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J . Phys. Chem. 1990, 94, 2144-2146
Structure and Reacthrlty of Bimetallic ConV, Clusters S. Nonose, Y. Sone, K. Onodera, S. Sudo, and K. Kaya* Department of Chemistry, Faculty of Science and Technology. Keio University, 3- 14- I Hiyoshi, Kohoku-Ku, Yokohama 223, Japan (Received: December 6 , 1989)
Bimetallic clusters composed of V and Co were synthesized by a laser vaporization method. The reactivity of these alloy clusters toward H2was determined by use of a fast-flow reactor in comparison with pure clusters Con. It was found that the Co12Vcluster is extraordinarily stable against H2adsorption whereas C013 exhibits substantial reactivity toward Hz. This result was explained in terms of the stable structure of Co12Vwhere the V atom is located at the center interacting with 12 Co atoms on the surface.
Introduction Small isolated metal clusters and semiconductor clusters represent a new class of compounds. They are expected to have chemical properties that span the range from molecular kinetics to real bulk surface catalysis. Because small clusters are composed mainly of surface atoms, the most characteristic chemical property of the clusters lies in the surface adsorptivity and/or reactivity which plays crucial roles in catalysis. Since the pioneer work of Smalley,lV2 Riley,3 and Kaldor4v5on the reactivity studies of several metal clusters, remarkable cluster size dependence on the reactivity toward molecules was found as a characteristic common to all kinds of clusters. These phenomena are understood as a mesoscopic nature of clusters based on the geometric and electronic structure of individual clusters. On the other hand, catalysts used in chemical industry are in general alloys composed of several metal It seems essentially important to gain microscopic insight into the mechanism of the catalysis on the alloy systems. However, practically nothing has been studied on the reactivity of alloy clusters. We have developed a new method of generating clusters composed of two different elements from two independent vaporization sources. In this method, clusters which contain two metal elements at any mixing ratio can be generated.9
( I ) (a) Geusic, M. D.; Morse, S. C.; O'Brien, S. C.; Smalley, R. E. Reo. Sci. Instrum. 1985, 56, 2123. (b) Morse, M. D.; Geusic, M. E.; Hearth, J. R.; Smalley, R. E. J . Chem. Phys. 1985, 83, 2293. (2) (a) Elkind, J. L.; Weiss, F. D.; Alford, J. M.; Laaksonen, R. T.; Smalley, R . E. J . Chem. Phys. 1988,88, 5215. (b) Alford, J. M.; Weiss, R. T.; Laaksonen, R . T.;Smalley. R. E. J . Phys. Chem. 1986, 90, 4480. (3) (a) Richtsmeier, S. C.; Parks, E. K.; Liu, K.; P o h ,L. G.;Riley, S. J. J . Chem. Phys. 1985, 82, 3659. (b) Parks, E. K.; Liu, K.; Richtsmeier, S. C.; Pob, L. G.;Riley, S. J. J. Chem. Phys. 1985,82, 5470. (c) Liu, K.; Parks, E. K.; Richtsmeier, S. C.; Pobo, L. G . ; Riley, S. J. J . Chem. Phys. 1985, 83, 2882. (d) Parks, E. K.; Nieman, G. C.; Pobo, L. G.; Riley, S. J. J . Phys. Chem. 1987,91, 2671. (e) Parks, E. K.; Nieman. G. C.; Pobo, L. G.;Riley, S. J. J . Chem. Phys. 1988, 88, 6260. (4) (a) Whetten, R. L.; Cox. D. M.;Trevor, D. J.; Kaldor, A. Surj. Sci. 1985, 156,8. (b) Kaldor, A.; Cox, D. M.; Trevor, D. J.; Zakin, M. R. 2. Phys. D 1986, 3, 195. (c) Zakin, M. R.; Cox, D. M.; Whetten, R. L.; Trevor, D. J.; Kaldor, A. Chem. Phys. Lett. 1987, 135, 223. (d) Zakin. M. R.; Cox, D. M.; Kaldor, A. J . Phys. Chem. 1987, 91, 5224. (5) (a) Cox, D. M.; Reichmann, K. C.; Trevor, D. J.; Kaldor, A. J . Chem. Phys. 1988, 88, I 1 1. (b) Cox, D. M.; Trevor, D. J.; Whetten, R. L.; Kaldor, A. J . Phys. Chem. 1988, 92,421. (c) Zakin, M. R.; Brickman, R. 0.;Cox, D. M.;Kaldor, A. J . Chem. Phys. 1988, 88, 5943. (d) Zakin, M. R.; Brickman, R. 0.; Cox, D. M.; Kaldor, A. J. Chem. Phys. 1988,88,6605. (e) Zakin, M. R.; Cox. D. M.; Brickman, R. 0.;Kaldor, A. J . Phys. Chem. 1989, 93, 6823. (6) Physical and Chemical Properties of Thin Metal Overlayers and Alloy Surfuces; Zehner, D. M., Goodman, D. M., Eds. Mat. Res. Soc. 1987, 83. (7) (a) Kwl, B. E.; Somorjai, G. A. In Catalysis; Anderson, J. R., Boudart, M., Eds.; Springer-Verlag: Berlin, 1985; Vol. 7, Chapter 3. (b) Bond, G. C. Heterogeneous Catalysis, 2nd ed.; Clarendon Press: Oxford, 1987. (8) Bonzel, H. P. Surf.Sci. Rep. 1987, 8, 43. (9) (a) Nonose, S.;Sone, Y.; Onodera, K.; Sudo, S.; Kaya, K. Chem. Phys. Lett. 1989. 164, 427. (b) Nonose, S.; Sone, Y.; Onodera, K.; Sudo. S.; Kaya, K . Manuscript in preparation
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l i
Figure 1. Schematic diagram of experimental apparatus. Clusters composed of two metal elements are synthesized via vaporization of two target rods by two pulsed YAG lasers. Chemical reactions are performed using a fast-flow reactor. The reaction products are ionized by ArF excimer laser and detected by a time-of-flight (TOF) mass spectrometer equipped with a reflectron.
In this work, in order to obtain information about the microscopic mechanism of chemisorption on a transition-metal surface on which another transition-metal element is coadsorbed, reactivity toward H 2 of bimetallic clusters Co,V, (n > m ) was measured under molecular beam conditions. Vanadium, which is located on the left-hand side of the periodic table, has a high reactivity toward H, in contrast to cobalt which has relatively low reactivity.' We found dramatic change in reactivity by the mixed-cluster formation. Especially, unexpected stability of C O , ~ V against H 2 adsorption was found in contrast to high reactivity of the genuine . is explained in terms of the geometric and cluster C O , ~ This electronic structural stability of the Co12Vcluster. Experimental Section A schematic diagram of the experimental apparatus is presented in Figure 1. Clusters containing two metal elements were synthesized via vaporization of two target rods by two pulsed YAG lasers (532 nm) in a He carrier gas. One target is a rod of Co, and another one is that of V. The distance between the two rods is 5 mm. In order to mix two metal elements homogeneously, two laser pulses were used to irradiate the two rods by the delay time of 4 ~s in synchronization with the flow speed of carrier gas. The mixing ratio of two metal elements can be roughly controlled by controlling the fluence of two pulsed lasers i n d e ~ e n d e n t l y . ~ Reactivity experiments toward H2 were performed using a fastflow reactor, into which the second H e pulse containing H, was i n j e ~ t e d . ~ . The ' ~ reaction products were ionized by an ArF excimer laser and detected by a time-of-flight (TOF) mass (IO) (a) Fuke, K.; Nonose, S.; Kikuchi, N.; Kaya, K. Chem. Phys. Lett. Kikuchi, N.; Fuke, K.; Kaya, K.
1988. 147, 479. (b) Nonose, S.; Sone, Y.; Chem. Phys. Lett. 1989, 158, 152.
0 1990 American Chemical Society
Letters
The Journal of Physical Chemistry, Vol. 94, No. 7, 1990 2745
Figure 2. TOF mass spectra of Co,,V, (n > m) clusters (from n = 7 to 14) mixed with (a) 100 Torr of H2 seeded in 1 atm of He and (b) only pure He as a reference. Peaks of the clusters are labeled according to the notation n-m, denoting the number of cobalt atoms (n) and substituted vanadium
atoms ( m ) . spectrometer equipped with a reflectron.” Ionization laser fluence was limited to less than 0.1 mJ/cm2 in order to avoid the effect of multiphoton processes. A LeCroy 9400 transient oscilloscope coupled with a microcomputer was used for averaging the T O F mass spectra.
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4.0
Results and Discussion
In Figure 2, the TOF mass spectra of Co,,V, clusters reacted with H2 are presented in comparison with the genuine Co,,V, distribution. The reactivity of the clusters was estimated from ~ ~ ~ ~ ~ ~ ~ ~ ~ the reactivity index R, defined e l s e ~ h e r e .It ~is ~already known that the reactivity of pure Co clusters Con exhibits a minimum value at n = 6 and increases gradually to n = 9. R, reaches a maximum at n = 10-18.1b The change in reactivity of bimetallic clusters Co,,V, is seen in Figure 3, taking n = 6 , 8 , IO, 11, and 13 as examples. As seen in the figure, when n equals 6 or 8, the reactivity exhibits a sudden onset by exchanging one V atom for a Co atom. By increasing the number of exchanged vanadium atoms m up to 3, the reactivity increases gradually and drops again when m becomes 4. This is quite similar to the case of AI,V, clusters where a sudden onset of adsor tivity is seen by adding one V atom to nonreactive AI, clustersJb In the case of small clusters, n < 10, all atoms are on the surface of the cluster. Hydrogen molecules have a chance Figure 3. The relative rate constant R, of Co,,V, (n > m ) clusters for to interact with any atoms of the cluster. It is known that the reaction with H2. Open circles, closed circles, open squares, open triangles, and closed triangles are R, of n = 6 , 8, IO, 1 I , and 13, respecheat of adsorption of a hydrogen molecule on solid vanadium tively. surface is much larger than that of cobalt. It is reasonable to conclude that the V atom on bimetallic Co,V, clusters plays the to 18 was already reported.lb As seen in Figures 2 and 3, when role of an active site for adsorption. Then, exchanging a V atom one V atom is substituted for Co13 to yield Co12V, reactivity for a Co atom of less (or non) reactive Con clusters induces a decreases remarkably. So, a substituted V atom must play a role sudden increase in reactivity. as an adsorption inhibitor to the rest of the Co atoms. This implies By exchanging more V atoms for Co atoms in these small that the structure of CoI2Vbecomes very rigid geometrically or clusters, the reactivity increases at first and then exhibits a sudden electronically, or both. In consideration of the fact that second decrease at m = 4 as seen in Figure 3. One cannot give a V atom substitution causes enormous enhancement of the reactivity quantitative explanation for this cluster size dependence of the again as seen in Figure 3, the first V atom which also could be reactivity because no information on geometric and electronic an active adsorption site should be shielded geometrically from structure has been obtained for these clusters, and we have to wait for more detailed information from photoelectron spectroscopy H2. The most probable structure for Co12V is either fcc or closest-packed hcp in which the V atom occupies the central and other means. position. Then, the interior V atom is surrounded by the exterior In the case of larger clusters, n 2 11, there are enough atoms 12 Co atoms. In order to explain the stability of the exterior Co to close the first shell. One atom out of the rest may occupy the atoms against adsorption, partial electron transfer from Co atoms central position which is more or less shielded from the exterior. to central V atom seems to be needed. The electron transfer causes The high reactivity of Con clusters with n ranging from n = 10 the deactivation of the exterior sites as well as the activation of the interior V atom for dissociative chemisorption of hydrogen. ( I 1 ) Katareav, V. 1.; Mamylin, B. A.; Shmikk, D.V.; Zaugin, V. A. Sou. Even though the V atom could be an active site for adsorption Phys.-Tech. Phys. (Engl. Transl.) 1972, 16, 1177.
J . Phys. Chem. 1990, 94, 2146-2154
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of H,, that V atom is guarded against contact with H, by the surrounding Co atoms and H2 cannot be chemisorbed on the V atom. They cause the decrease in reactivity of Co13by a V atom substitution, which explains the observed stability of Co12V. When more than one V atom is substituted for a Co atom in the CoI2V cluster, the reactivity is found to increase suddenly again. This comes from the fact that the second V atom is on the surface of the cluster. H2 can react with the surface V atom, to increase the reactivity for adsorption. Thus, this V atom plays the role of adsorption accelerator. When n is different from 13, but takes a value near 13, one V atom substitution generally causes a slight decrease of reactivity
and a second V atom induces the enhancement of adsorptivity. In consideration of the lack of rigid structure of clusters with n being other than 13, the observed results seem to support the particularly stable structure of Coi2V. A definite conclusion will be reached by the measurement of inner-shell electronic transition of the V atoms on Co,V, clusters. Acknowledgment. This work is supported by a Grant-in-Aid for Scientific Research on Priority Areas from the Ministry of Education, Science and Culture. S.N. expresses his gratitude for the donation of Fellowships of Japan Society for the Promotion of Science for Japanese Junior Scientists.
ARTICLES Nitrogen Molecule Actlvation by Excited States of Copper M. SPnchez-Zamora, 0. Novaro,*-tand M. E. Rdz lnstituto Mexican0 del Petrdeo, Investigacidn Bdsica de Procesos, A.P. 14-805, MPxico D.F. 07730, Mgxico (Received: September 14, 1988: In Final Form: September 14, 1989)
Ab initio molecular orbital studies that include variational (with a multiconfiguration reference state of 200 states) and perturbational (including over 3 million configurations) configuration interaction calculations were addressed to the interaction of nitrogen molecules with copper. The Cu ground state ,S and first two excited states ,P and ,D were studied as they interact in different geometrical approaches (including side-on and end-on geometries) with ground-state N2 molecules. The end-on approach is the most favorable, and even if none of the copper states are really effective in activating the triple bond of N,-which is only slightly relaxed by its interactions with copper-the 2P and ,D Cu states do attract N2, showing well-defined relative and absolute minima with respect to the energy of the isolated Cu and N2 moieties. These stable CU-NEN complexes are in good agreement with the experimental results obtained when a solid nitrogen matrix is substituted for rare-gas matrices in matrix isolation experiments for photoexcited reactions (Cu* + H2 + D2 CuH + CUD + H + D).
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1. Introduction The interaction of nitrogen molecules with copper itself has not received too much attention, in spite of recent matrix isolation studies' that will be referred to the next section. The general subject of dinitrogen attachment to metal atoms or clusters, however, has a wealth of experimental and theoretical studies concerning structural and vibrational aspects.2-40 From these studies, it is concluded in the first place that there exist three forms in which N2 attaches to the metal: end-on (Le., C,, symmetry), bent (i.e., C, symmetry), and side-on (i.e., C2, symmetry). Any geometrical arrangement for the molecule and the metal in these dinitrogen complexes is a combination of these three. End-on coordination is more common than the side-on form.2-8,11-'4.18-26,30,3k40 The bent structure complexes have near linear (end-on) geometries6 Actually structures with angles no smaller than 1 7 5 O for mononuclear complexes and larger than 17 1 O for polynuclear have been On the other hand, the spectroscopic studies show that the u-donating property of dinitrogen is the weakest among several ligands, but its *-accepting capacity is of medium magnitude, falling between that of carbon monoxide and of the organic cyanides and isocyanides.6-8 These donator-acceptor properties play a critical role in the reduction of dinitrogen ligands. Due to its limited a-transfer capacity, the back-donation of the d-orbital electronic density in the metal toward the empty 1rB orbitals of 'On sabbatical leave from Instituto de F k a UNAM.
the dinitrogen becomes the main contributing role for the strengthening of the metal-nitrogen bond. ( I ) Ozin, G. A.; Mattar, S. M. Private communication. (2) Jiqing, X.; Lijuan, X.; Xisheng, L.; Zhigi, 2. Jilin University Sci. Sin. 1981, 24, 35-45. (3) Chi-Ching, H. (Jiqing, X.) A Quantum-ChemicalTheory of Tramition Metal-Dinitrogen Complexes. In Proceedings of the 3rd International Symposium on Nitrcgen Fixation; Newton, W. E., Orme-Johnson, W. H., Eds.; University Park Press: Baltimore, 1980; Vol. I, pp 317-341. (4) Veillard, H. Nouu. J . Chim. 1978, 2, 215-224. (5) Siegbahn, P. E. M.; Blomberg, M. R. A. Chem. Phys. 1984, 87, 189-20 1. (6) Pelikin, P.; Boca, R. Coord. Chem. Rev. 1984, 55, 55-1 12. (7) Chatt, J.; Dilworth, J. R.; Richards, R. L. Chem. Reo. 1978, 78, 589-625. (8) Kobayashi, H.; Yamaguchi, M.; Yoshida. S.; Yonezawa, T. J. Mol. Catal. 1983, 22, 205-218. (9) Ziegler, T.; Rauk, A. Inorg. Chem. 1979, 18, 1755-1759. (10) Isshiki, Y.; Hirashita, N.; Oguchi, T.; Yokoyama, G.; Yamazaki, H.; Kambara, T.; Gondaira, K. I. Surf. Sci. 1981, 102, 443-462. ( I I ) Murrell, J. N.; AI-Derzi, A.; Leigh, G. J.; Guest, M. F. J . Chem. SOC., Dalton Trans. 1980, 1425. (12) Yamabe, T.; Hori, K.; Minato, T.; Fukui, K. Inorg. Chem. 1980, 19, 2154-2159. (13) Yamabe, T.; Hori, K.; Fukui, K.Inorg. Chem. 1982,21,2046-2050. (14) Messmer, R. P. Surf. Sci. 1985, 158, 40-57. (15) Anderson, A. B. Chem. Phys. Lett. 1977, 49, 550-554. (16) Wedler, G.; Steidel, G.; Borgmann, D. Surf. Sci. 1980, 100, 507-518. (17) Anderson, A.; Hoffmann, R. J . Chem. Phys. 1974, 61,4545-4559. ( I 8) Rap*. A. K. Inorg. Chem. 1984, 23, 995-996. ( 1 9) RappE, A. K. Inorg. Chem. 1986, 25, 4686-4691,
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