Photon Factory Activity Report 2019 #37 (2020)

BL-12C/2018G063

The effect of the Mn oxidation state on the activity of supported MnOx nanocluster
catalysts in the 1-phenylethanol oxidation reaction
Kyoko NAKAMURA, Nobuyuki ICHIKUNI*, Takayoshi HARA and Shogo SHIMAZU
Graduate School of Science and Engineering, Chiba University,
1-33 Yayoi-cho, Inage-ku, Chiba, 263-8522, Japan
1 Introduction
Manganese catalysts attract considerable attentions fromboth academia and industry due to the low cost in contrast
to the precious metal catalysts. Furthermore, manganeseoxide was chosen as the catalyst because it has an oxidation
ability. Our previous work also showed the good activity
for aerobic alcohol oxidation with supported MnOx
nanocluster, however, the effect of oxidation state on thecatalytic activity has not been clarified [1].
In this work, we prepared supported MnOx catalysts
with different supporting materials and investigated thelocal structure by utilizing XAFS. As a result, we foundthat adjusting the oxidation state of Mn could be a usefulapproach to prepare a highly efficient supported MnOx
nanocluster catalyst.
2 Experiment
MnOx nanocluster on SiO2 (Aerosil, #200) and Al2O3(Aerosil, Alumina C) support was prepared by a colloidal
method. Colloidal Mn particles were prepared by reducingMn(OAc)2 with NaBH4 in the presence of poly(Nvinylpyrrolidone) (PVP). Subsequently, the catalyst wasobtained by fixing colloidal particles on supporting
materials. Mn loading amount was regulated to 3 wt%. Mn
K-edge X-ray absorption fine structure (XAFS) data werecollected at BL-12C of IMSS-KEK PF (Proposal No.
2018G063) with a Si(111) double crystal monochromatorin transmission mode.

|FT|A
(e)
(d)
(c)
(b)
(a)
0
2
4
6
Radial distance (Å)

Normalized absorption

3 Results and Discussion
Fig. 1A shows FT spectra of k3-weighted Mn K-edge
EXAFS. The peak intensity of colloid derived catalysts (dand e) are smaller than that of reference compounds (a-c),
which indicated the nanosized manganese oxide formationin the catalysts.
B
(e)
(d)
(c)
(b)
(a)

6540
6560
6580
Photon energy (eV)Fig. 1. (A) FT of Mn K-edge EXAFS spectra and (B) MnK-edge XANES spectra for (a) MnO2, (b) Mn2O3, (c)Mn3O4, (d) Mn/SiO2, (e) Mn/Al2O3.

Mn K-edge XANES spectra of catalysts are notcompletely same as that of any bulk MnOx, suggesting that
the Mn species on catalyst exists with various oxidation
states as shown in Fig. 1B. Pattern fitting of XANES wascarried out in order to estimate the ratio of each component
and the results are shown in Table 1. In MnOx/Al2O3, theratio of MnO2 was increased compared to MnOx/SiO2.
Oxidation reaction of 1-phenylethanol was carried out
and the results are shown in Table 1. Prepared catalysts
were active for the oxidation of 1-phenylethanol using
molecular oxygen without any additives such as basereagents. We have already evaluated the cluster size effectfor the aerobic oxidation [1]. Therefore, high activity of the
prepared catalysts might come from the decrease of clustersize.
In addition, the high yield was obtained on MnOx/Al2O3.
Our previous studies have shown that good redox propertyof MnO2 is beneficial for the catalytic performance of 1phenylethanol oxidation [1]. Therefore, through the resultsof characterization and catalytic reaction, we concluded
that the formation of MnO2 on alumina support lead togood activity for this reaction.
Table 1 Acetophenone yielda and pattern fitting resultsb of
prepared catalysts

Catalyst
Mn/SiO2
Mn/Al2O3

Yieldc (%)
1343

Composition (%)
MnO2

Mn2O3

Mn3O4

MnO

83851
2331
39

100a

1-Phenylethanol (1 mmol), MnOx catalyst (0.10 g, S/C =18), solvent: p-xylene (5 mL), atmosphere: O2 (1 atm),
reaction temperature: 373 K, reaction time: 6 h.
b
Determined by XANES pattern fitting analysis.
c
Determined by gas chromatography using an internal
standard technique.
Acknowledgement
This study was supported by JSPS KAKENHI Grant
Number 17K06912.
Reference[1] K. Nakamura, N. Ichikuni et al., Catal. Today 352, 250(2020).
* ichikuni@faculty.chiba-u.jp