Effect of the nature of the active component and support on the activity of catalysts for the hydrolysis of sodium borohydride

Article abstract of DOI:10.1134/S0023158407010223

The effect of the nature of an active component and a support on the rate of hydrolysis of aqueous sodium borohydride solutions was studied. It was found that the activity of supported catalysts, which were reduced in a reaction medium of sodium borohydride, decreased in the order Rh > Pt ≈ Ru ? Pd regardless of the nature of the support (γ-Al₂O₃, a Sibunit carbon material, or TiO₂). The catalysts based on TiO ₂ exhibited the highest activity. As found by UV-vis diffuse reflectance spectroscopy, the composition and structure of the supported precursor of an active component depend on the nature of the support. It is likely that rhodium clusters with different reaction properties were formed on various supports under the action of a reaction medium.

Full text of DOI:10.1134/S0023158407010223

ISSN 0023-1584, Kinetics and Catalysis, 2007, Vol. 48, No. 1, pp. 168–175. © MAIK “Nauka /Interperiodica” (Russia), 2007.
Original Russian Text © V.I. Simagina, P.A. Storozhenko, O.V. Netskina, O.V. Komova, G.V. Odegova, T.Yu. Samoilenko, A.G. Gentsler, 2007, published in Kinetika i Kataliz,
2007, Vol. 48, No. 1, pp. 177–184.
YOUNG SCIENTISTS’
SCHOOL
Effect of the Nature of the Active Component and Support
on the Activity of Catalysts for the Hydrolysis
of Sodium Borohydride
V. I. Simaginaa, P. A. Storozhenkob, O. V. Netskinaa, O. V. Komovaa,
G. V. Odegovaa, T. Yu. Samoilenkoa, and A. G. Gentslera
a Boreskov Institute of Catalysis, Siberian Branch, Russian Academy of Sciences, Novosibirsk, 630090 Russia
E-mail: simagina@catalysis.nsk.su
b State Research Institute of Chemistry and Technology of Organoelement Compounds, Moscow, 111123 Russia
Received October 28, 2005
Abstract—The effect of the nature of an active component and a support on the rate of hydrolysis of aqueous
sodium borohydride solutions was studied. It was found that the activity of supported catalysts, which were
reduced in a reaction medium of sodium borohydride, decreased in the order Rh > Pt ≈ Ru ꢀ Pd regardless of
the nature of the support (γ-Al2O3, a Sibunit carbon material, or TiO2). The catalysts based on TiO2 exhibited
the highest activity. As found by UV–vis diffuse reflectance spectroscopy, the composition and structure of the
supported precursor of an active component depend on the nature of the support. It is likely that rhodium clus-
ters with different reaction properties were formed on various supports under the action of a reaction medium.
DOI: 10.1134/S0023158407010223
INTRODUCTION
active catalyst component was studied using UV–vis
diffuse reflectance spectroscopy.
A solution of sodium borohydride is a safe source of
pure hydrogen for portable fuel cells; it can generate to
7 wt % hydrogen. In the presence of catalysts, the lib-
eration of hydrogen occurs even at room temperature
[1]. The activity of chemically different catalysts pre-
pared by various methods in the reaction of NaBH4
hydrolysis has been studied in the last few years. In par-
ticular, the reactivity of ruthenium catalysts supported
on anion-exchange resins (A-26 and IRA-400) [2, 3]
and platinum group metals on various supports [4–6],
as well as highly dispersed nickel and cobalt [7, 8], was
studied. However, the activities of catalysts measured
in the cited literature and other publications are difficult
to compare with each other because the rate of hydro-
gen release depends on many parameters: temperature,
concentration of a sodium borohydride solution, pH of
the solution, design of the catalytic reactor, etc. More-
over, published data on the effect of the nature of an
active component and a support on the rate of hydroly-
sis of sodium borohydride in the presence of supported
metal catalysts formed immediately in a reaction
medium of sodium borohydride are inadequate.
EXPERIMENTAL
Aqueous solutions of RhCl3 · nH2O, H2PtCl6 · 6H2O,
PdCl2, and Ru(OH)Cl3 (OAO Aurat) were used for the
preparation of catalysts. Water was added to a calcu-
lated weighed portion of a support, and the contents
were stirred with a magnetic stirrer for 15 min. Then, a
solution of a corresponding metal salt with a required
concentration was added, and the mixture was addition-
ally stirred for 15 min at room temperature. Next, the
suspension was heated to 80°C, and an excess of water
was evaporated with continuous stirring. After the stage
of impregnation, the catalysts were dried in air at 110–
130°ë for 2 h. The calculated metal content was 1 wt %.
The carbon material Sibunit (KTITU, Siberian
Branch, Russian Academy of Sciences) [9] with SBET
=
530 m2/g and a particle size of 0.08–0.10 mm, γ-Al2O3
(OAO Katalizator) with SBET = 170 m2/g and the above
particle size, and powdered TiO2 samples, whose char-
acteristics are given in Table 1, were used as supports.
Rhodium catalysts on TiO2 sample nos. 1, 2, and 3 are
subsequently referred to as Rh/TiO2-1, Rh/TiO2-2, and
Rh/TiO2-3, respectively.
The aim of this work was to perform the synthesis
and a comparative study of chemically different sup-
ported catalysts in the reaction of NaBH4 hydrolysis:
rhodium, platinum, ruthenium, and palladium sup-
ported on γ-Al2O3; a Sibunit carbon material; or TiO2
To study the generation of hydrogen, 98% sodium
borohydride (Sigma-Aldrich) dissolved in distilled
water was used. The samples of NaBH4 were kept in a
(anatase). The role of the support in the formation of an desiccator in order to prevent hydrate formation by the
168

EFFECT OF THE NATURE OF THE ACTIVE COMPONENT
Table 1. Physicochemical properties of TiO2 samples
169
Calcination
temperature, °C
Phase composition
Dispersity, Å
Impurity
Sample
SBET, m2/g
(according to XRD data) (according to XRD data) concentrations, wt %
1
2
110
500
243
79
Anatase, 100%
110
140
–
Nb, 0.04
S, 0.03
Anatase, 96%, rutile, 4%
Fe, 0.22
Ca, 0.58
Nb, 0.25
S, 3.92
3
500
74
Anatase, 100%
150
Fe, 0.07
Ca, 0.01
Note: Sample 1 is a commercial product from Sigma-Aldrich (CAS 1317-70-0); sample 2 is a commercial sample from Solikamsk Mag-
nesium Works; and sample 3 is a commercial sample from Leninabad Rare Metals Combine (Tajikistan).
interaction with water vapor in air. The hydrolysis reac-
The diffraction patterns of samples were obtained
tion was performed at 40°C in a thermostated glass on a URD-63 diffractometer (Germany) using ëuKα
reactor equipped with a magnetic stirrer at a stirring radiation. The sizes of coherent scattering regions
rate of 800 rpm. A 10-ml portion of distilled water was
added to a 0.0465-g portion of sodium borohydride
placed in the reactor. Next, a catalyst was added to the
reactor; the catalyst amount was calculated to obtain a
specified metal/hydride molar ratio. The reactor was
sealed with a nozzle connected to a burette. The volume
of liberated hydrogen was measured using a 100-ml
burette. The qualitative analysis of the resulting gas was
performed using a Kristall-2000 chromatograph (OAO
IZhMZ Kupol). The normalized rate of hydrogen gen-
eration (r) was calculated using the equation
(CSRs) were calculated using the Scherrer equation
based on the halfwidths of the strongest diffraction
peaks: (101) and (110) for anatase and rutile phases,
respectively. The CSR size determination error was
~10%. At a low concentration of a particular phase (less
than 4%), it was impossible to evaluate reliably the
CSR size. The quantitative analysis of individual crys-
talline phases in the samples was performed with the
use of the PCW program.
The UV–vis diffuse reflectance spectra were mea-
sured in air at room temperature using a Specord M-40
spectrometer (Carl Zeiss Jena) with a standard diffuse-
reflectance attachment. The absorption of a support was
compensated for in the measurement of the spectra. In
the analysis of unreduced catalysts, a parent support
sample was used for compensation, whereas a support
sample treated with NaBH4 was used in the analysis of
catalysts reduced with sodium borohydride.
VH
2
----------
r =
,
t1/2m
where r is the rate of reaction ((ml H2) g–1 s–1); VH is
2
the volume of hydrogen (ml (NTP)) liberated in the
half-reaction time t1/2 (s); and m is the weight (g) of a
catalyst or an active component (Rh, Pt, Pd, or Ru) of
the catalyst.
RESULTS AND DISCUSSION
The specific surface area (SBET) was determined
from the thermal desorption of argon; the relative deter-
mination error was 10%.
The hydrolysis of sodium borohydride begins even
at room temperature, and the rate of hydrogen genera-
tion increases with temperature. The complete conver-
sion of NaBH4 cannot be reached because of an
increase in the pH of the solution [10]. In the presence
of the nanodispersed powders of platinum group met-
als, the rate of hydrogen generation increases dramati-
cally. However, the catalytic activity of metals is irre-
producible from experiment to experiment, and it varies
The concentrations of impurities in TiO2 were found
using inductively coupled plasma atomic emission
spectrometry on an Optima 4300 DV instrument (Ger-
many). For the determination of sulfur, acid extracts
from the samples in hydrochloric acid were taken. The
analysis for Nb, Fe, and Ca was performed after dis-
solving a weighed portion of TiO2 in H2SO4 with the over a wide range (20–30%) [11]. Dragieva et al. [12]
addition of HF. The relative determination error related this instability to changes in the size of metal
was 5%.
particles over a wide range and to the subsequent parti-
KINETICS AND CATALYSIS Vol. 48 No. 1 2007

170
SIMAGINA et al.
Hydrogen yield, %
100
The addition of nanodispersed rhodium powder, which
was prepared by the reduction of rhodium chloride in a
medium of sodium borohydride, allowed us to increase
the rate of hydrogen generation: a 70% yield was
reached even in 33 min. Only 6 min was required for
reaching the same conversion on the Rh/TiO2-1 cata-
lyst. Moreover, the presence of a supported catalyst
allowed us to reach a maximum yield of hydrogen.
Thus, we can conclude that the supporting of a metal
onto the surface of a carrier stabilizes rhodium nano-
particles and increases their activity. Figure 2 shows an
electron micrograph of supported rhodium particles
formed in a medium of sodium borohydride. These par-
ticles are characterized by a narrow size distribution
over the range 2–3 nm.
3
2
80
1
60
40
20
0
As can be seen in Table 2, the activity of Pt, Rh, Ru,
and Pd metals supported onto Sibunit, γ-Al2O3, and
TiO2 (anatase) decreases in the order Rh > Pt ≈ Ru ꢀ
Pd. This relation is obeyed with the use of all three test
supports. The order found in this work is somewhat dif-
ferent from that obtained by Prokopchik et al. [11, 13–
15]; in these publications, the activity of bulk systems
in the reaction of NaBH4 hydrolysis decreased in the
order Rh > Pt > Pd > Ru. It is also different from the
order Pt > Rh > Ru ≈ Pd, which was published by
Kojima et al. [4] and obtained with the use of titanium
dioxide as a support. This is likely due to differences in
catalyst preparation procedures.
0
100
200
300
400
500
Time, min
The rate of hydrogen release in the presence of
Pd-containing catalysts is low. As exemplified by the
Pd/C catalyst (henceforth, the symbol C refers to
Sibunit), it can be seen (Fig. 3) that it rapidly loses its
activity and the rate of generation approaches the level
of sodium borohydride decomposition in the absence of
Fig. 1. Hydrogen generation at 40°C (1) in the absence of a
catalyst and in the presence of (2) a bulk rhodium catalyst
and (3) a supported Rh/TiO -1 catalyst. The Rh/NaBH
2
4
molar ratio is 1 : 2000.
cle agglomeration in the course of the hydrolysis of a catalyst. With consideration for the well-known abil-
NaBH4.
The results of this work are indicative of the advan-
tages of supported catalysts used for the hydrolysis of
NaBH4 over monometallic powders. It can be seen in
Fig. 1 that the rate of hydrolysis of sodium borohydride
in the absence of a catalyst gradually decreased with
time; and the yield of hydrogen was 70% in 420 min.
ity of palladium to absorb considerable amounts of
hydrogen [16], it is believed that catalyst deactivation is
due to hydrogen adsorption on palladium with the for-
1
mation of the strong Pd–H bond [17].
In addition, the nature of not only an active compo-
nent but also a support affects the activity of supported
catalysts. A comparison between the rates of hydrogen
release per gram of rhodium with the use of various
supports (Fig. 4) shows that the activity of catalysts
supported on Sibunit and γ-Al2O3 differs only slightly
from the activity of bulk rhodium, which was prepared
by the reduction of rhodium chloride with a solution of
sodium borohydride. Moreover, the rate of hydrogen
release on Rh/γ-Al2O3 was even lower than that on the
bulk catalyst. To reveal the reason for this low activity,
we studied the electronic state of rhodium on the sur-
face of aluminum oxide using UV–vis diffuse reflec-
tance spectroscopy.
Table 2. Rates of hydrogen release ((ml H2) g–1 s–1, normal-
ized to 1 g of a catalyst) from aqueous NaBH4 solutions
at 40°C and a stirring rate of 800 rpm
Active component
Support
Rh
Pt
Ru
Pd
γ-Al2O3
Sibunit
TiO2-1
4.5
10.8
30.3
2.3
7.3
–
–
7.0
0.8
2.0
Figure 5 shows the UV–vis diffuse reflectance spec-
tra of a mechanical mixture of MgO and rhodium chlo-
14.3
16.1
1
Note: The metal/NaBH ratio was 1 : 2000 for Rh, Pt, and Ru cat-
4
The Pd–H bond energy is 240 kJ/(mol Pd) at the surface coverage
θ = 0.5 [18].
alysts; the Pd/NaBH ratio was 1 : 1000 for a Pd catalyst.
4
KINETICS AND CATALYSIS Vol. 48 No. 1 2007

EFFECT OF THE NATURE OF THE ACTIVE COMPONENT
171
20 nm
Fig. 2. Electron micrograph of the surface of an Rh/TiO catalyst.
2
Hydrogen generation rate, (ml H2) s–1
0.08
0.06
0.04
0.02
0
2
1
0
2000
4000
6000
8000
10000
Time, s
Fig. 3. Dependence of the rate of hydrogen release from aqueous NaBH solutions on reaction time at 40°C: (1) in the absence of
4
a catalyst and (2) in the presence of Pd/C. The Pd/NaBH molar ratio is 1 : 1000.
4
KINETICS AND CATALYSIS Vol. 48 No. 1 2007

172
SIMAGINA et al.
Absorption, %
–1
r, (ml H2) g s–1
Rh
7000
6000
5000
4000
3000
2000
1000
80
70
60
50
40
30
20
10
4
1
3
2
0
1
2
3
4
5
Fig. 4. Rate of hydrogen release (normalized to 1 g of rhod-
42 38 34 30 26 22 18 14 10
ν × 103, cm–1
ium) from aqueous NaBH solutions at 40°C in the pres-
4
ence of Rh-containing catalysts: (1) bulk Rh, (2) Rh/γ-
Fig. 5. UV–vis diffuse reflectance spectra: (1) a mechanical
mixture of MgO with RhCl · nH O, (2) γ-Al O ,
Al O , (3) Rh/C, (4) bulk Rh with the addition of a weighed
2
3
portion of TiO in the course of the reaction, and (5)
3
2
2 3
2
(3) unreduced Rh/γ-Al O , and (4) reduced Rh/γ-Al O .
Rh/TiO -1. The Rh/NaBH molar ratio is 1 : 2000.
2
3
2 3
2
4
ride (spectrum 1), parent aluminum oxide (spectrum 2),
unreduced Rh/γ-Al2O3 (spectrum 3), and Rh/γ-Al2O3
reduced with sodium borohydride (spectrum 4). As can
be seen, these spectra differ noticeably from each other.
The spectrum of the parent salt (1) exhibited absorption
bands in the region 28000–11000 cm–1. According to
Lever [19], these absorption bands can be attributed to
d−d transitions in the hexacoordinated Rh3+ ion. An
absorption band at 18900 cm–1 was the most intense of
these bands. Upon supporting RhCl3 onto the surface of
aluminum oxide, the position of this absorption band
shifted by 3700 cm–1 to the high-frequency region
(spectrum 3; absorption band at 22600 cm–1). Along
with changes related to d–d transitions, a prominent
high-frequency shift of the charge-transfer band and an
increase in the intensity of this band were also
observed. Thus, the electronic states of bulk and sup-
ported rhodium chloride differ significantly. This is
likely due to the strong interaction of the parent rhod-
ium salt with the surface of aluminum oxide up to the
formation of a new surface compound [20].
air [21]. It is believed that the occurrence of a portion
of rhodium in an oxidized state on the surface of
Rh/γ-Al2O3 is responsible for its lowest activity.
As can be seen in Table 2, a rhodium catalyst pre-
pared based on titanium dioxide exhibited the maxi-
mum activity. The rate of hydrogen generation on this
catalyst (Fig. 4) was higher than that on bulk rhodium
by a factor of more than 5. The addition of a weighed
portion of titanium dioxide to a reaction medium con-
taining an aqueous solution of rhodium chloride and
sodium borohydride allowed us to increase the rate of
hydrogen release by 30% (Fig. 4). It is likely that rhod-
ium metal particles, which were formed in solution in
the course of rhodium chloride reduction, were
adsorbed on the surface of titanium dioxide to produce
centers that are more active in the hydrolysis of sodium
borohydride. The high reactivity of rhodium on the sur-
face of titanium dioxide is related to the formation and
stabilization of supported nanodispersed rhodium
metal particles in a medium of sodium borohydride
(Fig. 2) and, probably, changes in the electronic proper-
ties of metal clusters because of their interaction with
the support. The effect of the strong interaction of rhod-
ium with titanium dioxide in catalysts reduced with
hydrogen at high temperatures is well known [22].
After the reduction of the Rh/γ-Al2O3 catalyst under
the action of a reaction medium (sodium borohydride),
structureless absorption was observed in the region
28000–11000 cm–1 (Fig. 5, spectrum 4), which sug-
gests the formation of rhodium metal. However, an
A comparative study of rhodium catalysts (Fig. 6)
absorption band at 24000 cm–1 and absorption above prepared based on commercial titanium dioxide sam-
42000 cm–1, which suggest the presence of Rh3+ ions, ples demonstrated that they exhibited lower activity
were observed against the background of nonselective than that of a catalyst based on TiO2-1 from Sigma-Ald-
absorption. It is likely that the presence of rhodium ions rich. The differences were likely due to lower specific
in the reduced sample can be explained by either the surface areas and the presence of impurities in commer-
stabilization of an oxidized state of rhodium in the cial samples (Table 1). The rate of hydrogen generation
resulting surface compound and the incomplete reduc- in the presence of the Rh/TiO2-3 catalyst was much
tion of it or the rapid reoxidation of rhodium metal in lower than that in the presence of Rh/TiO2-2, although
KINETICS AND CATALYSIS Vol. 48 No. 1 2007

EFFECT OF THE NATURE OF THE ACTIVE COMPONENT
Volume of H2, ml
173
100
80
60
40
20
0
1
2
3
0
500
1000
1500
2000
2500
3000
3500
Time, s
Fig. 6. Hydrogen generation at 40°C in the presence of supported Rh catalysts: (1) Rh/TiO -1, (2) Rh/TiO -2, and (3) Rh/TiO -3.
2
2
2
The Rh/NaBH molar ratio is 1 : 2000.
4
the texture characteristics of TiO2-2 and TiO2-3 were parent salt after supporting it onto the carrier surface.
similar. It is believed that the occurrence of sufficiently Taking into account this circumstance, we can perform
strong Brønsted acid sites [23] on the surface of sulfate- a more detailed analysis of spectral curves. The inten-
containing TiO2-3 (Table 1) affects the electronic state sity of an absorption band at 25000 cm–1 in the UV–vis
of rhodium metal clusters [24, 25], and this decreases diffuse reflectance spectrum of the Rh/TiO2-2 sample
the activity of Rh/TiO2-3 in the test reaction.
(spectrum 1) is much higher than the intensity of a sim-
ilar absorption band at 24800 cm–1 in the spectrum of
the Rh/TiO2-3 sample (2). In this case, an absorption
band at 20000 cm–1 in the spectrum of Rh/TiO2-3
exhibits a lower intensity and it is somewhat broadened
and shifted by 500 cm–1 to the high-frequency region
with reference to an analogous absorption band in the
spectrum of Rh/TiO2-2. Note that the absorption band
at 24800 cm–1 in the spectrum of the Rh/TiO2-3 sample
can be related to d–d transitions in the Rh3+ ion with a
certain degree of probability, whereas the intense
absorption band at 25000 cm–1 in the spectrum of
Rh/TiO2-2 cannot be related to only this transition.
To study the effect of the surface chemistry of ana-
tase titanium dioxide on the state of rhodium chloride,
we analyzed the UV–vis diffuse reflectance spectra
(Fig. 7) of rhodium chloride supported onto TiO2-2
(spectrum 1) and sulfate-containing TiO2-3
(spectrum 2) and the spectrum of a mechanical mixture
of MgO and RhCl3 (spectrum 3). Spectra 1–3 differ
noticeably from one another. Let us analyze them in
more detail. Initially, we compare the spectra of
Rh/TiO2-2 and Rh/TiO2-3 samples (1 and 2, respec-
tively) with the spectrum of a mechanical mixture of
MgO and RhCl3 (3). It is believed that the interaction
between the components of this mixture is insignifi-
Previously, a similar absorption band was observed
cant. The spectra of all of the test samples exhibit three in a study of the interaction of vanadium and copper
absorption bands in the region 28000–11000 cm–1. compounds with the surface of titanium dioxide. Its
According to Lever [19], these absorption bands can be appearance was related to the formation of a Ti–O–
attributed to d–d transitions in the hexacoordinated metal bond. According to published data [26, 27],
Rh3+ ion. However, unlike the spectrum of the mechan- absorption in the region of the spectrum under consid-
ical mixture of MgO and RhCl3, absorption bands due eration characterizes the degree of interaction of a
to the d–d transitions are better resolved in the UV–vis metal with the surface of the support (TiO2). In this
diffuse reflectance spectra of the supported samples. case, the higher the absorption band intensity at
Evidently, this is due to a decrease in the effect of 25000 cm–1, the higher the concentration of metal ions
charge transfer on the d–d transitions in the Rh3+ ion that form the above bond. The presence of sulfate ions
because of an increase in the degree of dispersity of the on the surface of TiO2 significantly decreases the inten-
KINETICS AND CATALYSIS Vol. 48 No. 1 2007

174
SIMAGINA et al.
component of the catalyst affects the rate of hydrogen
Absorption, %
generation. We found that the activity of catalysts on all
three test supports decreases in the order Rh > Pt ≈
Ru ꢀ Pd. It is likely that the lowest reactivity of a pal-
ladium catalyst is due to the formation of a strong Pd−H
bond. Of rhodium catalysts based on Sibunit, γ-Al2O3,
and TiO2, Rh/TiO2 exhibits the highest activity. It is
likely that the difference in the reactivity of rhodium
catalytic systems is related to the interaction of the pre-
cursor of an active component with the support.
According to UV–vis diffuse reflectance spectra, com-
plexes of the precursor of an active component with the
support are formed in the course of supporting rhodium
chloride. Differences in the composition and structure
of surface complexes depend on the nature of the sup-
port. The further reduction of these complexes in a
reaction medium of sodium borohydride results in the
formation of metal clusters, which exhibit various reac-
tion properties.
90
80
70
60
50
40
30
20
10
3
2
1
42 38 34 30 26 22 18 14
ν × 103, cm–1
ACKNOWLEDGMENTS
Fig. 7. UV–vis diffuse reflectance spectra of the samples:
(1) Rh/TiO -2, (2) Rh/TiO -3, and (3) a mechanical mixture
We are grateful to V.I. Zaikovskii, A.V. Ishchenko,
and S.V. Tsybulya for their assistance in studying the
catalysts. This work was supported by the Russian
Foundation for Basic Research (project no. 04-03-
32208) and the Norilsk Nickel Mining and Metallurgi-
cal Company.
2
2
of MgO with RhCl · nH O.
3
2
sity of this absorption band because of the interaction of
the metal with sulfate groups [28].
We believe that, upon supporting rhodium chloride,
it interacts with the surface of TiO2-2. If a rhodium salt
is supported onto sulfate-containing titanium dioxide
TiO2-3, it interacts with sulfate ions. This hypothesis is
supported by a gradual shift of the most intense absorp-
tion band at 18900 cm–1 in the spectrum of the parent
salt after supporting it onto the surface of the test sam-
ples of titanium dioxide. The shift of this absorption
band to the high-frequency region, which is indicative
of an increase in the ligand field, suggests a change in
the state of the first coordination sphere of the rhodium
ion. It is likely that the increase in the ligand field was
due to the replacement of one or more ligands in the
parent RhCl3 salt by the surface oxygen of the TiO2-2
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