J . Phys. Chem. 1989,93, 2485-2490
xI in eq C3 and C4 varies from xo (at t = 0) to x,,' (at t = tr), xI in eq C5 varies from x,,' (at t = tf) to (1 - 8 ) / 2 (at t =
and
Thus the relative field factor changes from E, = -1 at t = 0 to E , = E,' = O(l - 2 x 0 at t = tf to E,' = +1 at t = tl. Positive and negative values of the field factor indicate speeds that are respectively higher and lower than that in the SCE-free case. The extent of speed compensation depends on the relative balance of these two situations over the transit time of a given ion. If the switchover in sign occurs during the transit of the first arriver, its T O F might approach SCE-free TOF. In any case the last arriver is certain always to experience some speed compensation. In Figure 10 we show plots of E , and E,' against the position of the last arriver x1for case I and case 11. Three different values of x,,' (or xo) are used to test the role of the parameter xo; x,,' tl).
2485
= 0.3 (dashed line), 0.5 (dash dotted line), and 0.7 (dotted line). Mobilities given by eq 30 are used. Points a indicate xI = xo at t = 0. Points b indicate xI = xd when t = t f ,when the first arriver has reached its target electrode. Points c correspond to t = tl, when the last arriver reaches its largest electrode. The horizontal line represents the SCE-free value of E,, E: = 0. Speed compensation can take place when a given charge experiences both positive and negative deviations from the zero line during its TOF. In case I the positive first arriver, moving left to right, feels some compensation only for the dotted E,, E,' case. In case I1 the negative first amver, moving right to left, feels some compensation only for the dashed E,, E,' case. By contrast, the last arriver, moving from a to c, experiences significant though variable speed compensation in all cases.
Reaction of Carbon Dioxide with Gaseous Niobium and Niobium Oxide Clusters Li Song, Alexander Eychmiiller, R. J. St. Pierre, and M. A. El-Sayed* Department of Chemistry and Biochemistry, University of California at Los Angeles, Los Angeles, California 90024- 1569 (Received: September 6, 1988)
Small niobium and niobium oxide clusters (Nb, and Nb,Oy; x = 1-13, y = 1-4) were synthesized by laser vaporization in a supersonic molecular beam system and reacted with carbon dioxide and its oxygen isotopically labeled derivative (CO*z). The reaction between Nb, and COz gave Nb,O+ mass peaks for all x values observed: Nb,C02+ for x I 5 , Nb,(COz)z+ for x I 7, and Nb,(C02)3+ for x I 9. The relative apparent rate constant for the total reaction of Nb, with COz seems to increase almost smoothly with increasing cluster size. However, the relative yields of the oxide formation initially increased but later decreased for cluster sizes where the CO, addition becomes observed. These results are described in terms of a mechanism involving complex formation between Nb, and CO, leading to the formation of Nb,O + CO. Due to the exothermicity of the formation of the Nb-0 bond and CO, small clusters seem to boil off the CO molecule formed. As the size of the clusters increases, its internal degrees of freedom increase and its ability to retain and bind the resulting CO molecule(s) increases. This, together with the increasing ability of Nb, for large x to form Nb,(C02), (or Nb,O, to form Nb,Oy(COz),) by multiple collisions, opens up more channels by which Nb, (or Nb,O) can form products and thus lead to the obserxed increase in the relative overall reaction rate constant with increasing cluster size. An intense mass peak of NbO+ was observed to have a one-photon ionizing laser intensity dependence. The formation of NbO+ was described in terms of an evaporation mechanism from the larger oxygen-containingcluster product ions formed in the ionization region via a one-photon absorption process.
I. Introduction The reactivity of gaseous metal clusters has been a focus of interest in the past few years. Studies involving both the maingroup metals'-5 and the transition metalsGz6have been investi( I ) Cox, D. M.; Trevor, D. J.; Whetten, R. L.; Kaldor, A. J . Phys. Chem. 1988, 92, 421.
(2) Peterson, K.; Dao, P.; Castleman, A., Jr. J . Chem. Phys. 1983, 79, 777. (3) Dao, P.; Peterson, K.; Castleman, A., Jr. J . Chem. Phys. 1984,80, 563. (4) Rothe, E.; Sinha, D.; Ranjbar, F. J . Chem. Phys. 1982, 76, 5650. (5) Schulz, C.; Haugstatter, H.; Tittes, H.; Hertel, I. Phys. Reu. Lett. 1986, 57, 1703. (6) Whetten, R. L.; Cox, D. M.; Trevor, D. J.; Kaldor, A. Surf. Sci. 1985, 156, 8. (7) Richtsmeier, S. C.; Parks, E. K.; Liu, K.; Pobo, L. G.; Riley, S. J. J . Chem. Phys. 1985,82, 3659. (8) Parks, E. K.; Liu, K.; Richtsmeier, S. C.; Pobo, L. G.; Riley, S. J. J . Chem. Phys. 1985, 82, 5470. (9) Liu, K.; Parks, E. K.; Richtsmeier, S. C.; Pobo, L. G.; Riley, S. J . J . Chem. Phys. 1985, 83, 2882. (IO) Parks, E. K.; Nieman, G . C.; Pobo, L. G.; Riley, S. J. J . Phys. Chem. 1987, 91, 2671. (11) Parks, E. K.; Weiller, B. H.; Bechthold, P. S.; Hoffman, W. F.; Nieman, G. C.; Pobo, L. G.;Riley, S. J. J . Chem. Phys. 1988, 88, 1622. (12) Geusic, M. E.; Morse, M. D.; Smalley, R. E. J . Chem. Phys. 1985, 82, 590. (13) Zakin, M. R.; Cox, D. M.; Whetten, R. L.; Trevor, D. J.; Kaldor, A . Chem. Phys. Lett. 1987, 135, 223. (14) Morse, M. D.; Geusic, M. E.; Heath, J. R.; Smalley, R. E. J . Chem. Phys. 1985,83, 2293. (15) Hamrick, Y.;Taylor, S.; Lemire, G . W.; Fu, 2.-W.; Shui, J.-C.; Morse, M. D. J . Chem. Phys. 1988, 88, 4095.
0022-3654/89/2093-2485$01 S O / O
gated. There are a few recent review articles on the studies of reactions of the gas-phase metal cl~sters.~'-~O The ultimate goal is to understand the transition from metal clusters to the bulk (16) Whetten, R. L.; Zakin, M. R.; Cox, D. M.; Trevor, D. J.; Kaldor, A. J . Chem. Phys. 1986,85, 1697. (17) Cox, D. M.; Reichmann, K. C.; Trevor, D. J.; Kaldor, A. J . Chem. Phys. 1988,88, 1 1 I . (18) Zakin, M. R.; Brickman, R. 0.;Cox, D. M.; Kaldor, A . J . Chem. Phys. 1988,88, 3555. (19) Alford, J. M.; Weiss, F. D.; Laaksonen, R. T.; Smalley, R. E. J. Phys. Chem. 1986, 90, 4480. (20) Elkind, J. L.; Weiss, F. D.; Alford, J. M.; Laaksonen, R. T.; Smalley, R. E. J . Chem. Phys. 1988, 88, 5215. (21) St. Pierre, R. J.; El-Sayed, M. A . J . Phys. Chem. 1987, 91, 763. (22) Zakin. M. R.: Cox. D. M.: Kaldor. A. J . Phvs. Chem. 1987.91.5224. (23j St. Pierre, R.'J.;Chronister, E. L.;'Song, L.iEl-Sayed, M. A.J : Phys. Chem. 1987, 91, 4648. (24) St. Pierre, R. J.; Chronister, E. L.; El-Sayed, M. A. J . Phys. Chem. 1987, 91, 5228. (25) Zakin, M. R.; Brickman, R. 0.; Cox, D. M.; Kaldor, A. J . Chem. Phys. 1988, 88, 5943. (26) (a) Song, L.; Eychmuller, A,; El-Sayed, M. A. J. Phys. Chem. 1988, 92, 1005. (b) Song, L.; Eychmuller, A.; El-Sayed, M. A,, submitted for publication in J . Phys. Chem. (27) Kaldor, A,; Cox, D. M.; Zakin, M. R. Adu. Chem. Phys. 1988, 70, 211. (28) Kappes, M. M. Chem. Rev. 1988, 88, 369. (29) Whetten, R. L.; Schriver, K. E. In Gas Phase Inorganic Chemistry; Russel, D., Ed.; D. Reidel: Boston, 1988. (30) Morse, M. D. Chem. Reu. 1986, 86, 1049.
0 1989 American Chemical Society
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The Journal of Physical Chemistry, Vol. 93, No. 6, I989
surface and therefore to obtain more insight into the surface properties in catalysis and surface reconstruction. In order to do this, the reactivity of metal clusters is usually studied as a function of cluster size. Depletion studies have been carried out on the reactions of and C0.149'7 niobium clusters with H2,12J3919320 D2, 14-16~18 N2,l4*l5 In the reaction with H2, D2, and N2, niobium clusters show a strong size dependence, with Nb8, Nblo,and Nb16more inert than their neighbors.I4 The reactivities of niobium oxide clusters are quite different from those of the bare ~1usters.I~ In the reaction of N b clusters with CO, their relative reaction rate increases monotonically as a function of cluster size.14 It was also shown that the reactivity patterns of the cluster ions are similar to those of the neutrals, with the ions generally more reactive than the neutrals. The reactivity patterns were correlated to the ionization potentialsI6 and geometrical structures.11,21,23 According to some of the recent reports, there were isomers identified for neutral niobium cluster^'^^^^ and ionic clusters.18,20 In other experiments, the mass peaks for the products from the reactions were resolved and studied as a function of cluster size.21-26 In the reaction of niobium clusters with benzene21.22 and deuterated benzene,22it was found that the dehydrogenation probability has a minimum at x = 8 and 10 but a maximum at x = 5,6, and 11. This dehydrogenation probability is dependent on the ionization laser intensity24and the wavelength of the ionization laser.22 Most recently, it was reported that the Nb,' cluster ions have a similar product distribution and dehydrogenation probability in the reaction with benzene.25 It was previously concluded2I that the apparent correlation between the ionization potentials (IPS) of the neutral niobium clusters and their reactivity reflects a correlation between both the IP and the reactivity and the stabilities as well as the structures of these clusters. In this paper, we use a laser vaporization supersonic molecular beam expansion technique3I to study the product mass distribution of the reaction between C 0 2 and niobium and niobium oxide clusters. By analysis of the product distribution as a function of cluster size, a possible mechanism responsible for the observed products is proposed. 11. Experimental Section
The details of the experimental setup have been described elsewhere." Briefly, the cluster machine used is composed of three differentially pumped chambers. The typical pressure of the source chamber is 2 X lo-' Torr when the nozzles are closed and 3 X lom4Torr when the nozzles are pulsed. The pressures in the ionization region and in the detection region are maintained at 1 X lo-' and 5 X Torr, respectively. The niobium clusters are synthesized in a pulsed molecular beam system similar to that first described by Smalley et aL3I The evaporation is done by focusing the 355-nm (5 mJ per pulse, 6 4 s pulse duration, and focused to 1-mm spot size by a 50-cm UV lens) UV light from a Nd:YAG laser (Quanta-Ray DCR-1A model) onto the niobium rod. The main pulsed nozzle (a Lasertechniques LPV pulsed valve driven by their 203 pulsed valve driver) is fired just before the laser, and the timing is adjusted to maximize the cluster signal. The backing pressure of the main nozzle is typically 100 psi of high-purity helium (99.999%). The reactant is introduced through a pulsed valve (General Valve Corp., Series 9) into the reaction tube (6 mm in diameter, 6 cm in length). The retention time of the clusters in the reaction tube is approximately 120 ps. After leaving the reactor, the cluster reaction product and unreacted cluster mixture is expanded into a high-vacuum region where a sudden decrease in temperature and density prevents further reaction from occurring. The cluster expansion passes a 2-mm skimmer to form a well-collimated molecular beam. The molecular beam then is crossed by an ArF excimer laser (Lambda Physik, EMG 101 MSC, 193 nm, 6.4 eV) in the ionization region, and the photoions produced are accelerated along a path that is at 90' from their (31) Geusic, M. E.; Morse, M. D.; OBrien, S. C.; Smalley, R. E. Rev.Sci. Instrum. 1985, 56, 2123.
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the first triple molecular addition on Nb9. See text for the detailed experimental conditions. original path by using a constant electrical field into the field-free region of a 1.7-m time-of-flight (TOF) mass spectrometer. The ions are collected by a tandem microchannel plate detector. The signal is amplified by a video preamplifier (Pacific Instruments, 150 MHz) and digitized by a LeCroy transient digitizer (Model 8828). The spectrum is stored and later analyzed in an IBM AT computer. 111. Results and Discussion Figure 1 shows the difference spectrum between a product
spectrum and a control spectrum of Nb, and Nb,O,.obtained at low laser fluence (less than 1000 pJ/cm2) to eliminate multiple-photon processes. In the control spectrum, pure He (with a backing pressure of 5 psi) was pulsed into the reactor. The product spectrum was obtained when the pure helium was replaced by a mixture of 20% carbon dioxide seeded in the helium gas. All the other conditions were kept identical with those of the control spectrum. In the difference spectrum, a mass peak below the base line means that the corresponding species is reactive and therefore its mass peak intensity was depleted, while a mass peak appearing above that line indicates a newly formed product. From Figure 1, a very strong increase in the NbO' signal intensity is observed. The other main products are Nb,(C02), Nb,0(C02), and Nb,02(C02) for cluster size x 2 5. The amount of these products formed can be controlled by the C 0 2 concentration and the duration time of the pulsed reactor valve. Other less intense products are the double and triple molecular addition of C 0 2 to Nb, (or Nb,O,) with a threshold of x around 7 (or 8) and 9 for the double and triple molecular addition, respectively. These higher order molecular additions seem to be more probable on large clusters, but the product intensities are weaker than those resulting from the single molecular addition. Due to the fact that it is difficult to obtain niobium clusters without their oxides, it is very difficult to ascertain the amount of oxides formed during the reaction. This is due to the fact that niobium oxides are formed in the reaction between Nb, and C 0 2 but also react with C 0 2 themselves. Thus, the observed Nb,O, peaks are net results of both formation and reaction. In order to separate these two effects, we have studied the reaction of the heavy oxygen isotopically labeled carbon dioxide (CO*,). Mass peaks appearing, e.g., Nb,O* (or Nb,OO*), reflect the reaction products of Nb, (or Nb,O). The depletion of the Nb,O mass peaks reflects the reactivity of such oxide clusters. A typical difference spectrum resulting from the reaction of Nb, and Nb,O, with the isotopically labeled C 0 * 2 is given in Figure 2. This figure correctly represents the depletion of Nb, and Nb,O, and some of the product formation. The detailed
The Journal of Physical Chemistry, Vol. 93, No. 6, 1989 2487
Reaction of CO, with Nb, and Nb,O,
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discussion will be given in the respective sections below. A. The Formation of Cluster Oxides. Figure 2 shows the results of the reaction of Nb, and Nb,O, with CO*z. New features are observed beside the o n e seen for the reaction of normal carbon dioxide with these clusters. First, each niobium monoxide cluster peak becomes a doublet with Nb,O (formed in the vaporization source) and Nb,O* (formed from the chemical reaction with the isotopically labeled carbon dioxide). Each cluster dioxide now becomes a triplet with the following composition: Nb,02 (observed as depleted peaks), Nb,OO* (newly formed peaks), and Nb,O*, (newly formed peaks). Nb,OO* must be formed from the reaction of Nb,O with CO*,: Nb,O CO*2 Nb,OO* CO*
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Each of the niobium trioxide clusters has a quartet mass peak corresponding to the following species: Nb,03+ (formed in the vaporization source), Nb,020*+ (results of the reaction of Nb,02 with CO*,), Nb,OO*,+ (results of the reaction of Nb,O with CO*,), and Nb,0*3+ (results of the reaction of Nb, with CO*2). There is a minor peak observed for the niobium dimer oxide (Nb20) appearing when COz is introduced into the reactor. This dimer oxide peak becomes N b 2 0 * when the isotopically labeled carbon dioxide is used. This signal is totally absent in the control spectrum, indicating that it is formed either from the direct photoionization of the "hot" neutral N b 2 0 (Nb,O*) or from the evaporation of the larger oxygen-containing ionic cluster products. B. The Formation of Nb,(CU,), and Nb,Uy(CU2),. From Figure 1, we observe a threshold for the appearance of Nb,(COz), cluster products. Below this threshold, only Nb,O, are observed. There are many ways to show this threshold effect. In this paper we use the conversion efficiency defined as Zi(x)/AZ(x),where f i ( x ) is the intensity of the product i and A f ( x ) = I,(control) I,(product) is the absolute value of the depletion of the peak intensity of cluster x . For example, the conversion efficiency of Nb, into Nb,C02 is defined as the following:
where f(Nb,COz)r,ction is the intensity of the mass peak of Nb,C02 produced; and I(NbJmntm,and Z(NbJmdOn are the mass peak intensities of Nb, before and after the reaction, respectively. To be precise, this conversion efficiency needs to be corrected by multiplying by the ratio of the photoionization cross sections of Nb,C02 to that of Nb,, but these ionization cross sections are presently unknown.
Figure 3. Relative product yield plotted as a function of cluster size. The broken lines are the relative yields to form niobium cluster oxide from the reaction of Nb, with CO*,, and the scale is on the right-hand side of the figure. The solid lines are the relative yields for the formation of the single, double, and triplet C02 molecular addition products. The scale for the relative yields of these three molecular products is on the left of the figure.
Figure 3 shows the conversion efficiency of Nb,(C02), Nb,(CO,),, and Nb,(C02)3. From this plot, we see a threshold for the formation of Nb,(C02) around x = 5 , a threshold for the formation of Nb,(C02), around x = 7, and a threshold for the formation of Nb,(C0J3 around x = 9. Not only are the absolute intensities of the multiple molecular addition products weaker than the single molecular addition products, but the conversion efficiency also drops from the single to double and triple molecular addition. The multiple molecular addition products can be eliminated by reducing the mole fraction of C 0 2 in the reactor. In Figure 3, the conversion efficiency of Nb, into Nb,O* (or Nb,O*,) is also shown as a function of cluster size for comparison. It is interesting to note that as x increases, the efficiency of making the oxide increases at first; but, as the addition products begin to form, the relative oxide yield drops. This strongly suggests that the oxide formation and the molecular addition are competing with one another. Furthermore, as x increases, the addition type reactions take over. The overall relative reactivity of Nb, and Nb,O is measured by the overall relative rate of their disappearance. This is determined by using the following equation which has been previously derived for high reactant ( C 0 2 in our case) concentration where the concentration change of the reactant during the reaction is negligible.14 Rx = -1n ([Nbxlf/[NbxlJ = In ([Nbxli/[Nbxlf) = tkx[Al (1)
where R, is the rate of the reaction of cluster x , t is the retention time, [A] is the concentration of the reactant in the reactor, [Nb,lf and [Nb,li are the concentrations of the cluster Nb, after and before the reaction, respectively, and k, is the rate constant for the reaction of cluster x with the reagent. The rate constant k, is actually the sum of all rate constants of the reactions of cluster x . In this experiment, the mass intensity ratio before and after the reaction is calculated and the logarithm of that ratio is plotted as a function of cluster size. From a plot of this kind, it is easy to compare the relative reactivity (or relative reaction rate) of the clusters as a function of cluster size. Errors can be introduced by using the above technique due to the fact that the unreactive scattering cross sections of Nb, clusters with He are certainly different from that with CO,. As a result, the residence time of the clusters in the reactor will change when CO, is mixed with He. This will in turn change the arrival time of the Nb, clusters to the ionization region when COz is added. This will add an additional apparent depletion of the bare clusters Nb, and which is not due to the chemical reaction. In any case, eq 1 is essentially correct, but k, might contain a term kapparent which is proportional to the effect of the unreactive scattering cross section for the reagent molecules. The fact that the total intensity
2488
Song et al.
The Journal of Physical Chemistry, Vol. 93, No. 6, 1989
..
1
0
2
4
6
8
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CLUSTER SIZE
CLUSTER SIZE
Figure 4. Relative rate constant k , as a function of cluster size for both the pure niobium clusters and their cluster oxides with CO*z. The
relative rate constant generally increases as the cluster size increases.
loss of Nb, or Nb,O is much larger than the sum of the total mass peak intensity of the products might be attributed partially to the different ionization cross sections for the reacting and product clusters and partially to the presence of unreactive scattering processes. The quantity In [(Zc(x)/(Zp(x)] is plotted as a function of cluster size in Figure 4, where Zc(x) is the peak intensity of either Nb, or Nb,O in the control spectrum and Zp(x)is the peak intensity of Nb, or Nb,O in the product spectrum. From this plot we can observe that the apparent rate constant of the reactions of a certain cluster Nb, or Nb,O generally increases as a function of cluster size for both the niobium clusters and the niobium cluster oxides. From Figure 4,it can be seen that the changes in the relative reactivity of the cluster oxides with increasing cluster size are comparable to those for the pure niobium clusters Nb,. This means that the oxygen on each Nb,O does neither “poison” nor catalyze the niobium cluster in its reaction with CO,. C. Comparison of the Reaction of C02 with Niobium and Niobium Oxide Clusters. As for the niobium clusters Nb,, the conversion efficiency of the molecular addition reaction of COz to Nb,O is plotted as a function of cluster size in Figure 5. The addition of C 0 2 to Nb,O, mimics that of Nb,. At the concentration of C 0 2 used, the threshold for the formation of Nb,O(CO,) is x = 5 , for the formation of Nb,0(C02)2 it is x = 8, and for the formation of Nb,0(C02)3 it is x = 9. Above the thresholds the conversion efficiency first increases rapidly and then changes very little on larger clusters. A plot of the conversion efficiency of the molecular addition of COz to Nb,02 shows the same characteristics as that of the molecular addition of COz to the niobium cluster monoxides. There is a threshold at x = 5 for the single molecular addition, a threshold at x = 8 for the double molecular addition, and a threshold at x = 9 for the triple molecular addition. Another interesting plot is the conversion efficiency of Nb, into Nb,O* and Nb,0*2, in the reaction of Nb, with CO*,. This is also shown in Figure 3. Only clusters up to x = 9 were plotted due to the loss of resolution. From this plot, it can be seen that the oxygen abstraction channel is more dominant on small clusters and becomes less important on relatively large clusters, although up to x = 12 the formation of Nb,O*+ was still observed. Thus, the observed decrease in the apparent formation efficiency of the cluster oxides as the size of the cluster increases is a result of opening up new channels for the reaction of the larger clusters, e.&, leading to the formation of the Nb,0(C02), and Nb,(CO,),. D. Possible Reaction Mechanisms. 1. Abstraction us Addition Reactions. From the results of this experiment, there seems to be a correlation between the oxide formation conversion efficiency and the CO, molecular addition conversion efficiency. The latter has a threshold and increases very rapidly near the threshold and then changes very little for larger clusters. The former has a general tendency of decreasing with increasing cluster size.
Figure 5. Relative yields to make carbon dioxide molecular addition products to the niobium cluster oxides formed in the vaporization region. There are thresholds for the molecular addition shown in the figure. The higher order molecular addition is weaker than the single molecular
addition. However, its largest decrease takes place at the onset of the appearance of the CO, addition type reaction product. This suggests that both channels originate from the same type of mechanism. The above observation can be described by two different mechansisms. In the first one, the abstraction reaction is proposed to take place via an impulsive collision. As the cluster size increases, the collisions become more sticky, leading to addition type reactions. In the second mechanism, one proposes a sticky type collision leading to complex formation and a reaction forming Nb,O + CO for all the niobium clusters. Due to the exothermicity of the reaction, CO evaporates on small clusters (for x < 5) to cool off the resulting cluster products. It is difficult to distinguish between these two mechanisms. The use of a large range of pressures to study the collisional deactivation process is difficult to carry out as it greatly changes the operation characteristics of the nozzle beam and thus the cluster distribution. However, from the studies of Nb, with unsaturated cyclic hydrocarbon^,^^ sticky type collisions are found necessary to explain the observed addition and dehydrogenation reaction products. For the small clusters of Nb, in the reaction with benzene,21 addition type reactions are observed whereas dehydrogenation wcurs as the size of the cluster increases. It thus seems that reagents with double bonds are more likely to form a a-bond complex with metal atoms and clusters with empty d-orbitals. This suggests that the mechanism involving complex formation prior to reaction is more appropriate to describe the reaction between Nb, and CO,. Based on published values of the bond energies,32the reaction o f N b + CO, NbO C O has an exothermicity of -780 kJ/mol. An additional energy, corresponding to the bond energy of Nb-CO, will be given off if the product C O binds to Nb,. This energy can be estimated to be 120 kJ/mol based on the data This adds -900 kJ/moi to the internal energy of the cluster undergoing the reaction. The efficiency of the collisional deactivation of helium gas in the reactor (before the expansion into the ionization region) will determine the fate of these cluster products. If collisional deactivation is not very efficient, the number of vibrational degrees of freedom of small clusters is not sufficiently large to dissipate the thermal energy released from this reaction. As a result, the C O (CO*) part of the carbon dioxide molecule is “evaporated” from the cluster to cool and stabilize the cluster oxides formed. More collisions between the “cool” newly formed cluster oxide with COz (CO*,) molecules will undergo further oxygen abstraction (sequential reactions), resulting in the formation of Nb,02 (Nb,O*,) and Nb,03 (Nb,0*3). These results strongly suggest that these multiple oxygen-containing cluster
-
+
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( 3 2 ) CRC Handbook of Chemistry and Physics; Weast, R. C., Ed.; CRC Press: Boca Raton, FL, 1982. (33) Wedler, G.; Papp, H.; Schroll, G. Surf. Sci. 1974, 44, 463.
Reaction of C 0 2 with Nb, and Nb,O,
The Journal of Physical Chemistry, Vol. 93, No. 6, 1989 2489
LOG IONIZING LASER INTENSITY
Figure 6. Laser intensity dependence plot for Nb+ and NbO+ as a function of laser power at 193 nm.
products are formed in the reactor and not as a result of photochemistry in the ionization region. This is due to the fact that multiple collisions take place only in the reactor. As the size of the cluster increases, the vibrational and rotational degrees of freedom increase. This could allow for rapid dissipation of the energy released from the exothermic reaction between Nb, (Nb,O) and C02. As a result, the stability of the binding between the C O product and the large cluster oxides formed is enhanced (i.e., it increases their lifetime long enough to be detected). Therefore, the CO molecule might be bound to the cluster and an apparent molecular addition product is observed. On large clusters, the product might also have COz completely dissociated and bound as niobium oxides and niobium carbide. According to this mechanism, the C 0 2 in the Nb,COZ and the Nb,0,C02 clusters does not exist as C 0 2 but rather as bound C O (or C and 0) and 0; Le., the addition is of the dissociation type. It must be emphasized that this conclusion is reached, not on a structural ground, but simply on the observed dependence on the observed competition of forming Nb,O and Nb,C02 on the cluster size. Further structural studies are certainly needed to confirm this proposed mechanism. In the above mechanism, we blamed the evaporation of C O molecule(s) from small cluster products on the unthermalized heat of reaction of Nb, (or Nb,O,,) clusters with C 0 2 . However, part of the excess internal energy in the cluster products could also result from part of the excess energy of the one-photon ionization process. The threshold observed for the molecular addition might also be attributed to geometric factors. On the small clusters, the size of the cluster is not sufficiently large to "anchor" a linear triatomic carbon dioxide molecule. As the cluster size increases, there is more room for the triatomic molecules to be attached to the cluster molecule, and a molecular addition type reaction can take place. 2. The Mechanism of the Formation of the Nb@ Mass Peak. The dominance of the NbO+ mass peak resulting from the reaction of Nb, with C 0 2 is observed in Figure 1 and of the NbO*+ resulting from the reaction of Nb, with CO*z is observed in Figure 2. There are a number of possible mechanisms that might explain the appearance of the intense NbO+ and NbO*+ mass signal. Large amounts of NbO+ could be formed in the reactor Nb
+ C02
+
+ CO
(A)
+ Nb,-,CO
(B)
NbO
or Nb,
+ C02
+
NbO
Upon ionization NbO+ and Nb,-lCO+ should be observed, the first probability via the absorption of two photons34and the second by a one-photon absorption process. Figure 6 shows the laser intensity dependence of the NbO+ signal as well as the Nb+ signal. The latter is known to be formed by a two-photon absorption process, and indeed it gives a slope of approximately 2. Under the same laser intensity, the NbO+ shows a slope close to unity
and thus is formed via one-photon absorption. (However, one cannot exclude the possibility that NbO may have intense excited charge-transfer states at the one-photon energy which are readily saturated, leading to an apparent one-photon ionization process.) This, together with the fact that the Nb,,CO+ signal was never observed in these experiments (although Nb,CO+ was observed in the reaction of Nb, with COI4), suggests that mechanism B above cannot be correct. For mechanism A to be valid, one has to propose that only one photon is required to ionize the NbO formed in the reactor and that an unusually large amount of NbO is produced. N b could indeed be present in large amounts. One might expect that NbO to have a higher IP34than N b (on account of the high electronegativity of oxygen); its ionization can take place via a one-photon process only if NbO is formed in the reactor in an (vibrationally or electronically) excited state. The most serious objection to mechanism A is the fact that the intensity of the NbO*+ produced is so much higher than all any of the other oxides Nb,O*+. The large and sudden drop in the Nb,O*+ mass signal at x >1 strongly suggests that an unusual mechanism (nonlinear type) must be involved in its production. We propose that, for many oxygen-containing cluster products, both the reaction and the one-photon ionization processes could leave them with excess internal energy. Some of the e x m s energy could be removed by the evaporation of NbO in the reactor (if its ionization indeed requires only one photon) or NbO+ in the ionization region. Some cluster ions might be so "hot" that they need to evaporate NbzO or Nb20+. In strong support of the evaporation mechanism in forming NbO+ signal is the observation that more NbO+ was detected after the reaction with C0*2. (This increase exceeds the amount that could be formed due to the 1% C 0 2 in the isotopically labeled carbon dioxide.) Mechanism A should yield only NbO*+ (and not NbO'). The observed increase in NbO+ in the reaction with C0*2 could be conveniently explained as the evaporation of either clusters such as Nb,OO* or Nb,0CO*2 in the reactor or their ions in the ionization region. 3. The Mechanism ofthe Formation of Nb20+. As mentioned earlier, there is a small amount of Nb20+observed after the carbon dioxide is introduced into the reactor. The oxygen in this dimer oxide originates apparently from the carbon dioxide used. The dimer oxide signal is totally absent in the control spectrum. Since N b 2 0 neutral molecules cannot be ionized under the experimental conditions, these results suggest that the observed Nb20+is formed from the evaporation from the larger cluster products upon photoionization. Like NbO', part of the N b 2 0 +heat of evaporation might result from the unthermalized heat of the reaction itself. The following fragmentation mechanisms might be responsible for the signal observed: Nb,( C02) #,
hv
-
Nb,( C02)#,
[Nb,( CO2),] # # + NbzO+ Nb,-2(C0z),-l(CO)
(la)
N b 2 0 + {Nb,2(COz)n_lCO)+
(1b)
-.-+
+
+
hv
[Nb,( CO2),] # # + N b 2 0 + Nbx-2(C02)n-1 C O (2a)
Nb20
+
+
+ {NbX-2(CO2),,J+ + C O
+
(2b)
where # signifies thermally or internally hot cluster species. Since there was no (Nb,(CO,)CO)+ signal observed, mechanisms 2a and 2b are more probable. This is very reasonable because (2a) and (2b) should be more effective in cooling the large remaining fragment (i.e,, represent a lower energy channel and thus should have a larger RRKM rate constant35) than (la) and ( 1 b). They (34) So far there is no direct measurement of the IP of the monomer oxide to our knowledge. However, the IP of the niobium atom is known to be 6.88 eV. The IP of NbO is expected to be higher than that of N b due to the high electronegativity of the oxygen atom and thus should be higher than the photon energy of 193-nm radiation (6.42 eV). This argument is strongly supported by the observation in ref 26b that NbCN and NbBr both have ionization potentials higher than 6.42 eV. (35) Laidler, K. J. Chemical Kinetics; Harper and Row: New York, 1987.
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The Journal of Physical Chemistry, Vol. 93, No. 6,1989
Song et al.
are also more favored by entropy considerations than ( l a ) and (1 b). In this fragmentation process, channel 2b is expected to be more probable than channel 2a because the product carrying the positive charge on the dimer oxide is expected to be less stable than that with the charge on the larger fragment. The same argument applied for the reaction producing NbO'. The reason why the signal of NbO' is much larger than that of N b 2 0 +is probably due to the expectation that NbO' is produced from "hot" cluster oxide ions while Nb20+ is produced from "very hot" cluster ions. The latter should have much smaller density than the former. As proposed for the NbO', one may propose the following reaction for the formation of Nb20+: Nb2
+ C02
-
CO
+ [Nb20]#
6
, -co I
I
Nb20+
On the basis of this mechanism, the neutral dimer oxide formed in the reactor is internally "hot" and thus is ionized by one photon of the 193-nm laser light. (Under the same fluence condition the "cold" niobium dimer oxide formed in the vaporization region cannot be ionized by one-photon absorption.) This possibility cannot be ruled out from our experiment since this mechanism has also been proposed for the formation of NbO'. If the evaporation mechanism for the formation of NbO+ and N b 2 0 + is indeed correct, the fact that NbO+ is formed in much larger quantities can suggest either that its formation goes through a much lower energy channel or that it requires a much lower energy barrier. E. Changing Pressures and Ionizing Laser Wavelength. In order to distinguish between thermal and photochemical evaporation, we have studied the reaction of Nb, with C 0 2 under a variety of conditions. In one experiment, the 218-nm radiation (the second anti-Stokes stimulated Raman line of hydrogen gas pumped by the 266-nm light of the fourth harmonic output of a Nd:YAG laser) was used as the ionization source. In another experiment, the backing pressure of the main source nozzle was changed from 50 to 100 psi. The carrier gas of C 0 2 reactant in the reactor was changed from He to CH4 and SFs in a third experiment. The product yield as a function of cluster size was found to be essentially the same under all these types of small range of perturbations within our experimental error. It is difficult to answer the question we are asking. If one believes that the cooling results from the He gas that makes the niobium clusters (since its pressure is much higher than the carrier gas in the reactor), then a change of its pressure by a factor of 2 would not change the results within our experimental uncertainty. The absence of wavelength dependence could suggest either that thermal evaporation is dominant or that the ionized electron carries most of the excess energy above the ionization threshold, leaving the same amount of internal excitation energy which is independent of the ionization wavelength to the cluster or cluster product ions. IV. Summary of Conclusions In summary, the exothermic nature of the reaction of C 0 2 with niobium (or their oxide) clusters to form Nb-0 and N b C O bonds together with some of the excess energy received upon singlephoton ionization could explain all the observations. As x increases, the conversion efficiency for the oxide formation decreases as that for the molecular addition increases. This suggests that, for small clusters, evaporation of CO and other small species occurs (e.g., Nb, Nb2, NbO, and N b 2 0 in the reactor and NbO' and N b 2 0 + in the ionization region). Due to the relative stabilities of NbO' and Nb20+,the rate of formation of these species might be larger than that for the bare niobium species. For large clusters, less evaporation of C O in the reactor takes place, leading to Nb,(C02), or NbxOy(C02), molecular addition type products. The overall rate of the depletion of both the bare niobium and niobium oxide clusters is found to increase as the cluster size increases. This might be due to an increase in the number of channels for. the formation of the stable reaction products (e.g., addition of more and more COJ as well as an increase in the cross sections for unreactive scattering as the size of the cluster increases. Mechanistically, one might write down the scheme shown in Figure 7 to represent some of the processes that might be occurring
Figure 7. Schematics representing some of the possible mechanisms that could explain the laser mass spectrum of the products of the reaction between niobium clusters (Nb,) and C02 at sufficient concentration to allow multiple collisions in the reactor (but not in the ionization region). Arrows represent different processes: ionization (+), reaction (--+), evaporation of CO and evaporation of NbO'or Nb20+(--)). As the size of the cluster products or the cluster product ions increases, the smaller is the probability for the evaporation process. Species observed in the TOF spectrometer are underlined. (.-e+),
for the reaction of Nb, with C 0 2 (with similar equations for the reactions with Nb,?,). Another interesting observation was that we did not observe the reaction Nb,
+ C02
+
Nb,C
+0 2
For x = 1, the reaction has an exothermicity of approximately 3 k J / ~ n o Iwhich , ~ ~ is not favorable at all compared to the formation of Nb,O or Nb,O(CO). The above mechanism is based on the observation for the stable species formed during the time of our reaction and detection and at the C 0 2 pressure we have used. In this scheme, the reaction that forms a metal oxide and metal carbonyl with each niobium cluster is proposed to be the first step. Stabilizing the different clusters by evaporation takes place either in the reactor by evaporating CO (or NbO) or in the ionization region by evaporating NbO' or CO (if large excess energy above the ionization threshold is retained by the cluster product ions, which have retained a large amount of heat of the reaction of their neutrals in the reactor). This conclusion is based on the fact that intense NbO' mass peak is observed even at the lowest ionization laser intensity used. For clusters with x between 5 and 10, the difference between the IP and the one-photon energy is nearly 1 eV. This difference is larger for larger clusters. A good fraction of this energy is probably taken away by the ionized photoelectron. However, the final fate of the "hot" cluster neutral products formed thermally in the reactor will no doubt be determined by the amount of excess energy above the ionization threshold retained by the cluster ions after ionization. The fact that our results show that the large clusters seem to be able to stabilize the formation of more chemical bonds (in the addition type reaction) suggests that some or all of the excess energy must result from the exothermicity of the thermal reaction. Acknowledgmenf. This work is supported by the Office of Naval Research. A.E. thanks the Deutsche Forschungemeinschaft for a research scholarship. Registry No. Nb, 7440-03-1; C 0 2 , 124-38-9;niobium oxide, 1262700-8.
