Study of silica templates in the rice husk and the carbon–silica nanocomposites produced from rice husk

Journal of Physics and Chemistry of Solids 87 (2015) 58–63

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Study of silica templates in the rice husk and the carbon–silica nanocomposites produced from rice husk Yu.V. Larichev a,b,n, P.M. Yeletsky a, V.A. Yakovlev a a b

Boreskov Institute of Catalysis, Siberian Branch of Russian Academy of Sciences, Prospekt Akademika Lavrentieva 5, Novosibirsk 630090, Russia Novosibirsk State University, Pirogova Street 2, Novosibirsk 630090, Russia

art ic l e i nf o

a b s t r a c t

Article history: Received 4 May 2015 Received in revised form 23 July 2015 Accepted 30 July 2015 Available online 10 August 2015

Carbon–silica nanocomposites obtained by rice husk carbonization in a fluidized-bed reactor using a deep oxidation copper–chromium catalyst were studied. Dispersion characteristics of the silica phase in these systems were determined by small-angle X-ray scattering (SAXS) using the full contrast technique. SiO2 was found in the initial rice husk as compact nanoparticles having a wide size distribution. This distribution consists of a narrow fraction with particle sizes from 1 to 7 nm and a wider fraction with particle sizes from 8 to 22 nm. Oxidative heat treatment of rice husk in a fluidized bed in the presence of the catalyst decreased the fraction of small SiO2 particles and increased the fraction of large ones. It was demonstrated that the particle size of silica in the carbon matrix can be determined selectively for deliberate design of porous carbon materials with desired properties. & 2015 Elsevier Ltd. All rights reserved.

Keywords: A. Non-crystalline materials A. Nanostructures C. X-ray diffraction C. Electron microscopy D. Microstructure

1. Introduction Rice husk (RH) is a bulky agricultural waste with annual world production about 100 million tons [1,2]. RH is difficult to recycle due to high ash content and insufficient calorific content. Therefore, development of appropriate methods for its conversion into highly marketable products is a topical problem. RH is a promising renewable source of various organic (mixture of phenols, alcohols and organic acids [2–9]) and inorganic products, including siliconand carbon-containing materials, such as SiO2, SiС, Si, mesoporous silicates and others [1,2,10–25], as well as porous carbon–mineral and carbon materials with micro- and mesoporous texture [1,2,26–31]. Such a wide spectrum of products that can be produced from RH is related to its specific composition. According to the literature, RH comprises cellulose, hemicellulose, lignin, and inorganic moiety consisting mainly of silica [32]. The total content of other elements and compounds in RH does not exceed 1–3%. Thus, RH can be considered as a natural composite made of a biopolymer matrix and a reinforcing silica phase. For production of porous carbon-containing materials, RH is commonly carbonized with subsequent activation by chemical reagents [26,28–30]. As it was shown earlier [30,33], alkaline activation of a carbon–mineral composite derived from RH made it possible to produce microporous carbon materials with specific n

Corresponding author at: Boreskov Institute of Catalysis, Siberian Branch of Russian Academy of Sciences, Prospekt Akademika Lavrentieva 5, Novosibirsk 630090, Russia. E-mail address: [email protected] (Yu.V. Larichev). http://dx.doi.org/10.1016/j.jpcs.2015.07.025 0022-3697/& 2015 Elsevier Ltd. All rights reserved.

surface area close to its maximum possible value and pore volume as high as 3.0 cm3/g. Application of sodium or potassium carbonates instead of alkali allowed obtaining carbon materials with mesoporous texture [31]. In this case, the particle size of the silica phase is one of the key factors determining the porosity of resulting carbon materials, especially mesoporous ones, because silica actually serves as a template for pores formed in the carbon material after its removal. It is quite difficult to estimate directly the particle size of the silica phase using conventional methods (XRD, TEM) because such samples are usually X-ray amorphous and have disordered structure. Due to the lack of direct experimental data on the size of the silica template, optimal values of the textural properties of RH carbon materials should be selected empirically by variation of the process conditions [27,30,31]. The goal of the present study was to acquire direct data on the morphology and dispersion of silica particles in both rice husk and C/SiO2 nanocomposites obtained by its carbonization as a function of the process temperature. The obtained data could provide better understanding and control of processes underlying the formation of porous structure in carbon materials produced from rice husk.

2. Material and methods 2.1. Sample preparation The study was carried out with rice husk grown in the Krasnodar region of Russia. The husk was preliminary ground down to

Yu.V. Larichev et al. / Journal of Physics and Chemistry of Solids 87 (2015) 58–63

performed using a JEM-2010 transmission electron microscope (JEOL, Japan) with accelerating voltage 200 kV and resolution 0.14 nm. The chemical composition was determined from EDX spectra. For more wide coverage for EDX analysis it has been used electron beam with 50 nm diameter. The samples for the TEM study were prepared by ultrasonic dispersing in ethanol and consequent deposition of the suspension upon a “holey” carbon film supported on a copper grid.

Cyclone

RH

59

C/SiO2 Air

Fig. 1. Sketch of rice husk carbonization process in a fluidized-bed reactor.

the particle size below 1 mm. According to [27], this rice husk contained 19–25% lignin, 34–42% cellulose, and 17–22% hemicellulose. The ash content of the husk was 19.5%, and the SiO2 content in the ash was above 96%. The silica content in samples has been determined by ICP-OES using Optima 4300 analyzer (Perkin–Elmer, USA) in compliance with [34]. RH carbonization was performed in a steel reactor with an inner diameter of 75 mm and a catalyst bed height of 1000 mm. Its capacity was 3 kg RH/h. A deep oxidation catalyst IK-12-73 with the particle size of 2– 3 mm manufactured by Katalizator Ltd. [27] was used in the process. RH was fed to the reactor (Fig. 1) with air flow (the molar ratio of oxygen to carbon, α, was ∼2). Carbonization time τ was
1 s. After carbonization at 465, 550 and 600 °C, the resulting carbon–mineral material was collected in a cyclone. A more detailed description of the preparation procedure and investigation of the properties of carbon–silica composites (C/SiO2) can be found in [27]. The prepared carbon–silica composites are denoted as CS TTT, where TTT is the carbonization temperature. 2.2. Sample characterization 2.2.1. Nitrogen absorption (BET) Textural characteristics of the samples were determined by physical adsorption of nitrogen at the liquid nitrogen temperature using an ASAP-2400 automated volumetric adsorption unit (Micromeritics Instrument Corp., Norcross, GA, USA). Prior to analysis, the samples were calcined at 150 °C for 4 h at a pressure of
10  3 mm Hg. The analysis time was chosen individually for each sample. The resulting adsorption isotherms were used to calculate the specific surface area and total pore volume. 2.2.2. TRUE density measurement True density of rice husk and carbon–silica nanocomposites was measured pycnometrically. Volume of the liquid displaced by a porous sample was measured. The measurement was made at least three times for each sample, and the obtained values were averaged. The liquid used in these experiments was heptane. The measurement error was less than 4%. 2.2.3. X-ray diffraction X-ray diffraction analysis was carried out with an HCG 4-C diffractometer (Freiberger Präzisionsmechanik, Germany) with a graphite monochromator on the diffraction beam using a tube with a copper anode and wavelength λ equal to 1.5416 Å. 2.2.4. Electron microscopy Electron microscopy examination

of

the

samples

was

2.2.5. Small-angle X-ray scattering SAXS data were acquired using an S3 MICRO (HECUS) diffractometer with point collimation and Cu Kα radiation (wavelength is equal to 1.5416 Å). The electrical power of generator for X-ray tube is 50 W (1.0 mA at 50 kV). The diffraction patterns were measured in the vector h ranges from 0.01 to 0.6 Å  1, where h¼4π sin θ/λ. The samples were placed in a 1.5 mm glass capillary (outside diameter) with the wall thickness of 0.01 mm. For selective extraction of the scattering signal of dispersed SiO2 from the total scattering sample we developed an original full-contrast technique using fluorocarbon compounds [35,36]. A sample in the capillary was filled with a special liquid having a density of 1.4 g/ cm3, which roughly corresponds to the density of polysaccharide or a carbon matrix1. The application of such contrast makes it possible to selectively suppress scattering from the matrix and obtain a residual small-angle scattering signal only from silica, because it has higher density (2.2 g/cm3) and cannot be contrasted by this liquid. A mixture of perfluorobenzene with heptane in a proportion providing the desired density served as a masking liquid. A detailed description of the technique was reported in articles [35,36]. Processing of experimental data and calculation of the particle size distributions were based on the GNOM program from the ATSAS package [37,38].

3. Results and discussion Initial rice husk and the carbon–silica composites obtained by its carbonization at 465–600 °C were characterized by physicochemical methods. Fig. 2 displays a TEM image of the initial RH (inner layers). The EDX spectrum of the observed section shows intense signals from carbon, oxygen and silicon (Fig. 2, inset). Amorphous silica separated by thin carbon layers is observed in the TEM image (see arrows in Fig. 2). Typical sizes of such separated silica sections are found in the range from 2 to 15 nm. Due to low contrast between carbon and silica in the TEM images, it is difficult to reliably separate these phases for histogram preparation. Moreover, partial RH decomposition under the electron beam in the TEM chamber can influence the aggregation of silica particles and their sizes. For these reasons, the TEM data are only approximate and need addition verification. TEM images for the all C/SiO2 composites produced from RH at different carbonization temperatures are qualitatively similar to each other. Amorphous regions that are difficult to interpret are present in all TEM images of C/SiO2 samples. We assume that more ordered regions can include the carbon phase, whereas amorphous regions can include the silica phase. For example, Fig. 3 shows a TEM image of the CS550 sample, which also shows amorphous silica with islands of various carbon structures (see arrows in Fig. 3). Typical sizes of separated silica sections are about 1 Actually the intensity of X-ray scattering depends on the electron density differences. We used pycnometric density exclusively for convenience. These values are close to each other but not the same things. For amorphous or disordered samples are very difficult to measure electron density value instead pycnometric density. For this reason in practical applications is better to use pycnometric density and make additional density corrections if it need in experiment.

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Fig. 4. X-ray diffraction pattern for initial rice husk. Reflexes from cellulose structure shows as a bar chart.

22.8

0

CS 550 Amorphous silica Graphite-like carbon material

Intensity, arb.un.

Fig. 2. TEM image of initial rice husk. Inset shows EDX spectra from this image.

22.0

10

0

25.7

20

30

0

40

50

60

2θ θ ⁄ deg Fig. 5. X-ray diffraction patterns for carbon–silica sample CS 550, amorphous silica and graphite-like carbon material.

Fig. 3. TEM image of CS 550 sample. Inset shows EDX spectra from this image.

10 nm. This value is approximate because in this case is also difficult to separate reliably the carbon and silica phases. The EDX spectrum of the observed section shows intense signals from carbon, oxygen and silicon (Fig. 3, inset). In addition to these signals, the EDX spectrum shows various impurities, for example potassium, which seems to be distributed quite nonuniformly over the sample because its weight content in the ash is as low as ca. 2% [31]. At relatively low (
600 °C) treatment temperatures, alkali metal impurities are likely to interact with silica to yield silicates, while at higher heat treatment temperatures they may intercalate in the carbon structure [39–42]. Fig. 4 displays the diffraction pattern of the initial RH. Reflections observed for RH can be attributed to the cellulose structure [43–46]. No other reflections corresponding to silica are observed. This fact suggests that silica is in the amorphous state or in the form of polysiloxanes [27]. The RH carbonization results in disappearance of reflections typical of cellulose and appearance of a broad peak with the maximum at 22.8°. Fig. 5 shows the diffraction pattern of the carbon–silica composite CS550 that was produced by RH carbonization in a fluidized catalyst bed at 550 °C. A broad peak with the maximum at 22.8° can be assigned to amorphous silica [33,46,48]. For comparison purposes, Fig. 5 displays also diffraction patterns of amorphous silica having similar peak with the maximum at 22.0° [47] and graphitic carbon material [42] with a narrower peak with the maximum at 25.7°. The

peak observed for the CS550 sample is broader and shifted to the right with respect to the corresponding peak of SiO2. This may be caused by superposition of the reflection from disordered carbon [42,49]. Separation of these reflections from each other seems to be impossible. The diffraction patterns of the C/SiO2 composites obtained at other temperatures do not show essential differences. The absence of distinct reflections for the carbon-containing phase is quite expectable because graphitization of carbon materials proceeds at much higher temperatures [31,50–52]. Formal application of the Scherrer equation suggests that the size of particles in this sample is about 1.2 nm. It means that this sample is amorphous and is not suitable for such particle size measurement. So for correct silica particle sizes measurement we need to use other methods. Carbon materials obtained at low temperatures have disordered amorphous structure and low density (
1.5 g/cm3) [50– 52]. Table 1 lists main textural characteristics of the produced materials. According to Table 1, rise of the heat treatment temperature leads to an increase in the specific surface area and pore volume as well as in the density and residual content of silica in the sample. As we used atmospheric oxygen in our carbonization process, the temperature increase results in a carbon content decrease due to its combustion. The decrease of the carbon content in turn increases the density of the composites. These data can be Table 1 Textural properties of used samples. Samples

SBET (m2/g)

Vpore (cm3/g)

Density (g/cm3)

% (wt.) SiO2

Rice husk CS 465 CS 550 CS 600

n.d. 152 217 232

n.d. 0.15 0.20 0.22

1.54 1.78 1.86 1.95

19.5 58.7 70.7 81.8

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61

1.9

Intensity, counts/s

Density, g/cm

3

2.0

1.8 1.7 1.6

Rice husk (contrast) CS 465 (contrast) CS 550 (contrast) CS 600 (contrast) GNOM fit

10

1

0.1

1.5 10

20

30

40

50

60

70

80

0.00

90

0.04

0.08

% SiO2

h, Å

0.12

0.16

-1

Fig. 6. Linear approximation of the dependence between true density and silica content in the samples.

Fig. 8. SAXS data for rice husk and carbon–silica composites after their impregnation with masking liquid.

used to plot density of the material as a function of its silica content (Fig. 6). Supposing a linear dependence of the composite sample density on its silica content, one can calculate the average densities of the “carbon” and “silica” phases under the assumption that the samples consist of only these two phases. The average densities of the indicated phases were found to be equal to 1.41 and 2.10 g/cm3, respectively. Taking into account small size of the particles and possible impurities in the resulting material, the value of 2.10 g/cm3 agrees satisfactorily with the density of amorphous silica (2.2 g/cm3). The value of 1.4 g/cm3 agrees well with both the averaged densities of cellulose and lignin (1.50–1.55 and 1.30–1.42 g/cm3, respectively) and the corresponding density for charcoals (
1.4–1.5 g/cm3), which are produced by carbonization of cellulose-containing biomass at temperatures not higher than 600 °C [50–53]. The obtained densities of the “carbon” and “silica” phases can be used in the analysis of small-angle X-ray scattering (SAXS) data for these samples. Fig. 7 presents SAXS data for the initial RH. The slope of the scattering curve for the RH I(h) sample is proportional to
h  2.8. Hence, the particle form-factor cannot be determined strictly in this case, and it is difficult to find the particle size distribution. Such slope of the scattering curve can be typical, for example, of fractal aggregates, which may consist of cellulose fibers. The application of the masking liquid technique makes it possible to suppress virtually completely the scattering signal of the carbon-containing phase. Knowing the averaged density of the carbon-containing phase, one can use a masking liquid with similar density and thus selectively suppress the contribution of this phase to the total scattering [35,36]. When RH was impregnated

with the masking liquid having the density of 1.4 g/cm3, the SAXS signal corresponding mainly to the silica phase was obtained for the sample. As it is shown in Fig. 7, the scattering curve for the contrast matched sample has much lower intensity and a radically different slope (I(h)
h  4). Such slope of the scattering curve is typical for compact particles in two phase system. Application spherical form-factor for modeling such curves gets a good fit quality. If these particles really have strong anisometry (e.g. rodlike particles) we could not get a good fit of our experimental data by a spherical shape. For this reason we can conclude that silica particles in rice husk have a sphere-like shape. Moreover this result has a good agreement with TEM data (Fig. 2). Similar curves from the contrasted samples were obtained also for the other carbon–silica composites (Fig. 8). Although the fraction of the carbon-containing phase in C/SiO2 samples substantially decreased with respect to the initial RH (Table 1), the contribution of this phase to the total scattering remained quite significant. So, the masking liquid technique should be used for selective extraction of the SAXS signal from the silica phase. The SAXS curves measured after contrasting the samples were used to calculate the SiO2 particle size distributions (Fig. 9) under the assumption of a spherical form-factor of the particles. As the intensity of smallangle scattering is directly proportional to the absolute content of a dispersed phase, the content of different fractions of SiO2 particles in the samples can be roughly estimated from the calculated distributions. In addition, one can monitor changes in the dispersion composition of the initial silica “skeleton” of untreated RH under various heat treatment conditions. The obtained distributions indicate the presence of two main particle size fractions: numerous small particles (1–7 nm) and a minor amount of larger particles (
10–20 nm). Some particles may grow in size up to 26 nm upon the heat treatment temperature increase. However, the amount of such particles is insignificant, especially if the obtained distributions in the volume of the particles are recalculated

initial rice husk rice husk (contrast)

10 4

Dv(r), arb.un.

1

0.1

0.01 0.0

0.1

0.2

Rice husk CS 465 CS 550 CS 600

0.0000012

I(h)*h

Intensity, counts/s

100

0.0

0.1

0.2

-1

h, Å

Fig. 7. SAXS data for initial rice husk and rice husk after impregnation with masking liquid.

0.0000009

0.0000006

0.0000003

0.0000000

0

5

10

15

20

25

Particle sizes, nm Fig. 9. Volume particle size distributions for silica particles in rice husk and carbon–silica composites calculated from data in Fig. 8.

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Table 2 Particle sizes of silica particles in used samples. Samples

Rg (SiO2)1, nm

Dav (SiO2)2, nm

Dmax(SiO2)3, nm

Rice husk CS 465 CS 550 CS 600

5.78 70.12 6.25 70.15 6.57 70.11 7.21 70.16

4.74 5.94 6.11 7.55

22.4 23.2 25.8 26.2

1

Radii of gyration. Mean value of SiO2 particle sizes from SAXS distributions (Fig. 9); o Dav 4 ¼ Σnidi/Σni., where di the diameter of i-particles on the SAXS histogram, N the number points on the SAXS histogram. 3 Maximal values of SiO2 particle sizes from SAXS distributions (Fig. 9).

Careful washing or, on the contrary, additional RH doping with alkali metal compounds prior to heat treatment can retard or promote sintering of the silica template in the carbon matrix. Quantitative data on the particle size of the template would aid in the synthesis of carbon materials with controllable pore volume and specific surface area upon leaching.

4. Conclusions

2

to distributions in their number. The particle size distribution data demonstrate that the heat treatment decreases the number of small silica particles and increases the fraction and size of larger particles (Fig. 9). Table 2 lists the radii gyration (Rg) derived from the SAXS data (Fig. 8) as well as the average and maximum sizes of silica particles derived from the distributions (Fig. 9). These data show that the heat treatment of RH strongly increases the average sizes of silica particles in the produced samples. Also need to note that increasing of temperature treatment conduct not only to increasing silica particle size but increasing general specific surface of composites too (Tables 1 and 2). On the one hand increasing silica particle sizes means to decrease specific surface of silica phase. To other hand increasing of temperature treatment from 465 to 600 °C lead to enhance of carbon matrix specific surface [27,30,31]. Since general value of carbon–silica composites specific surface are depends from the both parts (as “silica” and “carbon” phases) finally we indeed observed increasing values specific surface of composites at increasing temperature of treatment. Resolution of the SAXS method is insufficient to elucidate whether silicon can be present in RH as siloxanes. However, the amount of siloxanes in RH appears to be quite small. This assumption is based on the following reasoning. Owing to their atomic size, siloxanes do not make an appreciable contribution to the intensity of small-angle scattering from RH. Nevertheless, in the case of substantial siloxane content, the CS465 sample would have an increased content of both the small and large silica particles compared to RH because the heat treatment would convert siloxanes to dispersed silica. It would make additional contribution to the observed distribution. The heat treatment was shown to decrease the number of small particles and increase the fraction of larger ones. This result suggests that the fraction of siloxanes in RH is negligible, and silicon is present in RH mainly as silica nanoparticles. It should be noted also that the effect of treatment temperature on the sintering process is nonlinear. For example, the treatment temperature increase from 465 to 550 °C has only a minor effect on the silica particle size (Fig. 9). Meanwhile, slight elevation of the temperature from 550 to 600 °C strongly increases the size of SiO2 particles (Fig. 9). The heat treatment temperatures roughly correspond to the range between THüttig and TTammann for silica (0.3  0.5 Tmelt (K) or from 330 to 730 °C). This implies high mobility of dispersed silica particles in the carbon matrix and indicates that their sintering most likely follows the particle migration mechanism. During the formation of silicates, alkali metal impurities can additionally increase the mobility of silica particles and broaden the particle size distributions due to lower melting temperatures compared to pure SiO2. Thus, the temperature treatment makes it possible to vary in wide range the size of silica particles in the indicated materials. In addition, it seems necessary to control the content of alkali metals in RH, because they can form silicates (for example, Na2SiO3 or K2SiO3) that are low-melting in comparison with pure silica.

The carbon–silica nanocomposites С/SiO2 were produced by oxidative carbonization of rice husk in a fluidized catalyst bed at 465, 550 and 600 °C. The properties of RH and С/SiO2 were studied by various physicochemical methods to reveal that the carbon phase in С/SiO2 and the SiO2 phase in С/SiO2 and RH are X-ray amorphous, carbon–silica nanocomposites have mesoporous structure and specific surface area of 152–232 m2/g, which increases with temperature. It was used for selective determination of dispersion characteristics of the silica phase by SAXS using a contrast technique. Silica was shown to occur in the rice husk mainly as compact nanoparticles having a wide size distribution. This distribution consists of a narrow fraction with particle sizes from 1 to 7 nm and a wider fraction with particle sizes from 8 to 22 nm. The presence of SiO2 in the form of siloxanes was negligible. Heat treatment of rice husk decreases the number of small particles and increases both the content and the size of larger silica particles. The growth of heat treatment temperature leads to an increase of the average particle size of the silica phase from 5.9 (465 °C) to 7.6 nm (600 °C). The study demonstrated that the particle size of silica in the carbon matrix can be determined selectively for deliberate design of porous carbon materials with specified properties.

Acknowledgements The authors are grateful to A.V. Ishchenko, D.A. Zyuzin and K.V. Obida for assistance in the investigations of the samples. The reported study was supported by RFBR, Research Project no. 14-0331851 mol_a and by MES (Russia).

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