITDR/NG CoCI A-X
A-X
I
I
I
I
1
1
23500
23000 cm-'
Figure 2. Action spectrum of the Ca/HCI complex obtained by looking at the fluorescence of the A211 state of CaCl (A = 620 nm) while sweeping the laser wavelength. The position of the Ca(4s2'So-4s4p'PI)
resonance line is also indicated. 1
550
I
60%n
I
650
Figure 1. Chemiluminescence spectra of CaCl. The points correspond to the excitation of the Ca/HCI complex excited by a laser out of resonance of the Ca line (A = 430 nm). The second spectrum results from the reaction of a calcium atom excited in the 4s4p'PI state with HC1.
mately 30 ps corresponding to the time taken by the supersonic metal stream to reach the observation region. This second laser pulse excites the molecular beam and the resulting emission is analyzed by a monochromator followed by a photomultiplier and a boxcar averager. By tuning the second laser close to the resonance line of the calcium atom (422 nm), fluorescence is observed in the red (around 600 nm). The fluorescent spectrum resulting from the excitation at 430 nm is given in Figure 1. It is compared to the laser-induced chemiluminescence spectrum due to the reaction between Ca(4s4p'P1) and HCI obtained in a beam gas configuration. The two spectra present the two characteristic maxima at 620 and 595 nm corresponding to the emission of the two states A and B of CaC1.3*4*bs The emission recorded with the laser at 430 nm presents all the characteristics of a chemiluminescence resulting from the excitation of a Ca/HC1 complex. First, the intensity increases with the Ar/HC1 pressure and disappears instantaneously when blocking the opening of the valve. Moreover, no difference is found between the time dependence of this signal and the time evolution observed when doing direct laser-induced fluorescence of CaC1. This last point is not only a further proof that the emitting product is CaCI, but it also indicates that this product is formed instantaneously. It is important to stress that, in order to observe the emission resulting from the excitation of the Ca/HC1 complex, the experimental conditions have to be carefully controlled. The evaporation laser produces not only neutral calcium atoms but also other species which can absorb and reemit light. For too large an intensity of the vaporization laser, other fluorescent signals are observed which can be discriminated from the known CaCl chemiluminescence displayed in Figure 1. These spurious signals increase when the background pressure in the main chamber is to 10 mbar. Their origin is believed to be increased from collisions with background gas. This is confirmed by looking at the time evolution which shows a slower rise time than the laser. Moreover, these signals are maximal for a delay of about 50 ps instead of 30 ps for the 1 to 1 complex, suggesting another heavier precursor. Consequently, in order to see the signal of the Ca/HCl complex, the laser vaporization power as well as the delay between the two laser pulses has to be carefully chosen to minimize the
contribution of these spurious signals to the Ca/HCI complex chemiluminescence. Figure 2 gives the action spectrum of the Ca/HCl complex recorded at the emission maximum at 620 nm, corresponding to the emission from the AZIIstate. This spectrum is obtained by tuning the second laser in the vicinity of the resonance line of Ca and extends over about 1000 cm-'. It appears as a broad structureless continuum. No important differences were observed when monitoring the fluorescence coming from either the A or B state. Such a wide structureless spectrum has been observed in the excitation of the Hg/C12 van der Waals cornplex.'*l2 It has been interpreted as a fast reaction without any activation barrier. This is likely to be the case here considering the large cross section of 68 A2 which has been measured for the chemiluminescent reaction of Ca(4s4p'PI) with HCl.4 Such a large cross section has led Rettner and Zare to interpret the reaction between excited Ca and HCI by a harpooning mechanisms4 Nevertheless, it has been shown that attachment of an s electron to HCI results in the dissociation into C1- H.I3,l4 The net effect of this attachment will be the same as for the harpooning reaction but selective for the s or pu electron. In the interpretation of Rettner and Zare (see ref 4), the covalent Ca(lP) HCI('Z+) surface crosses successively the Ca+(2S, ZD, and 2P) + HCI- (?Z') ionic surfaces for decreasing Ca-HCI distances. The first crossing at about 3.5 A is responsible for the large cross section. It ends on a symmetrical Ca+(%) + HC1-(2Z+) ionic intermediate which cannot keep the memory of the initial polarization. The conclusion was that the effect of the polarization on the A/B branching ratio could only originate from inner crossings in the entrance channel. If we now consider the excitation of the Ca/HC1 van der Waals complex, this process derives its oscillator strength from the Ca('S-'P) transition and no direct excitation of the ionic species is possible due to the Ca-HCl structure of the ground-state complex. The absorption of a photon thus excites the complex to a covalent potential surface correlating to Ca('P) + HCl('2') and because the surfaces corresponding to the various orbital orientations are no longer degenerate, the different orientations will correspond to different laser frequencies. As the typical equilibrium distance for such a complex would be of the order of 3-4 A, the laser absorption occurs close to the crossing with the Ca+(*S) + HC1-(2Z+) ionic surface and the reaction is expected
+
+
(10) Jouvet, C.; Soep, B. Chem. Phys. Lett. 1983, 96,426. (11) Jouvet, C.; Soep, B. J . Phys. (Les U h ,Fr.) 1985, 46, C1-313. (12) Jouvet, C.; Boivineau, M.; Duval, M. C.; Soep, B. J . Phys. Chem. 1987, 91, 5416. (13) ONeil, S. V.;Rosus, P.; Norcross, D. W.; Werner, H. J. J . Chem. Phys. 1986,85, 7232. (14) Gauyaq, J. P. Europhys. Lett. 1986, 1, 287.
J . Phys. Chem. 1988, 92, 4576-4578
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to proceed predominantly via this crossing. In this interpretation, whatever the orientation of the state populated by the laser or, equivalently, whatever the laser wavelength, no differences is expected on the branching ratio between the A and B states of CaCl because there is only one ionic symmetrical intermediate. Consequently the observation of a constant A/B branching ratio as a function of the laser wavelength is consistent with the interpretation of Rettner and Zare.4 The fact that the orbital orientation has no effect on the A/B branching ratio does not mean that the reactivity is the same for I:or IT orientation. Considering the symmetry of the ionic surface, the favored configuration for the reaction is pZ.15 This is particularly true in the present experiment starting from a van der (15) Grice, R.; Herschbach, D. R. Mol. Phys. 1974, 27, 159.
Waals complex because we have a very small impact parameter. Consequently, the action spectrum of Figure 2 reflects principally the excitation to the 2 covalent surface. As a conclusion, it should be interesting to investigate the crossings with other ionic surfaces by looking at more excited states of the complex. Another interesting point should be to look at the anisotropy of the chemiluminescence in order to get more information about the symmetry of the states involved in the reaction.2 Such a work is in progress. Acknowledgment. The success of this experiment is due to careful machining of the evaporation source by Philippe Ceraolo. We are grateful to Dr. J. M. Mestdagh for fruitful discussions. C.J.W. acknowledges support from the Royal Society under the European Science Exchange Scheme. Registry No. CaCI, 15606-71-0; Ca, 7440-70-2; HCI, 7647-01-0.
Kinetics and Mechanism of the Catalytic Oxidation of Methane over Lithium-Promoted Magnesium Oxide V. T. Amorebieta and A. J. Colussi* Department of Chemistry, University of Mar del Plata, 7600 Mar del Plata, Argentina (Received: April 19, 1988)
The kinetics of methane oxidation in CH4/02mixtures over 7% lithium-promoted magnesium oxide has been investigated by dynamic mass spectrometry between 800 and 1100 K. At low pressures, rates become first order and half-order with respect to methane and oxygen, respectively, directly revealing the reversible dissociative chemisorption of O2on the catalyst. Otherwise, the complex rate law applicable over the entire range of pressures studied here can be accounted for by assuming competitive Langmuir adsorption of both reactants followed by chemical reaction of CH4(g) with chemisorbed oxygen. The rate constant for methane oxidation exhibits Arrhenius behavior with E, = 86 kJ mol-'. Trapping of reactive species with iodine in a tandem reactor confirms the presence of methyl radical in the gas phase.
Introduction The fact that about two-thirds of the world's natural gas reserves are located in remote areas has prompted a widespread search for processes involving catalytic conversion of methane into fuels and chemical^.^-^ Since dimerization of methane is thermodynamically unfavorable both for enthalpic and entropic reasons 2CH4 C2H6 + H2 AHo = 15.6 kcal/mol, ASo = -2.9 eu partial oxidation would provide the required driving force: 2CH4 + ' / 2 0 2 --* C2H6 + H2O AHo = -42.2 kcal/mol, ASo = -13.5 eu Among the various schemes for converting methane into more valuable petrochemical feedstocks such as ethane and ethylene, direct oxidative coupling over heterogeneous catalysts has become a promising alternative considering the high cost of producing synthesis gas.2,6 Clearly, the major technical challenge of any method is to find a kinetically feasible process for methane oxidation which prevents losses of the presumably more reactive C2 products. Metal oxides have been widely investigated in this connection since the pioneer work of Keller and Bhasin.' The list includes
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Jones, C. A.; Leonard, J. J.; Sofranko, J. A. Energy Fuels 1987, 1, 12. Chem. Eng. News 1987, 65(37), 19. Ito, T.; Lunsford, J. H. Nature (London) 1985, 314, 721. Otsuka, K.; Jinno, K.; Morikawa, A. J . Caral. 1986, 1.00, 353. Sofranko, J. A.; Leonard, J. J.; Jones, C. A. J . Cutul. 1987, 103, 302. Gesser, H. D.; Hunter, N. R.; Prakash, C. B. Chem. Reu. 1985, 85,
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not only metals having two or more stable oxidation states but also the l a n t h a n i d e ~ . ~ ~ ~Very ~ ~ - "interestingly, Lunsford and co-workers have recently proposed the use of lithium-doped magnesium oxide, a catalyst which does not contain redox-active metal center^.^ The active sites are believed to be substitutional lattice oxide ion defects which generate methyl radicals via H-atom ab~traction.'~-'~ Ion size effects seem to confirm this view since whereas Na/CaO is also active, Na/MgO is much less efficient.16 Continuous and stepwise oxidation have been described,'~~ and it has been conjectured that the regeneration of active centers controls overall rates in all these systems.' In this paper we report a detailed kinetic investigation of methane oxidation in CH4/O2 mixtures over 7% Li-doped magnesium oxide which clearly reveals the reversible dissociative chemisorption of O2 on the solid and identifies the reaction of (7) Keller, G. E.; Bhasin, M. M. J. Coral. 1982, 73, 9. (8) Otsuka, K.; Liu, Q.; Morikawa, A. J . Chem. SOC.,Chem. Commun. 1986, 586. (9) Imai, H.; Tagawa, T.; Kamide, N. J . Caral. 1987, 106, 394. (10) Wang, J. X.; Lunsford, J. H. J . Phys. Chem. 1986, 90, 3890. (11) Lin, C. H.; Campbell, K. D.; Wang, J. X.; Lunsford, J. H. J. Phys. Chem. 1986, 90, 534. (12) Driscoll, D. J.; Martin, W.; Wang, J. X.; Lunsford, J. H. J . Am. Chem. SOC.1985, 107, 58. (13) Driscoll, D. J.; Lunsford, J. H. J. Phys. Chem. 1985, 89, 4415. (14) Campbell, K. D.; Morales, E.; Lunsford, J. H. J. Am. Chem. SOC. 1987, 109, 7900. (15) Aika, K. I.; Lunsford, J. H. J . Phys. Chem. 1978, 82, 1794. (16) Lin, C. H.; Ito, T.; Wang, J. X.; Lunsford, J. H. J. Am. Chem. SOC. 1987, 109, 4808.
0 1988 American Chemical Society
