22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Clusters synthesis on Ag-zeolite by pulsed discharges in water
B.G. Rodríguez-Méndez1, R. López-Callejas1, M.T. Olguín1, R. Valencia-Alvarado1, A. Mercado-Cabrera1,
R. Peña-Eguiluz1, A.E. Muñoz-Castro1 and A. de la Piedad-Beneitez2
1
Instituto Nacional de Investigaciones Nucleares, AP. 18-1027, 11801, México
2
Instituto Tecnológico de Toluca, AP. 890, Toluca, México
Abstract: The aim of this paper is present the research about the formation of clusters on
the surface of silver-modified natural zeolite (Ag-zeolite). When zeolite was in contact
with pulsed dielectric barrier discharge in water, it was found that a broad range of clusters
sizes were formed. We evaluate the particle size and frequency of occurrence regards the
effect of contact time with the plasma applied into water.
Keywords: zeolite, discharge, water, cluster
1. Introduction
Pulsed discharges are often used to generate nonthermal plasmas (NTP) in gas, liquid or gas/liquid due to
the formation of energetic electrons, maintaining a unique
combination of high electron mean energy with low heavy
particle (neutral and ion) temperatures. Pulsed discharges
generated in water produce as well electromagnetic
radiation, shock waves, electric fields, and these
characteristics are widely used in various emerging
environmental and (bio-) medical applications.
Some authors have showed a more efficient use of
plasma in combination with catalysts. The introduction of
a catalyst into the plasma region (plasma-catalytic
system) may change the electrical properties of the
plasma. Also, generate additional reactive species or
change the nature of the catalyst improving the efficiency,
selectivity and stability of the process. Sometimes a
synergistic effect is reported as the result of employing
plasma-catalysis, exceeding the effect of the catalyst and
plasma separately [1]. The kind of catalyst includes
platinum-based catalyst, TiO 2 , MnO 2 , Al 2 O 3 , and
zeolites.
A zeolite is a natural or synthetic aluminosilicate
material with a particular structural feature. Some of the
characteristics that distinguish zeolites from other porous
materials are their variety of composition and crystal
structure. These can accommodate large cations which
can be readily exchanged with other ones. In their
framework can be deposited transition metals, which
together with the zeolite matrix form a material with new
properties, resulting in the combination of the zeolite
properties with the physical and chemical properties of
the incorporated metal [2]. Certain transition metal ions
supported on zeolites have attracted lots of attention due
to their ability to form good homogeneous and
heterogeneous catalysts that may be reduced chemically
to metal atoms or metal clusters. Such systems usually
manifest pronounced catalytic activity in various media
[3]. In particular, the effect of silver addition in catalytic
materials was found to improve catalyst stability and
modify the product selectivity [4]. For bacterial
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decontamination treatments, silver inhibits microbial
growth by disrupting the bacterial cell membrane and
reacts to form toxic compounds into cells by means of
their ionic form [5].
Silver particles and silver ions (Ag+, Ag2+) are widely
used because are relatively less toxic to human cells.
Special and unique properties of zeolites conditioned with
silver ions (Ag-zeolites) have earned them extensive
industrial uses and applications in different fields such as
catalysis [6] and sensor fabrication [7], and as an
antibacterial agent [8]. However, in most of these uses,
the direct application of noble metal clusters is often
difficult due to their very small size (nm scale).
In this work, results about the formation of stable
clusters on the surface of natural mexican zeolite
conditioned with silver (Ag-zeolite), after contact with
pulsed discharges in water at different treatment times are
presented. We found that the relative size of these clusters
depends on the contact time with the electrical discharge.
2. Materials and methods
2.1 Electrical and mechanical setup
A system to generate pulsed discharges in water in the
range of 25 kV, 25 µs pulse width and repetitive rate of
500 Hz was applied to a coaxial insulated-wire-tocylinder discharge reactor. The wire was covered by an
insulator tube of alumina, and the concentric cylinder was
made of stainless steel (30 cm long and inner diameter of
2.54 cm). The water and the Ag-zeolite sample were put
between dielectric barrier and reactor wall.
2.2 Characterization of Ag-zeolite material
The mexican zeolite from Taxco, Guerrero, was
powdered, sieved and conditioned with AgNO 3 solution
to obtain a silver-modified natural zeolite (Ag-zeolite).
The diameter of the particle selected for experimentation
was approximately 2 mm and 10 mg. For the
characterization of the zeolitic material, scanning electron
microscopy (SEM) observations were carried out; the
samples were mounted directly on the holders and then
observed at 20 kV in a JEOL JSM-5900LV electron
1
microscope. Chemical analyses of the sample were
performed by energy dispersive spectroscopy (EDS).
2.3. Experimental setup
A pulse power supply was connected to the reactor for
produce a discharge into water. The voltage-current
signals were monitored with a digital oscilloscope
(Tektronix TDS2024) and were transmitted to a computer.
Ten milliliters of double distilled water at room
temperature (294 K) were placed into the reactor as
shown in Figure 1. After that, only one particle of Agzeolite was added to the water volume and maintained
until completing the experiment. Immediately after
discharge treatment, the water volume sample was
removed from the discharge reactor and the Ag-zeolite
was placed apart and kept at room temperature until
drying, and analyzed after 24 h.
Characterizations to the zeolitic material were made
before and after electric discharge process by SEM and
EDS analyses. Figure 3 shows the morphological
structure of Ag-zeolite obtained by SEM revealing the
typical crystal morphology of Ag-zeolite.
The elemental composition of Ag-zeolite samples
obtained by EDS is shown in Figure 4 and depicts profile
with elements present in Ag-zeolite. As expected,
composition includes O, Mg, Al, Si, K, and Ag.
(b)
1
2
12
4
3
11
5
6
7
1) anode, (2) dielectric
barrier, (3) distilled
water, (4) cathode, (5)
Ag-zeolite, (6) Teflon
insulator, (7) gas inlet,
(8) pulsed power
supply,
(9)
high
voltage probe, (10)
current transformer,
(11) oscilloscope, and
(12) computer.
(a)
Fig. 3. SEM micrograph of the Ag-zeolite: (a) general
morphology (X100) and (b) crystal morphology (X3000).
10
8
Positive
9
Negative
Fig. 1. Schematic diagram of the experimental layout.
3. Results
High voltage pulses of 25 kV amplitude, 30 µs
duration, and 500 Hz frequency were applied, and the
typical voltage and current waveforms are shown in
Figure 2.
1.5
2.5x104
1.0
2.0x104
0.0
1.0x104
5.0x103
-0.5
0.0
-1.0
-5
-5.0x10
0.0
-5
5.0x10
-4
1.0x10
-4
1.5x10
-4
2.0x10
Time [s]
Fig. 2. Voltage and current waveforms in the PDBD
reactor.
2
Current [A]
Voltage [V]
0.5
1.5x104
Fig. 4. EDS profile of Ag-zeolite.
Figure 5 shows the SEM micrographs of the Ag-zeolite
after different treatment times ((a), 1 min; (b), 5 min; (c),
10 min) with electric discharges. As shown in this figure,
after 1 min in contact with the discharge it can be
observed small particles. The EDS analysis in these
particles revealed Ag presence (see Table 1 and Figure
6). After 5 min it can observe small crystalline structures
(clusters) onto Ag-zeolite surface, however, an interesting
observation is that when treatment time is longer, about
10 min, complex structures with different sizes were
formed.
Table 1. Elemental composition of zeolite-cluster.
Element (Line)
Element wt. %
Atomic %
O (K)
37.91
73.73
Al (K)
1.40
1.61
Si (K)
8.73
9.68
Ag (L)
51.96
14.98
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4. Conclusions
In this research, we have found that silver clusters are
synthesized via electric discharges in water. This process
may be useful to find the way to obtain metal clusters of a
superior way to those of conventional methods. The
properties of these clusters remain the same for several
weeks. An important outcome of this research is the
successful synthesis of stable Ag clusters at room
temperature without additives, chemical reducing agents
or stabilizers and the size can be controlled by treatment
time for specific applications.
(a)
(b)
5. References
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Langenhove. Appl. Catal. B-Environ., 78 (2008)
[2] E. David. J. Achievements Mat. Manufacturing Eng.,
19, 1 (2006)
[3] S. Mentus, Z. Mojovic, N. Cvjeticanin and Z Tesic. J.
New Mat. Elect. Syst., 7 (2004)
[4] M. T. Schaal, A. C. Pickerell, C. T. Williams and J. R.
Monnier. J. Catal., 254 (2008)
[5] A. Spain. Reviews in Undergraduate Research, 2
(2003)
[6] M. Anpo, M. Matsuoka, H. Yamashita, W.S. Ju, S.E.
Park and Y.G. Shul. J. Ind. Eng. Chem., 6, 3 (2000)
[7] F. Manea, A. Pop, C. Radovan, P. Malchev, A.
Bebeselea, G. Burtica, S. Picken and J. Schoonman.
Sensors, 8 (2008)
[8] I. De la Rosa-Gómez, M.T. Olguín and D. Alcántara.
Appl. Clay Sci., 40 (2008)
(c)
Fig. 5. SEM micrographs of the appearance of clusters
onto Ag-zeolite surface: a) 1 min, (b) 5 min, and (c) 10
min in contact with discharge in water.
Fig. 6. EDS profile of Ag-zeolite cluster.
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