Investigation of Acid-Base and Sorption Properties of Surface of Metal Ceramic Composites

Transcription

1 ISSN , Russian Journal of Applied Chemistry, 2017, Vol. 90, No. 8, pp Pleiades Publishing, Ltd., Original Russian Text L.N. Skvortsova, L.N. Chukhlomina, T.S. Minakova, M.V. Sherstoboeva, 2017, published in Zhurnal Prikladnoi Khimii, 2017, Vol. 90, No. 8, pp SORPTION AND ION EXCHANGE PROCESSES Investigation of Acid-Base and Sorption Properties of Surface of Metal Ceramic Composites L. N. Skvortsova a *, L. N. Chukhlomina b, T. S. Minakova a, and M. V. Sherstoboeva a a Tomsk State University, pr. Lenina 36, Tomsk, Russia * lnskvorcova@inbox.ru b Department of Structural Macrokinetics, Tomsk Scientifi c Centre, Siberian Branch of Russian Academy of Sciences, Akademicheskii pr. 10/4, Tomsk, Russia Received August 12, 2017 Abstract The acid-base properties of the surface of composites based on boron, silicon, and sialon nitrides were investigated by the indicator method of Hammett and Tanabe with spectrophotometric indication. Identification of the surface sites of the composites under study was carried out. On the surface of composites based on boron and sialon nitrides the Lewis base sites dominate, while on the surface of samples based on silicon nitride the Brønsted acid sites. Sorption of dyes and oxalic acid by cermet materials was studied. It is shown that the adsorption activity depends on the nature and amount of surface active sites of the composites. DOI: /S The sources of water supply are subject to intensive contamination with water soluble organic materials (SOM), and their quality in many regions can not be considered satisfactory. Traditional water treatment methods often do not provide the required level of SOM removal. Effective methods of destruction of SOM are the new advanced oxidation processes (AOPs), which include heterogeneous and homogeneous photocatalytic processes involving hydroxyl radicals. Under the conditions of homogeneous photocatalysis the most popular photo-fenton system [1, 2] proposed in 1960 consists in the initiation of hydroxyl radicals by the addition of catalysts (transitional metal cations such as Fe 2+ and Fe 3+ ) in the solution containing hydrogen peroxide. De la Cruz N. et al. [3] succeeded in reducing a content by 97 98% of 32 organic pollutants (drugs, corrosion inhibitors, pesticides) in the treatment plants in Lausanne (Switzerland) in the photo-fenton process carried out under irradiation with ultraviolet (254 nm) in a practically neutral environment. As a result of previous studies, we found a high catalytic activity of iron-containing composites based on boron and silicon nitrides in the presence of hydrogen peroxide and(or) oxalic acid in the process of destruction of organic pollutants (oxalic acid, phenol, formaldehyde) due to the formation of photoactive systems of homogeneous catalysis (photo-fenton, ferrioxalate). At the same time, metal-ceramic composites include wide bandgap semiconductor compounds (BN, Fe 3 C, Al 4 C 3, SiC), which under exposure to UV radiation exhibit optical activity and can interact with oxygen, water or organic materials, which also leads to the formation of hydroxyl radicals and other highly reactive oxygen-containing compounds. The latter are capable of mineralizing adsorbed organic pollutants. Composites have a number of advantages: They are synthesized from the waste of metallurgical production by the method of self-propagating high-temperature synthesis (SHS) and do not cause secondary pollution, and consequently, the costs of additional post-treatment. The effectiveness of composites as photocatalysts is not only related to their chemical nature and physical characteristics (dispersion, porosity), but also depends significantly on the surface acid-base properties that determine their reactivity and adsorption selectivity in interaction with SOM. It suggests an importance to study the distribution of adsorption sites (DAS) with respect to acid-base type and force on the surface of the samples 1246

2 INVESTIGATION OF ACID-BASE AND SORPTION PROPERTIES OF SURFACE 1247 under study for the selection of optimal photocatalytic systems. The aim of this work is to evaluate the nature of the acid-base sites of the surface of metal-ceramic composites based on boron, silicon and sialon nitrides using the indicator method of Hammett and Tanabe with spectrophotometric indication and their sorption properties with respect to dyes and oxalic acid. EXPERIMENTAL Composite materials based on silicon, boron and sialon nitrides obtained by the AOP method in the Department of Structural Macrokinetics of the Tomsk Scientific Center of the SB RAS are investigated. Composites were prepared by nitriding ferrosilicon, ferroboron, and ferrosilicoaluminium in the presence of various blowing agents (urea, ammonium bicarbonate). Synthesis of boron and silicon nitrides and their characteristics are presented in [4, 5]. Sialon is a solid solution of variable composition Si 6-x Al x O x N 8 x. The phase composition of the materials was studied by X-ray diffraction (XRD) on a diffractometer Shimadzu XRD6000 (Japan), micro-x-ray spectral analysis of the composites was carried out on a Quantax 70 attachment for the HitachiTM-3000 scanning electron microscope. The acid-base properties of the surface of composites were studied by the ph-metry, which makes it possible to estimate the integral acidity [6]. The ph of the medium was measured by a ph meter PP-15 (Germany). First bidistilled water (10 ml) was introduced into the beaker and after the ph stabilization of the glass composite electrode PY-P11 (ph 0 ) 0.1 g of the sample was added. Then, the ph of the suspension of the samples was recorded over time with constructing curves that reflected the acid-base properties of the solid surface and demonstrated the predominance of acid or base sites. The distribution of surface sites with respect to acid-base properties was studied by the method of Hammett and Tanabe [7, 8] using 17 indicators with p Kα values in the range The optical density was determined using a spectrophotometer PE-5400B (Russia). The concentration of active sites of the force q equivalent to the amount of adsorbed dye was determined by the technique in [9, 10] and calculated by formula: where c ind and v ind are the concentration and volume of the indicator, m 1 and m 2 are sample weights for D 1 and D 2 measurements, D 0 is the optical density of the indicator solution before sorption, D 1 is the optical density of the indicator solution after sorption, D 2 is the optical density of the indicator solution in the blank test; The signs (+) and ( ) correspond to a multidirectional and unidirectional change in D 1 and D 2 relative to D 0 [7]. The distribution curves of the adsorption sites with respect to the acid strength on the surface of the samples were constructed in the coordinates q pka = f( pka ). The function of the acidity of the surface of the composites was calculated as the average statistical value of the set of all the sites ( q рkа ): рk а q рkа H 0 =, q рkа where q рkа is the number of sites of a given acid strength [7]. To assess the sorption properties of the composites, oxalic acid and dyes Methyl orange (MO), Methylene blue (MB), Bromothymol (BT), and Thymol blue (TB) were used. The composite (100 mg) was poured into 10 ml of a solution of a dye with a concentration of 5 10 mg L 1 or H 2 C 2 O 4 with a concentration of 45 mg L 1 and placed on a magnetic stirrer for 20 min. Therewith sorption was carried out in the light and in dark conditions (dark). The content of dyes in solution was monitored by spectrophotometric method (SP): MB 661 nm; MO 460 nm; BT, TB 430 nm. The concentration of oxalic acid was determined by potentiometric titration with KMnO 4 solution. The sorption R (%) of dyes and H 2 C 2 O 4 was estimated from their loss from the solution by the formula: (c in c res ) R = 100, c in where c in and c res are the initial and residual concentrations of dyes and H 2 C 2 O 4 in solution. RESULTS AND DISCUSSION The phase composition of the investigated materials and the content of iron in them are shown in Table 1. In addition to the phases of the main components, boron nitride (α-bn) and silicon nitride (β-si 3 N 4 ), the samples contain phases of ferroboron (FeB, Fe 2 B) and ferrosilicon

3 1248 SKVORTSOVA et al. Table 1. Phase composition and iron content in composite materials based on boron nitride, silicon and sialon Composite no. The basis of the composite Phase composition w(fe), % 1 α-bn BN, Fe, FeB + Fe 2 B, Fe 3 C, H 3 BO α-bn BN, Fe, FeB + Fe 2 B, Fe 3 C, H 3 BO β-si 3 N 4 Si 3 N 4, Fe, Fe х Si y α-bn, β-si 3 N 4 BN, Si 3 N 4, Fe, FeB β-si 3 N 4, α-bn Si 3 N 4, BN, Fe, FeSi 2, FeB + Fe 2 B Si 3 Al 3 O 3 N 5 Si 3 Al 3 O 3 N 5, Fe, Si 3 N 4, SiC (Fe x Si y ), which did not enter the nitriding process, phase of intermediate products: iron silicide (FeSi 2 ), as well as the phases of the combustion products: iron (Fe), iron carbide (Fe 3 C). The integral acidity of the surface was estimated by the ph-metry to determine whether the surface sites belong to the hydrated (Brønsted) or aprotic (Lewis) type. Figure 1 shows the curves of ph changes in aqueous suspensions of composites over time. It can be seen that for composites based on boron nitride (nos. 1, 2), a sharp change in ph is observed in the main region during the first 30 s after immersion of the sample in the deionized water, which indicates the presence of rapidly hydrated Lewis aprotic sites. A slow and smooth change in the ph of aqueous suspensions of composites based on silicon (no. 3) and sialon (no. 6) nitrides indicates the presence of already hydroxylated Brønsted sites. The acidity of the composite suspension on a mixed basis (no. 4) does not change over time and is close to the ph of the deionized water. This may be due to the fact that the ratio of acid and base sites on the surface of a given composite is similar to that of water (the ratio of the concentrations of hydrogen and hydroxyl ions). At the same time, dynamic exchange processes in the system practically do not lead to a change in the ph of the suspension. To determine the number of sites of a certain acid-base force, curves of the concentration distribution of surface active sites with a certain pk a value were plotted using a working set of dye indicators in a wide range of pk a. It was shown (Fig. 2) that the surface of composites based on boron (nos. 1, 2) and sialon (no. 6) nitrides is dominated by the aprotic main Lewis sites, in contrast to the surface of samples based on silicon nitride (no. 3) and on the mixed basis (nos. 4, 5), where the Brønsted acid sites predominate (Fig. 3). Fig. 1. The curves of ph changes of aqueous composites suspensions in time (s). Figures at curves correspond to numbers of composites in Table 1; the same for Figs. 2, 3. Fig. 2. Distribution of adsorption sites q (μmol g 1 ) on the surface of composites based on boron and sialon nitride.

4 INVESTIGATION OF ACID-BASE AND SORPTION PROPERTIES OF SURFACE 1249 To identify the surface active sites of the composites, separate phases were studied: ferroboron (FeB), boron nitride (BN), and metallic (carbonyl) iron. The distribution of the active sites of their surface is shown in Fig. 4. It can be seen that the Lewis base sites and the Brønsted protolytic sites form two phases: ferroboron and boron nitride. Brønsted acid sites of (pk a 0 7) are formed as a result of complete or partial hydration of iron, boron, and silicon, and the Brønsted base sites (pk a 7 14) as a result the hydration of Lewis aprotic sites. Metallic iron forms on its surface a weak acid site with pk a 6.4. In [11] in researching the ferrosilicon suspension it was established that the surface iron is oxidized by oxygen dissolved in the aqueous suspension to Fe 2+ and Fe 3+ ions, which in the neutral and slightly alkaline media are hydrolyzed to form the corresponding hydroxides. The hydroxides then crystallize into oxides. Silicon exists in the stable form of monosilicic acid. In the study of the surface active sites of iron(iii) oxide, the authors of [12] also found predominantly Brønsted weak acid sites. The functions of the acidity of the surface of composites and the initial phases are shown below. The values obtained confirm the weakly acid nature of the materials under study: Composite no FeB Fe Н Based on the results of the work and the literature data the surface sites of the investigated composites were identified (Table 2). Thus, the diversity of the Lewis and Brønsted sites on the surface of the composites under study suggests Fig. 3. Distribution of adsorption sites q (μmol g 1 ) on the surface of composites based on silicon nitride and on the mixed basis. q pk a Fig. 4. Distribution of adsorption sites q (μmol g 1 ) on the surface of individual phases of composites. Table 2. Surface active sites of composites under study Composite (basis) BN; Si 3 N 4 ; Si 3 Al 3 O 3 N 5 Si 3 N 4 ; BN Si 3 N 4 ; BN; Si 3 Al 3 O 3 N 5 Si 3 N 4 ; BN Si 3 N 4 ; BN Phase of the composite under study FeB; BN FeB; BN Fe; FeB; BN; (Fe х Si y ) Functional groups (FG) N: FeOH Н δ+ BО H δ+ SiO H δ+ FeB; BN Fe; FeB Fe 2 O 3 H 2 О =N : H OH Fe 3+ Si(OH) 4 рk а ~7 14 Acid-base site Lewis bases Brønstede sites Lewis acids strong acid sites weak acid sites

5 1250 SKVORTSOVA et al. Table 3. Degree of dark and light adsorption R of dyes and H 2 C 2 O 4 by composites. с МB = 5 mg L 1, с МО = с BTB = с ТB = 10 mg L 1, с Н2 С 2 О 4 = 45 mg L 1, m kt = 100 mg, V sol = 10 ml Composite no. light adsorption R, % dark adsorption МО МB BTB ТB Н 2 С 2 О 4 МО МB BTB ТB Н 2 С 2 О their high adsorption activity to a wide range of organic compounds. Table 3 shows the results of the estimation of dark and light sorption of dyes and H 2 C 2 O 4 by the samples under study. It can be seen that a high degree of sorption of the cationic dye MB (pk a 1.23) is observed for composites based on boron nitride and a mixed composites with the predominance of the BN phase (no. 4), which can be explained by the presence of a large number of Lewis bases (Fig. 3). The lesser sorption activity of composites to MO (pk a 3.56) and H 2 C 2 O 4 (pk a1 1.25; pk a2 4.27) is due to the small number of corresponding active sites on the surface of the materials. For the anionic dye TB, the lowest values of the sorption degree are observed, which is related to the minimum number of Brønsted weak base sites with pk a The degree of light adsorption of the investigated organic pollutants is higher than the dark one in the presence of samples based on boron nitride, which may be due to the presence of photoactive compounds (BN, B 4 C, FeB) on the surface of composites and high iron content. Taking into account the nature of surface functional groups of metal-ceramic composites, the mechanism of adsorption of the investigated organic substances was proposed: (1) Adsorption of Methylene blue (pka 1.23) (2) Adsorption of oxalic acid (рk а1 1.25; рk а2 4.27) (3) Adsorption of Methyl orange (рk а 3.56) (4) Adsorption of Thymol blue (рk а 8.80) /N : H OH + Hind /N : H... Ind + Н 2 О. (5) Adsorption of Bromothymol blue (рk а 7.30) /Fe 2 O 3 H 2 О + 2Hind /2Fe(OН)... 2 Ind, /Si(OH) 4 + Hind /Si(OН)... 3 Ind + Н 2 О. CONCLUSIONS (1) The general acid-base state of the samples under study was investigated by ph-metry. A sharp change in ph in the base region for boron nitride-based composites indicates the presence of rapidly hydrated Lewis aprotic sites, and a smooth ph change for samples based on silicon and sialon nitrides suggests the presence of already hydroxylated Brønsted sites.

6 INVESTIGATION OF ACID-BASE AND SORPTION PROPERTIES OF SURFACE 1251 (2) The indicator method of Hammett and Tanabe with the spectrophotometric indication was used to identify and quantify the surface sites of different acidity. It was found that the surface of samples based on silicon nitride is dominated by the Brønsted acid sites, and on the surface of samples based on boron nitride and sialon, the Lewis aprotic base sites formed predominantly by FeB and BN phases. (3) The adsorption activity of composites to dyes and H 2 C 2 O 4 was correlated with the nature and number of active sites on the surface of the material. ACKNOWLEDGMENTS The studies were carried out with the support of the Russian Foundation for Basic Research project no a. REFERENCES 1. Bacardit, J., Stotzner, J., and Chamarro, E., Ind. Eng. Chem. Res., 2007, vol. 46, pp Chen, L., Ma, J., Li, X., et al., Environ. Sci. Technol., 2011, vol. 45, no. 9, pp De la Cruz, N., Gimenez, J., Esplugas, S., et al., Water Res., 2012, vol. 46, pp Skvortsova, L.N., Chuklomina, L.N., Mokrousov, G.M., et al., Russ. J. Appl. Chem., 2012, vol. 85, no. 1, pp Skvortsova, L.N., Chuklomina, L.N., and Batalova, V.N., Russ. J. Appl. Chem., 2014, vol. 87, no. 11, pp Slizhov, Yu.G., Matveeva, T.N., and Minakova, T.S., Russ. J. Phys. Chem. A, 2012, vol. 86, no. 3, pp Nechiporenko, A.P., Donorno-aktseptornye svoistva poverkhnosti tverdofaznykh sistem. Indikatornyi metod (Donor-acceptor Properties of the Surface of Solid Phase Systems. Indicator Method), St. Petersburg: Lan, Nechiporenko, A.P. and Kudryashova, A.I., Zh. Prikl. Khim., 1987, vol. 60, no. 9, pp Pakhnutova, E.A. and Slizhov, Yu.G., Russ. J. Phys. Chem. A, 2014, vol. 88, no. 8, pp Nechiporenko, A.P., Burenina, T.A., and Koltsov, S.I., Zh. Obshch. Khim., 1984, vol. 35, no. 9, pp Dvoychenkova, G.P., Timofeev, A.S., Chernysheva, E.N., and Koval chuk, O.E., Min. Inform. Anal. Bull., 2015, no. 9, pp Karpova, S.S., Moshnikov, V.A., Mjakin, S.V., and Kolovangina, E.S., Semiconductors, 2013, vol. 47, no. 3, pp