J . Phys. Chem. 1992, 96, 6931-6937 intensities of these two peaks with caprolactam concentration show that they are most probably due to the monomer and the cyclic dimer.
Acknowledgment. We thank Mr. Raj Punwaney for technical assistance with this project. We are grateful to the Du Pont Company for their support of this work. We also thank Professor T. G. Spiro for useful comments and suggestions in the preparation of this manuscript.
References and Notes (1) Robin, M. B.; Bovey, F. A.; Basch, H. In The Chemistry of Amides; Zabicky, J., Ed.; Interscience Publishers: New York, 1970. (2) Kaya, K.; Nagakura, S. Theor. Chim. Acta 1967, 7, 117. (3) Kaya, K.; Nagakura, S. Theor. Chim. Acta 1967, 7, 124. (4) Nielsen, E. B.; Schellman, J. A. J . Phys. Chem. 1967, 71, 2297. (5) Harada, I.; Sugawara, Y.; Matsuura, H. J . Raman Spectrosc. 1975,
4, 91. (6) Sugawara, Y.; Harada, I.; Matsuura, H.; Shimanouchi, T. Biopolymers 1978, 17, 1405. (7) Mayne, L. C.; Ziegler, L. D.; Hudson, B. J . Phys. Chem. 1985,89, 3395. ( 8 ) Dudik, J. M.; Johnson, C. R.; Asher, S . A. J. Phys. Chem. 1985,89, 3805. (9) Song, S.;Asher, S.A.; Krimm, S.;Bandekar, J. J . Am. Chem. SOC. 1988, 110, 8547.
6931
(10) Krimm, S.;Song, S.;Asher, S.A. J. Am. Chem. Soc. 1989,111,4290. (11) Wang, Y.; Purrello, R.; Spiro, T. G. J . Am. Chem. Soc. 1989, 1 1 1 , 8274. (12) Mayne, L. C.; Hudson, B. J . Phys. Chem. 1991, 95, 2962. (13) Song, S.;Asher, S.A.; Krimm, S.;Shaw, K. D. J . Am. Chem. Soc. 1991, 113, 1155. (14) Wang, Y.; Purrello, R.; Jordan, T.; Spiro, T. J. Am. Chem. Soc. 1991, 113.6359. (15) Wang, Y.; Purrello, R.; Georgiou, S.;Spiro, T. J. Am. Chem. SOC. 1991, 113, 6368. (16) Lord, R. C.; Porro, T. J. Z . Elektrochem. 1960.64, 672. (17) Franzen, J. S.;Stephens, R. E. Biochemistry 1963, 2, 1321 (18) Krikorian, S.E. J . Phys. Chem. 1982, 86, 1875. (19) Phillips, D. L.; Lawrence, B. A.; Valentini, J. J. J . Phys. Chem. 1991, 95, 7570. (20) Phillips, D. L.; Lawrence, B. A.; Valentini, J. J. J. Phys. Chem. 1991, 95, 9085. (21) Miyazawa, T.; Shimanouchi, T.; Mizushima, S.I. J . Chem. Phys. 1956, 24, 408. (22) Warshel, A,; Levitt, M.; Lifson, S.J . Mol. Spectrosc. 1910.33, 84. (23) Chen, C. Y. S.;Swenson, C. A. J . Phys. Chem. 1969, 73, 2999. (24) Bertie, J. E.; Michaelian, K. H. J . Chem. Phys. 1982, 76, 886. (25) Bertie, J. E.; Michaelian, K. H.; Eysel, H. H.; Hager, D. J . Chem. Phvs. 1986. 85. 4779. 126) Chang,'Y. T.; Yamaguchi, Y.; Miller, W. H.; Schaefer, H. F., 111 J. Am. Chem. SOC.1987, 109,1245. (27) Hildebrandt, P.; Tsuboi, M.; Spiro, T. G. J . Phys. Chem. 1990, 94, 2274.
Chemistry and Kinetics of Size-Selected Cobalt Cluster Cations at Thermal Energies. 2. Reactlons with O2 B. C. Guo, K. P. Kerns, and A. W. Castleman, Jr.* Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802 (Received: December 19, 1991; In Final Form: April 10, 1992)
The size-dependent chemistry and kinetics of gas-phase reactions of Con+(n = 2-9) with O2are examined by using a SIDT-LV (selected ion drift tube with laser vaporization source) operated at thermal energies. Con+are observed to display a very high reactivity toward 02,and the clusters tend to undergo successive oxidation reactions. The bimolecular reaction rate constants measured for the primary reactions display a strong correlation between the size of the clusters and their reactivity. As in the case of reactions with other reactant molecules such as CO, clusters containing four and five cobalt atoms exhibit a higher reactivity toward oxygen than clusters of neighboring size. The primary reactions result mainly in a replacement of a Co atom by one 02,which suggests that the oxygen and cobalt atoms in the formed cobalt oxide clusters are bound together by strong chemical bonds. The formed oxide clusters are also very reactive toward oxygen. Except for a few cases, most of the oxide cluster reactions with oxygen proceed via either switching or attachment pathways. The successive oxidation reactions of Co,' virtually terminate when oxide clusters with stoichiometricstructures of (C00),(C0O2),+ (n = &3), c0204,5+, or c0&4,5+ are formed. Compared with the results obtained by other methods, the present work provides another important example demonstrating that SIDT-LV is a very effective technique to examine the reactions of size-selected metal cluster cations at thermal energies.
1. Introduction
Currently, there is intense interest in the chemistry of gas-phase transition metal Understanding the physical and chemical properties of these aggregates has important consequences for both fundamental and applied areas. For instance, metal clusters are viewed as representing a natural bridge between gas-phase molecules or atoms and solids. Hence, the evolution in the onset of metallic behavior from the gas phase to the condensed phase can sometimes be inferred from investigations of the sizedependent properties of the clusters. On the other hand, metal clusters of small finite size are also of direct interest, since they can play a significant role in both homogeneous and heterogeneous catalytic proce~ses.~3'A molecular-level understanding of the catalytic activity of a metal is expected to be of considerable value in the design of new generations of industrial catalysts such as supported catalysts, which are required to be cheap but more efficient and selective. The work described in this paper is part of a continuing effort under way in our laboratory8v9designed to unravel the chemistry
and kinetics of reactions of size-selected metal cluster ions under thermal reaction conditions. In a previous paper, we reported sizedependent reactivity of Co,' toward the CO molecule.8 From the reactions with CO, along with well-developed theories, we were able to gain some insight into the geometric structure of naked cobalt cluster cations. The present work examines the size-dependent chemistry and kinetics of cobalt cluster cation reactions with O2 using a selected ion drift tube with laser vaporization source (SIDT-LV) operated under well-defined energies. The work is motivated by the fact that cobalt is one of the most important catalysts in industry,lWl2especially in the process to convert methane directly into large hydrocarbon molecules. As indicated in a recent review,12 oxygen plays an essential role in the catalytic process, and most of the industrial catalysts use cobalt oxides as promoters to enhance the conversion efficiency. Hence, it is interesting to probe the interaction of cobalt with oxygen from the point of view of systems of varying size. It is also expected that such studies will provide valuable information on the microprocesses occurring on the surface of metal and metal oxide
0022-365419212096-6931 $03.00/0 0 1992 American Chemical Society
Guo et al.
6932 The Journal of Physical Chemistry, Vol. 96, No. 17, 1992
bearing particles that may influence the chemical conversion of various atmospheric constituents. Oxidation of a metal is one of the most common reactions occurring at its surface. During the past decade, there has been a great deal of research directed to elucidating the interactions of transition-metal cluster cations with oxygen. Most of the earlier studies were performed using ion beam and FT-ICR techniques. Armentrout and co-workers utilized an ion beam method to examine the oxidation chemistry of Mn2+,13Fel-3+, and Nb1-3+.'4 Using similar techniques, the groups of Anderson15and JarroldI6 studied reactions of Aln+with oxygen. The reactivities of a large number of transition-metal dimer and trimer cations toward oxygen also have been investigated by Freiser's group1' using FT-ICR experiments. FT-ICR experimentsI8also revealed a rich chemistry for the reactions of Cu,+ with 02.Recently, Bower's and our own group started to employ a SIDT-LV technique to probe the reactions of transition-metal cluster cations19and their oxygen-poor oxide clusters.20 The results have shown that the SIDT-LV technique is a very effective tool to explore the chemistry of bare metal and their coordinated clusters, especially at thermal conditions. As for cobalt cluster cations, in the past only the oxidation processes of the dimer and trimer have been examined.21*22 In the present work, we report not only the reactivity of large bare Co,+ toward oxygen but also the successive oxidation chemistry of those clusters. Moreover, the absolute oxidation reaction rate constants of Cont are measured quantitativelyand the reactivities tabulated. 2. Experimental Section The details of the apparatus and the experimental procedures have been published previously.23Only a brief description of the operational techniques and conditions used in the present work is given. Cobalt cluster cations are produced by the pulsed laser vaporization of a rotating/translating Co rod in an appropriately timed high-pressure He pulse.24 After exiting the source and passing through a skimmer, the formed ionic clusters are focused into the first quadrupole mass spectrometer,whereupon a specific size cobalt cluster cation is selected. The size-selected cluster ion is then refocused into an ion beam by a second group of ion lenses and injected into the drift tube reactor through a 1.0-"diameter orifice in the entrance plate. The drift tube reactor is comprised of a main cylindrical copper tube (9.0-cm i.d. by 2.9 cm long) and a pair of stainless steel plates as the entrance and exit plate. The reactor pressure measured by a capacitance manometer is set at about 0.6 Torr with He as the buffer gas. A drift voltage is also applied across the entrance and the exit plates to drift both the injected parent and the product ions toward the exit plate. After traveling across the drift tube, a small fraction of the reactant orifice of the exit plate and product ions diffuse through a 1.0-" into the high-vacuum chamber. Then, the ions are focused into the second quadrupole mass spectrometer equipped with a channeltron electron multiplier, where they are mass analyzed and detected. As in the previous work, a prior test experiment was conducted to find the energy ranges in which the injection and drift p m do not alter the thermal distribution of the injected clusters. On the basis of the results, we set the injection voltage below 0.7 V and the drift voltage below 0.5 V to ensure that the processes would not perturb the thermal reaction conditions. The experiments are conducted at room temperature.
3. Results 3.1. The Products from the Primary Rea&m. As mentioned in a previous paper: besides bare cobalt clusters, there are some cobalt oxide clusters produced directly from the source, and some of them have a mass close to that of certain bare metal clusters. To eliminate the potential interference of the oxide clusters with the chemistry and kinetics of the bare metal clusters, the resolution of the selection mass spectrometer was set as high as possible to ensure that only the bare metal cluster cation is able to be injected into the reactor. For instance, Figure 1 is a typical mass spectrum
50 0
70 0
1100
900
1300
1500
1700
MASS (AMti)
Figure 1. Mass spectrum of size-selected Co2' without adding O2 into the reactor.
TABLE I: Product Distribution from the Primary Reactions of Con+ with O2 Co.+ 2 3
products
cot CO2O2+ cot c020+
4
CO2O2+ CO3O2+
branching ratios 70%
30%
Cont 5 6
10%
60% 30%
7 8
100%
9
products CO4O2+
Cot CO5O2+ C0602+
co,02+
co*o*+
branching ratios 100% 10%
90% 100% 100% 100%
of the injected Co2+without adding oxygen into the reactor. The spectrum clearly indicates that the resolution of the mass spectrometer is high enough to select only Co2+from the wide cluster distribution. Similar experimental conditions are utilized to select other bare cobalt cluster cations for study. Table I lists the measured product distribution from the primary step reactions of Co,+ with 0,(n = 2-9) at a total reactor prwure of 0.6 Torr. The product branching ratios measured for the reactions are also listed in the table. The accuracy of the measured branching ratios is influenced by the quadrupole mass discrimination of the products of sequential reactions. In order to reduce the mass discrimination, we set the resolution of the second quadrupole mass spectrometer as low as possible but yet sufficiently high to separate the different products. Moreover, a -10.0-V bias voltage is applied to each of the two pairs of quadrupole rods to enhance the transmission of the product ions with the different masses. Since most of the products from the primary reactions react rapidly with oxygen, in order to obtain accurate product branching ratios, we kept the amount of the oxygen molecule addition as small as possible to prevent the primary products from undergoing further reactions, thereby leading to more accurate product branching ratios. On the basis of these factors, we estimated that the error for the branching ratio is smaller than 30%. As can be seen from Table I, there are two different types of primary reactions. One involves reactions of Con+(n = 2,3, and 6), which are able to produce multiple reaction products, whereas the other one is for C04,5,7-9' clusters, which can only generate one product via a switching pathway. Next, we use reactions of Co2+and Co,+ as examples to briefly discuss the difference in these two types of reactions. Co2+is one of three clusters that leads to multiple products. Figure 2 displays a mass spectrum acquired from the primary reaction of Co2+with 0,at an oxygen partial pressure of 1.3 mTorr. Two reaction channels are observed to occur in the process: One involves an attachment of one oxygen molecule onto Coz+ according to reaction 1, whereas the other one results in the decomposition of Co2+to yield Co+ based on the reaction channel (2). As seen from Table I, Co2+is the only bare cobalt cluster c02++ 0 2 = c0202+
+ o2= c o + + c o o ,
CO2+
(2) that is capable of producing an attachment product. However,
The Journal of Physical Chemistry, Vol. 96, No. 17, 1992 6933
Chemistry and Kinetics of Cobalt Cluster Cations N
U 0
A 50 0
70 0
900
1100
1300
1500
1700
M A S S (AMU)
Figure 2. Spectrum resulting from the reactions of size-selected Co2' with O2 at an oxygen partial pressure of 1.3 mTorr. TABLE 11: Product Distribution from the n tb Sequential Oxidation Reactions of Co.+ with 0, Co,O,+ Co20+ Co202+ co3O2+
C0402+
Co502+ co6O2+ Co702+
co802+
first step Co203+
second step CoZO5+
third step
fourth step
C0204+
C0303+
Co305+
Co304+ Co304+
Co404+ C0404+ c0504'
co6O4+ co,04+
Co506+ Co 0 +
c~:o::
Co706
160 0
C06Og+
Co,Ol0+
it is apparent that the decomposition of Co2+ is still the main channel for the reaction. No other products such as Co20+and COO+are observed. Unlike the example discussed above, the primary reaction of C O ~with + oxygen produces only one product. Figure 3a shows a mass spectrum resulting from this reaction. The partial pressure of oxygen is about 0.004mTorr in the reactor. It is apparent that the reaction with oxygen leads only to a replacement of one cobalt atom by an oxygen molecule and that no other reaction pathways occur. The primary reactions of other cluster ions with size of n = 5,7-9 follow the the same reaction pattern. These reactions are summarized in reaction 3. co,+
+ 0 2 = CO,~OZ+ + c o
200 0
220 0
240 0
260 0
MASS( A M U )
co608+ Co708+
180 0
(3) 3.2 The Product Distribution of the Seque-ntialReactiom. The products formed from the primary reactions contain only one or two oxygen atoms, and these oxygen-poor cobalt oxide clusters also are very reactive toward oxygen molecules. Table I1 summarizes the product distributions from the sequential oxidation reactions. In general, except for Co302+,most of the sequential step reactions mainly involve either a replacement of one Co atom by an oxygen molecule or an attachment of one oxygen molecule onto the oxygen-poor clusters. In the cases where multiple sequential reaction steps occur, the products from each of the sequential steps are determined from the relationshipof the product signals to the partial pressure of oxygen, along with various logic arguments. Next, we present two examples to demonstrate how to determine the products formed from the sequential reaction steps. The first example is the oxidation reaction of Co302+. Co302+ is formed from the primary reaction of Cod+ with oxygen as shown in Figure 3a, and it demonstratesa unique reaction pattern toward oxygen. Figure 3b is the mass spectrum resulting from the sequential reactions of CO~'with oxygen at 0.025 mTorr. Compared with Figure 3a, it is seen that two new product peaks, Co304+ and c0303', appear in the spectrum in addition to Co302+and Cod+. On the basis of the fact that the intensity ratio of Co304+ to Co303+does not vary appreciably with the change in the oxygen partial pressure, one can easily determine that both clusters are formed directly from the oxidation reaction of Co302+. The
Figure 3. (a, Top) Mass spectrum resulting from the primary reactions of size-selected Codt with O2 a t an oxygen partial pressure of 0.004 mTorr. (b, Bottom) Mass spectrum resulting from the successive reactions of size-selected CO~'with O2at an oxygen partial pressure of 0.025 mTorr.
following reaction channels (4) and ( 5 ) represent the formation process of the two clusters, respectively. Interestingly, Co302+ Co302++ O2 = Co304+ (4) CO302'
+ 0 2 = c0303++ 0
(5)
is the only cobalt oxide cluster that is able to break 0-0 bonds and coordinate only an oxygen atom onto itself. The formed c0303' is also reactive toward O2to form Co305+as indicated by reaction 6. However, both Co304+and Co305+are virtually Co303+ O2 = Co305+ (6)
+
inert toward oxygen. As for the dimer oxide cluster, the oxidation reactions terminate when C O ~ O ~are , ~formed. + The second example is the sequential oxidation reactions of Cos+. Figure 4 displays the mass spectra of reactions of Cos+ obtained at two different oxygen pressures. As indicated in Figure 4a, at an oxygen partial pressure of 0.01 mTorr, only the products C q 0 2 +and Co604+are formed. Since one collision can lead only to the attachment of one O2molecule, Co604+must be the product from a sequential reaction. As shown in Figure 4b, upon increasing the oxygen pressure to 0.32 mTorr, Co604+will attach an additional O2to form a new product, Cos06+,which, in turn, reacts with one more oxygen molecule to produce Co608+. It is apparent that when a cobalt oxide cluster with the stoichiometric structure of ( C O O ) ~ ( C O O ~is)formed, ~+ no further reactions occur. The same patterns are observed in the cases of tetramer, pentamer, and heptamer oxide clusters, and their oxidation reactions stop once (Co0)4(Co02)0,1,3+ form. 3.3. The Primary Reaction Rate Constants. As shown in the above, all the bare cobalt clusters but the dimer mainly undergo bimolecular reactions with 02.We have used the procedures, which were discussed in detail in a previous paper,23to measure the absolute reaction rate constants for these reactions. For an ion molecule reaction studied in the present work, the bimolecular rate constants can be expressed by the following equation In ( I / I ( O ) ) = -k[O,]t (7)
6934 The Journal of Physical Chemistry. Vol. 96, No. 17, 1992
Guo et al.
TABLE III: Measured Bimolecular Rate Constants and Calculated Collision Rate Constants of Co.' with 0," co,+ k,,,, cm3/s kL X cm3/s kcxc/kL 2 (5.1 f 0.2)X lo-" 5.89 0.09 3 (10.0f 0.3)X 5.68 1.76 4 (12.0f 0.4)x 1O-Io 5.56 2.16 5 (11.5 f 0.3)X 5.49 2.09 6 (7.6f 0.2) X 5.45 1.39 7 (9.7f 0.4) x 1O-Io 5.42 1.79 8 (9.5 f 0.3) X 1O-Io 5.40 1.76 9 12.0 x 10-10 5.38 2.23 I
k,,, is the measured bimolecular reaction rate constant; kL denotes the calculated Langevin collision rate.
1
400 0
420 0
440 0
460 0
480 0
500 0
MASS (AMU)
SCHEME I *
I
co,+ +
o2 -b
I
"'1
co-co,~2-co0'
+* co,-lo2+ 4
+ co
SCHEME II
1 +*+
co2+ + 0, +[ o - c ~ - c ~ - o \
co202+
(~2a)
of Cog+,the rate constant is obtained by using a single measurement. 400 0
420 0
440 0
460 0
480 0
500 0
MASS (AMU)
Figure 4. (a, Top) Mass spectrum resulting from the successive reactions of sizeselected with O2at an oxygen partial pressure of 0.01 mTorr. (b, Bottom) Mass spectrum resulting from the successive reactions of size-selected Cost with O2at an oxygen partial pressure of 0.32 mTorr.
a'
-
I
v
s - 2 0
- -3
v
C
0 .
-4 0 --4
I
nI
I -
0 0
1
I
\,
0 2
05
07
10
12
P R E S S U R E OF 02 (mTorr)
Figure 5. Semilog plots of Co2+and Cos+ intensities vs the partial pressure of O2
where Z and Z(0) are the intensities of the selected cobalt cluster cation with and without adding the O2reactant, k is the bimolecular rate constant, and t is the time the ion spends in the drift tube reactor. The time, t, is measured by comparing the time difference with and without the buffer gas of helium in the reactor and fKed for each of the selected clusters regardless of the partial pressure of oxygen. From eq 7, if t is fixed, one can obtain the bimolecular reaction rate constant k through the slope of a plot of In (Z/Z(O)) vs [O,]. The requisite data were obtained through measurements of the cobalt cluster cation intensity at a number of different pressures of 02.Figure 5 displays typical semilogarithmic plots of the ion intensities for C%+ and cO,+vs the partial pressure of oxygen. In order to reduce the statistical error, the cobalt cluster cation intensities are measured at least two times at each of the partial pressures of 0,; the intensities used in the plots are the average values of the multiple measurements. The measured reaction rate constants for Con+(n = 2-9) with O2are listed in Table 111. The errors listed in Table 111are based on one standard deviation from a linear least-squares fit to the experimental points such as the ones shown in Figure 5 . In the case
4. Discussion 4.1. Successive Oxidation Rerctiom of Con+. All the Con+ clusters studied in the present work demonstrate a high reactivity toward the oxygen molecule. One of the interesting findings from the present work is that the primary products CowlO2+are formed through a switching reaction procc%s as expressed by reaction 3. As mentioned in the Experimental Section, the reactions studied herein are thermal reactions, and the observed reactions must be exoergic. Hence, a replacement of a Co atom by one 0,molecule suggests that the bonding Cowl+to O2is very strong36and that the cobalt and oxygen atoms in those clusters are bound together by chemical interactions rather than the electrostatic forces. In other words, the formed CoF102+is not a simple ion molecule association complex, but a cobalt oxide cluster. Scheme I displays a possible mechanism accounting for the switching process. The bonding of O2to Con+creates a nascent adduct (Co,O2+)* with a large amount of excessive energy. In general, the nascent adduct with excessive energies can either decompose or be stabilized,dependins on its lifetime. If the adduct has a long enough lifetime to survive before colliding with the buffer gas, the adduct will eventually be stabilized. But if its lifetime is too short, the adduct will undergo fragmentation. The details of the competition between decomposition and stabilization processes of a nascent adduct have been discussed.25 In the case of the reactions shown in Scheme I, the nascent adducts may have such a short lifetime that they will d d t e before colliding with the buffer gas. According to Stevenson's rule,%the preferred ionic product in decomposition is the one having the lower ionization potential (IP). There have been some experimental data which suggest that the ionization potential of the CO atom is much higher than that of the cobalt clusters.27 Furthermore, the coordination of ligands can also reduce the IP of metal clusters.28 Therefore, the loss of a Co atom in the switching reaction is consistent with Stevenson's rule. As can be seen from the above section, the reactions of Cq,3,6+ with oxygen are able to produce multiple products. In the case of the dimer, Scheme I1 exhibits a possible mechanism for the reaction patterns shown in reactions 1 and 2. Basically, the nascent adducts have two isomers. The isomer (Co-CO02+)* would lead to loss of COO, and formation of Co+. Absence of COO2+formation in pathway S2b implies IP(Co0,) > IP(C0). On the other hand, the isomer (O-Co-Co-O+)* in pathway S2b may have a sufficiently long lifetime to survive, before colliding with buffer, which serves to remove excessive energy.
The Journal of Physical Chemistry, Vol. 96, No. 17, 1992 6935
Chemistry and Kinetics of Cobalt Cluster Cations SCHEME 111
4.2. Size-Dependent Reactivity of Con+. Table I11 lists the measured reaction rate constants of Con+with 02,along with the calculated collision rates for each of the Con+clusters with an O2 molecule. The calculated values are the Langevin collision ratesB given by the following equation
\
\ [ C O O - ( C O ~ O ) ] +*
_.)
CO,O+
+
kL = (2.33 x 1 0 - 9 ) ( ~ / ~ ) 1 / 2 COO
(S3b)
The trimer reaction is an unique case in that it can create a product with an odd number of oxygen atoms. The possible mechanism is displayed in Scheme 111. Like the dimer, two nascent adducts are formed. The formation process of Co202+ in pathway S3a is similar to reaction S1. (However, pathways S3a is also able to generate a small amount of Co+, presumably because the IP of Co is not substantially higher than the IP of Co202. The same mechanism and argument can apply to the reaction pattern of Cob+with oxygen.) Loss of COO,and formation of Co2O+in pathway S3b, may suggest that the IP of COO is higher than the IP of Co2O. The cobalt oxide clusters formed from both the primary and sequential reactions of Con+with O2are also very reactive toward oxygen. Except for Co302+,most of the oxide clusters follow a similar reactioin pattern. That is, the oxidation reactions of oxide clusters result in either a replacement of a Co atom by one O2 molecule or an attachment of 02,depending onto the number of oxygen atoms contained by the oxide clusters. As shown in Table 11, in general, the clusters having less oxygen atoms would undergo replacement of a Co atom by one oxygen atom, while clusters with more oxygen atoms tend to attach one O2molecule onto themselves. However, Co402+and C0,04+ undergo both switching and attachment reactions. The oxidation reaction will essentially terminate when the oxide clusters with stoichiometricstructures of (COO)~(COO~),+ (n = 0-3) are acquired. For the case of the dimer and trimer, the oxidation will stop when the oxide clusters C O ~ , ~or O C2,30s+ ~+ are formed. Scheme IV shows the possible mechanisms, which may account for the reactions of the cobalt oxide clusters with oxygen. The key to the mechanisms shown in Scheme IV is the existence of a bare Co atom at one of the termini of the formed nascent adduct. If there is a side Co atom in the formed adduct, in view of the high IP of the Co atom the adduct would be expected to lose a Co atom via pathway Ma. As for pathway S4b, the nascent adduct does not have a side Co atom. Presumably the lifetime of the complex is long enough to favor the formation of the attached product CO,O,,,+~+.The above arguments are at least consistent with the experimental results. That is, the clusters with few oxygen atoms have a greater chance to form a nascent adduct with a side Co atom, whereas each of the cobalt atoms in the clusters with more oxygen atoms is more likely to bond to at least one oxygen atom; thereby, the clusters do not lose a bare Co atom. As for Co402+and Co704+,which have a fair number of cobalt atoms, they can produce two kinds of the nascent adducts, one possesses a side cobalt atom while the other does not. Pathways S4a and S4b coexist in their reactions with oxygen. Hence, two products are formed. It should be noted that the proposed mechanisms might be just one of many possibilities that can account for the present results. Possibly there are other arguments that can be used to account for the results. For instance, the hot nascent oxide products are likely fluxional in structure, and the decomposition patterns might be controlled by the product channel energetics. Since one does not know the exact energetics of the nascent adducts and the final products, we do not speculate further.
(8)
where a is the polarizability of 02,and equal to 1.6 A,30 and p is the reduced mass in amu. The Langevin collision rates give an upper limit of the rates of ion-molecule reaction at thermal energies. Table I11 also lists the ratios of the measured reaction rate constants to the calculated collision rates. Surprisingly, all the ratios but the one for the dimer are larger than 1; that is, the measured rates are even larger than the calculated upper bounds. There are two possibilities to account for this observation. First, the measured reaction rate constants might have a large error. Second, it is likely that the calculated values represent underestimates of the true ones. As mentioned in a previous paper,23test studies had been conducted to compare the rate constants measured with this instrument to accepted published data. The agreement between the literature values and our measured data is found to be satisfactory. Typically the measured data compare with literature values to within 15%. Hence, we are very confident that the data measured with this SIDT-LV are accurate. Furthermore, we have measured some of the rate constants reported in the present paper three times to ensure that the reported data are reproducible. It appears that the reproducibility is excellent and that the error in the reproducibility is less than 10%. Thereby, we believe that the experimental errors are not the main sources to cause such a large difference. The Langevin formalism is based on an ion-induced dipole potential. There have been a large number of experimental results which indicate that the model is sometimes overly simplified” and that eq 2 can underestimate the collision rates of many ionmolecule reactions. As pointed out in Gioumousis and Stevenson’s original paper, the assumption of an isotropic polarizability (a) used in eq 8 might be too general. Thus, for molecules such as oxygen, which have a polarizability tensor that is far from isotropic, the model might give incorrect rates. In fact, comparison of measured rates with the calculated ones made by Stevenson and his worker^^^ has indicated that eq 8 underestimates the collision rates of H2+ and D2+ with oxygen by factors of more than 3. Indeed, trajectory studies made in our laboratory revealed that nonisotropic polarizabilities can lead to large enhancements in rates.32 Furthermore, the model treats the ion as a structureless point charge. In the case of cobalt cluster cations, the charge may be delocalized on the entire cluster instead of on a single Co atom. Thereby, the size of the cluster ion might also have an influence on the collision rate. Therefore, we conclude that eq 2 might underestimate the collision rates for the reactions of Con+with oxygen studied in the present work. Next, we consider the size-dependent reactivity of Con+toward oxygen. Figure 6 displays the changes in the reactivities of Con+ toward oxygen, with the size of the clusters. For purposes of comparison, the reactivities of Con+toward CO are also presented in Figure 6. Surprisingly, it is seen that the reactions with CO and O2 demonstrate a similar pattern, although their reaction mechanisms are much different. (Note: The reactions with O2 proceed via a bimolecular reaction process, but with CO via a three-body association process.8) In both cases, C04,s+are more reactive than their neighboring clusters of C O ~ , ~ ,There ~ + , have been numerous reports indicating that the tetramer and pentamer of many other transition metals also display a higher reactivity than clusters of neighboring size.33 Obviously, the argument that there are unusual electronic structures or electronic states existing in the two clusters cannot explain their anomalous reactivity, because the number of valence electrons and electronic structures is very different across the series of transition-metal elements. Therefore, in a previous paper8 we argued that the geometric structure of these metal clusters might play a crucial role governing their reactivity. As mentioned in the Introduction, through de-
6936 The Journal of Physical Chemistry, Vol. 96, No. 17, 1992
120 -
4
*
.
G
o
7
8
0
6 0 -
0
X Y
G
G 001 1
t 2
0 3
4
5 6 SIZE O F Con
9
10
+
Figure 6. Plots of the measured bimolecular reaction rate constants as a function of cluster size for oxygen ( 0 )and for CO (0);see ref 8.
termining the maximum coordination number of CO molecules on specific size cobalt cluster cations, along with well-developed theories, we have been able to gain some insight into their structures. On this basis, the tetramer is expected to have a tetrahedral structure and the pentamer to have trigonal bipyramid structure. Thus, we believe that the structures may have high reactivity sites toward oxygen, which are able to enhance the reactivity of the cobalt clusters. 4.3. Comparison with other Work. Reactions of cobalt dimer and trimer cations with oxygen have been examined by several groups. Freiser and his co-workers investigated the reactions of cobalt dimer and trimer cations with oxygen using an FT-ICR.22 The observed reaction patterns for those two clusters are different from the ones reported in the present paper. In Freiser’s work, Co2+is able to break 0-0 bonds to form Co2O+ rather than leading to h O , + as o k e d in the present work. One may argue that the difference may result from the difference in the pressures used in the two studies. The pressure is extremely low in the ICR cell but comparatively high in our drift tube reactor. In principle, decomposition processes may tend to occur under very low pressure conditions, whereas high pressures can enhance bonding of an entire molecule onto a metal cluster. However, consideration must be also given to the difference in the energetics of the cobalt clusters. As indicated in the Experimental Section, the reactions of cobalt cluster cations with oxygen proceed under well-defined thermal conditions in the drift tube reactor. Hence, the reactions are exoergic. In the case of the FT-ICR experiment, the dimer cluster is formed from the collision-induced dissociation process of cobalt dimer carbonyl ions. Recently, Armentrout’s group has conducted an experiment to test the energy-dependent reaction of Co2+with oxygen. Their work indicated that it is very likely that the cobalt dimer in the ICR work may be internally excited and that the formation of Co2O+from Co2+and O2is endothermic. As for the trimer, Freiser’s group observed Co2+,Co202+,and Co2O+,but Co+ along with C%02+and Co20+is observed in this work. The same reasons discussed above may explain the results of the trimer case as well. In the present work, Co302+is the only observed oxide cluster that can break 0-0 bonds to form Co303+. This might suggest that Co303+is a very stable cluster, in which the cobalt and oxygen atoms are strongly bound to each other. Some of Freiser’s work also suggests that c0303’ is very special. Several years ago, Freas and co-workers used a sputtering ion source in the presence of oxygen of 0.1-0.2 Torr to produce cobalt cation clusters.34 The most abundant clusters were [Co(CoO),]+ and [(COO),]+ clusters. Many of the clusters had an odd number of oxygen atoms. Obviously, this is in contrast to the present work. The discrepancy is easily explained in light of the energetics of the formed bare cobalt or oxide cluster ions. It is well-known that the sputtering source can produce clusters with a substantial amount of internal energy35and the existence of excited clusters can be expected to yield different reaction patterns. 5. Conclusions We report the results of experiments made to examine the chemistry and kinetics of size-selected cobalt cluster cations toward
Guo et al. oxygen using SIDT-LV operated under well-defined conditions. Compared with other techniques, SIDT-LV is shown to be a very effective method to study the thermal reactions of size-selected metal clusters. The major conclusions drawn from the present work are as follows: (1) All the cobalt cluster cations studied in the present work display a high reactivity toward the oxygen molecule and most of their reactions proceed via bimolecular reaction mechanisms. Except for C02,3,6+,which are able to produce multiple products, the other cluster reactions lead to a replacement of a Co atom by one O2molecule. The results suggest that the oxygen atoms are bound to the cobalt atoms by strong chemical bonds. The product patterns can be explained in light of the trends in the values of the IP of the clusters and related product species. (2) The cobalt oxide clusters tend to undergo successive oxidation reactions with oxygen, and the oxidation reactions virtually terminate when clusters with the stoichiometric formula (CoO),(C0O2),+ (n = 0-3) or C O ~ , ~ Oare ~ formed. ,~+ Except for a few cases, most of the reactions of the oxide clusters with oxygen proceed via either a switching pathway or an attachment pathway. (3) The effective bimolecular reaction rate constants are measured for all primary step reactions of Con+with 02,The rate constants show large variations with the size of the clusters. As in the cases of reactions with CO, Cod+ and Cos+ display a much higher reactivity toward O2than do clusters of neighboring size. Considerations suggest that the geometric structure is the major factor that is responsible for this trend.
Acknowledgment. Financial support by the Environmental Protection Agency, Grant R-8 17437-01-0, and DuPont Chemicals through an unrestricted grant to the Department of Chemistry of The Pennsylvania State University, are gratefully acknowledged. We also thank one of reviewers for his alternative mechanism of fragmentation reactions.
Reference$-and Notes (1) Kaldor, A,; Cox, D. M.; Zakin, M. R. In Euolution of Size Effects in Chemical Dynamics; Prigogine, I., Rice, S. A., Eds.;Wiley: New York, 1988; Part 2. (2) Physics and Chemistry of Small Clusters; Jena, P., Rao, B. K., Khanna, S.N., Eds.; Plenum: New York, 1987. (3) Metal Clusters;Trager, F., Putlitz, G. Z., Eds.; Springer: Berlin, 1986. (4) Gas Phase Inorganic Chemistry; Russell, D. H., Ed.; Plenum: New York, 1989. (5) Castleman, A. W., Jr.; Keesee, R. G. Chem. Reo. 1986, 86, 589. (6)Homogeneous Catalysis; Parshall, G. W., Ed.; Wiley-Interscience: New York, 1980. (7) Metal Clusters in Catalysis; Gates, B. C., Guczi, L., Knozinger, H., Eds.; Elsevier: New York, 1986. (8) Guo, B.C.; Kerns, K. P.; Castleman, A. W., Jr. J. Chem. Phys. 1992, 96, 8177. (9)Guo, B. C.; Kerns, K. P.; Castleman, A. W., Jr. Chemsitry and kinetics of size selected cobalt cation clusters at thermal energies: 3. Reactions with NH,. J. Chem. Phys., to be submitted. (10)New Horizons in Catalysis; Seiyama, T., Tanale, K., Eds.; Elsevier: New York, 1981. (1 1) Progress in C1 Chemistry in Japan; The Research Association for C, Chemistry, Eds.; Elsevier: New York, 1989. (12)Haggin, J. C&E News 1991, April 29, 22-24. (13)Armentrout, P. B.; Loh, S.K.; Ervin, K. M. J . Am. Chem. Soc. 1984, 106,1161. (14)Loh, S. K.; Hales, D. A.; Lian, L.; Armentrout, P. B. J . Chem. Phys.
1989,91, 6148. (IS) Rutta. S. R.; Anderson, S. L. J. Chem. Phys. 1988,89, 273 and references therein. (161 Jarrold, M. F.; Bower, J. E. J . Chem. Phys. 1987, 87, 1610 and references therein. (17) (a) J a c o h n , D. B.; Freiscr, B. S. J . Am. Chem. Soc. 1984,106,4623. (b) Jacobson, D. B.; Freiser, B. S.J . Am. Chem. Soc. 1985,107, 1581 and references therein. (18) Irion, M.P.; Selinger, A. Chem. Phys. Lett. 1989,158, 145. (19)Kemper, P. R.; Bowers, M. T. Int. J . Mass Spectrom. Ion Proc., in press. (20)Guo,B. C.; Kerns, K. P.; Castleman, A. W., Jr. Studies of Reactivity of Titanium Oxide Cation Clusters: Stability of the Clusters in the Presence of Oxygen at Thermal Energies. Inr. J. Mass Spectrom. Ion Proc., in press. (21) Hales, D. A.; Armentrout, P. B. J . Cluster Sci. 1990, I , 127. (22) Jacobson, D. B.; Freiser, B. S. J . Am. Chem. SOC.1986, 108, 27. (23)Guo, B. C.; Kerns, K. P.; Castleman, A. W., Jr. J. Phys. Chem. 1992, 96,4879. (24)Dietz, R.B.; Duncan, M. A,; Powers, D. E.; Smalley, R. E. J . Chem. Phys. 1981,74, 6511.
J . Phys. Chem. 1992,96, 6937-6941 (25) Tonkyn, R.; Ronan, M.; Weisshaar, J. C. J . Phys. Chem. 1988.92, 1992. (26) Stevenson, D. P. Discuss. Faraday Soc. 1951, 10, 35. (27) Armentrout, P. A. Personal communication, 1991. (28) Dao, P. D.; Peterson, K. I.; Castleman, A. W., Jr. J . Chem. Phys. 1984.80, 563. (29) Gipumousis, G.; Stevenson, D. P. J . Chem. Phys. 1958, 29, 294. (30) Handbook of Physics and Chemistry, 76th ed.; CRC Press: Boca Raton, FL, 1988. ‘ (31) Su,T.; Chesnavich, W. J. J . Chem. Phys. 1982, 76, 5183. (32) Schelling, F. J.; Castleman, A. W., Jr. Chem. Phys. Lett. 1984, 1 1 1 , 47. (33) (a) Zakin, M. R.; Brickman, R. 0.;Cox, D. M.; Kaldor, A. J . Chem.
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Phys. 1988, 88, 3555. (b) Zakin, M. R.; Brickman, R. 0.;Cox, D. M.; Kaldor, A. J. Chem. Phys. 1988, 88, 6605. (c) Upton, T. H.; Cox, D. M.; Kaldor, A. J . Phys. Chem. 1989, 93, 6823. (d) Irion, M.P.; Selinger, A.; Schnabel, P. Z . Phys. D. 1991, 19, 393. (e) Schnabel, P.; Irion, M. P.; Weil, K. G. J . Phys. Chem., submitted for publication. (f) Nakajima, A.; Kishi, T.; Sugioka, T.; Sone, Y.;Kaya, K., unpublished results. (34) Freas, R. B.; Dunlap, B. I.; Waite, B. A.; Campana, J. E. J . Chem. Phys. 1987, 86, 1276. (35) Begemann, W.; Dreihofer, S.; Meiwes-Broer, K. H.; Lutz, H. 0. Z . Phys. D. 1986, 3, 183. (36) Armentrout’s group has determined the bonding strength of Co,, + Co in an ion beam technique.” Their results indicate that the bonding energies for the bare cobalt cation clusters are either near or larger than 2.0 eV.
Application of Density Functional Theory to Infrared Absorption Intensity Calculations on Transition-Metal Carbonyls Liangyou Fan and Tom Ziegler* Department of Chemistry, University of Calgary, Calgary, Alberta, Canada T2N I N4 (Received: February 13, 1992)
Approximate density functional theory has been evaluated as a practical tool for calculations on infrared vibrational frequencies and absorption intensities of transition-metalcomplexes. The density functional schemes included the local density approximation (LDA) by Gunnarson (Phys. Rev. 1974, BIO, 1319) as well as a self-consistentnonlocal density functional method (LDAINL) in which the gradient-corrected exchange term by Becke (Phys. Rev. 1988, A38, 3098) and the gradient-corrected correlation term by Perdew (Phys. Rev.1986,833,8822) have been added to LDA. The LDA and LDA/NL schemes have been applied to calculations on the infrared vibrational frequencies and absorption intensities of Ni(C0)4and Cr(C0)6. The calculations were carried out with a double-l plus polarization basis set for C and 0 and a triple-f plus polarization basis set for Cr and Ni. The simple theoretical LDA model has been found to reproduce vibrational spectra of metal carbonyls adequately. The more sophisticated, and also more expensive, nonlocal scheme does not introduce important improvements in the calculated vibrational frequencies for Cr(C0)6and Ni(CO).+ The calculated frequencies are in general in better agreement with experiment than values obtained by ab initio Hartree-Fock calculations. Calculated atomic polar tensors and harmonic force fields are provided for both molecules.
Introduction IR spectroscopy can supply important information about the structure of transition-metal complexes. The force constants representing bond stretches and angular bendings afford in addition valuable clues to the nature and strength of the bond between transition-metal centers and coordinating ligands. The absolute IR absorption intensities finally provide information about charge delocalizations associated with ligand coordination to a metal center The experimental determination of general quadratic force fields in transition-metalcomplexes is usually hampered by insufficient experimental data. That is, the number of independent force constants in a molecule is usually much larger than the number of observable frequencies due to the same system. Thus,additional information must be obtained by observing frequencies due to an often large number of isotope-substituted species. Isotope substitutions in organometallics are laborious, and it is often not possible to generate the required number of species. Furthermore, the absolute IR intensities of metal carbonyls are measurable only for C-O stretching modes. The other bands are too weak in comparison with the dominating CO band to be accurately measured by experiment. Theoretical calculations are therefore expected to play an important part in supplementing experimental information. Unfortunately, theoretical studies on vibrational spectra are rather scarce for transition-metal complexes in contrast to the situation among classical main group molecules. There are few theoretical calculations on vibrational spectra and IR intensities of transition metals. The paucity of ab initio calculations in this field stems primarily from the fact that ex-
pensive high-level methods with extensive treatment of electron correlation are required. This makes ab initio studies on transition-metal systems less attractive than similar studies on organic molecules where the more economical Hartree-Fock method can be used. Vibrational studies should in principle be attractive for methods based on DFT. However, the required computer programs for such studies have only emerged over the past few years. Among the limited calculations based on density functional theory, Baerends and Rozendaa12calculated the frequencies of C-O and Cr-C stretches in Cr(CO), by the X, method. Rosch and Jbrg have used a similar method to calculate the frequencies of Ni-C and C-O stretches in Ni(C0)438as well as the symmetrical Fe-Cp stretch in ferrocene.3b Papai et al. r e p r o d u ~ e dthe ~ ~vibrational frequencies of Ni(C0)4, NiC2H4,and PdC2H4. Dunlap et al. calculated the dipole strengths of NiCO for Ni-C and C-O stretches.4b The frequencies calculated by DFT in these cases agree reasonably well with experiment. However, a complete set of IR intensities and a full general quadratic force field of transition-metal complexes calculated by DFT have not previously been published. We present here a full analysis of the vibrational spectra for Ni(C0)4 and Cr(CO), based on DFT. We have made some efforts to reproduce vibrational frequencies by the HartreeFock-Slater (HFS) method in a previous studyas The satisfactory agreement between experimentally observed frequencies and HFS calculations encourages us to extend our investigations to IR absorption intensities. We shall further make use of the more sophisticated density functional methods introduced over the past 10 years. These methods include the local
0022-3654/92/2096-6937$03.00/0 0 1992 American Chemical Society
