1996
The Journal of Physical Chemistry, Vol. 82, No. 18, 1978
T. Morimoto, Y. Yokota, and S. Kittaka
Adsorption Anomaly in the System Tin( I V ) Oxide-Water Tetsuo Morimoto, Yasuhiro Yokota, Department of Chemistry, Faculty of Science, Okayama University, Tsushima, Okayama 700, Japan
and Shigeharu Kittaka" Department of Chemlstry, Faculty of Science, Okayama Coliege of Science, I- 1 Ridaicho, Okayama 700, Japan (Received January 23, 1978; Revised Manuscript Received June 26, 1978)
The adsorption anomaly of water on Sn02,Le., the appearance of a discontinuity in the adsorption isotherm, was investigated by measuring the adsorption isotherm and the isosteric heat of adsorption on samples which had been treated in vacuo at 1073 K, in H2at 473 K, and in O2at 1073 K. The discontinuity disappeared when the samples were treated either in vacuo at 1073 K for a longer time or in H2at 473 K, and reappeared after treatment of the sample at 1073 K in 02. A corresponding change in the isosteric heat of adsorption, qst, was found; a plateau in the qst curve, which was observed on the sample treated at 1073 K for 4 h, disappeared after H2 treatment and a monotonically decreasing curve was obtained, while the succeeding O2treatment of the sample reproduced the plateau. This led to the conclusion that the removal of surface oxygens was responsible for the decay of the discontinuity. Crystallographic considerations on the Sn02surfaces strongly suggest that the discontinuity mentioned above occurs on the well-developed (100) plane of Sn02.
Introduction In the previous investigation' we found that the adsorption isotherm of water on Sn02 had a discontinuity in the range of relative pressure 0.02-0.03, and that this adsorption anomaly decayed when the sample was heated repeatedly in vacuo at higher temperatures. In accordance with this result, the isosteric heat of adsorption qst showed a characteristic behavior; the qst curve exhibited a plateau at the same coverage as the discontinuity appeared, indicating a strong interaction between adsorbed water molecules, and the plateau also decayed upon successive heat treatment of the sample as did the discontinuity. Furthermore, the adsorption data showed a good fit to the Hill-de Boer equation, which permitted calculation of the two-dimensional critical temperature of water (292 K). These facts led to the conclusion that the adsorption anomaly, which appeared in the Sn02-H20system, also could be interpreted by the two-dimensional condensation of water on the fully hydroxylated surface of Sn02,as in the ZnO-H20 system.2 The aim of the present work is to investigate in more detail the adsorption anomaly in the Sn02-H20 system in connection with the reproducibility of water physisorption sites being responsible for the appearance of the discontinuity. Another aim is to consider which crystal plane of Sn02 contributes to the occurrence of the adsorption anomaly.
Experimental Section Materials. The starting material used in this study was the same as that used in the previous work,l which was prepared by oxidizing metallic tin with HN03. The precipitate formed was washed with distilled water until the final pH of the filtrate reached 3, dried at 383 K for 24 h in an electric oven, and pulverized in an agate mortar. The powdered solid was calcined for 5 h at 1073 K to ensure complete decomposition of possible nitrogencontaminated compounds into oxides. Then it was washed with distilled water to remove soluble impurities until the conductivity of the filtrate approached that of distilled water, and dried at 383 K for 24 h. The sample was shown to have a well-crystallized rutile structure by X-ray analysis, and to have a N2 surface area of 7.71 m2g-l. The 0022-365417812082-1996$01.00/0
two gases, O2 and H2,used for the treatment of the sample were 99.99 and 99.999% pure, respectively. Measurement of Water Adsorption. The main techniques of water adsorption measurements were the same as those described previously.lr2 Here, the H2 and O2 treatments were added together with the determination of adsorbed H2and 02.These procedures were conducted successively on the same sample in an adsorption apparatus. Thus, after every treatment of the sample the surface area was determined by Nz adsorption, and the first and second adsorption isotherms of water at 298 K were measured. After measuring the first adsorption isotherm the sample was exposed to saturated water vapor for 12 h to ensure surface hydroxylation, evacuated for 4 h at 313 K under vacuum (1.5 X N mY2)to remove the physisorbed water, followed by the measurement of the second adsorption isotherm. Scanning Electron Micrograph. Scanning electron microscopy was carried out on the Sn02sample by the use of a scanning electron microscope, Model HHS-2R, produced by Hitachi Co. Ltd.
Results and Discussion Water Adsorption Isotherm on Sn02. In the present work two series of treatments of the sample were performed: in series 1the sample was pumped out at 1073 K under vacuum (1.5 X N mW2> for 12 h, followed by treatment in O2 at 1.3 kN m-2 at 1073 K for 3 h. In series 2 the sample was first heated under vacuum (1.5 X N m-2) at 1073 K for 4 h, next treated in H2 at 0.89 kN m-2 at 473 K for 2 h, and then placed in Oz at 2.0 kN m-2 at 1073 K for 3 h. The water adsorption isotherms obtained on the samples in series 1 are illustrated in Figure 1. For comparison, the data for the sample treated in vacuo at 1073 K for 4 h are cited from the previous paper.' As is seen in Figure 1, the treatment of the sample at 1073 K for 12 h results in a considerable decrease in the amount adsorbed from that of the sample treated for 4 h, and lowers the height of the jump appearing at the relative pressure of 0.02-0.03, similar to the case of successive treatment of the same sample at higher temperatures.' In contrast to this, the succeeding O2treatment of the sample increases the height 0 1978 American Chemical Society
The Journal of Physical Chemistry, Vol. 82, No. 18, 1978 1997
Adsorption Anomaly in SnO,-H,O RELATIVE PRESSURE 0. I 0.2
0
0.3
0 25 N
-P
n
=fE 0 2 0 n
a
015
0
v)
n
a
w 0 IO
s
d
0 05
0
04 06 PRESSURE, kN/rn2
02
0.8
10
12
Figure 1. Adsorption isotherms of water on SnO, (series 1) at 298 K. Solid and broken lines represent the first and second adsorption isotherms, respectively. Pretreatment was as follows: 1073 K, 4 h (0); 1073 K, 12 h (0);1073 K, 02 (a). RELATIVE PRESSURE 01 02
0
0.3
0 25 N
E
&
n
-1
2 0.20
n 0.15
cc
8 n a
w 0.10
2, -I 0
>
0.05
O
0
I
I
02
I
I
I
I
'
0.4 0.6 PRESSURE, kN/m*
I
0.8
I
'
I.o
I
I .2
Figure 2. Adsorption isotherms of water on Sn02 (series 2) at 298 K. Solid and broken lines represent the first and second adsorption isotherms, respectively. Pretreatment was as follows: 1073 K, 4 h (0); 473 K, H2 (0);1073 K, 0 2 (a).
of the jump almost to that of the sample treated at 1073 K for 4 h (Figure 1). It should be noted here that the initial parts of the three adsorption isotherms coincide almost exactly. After the jumps the isotherms exhibit considerable distances between them. In the experiments of series 2 the effects of H2 and O2 treatments of the sample on the adsorbability of water were tested. Adsorption isotherms obtained are given in Figure 2. Figure 2 shows that the H2 treatment of the sample causes the disappearance of the discontinuity in the isotherm and at the same time a marked decrease in adsorbed amount. On the other hand, the succeeding O2 treatment of the sample gives rise to an entire recovery of the adsorbed amount of water as well as the reappearance of the discontinuity in the isotherm, as in series 1. During the treatment of the sample in vacuo at 1073 K (series 1)the color of the sample changed from pale yellow to grey, while the O2treatment recovered the original color, as reported by Thornton and H a r r i ~ o nand , ~ Fuller and WarwickS4The same cycle of color change was observed
also in series 2: the H2treatment made the color dark grey, and the O2 treatment pale yellow. The ESR study of S n 0 2 showed that the outgassing of the sample under a high vacuum of 1.3 X N mW2produces surface oxygen vacancies, which increase with increasing treatment temperature. A quantitative investigation of the surface oxygen vacancies on the present sample shows the following results. The amount of H2 consumed in the reaction in series 2 was found to be 3.70 atoms nm-2, and that of O2 adsorbed in the experiments of series 1and 2 to be 1.98 and 2.50 atoms nm-2, respectively. If part of the surface oxygens of Sn02 are removed by the reaction with H2 to form water molecules, the loss of surface oxygen in series 2 should be 1.85 atoms nm-2. Accordingly, the amount of oxygen atoms needed to repair the surface defects should naturally be the same as the calculated one. However, the amount of O2actually consumed was 2.50 atoms nm-2,larger than that taken up by H2 treatment, probably because of partial removal of surface oxygens by initial treatment of the sample at 1073 K. Assuming that the real surfaces of Sn02 having a rutile structure are composed of equal proportions of (100) and (110) planes, the number of oxygen atoms in the electrically neutral surfaces can be computed to be 5.65 atoms nm-2, The observed values of both hydrogen atoms consumed for reduction and oxygen atoms adsorbed on the oxygen deficient surfaces are less than a half of this value. This estimation leads us to believe that these oxidation and reduction reactions are limited only in the Sn02 surface. In conclusion, it is reasonable to consider that the change in adsorbability of water on SnOa treated in various ways as stated above is strongly attributable to the amount of surface oxygen vacancies. A remarkable decrease in adsorbed water by treating the sample at a higher temperature or in an H2 atmosphere can be interpreted as follows. For the formation of two hydroxyls by dissociative adsorption of a water molecule, it is required that a surface oxygen atom, which is capable of receiving a hydrogen atom from the water molecule, is adjacent to a surface metal atom which receives a hydroxyl. Therefore, the removal of an oxygen atom from the surface will result in the decrease of two hydroxyls after chemisorption of water and consequently the decrease in physisorbed water on them. Furthermore, it should be noticed that a partial removal of surface oxygen takes place on the surface of Sn02,which is effective on the decay of the jump (Figures 1 and 2). Isosteric Heat of Physisorption of Water. In order to examine the effects of H2 and O2 treatments of the sample on the isosteric heat of physisorption of water, qat, the second adsorption isotherms were measured at various temperatures, 283,288,293, and 298 K, on the fully hydroxylated surfaces in series 2. The plots In P vs. 1/T gave a good linear relationship, from which qatwas calculated as shown in Figure 3. In this figure the data for the sample treated at 1073 K in vacuo for 4 h are cited from the previous paper.l Figure 3 shows that the shape of the qst curve varies drastically with the treatment of the sample. A plateau in the qatcurve, which appears in the range of coverage of 0.5-0.8 on the sample treated at 1073 K, disappears upon H2treatment, while the O2treatment reproduces a distinct plateau in the qst curve. In other words, the plateau behaves in concert with the discontinuity of the adsorption isotherm on treating the sample in H2 or in 02.These phenomena can best be understood in terms of the idea that uniform surfaces are damaged, when the sample is
1908
The Journal of F h y s b l Chemistry, Vol. 82,No. 18, 1978
T.
Mwlmoto. Y.
TABLE I: Relation between the Amounts of phvsisorbed and Chemisorbed Water on SnO. Surface pretreatment S.' Vm,? Vm,? VP Vc vh vc t vh, conditionb m' g-' mL m-l mL mP [H,O]nm-' [ O H ]nm-' [OH]nm-' [ O H ]nm-a I
3
Yokota.
and S.Kiltaka
Vp/(V, t vh)
Series 1 .
1073.4' 1073-12 1073-0,
6.82 7.14 8.16
0.232 0.149 0.203
0.080 0.061 0.072
2.15 1.64 1.92
1073-4e 473-H, 1073-0,
6.82 6.16 8.76
0.232 0.102 0.232
0.080 0.062 0.086
2.15 1.67 2.31
8.35 4.73 7.03
0.30 0.30 0.30
8.43 5.03 7.33
0.255 0.326 0.263
8.35 2.14 7.82
0.30
8.43
0.255
Series 2
S is the specific surface area. cited from previous work.'
*
0.30
8.14 0.284 The notation 1073-12represents the sample treated a t 1073 K for 12 h, etc. Data
J
I
0
0.02
0.04
0.08
0.06 0.10 0.12 0.14 VOLUME ADSORBED. n l ( ~ . t . O . l / r n '
I
0.16 018
Figwe 3. Isosteric heat of adswption. qa. of water on SnO, (wbs 2) as a function of h a amwnt of adsabed wale,. Pretreatment was as follows: 1073 K. 4 h (0);473 K, H, (0);1073 K 0,(0).A broken line represents h s heat of liquefaction of water. AROWS indicate the monolayer coverages of water molecules.
placed in a reducing atmosphere, by the removal of part of the surface oxygens, which makes the two-dimensional condensation of water impossible on them. Relation between Physisorbed and Chemisorbed Water on SnO,. Adsorption data of water on SnO, are listed in Table I. The values V,, and Vrnl are the monolayer capacities obtained from the first and second adsorption isotherms through the B-point method. Vp is the number of water molecules adsorbed on a unit surface area of SnO,, recalculated from V , . V, is the chemisorbed amount of water expressed by d H groups nm2, which is obtained from the difference between the values V,, and V.,.ZS Here, it is confirmed numerically that V, decreases upon reducing and recovers upon subsequent 0, treatment. In the present study the water content vh, which remains on the surface after the treatment of the sample, was not measured. However, it is seen from the previous work' that on samples evacuated a t temperatures higher than 1073 K, Vh is very small and almost the same irrespective of the pretreatment temperature of the sample, so that we can cite the data from the previous work without significant errors. Thus,by introducing the Vhvalues we can estimate the total amount of chemisorbed water, V, + V,, and herewith the ratio of the number of physisorbed water molecules in the monolayer to that of the underlying surface hydroxyls, as seen in the last column of Table I. The ratios thus calculated are found to be about 1:3 (H,OOH), as small as those reported in the previous w0rk.l Crystallographic Consideration of Hydroxylated Surface Structure of SnO,. The results obtained in the previous and present works lead to the conclusion that the discontinuity in the adsorption isotherm of water on Sn0, can be ascribed to the two-dimensional condensation of
Flgure 4. Scanning
electron micrograph
of SnO,.
water and that it decays upon the removal of surface oxygen and then recovers upon adsorption of oxygen atoms on oxygen deficient sites. The two-dimensional condensation has primarily been discussed in systems which are composed of adsorbents having energetically uniform surfaces such as those of gr:tphite7-"' and alkali and nonpolar adsorbates such as inert gases and hydrocarbons. In 1968 the Zn0-H20 system, composed of a polar adsorbent and a polar adsorbate, was found to give a discontinuity in the adsorption isotherm," and later it was proved to be an example of two-dimensional condensation., The phenomenon appearing in this unique system has been elucidated in terms of the formation of closed hydrogen bonding, that is a characteristic configuration of surface hydroxyls formed on the well-developed (10iO)15plane of ZnO, which makes the interaction of the surface with water molecules weak. Also in the present system Sn0,-H,O, it is reasonable to expect that the phenomenon involves a particular crystal plane of SnO,, as in the case of the system ZnO-H,O. However, it is hopeless to attempt to identify the well-developed plane on SnO,, because the particles used are so small that electron microscopy does not give sufficient evidence of surface crystallinity, as can be seen in Figure 4. Nevertheless, it is clear that this phenomenon takes place on the fully hydroxylated surface of a particular crystal plane of SnO,. Therefore, we can only consider which plane of the Sn02crystal plays an effective role in the appearance of the discontinuity, on the basis of the facts that X-ray analysis showed the present SnO, sample to be a well-developed rutile structure, and that the cleavage property of this substance is that (100) is imperfect, (110) more so, and (111) hardly distinct.16 The results of crystallographic consideration of fully hydroxylated surfaces of three planes of SnO, can be summarized in the following way. On the hydrated (110) plane, two kinds of hydroxyls are present: one is bonded
Adswplion Anomaly in Sn0,-H,O
LA
10101
Fkum 5. Planar (a) and slde (b) views of lh8 hydroxylated (100) plane of SnO,.
to one tin atom and the other to two tin atoms. Both kinds of hydroxyls form each row, existing alternately with each other along the c axis. Both kinds of hydroxyls point outward perpendicularly from the surface, so that they will behave as active sites for approaching water molecules, similar to the case of the (110) plane of rutile." The hydration of the (111) plane of SnOz produces a rather complicated structure. It forms uneven surface hydroxyls pointing outward from the surface, which also behave as active sites for water molecules similar to the case of the (110) plane. The hydrated (100) plane also has two kinds of hydroxyls, bonded to one or two tin atoms, but different in orientation from those on the (110) plane (Figure 5a and 5b). Hydrogen atoms in these hydroxyls point away from the tin atoms as a result of electrostatic repulsion between hydrogen and tin atoms. In addition, two effects should be taken into consideration in this case: one is the in-
The Jwml Of physical
Chemistry, Vol. 82, No. 18, 1978
1999
teraction of a hydrogen atom in a hydroxyl group with the oxygen atoms in the neighboring two hydroxyls, the other is a partial contribution of tetrahedral covalency of the oxygen atom. As a result, the hydrogen atom sinks somewhat into the interstice between two oxygen atoms. Thus, it follows that the surface hydroxyls are almost buried in this plane. If such circumstances are realized in the (100) Dlane of SnO.. the bondine force emanatine from this p i k e for water molecules should be considerahl; smaller than on the surface having hydroxyls pointing outward, which yields the possibility of the two-dimensional condensation of water. These considerations lead to the conclusion that among the three kinds of crystal planes which are most available on SnOz, only the (100) plane has the possibility, after full hydroxylation, to operate as an inert surface for the adsorption of water molecules and to cause two-dimensional condensation of water. As stated above, the ratio HzO/OH on Sn02surfaces is about 1:3, which is extraordinarily smaller than on other metal oxides. This is definitely due to the smaller V, values. This may be plausible if an adsorbed water molecule is located at the center of three hydroxyls in the (100) plane, where the neighboring two rows of hydroxyls are combined by thrusting their hydrogen atoms mutually into interstices between two neighboring oxygens, as described above. References a n d Notes (1) S. Klttaka. S. K a m m . and T. Morhlmlo. J. Warn. Soc.. F a d y Trans. 1. 74. 676 (1978). (2) M. Nagao. J. Fhys. Chem.. 75. 3822 (1971). (3) E. W. RDmlon and P. G. Hankm. J. chsm. Soc.. Fara&y Tram. 1 . 71, 461 (1975). (4) M. J. Fuller and M. E. W a m l d . J. Catal.. 39. 412 (1975). (5) Y. Mhckawa and S. Nakamra. @I. J. appr. Fnys.. 14.779 (1975). (6) T. Mwimlo. M. Nagao. and F. Tokuda. J . mys. Chem.. 73, 243 (1969). (7) J. P. O l M n and S. ROSS. Roc. I?. Sm.London. Ser. A , 265.447 (1962). 181 W. 0 . Machin and S. ROSS. Roc. ROV.Soc. London. Ser. A. 265 .4GC ,,OR,, _"" ~
W. R. Smim and D. 0. Fwd. J . mys. Wunn.. 89. 3587 (1965). 6. w. Davls and c. Pierce. J. mp. Chem.. 70, 1051 (1966). 8.6. FWe and W. 0 MchAan. J. Am. (xm. Soc.. 79. zsB9 (1957). ' s. ROSS and H. cbh. J . am. h m . sm..76.i29.i (1954). S. ROSS.J. P. MMer. and J. J. Mnchen. A&. Chem. Ser.. No. 33. 317 (1961). (14) T. Maimlo. M. Nagao. and F. Tckuda. EM. Warn. Soc. Jpn.. 41,
(9) (10) I 1 11 (12) (13)
qc1.2 I ,.LIc*\
,""" ,*"",.
(15) T. Mwimolo and M. Nagao. J. PhYJ. Chsm.. 78. 1118 (1974). (16) S. Dana and W. E. Ford. "A Textbook of Mineralogy". Wiby. New Y a k . N.Y.. 1960. p 480. (17) M. J. Jaycodr and J. C. R. W a b x . J. chem. Soc.. Faredsy Tmm. 1. 70. 1501 (1974).
