J. Phys. Chem. B 2004, 108, 14575-14578
14575
On the Formation of Silver Carbonyl Clusters in CO Droplets† Irene Rabin* and Wilfried Schulze Fritz-Haber-Institut der MPG, Faradayweg 4-6, D-14195 Berlin, Germany ReceiVed: February 19, 2004; In Final Form: April 6, 2004
Neutral silver cluster CO compounds have been produced by pick-up of silver atoms by large CO droplets. FTIR spectra in the CO stretching frequency region reveal the presence of both fully covered Agn clusters for n ≈ 45 and silver cluster carbonyl complexes Agn(CO)m for n e 15. Cationic Agn(CO)+ m 1 e n e 15 and m ) 1, 2, 3 were produced by electron impact ionization and identified mass spectrometrically. A post ionization fragmentation mechanism has been proposed to explain the abundance pattern of ionic distributions.
Introduction Both the chemistry of transition metal carbonyls and the interaction between transition metal surfaces and carbon monoxide have been subject to investigation for decades.1,2 In both cases, the interaction of the transition metal and CO is well described by the interplay between donation from 5σ orbitals of CO to transition metal s-orbitals and back-donation from d-orbitals of transition metal to antibonding 2π*-orbitals of CO.3 The charge transfer from CO onto metal causes CO bond shortening whereas the back-donation acts in the opposite way. In the majority of transition metal carbonyl complexes the value for the stretching frequency of CO lies below that for the free CO molecule (2143 cm-1) due to a considerable back-donation. Similarly, for transition metal surfaces the value of the CO stretching vibration at low CO coverage is lowered with respect to the monolayer value of ∼2110 cm-1.4 From this point of view the coinage metals present a special case. In particular the back-donation process is largely suppressed for silver, due to its low-lying d-states. As a result, only indications of weak chemisorption have been reported for fully CO-covered silver surfaces.5 Our previous study of fully CO-covered silver clusters revealed a change in their reactivity toward CO below the cluster size of 300 atoms per cluster. A decrease of the CO stretch frequency to a value of 2080 cm-1 for Agn, n ≈ 30, was interpreted in terms of the charge transfer from metal clusters to CO.6 Although in many cases the reactivity of the coinage metal clusters was found to be similar to that of the corresponding macroscopic surfaces,7 some reaction channels display a dependence on the cluster individual electronic structure. For example, closed shell cationic CunCO+ clusters were found to be more stable toward collision-induced dissociation than their open shell counterparts.8 Furthermore, a manifestation of the electronic odd-even effect was observed in the study of the photochemistry of carbonyl sulfide (OCS) adsorbed on small silver clusters.9 For the atomic interaction of silver with CO, the poor backdonation results in an essentially van der Waals molecule AgCO whose very existence is doubtful.10 Weakly bound neutral complexes Ag(CO)m (m ) 2, 3) have been observed in matrix isolation experiments.10-12 The picture is different for silver †
Part of the special issue “Gerhard Ertl Festschrift”. * To whom correspondence should be addressed. E-mail: rabin@ fhi-berlin.mpg.de.
carbonyl cations, which were observed experimentally both in the gas phase13 and in Ne matrices.10 Here a large degree of electrostatic interaction causes the stabilization of these positivelycharged complexes in which the CO stretching frequency exceeds even that of the charged CO molecule.14 No experimental investigation of small silver carbonyl clusters has been reported so far. Matrix isolation studies of the AgCO interaction led to the conclusion that cluster growth in the pure monoxide surrounding is arrested by the formation of stable silver carbonyls Ag(CO)m, m ) 2, 3. Furthermore, silver dimers were found non reactive toward CO in a fast flow reactor at room temperature.15 However, evidence for the existence of the small silver cluster carbonyls has been observed in the infrared spectra of the compounds produced by matrix isolation of silver in Ar/CO mixtures.6 In the present work we report for the first time the observation of silver cluster carbonyl cations Agn(CO)+ m (1 e n e 15; m ) 1, 2, 3) formed in a pure CO surrounding. The neutral complexes Ag(CO)m (n g 3, m g 3), produced by means of the recently developed pick-up technique, were studied optically and mass spectrometrically. Infrared spectra in the region of the CO stretching frequency show dependence on the mean size of the cluster distribution. Experimental Section The experiments were performed by combining mass spectrometry with optical spectroscopy. Ag clusters were produced by pick-up of silver atoms by large CO droplets. The experimental setup has been described previously.16 Silver is evaporated from a resistively-heated crucible operated in the temperature range 1000-1100 °C corresponding to a vapor pressure of about 10-2 - 10-1 mbar. CO clusters are formed by adiabatic expansion at 180 K through a conical nozzle with a throat diameter of 0.1 mm and an opening angle of 25°. Crossing the region of high silver partial pressure, carbon monoxide clusters accumulate Ag atoms. After passing through an additional cryopump the Ag+CO cluster beam enters an axially aligned singly focusing magnetic mass spectrometer.17 Ionization was achieved by impact of electrons with 10-200 eV energy. For optical spectroscopy white light from a 400 W Xe lamp traverses the cluster beam axis at the end of the pick-up region. Scattered and fluorescence light is detected at 90° and spectrally analyzed in the range 200-1000 nm by means of a monochromator connected to a charge coupled device (CCD).
10.1021/jp049234l CCC: $27.50 © 2004 American Chemical Society Published on Web 07/09/2004
14576 J. Phys. Chem. B, Vol. 108, No. 38, 2004
Figure 1. Mass spectra from the Ag/CO particles emerging from the pick-up source operated at a constant silver partial pressure of 2 × 10-1 mbar and with three different CO stagnation pressures P0: a) 10 bar, b) 7 bar, c) 5 bar. All the spectra were measured at an electron impact energy of 100 eV.
For IR experiments the Ag/CO clusters with specific size distribution are condensed for 30 min on KBr at temperatures of about 15 K. After preparation of the matrix the target is lowered into the absorption chamber of an FTIR spectrometer (Bruker IFS 113). Results and Discussion The study of silver cluster formation in noble gas droplets revealed that it is possible to produce bare silver clusters or mixed silver-X (X)Ar, Ne) compounds. The characteristic features of these two different cluster formation regimes are summarized below. 1). Production of the Bare Agn Clusters. The rare gas host shell is removed during the cluster formation process. A characteristic mass spectrum shows only a distribution of bare silver clusters. An emission spectrum taken at the interaction region displays bands of gas-phase dimers and trimers and it is independent of the noble gas employed.16 2). Production of Mixed AgnArm Compounds. The argon host shell is still present upon completion of the clustering process. In the characteristic mass spectrum one finds, in addition to a distribution of bare clusters, peaks due to the Ar+ m and AgnAr+ m species. In this case the emission spectrum corresponds to that of silver dimers and trimers in an Ar matrix.18 At first sight, silver cluster distributions grown in carbon monoxide droplets seem similar to those grown in argon. Mass spectra reproduced in Figure 1 demonstrate the already familiar dependence of the mean size of silver cluster distribution on a droplet host size: the larger the initial host droplet the larger is the nuclearity of the formed metal clusters. At a fixed nozzle temperature and geometry the host cluster size (i.e. CO, in this case) is determined by the stagnation pressure at which expansion takes place: larger clusters are formed at higher stagnation pressures. The evaluation of the average number of
Rabin and Schulze CO molecules per droplet is complicated by the lack of reliable scaling parameters for carbon monoxide. The exact numbers are, however, of lesser relevance for the results under discussion. According to the rough estimation of the lower limit of droplet size from the mass spectra of pure CO clusters (not shown) the average droplet size grows from njmin ) 320 at 5 bar to njmin ) 1000 at 10 bar. The Ag/CO spectra (Figure 1a, b, c) were obtained at different stagnation pressures P0 (10, 7, and 5 bar, respectively) with all other source parameters (nozzle temperature, jet geometry, silver vapor pressure) kept constant. The mass spectra were measured at 100 eV electron impact energy. Hence, the fragmentation induced by the ionization process is expected to shift the observed distributions to lower masses. Upon ionization s1-metal clusters cool by evaporation of neutral monomers and dimers until a stable ionic distribution is reached.19 For mixed clusters one expects a considerable evaporation of host shell particles following the ionization event. By analogy to the Ag/Ar system we would expect the localization of the positive charge on the metal cluster. The difference in the ionization energies between the CO droplet and metal cores leads to energy releases that might be sufficient to completely remove the carbon monoxide shell. The fragmentation extent of the remaining metal cluster can be determined only experimentally. At the moment however, we are interested only in the apparent relative changes that are not obscured by the underlying fragmentation. In Figure 1 the (a) and (b) spectra differ only in the size of the silver distributions Ag+ n with n e 45 and n e 30, respectively. The main intensity peaks at n ) 11 and n ) 9, respectively, where the odd-even alternation of the ion abundance is pronounced. The fact that odd cluster ions (i.e. with even number of electrons) are of higher intensity than their even neighbors points at the post ionization fragmentation of metal clusters. The pattern is well-known from previous mass spectrometric studies of silver clusters produced by sputtering,20 gas aggregation with subsequent electron impact21 and multiphoton ionization.22 In addition to the singly-charged ions, the doubly-charged ones are observed at “half masses”. The largest doubly-charged cluster ion detected in the top spectrum corresponds to Ag+ 49 while that of the spectrum (b) amounts only to . Both numbers agree well with the respective distribuAg2+ 33 + tions of singly charged Ag+ n . The Agn distribution depicted in Figure 1c with nmax ) 12 shows a less pronounced odd-even effect despite the high electron impact energy (main peaks). In the multitude of the lines between the main peaks we find no evidence for doubly-charged ions but a series of mixed clusters Agn(CO)+ m . This is a somewhat unexpected result if the formation of silver clusters follows the same principle as in an Ar droplet (regime 1). In the Ag/Ar system, reducing the host diameter would merely lead to a reduction of the mean silver cluster size. With this in mind we have reexamined the background of the mass spectra depicted in (a) and (b) and discovered trace amounts of mixed clusters as well. Moreover, attempts to produce silver clusters uncontaminated with CO or to detect free silver dimers and trimers by optical spectroscopy failed. To summarize, the average size of silver clusters formed in CO droplets displays the familiar dependence on host droplet size. However, mixed Agn(CO)m compounds could be observed under all experimental conditions. Thus, we believe that there is only one regime of cluster formation in the Ag/CO system in contrast to the Ag/Ar one. Further analysis of the mass spectra reveals that the composition (i.e. n and m numbers) of the charged Agn(CO)+ m clusters is markedly different from that of
On the Formation of Silver Carbonyl Clusters
J. Phys. Chem. B, Vol. 108, No. 38, 2004 14577 Figure 2b the absence of Ar+ m and doubly-charged ions from the spectrum is accounted for by the fact that they cannot be produced at an electron impact of 12 eV. The odd-even intensity variation of Ag+ n is largely suppressed but no dramatic rise of the Ag+ n intensity toward the high mass end is observed, indicating that only short chain fragmentation, if any, occurred. The intensity of AgnAr+ m (n ) 1, 2, 3) rises with of higher n are discernible in growing m. Moreover, AgnAr+ m the spectrum. The presence of these ions confirms the hypothesis of Ar shell evaporation following the ionization event. At the same time, the unchanged relative intensities of Ag+ and Ag+ 2 are in agreement with the fragmentation mechanism for the s1metal clusters established in previous studies.19 Thus, the overall fragmentation of a silver cluster grown in an argon droplet would evolve along the following lines: hν,e-
Figure 2. Influence of electron impact energy Ei on mass spectra from the Ag/Ar particles emerging from the pick-up source at a silver partial pressure of 10-1 mbar and Ar stagnation pressure of 10 bar corresponding to the droplet size of about 1000 at./cl.: a) Ei )100 eV, b) Ei )12 eV.
Figure 3. Influence of electron impact energy Ei on mass spectra from the Ag/CO particles emerging from the pick-up source at a silver partial pressure of 2 × 10-1 mbar and CO stagnation pressure of 5 bar: (a) Ei ) 100 eV, (b) Ei ) 12 eV.
AgnAg+ m . With argon at 100 eV electron impact energy we find the series AgnAg+ m with n ) 1-3, 1 e m e x, x . n, whereas Agn(CO)+ compounds contain one to three carbonyl groups m attached to a cluster Agn n ) 1, nmax. Figures 2 and 3 illustrate this difference. Figure 2a and Figure 3a depict portions of mass spectra of silver clusters formed in Ar and CO droplets, respectively, and ionized at 100 eV. Figure 2b and 3b show spectra from the same distributions obtained at 12 eV electron impact energy. Let us consider first the Ag/ Ar system. In the upper spectrum of Figure 2 we find in addition to the naked Ag+ n cluster ions (n e 10) the following series: + + Ar+ m (2 < m < 26), AgArm (1 < m < 10), Ag2Arm (1 < m < + 2+ 9), Ag3Arm (1 < m < 8), and Agn (9 e n e 13). The strong odd-even effect in the abundance pattern of the singly-charged ions and the presence of doubly charged clusters with n ) 13 allows us to conclude that in this mass range we observe a stable ionic distribution. At the same time the composition of mixed AgnAr+ m proves the presence of the Ar shell at the moment of + ionization. Note the low abundance of AgnAr+ m and Ag2 . In
+ + Me2n + 1Arm 98 Me2n+1 + mAr f Me2n-1 + Me2 hν,e-
+ Me2nArm 98 Me+ 2n + mAr f Me2n-1 + Me
(1)
In addition, we would like to stress that the emission spectrum measured at the exit of the pick-up source displayed both features of silver particles in an argon solid matrix and of free monomers and dimers. Let us now turn to the mass spectra of Ag/CO measured at the same ionization energies (Figure 3a and 3b). The initial ionic distribution of Agn+ is in the same range as that of the Ag/Ar. The intensity of the bare silver cluster ions alternates at oddeven n numbers and falls off gradually toward n ) 9. Each cluster peak is followed by a group of the respective carbonyls Agn(CO)+ m , m ) 1, 2, 3. Here the abundance of the carbonyl compounds is comparable with that of the naked silver cluster ions. Moreover, only intensities of odd Agn+ exceed those of the respective carbonyls by a factor of 2. An additional manifestation of the odd-even effect can be seen in the reversal of the relative intensity order in the series with 4 < n < 8 for n ) 4, 6, 8 IAgn(CO)1+ > IAgn(CO)2+ > IAgn(CO)3+, whereas for odd n ) 5, 7, 9 the order is reversed, that is, IAgn(CO)3+ > IAgn(CO)2+ > IAgn(CO)1+. The monomer, dimer, and trimer fall out of the series. There is a very small amount of AgCO+, whereas Ag(CO)2+ and Ag2(CO)2+ appear to be the most stable species in the atomic and dimer series. At 12 eV electron impact energy (Figure 3b) a soft rise of the intensity has replaced the gradual falloff toward the high mass end. However, the odd-even alternation characteristic of the naked silver cluster ions is still pronounced. Striking differences characterize the portions of spectra at n e 5. The relative intensities of dimer, trimer, and tetramer are decidedly lower than at 100 eV impact ionization. Also, atomic and dimer carbonyl compounds are practically absent from the spectrum. While Ag3+ loses its dominant position (cf. Figure 3a) its carbonyl Ag3(CO)2+ is represented now by a prominent peak. From the comparison of the mass spectra at 12 and 100 eV we conclude that Agn(CO)m, n > 3 were present in the neutral distribution whereas Agn+ (n ) 2, 3) as well as Agn(CO)m+, n ) 1, 2, m ) 1, 2, 3 are fragmentation products. This result demonstrates that the mechanism responsible for the ionizationinduced fragmentation of Agn(CO)m+ compounds should be different from the mechanism (1) established for the post ionization processes in the Ag/Ar system. There is not much experimental or theoretical work on silver carbonyl clusters in the size range under discussion. The picture is different with respect to atomic silver carbonyls where both neutral and charged species have received attention. Both
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Figure 4. Comparison of FTIR spectra in the 2230-1800 cm-1 region for pure solid CO (lower curve) and Ag/CO distributions produced at a silver partial pressure of 2 × 10-1 mbar and CO stagnation pressure of 5 bar (middle curve) and 10 bar (upper curve). The band under the peaks of free CO at 2092 cm-1 (upper curve) belongs to the CO stretch frequency of the CO monolayer adsorbed on silver clusters Agn, n > 30.
experimental and theoretical studies found carbonyl cations Ag(CO)m+ (m ) 1-4) to be stable compounds. The sequential bond energies obtained experimentally from collision-induced dissociation13 and by density functional theory calculations10 agree reasonably on the values for mono- and bicarbonyl cations. These values are 0.9 eV and 1.1 eV for the loss of the CO group from mono- and bicarbonyl cations, respectively. There is some disagreement on the data for tri- and tetracarbonyl where experimental values of 0.57 eV and 0.47 eV, respectively, exceed the calculated ones. The important message for us is: the threshold for CO loss from silver carbonyl and bicarbonyl is comparable to Ag loss from a silver cluster ion. No data for silver cluster carbonyls is available but we will assume that the electrostatic bonding model developed for the Ag(CO)m+ is valid for the interaction of cluster carbonyls and that the binding energies are comparable. In such a case the following mechanism could account for the observed fragmentation pattern: upon ionization rapid evaporation of CO groups produces charged mixed clusters Agn(CO)m+ m ) 3, 4. Further cooling occurs via the loss of either Ag or CO, which are competing processes. The loss of Ag2 is less favorable. e-
Agn(CO)m 98 Agn(CO)+ m f + Agn(CO)m-x
+ xCO f
+ Agn(CO)m-x-1 +C + Agn-1(CO)m-x + Ag
(2)
Summarizing the results of the mass spectrometric investigation we conclude that no free neutral silver clusters can be obtained in the case of silver cluster growth in carbon monoxide droplets (N. B. Only silver monomers have been detected in the emission spectra measured at the exit of the interaction zone.). In addition, we have studied the cluster distributions of the type reproduced in Figure 1 by means of FTIR spectroscopy. In Figure 4 the infrared spectra for the solid CO (lower solid line) and the spectra obtained for silver clusters grown in CO droplets are compared, the mean size of the silver distribution increasing from bottom to top. In addition to the absorption at 2143 and 2092 cm-1 of pure carbon monoxide, we see the large peaks at 1937 and 1967 cm-1 that have been assigned to the doubly degenerate C-O stretching mode of Ag(CO)3.10,11 The
peak at 2085 cm-1 which appears as a shoulder on the free CO absorption has also been attributed to Ag(CO)3 by the same authors. The band under the peaks of free CO at 2092 cm-1 (upper spectrum) has been ascribed to the CO stretch frequency for CO monolayer on Agn, n ≈ 30.6 The lowering of the CO stretch frequency as compared with that found on bulk silver (2110 cm-1) has been interpreted as evidence for the charge transfer from the silver cluster to the CO monolayer. Note that this broad band appears only in the upper spectrum that corresponds to the larger of the cluster distributions studied in the present work. Furthermore, we observe new sharp features that have not been explicitly reported so far. We attribute them to CO stretch frequencies from different silver carbonyl clusters Agn(CO)m (3 e n e 15, m ) 1, 2, 3). The majority of the new lines is found in the frequency region characteristic for Ag(CO)3. However, we also find a group of bands close to that of a free CO molecule that we tentatively attribute to the individual contributions from small silver cluster monocarbonyls, AgnCO (ne 8). In this size range, the properties of silver clusters are known to be strongly dependent on their characteristic electronic configurations. In other words, the differences in the behavior of adjacent clusters can be easily detected. Therefore we suggest that the appearance of satellite lines on both sides of the free CO line is a manifestation of the individual character of AgnCO interaction. Conclusions Under appropriate expansion conditions, the coagulation of Ag atoms in CO droplets leads to the formation of small silver cluster carbonyls, Agn(CO)m (n < 15). Mass spectrometric analysis revealed the existence of stable cationic species for n < 15 and m ) 1, 2, 3. In this case, post-ionization fragmentation processes result in competing CO or Ag atom evaporation. References and Notes (1) Campuzano, J. C. The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis; 1990; Vol. 3. (2) Lupinetti, A. J.; Strauss, S. H.; Frenking, G. Nonclassical Metal Carbonyls; Karlin, K. D., Ed.; John Wiley & Sons: New York, 2001; Vol. 49, p 1. (3) Frenking, G.; Froehlich, N. Chem. ReV. 2000, 100, 717. (4) Ryberg, R. AdV. Chem. Phys. 1989, 76, 1. (5) Hansen, W.; Bertolo, M.; Jacobi, K. Surf. Sci. 1991, 253, 1. (6) Froben, F. W.; Rabin, I.; Ritz, M.; Schulze, W. Z. fur Phys. D 1996, 38, 335. (7) Knickelbein, M. B. Annu. ReV. Phys. Chem. 1999, 50, 79. (8) Nygren, M. A.; Siegbahn, P. E. M.; Jin, C.; Guo, T.; Smalley, R. E. J. Chem. Phys. 1991, 95, 6181. (9) Brown, L. A.; Rayner, D. M. J. Chem. Phys. 1998, 109, 2474. (10) Liang, B.; Andrews, L. J. Phys. Chem. A 2000, 104, 9156. (11) McIntosh, D.; Ozin, G. A. J. Am. Chem. Soc. 1976, 98, 3167. (12) Chenier, J. H. B.; Hampson, C. A.; Howard, J. A.; Mile, B. J. Phys. Chem. 1988, 92, 2745. (13) Meyer, F.; Chen, Y.; Armentrout, P. B. J. Am. Chem. Soc. 1995, 117, 4071. (14) Barnes, L. A.; Rosi, M.; Bauschlicher, C. W., Jr. J. Chem. Phys. 1990, 93, 609. (15) Li, L.; Hackett, P. A.; Rayner, D. M. J. Chem. Phys. 1993, 99, 2583. (16) Ievlev, D.; Rabin, I.; Schulze, W.; Ertl, G. Chem. Phys. Lett. 2000, 328, 142. (17) Goldenfeld, I.; Frank, F.; Schulze, W.; Winter, B. Int. J. Mass Spectrom. Ion Processes 1986, 71, 103. (18) Ievlev, D.; Rabin, I.; Schulze, W.; Ertl, G. Eur. Phys. J. D 2001, 16, 155. (19) Brechignac, C.; Cahuzac, P.; Roux, J. P.; Pavolini, D.; Spiegelmann, F. J. Chem. Phys. 1987, 87, 5694. (20) Katakuse, I.; Ichihara, T.; Fujita, Y.; Matsuo, T.; Sakurai, T.; Matsuda, H. Int. J. Mass Spectrom. Ion Processes 1985, 67, 229. (21) Rabin, I.; Jackschath, C.; Schulze, W. Z. fur Phys. D 1991, 19, 153. (22) Kandler, O.; Athanassenas, K.; Echt, O.; Kreisle, D.; Leisner, T.; Recknagel, E. Z. fur Phys. D 1991, 19, 151.
