Experimental and theoretical study on initial stage of oxidation of metal clusters

概要

1. Introduction
Metal clusters, defined as assemblies of a few to hundreds of metal atoms, have gained current attention as promising candidates for novel catalysts because they exhibit unique chemical properties owing to specific geometric and electronic structures.[1] Another attractive feature of metal clusters as catalytic material is that their chemical properties can be tuned over a wide range by changing the size and composition of the clusters.

Interaction between oxygen molecules and metal clusters is very important because it is a key step for aerobic oxidation catalyzed by metal clusters[2] and it is related to the stability of the clusters under ambient conditions. In my doctoral study, I focused on oxidation processes of clusters of cobalt and iridium in the gas phase and studied stability and structures of the metal oxide clusters by means of mass spectrometry, photoelectron spectroscopy and ion mobility mass spectrometry with a help of density functional theory (DFT) calculations. The aim of this work is to attain molecular level understanding on the interaction between metal clusters and oxygen molecules and to gain an insight into rational development of atomically precise metal oxide clusters as novel catalysts in the real world.

2. Oxidation of cobalt cluster anions
Although stable compositions and geometric structures of cobalt oxide clusters have been reported,[3] there is no report on how the oxidation of cobalt clusters is initiated to finally form stable oxide clusters. I conducted experimental and theoretical studies on the early stage of the oxidation reaction of cobalt cluster anions Con − and geometric and electronic structures of the products ConOm − .

Experiments were conducted by using a home-made time-of-flight mass spectrometer (TOF-MS) coupled with a magnetic bottle photoelectron spectrometer (MB-PES). [4] Cobalt cluster anions Con − were generated by the irradiation of a focused output of the second harmonic of a Nd:YAG laser onto a cobalt rod under pulsed helium carrier gas. Cobalt cluster anions were subsequently introduced into a reaction cell filled with oxygen whose partial pressure was adjusted by changing the valve opening time tPV. Finally the anionic species were extracted into the TOF-MS by applying a pulsed high voltage. The photoelectron was detached from ConOm − by irradiating the fourth harmonic of a Nd:YAG laser and were detected by MB-PES. DFT calculations were performed at B3LYP/6-31+G(d) level.

Figure 2 shows the typical mass spectra of ConOm − recorded by changing tPV. The intensity distributions of ConOm − did not show even-odd alternation with respect to m, indicating that ConOm − are produced via dissociative adsorption of O2. The distributions of ConOm − were gradually shifted to larger m values with increase in the O2 pressure and reached those expected from the bulk composition (Co3O4) at a high O2 pressure. At relatively low O2 pressures, two types of magic compositions were observed; intensities of mass peaks indicted by triangles and circles (Figures 2c and 2d) dropped abruptly by removing or adding one O atom, respectively. In order to shed light on the origin of their magic behaviors, adiabatic electron affinities (AEAs) of ConOm − were determined from the onset of the photoelectron spectra. The AEAs increased monotonically with m, but increased abruptly at the magic compositions indicated by triangles. These results suggest the origin of the former magic clusters is high stability against thermionic emission. Structural change upon sequential oxidation was theoretically studied using Co7Om − (m ≤ 6) as an example. The optimized structures shown in Figure 3 demonstrated that O atoms are attached sequentially to the facets or edges of Co7 − while retaining the Co7 core structure. On the basis of these results, we propose that the latter magic ConOm − clusters correspond to stable intermediates of the oxidation reaction whose surfaces are fully covered by O atoms. The sudden drop of peak intensity by the addtion of a single O atom is associated with sudden decrease in the binding energy of O atom.

3. Iridium oxide clusters
It was reported that iridium cluster had a partially oxidized area on the surface in the aerobic condition and showed a higher selectivity in hydrogenation of the NO2 group. The selective hydrogenation was ascribed to preferential adsorption of the NO2 group onto the oxidized area and activation of the hydrogen on the nonoxidized area.[5] However, there is no experimental research on the atomic packing structure of isolated iridium oxide clusters although theoretical calculation predicted planar structural motifs for Ir5Om − (m ≥ 6). [6] Formation of planar motifs in iridium oxide clusters is in contrast to three dimensional motifs in cobalt oxide clusters although iridium and cobalt belong to the same group in the periodic table. Here, I conducted the ion mobility-mass spectrometry and DFT calculation on iridium oxide cluster anions IrnOm − in order to unravel the structural evolution as a function of m.

Experiments were conducted by using an ion mobility TOF-MS apparatus installed at Tohoku University. [7] Iridium oxide clusters were generated by irradiating a focused output of the second harmonic of a Nd:YAG laser onto an iridium rod under the O2-seeded helium carrier gas. Iridium oxide cluster anions were introduced into a drift cell filled with helium by pulsed voltage. Collision cross sections (CCSs) were determined from the arrival time in the cell. After drift cell, the target ions were analyzed by TOFMS. DFT calculations for structural calculation were performed at B3LYP/def-SV(P) level. Theoretical CCSs were calculated by using projection approximation method in MOBCAL. [8]

Figure 5 shows experimental CCS values of IrnOm − (n = 4–8) plotted as a function of m in the range of 0–14. The CCSs values for n = 4–6 increase monotonically with m as expected. The CCS values for n = 7 grow gradually with size as in the case for n = 4–6, but also take an irregularly smaller value than neighbors at m = 6. The CCS values for n = 8 increase monotonically in the range of m = 0–11, decrease suddenly at m = 11, and increase again in the range of m = 11–14. These results suggest the coexistence of structure isomers for Ir7O6 − and Ir8O11 − . Theoretical calculations on Ir8Om − indicates that oxide structure with on-top oxygen atoms have larger CCS values than that with bridging (μ2-bonded) oxygen atoms (Figure 6). Based on the theoretical calculation results, I proposed that oxygen-deficient (m ≤ 11) and oxygen-rich (m ≥ 11) clusters have on-top and bridging oxygen motifs, respectively. Oxygen atoms were bonded to the cubic Ir8 − core as on-top motif sequentially as the core structure was distorted in m ≥ 7, which is in sharp contrast to the case of Con − where oxygen atoms were bonded as μ2 or μ3 motifs (Figure 3). Then all oxygen atoms were bonded as bridging motif in m > 11 and Ir-Ir bonds of the core no longer existed in this region.

4. Conclusion
In this study, initial stages of oxidation process of cobalt and iridium cluster anions were investigated experimentally and theoretically. Oxygen atoms are bonded sequentially to the surface of cobalt cluster anions while retaining the core structure. In contrast, oxygen atoms are bonded sequentially to the terminal site of iridium cluster anions and on-top bonding motif suddenly underwent transformation into bridging μ2 bonding motif at a later stage of the oxidation. These contrasted behaviors may reflect the difference in the electron orbitals involved in the bonding.

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