S. Imai et al.
Applied Catalysis A, General 624 (2021) 118331
TiO
cis-4-octene at various reaction temperatures, respectively. The rates of
-octyne consumption and cis-4-octene production increased with in-
2
. Fig. 5(a) and (b) show time courses of the amounts of 4-octyne and
(Entry 5). Thermal catalysis of Pd nanoparticles was controlled by
introducing an Ag shell and cis-4-octene was produced selectively (Entry
6). However, the rate of the thermal reaction was about one-fifth of that
of the photocatalytic reaction (Fig. 3), suggesting that formation of AHS
4
crease in the reaction temperature, and the rate constants were deter-
mined on the basis of zero-order kinetics. From the slope of an Arrhenius
+
by photocatalytic reduction of H on Pd@Ag nanoparticles occurs more
plot shown in Fig. 5(c), the value of E
a
was determined to be 12 kJ
2
easily than that by dissociative adsorption of molecular H .
ꢀ 1
mol . Compared with values of E
a
of the hydrogenation over Cu/TiO
2
Effect of the light intensity on the cis-4-octene yield was examined
(Fig. S3). Larger yield of cis-4-octene was obtained under more intense
light; however, the yield gradually saturated just like a logarithmic
curve. In other words, a modest yield can be obtained under light in-
54 kJ mol 1) [20] and Pd@Cu/TiO
ꢀ
(25 kJ mol ) [21], the much
ꢀ 1
(
2
smaller value of E means that a Pd core and an Ag shell are a good
a
combination for selective hydrogenation of 4-octyne.
ꢀ 2
To obtain information about the side reaction, i.e., H
2
evolution, UV
in the
absence of 4-octyne was carried out at various temperatures, in which
only H was produced because there was no other electron acceptor.
Fig. 6(a) and (b) show the results and an Arrhenius plot, respectively,
tensity of several mW cm , which is almost equal to that of solar light.
irradiation to 2-propanol suspensions of Pd(0.2)@Ag(0.5)/TiO
2
3.6. Reusability and applicability of Pd@Ag/TiO2
2
Table 6 shows the results of re-use of Pd@Ag/TiO for hydrogenation
2
ꢀ 1
and E
tivity of Pd(0.2)@Ag(0.5)/TiO
by the value of E for 4-octyne hydrogenation being smaller than the
value of E for H evolution. Production of H means that AHS are not
a
of H
2
evolution was determined to be 17 kJ mol . High selec-
of 4-octyne and use of Pd@Ag/TiO for hydrogenation of other alkynes.
2
2
for 4-octyne hydrogenation is explained
After the first use (Entry 1), Pd@Ag/TiO still showed high performance
2
a
for 4-octyne hydrogenation in the second and third uses (Entries 2 and
a
2
2
3). cis-Selectivity was also achieved in hydrogenation of 2-hexyne (Entry
selectively used for hydrogenation. We think that efficiency of AHS
utilization (EHU) is important as well as hydrogenation selectivity and
will discuss EHU in the next section.
4). Pd@Ag/TiO2 also worked in hydrogenation of terminal alkynes to
corresponding alkenes (Entries 5-7). We noted that Pd@Ag/TiO was
2
–
not active for a carbon-nitrogen triple bond (C
–
N) (Entries 6). Inter-
estingly, a chloro group, which is typically a good leaving group, was
preserved in hydrogenation of alkyne (Entries 7). Since Pd is very active
for elimination of a chloro group from a hydrocarbon [10], this result
indicates that the Pd core was almost completely covered with the Ag
3
.4. Effect of Ag content on catalytic properties of Pd@Ag/TiO
2
To investigate the catalytic properties of Pd@Ag/TiO
various thicknesses of the Ag shell, hydrogenation of 4-octyne over Pd
0.2)@Ag(X)/TiO was carried out for 0.5 h and values of EHU on the
hydrogenation were compared. Fig. 7(a) and (b) show the amounts of
the reduction product (cis-4-octene and H ) and oxidation product
acetone) in 2-propanol suspensions. The amounts of cis-4-octene and
acetone monotonically decreased when the thickness of the Ag shell
increased. On the other hand, the amount of H evolved reached a
2
having
shell. These results show a wide applicability of the Pd@Ag/TiO pho-
2
(
2
tocatalyst in hydrogenation of alkynes to alkenes.
2
4. Conclusion
(
Pd@Ag/TiO2 having various Ag contents was prepared by using a
two-step photodeposition method and used for photocatalytic hydro-
genation of alkyne in an alcohol suspension without the use of H2 gas.
Almost quantitative conversion of 4-octyne to cis-4-octene was achieved
2
maximum at X = 1.0 and then decreased with increase in X. 4-Octyne
barely hydrogenated to cis-4-octene at X = 4.0, indicating that most of
the AHS were used for H
2
evolution. In addition, only H
2
evolution
over Pd(0.2)@Ag(0.5)/TiO , while evolution of H2 as the by-product
2
occurred over Pd-free Ag(0.5)/TiO
2
. From these results, values of EHU
barely occurred during the hydrogenation, resulting in high efficiency
for the hydrogenation were calculated and are summarized in Table 3,
of AHS in photocatalytic hydrogenation. Reactions at various tempera-
tures revealed that the activation energies for the photocatalytic hy-
indicating that the maximum value of EHU (81 %) was obtained at X =
0
.5. EHU was calculated from Eq. (6):
drogenation and H evolution over Pd(0.2)@Ag(0.5)/TiO2 were 12 kJ
2
ꢀ 1
ꢀ 1
mol and 17 kJ mol , respectively, which explains the high value of
EHU in the hydrogenation. With increase in the Ag content, the value of
n(cis-4-Octene)
EHU =
× 100
(6)
n(Acetone)
E
a
in the hydrogenation became close to that in H
in a decrease in the value of EHU in the hydrogenation. An applicability
test showed that the Pd@Ag/TiO photocatalyst can be used for hy-
2
evolution, resulting
Values of E
0.5)/TiO and Pd(0.2)@Ag(X)/TiO
for hydrogenation gradually increased when the thickness of the Ag
shell increased and drastically increased at X = 4.0. Since no hydroge-
nation occurred even at 328 K, E of Pd-free Ag(0.5)/TiO was not
determined, suggesting that the value of E is very large. The change in
for H evolution was small compared with the change in the E for
hydrogenation, and the catalytic properties of Pd(0.2)@Ag(X)/TiO
became close to those of Ag(0.5)/TiO as shown in Table 4. Interest-
ingly, the value of E for hydrogenation became larger than that of E for
evolution at X = 4.0.
a
for hydrogenation and H
2
evolution over Pd-free Ag
(
2
2
are shown in Table 4. The values of
2
E
a
drogenation of various alkynes to alkenes.
a
2
Declaration of Competing Interest
a
E
a
2
a
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
2
2
a
a
H
2
CRediT authorship contribution statement
3
.5. Effects of reaction conditions
Shota Imai: Conceptualization, Investigation, Writing - original
draft. Yasumi Kojima: Conceptualization, Investigation. Eri Fudo:
Investigation. Atsuhiro Tanaka: Validation, Writing - review & editing.
Hiroshi Kominami: Supervision, Validation, Writing - review &
editing.
Table 5 shows the effects of photoirradiation, co-catalyst and at-
mosphere of gas phase on hydrogenation of 4-octyne at 293 K. Photo-
irradiation to 4-octyne in the presence of Pd(0.2)@Ag(X)/TiO under Ar
for 1 h produced cis-4-octene almost quantitatively (Entry 1). No hy-
drogenation occurred under Pd(0.2)@Ag(X)/TiO -free and light-free
conditions (Entries 2 and 3). Entries 4-6 show the results of thermal
reactions at 293 K under H (1 atm) for 1 h. In the presence of Pd(0.2)/
TiO , 4-octyne was deeply hydrogenated to octane (Entry 4). No hy-
drogenation of 4-octyne occurred over Ag(0.5)/TiO even under H
2
2
Acknowledgements
2
This work was partly supported by JSPS KAKENHI Grant Numbers
20H02527. A.T. is grateful for financial support from the Faculty of
Science and Engineering, Kindai University.
2
2
2
8