1P037

Generation and reactivity of NiCr clusters(The University of Tokyo) ○Michał Piotr Kwiatkowski,Ken Miyajima, Fumitaka Mafuné
Introduction
Bimetallic alloy clusters of ubiquitous metals represent one of the most promising way towards new
catalytic systems. Metal mixing (alloying) can significantly modify the catalytic activity and selectivity.
However, how alloying modifies reactivity behavior is still an open question. Does it just change the cluster
distribution of a given metal which otherwise retains its reactivity, or does it change the local bonding
geometry (structure effect), or modify the reactivity of the metal atoms? This work is our first step toward
the study of cationic NiCr bimetallic clusters which contains 70 wt% of Ni and 30 wt% of Cr. Since Kaldor
and co-workers [1] published experimental data on generation of neutral NiCr alloy clusters in gas phase no
experimental data are available according to our best knowledge. Moreover, in contrast to several
theoretical published works, no data on NiCr oxide clusters in gas phase were published either.

Experimental
Experimental apparatus and measurement techniques have been described in details in previous work on
nickel oxide clusters [2]. Briefly, NiCr alloy clusters were generated by laser vaporization of Ni0.7Cr0.3
solenoid pulsed valve and NiCr+ cations weredetected by using TOF-MS. Reaction withmolecular

oxygen andnitrous

oxide

wereinvestigated, in order to reveal basic mechanism ofreactivity of these bimetallic alloy cluster cations

a)
Intensity (arb. unit)

alloy rod within the supersonic beam from awhich can be a potential substitute to the widely

used rare metal.



100 200 300 400 500 600 700 800 900 1000 1100 1200 1300

Results and discussion

Figure 1 shows the experimentally observed b)

M16

M17M18M1916, 313, 614, 515, 411, 812, 712, 613, 514, 415, 311, 710, 711, 612, 513, 414, 315, 211, 512, 413, 314, 2

correlated with the alloy source component ratio(Ni0.7Cr0.3).

10, 6

peak assignment of experimental data is well9, 7

cluster distribution of NixCry up to x + y = 23 of
total number of atoms in the clusters. Qualitative

Intensity (arb. unit)+We examined the reactions of NiCr+ cluster
cations with molecular oxygen and nitrous oxide atminimum concentration where oxide products
intensity is high enough to observe, and at highconcentration where reactions are dominated bymulti-collision with reactant gas.

880 900 920 940 960 980 1000 1020 1040 1060 1080 1100
Mass Number m/z

Fig.1 Mass spectra of the NixCry+ alloy clusters: a)experimental spectrum with range M 3-23 of total numberof cluster components, where M = x + y, b) spectrum withselected m/z range.

The initial stages of oxidation with 0.1 torr O2 are favored formation of the cluster dioxides:
NixCry+ + O2 → NixCryO2+(1)As the O2 partial pressure of the reactant gas increase, the multioxides NixCryOz tend to be formed. It is
well known in literature [3, 4] that the initial state of the oxidation of Crn+ and Nim+ clusters is the cluster
dioxides formation, and with rising O2 concentration the dominant products are switched to multioxides,
wich is similar to the NiCr case. However, the generation mechanism of Cr+ and Ni+ oxides is verified as
the dissociative chemisorption mechanism, e.g.:
Crn+ + O2 → Crn-1+ + Cr +O2 → Crn-1O+ + Cr + O(2)Comparing it with NiCr+ oxidants formation, where even the same type of oxide products are formed in
low and high oxygen concentration, no any evidence of dissocative chemisorption mechanism wasobserved in our experimental conditions. Moreover, as it is shown in reactivity map in Figure 2a for low
O2 concentration it seems to be a kind of “reactivity barrier” for the large clusters. Small NixCry+ clusters (x+yare much more reactive with molecular oxygen, which indicates the reactivity size dependence.
In the N2O case, oxygen transfer reaction was occurred:
NixCry+ + N2O → Nix-mCry-nO+ + N2 + Crn+/Nim+/NimCrn+

It is clear that the clusters oxides formed even atminimum N2O concentration dissociate by sequentiala) 10a)9loss of chromium or nickel atoms or “cluster

8fragments”, NimCrn, where n,m << x,y. Dissociative6Cr7chemisorption of the atomic oxygen on the cluster

32+1ensues. Relatively high ratio of small NixCry clustersfragments” by dissociative chemisorption in N 2O

54

surface heats the cluster strongly and dissociationin Figure 2b is a result of rising number of “cluster(3)0bb) 109)
8reaction, while rich mono-oxides of these small NiCr+

76Cr

clusters was observed.

5experiment4(experimental procedure was desribed in previous

2

Moreover,bypostheating

31

work as well [2]) were verifed that oxygen are+considered to bind strongly to NiCr , wich indicate that
NiCr+ oxides are stable structures even at hightemperature conditions up toC

NiCr cluster in gas phase have deffinitly different
catalytical properties than their monocomponents Ni
and Cr clusters, which make these bimetallic clusters

00

12345

6789 10 11 12 13 14 15 16 17 18

Ni
0.5

1.0

1.5

N/A

Fig.2 Color maps for the bare NixCry+ alloy clusters ratioof clusters integrated intensity: a) ratio with/without
reaction with O2 IwithO2 /IwithoutO2, b) ratio with/without
reaction with N2O = IwithN2O /IwithoutN2O. Partial pressurefor O2 and N2O was 0.1 and 1 torr, respectively.

developmental subject of future study.
1. E.A. Rohlfing, D.M. Cox, R. Petkovic-Luton, A Kaldor, “Alloy cluster beams: nickel/chromium and nickel/aluminium”, J
Phys.Chem. 1984, 88(25), 6227-6231.

2. K. Sakuma, K. Miyajima, F Mafuné, „Oxidation of CO by nickel oxide clusters revealed by post heating”, A,13, 117 16 , 3 6 3265.
3. J B Griffin, P B Armentrout, “Guided ion beam studies of the reactions of Cr n+ (n = 2-18) with O2: chromium cluster oxide anddioxide bond energies”, J Phys Chem , 1 8, 1 8 1 , 8 6 -8074.
4. D. Vardhan, R. Liyanage, P B Armentrout, “Guided ion beam studies of the reactions of Nin+ (n = 2–18) with O2: Nickel cluster oxideand dioxide bond energies”, J. Chem. Phys. 2003, 119(8), 4166-4178