, Volume: 19( 3) DOI: 10.37532/0972-768X.2021.19(s1).001
New Methodology to Determine the Surface Energy, Specific Interactions and Acid-Base Properties of Titanium Dioxide by Inverse Gas Chromatography
- Tayssir Hamieh Satie-Ifsttar, Université Gustave Eiffel, Campus de Marne-La-Vallée, E-mail:[email protected]
Inverse gas chromatography was used to characterize the thermodynamic surface and interfacial properties of titanium dioxide catalyst. The dispersive component of the surface energy of TiO2 was determined by using the various models of surface areas of n-alkanes. An excellent linear relation correlating the differential enthalpy and entropy change of adsorption to the carbon atom number was obtained. A new method to separate the polar and non-polar contributions of the enthalpy of adsorption was proposed. The values of polar contributions of differential enthalpy of the probes were classified in increasing order: Benzene
Inverse gas chromatography was used to characterize the thermodynamic surface and interfacial properties of titanium dioxide catalyst. The dispersive component of the surface energy of TiO2 was determined by using the various models of surface areas of n-alkanes. An excellent linear relation correlating the differential enthalpy and entropy change of adsorption to the carbon atom number was obtained. A new method to separate the polar and non-polar contributions of the enthalpy of adsorption was proposed. The values of polar contributions of differential enthalpy of the probes were classified in increasing order: Benzene<Tetrahydrofuran<Ethyl acetate<Chloroform<Acetone<Acetonitrile<Nitromethane This order was confirmed by that of the relative polarities of the polar probes. We obtained an interesting linear relation of the polar enthalpy of adsorption versus the relative polarity of molecules. It was proved than the TiO2 surface exhibits an amphoteric acid base character with stronger basicity. The determination of the specific surface enthalpy and entropy of adsorption and allowed to evaluate the acid-base constants of this catalyst. It was proved that the acid constant KA=2.73, the basic constant KD=10.82 and their ratio KD/KA=3.97 clearly showed a catalyst surface rather basic than acidic. The comparison with the results obtained by zetametry confirmed those obtained by inverse gas chromatography. It was proved that the method of the vapor pressure of molecules adsorbed on the Titania surface gave the most accurate acid base constants relative to the methods of molecule polarizability and morphological index of polar and n-alkane molecules.
Titania; Surface energy; Enthalpy and entropy of adsorption; Acid base constants; Donor and acceptor number
Titanium dioxide, TiO2, has been widely employed in our lives as the common compound of titanium. Besides titanium (IV) oxide and Titania, TiO2 is also termed E171 in food coloring and titanium white or pigment white 6 in building paints [1-3]. In addition, the powder form of TiO2 possesses a hydrophilic character [3,4]. Titania embraces various merits and certain optical, electronic and physiochemical properties which makes it convenient for a wide range of applications. TiO2 is of low cost, available, biologically and chemically inert, stable and corrosion resistant. It is a material of good biocompatibility and strong oxidizing power. In addition, it is efficient as an anti-bacterial agent, in the environmental purification due to its high photocatalytic activity and in the UV protection [2-6]. It seems very interesting to study the physicochemical properties of the Titania catalyst known as Degussa P25. TiO2 P25 has been extensively used thanks to its high photocatalytic activity. In fact, it was mentioned that it is not easy to find a photocatalyst of greater photoactivity as compared to P25. Based on such information, P25 is described as the ‘de-facto standard titania photocatalyst’ and has been commonly employed as a ‘benchmark’ for photocatalysts. It has been also demonstrated that in several reactions, this mixed-phase Titania exhibited a higher photoactivity than pure anatase. Many papers have been issued on the photocatalytic reactions involving Degussa P25 TiO2 since 1990 [7-9]. We propose in this paper to determine the surface properties of commercial Degussa P25 TiO2 particles constituted of 80% anatase and 20% rutile; more particularly, we are interested in the determination of specific interactions of adsorption, Lewis acid base and surface energy of this catalyst by inverse gas chromatography (IGC) at infinite dilution. Organic model molecules were used in this study such as n-alkanes (from n-pentane to n-nonane) and some polar adsorbates such as benzene, chloroform, acetonitrile, nitromethane, ethyl acetate, acetone and tetrahydrofuran. These molecules adsorbed on Titania catalyst allow obtaining by IGC the dispersive component of the surface energy, specific enthalpy and entropy of adsorption and the acid base constants in Lewis terms of TiO2.
IGC technique, methods and materials
IGC is the most important technique to characterize the surface properties of solid substrates such as oxides, polymers or copolymers in bulk or adsorbed phases IGC. This powerful technique first used by Conder and Young in 1970’s [10-14] took an important development after 1980 where many researches were devoted to the physicochemical determination of oxides, glass fibers and polymers [15-23].
Polar molecules and n-alkanes of known properties are injected in the column containing the solid. The retention times of these probes, measured at infinite dilution, allow us to determine the interactions between the organic molecules and the solid, if we suppose that there are no interactions between the probe molecules themselves. Measurements were carried out with a DELSI GC 121 FB Chromatograph equipped with a flame ionization detector of high sensitivity. The retention data were obtained with a stainless-steel column of length 20 cm and 2 mm internal diameter packed with 1 g of titania powders. The net retention volume Vn was calculated by following the same methodology used in other papers [24-26].
On the other hand, the method used to obtain specific enthalpy of interaction between a probe and a solid is that developed by Saint-Flour and Papirer [17-19] who obtained a straight line when plotting RTlnVn against lnP0 where P0 is the vapour pressure of the probes. In the case of polar probes injected into the column, specific interactions of adsorption ΔGsp can be calculated by using this method.
Polar molecules used to determine the specific interactions with the solid substrates are characterised by their donor (DN) and acceptor (AN) numbers . The acidic KA and basic KD constants characterising the solid substrate were calculated by using Papirer et al. method . The dispersive component of the surface energy of solids was determined by using the well-known relationship of Fowkes [28,29].
We used another relation proposed by Dorris and Gray  for the calculation of of a solid.
Materials and Solvents
The titania catalyst was obtained from Degussa in powder form. The titania analysed here, had a specific surface area obtained by BET method by adsorption of nitrogen on titania, 59 m2/g.
Model organic molecules
Classical organic molecules, characterized by their donor and acceptor numbers [25,29], were used in this study. Corrected acceptor number AN'=AN-ANd, given by Riddle and Fowkes  who subtracted the contribution of Van der Waals interactions (or dispersion forces), was used in this paper. We used here the values of AN' and AN' of different polar molecules adopted by Hamieh et al. [24-26].
The different solvents used for IGC measurements were chosen for acid, base and amphoteric properties necessary to determine the acid-base characteristics of the titanium dioxide. All probes (Aldrich) were highly pure grade (i.e., 99%). The probes used were n-alkanes (pentane, hexane, heptane, octane, and nonane); amphoteric solvents: acetonitrile, acetone; basic solvents: ethyl acetate, tetrahydrofuran (THF) and acidic solvent: chloroform and nitromethane.
The IGC measurements were performed on a DELSI GC 121 FB chromatograph equipped with a flame ionization detector. Dried nitrogen was the carrier gas. The gas flow rate was set at 20 mL/min. The injector and detector temperatures were maintained at 200 ºC during the experiments. To achieve infinite dilution, each probe was injected with 1 μL Hamilton syringes taken from the vapor above the liquid solvent surface and emptied into air, in order to approach linear condition gas chromatography, equipped with a split system. In such a way that the interactions between probe molecules can be considered to be negligible and only the interactions between the surface of the solid and an isolated probe molecule are important. The column temperatures were 40 to 120 ºC, varied in 10 ºC steps. Each probe injection was repeated three times, and the average retention time, tR, was used for the calculation. The standard deviation was less than 1% in all measurements. In all experiments, the real retention time was systematically calculated based on the first order retention time taking into account the peak asymmetry. The packed columns were then preconditioned (at a temperature equal 130 ºC and under a nitrogen flow rate) overnight to remove any residual solvent left in the packing material.
Therefore, the unitless ratio KA/KD (from the values given by Lee et al.  is equal to KA/KD=0.36 or KD/KA=2.78. This result approaches the result that we obtained by using the vapor pressure method (KD/KA=3.97) but it is very different from the last method using the topological index. Bogillo and Voelkel  also studied the surface properties of the titanium dioxide with is modified forms. By using the vapor pressure method they obtained KA/KD=-0.138 mol/kcal and with the polarizability method the ratio was KA/KD=0.464 mol/kcal. These values can be converted to the following unitless ratios KD/KA=-18.16 and KD/KA=5.40. The negative value obtained by Bogillo and Voelkel cannot be admitted and probably they committed a mistake in their calculations. However, the second value obtained by the polarizability method, can be approached to the value obtained by our study.
It can be conclude that the acid-base surface properties of titanium dioxide strongly depend on the chosen methods of the determination of the specific free enthalpy of interaction between the polar molecules and the solid substrate. The method using the vapor pressure of molecules gives the more accurate values of KA and KD. However, the various results obtained by the different IGC methods can be only considered as qualitative. In this study, we deduced that the titanium dioxide exhibits more basic than acidic surface.
In this study, we determined the acid base interactions and the surface properties of titanium dioxide catalyst by inverse gas chromatography at infinite dilution. The curves of lnVn=f(1/T) allowed to obtain the differential heat, the enthalpy and entropy of adsorption of different organic probes adsorbed on the titanium oxide surface. We separated the two polar and dispersive contributions of polar molecules adsorbed on the Titania surface. For the first time, we gave a linear relation between the polar enthalpy of adsorption of probes and their relative polarities with an excellent accuracy. The different molecular models of the surface areas of n-alkanes were applied to calculate the dispersive component of the surface energy of TiO2 catalyst. The obtained results clearly showed a linear dependency of against the temperature for all used model of surface area of n-alkanes. The application of Saint-Flour and Papier method allowed determining the specific free energy of adsorption as a function of the temperature and then to deduce the specific enthalpy and entropy of polar probes adsorbed on titania surface. It was proved that titanium dioxide surface is an amphoteric surface with stronger basic character. The acid KA and base KD constant of this catalyst were determined KA=2.73, KD=10.82 and KD/KA=3.97. The results obtained by zetametry for the dispersion of titania particles in organic liquids allowed to obtain the acceptor ANS´and donor DNS´ numbers of the titania catalyst: DNS´=46, ANS´=11 and DNS´/ANS´=4.18. These results proved that DNS´=4.25 KD and ANS´=4.03 KA showing an acid base character of TiO2 catalyst in organic liquid medium obtained in zetametry, larger than that obtained in inverse gas chromatography at infinite dilution. The comparison of the method based on the vapor pressure to those using the deformation polarizability and the topological index led to conclude that all method gave an identical order of the interaction force of the various polar probes and similar acid base constant ratio KD/KA. The method using the vapor pressure seems to be the best one, because it is based on the dependency of the vapor pressure of molecules on the temperature and the obtained results by this method are more accurate.
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Conflicts of Interest/Competing Interests
There is no conflict of interests/Competing interests
Availability of Data and Material
Tayssir Hamieh, PhD, PhD, HDR, ENG (Conceptualization: Equal; Formal analysis: Equal; Funding acquisition: Equal; Investigation: Lead; Methodology: Lead; Project administration: Lead; Resources: Equal; Supervision: Equal; Validation: Equal; Writing-original draft: Lead; Writing review and editing: Lead)
Fatima Al-Ali, PhD (Conceptualization: Equal; Formal analysis: Equal; Investigation: Supporting; Methodology: Supporting; Validation: Supporting; Writing original draft: Supporting)
Ali Ali Ahmad, PhD Student (Formal analysis: Equal; Investigation: Supporting; Methodology: Supporting; Validation: Equal; Writing original draft: Supporting)
Khaled Chawraba, PhD Student (Formal analysis: Equal; Investigation: Supporting; Methodology: Supporting; Validation: Equal; Writing original draft: Supporting)
Joumana Toufaily, PhD, HDR (Conceptualization: Equal; Formal analysis: Equal; Funding acquisition: Supporting; Investigation: Supporting; Methodology: Supporting; Resources: Equal; Validation: Equal; Writing original draft: Supporting)
Zahraa Youssef, PhD (Conceptualization: Supporting; Formal analysis: Supporting; Investigation: Supporting; Methodology: Supporting; Validation: Supporting; Writing original draft: Supporting)
Nabil Tabaja, PhD (Conceptualization: Supporting; Formal analysis: Supporting; Investigation: Supporting; Supervision: Supporting; Validation: Supporting; Writing original draft: Supporting)
Thibault Roques-Carmes, PhD, HDR (Conceptualization: Equal; Formal analysis: Equal; Investigation: Supporting; Methodology: Supporting; Project administration: Supporting; Supervision: Equal; Validation: Equal; Writing original draft: Supporting)
Jacques Lalevée, PhD, HDR (Conceptualization: Equal; Formal analysis: Equal; Investigation: Supporting; Methodology: Supporting; Project administration: Supporting; Supervision: Equal; Validation: Equal; Writing original draft: Supporting).