- Email: [email protected]
CO reactivity of small transition-metal clusters: Nin and Nbn
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•iiii•iiiii•{{!ii•i!ii!•ii!iii{{iiiii{i•i•iiiSii•i!i! !i!i ELSEVIER
Surface Science 331-333 (1995) 231-236
CO reactivity of small transition-metal clusters:
Ni n
and N b
n
L. Holmgren *, M. Andersson, A. Ros6n Department of Physics, Chalmers University of Technology and G6teborg University, S-412 96 G6teborg, Sweden Received 5 August 1994; accepted for publication 3 November 1994
Abstract The size-dependent reactivity of neutral Ni and Nb clusters towards CO has been investigated. A beam of clusters is let through a low-pressure reaction cell containing CO and the clusters experience only a few collisions with CO molecules. The reaction products are detected with laser ionisation and time-of-flight mass spectrometry. Absolute sticking probabilities (S) of the first CO molecule on Ni9-Ni4o and Nba-Nb47 have been determined. For Ni, S exhibits a smooth size dependence with a broad maximum of S = 0.8-0.85 for cluster sizes n = 12-18. For larger clusters S seems to level off at a constant value of ~ 0.5. For Nb, the CO reactivity shows a strong size dependence for n < 13 with a distinct minimum in reactivity for n = 10. For cluster sizes Nb13-Nb2o S increases smoothly and for n > 20 S is almost constant close to 1.0. We compare our results with previous investigations on CO reactivity of transition-metal clusters as well as with the reactivity of corresponding bulk surfaces. Keywords: Carbon monoxide; Chemisorption; Clusters; Nickel; Niobium; Sticking
I. Introduction Clusters can be useful as models for surfaces and as such they have been employed by theoreticians for some time in the analysis of surface processes [1]. The cluster approach focuses on the properties of particular surface sites, by including the local geometry, and enables the use of high-accuracy quantum calculations. In recent years the development of the laser vaporisation source has enabled experiments on free clusters of basically all solid elements. The single crystal surface in ultra-high vacuum (UHV), which for a long time has served as a useful model for a
* Corresponding author. Fax: +46 31 772 3496; E-mail: [email protected].
real polycrystalline surface, can now be complemented with the free cluster in UHV as model system. The latter model can in many respects be a preferable one for a catalytic surface, especially as many catalysts have rough surfaces or consist of small metal particles deposited on a substrate. Access to experimental results on free clusters also enables a direct comparison with theoretical investigations performed with clusters as model systems. The chemisorption of carbon monoxide on transition-metal atoms, clusters and surfaces has been the subject of extensive studies. This is due to the great technological importance of CO reactions on transition metal surfaces and the rich chemistry of the transition metal carbonyls. In this paper we discuss the chemisorption of CO on free, neutral clusters of two transition metals, Nb and Ni, located to the left and right, respectively, in the periodic table. The
0039-6028/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved
SSDI 0 0 3 9 - 6 0 2 8 ( 9 5 ) 0 0 0 9 9 - 2
232
cluster source
L. Holmgren et al. / Surface Science 331-333 (1995) 231-236
t
ions to reaction time-of-flight cell spectrometer I I - - b. cluster
m~ skim
f
laserlight
Fig. 1. Schematic of the set-up used in the cluster reactivity experiments. cluster reactivity is compared with reactivity properties of corresponding bulk surfaces, especially for Ni where extensive data is available. W e also compare our results with previous investigations on CO reactivity of small transition metal clusters [2,3].
2. Experimental methods A very brief description of the experimental set-up will be given here, as further details have been
presented elsewhere [4]. The set-up consists of two vacuum chambers, one for the production and one for the reaction and detection of clusters, as shown in Fig. 1. A pulsed flow of metal clusters with He as carrier gas is produced in a laser vaporisation source. Each pulse contains a wide size distribution of neutral clusters, ranging from a few atoms to several hundred atoms. The c l u s t e r / H e beam is cooled via a supersonic expansion and enters the second chamber through a 1-mm-diameter skimmer, which skims off most of the He. In the second chamber the cluster beam passes through a reaction cell containing CO before entering the ionisation region, where the clusters are ionised with pulsed laser light and detected in a linear time-of-flight mass spectrometer. The light from an A r F excimer laser (photon energy 6.4 eV) is used for photoionisation, and the light intensity is kept low, 6 0 - 8 0 / I J / c m 2 / p u l s e , to prevent multiphoton absorption. The CO gas is leaked into the reaction cell, maintaining a constant pressure in the cell of ~
,.,I.L! L [LL_LLLLL , L LLL.L. 4
5
6
7
8
9
LLL_LL
i0 11 12 13 14 15 16 17 18 19 20 21
Fig. 2. Mass spectra of Nbn reacting with CO. The mass spectrum in the top panel was recorded without CO in the reaction cell, but a small amount of NbnO impurities formed in the cluster source are present. In the middle and lower panels the CO pressures were 4.9 × 10-4 and 9.6 x 10-4 mbar, respectively. The mass of a CO molecule is 28 u and the mass of a Nb atom is 93 u. Thus, peaks representing clusters with one and two adsorbed molecules appear shifted to the right with ~ 0.3 and ~ 0.6, respectively, of the distance between two pure cluster peaks.
L. Holmgren et al. / Surface Science 331-333 (1995) 231-236
1 0 - 4 - 1 0 -3 mbar. The cell is a 50 mm long cylinder with 1 mm entrance and exit apertures. The small apertures ensures a high pressure ratio ( > 10 3) between the reaction cell and the surrounding chamber. At the CO pressures used in these experiments the average number of collisions experienced by the clusters in the cell, varies from less than one to about two collisions. The number of collisions per cluster is described by a Poisson distribution. The cluster beam is traveling with the sonic velocity of He, which means that the average impact energy in the cluster-CO collision is approximately 30 k J / m o l (nearly independent of cluster size). To check for possible variations in ionisation efficiency of pure and reacted clusters control experiments were performed, where the clusters were ionised with 6.0-eV photons from a dye laser. No significant variations in detected cluster intensities were observed compared with the 6.4-eV photon measurements. In collisions with reactive gas molecules a significant number of clusters, especially for the small sizes, will be scattered out of the cluster beam. To be able to correct for this, experiments were also performed with inert gas in the reaction cell, in order to estimate the scattering of different clusters as a function of pressure. In the mass spectra peaks appear for all cluster sizes, i.e. bare clusters as well as clusters with adsorbed CO molecules, as can be seen in Fig. 2. Thus, the relative abundance of pure and reacted clusters can be determined for each CO pressure in the reaction cell. The average number of collisions experienced by a cluster passing through the cell at a certain pressure was determined using a hard sphere model for the clusters and gas molecules. The radius of the cluster, R,, was calculated by assuming that the cluster has the same density as the bulk metal and by adding to this radius a constant value, 3 = 0.5 o A, to account for electron spillout. Thus, for an n-atom cluster: Rn=ratomnl/3-t-O. F o r the CO molecule a hard-sphere value of 1.90 ,~ was used for the radius [5]. To evaluate the sticking probability (S) in a cluster-molecule collision the product abundance versus average number of collisions data is fitted to a first-order kinetic model with S as the fitting parameter. It should be noted that the sticking probability
233
1.0 ~, 0.8
~ 0.6 e,
",7, 0.4 ¢.9 r/3
Ni n / CO
0.2
l'O
;5
1o
do
Number of Ni atoms in the Cluster Fig. 3. The sticking probability of the first CO molecule on Ni,.
determined in this way is the combined probabilities of the molecule to adsorb on the cluster in the collision and to stay adsorbed throughout the ionisation process. Unimolecular decay of formed products will occur, with a rate mainly depending on the number of vibrational degrees of freedom within the cluster, the temperature of the cluster and the chemisorption energy. Thus, this rate compared with the time between product formation and detection will determine whether a significant fraction of the products will decompose.
3. Results Absolute sticking probabilities of the first CO molecule on Ni, and Nb, clusters of sizes n = 9 - 4 0 and n = 4-47, respectively, have been determined and are shown in Figs. 3 and 4. The sticking of CO 1.2
i
i
i
l
l
1.0
~ 0.8 T
~
~
0.6
Tr
Nb n / CO
Trl .~
r~
0.4
Iv
0.2 0
J 5
li0
li5
i 20
= 25
i 30
J 35
i 40
t 45
Number of Nb atoms in the Cluster Fig. 4. The sticking probability of the firstC O molecule on Nb,.
234
L. Holmgren et al. / Surface Science 331-333 (1995) 231-236
on Nin exhibits a relatively smooth size dependence with a maximum sticking probability of 0.80-0.85 for n = 12-18. For larger cluster sizes S decreases with n, but seems to level off at a constant value of approximately 0.5 for n > 35. The sticking of CO on Nb clusters shows a strong size dependence for n < 13. For the smallest clusters, n = 4-9, there is a rapid increase in S with n. This increase is abruptly broken with a distinct minimum at Nb m and also Nb12 is less reactive than its nearest neighbours. For larger cluster sizes S increases slowly to n -- 20-25, with only small fluctuations, and for n > 25 S is approximately constant at a high value close to 1.0. Note that in measuring the sticking probability of a molecule onto a free cluster it is not unphysical to obtain S > 1, since S, as defined here, is the ratio between the reactive and geometrical cross sections. Thus, a sticking exceeding unity will be obtained if the cluster is able to trap molecules passing close to but not colliding with the cluster (in the hard-sphere picture). For clusters sizes smaller than those shown in Fig. 3 and 4 the determination of S is difficult or uncertain due to cluster ionisation potentials close to 6.4 eV, substantial scattering of clusters and generally low reactivity. For larger cluster sizes there is also an increased uncertainty in the evaluation of S, due to decreasing cluster intensities and limited mass resolution. The error bars shown in the figures represent the estimations of uncertainties in the recording and evaluation of experimental data. These errors are relative errors for one cluster size compared to others. In addition to this there is also an error in the absolute scale, independent or only weakly dependent on cluster size. This latter error originates mainly from uncertainties in the CO-pressure measurement and in the methods for correcting for scattering of clusters out of the beam. The absolute error is estimated to + 10-15% of the measured value, with the larger limit for the smallest clusters. Also the choice of geometrical cross section for the clusters will influence the determined sticking probabilities.
4. Discussion The CO reactivity of transition metal clusters have previously been studied by Morse et al. [2] and
Cox et al. [3]. Both groups found that most transition-metal clusters containing three or more atoms readily react with CO. Generally, they found that the CO reactivity is a rather smooth function of cluster size, without the dramatic size variations observed in the H 2 and N 2 reactivity. We begin the comparison between our results and the results of the other groups by describing similarities and differences in the experimental methods. The means for production and detection of clusters, i.e. laser vaporisation and photoionisation followed by time-of-flight mass spectrometry, were the same. However, the reactions towards CO were performed under different conditions. In the previous experiments the reaction cell was attached directly to the cluster source and the reactions took place after the cluster formation but before the supersonic expansion of the cluster beam. The reactive gas, at low concentrations, was seeded into the He carrier gas and, thus, clusters and reaction products were thermalised in collisions with carrier gas atoms. In our experiment the reactor contains only the reactive gas, at a low pressure, and we probe the results of individual cluster-molecule collisions. Morse et al. have measured the CO reactivity of Nb n for n ~< 18, and Cox et al. for n ~< 10. Both groups found the atom and dimer inert. Morse et al. found the reactivity to increase slowly, almost monotonically with n except for small dips at n = 7, 10, 12 and 16. Cox et al. observed a high CO reactivity of N b n compared with clusters of most other transition metals. They found that all studied cluster sizes of Nb n had about the same reactivity with the exceptions of Nb 3 and Nbl0, which were about 20% less reactive. Our results for niobium clusters differ from the earlier results in two ways. Firstly, we see a sharp increase with n in the reactivity of small clusters, n = 4-9, which is much steeper than what Morse et al. observed. Secondly, in our measurements Nb m appears relatively much more inert compared with neighbouring cluster sizes. As regards the first discrepancy we believe that this can be an effect of the inability of the smaller clusters to accommodate the chemisorption heat and form stable products in the absence of a thermalising buffer gas. Similar effects could make Nb m seem less reactive in our measurements. If we assume that CO has a low binding energy on this cluster, the buffer gas colli-
L. Holmgren et aL / Surface Science 331-333 (1995) 231-236
sions and the cooling in the supersonic expansion in the previous experiments can significantly help stabilising this weakly bound product. A possible explanation of the strong size dependence in the CO reactivity of Nb~ around n = 10 might be found by analysing the electronic structure of the metal-CO bond. The chemisorption of CO on metal surfaces is generally assumed to involve transfer of electrons to the CO 2zr antibonding orbital, which weakens the C - O bond. However, there is also a flow of charge from the CO 50- orbital to the metal, which is thought to strengthen the C - O bond. The filling of electrons in the 27r orbital is the dominating flow and a strong metal-CO bond is accompanied by a weak C - O bond. Having this in mind it might be possible to explain the observed Nbn reactivity with charge transfer arguments. Previous investigations on the ionisation potentials (IP) of Nb~ showed that the IP exhibits a significant size dependence around n = 10 [6-8]. For n ~< 8 the IP is high ( > 5.3 eV), at n = 9 the IP drops to ~ 4.8 eV followed by an increase to ~ 5.5 e V a t n = 1 0 and a local minimum of ~ 4 . 7 e V a t n = 11. For n = 12-20 the IP is again a rather smooth function of n except for a minimum at n = 15. This means that clusters like Nb 9 and Nbl~, which have low IP, can transfer electrons more easily to the CO molecule and form a strong metalCO bond, than Nbl0 can with its higher IP. Similar charge transfer arguments have been used to explain the variations in D 2 reactivity of Nb [6,9] as well as Fe [10] clusters, since the dissociative chemisorption of D 2 also involves transfer of electrons to an antibonding orbital in the D 2 molecule. Experimentally the D 2 and N 2 reactivity of Nb~ showed minima for n = 8, 10, (12) and 16 [2,9]. For Nb9, Nbll and Nb12isomers with different IP a n d / o r D 2 reactivity have been identified [7,9,11,12]. It has also been discussed if the variations of properties of Nb~ is an effect of structural changes and the start of formation of a layer of surface atoms surrounding one (or several) 'inside' atoms [8]. The structural changes would then be reflected in both electronic and chemical properties. The Ni~-CO reactivity was measured by Cox et al. who studied Nil-Nil3. They found the atom and dimer unreactive, some variations in the reactivity for n = 3-5, an almost constant reactivity for n =
235
6-11 and an increase in relative reactivity with about 30% for n = 12 and 13. The overall reactivity was found to be about 50% lower for Ni n than for Nb n. The results of Cox et al. agree with ours in the way that Nil2 and Nil3 are more reactive than Ni9-Nill, with the difference that we observe an increase in reactivity for n = 9-11 not present in the data of Cox et al. Again this could be an effect of low CO binding energy on these clusters, combined with the less efficient product stabilisation, especially for the smaller clusters, in our experiment. However, as long as the binding energy is not known, it is not possible to draw any certain conclusions. In the previous experiment as well as in our measurements it was found that the CO reactivity was generally higher for Nb, than for Nin, although, in our case, this difference is best observed in the larger clustersize range. Compared with Nb n both IP and CO reactivity of Ni, exhibit a rather smooth size dependence. For Nig-Nil~ there is a decrease in IP, but for larger sizes the variations are very limited [13]. Thus, for the small sizes the increase in S for CO on Ni, coincides with a decrease in IP, whereas the slow decrease in S for n > 15 does not correlate with any IP variations. Our conclusion is that the electronic structure of the clusters, e.g. characterised by the IP, significantly influences the CO reactivity, as can be seen in the distinct variations in S for Nb9-Nb12 and the smooth size dependence for Ni,. However, the IP alone cannot be expected to explain all variations in reactivity in various cluster size ranges and on different metals. Since clusters for a long time have been used as theoretical models for surfaces it is interesting to compare the cluster reactivity with the reactivity of the corresponding bulk surface. Sufficiently large clusters can be expected to attain properties similar to those of the bulk. It can be interesting to see if this seems to occur even at small cluster sizes, as the ones studied here. The adsorption of CO on Ni surfaces has been studied extensively and kinetic data is available for several single crystal surfaces [14-20]. At room temperature conditions the CO molecule is assumed to adsorb via a precursor state with an initial sticking probability of 0.75-0.95 [1518] and the chemisorption energy is 108-136 k J / m o l [16,18-20]. Molecular beam studies of CO sticking on Ni(100) and N i ( l l l ) showed that an increase in
236
L. Holmgren et al. / Surface Science 331-333 (1995) 231-236
impact energy and an oblique angle of incidence caused a decrease in S [15,17]. In our cluster experiment there is a rather wide distribution in collision energy and the collisions could be anything from head on collisions to a slight touch between the "hard-sphere surfaces". These less well-defined experimental conditions complicates a detailed comparison. W e can only note that the sticking probability of CO for the most reactive nickel clusters, n = 1 2 18, is similar to the bulk value, while the sticking probability is somewhat lower for larger cluster sizes. Much less data is found in the literature concerning CO reactions on Nb surfaces and we have not found any data on sticking probabilities. However, we note that a sticking probability close to unity, which was found for large Nb n, has also been measured for surfaces of several left-hand-side transition metals [14]. We conclude by discussing briefly the relative bond strengths of C O / N b n and C O / N i n . In our experiments we found that increasing the intensity of the ionising laser light caused a decrease in the relative abundance of CO products for Ni~. W e believe that this is an effect of multiphoton absorption, inducing heating of the cluster and subsequent desorption of adsorbed CO molecules. For Nb~ this effect was much less apparent, which seems to indicate that CO is more strongly bound to Nb~ than to Nin. We cannot in our experiments determine whether the chemisorption occurs dissociatively or not on the cluster surface. However, the stronger bonding between CO and Nb~ than between CO and Nin could indicate that CO chemisorbs dissociatively on Nb~ but nondissociatively on Ni~, following what is characteristic of the corresponding bulk surfaces [141.
Acknowledgements W e gratefully acknowledge financial support from the N U T E K / N F R Materials Research Consortium "Clusters and Ultrafine Particles", the Swedish Re-
search Council for Engineering Sciences (TFR) and the Swedish Council for Planning and Coordination of Research (FRN).
References [1] G. Pacchioni, P.S. Bagus and F. Parmigiani, Eds., Cluster Models for Surface and Bulk Phenomena (Plenum, New York, 1992). [2] M.D. Morse, M.E. Geusic, J.R. Heath and R.E. Smalley, J. Chem. Phys. 83 (1985) 2293. [3] D.M. Cox, K.C. Reichmann, D.J. Trevor and A. Kaldor, J. Chem. Phys. 88 (1988) 111. [4] J.L. Persson, M. Andersson and A. Ros6n, Z. Phys. D 26 (1993) 334. [5] A. Roth, Vacuum Technology, 2nd ed. (North-Holland,Amsterdam, 1982). [6] R.L. Whetten, M.R. Zakin, D.M. Cox, D.J. Trevor and A. Kaldor, .I. Chem. Phys. 85 (1986) 1697. [7] M.B. Knickelbein and S. Yang, J. Chem. Phys. 93 (1990) 5760. [8] K. Athanassenas, D. Kreisle, B.A. Collings, D.M. Rayner and P.A. Hackett, Chem. Phys. Lett. 213 (1993) 105. [9] M.R. Zakin, R.O. Brickman, D.M. Cox and A. Kaldor, J. Chem. Phys. 88 (1988) 3555. [10] R.L. Whetten, D.M. Cox, D.J. Trevor and A. Kaldor, Phys. Rev. Lett. 54 (1985) 1494. [11] Y. Hamrick, S. Taylor, G.W. Lemire, Z.-W. Fu, J.-C. Shui and M.D. Morse, J. Chem. Phys. 88 (1988) 4095. [12] M.B. Knickelbein and S. Yang, J. Chem. Phys. 93 (1990) 1476. [13] M.B. Knickelbein, S. Yang and S.J. Riley, J. Chem. Phys. 93 (1990) 94. [14] J.C. Campuzano, in: The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis, Vol. 3, Eds. D.A. King and D.P. Woodruff (Elsevier, Amsterdam, 1990) p. 389, and references therein. [15] S.L. Tang, M.B. Lee, J.D. Beckerle, M.A. Hines and S.T. Ceyer, J. Chem. Phys. 82 (1985) 2826. [16] H. Froitzheim, U. K6hler and H. Lammering, Phys. Rev. B 34 (1986) 2125. [17] M.P. D'Evelyn, H.P. Steinriick and R.J. Madix, Surf. Sci. 180 (1987) 47. [18] N. AI-Sarraf, J.T. Stuckless, C.E. Wartnaby and D.A. King, Surf. Sci. 283 (1983) 427. [19] J. Bauhofer, M. Hock and J. Kiippers, Surf. Sci. 191 (1987) 395. [20] N. AI-Sarraf, D.A. King, Surf. Sci. 307-309 (1994) 1.
•iiii•iiiii•{{!ii•i!ii!•ii!iii{{iiiii{i•i•iiiSii•i!i! !i!i ELSEVIER
Surface Science 331-333 (1995) 231-236
CO reactivity of small transition-metal clusters:
Ni n
and N b
n
L. Holmgren *, M. Andersson, A. Ros6n Department of Physics, Chalmers University of Technology and G6teborg University, S-412 96 G6teborg, Sweden Received 5 August 1994; accepted for publication 3 November 1994
Abstract The size-dependent reactivity of neutral Ni and Nb clusters towards CO has been investigated. A beam of clusters is let through a low-pressure reaction cell containing CO and the clusters experience only a few collisions with CO molecules. The reaction products are detected with laser ionisation and time-of-flight mass spectrometry. Absolute sticking probabilities (S) of the first CO molecule on Ni9-Ni4o and Nba-Nb47 have been determined. For Ni, S exhibits a smooth size dependence with a broad maximum of S = 0.8-0.85 for cluster sizes n = 12-18. For larger clusters S seems to level off at a constant value of ~ 0.5. For Nb, the CO reactivity shows a strong size dependence for n < 13 with a distinct minimum in reactivity for n = 10. For cluster sizes Nb13-Nb2o S increases smoothly and for n > 20 S is almost constant close to 1.0. We compare our results with previous investigations on CO reactivity of transition-metal clusters as well as with the reactivity of corresponding bulk surfaces. Keywords: Carbon monoxide; Chemisorption; Clusters; Nickel; Niobium; Sticking
I. Introduction Clusters can be useful as models for surfaces and as such they have been employed by theoreticians for some time in the analysis of surface processes [1]. The cluster approach focuses on the properties of particular surface sites, by including the local geometry, and enables the use of high-accuracy quantum calculations. In recent years the development of the laser vaporisation source has enabled experiments on free clusters of basically all solid elements. The single crystal surface in ultra-high vacuum (UHV), which for a long time has served as a useful model for a
* Corresponding author. Fax: +46 31 772 3496; E-mail: [email protected].
real polycrystalline surface, can now be complemented with the free cluster in UHV as model system. The latter model can in many respects be a preferable one for a catalytic surface, especially as many catalysts have rough surfaces or consist of small metal particles deposited on a substrate. Access to experimental results on free clusters also enables a direct comparison with theoretical investigations performed with clusters as model systems. The chemisorption of carbon monoxide on transition-metal atoms, clusters and surfaces has been the subject of extensive studies. This is due to the great technological importance of CO reactions on transition metal surfaces and the rich chemistry of the transition metal carbonyls. In this paper we discuss the chemisorption of CO on free, neutral clusters of two transition metals, Nb and Ni, located to the left and right, respectively, in the periodic table. The
0039-6028/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved
SSDI 0 0 3 9 - 6 0 2 8 ( 9 5 ) 0 0 0 9 9 - 2
232
cluster source
L. Holmgren et al. / Surface Science 331-333 (1995) 231-236
t
ions to reaction time-of-flight cell spectrometer I I - - b. cluster
m~ skim
f
laserlight
Fig. 1. Schematic of the set-up used in the cluster reactivity experiments. cluster reactivity is compared with reactivity properties of corresponding bulk surfaces, especially for Ni where extensive data is available. W e also compare our results with previous investigations on CO reactivity of small transition metal clusters [2,3].
2. Experimental methods A very brief description of the experimental set-up will be given here, as further details have been
presented elsewhere [4]. The set-up consists of two vacuum chambers, one for the production and one for the reaction and detection of clusters, as shown in Fig. 1. A pulsed flow of metal clusters with He as carrier gas is produced in a laser vaporisation source. Each pulse contains a wide size distribution of neutral clusters, ranging from a few atoms to several hundred atoms. The c l u s t e r / H e beam is cooled via a supersonic expansion and enters the second chamber through a 1-mm-diameter skimmer, which skims off most of the He. In the second chamber the cluster beam passes through a reaction cell containing CO before entering the ionisation region, where the clusters are ionised with pulsed laser light and detected in a linear time-of-flight mass spectrometer. The light from an A r F excimer laser (photon energy 6.4 eV) is used for photoionisation, and the light intensity is kept low, 6 0 - 8 0 / I J / c m 2 / p u l s e , to prevent multiphoton absorption. The CO gas is leaked into the reaction cell, maintaining a constant pressure in the cell of ~
,.,I.L! L [LL_LLLLL , L LLL.L. 4
5
6
7
8
9
LLL_LL
i0 11 12 13 14 15 16 17 18 19 20 21
Fig. 2. Mass spectra of Nbn reacting with CO. The mass spectrum in the top panel was recorded without CO in the reaction cell, but a small amount of NbnO impurities formed in the cluster source are present. In the middle and lower panels the CO pressures were 4.9 × 10-4 and 9.6 x 10-4 mbar, respectively. The mass of a CO molecule is 28 u and the mass of a Nb atom is 93 u. Thus, peaks representing clusters with one and two adsorbed molecules appear shifted to the right with ~ 0.3 and ~ 0.6, respectively, of the distance between two pure cluster peaks.
L. Holmgren et al. / Surface Science 331-333 (1995) 231-236
1 0 - 4 - 1 0 -3 mbar. The cell is a 50 mm long cylinder with 1 mm entrance and exit apertures. The small apertures ensures a high pressure ratio ( > 10 3) between the reaction cell and the surrounding chamber. At the CO pressures used in these experiments the average number of collisions experienced by the clusters in the cell, varies from less than one to about two collisions. The number of collisions per cluster is described by a Poisson distribution. The cluster beam is traveling with the sonic velocity of He, which means that the average impact energy in the cluster-CO collision is approximately 30 k J / m o l (nearly independent of cluster size). To check for possible variations in ionisation efficiency of pure and reacted clusters control experiments were performed, where the clusters were ionised with 6.0-eV photons from a dye laser. No significant variations in detected cluster intensities were observed compared with the 6.4-eV photon measurements. In collisions with reactive gas molecules a significant number of clusters, especially for the small sizes, will be scattered out of the cluster beam. To be able to correct for this, experiments were also performed with inert gas in the reaction cell, in order to estimate the scattering of different clusters as a function of pressure. In the mass spectra peaks appear for all cluster sizes, i.e. bare clusters as well as clusters with adsorbed CO molecules, as can be seen in Fig. 2. Thus, the relative abundance of pure and reacted clusters can be determined for each CO pressure in the reaction cell. The average number of collisions experienced by a cluster passing through the cell at a certain pressure was determined using a hard sphere model for the clusters and gas molecules. The radius of the cluster, R,, was calculated by assuming that the cluster has the same density as the bulk metal and by adding to this radius a constant value, 3 = 0.5 o A, to account for electron spillout. Thus, for an n-atom cluster: Rn=ratomnl/3-t-O. F o r the CO molecule a hard-sphere value of 1.90 ,~ was used for the radius [5]. To evaluate the sticking probability (S) in a cluster-molecule collision the product abundance versus average number of collisions data is fitted to a first-order kinetic model with S as the fitting parameter. It should be noted that the sticking probability
233
1.0 ~, 0.8
~ 0.6 e,
",7, 0.4 ¢.9 r/3
Ni n / CO
0.2
l'O
;5
1o
do
Number of Ni atoms in the Cluster Fig. 3. The sticking probability of the first CO molecule on Ni,.
determined in this way is the combined probabilities of the molecule to adsorb on the cluster in the collision and to stay adsorbed throughout the ionisation process. Unimolecular decay of formed products will occur, with a rate mainly depending on the number of vibrational degrees of freedom within the cluster, the temperature of the cluster and the chemisorption energy. Thus, this rate compared with the time between product formation and detection will determine whether a significant fraction of the products will decompose.
3. Results Absolute sticking probabilities of the first CO molecule on Ni, and Nb, clusters of sizes n = 9 - 4 0 and n = 4-47, respectively, have been determined and are shown in Figs. 3 and 4. The sticking of CO 1.2
i
i
i
l
l
1.0
~ 0.8 T
~
~
0.6
Tr
Nb n / CO
Trl .~
r~
0.4
Iv
0.2 0
J 5
li0
li5
i 20
= 25
i 30
J 35
i 40
t 45
Number of Nb atoms in the Cluster Fig. 4. The sticking probability of the firstC O molecule on Nb,.
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L. Holmgren et al. / Surface Science 331-333 (1995) 231-236
on Nin exhibits a relatively smooth size dependence with a maximum sticking probability of 0.80-0.85 for n = 12-18. For larger cluster sizes S decreases with n, but seems to level off at a constant value of approximately 0.5 for n > 35. The sticking of CO on Nb clusters shows a strong size dependence for n < 13. For the smallest clusters, n = 4-9, there is a rapid increase in S with n. This increase is abruptly broken with a distinct minimum at Nb m and also Nb12 is less reactive than its nearest neighbours. For larger cluster sizes S increases slowly to n -- 20-25, with only small fluctuations, and for n > 25 S is approximately constant at a high value close to 1.0. Note that in measuring the sticking probability of a molecule onto a free cluster it is not unphysical to obtain S > 1, since S, as defined here, is the ratio between the reactive and geometrical cross sections. Thus, a sticking exceeding unity will be obtained if the cluster is able to trap molecules passing close to but not colliding with the cluster (in the hard-sphere picture). For clusters sizes smaller than those shown in Fig. 3 and 4 the determination of S is difficult or uncertain due to cluster ionisation potentials close to 6.4 eV, substantial scattering of clusters and generally low reactivity. For larger cluster sizes there is also an increased uncertainty in the evaluation of S, due to decreasing cluster intensities and limited mass resolution. The error bars shown in the figures represent the estimations of uncertainties in the recording and evaluation of experimental data. These errors are relative errors for one cluster size compared to others. In addition to this there is also an error in the absolute scale, independent or only weakly dependent on cluster size. This latter error originates mainly from uncertainties in the CO-pressure measurement and in the methods for correcting for scattering of clusters out of the beam. The absolute error is estimated to + 10-15% of the measured value, with the larger limit for the smallest clusters. Also the choice of geometrical cross section for the clusters will influence the determined sticking probabilities.
4. Discussion The CO reactivity of transition metal clusters have previously been studied by Morse et al. [2] and
Cox et al. [3]. Both groups found that most transition-metal clusters containing three or more atoms readily react with CO. Generally, they found that the CO reactivity is a rather smooth function of cluster size, without the dramatic size variations observed in the H 2 and N 2 reactivity. We begin the comparison between our results and the results of the other groups by describing similarities and differences in the experimental methods. The means for production and detection of clusters, i.e. laser vaporisation and photoionisation followed by time-of-flight mass spectrometry, were the same. However, the reactions towards CO were performed under different conditions. In the previous experiments the reaction cell was attached directly to the cluster source and the reactions took place after the cluster formation but before the supersonic expansion of the cluster beam. The reactive gas, at low concentrations, was seeded into the He carrier gas and, thus, clusters and reaction products were thermalised in collisions with carrier gas atoms. In our experiment the reactor contains only the reactive gas, at a low pressure, and we probe the results of individual cluster-molecule collisions. Morse et al. have measured the CO reactivity of Nb n for n ~< 18, and Cox et al. for n ~< 10. Both groups found the atom and dimer inert. Morse et al. found the reactivity to increase slowly, almost monotonically with n except for small dips at n = 7, 10, 12 and 16. Cox et al. observed a high CO reactivity of N b n compared with clusters of most other transition metals. They found that all studied cluster sizes of Nb n had about the same reactivity with the exceptions of Nb 3 and Nbl0, which were about 20% less reactive. Our results for niobium clusters differ from the earlier results in two ways. Firstly, we see a sharp increase with n in the reactivity of small clusters, n = 4-9, which is much steeper than what Morse et al. observed. Secondly, in our measurements Nb m appears relatively much more inert compared with neighbouring cluster sizes. As regards the first discrepancy we believe that this can be an effect of the inability of the smaller clusters to accommodate the chemisorption heat and form stable products in the absence of a thermalising buffer gas. Similar effects could make Nb m seem less reactive in our measurements. If we assume that CO has a low binding energy on this cluster, the buffer gas colli-
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sions and the cooling in the supersonic expansion in the previous experiments can significantly help stabilising this weakly bound product. A possible explanation of the strong size dependence in the CO reactivity of Nb~ around n = 10 might be found by analysing the electronic structure of the metal-CO bond. The chemisorption of CO on metal surfaces is generally assumed to involve transfer of electrons to the CO 2zr antibonding orbital, which weakens the C - O bond. However, there is also a flow of charge from the CO 50- orbital to the metal, which is thought to strengthen the C - O bond. The filling of electrons in the 27r orbital is the dominating flow and a strong metal-CO bond is accompanied by a weak C - O bond. Having this in mind it might be possible to explain the observed Nbn reactivity with charge transfer arguments. Previous investigations on the ionisation potentials (IP) of Nb~ showed that the IP exhibits a significant size dependence around n = 10 [6-8]. For n ~< 8 the IP is high ( > 5.3 eV), at n = 9 the IP drops to ~ 4.8 eV followed by an increase to ~ 5.5 e V a t n = 1 0 and a local minimum of ~ 4 . 7 e V a t n = 11. For n = 12-20 the IP is again a rather smooth function of n except for a minimum at n = 15. This means that clusters like Nb 9 and Nbl~, which have low IP, can transfer electrons more easily to the CO molecule and form a strong metalCO bond, than Nbl0 can with its higher IP. Similar charge transfer arguments have been used to explain the variations in D 2 reactivity of Nb [6,9] as well as Fe [10] clusters, since the dissociative chemisorption of D 2 also involves transfer of electrons to an antibonding orbital in the D 2 molecule. Experimentally the D 2 and N 2 reactivity of Nb~ showed minima for n = 8, 10, (12) and 16 [2,9]. For Nb9, Nbll and Nb12isomers with different IP a n d / o r D 2 reactivity have been identified [7,9,11,12]. It has also been discussed if the variations of properties of Nb~ is an effect of structural changes and the start of formation of a layer of surface atoms surrounding one (or several) 'inside' atoms [8]. The structural changes would then be reflected in both electronic and chemical properties. The Ni~-CO reactivity was measured by Cox et al. who studied Nil-Nil3. They found the atom and dimer unreactive, some variations in the reactivity for n = 3-5, an almost constant reactivity for n =
235
6-11 and an increase in relative reactivity with about 30% for n = 12 and 13. The overall reactivity was found to be about 50% lower for Ni n than for Nb n. The results of Cox et al. agree with ours in the way that Nil2 and Nil3 are more reactive than Ni9-Nill, with the difference that we observe an increase in reactivity for n = 9-11 not present in the data of Cox et al. Again this could be an effect of low CO binding energy on these clusters, combined with the less efficient product stabilisation, especially for the smaller clusters, in our experiment. However, as long as the binding energy is not known, it is not possible to draw any certain conclusions. In the previous experiment as well as in our measurements it was found that the CO reactivity was generally higher for Nb, than for Nin, although, in our case, this difference is best observed in the larger clustersize range. Compared with Nb n both IP and CO reactivity of Ni, exhibit a rather smooth size dependence. For Nig-Nil~ there is a decrease in IP, but for larger sizes the variations are very limited [13]. Thus, for the small sizes the increase in S for CO on Ni, coincides with a decrease in IP, whereas the slow decrease in S for n > 15 does not correlate with any IP variations. Our conclusion is that the electronic structure of the clusters, e.g. characterised by the IP, significantly influences the CO reactivity, as can be seen in the distinct variations in S for Nb9-Nb12 and the smooth size dependence for Ni,. However, the IP alone cannot be expected to explain all variations in reactivity in various cluster size ranges and on different metals. Since clusters for a long time have been used as theoretical models for surfaces it is interesting to compare the cluster reactivity with the reactivity of the corresponding bulk surface. Sufficiently large clusters can be expected to attain properties similar to those of the bulk. It can be interesting to see if this seems to occur even at small cluster sizes, as the ones studied here. The adsorption of CO on Ni surfaces has been studied extensively and kinetic data is available for several single crystal surfaces [14-20]. At room temperature conditions the CO molecule is assumed to adsorb via a precursor state with an initial sticking probability of 0.75-0.95 [1518] and the chemisorption energy is 108-136 k J / m o l [16,18-20]. Molecular beam studies of CO sticking on Ni(100) and N i ( l l l ) showed that an increase in
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impact energy and an oblique angle of incidence caused a decrease in S [15,17]. In our cluster experiment there is a rather wide distribution in collision energy and the collisions could be anything from head on collisions to a slight touch between the "hard-sphere surfaces". These less well-defined experimental conditions complicates a detailed comparison. W e can only note that the sticking probability of CO for the most reactive nickel clusters, n = 1 2 18, is similar to the bulk value, while the sticking probability is somewhat lower for larger cluster sizes. Much less data is found in the literature concerning CO reactions on Nb surfaces and we have not found any data on sticking probabilities. However, we note that a sticking probability close to unity, which was found for large Nb n, has also been measured for surfaces of several left-hand-side transition metals [14]. We conclude by discussing briefly the relative bond strengths of C O / N b n and C O / N i n . In our experiments we found that increasing the intensity of the ionising laser light caused a decrease in the relative abundance of CO products for Ni~. W e believe that this is an effect of multiphoton absorption, inducing heating of the cluster and subsequent desorption of adsorbed CO molecules. For Nb~ this effect was much less apparent, which seems to indicate that CO is more strongly bound to Nb~ than to Nin. We cannot in our experiments determine whether the chemisorption occurs dissociatively or not on the cluster surface. However, the stronger bonding between CO and Nb~ than between CO and Nin could indicate that CO chemisorbs dissociatively on Nb~ but nondissociatively on Ni~, following what is characteristic of the corresponding bulk surfaces [141.
Acknowledgements W e gratefully acknowledge financial support from the N U T E K / N F R Materials Research Consortium "Clusters and Ultrafine Particles", the Swedish Re-
search Council for Engineering Sciences (TFR) and the Swedish Council for Planning and Coordination of Research (FRN).
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