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164
MnOx,SiO2,TiO2/Ti AND CoOx,SiO2,TiO2/Ti COMPOSITES
FORMED BY COMBINATION OF METHODS OF PLASMA
ELECTROLYTIC OXIDATION AND IMPREGNATION
M.S. Vasilyevaa,b
, V.S. Rudneva,b,*
, A.Yu. Ustinova,b
, M.A. Tsvetnova
a Far Eastern Federal University, Vladivostok, Russia
b Institute of Chemistry, Far-Eastern Branch, Russian Academy of Sciences,
Vladivostok, Russia
Abstract
Silicon-containing oxide layers deposited on titanium using the plasma
electrolytic oxidation (PEO) method were modified with manganese and cobalt
compound through impregnation followed by annealing. The MnOx,SiO2,TiO2/Ti
composites catalyze the CO oxidation into CO2 at temperatures above 100°C, whereas
the CoOx,SiO2,TiO2/Ti composites do the same above 200°C. Nanosized particles
have been found on the surface of the composites under study: for CoOx,SiO2,TiO2/Ti
granules of a diameter of a few dozen nm; for MnOx,SiO2,TiO2/Ti nanowhiskers
consisting predominantly of manganese oxides. The presence of manganese-
containing nano-whiskers substantially increases the catalyst specific surface, thus
facilitating the attainment of higher degree of transformation of initial gaseous
substances.
Keywords: plasma electrolytic oxidation; SiO2,TiO2 coating; manganese oxides;
cobalt oxides; CO oxidation.
Introduction
Recently, transition metal oxides, including those deposited on various
substrates (silica gel, aluminum and titanium oxides, metals, and alloys), have been
extensively applied as heterogeneous catalysts in chemical and oil processing industry
as well as in cleaning industrial gaseous wastes and automobile transport exhaust
gases (14). Application of metals as substrates enables one to manufacture
mechanically and thermally resistant catalysts of different shapes, including those of
complex cellular structures, and catalysts with high thermal transfer coefficients: the
latter is crucial, in particular, in producing metallic microreactors for coupled
reactions (35).
Plasma electrolytic oxidation (PEO) is one of unconventional and promising
methods used for fabrication of oxide catalysts on metal substrates (6–16). The PEO
method, which comprises the buildup of anodic oxide layers on metals and alloys in
the near-anode area of spark and microarc electric discharges, enables one to obtain
oxide layers consisting not only of the treated metal oxide (as during conventional
anodization), but also of electrolyte components (17–19). In many cases, PEO layers
are characterized by developed and defect surface and can be efficiently impregnated
in aqueous salt solutions (6–9). In the process of catalysts fabrication through
combination of the PEO method with impregnation and annealing, the PEO layer
165
serves as an oxide one or as the secondary substrate (here, metal or alloy constitute
the primary substrate).
To deposit the catalytically active substance by the impregnation/annealing
method, it appears efficient to use PEO layers formed in the silicate electrolyte
(Na2SiO3) having relatively high water absorption (9), specific surface area, and
porosity (20). Manganese and cobalt oxides are known to be among the most active
catalysts of CO oxidation (2125).
The objective of the present work was to study the structure and catalytic
activity of MnOx,SiO2,TiO2/Ti and CoOx,SiO2,TiO2/Ti composites obtained by
impregnation of silicon-containing oxide layers deposited on titanium using the
plasma electrolytic oxidation (PEO) method followed by annealing.
Materials and methods
Titanium plates (25 mm × 5mm × 1 mm in size) of VТ1-0 alloy were used for
plasma electrolytic oxidation. Prior to anodizing, the samples were polished
mechanically and chemically with a mixture of concentrated acids HF: HNO3 (1:3
volume ratio) at 60–80oС for 2–3 s, and then rinsed in distilled water and dried in air.
Distilled water and commercial reagents of the grades specified below were
used in solutions preparation: Na2SiO3·9H2O (pure grade), Mn(NO3)2·4H2O
(analytically pure grade), Co(NO3)2 (pure grade).
Oxide coatings were formed galvanostatically on anode-polarized titanium in
electrolyte. A coil pipe cathode of stainless steel (diameter 0.5 cm) was pumped with
cold water.
The PEO process was carried out in a thermally stable glass vessel of a volume
of 1000 ml. The electrolyte in a vessel was stirred using a magnetic stirrer. A TER4-
63/460H thyristor unit (Russia) with unipolar pulse current was used as a power
source.
The silicon-containing oxide coatings on titanium were formed at an effective
current density of 0.1 A/cm2 or 0.2 A/cm
2 in an aqueous 0.1 M Na2SiO3 electrolyte
for 10 min. After the PEO process, the samples were rinsed with distilled water and
dried in air at room temperature.
Deposition of the active substance was carried out by impregnation of
SiO2+TiO2/Ti composites in aqueous 0.1 M Mn(NO3)2 or 0.1 M Со(NO3)2 solutions
with subsequent annealing in air in a muffle furnace at 500°C for 4 hours.
To obtain the information on the coatings morphology and element
composition, the methods of X-ray diffraction analysis (XRD), transmission electron
microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) were used. In the
first method, a ZEISS ULTRA 55 scanning electron microscope with a Thermo
Scientific energy-dispersive X-ray microanalysis (EDX) system was used. The depth
of the analyzed layer was about 1 µm. Simultaneously, the surface electron
microscopy images were obtained using the above device. To obtain TEM images, a
Hitachi H-8100 electron microscope with an EDAX DX-4 energy-dispersive X-ray
detector was used. The XPS spectra were recorded on a Specs high-vacuum device
(Germany) using a 150-mm electrostatic hemispheric analyzer. MgK radiation was
used for ionization. The depth of the analyzed surface layer was about 25 nm. The
spectra calibration was performed on C1s-lines for hydrocarbons, whose energy was
taken to be equal to 285.0 eV. Bombardment with argon ions having the energy of
166
5000 eV was applied for surface layers etching. The etching rate was about 0.1 Å/s;
the etching time was 5 minutes.
The phase composition was determined by the method of X-ray diffraction
analysis (XRD) on a D8 ADVANCE diffractometer (Germany) in CuK radiation.
Identification of the compounds contained in the samples under study was performed
in the EVA automatic search mode using the PDF-2 database.
The catalytic activity of the cobalt- and manganese-containing composites was
investigated in the CO oxidation reaction using a BI-CATflow catalytic installation
(Institute of Catalysis, Siberian Branch, Russian Academy of Sciences, Novosibirsk).
Four planar titanium plates (25 mm х 25mm x 1 mm) with the cobalt compounds
sprinkled with quartz filling agent were placed into a quartz reactor of a volume of 3
cm3. The starting reaction mixture consisted of 79% Ar, 20% O2, and 1% CO. The gas
flow rate was 70 ml/min. The measurements were performed in the range from the
room temperature to 500°С. The rate of the temperature change in the reactor was
10°Сmin-1
. The composition of the gas mixture was determined in 30 min after
establishment of the specified temperature in the reactor. The quantitative
determination of the composition of the gaseous products was performed by means of
a РЕМ-2М gas analyzer (Institute of Catalysis, Siberian Branch, Russian Academy of
Sciences, Novosibirsk).
The coatings specific surface area, volume, and average pore size were
determined from the isotherms of N2 adsorption at T=77 K. Measurements were
carried out on a Sorbtometer-M device. The obtained experimental data were
processed by the BET method, t-method based on Gregg and Sing standard
adsorption, and BJH (Barrett, Joyner and Halend) method (26).
Results and discussion
According to the data of energy-dispersive X-ray spectral analysis that
provides the averaged element composition for the layer of a thickness of 1 µm, the
surface layer of both modified coatings contains small amounts of carbon and
titanium and significant amounts of oxygen and silicon (Table 1). Cobalt-containing
surface layers contain 9 at. % Co, while manganese-containing ones 12 at. % Mn.
Thus, it is evident that cobalt and manganese compounds are present on the surfaces
of respective samples. High silicon concentration in surface and bulk layers enables
one to assume the presence of amorphous silicon compounds (probably, silica) in
coatings.
Table 1 Phase and element composition of the surface part (thickness ~1 µm) of Co-
and Mn-containing oxide layers on titanium from the data of energy-dispersive X-ray
spectral microanalysis
Composite Phase
composition
Element composition
(at %)
С O Si Ti Co Mn
СоОх,SiO2,TiO2/Ti TiO2 (rutile,
anatase)
2.6 66.2 19.5 3.1 8.7 -
MnОх,SiO2,TiO2/Ti TiO2 (rutile),
Mn2O3
3.1 63.8 17.3 3.5 - 11.9
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According to the data of X-ray diffraction analysis, titanium dioxide in the
rutile modification and manganese oxide Mn2O3 crystallize on the surface of
manganese-containing samples (Table 1). The thermal decomposition of manganese
nitrate is known (27) to proceed according to the scheme:
t=180°C
Mn(NO3)2 MnO2 + 2NO2 + O2
At higher temperatures (530600°C), MnO2 transforms into Mn2O3. In our
case, the formation of the crystalline Mn2O3 in the process of annealing at lower
temperature (500°C) must be the result of interaction between the precursor and the
surface of silicon-containing oxide layers. According to (28), manganese cations
interact with the silica surface yielding, due to heating of such systems, different
manganese oxides and silicates.
In our case, small MnO2 reflections in X-ray images or their absence at studies
of manganese-containing coatings annealed at temperatures up to 500°C must be
related to the oxide predominant amorphous state. The latter assumption is
corroborated by the results of X-ray photoelectron spectroscopy (XPS, the analyzed
layer thickness ~3 nm). The Mn 2p binding energy Eb (642.0 eV) for the manganese-
containing coating on titanium annealed at 500°C indicates to the fact that manganese
must be present there as MnO2 (Table 2, Fig. 1a). Upon the surface etching, the Mn
2p changes to 641.5 eV and the spectrum shape undergoes virtually no changes,
which enables one to conclude on the manganese presence as Mn2O3, Fig. 1b.
On the basis of the presented data in Table 2 one can conclude that on the
surface of the Mn-containing coatings oxygen is present in several states: in the
structure of SiO2 (Еb O (1s) = 533.6 eV) and of MnOx (Еb O (1s) = 529.8 eV). Higher
manganese content and lower oxygen content, as compared to those in deeper layers,
were observed during the coatings study (compare the energy-dispersive analysis
(Table 1) and XPS (Table 2) data).
168
Fig. 1 Mn 2p XPS spectra of surface layers of MnОх,SiO2,TiO2/Ti composite.
Table 2 Characteristics of the surface of Mn-containing coating from the data of XPS
Surface
type
Surface chemical composition
Si (2p) C (1s) O (1s) Mn (2p3/2)
Еb
(eV)
C
(at%)
Еb
(eV)
C
(at%)
Еb
(eV)
C
(at%)
Еb
(eV)
C
(at%)
Before
etching 104.2 19.5 285.0 7.7
529.8
533.6
22.4
35.1 642.0 13.2
After
etching 104.2 21.7 285.0 3.1
529.6
533.4
20.9
37.1 641.5 16.2
Only crystalline TiO2 in rutile and anatase modifications was found on the
surface of cobalt-containing samples (Table 1). According to the literature data [29],
at 200300°C decomposition of cobalt nitrate is accompanied with formation of
Co3O4, which transforms into CoO only at temperatures above 900°C. The absence of
cobalt-containing crystalline phases can be related to the fact that upon deposition
cobalt ions could also interact with the silica surface and spread over the substrate
surface without formation of an individual surface phase or diffuse through pores
inside the oxide coating. Indeed, according to the XPS data, in the surface layers
(thickness ~3 nm) of Co-containing coatings one observes several-fold lower amounts
169
of cobalt and, simultaneously, higher amounts of carbon and silicon than in average
over the outer coating layer of a thickness of 1 µm (compare Tables 1 and 3).
Table 3 Characteristics of the surface of Co-containing coating from the data of XPS
Surface
type
Surface chemical composition
Si (2p) C (1s) O (1s) Со (2p3/2)
Еb
(eV)
C
(at%)
Еb
(eV)
C
(at%)
Еb
(eV)
C
(at%)
Еb
(eV)
C
(at%)
Before
etching 103.2 24.5
285.
0 26.9
530.7
533.0
6.4
39.2
781.
0 3.0
After
etching 103.4 30.7
285.
0 11.6
529.9
532.7
5.8
46.8
781.
7 5.2
As regards the cobalt state, in accordance with the XPS data, it must be present
as the diamagnetic oxide Co2O3, which could be indicated by the absence of shake-up
satellites in the 2p spectrum of Co, whereas in the subsurface layer (upon etching) one
observes cobalt oxidized to higher degree, and that is mostly or completely
paramagnetic (Fig. 2). On the basis of the presented data in Table 3 one can conclude
that on the surface of the Co-containing coatings oxygen is present in several states: in
the structure of SiO2 (Еb O (1s) = 533.0 eV) and of CoOx (Еb O (1s) = 530.7 eV).
170
Fig. 2 Co 2p XPS spectra of surface layers of CoOx,SiO2,TiO2/Ti composite.
Using the method of scanning electron microscopy, significant differences in
the morphology of the coatings under study were established as well (Fig. 3).
171
Fig. 3 SEM images of the surface of MnОх,SiO2,TiO2/Ti (a, b) and
CoOx,SiO2,TiO2/Ti (c, d) composites.
Numerous areas covered by a dense layer of nanostructures of a diameter of
less than 50 nm and a length of up to 1 µm (nanowhiskers) are present on the surface
of manganese-containing oxide coatings (see Figs. 3ab). Nanowhiskers are localized
predominantly around pores and in the grooves between coral-like structures (Fig. 3a).
Nanowhiskers are preserved on the surface in the unchanged form even upon
prolonged annealing at 500°C.
Estimation of the nanowhiskers composition by the X-ray structural analysis
method demonstrated that they contained manganese (1423.0 at%), silicon (up to
15.0 at. %), oxygen (5565 at%) as well as insignificant amounts of titanium (3.05.0
at%) and carbon (3.06.0 at%). However, the presented data do not allow concluding
on the whiskers chemical composition (whether they consist of individual silicon and
manganese oxides or their structure is of a mixed character): since the whiskers
thickness is very small, relatively high contents of silicon and titanium can be due to
their presence in lower coating layers captured by the probing beam.
The transmission electron microscopy (TEM) with the X-ray spectral
microanalysis allowed more accurate determination of the structure and composition
of the obtained nanocrystals. According to TEM images (Fig. 4), nanowhiskers are
heterogeneous with respect to thickness and have numerous defects. The X-ray
spectral microanalysis demonstrated the absence of silicon and titanium in these
objects and the presence of oxygen and manganese at the atomic ratio O : Mn = 2.5 :
1. Nanowhiskers formed on the coating surface must consist of manganese oxides and
contain, possibly, MnO2 and Mn2O3.
Fig. 4 TEM image of whiskers on the surface of the MnОх,SiO2,TiO2/Ti composite.
The above nanostructures were not found on the surface of Co-containing
coatings (Figs. 3cd). Their surface has flat areas, coral-like structures and grooves,
and pores as well as areas consisting of massifs of nanosized round particles.
The parameters of the porous structure of Mn- or Co-containing samples
calculated on the basis of N2 adsorption isotherms are shown in Table 4. In both cases,
the specific surface areas of the formed composite coatings are rather low because of
small thickness of the oxide coating (not larger than 10 µm). For the Mn-containing
coating, the specific surface area is equal to 0.388 m2/g, whereas for the Co-
172
containing one it is an order of magnitude smaller and constitutes just 0.049 m2/g.
Probably, the larger specific surface area of Mn-containing coatings is due to the
presence of nanowhiskers of manganese oxides. Besides, Mn-containing coatings are
characterized by higher porosity than Co-containing ones, whereas the pore size is, in
opposite, several-fold smaller. The pore radii of the Mn-containing coating calculated
on the basis of the cylindrical pore model are around ~200 nm, which is comparable
with the distance between nanowhiskers (Fig. 3b). For the Co-containing coating, this
value is around 600 nm. In the latter case, it appears difficult to assign pores of this
size to some components of the coating because of its complex heterogeneous
structure (Fig. 3cd). Here, the pore lengths are virtually equal in both cases and
comparable to the thickness of the silicon-containing coating (10 µm), which
indirectly indicates to analysis of both the outer layer formed on the coating surface as
a result of thermal decomposition of manganese or cobalt nitrates and the bulk of the
oxide coating.
Table 4 Characteristics of the porous structure of Mn- and Co-containing coatings
Composite
Specific
surface
area
(m2/g)
Volume of
pores
(cm3/g)
Pore
radius
(nm)
N (g-1
) l (µm)
СОх,SiO2,TiO2/Ti 0.049 0.044 575.4 2.08∙1010
8.1
MnОх,SiO2,TiO2/Ti 0.388 0.062 192.6 2.92∙1011
7.3
Note: N – pore quantity in 1 g of coating (on the cylindrical pore model); l – pore
length (on the cylindrical pore model).
The formed composites were studied in the catalytic oxidation of carbon
monoxide one of the most harmful toxic substances present in exhaust gases of
combustion engines and waste and ventilation gas emissions.
Without the surface modification by manganese and cobalt oxides, the
SiO2,TiO2/Ti composite has low activity in the CO oxidation reaction. СО oxidation
in the presence of this composite starts in the temperature range around 500°C. Under
the experimental conditions, the СО oxidation at this temperature does not exceed
15% (Fig. 5).
173
Fig. 5 The temperature dependence of CO conversion for SiO2,TiO2/Ti (curve 1),
CoOx,SiO2,TiO2/Ti (curve 2) and MnOx,SiO2,TiO2/Ti (curve 3).
The MnOx,SiO2,TiO2/Ti and CoOx,SiO2,TiO2/Ti composites are rather active
catalysts of СО oxidation to СО2. They manifest the catalytic activity in the CO
oxidation at temperatures above 100 and 200°C, respectively (Fig. 5). In other words,
the manganese-containing structures manifest higher activity in comparison with the
cobalt-containing ones. Under the experimental conditions (gas flow rate, gas
concentration, and area of catalyst loaded into the reactor), the CO half-conversion
temperature T50 for manganese oxide composites is equal to 150 °C, which is by
100°C lower than for cobalt oxide composites.
The activity of oxide catalysts is known to depend not only on the nature of
deposited oxides, but also on the composition, structure, and porosity parameters of
catalysts, which, in their turn, are determined by the formation conditions [1].
As was mentioned in some works, cobalt oxides are more active catalysts of
CO oxidation than manganese oxides [1, 21, 30]. In a number of cases, cobalt oxides
are capable to catalyze this reaction even at negative temperatures (°C) [24]. Higher
activity of the formed MnOx,SiO2,TiO2/Ti composites in comparison with
CoOx,SiO2,TiO2/Ti must be determined as by higher manganese content compared to
cobalt (13.2 Mn; 3.0 Со at%, respectively, see Table 2, 3) as by more developed
surface related, in particular, to the presence of manganese-containing nanowhiskers.
Conclusions
The MnOx,SiO2,TiO2/Ti and CoOx,SiO2,TiO2/Ti obtained by impregnation of
silicon-containing PEO-layers on titanium with subsequent annealing are catalytically
active in CO oxidation. The manganese-containing layers catalyze the CO conversion
into CO2 at temperatures above 100°C, whereas the cobalt-containing structures
manifest the activity at temperatures above 200°C. Nanosized particles were found on
the surface of both composites under study. Granular particles of a diameter of a few
dozen nanometers were present on the surface of cobalt oxide layers on titanium,
while nanowhiskers consisting predominantly of manganese oxides and characterized
by rather high stability in the studied temperature range were found on the surface of
manganese oxide layers.
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