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The Journal of Physical Chemistry, Vol. 82, No. 9, 1978
The heat of dissociation was determined as a function of coverage by substituting the values for the monomer fraction a t 77 and 100 K into the equation -AH,, = R d In & / d ( l / T ) R d In (monomer fraction)02/d(l/T) (1) where KOis the equilibrium constant of the dissociation ((NO), = 2NO) a t coverage 0, and is expressed as (monomer fraction): (total amount adsorbed) to a good approximation in this case.5 The value of AHdi,(3.2 f 0.3 kcal/mol) thus determined was almost independent of the surface coverage and comparable to the value (3.7 f 0.2 kcal/mol) in the liquid phase6 and the value (2.5 f 0.2 kcal/mol) in the gas phase.7 The line width of the spectrum broadened remarkably a t 120 K as shown in Figure IC. The relative peak height decreased rapidly with the increase of coverage. For instance, the peak height a t 8 = 0.6 was less than one tenth of that a t 8 = 0.1. The line width broadening of the spectrum became even more pronounced above 140 K. The number of spins could not be determined accurately above this temperature because of a long tailing of the spectrum. Since the monomer fraction above 120 K could not be determined experimentally, it was estimated from the heat of dissociation and the value of the monomer fraction a t 77 K, as a function of the coverage. Results are shown in Figure 2 by broken lines. The estimated monomer fraction is less than 0.05 even a t 150 K in the region of 0.2 < 8 < 1. This means that the existence of the NO monomer is neglected and a model of dimer adsorption can be adopted, to a first approximation, for the analysis of the isotherm obtained below 150 K as in the succeeding papera4
Furuyama et al.
Chemisorption of nitric oxide was also observed to occur simultaneously with the physisorption on the MgO surface for the initial dose of the NO gas, even a t temperatures as low as 100 K. However, it did not increase appreciably by an additional dose of the gas. After evacuating the physisorbed NO at room temperature, the spectrum shown in Figure Id was obtained. The spectrum shape and g values were quite similar to those of the NO;- species assigned by LunsfordQ3The amount of the chemisorbed species, however, was quite small as compared with that of the physisorbed. Therefore, the effect of chemisorption was neglected in the determination of the thermodynamic data of physisorption a t temperatures below 150 K in this and in the succeeding paper.4
Acknowledgment. This work was supported in part by the Grant-in-aid for Research (Contract (No. 943007 to R.H. and S.F., No. 054182 to S.F., and No. 911503 to T.M.) from the Ministry of Education of the Japanese Government. The authors thank Professor Yuzaburo Fujita (Okayama Univeristy) for his helpful discussion. References and Notes A. Lubezky and M. Folman, Trans. Faraday Soc., 67, 3110 (1971). A. J. Woodward and N. Jonathan, J. Phys. Chem., 75, 2930 (1971). J. H. Lunsford, J . Chem. Phys., 46, 4347 (1967). S. Furuyama, H. Fujii, M. Kawamura, and T. Morimoto, J. Phys. Chem., following article in this issue. (5) KBis defined as (V,,(NO),)*/ V,,((N0)2)o, where ,V , is the adsorbed amount. Since Vad,(NO)B is very small in the present experiment (see Figure 2), V,d,((N0)2)s is practically the same as V,,,(total)O. = (monomer Therefore, KOis expressed as ( V,d,(NO),)2/ Vado(total)B fraction): V,d,(total)B to a good approximation. Furthermore, since V,d,(total)o is not a function of temperature, d In KB/d(l/T) can be replaced by d In (monomer fraction)t/d(l/ T ) as in eq 1. (6) A. L. Smith and H. L. Johnston, J. Am. Chem. Soc., 74, 4696 (1952). (7) C. E. Dinerman and G. E. Ewing, J . Chem. Phys., 53, 626 (1970). (1) (2) (3) (4)
Physisorption of Nitric Oxide, Carbon Monoxide, Nitrogen, and Oxygen by Magnesium Oxide Powder Shozo Furuyama,* Hidemitsu Fujii, Moriyasu Kawamura, and Tetsuo Morimoto Department of Chemistry, Faculty of Science, Okayama University, Tsushima, Okayama 700, Japan (Received December 6, 1977)
Physisorption isotherms of the title gases were measured in the temperature range 77-150 K on MgO powder. The gases were assumed to be adsorbed in a preferred position. (NO)2,the adsorbed form of nitric oxide, was on a one and a half unit cell, while the other gases were on a one unit cell, under the assumption that the surface consisted of the (100) plane. The maximum qst values obtained were 5.6, 3.8, 2.6, and 2.3 kcal/mol for (N0)2/2, CO, N2,and 02,respectively. From these, reasonable values (-1.8 kcal/mol and 5 X lo9 Vjm) were estimated for AHdi,(NO = (N0)2/2)and for the strength of the electrostatic field of the MgO surface. The Stherm’s of (N0)2/2,N2,and O2were almost constant in the range 0.2 < 0 < 0.9 with values of 10,16-19, and 22-24 cal/mol deg, respectively, at their boiling points. That of CO at 100 K, however, decreased from 20 to 13 caljmol deg with an increase of 0. A statistical mechanical analysis showed that (NO), librated, while N2 and O2 could rotate relatively freely on the MgO surface. The degree of motional freedom for CO was close to that of N2 at small 0, but approached that of (NO)2 at large 8.
Introduction The physisorption of nitric oxide shows a special feature that dimerization takes place on the surface of adsorbents. IR spectroscopy revealed that most of the nitric oxide molecules are adsorbed as dimers in the cis form on alkali halide films a t 77 K.lr2 According to the ESR study, the ratio between amounts of monomer and dimer depends on the nature of the adsorbent surface, the surface coverage 0022-3654/78/2082-1028$01.00/0
of the adsorbates, and t e m p e r a t ~ r e . ~The ? ~ adsorption isotherm has been measured and analyzed by statistical mechanics by several worker^.^-^ The thermodynamic properties of physisorbed nitric oxide, however, have not been fully elucidated yet. In the present work, the adsorption of nitric oxide on magnesium oxide powder was measured in the temperature range 90-150 K, and analyzed by statistical mechanics.
0 1978 American Chemical Society
Physisorption of NO, CO, N, and
I-
1rL
NO
m $
9
12
J
L
GO
02
-
10
8
5 ,
TABLE I: Monolayer Capacity and Surface Area vm Surface (mL STP/d area (mz/g) AdsorAdsorbent bate BET B BETa Ba Bb MgOllOOA NO 28.3 29.0 95 97 104 N, 0,
-c 9
MgOlOOOB
4
N,
22.2 20.6 9.1 7.4 6.3
0,
5.5
NO CO
2
- 2 - 1
0
1
2
0
1
2
0
1
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The Journal of Physical Chemistry, Vol. 82, No. 9, 1978
O2 by MgO
2
- 2 . 1
0
1
21.4 21.4 8.3 6.0 6.0 6.0
97 78 31 33 27 21
93 77 28 27 24 21
102 102 30 29 29 29
2
log Pe (Torr)
Figure 1. Adsorption isotherms of NO, N2, 02, and CO on MgO 1000B.
Calculated with
Calculated with the values of a,. the values of ap (see text). 6
Magnesium oxide was chosen as the adsorbent because of its simple structure (NaC1type), and its large surface area (25-100 m2/g). Adsorptions of CO, Nz, and 0, were also measured for comparison. 5
Experimental Section Materials. Magnesium oxide powder was synthesized by the following method. Crude magnesium methylate was produced by the reaction of magnesium ribbon (99.9%) with reagent grade methanol a t 0-50 "C. After being recrystallized twice, the purified magnesium methylate was dissolved in methanol and hydrolyzed slowly by contact with water vapor in a closed vessel. The magnesium hydroxide formed was filtered, dried a t 100 "C for 1 day, and calcined a t 500 "C for 10 h in an atmosphere of Nz. It was finally ground into powder of uniform size (150-270 mesh) and stored in a desiccator filled with Nz gas. Two kinds of samples were prepared from it. The MgO llOOA was prepared by outgassing the MgO powder a t 1100 "C for 4 h a t Torr. The MgO lOOOB was prepared by calcination a t 1000 "C in air for 1 day and by succeeding Torr. The BET (N,) outgassing a t 1000 "C for 2 h at area and mean crystallite size were 97 m2/g and 100 A and 27 m2/g and 500 A for the MgO llOOA and MgO 1000B, respectively. Nitric oxide gas (99.9%), supplied from Nippon Tokushu Gas Co., was purified by evacuating noncondensable gases a t 77 K, after having been passed over activated molecular sieve 13X. Commercial nitrogen (99.999%), oxygen (99.9% ), and carbon monoxide (99.9% ) gases were used without further purification. Apparatus and Procedure. A conventional vacuum apparatus equipped with a quartz adsorption cell was used for the isotherm measurement. The pressure was measured with a Datametrics Baracell pressure sensor 570D-100T connected to an Electronic manometer 1173. Correction for the thermal transpiration effect was made by use of previous datag and the Takaishi-Sensui equation.1° The accuracy of the pressure determination was within 0.2, 1.2, and 3% in the ranges 100-1, 1-0.1, and 0.1-0.01 Torr, respectively. The adsorption temperature was controlled within f0.05 "C in the range 100-150 K, with a laboratory made cryostat driven by a modified Ohkura temperature controller EC 53/2. Its absolute temperature was determined with a methane or ethylene vapor pressure thermometer. Results The adsorption isotherms of NO, N,,and 0, were measured on MgO llOOA and those of NO, N,, O,, and CO were on MgO 1000B, in the temperature range 77-148 K. Since the shapes of the isotherms were very similar on the two samples, only the isotherms measured on lOOOB are shown in Figure 1. (The isotherms on MgO llOOA are
c '
4
5
Y
v
3 3
0-
2
- -AH? 1
0.5
e
1
1.5
Figure 2. The qstvs. 6' plots of (N0),/2, N2, O,, and CO. The solid lines are for MgO 1100A and the broken lines are for MgO 10008.
plotted in Figure l', available as supplementary material. See paragraph a t end of text regarding supplementary material.) The values of the monolayer capacities determined by the B E T method, V,(BET), and by the B point method, V,(B), are summarized in Table I. These, when combined with the values of the generally used adsorption cross section, a. (12.5, 16.2, 14.1, and 16.8 A2 for NO, N,,02,and CO, respectively), gave the surface areas summarized in columns 5 and 6 of Table I. The isosteric heat of adsorption, qst, was calculated from the Clausius-Clapeyron equation by applying least-squares treatments, and plotted against the surface coverage (0 = Vad/ V(B)) in Figure 2 (in Figures 2, 3, and 5, nitric oxide is denoted by (N0I2/2, instead of NO, for reasons mentioned later). All of the qst)s decreased slightly, a t first, with an increase of 0, and then increased to reach maximum values. Finally they decreased rapidly to the heat of liquefaction except for the case of NO measured a t 90-113 K where the qst decreased to the heat of sublimation (see Figure 1). The differential molar entropy of the adsorbate, sad, was calculated from the usual equation
-
Sad = Sgo r'
R hl ( 7 6 0 / P , )- qst/T
(1) where sBo is the molar entropy of the gas at 1 atm and P, the equilibrium pressure (in Torr). Results at 121,77,90, and 100 K for (N0)2/2,N2, 02,and CO, respectively, were
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The Journal of Physical Chemistry, Vol. 82, No. 9, 1978
Furuyama et al. 6.338
(a)
(b)
Figure 4. (a) The molecular dimension of (NO),. The van der Waals radii of N (1.5 A) and 0 (1.40 A) were obtained from ref 16. (b) A schematic representation of the adsorption configuration of (NO),.
t
0
I
I
0.5
1
e
1
1.5
Figure 3. The Sad vs. 0 plots of (N0),/2 (121 K), N, (77 K), 0,(90 K), and CO (100 K). The solid lines are for MgO 1lOOA and the broken lines are for MgO 10008.
plotted against 0 in Figure 3.
Analysis and Discussion of Results The ESR measurements revealed that the monomer fraction of nitric oxide adsorbed on MgO was less than 5% in the range 0 > 0.2 even a t 150 KS4 In the following analysis, therefore, the existence of the NO monomer is neglected and the adsorbate is assumed to be only in the dimer form. However, the values of qst and sad for nitric oxide refer to (N0),/2, instead of (NO),, for convenient comparison with the data of Nz and CO. Although the BET (N,) surface area and crystallite sizes of the MgO llOOA and lOOOB samples differed, the corresponding adsorption data were quite similar to each other as can be seen in Figures 2 and 3. We will treat the adsorption data for the two samples as completely identical in the following analysis. Size and Geometry of the Adsorption Site. Table I shows that the values of V,(B) for Nz, O,, and CO are almost the same, but they are smaller than the values for (N0),/2 by a factor of 1.3-1.4. The surface area calculated by combining the values of Vm(B)with the values of uo, however, scatter widely (column 6). It is concluded that the values of a. are inapplicable but the same value for the 02,and CO adsorption cross section must be used for Nz, on the MgO surface. This conclusion is in accordance with the conclusion obtained by Walker and Zettlemoyer,ll and Anderson’, that “a molecule is physisorbed in a preferred position on the MgO surface”. The surface of the MgO has been assumed to consist of the (100) plane.13 The dimension of a one unit cell on the (100) plane of the MgO surface is 17.7 A2, which is quite suitable for accomodating an admolecule of N,, O,, and CO. The value of up (the adsorption cross section of the preferred position) for these three gases may be 17.7 A,. Since the value of Vm(B)for (N0),/2 is 1.3-1.4 times larger than those of Nz, 02,and CO, the value of up for (NO), should be 17.7/(1.3-1.4) X 2 = 25-27 A2. This value is close to the dimension of a one and a half unit cell (26.5 A2) of the (100) plane and fits the molecular dimension of (NO), given in Figure 4a.I4J5 Therefore, the one and a half unit cell may be the adsorption site for (NO), on the (100) plane of the MgO surface. The surface areas calculated from the values of Vm(B) and up are summarized in column 7 in Table I.
Agreement between the values of surface areas of the different gases is quite good both on the MgO llOOA and 1000B. Heat of Adsorption. Figure 2 shows that qst increases in the order of 0, C Nz < CO < (N0),/2 below 0 = 0.9. This situation may be explained as follows. The qst can be divided into five terms (E,, E,, E,, E,, and AHdim)to a first approximation. Here the first three terms are the components of the qst which are determined principally by the polarizability (a),quadrupole moment (Q), and dipole moment ( p ) of the adsorbate, respectively.16J7 E, is the component originating from the repulsion force, and A&m is the heat of dimerization of NO into (N0),/2. Since the magnitudes of a for the gases do not differ much from each other,18 we may neglect the E , for comparison of the qst. Further we may also neglect the E,, because its absolute value is very small and presumably almost the same for these four gases.16 Then, the largest value of the qst for (N0),/2 is ascribed solely to the combined con. second largest tribution from the E,, E,, and mdimThe value of qst for CO is due to the sum of E Q and E,. Since the values of Q and p for NO and CO are close to each other,18 the difference between their heats of adsorption, Aqst, may be attributed to the value of -mdi,,,.Actually, the value of Aqst (1.8 kcal/mol, irrespective of 0) is in harmony with the value of -AHdim (1.6 kcal/mol) determined by the ESR technique in the previous and with the value (1.8 kcal/mol) determined in the liquid phase.lg The difference between the maximum values of qst for CO and Nz is 1.2 kcal/mol. This difference, to a first approximation, may mainly be ascribed to the dipole-polar surface interaction (E,) of CO, because the Nz gas does not possess a dipole moment and the magnitudes of the quadrupole moment of the two gases are close to each other.ls The interaction energy between a dipole and an electrostatic field (F)is calculated from the equation16
E,, = -pF
(2)
By substituting the value of p (0.11 D) for CO and that of E , (1.2 kcal/mol) into eq 2, we obtained the value of 5 X lo9 V/m (1V/m = (3 X 104)-1esu) for the strength of the electrostatic field of the MgO surface. This is, of course, an approximate estimate but fairly consistent with the values of F obtained for alkali halides (6-9 V/m).’O The smallest value of the q,,(max) for 0, arises from its lack of both dipole and quadrupole moments. The distinct maxima in the qst vs. 0 curves of (NO), and CO suggest that a strong attractive potential operates between admolecules. A configuration of the adsorbed (NO), molecules in Figure 4b is compatible with this suggestion, since the dipoles are directed opposite to each other. In the case of CO, the direction of the dipole may also be mutually opposite. Entropy of Adsorbate. The differential molar entropy of an adsorbate can be divided into two components, the thermal entropy, and the configurational entropy,
Physisorption of NO, CO, N, and
The Journal of Physical Chemistry, Vol. 82, No.
0,by MgO
,
,
9, 1978 1031
deg. It is concluded that the adsorbed (NO), molecule does not rotate freely but librates on the MgO surface a t 121
K.
-
OO
0.5
1
e
Figure 5. The Qtherm vs. 8 plots of (N0)*/2 (121 K), N, (77K), 0,(90 K), and CO (100 K). The solid lines are for MgO 1IOOA and the broken lines are for MgO 1000B.
Sconf. The Sconfof the localized adsorption may be calculated, to a first approximation, from the following equation:
(3) Here W is the number of the configuration, and N, and N , are the number of the adsorption sites and the number of adsorbate molecules on the adsorption surface. The Scad is plotted against 8 in Figure 3. The Sthem is, then, obtained from the $therm = Sad - Sconfrelation and plotted against 8 in Figure 5. Figure 5 shows that Stherm decreases in the order of O2 > Nz = CO > (N0),/2. It is also shown in Figure 5 that the Sther, for CO decreases rapidly with an increase of 8, while the for (N0)2/2, N,, and 02 remain almost constant in the range 0.2 < 8 < 0.8. The $therm of the localized adsorption can be further divided into the entropies of electronic state, vibration, rotation, and external vibration designated as soeleC, sov,b, sorot,and soextvib. Here the external vibration means the vibrations of admolecule toward the vertical and horizontal directions against the adsorbent surface. Among these and So,ib can be estimated unambigentropies, the soelec uously. The values of soelec for 0, is R In 3 = 2.2 cal/mol deg, while those for (N0),/2, N, and CO are 0 cal/mol deg. The Sovib, calculated from the usual equation,,l is negligibly small for N2, O,, and CO, but has a value of 1.0-1.9 cal/mol deg for (N0),/2 a t 121 KaZ2 The magnitude of sorotdepends on the degree of rotational freedom. Here we assume that the adsorbed (NO), molecule rotates freely on the surface. The sorotof a free rotating nonlinear molecule is calculated from the following equation:21 so free rot = 0.5R In (IAIBIc X
loll7)
+
1.5R In T - R In u - 1.03
(4)
where cr is the symmetry number of rotation. The moment , (NO), is evaluated as 776 X g3 of inertia, I A I ~ I cfor cm3 from the molecular geometry given in Figure 4a. Thus, rot = 9.3 cal/mol deg for (N0),/2 we have a value of sofree with cr = 2 a t 121 K. The value of soext vi), for (N0),/2 cannot be estimated exactly but is probably larger than 3 cal/mol deg.23 Accordingly, we have the following relation: -
Sthenn(calcd) = (sovib + so free 13 cal/mol deg
rot -k Soextvib)
This is definitely larger than the Stherm(obsd)= 10 cal/mol
The sofree rot of a linear molecule is calculated from the usual equation.,l The values for N, (77 K), 0, (90 K), and CO (100 K) are 7.0, 8.1, and 9.1 cal/mol deg, respectively. The values of soextvib of those three gases may fall in the range 5-19 cal/mol deg.23 Therefore, the estimated values + sofreerot + soextvib) amount to 12-26 and of Sther, (soelec 15-29 cal/mol deg for N, and 0,. These do not differ much from the observed values of Sther,, 16-19 and 22-24 cal/mol deg. It is concluded that the adsorbed N2 and O2 molecules possess a rather large, if not complete, freedom of rotation. As already mentioned, the &herm for CO decreases with the increase of 8. Its magnitude is close to those of N2 and 0, in the range 8 < 0.4, while it approaches that of (N0),/2 around 6' = 0.8. This indicates that the adsorbed CO molecules can rotate relatively freely at lower coverage, but not a t higher coverage, on the MgO surface. This difference between CO and (NO), may be due to their different adsorption geometries. (NO), occupies a multiple-centered site (see Figure 4b), while CO occupies a single site. It is presumed that the multiple-centered site restricts the molecular motion more strongly than the one-centered site. Accordingly, the $therm of (N0),/2 is smaller than that of CO at lower coverage. However, at higher coverage, the dipole-dipole interaction begins to restrict molecular motion, and the &her, value of CO approaches that of (NO)z/2Difference between V,(BET) and V,(B) of CO. Finally we should mention the large difference between the values of V,(BET) and V,(B) for CO on MgO 1000B. For other gases, the two values are reasonably comparable to each other. Since the relationship, Vm(CO)= Vm(N,) = Vm(02) = V,(N0)/1.35, is very consistent with the model of the adsorption in a preferred position, the value of Vm(B) might be correct. The characteristic variation of $therm with the coverage might be the cause of the wrong value of V,(BET) for CO on MgO 1000B.
Acknowledgment. This work was supported in part by the Grant-in-Aid for Research (Contract No. 054182 to S.F., and No. 911503 and 011004 to T.M.) from the Ministry of Education of the Japanese Government. The authors thank Professor Tetsuo Takaishi (Rikkyo University) for his helpful discussion. Supplementary Material Available: A plot of the adsorption isotherms on MgO 1100A, Figure 1' (1page). Ordering information is available on any current masthead page. References and Notes (1) A. Lubezky and M. Folman, Trans. Faraday Soc., 67,3110 (1971). (2) A. J. Woodward and N. Jonathan. J. Phvs. Chem.. 75.2930 (1971). (3) (a) J. H. Lunsford, J . Phys. Chem., 72:2141 (1967);'(b) J.
