Effect of protonation on the photocatalytic activity of the layered titanate Rb2Nd2Ti3O10

Article abstract of DOI:10.1134/S1070363217110317

It was found that the contact of the layered oxide Rb₂Nd₂Ti₃O₁₀ with an aqueous solution gives rise to a protonation process, which leads to lowering the oxide photocatalytic activity in reactions of hydrogen evolution from a water-alcohol solution.

Full text of DOI:10.1134/S1070363217110317

ISSN 1070-3632, Russian Journal of General Chemistry, 2017, Vol. 87, No. 11, pp. 2728–2729. © Pleiades Publishing, Ltd., 2017.
Original Russian Text © I.A. Rodionov, S.A. Fateev, I.A. Zvereva, 2017, published in Zhurnal Obshchei Khimii, 2017, Vol. 87, No. 11, pp. 1906–1907.
LETTERS
TO THE EDITOR
Effect of Protonation on the Photocatalytic Activity
of the Layered Titanate Rb2Nd2Ti3O10
I. A. Rodionov*, S. A. Fateev, and I. A. Zvereva
St. Petersburg State University, Universitetskaya nab. 7–9, St. Petersburg, 199034 Russia
*e-mail: i.rodionov@spbu.ru
Received July 6, 2017
Abstract—It was found that the contact of the layered oxide Rb2Nd2Ti3O10 with an aqueous solution gives rise
to a protonation process, which leads to lowering the oxide photocatalytic activity in reactions of hydrogen
evolution from a water-alcohol solution.
Keywords: layered oxide, titanate, hydration, protonation, photocatalysis, hydrogen
DOI: 10.1134/S1070363217110317
Layered perovskite-like titanates A2Ln2Ti3O10 (A
being alkaline metal, Ln being lanthanide) related to
Ruddlesden–Popper phases are promising photo-
catalysts [1]. Their high photocatalytic activity in
water decomposition is often referred to the possibility
of intercalating water molecules into the interlayer
space, which is considered as a reaction zone along
with the crystal surface [2, 3]. When the cation radius
of the alkali metal A in the composition of A2Ln2Ti3O10
titanates increases, their ability to water intercalation
and simultaneously the photocatalytic activity increase
[4]. In this regard, Rb-containing phases are of
particular interest for the study.
Layered titanate Rb2Nd2Ti3O10 was synthesized by
the ceramic technique from the corresponding simple
oxides and carbonates TiO2 (≥99.9%), Nd2O3 (≥99.9%),
and Rb2CO3 (≥99.8%). The reagents were taken in
stoichiometric ratios in accordance with reaction (1):
Rb2CO3 + Nd2O3 + 3TiO2 = Rb2Nd2Ti3O10 + CO2↑. (1)
Rubidium carbonate was taken in excess of 40% to
compensate for its losses due to evaporation at high
temperature. The charge was ground in an agate
mortar, pressed into tablets, and calcined in air
according to the temperature program 700°C, 6 h +
1000°C, 10 h. To obtain three stable hydrated-protonated
forms of the compositions HRbNd2Ti3O10·0.8H2O,
H1.5Rb0.5Nd2Ti3O10·0.6H2O, and H2Nd2Ti3O10·0.4H2O,
weighted quantities of 0.3 g of initial samples were
added to 50 mL of distilled water and aqueous solu-
tions of hydrochloric acid with concentrations of 0.016
and 0.1 M, respectively. The resulting suspensions
were stirred for 24 h and then the precipitates were
separated by centrifugation.
We have found that the layered titanate Rb2Nd2Ti3O10
on contact with aqueous solution is not only subjected
to the intercalation of water in the interlayer space, but
also to protonation, i.e. to the substitution of protons
for the interlayer rubidium cations. As a result a
number of hydrated-protonated phases with the common
formula HxRb2–xNd2Ti3O10·yH2O are formed, each
being stable within a certain range of solution acidity.
In this case amount of intercalated water (y) decreases
in the series of these phases as the protonation degree
(x) increases, which can affect their photocatalytic
properties. The aim of this work was to study
photocatalytic activity of the new layered titanate
Rb2Nd2Ti3O10 and its dependence on intercalation and
ion exchange processes occurring on the contact of the
layered oxide with aqueous solutions.
The phase composition was controlled at each stage
of the synthesis by the X-ray phase analysis (Rigaku
Miniflex II, CuKα). The hydration and protonation
degrees were confirmed by the thermogravimetric
method (TG 209 F1 Libra, Netzsch) and energy-
dispersive X-ray microanalysis (Oxford Instruments
INCAx-act). The band gap energy was determined by
the diffuse reflection spectroscopy (Shimadzu UV-
2728

EFFECT OF PROTONATION ON THE PHOTOCATALYTIC ACTIVITY
2729
2550, ISR-2200) using Kubelka-Munk transformation.
Photocatalytic activity was studied in the model
reaction of hydrogen evolution from ethyl alcohol
aqueous solution with a mole fraction of 1%.
Simultaneously with hydrogen evolution ethanol is
oxidized in such a system to acetaldehyde in the
stoichiometric amount. The photocatalytic experimental
technique is described in detail in [4]. The irradiation
was carried out with a DRT-125 UV lamp using a
liquid light filter cutting off radiation with wave
lengths less than 220 nm. The amount of evolved
hydrogen was determined by gas chromatography
(Shimadzu GC-2014 with a system of online sampling,
thermal conductivity detector, carrier gas argon).
Volume of the reaction suspension was 50 mL, photo-
catalyst content in the suspension 1 mg/mL.
the number of absorbed photons. However, it should
lower the reactivity of photogenerated electrons and
holes owing to a displacement of edges of conduction
and valence bands. This also may contribute to the
observed decrease in the photocatalytic activity.
Thus, the substitution of protons for rubidium
cations in the interlayer space of layered oxide
Rb2Nd2Ti3O10 leads to lowering its photocatalytic
activity, therefore it is advisable to use this oxide as a
photocatalyst only in alkaline media, where the
protonation occurs to a lesser extent.
ACKNOWLEDGMENTS
This work was financially supported by the Russian
Foundation for Basic Research (grant no. 16-33-
60044). I.A. Rodionov expresses his gratitude to the
Council for the grants of the President of the Russian
Federation (SP-2176.2016.1).
The maximal rate of hydrogen evolution of
8.7 μmol/h was observed for the sample HRbNd2Ti3O10·
0.8H2O. It should be noted that this metastable form of
the layered oxide has the lowest protonation degree
and is almost immediately formed on contact of initial
Rb2Nd2Ti3O10 with distilled water. However more
substituted samples H1.5Rb0.5Nd2Ti3O10·0.6H2O and
H2Nd2Ti3O10·0.4H2O showed significantly lower
photocatalytic activity of 1.4 μmol/h and 1.0 μmol/h,
respectively. The rate of hydrogen evolution for the
P25 Evonik-Degussa commercial photocatalyst TiO2
in similar conditions was 3.5 µmol/h.
The work was fulfilled using equipment “Research
Center for X-ray Diffraction Studies,” “Center of
Thermogravimetric and Calorimetric Research,” “Inter-
disciplinary Resource center for Nanotechnology,” and
“Educational Resource Center of Chemistry” of St.
Petersburg State University.
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On the one hand, the observed trend is correlated
with a decrease in the amount of sorbed water with
increasing protonation degree and thus is an indirect
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0.4H2O is also observed. As in the range of 3.54–
3.60 eV there are no radiation bands of the lamp in
use, we can conclude that this change has no effect on
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