J. Phys. Chem. 1991,95,6833-6835
Structure and Reactivity of Blmetalllc Co,V,
+
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Cluster Ions
Atsushi Nakajima, Takashi Kishi, Tsuneyoshi Sugioka, Yasutomo Sone, and Koji Kaya* Department of Chemistry, Faculty of Science and Technology, Keio University, 3- 14- I Hiyoshi, Kohoku- ku. Yokohama 223, Japan (Received: February 4, 1991; In Final Form: April 23, 1991)
The reaction of bimetallic Co,V,+ cluster ions, generated by laser vaporization, toward hydrogen was investigated by using a fast-flow reactor. Compared with pure Con+cluster ions, the reactivity of Co,,V+ cluster ions exhibits a drastic decrease at n 1 13 similar to that of neutral Co,V, clusters. The result can be explained in terms of their geometrical structure; at n 1 13, the V atom is located at the center of the cluster and is surrounded by Co atoms.
1. Introduction
The physical and chemical properties of bulk metal alloys have long been studied in the field of catalysis. With the development of laser vaporization and molecular beam techniques,'i2 metal cluster research in gas phase has been studied and reactions of molecules with clusters have extensively been r e p ~ r t e d . ~ - ~ However, practically nothing has been studied on the metal alloy system. Recently we have developed a new method of generating bimetallic clusters from two independent laser vaporization sources,( in which two metal elements can be mixed in any proportion. This method enables us to investigate the properties of bimetallic clusters and compare them to those of the bulk metal alloys. As reported previously,( we have found a drastic change in reactivity toward H2 of cobalt (Co)-vanadium (V) bimetallic clusters, Co,,V, ( n > m ) . Especially, when one V atom is substituted in Co13to yield CoI2V,its reactivity decreases remarkably. The stability of CoI2Vhas been explained by the geometrical structure; the V atom is located at the center interacting with 12 Co atoms on the surface. Recently, the influence of the charge on Nb, and Fen clusters for the kinetics of the chemisorption reaction has been reported by Kaldor and co-workers under identical reactor conditions?** They have found that the excess positive charge on clusters has a profound influence on their reaction rate for the size of 1-30 atoms. The charge effect on the reactivity should be related to the electronic properties. While it is clear that the electronic properties of clusters should play a critical role in determining their chemisorption reactivity, the geometrical structure of clusters should also be important, as pointed out by Smalley and coworker~.~ Since the geometrical structure may control (or may be controlled by) the electronic properties at a certain size of cluster, the comparison between neutral and ionic clusters can reveal the electronic and geometrical factor for the chemisorption reaction. In this work we examine the charge effect of Co-V bimetallic clusters on the chemisorption reaction in comparison between Co,V, and Co,V,+ to obtain insight into their electronic and geometrical structures. Bimetallic clusters are composed of two different atoms that have different ionization potentials (IP). Therefore, the positive charge of the bimetallic cluster cations should be localized at a constituted atom or a part of clusters, where its IP is the lowest in the cluster. The localization of the ~~
~~
(1) Powers, D. E.; Hansen, S.G.; Geusic, M.E.; Pulu, A. C.; Hopkins. J.
B.; Dietz, T. 0.;Duncan, M.A,; Langridge-Smith, P. R. P.; Smalley, R. E. J. Phys. Chcm. 1982,86,2556. (2) Bondyky. V. E.; English, J. H. J . Chcm. fhys. 1981, 74, 6978. (3) Elkind, J. L.; Weis, F. D.; Alford, J. M.; Laaksonen, R. T.; Smalley, R. E. J . Chem. Phys. 1908, 88, 5215, and references cited therein. (4) Fayet, P.; Kaldor, A.; Cox, D. M. J . Chcm. Phys. 1990,92,254. and references cited therein. (5) Parks, E. K.; Nieman, G. C.; Pobo, L. G.; Riley, S. J. J . Chem. Phys. 1988,88, 6260, and references cited therein. (6) Nonose, S.;Sone, Y.;Onodera, K.; Sudo, S.;Kaya, K. J . Phys. Chem. 1990. 94. 2 7. M - _, _ . ... (7) Zakin, M. R.; Brickman. R. 0.;Cox, D. M.; Kaldor, A. J. Chem. Phys. 1988,88, 3555. (8) Zolrin, M. R.; Brickman, R. 0.;Cox, D. M.; Kaldor, A. J. Chem. Phys. 1988,88,6605.
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positive charge should strongly be related to the chemisorption reaction; therefore, we discuss the charge effect on reactivity in Co-V bimetallic clusters and their geometrical structure. 2. Experimental Section
Details of the experimental apparatus have been reported previ0usly.6,~ To generate bimetallic cluster ions containing cobalt (Co) and vanadium (V) atoms, two target rods of Co (Nilaco Corp., 99.998%) and V (Nilaco Corp., 99.8%) were laser-vaporized in a He camer gas (5-10 atm, 99.9999%); two rods were vaporized by two pulsed YAG lasers (532 nm, Quantaray DCR-2A and DCR-2), respectively. Each laser was focused on the surface of the rotating rod through a lens (f=60 cm). After thermalization with He, cluster ions were injected into a fast-flow reactor,I0into which a sample gas of H2was mixed with He (5-10% H2in He). Mass analysis of the reaction products was performed by a time of flight (TOF) mass spectrometer equipped with reflectron. The cluster ions in the beam were extracted into the TOF tube by bias (-2 kV, rise time = 100 ns) by applying a fast high-voltage pulse (Velonex Model 350 and V1742). The ions were detected by a dual multichannel plate, and the signal was amplified and stored in a transient oscilloscope (LeCroy 9400, 100-MHz sampling rate) coupled with a microcomputer (NEC PC-9801). The mass resolution, m/Am, was typically 300-400. The repetition rate was 7-8 Hz, and a mass spectrum was obtained by averaging typically 700-800 outputs.
3. Results and Discussion Figure 1 shows the TOF mass spectra of Co,,V,+ cluster ions in the range n = 2-16. In the TOF mass spectrum, the bimetallic cluster ions, composed of a specific number of atoms, can be observed in groups. Since the mass units of Co and V are 59 and 51 amu, respectively, an exchange of a Co atom for a V atom leads to a decrease of 8 amu in the cluster. That is, the mass peak is shifted to a lower mass by the exchange. Figure 2 shows the enlarged TOF mass spectra of Co,V,+ at n = 4 and 5 before and after an adsorption reaction with hydrogen molecules, H2. When a cluster ion reacts with H2 (5-10% H2 in He) through the fast-flow reactor, the cluster ion makes a H2adduct ion. As shown in Figure 2b, the adduct can be observed in the spectrum, and the cluster ions almost quantitatively resulted in their adduct ions with the reactant gas. The reactivity of the cluster ions toward Hz was estimated from the reactivity index R, defined elsewhere." As reported previously,iz the relative reactivity of pure Co cluster ions, Co,', exhibits a local small maximum at n = 5 and a broad maximum around n = 15 in the H2chemisorption reaction. The change in reactivity of ( n = 2-19, m = 0-2) is shown in bimetallic clusters Co,,V,+ Figure 3. The relative reactivity was obtained by averaging several (9) Nakajima, A,; Kishi, T.; Sugioka, T.; Sone, Y.;Kaya, K. Chem. fhys.
-. .
Left. 1991. 297. ---177. --
(IO) Geusic, M. E.; Mom. M. D.; O'Brien, S.C.; Smalley. R. E. Rw. Sci.
Instrum. 1985, 56, 2123. (1 1) Morse, M. D.; Geusic, M. E.; Heath, J. R.; Smalley, R. E. J . Chcm.
Phys. 1985.83, 2293. (12) Nakajima. A.; Kishi, T.; Sone, Y.;Nonose, S.;Kaya, K. 2.f h y s . D 1991, 19, 385.
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6834 The Journal of Physical Chemistry, Vol. 95, No.18, 1991
3 1.2
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500 700 MASS NUMBER (dz)
100
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Figure 1. Time of flight mass spectra of ComV,+ (n > m ) cluster ions (for n = 2-14). Peaks of the cluster ions are labeled according to the
notation n-m,denoting the number of cobalt atoms (n)and that of the vanadium atoms (m).
4
CcnVdI+
260 280 MASS NUMBER (dz)
240
300
Figwe 2. Time of flight mass spectra of Co,V,+ (n > m) cluster ions at n = 4 and 5 mixed with (a) only He as a reference and (b) 50 Torr of H2seeded in 1 atm of He. Peab of the cluster ions are labeled according to the notation n-m-I, denoting the number of cobalt atoms (n),that of vanadium atoms (m), and that of hydrogen atoms (0. Since hydrogen atoms are adsorbed onto the cluster ions in a unit of a Hz molecule, only even numbers of hydrogen atoms can be observed in the adduct ions.
sets of data. The uncertainty of the reactivity of Co,lV+ and CoP2V2+was estimated to be about k0.05as one standard deviation on the basis of the reactivity of Con+. 3.1, Change in Reactivity of Small Cluster Size ( a = 2-10). As shown in parts a and b of Figure 3, the change in reactivity of Co,V,+ is relatively small in the small cluster size, except for n = 4 and 5. As reported previously,6 neutral Co-V,,, clusters become reactive by an exchange of Co atom for V atom in small cluster size (n = 5-8). We have explained the result by an analogy of reactions on metal surfaces; the heat of adsorption of hydrogen molecules on solid vanadium surfaces is much larger than that of cobalt, and in neutral bimetallic Co,V,,, clusters the V atom works as an active site for adsorption reaction. Jn the case of bimetallic Co,,,,V,+ cluster ions, however, the positive charge seriously affects the reactivity. Since the ionization potential energy of the V atom (6.74 eV) is smaller than that of the Co atom (7.86 eV), the positive charge is localized on the V atom in small Co,V,,,+ cluster ions. In our experimental setup, the clustering process is that either atoms or atomic ions are mixed in the He carrier gas and the bimetallic clusters grow homogeneously (successive nucleation from atoms).I3 Since it would be difficult for either element to form a unit excluding the other element in small bimetallic clusters, the localization of the positive charge depends on their ionization potentials of atoms. In the adsorption reaction of hydrogen molecules, electron donation from a metal atom (or cluster) to an adsorbing molecule is known to be important.'** When the V atom exists as positively charged (13),Sone, Y.;Horhino, K.; Naganuma, T.; Nakajima, A,; Kaya, K.
Unpublished rwulta.
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0.6
0.4
0.4
g 0.2
0.2
4 0.0
0.0 I
* n=15 11-16
+ n.17
1 I
I
* n-18 n=19 1 --t
.
co: co,,v' C0,J; co:, CO,lV' co,.*v; Figure 3. Relative reactivity in the series of C o , V + (n = 2-19, m = 0-2) cluster ions toward Hz: (a) n = 2-6; (b) n = 7-10; (c) n = 11-14; (d) n = 15-19. In C o,' and Cos+, the substitution of a Co atom for a V atom leads to a large increase in reactivity, whereas in Con+(n 1 13) the substitution leads to a large decrease in reactivity.
0
4
220
j 0.8
V+ ion, the electron density on the V atom decreases and the V+ atom cannot work as an active site. At relatively larger cluster size of n = 10, the reactivity somewhat increases by the exchange of a Co atom for a V atom. At a large n, a unit of several Co atoms can form partially in the cluster. The ionization potential energy of CO,cluster14l5becomes smaller than that of the V atom (at n 2 3), and then the positive charge is now localized on parts of the Conclusters. The V atom can then exist as a neutral atom and work as an active site, similar to neutral CO,,,,~,,, clusters. On the other hand, the reactivity of Co3V+and CO~V+ is high compared to that of C O ~and + Cos+.As pointed out previously,12 the reactivity of positively charged cluster ions consisting of a transition metal, for example, Nb,,+,' Fe,,+? and V,,+,16 is generally high at the cluster size of n = 4 and 5. In the chemisorption reaction of Con+toward H2, the reactivity at n = 4 and 5 is not distinctly high, but the reactivity toward N2,CH,, and C2H, is high as well as that of Cold+and C015+.12*17 High reactivity at n = 4 and 5, therefore, seemsto be a common feature in positively charged clusters. The common feature implies that their geometric structure, for example, trigonal (bi)pyramid, governs the high reactivity for the adsorption reaction. Such a geometric structure is likely to lead to the activation in the adsorption reaction of C%V+ and Co,V+, but one cannot give a quantitative explanation for reactivity at n = 4 and 5 . 3.2. Change in Reactivity of Large Cluster Size (a = 11-19). Parts c and d of Figure 3 show the changes in reactivity of Co,V,,+ cluster ion at large cluster size (n = 11-19). Compared with pure Con+cluster ion, the reactivity of CoPIV+cluster ion distinctly exhibits a decrease at n 2 13, though it exhibits a slight increase at n = 11 and 12. The changes in reactivity at n 1 13 are quite similar; the first V atom substitution leads to a decrease in reactivity of pure Con+ion, whereas the second V atom substitution leads to an increase in reactivity of cluster ion. This implies that a fmt-substituted V atom plays a role as an adsorption inhibitor and the second V atom as its accelerator. This phenomenon is almost the same for neutral Co,,,,V,,, clusters,6 and the reactivity change in Co13+,CO,~V+, and CollV2+can be exd below. plained qualitatively by the geometric structure, as d Furthermore, the similarity between charged Co,V,,,+ and neutral Co,V,,, also suggests that the reactivity changes can be (14) (15) (16)
Parks, E.K.;Mots, T. D.;Riley, S.J. J. Chem. Phys. 1990,92,3813. Yang, S.;Knickelbein, M.B. J . Chem. Phys. 1990,93, 1533. akin, M.R.;Cox, D. M.;Brickman, R. 0.;Kaldor, A. J . Phys.
. -.
Chem. 1989. 93.6823. - . ---(I7)Bhcat, P. J.; Pettiette, C. L.; Yang, S.;Zheng, L A ;Craycraft, M. J.; Smalley, R. E. J. Chem. Phys. 1986, 85, 4747.
J . Phys. Chem. 1991,95,6835-6842 attributed to the geometric structure, rather than to the electronic structure. In the casc of large cluster ions, n 1 11, there are enough atoms to close the first shell. One atom of the rest may occupy the central position which is shielded from the exterior. In particular, at n L 13 the reactivity decreases remarkably as shown in Figure 3c. The remarkable decrease in reactivity indicates that the first shell is rigidly constructed at n = 13 for the first time. Then, the most probable structure is either fcc or close-packed hcp in which the V atom occupies the central position. When one more V atom is substituted for a Co atom in the Co12V+cluster, the reactivity is found to increase suddenly. This
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reactivity change can be also explained by the geometrical structure as mentioned above. Since the second V atom must be located on the surface of the cluster ion, H2 can react with the surface V atom. Further investigation on the IP measurement is in progress in our group to examine the relation between IP and reactivity. Acknowledgment. We are grateful to Dr. S. Nonose (the University of Tokyo) for his contribution of the first stage of the study and the stimulating discussion. We acknowledge financial support of a Grant-in-Aid for Scientific Research for Priority Area by the Ministry of Education.
Electronic Spectra of PMhalonltriie Isolated in an Argon Matrix Bryce E. Wilhuus~n,**~ Tbomas C. VanCott,ks Janna L. Rose,Lll Andreas Schrimpf,kl Marceli Koralewski,k# and Paul N. Schatz*** Chemistry Department, University of Virginia, Charlottesville, Virginia 22901, and Chemistry Department, University of Canterbury, Christchurch I . New Zealand (Received: March 1, 1991)
The absorption, emission, and magnetic circular dichroism of phthalonitrile (1,2-dicyanobenzene) isolated in an argon matrix are reported between 19000and 82000 cm-'.These spectra are interpreted in terms of parent transitions involving the benzene ring and cyano substituents. The matrix isolation technique affords well-resolved vibrational structure, which permits the determination of vibrational frequencies for the two lowest lying singlet excited states. Contrary to the conclusions of earlier workers, the spin-consewing transitions are Franck-Condon allowed.
Introdwtioa Phthalonitrile (Pn, 1.2-dicyanobenzene) is a relatively simple derivative of the prototypical aromatic molecule benzene, and it is of interest to understand how the cyano substituents influence the electronic properties of the parent molecule. In this paper we report absorption, emission, and magnetic circular dichroism (MCD) spectra of Pn isolated in argon matrices (Pn/Ar) over the range 19000-82OOO cm-'. There have in fact been few reports of the electronic spectra of Pn. Takei and Kanda' reported the phosphorescence, at 90 K, of Pn dissolved in ethanol and cyclohexane, and the absorption spectra of Pn in the gas phase, in ethanol, and in cyclohexane Over the range 34OOCb38000 cm-I. Barraclough et ala2reported the vapor-phase absorption spectrum over the same range. Much more recently, Toselli et ala3measured the absorption spectra of Pn in various solvents between 32 000 and 36 OOO cm-I. All of these studies concemed only the lowest energy valence transitions of Pn,and were obtained under conditions that yield relatively broad bands. In contrast, we are able to obtain spectra from the visible into the vacuum ultraviolet region, and by using matrix-isolation techniques, we are able to obtain spectra that are very much better resolved. We are thus able to make detailed vibronic assignments of most transitions and can determine the degree of "allowedness'' of the lower singlet excited states. Experimentnl Section Pn was obtained from Eastman Kodak and used without further purification. Matrices were prepared by subliming Pn from a quartz Knudsen cell and codepositing the vapor with a large excess University of Canterbury. 'University of Virginia. I h n t addrsu: Walter R e d Army Institute of Research Department of Rctroviral Research, Suite 200, 13 Taft Court, Rockville, MD 20850. 'Rewnt addrsu: Rayovac Corporation, Madison, WI. h n t addras: Fachkreich Phyiik der Phillip Univeraitat, D-3550 Marburg, Federal Republic of Germany. # Institute of Physics, Adam Mickiewicz University, Grunwaldzka 6, 60-780 Poznan, Poland.
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of argon onto a cryogenically cooled (typically 5-10 K) LiF or c-cut sapphire window. The matrices were not annelaed before data collection. For emission studies, the samples were cooled by using a closed-cycle helium refrigerator (CTI-Cryogenics) operating at 10 K. Spectra were measured by using a SLM spectrometer at a resolution of 0.5 nm. Absorption spectra of the same samples were obtained with a CARY-2145 spectrophotometer at a resolution of 0.05 nm. Samples for the measurement of MCD were prepared in the bore of a superconductingmagnet (Oxford Instruments). Sample temperature, magnetic field, and spectral resolution were, respectively, - 5 K, 3.3 T, and 0.4 nm. MCD and absorption spectra below 47000 cm-' were obtained simultaneously by using a spectrometer that has been described previously.4 Spectra in the vacuum ultraviolet region were measured at the Synchrotron Radiation Center of the University of Wisconsin by using the 1-GeV electron storage ring ('Aladdin") and a 1-m AI SeyaNamioka monochromator with a 1200 lines/mm AI grating overcoated with MgF2. All spectra were recorded digitally and analyzed by computer. Calibrations of the absorbance and MCD were achieved by reference to the spectra of a standard solution of d-10-camphorsulfonic acid? Depolarization of light by our matrix samples was determined by comparing the CD spectrum of the standard placed after the sample with that obtained in the absence of the matrix and was found to be negligible.
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Results
The absorption, emission, and MCD spectra over the range 19OOO-82000 cm-' are reproduced in Figure 1. The spectra at ( I ) Takei, K.; Kanda, Y. Speclrochim. Acta 1962, 18, 1201-16. (2) Barraclough, C. 0 . ; Bisaett, H.; Pitman, P.; Thistlewaite, P. J. Aust. J . Chem. 1977,30, 753-65. (3) Toselli, N. B.; Anunziota, J. D.; Silber. J. J. Spcrrochlm. Acta 3988, 4 4 4 157-64. (4) Rose, J.; Smith, D.; Williamson, 8.E.; Schatz, P. N.; O'Brien, M. C. M.J . Phys. Chcm. 1986, 90,2608-15. (5) Chen, 0. C.; Yang, J. T. Anal. k t r . 1977, 10, 1195-207.
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