Table 2 Dehalogenation of various aromatic halides
Run
Substrate
System
Time/h
Conv. (%)
Yield (%)
1
2
Photo
H2
1.5
1.5
99
74
97
72
Scheme 1 The reaction sequence for protons on the metal particles of
(a) Pd1Pt5@TiO2 and (b) Pd1 + Pt5@TiO2 catalysts.
3
4
Photo
H2
1
1
499
52
99
51
by photooxidation of 2-PrOH are consumed by dehalogenation
(eqn (3) and (4)) and reaction with NaOH (eqn (5)), respectively.
The H+ balance is therefore expressed by the ratio of the
amount of formed toluene to that of formed acetone,
5
6
Photo
H2
1
1
499
58
99
57
7
8
Photo
H2
1
1
499
77
99
76
H+ balance = [toluene formed]/[acetone formed]
(8)
Table 1 summarizes the H+ balance for the respective systems
obtained by photoreaction for 1 h. The values for Pdx@TiO2
are almost 1 (runs 1–4), indicating that H+ formed by alcohol
oxidation are consumed quantitatively by dehalogenation on
Pd (eqn (6)).3 The values for PdPt@TiO2 are also almost 1
(runs 5–9), suggesting that H+ are also consumed quantitatively
although H+ are reduced on Pt (eqn (7)). The results suggest
that, as shown in Scheme 1a, the H atom formed on Pt is
transferred to the adjacent Pd site (H–Pd formation), which is
probably due to the stronger H–Pd interaction.9
9
10
Photo
H2
1
1
499
76
97
75
11
12
Photo
H2
2
2
97
51
96
49
13
14
Photo
H2
2
2
92
60
87 (toluene)
37 (toluene)
In summary, we found that UV irradiation of TiO2 loaded
with Pd–Pt alloy promotes efficient dehalogenation. This
offers crucial advantages: (i) safe alcohols can be used as a
hydrogen source; and, (ii) the reaction proceeds much faster.
The alloy catalyst that facilitates efficient production of
active species (Hꢀ–Pd) may also promote other hydrogenation
reactions, and the work along these lines is currently in
progress.
H–Pt - H–Pd
(9)
The H atom is reduced to the hydride species (Hꢀ–Pd) by eꢀ,
which promotes dechlorination (eqn (4)).
H–Pd + eꢀ - Hꢀ–Pd
(10)
As shown in run 14, the H+ balance for the Pd1 + Pt5@TiO2
system is only 0.44. This is because the absence of Pd around
the Pt site suppresses the H transfer from Pt to Pd and results
in removal of H atoms by H2 formation (Scheme 1b).10
This work was supported by a Grant-in-Aid for Scientific
Research (No. 23360349) from the Ministry of Education,
Culture, Sports, Science and Technology, Japan (MEXT).
H–Pt + H–Pt - H2m + 2Pt
(11)
Notes and references
Change in H2 amount during reaction confirms this
(Fig. S7, ESIw). Pd1 + Pt5@TiO2 promotes H2 formation
and dechlorination simultaneously; however, Pd1Pt5@TiO2
scarcely produces H2 until p-chlorotoluene disappears. The
alloy sites are therefore necessary for efficient H transfer from
the Pt to Pd sites.
1 (a) F. Alonso, I. P. Beletskaya and M. Yus, Chem. Rev., 2002, 102,
4009; (b) V. V. Grushin and H. Alper, Chem. Rev., 1994, 94,
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3 K. Fuku, K. Hashimoto and H. Kominami, Chem. Commun.,
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4 Y. Shiraishi, Y. Sugano, S. Tanaka and T. Hirai, Angew. Chem.,
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5 M. Legawiec-Jarzyna, A. Srebowata, W. Juszczyk and
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6 L. Vegard, Z. Phys., 1921, 5, 17.
7 B. Herbert and J. Michaelson, J. Appl. Phys., 1977, 48, 4728.
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The above results reveal that, on the alloy site (Scheme 1a),
efficient reduction of H+ by eꢀ on Pt, smooth H transfer to
Pd, and reduction of H by eꢀ occur sequentially. This sequence
produces a large amount of Hꢀ–Pd species and promotes
efficient dehalogenation. It must be noted that, as shown in
runs 15 and 16 (Table 1), the alloy catalyst is reusable for
dehalogenation at least two times without loss of activity and
selectivity. In addition, as shown in Fig. 1b, the size of Pd–Pt
alloy particles scarcely changes even after the reaction.
The alloy catalyst is tolerant for photocatalytic dehalogenation
of various aromatic halides (Table 2). Pd1Pt5@TiO2 produces
the corresponding dehalogenation compounds with very high
yield, and the catalytic activities are much higher than those
obtained with Pd1@TiO2 using H2.
9 M. Yamauchi, H. Kobayashi and H. Kitagawa, ChemPhysChem,
2009, 10, 2566.
10 B. Ohtani, K. Iwai, S. Nishimoto and S. Sato, J. Phys. Chem. B,
1997, 101, 3349.
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 7863–7865 7865