Determination of Surface Properties of Various Oxides

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Determination of Surface Properties of Various Oxides and Sulfides by Inverse Gas Chromatography Slobodan K. Milonji a a The Vina Institute of Nuclear Sciences, Chemical Dynamics University, Belgrade, Serbia Online Publication Date: 01 October 2009

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Chromatography',Materials and Manufacturing Processes,24:10,1086 — 1089 To link to this Article: DOI: 10.1080/10426910903032147 URL: http://dx.doi.org/10.1080/10426910903032147

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Materials and Manufacturing Processes, 24: 1086–1089, 2009 Copyright © Taylor & Francis Group, LLC ISSN: 1042-6914 print/1532-2475 online DOI: 10.1080/10426910903032147

Determination of Surface Properties of Various Oxides and Sulfides by Inverse Gas Chromatography Slobodan K. Milonjić The Vincˇ a Institute of Nuclear Sciences, Chemical Dynamics University, Belgrade, Serbia The surface properties of various oxides (ZrO2 and Fe3 O4 ) and sulfides (HgS and FeS) were investigated by the inverse gas chromatography method (IGC). The retentions of several organics were measured in the temperature range from 330 to 500 K. The dispersive component of the surface free energy (sd ) of studied adsorbent surfaces was estimated using retention times of different nonpolar organics in the infinite dilution region. The sd values decrease with temperature increase. The obtained results proved that IGC is an efficient and successful technique for the characterization of surface properties of these kinds of materials.

Downloaded By: [Milonji, Slobodan K.] At: 20:54 11 September 2009

Keywords Inverse gas chromatography; Oxides; Sulfides; Surface free energies; Surface properties.

1. Introduction The potentialities of inverse gas chromatography (IGC) for physicochemical characterization of different solids have been known for many decades [1–3]. Contrary to conventional gas chromatography (GC), where the stationary phase (sorbent) serves to separate and identify various gaseous components, IGC uses specific solute probes (organics) to determine the interaction between solutes and the solid column packing material under investigation. IGC can be conveniently carried out with a simply modified commercial gas chromatograph in a wide range of temperatures. Measurements may be carried out both at zero and finite surface coverage. In the case of zero coverage (Henry’s law region), vapors of testing adsorbates (probes) are injected onto the column filled with inorganic or organic solid. In this case, the testing probes interact with strong active sites on the examined solid surface and the lateral interaction is minute, if not negligible. Because of these advantages as well as its simplicity, accuracy, and speed, IGC has been frequently applied to surface characterization of porous and nonporous solid materials, such as polymers, activated carbons, carbon fibers, oxides, non-oxide ceramics, graphites, zeolites, hydroxyapatites, clays, and other materials. Only several review articles on this subject, published recently, have been cited [4–7]. The surface energy of solid materials is an important property that controls many processes such as adsorption, catalysis, friction, adhesion, dispersion stability, and wetting by liquids (wettability). Industrial importance of inorganic oxides and metal sulfides as adsorbents, catalysts, sensors, fillers, abrasive materials, etc. are well recognized.

Received November 7, 2008; Accepted January 14, 2009 Address correspondence to Slobodan K. Milonjić, The Vinˇca Institute of Nuclear Sciences, Belgrade 11001, Serbia; E-mail: smiloni@ vin.bg.ac.yu

This article is a continuation of our work devoted to the study of the adsorption of various organics on natural magnetite, zirconia, mercury, sulfide, and iron (II) sulfide [8–12]. The aim of the present study is to determine the dispersive component of the surface free energy of the named solids by IGC method. 2. Experimental 2.1. Materials Four materials, used as adsorbents, were examined. Natural magnetite, Fe3 O4 , from Baljevci na Ibru Mines, Serbia, was ground and sieved, and a 0.160–0.250 mm fraction was used. The fraction was washed with doubly distilled water and dried for 24 hours at 383 K. The content of the initial natural magnetite used in this work, determined by chemical analysis, was as follows: Fe2 O3 . FeO = 80.10%; SiO2 = 8.40%; Al2 O3 = 0.79%; CaO = 1.20%; NiO = 0.16%; and MgO = 0.99%. The specific surface area of the fraction was 2.0 m2 g−1 . In experiments, three magnetite samples were used: initial (untreated) magnetite, magnetite treated with hydrochloric acid, and magnetite coated with Carbowax 20 M (1% in chloroform). These samples are designated as Fe3 O4 # 1, Fe3 O4 # 2, and Fe3 O4 # 3. Zirconium oxide, ZrO2 , was prepared by precipitation of ZrOCl2 solution with ammonia, described previously [8]. The precipitant was dried in air at 383 K for the period of 10 days. The material was ground and sieved. The fraction of 0.250–0.350 mm was split into three portions and each was thermally treated for 4.5 h in air, at 523, 873, and 1173 K, respectively. The specific surface areas of the samples were 220, 34, and 13 m2 g−1 for materials heated at 523, 873, and 1173 K, respectively. Throughout this work these adsorbents are referred to as ZrO2 # 1, ZrO2 # 2, and ZrO2 # 3, respectively. Mercury sulfide, HgS, was prepared by precipitation of HgCl2 solution with H2 S, described previously [9]. The resultant precipitate was filtered, thoroughly washed

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SURFACE PROPERTIES OF VARIOUS OXIDES AND SULFIDES


with deionized water, dried, ground, and sieved. The specific surface area of the used fraction (0.100–0.150 mm) was 3.2 m2 g−1 . Iron (II) sulfide, FeS, was a commercial product (Kemika, Zagreb, Croatia). The specific surface area of the used fraction was 0.5 m2 g−1 . The same adsorbents have been used to study the adsorption of several organics by IGC [8–12]. The adsorbates (probes), obtained from various commercial sources, were of analytical reagent grade. N -pentane, n-hexane, n-heptane, and n-octane were used as nonpolar probes. These probes are commonly used for determination of the dispersive component of the surface free energy of solids by IGC method. 2.2. Instrumentation and Columns Specific surface areas of the obtained materials were determined by a Ströhlein area meter using the single point nitrogen adsorption method. The adsorption of nitrogen gas was performed at 77 K. To determine crystallinity, X-ray diffractograms of the used adsorbents were recorded by a Siemens Kristalloflex 4 with a GM counter. The chromatographic experiments were performed with Perkin-Elmer model F-17 and 881 gas chromatographs equipped with a flame ionization detector (FID). High purity nitrogen was used as a carrier. The column inlet pressure was measured by a precision manometer, and the outlet pressure (assumed to be atmospheric) by a precision barometer. All other relevant parameters were measured and controlled with precision required for physicochemical measurements. Stainless steel columns (2 m × 2.2 mm I.D.) previously washed with both polar (ethanol) and nonpolar (n-hexane) solvents were packed with the selected adsorbents. After the packing, each column was conditioned overnight at 523 K in a stream of nitrogen (flow rate 5 cm3 min−1 ). Each adsorbate was injected as vapor of a volume less than 0.1 L at least three times and the retention times were averaged. The system dead-time was assumed to be equal to the retention time of methane at the column temperature. The experiments were carried out in the temperature range of 332–493 K. To ensure equilibrium conditions for the adsorption, the nitrogen flow rate was varied in the range from 10 to 20 cm3 min−1 . The carrier gas flow rate was measured by a soap bubble flowmeter at the detector outlet and corrected for pressure drop in the column by using the James–Martin gas compressibility factor (j). The peaks were characterized by a good symmetry with retention volumes independent of the vapor sample size, indicating that the Henry’s law region was reached. 3. Results and discussion The adsorbate net retention volumes, VN , were calculated from the expression VN = tr − to Ff T /Tf jpo − pw /po

(1)

Table 1.—Standard free energy change, −Ga (kJ mol−1 ), at 373 K for listed n-alkanes with natural magnetite samples. Adsorbent

n-C5

n-C6

n-C7

n-C8

Fe3 O4 # 1 Fe3 O4 # 2 Fe3 O4 # 3

11.10 15.65 11.87

12.98 18.42 12.20

15.33 20.55 12.64

– 25.25 13.36

where tr is the adsorbate (probe) retention time, to is the retention time of a nonretained compound (in our case methane), Ff is the flow rate measured with a soap bubble flowmeter at temperature Tf , T is the column temperature, po is the column outlet pressure (taken as barometric), pw is the vapor pressure of water at Tf , and j is the James–Martin gas compressibility correction factor. The surface partition coefficient, Ks , defined as the net retention volume per unit of the adsorbent surface area As ), was then calculated as Ks = VN /As

(2)

where As is the product of the specific surface area, S, and the mass of adsorbent in the column. Knowing the partition coefficients, the standard free energy change of adsorption, Goa , can be calculated from the relation [13, 14] Goa = −RT lnKs ps/g /s

(3)

where R is the gas constant, ps/g is the adsorbate standardstate vapor pressure, and s is the two-dimensional standard-state (surface) spreading pressure of the adsorbed gas. The standard reference state is taken [13, 15] as ps/g = 101 kNm−2 (101 kPa) and s = 0338 mNm−1 . The data for the standard free energy change of adsorption of n-alkanes on magnetite, zirconia, and metal sulfides are given in Tables 1–3, respectively. For the sake of brevity, Goa values at other investigated temperatures are not presented in the article. As expected, Goa of the n-alkane homologue series linearly increases with the increasing chain length, molecular weight, boiling points, and molar refractions. This trend is valid for all temperatures and all investigated adsorbents. For a given adsorbate, the standard free energy change of adsorption is the sum of energies of adsorption attributed to dispersive and specific interactions. Adsorption of nonpolar adsorbates, as n-alkanes, is caused by dispersive interactions, whereas for polar adsorbates both London and

Table 2.—Standard free energy change, −Goa (kJ mol−1 ), at 473 K for listed n-alkanes with prepared zirconia adsorbents. Adsorbent

n-C5

n-C6

n-C7

n-C8

ZrO2 # 1 ZrO2 # 2 ZrO2 # 3

13.49 11.01 12.06

17.86 13.88 14.78

22.25 17.14 18.32

– 20.38 21.95

´ S. K. MILONJIC

1088 Table 3.—Standard free energy change, −Ga (kJ mol−1 ), for listed nalkanes, at 393 and 373 K, with HgS and FeS, respectively. Adsorbent

n-C5

n-C6

n-C7

n-C8

HgS FeS

– 15.20

17.73 18.32

21.44 22.02

25.14 22.94

sd = G2CH2 /4CH2 N 2 a2CH2

acid-base interactions contribute to Goa : Goa = Gda + Gsa

(4)

where Gda and Gsa are the dispersive and the specific component of the standard free energy change of adsorption, respectively. For n-alkanes, Goa = Gda and changes with the number of carbon atoms in their molecules. The increment of adsorption energy corresponding to methylene group, GCH2 , may be calculated from: Downloaded By: [Milonji, Slobodan K.] At: 20:54 11 September 2009

interaction, and the probe-probe interaction can be neglected under zero surface coverage (i.e., infinite dilution of the gaseous probe). Dorris and Gray [16] have used the incremental amount of the free energy of adsorption, corresponding to the adsorption of one CH2 group, to determine the dispersive component of surface free energy

GCH2 = −RT lnVN n /VN n+1

(5)

where VN n and VN n+1 are the net retention volumes of two consecutive n-alkanes having n and n + 1 carbon atoms, respectively. GCH2 is independent of the chosen reference state of adsorbed molecule. Table 4 shows the GCH2 values for n-alkane series adsorbed on studied adsorbents at various temperatures. For all investigated adsorbents, GCH2 values decrease with increasing temperature, as expected. Intermolecular interactions in an adsorbate/adsorbent system may be dispersive and specific, which corresponds to the dispersive, sd , and the specific component, ss , of the free surface energy, s , of adsorbents s = sd + ss

(6)

An interaction between adsorbates (n-alkanes) and adsorbent surfaces is mainly attributed to the dispersive

where CH2 is the surface free energy of a solid containing only methylene groups such as polyethylene CH2 = 36.8– 0.058 T ( C) mJm−2 ), N is Avogadro’s number, and aCH2 is the cross-sectional area of an adsorbed CH2 group (0.06 nm2 ). The calculated sd values for all investigated adsorbents are reported in Table 5. For all investigated adsorbents, the dispersive component decreases with an increase in temperature, following the decrease in the net surface energy with increasing temperature. For acid treated magnetite (Fe3 O4 # 2), the sd value was found to increase linearly with the temperature decrease, in the temperature range (from 373 to 423 K): sd = 365.6– 0.825 T. The sd value (119.8 mJm−2 ) at 298 K was obtained by extrapolation of the above equation. For zirconium samples, the sd values were also found to increase linearly with the temperature decrease: Zirconia # 1 (from 433 to 493 K): sd = 304.6–0.406 T. The sd value (183.6 mJm−2 ) at 298 K was obtained by extrapolation. Zirconia # 2 (from 413 to 493 K): sd = 236.6–0.374 T. The sd value (125.1 mJm−2 ) at 298 K was obtained by extrapolation. Zirconia # 3 (from 413 to 473 K): sd = 263.9–0.420 T. The sd value (138.7 mJm−2 ) at 298 K was obtained by extrapolation. For mercury sulfide and iron (II) sulfide, the sd values were found to increase linearly with the temperature decrease: HgS (from 393 to 473 K): sd = 198.1–0.300 T. The sd value (108.7 mJm−2 ) at 298 K was obtained by extrapolation. Fe(II)S (from 332 to 373 K): sd = 455.4–1.104 T. The sd value (126.4 mJm−2 ) at 298 K was obtained by extrapolation.

Table 4.—The GCH2 (kJ mol−1 ) values for investigated adsorbents at various temperatures.

Table 5.—The sd (m Jm−2 ) values for investigated adsorbents.

Adsorbent

Adsorbent

Temperature Fe3 O4 #1 Fe3 O4 #2 Fe3 O4 #3 ZrO2 #1 ZrO2 #2 ZrO2 #3 HgS

333 353 373 393 408 413 423 433 443 453 473 483 493

– – 2.12 – – – – – – – – – –

– – 3.20 2.56 2.19 – 1.77 – 1.45 – – – –

0.91 – 0.50 – – – – – – – – – –

– – – – – – – 4.70 – 4.53 4.38 – 4.23

(7)

– – – – – 3.74 – 3.58 – 3.41 3.12 – 3.02

– – – – – 3.91 – 3.61 – 3.60 3.30 – 2.91

FeS

– 3.80 – 3.56 – 2.58 3.70 – – – 3.56 – – – 3.42 – – – 3.29 – 3.08 – – – – –

Temperature Fe3 O4 #1 Fe3 O4 #2 Fe3 O4 #3 ZrO2 #1 ZrO2 #2 ZrO2 #3 HgS

333 353 373 393 408 413 423 433 443 453 473 483 493

– – 26.3 – – – – – – – – – –

– – 59.8 38.3 28.0 – 18.3 – 12.3 – – – –

4.8 – 1.5 – – – 0.53 – – – – – –

– – – – – – – 129.1 – 119.9 112.1 – 104.6

– – – – – 81.8 – 74.9 – 68.0 56.9 – 53.3

– – – – – 89.4 – 82.6 – 75.7 63.6 – 49.5

FeS

– 84.4 – 74.1 – 38.9 80.0 – – – 74.1 – – – 68.4 – – – 63.3 – 55.4 – – – – –

SURFACE PROPERTIES OF VARIOUS OXIDES AND SULFIDES


To our best knowledge, there are no literature data on the dispersive component of the surface free energy for magnetite, zirconia, mercury, and iron (II) sulfides for comparison. The sd values for zirconia, acid treated magnetite, mercury, and iron (II) sulfides samples are much higher than those for silica [17], hydroxyapatite [18], MgY and NH4 Y molecular sieves [19], silicon nitride nanopowder [20], and several construction materials (Marble Sandstone, Granite, Brick) [21]. The sd values for zirconia samples are similar to those obtained for chromia [22]. It has been shown that the sd values obtained for sepiolite are rather low when compared to those obtained for other clay types (kaolinite, illite, bentonite) [23, 24]. Recently, Wang and coworkers [25] have reported the sd value (120 mJm−2 ) for carbon nanotubes, which is similar to those found for metal oxides. They have also shown reduction in sd of carbon nanotubes caused by chemical modification. 4. Conclusions The efficiency of IGC in determination of the dispersive component of surface free energy of magnetite, zirconia, mercury sulfide, and iron (II) sulfide was demonstrated. Nonpolar probes (n-alkanes) were used to monitor the change in nonpolar component of the surface free energy of adsorbents in the temperature range between 330 and 500 K. The acid treated magnetite exhibits a higher sd value (59.8 mJm−2 ) than unmodified (26.3 mJm−2 ) and treated with PEG (Carbowax 20 M) (1.5 mJm−2 ) magnetite. The sd values (129.1, 74.9, and 82.6 mJm−2 at 433 K) for zirconia samples are much higher and change with thermal treatment. The sd value for mercury sulfide is higher than the iron (II) sulfide one at the same temperature. The calculated sd values for all used adsorbents decrease with temperature increase.

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Acknowledgments The author is grateful to the Ministry of Science and Technological Development of the Republic of Serbia for financial support (Project No. 142004).

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