Photocatalytic Generation of Hydrogen from Organic Substances Using Iron-Containing Composites under the Conditions of UV and Visible Irradiation

Article abstract of DOI:10.1134/S1070427220070034

Morphological features of Fe-containing composites based on silicon, titanium, vanadium, and boron nitrides and prepared by self-propagating synthesis were studied using scanning electron microscopy, and the content of elements (B, Si, N, Ti, V, O, Fe, C, Al, Mg, Ca) on their surface was quantitatively assessed using an attachment for EDX microanalysis. The optical properties of the composites were studied, and the band gap values for the semiconductors incorporated in the ceramic matrix were determined. A comparative evaluation of the performance of the composites in H₂ generation from solutions of organic substances (НСООН, Н₂С₂О₄, malic acid, sucrose) under UV and visible irradiation was conducted. The efficiency of the photocatalytic generation of H₂ under the action of visible light revealed the associated both of the composition of the ceramic matrix containing semiconductor compounds and of the occurrence of the photo-Fenton process in the solution in the presence of H₂O₂.

Full text of DOI:10.1134/S1070427220070034

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2020, Vol. 93, No. 7, pp. 960–966. © Pleiades Publishing, Ltd., 2020.
Russian Text © The Author(s), 2020, published in Zhurnal Prikladnoi Khimii, 2020, Vol. 93, No. 7, pp. 945–951.
CATALYSIS
Photocatalytic Generation of Hydrogen from Organic
Substances Using Iron-Containing Composites
under the Conditions of UV and Visible Irradiation
a,
a
b
a
L. N. Skvortsova *, I. A. Artyukh , K. A. Bolgaru , and I. A. Pichikov
a
National Research Tomsk State University, Tomsk, 634050 Russia
b
Tomsk Scientific Center, Siberian Branch, Russian Academy of Sciences, Tomsk, 634021 Russia
e-mail: lnskvorcova@inbox.ru
*
Received November 8, 2019; revised January 22, 2020; accepted February 8, 2020
Abstract—Morphological features of iron-containing composites based on silicon, titanium, vanadium, and
boron nitrides and prepared by self-propagating synthesis were examined by scanning electron microscopy, and
the content of elements (B, Si, N, Ti, V, O, Fe, C, Al, Mg, Ca) on their surface was quantitatively estimated using
an attachment for EDX microanalysis. The optical properties of the composites were studied, and the band gap
values for the semiconductors incorporated in the ceramic matrix were determined. A comparative evaluation of
the performance of the composites in hydrogen generation from solutions of organic substances (НСООН, Н С О ,
2
2
4
malic acid, sucrose) under UV and visible irradiation was made. The efficiency of the photocatalytic generation
of hydrogen under the action of visible light is shown to be associated both with the composition of the ceramic
matrix containing semiconductor compounds and with the occurrence of the photo-Fenton process in the solution
in the presence of hydrogen peroxide.
Keywords: iron-containing metal–ceramic composites, semiconductors, heterogeneous photocatalysis,
homogeneous photocatalysis, hydrogen power engineering
DOI: 10.1134/S1070427220070034
The efficiency of the solar energy utilization in
hydrogen generation processes using semiconductor
catalysts is yet insufficient. Systems active under visible
irradiation are being studied. For example, systems
with titanium dioxide [1], absorbing visible light, are
(λ = 450 nm). Photocatalysts combining several
semiconductors, in particular, CdS–TiO and SnO –
2
2
TiO [6], show promise for expanding the range of the
2
active light toward visible region.
Iron-containing photo-Fenton process catalysts are of
particular interest as economically and environmentally
attractive materials. We prepared stable iron-containing
catalytic composites by self-propagating high-
temperature synthesis in the autowave nitridization
of iron alloys. The iron–ceramic materials showed
high performance in oxidative degradation of organic
contaminants with simultaneous hydrogen evolution
under the action of UV irradiation [7].
being developed. The efficiency of the H generation
2
from water under the action of UV/visible irradiation
was significantly enhanced by using metals (Cu,
Au, Pt) as TiO cocatalysts [2]. Metal chalcogenides
2
are good materials for photocatalytic generation of
hydrogen, because they exhibit activity in the far-UV
and visible range. Effective Bi S /SnS [3] and In/SnS
2
3
[4] composites for hydrogen generation from water
were suggested. Markovskaya et al. [5] developed
semiconductor photocatalysts based on nanostructured
ZnS–CdS solid solutions applied onto a porous support
Previously we demonstrated the possibility of
modifying the ceramic matrix of composites based
on silicon nitride and SiAlON by introducing into its
formulation semiconductor compounds (SiC, TiN) in
the synthesis step to enhance the photocatalyst activity
and shift it toward the visible range [8]. The composites
(
silica gels, galloisite nanotubes) for the H generation
2
from aqueous solutions of inorganic electron donors
Na S/Na SO ) under irradiation with visible light
(
2
2
3
960

PHOTOCATALYTIC GENERATION OF HYDROGEN
Table 1. Characteristics of the composites and separate semiconductors
961
Composite
SiN
Phase compositiona
Eg, eV
Iron content, wt %
1.8–2.6
β-Si N , α-Fe, β-Si Al O N , SiC, Fe Si
у
3.1 [11]
3
4
3
3
3
5
х
2
.4 [11]
TiN
VN
TiN, β-Si N , α-Si N , α-Fe, Fe Si
у
3.3 [11]
2.0–4.7
15–19
3
4
3
4
х
VN, α-Fe, V N
5.4
2
3
.6
5.3
.8
BN
α-BN, α-Fe, FeB, Fe B
5–35
2
3
a
Semiconductor phases are printed bold.
showed high performance in Н generation in the course
of titanium metal. The absence of the SiAlON phase is
caused by its dissolution in the crystal lattice of silicon
nitride. At 20 wt % dilution of ferrosilicoaluminum with
titanium, the TiN phase prevails, and the phases of α-Fe
and iron silicides FeхSiy are also present. The VN sample
is a product of ferrovanadium nitridization; therefore, its
phase composition includes only the synthesis products.
The phase composition of the composite based on BN,
prepared by the combustion of ferroboron in nitrogen,
includes α-BN with the hexagonal lattice, α-Fe, and
2
of decomposition of sacrificial reagents (Н С О ,
2
2
4
НСООН, malic acid) under the conditions of UV
irradiation.
This study was aimed at comparative evaluation of
the photocatalytic activity of modified composites in
the course of Н generation from sacrificial reagents
2
under the conditions of UV and visible irradiation
and at determination of the photocatalytic activity of
an iron–ceramic sample based on a wide-band-gap
semiconductor, vanadium nitride.
unchanged ferroboron phases (FeB, Fe B). Thus, the
2
ceramic matrices of the composites studied contain
wide-band-gap semiconductors (Si N , SiC, TiN, VN,
EXPERIMENTAL
3
4
BN).
Synthesis of the modified composites and analysis
of their phase composition were performed at the
Department of Structural Macrokinetics, Tomsk
Scientific Center, Siberian Branch, Russian Academy of
Sciences. The composites were prepared in the autowave
heating mode using iron alloys (ferrosilicoaluminum,
ferrovanadium, ferroboron) as precursors. To enhance
the photocatalytic activity of the materials, the ceramic
matrix was modified by introducing semiconductor
compounds (SiC, TiN). The procedures for preparing
composites based on nitrides by self-propagating high-
temperature synthesis are described in [9].
The surface morphology was studied with a Hitachi
TM3000 scanning electron microscope at an accelerating
voltage of 15 kV under the conditions of low vacuum
–
2
(
electron gun: 5 × 10 Pa; sample chamber: 30–50 Pa).
Elemental analysis of the materials was performed with
a Quantax 70 attachment for EDX microanalysis.
The optical properties of the VN and BN samples
were studied with a UV-Visible Evolution 600
spectrophotometer (Thermo Scientific, the United
States) using MgO as a reference. We recorded the
diffuse reflectance spectra, which were transformed into
the electronic absorption spectra using the Kubelka–
X-ray diffraction analysis of the materials was
performed with a Shimadzu XRD6000 diffractometer
2
Munk function [F = (1 – R) /2R, where R is the
diffuse reflection coefficient]. The band gap width was
determined from the edge of the main optical absorption
band. Then the electronic absorption spectra were
transformed in the coordinates absorption coefficient
[F(R)Е]–photon energy (hν). The quantity Eg was
determined by extrapolation of the linear portion of the
(
Japan) (Table 1). The SiN sample prepared by
nitridization of ferrosilicoaluminum with the addition of
shungite as modifier consists of the two major phases,
β-Si N and β-Si Al O N with the prevalence of the
silicon nitride phase, and contains also the phases of SiC,
α-Fe metal, and intermediate nitridization products, iron
silicides Fe Si . The TiN composite was prepared by
3
4
3
3
3
5
2
0.5
dependence [F(R)Е] /[F(R)Е] = f(hν) to intersection
with the hν axis.
х
y
nitridization of ferrosilicoaluminum with the addition
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 93 No. 7 2020

962
SKVORTSOVA et al.
(b)
(a)
3
μm
3
μm
(d)
(c)
3
μm
3
μm
Fig. 1. Electron micrographs of the surface of the composites (a) BN, (b) SiN, (c) VN, and (d) TiN.
–
1
A DRL-250 high-pressure mercury lamp with the
reactor at a constant rate (10 mL min ), and the gas
mixture at the outlet was fed to the flow meter with a
soap solution to monitor the gas flow rate. Samples for
analysis were withdrawn into a closable 100-mL gas
pipette after its 20-min flushing with a nitrogen–gas
mixture. The gas mixture was collected for 10 min and
analyzed by gas chromatography with a Kristall 5000-
range 240–1100 nm and the strongest line at 254 nm
served as a UV radiation source. A DIORA 30 LED
lamp (Fiztekh-Energo, Tomsk, Russia) with the range
4
10–750 nm and the strongest lines at 450 and 600 nm
was used for visible irradiation. The generated hydrogen
in the gas mixture of the reaction systems was collected
using an installation described in [10].
1
device (Khromatek, Russia, 2007), and the catalyst
performance in generation of molecular hydrogen was
evaluated. The random error did not exceed 10%.
Photocatalytic generation of hydrogen. A 200-
mg portion of the composite was placed in a 100-
mL quartz reactor and poured over with 20 mL of a
model solution of a sacrificial reagent, after which
As sacrificial reagents we used aqueous solutions of
carboxylic acids (НСООН, Н С О , malic) and sucrose,
which proved to be effective in hydrogen generation in
the presence of Fe-containing composites under UV ir-
radiation [8].
2
2
4
0
.2 mL of 0.1 М Н О was added if necessary. The
2 2
reactor was hermetically sealed and placed on a
magnetic stirrer arranged in front of the radiation
source. The stirred composite/solution suspension
was bubbled with nitrogen from a cylinder prior to
irradiation to remove gases adsorbed by the catalyst.
In the course of irradiation, nitrogen was fed to the
RESULTS AND DISCUSSION
In the BN sample, light particles of the metal phase
of the 0.45–2.1 μm size are concentrated in the center,
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PHOTOCATALYTIC GENERATION OF HYDROGEN
963
Table 2. Comparison of the performance of the composites in Н generation from solutions of sacrificial reagents under UV
2
and visible irradiation
-1
–1
Performance, μmol g h
System, M
UV irradiation
TiN
visible irradiation
TiN
SiN
755
VN
756
561
259
SiN
660
201
157
VN
1070
759
0
0
1
.5Н С О /0.001H O
644
190
145
62
2
2
4
2
2
.5НСООН/0.001H O
Not studied
276
541
342
2
2
% C H O (sucrose), рН 2
225
1
2
22 11
0
.5НСООН/0.001H O (without
13
11
2
2
composite)
2
and dark particles of boron nitride of the 2–7 μm size are
uniformly distributed over the surface (Fig. 1a).
we plotted the dependence [F(R)Е] = f(hν). Vanadium
nitride is an indirect-band-gap superconductor, and we
0
.5
used for it the dependence [F(R)Е] = f(hν). The band
gap width for the semiconductor phases was determined
from the edge of the main optical absorption band
Granules of the SiN and VN composites have a
round shape and are agglomerates of particles with the
grain size covering a wide range: 1–5 μm for SiN and
(Figs. 2b, 2d).
1
–35 μm for VN (Figs. 1b, 1c). The TiN composite
The band gap width in semiconductors (Table 1)
mainly consists of particles in the form of plates and
large and small faceted tubes of 2–11 μm size (Fig. 1d).
Elemental microanalysis shows that the phase having
the shape of faceted tubes is TiN. The iron metal phase
is distributed more uniformly over the surface of the SiN
and VN composites. Along with the main phase-forming
elements, the composites contain impurity elements, in
particular, Al, C, Mg, and Ca (Fig. 1).
present in the ceramic matrix of the composites is
smaller than the photon energy of the UV radiation
source (4.5 eV). The electronic absorption spectra and
the band gap widths of the semiconductor phases of the
composites under consideration also suggest that these
composites should exhibit activity in the far-UV and
visible ranges.
Partial dissolution of iron metal from the sample
surface in the presence of Н О creates the conditions for
the occurrence of the photo-Fenton process generating
hydroxyl radicals [11, 12]. The classical Fenton process
is based on the reaction of hydrogen peroxide with
bivalent iron ions in an acid solution; in this process,
H O is an oxidant and Fe is a catalyst. The mechanism
of the Fenton process with the generation of hydroxyl
radicals is usually presented as follows:
The optical properties of the SiN and TiN composites
were studied and described previously [8]. In this study,
we recorded the diffuse reflectance spectra of the samples
based on boron and vanadium nitrides, which were
transformed using the Kubelka–Munk function (Fig. 2).
2
2
The composites are characterized by relatively low
absorbance in the middle- and near-UV range (280–
2+
2
2
4
00 nm) compared to the SiN and TiN samples, for
which the F(R) values in this range of the spectrum are
–1.5 [8]. The absorption band of the BN composite in
1
2+
3+
•
–
Fe + H O → Fe + HO + OH .
(1)
aq
2
2
aq
the region of 250 nm (Fig. 2a) belongs to the unchanged
ferroboron. The VN composite absorbs in the near-
UV and range and in the range 280–300 nm, which is
The reaction is accelerated under UV irradiation
leading to the conversion of Fe3+ ions to Fe2+ ions with
the release of an additional amount of hydroxyl radicals:
associated with the presence of the VN and V N phases.
2
The electronic absorption spectra were transformed in
the coordinates absorption coefficient–photon energy
Fe3+ + H O + hν → Fe2+ + HO• + H+.
(2)
aq
2
aq
(hν). Semiconductor compounds present in the BN
composite are characterized by direct electron transfer
from the valence band (VB) to the conduction band
Thus, the hydrogen generation from organic
compounds is possible under the conditions combining
heterogeneous and homogeneous photocatalysis.
(CB); therefore, to determine the band gap width (Eg),
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 93 No. 7 2020

964
SKVORTSOVA et al.
(a)
(b)
λ,nm
hν,eV
(d)
(c)
hν,eV
λ,nm
Fig. 2. (a, c) Electronic absorption spectra and (b, d) plots of the absorption coefficients vs. photon energy for the composites (a, b) BN
and (c, d) VN.
The highest performance of the catalysts is observed
with hydroxyl radicals with the release of hydrogen.
According to [15], photochemical decomposition
in Н generation from oxalic acid solutions with the
2
addition of Н О (Table 2). In the presence of the VN
of formic acid can occur with the release of СО and
2
2
2
sample, the efficiency of the Н evolution under visible
Н . Formic acid remains one of promising sacrificial
2
2
irradiation is considerably higher than under UV irra-
diation. The composite based on vanadium nitride con-
tains the largest amount of iron (Table 1); its dissolution
results in the formation of the photoactive ferrioxalate
reagents.
The SiN and TiN composites are characterized
by low and virtually equal content of surface iron
(
Table 1). On the other hand, the sample based
3
–
complex [Fe(С О ) ] , which undergoes photolysis in
2
4 3
on silicon nitride exhibits considerably higher
performance in the Н2 generation under visible
irradiation in all the systems. This fact suggests the
2
+
the range 400–500 nm and generates Fe and hydroxyl
radicals with high quantum yield (1.24) [13]:
participation of the semiconductor SiC phase (E =
g
Fe(C O ) ] + hν → Fe2+ +2C O + C O ,
3
−
2−
−
•
(3)
(4)
(5)
[
2
4 3
2
4
2
4
2
.4 eV) in the photolytic process, as this phase absorbs
−
•
−•
C O → CO + CO ,
in the region of the most intense radiation of the visible
light lamp (450 nm), and the role of the heterogeneous
photocatalysis. The fact that the efficiency of the Н2
generation from solutions of sacrificial reagents with
the addition of Н О under UV irradiation differs
2
4
2
2
•
2−
4
3
−
[
Fe(C O ) ] + CO− → Fe2+ + CO + 3C O
2
4 3
2
2
2
Fe2+ + H O + 3C O → [Fe(C O ) ] + HO + HO . (6)
2–
3−
–
•
2
2
2
4
2
4 3
2
2
According to [14], carboxylic acids are oxidized
insignificantly for all the composites also counts in
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PHOTOCATALYTIC GENERATION OF HYDROGEN
965
Table 3. Results of photocatalytic generation of Н from solutions of carboxylic acids under visible irradiation with the addition
2
of Н О and without it
2
2
Performance, μmol g–1 h–1
without H O
2
Composite
System, M
H O
2
2
2
VN
VN
BN
SiN
TiN
0.5НСООН
759
75
623
60
0.5НООС–СН –СН(ОН)–СООН (malic acid)
2
0.05Н С О
4
489
201
145
315
386
228
2
2
0.5НСООН
0.5НСООН
favor of the heterogeneous photocatalysis. This may be
associated with the presence of semiconductor phases
when generating Н from oxalic acid solutions with
2
the addition of Н О . In the presence of the composite
2
2
with close E values (3.1–3.4 eV) in the ceramic matrix.
based on vanadium nitride, containing a large amount of
iron, the efficiency of the Н release from Н С О un-
g
It was interesting to evaluate how the photo-Fenton
process influences the rate of the evolution of molecular
2
2
2
4
der visible irradiation is considerably higher than under
UV irradiation, which is caused by high photoactivity
Н from organic substances under the conditions of
2
3–
of the formed ferrioxalate complex [Fe(С О ) ] in the
visible irradiation in the presence of iron–ceramic
composites. To this end, we performed a photocatalytic
2
4 3
range 400–500 nm. High efficiency of the photocatalytic
generation of Н in the presence of the SiN composite
experiment on Н evolution from carboxylic acids in the
2
2
presence of composites with the addition of Н О and
under visible irradiation (range 450–600 nm) is associ-
ated with the presence of the semiconductor SiC phase
(band gap 2.4 eV) in the ceramic matrix.
2
2
without it (Table 3). The presence of Н О influences
2
2
the efficiency of the photocatalytic generation of Н2
from carboxylic acids ambiguously. For the composites
with high iron content (VN, BN), the addition of Н О
FUNDING
2
2
appreciably increases the Н evolution rate, suggesting
2
The study was performed within the framework of the
government assignment for the Tomsk Scientific Center,
Siberian Branch, Russian Academy of Sciences (no. 0365-
the role of homogeneous catalysis involving the photo-
Fenton reaction. On the contrary, when using composites
with low iron content (SiN, TiN), the presence of Н О
2
2
2
019-0005) and of the research project supported by the
decreases the production of molecular hydrogen.
Probably, Н О as an electron acceptor competes for the
Program for Increasing the Competitiveness of the Tomsk
State University.
2
2
electrons of the catalyst conduction band and decreases
the Н generation efficiency. These relationships confirm
2
CONFLICT OF INTEREST
The authors declare that they have no conflict of interest.
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2
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