Langmuir 1994,10,685-691
685
Adsorption of H2,02, CO, and C 0 2 on a y-Alumina: Volumetric and Calorimetric Studies M. Cabrejas Manchado, J. M. Guil,' A. P6rez MasiA, A. Ruiz Paniego, and J. M. Trejo Menayo Instituto de QuEmica m i c a "Rocasolano", CSIC, Serrano, 119,28006 Madrid, Spain Received June 29,1992. In Final Form: December 20,1993@ Volumetric and microcalorimetricmeasurements of the adsorption of Hz,0 2 , CO, and CO2 on yA1209 have been carried out. Amounts adsorbed at different temperatures in the range 195-673 K have been determined as a function of pressure and time. Heats of adsorption vs coverage have been measured at 315 K, and in some cases also at 263 K. Hydrogen and oxygen adsorbed weakly on y-alumina; an activated, somewhat stronger adsorption of these two adsorbates appeared at 300-400 K, and a highly activated oxygen reaction with the alumina surface began at around 573 K. Carbon monoxide adsorption was stronger than that of hydrogen or oxygen, and reached higher coverages. CO adsorption was mostly due to a weak surface carbonyl bond although fotmate species also appeared on the surface. Carbon dioxide adsorptionon y-alumina produced three energeticallydifferent specieswhich were identifiedwith the help of infrared spectroscopydata as (1)very strongly held surface carbonate, (2)surface bicarbonate formed by reaction with, and whose number therefore depends on the number of, hydroxyl groups, and is slowly converted into carbonate, and (3)weakly adsorbed COZ. Adsorption calorimetry results give support to the conclusions obtained from the analysis of volumetric isotherms, showing that isotherms themselves can yield valuable information on several details of the adsorption process.
Introduction Amorphous oxides of high specific surface area constitute the support of many metallic catalysts, y-alumina being most widely used. The exposed metal area is usually determined by selective chemisorption of various adsorbates, whose adsorption on the support must also be known in order to obtain the true metallic surface area. Furthermore, it is well-known that also the alumina support plays a role in some catalytic reactions' and that alumina aloneis a usualcatalyst. Therefore,the catalyticproperties of alumina are actively studied in many laboratories. The catalytic properties of y-alumina and other oxides have been related to the acidlbase characteristics of their surface and to the metal-oxygen bond strengthe2p3It has been demonstrated that carbon dioxide poisons the alumina sites active in hydrogen and hydrocarbon exchange reactions, whereas it does not affect olefin isomerization ~enters.~*6 Carbon dioxide and also carbon monoxide are used as probe moleculesto characterize the acid and basic centers of alumina and other oxide Catalysts? Besidesthis application,carbon dioxide adsorption is used for determiningthe exposed surfacearea of oxide catalysts, eg. molybdenum oxide, supported on y-alumina."lo While the identification of surface species, mainly by infrared spectroscopy, has been actively pursued, many
* To whom correspondenceshould be addressed.
AbstractpublishedinAdvance ACSAbstracts, February 1,1994. (1)Gates, B.; Katzer, J. R.; Schuit, G. C. A. Chemistry of Catalytic proCesse~;McGraw Hilk New York, 1979;Chapter 3. (2)hthwisch, D. G.; Dumesic, J. A. Langmuir 1986,2,73-79. (3)Tanabe, K. In Catalysis Science and Technology;Anderson, J. R., Boudart, M., Eds.; Springer-Verb New York, 1981;p 231. (4)h y n e k , M. P. J. Phys. Chem. 1976,79,1280. (6)Della Gam, G.; Fubini, B.; Ghiotti, G.; Morterra, C. J. Catal. 1976, 43,90. Morterra,C.; Zechina, A.;Coluccia, S.;Chiorino, A. J.Chem.SOC., Faraday Trans. 1 1977,73,1644. (6) Morterra, C.; Magnaccn, G.; Cerrato, G.; Del Favero, N.; Filippi, F.; Folonari, C. V. J. Chem. Soc., Faraday Trans. 1 1993,89,136. (7)Segawa, K.; Hall, W.K. J. Catal. 1982,77,221. (8)O'Young, C.-L.; Yang, C.-H.;DeCanio, 5.J.; Patel, M. S.;Storm, D. A. J. Catal. 1988,113,307. (9)McKenzie, A. L.;Fishel, C. T.; Davis, €2. J. J.Catal. 1992,138,547. (10)Mulcshy,F. M.; Kozminski,K. D.; Slike,J. M.; Ciccone,F.;Scierka, 5.J.; Eberhardt, M. A.; Houlla, M.; Hercules, D. M. J. Catal. 1993,139, 688. 0743-746319412410-0685$04.50/0
difficulties have been encountered to determine the strength and number of surface active centers from C02 adsorption. For all the above reasons, it is important to obtain data on the adsorption capacity and energetics of yalumina for the adsorbates commonly used in the characterization of supported metal and metal oxide catalysts, these adsorbates being also reactants in different catalytic processes. However, information on the adsorption syst e m studied in this paper is scarce. This applies especially to the energetic aspects of the process, i.e. to calorimetric measurements. This work is part of a research project devoted to the characterization of several group VI11 metal supported catalysts and to their application in simple hydrogenation and oxidation catalytic reactions. Here the adsorption of hydrogen, oxygen, carbon monoxide, and carbon dioxide on y-alumina is reported. Volumetric measurements over a wide temperature range were carried out to obtain adsorption isotherms and complementary kinetic adsorption data. The differential heat of adsorption as a function of coverage was determined by adsorption microcalorimetry. The use of two different techniquesallowsus to establish correlations between calorimetric and volumetric results. Besides, infrared spectroscopy was used to confirm our interpretation of carbon monoxide and carbon dioxide adsorption results. Particular attention is dedicated to C02 due both to its importance in catalyst characterization and to the variety of surface species produced upon its adsorption on y-alumina. Experimental Section
Materials. Hydrogen, oxygen, carbon monoxide, and carbon dioxide (SociedadEspailola del Oxfgeno, Spain),99.995% pure, were used as adsorbates. Hydrogen was introduced into the apparatusthrough a palladium thimble heated to ca. 800 K. The other gases were distilled several times inside the apparatus, using a liquid nitrogen cold trap and rejectingboth the initial gas fraction and the last condensates. y-Alumina (GirdlerT-126)was used as adsorbent. It was fiit heated in air at 973 K for 4 h and then in a static hydrogen Q 1994 American Chemical Society
686 Langmuir, Vol. 10, No. 3,1994 atmosphere (-0.1 MPa) at 773 K for 2 h. Nitrogen adsorption at 77 K yielded the followingresulk BET specific surface area, 149 m2 9';pore volume, 0.56 cms 9'; mean pore diameter, 7.5 nm. Adsorption Volumetry. Adsorption measurements were performed in a conventionalvolumetricapparatus. Dead volumes were carefully calibrated either by mercury weighing or by helium expansions. This, together with the use of capacitance manometers (Baratron 310,MKS, U.S.A.) ensured a high degree of precision. Reproducibility, measured by cumulative helium expansions, was always better than 0.2 pmol. Before each experiment the sample was outgaseed overnight in a vacuum better than 1 mPa at 773 K. Two types of experiments were performed (i) Isotherms were determined in the usual way by measuring amounts adsorbed at increasing pressures, up to about 16 kPa (25kPa in some cases). Pressures were read after a standard waiting time of 40min. (ii) Adsorption kinetic experimentswere carried out by monitoringthe adsorption of a dose of gas as a function of time, up to 80 min. The fiial pressure was about 26 kPa. Sometimes a second kinetic experiment was performed after outgassing for a period of 15 min at the temperature of the experiment. Both types of experiments were carried out at different temperatures in the range 195-673 K. MicrocalorimetricMeasurements. A heat-flow microcalorimeter of the Tian-Calvet type (Model BT, Setaram, France) was used to determine differential heats of adsorption by measuring the heat evolved in the adsorption of a given amount of adsorbate. For this purpose the calorimeter cella are part of a precise volumetric apparatus similar to that described above. The limit of detection of the calorimeter is about 0.2 mJ or 2 pW. The heat/voltage proportionality constant of the microcalorimeter was calibrated by the Joule effect. The correction for the heat evolved in the gas compression associated to the gas entrance in the cell was determined by previous experiments with helium, typical values being about 25 mJ at the maximum pressure increment at the end of the isotherms. Reproducibility of the calorimetric measurements, estimated from the mean deviation of series of these helium expansion experiments, was in the order of 2 mJ. The temperature range of the calorimeter was 77-473 K. Most experiments were made at 315 K, with a few at 263 K. The sample pretreatment and experimental procedure followed in the calorimetric measurements were the same used for volumetric isotherms. Infrared Spectra. Infrared spectra were recorded at beam temperature using an FT-IR spectrometer with DTGS detector (Model 1600 Series FTIR, Perkin-Elmer) in the wavenumber range 4400-1000 cm-l. The infrared cell, equipped with NaCl windows, was connected to a high vacuum apparatus with a gas dosingsystem and allowed sample outgassing at high temperature. An amount of 7 mg of powdered y-alumina was pressed at 250 MPa into a disk of around 1.2 cm diameter. The same sample pretreatment as with the isotherms was followed. Increasing amounts of COz at increasing pressures were adsorbed at room temperature. Afterward, the sample was outgassed for 15 min at room temperature, the cell was isolated from vacuum, and successive IR spectra were obtained over several days. Amounts adsorbed are expressed as micromoles of molecules per gram of y-alumina dried under vacuum at 773 K.
Results and Discussion Hydrogen Adsorption. Figure 1 shows several hydrogen adsorption isotherms on y A 1 2 0 3 . The amount adsorbed increased smoothly with increasingpressure, and also with decreasing temperature, except for experiments in the range 323-423 K where isotherms were practically coincident. A slight change in shape took place in the middle of this temperature range: the isotherm at 373 K shows a more pronounced knee as compared with those at 323 and 423 K. These two facta suggest the existence of a weak adsorption at low temperatures that becomes an activated, somewhat stronger adsorption at 300-400 K. The isobar displayed in Figure 2 shows more clearly the whole picture. The two broken lines completepossible
Cabrejas Manchado et al.
/' /
x
p I kPa
Figure 1. Hydrogen adsorption isotherms on y-AlaOa: X, 196.5 K; +, 242.0 K; A, 273.2 K.;c ] , 323.5 K;0,373.5K 0,423.2K; 0,473.2 K; V, 573.2 K; *, 637.2 K. paths of these two processes. At low temperatures, the equilibrium of the stronger state cannot be reached due to its significant activation energy, while at high temperatures the weak adsorption is clearly displaced toward desorption. Hydrogen adsorption kinetic experiments were also performed at 273,323, and 374 K. The amount adsorbed reached a plateau after 30-40 min, which justifies the waiting time selected to measure the isotherms. In readsorption kinetic experiments the plateau was reached after =20 min. The amount adsorbedafter 80 min, around 1.1 pmol gl, was approximately the same for the three readsorption experiments. This denotes an increase of irreversible adsorption, calculated as the difference between adsorption and readsorption runs, from around 31 % of the total uptake at 273 and 323 K, to -39% at 374 K; the increase is in fact more important since in the last case the intermediate outgassing was made at higher temperature. All these kinetic experimentsconfirmed the change from weak adsorption to a somewhat stronger process in the temperature range 300-400 K as shown by the isochron at 80 min (open circles in Figure 2). It runs above the isobar because the equilibrium pressure was higher, 25 kPa as compared with 13.3 kPa for the isobars. The maximum amount adsorbed, about 3 pmol g',is somewhatlower than the number of sites found for H2-D2 exchange, 5-10 pmol gl,ll which means that probably the same sites are involved in the two processes. The calorimetric heat for hydrogen adsorption at 263 and 315 K is shown in Figure 3. Experimental points ~~
~
(11) Larson, J. G.;Hall, W.K.J . Phys. Chem. 1965,69, 3080.
Adsorption of H2, 02,CO, and C02 on a r-A120s
Langmuir, Vol. 10, No.3, 1894 687
I b
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-
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-
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Figure 4. Oxygen adsorption on y-AlaO~: 0,isobar at 26.7 kPa; 0, isochron at 80 min and -25 kPa. T/K
Figure 2. Hydrogen adsorption on y-Al2Oa: 0 , isobar at 13.3 P a ; 0, isochron at 80 min and =25 kPa.
rN
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Figure 3. Differentialheat of adsoqhion of hydrogenon y-Al2Oa. (0,263K,0 , 315 K.)
0 0
-
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5
present a high scattering due to the low values of the amount adsorbed and the heat evolved in each dose. Similar heats of adsorption were obtained at both temperatures. The experimental scattering does not allow detection of the difference of adsorption heat for the two postulated processes, that otherwise should be small, since at 316 K the stronger adsorption contributes very little to the total uptake. The mean value is 30 kJ mol-' of Hz, which confirms a weak adsorption as already inferred from the analysis of the isotherms. Oxygen Adsorption. The shape of the oxygen adsorption isotherms was similar to that of the hydrogen ones, although the amounts adsorbed were about 3 times higher. Figure 4 presents the isobar at 27 kPa. A change from a weak to a stronger adsorption, in the range 340380 K,is not so clear as with hydrogen. Oxygen adsorption kinetic experimenta are displayed in Figure 5. Two consecutive experiments, with an intermediate 1Bmin outgassing at the temperature of the
3
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:
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t
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OO
t lmin
Figure 5. Oxygen adsorption on y-Al2Oa: f i t (open symbols) apd second (full symbols) kinetic experimenta at -26 Wa; A, 273.2 K 298.0 K;0 , 323.4 K 9; 348.0 K;0,375.0 K 0 , 473.2 K;V, 573.6 K.
experiment, were performed at most temperatures (open and full symbols, respectively).
Cabrejas Manchado et al.
688 Langmuir, Vol. 10, No. 3, 1994
U
0 0
lo/
I 2 4 6
OO
n,dt pmol O2g-'
Figure 6. Differential heat of adsorption of oxygen on Y-AlzOa: 0,263 K 0,315 K.
At 273 K most of the adsorption was instantaneous, although a slow process still continued when the experiment was concluded. Outgassing was not complete at this low temperature, and a certain amount of irreversible adsorption was apparent when comparing first and second kinetic experiments. However, at 298 and 323 K first and second adsorption kinetic experiments practically coincided, which means that adsorption was almostcompletely reversible: intermediate outgassing at this temperature removed all the oxygen adsorbed on the alumina, due to the weakness of the surface bond. Besides, the short time needed to reach adsorption equilibrium at these somewhat higher temperatures proves that the formation of this weak bond involves a low activation energy, only noticeable at 273 K in the temperature range of our experiments. The experiment at 348 K supports the onset of a somewhat stronger adsorption in this temperature range, as suggested by the analysis of the adsorption isobar (Figure4)) since the first kinetic experiment did not reach a plateau and showed a fraction of the adsorption to be irreversible. A somewhat higher temperature, 375 K, was sufficientto achieve equilibrium and to produce a complete intermediate outgassing as shown by the coincidence of first and second kinetic experiments (Figure 5). The kinetic experiment at 573 K clearlyreveals the onset of another, much stronger process, fully irreversible, and with a high activation energy. It is to be noted that the sample was previously treated in hydrogen at 773 K. Hydrogen is dissociatively adsorbed at high temperature on Lewis acid-base sites composed of coordinatively unsaturated aluminum and oxygen, forming Al-H and an adjacent OH.11-13 The new oxygen process that appears at 573 K could be ascribed to the reverse reaction of that produced in the hydrogen treatment at high temperature. In the second experiment no adsorption at very short times was observed, the slow uptake not being due to readsorption but to the persistence of the new oxygen process. Oxygen adsorption heats at 263 and 315 K are displayed in Figure 6 as a function of coverage. They have a nearly constant value of around 15kJ mol-' at both temperatures, which corresponds to the adsorption process at low temperature since the experiments were made below 350 K. The mean value obtained is lower than that for hydrogen, in spite of the previous hydrogen treatment of the sampleat 773 K, although there are more surface sites for oxygen adsorption than for hydrogen adsorption. Carbon Monoxide Adsorption. Figure 7a displays a carbon monoxide isotherm at 315 K on the y-alumina sample. Initial adsorption increases steeply with equi(12) Carter, J.; Lucchesi, P.J.; Comeil, p.;Yates, D. J. C.; Sinfelt, J. H. J. Phys. Chem. 1966,69,3070. (13) Ioka, F.; Sakka, T.; Ogata, Y.; Iwaeaki, M. Can. J. Chem. 1993, 71,663.
-3$
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00
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I
1
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Figure 7. Adsorption of carbon monoxide on y-AltOa at 316 K (a) isotherm; (b) differential heat of adsorption.
librium pressure, Le. the CO-surface bond is strongenough to retain in the first doses all the adsorbate on the surface under negligible equilibrium pressure, in contrast to hydrogen or oxygen adsorption which requires some pressure right from the first dose to attain a measurable uptake. The amount adsorbed is about 3 times higher than for oxygen under similar conditions. In adsorption kinetic experiments at 323 K, a welldefined plateau was reached in less than 5 min, which implies a process with low activation energy. In a subsequent readsorption kinetic experiment at 323 K the uptake reached 90% of the amount adsorbed in the first experiment, showing the existence of a small amount of CO strongly bound to the alumina surface. This irreversible adsorption approximately coincides with the carbon monoxide uptake in the isotherm at very low equilibrium pressures (Figure 7a). The adsorption heats displayed in Figure 7b confirm the existence of a number of sites of higher adsorption energy as compared with those for hydrogen or oxygen. They decreased with increasing coverage, and two seg ments with different slopes can be distinguished. A value of 64 kJ mol-', assumed to be constant, had been obtained from measurements of integral heat of adsorption of CO on y-Al203;l4 it agrees with our values at the lowest coverages. It is commonly accepted that carbon monoxide adsorption on y-AlzOs takes place initially through a weak carbonyl bond which is easily converted into formate, identified by infrared spectra, by reaction with a surface hydroxyl gr0up.2~6Formate species can further react with another hydroxyl group or oxygen ion to yield bicarbonate or carbonatespecies,respectively,but these processes occur (14) Garroue, E.; Ghiotti, G.;Giamello, E.; Fubini, B. J. Chem. SOC., Faraday Trans. 1 1981, 77,2613.
Adsorption of Hz,0 2 , CO, and COS on a y-Al203
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pmol gl, was adsorbed at negligible equilibrium pressure, which denotesthe existenceof a large number of adsorption sites of high adsorption energy. As will be discussed later, a true chemical bond is formed. After this initial region, the isotherm showed a well-defined knee leading to a final region, where the amount adsorbed increased almost linearly with pressure, in which the adsorption is energetically weak and therefore requires a higher gas phase pressure to take place. This final region is comparable to the entire isotherms of hydrogen or oxygen and to the last part of the carbon monoxide isotherm. The readsorption isotherm runs nearly parallel to the first isotherm at equilibrium pressures higher than 5 P a . The difference, 90pmol g1,corresponds to the irreversible adsorption, and coincides well with the initial amount adsorbed at zero equilibrium pressure at the beginning of the first isotherm, ca. 100 pmol g', showing that this amount was not removed by the intermediate outgassing. Therefore, one would expect readsorption to take place only on weaker sites, i.e., at finite equilibrium pressures. However, this is not the case: a certain initial amount, =40 pmol gl, of carbon dioxide was readsorbed at very low equilibrium pressures, Le., becoming strongly bound to the alumina surface. It cannot be readsorption on sites of high adsorption energy emptied by outgassing since, as established above, evacuation from these sites did not occur. Consequently, one has to postulate that either a part of the species outgassed left a modified surface ready to adsorb COz strongly or that strong adsorption sites were produced during the time interval elapsed between the measurement of the two isotherms. Adsorption heat vs coverage is presented in Figure 8b for both adsorption and readsorption experiments. To plot the readsorption points (full symbols), the nad axis has been displaced 90pmol g-' to the right (upper part of Figure 8b) since this is the amount that was calculated from the difference between the two isotherms as irreversible adsorption remaining on the surface. In this way, results from both experiments may be compared and discussed at, in principle, similar surface coverages. The analysis of the curves in Figure 8b enables us to identify three regions, designated as 1, 2,and 3,which correspond to three energetically different types of adsorption. Region 1 (up to 15 pmol g-9. This region only appears in the first experiment (Figure 8b, open symbols). It corresponds to the beginning of the vertical branch of the volumetric isotherm (Figure 8a). Adsorption heats are very high (130-105 kJ mol-') and clearly show surface heterogeneity for carbon dioxide adsorption,which implies a surface mobility sufficient to allow C02 molecules to reach the strongest adsorption sites where they preferentially remain. The existence of an almost completely mobile monodentate carbonate had been postulated from adsorption entropy calculations to explain changes with temperature of the infrared spectra for carbon dioxide adsorption on alumina? Region 2 (15-130 pmol g - I ) . The curve Qad-nad for the first adsorption experiment shows here a steep decrease from 105 down to 50 kJ mol-'; the corresponding amount adsorbed comprises up to the knee of the isotherm and consists mainly of irreversible adsorption, as discussed above. In this region 2 lies the initial part of the readsorption experiment (Figure 8b, full symbols), with heats of 95-50 kJ mol-l. It corresponds to the vertical portion of the readsorption isotherm. It is important to mention that this part of the curve Qad-nad of the readsorption exper-
1
00
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30
20
plkPa
100
0 I
1
1
200
nod lpmol CO2g-1
Figure 8. Adsorption of carbon dioxide on yAl,Os at 315 K (a)isotherm;(b) differentialheat of adsorption. (Opensymbols, first experiment; full symbols, readsorption experiment.)
to a much lower e ~ t e n t . ~ *The ~ ~curve * l ~ nad-qad in Figure of strong 7b shows a small amount, up to 4 pmol gl, adsorption, which may correspond to formate, whose production is restricted by the number of hydroxyl groups still remaining on the surface after the treatment at 973 K. Subsequently adsorption due to a weaker CO-Al203 interaction, probably through a surface-carbonyl bond, ensues.2 Carbon Dioxide Adsorption. Two carbon dioxide adsorption isotherms were obtained at the same temperature, 315 K, in the following way: a first isotherm was measured; the sample was then outgassed at 315 K for 15 min and kept under static vacuum for 23 days at 315 K; then a second (readsorption) isotherm was determined. The objective was to detect a possible change in the irreversibly adsorbed C02 after a long period of time. Figure 8a shows both the adsorption isotherm (open symbols) and the readsorption isotherm (full symbols). A large amount of carbon dioxide, about 13 times the carbon monoxide uptake under comparable experimental conditions, was taken up by the alumina in the first adsorption isotherm (Figure 8a). However, only a small fraction of the alumina surface becomes covered, as previously reported.6JOJ7 An initial uptake, around 100 (15) Trillo, J. M.;Munuera, G.;Criado, J. M. Cutal. Rev. 1972, 7,51. (16) Fredriksen, G.R.;Blekkan, E.A.; Schanke, D.;Holmen, A. Ber. Bensen-Gee Phye. Chem. 1999,97, 308. (17) Parkyne,N. D.J. Chem. SOC.A 1969,410. Parkyns, N. D.J.Phys. Chem. 1971, 75, 526.
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surfacehave reacted with C02, since formation of carbonate iment does not coincide with the corresponding portion species on them at 315 K is activated, i.e. it needs time. of that of the first experiment. Region 3 (from 130 pmol g-l up). At these higher Region 2 is ascribed to bicarbonate species formed by coveragesthere is a slow uniform decreaseof the adsorption C02 adsorption on aluminum ion-hydroxyl group pairs. heat as coverage increases. This region 3 corresponds to Their number is therefore given by the amount of hydroxyl the final part of the isotherm. The existence of a large groups on the alumina surface, which depends on the number of weak adsorption sites, as deduced from the thermal treatment of the ~ a m p l e . ~ pThey ~ J ~ practically analysis of the isotherms, is thus confirmed by the disappeared after heating to 1073 K, since infrared bands calorimetricresults. Isosteric heats of adsorptionof carbon of hydroxyl groups were not observed, and, of course, dioxide on alumina had been determined from adsorption bicarbonate bands did not appear upon addition of carbon isotherm^.^ Since a measurable equilibrium pressure, dioxide.17 After heating at 973 K, as in our case, most which only occurs at the end of region 2 and in the entire hydroxyl groups were r e m o ~ e d . ~ tTherefore, ~J~ the adregion 3, has to exist to calculate isosteric heats,18 only sorption between 15 and 130 pmol g1gives approximately values for coverages higher than ca. 100 pmol g1were the amount of hydroxyl groups still remaining on the obtained. They coincide well with our values of region 3 surface of the sample. Carbon dioxide adsorption also (Figure 8b). discloses a definite surface heterogeneity for bicarbonate formation. The main part of the bicarbonate species In this region the qad-nad plot for the readsorption remain on the surface after outgassing at 315 K, although experiment (Figure 8b) coincides very well with that of they are removed at higher temperature^.^ However, 40 the first adsorption run, so that the uptake can be ascribed pmol g-l of bicarbonate species is produced in the to readsorption on the same sites which became free in the readsorption isotherm in region 2. To explain these results, intermediate outgassing. we postulate a slow evolution of a certain number of These results can be interpreted with the help of infrared bicarbonate to carbonate species during the time interval spectroscopic data from the literature, which show that elapsed between the two isotherms, only a portion of the direct formation of carbonate and bicarbonate species bicarbonate, viz. those with weaker, but not the weakest, occurs when carbon dioxide is adsorbed on a cation and an oxide ion or hydroxyl group, r e s p e ~ t i v e l y . ~ ~The ~ ~ J ~ J ~surfacebond being involved in the change. Carbon dioxide readsorption produced again bicarbonate species on the number and nature of the species produced strongly hydroxyl groups that had become free (region 2 in the depend on sample pretreatment.5p6J7Jg readsorption isotherm, full symbols of Figure 8b). Surface carbonate has been postulated either as a The final part of the nad-qad curve, region 3, in both monodentate or bidentate species on an exposed oxide adsorption and readsorption isotherms corresponds to i0n.~~6~6J7~~0 It begins to desorb at 423 K,and is still present weak, Le. strongly pressure-dependent adsorption of at 773 K,4 but is completely removed by pumping at this C02,4J7which displays a smooth heterogeneity, as can be temperature.19 seen in Figure 8b. This is in agreement with the variety At room temperature, the formation of bicarbonate of sites, probably specific, postulated to exist for the C02 groups takes precedence over that of carbonate groups in interaction with the alumina surface in order to explain hydroxylated alumina surfaces, but both species appear the infrared frequency dependence on the pretreatment simultaneously upon partial dehydro~y1ation.l~On the temperature." contrary, in experiments at 375 K it is assumed that In order to substantiate the assumption of a slow bicarbonate entities are only formed after the most evolution of bicarbonate species to surface carbonate a energetic sites that generate carbonate species are full, similar experiment was performed, following by infrared since carbonate production is a~tivated.~ Other authors spectroscopythe change with time of C02 surface species. have shown that carbonateinfrared bands were less intense in the beam-temperaturespectra (310K) than in that taken The infrared results for the adsorption experiment at 660 K.2 Surfacebicarbonatesare progressively removed closely followed those already reported in the literature by outgassing between room temperature and 423 K.4 (see references above). Two species are clearly present from the beginning: (i) surface bicarbonate as revealed A third kind of adsorption of weakly held carbon dioxide molecules,linearly bound to aluminum ions, and which is by three bands at 1652,1446-1450, and 1230 cm-l, and a strongly pressure dependent, has been d e ~ c r i b e d . ~ ~well-defined ~ ~ ~ ~ ~ ~band at 3616 cm-l, whose intensity increased Three a-coordinated linear structures, on distinctly corapidly up to a pressure of 2 kPa and remained practically ordinated A1 ions, have been postulated to exist for the constant at higher pressure; their appearance is accompanied by an intensity reduction of alumina OH bands at weak adsorption.S16 ca. 3773 cm-l, and less clearly of those at around 3725 and Besides those outlined above, other species have been 3665 cm-1;6-8*21 (ii) weak C02 adsorption which produces reported: uncoordinated ionic carbonate formed at a slow bands at 2358 and 2344 cm-l of increasing intensity with rate in a symmetricalelectrostatic environment, probably increasing COz pressure. Besides, other bands were regions of high surface dehydration;4J7J9"organic-type" detected. Shoulders at 1590 and 1400 cm-l, indicative of carbonate in a bridged structure;516J7 and even two types monodentate carbonate species, are seen with difficulty of surfacebicarbonates,depending on the types of exposed because they are swamped by the strong bicarbonate aluminum ion and OH group inv0lved.5>~ bands. They appeared with the first dose and remained We think that in region 1 of the nad-qad plot (Figure 8b) practically constant. The presence of two wide, poorly carbon dioxide is adsorbed as surface carbonate. Indeed, defined bands at 1800-1700 and 134Ck1260 cm-l that outgassing at 315 K did not remove it. Certainly, only a decreased with time, could be ascribed to formation of minute fraction of the aluminum-oxygen ion pairs on the surface bidentate carbonates. Afterward, as already indicated, outgassing during 15 (18)Rouquerol, F.; Rouquerol, J.; Della Gatta, G.; Letoquart, C. Thermochim. Acta 1980,39,161. min at room temperature was performed and IR spectra (19) Gregg, S. J.; Ramsay, J. D.F. J. Phys. Chem. 1969, 73, 1243. were obtained in successivedates. The first spectrum after (20) Lercher, J. A.; Colombier, C.; Noller, H. J. Chem. Soc., Faraday pumping showed an almost complete disappearance of Trans. 1 1984,80,949. (21) Okamoto, Y.; Imanaka, T.J. Phya. Chem. 1988,92,7102. the weakly adsorbed C02 bands and a small decrease of
Adsorption of H2,0 2 , CO, and C02 on a y-A1203
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jointly with some bicarbonate species. Development of ionic carbonate could also occur with its characteristic band at around 1450 cm-l overlapping that of surface bicarbonate, at 1462 em-'. On the whole, the picture coincides with that postulated above to explain the C02 readsorption results.
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1600
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1200
Figure9. Infrared spectraof carbon dioxide adsorbed on y-AltO8 at room temperature,as a function of time elapsed after vacuum isolation subsequent to 15 min of outgassing: (a) 2 min; (b) 23 h; (c) 45 h; (d) 136 h; (e) 192 h.
the bicarbonate bands. We now focus on the 1900-1200' cm-l region. Spectra are presented in Figure 9 after subtraction of the spectrum of the y-alumina sample outgassed at 766 K. A decrease of bicarbonate species (bands at 1652,1450,and 1230 cm-l) and a simultaneous increase of the surface monodentate carbonate bands, at 1583 and 1398 cm-', occurred with time. After 8 days (spectrum e) a stable situation seems to have been reached: surfacemonodentate carbonate is clearlypresent
Conclusions Hydrogen adsorbs weakly on y-alumina at low temperatures, a somewhat stronger adsorption appearing at 300-400 K. The number of hydrogen adsorption sites nearly coincides with the number of H B D ~exchange centers, since they are possibly the same. Oxygen presents adsorption characteristics similar to those of hydrogen adsorption. A different, much stronger, process, which can be identified as the reverse reaction of that produced in the hydrogen pretreatment at 773 K, appears at around 573 K. Carbon monoxide adsorption is stronger than that of hydrogen and oxygen and takes place to a higher extent. Surfaceformate is present, althoughmost of the adsorption takes place through a weak surface carbonyl bond. Three energetically different species are formed upon carbon dioxide adsorption on y-alumina. They have been identified with the help of infrared spectroscopicdata from the literature and our own results as: very strongly held surface monodentate carbonate; surface bicarbonate species, whose number depends on the number of previously existing hydroxyl groups, and that are slowly converted into monodentate carbonate (may be also into a certain amount of ionic uncoordinated carbonate); and weakly adsorbed C02. Heat of adsorption vs coverage data for this probe molecule evidence its usefulness in characterizing the number and strength of surface centers. Adsorption calorimetric results give support to the conclusions obtained from the analysis of volumetric isotherms, showing that isotherms themselves can yield information on different details of the adsorption process. Acknowledgment. This work was partially supported by the DGICYT, Spanish Ministry of Education and Science, under Projects PR84-0095and PB87-0327.
