2D14
Reactivity of cationic niobium carbide clusters with N2O(The University of Tokyo) Mushtaq Sobhan, Ken Miyajima, Fumitaka Mafune

Introduction
In the last two decades metal carbide clusters are extensively studied due to the fact that they cancatalyse reactions in a manner similar to platinum group metals (Pt, Ru, Rh, Pd, Os and Ir), which isexplained as being due to modification of the surface electronic properties by the addition of carbon.
Niobium carbide clusters has been investigated by several groups using methane gas as a carbon
source. It can also be prepared in the gas phase by the double laser vaporization. Using thisprocedure the reactivity of niobium carbide (NbC) clusters has been previously investigated with H2gas by Miyajima et al. [1]. In that study the authors found that NbnCm clusters with composition ratio
n:m=1:5 were relatively inert to H2 gas. To further investigate the composition dependent reactivityof NbnCm+ clusters we measured the reactivity of NbnCm+ clusters with N2O gas.
Experiment
Niobium carbide clusters were formed in a supersonic double laser ablation source coupled to areflectron-equipped time-of-flight mass spectrometer. The niobium and graphite rods were irradiatedwith the focused laser pulses (∼10 mJ/pulse) at a wavelength of 532 nm from Quanta Ray GCR-170and Continuum Surelite II Nd3+:YAG lasers for generating the plasma. The evaporated niobium
atoms and carbon atoms were cooled in the gas phase by the He gas (>99.99995%), forming NbnCm+
clusters. The clusters then entered into the collision cell filled with N2O gas. The stagnation pressureof He carrier gas used was 5 atm or 9 atm which affects strongly towards the cluster distribution.

Results and discussion
Mass Spectra of NbnCm+: In this study we produced two different mass distributions to observe theFigure 1: Mass distribution of NbnCm+ clusters (a) Nb4C4 as the highest intensity condition (b) NbnC3
product rich condition.

reactivity with N2O gas. Figure 1 shows the two types of mass distribution produced in two different
conditions. In one distribution (Figure 1a; He 5 atm) ions Nb2C3+, Nb3C3+, Nb4C4+, Nb5C3+, Nb6C1+,Nb6C6+ show high abundance and in another type of distribution (Figure 1b, He 9 atm) NbnC3+(n=2-8) show high abundance.
Reactivity with N2O: The reactivity of NbnCm+ clusters were examined for both mass distributions.
We observed that the same cluster for both the mass distributions behaved differently to N2O gas at
the same pressure. Figure 2 shows the color maps of intensity ratios of two types of mass distribution.
Generally NbnCm clusters decreased with N2O gas addition. However, some clusters, such as Nb2C3
and Nb3C2 are inert compared to the adjacent compositions. Additionally, in the NbnC3 product richcondition Nb2C3, Nb3C4, Nb5C7 and Nb6C7, Nb8C7 were found relatively inert. Furthermore, NbnC3clusters have reacted more compared to the other clusters. This trend is very different to the result ofNb4C4 as the highest intensity condition. This observation implies that geometrically different
isomers are formed due to the different cluster formation environment. In NbnC3 product richcondition, Nb clusters react to the neutral C3 cluster in a growth region while in another condition
Nb and C are well mixed during the cluster formation. In order to verify the production of different
isomers photoionization experiment can be utilized to obtain their ionization potentials (IP) [2].
Previous IP measurement in our group was conducted under the NbnC3 rich condition, and the valuesof IPs were close to the calculated ones for 2x2x2 cubic structure in the case of Nb5C3. We arefurther investigating the IP measurement of Nb4C4 as the highest intensity condition which will bepresented in the conference.

(a)

Number of Nb atoms
34567

Number of Nb atoms(b)

820011234567

Number of C atoms

Number of C atoms

22345678

1.0Inert

0.8

340.6

50.4

60.2

Reactive

78899N/A

Figure 2: Color maps for two different production conditions of NbnCm+ clusters (a) Nb4C4 as thehighest intensity condition (b) NbnC3 product rich condition. Values are calculated from the ratio ofcluster intensity: ratio = I with N2O /I without N2O.

References:[1] Ken Miyajima, Naoya Fukushima, Fumitaka Mafuné, J. Phys. Chem. A 2009, 112, 5774.
[2] Naoya Fukushima, Ken Miyajima, Fumitaka Mafuné, J. Phys. Chem. A 2009, 113, 2309.