Topics in Current Chemistry (2019) 377:27
1 3
activity of the resulting samples with a light intensity of 4.5 mW cm
−2
was assessed.
It was observed that the activity in the dehydrogenation of FA was strongly dependent on the foreign metal used. The best performance was achieved with Pd@Ag5%tetrahedron–TiO 2 , with a conversion of 98.7%, while 63.2% and 35.5% of conversion was achieved with Cu- and Au-modified Pd-tetrahedron–TiO 2 , respectively.
A sample modified with Pt displayed reduced activity. According to the results of
DRIFT analysis and the work function of the metals used, this catalytic trend was
explained mainly in terms of CO interaction strength and poisoning effect.
Ago et al. [79] also reported on the photocatalytic activity of PdAg–TiO 2 catalysts (AgPd@Pd/TiO 2 ). In that case, the desired composition of the metal phase
was achieved by dealloying the AgPd core. Samples with various compositions (i.e.,
Ag 100−x Pd x @Pd/TiO 2 , with x = 7, 10, 15) were prepared by microwave heating at
100 °C during 30 min, 1 h, and 2 h, respectively. The best composition of the catalysts was determined to be Ag 93 Pd 7 @Pd/TiO 2 . Furthermore, the effect of TiO 2 was
also analyzed by using anatase (A) and P25 (P). The characterization of the catalysts
indicated that 0.8 nm-thick Pd shells were achieved on the AgPd cores for both TiO 2
supports, and the composition, sizes, and morphology did not depend on the TiO 2
used. The photocatalytic activity was monitored by measuring the gas generated
while irradiating with a Xe lamp and heating at various temperatures (from 27 °C to
90 °C). The profiles of the gas evolution with and without illumination are plotted in
Fig. 4. Analysis of the profiles indicated that the initial reaction rate was improved
by a factor of 1.5–1.6 for AgPd@Pd/TiO 2 (A) and AgPd@Pd/TiO 2 (P) at 27 °C,
respectively, while that factor was 1.1–1.2 at 90 °C. Such differences observed with
the temperature were explained on the basis of the migration of photogenerated
electrons from TiO 2 to Pd. At low temperatures, the photogenerated electrons transfer from the conduction band of TiO 2 to Pd shell with larger work function (5.1, 4.7,
and 4.0 eV for Pd, Ag, and TiO 2 , respectively). The electron-rich Pd species formed
upon irradiation were responsible for enhancement of FA decomposition ability.
However, at higher temperatures, photogenerated electrons have a higher migration
rate, but, at the same time, electron–hole pairs recombination is also favored, which
eventually results in a lower number of electrons reaching the surface of Pd. As for
the TiO 2 support, it was observed that AgPd@Pd/TiO 2 (A) displayed better activity
than AgPd@Pd/TiO 2 (P) under both light and dark conditions, which was ascribed
to the slower electron–hole recombination rate of anatase in comparison to the rutile
phase present in P25, as well as to its higher specific surface area and strong interaction of anatase phase with AgPd@Pd particles.
Apart from the most commonly investigated PdAg-based catalysts, other compositions have been studied for this application. For instance, Xue el al. [80]
reported selective photocatalytic decomposition of FA over AuPd nanoparticle-decorated TiO 2 nanofibers under simulated sunlight irradiation, which were expected
to combine the optimum ability of Pd to boost the decomposition of FA with the
surface plasmon resonance (SPR) of Au, as well as the optimized features of the
three-dimensional (3D) TiO 2 structure. Samples with various Au/Pd ratios were
prepared by electrospinning from a solution containing HAuCl 4 , Pd(C 2 H 3 O 2 ) 2 ,
tetrabutyl titanate [Ti(OC 4 H 9 ) 4 ], and poly(vinylpyrrolidone) (PVP). Pd 1 /TiO 2 and
Au 1 /TiO 2 were synthesized as control samples. Among those investigated, sample
200
Reprinted from the journal
1 3
activity of the resulting samples with a light intensity of 4.5 mW cm
−2
was assessed.
It was observed that the activity in the dehydrogenation of FA was strongly dependent on the foreign metal used. The best performance was achieved with Pd@Ag5%tetrahedron–TiO 2 , with a conversion of 98.7%, while 63.2% and 35.5% of conversion was achieved with Cu- and Au-modified Pd-tetrahedron–TiO 2 , respectively.
A sample modified with Pt displayed reduced activity. According to the results of
DRIFT analysis and the work function of the metals used, this catalytic trend was
explained mainly in terms of CO interaction strength and poisoning effect.
Ago et al. [79] also reported on the photocatalytic activity of PdAg–TiO 2 catalysts (AgPd@Pd/TiO 2 ). In that case, the desired composition of the metal phase
was achieved by dealloying the AgPd core. Samples with various compositions (i.e.,
Ag 100−x Pd x @Pd/TiO 2 , with x = 7, 10, 15) were prepared by microwave heating at
100 °C during 30 min, 1 h, and 2 h, respectively. The best composition of the catalysts was determined to be Ag 93 Pd 7 @Pd/TiO 2 . Furthermore, the effect of TiO 2 was
also analyzed by using anatase (A) and P25 (P). The characterization of the catalysts
indicated that 0.8 nm-thick Pd shells were achieved on the AgPd cores for both TiO 2
supports, and the composition, sizes, and morphology did not depend on the TiO 2
used. The photocatalytic activity was monitored by measuring the gas generated
while irradiating with a Xe lamp and heating at various temperatures (from 27 °C to
90 °C). The profiles of the gas evolution with and without illumination are plotted in
Fig. 4. Analysis of the profiles indicated that the initial reaction rate was improved
by a factor of 1.5–1.6 for AgPd@Pd/TiO 2 (A) and AgPd@Pd/TiO 2 (P) at 27 °C,
respectively, while that factor was 1.1–1.2 at 90 °C. Such differences observed with
the temperature were explained on the basis of the migration of photogenerated
electrons from TiO 2 to Pd. At low temperatures, the photogenerated electrons transfer from the conduction band of TiO 2 to Pd shell with larger work function (5.1, 4.7,
and 4.0 eV for Pd, Ag, and TiO 2 , respectively). The electron-rich Pd species formed
upon irradiation were responsible for enhancement of FA decomposition ability.
However, at higher temperatures, photogenerated electrons have a higher migration
rate, but, at the same time, electron–hole pairs recombination is also favored, which
eventually results in a lower number of electrons reaching the surface of Pd. As for
the TiO 2 support, it was observed that AgPd@Pd/TiO 2 (A) displayed better activity
than AgPd@Pd/TiO 2 (P) under both light and dark conditions, which was ascribed
to the slower electron–hole recombination rate of anatase in comparison to the rutile
phase present in P25, as well as to its higher specific surface area and strong interaction of anatase phase with AgPd@Pd particles.
Apart from the most commonly investigated PdAg-based catalysts, other compositions have been studied for this application. For instance, Xue el al. [80]
reported selective photocatalytic decomposition of FA over AuPd nanoparticle-decorated TiO 2 nanofibers under simulated sunlight irradiation, which were expected
to combine the optimum ability of Pd to boost the decomposition of FA with the
surface plasmon resonance (SPR) of Au, as well as the optimized features of the
three-dimensional (3D) TiO 2 structure. Samples with various Au/Pd ratios were
prepared by electrospinning from a solution containing HAuCl 4 , Pd(C 2 H 3 O 2 ) 2 ,
tetrabutyl titanate [Ti(OC 4 H 9 ) 4 ], and poly(vinylpyrrolidone) (PVP). Pd 1 /TiO 2 and
Au 1 /TiO 2 were synthesized as control samples. Among those investigated, sample
200
Reprinted from the journal