K.-i. Shimizu et al. / Applied Catalysis A: General 421–422 (2012) 114–120
119
surfaces has been confirmed in several systems using surface sci-
ence techniques [38,39]. Gong and Mullins [38] and Friend and
co-workers [39] independently reported that an oxygen atom on
Au(1 1 1) acted as basic co-catalyst that promotes various organic
reactions at low temperatures. These examples, coupled with find-
ings in the present study, suggest that cooperation of metal surface
and the surface oxygen as a basic co-catalyst may open new pos-
sibilities for the tailoring of supported metal catalysts for green
organic reactions.
dry
Ag/SiO2-17+H2O
4. Conclusion
Ag/SiO2-17
SiO2+H2O
Based on the well established fact that oxygen-adsorbed Ag sur-
faces show higher reactivity for water dissociation than clean Ag
surfaces, we have developed the SiO2-supported Ag particles with
the surface oxygen atoms (Oad) as a highly effective heterogeneous
catalyst for hydration of nitriles. A linear correlation between TOF
and the coverage of Oad is shown, which indicates that Oad is indis-
pensable for this catalytic system. The Ag/SiO2 catalyst with Ag size
of 17 nm and oxygen coverage of 22% shows the highest TOF.
The catalyst has a high synthetic utility, because it is reusable
and shows high activity for selective hydration of aliphatic,
aromatic, heteroaromatic, and ␣,-unsaturated nitriles to the cor-
responding amides. Fundamental studies suggest a cooperative
mechanism between metallic Ag and Oad (oxygen atoms adjacent
to the active Ag sites), and Oad as a Brønsted base site should play an
important role in the dissociation of water. These findings demon-
strate that a surface science driven strategy is useful to design a new
Ag catalyst for green organic reactions involving water dissociation
as a critical step.
300 400 500 600 700 800 900
Wavenumber/cm-1
Fig. 6. Raman spectra of Ag/SiO2-17 and SiO2.
When Ag/SiO2-17 (ca 10 mg) was soaked with water (ca 50 mg),
new bands centered round 480 and 810 cm−1 and shoulder bands
around 550 and 640 cm−1 were observed. The bands at 480 and
550 cm−1 can be assigned to Ag O stretching vibration of adsorbed
OH on Ag, and the bands at 810 cm−1 can be assigned to AgO
H
bending vibration of adsorbed OH [31–34]. Treatment of SiO2 with
water in the same manner did not result in the appearance of
these bands. After subsequent drying of the wet Ag/SiO2-17 sam-
ple for 45 min under ambient condition, the state of the sample was
changed from wet slurry to dry powder, and the Raman bands due
to AgOH species were nearly absent. These results show that water
dissociation occurs on the Ag metal surface of Ag/SiO2-17, follow-
ing the reaction path H2O + Oad → 2OHad, but the OHad species are
unstable under the ambient condition.
Acknowledgments
This work was supported by the Japanese Ministry of Education,
Culture, Sports, Science and Technology via Grant-in-Aids for Scien-
tific Research B (23360354) and for Young Scientists A (22686075).
The X-ray absorption experiment was performed with the approval
of the Japan Synchrotron Radiation Research Institute (Proposal No.
2010B1447). The authors thank Mr. Masanori Hashimoto (Honda
R&D Co., Ltd.) for his help in STEM measurements.
Based on the above-mentioned facts and our mechanistic
results, a possible reaction mechanism of nitrile hydration by
Ag/SiO2-17 is shown in Fig. 7. Nitrile and the catalyst surface are
in equilibrium with the nitrile-Ag adsorption complex (step 1). The
Oad site as a Brønsted base abstracts proton from H2O, resulting in
the formation of H␦+ on the Oad site and HO␦− on the Ag site (step
2). Nucleophilic addition of OH␦− species to the nitrile carbon atom
of adsorbed nitrile species occurs to give the amide. This step, as the
rate-limiting step, occurs via a negatively charged transition state
Further mechanistic study is in progress to clarify the mechanism.
surfaces toward water dissociation has been confirmed using DFT
calculations or surface science techniques [26,35–37]. For example,
Shavorskiy et al. reported that small amount of adsorbed oxygen
induced dissociation of water on the surface of five metals (Ru,
Rh, Pd, Ir, Pt) [36]. The Oad-enhanced reactivity of precious metal
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
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