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

167

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.

References

174

1. Boreskov GK: Heterogeneous catalysis. Мoscow, Nauka, 1988 (In Russian).

2. Satterfield CN: Heterogeneous catalysis in practice. New York, McGraw Hill Book

Comp., 1980.

3. Popova NM, Burmestrov SV: Application of catalysts on metal substrates in

catalysis, in: Catalytic hydrogenation and oxidation, Alma-Ata, Nauka Kaz. SSR,

1975, pp. 140149 (In Russian).

4. Brück R: Development status of metal substrate catalysts, in: Bode H (Ed.),

Materials aspects in automotive catalytic converters. Weinheim, Wiley-VCH, 2002,

pp. 1930.

5. Makarshin LL, Parmon VN: Microchannel catalytic systems for hydrogen

energetics. Russ. J. Gen. Chem. 2007 77 (4) 676684.

6. Tikhov SF, Chernykh GV, Sadykov VA, Salanov AN, Alikina GM, Tsybulya SV,

Lysov VF: Honeycomb catalysts for clean-up of diesel exhausts based upon the

anodic-spark oxidized aluminum foil. Catal. Today 1999 53 (4) 639646.

7. Vasilyeva MS, Rudnev VS, Ustinov AYu, Korotenko IA, Modin EB, Voitenko

OV: Cobalt-containing oxide layers on titanium, their composition, morphology, and

catalytic activity in CO oxidation. Appl. Surf. Sci. 2010 257 (4) 1239–1246.

8. Vasilyeva MS, Rudnev VS, Ustinov AYu, Kuryavyi VG, Sklyarenko OE,

Kondrikov NB: Nickel- and copper-containing oxide films on titanium. Russ. J. Inorg.

Chem. 2009 54 () 17081712.

9. Vasil’eva MS, Rudnev VS, Sklyarenko OE, Tyrina LM, Kondrikov NB: Titanium-

supported nickel–copper oxide catalysts for oxidation of carbon (II) oxide Russ. J.

Gen. Chem. 2010 80 (8) 1557–1562.

10. Meyer S, Gorges R, Kreisel G: Preparation and characterisation of titanium

dioxide films for catalytic applications generated by anodic spark deposition. Thin

Solid Films 2004 450 (2) 276–281.

11. Patcas F, Krysmann W: Efficient catalysts with controlled porous structure

obtained by anodic oxidation under spark-discharge. Appl. Catal. A. 2007 316 (2)

240–249.

12. Patcas F, Krysmann W, Honicke D, Buciumana F-C: Preparation of structured

egg-shell catalysts for selective oxidations by the ANOF technique. Catal. Today

2001 69 (14) 379–383.

13. Yu X, Chen Li, He Y, Yan Z: In-situ fabrication of catalytic metal oxide films in

microchannel by plasma electrolytic oxidation. Surf. Coat. Tech. 2015 269 30–35.

14. Shtefan VV, Smirnova AYu: Electrochemical Formation of Cerium-containing

oxide coatings on titanium. Russ. J. Appl. Chem. 2013 86 (12) 1842–1846.

15. Liu D, Jiang B, Zhai M, Li Q: Cu (II)-containing micro-arc oxidation ceramic

layers on aluminum: formation, composition, and catalytic properties. Mater. Sci.

Forum. 2011 695 2124.

16. Bayati MR, Moshfegh AZ, Golestani-Fard F: Synthesis of narrow band gap

(V2O5)x–(TiO2)1-x nano-structured layers via micro arc oxidation. Appl. Surf. Sci.

2010 256 (9) 29032909.

17. Chernenko VI, Snezhko LA, Papanova II: Preparation of coatings by plasma

electrolytic oxidation. Leningrad, Khimiya, 1991 (in Russian).

18. Walsh FC, Low CTJ, Wood RJK, Stevens KT, Archer J, Poeton AR, Ryder A:

Trans. Inst. Metal Finish. 2009 87 (3) 122–135.

175

19. Jiang BL, Wang YM, in: Dong H (Ed.), Surface Engineering of Light Alloys.

Oxford-Cambridge-New Delhi, Woodhead Publishing Limited, 2010, pp. 110–154.

20. Vasilyeva MS, Artemyanov AP, Rudnev VS, Kondrikov NB: The porous

structure of silicon-containing surface layers formed on titanium by plasma-

electrolytic oxidation. Protection of metals and physical chemistry of surfaces 2014

50 (4) 499–507.

21. Gorohovatskiy YaB (Ed.): Catalytic properties of substances. Kiev, Naukova

Dumka, 1977 (in Russian).

22. Zaki MI, Hasan MA, Pasupulety L, Fouad N, Knozinger H: CO and total

oxidation over manganese oxide supported on CH4 and catalysts ZrO2, TiO2,

TiO2Al2O3,SiO2Al2O3. New J. Chem. 1999 23 11971202.

23. Voß M, Borgmann D, Wedler G: Characterization of alumina, silica, and titania

supported cobalt catalysts. J. Catal. 2002 212 (1) 10–21.

24. Jansson J, Palmqvist AEC, Fridell E, Skoglundh M, Österlund L, Thormählen P,

Langer V: On the catalytic activity of Co3O4 in low-temperature CO oxidation. J.

Catal. 2002 211 (2) 387397.

25. Thormählen P, Skoglundh M, Fridell E, Andersson B: Low-temperature CO

oxidation over platinum and cobalt oxide catalysts. J. Catal. 1999 188 (2) 300310.

26. Gregg S, Sing K: Adsorption, surface area and porosity, second ed. London, New

York, Paris, San Diego, San Francisco, San Paulo, Sydney, Tokyo, Toronto,

Academic Press Inc., 1982.

27. Gallagher PK, Schrey F, Prescott B: The thermal decomposition of aqueous

manganese (II) nitrate solution. Thermochim. Acta 1971 2 (5) 405412.

28. Nohman AKH: Precursor-support interactions in silica-supported manganese

oxide catalysts. Monatsh. Chem. 2004 135 (3) 269–278.

29. Wang Ch-B, Lin H-K, Tang Ch-W: Thermal characterization and macrostructure

change of cobalt oxides. Cat. Lett. 2004 94 (12) 6974.

30. Boreskov GK, Marshneva GI: Mechanism of oxidation of carbon monoxide on

transition metal oxides. Doklady AN SSSR 1973 213 (1) 112115 (In Russian).