G Model
CATTOD-10284; No. of Pages7
ARTICLE IN PRESS
L. Clarizia et al. / Catalysis Today xxx (2016) xxx–xxx
2
observed for organic species strongly adsorbed on the catalyst sur-
face such as methanol, glycerol, and formic acid.
In the present work, investigations were extended to the devel-
opment of a suitable mathematical model capable of simulating
measurements at a wavelength of 585 nm. The concentration
of formaldehyde was determined by the colorimetric method by
Bricker and Vail [28], based on the use of chromotropic acid. The pH
of the solution was monitored by means of an Orion 420 p pH-meter
(Thermo).
hydrogen production over the same catalyst (nano-Cu(s)/TiO ),
2
developed in the previous study, when glycerol or methanol were
adopted as sacrificial agents [17].
Irradiance measurements were carried out on the external wall
of the reactor at different heights through a digital radiometer
(Delta Ohm HD 2102.1). Two sets of measurements were done, the
first on an empty reactor and the second with the catalyst slurry
inside it. From each set of data it was possible to estimate the total
power emerging from the reactor, in both the absence and the pres-
ence of absorption by the slurry. The difference between these two
values was assumed as the power absorbed by the catalyst.
2
. Material and methods
2.1. Materials
All organic compounds used as sacrificial agents, TiO nanopow-
2
der (commercial grade, Aeroxide TiO -P25, average particle size
2
2
−1
2.4. Kinetic model
2
1 nm, specific surface area 50 ± 15 m g , 80/20 anatase/rutile),
cupric sulfate pentahydrate (CuSO ·5H O,>98%) and chromotropic
4
2
A simplified network of reactions was singled out by considering
that, upon irradiation of Cu(s)/TiO2 catalyst nanoparticles, a couple
of charge carriers is generated (r ):
acid disodium salt dihydrate (C H Na O S ·2H O, 78.5%) were
1
0
6
2
8
2
2
purchased from Sigma Aldrich. Glycerol (≥99.5%) was purchase
from Sigma Aldrich, whereas methanol (99.9%) was purchased
from Carlo Erba Reagents. Doubly glass–distilled water was used
throughout this study.
1
hꢁ
Cu(s)/TiO →e + h+
−
(r1)
2
˚
UV
˚VIS
V
2.2. Photocatalytic procedure
reaction rate : G =
· Qa,UV
+
Qa,VIS
V
Photocatalytic runs were carried out in an annular glass batch
The rate of reaction r , which is a photochemical step, was
1
◦
reactor (0.300 L) thermostated at 25n C and equipped with a mag-
netic stirrer. On the top of the reactor, an inlet allowed to feed
reactants and nitrogen gas, and an outlet was used to collect liquid
and gaseous samples at varying reaction times [30].
accounted for by the products between quantum yields, in both
the UVA (˚UV ) and the visible range (˚VIS), and the respective
average volumetric rates of photon absorption[34] by the catalyst
suspension (Qa,UV /V, Qa,VIS/V), where V is the volume of irradi-
ated solution (V = 0.280 L).
The reactor was endowed with a high-pressure mercury vapor
lamp by Helios Italquartz (power input: 125 W), principally emit-
ting at 305, 313, and 366 nm (manufacturer’s data). The effective
Charge carriers may recombine through radiative or non-
radiative processes (r ):
2
0
i
−6
−6
irradiances (Iꢀ ) at 305, 313 and 366 nm are 2.56 × 10 , 2.70 × 10
k
r
−
+ h+ →heat and light
−
6
−1
Einstein s , respectively. The lamp was located
e
(r2)
and 3.30 × 10
inside a glass cooling jacket in the center of the reactor and sur-
rounded by the reacting solution.
ꢀ
ꢁ ꢀ ꢁ
+
−
reaction rate : kr ·
h
·
e
For each run a fixed amount of TiO -P25 nanopowder was ini-
2
As reported by others [18], reaction r is regulated by a second-
order kinetic law in which kr is the electron/hole recombination
reaction constant.
2
tially suspended in an unbuffered doubly distilled aqueous solution
containing the sacrificial species.
In order to avoid the undesired reaction between dissolved
oxygen and photogenerated electrons, before starting the pho-
tocatalytic experiment, a nitrogen stream was bubbled into the
solution for 30 min. After this period, cupric sulfate pentahydrate
was quickly added to the mixture.
Otherwise, charge carriers can be scavenged. In particular, pho-
togenerated holes can also react with the adsorbed sacrificial
∗
organic compound (S ), which successively oxidizes on the cata-
lyst surface (r –r5), where reaction r4 is the rate-determining step
4
for substrate oxidation:
Moreover, throughout the photocatalytic runs, nitrogen was
−
1
S + ∴ ꢀ S∗
∗
continuously fed at a flow rate of 0.3 L min in order to prevent
the air inlet into the reactor.
(r3)
CT · K
· [S]
S∗] =
ads
[
(
1 + K · [S])
2.3. Analytical procedures
ads
k
+
At different reaction times (≥5 min), gaseous samples were col-
S∗ + h+ →S + H+
h
•∗
(r4)
(r5)
lected by means of Tedlar gas sampling bags (1 L) and injected into
the gas-chromatograph to record the rate of hydrogen generation.
For this purpose, a gas chromatograph (Agilent 7820A) was used
equipped with a HP-PLOT Molesieve 5A column (Agilent) and a TCD
detector using argon as carried gas. Liquid samples, collected at dif-
ferent reaction times, were filtered on regenerated cellulose filters
ꢀ
ꢁ
+
∗
reactionrate : kh
+
h
[S ]
•
+H /fast
•∗
∗
+
S
+ H
→
∴ + Sox + 2H
(
pore diameter 0.20 m, Scharlau). The filtrate was used to mea-
As previously proposed [19], the direct reaction between pos-
sure concentrations of total dissolved copper and formaldehyde
produced through the oxidation of methanol.
itive holes and organic substrates is only possible if organics are
strongly adsorbed on the catalyst surface. The adsorbed species
∗
In order to evaluate the concentration of total dissolved cop-
per (cupric and cuprous species), a colorimetric method using
an analytical kit (Macherey-Nagel) based on oxalic acid bis-
cyclohexylidenehydrazide (cuprizone) was adopted. An UV/vis
spectrophotometer (Cary 100 UV–vis Agilent) was used for the
concentration [S ] may be obtained by a Langmuir-type model for
−
1
adsorption, as reported for the equilibrium r in which K (M ) is
3
ads
1
q (g L ). The term CT was calculated through the formula CT = q · N,
Please cite this article in press as: L. Clarizia, et al., Kinetic modeling of hydrogen generation over nano-Cu(s)/TiO2 catalyst through