THALES Project No. 65/1181

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1 THALES Projet No. 65/8 Time distribution of adsorption ative sites on heterogeneous solid surfaes through experimental measurements of adsorption energies, loal adsorption isotherms and the energy distribution funtion by reversed-flow gas hromatography Researh Team Fani Roubani Kalantzopoulou, Professor, National Tehnial University, Athens, Greee Sofia Margariti, Ioannis Bassiotis, Vassilios Siokos, (Researh Students), National Tehnial University, Athens, Greee Introdution The Reversed Flow Gas Chromatography (RF-GC) is a well known dynami method ombining simpliity with auray Katsanos [5] and Katsanos et al. [6]. This method is useful espeially for omplex systems, where surfae heterogeneity plays a very important role and must be taken into aount in the physiohemial interpretation of the adsorption proess, Metaxa et al.[3], Kalogirou et al.[3]. This method based on two mass-balane equations, one in the gas region and the other in the solid bed one, is useful for evaluating adsorption isotherms beause it gives modelindependent results, Sotiropoulou et al. [2]. The RF-GC method is also desribed and evaluated for the determination of many physiohemial parameters in homogeneous gas phase, Roubani-Kalantzopoulou et al.[] and in heterogeneous as well, Yun et al. [22], Karagiorgos et al. [] and Birbatakou et al. []. The RF-GC method has been applied reently for the measurement of some loal parameters onerning adsorption energies, ε, with respet to time, Katsanos et al.[7,8], gaseous adsorbate onentrations above the solid surfaes, y, loal monolayer apaities,, that is the imum adsorbed onentrations of the probe gases on the olletion of the adsorption sites i being ative at time t and loal adsorption isotherms, θ i (p,t,ε), Roubani-Kalantzopoulou et al. [5]. In addition, the probability density funtion, φ(ε), Roubani-Kalantzopoulou et al. [6] (a modified funtion, of the former f(ε), Katsanos et al.[9] and Siokos et al. [9], of adsorption energies for some probe gases on heterogeneous solid surfaes, as well as the lateral moleular interation energies (dimensionless), βθ i, have been determined through the same experimental results, Katsanos et al.[]. The term loal means with respet to time, that is involves only a limited olletion of adsorption sites ative at time t. This artile onerns adsorption ourring at gas-solid interfaes, when the solids are heterogeneous in nature, as well determination of adsorption energies and related parameters of seven gaseous substanes (five organi in the presene or not of an

2 inorgani, O 3 or NO 2 ) on ten heterogeneous solid adsorbents. Thus, a time resolved hromatographi study is done for the haraterization of all these gas-solid interfaes. Experimental The experimental set-up and the proedure an be found in the invited review artile written reently by Roubani- Kalantzopoulou [7]. The light hydroarbons, C 2 H 2, C 2 H, C 2 H 6, C 3 H 6 and -C H 8, as well as the nitrogen dioxide, were obtained from Air Liquide and had a purity of 99.% %. The arrier gas was nitrogen % from Air Liquide, dried by silia gel. Ultra-highpurity hydrogen from the same ompany was also used as reatant as well as arrier gas for the study of heterogeneous atalyti reations. The O 3 was prepared in the laboratory by using a laboratory ozonizer (Ozoneur OZ- L of OZONIA Int.) and had a onentration of 3.3 g/nm 3. The solids Cr 2 O 3, CdS, TiO 2 and PbO 2 were pro-analysis produts of Merk. The solids ZnS, CaO, MgO and SiO 2 were produts of Fluka with a high purity (>99%). The atalysts used in atalyti studies were NiO blak from Fluka (nikel ontent: ~76%), and Co 3 O from Merk (obalt ontent: ~8%). Theory The RF-GC method does not depend on analytial or numerial solutions of the lassial integral Eq. () (f. Refs Margariti et al. [,2]), Tylipaki et al.[2], Roubani- Kalantzopoulou et al. [7], but on a time funtion of the extra hromatographi peaks obtained by short flow-reversals of the arrier gas, as Eq. (2) shows: Θ ( p, T ) = θ i ( p, T, ε ) f ( ε ) dε () / M ' H = g( l, t) = Ai exp( Bi t) (2) where Θ(p,T) is the overall experimental adsorption isotherm, θ i (p,t,ε) the loal isotherm, f(ε) the probability density funtion for the adsorption energies, H is the peak height, M the response fator of the detetor, A i the pre-exponential fators, and B i the exponential oeffiients of time t, when flow reversals were performed. The main equation of the experimental tehnique as well as of the orresponding model is Eq. (2). The relevant mathematial model with the alulation of A i and B i values from the experimental points H, t, and their physial ontent have been reported many times. Finally, the neessary relations for the alulation of ε,, θ i (θ t ), φ(ε) and β from Eq. (2), for the adsorption of gases on heterogeneous surfaes as a funtion of time, have been obtained through the ombination of Eq. (2) and the Jovanovi isotherm model written in the form as Eq. () of Ref. [7] shows: i=

3 θ ( p, T, ε) = exp( K p) (3) where K = K ( T ) exp( ε / RT ) () R being the gas onstant, and K υ S 3 h ( T ) = (5) 3 / 2 5 / 2 (2π m ) ( kt ) b ( T ) g Here, K is the Boltzmann s onstant; m the moleular mass of the adsorbate, h the Plank s onstant and u s (T)/b g (T) the ratio of two partition funtions, namely the one for the adsorbed moleule, u s (T), and that for the rotations-vibrations in the gas phase b g (T). This ratio equals approximately one. More preisely, the relations for the alulation of ε(kj mol - ), from experimental data are given by Eqs (6)-(8): ο [ ln( KRT ) ln( RT ) ln K ] (mol g - ) and θ t ε = RT (6) = S + K R T S y (7) S θ t = (8) K R T y Aording to Jaronie and Madey [7], the probability density funtion f(ε) of Eq. () desribing the adsorption energy distribution, is defined as the derivative of the number of adsorption sites with respet to the adsorption energy. Sine the number of adsorption sites is proportional to the loal monolayer apaity of Eq. (7), f(ε) may be defined as: Finally one obtains [23]: / t f ( ε ) = = ε ε / t (9) ( / t) 2 KRT ( ) s + s / y t s / y f ( ε ) = RT KRT / t KRT () The modified funtion φ(ε) is produed if f(ε) is multiplied by θ and the produt divided by. φ(ε;t) = θ f(ε) / ()

4 As regards the lateral interations of the adsorbed moleules, the dimensionless parameter β Margariti et al.[2] an be obtained: β = zω /RT (2) ω being the lateral interation energy and z the number of neighboring atoms for eah adsorption site. Thus, the θzω is the differential energy of adsorption added to ε due to lateral interations. Results and disussion The alulations of the new physiohemial quantities ε,, θ t, φ(ε) and β pertaining to heterogeneous surfaes, start from the diffusion bands of RF-GC experiments {f. Refs [, 7]} by reording the pairs H, t and alulating the preexponential fators A, A 2, A 3 and A, and the time oeffiients B, B 2, B 3 and B. By entering the pair values of H, t into the DATA lines of a non-linear regression analysis GW- PC program the physiohemial parameters and funtions exposed and defined in the setion Theory are alulated and printed. The adsorption energy ε versus experimental time, for the systems propylene/hromium oxide and propylene/ozone/hromium oxide (dot line) at K, in nitrogen atmosphere is presented in Fig.. The gas moleules at time t are exlusively adsorbed on sites i all of the same energy ε i. The ima and minima represent rather transition adsorption energies before their final leveling off with time. The loal adsorption isotherm, θ i, versus adsorption energy, ε i, for the systems propylene/zin sulfide and propylene/ozone/zin sulfide (dot line) at K, in nitrogen atmosphere is shown in Fig. 2. The shape of these urves resembles that given by Adamson and Ling, as exemplified by Rudzinski and Everret [8]. The main differene is that, in this ase, all the physiohemial quantities are obtained experimentally and not analytially. Conerning the dependene of distribution funtion of adsorption energies on the experimental time for the systems ethane/admium sulfide and ethane/ozone/admium sulfide (dot line) at the same onditions as above, three types of adsorption ative sites are evident, Fig. 3. It s impressive that similar results an be arried out from the orresponding plot in Fig. 8, onerning the heterogeneous atalyti hydrogenation of - butene on nikel oxide atalyti bed at 26 C. The lateral moleular interation energy βθ i (dimensionless) as a funtion of time for the system -butene/(ozone)/silion oxide in Fig. 5 shows that a similar behavior is observed and the plots also show three kinds of ative sites for adsorption. The same results are obtained from the plots in Fig., where a time resolved analysis regarding loal isotherms is presented for the system -butene/(ozone)/hromium oxide. The dependene of distribution funtion of adsorption energies in relation with the adsorption energies themselves for the systems ethylene/magnesium oxide and ethylene/ozone/magnesium oxide at K is presented in Fig. 6. A Gaussian urve is observed (the slight deviations from this behavior orrespond to the beginning and the end of the experimental results). In Fig. 7, the time resolved analysis of the loal monolayer apaities,, is strongly depending on the experimental temperature.

5 ε (kj mol - ) (Fig.) θ (dimensionless),,9,8,7,6,5,,3,2,, ε (kj mol - ) (Fig.2) φ(ε;t) ( -2 mol kj - min - ) (Fig.3) θ (dimensionless),8,6,, (Fig.) 2,,8,6,,2,,8,6,,2, βθ (Fig.5) φ(ε;t) (-2mol kj- min -) ε (kj mol - ) (Fig.6) Cs (µmol g - ) (Fig.7) φ(ε;t) ( -2 mol kj - min - ) (Fig.8) Figures -8. Physiohemial quantities for surfae haraterization of gas-solid interfaes, through a suitable time -resolved hromatographi analysis.

6 Aknowledgments Support of this work by the National Tehnial University of Athens with the ontrat 65/8/22 under the researh program THALES is gratefully aknowledged. Referenes. Birbatakou, S., Pagopoulou, I., Kalantzopoulos, A. and Roubani-Kalantzopoulou, F. : J. Chim. Phys., 95, 28, Jaronie, M., and Madey, R.: Physial adsorption on heterogeneous solids, Elsevier, Amsterdam, Kalogirou, E., Bassiotis, I., Artemiadi, Th., Margariti, S., Siokos, V. and Roubani- Kalantzopoulou, F.: J. Chromatogr. A, 969, 8, 22. Karagiorgos, G. and Roubani Kalantzopoulou, F.: Z. Phys. Chem, 23, 23, Katsanos, N. A.: Flow Pertubation Gas Chromatography, Dekker (Marel), New York, Basel, Katsanos, N. A, Thede, R., and Roubani-Kalantzopoulou, F.: J. Chromatogr. A, 795, 33, Katsanos, N. A, Arvanitopoulou, E., Roubani-Kalantzopoulou, F. and Kalantzopoulos, A.: J. Phys. Chem. B, 3, 52, Katsanos, N. A., Rakintzis, N., Roubani Kalantzopoulou, F., Arvanitopoulou, E. and Kalantzopoulos, A.: J. Chromatogr. A, 85, 3, Katsanos, N. A., Iliopoulou, E., Roubani-Kalantzopoulou, F., and Kalogirou, E.: J. Phys. Chem. B, 3, 228, 999. Katsanos, N.A., Roubani Kalantzopoulou, F., Iliopoulou, E., Bassiotis, I., Siokos,, V., Vrahatis, M. N. and Plagianakos, V. P.: Colloids and Surfae A, 2, 73,22. Margariti, S., Siokos, V. and Roubani Kalantzopoulou, F.: J. Chromatogr., A, 8, 23, Margariti, S., Bassiotis I. and Roubani-Kalantzopoulou F., J. Coll. Interf. Si., 27, 2 3. Metaxa, H., Kalogirou, E. and Roubani Kalantzopoulou, F.: Russian J. Phys. Chem., 73, 2, 999. Roubani Kalantzopoulou, F., Kalogirou, E., Kalantzopoulos, A., Metaxa, H., Thede, R., Katsanos, N. A and Sotiropoulou, V.: Chromatographia, 6, 6, Roubani Kalantzopoulou F., Bassiotis, I., Artemiadi, Th., Margariti, S., Arvanitopoulou, E. and Katsanos, N.A.: Fresenius Environmental Bulletin, (No ), 98, 2 6. Roubani Kalantzopoulou, F., Artemiadi, Th., Bassiotis, I., Katsanos, N. A and Plagianakos, V., Chromatographia, 53, 35, 2 7. Roubani-Kalantzopoulou, F.: J. Chromatogr. A, 37, -2, 2 8. Rudzinski, W. and Everett. D.H.: Adsorption of Gases on Heterogeneous Surfaes, Aademi Press, London, haps., 5., Siokos, V., Kapolos, J. and Roubani Kalantzopoulou, F.: Z. Phys. Chem. 26, 33, Sotiropoulou, V., Vassilev, G. P., Katsanos, N. A, Metaxa, H. and Roubani- Kalantzopoulou, F.: J. Chem. So. Faraday Trans., 9, 85, Tylipaki, H., Margariti, S., Bassiotis., I. and Roubani-Kalantzopoulou F.: Chromatographia, 59, ¾, Yun, X., Long, Z., Kou, D., Lu, X. and Li H., J. Chromatogr. A, 736, 5, 996