Size effects in metal cluster-ion chemistry

In ternational Journal o f Mass Spectrometry and Ion Processes, 121 (1992) 1--47

1

Elsevier Science Publishers B.V., A m s t e r d a m

Review Size effects in metal cluster-ion chemistry Manfred P. Irion lnstitut J~r Physikalische Chemic, Technische Hochschule Darmstadt, Petersenstra.Be20, W-6100 Darmstadt (Germany) (First received 4 May 1992; in final form 17 July 1992)

ABSTRACT In a special Fourier-transform ion cyclotron resonance mass spectrometer, the chemical reactions of different metal cluster ions with a variety of reactive gases have been observed at room temperature as a function of cluster size. The duster ions generated by sputtering with Xe + primary ions are transported via ion lenses to the analyzer cell, where they can be stored for up to 100s. For noble metals, mere adsorption of 1--4molecules is the prevalent reaction type. In the case of the more reactive transition metals, addition to the cluster is typically accompanied by dehydrogenation. Several drastic size-specific effects are discussed, which depend not only on the metal alone but also on the complete system of metal cluster ion and reactive gas. For some systems, only a few sizes are reactive with the majority being inert. For others, the reverse is true and the largest number of sizes is maximized. In addition, there are systems where reactivity either oscillates with increasing size or varies smoothly. For some ion/molecule reactions, the absolute rate constants have been measured. Reactions that do not proceed spontaneously (oxidation of copper clusters, methane activation) are induced by resonantly exciting the ions to a higher kinetic energy. The Fe~- ion is distinguished by a strong reactivity towards NH3 as well as towards C2 H4, etc., whereas Ni~ proves totally inert. This suggests that the tetramer cannot have the same structure in both cases. For the first time, the catalytic activity of a naked gas-phase metal cluster could be proven. Fe~ was shown by a complex (MS) 5 experiment to synthesize benzene from three adsorbed ethene molecules.

Keywords: FT-ICR; SIMS; metal clusters; size-specific chemistry; kinetics. I. I N T R O D U C T I O N

Clusters can be defined as aggregations of a limited number of identical species, such as atoms or molecules. By observing the development of their physical and chemical properties with increasing size, one should be able to model the transition from a single particle in the gas phase to a multitude of them, constituting the solid phase. Aggregates of metal atoms are of special interest since they are often characterized by a high degree of coordinative unsaturation with a large number of dangling bonds. Thus, they are reactive Correspondence to: M.P. Irion, Institut fiir Physikalische Chemic, Technische Hochschule D a r m s t a d t , Petersenstrasse 20, W-6100 D a r m s t a d t , G e r m a n y . 0168-1176/92/$05.00

© 1992 Elsevier Science Publishers B.V. All fights reserved.

2

M.P. lrion/Int. J. Mass Spectrom. Ion Processes 121 (1992) 1-47

enough to be used as homogenous [1] or heterogeneous [2] catalysts. In these applications, however, they are mostly either surrounded by ligands or supported by a solid and, therefore, cannot exhibit the behaviour typical of the pure naked clusters [3]. Pure naked metal clusters without any remaining chemical bonds might serve not only as model substances for the surfaces of heterogeneous catalysts but also represent very active catalysts in themselves. Therefore, knowledge of their chemical behaviour as a function of size is important. The advent of supersonic beam technology [4(a),(b)], combined with laser vaporization of metals [4(b),5] enabled for the first time studies on pure naked metal clusters in the gas phase. Thus, since 1981, a wealth of information has been accumulated on neutral metal clusters. Physical properties, such as ionization energies and electronic structures [6,7] as well as chemical reactivity [7,8] have been investigated as a function of cluster size. Recently, such results have been the subject of several conferences [9] and excellent reviews [10]. The usual experimental procedure is to vaporize a metal in a helium flow with a pulsed laser, expose the growing clusters to a reactive gas and photoionize the whole mixture with a UV laser. Reactants and products are usually identified by time-of-flight (TOF) mass spectrometry. The disadvantages of this technique are twofold: first, clusters are generated as size distributions to start with. Selection of a single size is extremely problematic and has only recently been achieved [11]. Second, ionization, as required by mass spectrometric analysis, may easily fragment the reactant ions, thus making it difficult to extract any reliable information on cluster size [12]. Even though interest initially centers around neutral clusters, it would appear advantageous to start with the charged clusters and investigate their chemical reactions. In this way, the reactants can be clearly defined and a unique relationship established with subsequent products. In addition, ions may be stored for the duration of the experiment. For these reasons the chemistry of metal cluster ions only is reported here. Charge should have a decreasing impact with increasing size and may only affect rate constants but not the chemistry involved [14]. Thus, there is a slight chance that the results of ion chemistry may be transferable to the chemistry of neutrals. A comparison of both kinds of results should yield valuable insights into the influence of charge on a metal cluster in any case. The following discussion describes chemical reactions of metal cluster cations, generated by sputtering and stored in a specially designed Fourier-transform ion cyclotron resonance mass spectrometer (FT-ICR MS). The study of several different combinations of metal clusters and reactive gases suggests the observed chemistry to be determined not by the metal alone but by the entire system. Surprising size-specific effects are found but not easily interpreted. During the time span of their storage in

M.P. lrion/Int. J. Mass Spectrom. Ion Processes 121 (1992) 1--47

~'~ . . . . . . . . . . .

PumpI

Pump2

L2

3

G !

-o

U

Magnet

Fig. 1. Experimental arrangement of the FT-ICR MS with external secondary ion source, consisting of the elements: SC, sample chamber; X, Xe ÷ beam; T, target; DP, differential pumping tube; LI, L2, einzel lenses; SI, $2, sets of steerer plates; E, extractor electrode; G, control grid; IC, ICR cell; PV, pulsed valve; W, window.

the ICR cell, the cluster ions are size-selected, equilibrated to near room temperature and finally reacted with an added gas. Thus, absolute rate constants are derived for the spontaneous reactions. Certain reactions demand translational excitation of the ions in order to proceed. An example of true catalytic activity of a gas phase bare metal cluster ion is even demonstrated and is proven by a complex experiment of five sequential stages of mass spectrometry ((MS)S). II. EXPERIMENTAL CONDITIONS

11.1. Description of experimental design and procedure The applied experimental set-up is reproduced in Fig. 1. It has already been the subject of a detailed description [15]. Naked metal cluster cations are continuously generated in an external sample chamber (SC) by bombarding the respective metal target (T) with a beam of 20 keV Xe ÷ primary ions [16] from a duoplasmatron (X). They are accelerated to a kinetic energy of about 2 keV and guided by a system of steerer plates and einzel lenses through magnetic fringing fields toward the ICR cell of a home-built FT-ICR MS. The cell is in the shape of a cube with a side-length of 80 mm and is located in the homogeneous region of a superconducting 7.05T magnet (Oxford Instruments Ltd). A grid (G) in front of the cell is used to electrostatically decelerate the ions to near ground potential and to allow a time-selected segment of them to enter the cell. D.c. voltages and pulses are provided by a

4

M.P. lrion/Int. J. Mass Spectrom. Ion Processes 121 (1992) 1-47

Spectrospin data system with an ASPECT-3000 mini computer, which also performs the Fourier transformation. Two piezoelectric valves are mounted at a distance of 90 m m to introduce inert, as well as reactive gases. With neon pulsed in up to a maximum of about 10-4mbar, the external cluster ions lose excess kinetic and internal energy through collisions and are finally trapped within the cell for time periods of up to 100 s. Normally, the background pressure is below 10 -9 mbar. Reactant gases may be admitted through a pulsed valve, in cases where the pressure rise does not need to be known exactly. For quantitative measurements, the gases are added with continuous-leak valves whereby the pressure increases up to a steady state value in the range 5 x 10 -8 to 1 x 10-Tmbar. To obtain a quick overview of the reactivity of certain metal cluster ions, experiments are carried out on size distributions of the ions trapped initially. The following section presents some examples of these, illustrated by wide-band FT mass spectra. For more detailed knowledge, it is necessary to isolate in the ICR cell, ions of a single size and sometimes even of a single isotope. In order to obtain information on the structures of the ions, collision-induced dissociation (CID) may be initiated, even with single isotopes. The option of high mass resolution helps to distinguish too closely-spaced ion signals. Finally, the evaluation of absolute rate constants is demonstrated. I1.2. Production o f different metal cluster ions

In general, sputtered metal cluster ions are characterized by a pseudoexponential intensity decay with increasing cluster size [17]. This is demonstrated by the mass spectra obtained with our instrument [18] which cover all the clusters whose chemistry is reported in this review. The mechanism of cluster formation by fast ion bombardment of a surface is not known exactly. They are either pre-formed within the bulk or grow in a plasma a few hundred picometers above the surface [19]. In either case, the probability of their formation will decrease with increasing size. Superimposed upon that decay are intensity irregularities caused by the special stability of closed shells, as evident in the case of metals of groups Ib and III of the Periodic Table. H.2.1. M e t a l s with characteristic intensity variations." Cu + , A g + , A u + and

In general, clusters with closed electronic shells have proven to be the most stable. Cluster sizes at which this condition is fulfilled, are called "magic numbers". The spherical jellium model predicts these to occur for 2, 8, 18, 20, 34, 4 0 , . . . , valence electrons [20]. For metals with one valence electron, such as the alkali or the coinage metals, magic numbers are to be expected with 3, 9, 19, 21, 35, 4 1 , . . . , atoms. Figures 2(a), 2(b) and 2(c) show the most intense

M.P. lrion/Int. J. Mass Spectrom. Ion Processes 121 (1992) 1--47

Cu~ 13

i 0

Jl

it.,,

500

....

1000 m/z (a)

1500

2(X)O

9

Agn+ 13

i 0

L,, i, i 2000

x511B7

m/z

l,,9

/,000

55

6000

(b)

x10

59 i

0

27

5000

~

m/z

10000

(e) Fig. 2. (opposite and above).

77

15000

5

6

M.P. Irion/lnt. J. Mass Spectrom. Ion Processes 121 (1992) 1--47

÷

Inn

35 I,

z0b0

p

61

0b0° , 60b0

,~

80b0

8,0

90

10-3

10 00

(d)

Fig. 2. (a) FT mass spectrum of sputtered copper cluster cations, (b) FT mass spectrum of sputtered silver cluster cations, (c) FT mass spectrum of sputtered gold cluster cations, (d) FT mass spectrum of sputtered indium cluster cations.

Cu,+ , Ag,+ and Au,+ signals for cluster sizes in rough accordance with these numbers. Indium, with three valence electrons, should exhibit magic numbers at 7 and around 14 atoms, corresponding to 20 and 41 (close to 40) electrons. The quite strong signals o f In~- and InCa in Fig. 2(d) appear to confirm the preceding statement. In conclusion, the magic numbers observed here agree well with those reported by others, using different techniques for cluster generation and detection [21]. Our size distributions represent true equilibrium distributions where all less-stable cluster ions decompose before detection. The gold-cluster spectrum contains the highest mass, A u ~ , that we were able to detect (16.742 u), but the largest cluster that we observed in terms o f atomic number was In~03 (11.826 u). H . 2 . 2 . M e t a l s w i t h the t y p i c a l p s e u d o - e x p o n e n t i a l i n t e n s i t y d e c a y : V. + , F e + , Co + , Ni + and Pd +

The noble metal clusters described in the previous section are quite inert toward reactive gases. To discover the rich and varied chemistry that generally characterizes metal clusters, one must switch to less noble transition metals. Figures 3(a)-3(e) display the mass spectra of Vn+ , Fe + , Co + , Ni + and Pd + clusters exemplifying the simple, m o n o t o n o u s pseudo-exponential intensity decay with increasing size. Vanadium clusters proved extremely reactive, like the other members of this group, giving oxides and nitrides with residual traces of air in the vacuum. This apparently limited the m a x i m u m achievable size to n = 15, which for the other clusters extended up to n = 25.

M.P. lrion/lnt. J. Mass Spectrom. Ion Processes 121 (1992) 1-47

200

z,O0 m/z

600

800

1001

(a)

Feg 7

x5

19

•

r

2OO

500

m/z

800

1100

(b)

+

10

6

. . . .

soo . . . .

xlO I , 20

iobo'

(c)

Fig. 3. (overleaf and above).

m/z'

'lsbo

'

'

2~G

8

M.P. lrion/Int. J. Mass Spectrom. Ion Processes 121 (1992) 1-47

+

Nio

x7

x 35

6

ii

s6o

lObO

m/z (d)

Pdn+ 2

,

.Jl ,

,

,

,

.

.

.

.

10

.

.

1000

.

.

,

m/z

,

,

, ,

I

,

2000

,

,

,

'

' '

, ' , - i

300C

(e)

Fig. 3. (a) FT mass spectrum of sputtered vanadium cluster cations, (b) FT mass spectrum of sputtered iron cluster cations, (c) FT mass spectrum of sputtered cobalt cluster cations, (d) FT mass spectrum of sputtered nickel cluster cations, (e) FT mass spectrum of sputtered palladium cluster cations.

11.3. Selection of certain ion masses for chemistry studies Sometimes, it becomes necessary to study a specific ion mass separately, without interference from neighbouring ions. In these cases, all ions of unwanted masses are eliminated by exciting them with r.f. ejection pulses, so that they hit the ICR cell walls. Figure 4 contains our standard pulse sequence for this purpose. First, the cell is cleared of all residual ions by a quench pulse and, at the same time, neon is pulsed in. From the resulting collisions, the external cluster ions that enter the cell during the gate pulse are already partially thermalized and trapped. This is followed by the ejection pulses, after

M.P. lrion/Int. J. Mass Spectrom. Ion Processes 121 (1992) 1-47

9

which one specific cluster size remains isolated in the cell. Reactive gases are now introduced, either through continuous-leak valves or the second pulsed valve. In case the ions left in the cell do not react spontaneously, they may be forced to do so by excitation with an r.f. activation pulse at their resonance frequency. After a variable reaction-time delay, the r.f. excitation pulse follows, initiating the transient response from all the ions present. Fourier transformation of the transient finally yields the mass spectrum. 11.4. Temperature of the cluster ions in the ICR cell For the investigation of cluster ion chemistry it is essential to know, especially for quantitative measurements, the actual thermodynamic state, i.e. the temperature of the stored ions. The importance of this point can be seen in the conflict between the groups of Smalley and Jarrold concerning silicon cluster ions. In an FT-ICR apparatus, Smalley and co-workers found certain cluster sizes to be totally unreactive [22(a)], whereas in the ion drift tube experiments of Jarrold and co-workers all clusters reacted with no sizespecificity [23]. At first this discrepancy was attributed to different structural isomers and should have been resolvable by annealing [22(b)], but now it has been realized that temperature may be the all-deciding factor. Although this problem has not yet been finally settled, it appears that in the FT-ICR experiment the silicon clusters produced by laser vaporization might have had a mean temperature of 700 K [23(c)]. The preparation technique exerts a distinct influence upon the temperature of the cluster ensemble, provided that its constituents may be characterized by a common temperature at all. Clusters produced by laser vaporization/supersonic expansion are in internal equilibrium during the process of formation, when they suffer a large number of collisions with the buffer gas. After expansion into vacuum, they are no longer in a well defined state. Photoionization, as often applied, may also increase their energy. Thus, the above-quoted preparation method does not necessarily yield clusters at room temperature. Sputtering produces very hot clusters that can reach internal equilibrium by evaporating atoms, but the common temperature is equally not well defined. The clusters must interact with a heat bath at constant temperature to reach better-defined thermal conditions and a uniform temperature. With a modification of our standard pulse sequence, as shown in Fig. 4, we were able to derive a measure for this value in our experiment. We inserted another variable delay between the gate and the r.f. ejection pulse, called "additional equilibration time" (Fig. 5). The cluster ions that have just entered the cell have undergone a number of collisions with neon gas pulsed-in for trapping and are thus partially thermalized to its temperature (room temperature). By extending the additional time delay they are made to undergo

10

M.P. lrion/lnt. J. Mass Spectrom. Ion Processes 121 (1992) 1--47

Quench

U--

b PnezovaLve I ( t r a p p i n g gas) GaLe

t

EJectbon

-

-

-

. . . . . . . . . .

Plezova[ve 2 Creactlve gas]

U

Rctavatton

I

(CID)

Iq

Excutatnon

~R

~i

|

Rcqulsltlon

^v

I

~,

2000

J

I

~,,

2800 2900

I II

I

3S~0 3905 3903

40~0

TameCtqp)

Fig. 4. FT-ICR pulse sequence for standard applications.

an even higher number of collisions with the bath-gas xenon, leaked into the vacuum (at about 1 x 10 -7 mbar) by the operating duoplasmatron. We measured, as a function of the additional equilibration time, the initial rates for a reaction already familiar to us, the dehydrogenation o f N H 3 by Fe~-

Quench

U

LJ

Plezovaive 1 (traDpqng gas] GaLe I=

~"

-

~t

Ejection

Excntaluon

Rcqu,sltlon

i

0 5

I

2000

~v

I 5000

t

hv

5800

11, 9800

9805 9803

Fig. 5. FT-ICR pulse sequence, modified for temperature estimation.

, 10000 T , m e ( t 4 p ) ms

11

M.P. lrion/lnt. J. Mass Spectrom. Ion Processes 121 (1992) 1--47 { t / < k > t = O ) - 100

90 80 .

70 60 50 40

Fe;

30

NH3 _H 2

Fe4.NH+

~

20 10 I

I

I

I

I

I

I

I

I

1

2

3

4

5

6

7

8

9

Additional Equilibration Time Is]

Fig. 6. Initial rates for the dehydrogenation of ammonia by Fe;.

ions [24] (see section IV.1.2.1): Fe~- + NH 3 ~ Fe4(NH) + + H 2

(1)

As is evident from Fig. 6, these rates first drop exponentially with increasing equilibration time, as would be expected for cluster ions still in the process of cooling. After about 5s, however, the temperature dependence has approached a constant at a little more than half the value immediately after trapping. By this time, trapped cluster ions have achieved complete equilibrium. The measured initial rates have dropped by a factor of about two over the whole time span. Thus, if the activation energy of reaction (1) is not unexpectedly low, the cluster temperature directly after trapping must have been less than: (room temperature + 20°C). We conclude that cluster ions originally produced by sputtering at a high temperature, can be cooled to a value between room temperature and, at most, 40°C within a period not longer than 5 s.

11.5. The option of CID In general, mass spectrometry is not the best technique for obtaining direct information on the structure of ions. However, it can yield some more-indirect information through the option of CID. Although not as efficient as X-ray diffraction applied to a crystal, it is often used in mass spectrometry for structure determination. In 1982, this universal process was

M.P. Irion/lnt. J. Mass Spectrom. Ion Processes 121 (1992) 1-47

12

first applied to FT-ICR MS by Freiser and co-workers [25], who also continued to investigate it systematically [26]. For a CID experiment, the ion of interest is first isolated from all the ions stored in the ICR cell, as described in the preceding paragraph. The selected ions are excited by an r.f. pulse at their resonance frequency to a variable amount of translational energy, given in electronvolts by eqn. (2) T - Er2f e2 t2 8m

(2)

where Erf. is the electric field in volts per meter, e is the elementary charge, t is the irradiation time in seconds and m is the mass in kilograms. The excitation of ions, resulting in an expansion of their cyclotron orbits, is limited to a value below that which would cause them to be ejected from the ICR cell. Thus, the finite distance of the cell plates puts an upper limit to the achievable ion kinetic energy: Tmax=

e2 r 2 B 2 2m

(3)

where r is the ICR cell plate distance in meters and B the magnetic field strength in teslas. To attain the maximum translational energy, the cell size, as well as the magnetic field strength, need to be maximized. Under these conditions, excitation up to the high-energy regime of ion beam instruments ( > 1 keV) is possible. In 1983 this was demonstrated by Bricker et al. who fragmented, in a cubic ICR cell of 50 m m side-length at 3 T, the stable CrH~molecular ion [27]. In our cubic cell, of 80 m m side-length at 7 T, ions of 100 u can be excited to a laboratory energy, ELab, of up to 37.8 keV, provided that they are positioned in the center of the cell. Ions that have been accelerated to higher kinetic energies undergo an increased number of collisions with the neutral particles present and finally dissociate: inert gases or the sample gas itself are commonly used for this purpose. In order to obtain reliable dissociation thresholds, "single collision conditions" must be obeyed [28(a)]. For a comparison of different collision gases, the laboratory energy is converted to the "center-of-mass" energy, Ecru, which defines the maximum energy transfer in a collision of a molecule, M, with an ion, I, and is given, for the masses mi, by the equation Ecm = ELab mm~M + mr

(4)

Tandem mass spectrometry, as it is used to clarify the paths of complicated ion/molecule reactions (see example in section V.2), relies on the tool of CID. In this variant, abbreviated (MS) n, multiple stages of mass spectrometry are applied to generate a desired type of ion by chemical means, isolate it, follow

M.P. lrion/lnt. J. Mass Spectrom. Ion Processes 121 (1992) 1-47

Cus" cu; ,hlu

II

..63Cu~.OSCuZ

13

~

63Cu,.6SCus" ÷

O~Cus65Cu~,

÷

........

d

S~Cu665Cu 3 .. . . . . .

.....

+

63Cu765CU 2

6~C%65Cu"

...---6]CU8 + I

l

.~

i 0

l 520

l

I 5t, O

I

l 560

I

I 580

n mlz

Fig. 7. Isotope-selective C I D of the Cu9÷ cluster with xenon as collision gas.

its decay by CID, isolate the daughter ions, follow their reactions, etc . . . . In a conventional mass spectrometer, n separate sectors would be needed to conduct such an experiment. Using ion storage, the different steps are performed sequentially in time rather than in space. The maximum number of achievable steps depends upon the number of parent ions stored originally and the overall efficiency of the (MS)" process [28(b)]. According to a recent review, it is limited in practice to n = 5 or, at most, n = 6 [28(c)]. However, already in 1984, (MS) 5 was demonstrated on organic ions at 1.4 T [28(d)] and at 7 T, even more than (MS) 2° should, in theory, be possible [28(e)].

H.6. Isotope-selective CID on the Cu~- ensemble The mass spectrum of Cu~ cluster ions in Fig. 2(a) contains broadened patterns caused by the two naturally occurring copper isotopes 63Cu (0.6917) and 65Cu (0.3083) [29]. If CID is to be applied to Cu~- as the parent ion, the r.f. activation pulse has to have a perfectly rectangular shape, or the pattern will not completely disappear with increasing energy. This disadvantage can be turned into an advantage by isolating ions of a specific isotopic mass and then exciting them. For that purpose, a phase-inverted r.f. pulse is added to the normal ejection pulse, so that these ions are not eliminated. The result can be seen in Fig. 7: single isotopes from the 63CU9_n65CUn+ pattern decay into their respective daughters 63Cu8-n 65Cu~ and 63CU9_n 65Cu,- l [15(b)].

14

M.P. Irion/Int. J. Mass Spectrom. Ion Processes 121 (1992) 1-47

11.7. Isolation of a s&gle •otopic mass: the ion S6Fe4(NH) +

Studying the reaction of Fe~- ions with NH3 (see Section IV.1.2.1), we encountered a similar problem. The sputter target consisting of natural iron with the isotopes 54Fe (0.0580), 56Fe (0.9172), 57Fe (0.0220) and 58Fe (0.0028) [29] produced ion signals that were, with increasing cluster size, not more than 1 u apart. In this situation, a further refinement of the ejection technique like "soft ejection" [30(a)] or front-end-resolution enhancement using tailored sweeps (FERETS) [30(b)] was required, allowing the isolation of these single isotopic ion signals close to + 0.5 u at 240 u without exciting the isolated ions. Figure 8 shows an example: with standard ejection, the ion of interest, 56Fea(NH)+, is still surrounded by neighbouring peaks 54'57Fe56Fe3(NH)+; however, with special soft ejection it is completely free of them [30(c)]. 11.8. Making use of the ultrahigh mass resolution

In the process of sputtering, the 56Fe4H+ ion was also generated, probably due to hydrogen contamination of the solid iron. It has such a similar mass to the isotope 57Fe56Fe~- that only a mass spectrum with a resolution of about 80.000 would resolve the two peaks (Fig. 9). To achieve this high mass resolution, the range of excited masses has to be kept very narrow. In this case, it extends only over 4 u. 11.9. Measurement o f absolute rate constants

Ion/molecule reactions in an ICR cell are generally governed by pseudo first-order kinetics, as the reactive gas is present in great excess over the ion concentration. While the reactive gas is continuously admitted, the associated rate constants are measured by first selecting the reactant ions in the ICR cell and, after a variable time delay, detecting products as well as left-over reactants. Monitoring ion intensities as a function of this time delay yields the pseudo first-order rate constant k' = kP, where k is the absolute rate constant and P is the actual pressure of the reactive gas. Thus, in order to measure absolute rate constants, it is necessary to determine the actual pressure with sufficient accuracy. This is traditionally done by correcting an ion gauge reading with the ionization cross-section of the gas and the pressure gradient between the ion gauge and the ICR cell [31(a)]. Usually, the reaction CH4+ + C H 4 ~ CH~ + C H 3

(5)

is chosen as a reference, since its rate constant is temperature independent and has often been measured using various methods [31(b)]. However, it is more advantageous to determine the reactive gas pressure

M.P. lrion/Int. J. Mass Spectrom. Ion Processes 121 (1992) 1-47

15

56Fe4(NH}"

Standard Eje~ion

S4Fe S6F~e'~NH)÷

s7FeS6Fe3(NH)*

T.l.l,~l,i.l.l~l,J.l,1,i.i,i,lii.i,"'l.l.l"l 233. ~

257. ~

Z4t • g

2¢S. ~

2~,9.

m/z

S6Fe4(NH)*

"' Soft Ejection"

""

I

"

I

'-I

1

i ~

253.~

I

'

I~

I

"1

'

|~

I

'

2,57,6

I

"'

I

''1

241.6

1

1~

I

r']l

245.0

1'

I

'

I

"

I

"

249.9

m/z

Fig. 8. Demonstration of "soft ejection", applied to the S6Fe4(NH)+ cluster, as opposed to "standard ejection".

directly in the ICR cell. To do that, the reactive gas is itself ionized by electron impact from the internal electron gun and the pseudo first-order rate constant for the molecular ions reacting with the neutral gas molecules, k ~ is measured. The absolute rate constants, kM~ are tabulated for most ion/

16

M.P. lrion/Int. J. Mass Spectrom. Ion Processes 121 (1992) 1-47

S6Fe~

i Fe4+

STFeSeFe~ 3~"i5.~=eH ~'* A,-~ = 80000

S4FeS6Fe3 i 222 .

.

.

.

223 m/z ,

224 ,

.

.

.

225 ,

-

Fig. 9. High resolution FT mass spectrum around the 56Fe~- cluster. molecule reactions [32], thus enabling the calculation of the reactive gas pressure actually present in the ICR cell: k~l = kMIP

(6)

If the reaction of the cluster ion (CI) is measured under the same conditions as that of the reactive-gas molecular ion (MI), the desired absolute rate constant is calculated according to kc I _

k~l kMl p - k~[ k~ I

(7)

In the exemplary case of the typical metal cluster ion reaction of ethene addition/dehydrogenation, as discussed in sections IV.1.2. to IV.1.5, the ion/molecule reaction C2H ~- + C2H 4 "--}C3H ~- + C H 3

(8)

is used where kMt = (4.3 + 1.5) × 10-1°cm3s -I [33]. III. STUDIES OF METAL CLUSTER-ION CHEMISTRY

As mentioned in the Introduction, the interest is originally centered on the chemistry displayed by the neutral clusters. Thus, the majority of work has indeed been conducted in this field [7-10]. However, for the reasons already discussed, that approach will be totally neglected here. Different methods are in use to investigate the chemical behaviour of charged metal clusters. Actually, the same experimental configuration as for the study of neutrals

M.P. lrion/lnt. J. Mass Spectrom. Ion Processes 121 (1992) 1--47

17

may be employed, except that ions are now detected directly by pulsed extraction in the TOF MS [13,14]. Other possibilities are drift tube arrangements with quadrupole filters for size selection and detection [34]. The technique of FT-ICR MS offers by far the biggest advantages for the detailed investigation of metal cluster-ion chemistry, which have been outlined more specifically in the Experimental section. Realizing the specific advantages, Freiser and co-workers developed, quite some time ago, an excellent method for preparing metal cluster ions directly in an ICR cell by irradiation of a metal surface with a laser beam [35]. Without additional aggregation, as induced by a high pressure helium environment, this approach was unfortunately limited to the smaller-sized clusters (dimers, and trimers) [36]. Later, Reents and co-workers studied the reactions of small semiconductor cluster ions generated by a related method [37]. Smalley and co-workers were the first to generalize this approach to many different metals and a large size range by supplementing laser vaporization with collisional cooling. In 1986, they coupled supersonic beam production of metal cluster ions with FT-ICR detection [38]. When using this arrangement to store Nb~+ (n = 3-25) clusters in the presence of H 2 at varying pressures and reaction times, adsorption was found to occur producing Nbn H~y where y < n. Nb~0 and Nb~ appeared extremely unreactive, while Nb~ and Nb~9 seemed to possess two structural isomers, of which one was inert [39]. No explanation for this size-specificity was given, except for the general statement that a purely electronic model was insufficient. Such a model has been proposed for the apparent reactivity/ionization potential (IP) relationship that had been noted in some cases [40]. For Nb + clusters reacting with H2 it was concluded that structure was the most important factor. Zakis and co-workers studied the reactions with D 2 in a flow tube and obtained differing results. They found two structural isomers for Nb~2, one more and the other less reactive. A strong anti-correlation of reactivity and IP was seen which was rationalized by cluster valence electronic structure. In comparison, the neutral Nbn clusters did not behave too differently [41(a)]. Reaction of Nb + clusters with 02 studied in a drift cell did not show any size-specificity [41(b)]. More recently, Smalley et al. applied the supersonic cluster beam/FT-ICR approach to study adsorption of NH 3 by Si~+ clusters to yield SinNH / . They identified Si~3 as the most reactive cluster while Si~9 and Si~5 appeared inert. Again structural isomers were found [22]. As mentioned earlier, these results are strongly questioned by Jarrold and co-workers, who cannot confirm any size-specificity for the reactions of silicon cluster ions with ammonia, water or ethene [23]. In 1989, Irion and co-workers presented a new approach to the study of the thermal reactions of metal cluster ions based upon coupling a secondary ion

18

M.P. lrion/Int. J. Mass Spectrom. Ion Processes 121 (1992) 1-47

source with FT-ICR detection [15]. Some results obtained by this method are discussed below. IV. CHEMICAL BEHAVIOUR OF DIFFERENT METAL CLUSTER IONS EXPOSED TO A VARIETY OF REACTIVE GASES IN AN FT-ICR MS In the following discussion, reactions of metal cluster ions with added gases are reported that have been divided into those proceeding spontaneously and the others needing additional ion activation. Practically all spontaneous reactions start with an addition to the cluster and then proceed to the elimination of hydrogen or the synthesis of a larger molecule. It is important to remember that all spontaneous reactions are performed with cluster ions trapped for a long time and thermalized to nearly room temperature (see section II.4). In 1988, Kaldor et al. stated that all chemical reactions of gas phase naked metal clusters studied up to that date could be characterized as addition reactions, perhaps followed by dehydrogenation, but still "the analogs of molecular adsorption onto metal surfaces" [42]. As is evident from this chapter, that statement describes the situation very well, even for most results reported until recently. This only changed in 199 l, when for the first time a charged gas-phase naked metal cluster was discovered to catalyze the synthesis of a larger molecule from smaller ones added onto it. The process is explained in greater detail in a later section of this review. IV.1. Spontaneous addition reactions The general prerequisite to reaction is addition of the substrate to the cluster ion. Mere addition of a molecule not accompanied by any other chemistry usually requires collisional or radiative cooling to stabilize the product. The first option involves using fairly high pressure and is, therefore, not so well suited for study by FT-ICR. Because of the rather long time involved, the second cooling mechanism is much more appropriate for FT-ICR conditions, as a recent study shows [43(a)]. Addition to metal clusters without any subsequent chemistry is demonstrated here for a few examples. The largest number of systems was found to belong to the category of addition accompanied, or followed by, some other kind of chemistry, generally dehydrogenation. These are listed in a somewhat empirical manner according to the diverse types of reactive behaviour characterizing them. Reactivity may vary drastically as a function of cluster size. In some systems only a few sizes react readily, but most of them are totally inert. IV.I.1. Systems with mere addition o f intact molecules: Pd+/D2, (Ag +, Au+ )/(NH3, C6H6) and C0+/C6H6 As mentioned above, mere addition of a molecule onto a metal cluster ion is better studied in mass spectrometers operating in the high pressure region

M.P. lrion/Int. J. Mass Spectrom. Ion Processes 121 (1992) 1-47

Pd7+

~,.~ ~-~,,

.,~

,,,,

750

I P[J++ D2]

:,

,~

.:.'-~-,~..,.

800

19

Pds+

~

,

.~

850

.,,

,

.,., m/z

Fig. 10. Reaction of Pd¢ and Pd; cluster ions with deuterium. (millibar range). At pressures of 10 -5 to 10 -8 mbar, the most probable stabilization process is emission of IR radiation [43(b)]. Examples of this mere association process are shown with D2 addition onto Pd~ and C 6 H 6 o r NH3 addition onto Ag+, Au+ or even Co + . Whenever a substrate molecule is added without chemical modification, the reaction appears to be reversible, i.e. the adducts formed, decay again while being stored in the ICR cell. Only physisorption occurs, as collisions at room temperature suffice to break the cluster-substrate bond. IV.I.I.1. The Pd+/D2 system: mere addition o f deuterium. Adsorption of

hydrogen by neutral metal clusters is one of the first and most thoroughly studied reaction systems [44(a)]. Rate constants, as well as saturation values, have been measured and did not show much size-specificity. As palladium metal is known to store large amounts of hydrogen gas, we decided to study, in an exemplary manner, the reactions of Pd~- and Pd~- cluster ions with D2 [30(c)]. The only reason we did not investigate the larger sizes was their extremely complex isotope patterns. The element consists of the six naturally occurring isotopes, l°2Pd (0.0096), l°4Pd (0.1097), 1°sPd (0.2223), l°6pd (0.2733), 1°8pd (0.2671) and II°pd (0.1181) [29]. An amazing size effect was observed, when both types of ion were isolated together and exposed to deuterium at about 1 x 10 -7 mbar (Fig. 10). After a reaction time of 8 s, Pd~ions appeared to have added three deuterium atoms while Pd~- ions appeared inert. The formation of Pds D~- product "would mean that a metal cluster had dissociatively chemisorbed hydrogen. Such an effect would be improbable because of its high activation energy and indeed it has not yet been observed.

M.P. lrion/lnt. J. Mass Spectrom. Ion Processes 121 (1992) 1-47

20

I

NH

Ag n+

adducts

3

11

I'"

15

I

20

1

i

.

i , . . , l ,, , , l ,. ,, I , , , , l , , , , [ , ,

L,00

700

,p.,, ,i,..,

1000

I , .,, i , , , , r , , , , l

1300

, , , , i, , . , i , , , , i , ~ , ,

mlz

1600

i,.,,i,,,,

1900

T,,. ,F,,,, i,,,, [ ~ 1

2200

,, . . l . . . .

2500

Fig. 11. Overview FT mass spectrum of adduct ions formed by the addition of ammonia onto Ag.+ (n = 3-21) cluster ions.

However, a closer look at the mass spectrum in Fig. 10 reveals that the isotope pattern of Pd~- has broadened by the mass of one D2 molecule (4 u) upon deuterium adsorption. Thus, must probably, a mixture of Pd8 D~- and Pd 8D~is formed which we could not resolve any further, even with the very high resolution of the FT-ICR technique. The observed size specificity is not understood at this time and it was not reported in a study of neutral Pd, cluster chemistry [44(b)]. The reactivities of Pd 7 and Pd8 were found to be very similar; only Pd 9 proved relatively inert. The maximum ratio of adsorbed deuterium atoms to palladium atoms was about 2.5:1 in that size range, which is much higher than in our experiment (0.5 : 1 at the most). The reason for this is the low-pressure conditions of the FT-ICR experiment in general, which we did not exploit to the highest possible value. IV.I.I.2. The (Ag+,Au.+)/(NH3, C6H~) systems." mere addition of ammonia or benzene. Ag,+ or Au + cluster ions (n = 3-21, complete mass spectra in Figs. 2(b), 2(c)) exposed to ammonia or benzene vapour at about 10-6mbar, add A u n (C 6 H 6),,+ u p to m = 3 intact molecules to give Ag, (NH 3)+, Au, (NH 3)+ or A g . ( C 6H6) + , respectively. Figure 11 demonstrates this for NH3 adsorption onto Ag~ cluster ions. Obviously, the occurring physisorption cannot provide enough energy to overcome the barriers to chemical reaction [45]. IV.l.l.3. The Con4-/C6H6 system: mere addition of benzene. Co + cluster ions generated in the size range n = 2-28 (Fig. 3(c)) and trapped in the presence of benzene vapour at about 10-6mbar attach with no size-specificity, up to

M.P. lrion/lnt. J. Mass Spectrom. Ion Processes 121 (1992) 1--47

21

m = 3 intact molecules to yield C o n ( C 6 H 6 ) m + . Thus, they react similarly to the noble metal and the smaller nickel clusters. The absence of any dehydrogenation again confirms the relatively low general reactivity of cobalt clusters noted earlier [45]. IV.1.2. Systems with a minimal number of reactive sizes." Fe+/NH3, Fen+/ C2H4 and Fe +/c-C3H 6 Although it has been stated that the chemistry observed is mostly determined by the complete system of metal cluster and reactive gas, the group of systems listed in this section seems to contradict that conclusion. This special class with reactive sizes as the exception appears to be dominated by iron cluster ions, which were the only ones to fit clearly into this category. First, the system of iron cluster ions with ammonia as the reactive gas is examined, as it represents a nice, clear example of that unique behaviour where just one single size out of the total range reacts in any way. Next, dehydrogenation of ethene and cyclopropane is described, where two and three different sizes respectively proved capable of producing a reaction, albeit in a different manner. In this context, it might be interesting to note that dehydrogenation of bound ethane to a C2H4 ligand occurs with the same single size iron cluster (the tetramer) as that of bound ammonia to an NH ligand. The addition of a single hydrogen atom to the bare metal cluster ion to yield Fe4 H + completely inhibits its reactivity. In contrast, all cluster sizes larger than the tetramer react with acetylene by binding C2 ligands; the size before (the trimer) is inert, but it may be activated by the addition of a hydrogen atom to yield Fe3H + [46]. IV.1.2.1. The Fe+ /NH3 system." dehydrogenation of ammonia. When a distribution of thermal Fe + ions (n = 2-13; Fig. 3(b)) is exposed to ammonia at about 10-7mbar, F e f ions turn out to be unreactive while Fe~ ions slowly decay to Fe~. The larger clusters Fe+_13attach up to four intact NH3 units. Fe~-, however, deviates from that pattern in dehydrogenating the ammonia to yield Fe4(NH) + as a first step, as shown in reaction (1). The cluster must still have a second site for dehydrogenation, as the reaction is repeated to form Fe4(NH)~-. From that step on, only intact NH 3 molecules are added further as in the case of cluster ions Fe~-_~3.This may occur with and without accompanying loss of an iron atom, leading to Fe3(NH)E(NH3)- and Fe~(NH)z(NH3) + with a probability of 65% and 35% respectively. The respective end products of the sequence are Fe3(NH)E(NH3) J- and Fe4(NH)z(NH3) + [30(c),45]. The complete reaction scheme is reproduced in Fig. 12. Since the Fe + - N H bond strength must exceed the dissociation energy of H2-NH, comparison of this value with the measured bond strength of single-bonded F e + - N H leads to the conclusion that Fe~--NH must be

22

M.P. Irion/lnt. J. Mass Spectrom. Ion Processes 121 (1992) 1-47

NH$[j-M2 Fe4(NH)+ ,re, o)

NN3~/-)'I;~ NH3

Fe4(NH)2 ~ (~s,)

~4(NH)2(NH3) + ( :k~'1)

Fe'dNH)2(NH3} + c2,s)

Fe4CNH)2(NH3)2"' ~2,~)

Fes(NHI2INH312 + c~.e)

Fig. 12. Reaction sequence of Feg cluster ions with ammonia.

double-bonded [47]. This is in good agreement with the observation of two binding sites on naked Fe~ for NH3 dehydrogenation. The behaviour of small iron clusters towards ammonia is of some relevance to the understanding of industrially-applied ammonia synthesis. Here, intermediates are postulated consisting of an NH-group bound to the metal atoms of the heterogeneous catalyst surface, similar to the ionic gas phase intermediates discussed above [48]. Parks et al. studied the reactions of neutral Fe, clusters up to n = 100 with ammonia and found only adsorption of intact molecules with a smooth cluster-size dependence [7(c)]. Nothing special was observed about Fea, thus, in the case of the small clusters, the positive charge exerts a drastic influence. In one way, Fe~- represents an exceptional case, since other cluster ions such as Si,+ [22] or GaxAs~- [49] do not dissociate the ammonia either.

IV.1.2.2. The Fe+IC2H4 system: dehydrogenation of ethene. The same Fe + (n = 2-13) cluster ion distribution exhibits a pronounced size effect when it is reacted with ethene at about 10 - 7 mbar. Under these conditions, the cluster ions FeJ-, Fe~- and Fe~_13 appear totally inert. Only Fe~- and FeJ- are capable of reaction. The chemistry occurring is characterized mainly by a sequential addition and dehydrogenation of ethene molecules. Fe + forms Fe4(C2H2) + adduct clusters where m = 1-4 acetylene units attach to metallic nucleus. In comparison, FeJ- appears less reactive, adding only m = 1 or 2 C2H2 groups onto Fes(C2H2) + . CID experiments on the isolated adduct ions lead to the conclusion that the ligands bound to Fe~- isomerize to a benzene precursor, whenever their number is greater than or equal to three. This catalytic process is presented in greater detail in section V [50]. Zakin et al. studied the adsorption of Dz onto Fe + (n -- 1-31) clusters in a flow tube and also found Fe~- and Fe~- to be especially reactive, as well as Fe~8 and Fe~3_31• They discuss, again in terms of a reactivity/IP relationship, the behaviour of the larger clusters but not that of the smaller sizes. In comparison with the neutral Fe,

23

M.P. lrion/Int. J. Mass Spectrom. Ion Processes 121 (1992) 1-47

Fe4(C2H~).+

C2H4

>

~4(C2H2)., +.i

- H2

[ c m 3 s - I mdecule-1 ]

110- 9

lO-m

10-11

10-12

I

I

I

I

0

1

,"

3

m

Fig. 13. Absolute rate constants for the sequential dehydrogenation of ethene by Fe~ cluster ions. clusters, charge is observed to have a substantial effect, drastically enhancing the rates of Fe~_6 [51]. For the consecutive dehydrogenation steps Fe4(C2H2)m+ + C2H4 --* Fe4(C2H2)++I + H 2

(9)

the absolute rate constants (see section II.8) have been determined to be: m

k m (cm 3 s- t ) [52]

0 1 2 3

2.1(___0.6) 4.5(___0.9) 5.3(___0.4) 6.9(+0.3)

x x x x

10 -I° 10 -tl 10 -II 10 -12

They are also displayed in Fig. 13, from which it is evident that the adduct with three acetylene units is formed markedly faster than suggested by a linear progression o f the other rate constants. This obvious rate increase for the stoichiometry o f FenC6H ~- confirms our earlier hypothesis o f benzene precursor formation upon reaction of Fe~- with ethene. These conclusions are also in accordance with another observation, namely Fe4(C2H2)~ clusters, where m < 3, are able to bind a dehydrogenated ammonia fragment, yielding

24

M.P. Irion/Int. J. Mass Spectrom. Ion Processes 121 (1992) 1-47

Fe4(C2H2)mNH +. Those where m = 3 have lost this capability and attach only intact NH3 molecules [52].

IV. 1.2.3. The Fe+/c-C 3H6 system." dehydrogenation of eyclopropane. When the Fe + (n = 2-13) cluster distribution is stored in the presence of cyclopropane at about 10 -7 mbar, only the ions Fe~-, Fe] and Fe~- react. FeJ- cleaves the molecule, attaching up to four carbene groups to give Fe3(CHz) + (m = 1-4). The larger reactive cluster ions bind the molecule with hydrogen elimination again. They adsorb a C3H 4 unit and then a C3H2 unit to yield Fe4(C3H4) (C3 H2) + or Fe 5(C 3H4) (C3 H2) + respectively. Fe~- concludes this sequence by the addition of a carbene to form Fe4(C3H4) (C3H2) (CH2) +, revealing its ability to accommodate just two C3 groups. Fe~ terminates the sequence by adding another C3H 2 group to yield Fes(C3H4) (C 3H2)~-, demonstrating a larger number of binding sites suited for dehydrogenation. It is also spacious enough to accommodate three C3 groups. For the three sequential reaction steps Fe 4+

k~ ,

Fe4(C3H4)+

k2 ,

Fe4(C3H4 ) (C3H2) + k3 Fe4(C3H4 ) (C3H2) (CH2)+

the absolute rate constants (see section 11.8) have been measured to be as follows: I

kt (cm 3s-I) [52]

0 1 2

2.0(+0.4) x 10 -l° 1.8(+0.3) × 10 -l° < 10 -12

With cyclopropane, an adduct cluster of Fe4C6H ~- stoichiometry is also formed. Obviously, the degree of dehydrogenation is controlled by the tendency to achieve this stoichiometry. The rates of the first and second reaction step are nearly equal, although the number of C-H bonds to be split is two compared with four. This again indicates that the formation of the stable benzene precursor acts as a driving force [50].

IV. 1.3. Systems with a multitude of reactive sizes." Ni,+/C2 H4, Ni +/C6 Ha and

Co+/C,H~ Most systems investigated belong to this class of reactive behaviour whereby certain cluster sizes are distinguished by an extreme inertness. In general, these inert cluster sizes tend to be found amongst the smaller sizes their number ranging between two and five. Usually, the larger cluster sizes are highly reactive.

M.P. lrion/Int. J. Mass Spectrom. Ion Processes 121 (1992) 1-47

25

Nis+ + H2C=EH2 (Xe,~300K) Nt_t-_ kL'*

I~li° I r . l - L ~ ÷

1 0 ""''""'""~"""

550

400

m]z

450

500

Fig. 14. StackedFT mass spectra of the products from the reactionof isolated Ni~ clusterions with ethene as a function of time. IV. 1.3.1. The Ni + /C 21t4 system: dehydrogenation of ethene. When Ni~ clusters in the range n = 2-15 (see complete mass spectrum in Fig. 3(d) are trapped in the presence of ethene at about 10-Tmbar, nearly all sizes react by binding C2H 2 ligands from the dehydrogenation of C 2 H 4 to form Nin(C2H2) + . Ni~ions which were isolated in the ICR cell and stored for a maximum of 40 s had up to nine C2 H2 units attached to them. However, Nis(CzH 2)~- is not necessarily the end of the sequence, as the reaction was not pursued for an extended time. The course of reaction as a function of storage time is shown in Fig. 14. During this long-time scale experiment, the smaller clusters NiJ-~ appear to grow continually, which could be caused by collisions with C2 H4 molecules. Thus, Ni + and Ni~- reveal themselves as inert, while Ni~ just attaches one C2 H2 ligand to give Ni 2(C 2H2) +. Using deuterated ethenes, hydrogen loss was seen to occur from the 1,2-position, which means the ethene must be adsorbed with its double bond parallel to the cluster surface. Clusters larger than Ni~2 are even more reactive than the smaller ones, exhibiting a second size-specific effect. They strip the ethene of all hydrogen atoms, forming the carbides Ni,C~- or Ni, C + and ultimately the stable end products Ni,C~- when longer times are used [53]. IV.1.3.2. The Ni +/C6H6 system: transition from mere addition to dehydrogenation of benzene with increasing cluster size. Nickel clusters behave in a similar way towards benzene, as displayed in Fig. 15. Ni~- does not react, but Ni~ and Nig bind intact molecules to yield Ni3,4(C6H6) + up to m = 3. For these

M.P. Irion/Int. J. Mass Spectrom. Ion Processes 121 (1992) 1-47

26 NI'"

"

Ni X ÷

•

NI X ;

J i

4-

X"

C6H 6

Y"

C6Ht.'

', Nt2 •

i,2

N,,x"

" •

N

-

"

NI4X÷

NI;

-

"

NI 5 X ÷ ~

•

Ni6 X+

4-

Ni4X 2

Nlt X3 +

NI s XY +

Ni5 X z y ÷

'~ NI5XY3 ÷

Ni6XY"

Ni6X2Y*

- [Ni6XY3 ÷]

•,

[N,5XYt* ]

i

NIl;

"

Fig. 15. Pattern of Ni.+ (n = 1-6) cluster ion reactions with benzene. reversible reactions, a pressure of about 5 x 10-6mbar is needed. Larger clusters attach the first molecule still intact, but start to exhibit dehydrogenation from the second one onwards, which is then bound irreversibly. Products Ni,~5(C6H6) (c6n4)m+ (m = 1-4) are formed. This means that with both reactive gases, the same distinct size-specific effect occurs. A tentative explanation is to assume that, after Nig, clusters change from a two- to a three-dimensional structure. Ni~ seems to provide nine binding sites for ethene adsorption, Ni~ only one. As proposed earlier, a trigonal bipyramid with nine bridge positions is a viable candidate for the structure [53]. Furthermore, a spatial arrangement of nickel atoms should be better suited to accommodate benzene molecules in those positions required for dehydrogenation [45]. The effect whereby ligands influence the degree of dehydrogenation is also seen in atomic metal-ligand systems. Thus, the smaller clusters typically bind benzene molecules via physisorption where collisions during the trapping time are sufficient to break the bond. Figure 16 shows, as a function of time, what happens when the Ni4(C6H6) + adduct is isolated in the cell under the conditions where it is formed. First it splits off one and then even two benzene molecules, only to attach these again later on to yield the stable end product Ni4(C6H6)~-. The reversibility of benzene addition onto thermal nickel cluster ions is clearly demonstrated.

IV.1.3.3.

The Co+/C2H4 system: dehydrogenation of ethene. When Co+(n = 3-16) cluster ions (Fig. 3(c)) are exposed to ethene at about 5 × 10 -8 mbar, binding under dehydrogenation is again the prevailing reaction, as Fig. 17 reveals. Whereas for Co~- ions, no product could be identified, C o l , Co~- and Co~- proved completely inert. The most reactive ions were Co~- and Co~-, forming adducts up t o C 0 4 ( C 2 H 2 ) ~ - and C05(C2H2)~- respectively. The larger clusters from C0~2 and upwards abstracted more hydrogen from the ethene, yielding Co, C~- and ConCz(CzH2) +. Generally, cobalt clusters appeared not only less reactive than nickel clusters but also different in their

28

M.P. lrion/lnt. J. Mass Spectrom. Ion Processes 121 (1992) 1-47

> ~en(C2H2m_2)~ H2 ( m = 1 - 3 )

Fen* * C2H2m

k [ cm3 s - l ] 10-9

x)-~)

_

-.- Eu,~e

'~ Jk

-x- Ethane 10-11

I

I

I

I

I

I

I

I

|

|

3

4

5

6

7

8

9

10

II

12

Fig. 18. Absolute rate constants for the sequential dehydrogenation of acetylene, ethene and ethane by Fe,+ (n = 3-12) cluster ions.

IV. 1.4.1. The Fe,+/C2H2 system." dehydrogenation of acetylene. Fe~+ (n = 2-13) cluster ions of the same distribution as before (Fig. 3(b)) that are stored in the presence of acetylene at about 1 × 10-7mbar are again found to bind the molecule in question under dehydrogenation. C2 ligands are adsorbed to form adduct ions of the type FenCf,, up to m = 3. Figure 18 displays, as a function of cluster size, the absolute rate constants for the reaction Fe~+ + C2H2 --, Fe, C~- + H2

(10)

as well as for the reactions with C2H 4 and C2H 6. As is evident, the reaction with acetylene is not characterized by a distinct size-specificity, in contrast to those with ammonia [45,47], ethane [46] or ethene [50]. With increasing size the reactivity starts to oscillate between values differing by a factor of ten. This course of reaction is superimposed by a slow rise to near the collision rate. The reactions are the fastest we have measured so far [46].

IV. 1.4.2. The Pd+/c2n 4 system: dehydrogenation of ethene. When Pd + (n = 29) cluster ions (complete mass spectrum in Fig. 3(e)) are exposed to ethene at about 10-Tmbar, binding under dehydrogenation again takes place. At reaction times of up to 4 s, no more than one C2H2 ligand is adsorbed to yield Pd, C2H/. As in the system discussed before, the reactivity exhibits here also an oscillating pattern of behaviour. As Fig. 19 reveals, P d [ and Pd~- ions appear highly reactive since after only 2 s, only the product peaks are seen. Pd~-, in contrast, proves much less reactive, showing after 4 s a prominent product and a strong parent ion peak. At the same time, Pd~- exhibits an even greater inertness with a tiny product peak by the side of a well-developed

M.P. lrion/lnt. J. Mass Spectrom. Ion Processes 121 (1992) 1-47

29

fi+ i+ if ? 21;/S •

300

,

t.O0

,

i

.

.

500

.

.

.

.

.

i

600

.

.

.

.

.

v

700

.

.

.

.

,

800

-

, -

•

,

900 m/z

Fig. 19. Overview FT mass spectra of ions during the dehydrogenation of ethene by Pd~+ (n = 2-9) cluster ions as a function of time.

parent ion peak. Finally, total inertness is reached with Pd~-, where no product ion peak is visible even after 4 s. Pd~- is again very reactive, as only its product peak appears at that time. The larger clusters, Pd~- and Pd~-, are the most reactive of all. After a reaction time of only 1 s, no parent ions can be detected [30(c)].

IV.1.5. Systems with reactivity varying smoothly as a function of cluster size: Fen+/NeH4, V.+/C2H4 and Vn+/C6H6 Some systems are characterized by a reactivity increasing smoothly with cluster size. They usually exhibit no dramatic size-specific effects. Examples are reported of two types of metal clusters, one of which behaves in a manner slightly atypical. The observed smooth increase in reactivity is superimposed by some kind of a size-specific effect, where for one cluster size, the capability to dehydrogenate a bound molecule appears all of a sudden exhausted.

IV.1.5.1. The Fe~+IN2H4 system." reactions with hydrazine. All ions of the Fe~(n = 2 - 1 3 ) cluster distribution prove enormously reactive toward hydrazine at about 1 × 10 -8 mbar. If the reactivity is defined by the disappearance of the reactant cluster ion according to the net reaction Fe~+ + N2H 4 ~ products

(11)

its smooth course as a function of cluster size is displayed in Fig. 20. It is

M.P. lrion/Int. J. Mass Spectrom. Ion Processes 121 (1992) 1-47

30

r 2 . 1 0 ~a [ m 2]

k tel 90

8.8

80 70 60 5() 40

3.3

30 20 10 I

I

!

I

I

I

I

I

I

I

I

3

4

5

6

7

8

9

10

11

12

13

N

Fig. 20. Relative rate constants for the reactions of Fe~+ (n = 2-13) cluster ions with hydrazine as a function of cluster size. The simulation of the reaction cross-section underneath using the simple liquid drop model [54(a)] seems in reasonable agreement.

apparent that there is no size-specific effect as in the case of ammonia, where only Fe~- reacted (see section IV. 1.2.1). The reactivity towards hydrazine rises smoothly with increasing cluster size in the range Fe~_8 and stays nearly constant between Fe~ and Fe~2. The drop between Fe~ and Fe~3 may be explained by the effect of geometrical shell closing at n = 13, which generally causes clusters to be less reactive. The detailed pattern of Fe + cluster reactions with NEH 4 is very complicated, as Fig. 21 shows. A whole multiplicity of different reactions occurs which also lead to a huge variety of products. Their c o m m o n feature is a hydrazine fragment added to the cluster ion. Addition of an intact molecule only occurs exceptionally, requiring either a cluster with many degrees of freedom, such as F e 6 N f , or a simultaneous chemical change. This can be loss of an iron atom or an F e l l 2 unit, which was shown to be stable [54(b)]. Obviously, addition of a hydrazine molecule onto a bare metal cluster produces heat that must be used up or it will initiate further chemical reactions. The most frequent reaction of the larger cluster sizes leads to FenNf product ions. In this way, a n N 2 ligand may be attached to naked Fe + clusters, which is not possible by simply trapping them in N2 gas. To facilitate understanding of the experimental observations, the multiplicity of reactions occurring are subdivided into the following types of reaction: (1) addition of hydrazine under dissociation

31

M.P. lrion/lnt. J. Mass Spectrom. Ion Processes 121 (1992) 1-47 Fen(L)*: n=6 |

n=,4-...13

.~ Fen(N 2) +

In=4r5,

6

I

) ' FeeL(N2H2) +

i

~- Fe6L(N2H4~ +

i

[n=5t6 n=2...5

~= Fen(NH)+

n=2 n:2

~> Fe2(NH)3 ÷

it=2

~--"Fe2(NH)2(NH2 )+

~- FenL(NH)*

Fen ÷

n=2

) Fe =(NH2) +

n=3

:> Fe3(N2H2) * n=3

NzH4

~ FenL(NH2]*

n=2 :~' Fe2(NH)(NH2)2 +

n=4,5

> Fen_I(N2H2) +

n=4,5

n=3.4

~_ Fen_l(N 2H4) +

n=4

:~ Fen_IL(NH) +

), Fe3L(NH2) +

Fig. 21. Pattern of Fe.+ (n = 2-13) cluster ion reactions with hydrazine. symmetric dissociation Fe~ + N2H4 ~ F e . N H f + N H 2

(12a)

asymmetric dissociation Fe.+ + N2H4 ~ F e . N H + + NH 3

(12b)

(2) addition of hydrazine under dehydrogenation single dehydrogenation Fe~ + N2H 4 ~ Fe.N2H f + H2

(13a)

double dehydrogenation Fe~ + N2H4 ~ Fe.N + + 2H~

(13b)

single dehydrogenation under loss of an iron atom Fe~ + NzHa ~ Fe._~NzH~- + Fell 2

(13c)

(3) addition of intact hydrazine simple addition Fe6Nf + N2H 4 ---}Fe.N2(NzH4) +

(14a)

addition under loss of an iron atom Fe + + N2H4 --} Fe._~N2H~- + Fe

(14b)

Thus, if hydrazine addition to the clusters is followed by a double dehydrogenation, product ions are generated containing a nitrogen molecule, i.e. F e . N f . As already mentioned, it was not possible to prepare these ions by a simple adsorption o f N 2 onto naked Fe + clusters. To obtain some information

32

M.P. lrion/lnt. J. Mass Spectrom. Ion Processes 121 (1992) 1-47

Int tel 100 9O 80 7O 60

~2 +

50 40 30 2O 10

10

20

30

40

50

6O

Ecru EeV] Fig. 22. Ion abundances during CID of Fe6N~- adduct ions in the presence of xenon collision gas as a function of collision energy. on the bond between the iron cluster nucleus and the N2 unit, we subjected as a typical example, Fe6Nj- ions to CID with xenon as a collision gas. Figure 22 contains the intensities o f all ions involved in the CID process as a function o f collision energy. For EcM values of up to 70 eV, only adduct clusters with a reduced iron core, Fe, Nj- (n = 4-5), and smaller iron clusters Fe+(n = 2-5) are produced. Fragments with just one nitrogen atom do not a p p e a r - - t h e N2 ligand stays always intact. The bond between the nitrogen atoms is retained to the extent that they are not completely dissociated on the cluster. However, the N2 unit is not a weakly bonded nitrogen molecule either, as the fairly high desorption energy reveals. An intermediate case m a y be assumed with bonds o f almost equal strength from the nitrogen atoms to each other as well as to the metal cluster [47].

IV.1.5.2. The Vn+/C2H4 system: dehydrogenation of ethene. The most reactive clusters we have ever studied are those o f the transition metals o f group V in the Periodic Table, especially V + and Ta + ions. Although using our method they are, in principal, generated up to M ~ (mass spectrum in Fig. 3(a)), in practice, reactions can be carried out only for the smaller sizes. The reason is their extremely high reactivity towards traces o f air left as a residual gas in our vacuum system. The background vacuum in our ICR cell was below 1 × 10 -8 mbar, but that did not prove clean enough for these delicate clusters. If we attempted to store them for more than 8 s, only oxides could be detected.

M.P. lrion/lnt. J. Mass Spectrom. Ion Processes 121 (1992) 1-47 i

n:2

÷

= VzC6H~.~ V 2 C 1 2 H e

3

+@

V3C~H~÷

33

÷

÷ ~V3C~sH~z + = V3C~zHa

v,:0=;--v,c,,.; s.

VsC~VsC~zH/

6 = V61::~

. V6C~zHz

I 7= vTco+ =vTc,2M~

H

÷

Fig. 23. Pattern of the reactions o f V .+ (n = 2-7) cluster ions with benzene and of V=+ (n = 3-6) cluster ions with ethene.

This precluded detailed investigations of their chemistry at long reaction times and limited the accessible size range (tantalum clusters behaved quite similarly). It is not too surprising that they induce dehydrogenation of bound ethene all the way down to V, CJ- clusters. In actual fact, the V~- cluster produces a 1 • 1 mixture ofV3C ~- and V3C2H~-. For the larger clusters, up to V~-, only the V,C~- carbides were identified. In addition, V~- and V~- yielded adducts of the type V,C~-. Long time-scale experiments would certainly have revealed adducts with an even larger number of C2 ligands, but could not be carried out because of the problems with oxygen contamination [45].

11,'.1.5.3. The Vn+IC6H6 system: dehydrogenation of benzene. V~(n = 2-7) clusters react with benzene vapour at about 10 -7 mbar exclusively by dehydrogenation, which grows stronger with increasing size. The V ÷ ion exhibits reversible adsorption of up to two benzene molecules, forming the probable sandwich complex V(C6H6)+. Figure 23 shows the reactions with C r H 6 and C 2 H 4 for the various cluster sizes. The dimer attaches up to two C 6 H 4 units, the trimer up to three of them. The tetramer attaches a C 6 H 2 unit and then an intact C r H 6 molecule. The behaviour of the pentamer is most remarkable. It strips the first added benzene molecule of all hydrogen atoms yielding V,C~-, (in agreement with the continuously rising reactivity in this sequence) but then continues by adsorbing an intact molecule in a reversible reaction. Obviously, the first step of radical dehydrogenation has completely exhausted the reactivity of the cluster. The hexamer and heptamer even dehydrogenate the second attached benzene molecule [45]. Kaldor and co-workers investigated the reactions of V~ (n = 1-19) clusters with C 6 D 6 in a flow tube reactor. They found all the sizes that we studied to be less reactive, e.g. V~- and VJ- to be totally inert. The V~- to V~- ions were the only ones to completely strip the benzene molecule of all hydrogen atoms,

34

M.P. Irion/lnt. J. Mass Spectrom. Ion Processes 121 (1992) 1-47

yielding V,C~-, in agreement with our results. In addition, with most clusters, a mixture of VnC6D ~- and VnC6D ~- was obtained at each value of n [13]. Our explanation for these deviations is the effect of temperature. For the clusters in their experiments the temperatures prior to reaction could have been distinctly lower than for ours. Supersonic expansion may cool clusters to a temperature in the range 30-100 K, whereas we checked the cluster ions in our experiments to be roughly at room temperature (section 11.4). In a comparable study of Nb,+ (n = 1-13) cluster ion reactions with C6H 6 and C6D6, the same authors found the adsorption rates to be independent of cluster size and the ability for dehydrogenation to vary strongly with it. It is remarkable that the clusters Nb~_6 (and Nb + ) produced a high percentage of the carbides Nb, CJ-. Clusters larger than Nb~- also gave the pure adducts Nb, C6H~- and Nb, C6D~-. The authors affirm that dehydrogenation intensifies with increasing cluster ion temperature, which is presently not very well known. They also stress that it becomes extremely problematic to interpret product analyses, when using detection by photoionization [55].

IV.2. Enforced reactions The metal cluster-ion reactions previously discussed all occurred spontaneously. Thus, product ions could be detected immediately after the reactive gas had been admitted to the ions trapped and thermalized in the ICR cell. Ions that do not react spontaneously, can be resonantly excited with an r.f. activation pulse after their selection (see pulse sequence in Fig. 4). They are thereby forced to undergo many collisions with the reactive gas and can either overcome barriers to endothermic reactions or suffer CID. An example will be given for both these cases. Even though the multiple collision process taking place is not too well defined, thresholds may even be derived with some caution [56].

IV.2.1. Oxidation of Cu*, by 02 When a distribution of Cu,+ (n = 1-31) cluster ions (mass spectrum in Fig. 2(a)), that have been trapped and thermalized in the ICR cell is exposed to oxygen at about 10 -7 mbar, no reaction occurs at all. Obviously the oxidation with 02 is an endothermic reaction that needs activation to proceed. Figure 24 shows a plot of stacked mass spectra of daughter ions produced by the excitation of isolated Cu~ parent ions as a function of center-of-mass collision energy. Mainly, CID is observed to yield the smaller clusters Cu~-, Cu~-, Cu~and Cu~. Also, the reaction products CusO~- and Cu30 ÷ are detected, the former already at a low energy (Ecm > 1.5 eV), the latter only from higher energies upwards (Ecru > 3.5 eV). Here, we attempted to excite all the isotopes of Cu~-, but it is evident that our r.f. pulse does not have the necessary ideal

M.P. Irion/lnt. J. Mass Spectrom. Ion Processes 121 (1992) 1-47

35

Ecru 3 1.5, 0 eV 2OO

3OO

~OO

5~

6~

mlz

Fig. 24. Stacked FT mass spectra of the CID products of isolated Cu9+ cluster ions in the presence of 02 as a function of collision energy.

rectangular shape. With increasing energy, the inner ion peaks are more diminished than the outer ones. In this case, it is possible to limit the excitation to single isotopic ions (see section 11.6). All fragment and product ions were separately isolated in the cell to study their CID patterns, from which the Cu9+ decomposition path shown in Scheme 1 resulted. It is interesting to note that the even-odd intensity alternation seen in the mass spectrum of the naked Cu,+ cluster ions is further reinforced by the reaction with oxygen; even-sized clusters are minimized, while oxides are obtained from odd sized clusters only. A common feature is fragmentation to Cu,+2 with loss of stable C u 2 [57]. The oxides decay preferentially by loss of another stable neutral, Cu20. Characteristically, CID of the oxides no longer produces any naked metal clusters. The result suggests that the oxygen Cu8 +

-Cu; CuS+ Cu9 +

~- Cu7 + -Cu2

- Cu3+ -Cu2

-Cu

= Cu2+

-Cu2~

-Cu20

Cu+

i-Cu20 Cu5O2+ -Cu 2

~

= Cu2O+

cu3o + -Cu 2~

Scheme 1. Decay pattern of Cu.+ cluster ions during CID.

-Cu

M.P. lrion/Int. J. Mass Spectrom. Ion Processes 121 (1992) 1-47

36

is not just sticking to the surface of the copper cluster. Instead, the reaction with 02 may cause its complete rearrangement, such that the electronegative oxygen atoms try to group around themselves as many copper atoms as possible. In this way, very stable structures might be formed, analogous to the well-known proton hydrates [58]: + +

O-- C

and

l

Of course, the observation that CID of the Cu, O~ clusters did not produce any bare Cu~ might also be interpreted as meaning that the dissociation energies, D, are related according to D ( C u + - O ) > D(Cu,_,O+-Cu). In a recent extensive CID study of copper oxide clusters, however, Gord et al. found similar evidence for "ion structures with oxygen distributed throughout the species rather than simply adsorbed to the surface of a pure metal cluster ion" [60].

IV.2.2. Activation of methane by [Ni]-]* We found Ni~- cluster ions to be especially reactive in dehydrogenating ethene, but towards methane they proved totally unreactive, even at high pressure. It was only possible to induce the endothermic reactions through translational excitation of the ions. For Ni~- clusters isolated in the cell and exposed to CH4 at about 1 x 10-6mbar, the course of ion abundances as a function of center-of-mass collision energy is given in Figure 25. The intensity of the parent ion quickly falls with increasing energy, while those of the product ions either pass through a maximum (Ni~-, NisC + and Ni3C+/ Ni3CH ÷ ) or increase gradually (Ni~-, Ni4 C+ ). Thus, the main process is CID to smaller-sized metal cluster ions. Addition/dehydrogenation, yielding the carbides, occurs only to a very small extent. For NisC + formation, a minimal collision energy of 2.0 _ 0.5 eV is required [53]. It is not surprising that methane resists activation by thermal Ni~ cluster ions. Neutral Fe,, AI,, Nb, [61] or Rh, [62(a)] clusters showed no sign of CH4 activation either. Successful activation was reported only for Pd, clusters at low reaction rates [44(b)], and for Pt, clusters at a higher efficiency [62(b)]. Of interest here also, the small cluster sizes Pt2-5 were found to be the most reactive. A solid nickel surface catalyzes the process of steam reformation ( C H 4 - q - H 2 0 ~ CO q-3H2) at high pressure and temperature, but proves much less reactive towards methane under UHV conditions. Apart from the very low adsorption probability, a barrier to the dissociative chemisorption of C H 4 o n a Ni(! 11) surface has been found by a molecular beam experiment.

M.P. lrion/Int. J. Mass Spectrom. Ion Processes 121 (1992) 1--47

37

90

o • 0 •

Nis÷ [H÷l NisC* Ni~+ NkC*

t Ni3CH÷/Ni~C÷

50 .~_

I .0 ~

~

// ,O 0~

10

0

2

t~

6

8

10

12

Ecru leVI

Fig. 25. Ion abundances during the resonant excitation o f isolated Ni~ ions in the presence of CH4 as a function of collision energy.

This could be overcome by supplying translational energy to the methane. The adsorbed species was identified as the CH 3 radical and the activation process as C - H bond dissociation [63]. Also we have overcome this barrier by increasing the kinetic energy of one of the reactants, in this case the Ni~- ions. In this case, most of the energy is needed to break the first C - H bond. Once this has happened the energy gained upon formation of the Ni~+ - C H 3 bond is sufficient to remove the remaining hydrogen atoms. Our quoted threshold is higher than that reported for CH4 activation (about 0.5 eV) on Ni(100) and Ni(110) surfaces [64]. The discrepancy may be due to the already mentioned uncertainty of the CID method. Methane activation is of interest both scientifically and economically because of the need to transform a small and quite unreactive hydrocarbon into more useful products, such as methanol or the larger hydrocarbons (via

38

M.P. lrion/Int. J. Mass Spectrom. Ion Processes 121 (1992) 1-47

Fischer-Tropsch synthesis). It may also be achieved with simple metal ions, either pure (M +) or bound to a fragment (ML+). Schwarz has recently published an extensive review article on this important subject [65]. V. GAS-PHASE CATALYSIS BY Fe+ CLUSTER IONS

Naked metal clusters are characterized by such a high degree of coordinative unsaturation that they should not only serve as model substances for heterogeneous catalysis, but should also represent highly selective and efficient catalysts themselves. In many proposals, the search for such new catalysts is given as a reason for the investigation of their chemical behaviour as a function of cluster size. However, in spite of numerous efforts along these lines [3,9(a),9(b)] no synthetic reaction catalyzed by a gas-phase naked metal cluster had been reported up to 1991. As mentioned in the introduction to section IV, in that year clear evidence of such catalytic activity was obtained for the first time [50].

V.1. Experimental evidence In section IV.1.2.2, it was demonstrated that Feg clusters react with C2H 4 by attaching up to four C2 H2 units. We assume the formation of a benzene precursor when the ligands have achieved C 6 H 6 stoichiometry. To prove this hypothesis, it was necessary to study the behaviour of the adduct ions when subjected to CID with xenon leaked into our vacuum up to a pressure of about 10-7mbar, by the operating Xe + ion gun (section II.5). Thus, Fe4(C2H2)~ions decay into carbide ions at low energy (Ecm ~ 3 eV), but lose iron atoms as the energy is increased (Ecm > 14eV). In contrast, Fe4(C2H2) ~- ions yield pure Fe~- at low energy (Ecm ~ 3.4 eV), and all carbon and hydrogen atoms are simultaneously removed. Increasing the energy causes dehydrogenation (Fig. 13)(Ecru > 6eV) and also elimination of iron atoms (E~m> 16eV). Fea(C2H2) ~- ions lose o n e C 2 H 2 ligand at the lowest energy (Ecm ~ 2eV); at higher energies three (ECru~ 4 eV), and finally all four (Ecru > 6 eV). Figure 26 displays an overall mass spectrum for all the ions involved in this CID experiment. It is amazing that no fragment with two C2H2 ligands appears at all! Thus, the metal-carbon bond is weak, when C 6 H 6 stoichiometry is completed on the Fe~- cluster. In these cases, there is strong evidence for the neutral molecule liberated being benzene. If C6H 6 stoichiometry is not complete, the metal bonds within the cluster are weaker than the metalcarbon bond. A measurement of the absolute rate constants for the four single steps of ethene dehydrogenation nicely confirms this [52]. Normally, they should fall linearly with the number of C2 H2 ligands already bound to the cluster. In the experiment they are quite high for the step completing C6H 6

M.P. lrion/lnt. J. Mass Spectrom. Ion Processes 121 (1992) 1-47

Fe4.(C2H2)m

rn

,

=

0

, ..........

~

1

. . . . . . . .

, . . . . . . . . .

3

, . . . ,

39

rr,

4

. . . . . . . .

,

r,

,

,

m/z

Fig. 26. Overview FT mass spectrum of products from CID of isolated Fe4(C2H2)~ adduct ions with xenon collision gas.

stoichiometry and remarkably low for further C2 H2 addition. As discussed in section IV.1.2.3, Fe~- also binds two molecules of cyclopropane under dehydrogenation with the ligands forming a benzene precursor. All the evidence reported so far is sufficient to enable us to draw a catalytic cycle, where Fe~- converts ethene into benzene (Scheme 2).

CeHe

t~Q~CeHe ~ Fe4(C2H2)3 + H2~

H2

Fe4(C2H2) +

Fe,(C2H2)2..~~.~ ~ C2H4 C2H, H2

Scheme 2. Catalytic cycle of Fe~- cluster ions, transforming ethene into benzene.

40

M.P. lrion/lnt. J. Mass Spectrom. Ion Processes 121 (1992) 1--47

In 1981, such catalytic cycles in the gas phase were proven for the first time in an analogous procedure, however, with the simple metal ion Fe ÷ . The hydrocarbons C3 Hs, C3 H6, C3 H4, C2 Hr, C2 Ha and C2 H2 as well as CO, were oxidized by N20 [66]. The ethane oxidation was recently studied in more detail by Schwarz and co-workers, using FT-ICR techniques as well [67]. This group also achieved benzene formation on Fe ÷ , when the CO ligands in Fe(CO)~+<5ions were replaced by C2H2 to yield Fe(C2H2)~,4. In this case, C6H6 was also liberated by CID and detected directly via neutralization-reionization mass spectrometry [68]. Recently, Pan et al. have examined the reactions of Ir~ and Co~+ clusters (n = 1-4) from electron impact of M4(COh2 in an FT-ICR MS. With cyclohexane, they predominantly found binding of benzene up to Ir4(C6H6)~- and Co4(C6H6) + , albeit not in a catalytic reaction [69]. V.2. Proof by mass spectrometry with multiple steps." ( M S ) n As has been shown, the exposure of naked Fe~- ions to either ethene or cyclopropane yields the complex Fe 4[C6H6]+. Upon CID of this adduct the naked Fe~ ions are regenerated which can, in principle, start a new cycle. If this could be demonstrated, it would prove beyond doubt the assumption of catalytic benzene precursor formation on Fe~-. Only a high-performance tandem mass spectrometric experiment is suited for this special task. Through an optimization of the experimental conditions for the ion transfer to the ICR cell, we achieved such an increase in the intensities of stored and detected ions and detected ions that several successive reaction steps could be carried out with them. Thus, we performed the following complex (MS) s experiment with five sequential mass spectrometric steps in the FT-ICR instrument: Fe+ rsol. Fe~- +C2H4/-H2 ) Fe4(C2H2)m+ (m = 1-3) (MS) I lsol. (MS)~ Fe4(C2H2)~- CID(I) Fe~ +C2H,/-"2 (MS)3

)

)

Fe4(C2H2)m+ (m = 1-3) (Ms), isoL Fe4 (C 2H2)J- CID ¢,) Fe+ (MS)') Figure 27a shows the Fe + ion signal isolated in the first step. From their reaction with ethene, the spectrum of the adducts Fe4(CEH2)+ (m = I-3) in Fig. 27(b) is obtained. From these, the Fe4(C2H2)~- ion is selected (Fig. 27(c)) and its CID with xenon is induced at a collision energy of Ecm ~ I I eV. The residual mother ion, as well as the product of this decay (naked Fe + ), can be seen in Fig. 27(d). Finally, the whole sequence is repeated once again in Figs. 28((~i)-(d)). In such an experiment the ion intensity inevitably decreases with each step, but here it is three to four times higher than that of the background noise, even at the final step.

M.P. lrion/lnt. J. Mass Spectrom. Ion Processes 121 (1992) 1-47 Fe4 ~

(a)

X2

41

~4¢ ~ -mH:.

, .=

~"'200' 2 3 0 " '260 " " ' 2 9 0 ' ' ' '320 ~''' .1.s

•

i . ,

F e . ( C 2 H : , ) 3"

.=

.

200

i

.

,

'

.

,

-

230

,

.

,

-

i

.

260

l

.

l

.

,

.

,

I.

290

- i . ,

- i ' ,

320

-

m~

,.

i

• i.

200

i.

i . i .

i,

230

(b)

Fe4(C:,H2}m+

d.

,_

i - i . l , l . i . i . i . r . i ,

260

290

320

CtO

(c)

i.~

m/z

(d)

Fe4.(C2H2) ~

=

,.

i.

i

a

.

-

-r

m/z

200

230

260

290

320

m/z

Fig. 27. (a) Mass peak of Fe~- cluster ions isolated from the original Fe~+ cluster distribution (lst generation), (b) FT mass spectrum of the adducts Fe4(C2H2)m+ (m = 1-3) from the reaction of Fe2 with C2H4 (lst generation; ion intensities were increased by a factor of 2.0), (c) Mass peak of isolated adducts Fe4(C2 H2)~ (1 st generation; ion intensities were increased by a factor of 1.5), (d) FT mass spectrum of mother and daughter ions from CID of Fe4(C2H2)~- with xenon collision gas where P ,~ 10-7 mbar and E,m ~ 11 eV (reproduction of Fe~- ; 1st generation).

Thus, the Fe~ ions originally produced have been run through the same reaction cycle twice! This demonstrates that a naked gas-phase metal cluster can indeed act as a catalytic center and repeatedly build up a larger molecule from smaller units [70]. VI. CONCLUSIONS

In this review the chemistry of bare metal cluster ions has been described for a variety of combinations of metals and reactive gases. The cluster ions were generated by sputtering and transferred to the ICR cell of an F T - I C R mass spectrometer. Through interaction with xenon as a bath gas, they are cooled so that, with reactive gases admitted directly into the analyzer vacuum, ion/molecule reactions can be studied at nearly room temperature. In certain cases, careful pressure calibration allows the determination of absolute rate constants. Ions of a specific mass-to-charge ratio are selected in the ICR cell by the elimination of undesired ions through r.f. ejection pulses. Some indirect information on ionic structure is accessible via CID. As expected, noble metal clusters generally show quite inert behaviour,

42

M.P. lrion/Int. J. Mass Spectrom. Ion Processes 121 (1992) 1-47

-2.5

Fe4+

(a)

x2

FI4•

.C:H, ~ RI4(C2H2)m+

2

m:

. . . . . . . i .

=L~

1.

i . i . 1

200

(b)-

-mH 2

3

. . . . . . . . . . . . . . . . . . . . . . . . . u r i . , .

1 " I

230

r

l

'

l

260

'

l

"

1"

290

I"

1 " I "

I"

I

• l

m/z

320

(c)

.i

. i , , , i , l - , - i , l , i

200

rx 1.5

230

' l ' r , l , l , r , l , l - ,

260

290

320

C2H

2) 3 +

m/z

(d)

CIO Fe+(

Fe,,.( C 2 H 2) 3 ÷

~eZ ......

~

...........

i

. . . . . . .

UL

.....

lie

~

-. I . I . l ' l . , r , - i . , . , . , ' l ' l ' , ' , ' l . l . r ' ,

- , . , - , ' l ' , ' , ' , - , - , . i . , ' , - , - , ' r ' , . , ' ,

200

230

260

290

320

m/z

200

230

260

290

320

m/z

Fig. 28. (a) Mass peak of isolated Fe~ cluster ions (2nd generation; ion intensities were increased by a factor of 2.5), (b) FT mass spectrum of the adducts Fe4(C2H2)~+ (m = 1-3) from the reaction of Fe~- with C2 H+ (2nd generation; ion intensities were increased by a factor of 2.0), (c) Mass peak of isolated adducts Fe4(C2H:) ~ (2nd generation), (d) FT mass spectrum of mother and daughter ions from CID of Fe+(C2H2)~- with xenon collision gas where P ~ 10 -7 mbar and E~m ~ 11 eV (reproduction of Fe~- ; 2nd generation).

attaching molecules without chemical modification. Contrary to this, typical transition metals are characterized by addition combined with dehydrogenation, whereby usually one hydrogen molecule is lost per substrate molecule. Very reactive, or simply larger clusters (Vn+ or Ni,+> 12) can even produce carbide ligands from hydrocarbons. Most metal cluster-reactive gas systems exhibit characteristic size-specific effects which are not yet understood. F o r some, only a minimal n u m b e r of cluster sizes react and the majority are inert, while for others the exact opposite is true. There are also systems where reactivity oscillates or varies smoothly with cluster size. The Fe~- ion exhibits extraordinary reactivity towards N H 3, as well as towards C2H4. It is the only Fe,+ cluster in the range n = 3-13 that can act as a catalytic center to transform adsorbed/dehydrogenated ethene into benzene. F o r Fe,+ clusters, the tetramer seems to be a distinguished size. However, the Ni,+ tetramer is inert both towards C2H4 and C r H 6. While we had speculated that the dramatic reactivity rise from Ni~- to Ni~- may be caused by a transition from two- to three-dimensional structure, Fe~- obviously does not fit into that scheme. With only a two-dimensional structure it could scarcely

M.P. lrion/lnt. J. Mass Spectrom. Ion Processes 121 (1992) 1-47

43

accommodate four ligands to be dehydrogenated. Model calculations could possibly provide a clearer picture but do not yet exist for transition metal cluster ions. Recently, a theoretician has promised to work on this problem [71]. At present, the number of studies reported in the literature on the chemistry of transition metal cluster ions is steadily increasing. In 1987, W6ste and co-workers studied the reactions of sputtered Ni~+ (n = 1-13) clusters with CO in a triple quadrupole mass spectrometer. No dramatic size-specific effects were seen, just the addition of carbonyl ligands accompanied by some degree of cluster and ligand fragmentation. The Ni~- and Ni~- ions did not prove especially inert. Molecular structure was inferred by counting cluster valence electrons [34(a)], according to a theory by Lauher [72]. Castleman and coworkers applied the same approach to explain the saturation behaviour of Co~+ (n = 2-8) clusters with CO in a selected-ion drift tube yielding Con (CO)m+~20• In agreement with our findings Co~- and Co~- were observed to be especially reactive [73(a)], as was also the case with oxidation with 02 [73(b)]. Using the theory mentioned above they propose a tetrahedral and a trigonal bipyramidal structure respectively. Nakajima et al. found a preferred reactivity for the mixed clusters Co3V + and Co4 V+ , leading them to the conclusion that the reactivity of transition metal cluster ions was generally high for the tetramer and the pentamer [74]. Our results clearly show that such a statement cannot be made in this generality. In the Ni~/C 2 H4 system it was only the pentamer that proved reactive, in the Fe~+/C: H 6 system it was only the tetramer. On the contrary, the Fen+/C 2H2, N2 H4 systems had no size of a preferred reactivity. In a communication, that sputtered Ni+(n = 1-10), Pd.+(n = 1-4) and Pt,+ (n = 1 or 2) ions are reported to react with n-butane both by dehydrogenation and cracking, where the first proceeds without, and the second with a barrier [75]. Another report on the reactions of diverse transition metal clusters M~+ (n = 1-4) with C 2 H a describes addition of a CHx fragment, which we have never seen. In this experiment the temperature of the sputtered clusters is not well controlled [76]. Finally, in an early investigation of deposited Ag + clusters, the critical size for photographic film development was identified to be Ag4+ , which again demonstrates the special role of the tetramer in certain systems [77]. ACKNOWLEDGMENTS I thank Dr. Adrian Selinger and Dipl.-Ing. Patrick Schnabel for the measurements and Prof. Dr. Konrad G. Weil for numerous discussions. Financial support is gratefully acknowledged from Deutsche Forschungsgemeinschaft, Fonds der Chemischen Industrie und R6hm-Stiftung.

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M.P. Irion/lnt. J. Mass Spectrom. 1on Processes 121 (1992) 1-47

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