Effect of Cluster Size on Chemical and Electronic Properties - ACS


Sep 21, 1990 - Exxon Research and Engineering Company, Route 22 East, Clinton Township, Annandale, NJ 08801. 1 Current address: Ecole Polytechnique ...
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Chapter 17

Effect of Cluster Size on Chemical and Electronic Properties 1

D. M. Cox, A. Kaldor, P. Fayet, W. Eberhardt, R. Brickman, R. Sherwood, Z. Fu, and D. Sondericher

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Exxon Research and Engineering Company, Route 22 East, Clinton Township, Annandale, NJ 08801

Dependent on the exact number of metal atoms in a transition metal cluster, small clusters (nEA. Also the c l u s t e r IPs (EAs) are s i g n i f i c a n t l y greater(less) than the bulk work function. As w i l l be discussed below, recent experiments probing the e l e c t r o n i c structure of mercury (8) and platinum clusters(9) show that the small (n Fe D2

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Note that the although we expressed the product formed as Fe D2, the experiments are carried out at or s l i g h t l y above room temperature 295K, and a l l indications are that only d i s s o c i a t i v e l y chemisorbed hydrogen i s present. n

In Novel Materials in Heterogeneous Catalysis; Baker, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

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Now l e t us examine Feio more c l o s e l y . Note that as the hydrogen concentration i s increased the bare Feio ion signal begins to decrease and i s almost e n t i r e l y depleted at the highest concentration. Fee the other hand shows l i t t l e evidence of depletion, and other c l u s t e r s show varying l e v e l s of r e a c t i v i t y towards D2. This i s but one instance of strong s i z e s e l e c t i v e behavior observed f o r t r a n s i t i o n metal c l u s t e r s . Next we note that f o r Feio at the highest concentration a new product peak FeioDio is detected. The Feio c l u s t e r saturates at t h i s level of deuterium (hydrogen) uptake and does not chemisorb additional deuterium. As shown elsewheref21-22). the natural logarithm of the r a t i o of the bare c l u s t e r ion signals I / I i s then a measure of the r e l a t i v e rate constant f o r reaction of the c l u s t e r with reagent R (hydrogen in t h i s instance), where I i s the bare c l u s t e r ion signal with reagent added and I i s the bare c l u s t e r ion signal with helium only added. From data such as that shown in Figure 2, the r e l a t i v e rate constants f o r the reaction of metal c l u s t e r s with a reagent are calculated. Such data f o r the iron c l u s t e r r e a c t i v i t y towards di-hydrogen i s plotted in Figure 4, and that f o r the reaction of methane with platinum and palladium c l u s t e r s i s shown in figure 5. o n

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Deposition of Monosized Clusters. In order to study the properties of deposited monosized c l u s t e r s we have the c a p a b i l i t y to generate nanoamps (nA) of mass selected metal c l u s t e r ions. The experimental apparatus i s shown schematically in figure 3.(23) The metal c l u s t e r ions are produced by sputtering of a metal substrate with high energy rare gas (Ar+, Kr+ or Xe+) ions. The c l u s t e r ions are energy and mass selected and then directed onto an appropriate substrate. Typical operating conditions are as follows: Xe+ at 25KeV, 5ma; c l u s t e r beam s i z e 0.25 cm ; c l u s t e r beam currents several nA down to a few tenths of nA depending on the c l u s t e r size; surface coverage by c l u s t e r s i s t y p i c a l l y l i m i t e d to l e s s than 15% area coverage, i . e . c l u s t e r f l u x on surface i s between 5x1013 and 1x10*4 c n r ; substrates are kept at room temperature; and vacuum during deposition i s maintained at about 10-6 t o r r . The advantage of t h i s technique i s that continuous beams are produced but one major disadvantage i s that the i n t e n s i t y of larger c l u s t e r ions drops o f f r a p i d l y . So although the sputtering techinque i s general (most s o l i d substrates can be sputtered), i t i s d i f f i c u l t to obtain s u f f i c i e n t i n t e n s i t y to make sample deposits of l a r g e r (n>10) c l u s t e r s in a reasonable (say 2 hours) time. We have studied the mass selected deposited c l u s t e r s using a v a r i e t y of techniques including scanning transmission electron microscopy, scanning tunnelling microscopy, and UV photoemission. Some of t h i s work w i l l be described in the l a s t section. 2

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Studies of Gas Phase Clusters H-H Bond A c t i v a t i o n . The r e a c t i v i t y of iron c l u s t e r s toward di-hydrogen i s shown in figure 4. Also plotted in figure 4 are electron binding energies (IPs) of iron c l u s t e r s . Note that the

In Novel Materials in Heterogeneous Catalysis; Baker, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

Downloaded by UNIV OF SYDNEY on May 4, 2015 | http://pubs.acs.org Publication Date: September 21, 1990 | doi: 10.1021/bk-1990-0437.ch017

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Figure 2. Time-of-flight mass spectra of iron c l u s t e r s as a function of increasing deuterium p a r t i a l pressure i n the reactor The upper panel shows the reference mass spectrum obtained when helium only i s pulsed into the reactor. The lower curves show the e f f e c t of increasing deuterium pressure. (Reprinted with permissionfromRef. 2. Copyright 1985 American Institute of Physics.)

ο Figure 3 . A schematic of the apparatus used to produce mass selected c l u s t e r s . In Novel Materials in Heterogeneous Catalysis; Baker, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

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r e a c t i v i t y i s plotted on a log scale and the electron binding energies on a l i n e a r scale. Thus we see that the r e a c t i v i t y can vary by several orders of magnitude for only a few atom size change. Note also that the highest r e a c t i v i t y i s observed for the larger clusters n>23.(2.24) In fact nearly constant and high r e a c t i v i t y has been measured by the Argonne group from n=25 out to n=270.(3) For clusters larger than about 8 atoms we f i n d a strong correspondence between the measured r e l a t i v e r e a c t i v i t y and that which can be inferred from electron binding energies.(2.24) For the smaller c l u s t e r s , j u s t the opposite i s observed.(24) In i t s simplest terms the size selective behavior may be r a t i o n a l i z e d as resulting from the competition between Pauli repulsion which creates a barrier to di-hydrogen chemisorption and a t t r a c t i v e p a r t i a l charge transfer interactions. For the larger metal clusters with lower IPs, charge transfer from the metal to hydrogen i s the dominant a t t r a c t i v e interaction; whereas for the smaller, higher IP, clusters the dominant attractive interaction i s hydrogen to metal charge donation.(25) A similar global correspondence has been observed for di-hydrogen chemisorption on niobium and vanadium, the only other t r a n s i t i o n metal clusters for which both IP and r e a c t i v i t y measurements are presently available.(5-7) As noted above (with the exception of the coinage metals) most t r a n s i t i o n metal clusters containing more than about 25 metal atoms exhibit a high, nearly size independent r e a c t i v i t y towards di-hydrogen. This suggests that for clusters larger than about 25 atoms non-activated d i s s o c i a t i v e hydrogen chemisorption occurs (or at least the activation barrier is s u f f i c i e n t l y reduced that f a c i l e chemisorption occurs at near room temperature), quite analogous to what occurs on on many t r a n s i t i o n metal surfaces. It should be pointed out that the correspondence noted above is simply that, a correspondence. There are certain size c l u s t e r s , e.g. Nbi6, F e n , In the d i f f e r e n t metal systems for which t h i s correspondence does not seem to apply. This suggests that i t i s unlikely that the strong size dependence observed for activation of di-hydrogen shown in figure 4 can be explained by a model based s o l e l y on charge donation. We expect that a complete explanation w i l l not be shortly forthcoming and l i k e l y w i l l require s i g n i f i c a n t l y more experimental investigation of structure and s t a b i l i t y of the clusters as a function of s i z e . With additional structural data we hope that a detailed theoretical understanding can be obtained. C-H Bond Activation. We have also examined the chemisorption of various hydrocarbons on d i f f e r e n t t r a n s i t i o n metal c l u s t e r s . In t h i s section we describe results obtained for methane activation on neutral clusters. F i r s t , we note that under our experimental conditions (low pressure, near room temperature, short contact time) methane activation readily occurs only on s p e c i f i c type and size metal c l u s t e r s . For instance, we detect no evidence that methane reacts with iron(12), rhodium(26) or aluminum(27) c l u s t e r s , whereas as shown in figure 5 strong size selective chemisorption i s

In Novel Materials in Heterogeneous Catalysis; Baker, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

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Figure 4. Comparison of the r e l a t i v e r e a c t i v i t y of iron clusters towards hydrogen (right hand scale and c i r c l e s ) with the electron binding energy ( l e f t hand scale and v e r t i c a l lines) as a function of number of iron atoms in the cluster. (ReprintedfromReference 24. Copyright 1988 American Chemical Society.)

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Figure 5. Relative r e a c t i v i t y of platinum (upper panel) and palladium (lower panel) clusters towards methane.

In Novel Materials in Heterogeneous Catalysis; Baker, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

Downloaded by UNIV OF SYDNEY on May 4, 2015 | http://pubs.acs.org Publication Date: September 21, 1990 | doi: 10.1021/bk-1990-0437.ch017

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found f o r small platinum(28) and palladium(£9) c l u s t e r s , but the size selective behavior i s d i f f e r e n t f o r the two metals. As shown small (n=2-5) Pt clusters are the most reactive, whereas the mid-sized (n=7-16) Pd clusters are the most reactive. In both instances the larger clusters become less reactive. Again a simple charge transfer model can crudely r a t i o n a l i z e the gross behavior as follows: Both platinum and palladium clusters have high IPs and the small clusters appear to have the highest IPs of any metal system we have investigated. In t h i s instance the methane to metal charge transfer may dominate f o r a l l sizes. This i s not unexpected since ( i ) the larger c l u s t e r IPs may never becomes s u f f i c i e n t l y small f o r donation of charge from the metal to occur, and ( i i ) the lowest unoccupied molecular orbital of methane i s at s i g n i f i c a n t l y higher energy than that of hydrogen, i . e . as S a i l l a r d and Hoffman(25) have pointed out methane should be s i g n i f i c a n t l y poorer charge acceptor than hydrogen but i s as good or better charge donator. Such arguments also q u a l i t a t i v e l y explain the non-reactivity of iron and rhodium (lower IP systems) c l u s t e r s . Hydrogen (Deuterium) Saturation. In addition to k i n e t i c studies, we also measure hydrogen saturation, i . e . the maximum number of hydrogens which can be bound to a metal c l u s t e r as a function of the number of metal atoms in the c l u s t e r . Our r e s u l t s show that a very large number of hydrogens can be bound to small t r a n s i t i o n metal clusters.(29-30) Figure 6 shows the results of deuterium chemisorption on c a t i o n i c clusters of three metals, rhodium, platinum and n i c k e l . Let us examine the 10 atom c l u s t e r f o r each of the metals. The deuterium saturation levels are 32, 22 and 16 deuteriums f o r R h i o , P t i o and N i i o > respectively. By comparison Feio saturated at 10 hydrogen atoms (see figure 2) . As can be seen from figure 6, the dimer cations of Rh, Pt and Ni have the highest (D/M) ax ratios of 8, 5 and 5, respectively. For neutral iron clusters (H/M) x was only 1.1(31)» but f o r neutral clusters of vanadium,(32) niobium(32) and tantalum(33) (H/M) x of 1.4 has been measured. Thus hydrogen saturation displays not only a strong dependence on c l u s t e r size, but also a pronounced dependence on the metal type. These results suggest that small (