J. Phys. Chem. 1988, 92, 421-429
421
Aluminum Clusters: Ionization Thresholds and Reactlvtty toward Deuterium, Water, Oxygen, Methanol, Methane, and Carbon Monoxide D. M. Cox, D. J. Trevor,+R. L. Whetten, and A. Kaldor* Exxon Research and Engineering Co.. Annandale, New Jersey 08801 (Received: July 8, 1987)
Using the pulsed cluster beam flow reactor technique, we have measured the reactivity of aluminum clusters toward several molecules under thermal conditions. For each different molecule we observe that the reactivity exhibits a unique dependence on the number of aluminum atoms in the cluster. The overall reactivity of aluminum clusters toward different molecules is ordered roughly as O2 > CH30H > CO > D20> D2 > CH4 with CH4 showing no reaction under these experimental conditions. In addition, we are able to place upper and lower bounds on the ionization thresholds for the smaller aluminum clusters.
Introduction Recent reports'-3 have shown that chemisorption of a reactant gas (H2) on metal clusters is remarkably sensitive both to the number, n, of metal atoms (cluster size) as well as to the metal type (e.g., Nb, Co, Fe). For example, particular niobium clusters, Nb8, Nblo, and Nb16,exhibit significantly less reactivity toward H2 than other niobium clusters. For cobalt, Co, and COTare the least reactive clusters. For iron clusters reactivity toward hydrogen is an oscillatory function of the number of atoms in the c l ~ s t e r . ~ , ~ This oscillatory behavior is found to anticorrelate well with iron cluster photoionization thresholds4 and a simple model based upon charge transfer for hydrogen activation was proposed to explain this anticorrelation.2a Pronounced size-selective variation in ionization threshold energy and hydrogen chemisorption has recently been reported for Nb2b,5and VZbclusters, and as first observed for iron clusters, a good anticorrelation is obtained between the reactivity and the ionization threshold energy for clusters containing more than eight atoms. Nitrogen exhibits strong cluster size dependent chemisorption with N b and Co.lb Carbon monoxide chemisorption is facile for most transition-metal clusters exhbiting little variation with cluster size.laV6 Similarly iron clusters exhibit facile chemisorption of O2and dissociative chemisorption of H2S, but no reaction with CH4 could be observed under identical experimental condition^.^ In this paper we have investigated how the reactivity of gasphase aluminum clusters Al, ( n = 1-30) varies for a series of different molecules H2, D2, D20, 02,CH30H, CH4 and CO. Al, reactivity is found to be extremely sensitive both to reactant molecule and to cluster size n, with several orders of magnitude variation in reactivity observed for particular combinations of these parameters. So far it has been the most versatile system studied. Experimental Method The experimental techniques have been described in several previous publications2,68 and will be discussed only briefly here. Metal clusters are synthesized by pulsed-laser vaporization of metal atoms from a continuously rotating and translating metal rod inside the throat of a high-pressure pulsed nozzle. The vaporizing laser is synchronized to fire during the time a high-pressure helium pulse passes by the rod surface. The hot metal vapor, entrained in the helium, partially condenses to form clusters and cools to near ambient (350-500 K) during flow down a narrow tube extending from the vaporizing zone. Attached to the end of this tube is a larger diameter reactor tube into which a second pulse containing either the reactant gas seeded in helium or pure helium is injected. The average pressure in the reactor is estimated to be on the order of 10 Torr. The secondary reactant pulse is adjusted to be about 'I4 as intense as the primary helium pulse. The partial pressure of reactant in helium prior to injection is varied from 0.02% for highly reactive molecules up to 63% for the least reactive molecules. In one instance experiments were +Present address: AT&T Bell Laboratories, Murray Hill, N J 07974.
0022-3654/88/2092-0421$01.50/0
TABLE I: Bounds for Ionization Threshold Energies for Aluminum Clusters
lower bound, eV 6.0 6.42 near, but >6.5 6.42 6.0 very near 6.42 near, but 6.5
6.5 6.42 very near 6.42 near, but 15 extremely weak relative to the bare cluster signal. Since y > 3 product peaks are detected on all reactive (n = 6-21) clusters except A16, we propose that A16Dy,y > 2, either are not formed or, if formed, are thermodynamically unstable and decompose, are not detected (ionization threshold higher than 7.87), or are unstable upon ionization. The main differences between the results obtained with the small reactor and the large reactor are (a) the appearance of Al,D+ only in the former case, (b) the evidence of weak reactivity for clusters out to 15 atoms in size with the small reactor, (c) a decreased reactivity on A16 and A17 compared to Ala (as well as larger clusters) with the small reactor. From these observations and energetic considerations we suggest that Al,D+ results from direct ionization of A1,D present in the beam and that A1,D is produced only when the smaller diameter reactor is installed. The difference introduced by the small reactor is the enhanced probability of continued cluster growth while in the reactor, since next to He and Dz, A1 and A12 are the next most abundant components in the reactor. Injection into a larger diameter reactor will dilute the cluster and metal atom concentration more than injection into a smaller diameter reactor. Thus A1,D may be produced in the reactor by a reaction sequence such as Al,
+ A1
Al,D2
-
+ D2
A1,DZ
A1,D
(5)
+ AID
(6)
The bond dissociation energy (BDE) of the AlH molecule is about 3 eV,I8 whereas the bond strength between atomic hydrogen and bulk aluminum is on the order of 2 eV.19 This decrease in bond strength as the bulk is approached infers that BDE (A&-H) should be less than the BDE (AI-H), thus making reaction 6 somewhat exothermic (reaction 6 would be nearly thermoneutral if BDE(A1,-H) = BDE(A1-H)). We propose that this exothermicity is sufficient that reaction 6 will be competitive with Al,D,
+ A1
+
Al,+I
+ Dz
(7)
which is exothermic (BDE(A1,) = 1.5 eV18 (cohesive energy of bulk aluminum is 3.4 eV/atomzO). The production of Al,D+ by dissociative ionization A1,D2
hv = 7.87 eV
Al,,D+
+ Al,
appears not to be a likely explanation since this mechanism is expected to be independent of whether a large or small diameter reactor is used. However, let us examine this process more closely. The dissociative ionization process does require net energy input. This consists of the sum of the IP(AlP1D) and the bond dissoAl,ID + AID. The IPS of Al,,D ciation energy for A1,D2 are assumed to be >6.42 eV since such ions are not detected with
-
(17) Clusters containing an odd (as well as an even) number of D atoms are easily produced when deuterium is added to the carrier gas because deuterium is also decomposed in the vaporization region producing highly reactive atomic species. The D2/He ratio in the mix was 0.22. (18) Huber, K. P.; Herzberg, G . Molecular Spectra and Molecular Structure IV. Consranis of Diaromic Molecules; Van Nostrand Reinhold: New York, 1979. (19) Hjelmberg, H. Surf.Sci. 1979, 81, 539. (20) Kittel, C. Solid Scare Physics, 5th ed.; Wiley: New York, 1976.
0
5
10
15
20
25
30
AI Atoms Per Cluster
Figure 6. The relative rate constant ( R / F ) of A1 clusters for reaction with D20plotted as a function of cluster size. The uncertainty in R / F is again about &20%of full scale or f40. Ionization conditions the same as given in Figure 5, and D 2 0 / H e ratio in mix was varied from 0.004 to 0.0004.
low fluence 6.42-eV ionizing photons. The cluster metal bond energy can reasonably be expected to fall between 1.5 and 3.4 eV, the dimer BDE and the bulk cohesive energy, respectively. Thus a minimum photon energy of 7.92 eV is likely to be necessary for this dissociative ionization process close to the ionizing laser photon energy of 7.87 eV. Dissociative ionization may be possible if the clusters are not sufficiently cold. Since the AI-H bond is even stronger than the metal-metal bond, a dissociative ionization process such as Al,D2
hu
Al,D+
+D
(9)
is even less likely than reaction 8. Although multiphoton adsorption processes cannot be completely ruled out, we argue that they should be less probable under the present operating conditions, because (a) both ArF and F2ionizing laser intensities are purposely kept as low as possible and are comparable so we might reasonably expect nearly the same fragmentation from either laser (2 photons total energy be 12.8 or 15.8 eV). (b) the disappearance of signal on Al,, Al,, A15, A1, using low fluence 6.42-eV ionizing radiation supports the contention that the ionization thresholds of these clusters are greater than 6.42 eV and that multiphoton processes are probably not important contributors to ion production. (c) the cluster distribution remains relatively constant in shape for factors of 4 variation in ionizing laser intensity around the typical operating conditions. Increasing the ionizing laser intensity by a factor of 100 or greater (focusing) does greatly perturb the cluster distribution with a significant (nearly all) loss of large cluster ion signal and a concomitant increase in the small cluster ion signal (particularly on the atom). Lastly, we suggest that the changes observed in the reactivity pattern between the small and large reactor may be explained if additional cluster growth such as Al, Al,
+ A1 + A12
---*
+
Al,+1
(10)
Aln+2
occurs in the small reactor but is greatly reduced with the large diameter reactor. Thus with the small reactor Al, bare cluster signal may include an extra component which results from growth in the reactor from A , and A15 (nonreactive clusters) which have combined with a dimer (or two atoms) or a single atom, respectively. Such effects lead to an apparent reduction in A16 reactivity compared to that in the large diameter reactor where this process does not occur. Assuming a similar process is occurring on Ala, for example, Al, would get extra signal from A16 and/or A17 (both reactive clusters). This component of the A18
The Journal of Physical Chemistry, Vol. 92, No. 2, 1988 ,
,
,
,
(
,
,
,
,
,
1
1
1
1
, , , ,
,
,
,
Cox et al.
)
CHBOH 2000
1500
1000
500
0 0
5
15 20 AI Atoms Per Cluster
10
25
Figure 7. The relative rate constant (R/F) of AI clusters for reaction with methanol plotted as a function of cluster size. The uncertainty is about Et2055 of full scale or k400. Ionization condition the same as in Figure 5 , and CH30H/He ratio in mix was varied between 0.0002 to
0.0008.
bare cluster signal should react more readily than A18, Le., Al, and A17 react away before forming Al8. If so, the A18 reactivity appears to be enhanced relative to its value in the large diameter reactor. 2. Water Chemisorption. With water, the major product ion peaks observed at low extent of reaction are of mass equivalent to A1,(D20), (n 2 9; m = 1, 2, 3) with the m = 1 peak largest. We see no evidence of fragment ion peaks of the form A1,0+, Al,,DO+ or Al,,O+.zl The reactivity of aluminum clusters toward D 2 0 as a function of size is shown in Figure 6. A sharp increase in reactivity begins at A19 peaking at AIlo,while A12-A18 are highly unreactive. An oscillatory behavior of the reactivity is observed in which a local minimum occurs near All4 followed by a second maximum around AIl7and Al18. The peak reactivity toward D 2 0 on All,, is a factor of 50 larger than the peak reactivity toward D2 which occurs for A16. 3. Methanol Chemisorption. As shown in Figure I, most aluminum clusters react even more strongly with methanol than with D2 or D 2 0 . Interestingly, Al, and AlI4are found to be the least (but still highly) reactive toward methanol. The product peaks have masses equivalent to A1,(CH30H), with m = 1, 2 , or 3 with m = 1 the most intense at low extent of reaction. The peak reactivity for methanol occurs for AIl6 and All7 and is about 10 times more reactive than the peak reactivity for D 2 0 on All, and 500 times more reactive than the peak reactivity for Dz on AI6. At higher methanol partial pressure a very complicated mass spectrum with myriad product peaks of the general form AI,C,H,O, is observed suggesting that a gas-phase chemical reaction or aggregation may be occurring. Further work will be necessary before more can be said. 4 . Oxygen Chemisorption. In Figure 8 , the reactivity of aluminum clusters toward molecular oxygen is plotted as a function of cluster size.21 Surprisingly the aluminum cluster reactivity toward oxygen depends sensitively on cluster size. Namely, the atom and dimer are highly reactive toward O2but a sharp decrease in reactivity is observed at A13, followed by a nearly monotonic increase in reactivity with increasing cluster size until the 2530-atom clusters finally become as reactive as the dimer. The dominant product ion peak, detected at even the lowest oxygen partial pressures, is A1302. This product peak apparently is not produced via the usual chemisorption process, addition of an oxygen molecule to AI3. The justification for this statement lies in the observation that the bare A13 ion peak does not show any depletion upon addition of O2to the reactor while both the (21) Typically the monooxide can be observed as a small satellite peak on all aluminum clusters without any addition of O2 or other reactant into the system. These monooxide species apparently result from the small oxide present on the aluminum rod itself. The monooxide peak are also reactive.
5
0
30
10 15 20 AI Atoms Per Cluster
25
Figure 8. The relative rate constant ( R / F ) of AI clusters for reaction with oxygen plotted as a function of cluster size. The uncertainty in R / F is about f20% of full scale or f1000. Ionization conditions the same as Figure 5, and 02/He ratio in mix was varied from 0.0002 to 0.0006.
atom and the dimer exhibit very pronounced depletion. We propose that in this instance the A1302 product is produced by a reaction sequence such as A12
+0 2
A1
+ 0,
and/or
+
-
A1202
A102
+ A1
+ A12
+
-
A1302
A1302
This appears plausible in light of the fact that the atom and dimer concentrations are the largest of all metal species in the reactor, and also are the most reactive. The product A1202 is not expected to be observable in the photoionization mass spectrum because its 9.9 f 0.5 eV ionization potential22is significantly above the highest ionizing laser photon energy of 7.87 eV. Similarly the nonobservation of AlO, suggests that it also has an ionization potential above 7.87 eV. This is possibly the result of a resonant stabilization when bonding to two oxygen atoms, forming partial double bonds which have an average bond strength of about 109 kcal/mol compared to 120 kcal/mol for .A1=0.23 It is interesting that reactions of bare aluminum cluster ions, Al,', n = 1-3, with molecular oxygen show the dimer ion to be the most reactive while the trimer ion is very n ~ n r e a c t i v esimilar ~~ to that observed above for the neutral species. However, the chemistry is not the same in the two cases. The ion reactions are of the form Al,+
+ O2
-
A1,0+
+0
whereas the reaction of neutral A1 and Alp with O2 is likely to be a simple addition reaction, since the heats of formation23of small aluminum oxide molecules strongly favor addition over substitution reactions. For the atom, the substitution reaction is nearly thermal neutral due to the almost identical bond strengths of O=O and Al==O and the ability of an aluminum atom to bond strongly to two oxygen atoms. Since the pressure in the chemical reactor is relatively low, collisional stabilization may be an important consideration for these smaller molecules. For Alz, the addition reaction AI2 + O2 AI2O2 is strongly favored on energetic groundsz3compared to either substitution reaction, A1, -+
(22) (a) Drowart, J.; DeMaria, G.; Bums, R. P.; Inghram, M. G. J . Chem. Phys. 1960, 32, 1366. (b) Ho, P.; Burns, R. P. High Temp. Sci. 1980, 12, 31. (c) Fu, C. M.; Burns, R. P. High Temp. Sci. 1976, 8, 353. (23) Chase, Jr., M. W.; Davies, C. A.; Downey, Jr., J. R.; Fruip, D. J.; McDonald, R. A.; Syverud, A. N. JANAF Thermochemical Tables, Third Edirion (J. Phys. Chem. Ref. Dara, 1985, 14, Supplement No. 1, 1985). values are 249 kJ/mol for 0; 329.7 kJ/mol for Al; 66.9 kJ/mol for AIO; -86 kJ/mol for A102;487 kJ/mol for Alz; -145 kJ/mol for A120;-394.6 kJ/mol for (A10)2. (24) Hanley, L.; Anderson, S . Chem. Phys. Left. 1986, 129, 429.
The Journal of Physical Chemistry, Vol. 92, No. 2, 1988 421
Reactivity of Aluminum Clusters
+
-
-
+
+
O2 A120 0 or A1202 A102 Al. This suggests that exothermic reactions of O2 should occur with all aluminum clusters. It is very surprising that the reactivity drops dramatically at A13, and only gradually increases with increasing cluster size until finally becoming comparable with that of the dimer at the 25-30-atom cluster. This suggests that kinetic effects must be important and that the reactions are not dominated solely by thermodynamic considerations. Jarrold and Bower25have studied the reactions of mass-selected aluminum cluster ions, Al,', n = 4-25, with oxygen. In their experiments no oxygen-containing product ions were observed for center of mass collision energies of 1 eV, but oxygen was found to cleave aluminum clusters according to the reaction Al,+
+
0 2
-
Al,'
+ Al,,O2
For n = 4-6, Al' is the only product observed, but for n = 7-12 the pattern of Al,' observed suggests that Also2 (or A120 + Al,O) is the major neutral product. At Al13+,A1402or 2(A120) become the dominant neutrals. For clusters larger than n = 16, loss of Alloor equivalent becomes an important channel. If thermodynamics is the determining factor then units with composition (A1402), (Also2), and (AlloO2)may be particularly stable. The observation that aluminum cluster ions fission upon reaction with molecular oxygen raises a question regarding the assumption that only simple addition reactions are occurring upon reaction of O2 with neutral aluminum clusters. The exothermity of the neutral reactions will differ from that of the ion reactions only by the difference in ionization potentials of the parent ion and its fragment ion. As can be seen in Table I, this difference is bracketed to be less than 0.5 eV. In addition, the ion-molecule reactions are carried out with center of mass collision energies up to 3 eV. Such a high kinetic energy would tend to promote fission. The higher pressure of the flow reactor will help stabilize the collision complex and thus promote addition reactions. We observe no evidence for fission-type product ions in the mass spectrum. Nevertheless, fission reactions of the neutral clusters with O2 cannot be completely ruled out strictly on energetic grounds. 5. Methane Chemisorption. We observe no evidence for chemisorption of methane on aluminum clusters or on the atom or dimer since neither product ions nor bare cluster depletion are observed. A reaction between the atom and methane was initially thought to be possible since it had been reported that ground-state aluminum atoms insert into the methane C-H bond.26 However, a recent study revealed no evidence for such a reaction, although a photochemical initiated reaction was found to be both facile and photorever~ible.~' 6. Carbon Monoxide Chemisorption. The reactivity of aluminum clusters toward C O behaves qualitatively similar to that observed for deuterium/hydrogen in the sense that Al, is the most reactive cluster. However, in contrast to hydrogen, A16 reactivity with C O is about 100 times greater than that with D2. All other clusters n 1 2 do react with C O with a reactivity that is about a factor of 2 less than for A&. In addition, only bare cluster depletion is observed, no Al,(CO), product peaks are detected. The nonobservation of Al,(CO), product peaks is attributed to one or more effects. First, adduct species which are formed and escape intact from the reactor must still survive for an additional 400-500 ps, a time sufficient for them to travel the 85 cm from the reactor to the ionization zone. For AI,-CO complexes which contain significant internal energy, C O may desorb during this transit time. Depletion of the bare cluster would still be observed if the thermal dissociation products (presumed to be AI, and CO) leave the detection zone. Second, even if A1 clusters still have chemisorbed CO when they arrive in the ionization zone, irradiation by the ionizing laser may result in dissociation or dissociative ionization, processes which could greatly reduce the parent ion signal. Third, chemisorption of CO on many metals leads to (25) Jarrold, M. F.; Bower, J. E. J . Chem. Phys. 1986, 85, 5373. (26) Klabunde, K. J.; Tanaka, Y. J . Am. Chem. SOC.1983, 105, 3544. (27) Parnis, J. M.; Ozin, G. A. J . Am. Chem. SOC.1986, 108, 1699.
TABLE I 1 Tabulation of Molecular Orbital Energies and Energy Differences from EF"(eV) molecule CH4
D2
co H20 CH,OH 02(triplet)
orbital energy,* eV
acceptor
donor
orbital
eA* - E f
EF - eA
LUMO HOMO LUMO HOMO LUMO HOMO LUMO HOMO LUMO HOMO HOMO
17.5 -14.7 7.2 -16.2 3.5 -15.1 11.0 -1 1.7 13.1 -11.8 -14.5
21.75 10.45 1 1.45 11.95 7.75 10.85 15.25 7.45 17.35 7.55
" E Fis taken as 4.25 eV (ref 20). bValues from ref 31.
a shift (increase) in the work function relative to the bare metah2* As discussed earlier, the ionization thresholds of small aluminum clusters are close to 6.42 eV with some having thresholds above but most falling below this value. If addition of a single C O molecule raises the ionization threshold about 1.5 eV, such species could become virtually undectable via single photon ionization with 7.87-eV photons. The chemisorption of C O toward many different transition metals is discussed in detail elsewhere.,
Chemisorption Model In order to rationalize the observed trends in relative reactivity of aluminum clusters between different molecules, we appeal to simple molecular orbital considerations, in which the strength of the adsorbate-metal cluster i n t e r a c t i ~ nis~ assumed ~ . ~ ~ to depend primarily upon metal charge donation (donor adsorbates). In such instances the interactions will depend upon P*'/(eA* - EF)
or P2/(EF - e A )
(12)
respectively, where eA* denotes the energy of the adsorbate LUMO into which electron density is transferred from the cluster HOMO located at EF,eA is the adsorbate H O M O from which electron density is transferred to the cluster LUMO also assumed to be near EF, and P2 and P*2 are the respective interaction matrix elements. In order to examine coarse systematic trends, the values for orbital energies3Iof the molecular LUMO and H O M O S for CH,, CO, and H 2 0 are summarized in Table 11. Assuming H,, 02, the Fermi energy for aluminum is 4.25 eV, the value of the bulk work function32the differences eA* - EF and E , - eA are summarized in Table 11. We find that trends observed in the experimental reactivates between the different molecules can be crudely correlated with the inverse of the energy denominators in Table 11. For example, assuming CH,, D2, and C O behave mostly like acceptor molecules, the energy denominators are 22, 11.5, and 7.75 eV, respectively, whereas assuming that H 2 0 and C H 3 0 H are donor molecules, the energy denominators are 7.45 and 7.55 eV, respectively. Thus considering only molecular orbital energies CO, D20, and C H 3 0 H would be predicted to react more readily than H 2 and CH,, consistent with our experimental observations that the CH4 and D2 are much less reactive than CO, D 2 0 , or C H 3 0 H . Note that large clusters are unreactive toward both CH, and D2 and that their reactivity pattern differs only for clusters which (28) A change in the work function of up to 1.8 eV has been reported for CO chemisorption on ruthenium (Klein, R. Surf.Sci. 1970, 20, 1.) while changes on the order of 1 eV have been reported for Pd (Wedler, G.; Dapp, H.; Schroll, G. Surf.Sci. 1974, 44, 463.); Ni (Shayegan, M.; Williams, E. D.; Glover 111, R. E.; Park, R. L. Surf. Sci. Left. 1985, 154, L239.) and Re (Klein, R. Surf. Sci. 1970, 20, I.). (29) (a) Shustorovich, E. Surf. Sci. Rep. 1986, 6, 1 and references therein. (b) Shustorovich, E.; Baetzold, R. C.; Muetterties, E. L. J. Phys. Chem. 1983, 87, 1100. (30) Saillard, J. Y.; Hoffmann, R. J . Am. Chem. SOC.1984, 106, 2006. (31) Jorgensen, W. L.; Salem, L. The Organic Chemist's Book of Orbitals; Academic: -New York, 1973. (32) Weast, Ed. J. CRC Handbook of Chemistry and Physics, 64th Ed.; CRC: Boca Raton, FL, 1984.
428
Cox et al.
The Journal of Physical Chemistry, Vol. 92, No. 2, 198'8
contain 6-21 atoms which exhibit some reaction toward Dzbut not toward CH,. Considering both D2 and CHI as acceptor molecules, the M O energy difference criteria predicts that D2 should be more reactive than CH,, as observed for these smaller clusters. The cluster size dependence of hydrogen chemisorption will also reflect such effects as shifts in Fermi energy, orbital symmetry and ordering, heats of adsorption, and metal-CO bond energies as a function of cluster size. As pointed out by Saillard and H ~ f f m a n nthe , ~ ~interaction matrix element in the numerator of eq 12 can have a strong influence and should not be neglected. For example, they find that @*2 for the LUMO of both H2 and CH, is considerably larger than p2 of the HOMO, thus enhancing the acceptor character for both H, and CH,. Similarly, the H O M O S for donor molecules, DzO and CH30H, are expected to interact more strongly with an unoccupied (or partially occupied) metal orbital near the Fermi level than their L U M O s interact with the H O M O of metal cluster. Thus in the context of this simple picture, aluminum clusters would be predicted to react more readily with D,O and CH30H than with H2 and CH, and differences between DzO and CH30H possibly can be attributed to variations in the interaction matrix elements. In this picture CO is treated as an acceptor molecule since its energy denominator is comparable to that of H 2 0 and C H 3 0 H , and as would be predicted from this crude model, its reactivity is comparable to theirs. Reactions of oxygen with aluminum clusters may be expected to be quite facile since ground-state 0, is a triplet and presumably M, interactions even would undergo quite facile acceptor, A with occupied metal orbitals of the appropriate symmetry. Thus the overall ordering of the aluminum cluster reactivity with different molecules can be rationalized in a simple way. The variation of reactivity with cluster size similarly may reflect the fact that each cluster will have a unique set and number of orbitals. The symmetry and energy of the filled and unfilled orbitals can be expected to influence the individual cluster electronic and chemical properties. Such properties should be examined theoretically. Recent calculations9 probing the chemisorption of molecular hydrogen on small aluminum clusters (n = 2-6) show that only Al, has an exothermic heat of adsorption, consistent with the experimental observation that the onset of hydrogen chemisorption begins at AI6. Extension of such calculations to larger size clusters and different chemisorbed species is encouraged. Obviously the adsorbate-metal orbital energy levels are not the only factors to be considered, since they only reflect the energy differences between occupied/unoccupied molecule and metal cluster orbitals and do not indicate the strength (or lack thereof) of the interaction matrix elements. It has been shownzgathat one 2-4. This suggests that acceptor might typically expect @*'/B2 bonding will be favored unless the energy denominator for the donor interaction is significantly smaller than in the acceptor case. In light of the simple correlations pointed out above, it does appear that for these specific cases adsorbate and metal orbital energetics can serve as a useful guide toward beginning to understand such adsorbate-cluster interactions.
-
-
Reactions on Aluminum Surfaces Much effort has been expended in studies of reactions of molecules with aluminum surfaces. In this paper we are particularly concerned with results that have been obtained at low coverage. So, rather than attempting to review this extensive literature, we chose particular works as examples with which to elucidate particular points. For instance, studies of the interaction of methanol with aluminum surfaces have reported that methanol molecularly chemisorbs at low exposure and low temperature (223 At 223 K methanol was irreversibly and instantaneously taken up until for surface coverages less than about K).34335
(33) Rogers, Jr., J. W.; Hance, R. L.; White, J. M. Surf.Sci. 1980, 100, 388. (34) AI-Mawlawi, D.; Saleh, J. M. J . Chem. Soc., Faraday Trans. I , 1981, 77, 2965. (35) Tindall, 1. F.; Vickerman, J. C. Surf. Sci. 1985, 149, 577.
TABLE III: Reactivity of Aluminum Clusters toward Different Molecules molecule
reactivity (R/F)'
4
D2 D2O
co
most reactive cluster(s) none AI,
200
AIIO,A117-20
400
CH,OH
2000
A16 Allo, All+I,
0 2
6000
A12, Al, AI, (n 1 25)
obsd product none AInD2 ( 6 5 n 5 15) AIn(D2O)j (n t 8 , y = 1-3) none AIACH@H), (n t 3 , y = 1-2) A1302, Aln(02)y (n t 7, y = 1-2)
"Values for the most reactive cluster. For n t 15 neither depletion of the bare clusters nor appearance of product peaks is observed in chemical reactor studies. Seeding D2directly in helium carrier products are detected out to n = 21 (see text).
50%.36Further methanol absorption led to evolution of gas-phase products of which CH4 was the most abundant, but H2 and C O were also detected. Our cluster results at low extent of reaction are consistent with this picture in the sense that the methanol is chemisorbed but CHI and H2are unreactive for the larger clusters. Conflicting reports exist regarding C O chemisorption on aluminum surfaces. It has been reported that C O does3' and does not38 adsorb on clean and perfectly reconstructed aluminum surfaces. Subsequent agreed that the C O chemisorption which was observed could be due to undetected defects in the surface. Most recently, Paul and Hoffman'"' have reported electron energy loss spectra for C O on Al( 100) at 80 K and from observation of C O desorption at 125 K have calculated an adsorption energy of 8 kcal/mol. Our results show that aluminum clusters are reactive toward CO. This is consistent with the surface work in that small clusters may be perfectly clean, but almost by definition their surface must consist almost entirely of defect sites. The fact that no product peaks are observed may reflect the weak adsorption strength of C O on AI consistent with recent experimental measurement'''' and theoretical predictions4' that C O only weakly bonds to aluminum clusters. The interaction of water with aluminum surfaces is also quite facile with an initial (dissociative) adsorption followed by an oxidation phase with evolution of hydrogen.,, This is observed for clusters larger than A19 in our experimental work. An unexpected result is that the small clusters are relatively inert toward water chemisorption. A recent theoretical treatment43of the interaction of a water molecule with an aluminum surface has shown that although dissociation of chemisorbed H 2 0to 0 + H2 is thermodynamically favorable, spontaneous dissociation of a single monomer is a highly activated process. Thus the isolated monomer is expected to be stable on the metal surface. They suggest that the fact that H 2 0 dissociation is apparently spontaneous on many metals is probably an indication of a complex reaction path that involves more than one H,O molecule. This treatment shows that the bonding is accompanied by a net charge donation (-0.1 electron) to unoccupied cluster levels, Le., a facile M interaction. donor A Molecular hydrogen reportedly does not chemisorb on clean aluminum surfaces at room t e m p e r a t ~ r e , ,consistent ~ with the
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(36) The percentage coverage is defined in ref 33 as the ratio of the volume of methanol adsorbed to the volume of krypton adsorbed on the clean film. (37) (a) Flostroem, S. A.; Martinson, C. W. B. Appl. Surf.Sci. 1984, IO, 115. (b) Khonde, K.; Darville, J.; Gilles, J. M. Vacuum 1981, 31, 499. (c) Chiang, T. C.; Kaindl, G.; Eastman, D. E. Solid State Commun. 1980, 36, 25. (38) Bargeron, C. B.; Hall, B. N. Surf.Sci. 1982, 119, L319. (39) Khonde, K.; Darville, J.; Gilles, J. M. Surf.Sci. 1983, 126, 414. (40) Paul, J.; Hoffmann, F. Chem. Phys. Lett. 1986, 130, 160. (41) Bagus, P.S.; Bauschlicher, Jr., C. W.; Nelin, C. J.; Laskowski, B. C.; Seel, M. J. Chem. Phys. 1984, 81, 3594. (42) Ding, M. Q.; Williams, E. M. Surf.Sci. 1985, 160, 189. (43) Muller, J. E.; Harris, J. Phys. Reu. Lett. 1984, 53, 2493. (44) (a) FlodstrBm, S. A.; Peterson, L.G.; Hagstrom, S. B. M. J. Vac. Sci. Techno/. 1976, 13, 280. (b) Pellerim, F.; LeGressus, C.; Massignon, D. Surf. Sci. 1981, 111, L705.
J . Phys. Chem. 1988, 92, 429-433 weak chemisorption observed for clusters. Oxygen reacts readily with aluminum surfaces.45 The surprising feature from this work is that certain small clusters are much less reactive (but still quite reactive) toward O2 (Figure 7) compared to the atom, dimer, and larger (n > 25) clusters.
Summary and Conclusions The reactions of gas-phase aluminum clusters with different molecules have been studied. Very dramatic cluster size selective reactivities are observed. This work shows that the metal cluster reactivity depends not only on cluster size for a particular metal-molecule combination but also on the molecule for a particular metal. For the reactions studied, the relative reactivity for the most reactive cluster for different reactants is summarized in Table 111. As can be seen, several orders of magnitude variation in reactivity is observed between different reactant molecules with a general trend in reactivity CH4 < H2 < D 2 0 < C O < C H 3 0 H < 0,. As seen from the reactivity plots, e.g., Figures 5-8, a given size cluster may be extremely reactive toward one molecule but unreactive toward another. In Table 111 the primary product peaks observed at low extent of reaction are also tabulated. In the majority of the cases the
429
product peaks correspond to the mass of the associated cluster plus molecule. For example, with D2, D20, 02,and C H 3 0 H the first adducts detected are equivalent to masses A1,(D2), Al,(D20), A1,(02), and Al,(CH30H), respectively. For hydrogen chemisorption onto metal clusters it has been argued that dissociative chemisorption is most 1ikely.I” In particular, it has been argued6*& that chemisorption bonds must be in excess of about 13 kcal/mol in order for the product species to be collisionally stabilized prior to undergoing unimolecular decomposition. Thus the nonreactivity of aluminum clusters (toward CH4 for example) may simply reflect the weakness of the chemisorption bonds. Such effects are examined in more detail for reactions of C O with several transition metals as well as aluminum.6 In summary, this work indicates that relative reactivities exhibit large nonmonotonic variations as a function of cluster size and as a function of molecular type. In cases where the aluminum surfaces are unreactive the larger clusters are also found to be unreactive. For the reactive clusters unexpected and large variations in reactivity arise for clusters smaller than 20-30 atoms in size. The ratios of relative reactivities are expected to be a good measure of the ratios of rate constants. Registry No. Al, 7429-90-5; D2,7782-39-0; H20,7732-18-5; 02, 7782-44-7; CH,OH, 67-56-1; CH4, 74-82-8; CO, 630-08-0.
(45) For a recent review of oxygen chemisorption on aluminum surfaces see: Batra, I. P.; Kleinman, L. J . Electron Spectrosc. Relar. Phenom. 1984, 33, 175.
(46) Geusic, M. E.; Morse, M. D.; O’Brien, S . C.; Smalley, R. E. Reu. Sci. Insrrum. 1985, 56, 2123.
Initial Radiolysis Effects on the T2-02 Gas Reaction R. A. Failor,* P. C . Souers, Lawrence Livermore National Laboratory, Livermore, California 94550
and S . G. Prussint Department of Nuclear Engineering, University of California, Berkeley, California 94720 (Received: December 12, 1986; In Final Form: August 6, 1987)
Model calculations were performed to examine the effects of self-radiolysis of pure T2on the reaction of T2 and 02.The pressure dependence of the steady-state concentrations of the major T2 self-radiolysis products and the times required to reach steady state are examined in detail. The largest variations from the modeling results obtained by assuming initially pure T2and O2are found with low total tritium concentrations where self-radiolysis produces the largest T/T2 concentration ratios. In general, the effects of inclusion of T2self-radiolysis effects are important only during the first few seconds following initiation of the T2 O2 reaction.
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Introduction The rate of oxidation of molecular tritium to tritiated water is of great concern for the safe operation of tritium handling and nuclear fusion facilities. Two of the main reasons for concern are the increased health hazard of tritiated water vapor over the molecular form and the difference in chemical behavior of the two forms in tritium containment and removal systems. Tritiated water can be produced by the tritium oxidation reaction at room temperature because of radiolysis effects from the tritium fi decay. This stands in contrast to the reaction of H2and 02,which usually requires elevated temperatures to provide the intermediate free radicals. In a previous paper,’ we modeled the homogeneous gas-phase reactions of small amounts of tritium in oxygen at 298 K and with a total pressure of 1 atm. In that report we assumed the initial mixture contained only oxygen and tritium molecules and that the 6 radiolysis of the gas mixture began at the time of mixing. Consultant to Lawrence Livermore National Laboratory.
0022-3654/88/2092-0429$01.50/0
This is equivalent to instantaneous mixing of T2 and O2with the tritium stripped of its radiolysis products. It is possible to conceive of removing the atomic radiolysis products by adsorption on metal with a high surface area and removing the ionic radiolysis products with a high electric field. However, complete removal of all the tritium radiolysis products is a limiting case that would be difficult to accomplish in practice. Here, we consider the limit in which T, gas, in equilibrium with its radiolysis products, is instantaneously mixed with oxygen. We use the term self-radiolysis to mean the production of ions and atoms in initally pure T2 due to the slowing down of the tritiumdecay @ particle and all secondary electrons. We assume there are no effects of vessel walls which would result in losses and reactions of the active species. Although, this is a limit that is also difficult to accomplish in practice, this study permits assessment of the extent to which the mechanism and rate of TzO ( 1 ) Failor, R. A,; Souers, P. C.; Prussin, S. G . J . Phys. Chem. 1986, 90, 5914.
0 1988 American Chemical Society
