Relationships in formation of silicon nitride-supported metal nanoparticles

Article abstract of DOI:10.1134/S1070427210050010

The state of the particles of silicon nitride-supported metal (platinum, palladium, silver) depending on the support preparation conditions and the active component loading technique was examined in relation to the structural features and phase composition of the support. The common and different features in formation of the metal nanoparticles on the silicon nitride surface were identified. Pleiades Publishing, Ltd., 2010.

Full text of DOI:10.1134/S1070427210050010

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2010, Vol. 83, No. 5, pp. 755−767.  Pleiades Publishing, Ltd., 2010.
Original Russian Text  I.A. Kurzina, 2010, published in Zhurnal Prikladnoi Khimii, 2010, Vol. 83, No. 5, pp. 705−717.
INORGANIC SYNTHESIS AND INDUSTRIAL
INORGANIC CHEMISTRY
Relationships in Formation
of Silicon Nitride-Supported Metal Nanoparticles
I. A. Kurzina
Tomsk State University of Architecture and Civil Engineering, Tomsk, Russia
Received October 20, 2009
Abstract—The state of the particles of silicon nitride-supported metal (platinum, palladium, silver) depending
on the support preparation conditions and the active component loading technique was examined in relation to
the structural features and phase composition of the support. The common and different features in formation of
the metal nanoparticles on the silicon nitride surface were identified.
DOI: 10.1134/S1070427210050010
An essential prerequisite for transition to advanced
technologies consists in development of novel materials
whose functional characteristics are determined by the
properties of the specially created microareas, as well as
by the processes occurring at the atomic and molecular
levels in monolayers and nanobulks. One of the major
challenges faced by nanotechnology researchers is
stabilization of nonequilibrium nanoparticles without
major loss of their high reactivity. In this context,
a topical task consists in directed synthesis of stable
and active metallic nanoobjects whose high functional
activity is preserved in catalytic processes, especially
at high temperatures. This challenge is typically
solved via the use of various stabilizing matrix
supports. However, this route is associated with certain
complications, e.g., agglomeration of the particles and
entrainment of the active phase, which hinder further
use of these materials as catalytically active systems.
Moreover, under expo-sure of the nanosystems to
increased temperatures and/or redox reactant mixtures,
weak interaction of nanoparticles with the support
surface may lead to the loss of the functional properties
by the nanoheterogeneous system. At the same time,
the physicochemical and functional properties of the
nanoparticles involved in the chemical reaction may
be affected not only by the particle size but also by
the nature of the matrix support and reaction medium.
In this context, there is an urgent need to develop
approaches to preserving high reactivity of the
nanoparticles stabilized by support matrices so as to
make them suitable as active components of catalytic
systems in specific processes.
For metal (in particular Pd, Pt, and Ag) particles,
exhibitingcatalyticactivityinhigh-temperatureprocesses
(deep oxidation of hydrocarbons, hydrogenation of
carbon monoxide, partial oxidation of alcohols, etc.),
suitable supports are conventionally found in oxide
systems: silica, alumina, aluminosilicates [1]. However,
such systems often lose their activity via agglomeration
and entrainment of the active phase. Prolonged activity
of nanoparticles under high-temperature redox reaction
conditions can be achieved with an alternative support,
silicon nitride. It possesses high thermal conductivity, as
well as high strength and corrosion resistance and low
oxidation rate, which properties are of prime importance
for high-temperature exothermic catalytic processes
(deep oxidation of methane, partial oxidation of ethylene
glycol to glyoxal) [2].
All the basic principles of preparation of supported
metal systems, described in literature, concern oxide
supports. Interaction of platinum/palladium/silver
complexes with the oxide (Al2O3 and SiO2) surface,
755

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KURZINA
associated with metal–support bonding, were studied
in detail, while data on formation of Pt, Pd, and
Ag particles on non-oxide supports are scarce. The
approaches practiced in synthesis on oxide systems are
not absolutely applicable to the preparation of ceramic-
supported systems. Of key importance for the catalytic
properties of metal nanoparticles, along with the
nature of the support, will be the deposition conditions
(chemical nature of the precursor of the particles,
polarity of the medium, nature of the solvent, and heat
treatment conditions).
amount of Pt(II) bis-acetylacetonate dissolved in
toluene. We examined (0.12 wt% Pt)/Si3N4-I, (0.55 wt%
Pt)/Si3N4-I, and (0.87 wt% Pt)/Si3N4-I samples. The
notations for the platinum systems include the initial
amounts of the metal on the support [7]. The systems
with close palladium loadings (~0.5 wt%) were
prepared by impregnating the supports with a solution
of Pd bis-acetylacetonate in toluene (samples Pd/
Si3N4-I, Pd/Si3N4-II, Pd/Si3N4-am, Pd/Si3N4-cal) [6,
8, 9]. To elucidate the influence of the nature of the
solution, we prepared two catalysts by impregnating
silicon nitride (Si3N4-I) with a solution of Pd(II)
acetate in toluene (Pd/Si3N4-I-t) and water (Pd/ Si3N4-
I-w). Upon interaction of the supports with solutions
of palladium or platinum salts and drying at 80°C the
samples were exposed to an argon stream for 2 h at
500°C, subsequently at 350°C for 2 h to an oxygen
stream, and finally to a hydrogen stream at 500°C
for 2 h [8 ]. The content of the active component
(palladium, platinum) on the support was determined
on an ICP atomic emission spectrometer accurately to
within 2%.
Here, we examined the state of the supported metal
particles (platinum, palladium, silver) in relation to the
structural features, phase composition of silicon nitride,
and active component loading technique. Also, we
tested the prepared materials as candidate catalysts for
selective and deep oxidation of organic substances.
EXPERIMENTAL
We used silicon nitride samples with different
specific surface areas and phase compositions (Table 1):
commercially available crystalline materials (Si3N4-I,
Si3N4-II) and samples prepared by self-propagating
high-temperature synthesis technique [3–5] (Si3N4-
III). Also, we used silicon nitride samples comprising
amorphous fraction, Si3N4-am. This material contained
~20 wt% crystalline fraction of silicon nitride with
the α/β phase ratio of ~2.3. The Si3N4-cal support
was amorphous silicon nitride calcined in a nitrogen
atmosphere at 1400°C for 2 h, comprising amorphous
fraction (up to 20 wt%) with the α/β phase ratio of up to
6.6 (Table 1) [6].
The support for silver catalysts was found in the
Si3N4-III sample. The active component was deposited
onto the silicon nitride surface (as calculated for 5 wt%)
by two methods: (1) fractional deposition including
vigorously stirring of the silver nitrate solution with the
support, silver reduction with sodium tetraborohydride,
and 20-h settling of the resulting mixture, followed by
centrifugation and calcination of silicon nitride with
the reduced silver in an O2 stream at 400°C (Ag/Si3N4-
III-FR sample) and (2) impregnation from an organic
medium, including preparation of silver trifluoroacetate
crystals, dissolution of silver trifluoroacetate in
toluene, impregnation of the support with the resulting
solution for 2 h, evaporation of toluene, and sequential
calcination at a rate of 1 deg min–1 to 500°C in an
oxygen stream (Ag/Si3N4-III-OI sample). The content
of the active component (silver) on the support surface
was monitored by X-ray fluorescence spectroscopy
(XFS) on a Quant’X (XFS) spectrometer.
The platinum catalysts were prepared by
impregnating the Si3N4-I support with the appropriate
Table 1. Main characteristics of the silicon nitride support for
metal particles
Average size
of support
particles
Content of α- and
β-silicon nitride phases,
%
Ssp,
m2 g–1
Support
The catalytic performance of the palladium and
platinum systems in deep oxidation of methane were
examined in a continuous quartz reactor at temperatures
within 25–650°C in two modes: (1) on freshly prepared
samples at linearly increased temperature (3 deg min–1)
and (2) on catalysts, preliminarily exposed to the
reaction mixture for 3 h at 650°C (temperature increase
Si3N4-I
6
2
1 μm
α 85, β 15
Si3N4-II
1 μm
β 86, α 13, Si 1
Amorph. > 80, α/β ~2.3
Amorph. < 20, α/β ~6.6
β 85, α 15
Si3N4-am 66
Si3N4-cal 28
1 μm
1 μm
Si3N4-III
2
0.5–1 mm
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757
at a rate of 1 deg min–1). In our experiments we used
the CH4/O2/N2 reaction mixture with the percent ratio
of 2.5/19.5/78 at the feeding rate of 100 ml min–1. The
composition of the reactants at the reactor outlet was
monitored by mass spectrometry.
(a)
Pt (0.9 %)
The catalytic activity of the synthesized silver systems
was tested for partial oxidation of ethylene glycol (EG)
to glyoxal (GO) using a continuous fixed-bed catalytic
setup (inner diameter of the reactor 16 mm) [10]. The
catalyst bed height was 20 mm; the reaction mixture
eV
eV
had the molar composition (CH2OH)2/H2O/O2/N2
=
1.0/3.4/1.0/10.0, and the contact time was 0.06 s. The
catalytic activity was determined at temperature varied
within 500–620°C.
Pt (0.5 %)
eV
The elemental composition of the catalyst surface
and electronic properties of platinum and palladium
before and after deep oxidation of methane were
Pt (0.12 %)
Pt 0.12 wt%
Eb
(b)
40 nm
20 nm
(a)
(c)
(e)
(b)
(c)
40 nm 40 nm
(d)
40 nm 40 nm
(f)
Fig. 2. XPS data for catalysts with different Pt loadings. (I)
Intensity and (Eb) binding energy, eV. (a) Evolution of the Pt4f
spectra, (b) [Pt4f]/[Si2p] ratio, and (c) Pt content, at.%, on the
catalyst surface. (1) Before and (2) after the reaction.
Fig. 1. TEM images of Pt/Si3N4-I catalysts (a, c, e) before and
(b, d, f) after deep oxidation of methane.
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758
Table 2. Surface properties of silicon nitride-supported platinum catalysts Pt/Si3N4-I
Platinum loading, wt %a Pt4f7/2 binding energy, eV a
[Pt4f|/[Si2p]
KURZINA
Average particle size, nm
BR
AR
BR
AR
BR
AR
BR
AR
0.12
0.55
0.87
0.12
0.52
0.83
72.05
71.4
71.4
71.1
71.1
71.1
0.008
0.026
0.032
0.003
0.007
0.012
2
3
6
11
11
16
a BR and AR refer to the catalysts before and after deep oxidation of methane, respectively.
determined by X-ray photoelectron spectroscopy
(XPS) on an ESCALAB 200R spectrometer with
MgKα radiation (hν = 1253.6 eV). The charging effect
during photoemission was taken into account using the
internal reference method, based on the N1s spectrum
of the support with the binding energy Eb = 397.6 eV.
The phase composition and structural parameters of
the catalyst samples were determined by X-ray phase
analysis (XPA) on an XRD-6000 diffractometer using
CuKα radiation. The relative content of the phases in
silicon nitride was estimated from the relative intensities
of the corresponding lines in the diffraction patterns [8].
The structural features of the support and the catalysts
were examined by raster electron microscopy (REM) on
SEM 515 Philips and Tescan LMU II instruments, as
well as by transmission electron microscopy (TEM) on
a Philips CM 30 instrument.
TPD experiments the samples were exposed to a helium
stream at temperatures within 25–900°C for 6 h. The
TPD spectra were recorded for the samples exposed
to an oxygen stream (feed rate 100 ml min–1) by the
following scheme: T = 25°C, 5 min; T = 200°C, 5 min;
T = 800°C, 1 h. The TPD spectra were taken in a helium
stream (100 ml min–1) within 25–800°C at the heating
rate of 5 deg min–1; the desorbed gases were analyzed
by mass spectrometry.
The CO adsorption on the catalyst surface was studied
by IR spectroscopy on a Bruker FTIR-22Vspectrometer.
The IR spectra were measured within 4000–400 cm–1 at
1 cm–1 resolution. Before adsorption the samples were
heated at 433 K for 4 h. The CO adsorption was carried
out at 25°C for 40 min. After recording the spectra of
the adsorbed species at the adsorption temperature the
samples were heated under evacuation and kept at the
appropriate desorption temperature for 40 min. The
spectrum was recorded for the samples cooled to room
temperature, after which the above-described procedure
The reaction of oxygen with the Si3N4-I support and
palladium catalyst surfaces was studied by temperature-
programmed desorption (TPD) of oxygen. Before the
(a)
(b)
Fig. 3. Deep oxidation of methane on Pt/Si3N4-I catalysts: (a) temperature corresponding to 50% conversion T50, °C, for (1) freshly
prepared systems and (2) systems exposed to the reaction mixture and (b) variation of the methane conversion φ, %, with temperature
T,°C, for the catalysts exposed to the reaction mixture. The Pt content, wt%: (1) 0.12, (2) 0.55, and (3) 0.87.
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759
was repeated at a higher temperature. The experimental
results are represented by a series of the IR spectra of the
adsorbed CO layers at different desorption temperatures
(T 100, 200, 300°C).
of the metallic state and exceeds the binding energy of
the latter by 0.2 eV only (Fig. 2). The observed shift of
the Pt4f7/2 line is smaller than that recorded in oxidation
of platinum (> 1 eV), which suggests the metallic nature
of the supported particles. Such increase in binding
energy is characteristic for heterogeneous catalysts
containing metal nanoparticles on the support surface,
especially those with a size of 2–5 nm. The observed
effect is associated either with a slight deficiency of
electron density on the metal crystallites, resulted from
strong interaction of the supported metal nanoparticles
with support surface segments, or with relaxation of the
hole yielded by photoemission (final-state effect) [11].
Silicon Nitride-Supported Platinum-Containing
Systems. Table 2 summarizes the main physicochemical
properties of the surface of the platinum catalysts
prepared. Chemical analysis showed that, for the freshly
prepared samples, 0.12, 0.55, and 0.87 wt% platinum
was supported on silicon nitride. The TEM examinations
of Pt/Si3N4-I revealed the presence of nanocrystalline
Pt particles on the surface of the freshly prepared
systems (Fig. 1). The (0.12 wt% Pt)/Si3N4-I sample
was characterized by the average particle size of 2 nm
(Table 2, Fig. 1a). The metal particles for the remaining
catalysts were slightly larger: The average particle size
for the (0.55 wt% Pt)/Si3N4-I sample was 3 (Table 2,
Fig. 1c), and that for (0.87 wt% Pt)/Si3N4-I, 6 nm (Table
2, Fig. 1e), respectively.
The synthesized platinum catalysts were tested in
deep oxidation of methane. Figure 3 shows how the
methane conversion varies with increasing temperature
for the freshly prepared samples and for the systems
that were preliminarily exposed to the reaction mixture
at 650°C for 3 h. It is seen that (0.12 wt% Pt)/Si3N4-I
sample is characterized by a low activity: Exhaustive
conversion of methane was not achieved up to 650°C,
and 50% conversion was observed only at 610°C
(Fig. 3a). The catalysts containing 0.55 and 0.87 wt%
Pt afford higher methane conversions than the sample
with the lowest metal loading (Fig. 3a). For (0.55
wt% Pt)/Si3N4-I and (0.87 wt% Pt)/Si3N4-I the 50%
conversion of methane was achieved at 478 and 468°C,
respectively.However,theactivityofallthethreesystems
decreases upon exposure to the reaction mixture. For the
Figure 2a shows the Pt4f core-level XPS spectra
of the Pt-containing systems examined. It is seen that,
before catalysis, the Pt4f XPS line grows in intensity
with increasing amount of the supported platinum,
thereby increasing the [Pt4f]/[Si2p] line ratio (Table 2;
Figs. 2a and 2b). The binding energy for the (0.12 wt%
Pt)/Si3N4 sample exceeded that for bulk platinum metal
(72.05 against 71.2 eV) (Fig. 2a). At the same time, the
position of the Pt4f7/2 lines (Eb = 71.4 eV) for (0.55 wt%
Pt)/Si3N4 and (0.87 wt% Pt)/Si3N4 is very close to that
(a)
(b)
Fig. 4. Conversion φ, %, of methane on the silicon nitride-supported palladium catalysts vs. temperature T,°C. (a) Freshly prepared
systems and (b) samples preliminarily exposed to the reaction mixture for 3 h at 650°C. (1) Pd/Si3N4-I, (2) Pd/ Si3N4-cal, (3) Pd/ Si3N4-
am, and (4) Pd/ Si3N4-II.
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(a)
(0.12 wt% Pt)/Si3N4 catalyst the 50% methane conversion
was achieved at 615°C only, and for the samples with
a higher metal loading, at 550–560°C (Fig. 3b).
To explain the observed deactivation, we examined
the surface properties of the platinum catalysts upon
deep oxidation of methane. Chemical analysis showed
(Table 2) that, for the sample with the lowest metal
loading, the amount of the deposited platinum remains
unchanged after catalysis (0.12 wt% Pt). For the
remaining two catalysts, it slightly decreased to 0.52
and 0.83 wt%, respectively (Table 2). This is due to
entrainment of the metallic phase (in the amount of no
greater than 5%) with the reaction products from the
support surface.
50 nm
(b)
The TEM data suggest that, during the catalytic
reaction, the metal particles on the samples surface
were agglomerated into crystallites whose size strongly
exceeded that for the freshly prepared systems. This is
clear from the TEM images shown in Figs. 1b, 1d, and
1f. On the surface of the (0.12 wt% Pt)/Si3N4-I catalyst
there were particles with sizes up to 30 nm (Table 2). At
the same time, the crystallites for the remaining samples
measured up to 70 nm (average size 11 and 16 nm,
respectively; Table 2).
5 nm
(c)
An XPS examination of the Pt/Si3N4 samples
after methane oxidation showed that the platinum
particles were not oxidized during the catalysis. After
the reaction all the catalysts were characterized by the
Pt4f7/2 binding energy of 71.1 eV, which is close to Eb
for the bulk platinum metal (Table 2, Fig. 2). However,
Table 2 shows that the [Pt4f]/[Si2p] ratio decreases due
to a decrease in intensity of the Pt4f peaks (Fig. 2a). The
most pronounced decrease was observed for the sample
with the maximal platinum loading, (0.87 wt% Pt)/Si3N4.
These changes in the Pt4f XPS signal and [Pt4f]/[Si2p]
ratio relative to the initial state may be associated with
entrainment of platinum and/or agglomeration of the
metal particles during the catalytic reaction. It should
be noted that both factors apply to the case of catalysts
with the maximum and medium platinum loadings.
At the same time, for the (0.12 wt% Pt)/Si3N4 sample
a decrease in the [Pt4f]/[Si2p] ratio is associated with
agglomeration of the metal particles on the support
surface solely.
5 nm
Fig. 5. TEM images of the silicon nitride-supported palladium
particles: (a) Pd/Si3N4-I sample before the catalytic reaction;
(b) palladium particles supported on Si3N4-II (β-phase) after
deep oxidation of methane; and (c) Pd/Si3N4-am after the
catalytic reaction.
revealed for silicon nitride-supported catalysts
with 0.045, 1, and 2.2 wt% Pt in high-temperatures
selective oxidation of methane [15]. Presumably, at
high temperatures, silicon nitride-supported platinum
catalysts can be involved in two processes under
the reaction conditions: sintering (agglomeration)
Agglomeration, like entrainment of the metal
under catalytic oxidation conditions, is also known for
other platinum catalysts [12–14]. A decrease in the
[Pt4f]/[Si2p] ratio, associated with the Pt loss, was
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of nanosized metal particles into large particles and
oxidation of large particles to PtOx, followed by
sublimation at high temperature [5, 11–14].
(a)
Silicon Nitride-Supported Palladium-Containing
Systems. Table 3 summarizes the main characteristics
of the palladium-containing catalysts examined. The
technique proposed for palladium particle preparation
allows formation of the Pd nanoparticles on the silicon
nitride surface. Catalytic examinations showed that the
Pd/Si3N4-I sample was active and stable both in the initial
state and after 3-h exposure to the reaction mixture. The
50% conversion of methane was achieved at 360°C, and
exhaustive conversion, at 400°C (Table 3, Fig. 4). By
contrast, the palladium catalyst supported on Si3N4-II
(with β-phase dominating) was completely deactivated
during the reaction, and exhaustive conversion of
methane was not achieved throughout the temperature
range examined. Comparison of the catalytic properties
of the palladium catalysts supported on silicon nitride
comprisinganamorphousfractionshowedthefollowing.
The half conversion of methane was achieved with Pd/
Si3N4-am and Pd/Si3N4-cal samples at 430 and 370°C,
respectively, which temperatures are comparable with
that for Pd/Si3N4-I. However, upon exposure to the
reaction mixture the catalytic activity decreased for both
systems.
(b)
Fig. 6. TPD spectra of О2 from palladium-containing catalysts
in relation to the sample–oxygen interaction temperature T,
°C. (D) O2 desorption, arb. units. (а) Adsorption at 25°С and
(b) oxidation at 800°C. (1) Pd/Si3N4-I-a and (2) Pd/Si3N4-I-t;
the same for Fig. 7.
The difference in the catalytic properties is not
Table 3. Silicon nitride-supported palladium catalysts
Pd3p5/2 binding
energy, eV
Average size of Pd Temperature corresponding to
particles, nm 50% conversion of CH4, °C
Metal particle
precursor
Pd loading,
wt%
Sample
BR
AR
BR
AR
Ia
IIa
Pd/Si3N4-I
Pd/Si3N4-II
Pd(C5H702)2,
toluene
0.49
0.42
335.9
335.8
337.3
337.0
4.3
5.6
5.8
5.7
360
480
360
–
Pd(C5H702)2,
toluene
0.43
335.3
337.1
2.9
3.2
430
530
Pd/SiN-am
Pd/SiN-cal
Pd(C5H702)2,
toluene
0.45
0.58
335.9
335.5
337.3
337.3
3.2
4.3
3.6
4.8
370
307
470
317
Pd(C5H702)2,
toluene
Pd/Si3N4-I-t
Pd/Si3N4-I-w
Pd(CH3C02)2,
toluene
Pd(CH3C02)2,
water
0.53
335.8
337.4
49
8.3
427
602
a I and II refer to the freshly prepared samples and those preliminarily exposed to the reaction mixture for 3 h at 650°C, respectively.
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associated with agglomeration of the particles and/or
oxidation of palladium clusters; the binding energies
for all the systems upon the catalytic reaction were
close to that for the oxidized state of palladium (PdO)
(Table 3). However, the TEM examinations of the
Pd/Si3N4-II, Pd/Si3N4-am, and Pd/Si3N4-cal samples
after the catalytic reaction revealed formation of
amorphous SiOx on the metal particle surface, which is
due to oxidation of the β-phase of silicon nitride and
the presence of amorphous fraction (Figs. 5b and 5c).
The “encapsulation” of the palladium particles by the
amorphous layer under catalytic reaction conditions is
responsible for a decrease in the activity of the metal
particles [8, 9]. Enhanced activity and stability of
palladium supported on crystalline Si3N4-I (α-Si3N4)
can be associated with modification of the electronic
properties of Pd-x particles via specific interaction of the
active phase with α-phase of silicon nitride. The Pd3p5/2
binding energy for the Pd/Si3N4-I catalyst significantly
exceeds that for other systems examined and the Pd-
metal catalyst (335.3 eV) [9]. Thus, examination of
the physicochemical properties of the palladium-
containing systems supported on silicon nitride samples
with different phase compositions and characteristics
showed the following. The most active and stable in
deep oxidation of methane is the catalyst supported on
silicon nitride dominated by α-phase. An increase in
the proportion of amorphous fraction and β-phase leads
to deactivation of the catalyst. The resulting particles
are both active and stable; they measure 5 nm, on the
average (Fig. 5a).
cm–1
cm–1
Fig. 7. IR spectra of CO adsorbed at 25°C on the surface
of palladium catalysts, recorded at different desorption
temperatures. (A) Adsorption, arb. units, and (ν) wavenumber.
prepared catalyst and the sample exposed to the reaction
mixture, half conversion of methane was achieved at
307–317°C (Table 3), and exhaustive conversion, at
377°C. The catalyst prepared with the use of an aqueous
solution was less active toward deep oxidation of
methane. For the freshly prepared Pd/Si3N4-I-a sample
the 50% conversion of CH4 was achieved at 427°C only
(Table 3), and exhaustive conversion, at 650°C. Upon
exposure to the reaction mixture the activity of the Pd/
Si3N4-w catalyst was significantly decreased (50%
conversion at 602°C).
The catalytic properties of the palladium-containing
systems are also affected by the chemical nature of the
precursors of reactive sites as well as by the preparation
conditions. In this connection, we examined the
physicochemical properties of the palladium-containing
systems supported on α-Si3N4 (Si3N4-I) in relation
to their preparation technique. In the course of deep
oxidation of methane we examined the physicochemical
and catalytic properties of the silicon nitride-supported
palladium catalysts with the use of aqueous (Pd/Si3N4-
I-a) and toluene (Pd/Si3N4-I-t) solutions of palladium
acetate.
As follows from chemical analysis, TEM, and
XPS data, Pd/Si3N4-t and Pd/Si3N4-w catalysts are
characterized by identical active phase loadings (0.58–
0.53 wt%), average particle sizes (5–9 nm), and valence
states of the Pd particles, which factors do not govern
the catalytic properties of the palladium-containing
systems. The differences in the activities of the Pd-
containing catalysts can be manifested in the stage of
surface interaction of the reactants and in the adsorption
and oxidation properties of Pd.
We showed that, all other conditions being the
same, the catalytic properties of the palladium systems
prepared with the use of aqueous and organic solutions
of Pd acetate are different. The Pd/Si3N4-I-t sample is
highly active and stable. For example, with the freshly
The interaction of oxygen with the support and
palladium-containing system surfaces was examined
by the O2 TPD technique (Fig. 6). On the surface of
neat support oxygen was adsorbed as a weakly bound,
presumably molecular, species which was exhaustively
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desorbed at 60°C. The TPD spectra of oxygen from
(a)
supported palladium catalysts contain three peaks:
a broadened desorption peak with Tmax = 90°C and
two high-temperature peaks [Tmax (Pd/Si3N4-I-w) 630,
690°C and Tmax (Pd/Si3N4-I-t) 700, 760°C].
Compared to neat support, the catalysts exhibit an
increase in the low-temperature peak area (Tmax = 90°C)
in the spectra. This is associated with superposition of
the desorption peaks of weakly bound oxygen from the
support and the palladium particle surfaces (Fig. 6).
It should be noted that the low-temperature peak of
the Pd/Si3N4-t catalyst has a larger area than does Pd/
Si3N4-w. This is due to the presence of the reactive
sites characterized by higher adsorbate–adsorbent
interaction energy. The shapes and positions of the
maxima for the high-temperature desorption peaks of
20 μm
oxygen are determined by the preparation conditions for
(b)
the palladium-containing catalysts. For the Pd/Si3N4-
I-w sample we observed a proportional growth of the
thermal desorption peak areas at Tmax = 630 and 690°C
dav = 83.0 nm
with increasing temperature of oxygen exposure. At the
same time, for the Pd/Si3N4-I-t sample the desorption
peaks are shifted to higher temperatures (Tmax 700
and 760°C), and an increase in the temperature of the
catalyst–oxygen interaction is accompanied by that
in the areas of the TPD peaks, with the peak at Tmax
=
760°C dominating. The high-temperature peaks in
the TPD spectra of the catalysts are associated with
diffusion and subsequent desorption of oxygen from
the crystal lattice of the metal oxide (PdOx) which is
thermodynamically unstable above 680°C [16, 17]. The
appearance of several thermal desorption peaks may
be associated with formation of a variety of palladium
oxide phases or an oxide–metal mixture with different
oxygen–metal binding energies.
Fig. 8. REM images of the surface of the silver catalyst
supported on granulated silicon nitride (Ag/Si3N4-III-OI).
(a) Before catalysis and (b) corresponding particle size
distribution d, nm.
an absorption band near 2075 cm–1 and that at 2000–
1800 cm–1, comprising three components (1972, 1914,
1829 cm–1). As known [16], bands above 2000 cm–1 are
characteristic for adsorption of linear CO molecules,
while the adsorption of bridged CO molecules is
manifested below 2000 cm–1. Weaker, compared to
high-coordination sites, bonding for low-coordination
To examine the adsorption properties and structural
characteristics of the silicon nitride-supported
palladium particles, we studied the CO adsorption by
IR spectroscopy. The resulting spectra (Fig. 7) contain
Table 4. Characteristics of the silicon nitride-supported silver catalysts
Sample
Particle size, nm
Ag content, wt%
Catalytic propertiesa
KEG
mol%
,
SGO
mol%
BR
AR
BR
AR
Ag/Si3N4-III-DV
Ag/Si3N4-III-PO
109
83
222
94
0.45
5
0.15
5
97.8
23.8
45.6
80.9
a
KEG is conversion of ethylene glycol, and SGO, selectivity with respect to glyoxal.
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764
KURZINA
sites is manifested at higher frequencies [16]. The band
at 2075 cm–1 is associated with adsorption of linear
CO molecules in the Pd(111) plane, and those at 1914
and 1829 cm–1, with adsorption of bridged CO in (111)
plane interstices [18]. The band at 1972 cm–1 belongs
to bridged CO molecules in the (100) plane and the
molecules adsorbed on the defect edges and steps of
metal clusters that mainly contribute to this band [18,
19].
Systems. Table 4 presents the characteristics of the
silicon nitride-supported silver systems. The X-ray
phase analysis and XFS examination revealed weak
interaction between the support and the active phase:
The amount of the actually deposited supported silver
was <1% against 5% as calculated before deposition
(Table 4). Synthesis of Ag/Si3N4-III-FR catalyst
by fractional reduction allows formation of silver
throughout the surface of the support in the form of
11–480-nm particles with the average size of 109 nm.
Notably, these particles are localized on chips, edges,
and protruding faces of the support. Like with dispersed
silicon nitride [20], of much significance for formation
of the active component nanoparticles on the support
surface are the edges and the contact sites between the
intergrown particles in the support material, i.e., energy-
uncompensated sites of metal particle formation.
The intensity of the IR bands is determined by the
preparation conditions for the palladium systems. By
contrast to Pd/Si3N4-I-w, the Pd/Si3N4-I-t catalyst is
characterized by the highest intensities of all the bands
in the IR spectra (Fig. 7). With increasing desorption
temperature for Pd/Si3N4-I-t, the absorption bands at
2075 and 1972 cm–1 disappear above all, and at the
desorption temperature of 200°C the spectrum contains
only the IR bands at 1914 and 1829 cm–1. The peaks
associated with CO adsorption completely disappear at
300°C. For Pd/Si3N4-I-w catalyst all the IR bands tend
to decrease in intensity proportionally to increasing
desorption temperature and completely disappear
at 200°C. The IR-spectroscopic data we obtained
suggest that the Pd/Si3N4-I-t catalyst, active in methane
oxidation, comprises palladium clusters with the Pd(111)
orientation dominating, and the CO adsorption sites are
characterized by higher interaction energies.
Figure 8a shows the REM image of the surface of the
sample prepared by deposition of silver trifluoroacatate
from a toluene solution (Ag/Si3N4-III-OI). It is seen
that the active component particles are uniformly
distributed over the support surface, with the particle
size of ~83 nm dominating. For the majority of the
particles, the size ranges form 23 to 400 nm (Fig. 8a).
It should be noted that the particle size distribution
(Fig. 8b) is narrower than that for the catalyst prepared
from aqueous medium. This fact suggests a uniform
size of the resulting particles. Importantly, under these
preparation conditions there was no loss of the active
component. By contrast to aqueous solution of the silver
particle precursor, an organic solvent affords exhaustive
deposition of silver (5 wt%, Table 4).
Thus, the TPD and IR examinations of the palladium
systems showed that the Pd/Si3N4-I-t and Pd/Si3N4-
I-w catalysts significantly differ in the structure
and adsorption properties of the nanoparticles. This
affects the adsorption forms of reactants, as well as
the oxidative properties and mechanism of the surface
reactions. The structure and physicochemical properties
of the active sites on the surface of the particles formed
under the preparation conditions are directly affected by
the structure of the palladium complexes adsorbed on
the support during preparation, as well as by the atomic
composition and acid-base properties of the support
surface. This suggests the key importance of the nature
of the solvent in formation of palladium particles active
in deep oxidation of methane. The presence of water may
affect the decomposition of the palladium complexes
on the support surface, and the Pd yielded by reduction
exhibits a comparatively low catalytic activity. Also, the
solvent can strongly modify the surface characteristics
of the support.
We examined the catalytic properties of the prepared
silver systems in relation to the oxygen content in the
reaction mixture for O2/EG ratios within 0.8–1.3 at 580
and 560°C. Also, we studied the catalytic behavior of
the systems at 530–600°C for the O2/EG ratio of 1.2. We
found (Table 4) that the Ag/Si3N4-III sample prepared
from toluene is the most active among the catalysts
examined. The selectivity with respect to GO throughout
the O2/EG ratio range examined was 45–50%, the EG
conversion being 80–81%.
The catalytic activity of the system prepared by us is
comparable with that of aluminosilicate-supported silver
catalysts [10, 21]. The supportedAg catalysts are slightly
superseded in catalytic activity by polycrystalline silver
which is conventionally used as catalyst for synthesis of
glyoxal by catalytic oxidation of ethylene glycol [21]
Silicon
nitride-Supported
silver-containing
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RELATIONSHIPS IN FORMATION OF SILICON NITRIDE-SUPPORTED METAL NANOPARTICLES
765
(conversion 95–100 against ~90% and selectivity with
respect to glyoxal, 45–55 against 55–65%). However,
supported catalysts offer major advantages over bulk
analogs in terms of enhanced sintering resistance of the
active surface under high-temperature conditions, less
carbon deposition, etc. Owing to the unique properties of
silicon nitride (in particular, high thermal conductivity)
utilized in the catalytic systems prepared by us, is it
possible to synthesize glyoxal over a broader, compared
to other supported Ag catalysts, temperature range (up
to 650°C) without significant loss of selectivity with
respect to the target product and without formation of
carbon deposition products.
medium is the most stable and active. Enhanced
selectivity with respect to glyoxal can be associated with
uniform size of the active component particles which
are evenly distributed over the support surface. An
important finding of our study is that we did not reveal
any significant accumulations of carbon deposition
products on the system surface. This fact suggests an
essential role played by the nature and structural features
of the support in carbon deposition adversely affecting
the activity of the supported and bulk catalysts in partial
oxidation of ethylene glycol.
General relationships in rormation of metal
Particles. As known, chemical interaction between the
support and the metal are essential in formation of active
and stable nanoparticles [9, 11, 14, 22, 23]. Of special
significanceisthechemicalaffinityinthecaseofplatinum
catalysts supported both on the oxide surface [12–15,
23] and on silicon nitride. For example, the activity of
silica-supported catalysts is lost via formation of PtO2
and it subsequent sublimation in selective oxidation of
methane. By contrast, alumina-supported Pt catalysts
are fairly stable and active due to formation of a stable
spinel structure (PtAl2O4), which stabilizes platinum
on the support surface. The lack of chemical affinity of
platinum for silicon nitride is responsible for a major
(up to 70%) loss under the preparation conditions. It was
shown that the performance characteristics of the silicon
nitride-supported platinum systems strongly depend
on the degree of dispersion of the active component.
Weak chemical affinity of platinum to silicon nitride
leads to weak interaction with the support and favors
agglomeration and subsequent deactivation of the
materials.
To explain the difference in the catalytic properties
of the systems prepared from aqueous and organic
media, we examined their morphological properties
after the catalytic processes. On the Ag/Si3N4-III-FR
surface there were particles with the average size of
222 nm, nonuniformly distributed over support surface.
Comparison of the morphologies of the samples before
and after the catalysis revealed agglomeration of the
active phase, accompanied by a twofold increase in
the average particle size. On the support surface there
were agglomerates with the size of up to 2 μm, formed
from smaller particles occurring in close proximity to
one another under the reaction conditions. The XFS
examination showed that, in the sample prepared from
aqueous solution, agglomeration is paralleled by major
entrainment (up to 70%) of catalytically active phase;
after catalysis the amount of the active component was
0.15 wt% (Table 4).
In the case of Ag/Si3N4-III-OI sample prepared
from organic medium we did not reveal any significant
changes in morphology. The average particle size was
94 nm. Also, there were a small number of agglomerates
ranging in size from 250 to 400 nm, whose proportion
did not exceed 0.02. Thus, the silver particles prepared
by deposition from an organic solvent exhibit high
stability under the catalytic process conditions. The lack
of large particles with sizes above 500 nm is consistent
with the fact that the sites of deep oxidation of ethylene
glycol to CO2 can be formed with participation of large
silver particles. It should also be noted that, for the
Ag/Si3N4-III-OI sample, the XFS examination did not
reveal any significant loss of the active component after
the reaction mixture exposure. The content of the active
component on the support remained unchanged.
We showed that the properties of the palladium
catalysts are determined by the phase composition of
silicon nitride and chemical affinity. For example, the
Pd catalyst supported on silicon nitride with the α-phase
dominating is the most active and stable. The palladium
metal particles have an average size of 4 nm and are
uniformly spread over the support surface (Table 3,
Fig. 5a). The Si3N4-I-supported palladium particles are
agglomeration-resistant, as validated by the TEM and
XPS data. High activity and stability of the Pd catalyst
can be associated with the chemical interaction between
Pd and the silicon nitride support. According to the XPS
data, the Pd3d5/2 binding energy for freshly prepared
Pd/Si3N4-I catalyst exceeds that for neat Pd (335.9
against 335.2 eV). However, the shapes of the Pd3d5/2
XPS peaks and the corresponding Auger spectrum are
Thus, the catalyst deposited from nonaqueous
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KURZINA
comparable with those of metallic palladium rather
than of oxidized Pd2+ state. After the catalytic reaction
the Pd3d5/2 binding energy (337.3 eV) corresponds to
the partially oxidized state of Pd2+ (PdO). The Pd3d5/2
binding energy of 336.1 eV instead of 335.3–335.4 eV
for Pd metal was also revealed for (0.75 wt% Pd)/α-Si3N4
catalyst [23], which value exceeds that for SiO2- and
Al2O3-supported Pd. It should be noted that the binding
energy for Pd/α-Al2O3 corresponds to that for neat Pd
metal (335.2 eV). This is presumably associated with
modification of the electronic and structural properties
of Pd metal particles due to specific interaction of
palladium with the α-phase of silicon nitride. A change
in the electron density of the metal particles affects
the adsorption of the reactants and the surface reaction
mechanism. An increase in the electron density on the
metal particles favors oxygen adsorption as species
active in deep oxidation of methane. High activity of
the palladium catalyst can also be due to a high rate
of the redox (Pd  PdO) reactions proceeding during
catalysis.
found that the hydrophobic/hydrophilic balance in the
precursor solutions can be of decisive importance in
formation of active particles. The presence of water
can affect the chemical/elemental composition of the
surface. Formation of palladium and silver particles in
an aqueous solution proceeds by a mechanism different
from that operating in the case of organic solution.
This is responsible for formation of nanoparticles with
different redox, adsorption, and catalytic properties.
The use of an organic medium as solvent in the stage of
preparation of the catalytic systems allows formation of
an active catalyst for partial oxidation of ethylene glycol
into glyoxal and deep oxidation of methane.
CONCLUSIONS
(1) The presence of amorphous fraction and β-Si3N4
modification in the support leads to formation of
palladium particles that exhibit low activity in deep
oxidation of methane. The most active and stable in deep
oxidation reaction are palladium catalysts supported on
Si3N4 with the α-phase dominating.
The specific interaction between palladium and
α-Si3N4, yielding active particles, can be associated
with the elemental composition of silicon nitride
surface. Analysis of the Si2p signal in the XPS spectra
of α-Si3N4 showed that the chemical state of silicon
in the SiOx oxide contributes with 13% to the peak.
It was calculated that the [N1s]:[Si2p] ratio of 1.36
does not correspond to the stoichiometry of neat Si3N4
([N1s]:[Si2p] = 1.33). The surface oxygen enters in the
composition of either surface nonstoichiometric silicon
oxide or silicon nitride. The available structural data on
α-Si3N4 suggest that this is, in fact, silicon oxynitride in
which some nitrogen atoms are replaced by oxygen and
there exist nitrogen of silicon vacancies [24]. A number
of studies also showed that the α-Si3N4 structure could
be stabilized by the appearance of Si3+ in the valence-
compensated system with oxygen atoms [25]. This may
be specifically responsible for a higher stability of the
α-Si3N4 phase at high partial oxygen pressures and low
temperatures [26, 27]. The presence of oxygen in the
α-Si3N4 structure leads to strong chemical interaction
between palladium and the support.
(2) The redox, adsorption, and catalytic properties
of silicon nitride-supported palladium and silver
nanoparticles are strongly dependent on the nature of
the precursor solution utilized.
(3) The Pd and Ag systems prepared from an organic
medium are highly active and stable in deep oxidation
of methane and partial oxidation of ethylene glycol into
glyoxal and are promising for preparation of catalytic
composition thereof.
ACKNOWLEDGEMENT
This study was financially supported in part by the
Russian Foundation for Basic Research (project no. 09-
03-00604a) and Federal Target Program “Scientific and
Pedagogic Personnel of Innovative Russia.”
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