J. CHEM. SOC. FARADAY TRANS., 1994, 90(2), 363-367 Heat of Water Chemisorption on a-Al,O, at 200-400 OC Pier Francesco Rossi,* Giovanni Oliveri and Marta Bassoli lstituto di Chimica (CNR),Facolta di lngegneria , Universita di Genova , Fiera del Mare, P.1e.J.F. Kennedy, Pad. D, 16129 Genova, Italy The differential heats and the adsorption isotherms of water on a-Al,O, have been measured at five different temperatures in the range 200-400 "C using a microcalorimetric-volumetric apparatus. The amounts of water adsorbed and integral heats decreased with increasing temperature. At all temperatures the differential heats decreased with increasing amount of chemisorbed water, suggesting an ordinary surface heterogeneity. At very low coverages, the heats of adsorption at all temperatures are >200 kJ mol-', at higher coverages they are < 200 kJ mol-'.Two types of adsorption were considered: dissociative adsorption and coordination of molecular water. We also carried out some thermokinetic investigations on heat emission as a function of time at increas- ing equilibrium water pressures at 200°C and 400"C,and at different adsorption temperatures at 0.05 Torr equilibrium water pressure. The complex interactions of water vapour with the surface of different oxides have been studied extensively by many methods, such as gravimetric,' infrared2*, and calorimetric The chemisorption of water vapour on dehy- droxylated polymorphous aluminas has been studied at dif- ferent The adsorption process, at room temperature, consisted of both physisorption and chemisorp- tion.However, at higher temperature, the chemisorption of water vapour prevailed and it occurred by two different mechanisms : irreversible dissociative and reversible coordi- native adsorption, with different chemisorbed species of molecular water. The literature data for the heats of adsorp- tion of water on aluminas at temperatures >2OO"C appear really poor. For this reason, we carried out adsorption mea- surements of water on a-A120, in the temperature range 200-400 "C. The purpose of the study is to probe the distribution of different strength sites on a-Al,O, surfaces as a function of temperature. Experimental Materials The sample of metal oxide (ca. 2 g) used was a-Al,O, (99.999%) supplied by Aldrich (USA).The sample was immersed in the liquid water at room temperature for 48 h to give maximum hydration and then dried at 110°C. The spe- cific surface area of the a-Al,O, ,degassed at 500"C and in a vacuum of lo-' Torr, was found to be 15.37 m2 g-' on the basis of N, adsorption data. Microcalorimetric Apparatus The microcalorimetric assembly consisted of a heat-flow microcalorimeter (Calvet type, measuring temperature with an accuracy of fO.l "C) connected to a volumetric apparatus to measure adsorption. The measurements were carried out at 200, 250, 300, 350 and 400°C. The microcalorimeter Cali- bration was performed using a standard cell of alumina with Pt resistance, supplied by Setaram (Lyon, France).The reference cell and the laboratory cell, supplied by Glass-Emery (Genoa, Italy) were made of quartz, equipped with inner walls to reduce the geometrical volume. The water was purified by double distillation after passage through an ion-exchange resin. The microcalorimetric measurements were carried out by means of an IBM-AT computer, interfaced to the micro- calorimeter with A/D converter. The apparatus scheme has been shown elsewhere.' Prior to adsorption, the sample of ct-A120,, was degassed under a vacuum of low5Torr at 500°C for 5 h (to eliminate most of the chemisorbed water without possible sintering of the alumina surface)." After this pretreatment, the calorimetric cell, containing a-A120, under vacuum, was inserted into the microcalorimeter at 200°C and was connected to the volumetric Pyrex glass line for vapour adsorption.A first water adsorption up to surface saturation of the sample was performed (run 1). The alumina sample was then degassed again inside the micro- calorimeter and a second water adsorption was performed at 200°C to measure the reversible fraction of the adsorbate at the same temperature (run 2). The procedure was repeated on other samples at 250, 300, 350 and 400°C to give the reversible fractions of the adsorb- ate at these temperatures. Processing of the microcalorimetric data with a computer provides a good characterization of the water adsorption process. The peak areas and volumetric isotherms give the total heat and total adsorption, respectively, as a function of pressure.The integral heats for the successive increments are analytically fitted and the differential heats, qdiff, are then obtained by differentiation of the integral function with respect to the amount adsorbed n,. A study of the thermo- kinetics of heat evolution provides additional insight into the adsorption mechanisms. Results and Discussion Fig. l(a) shows the primary calorimetric isotherms (run 1) and Fig. l(b) the secondary calorimetric isotherms (run 2) at the five adsorption temperatures. Fig. 2 shows the corre-sponding volumetric isotherms. An increase in experimental temperature always involved a decrease in both heat released and amounts adsorbed. The first run appears to consist of an irreversible chemisorption followed by a pressure-dependent branch, because, at very low pressure, the water vapour was irreversibly chemisorbed on the strongest (Lewis acid) sites, while at higher pressure, the water was adsorbed reversibly on intermediate sites.In Fig. 2(c) we show the irreversibly adsorbed amounts of water (run 1 -run 2) as a function of experimental tem- perature. We note that this amount decreases slightly with increasing temperature (i.e.from 0.47 pmol m-2 at 200°C to 0.22 pmol m-at 400 "C).The volumetric isotherms of run 1 364 0.50 N 0.25 E OF--- 0.251 II Ih \ \" I I I 0 1 2 3 4 5 PP-orr Fig. 1 Calorimetric isotherms: (a) run 1, empty symbols; (b) run 2, filled symbols.0,200; 0,250; A, 300; 0,350 and V, 400°C. seem to show a Langmuir-like trend and none of the iso- therms has the BET type I1 behaviour as we observed, for example, for the physisorption of H20 on high-energy sur- faces." We observed that a part of the irreversibly bound water at 200°C becomes reversibly bound at higher tem- peratures. The initial adsorption decreases with increasing temperatures and at all temperatures, the heat released and amounts adsorbed from run 1 are higher than those for the reversible adsorption (run 2). Such data confirm the complexity of the chemisorption of water vapour on a-A120, at high temperatures. In particular, we think that in the temperature range 200-400"C, the simultaneous existence of different adsorption mechanisms is present.In fact when the sample of a-Al,O, is degassed under vacuum at different temperatures, the surface hydroxy groups react to form oxygen bridges and water according to scheme (1):l2 0 1 2 3 4 5 p/rorr Fig. 2 Volumetric isotherms: (a) run 1, empty symbols; (b) run 2, filled symbols; (c) run 1 -run 2. Symbols as in Fig. 1. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 140 100 EE--. .-0 c .-> -Q 60 \ 20 II I I I 15 75 135 195 255 time/mi n Fig. 3 Heat emission peaks as a function of increasing water vapour equilibrium pressures at 200°C: (a) 0.001, (b) 0.1, (c) 0.2 and (d) 1.6 Torr. (y axis refers to the galvanometer deviation of the micro- calorimeter assembly.) On contact with water vapour, the oxygen bridges react to re-form surface OH groups (dissociative chemisorption), according to reaction (2): HH The coordinative chemisorption of water vapour onto unsaturated aluminium ions can be presented by reaction (3)9 ,*I, I Such adsorption mechanisms, lead to the irreversible and reversible phases, and their balance depends on both cover- age and temperature.We think that at very low coverages, irreversible chemisorption due to the dissociative mechanism of water adsorption is present and at relatively higher cover- ages reversible chemisorption due to the coordinative mechanism is prevalent. Thermokinetic Study Semiquantitative information may be obtained from the peak shapes of heat emission from water adsorbed on alumina.In Fig. 3 we show some significant heat emission peaks with increasing coverage (increasing equilibrium pressure) of J. CHEM. SOC. FARADAY TRANS., 1993, VOL. 90 water vapour at 200°C as a function of time.8 Each peak corresponds to admission of a single vapour dose, and there- fore to a point on the calorimetric isotherms [Fig. l(a)]. Moreover Fig. 3(a)shows that the initial adsorption appears to be activated. The heat evolution rate varies with coverage in a complex manner: the peak shape does not show a regular increase in heat emission rate with coverage. Two factors appear to be in competition: the general trend towards instantaneous adsorption and a significant increase in activation energy with coverage [Fig.3(b)and (c)].At high coverages, the peak shape becomes close to that typical of reversible phenomena [Fig. 3(d)]. In conclusion (a)represents a slow, clearly activated, irreversible adsorption which is also apparent from the slow (many minutes) decrease in the gas pressure on the adsorbent whereas (d), represents a typical reversible phenomena of fast heat evolution, (b) and (c) show the presence of both phenomena. In Fig. 4 we show the rate of heat emission at 400°C for different coverages. The conclusions are similar to those of Fig. 3, but the emission peak shapes vary more slowly from (4to (4.In Fig. 5, we show the rate of heat emission at the same equilibrium pressure (0.05 Torr) (i.e. same coverage) at five different temperatures.The peak shape varies from (a) to (e): that of (a), 200°C is rounded (activated process) and they become sharper with increasing temperature (fast heat evolution). The increase of temperature, at the same equi- librium pressure, seems to oppose the activated irreversible process and to promote fast reversible adsorption. Surface Rehydroxylation In Fig. 6 the differential heats of adsorption on the outgassed surface (run 1) are reported as a function of coverage for the five temperatures and in the inset the variation of the integral heat of adsorption (Qi,J with coverage is shown. Such trends (at very low coverage the adsorption mainly represents the irreversible fraction) are characterized by five curves whose convexity decreases as the temperature in-creases.In fact, the distribution of the strengths of the active sites changes over the temperature range 200-400°C. On introduction of the first dose of water vapour to a-Al,O,, the average value of the differential heat is 270 kJ mol- ' for the five temperatures. This value decreases very steeply to near the heat of water vaporization (43.8 kJ mol-','3 or 10.51 kcal loo i 365 14C 100 E E 1 0.-c .-m > U 60 20 15 75 135 195 255 time/min Fig. 5 Heat emission peaks as a function of adsorption temperature at the same water vapour equilibrium pressure (0.05 Torr): (a)200, (b) 250, (c) 300,(d)350 and (e) 400 "C mol-').'4 At very low coverage, the value of qdiff (270 kJ mol-'), can be assigned to irreversible dissociative chemi- sorption.At higher coverage, &iff = 200-90 kJ mol- ', reversible coordination of molecular water is present. This is a region of gradually decreasing heats, representing adsorption on sites of intermediate strength. Finally a region of low differential heat ( <90 kJ mol-') follows, corresponding to H-bonded water. Our initial values of qdiff of water adsorption on a-Al,O, are in fairly good agreement with those measured at 150°C by Della Gatta (251 kJ mol-')' and are higher than those measured at 23 "C by Yung-Fang Yu Yao (>125 kJ mol-'), but these measurements were carried out on single crys- tals. However, our measurements of differential heats are 300 I I 1.5 01 I I I 15 75 135 195 255 0 1 2 t ime/min n,/pmol m-2 Fig.4 Heat emission peaks as a function of increasing water Fig. 6 Differential heats of adsorption (run 1). In the inset, integral vapour equilibrium pressure at 400°C: (a) 0.2, (b)0.4, (c) 0.7 and (d) heats of adsorption (run 1). (0) 250, (A) 300, (0)350 and 200, (O), 2.0 Torr (V)400 "C. 366 higher than those measured by Venable (84kJ rnol-')l6 and those measured by Hendriksen (ca. 109 kJ mo1-1),'4 but they both used immersion calorimetry at 25 "C. We can try to explain the reason why the differential heat curves show a rapid decrease with increasing coverage and temperature. There are two possible causes for this drastic decrease in differential heats: the extreme heterogeneity of the surface and repulsive interactions between adsorbate mol- ecules.As the temperature increases, these two factors have an increasing effect on the differential heats.I6 In Fig. 7, we show a theoretical study of the differential heats for derivation of the integral heats of adsorption. We carried out the following mathematical operations on an IBM Computer AT:'7 (a) Interpolation of the curves of the integral heats by means of a polynomial; (b)calculations of the polynomial coefficients by means of a mean-square method for a best fit to the experimental data; (c) polynomial derivation; (d)determination of the curve of differential heats. The polynomial type that best approaches the experimental differential heat curves is a polynomial of the fourth degree.We can verify that, at all experimental temperatures, the curves show an exponential decrease, apart from the varia- tion of the convexity of 350°C and at 400"C,towards the surface saturation. In Fig. 7, we can see that the increasing temperature affects the trend of the qdiff(and hence the trend in the site strength distribution) with coverage. In particular, it seems that the trend at 200°C and at 250°C is very similar; whilst at 300"C, the slope of the curve clearly changes and the trends at 350°C and 400°C are again different but very similar to each other and fall very rapidly towards the value of the heat of water vaporization. We can attempt to explain the trend of the differential heats at 350°C and at 400°C.Water molecules adsorbed on stronger sites must be immobile especially at lower tem- perature, while those on the less active sites of hydrogen- bonded hydroxy groups will have a good possibility of moving. As the temperature increases, the alumina surface becomes more porous to water vapour and so more water molecules penetrate into the bulk, producing a higher inter- action. Reversible Phases Differential heats of adsorption for run 2 are shown in Fig. 8 for all five temperatures. Such reversible phases seem to 300 -200-E 3 1-p" 100 0 0.5 1.o 1.5 2.0 na/pmol m-2 Fig. 7 Differential heats of adsorption obtained from derivation of a fourth-degree polynomial. (a) 200, (b) 250, (c) 300, (6)350 and (e) 400 "C.J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 300 E na/pmol m-2 I I 01 II I I I I I 0 0.5 1.o 1.5 2.0 n./pmol m-Fig. 8 Differential heats of adsorption (run 2). Inset: integral heats of adsorption (run 2). (m) 200, (0)250, (A)300, (+) 350 and (V) 400 "C. consist of molecular water chemisorbed on the dehydroxy- lated surface. Comparison of the differential heats of reversible adsorp- tion at the different temperatures, gives the following infor- mation on molecular water chemisorbed on the dehydroxylated surface: (a) At very low coverage, the same initial average value is always obtained (190 kJ mol-'); (b) the adsorption temperature does not seem to have much effect on the curve shape (exponential trend).We note that the number of available sites for the first adsorptions (run 1) are not yet useful for the second adsorp- tion (run 2). Furthermore the differential heats of run 2, at all five temperatures, fall in a very narrow range of values. As the temperature increases, the surface sites of a-Al,O, are previously saturated with a lower amount of adsorbed water and the differential heats decrease steeply from 190 kJ mol-' to a value near to that of the heat of water vaporization (43.8 kJ mol- '). Conclusions Surface calorimetry is a powerful technique for studying the chemistry of a surface and its interactions with different mol- ecules. We have shown that the active sites of or-Al,O, react again with water vapour at 400°C and the distribution of different strength sites changes on increasing the temperature from 200 to 400°C.Moreover, all our experimental data confirm the complex- ity of the adsorption process of water vapour on alumina at high temperature. References 1 R. B. Gammage, E. L. Fuller Jr. and H. F. Holmes, J. Phys. Chem., 1970,74,4276. 2 E. Borello, G. Della Gatta, B. Fubini, C. Morterra and G. Ven- turello, J. Card., 1974,35, 1. 3 J. B. Peri, J. Phys. Chem., 1965,69, 21 1. 4 E. McCafferty and A. C. 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