Thin layer of Ni-modified 13X zeolite on glassy carbon support as an electrode material in aqueous solutions

Article abstract of DOI:10.1134/S0036024407090208

A new type of an electrode material, zeolite modified by the incorporation of Ni or NiO clusters into its cavities, was synthesized by multiple impregnation of zeolite 13X with a Ni-acetylacetonate solution followed by solvent evaporation and thermal degradation of the nickel compound. Samples with a Ni/13X mass ratio within the range 0.2-1.0 were synthesized. Modification by both Ni and NiO clusters, depending on whether the atmosphere was reducing (H₂) or oxidizing (air), respectively, was used to finish the sample. After modification, the zeolite kept its original crystallographic structure, as proven by X-ray diffractommetry. The dimensions of the incorporated clusters were limited by the diameter of the zeolite cavities (reaching 1.3 nm). This material, homogenized with 10 wt % of nanodispersed carbon, was bonded in the form of a thin layer to a glassy carbon disc by means of Nafion and used as an electrode material in an aqueous 0.1 M NaOH solution. The cyclovoltammograms of this thin-layer electrode resemble those of a smooth nickel electrode in alkaline solutions.

Full text of DOI:10.1134/S0036024407090208

ISSN 0036-0244, Russian Journal of Physical Chemistry A, 2007, Vol. 81, No. 9, pp. 1452–1457. © Pleiades Publishing, Ltd., 2007.
COLLOID CHEMISTRY
AND ELECTROCHEMISTRY
Thin Layer of Ni-Modified 13X Zeolite on Glassy Carbon Support
1
as an Electrode Material in Aqueous Solutions
a
b
c
Z. Mojovi c´ , S. Mentus , and I. Krsti c´
a
b
IChTM Center of Catalysis and Chemical Engineering, Njegoseva 12, Belgrade
Faculty of Physical Chemistry, Belgrade University, Studentski trg 12, Belgrade
c
Faculty of Chemistry, Belgrade University, Studentski trg 12, Belgrade
e-mail: slavko@ffh.bg.ac.yu
Abstract—A new type of an electrode material, zeolite modified by the incorporation of Ni or NiO clusters
into its cavities, was synthesized by multiple impregnation of zeolite 13X with a Ni-acetylacetonate solution
followed by solvent evaporation and thermal degradation of the nickel compound. Samples with a Ni/13X mass
ratio within the range 0.2–1.0 were synthesized. Modification by both Ni and NiO clusters, depending on whether
the atmosphere was reducing (H ) or oxidizing (air), respectively, was used to finish the sample. After modifica-
2
tion, the zeolite kept its original crystallographic structure, as proven by X-ray diffractommetry. The dimensions
of the incorporated clusters were limited by the diameter of the zeolite cavities (reaching1.3 nm). This material,
homogenized with 10 wt % of nanodispersed carbon, was bonded in the form of a thin layer to a glassy carbon
disc by means of Nafion and used as an electrode material in an aqueous 0.1 M NaOH solution. The cyclovolta-
mmograms of this thin-layer electrode resemble those of a smooth nickel electrode in alkaline solutions.
DOI: 10.1134/S0036024407090208
Zeolites are aluminosilicates having a porous cavity 14]. There is evidence that the solid aluminosilicate
structure consisting of nanometer-sized cages intercon- framework hinders metal nanocluster agglomeration
nected by channels into a solid crystal framework with [15].
2
–1
a huge inner surface area of several hundred m g . They
are widely used as catalysts, ion-exchangers, adsor-
bents, and molecular sieves, and present a specific sup-
port for the incorporation of metal or metal-oxide clus-
ters, as well. A number of papers have shown that alka-
line cations contained in the zeolite structure may, by
the ion-exchange procedure, be replaced with transition
metal cations, which may further be reduced to metal
atoms inside zeolites cages. The stoichiometric compo-
sition of zeolites limits the maximum number of metal
atoms per zeolite cage, so only clusters of nanometer
dimensions can be obtained. Metal particles in zeolites
can grow up from an isolated atom to that of a cluster
containing a few tens of atoms. The catalytic effective-
ness of such systems in numerous, particularly organic,
chemical reactions is well known [1–7]. Detailed anal-
ysis of the size and location and chemical surrounding
of metal clusters in zeolites was performed with the aim
of explaining their catalytic properties. High-resolution
electron microscopy techniques are most suitable for
this investigation [8–12]. The complete characteriza-
tion of metal aggregates consists in the determination
of metal dispersion (particle size and particle location),
as well as the determination of the atomic and elec-
tronic structure of the particles. Clusters containing 5–
Zeolites and their inclusion complexes were also the
subject of electrochemical investigations. Xu et al. [16]
investigated the oxidation of hydrazine, alcohol, and
hydroquinone in water solutions by means of an elec-
trode material consisting of a zeolite–carbon mixture
fixed on an electronically conductive support. In several
cases, such a zeolite fixed on an electronically conduc-
tive support has been used to study the kinetics of redox
2+
reactions within its cavities. For example, MV/MV
2+
3+
(
MV = methylviologen) [17] and Fe /Fe [18] redox
pairs were studied in this particular way. Finally, plati-
num-modified zeolite as a microscopic bipolar elec-
trode suspended in a low conducting aqueous medium
was studied by Rolison et al. [19] for water splitting.
The present authors incorporated metal clusters in
zeolite cavities by impregnation of zeolites by solutions
of thermodegradable complex compounds—metal
acetylacetonates. They used such modified zeolites as
thin-layer electrodes after the addition of carbon black,
which provided their electronic conductivity [20, 21].
In this work, the recently developed procedure of
zeolite impregnation by Ni-acetylacetonate followed
by the thermal decomposition of acetylacetonates was
used to incorporate NiO clusters within zeolite cavities.
Zeolite NaX (faujasite type) was used as a supporting
material. NiO-modified zeolite was investigated as an
electrode material in an aqueous electrolyte solution.
Zeolites are expected to both enable the obtaining of an
1
2 Pt atoms are also identified by extended X-ray
adsorption fine-structure spectroscopy (EXAFS) [13,
1The text was submitted by the authors in English.
1
452

THIN LAYER OF Ni-MODIFIED 13X ZEOLITE ON GLASSY CARBON SUPPORT
Relative residual mass, %
1453
effective nickel catalyst in the form of nano-dispersion,
and to prevent its agglomeration during use.
Air
H
2
8
4
0
0
EXPERIMENTAL
Nickel(II) acetylacetonate [Ni(acac) ] was synthe-
2
sized for the purposes of this paper. The zeolite NaX
alternatively labeled 13X (Linde Co.) was heated to
3
50°C to remove adsorbed water and cooled to room
temperature in dry atmosphere. Then it was soaked
with a dilute acetone solution of Ni(acac) . The wet
2
sample was previously dried to 90°C to evaporate ace-
tone and further to 400°C with the purpose of both
decomposing nickel acetylacetonate and removing its
gaseous decomposition products. Figure 1 illustrates
the importance of the composition of the atmosphere in
this procedure. In this way, working in air, nanodis-
persed NiO clusters were incorporated into zeolite
cages. The impregnation–decomposition procedure
0
200
400
600
t, °C
Fig. 1. Residual mass versus temperature for Ni(acac)
heated at a rate of 15 K/min in oxidizing (air) and reducing
H ) atmospheres.
2
(
2
was repeated until the weight ratios NiO/zeolite of 0.2, in a sufficiently broad voltage range to encompass
0
.4, and 1.0 were reached. After this procedure, the water electrolysis.
modified zeolites changed color from white to dark
gray.
RESULTS AND DISCUSSION
The thermogravimetric (TG) analysis of both pure
and Ni(acac) -impregnated zeolite samples was per-
2
Thermogravimetry of Ni(acac) . In order to explain
2
formed using a SDT 2090 (TA Instruments) device.
properly weight changes caused by heating of zeolite
XRD diagrams of the original and modified zeolites impregnated with Ni(acac) , the thermal stability of
2
were taken by means of a Philips PW 1710 diffractome- pure Ni(acac) was analyzed thermogravimetrically in
2
ter using CuK radiation.
air and hydrogen atmospheres. The thermograms
recorded using either oxygen or hydrogen atmospheres
are presented in Fig. 1. The initial weight loss of 19%
α
In order to use the modified zeolite as an electrode
material, the zeolite sample was homogeneously dis-
persed in a solution of Nafion (5 wt %) in a mixture of
isopropyl alcohol and distilled water by means of an
ultrasonic bath. 10 wt % of Vulcan XC72 (Cabot Corp.)
carbon black was added to the initial suspension to pro-
vide the electronic conductivity of the sample. A droplet
of this suspension was placed on the surface of a glassy
carbon disc of a rotating electrode. After the removal of
the solvent by evaporation at 90°C, the zeolite particles
properly dispersed in the form of a thin layer of the den-
at 100°C under both air and H flow can be associated
2
with acetone excess. The presence of this amount of the
solvent is necessary for maintaining the solubility in
acetone; otherwise, dimeric and trimeric molecules
insoluble in acetone may be formed [23]. Under hydro-
gen flow, the main weight loss occurs suddenly in one
step at 230°C leaving 15% of the initial weight (against
the theoretically expected 18%, which indicates some
extent of evaporation losses). Under air flow, the
weight–temperature curve shows two distinguished
steps, indicating the sequenced loss of two molecules of
acetylacetonate. Thermal decomposition becomes
complete at 430°C, leaving 23% of the initial weight,
–2
–2
sity 12 mg cm , i.e., about 3.8 mg NiO cm , finally
resulted in a pasted deposit on the glassy carbon sup-
port. An analogous way of the preparation of such a
powdered materials for electrochemical investigations
was already published elsewhere, for instance, in the
case of galvanostatic charging and discharging of Li-
intercalate powders [22].
which corresponds to the transformation of Ni(acac) to
2
nickel oxide. Specifically, the theoretically expected
residual mass in weight percents, assuming NiO to be
the final product, amounts to 23.4.
For electrochemical investigations in a three-
electrode all-glass cell compartment, a glassy carbon
During the procedure of NiO incorporation, it was
disc covered by a layer of a homogeneous mixture of assumed that the processes observed in the case of pure
Ni-modified zeolite along with 10 wt % carbon black Ni(acac) and illustrated in Fig. 1 occur in the same way
2
was used as a working electrode. The reference elec- in the case of zeolite impregnated by Ni(acac) . Based
2
trode was Ag/AgCl in 1 M KCl, while a platinum foil on the thermogravimetric investigation shown in Fig. 1,
served as the counter electrode. The electrolyte was an Ni-modified zeolite was obtained by heating impreg-
aqueous solution 0.1 M NaOH. The electrolyte solution nated zeolite to 400°C in air. This procedure led to the
was purged by nitrogen stream. The device used for incorporation of NiO. The effects of NiO incorporation
electrochemical measurements was a 757 VA Compu- were further observed by XRD and electrochemical
trace Metrohm. The polarization curves were recorded methods.
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 81 No. 9 2007

1
454
MOJOVIC et al.
Œ
(
c)
in the diffractograms of modified zeolites too. This
indicates that the procedure did not destroy the original
crystal structure of zeolite.
Zeolite with lower nickel content (20%) prepared in
air was subsequently heated in hydrogen atmosphere to
reduce NiO to Ni. According to JCPDS (Joint Commit-
tee on Powder Diffraction Standards), the diffraction
lines of crystalline nickel correspond to the plane dis-
tances d = 0.204 and 0.177 nm. In the diffractogram of
d = 0.209 nm d = 0.148 nm
d = 0.126 nm
0
0
20
40
b)
d = 0.204 nm
60
80
(
1
3X–20% Ni at these positions, relatively narrow peaks
appear, which indicates crystal arrangement of incorpo-
rated nickel.
d = 0.177 nm
The zeolite sample with a higher Ni content pre-
pared by thermal decomposition of Ni(acac) in air, as
2
expected from the TGA measurements in Fig. 1, shows
reflections at d = 0.209, 0.148, and 0.126 nm, which,
according to JCPDS, belong to NiO.
20
40
60
80
(
a)
The mean size of the incorporated clusters was cal-
culated by means of the Scherer formula
L = Kλ/bcosθ ,
0
10
20
30
40
50
60
where L is the mean diameter along the vertical to the
plane of diffraction; K is the size parameter (usually
2
θ, deg
0
.9); λ is the wavelength of the X-rays, Å; b is the peak
Fig. 2. Comparative X-ray diffractograms of zeolite 13X
width at half height; and θ is the Brag angle.
(
a), 13X–20% Ni ((b) prepared in hydrogen atmosphere)
This calculation did not find significant differences
between cluster sizes expected on the basis of the two-
fold difference in weight percent of NiO, which may
indicate a rather approximate applicability of this
method in this case.
and 13X–40% NiO (c).
XRD analysis of the Ni-modified zeolite. The content
of adsorbed water in zeolite, providing water vapor
pressure equal to that of pure water, is about 20% by
Cyclic voltammetry and RDE experiments. An elec-
weight. One may assume that this amount corresponds trode used for the first time is always rather inactive. To
to full volume occupation of zeolite cavities. The vol- achieve full CV response, 20–30 potentiodynamic
ume occupation of zeolite cavities by NiO in NiO-mod- cycles must usually be performed. Regardless of
ified zeolite samples may be estimated owing to the whether reduced or oxidized Ni-modified zeolite is
density ratio between NiO and H O. Namely, this ratio used for electrode preparation, the same steady-state
2
is 6.67/1. Thus, one may assume that the total cavity CV curves are always observed. This is due to the high
volume occupation by NiO requires its weight percent- reactivity of nickel in the nanodispersed form, which
age of 134%. Consequently, the percentages achieved obviously undergoes fast transformation into hydroxide
in this work, 20, 40, and 100%, fall into the range of when dipped into an aqueous solution.
partial volume occupation.
The steady-state cyclovoltammograms of the thin-
The diffractograms of pure 13X zeolite, nickel- layer electrode involving Ni-modified zeolite with an
modified zeolite up to 20% (13X–20% NiO), and Ni/zeolite ratio of 0.4 as an electrode material, in the
nickel-modified zeolite up to 40% (13X–40% NiO) are potential range –1 to + 1 V, recorded in deaerated 0.1 M
presented in Fig. 2. Comparison of the diffractograms NaOH solution are shown in Fig. 3. Over a wide poten-
in Fig. 2 shows that all diffraction lines characteristic of tial range from –1.4 to 0.5 V, this electrode material
the original zeolite appear with the same intensity ratios shows a purely capacitive behavior, manifesting itself
in a practically potential-independent distance between
the current values at common potentials but opposite
The mean size (L, nm) of nickel moieties in 13X zeolite cav- polarization directions. The capacitance current is pro-
ities calculated on the basis of X-ray diffractometric data
portional to the scan rate (I = vC , where vis the scan
c
dl
rate) and, in Fig. 3, corresponds to a C value of
dl
Sample
d, nm
2θ, grad b, grad
L, nm
–
2
approximately 0.1 F cm . To reduce the capacitive
background current, one should use a relatively low
scan rate. In Fig. 4, the voltammograms recorded at a
1
1
3X–20% Ni
0.204
44.4996
51.7538
43.3062
62.9648
0.334
0.436
0.325
0.413
0.257
0.203
0.263
0.226
0
.177
0.209
.148
–
1
scan rate of 5 mV s are presented, illustrating simul-
taneously a rather slight influence of NiO percentage on
electrode activity.
3X–40% NiO
0
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 81 No. 9 2007

THIN LAYER OF Ni-MODIFIED 13X ZEOLITE ON GLASSY CARBON SUPPORT
j, mA cm–2 j, mA cm–2
1455
4
3
2
1
0
0
0
0
0
8
4
0
0
0
1
2
3
–
40
–
10
–
1
0
1
–
1
0
1
E, V (vs. Ag/AgCl)
E, V (vs. Ag/AgCl)
Fig. 3. Cyclic voltammograms (the 30th cycle and the next
ones) of NiO-modified zeolite, with 40% of NiO, recorded
in deaerated 0.1 M NaOH solution at a scan rate 50 mV/s.
Final anodic potential was progressively enlarged from –0.1
to 1.0 V in steps of 0.1 V.
Fig. 4. Cyclic voltammograms of NiO-modified zeolite,
with 20 (1), 40 (2), and 100% of NiO (3), recorded in deaer-
ated 0.1 M NaOH, at a scan rate 5 mV/s.
The processes which take place on the NiO-modi- theoretical charging/discharging capacity of this elec-
–
2
fied zeolite electrode may be identified by comparing trode (approximately 4 mC cm ).
them with the known processes taking place on other
On a smooth nickel electrode [24–27], the corre-
electrodes in the same solutions.
sponding anodic peak is clearly separated from the cur-
rent of the oxygen evolution reaction, appearing at
higher anodic potentials. However, at our NiO-modi-
fied zeolite electrode, the separation is absent and the
current of oxidation of Ni(II), at anodic potentials
higher than 0.8 V, passes gradually to the current of
oxygen evolution, also visible by gas-bubble liberation.
On the platinum surface, the hydrogen evolution
reaction may appear at potentials close to –1 V. At the
same potential, the NiO-modified zeolite electrode is
obviously inactive for this reaction, and cathodic poten-
tial extension beyond –1.5 V is required in order to
induce visible hydrogen evolution.
On the anodic side, in a limited potential range
.55–0.8 V, one may see a pair of anodic–cathodic
0
–
2
peaks. As is well known from the literature [24–27], in
the potential range 0.55–0.8 V vs. SCE, (0.4–0.65 V vs.
Hg/HgO) on a smooth Ni electrode, reversible Ni(II) ⇔
Ni(III) redox transformation occurs, being the basic
reaction of the nickel cathode of commercial power
sources [28]. By analogy with smooth nickel elec-
q, mC cm
3
2
1
0
Anodic
trodes, the processes on the 13X-Ni electrode can be
4
0
Cathodic
attributed to the following reactions:
–
–
β-Ni(OH) + OH ⇔ β,γ-NiOOH + H O + e .
2
2
From the coulonometric analysis of the cyclovoltam-
mograms in Fig. 3 limited to a potential range 0–0.8 V,
both the charging and discharging capacity of the
Ni-modified zeolite electrode were determined depend-
ing on the anodic potential, and this dependence is
shown in Fig. 5. Owing to the actual surface coverage
by modified zeolite, these values are close to 75% of the
0
.5
0.6
0.7
E, V (vs. Ag/AgCl)
Fig. 5. Faradaic capacity of 13X–40% NiO electrode in
.1 M NaOH as a function of final anodic potential in the
region of Ni(OH) /NiOOH redox reactions.
0
2
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 81 No. 9 2007

1
456
MOJOVIC et al.
Œ
j, mA cm–2
a current plateau appears, but with a certain bias toward
the potential axis. The non-biased plateaus are other-
wise characteristic of oxygen reduction on a smooth
platinum electrode [33]. For the rotation rate 600 rpm,
the plateau height amounts to 5 mA cm , which over-
estimates the plateau current observed on smooth plat-
inum in the same solution by approximately 30% [33].
Furthermore, the dependence of current on rotation rate
is less pronounced than expected for diffusion-con-
trolled processes. Namely, the Koutecky-Lewich
1
/j, mA–1 cm2
–
2
4
2
0 0.20
0
.15
(
Fig. 6, inset) has a slope much lower than that corre-
0
0.05 0.10 0.15
–
–
sponding to both the 2e and 4e routes of oxygen
reduction, and in addition, does not cross the origin of
the coordinate system. Most probably, these features
are due to the rough nature of the zeolitic electrode, as
well as to the aggravation of mass transport of both
reactants and products through it.
–
1/2 1/2
0
0
ω
, s
CONCLUSIONS
Impregnation of 13X zeolite by a Ni-acetylaceto-
nate solution in acetone followed by thermal degrada-
tion in air resulted in a new material—modified zeolite
13X with NiO clusters incorporated into the cavities of
the original zeolite structure. NiO can be reduced to Ni
by heating in hydrogen atmosphere. Through the addi-
tion of 10% of carbon black and bonded by polymeric
glue (Nafion) to an electronically conductive support
–
0.8
0
0.8
E, V (vs. Ag/AgCl)
Fig. 6. Cyclic voltammogram of 13X–20% NiO obtained
with rotating disc electrode in oxygen saturated 0.1 M
NaOH solution at sweep rate 5 mV s–1 and rotation rates
00, 600, 1000, and 1200 rpm. Inserted picture: the
Koutecky–Levich plot of the voltammograms for a common
potential of –0.6 V.
(glassy carbon, etc.), this material was used as an elec-
4
trode material. Its electrochemical performance was
studied in both deaerated and oxygen-saturated 0.1 M
NaOH aqueous solutions. Performance characteristic
of the Ni(OH) /NiOOH redox pair, as well as good cat-
2
In this potential range, oxygen evolution proceeds on
the basis of the following equation [29]:
alytic activity toward both oxygen reduction and evolu-
tion were observed. Contrary to expectations, an
increase in NiO content above 20 wt % did not lead to
a proportional enhancement of electrochemical perfor-
–
–
4
OH = O + 2H O + 4e .
2 2
This is in accordance with the data that NiOOH is a mance. It is reasonable to expect that the solid alumino-
very good catalyst for oxygen evolution [30–32]. Con- silicate framework may protect nickel clusters from
trary to this, on gold, oxygen evolution is much slower, agglomeration if subjected to numerous charging–dis-
unless special cathodic treatment is applied [29]. This charging cycles.
is only to illustrate the similar catalytic activity of the
modified zeolite electrode; however, it is very likely
that this electrode will not mechanically withstand
extensive gas evolution.
ACKNOWLEDGMENTS
The work was supported by the Ministry of Science
If the polarization is reversed upon the attainment of and Environmental Protection of the Republic of Serbia,
a final potential higher than 0.8 V, a cathodic peak cen- contracts no. TR6712B (Z.M) and no. 142047 (M.S).
tered at the potential of –0.5 V appears. This peak may
be attributed to the oxygen reduction reaction, involv-
ing oxygen which remains within the zeolite cavities
thanks to a slow liberation of oxygen from zeolite cav-
ities. This conclusion is supported by the fact that this
peak significantly decreases if rotation of the electrode
is switched on.
To additionally prove that we are dealing here with
oxygen reduction, the voltammogram in oxygen-satu-
rated solution was recorded as well using the rotation
disk technique. The corresponding diagrams are given
in Fig. 6. As expected, instead of a peak around –0.5 V,
One of the authors (M.S) thanks the European Com-
munity for financial support (contract no. ICA2-CT–
2
002–10001).
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