J. Phys. Chem. 1994,98, 9350-9353
9350
Gas Phase Antimony/Magnesium/Oxygen Clusterst H. T. Deng, Y. Okada, M. Foltin, and A. W. Castleman, Jr.’ Department of Chemistry, 152 Davey Laboratory, The Pennsylvania State University, University Park, Pennsylvania 16802 Received: April 29, 1994@
Antimony/magnesium/oxygen clusters are produced by a gas aggregation source, in which a mixture of antimony and magnesium is vaporized and reacted with N2O introduced in helium carrier gas. The resulting product distribution is detected by a time-of-flight mass spectrometer following ionization with a KrF excimer laser. Four types of cluster products are observed: Sb,+, SbxMgyOZ+,Sb,Mgy+, and MgyO,+. The mass spectral intensity distributions display enhanced abundances for MgzO+, S b 2 ~ M g 3 0 + Sbl4Mg20+, , Sb4Mg+, SbsMg2+, and SbsMg2+. The experimental observation of Mg20+ and Mg30+ shows that the suboxides of group 2 are stable species, consistent with theoretical predictions. The binding abilities of antimony clusters tomagnesium and magnesium oxides are found to be dependent on cluster size. When the number of antimony atoms in the clusters is smaller than 6, SbxMgyO:+ are the main products dominating the mass distribution. On the other hand, when the cluster size of Sb, is larger than 6, only S b / M g alloy clusters are observed. The unusual stabilities of Sbz+Mg30+ and Sbl+MgzO+ clusters are evidently due to the formation of covalent bonds between S b and M g atoms. In Sb4Mg+, SbsMg2+, and SbaMgz+ alloy clusters, however, the M g atom donates two electrons to the skeleton of the S b clusters in order to satisfy Wade’s rules. The structures of these stable clusters can be predicted by the polyhedral skeletal electronic pair theory.
Introduction The abundance distributions of clusters obtained through mass spectrometry directly reflect their stabilities and easeof ionization and are generally related to their electronic and/or geometric structures. The enhanced abundances are referred to as magic numbers and in the case of metal and metal alloy clusters are sometimes well explained by the jellium (electron) shell In other cases these metal clusters, including clusters of metal oxides and other metal compounds, are accounted for by invoking the polyhedral skeletal electronic pair theory (PSEPT)3-5and/or geometric models.69’ For example, the jellium model is successfully applied to explain the magic numbers of alkali metals, coinage metals, and group 13 metal clusters. However, the electronic structures no longer dominate the mass spectra distributions for group 14 and 15 metal clusters due to their large number of valence electrons, and PSEPT has been proposed to explain the magic numbers. For lead clusters, their stabilities are consistent with considerations of their expected geometric structures. Studies of alloy clusters prepared by substituting various atoms into the clusters and examining their stability provide important information about the electronic and geometric properties of these cluster systems. In the case of alloys among metals which individually display substantial free electron behavior,s the observed magic numbers still obey the jellium shell model. For Cu/Pb alloy clusters, either electronic properties or geometric structures can account for their observed stabilities and mass spectral abundances, depending on the relative compositions of Cu and Pb. Previously, alloy clusters containing group 14 and 15 metals have been studied in detail. Cs/Sn and Cs/Pb,lsl3 Na/Bi,I4 and Na/Sbls clusters have been reported to have magic numbers for stoichiometries corresponding to known Zintl ions of these post-transition metals. It is suggested that the alkali metal atoms merely donate electrons to alloy clusters and have no influence on their geometric structures. Studies of Sn/Bi and Pb/Sb,I6J7 Sn/As,I7 and Bi/Sbls systems show that magic numbers arise for species that are isoelectronic with stable analogues of known Zintl ions in the condensed phase, and both atoms participate in forming the geometric structures of those Dedicated to C. N. R. Rao on the occasion of his 60th birthday. Abstract published in Advance ACS Absfracts, August 15, 1994.
0022-3654/94/2098-9350$04.50/0
clusters. The same kind of Zintl ions are also found in alloy clusters of group 13 and 15,Ig group 14 and 16,20and group 15 and 16.20 As for Cu/Sb and Cu/Bi alloy clusters,2l two types of magic numbers are observed and consistent with the jellium model and PSEPT, respectively. Heretofore, no studies have been reported on the alloy clusters of group 2 and group 15. Group 2 metal atoms have an electronic closed shell structure (nsz), and it has not been known whether this may have some different influence on the stability of the alloy clusters compared to those comprised of metals of other groups. One objective of the present work is to examine the mass spectral distributions of Mg/Sb alloy clusters and to see whether the jellium model or PSEPT can be applied to explain the magic numbers of this system. In the past few years, numerous studies have been done on oxide c l ~ s t e r s . ~ ~Martin10J2.28-30 -2~ studied the CsO and CaO, BaO clusters and calculated the structures of C S ( C S ~ O ) ~ + clusters.IO A1,0, clusters were studied by using CID and other meth0ds.3393~ The reactivities of small transition metal oxide c l u s t e r ~ 3 and ~ 3 ~mixed oxide clusters39have also been investigated. MgO clusters have been studied by our group in detai1.4M3 Detailed ab initio calculations on Li30, Li40, Na30, and N a 4 0 suggest that M 3 0 molecules prefer a D3h structure while M 4 0 prefers a Td symmetry structure; the existence of these species has been proven by experimental ob~ervation.~1-32-~7-49 These suboxides have unusual stoichiometries which suggest violation of the octet rule. Due to the similar properties of magnesium and sodium, the structure and binding energies of magnesium suboxides have also been of interest. They also have been studied through a b initio calculationsg which have shown that Mg30 and Mg40 are stable species and prefer to have D3h and DMsymmetries, respectively. However, to the best of our knowledge, there is no experimental evidence for the existence of Mg30 and Mg40. Attempting to observe stable hypervalent magnesium oxides comprised another objective of the present investigation. The third objective of the work is to examine the stabilities of Sb-Mg-0 clusters. Studies of metal oxide clusters have been very important because many important catalysts consist of metal particles dispersed on oxide s ~ r f a c e s . ~ ~There *s are strong interactions between the metal particles and the oxide particles 0 1994 American Chemical Society
Gas Phase Antimony/Magnesium/Oxygen Clusters
The Journal of Physical Chemistry, Vol. 98, No. 37, 1994 9351
in these systems.46 Studies of metal-oxide clusters will be helpful in understanding the structures and binding energies of these systems. Compared with antimony, which cannot be oxidized at ambient temperature, magnesium can readily form oxides. Through analysis of the Sb/Mg/O cluster distributions, we can find out whether the cluster size influences the binding ability of antimony clusters with magnesium oxides. Herein we report recent results on the system comprised of Sb/Mg/O clusters and address the findings to the objectives mentioned above.
Experimental Section Details of the apparatus are described elsewhere.' Briefly, a gas aggregation sources in used in the present work. A mixture of antimony and magnesium is vaporized and is then reacted with NzO, which is carried in helium gas. Inside a liquid nitrogencooled source chamber, the S b and Mg metal mixture is resistively heated in a boron-nitridecrucible to 750-850 O C and evaporated into a flow of He gas (5.0 slm) with 15 sccm of NzO added. A 0.062 in. hole in the apex of a cone serves to sample a small fraction of the flow, and most of the gas in pumped out in front of the cone. The clusters are then introduced into a low-pressure ionization region between electrostatic grids of a time-of-flight mass spectrometer. The neutral clusters are photoionized by a slightly focused beam of light from a KrF excimer laser (4.98 eV). The laser beam (pulse width 20 ns) is focused with a convex lens to obtain 170-330 mJ/(cm2 pulse) in the ionization region, enabling detection of the desired cluster ions. The ionized clusters are accelerated with double stage acceleration grids and detected by a microchannel plate particle multiplier (MCP) after traversing a 186 cm differentially pumped field-free region. The ion signals from the MCP are accumulated using a 100 MHz transient digitizer and the time-of-flight spectra are transferred to a microcomputer for analysis.
Results and Discussion Before studying the metal-oxide cluster system, pure S b metal and Mg metal were examined individually to probe their distributions and magic numbers. Under the present experimental conditions, no pure Mg clusters are observed beyond Mg2+, even without addition of N20 reactant gas. N20 can greatly enhance the intensities of Mg and Mg/O clusters. The peak intensity observed for MgzO+ clusters is unusually high, in agreement with the observations by Ziemann et alaw3 As for S b clusters, magic numbers are observed at n = 5 and 7,which have 4n + 4 and 4n 6 skeletal electrons corresponding to nido and arachno structures,50 respectively. In contrast to magnesium, antimony does not react with N20 under the conditions of the present experiments. In order to investigate cluster distributions of SbxMgyO+,pure S b and Mg was mixed at a 1 :1 ratio. Without NzO, the intensities of MgO and S b clusters are very small, and no Sb,Mgy+ (except SbMg) or Sb,MgyO+ clusters are observed. After 15 sccm of N2O is introduced into the flow tube, reaction products are observed; the recorded mass spectra are displayed in Figures 1-3 for different mass regions. It is found that further increasing the concentration of N20 to 600 sccm has no significant impact on the resulting mass distribution. From Figure 1, both Mg30+ and MgzO+ are observed, and the peak intensity of MgzO+ is unusually high. This is the first observation of suboxides of group 2 metals of which we are aware, showing that the Mg30+and MgzO+ are stable species in the gas phase. Although only cations can be detected in the present experiment, little fragmentation is found in these type of strongly bonded systems. Furthermore, considering the low pressure in the region after ionization, collision-induced dissociation is not expected to be very significant. Hence, it is reasonable to imply
450
560
Figure 1.
670 780 890 FLIGHT TIME ( m i c r o s e c o n d s )
IO00
Time-of-flightof mass spectrum of Sb/Mg/O clusters for the
mass range from 20 to 100.
1380
1100
Figure 2.
1660 I940 2220 FLIGHT TIME ( m i c r o s e c o n d s )
2500
Time-of-flightof mass spectrum of Sb/Mg/O clusters for the
mass range from 200 to 600. The peak marked by * is due to an impurity.
- -K
IX
+
2500
2700
2900 3100 3300 FLIGHT TIME ( m i c r o s e c o n d s )
Figure 3. Time-of-flightof mass spectrum of Sb/Mg/O mass range from 600 to 1200.
3500
clusters for the
that the neutral Mg30 and MgzO are also stable species which can be ionized and lead to the observed species Mg30+ and MgzO+. Magnesium-magnesium bonding contributes significantly to the stability of M g 3 0 according to a b initio calc~lations.~ In Figures 2 and 3,three types of clusters are observed, namely, Sb,+, Sb,Mgy+, and Sb,Mg,O,+. Allof theobservedcombinations between Sb, and Mg,O, are listed in Table 1. It is clear that (i) all Sb, clusters can combine with one or two Mg atoms to form
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The Journal of Physical Chemistry, Vol. 98, No. 37, 1994
TABLE 1: Experimentally Observed Combinations of Sb, and M g P l Clusters' Sb Sb2 Sb3 Sb4 Sbs Sba Sb, Sbs Sb9 Mg + + + + + + + + + + Mg2 + + + +++++ + + + + + + Mg3 + + + + Mg4 MgO + + + Mg20 ++ ++ ++ + Mg,O + ++ ++ ++ + + + Mg3O2 + Mg4O + + ++ Mg02 + + a + = combining clusters; ++ = combining clusters exhibiting magic numbers. Sbl-9Mgl-2 cluster ions (except for SbMg2 clusters); (ii) when the size of the S b clusters is smaller than six, Sb,MgyO, are observed in the mass spectrum but no Sbl-5Mg3-4 clusters are present; (iii) when the size of S b clusters is larger than five, all S b clusters can bind one to four Mg atoms but no Sb,MgyO, clusters are formed, which indicates that Sb,Mgy clusters are not reactive with N2O. Therefore, it is reasonable to suggest that the Sb,MgyOz+ clusters come from the combination of Sb, with MgyOz clusters. The experimental findings indicate that the binding abilities of Sb, clusters to MgyO, clusters changes at a duster size of six. It is known that the reactivities of clusters vary with size in many cluster systems. The above results show that small Sb, ( x < 6 ) clusters facilely bind to suboxides, while the large clusters mainly accept the electrons from magnesium. The magic numbers in Sb,MgyO+ clusters appear in the form of Sb24Mg30+ and Sb14MgzO+. When x = 5 , SbsMg40+ clusters have an unusually high peak intensity. It seems that Mg2O and M g 3 0 are reactive and may combine with the small antimony clusters. Ab initio calculations show that stable geometric structures of MgzO and M g 3 0 are linear and planar structures, respectively, with the oxygen atom in the center. The partial charges are 0.9 and 0.66 for Mg atoms in Mg2O and Mg30, re~pectively.~ According to these structures, the oxygen atom in Mg30+and MgzO+ clusters draws the electrons from the Mg atoms and the Mg atoms still have one electron left which can be used to share with other atoms. In the Sb, clusters with small size, nonbonding electrons mainly localize on each S b atom, causing these S b atoms to have dangling bonds and to form covalent bonds with polarized Mg atoms. This is the reason that Sb,MgyO+ clusters have unusually high intensities. However, nonbonding electrons in S b clusters have higher delocalization with an increase of cluster size. Therefore, large S b clusters mainly can accept electrons from Mg rather than combine with Mg atoms or Mg suboxides. Considering the influence of geometric structure, Sb3+ clusters have a planar triangle and Sb4+ clusters have a tetrahedral s t r u ~ t u r e . They ~ ~ all have a triangular plane which can combine three Mg atoms of Mg30+ clusters, so the stable geometric structures also favor the formation of Sb34Mg30+ clusters in gas phase. The same considerations can be applied to SbsMg,O+ clusters. Unlike Na10 and Li40, which can exist in the gas phase, Mg40+ is not observed in the mass distribution and the calculated structure of M g 4 0 has D2d symmetry. The structure of Sb5+ is theoreticallys1suggested to have a square pyramid structure; therefore, the covalent bonds are formed between S b atoms and Mg atoms of both clusters leading to the formation of Sb5Mg40. From Table 1,it is found that the number of Mg atoms increases in Sb,Mgy+ clusters with an increase in the size of S b clusters. Compared to the Mg atom, the S b atom is larger, more polarizable, forms stable dimers, and easily forms clusters. The space occupied by the Sbclusters determines the number of Mg atoms which can be taken. This reasoning suggests that the larger the cluster size, the larger the backbone and the more Mg atoms that can be accommodated. The magic numbers for Sb,Mgy+ clusters appear
Deng et al. a t Sb4Mg+, SbsMg2+, and SbsMgz+. In the mass spectral distribution, Sbs' is not observed, although Sb6+ clusters have the expected number of electrons which satisfy Wade's rules. The reason that SbdMg+ is chosen to be the magic number is that the intensity of Sb.+Mg+is unusually high compared to the other Sb,Mg peaks. If the Mg atom donates two electrons to this system, the valence electrons can be counted as 5x 2y. This yields 22 electrons for the Sb4Mg neutral cluster. This number does not correspond to a jellium shell closing but does correspond to the known Zintl ion Sb42-, which has 4n + 6 skeletal electrons and an anachno structure. When one Mg atom attaches to this anachno structure and takes a vertex position, a square pyramid structure can be formed and represented as the structure of SbdMg. The same ideas are applied to the SbsMgz+ cationic and Sb6Mgz neutral clusters. The SbsMgz+ cation can be considered as an isoelectronic species to Sbs3-, which has 28 (4n + 8) electrons and the hypo structure. When two Mg atoms attach to Sb5, a pentagonal pyramid structure can be suggested for Sb5Mgz+. SbsMg2 neutral clusters have 34 electrons, corresponding to Sbs3anions which have 4n 10 skeletal electrons and also can be represented as a deltahedral structure. Finally, in passing it is worth noting that 34 electrons is a jellium shell closing for the WoodsSaxon potential.52
+
+
Conclusions Four kinds of clusters MgyOp+,Sb,+, Sb,MgyO+ and Sb,Mgy+ are formed by vaporizing a Sb-Mg mixture in a gas aggregation source and allowing it to undergo oxidation. The observation of Mg30+and MgzO+ in the mass spectra proves that the suboxides of group 2 are stable species. This provides experimental confirmation of earlier theoretical predictions. In the case of mixed metal oxide systems, we have observed that certain ones are especially prominent. Generally, Sbclusters are found to facilely combine with the Mg suboxides and Mg. Thesizeof the Sb,clusters directly determines the binding abilities to Mg or Mg-0 clusters. Magic numbers for Sb,MgyO+ clusters are observed in SbzdMg30+ and SblAMg20+,which are related to the formation of covalent bonds between S b and Mg atoms. Finally, as far as Sb-Mg interactions are concerned, certain species are found to be particularly stable. The Mg atom donates two electrons to form very stable Sb4Mg+, SbsMg2+, and Sb6MgZ+ clusters. The total number of electrons in these clusters satisfies Wade's rules, and the cluster structures can be predicted by the polyhedral skeletal electron pair theory which accounts for their appearance as magic numbers in the experiments.
Acknowledgment. The authors thank Dr. Y. Yamada and Dr. S. Wei for helpful discussions. Financial support by the Division of Chemical Sciences, Office of Basic Energy Sciences, Office of Energy Research of the U S . Department of Energy, Grant DE-FG02-92ER14258, is gratefully acknowledged. References and Notes (1) Knight, W. D.; Clemenger, K.; de Heer, W.; Saunders, W.; Chou, M.; Cohen, M. Phys. Rev.Lett. 1984, 52, 2141. (2) Cohen, M. L.; Chou, M. Y.; Knight, W. D.; de Heer, W. A. J . Phys. Chem. 1987, 91, 3141. (3) Mingos, D. M. P.; Slee, T.; Zhenyang, L. Chem. Rev. 1990, 90, 83. (4) Corbett, J. D. Chem. Rev. 1985,85, 383. (5) Corbett, J. D.Prog. Inorg. Chem. 1976, 21, 129. (6) LaiHing, K.; Wheeler, R. G.; Wilson, W. L.; Duncan, M. A. J. Chem. Phys. 1987,87, 3401. (7) Farley, R. W.; Ziemann, P.; Castleman, A. W., Jr. 2.Phys. D 1989, 14, 353. ( 8 ) Yamada, Y.; Castleman, A. W., Jr. J . Chem. Phys. 1992,97,4543. (9) Boldyrev, A. I.; Shamovsky, I. L.; von R. Schleyer, P. J . Am. Chem. SOC.1992, 114, 6469. (10) Martin, T. P. J . Chem. Phys. 1984, 81, 4426. ( 1 1 ) Martin. T. P. J . Chem. Phvs. 1985. 83. 78. ( l 2 j Martin; T. P. Angew. Chem., Int. Ed. Engl. 1986, 25, 197. (13) Martin, T. P. 2.Phys. D 1986, 3, 211.
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