570
J. Phys. Chem. 1981, 85, 570-574
Thermal Desorption Study of Surface Hydroxyls on Chromic Oxide Kunimltsu Morlshlge, Shigeharu Kittaka, Shulchi Iwasakl, Department of Chemistry. Faculty of Science, Okayama College of Science, 1- 1 Rldaicho, Okayama 700, Japan
and Tetsuo Morlmoto Department of Chemistry, Faculty of Science, Okayama UnlversW, Tsushima, Okayama 700, Japan (Received; July 3 1, lS80)
The variety of surface hydrolxyls on CrzO3 was investigated by means of thermal desorption, infrared absorption spectroscopy, and adsorption of water on hydroxylated Crz03by using five samples which differ in preparation method. Four distinct peaks appear around 170 (peak I), 260 (peak 111, 305 (peak 111), and 410 "C (peak IV) in the thermal desorption spectra of the original samples. The corresponding surface hydroxyls of peaks I and I1 can be regenerated reversibly upon rehydration at room temperature, but the reproduction of the corresponding surface hydroxyls of peak IV requires a thermal-activation process in water vapor. On the contrary, the correspondingspecies of peak I11 could never be regenerated upon rehydration. The surface hydroxyls on CrzO3 which cause the occurrence of a jump at a relative pressure of 0.04 in the water physisorption isotherm, that is, the two-dimensional condensation of water, were identified to be those giving peak N in the thermal desorption spectra. Furthermore, they also give rise to a broad band with a peak at 3400 cm-l in the infrared absorption spectra. Introduction Surface hydroxyls on powdered metal oxides are essentially heterogeneous in the sense that there exist a variety of surface hydroxyls on different crystal planes, corners, and edges and on crystal defects. Different kinds of surface hydroxyls may interact with water molecules in different manners. Recently, we have measured the water physisorption isotherms and the thermal desorption spectra of surface hydroxyls on ZnO' and Sn02,2and it was found that on these oxides two types of surface hydroxyls are present, each of which has constant desorption energy; one of them is associated with the occurrence of the two-dimensional condensation of water. Very recently, a clear jump, which is due to the twodimensional condensation of water, has been observed at a relative pressure of 0.04 in the water physisorption isotherm on Cr203.3 This suggests the existence of the peculiar surface hydroxyls which are formed on the Cr203 sample. The purpose of this work is to elucidate the variety of surface hydroxyls on Cr2O3, as well as the property of the surface hydroxyls relevant to the occurrence of the two-dimensional condensation of water. The thermal desorption spectra of surface hydroxyls and the water physisorption isotherms on five CrzO3 samples, which differ in preparation method, are measured. We also investigate the correspondence of each thermal desorption peak of the surface hydroxyls with the infrared absorption bands in the OH stretching vibration region. Experimental Section Materials. Five Cr203samples were used in this study. The details of the preparations have been described earliert3 Samples A-l, A-2, and A-3 were obtained by calcining raw Crz03sample at 700,900, and 1100 "C for 5 h in air, respectively, the latter being prepared by thermal decomposition of (NH4)2Cr207at 300 "C in a stream of NF Cr(N03)3was hydrolyzed with ammonia water, and the resulting precipitates were calcined at 900 "C for 5 h in (1) K. Morishige, S. Kittaka, T. Moriyasu, and T. Morimoto, J. Chern. Soc., Faraday Trans. 1,76,738 (1980). (2) K. Morishige, S. Kittaka, and T. Morimoto, Bull. Chern. SOC.Jpn., 53, 2128 (1980). (3) S. Kittaka, J. Nishiyama, K. Morishige, and T. Morimoto, Colloid Surfaces, in press.
air (sample B-2). Sample C-2 was prepared by calcining the raw Cr2O3 sample a t 900 "C for 5 h in air, the latter being obtained from thermal decompsition of Cr2(C204)3 at 600 "C in air. These materials were thoroughly washed with distilled water, electrodialyzed, and then dried at 80 "C for 2 days. The specific surface areas of samples A-1, A-2, A-3, B-2, and C-2, which were determined from BET analysis of Nzadsorption data at the temperature of liquid nitrogen, were 21.8, 14.5,3.28, 10.1, and 6.92 m2 g-l, respectively. Water Physisorption Isotherms and Thermal Desorption Spectra. Preliminary experiments confirmed that properties of surface hydroxyls on CrzO3 were sensitive to the pretreatment conditions of the sample. Before the measurement of water physisorption isotherms, therefore, CrzO3 samples were pretreated under the following conditions: condition 1, as prepared; condition 2, evacuation torr for 2 h and subsequent expoat 350 "C and 2 X sure to saturated water vapor at room temperature for 15 h (350 "C evacuation-room-temperature rehydration); torr for 2 condition 3, evacuation at 400 "C and 2 X h and subsequent exposure to saturated water vapor at room temperature for 15 h (400"C evacuation-room-temperature rehydration); condition 4,evacuation a t 450 "C torr for 2 h and subsequent exposure to and 2 X saturated water vapor at room temperature for 15 h (450 "C evacuation-room-temperature rehydration); condition 5, treatment under condition 4 followed by exposure to water vapor of 20 torr at 450 "C for 15 h (450 "C evacuation-450 "C rehydration). These samples were evacuated at 40 "C for 2 h to remove the physisorbed water, after which the water physisorption isotherm was measured at 25 "C by using a Sartorius vacuum microbalance; 0.37 g of the sample was employed in the present work, and the resulting bed thickness was ca. 2 mm. An oil manometer was used for the measurement of water vapor pressure. The adsorption equilibrium could be usually attained within 20 min, and the resulting isotherm was reversible. After measurement of water physisorption, the sample was evacuated a t room temperature for 2 h and then heated to 500 "C at a rate of 5/a "C m i d under vacuum, the resulting weight loss being recorded to obtain a thermal desorption spectrum of surface hydroxyls. Both the pretreatment of the sample and the measurement of water
0022-3654181l2O85-~570$0l.25/0 0 1981 Amerlcan Chemical Society
3
2.0-
C
n
d
t-
7
3
0
I
I
..'
m
0
Figure 1. Thermal desorption spectra of surface hydroxyls on A-2, together with those on A-1 and A-3. Pretreatment conditions are as follows: (a) original samples; (b) evacuation at 350 "C for 2 h and then rehydration at room temperature; (c) evacuation at 400 "C for 2 h and then rehydration at room temperature; (d) evacuation at 450 "C for 2 h and then rehydration at room temperature. Shaded circles in part d represent the sample evacuated at 450 "C for 2 h and then rehydrated at 450 O v. m
physisorption and thermal desorption were conducted in one vacuum microbalance system. Infrared Spectra. The infrared spectra were measured by using an infrared spectrophotometer, Type IR-G, manufactured by Nippon Bunko Co. D20 (Merck, 99.75%) was used without further purification.
Results and Discussion Thermal Desorption Spectra of Surface Hydroxyls. Figure 1shows the thermal desorption spectra of surface hydroxyls on samples A-1-3, where the desorption rate (-dw/dT) has been obtained by the graphical differentiation of the original thermogravimetric-analysiscurve. As is demonstrated in Figure la, the thermal desorption spectra of the original samples exhibited four peaks, around 170 (peak I), 260 (peak 111,305 (peak 1111, and 410 "C (peak IV). Peak-maximum temperatures of peaks I, 11, and IV shifted to higher temperatures in samples of larger surface area, indicating an occurrence of readsorption of desorbed gas. Such a phenomenon was not observed on peak 111. Sample A-1, obtained from calcination a t 700 "C, exhibited an obscure thermal desorption spectrum having broad peaks. When the calcination temperature was raised to 900 "C (sample A-2), surface crystallinity of the sample increased, as is revealed by a good resolution between peaks, accompanied by an increase of peak 11. Calcination of the sample at 1100 "C (sample A-3) resulted in a decrease of peaks I and 11, and a remarkable increase of peak IV,as well as an appearance of a new peak around 75 "C, which is supposed to be due to the removal of strongly physisorbed water molecules. The SEM photomicrograph~~ revealed that our Cr203samples did not form a simple shape such as very thin laminae of hexagonal or octagonal contour which had been reported by Zecchina et al.,4 but exposed several crystal planes of polyhedral grains. Therefore, one cannot discuss the present results (4) A. Zecchina, S. Coluccia, E. Guglielminotti, and G. Ghiotti, J.Phys. Chern., 75, 2774 (1971).
on the basis of a single crystal plane. As these desorption peaks are considered to arise from thermal desorption of surface hydroxyls attached to the different crystal planes of a-Cr203,respectively,6 the above-mentioned behavior of each peak against the calcination temperature represents successive steps of the growth of external crystal planes of Cr203microcrystals. Upon 350 "C evacuation-room-temperature rehydration of sample A-2, peak I11 disappeared, while the others remained unchanged, as is shown in Figure lb. Peak IV started to decrease by 400 "C evacuation-room-temperature rehydration (Figure IC). The 450 "C evacuationroom-temperature rehydration caused a complete disappearance of peak IV and an increase of peaks I and I1 (Figure Id). When this sample was rehydrated at 450 "C, peaks I and I1 decreased to the original level, while peak N reappeared (Figure Id). These results indicate that the surface hydroxyls giving rise to peaks I and I1 can be regenerated reversibly upon rehydration at room temperature, but the reproduction of the surface hydroxyls giving rise to peak IV requires a thermal-activation process in water vapor. Similar desorption peaks, peaks I-IV, appeared in the thermal desorption spectra of samples B-2 and (2-2, though the preparation methods differed from that of sample A (Figures 2 and 3). Sample B-2 showed small peaks I and IV, while sample C-2 exhibited a desorption spectrum similar to that of sample A-3 having large peak IV. Behavior of each desorption peak of these two samples against evacuation-rehydration pretreatment was the same as that of sample A. Peak I11 disappeared upon 350 "C evacuation-room-temperature rehydration, and at the (5) When sample A-3 was heated in Hz(50 torr) at 350 "C for 1 h and then rehydrated at room temperature, desorption peaks I, 11, and IV appeared in the same manner as the sample treated under condition 2 (350 "C evacuation-room-temperature rehydration). This means that these desorption peaks arise from removal of the surface hydroxyls on the stoichiometric Crz03surface. Refer to the following paper dealing with excess oxygen of Chromia: M. P. McDaniel and R. L. Burwell, Jr., J. Catal., 36, 394 (1975).
572
lhe Journal of Physical Chemistry, Vol. 85, No. 5, 1981 2.0.
Morlshlge et ai.
a
b
c
-1
$1 I-
2.0
I
E
A d
C
P
I
0
100
0
300
200
400
I 0
1
100
300
200
400
I
T ("C) Flgure 2. Thermal desorption spectra of surface hydroxyls on 8-2. Pretreatment conditions are as follows: (a) orlglnal sample; (b) evacuation at 350 OC for 2 h and then rehydration at room temperature: (c) evacuation at 400 O C for 2 h and then rehydration at room temperature; (d) evacuation at 450 OC for 2 h and then rehydration at room temperature. Shaded clrcles in part d represent the sample evacuated at 450 O C for 2 h and then rehydrated at 450 OC.
2.t
a
b
m
E
1.c
-'m
z Q)
-
0
I-
p
2.c
d
C
I
0
I
nD
H n
I.o
O,
100
200
I
300
100
400
200
300
400
BOO
TI 3 Figure 3. Thermal desorption spectra of surface hydroxyls on C-2. Pretreatment conditions are as follows: (a) original sample: (b) evacuatlon at 350 OC for 2 h and then rehydration at room temperature: (c) evacuation at 400 OC for 2 h and then rehydration at room temperature; (d) evacuation at 450 O C for 2 h and then rehydration at room temperature. Shaded clrcles In part d represent the sample evacuated at 450 O C for 2 h and then rehydrated at 450 OC.
same time peak I1 increased. Therefore, it may be considered that both of the corresponding species of peaks I1 and I11 coexisted on one crystal plane and the vacant sites created by removal of species for peak I11 were rehydrated at room temperature to give the surface hydroxyls for peak 11. As is demonstrated most clearly in Figure 3d, the disappearance of the surface hydroxyls for peak IV was accompanied by an increase of an extremely broad, shallow desorption peak ranging from room temperature to 350 "C. The latter surface hydroxyls with significantly heteroge-
neous desorption energy decreased upon rehydration at 450 "C,while the surface hydroxyls for peak IV regenerated. Such an interconversion between these two peaks, peak IV and the extremely broad peak ranging from room temperature to 350 O C , indicates that these two kinds of surface hydroxyls correspond to two different adsorption states on an identical crystal plane. Therefore, it can be concluded that there exist three kinds of surface hydroxyls (peaks I, 11, and IV) on crystalline Cr203 samples, regardless of preparation method. This fact suggests that
The Journal of Physical Chemistry, Vol. 85,No. 5, 1981 573
Surface Hydroxyls of Cr203
PIP.
P/Po
Figure 4. Water physisorptionisotherms on A-2, together with those on A-1 and A-3. Pretreatment conditions are as follows: (a) original samples; (b) evacuation at 350 O C for 2 h and then rehydration at room temperature; (c) evacuation at 400 OC for 2 h and then rehydration at room temperature; (d) evacuation at 450 OC for 2 h and then rehydratlon at room temperature. Triangles in part d represent the sample evacuated at 450 OC for 2 h and then rehydrated at 450 OC.
.
I Oo
0.1
02
o
od
P/
1 0.1
01
0.1
0-4
Po
Flgure 5. Water physisorptionisotherms on 8-2. Pretreatment conditions are as follows: (a) original sample; (b)evacuation at 350 OC for 2 h and then rehydration at room temperature: (c) evacuation at 400 OC for 2 h and then rehydration at room temperature; (d) evacuation at 450 OC for 2 h and then rehydration at room temperature. Triangles in part d represent the sample evacuated at 450 O C for 2 h and then rehydrated at 450 OC.
the Cr203microcrystals are mainly surrounded with three kinds of external crystal planes, being consistent with the crystallographic consideration by Stone and Vickerman.6 Water Physisorption Isotherms. The water physisorption isotherms on the Cr203samples, on which the thermal desorption spectra of surface hydroxyls were measured, are shown in Figures 4-6. The isotherms on the original samples showed a jump due to the two-dimensional condensation of water at a relative pressure of 0.04, which is common to all five samples. The height of the jump decreased in the order of A-3 > C-2 >> A-2 > A-l > B-2. It is evident from comparison with the thermal desorption spectra in Figures la, 2a, and 3a that this order is identical with that of desorption peak IV. This jump started to decrease upon 350 "C evacuation-room-temperature rehydration and became remarkably small upon 400 "C evacuation-room-temperature rehydration. The 450 OC evacuation-room-temperature rehydration caused a complete clisappearance of this first jump at the relative
Figure 6. Water physisorption isotherms on C-2. Pretreatment conditions are as follows: (a) original sample: (b) evacuation at 350 O C for 2 h and then rehydration at room temperature: (c) evacuation at 400 OC for 2 h and then rehydration at room temperature: (d) evacuation at 450 OC for 2 h and then rehydratlon at room temperature. Triangles in part d represent the sample evacuated at 450 OC for 2 h and then rehydrated at 450 OC.
pressure 0.04, as well as a simultaneous appearance of the second jump in the range of a relative pressure of 0.14-0.20. Sample B-2 exhibited both the fmt and second jumps even in the untreated condition. As far as the first jump is concerned, however, we could not find any fundamental difference in the behaviors on evacuation-rehydration treatments between this B-2 sample and the others. When the Cr203samples evacuated at 450 "C were rehydrated at 450 OC,' the first jump reappeared and the second jump decreased or disappeared. Such a behavior of the first jump upon evacuation-rehydration pretreatment is fully consistent with that of peak IV in the thermal desorption spectra of surface hydroxyls. This means that the surface hydroxyls on Cr203giving rise to desorption peak IV cause the occurrence of the first jump at the relative pressure 0.04 in the water physisorption isotherms, that is, the two-dimensional condensation of water. Though the second jump could not directly be correlated with any desorption peaks, peaks I-IV, it appeared most remarkably on the sample on which desorption peak IV disappeared. As was described in the preceding section, when the vacant sites created by removal of the surface hydroxyls for peak N were exposed to water vapor at room temperature, the surface hydroxyls giving rise to the exceedingly broad desorption peak ranging from room temperature to 350 OC were formed. The second jump seems to be relevant to such surface hydroxyls. The Cr203sample prepared by Carruthers et aL8 showed a simple water physisorption isotherm of Type I1 according to Brunauer's classification. Apparently the peculiar surface hydroxyls giving rise to desorption peak IV, which cause the twodimensional condensation upon water adsorption, were absent on their sample. From the sharp desorption peak it may be clear that the surface hydroxyls for peak I1 are homogeneous. As is revealed by the absence of the jump in water physisorption isotherm which can be correlated to the surface hydroxyls for peak 11, however, any attractive interaction did not work between water molecules (7) Sample A-3 was evacuated at 450 O C for 2 h and then rehydrated at various temperatures, after which the water physisorption isotherms were measured at 25 "C: rehydration at 200 OC resulted in a complete disappearance of the second jump and a faint reappearance of the first jump. The f i s t jump appeared most clearly upon rehydration at 4OC-500 OC.
(6) F. S. Stone and J. C. Vickerman,R o c . R. SOC.London, Ser. A, 354, 331 (1977).
(8) J. D. Carruthers,D. A. Payne, K. S.W. Sing, and L. J. Stryker, J. Colloid Interface Sci., 36, 205 (1971).
574
The Journal of Physical Chemlstry, Vol. 85, No. 5, 1981
1
t
2 cn
4
fz U
K I-
WAVENUMBER (am-')
Flgure 7. (a) Infrared spectra of H20 adsorbed A-1 sample: curve 1, after evacuation at room temperature for 1 h; curve 2, after evacuation at 400 "C for 1 h and then exposure to water vapor (10 torr) at 400 "C for 2 h, followed by evacuation at room temperature for 2 h; curves 3-7, evacuatlon times and temperatures being 1 h at 160 "C,1 h at 210 "C,1 h at 250 "C,1 h at 300 "C,and 1 h at 400 "C,respectively. (b) Infrared spectra of D20 adsorbed A-1 sample: curve 1, after evacuation at room temperature for 1 h; curve 2, after exposure to D,O vapor (10 torr) and then evacuation at room temperature for 1 h; curve 3, after evacuation at 440 "C for 1 h; curve 4, after exposure to D20 vapor (10 torr) at room temperature for 15 h and then evacuation at room temperature for 1 h; curve 5, after exposure to D20vapor (10 torr) at 400 "C for 2 h and then evacuation at room temperature for 1 h; curve 6, after exposure to H20vapor (10 torr) for 10 min and brief evacuation at room temperature.
physisorbed on the peak I1 surface hydroxyls, being different from the situation on the peak IV surface hydroxyls. Infrared Spectra. Sample A-1 was mainly used in the measurement of the infrared spectra of surface hydroxyls on Cr203,because of its highest surface area. This sample was evacuated at successively elevated temperatures up to 400 "C. The transparency against the infrared beam in the region of the OH stretching vibration of the original sample A-1 was unstable during the first cycle of this heat evacuation treatment. Therefore, the sample which was evacuated at 400 "C for 1 h was then rehydrated at 400 "C apd subjected to the second cycle of heat evacuation treatment, followed by the measurement of infrared
Morlshige et al.
spectroscopy. As is illustrated in Figure 7a, the original sample A-1 exhibited two shoulder peaks around 3600 and 3490 cm-l, as well as three absorption bands around 3400, 3320, and 3250 cm-l overlapped with each other. After 400 "C evacuation-400 "C rehydration, the strength of the absorption band at 3600 cm-l increased, compared with that of the original sample, while the other bands remained unchanged. When this sample was evacuated a t 210 "C for 1h, the band around 3250 cm-l disappeared. The band around 3320 cm-l remarkably decreased by evacuation at 250 "C for 1h and then completely disappeared by evacuation at 300 "C for 1h. Two bands around 3600 and 3400 cm-l remained almost unchanged after evacuation at 300 "C. When the evacuation temperature was raised to 400 "C, both the 3600- and 3400-cm-' bands started to decrease. The infrared band around 1600 cm-' due to the H20 bending frequency could be completely removed by evacuation at room temperature, differing from the resulta of Zecchina et al.4 From comparison with the thermal desorption spectra of Figure 1,it is possible to conclude that two bands around 3250 and 3320 cm-' are assigned to the OH stretching vibration of the surface hydroxyls for peaks I and 11, respectively. Next, we examined the effect of rehydration temperature upon the infrared absorption bands in order to identify the band ascribed to the surface hydroxyls for peak IV. Deuteration of surface hydroxyls on the original sample with D20 vapor resulted in a shift of the bands around 3600,3400, and 3250 cm-l into 2660,2510, and 2400 cm-', respectively, as is revealed in Figure 7b. Zecchina et al.4 have observed three main peaks around 2675,2550,2430 cm-l ascribed to the OD stretching vibration on an achromia sample. When the deuterated sample was evacuated at 440 "C for 1h and then rehydrated with D20 vapor at room temperature, the strength of the 2510-cm-' band decreased, while that of the 2400-cm-l band increased. Further rehydration of this sample with D20 vapor at 400 "C caused an increase of the strength of the 2510-cm-' band, as well as a decrease of that around 2400 cm-'. The dependence of this 2510-cm-' band (equivalent to the 34OO-cm-' band in the OH stretching vibration) upon rehydration temperature strongly indicates that this band can be assigned to the OD stretching mod4 of the surface hydroxyls giving rise to desorption peak IV, which is relevant to the occurrence of the first jump a t the relative pressure 0.04 in the water physisorption isotherm on Cr203. Deuterated samples A-2 and B-2 also exhibited this 2510-cm-' band.g (9) The low surface area of sample A-3 and the low transparency against the infrared beam of sample C-2 prevented the infrared measurement of these two samples.
