Thermal analysis of activated carbons modified with silver metavanadate

Thermal analysis of activated carbons modified with silver metavanadate

Thermochimica Acta 541 (2012) 42–48 Contents lists available at SciVerse ScienceDirect Thermochimica Acta journal homepage: www.elsevier.com/locate/...

658KB Sizes 2 Downloads 26 Views

Thermochimica Acta 541 (2012) 42–48

Contents lists available at SciVerse ScienceDirect

Thermochimica Acta journal homepage: www.elsevier.com/locate/tca

Thermal analysis of activated carbons modified with silver metavanadate Joanna Goscianska, Piotr Nowicki, Izabela Nowak, Robert Pietrzak ∗ Faculty of Chemistry, Adam Mickiewicz University in Pozna´ n, Grunwaldzka 6, 60-780 Pozna´ n, Poland

a r t i c l e

i n f o

Article history: Received 27 January 2012 Received in revised form 17 April 2012 Accepted 21 April 2012 Available online 28 April 2012 Keywords: Activated carbon Waste materials Silver metavanadate Structure/texture Supports for silver Thermogravimetric analysis

a b s t r a c t The effect of silver metavanadate doping on physicochemical properties and thermal behaviour of the activated carbons obtained from waste materials was investigated. The carbonaceous supports were subjected to carbonisation at 400 or 600 ◦ C. The samples carbonised at 600 ◦ C have much more developed surface area and porous structure than the analogous samples obtained at 400 ◦ C. Impregnation of activated carbons with silver metavanadate leads to a decrease in their surface area and pore volume. According to thermal analysis (TG, DTG) in the samples containing 1 and 3 wt.% of silver metavanadate, AgVO3 is fully decomposed to do vanadium oxide and Ag, with no intermediate products, while in the samples containing 5 wt.% AgVO3 , this salt is decomposed to vanadyl species as intermediate compounds at 350 ◦ C before the formation of V2 O5 at 500 ◦ C. Moreover, in all samples impregnated with silver metavanadate the nanoparticles of silver undergo crystallisation leading to reduction of Ag+ ions from the vanadium salt to Ag0 . © 2012 Elsevier B.V. All rights reserved.

1. Introduction The activated carbons with functional properties have been extensively studied in view of their important roles in diverse fields of application. Their high surface area and well developed porous system are the main advantages making them attractive for catalytic [1–4] and adsorption processes [5–8]. They can be produced from different precursors of organic origin including wood, peat and fossil coals of different degree of coalification, from brown coals to anthracites [9–12]. From the economical and ecological point of view, the activated carbons obtained from waste materials, such as fruit shells, sawdust, plastics, tyres, resins or agricultural waste, make an excellent alternative to those produced from fossil fuels. It is not only a good method of utilisation of slowly biodegrading wastes, but also a nice way of their conversion into valuable products. Doping of the activated carbons with single or multicomponents metal and metal oxide system brings about significant modifications in their thermal stability, reactivity, physicochemical and catalytic properties [13–18]. When nanoparticles of metals are deposited on the surface of the carbonaceous supports, their thermal decomposition is dependent on many factors, such as preparation method, the calcination conditions, nature of the matrix and also on the extent of loading [14,16]. Metal particles usually interact with the surface in the form of chemical bonds or relatively strong physical adhesion to create the effective bonding

force. However, this effective bonding force will be altered with the change of environmental conditions due to the temperature dependence of the surface groups on the activated carbons, and therefore influence the thermal stability of the composite particles. Thermal characterisation of silver vanadates using DTA and TG techniques has not been reported so far. Characterisation of different phases of four vanadates when thermally treated at different temperatures, by IR and X-ray diffraction techniques, has been studied by Brisi [19] who showed that the crystal structure of silver orthovanadate is orthorhombic and by Sinhamahapatra and Bhattacharyya [20] that silver metavanadate does not undergo any substantial change before melting of the substance. The impact of AgVO3 on the thermal behaviour of the supports has not been fully explained yet, and we decided to concentrate on this subject. We found out that modification of support surfaces with silver metavanadate is an important issue because these materials could be applied in rechargeable high-energy density lithium batteries and photocatalysts due to the excellent electrochemical and photophysical properties of AgVO3 [21,22]. The present work reports a study on the influence of silver metavanadate doping on surface properties and thermal stability of the activated carbons obtained from plum stones. The techniques employed were powder X-ray diffraction, nitrogen adsorption, elemental and thermogravimetric analysis. 2. Experimental 2.1. Sample preparation

∗ Corresponding author. Tel.: +48 618291476; fax: +48 618291505. E-mail address: [email protected] (R. Pietrzak). 0040-6031/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tca.2012.04.026

The starting plum stones PS (ash = 0.4 wt.%, moisture = 0.7 wt.%, matter = 80.6 wt.%, Cdaf = 46.4 wt.%, Hdaf = 5.5 wt.%, volatile

J. Goscianska et al. / Thermochimica Acta 541 (2012) 42–48

43

Fig. 1. Elemental analysis (C, H, O) of the investigated samples (wt.%).

Ndaf = 0.1 wt.%, Odaf = 48.0 wt.%) were ground with a roller mill, sieved to a uniform size range of 1.5–2.5 mm and dried at 110 ◦ C for 24 h. The raw plum stones were subjected to pyrolysis (C). Process was carried out in a horizontal furnace under a stream of argon with a flow rate of 0.170 L/min. The sample was heated (10 ◦ C/min) from room temperature to the final carbonisation temperature of 400 (C4) or 600 ◦ C (C6). In the final temperature, sample was kept 1 h and then cooled down in an inert atmosphere. The chars obtained (PSC4 and PSC6) were then subjected to chemical activation by KOH at 700 (A7) or 800 ◦ C (A8). Activation was performed with an alkali/char weight ratio of 4/1 for 30 min, in argon atmosphere (flow rate 0.330 L/min). The activated carbon obtained (PSC4A7, PSC4A8, PSC6A7 and PSC6A8) were washed first with 5% solution of HCl and later with distilled water until the pH of the washed solution was about 6–7, and then were dried at 110 ◦ C for 24 h. 2.2. Modification of activated carbons obtained Incipient wetness technique was used to impregnate all the activated carbons with an alcoholic solution of silver

metavanadate (AgVO3 , Alfa Aesar) in the amount necessary to obtain 1, 3 and 5 wt.% Ag loading. The amount of solution was calculated with respect to the pore volume of the supports. Following the impregnation, the catalysts were successively dried at 100 ◦ C for 5 h. 2.3. Sample characterisation 2.3.1. Elemental analysis The elemental analysis (C, H, N, S) of the products obtained at each stage of the processing was performed on an elemental analyser CHNS Vario EL III (Elementar Analysensysteme GmbH, Germany). 2.3.2. Nitrogen sorption Characterisation of the pore structure of obtained samples was performed on the basis of low-temperature nitrogen adsorption–desorption isotherms measured on a sorptometer Quantachrome Autosorb IQ. Prior to adsorption measurements, the samples were degassed in vacuum at 150 ◦ C for 10 h. Surface area and pore size distribution were calculated by BET (the relative pressure p/p0 range taken into account in the BET calculations was

Table 1 Porous structure of the activated carbons. Sample

Total surface area (m2 /g)

Total pore volume (cm3 /g)

Average pore diameter (nm)

PSC4A7 1% Ag-V/PSC4A7 3% Ag-V/PSC4A7 5% Ag-V/PSC4A7

1796 1660 1685 1655

0.99 0.95 0.97 0.96

1.98 2.28 2.31 2.36

PSC4A8 1% Ag-V/PSC4A8 3% Ag-V/PSC4A8 5% Ag-V/PSC4A8

2175 1738 1674 1477

1.09 1.02 0.97 0.88

2.05 2.32 2.36 2.39

PSC6A7 1% Ag-V/PSC6A7 3% Ag-V/PSC6A7 5% Ag-V/PSC6A7

2893 2616 2499 2411

1.39 1.46 1.41 1.37

1.91 2.24 2.26 2.27

PSC6A8 1% Ag-V/PSC6A8 3% Ag-V/PSC6A8 5% Ag-V/PSC6A8

2752 2401 2337 2231

1.37 1.33 1.31 1.26

2.00 2.22 2.24 2.25

44

J. Goscianska et al. / Thermochimica Acta 541 (2012) 42–48

Intensity, a.u.

* *

*

* *

* *

* *

*

* *

*

5%Ag-V/PSC4A7

*

*

* *

*

* *

*

3% Ag-V/PSC4A7

*

*

*

*

*

Intensity, a.u.

*

*

* * **

1% Ag-V/PSC4A7

*

* *

* * *

*

5%Ag-V/PSC4A8

* *

**

* *

* *

*

*

3% Ag-V/PSC4A8

* *

*

40

2 θ,

Intensity, a.u.

*

* *

* *

** * *

*

*

*

*

*

*

*

50

20

60

30

40

o

*

*

*

*

50

2 θ,

*

*

5%Ag-V/PSC6A7

*

*

3% Ag-V/PSC6A7

*

*

* * *

1% Ag-V/PSC6A7

*

* * * *

* *

* * *

*

60

o

5%Ag-V/PSC6A8

* *

* *

*

30

40

2 θ,

50

60

o

*

3% Ag-V/PSC6A8

* *

*

*

1% Ag-V/PSC6A8

*

*

PSC6A7 20

* PSC4A8

Intensity, a.u.

30

*

1% Ag-V/PSC4A8

PSC4A7 20

*

* PSC6A8

20

30

40

2 θ,

50

60

o

Fig. 2. X-ray diffractograms of activated carbons (PSC4A7, PSC4A8) modified with silver metavanadate thermally treated at 100 ◦ C (* represent crystalline phases of AgVO3 ).

0.006–0.2) and BJH methods, respectively. Total pore volume and average pore diameter were determined as well.

2.3.3. Surface oxygen groups The surface oxygen functional groups were determined by the Boehm method [23,24].

2.3.4. Powder X-ray diffraction (XRD) Prior to the XRD study, selected impregnated samples had been subjected to calcination in argon for 4 h at 400 ◦ C (temperature rate 2 ◦ C/min). The prepared materials were characterised by Xray diffraction (XRD) using a D8 Advance diffractometer (Bruker) (CuK␣ radiation,  = 0.154 nm) with a step size of 0.05◦ in the highangle range.

Table 2 Acid–base properties of the activated carbons. Sample

Acidic groups (mmol/g)

Basic groups (mmol/g)

Total content of surface oxides (mmol/g)

PSC4A7 1% Ag-V/PSC4A7 3% Ag-V/PSC4A7 5% Ag-V/PSC4A7

1.36 1.93 1.70 1.82

0.62 0.40 0.86 1.20

1.98 2.33 2.56 3.02

PSC4A8 1% Ag-V/PSC4A8 3% Ag-V/PSC4A8 5% Ag-V/PSC4A8

1.20 1.65 1.57 1.71

0.65 0.43 0.86 1.38

1.85 2.08 2.43 3.09

PSC6A7 1% Ag-V/PSC6A7 3% Ag-V/PSC6A7 5% Ag-V/PSC6A7

1.91 1.85 1.79 1.72

1.28 0.64 0.99 1.49

3.19 2.49 2.78 3.21

PSC6A8 1% Ag-V/PSC6A8 3% Ag-V/PSC6A8 5% Ag-V/PSC6A8

1.09 1.67 1.60 1.52

1.04 0.57 1.16 1.52

2.13 2.24 2.76 3.04

J. Goscianska et al. / Thermochimica Acta 541 (2012) 42–48

- Ag

0

- Ag

• •



5% Ag-V/PSC4A7

• •



20

3% Ag-V/PSC4A7

• •

30



• •

5% Ag-V/PSC4A8



3% Ag-V/PSC4A8



1% Ag/PSC4A8



1% Ag-V/PSC4A7 40

2 θ,

50

20

60

30

2 θ,

50

60

o

0

- Ag

0

• - V 2O 5





5% Ag-V/PSC6A7



Intensity, a.u.





• • • •

5% Ag-V/PSC6A8



3% Ag-V/PSC6A8



20

40

o

• - V2O5

Intensity, a.u.







- Ag

0

• - V2O5

Intensity, a.u.

Intensity, a.u.

• - V 2O 5

45

30



3% Ag-V/PSC6A7



1% Ag/PSC6A7 40

2 θ,

50

1% Ag-V/PSC6A8

60

o

20

30

40

2 θ,

50

60

o

Fig. 3. X-ray diffractograms of activated carbons (PSC6A7, PSC6A8) modified with silver metavanadate thermally treated at 400 ◦ C (• represent crystalline phases of V2 O5 ,  – Ag0 ).

2.3.5. Thermal analysis Thermogravimetric analysis was performed on an SETSYS 12 made by Setaram. The samples (10 mg, particle size below 0.06 mm) were heated at the rate 10 ◦ C/min, in the helium atmosphere. Analysis lasted for 100 min and the temperature during the decomposition varied from 20 to 1000 ◦ C.

3. Results and discussion Elemental analysis (Fig. 1) of the activated carbons obtained from plum stones has shown that pyrolysis and chemical activation lead to important changes in the composition of the precursor. The content of elemental carbon in the samples studied varied from 91.2 to 94.6 wt.%, the content of hydrogen – from 0.3 to 0.5 wt.% and that of oxygen from 5.1 to 8.3 wt.%. These changes are a result of high temperature treatment. It is well known that upon the heating process, the least stable fragments of the precursor’s structure undergo decomposition leading to formation of side products rich in hydrogen and oxygen, such as water and simple hydrocarbons. The results presented in Table 1 clearly illustrate that the activation of plum stones by KOH leads to activated carbons of well-developed surface area and porous structure with dominant micropores. As seen, the greatest effect on the textural parameters of the unmodified carbonaceous supports definitely has the temperature of the pyrolysis. The samples obtained by activation of the plum stones pyrolysed at 600 ◦ C have much more developed surface area and porous structure, than the analogous samples

obtained from plum stones pyrolysed at 400 ◦ C (e.g. SBET of PSC6A7 sample is almost 1000 m2 /g higher than for sample PSC4A7). As follows from further analysis of Table 1, the impregnation of the activated carbons with silver metavanadate leads to a decrease in their surface area and pore volume. These phenomena are probably caused by two factors: (a) a pore blocking being a result of the interaction between metals and carbon matrix and (b) generation of a large number of the surface oxygen complexes. The decrease in surface area after modification with silver metavanadate is the most pronounced for all samples Ag-V/PSC4A8, whose SBET values are 20–32% lower than the corresponding values for the unmodified sample PSC4A8. The smallest influence of the impregnation on the textural parameters was noted for all Ag-V/PSC4A7 samples, for which the decrease in the surface area varies in the range from 6 to 8%. According to the results of Boehm titration (Table 2), the activated carbon samples obtained by activation of plum stones are characterised by very high content of surface oxygen groups. The total content of surface groups varies between 1.85 and 3.19 mmol/g. From the contents of the acidic and basic surface functional groups, one can conclude that the majority of the support samples studied have surfaces of acidic character. The strongest acidic character was observed for sample PSC4A7, which contains twice more acidic groups than basic ones. Only sample PSC6A8 shows intermediate acid–base properties, because it contains comparable amounts of acidic and basic species. The impregnation of the supports with silver metavanadate causes significant changes in the number of groups, both of acidic

J. Goscianska et al. / Thermochimica Acta 541 (2012) 42–48

DTG signals + constant

DTG signals + constant

46

PSC4A7 1% Ag-V/PSC4A7 3% Ag-V/PSC4A7 5% Ag-V/PSC4A7 200

400

PSC4A8 1% Ag-V/PSC4A8 3% Ag-V/PSC4A8 5% Ag-V/PSC4A8 600

800

1000

200

400

600

800

1000

800

1000

Temperature [°C]

o

DTG signals + constant

DTG signals + constant

Temperature [ C]

PSC6A7 1% Ag-V/PSC6A7 3% Ag-V/PSC6A7 5% Ag-V/PSC6A7 200

400

PSC6A8 1% Ag-V/PSC6A8 3% Ag-V/PSC6A8 5% Ag-V/PSC6A8 600

800

1000

o

Temperature [ C]

200

400

600 o

Temperature [ C]

Fig. 4. DTG curves of activated carbons obtained from plum stones modified with silver metavanadate.

and basic character, indicating the domination of redox character of Ag containing materials. According to the data from Table 2, the intensity of these changes depends to some degree on the conditions of supports preparation. The most pronounced changes (especially in the content of the basic groups) are observed for the carbons obtained from samples PSC4A7 and PSC4A8, while the smallest effect of modification with silver metavanadate on the content of surface oxides was noticed for the all samples AgV/PSC6A7. Figs. 2 and 3 show the XRD pattern of the activated carbons (PSC4A7, PSC4A8, PSC6A7, PSC6A8) modified with silver metavanadate, thermally treated at 100 and 400 ◦ C. The reflections from the crystalline phases (Fig. 2), which can be formed on the surface during the activation process were not observed for unmodified carbonaceous supports. Introduction of silver metavanadate resulted in the appearance of reflections characteristic of AgVO3 at 2 = 25.6, 28.2, 29.8, 32.8, 34.3, 35.7, 37.9, 39.5, 44.1, 45.3, 50.8, 54.7 [20].

The XRD patterns recorded for the samples modified with silver metavanadate and calcined for 4 h at 400 ◦ C (Fig. 3) show the reflections from metallic silver at 2 = 38.1 and 44.1 corresponding to (1 1 1) and (2 0 0) planes, respectively. The peaks can be readily indexed to a face-centred cubic structure of silver, in consistence with available literature (JCPDS, File No. 4-0783) [25,26]. The XRD patterns also show low-intense reflections from V2 O5 at 2 = 25.2, 27.3, 28.2 and 31.7 [27]. The intensity of all reflections increases with increasing percentage content of silver and vanadium on the surface of carbon supports. In order to characterise the behaviour of the activated carbons modified with silver metavanadate at elevated temperatures, they were subjected to thermogravimetric measurements. The results obtained for all the samples are shown in Figs. 4 and 5 in the form of DTG and TG curves. The peak at 60–100 ◦ C, present in all DTG curves (Fig. 4) is assigned to elimination of the adsorbed water. The DTG curves of the carbon supports reveals one wide peak at about 700 ◦ C. According to the literature data [28,29], this peak

J. Goscianska et al. / Thermochimica Acta 541 (2012) 42–48

47

0,0

0,0

-0,4

-0,8 Mass loss [%]

Mass loss [%]

-0,6

-1,2

-1,2

-1,6

-2,0 -1,8

-2,4

-2,4

PSC4A7 1% Ag-V/PSC4A7 3% Ag-V/PSC4A7 5% Ag-V/PSC4A7 200

400

-2,8 600

800

PSC4A8 1% Ag-V/PSC4A8 3% Ag-V/PSC4A8 5% Ag-V/PSC4A8 200

1000

Temperature [ C]

400 600 Temperature [ C]

800

1000

800

1000

0,0

0,0

-0,2 -0,4 Mass loss [%]

Mass loss [%]

-0,5

-1,0

-0,6 -0,8 -1,0

-1,5

-1,2

-2,0

PSC6A7 1% Ag-V/PSC6A7 3% Ag-V/PSC6A7 5% Ag-V/PSC6A7

-1,4

PSC6A8 1% Ag-V/PSC6A8 3% Ag-V/PSC6A8 5% Ag-V/PSC6A8

-1,6

200

400 600 Temperature [°C]

800

1000

200

400

600

Temperature [ C]

Fig. 5. TG curves of activated carbons obtained from plum stones modified with silver metavanadate.

is assigned to the process of secondary devolatilisation in which hydrogen and methane are released from the sample under decomposition. Because of the temperatures of pyrolysis (400 or 600 ◦ C) as well as those of activation (700 or 800 ◦ C), the DTG curves of all initial activated carbon samples do not show peaks in the range 400–450 ◦ C assigned to primary devolatilisation, in which volatile and tarry substances not removed at the stage of material preparation are released. This is also reflected in the TG curves (Fig. 5) illustrating the mass losses of the samples. The mass loss is low till ∼600 ◦ C and above this temperature it increases. The character of TG curves depends on the secondary devolatilisation and the lack of hydrogen (Fig. 1) needed for compensation of free radicals generated upon sample heating. For the samples modified with AgVO3 , the TG curves indicate that the mass-loss is very slow and almost constant up to 450 ◦ C, which is consistent with the literature data [20]. Introduction of 1 or 3 wt.% of silver metavanadate on the surface of initial activated carbon samples results in the appearance of additional peaks on the DTG curves, at temperatures from the range 260–370 ◦ C. These peaks correspond to the complete

decomposition of silver metavanadate to silver and vanadium oxides. According to the literature data, in the range 200–290 ◦ C the silver nanoparticles undergo crystallisation so the Ag+ ions present in vanadates salt are reduced to metallic form [25,26]. It should be mentioned that the peak at about 700 ◦ C in the DTG curves of the samples modified with silver metavanadate might represent the melting of vanadium oxide. Thermal decomposition of samples PSC4A7, PSC4A8 and PSC6A8, modified with 5 wt.% AgVO3 takes place in a different manner. The DTG curves recorded for these samples show peaks in the ranges 200–290 ◦ C, 300–400 ◦ C and 410–540 ◦ C. The signals peaking at about 260 and 350 ◦ C correspond to the thermal decomposition of silver metavanadate to different vanadyl species (i.e. Ag4 V2 O7 ) as an intermediate compounds. As follows from the DTG curves, similarly as for the samples modified with 1 or 3 wt.% AgVO3 , also in those modified with 5 wt.% AgVO3 the nanoparticles of silver undergo crystallisation, a part of Ag+ ions present in the vanadium salt is reduced to Ag0 . At higher temperatures vanadyl species are converted to V2 O5 , as evidenced by the signal peaking at about 500 ◦ C (except for sample PSC6A7). In higher

48

J. Goscianska et al. / Thermochimica Acta 541 (2012) 42–48

temperatures, secondary devolatilisation and melting of vanadium oxide take place. 4. Conclusions The impregnation of activated carbons obtained from plum stones with silver metavanadate results in a decrease in the specific surface area of the carbons as a consequence of the metals and carbon matrix interactions and generation of a large number of surface oxygen complexes. In the samples modified with 1 and 3 wt.% silver metavanadate, the decomposition of AgVO3 to vanadium oxide takes place with no intermediate product formation, while in the samples modified with 5 wt.% silver metavanadate, AgVO3 decomposes to vanadyl species as an intermediate compounds at 350 ◦ C before the formation of V2 O5 at 500 ◦ C. In all activated carbon samples modified with AgVO3 , with increasing temperature the silver nanoparticles undergo crystallisation and a part of Ag+ ions present in AgVO3 are reduced to Ag0 , which is manifested by the appearance of a signal peaking at ∼260 ◦ C. Acknowledgements This work was supported by The Polish Ministry of Science and Higher Education project No. N N204 277537. Piotr Nowicki wishes to thank the Foundation for Polish Science for fellowship. References [1] J.L. Figueiredo, M.F.R. Pereira, The role of surface chemistry in catalysis with carbons, Catal. Today 150 (2010) 2–7. [2] S.V. Mikhalovsky, Y.P. Zaitsev, Catalytic properties of activated carbons. I. Gasphase oxidation of hydrogen sulphide, Carbon 35 (1997) 1367–1374. [3] X. Wu, A.K. Kercher, V. Schwartz, S.H. Overbury, T.R. Armstrong, Activated carbons for selective catalytic oxidation of hydrogen sulfide to sulfur, Carbon 43 (2005) 1087–1090. [4] L. Wang, B. Cao, S. Wang, Q. Yuan, H2 S catalytic oxidation on impregnated activated carbon: experiment and modeling, Chem. Eng. J. 118 (2006) 133–139. [5] P. Nowicki, R. Pietrzak, H. Wachowska, Sorption properties of active carbons obtained from walnut shells by chemical and physical activation, Catal. Today 150 (2010) 107–114. [6] P. Nowicki, H. Wachowska, R. Pietrzak, Active carbons prepared by chemical activation of plum stones and their application in removal of NO2 , J. Hazard. Mater. 181 (2010) 1088–1094. ˛ [7] K. Laszlo, P. Podko´scielny, A. Dabrowski, Heterogeneity of activated carbons with different surface chemistry in adsorption of phenol from aqueous solutions, Appl. Surf. Sci. 252 (2006) 5752–5762. [8] F.C. Wu, R.L. Tseng, High adsorption capacity NaOH-activated carbon for dye removal from aqueous solution, J. Hazard. Mater. 152 (2008) 1256–1267. [9] S. Bashkova, T.J. Bandosz, The effects of urea modification and heat treatment on the process of NO2 removal by wood-based activated carbon, J. Colloid Interf. Sci. 333 (2009) 97–103.

[10] P. Nowicki, R. Pietrzak, H. Wachowska, Influence of metamorphism degree of the precursor on preparation of nitrogen enriched activated carbons by ammoxidation and chemical activation of coals, Energy Fuels 23 (2009) 2205–2212. [11] R. Pietrzak, K. Jurewicz, P. Nowicki, K. Babeł, H. Wachowska, Nitrogen-enriched bituminous coal based active carbons as materials for supercapacitors, Fuel 89 (2010) 34573467. [12] P. Nowicki, R. Pietrzak, H. Wachowska, Siberian anthracite as a precursor material for microporous activated carbons, Fuel 87 (2008) 2037–2040. [13] B.Y. Jibril, A.Y. Atta, Activated carbon incorporated with first-row transition metals as catalysts in hydrogen production from propane, Int. J. Hydrogen Energ. 36 (2011) 5951–5959. [14] M. Seredych, S. Bashkova, R. Pietrzak, T.J. Bandosz, Interactions of NO2 and NO with carbonaceous adsorbents containing silver nanoparticles, Langmuir 26 (2010) 9457–9464. [15] G. Shao, L. Liu, T. Ma, F. Wang, T. Ren, Z. Yuan, Synthesis and characterization of carbon-modified titania photocatalysts with a hierarchical meso-/macroporous structure, Chem. Eng. J. 160 (2010) 370–377. [16] S. Bashkova, D. Deoki, T.J. Bandosz, Effect of silver nanoparticles deposited on micro/mesoporous activated carbons on retention of NOx at room temperature, J. Colloid Interf. Sci. 354 (2011) 331–340. [17] T. Tuan, N. Son, H. Dung, N. Luong, B. Thuy, N. Anh, N. Hoa, N. Hai, Preparation and properties of silver nanoparticles loaded in activated carbon for biological and environmental applications, J. Hazard. Mater. 192 (2011) 1321–1329. [18] J. Goscianska, I. Nowak, P. Nowicki, R. Pietrzak, The influence of silver on the physicochemical and catalytic properties of activated carbons, Chem. Eng. J. 189-190 (2012) 422–430. [19] C. Brisi, The crystal structure of the orthovanadates of cobalt and zinc, Ricerca Sci. 30 (1960) 1339–1342. [20] P. Sinhamahapatra, S. Bhattacharyya, Physico-chemical properties of catalysts: thermal analysis, infrared spectroscopy, X-ray diffraction and magnetic susceptibility of the vanadates of zinc, manganese and silver, J. Therm. Anal. Calorim. 9 (1976) 279–294. [21] G. Li, K. Chao, C. Ye, H. Peng, One-step synthesis of Ag nanoparticles supported on AgVO3 nanobelts, Mater. Lett. 62 (2008) 735–738. [22] K.J. Takeuchi, A.C. Marschilok, S.M. Davis, R.A. Leising, E.S. Takeuchi, Silver vanadium oxides and related battery applications, Coordin. Chem. Rev. 219-221 (2001) 283–310. [23] H.P. Boehm, E. Diehl, W. Heck, R. Sappok, Surface oxides of carbon, Angew. Chem. Int. Ed. 3 (1964) 669–677. [24] H.P. Boehm, Some aspects of the surface chemistry of carbon blacks and other carbons, Carbon 32 (1994) 759–769. [25] M.A.M. Khan, S. Kumar, M. Ahamed, S.A. Alrokayan, M.S. Alsalhi, Structural and thermal studies of silver nanoparticles and electrical transport study of their thin films, Nanoscale Res. Lett. 6 (434) (2011) 1–8. [26] M.A.M. Khan, S. Kumar, M. Ahamed, S.A. Alrokayan, M.S. Alsalhi, M. Alhoshan, A.S. Aldwayyan, Structural and spectroscopic studies of thin film of silver nanoparticles, Appl. Surf. Sci. 257 (2011) 10607–10612. [27] W.H. Shaheen, I.H.A. El Maksod, Thermal characterization of individual and basic copper carbonate and ammonium metavanadate systems, J. Alloys Compd. 476 (2009) 366–372. [28] G. de la Puente, M.J. Iglesias, E. Fuente, J.J. Pis, Changes in the structure of coals of different rank due to oxidation-effects on pyrolysis behaviour, J. Anal. Appl. Pyrol. 47 (1998) 33–42. [29] R. Pietrzak, H. Wachowska, Thermal analysis of oxidised coals, Thermochim. Acta 419 (2004) 247–251.