3
76
F.J. López-Tenllado et al. / Applied Catalysis A: General 505 (2015) 375–381
catalyst of choice is TiO2 typically, though some other such as
ZnO [12], CdS/TNTs nanocomposites [13], Fe-g-C N , and titanium
in [30]. It was separated in three portions and submitted to three
different calcination treatments at 500 (2 h), 800 (2 h) or 900 C
◦
3
4
silicate zeolite hybrid materials [14] or encapsulated polyoxomet-
alate [15] have been described.
(4 h), respectively, with the aim of obtaining samples with different
textural properties. Platinum was then incorporated for obtain-
ing different nominal loadings (0.02%, 0.04% or 0.06%, respectively)
by incipient wetness from an aqueous solution of [Pt(NH )](OH) .
In a previous paper [16] 24 different titania-based systems were
tested for gas-phase selective photo-oxidation of propan-2-ol to
acetone and liquid-phase transformation of 2-buten-1-ol (crotyl
alcohol) to 2-butenal (crotonaldehyde). In general, selectivities to
acetone with all the systems were very similar (ca. 50–60%) and
quite constant with conversion, the exception being platinum-
containing solids which showed quite high selectivity values to
acetone (in the 78–80% range at 22–28% conversion). As for the
crotyl alcohol transformation, the catalysts differed on the time
required to achieve a certain conversion level but for the same
conversion values all systems exhibited similar selectivities to cro-
tonaldehyde, the highest crotonaldehyde yield being, under our
experimental conditions, ca. 35%. This suggests that selectivity is
somehow associated to the semiconductor of choice. With a view
to increase the carbonyl compound yield, in the present manuscript
3
2
◦
◦
Solids were dried at 105 C for 12 h and finally calcined at 500 C
for 4 h. Nomenclature includes a suffix (5, 8 or 9) referring to ceria
◦
calcination temperature (500, 800 or 900 C, respectively) and the
Pt weight % in the case of the metalized samples.
Finally, Degussa P25 titania was used as a common reference
material for photocatalysis.
Some of the features concerning characterization of the solids
are given in Table 1.
2.2. Photocatalytic experiments
2.2.1. Gas-phase selective photo-oxidation of propan-2-ol to
acetone
the study is expanded to two other semiconductors: Bi WO6 and
2
CeO . Though comparatively less explored than TiO , both sys-
tems have been successfully used in photocatalytic processes.
Experimental device and reaction conditions were described
2
2
−
1
elsewhere [16]. Basically, 15 mL min
of O2 previously bubbled
◦
Therefore, for instance, Bi WO6 has been described in photo-
through propan-2-ol at 0 C was allowed into the photocatalytic
reactor, in which 30 mg of catalyst had been placed. The fix bed of
the catalyst was in contact with the gas flow. UV light (UV Spotlight
source LightningcureTM L8022, Hamamatsu, maximum emission
at 365 nm) was focalized on the sample compartment through an
optic fiber. Radiant flux in the catalyst compartment was measured
2
catalytic mineralization of rhodamine B [17–19], methyl orange
[
20] or 4-chlorophenol [21] as well as selective photo-oxidation
of glycerol to dihydroxyacetone [22] or several benzylic alco-
hols to the corresponding carbonyl compounds [23]. As for CeO2,
some examples of its photocatalytic application include degra-
dation of halogenated herbicides [24], organic acids [25] or dyes
−
2
to be 1 W cm (Newport UV-meter 818P-015-19 Model) and the
2
[
26], propylene epoxidation [27], and selective oxidation of benzyl
spot area 1.33 cm . Reactor was on-line connected to a HP6890
alcohols [28,29].
chromatograph equipped with a six-way valve, a HP-PLOTU column
(30 m long, 0.53 mm ID, 20 mm film thickness) and a Ni methana-
tor (Agilent Part Number G2747A) which allowed us to determine
the percentage of CO2 resulting from mineralization. Temperature
at the photoreactor was controlled by water thermostated at 10 C.
2
. Experimental
◦
2.1. Synthesis and characterization of the solids
The reaction lasted for 5 h.
The synthesis and characterization of the different bismuth
tungstate solids was described previously [17,18]. Briefly, the
Bi WO6 solids were obtained from a mixture of a solution of
2
2.2.2. Liquid phase selective photo-oxidation of 2-buten-1-ol
(crotyl alcohol) to 2-butenal (crotonaldehyde)
Bi(NO3)3 in glacial acetic acid and an aqueous solution of Na WO4
Experimental device and optimization of reaction conditions
were described in a previous study [16]. Under standard condi-
tions, 3 mL of a stock solution 0.01 M of crotyl alcohol (97% trans) in
acetonitrile was introduced in the 30 mL double-mouthed heart-
shaped reactor. After 5 min bubbling through the solution pure
2
at pH 2. The resulting suspension was submitted to hydrothermal
◦
treatment (140 C for 20 h) followed by filtration, washing, and dry-
◦
◦
ing overnight at 120 C. The solid was then calcined at 300 C for
h, thus obtaining the system labeled as Bi WO6 (2). Solid named
4
2
−
1
as Bi WO (9) was synthesized in a similar way but adjusting
oxygen (10 mL min ), 0.15 mL were sampled, filtered, injected in
the GC and taken as initial concentration (C0). The catalyst 18 mg
(6 g/L) was then added, the system closed and stirred (900 rpm)
and after 15 min of adsorption in the dark, light was allowed into
the reactor. UV light (UV Spotlight source LightningcureTM L8022,
Hamamatsu, maximum emission at 365 nm) was focalized on the
sample compartment through an optic fiber. Radiant flux in the
2
6
the pH of the Bi(NO ) –Na WO mixture at a value of 9. Incor-
3
3
2
4
poration of titanium was carried out through addition of TiO2 sol
formed from titanium isopropoxide in isopropanol) [17] before
(
the hydrothermal method at the required level as to obtain a final
composition 5% molar TiO /Bi WO .
2
2
6
The incorporation of gold was carried out by a photodeposi-
tion method. For this purpose, solutions of HAuCl4 in isopropanol
−
2
catalyst compartment was set up at 500 mW cm (measured with
a Newport UV-meter 818P-015-19 Model) and the spot area on
(
0.3 M) with the corresponding Au content were prepared (0.25 wt%
2
nominal). Suspensions of 1 g of catalyst in 250 mL of the previous
solution were prepared and sonicated, after which the pH value was
adjusted to 9 by KOH addition. Then, the suspensions were irradi-
ated by visible light for 2 h, under a N2 atmosphere and, finally,
the powder material was filtered, repeatedly washed, and dried
overnight.
Catalyst nomenclature includes the pH of the synthesis in brac-
kets. Moreover, when applicable, a suffix indicates the presence of
gold (Au) or the incorporation of 5% molar TiO2 (Ti5).
the liquid surface was 2.54 cm . Water used for cooling was ther-
◦
mostated at 10 C. Reaction profiles were obtained for the same
stock solution, each point corresponding to one individual reac-
tion. Once the system had been irradiated for the selected time,
the whole suspension was filtered (15 mm diameter, 0.45 m pore
size, Sartorius re. 17559) and analyzed by GC (HP6890 chromato-
graph equipped with an automatic injector and a HP-PLOTU column
−30 m long, 0.53 mm ID, 20 mm film thickness). The main reac-
tion products identified in the liquid phase were crotonaldehyde
(cis and trans) and acetaldehyde. Moreover, some other very minor
species (e.g. formaldehyde and methanol) were detected at trace
levels.
As regards ceria-based systems, they were prepared from a CeO2
sample supplied by Rhodia with ≥99.5% and 118 as purity and BET
specific surface area, respectively. This ceria was previously studied