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Acidity of a microporous amorphous alumina measured by intermittent temperature-programmed desorption of ammonia
Applied Catalysis A: General, 98 (1993) 61-70 Elsevier Science Publishers B.V., Amsterdam
61
APCAT AZ483
Acidity of a microporous amorphous alumina measured by intermittent temperature-programmed desorption of ammonia J.P. Joly, M. Khalfallah, D. Bianchi and G.M. Pajonk Universitk Claude Bernard Lyon 1, Institut des Sciences de la Mat&e, Laboratoire des Matgriaux et Pro&d& Catalytiques, 43 Boulevard du 11 Novembre 1918, 69622 Villeurbanne Ckdex (France) (Received 28 September 1992, revised manuscript received 18 January 1993)
Abstaact
Mathematical models used to achieve the deconvolution of temperature-programmed desorption (TPD ) spectra from heterogeneous surfaces generally rely on assumptions about the pre-exponential factor or on the variation of the activation energy of desorption with coverage. In contrast, intermittent temperature-programmeddesorption (ITPD) is a method that allows the apparent desorption activation energies EL,to be determined without making such restrictive assumptions. ITPD has been applied to ammonia, used as a basic probe molecule desorbed from an amorphous, microporous alumina sample. Five different chemisorbed states have been evidenced. These five states of adsorbed ammonia are characterized by apparent Ed values ranging from 109 to 234 kJ/mol. These results are compared with those of differential microcalorimetry found in the literature and the consequences of the discovery of discrete ammonia adsorbed states are discussed. Keywords: acidity; ammonia; desorption
amorphous
alumina; heat of adsorption;
temperature-programmed
INTRODUCTION
TPD of basic probe molecules is a method which, in principle, allows the number and strength of acid sites at a solid surface to be determined [ 1,2]. Nevertheless, the deconvolution of TPD spectra from strongly heterogeneous surfaces needs models that generally involve assumptions about the pre-exponential factor or on the variation of the activation energy of desorption with coverage [ 3,4]. In addition, this problem is more complicated if coupling between the desorption and molecular pore diffusion occurs [ 5,6]. In contrast, Correspondence to: Dr. J.P. Joly, Universite Claude Bernard Lyon 1, Institut des Sciences de la Matiere, Laboratoire des Materiaux et Pro&d& Catalytiques, 43 Boulevard du 11 Novembre 1918, 69622 Villeurbanne Cedex, France. Tel. ( + 33)72448560, fax. ( + 33)78892583.
0926-860X/93/$06.00
0 1993 Elsevier Science Publishers
B.V. All rights reserved.
62
J.P. Joly et al. / Appl. Catal. A 98 (1993) 61-70
ITPD is a method that allows the measurement of the apparent desorption activation energies Ed with very few prior assumptions [ 7,8] about the desorption process. ITPD was proposed ten years ago by one of the authors as a modified TPD method to measure the mobility of labile oxygen species absorbed on mixed oxide catalysts. It has been recently applied to the desorption of ammonia from the surface of a Y zeolite: a model with pore diffusion and readsorption on the one hand and an experimental study on the other hand [ 91, showed that ITPD is also suitable in the case of microporous materials. ITPD [7,9] consists of a TPD with an unusual saw-toothed heating programme, so that the resulting desorptions are interrupted and resumed several times. Application of Arrhenius’s law at the very beginning of each desorption, i.e., in a range of quasi-constant coverage, provides the apparent activation energy of desorption Ed for this particular coverage. The corresponding preexponential factor can be assessed from the intercept of the Arrhenius line. ITPD enters the group of TPD differential analysis discussed in a recent review by Zhdanov [lo]. ITPD is especially convenient when the deconvolution of the complete TPD curve is difficult to achieve. Ammonia, which can be used to study the acidity of solids with narrow pores [ 111, was chosen as a basic probe molecule. A sample of amorphous alumina, prepared through a sol-gel procedure [ 121, was first submitted to a normal TPD with a high initial coverage, second to an ITPD, and finally to normal TPD’s with different initial coverages. The corresponding results are depicted in this paper. EXPERIMENTAL
A xerogel of alumina, microporous and X-ray amorphous, was prepared according to ref. 12 using A1N08-9Hz0 dissolved in water-free methanol, and reacted with gaseous ammonia to precipitate the hydroxide. When it was heated in air or in vacuum for 15 h, the amorphous structure of the alumina remained intact up to 1023 or 1073 K. The BET surface area of alumina was 240 m2/g after a heat treatment in vacuum at 673 K. It exhibited a monomodal distribution of pore radii with a mean pore radius of 1 nm. A sample of 0.3 g of alumina was degassed for 1 h at 723 K under a vacuum of approximately 10F3 Pa. The degassing temperature was reached by a linear temperature increase of 4 K/min. Then the sample was cooled down to 373 K under vacuum. Ammonia adsorption (2.35*104 Pa) was performed at 373 K for 15 min followed by degassing at the same temperature for 1 h. This treatment was applied in order to study the desorption of ammonia from the strong acid sites only. Finally, the sample was cooled under vacuum to 298 K, and the ITPD procedure was applied. The ammonia desorption flux was measured by a MS10 AEI mass spectrometer set on m/e = 15 to avoid any interference of
63
J.P. Joly et al. / Appl. Catal. A 98 (1993) 61-70
desorbed water (m/e= 16,17 and 18). The heating rate was 4 K/min and the final desorption temperature never exceeded 823 K. RESULTS AND DISCUSSION
Fig. 1 shows the complete TPD spectrum registered after adsorption and desorption of ammonia at 373 K. It consists of a wide peak, the deconvolution of which is very difficult. As stressed by Soler and Garcia, the mathematical fitting of TPD spectra without previous knowledge of the desorption rate preexponential factor can lead to incorrect results [ 131, even in the case of desorption without re-adsorption. For the above reasons a full ITPD investigation was carried out and the result is shown in Fig. 2. For each run, the final desorption temperature is indicated by a lower case letter. The corresponding Arrhenius plot is shown in Fig. 3 and consists of a set of straight lines at lower Flux
(mmol/h)
0
2
1
3
Time (h)
Fig. 1. DTP under vacuum of ammonia adsorbed at 373 K. Temperature at the maximum: 440 K. Flux (mmollh)
300
500
‘O”
T(K)
Fig. 2. ITPD of ammonia previously adsorbed at 373 K. Letters a-q denote the various desorptions at 4”K/min. Their places indicate the temperatures at which the heating programme was interrupted to cool the sample readily.
J. P. Joly et al. / Appl. Catal. A 98 (1993) 61- 70
64
-13& 1.2
2
2 .a 103/T (K-1)
Fig. 3. Arrhenius plot corresponding to Fig. 2. The lower case letters (a-q) indicate the corresponding curves in Fig. 2.
temperatures as is required by the theory, i.e., small desorption rate and quasiconstant surface coverage. The application of this analysis to a porous solid has been previously discussed [ 91. In this case, for which re-adsorption is likely to occur readily, the apparent activation energy of desorption Ed is the heat of adsorption E. The requirement of quasi-constant coverage applies to each state separately. This condition is met in our experiments except for the upper part of the graph where the coverage is no longer constant and for the s-shaped curves (e, h and m in Fig. 3) which indicate that the current desorption state is exhausted. In these cases, Arrhenius plots are not straight lines and cannot be used to derive the apparent activation energy of desorption. The apparent activation energy of desorption as a function of surface coverage can be obtained from the slopes of the lines in the Arrhenius plots on the one hand, and from the surfaces under the ITPD curves on the other hand. Fig. 4 shows five states of adsorbed ammonia, each state being characterized by a definite value of Ed. Successive values of Ed for the same state remain in the range Ed k 8 kJ/mol, which is generally the reproducibility we observed in our experimental conditions for porous catalysts [ 8,9]. The greater scattering of energies around 234 kJ/mol is due to the fact that the corresponding state is present in a very small amount. The heterogeneity of the acidic surface of the xerogel is then clearly established and is stated more precisely by ITPD. Table 1 presents the value of Ed corresponding to Fig. 4. It has been verified that the sum of the amounts of ammonia desorbed by ITPD matches the amount determined from the whole TPD spectrum (Fig. 1) within an error of 8%. The total amount of ammonia adsorbed irreversibly at 373 K is 0.17 mmol/g of sample or 0.7 pmol/m’. The TPD spectra are reproducible after a heat treatment of the alumina sample at 773 K. The assessment of the pre-exponential factor A is possible from the straight section of the Arrhenius plots as long as the coverage in the considered state
J.P. Joly et al. / Appl. Catal. A 98 (1993) 61-70
65
Ed #.ihIO~) 260
-
0
20
40
60
80 %
100
of NH3 desorhed
Fig. 4. Apparent activation energy of desorption as a function of desorbed ammonia. The lower case letters (a-q) correspond to those in Fig. 3.
TABLE 1 Apparent activation energies of desorption and relative populations of the states of superficial ammonia irreversibly adsorbed at 273 K for 15 min
Ed (kJ/mol)
109 125 167 201 234
Order of magnitude of the pre-exp. factor (9-l)
Desorbed ammonia range (% )
Ammonia density range (pmol/m2 1
10” 10’2 10’3 10’4 10’3
O-10 10-55 55-78 78-95 95-100
0.7 -0.63 0.63 -0.32 0.32 -0.15 0.15 -0.035 0.035-O
is not too close to 0 (nor to 1 in the case of re-adsorption) [ 91. The order of magnitude of the pre-exponential factor was derived from the equation ln A=ln(rd/N,)
+ (&/RT)
where rdand No are the desorption rate (in mol/s ) and the adsorption capacity of the corresponding state (in mol) , is the desorption temperature and R is the perfect gas constant. The results presented in Table 1 show that the order of magnitude of A is not too far from that corresponding to the case without readsorption, i.e., 1013s-l. Nevertheless, it is not excluded that other desorption mechanisms may produce this value. It is interesting to compare differential heats E for ammonia adsorption reported in the literature with our results. Recent data on the subject are scarce and often concern y-alumina. Some scattering in the results is expected be-
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J.P. Joly et al. / Appl. Catal. A 98 (1993) 61-70
cause it is known that the dehydrating treatment of alumina strongly influences the number and the strength of acid centres. Auroux and Gervasini [ 141 reported results for ammonia adsorption at 423 K on a commercial (R.P. ) y-alumina outgassed at 673 K for 2 h before experiments. They found that the heat of adsorption decreased from 220 to 55 kJ/ mol when the superficial concentration of ammonia increased from 0.1 to approximately 1.7 ,umol/m2. These results agree with ours taking into account that we only analyzed the strong acid sites, the density of which were 0.7 poll m2. However, their heat curve, i.e. E versus coverage, presented a single point of inflexion. This could mean that the distribution of acid strength was continuous. However, it is possible that the increments of adsorbed ammonia were too large (10 points in the curve) to allow an eventual discrete type of strength distribution to be seen. Gervasini et al. [ 151 very recently found a more resolved heat curve with a commercial non-porous y-alumina of 208 m2/g. The heat of adsorption at 423 K decreased from 217 to 60 kJ/mol while the superficial density of ammonia increased from 0.15 to 2.5 mol/m2. The heat curve presents, in the higher heat region, several inflexion points that might indicate the presence of some ammonia adsorption states with discrete acid strengths. In an earlier paper, Stone and Whalley [ 161 measured the differential heat of adsorption of ammonia at 303 K. They used a y-alumina of 240 m”/g, prepared from aluminium isopropoxide in the absence of alkali metal ions and calcined at 773 K. Their heat curve decreased from 156 to 50 kJ/mol when ammonium coverage increased from 0.13 to 3.6 pmol/m2. Taking into account the coverage, these results are in satisfactory agreement with ours. However, the resolution of the calorimetric procedure was not sufficient to show if any discrete adsorption states of high energy were present. The above discussion shows an overall agreement between the heats of adsorption at comparable coverage on y-alumina and on the amorphous alumina we studied. This suggests that, for y-alumina, the distribution of strong acid strength among the acid centres might be discrete as in the case of the amorphous alumina we studied. Similarities in the surface properties of crystallized y-alumina and of an amorphous alumina have been evidenced by several experimental methods [ 171. The difficulty of finding clear evidence of the discontinuous character of the acid strength distribution could stem from the rather low resolution of conventional TPD or differential calorimetry. Efforts to increase the resolution of these methods could help to clear up this point. Since ITPD is not the usual way to study the acidity of solid surfaces, other more conventional TPD’s have been carried out for comparison with the precedimg results. Different surface coverages were obtained by adsorbing controlled amounts of ammonia on a bare surface. Ammonia doses proceeded from a constant volume at 295 K at eight different pressures in the range 151-2.32 lo4 Pa. The temperature of the sample was 295 K and the final pressure was
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J.P. Joly et al. / Appl. Catal. A 98 (1993) 61-70
very low, indicatingthat all the ammoniain contact with the solid wasvirtually adsorbed, excepted at the highestpressure.Adsorption was completed within 1 or 2 min. This shows that the adsorption energy is low and much smaller than the desorption activation energiesfrom the strong acid sites. On adsorbing ammoniaby this experimentalprocedure,the strongestadsorbedstatesare expectedto form preferentially,as in differentialmicrocalorimetry.After each adsorption, a conventional TPD was carried out up to 773 K. The resultsare presentedin Fig. 5 and in Table 2. It can be seen that, as expected, the lower the surfacecoverage,the strongerthe adsorption. An important displacementof the spectrummaximumwith coverageis observed.The pressureconditions for TPD a in Table 2 are similarto those employed for the TPD presentedin Fig. 1 but the amount of ammoniais 2.5 times FLUX
(mmollh)
T
0.25
300
400
500
600
700
800
T (K)
Fig. 5. DTP spectra at 4 K/min of ammonia adsorbed at 293 K. a-h correspond to various amounts of ammonia adsorbed, see Table 2. TABLE 2 Temperatures of the maximum of the spectra in Fig. 5 and quantities of ammonia adsorbed at 295 K TPD
T maximum (K)
Quantity of ammonia (m mol/g)
380 453 523 568 598 635 693 708
0.450 0.243 0.195 0.157 0.097 0.044 0.024 0.014
J. P. Joly et al. / Appl. Catal. A 98 (1993) 61- 70
68
larger because the adsorption temperature is lower (295 instead of 373 K). The excess of adsorbed ammonia is essentially in the lowest temperature state that is not stable at 373 K, as indicated by the temperature at the maximum. This confirms that, apart from strong acid sites evidenced by ITPD in this work, there is a relatively large amount of weak acid sites as reported in the calorimetric studies mentioned previously. The superposition of spectra, as shown in Fig. 5, provides an insight into surf&e heterogeneity because peak maxima and shoulders should gather around a small number of temperatures that are characteristic (in constant experimental circumstances) of the adsorption states. However, this pattern is complicated by maximum shifts due to strong peak overlapping. In an extreme case, the overlapping of two peaks which are close to each other and of similar magnitude may produce a wide single peak with a maximum at an intermediate temperature. This seems to be the case for TPD c. When it is excluded, the maximum temperatures of the TPD peaks gather around five values (380,453, 523,598 and 708 K) in agreement with ITPD results. Incidentally it is interesting to examine what results would be obtained through the classical equation at the maximum temperature, which is frequently used to assess Ed:
E,/RT’=
(A/cl!)exp( -E,JRT),
(where (Y is the heating rate and A is taken from Table 1). This equation is correct in the case of a first-order desorption without re-adsorption and gives approximate results in numerous other cases [18,19] provided that pore diffusion is not involved. The results are 97,124,154,187, and 210 kJ/mol for the five states, respectively. These energies, though generally smaller than those obtained by ITPD, may be considered satisfactory due to the difficulty in locating the right peak temperatures in the set of TPD’s with various coverages. However, it must be underlined that the shape and the position of the TPD peak generally depends on the experimental conditions. Thus, the above agreement between apparent activation energies from ITPD and from the positions of the peaks stems from the particular conditions we used. ITPD analysis, which is more independent of the mechanism of desorption [ 91, does not present this kind of difficulties. It is worth noting that the set of TPD’s presented in Fig. 5 is similar to that obtained almost three decades ago by Amenomiya et al. [20] for an alumina prepared by calcination under air at 873 K of aluminium hydroxide [ 211. Their experimental procedure of ammonia adsorption was different to ours: in their case different ammonia coverages were obtained by means of different outgassing temperatures instead of by different adsorption doses which were used in our case. The results confirm that there are some similarities between the acid sites on an alumina dehydrated under air and those detected in our work under vacuum. Nevertheless, a more detailed conclusion is not possible be-
J. P. Joly et al. / Appl. Catal. A 98 (1993) 61- 70
69
cause of the lack of precise data about the heats of ammonia adsorption on y-alumina. Under the assumption that the alumina used by Amenomiya et al. [ 201 is not very different from those mentioned above for more recent studies [ 14,151, it appears that the reported activation energies of desorption (75 and 29 kJ/mol for coverages of 0.19 and 3.32 ,umol/m2, respectively) are underevaluated. A further conclusion that can be drawn from the results presented in Fig. 5 is that adsorption on a bare surface is less selective than a desorption by ITPD from an initially saturated surface. For example, it can be seen than the adsorption in the state corresponding to a peak at 598 K occurs before the adsorption in the state corresponding to a peak at 708 K is completed. This observation may have consequences in calorimetry which occasionally provides poorly defined plateaus for adsorption states in the heat curves for ammonia adsorption on a solid. This may be explained by the fact that the kinetics of desorption are dominated by activation energies that are rather different while the kinetics of adsorption is governed by very low activation energies that are similar. As a result adsorption heats for each state determined by calorimetry may be somewhat underevaluated. The existence of various strongly adsorbed species on alumina has been evidenced by IR. Thus, according to a review published by Kniizinger [ 221 at least four adsorbed ammonia species have been identified beside weakly held ammonia. The four states of adsorbed ammonia are assigned to strong adsorption sites, mainly Lewis ones ( Al3+ ) , and possibly acid-base pairs involving 02surface ions as well. To our knowledge it is the first time that the quantities of adsorbed ammonia in five adsorbed states on an alumina sample and the corresponding adsorption energies have been determined. Moving on from the discovery of five energetically distinct states of ammonia adsorption on amorphous alumina, it should be possible to identify the corresponding acid centres. Unfortunately, TPD methods give very little information on this subject. Following the general agreement stated in the literature on the nature of the strong acid centres of y-alumina [ 23,241, and considering the recent experimental confirmation that no pyridinium ions are observed when pyridine is adsorbed on an amorphous alumina [ 171, it is likely that the acid centres for the adsorption states we observed are Lewis-type sites. This point is supported by theoretical studies of molecular models which show that numerous Lewis-type sites may be formed upon dehydrating the surface of alumina [ 25 1. CONCLUSION
In conclusion, this study on the adsorption/desorption of ammonia on an amorphous, microporous alumina confirms that the ITPD is useful for characterizing the distribution of strength among the superficial acid centres, even
J.P. Joly et al. / Appl. Catd. A 98 (1993) 61-70
70
in the case of a microporous solid. The efficiency of ITPD in showing various adsorption states is better than that of most classical TPD methods or differential calorimetry methods found in the literature. In addition, the discovery of five distinct strongly adsorbed states of ammonia on an amorphous alumina and the comparison with results published in the literature for y-alumina shows the importance of investigating in detail the distribution of acid strength for various alumina preparations. This could provide experimental data useful for the identification of the acid centres. ACKNOWLEDGEMENT
The authors are grateful to A. Brachet for valuable participation in the experimental work. REFERENCES 1 2 3 4 5 6 7 8 9 10 11
12
13 14 15 16 17 18 19 20 21 22 23 24 25
C.V. Hidalgo, H. Itoh, T. Hattori, M. Niwa and Y. Murakami, J. Catal., 85 (1984) 362. G.I. Kapsutin, T.R. Brueva, A.L. Klyachko, S. Beran and B. Wichterlova, Appl. CataI., 42 (1988) 239. Y. Tokoro, T. Uchijimaand Y. Yoneda, J. Catal., 56 (1979) 110. H.G. Karge and V. Dondur, J. Phys. Chem., 94 (1990) 765. D.M. Jones and G.L. Griffin, J. CataI., 80 (1983) 40. L. Forni, E. Magni, E. Ortoleva, R. Monaci and V. Solinas, J. Catal., 112 (1988) 444. J.P. Joly, C.R. Acad. Sci. Paris, 295 (Sbrie II) (1982) 717. M.C. Bacchus-Montabonel and J.P. Joly, J. Chem. Sot., Faraday Trans., 82 (1986) 3601. J.P. Joly and A. Perrard, Appl. Catal. A, 96 (1993) 355. V.P. Zhdanov, Surf. Sci. Rep., 12 (1991) 183. A. Auroux, J.C. Vedrine and P.C. Gravelle, in J. Rouquerol and K.S.W. Sing (Editors), Adsorption at the Gas-Solid and Liquid-Solid Interfaces, (Studies in Surface Science and Catalysis, Vol. 31), Elsevier, Amsterdam, 1982, p. 305. G. Tournier, M. Lacroix-Repellin, G.M. Pajonk and S.J. Teichner, in B. Delmon, B. Grange, P.A. Jacobs and P. Poncelet (Editors), Preparation of Catalysts IV, Elsevier, Amsterdam, 1987, p. 333. J.M. Soler and N. Garcia, Surf. Sci., 124 (1983) 563. A. Auroux and A. Gervasini, J. Phys. Chem., 94 (1990) 6371. A. Gervasini, G. BeIIussi, J. Fenyvesi and A. Auroux, 10th Intern. Congress on Catalysis, Budapest, 1992. F.S. Stone and L. WhaIIey, J. Catal., 8 (1967) 173. P. Nortier, P. Fourre, A.B. Mohammed Saad, 0. Saur and J.C. Lavahey, Appl. Catal., 61 (1990) 141. P.A. Readhead, Vacuum, 12 (1962) 203. R.J. Cvetanovic and Y. Amenomiya, Adv. Catal., 17 (1967) 103. Y. Amenomiya, J.H.B. Chenier and R.J. Cvetanovic, J. Phys. Chem., 68 (1964) 52. Y. Amenomiya and R.J. Cvetanovic, J. Phys. Chem., 67 (1963) 144. H. Knozinger, Adv. Catal., 25 (1976) 184. B.A. Benesiand B.H.C. Winquist, Adv. Catai., 27 (1978) 98. W. Kania and K. Jurczyk, Appl. Catal., 61 (1990) 27. S. Yoshida, in J.B. Moffat (Editor), Theoretical Aspects of Heterogeneous Catalysis, Van Nostrand-Reinhold, New York, 1990, p. 506.
61
APCAT AZ483
Acidity of a microporous amorphous alumina measured by intermittent temperature-programmed desorption of ammonia J.P. Joly, M. Khalfallah, D. Bianchi and G.M. Pajonk Universitk Claude Bernard Lyon 1, Institut des Sciences de la Mat&e, Laboratoire des Matgriaux et Pro&d& Catalytiques, 43 Boulevard du 11 Novembre 1918, 69622 Villeurbanne Ckdex (France) (Received 28 September 1992, revised manuscript received 18 January 1993)
Abstaact
Mathematical models used to achieve the deconvolution of temperature-programmed desorption (TPD ) spectra from heterogeneous surfaces generally rely on assumptions about the pre-exponential factor or on the variation of the activation energy of desorption with coverage. In contrast, intermittent temperature-programmeddesorption (ITPD) is a method that allows the apparent desorption activation energies EL,to be determined without making such restrictive assumptions. ITPD has been applied to ammonia, used as a basic probe molecule desorbed from an amorphous, microporous alumina sample. Five different chemisorbed states have been evidenced. These five states of adsorbed ammonia are characterized by apparent Ed values ranging from 109 to 234 kJ/mol. These results are compared with those of differential microcalorimetry found in the literature and the consequences of the discovery of discrete ammonia adsorbed states are discussed. Keywords: acidity; ammonia; desorption
amorphous
alumina; heat of adsorption;
temperature-programmed
INTRODUCTION
TPD of basic probe molecules is a method which, in principle, allows the number and strength of acid sites at a solid surface to be determined [ 1,2]. Nevertheless, the deconvolution of TPD spectra from strongly heterogeneous surfaces needs models that generally involve assumptions about the pre-exponential factor or on the variation of the activation energy of desorption with coverage [ 3,4]. In addition, this problem is more complicated if coupling between the desorption and molecular pore diffusion occurs [ 5,6]. In contrast, Correspondence to: Dr. J.P. Joly, Universite Claude Bernard Lyon 1, Institut des Sciences de la Matiere, Laboratoire des Materiaux et Pro&d& Catalytiques, 43 Boulevard du 11 Novembre 1918, 69622 Villeurbanne Cedex, France. Tel. ( + 33)72448560, fax. ( + 33)78892583.
0926-860X/93/$06.00
0 1993 Elsevier Science Publishers
B.V. All rights reserved.
62
J.P. Joly et al. / Appl. Catal. A 98 (1993) 61-70
ITPD is a method that allows the measurement of the apparent desorption activation energies Ed with very few prior assumptions [ 7,8] about the desorption process. ITPD was proposed ten years ago by one of the authors as a modified TPD method to measure the mobility of labile oxygen species absorbed on mixed oxide catalysts. It has been recently applied to the desorption of ammonia from the surface of a Y zeolite: a model with pore diffusion and readsorption on the one hand and an experimental study on the other hand [ 91, showed that ITPD is also suitable in the case of microporous materials. ITPD [7,9] consists of a TPD with an unusual saw-toothed heating programme, so that the resulting desorptions are interrupted and resumed several times. Application of Arrhenius’s law at the very beginning of each desorption, i.e., in a range of quasi-constant coverage, provides the apparent activation energy of desorption Ed for this particular coverage. The corresponding preexponential factor can be assessed from the intercept of the Arrhenius line. ITPD enters the group of TPD differential analysis discussed in a recent review by Zhdanov [lo]. ITPD is especially convenient when the deconvolution of the complete TPD curve is difficult to achieve. Ammonia, which can be used to study the acidity of solids with narrow pores [ 111, was chosen as a basic probe molecule. A sample of amorphous alumina, prepared through a sol-gel procedure [ 121, was first submitted to a normal TPD with a high initial coverage, second to an ITPD, and finally to normal TPD’s with different initial coverages. The corresponding results are depicted in this paper. EXPERIMENTAL
A xerogel of alumina, microporous and X-ray amorphous, was prepared according to ref. 12 using A1N08-9Hz0 dissolved in water-free methanol, and reacted with gaseous ammonia to precipitate the hydroxide. When it was heated in air or in vacuum for 15 h, the amorphous structure of the alumina remained intact up to 1023 or 1073 K. The BET surface area of alumina was 240 m2/g after a heat treatment in vacuum at 673 K. It exhibited a monomodal distribution of pore radii with a mean pore radius of 1 nm. A sample of 0.3 g of alumina was degassed for 1 h at 723 K under a vacuum of approximately 10F3 Pa. The degassing temperature was reached by a linear temperature increase of 4 K/min. Then the sample was cooled down to 373 K under vacuum. Ammonia adsorption (2.35*104 Pa) was performed at 373 K for 15 min followed by degassing at the same temperature for 1 h. This treatment was applied in order to study the desorption of ammonia from the strong acid sites only. Finally, the sample was cooled under vacuum to 298 K, and the ITPD procedure was applied. The ammonia desorption flux was measured by a MS10 AEI mass spectrometer set on m/e = 15 to avoid any interference of
63
J.P. Joly et al. / Appl. Catal. A 98 (1993) 61-70
desorbed water (m/e= 16,17 and 18). The heating rate was 4 K/min and the final desorption temperature never exceeded 823 K. RESULTS AND DISCUSSION
Fig. 1 shows the complete TPD spectrum registered after adsorption and desorption of ammonia at 373 K. It consists of a wide peak, the deconvolution of which is very difficult. As stressed by Soler and Garcia, the mathematical fitting of TPD spectra without previous knowledge of the desorption rate preexponential factor can lead to incorrect results [ 131, even in the case of desorption without re-adsorption. For the above reasons a full ITPD investigation was carried out and the result is shown in Fig. 2. For each run, the final desorption temperature is indicated by a lower case letter. The corresponding Arrhenius plot is shown in Fig. 3 and consists of a set of straight lines at lower Flux
(mmol/h)
0
2
1
3
Time (h)
Fig. 1. DTP under vacuum of ammonia adsorbed at 373 K. Temperature at the maximum: 440 K. Flux (mmollh)
300
500
‘O”
T(K)
Fig. 2. ITPD of ammonia previously adsorbed at 373 K. Letters a-q denote the various desorptions at 4”K/min. Their places indicate the temperatures at which the heating programme was interrupted to cool the sample readily.
J. P. Joly et al. / Appl. Catal. A 98 (1993) 61- 70
64
-13& 1.2
2
2 .a 103/T (K-1)
Fig. 3. Arrhenius plot corresponding to Fig. 2. The lower case letters (a-q) indicate the corresponding curves in Fig. 2.
temperatures as is required by the theory, i.e., small desorption rate and quasiconstant surface coverage. The application of this analysis to a porous solid has been previously discussed [ 91. In this case, for which re-adsorption is likely to occur readily, the apparent activation energy of desorption Ed is the heat of adsorption E. The requirement of quasi-constant coverage applies to each state separately. This condition is met in our experiments except for the upper part of the graph where the coverage is no longer constant and for the s-shaped curves (e, h and m in Fig. 3) which indicate that the current desorption state is exhausted. In these cases, Arrhenius plots are not straight lines and cannot be used to derive the apparent activation energy of desorption. The apparent activation energy of desorption as a function of surface coverage can be obtained from the slopes of the lines in the Arrhenius plots on the one hand, and from the surfaces under the ITPD curves on the other hand. Fig. 4 shows five states of adsorbed ammonia, each state being characterized by a definite value of Ed. Successive values of Ed for the same state remain in the range Ed k 8 kJ/mol, which is generally the reproducibility we observed in our experimental conditions for porous catalysts [ 8,9]. The greater scattering of energies around 234 kJ/mol is due to the fact that the corresponding state is present in a very small amount. The heterogeneity of the acidic surface of the xerogel is then clearly established and is stated more precisely by ITPD. Table 1 presents the value of Ed corresponding to Fig. 4. It has been verified that the sum of the amounts of ammonia desorbed by ITPD matches the amount determined from the whole TPD spectrum (Fig. 1) within an error of 8%. The total amount of ammonia adsorbed irreversibly at 373 K is 0.17 mmol/g of sample or 0.7 pmol/m’. The TPD spectra are reproducible after a heat treatment of the alumina sample at 773 K. The assessment of the pre-exponential factor A is possible from the straight section of the Arrhenius plots as long as the coverage in the considered state
J.P. Joly et al. / Appl. Catal. A 98 (1993) 61-70
65
Ed #.ihIO~) 260
-
0
20
40
60
80 %
100
of NH3 desorhed
Fig. 4. Apparent activation energy of desorption as a function of desorbed ammonia. The lower case letters (a-q) correspond to those in Fig. 3.
TABLE 1 Apparent activation energies of desorption and relative populations of the states of superficial ammonia irreversibly adsorbed at 273 K for 15 min
Ed (kJ/mol)
109 125 167 201 234
Order of magnitude of the pre-exp. factor (9-l)
Desorbed ammonia range (% )
Ammonia density range (pmol/m2 1
10” 10’2 10’3 10’4 10’3
O-10 10-55 55-78 78-95 95-100
0.7 -0.63 0.63 -0.32 0.32 -0.15 0.15 -0.035 0.035-O
is not too close to 0 (nor to 1 in the case of re-adsorption) [ 91. The order of magnitude of the pre-exponential factor was derived from the equation ln A=ln(rd/N,)
+ (&/RT)
where rdand No are the desorption rate (in mol/s ) and the adsorption capacity of the corresponding state (in mol) , is the desorption temperature and R is the perfect gas constant. The results presented in Table 1 show that the order of magnitude of A is not too far from that corresponding to the case without readsorption, i.e., 1013s-l. Nevertheless, it is not excluded that other desorption mechanisms may produce this value. It is interesting to compare differential heats E for ammonia adsorption reported in the literature with our results. Recent data on the subject are scarce and often concern y-alumina. Some scattering in the results is expected be-
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J.P. Joly et al. / Appl. Catal. A 98 (1993) 61-70
cause it is known that the dehydrating treatment of alumina strongly influences the number and the strength of acid centres. Auroux and Gervasini [ 141 reported results for ammonia adsorption at 423 K on a commercial (R.P. ) y-alumina outgassed at 673 K for 2 h before experiments. They found that the heat of adsorption decreased from 220 to 55 kJ/ mol when the superficial concentration of ammonia increased from 0.1 to approximately 1.7 ,umol/m2. These results agree with ours taking into account that we only analyzed the strong acid sites, the density of which were 0.7 poll m2. However, their heat curve, i.e. E versus coverage, presented a single point of inflexion. This could mean that the distribution of acid strength was continuous. However, it is possible that the increments of adsorbed ammonia were too large (10 points in the curve) to allow an eventual discrete type of strength distribution to be seen. Gervasini et al. [ 151 very recently found a more resolved heat curve with a commercial non-porous y-alumina of 208 m2/g. The heat of adsorption at 423 K decreased from 217 to 60 kJ/mol while the superficial density of ammonia increased from 0.15 to 2.5 mol/m2. The heat curve presents, in the higher heat region, several inflexion points that might indicate the presence of some ammonia adsorption states with discrete acid strengths. In an earlier paper, Stone and Whalley [ 161 measured the differential heat of adsorption of ammonia at 303 K. They used a y-alumina of 240 m”/g, prepared from aluminium isopropoxide in the absence of alkali metal ions and calcined at 773 K. Their heat curve decreased from 156 to 50 kJ/mol when ammonium coverage increased from 0.13 to 3.6 pmol/m2. Taking into account the coverage, these results are in satisfactory agreement with ours. However, the resolution of the calorimetric procedure was not sufficient to show if any discrete adsorption states of high energy were present. The above discussion shows an overall agreement between the heats of adsorption at comparable coverage on y-alumina and on the amorphous alumina we studied. This suggests that, for y-alumina, the distribution of strong acid strength among the acid centres might be discrete as in the case of the amorphous alumina we studied. Similarities in the surface properties of crystallized y-alumina and of an amorphous alumina have been evidenced by several experimental methods [ 171. The difficulty of finding clear evidence of the discontinuous character of the acid strength distribution could stem from the rather low resolution of conventional TPD or differential calorimetry. Efforts to increase the resolution of these methods could help to clear up this point. Since ITPD is not the usual way to study the acidity of solid surfaces, other more conventional TPD’s have been carried out for comparison with the precedimg results. Different surface coverages were obtained by adsorbing controlled amounts of ammonia on a bare surface. Ammonia doses proceeded from a constant volume at 295 K at eight different pressures in the range 151-2.32 lo4 Pa. The temperature of the sample was 295 K and the final pressure was
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J.P. Joly et al. / Appl. Catal. A 98 (1993) 61-70
very low, indicatingthat all the ammoniain contact with the solid wasvirtually adsorbed, excepted at the highestpressure.Adsorption was completed within 1 or 2 min. This shows that the adsorption energy is low and much smaller than the desorption activation energiesfrom the strong acid sites. On adsorbing ammoniaby this experimentalprocedure,the strongestadsorbedstatesare expectedto form preferentially,as in differentialmicrocalorimetry.After each adsorption, a conventional TPD was carried out up to 773 K. The resultsare presentedin Fig. 5 and in Table 2. It can be seen that, as expected, the lower the surfacecoverage,the strongerthe adsorption. An important displacementof the spectrummaximumwith coverageis observed.The pressureconditions for TPD a in Table 2 are similarto those employed for the TPD presentedin Fig. 1 but the amount of ammoniais 2.5 times FLUX
(mmollh)
T
0.25
300
400
500
600
700
800
T (K)
Fig. 5. DTP spectra at 4 K/min of ammonia adsorbed at 293 K. a-h correspond to various amounts of ammonia adsorbed, see Table 2. TABLE 2 Temperatures of the maximum of the spectra in Fig. 5 and quantities of ammonia adsorbed at 295 K TPD
T maximum (K)
Quantity of ammonia (m mol/g)
380 453 523 568 598 635 693 708
0.450 0.243 0.195 0.157 0.097 0.044 0.024 0.014
J. P. Joly et al. / Appl. Catal. A 98 (1993) 61- 70
68
larger because the adsorption temperature is lower (295 instead of 373 K). The excess of adsorbed ammonia is essentially in the lowest temperature state that is not stable at 373 K, as indicated by the temperature at the maximum. This confirms that, apart from strong acid sites evidenced by ITPD in this work, there is a relatively large amount of weak acid sites as reported in the calorimetric studies mentioned previously. The superposition of spectra, as shown in Fig. 5, provides an insight into surf&e heterogeneity because peak maxima and shoulders should gather around a small number of temperatures that are characteristic (in constant experimental circumstances) of the adsorption states. However, this pattern is complicated by maximum shifts due to strong peak overlapping. In an extreme case, the overlapping of two peaks which are close to each other and of similar magnitude may produce a wide single peak with a maximum at an intermediate temperature. This seems to be the case for TPD c. When it is excluded, the maximum temperatures of the TPD peaks gather around five values (380,453, 523,598 and 708 K) in agreement with ITPD results. Incidentally it is interesting to examine what results would be obtained through the classical equation at the maximum temperature, which is frequently used to assess Ed:
E,/RT’=
(A/cl!)exp( -E,JRT),
(where (Y is the heating rate and A is taken from Table 1). This equation is correct in the case of a first-order desorption without re-adsorption and gives approximate results in numerous other cases [18,19] provided that pore diffusion is not involved. The results are 97,124,154,187, and 210 kJ/mol for the five states, respectively. These energies, though generally smaller than those obtained by ITPD, may be considered satisfactory due to the difficulty in locating the right peak temperatures in the set of TPD’s with various coverages. However, it must be underlined that the shape and the position of the TPD peak generally depends on the experimental conditions. Thus, the above agreement between apparent activation energies from ITPD and from the positions of the peaks stems from the particular conditions we used. ITPD analysis, which is more independent of the mechanism of desorption [ 91, does not present this kind of difficulties. It is worth noting that the set of TPD’s presented in Fig. 5 is similar to that obtained almost three decades ago by Amenomiya et al. [20] for an alumina prepared by calcination under air at 873 K of aluminium hydroxide [ 211. Their experimental procedure of ammonia adsorption was different to ours: in their case different ammonia coverages were obtained by means of different outgassing temperatures instead of by different adsorption doses which were used in our case. The results confirm that there are some similarities between the acid sites on an alumina dehydrated under air and those detected in our work under vacuum. Nevertheless, a more detailed conclusion is not possible be-
J. P. Joly et al. / Appl. Catal. A 98 (1993) 61- 70
69
cause of the lack of precise data about the heats of ammonia adsorption on y-alumina. Under the assumption that the alumina used by Amenomiya et al. [ 201 is not very different from those mentioned above for more recent studies [ 14,151, it appears that the reported activation energies of desorption (75 and 29 kJ/mol for coverages of 0.19 and 3.32 ,umol/m2, respectively) are underevaluated. A further conclusion that can be drawn from the results presented in Fig. 5 is that adsorption on a bare surface is less selective than a desorption by ITPD from an initially saturated surface. For example, it can be seen than the adsorption in the state corresponding to a peak at 598 K occurs before the adsorption in the state corresponding to a peak at 708 K is completed. This observation may have consequences in calorimetry which occasionally provides poorly defined plateaus for adsorption states in the heat curves for ammonia adsorption on a solid. This may be explained by the fact that the kinetics of desorption are dominated by activation energies that are rather different while the kinetics of adsorption is governed by very low activation energies that are similar. As a result adsorption heats for each state determined by calorimetry may be somewhat underevaluated. The existence of various strongly adsorbed species on alumina has been evidenced by IR. Thus, according to a review published by Kniizinger [ 221 at least four adsorbed ammonia species have been identified beside weakly held ammonia. The four states of adsorbed ammonia are assigned to strong adsorption sites, mainly Lewis ones ( Al3+ ) , and possibly acid-base pairs involving 02surface ions as well. To our knowledge it is the first time that the quantities of adsorbed ammonia in five adsorbed states on an alumina sample and the corresponding adsorption energies have been determined. Moving on from the discovery of five energetically distinct states of ammonia adsorption on amorphous alumina, it should be possible to identify the corresponding acid centres. Unfortunately, TPD methods give very little information on this subject. Following the general agreement stated in the literature on the nature of the strong acid centres of y-alumina [ 23,241, and considering the recent experimental confirmation that no pyridinium ions are observed when pyridine is adsorbed on an amorphous alumina [ 171, it is likely that the acid centres for the adsorption states we observed are Lewis-type sites. This point is supported by theoretical studies of molecular models which show that numerous Lewis-type sites may be formed upon dehydrating the surface of alumina [ 25 1. CONCLUSION
In conclusion, this study on the adsorption/desorption of ammonia on an amorphous, microporous alumina confirms that the ITPD is useful for characterizing the distribution of strength among the superficial acid centres, even
J.P. Joly et al. / Appl. Catd. A 98 (1993) 61-70
70
in the case of a microporous solid. The efficiency of ITPD in showing various adsorption states is better than that of most classical TPD methods or differential calorimetry methods found in the literature. In addition, the discovery of five distinct strongly adsorbed states of ammonia on an amorphous alumina and the comparison with results published in the literature for y-alumina shows the importance of investigating in detail the distribution of acid strength for various alumina preparations. This could provide experimental data useful for the identification of the acid centres. ACKNOWLEDGEMENT
The authors are grateful to A. Brachet for valuable participation in the experimental work. REFERENCES 1 2 3 4 5 6 7 8 9 10 11
12
13 14 15 16 17 18 19 20 21 22 23 24 25
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