SORPTION OF NITRIC OXIDEBY A
Dec., 1960
would require that the anions be in contact with the nitrogen atom of the quaternary ammonium ion, the butyl groups moving out of the way to permit this. Considering the rather crude model used for the electrostatic free energy calculation the agreement must be regarded as satisfactory. The situation is strikingly different as far as the enthalpies and entropies are concerned. A test of the continuum theory for these quantities requires no detailed model. Using experimental AFO's we simply write,* assuming independence of ion size parameters with changing temperature AS0 = AFO (d In D/dT) = ( A F o / T ) ( d In D/d In AH0 = AFo (1 d In D/d In T )
+
T)
2
~GEL~
x
~
~
~
1903
Table I11 shows that agreement is quite poor. This is particularly true of the entropy in which the trend predicted is actually the reverse of the observed one. Also the observed positive entropy in pure benzene is not accounted for by the continuum theory. Clearly the discontinuous nature of the solvent and its specific interactions with the electrolyte must be taken into account. TABLE IV TESTO F
THE x1
CONTINUUM
THEORY' ASOC~IO. (e.".
(25') ; hsOAND AHo AHOoala. (kosl.1
0.0 -4.8 .005 -3.7 .010 -2.7 .015 -2.0 For all solvents d In D/d In T = -0.25.
4.3 3.4 2.5 1.8
and use the values of AFO derived from Table 11. a Calculated enthalpies and entropies are shown in Table IV. This is the test previously applied to Acknowledgment.-We would like to thank the the ion-ion pair equilibrium.6 National Science Foundation for the financial supComparison with the experimental quantities in port of this work.
SORPTION ,4ND MAGNETIC SUSCEPTIBILITY STUDIES ON NTRIC OXIDE-ALUMINA GEL SYSTEMS ,4T SEVERAL TEMPERATURES BY AAGESOLBAKKEN' AND LLOYD H. REYERSON School of Chemistry, University of Minnesota, Minneapolis, Minnesota Received June 18, 1960
Sorptions of nitric oxide by alumina gel were determined a t 181, 102, 207 and 273°K. Magnetic susceptibility of the sorbed nitric oxide was followed by a movable magnet. A rapid physical adsorption of the nitric oxide was in each case followed by a slow chemisorption. This was definitely shown by the magnetic studies, for the susceptibility rose rapidly during the physical adsorption and then remained almost constant or fell slightly during the long period of chemisorption. Desorption of the physically sorbed nitric oxide causes the magnetic suscentibility to fall tzothe starting point. The slow desorption of the chemisorbed gas produces no further change in the susceutibility of the system. Here is a system exhibiting both physical and chemisorption under the same conditions. Further, the rate of chemisorption was found to be faster a t lower temperatures, indicating B negative energy of activation if calculated in the usual manner. The data indicate that the transmission coefficient for the chemisorption is very low.
Introduction Early studies in this Laboratory2on the magnetic susceptibility and sorption of nitrogen dioxide on alumina gel had strongly indicated that NO2 was chemisorbed by alumina gel, and the magnetic studies indicate that aluminum nitrate was formed on the surface. More recent studies3 on the sorption of nitric oxide on silica gel showed marked differences between the behavior of NO2 and KO on this gel. Where the NO2 was physically sorbed and dimerized on the gel, the KO was physically sorbed but showed definite 2rIna/r character until the surface was nearly covered with a monolayer a t lower temperatures. These results suggested that the sorption of NO should be studied on alumina gel under conditions similar to those reported on silica1 gel. The following experiments show markedly different results from those previously reported. Experimental Using the same sorption and magnetic equipment as described earlier,3 isotherm and magnetic susceptibilities
-
(1) Graduate Norwegian Fellow at the University of Minnesota from the Norwegian Institute of Technology, Trondbeim, Norway. ( 2 ) L. H. Reyeraon and John Wertz, THISJOURNAL, 63, 234 (1949). (3) Aage Solbaliken and Lloyd H. Reyeraon, ibid., 63, 1G22 (1958).
were determined a t 181, 192,207 and 273°K. The alumina gel was prepared by the same identical method as in the former study.2 Its area, as determined by the BET-nitrogen method, was found to be 368 m.2/g. In contrast, the silica gel used in the recent worka had an area of 562 m.*/g. The same high purity nitric oxide was sorbed and a t no time during the whole investigation did the sorbed gas show any color, as reported by J. H. deBoer on work done in the laboratories of the States' Mines in Holland.' The very first experiments a t 192'K. showed that a very different process was going on than had been previously observed. A rapid physical adsorption occurred which was followed by a very slow chemisorption. The magnetic susceptibility rose rapidly, following the physical adsorption, until the slow chemisorption began. The susceptibility then remained almost constant or fell slightly during chemisorption. This showed that the physically adsorbed gas behaved in a way similar to that adsorbed by silica geLs The chemisorption which followed was of a very different character from previous chemjsorptions observed in this Laboratory. The initial rate de ended on the pressure of the gas and the temperature. sowever, for similar pressures, the rate increased as the temperature was lowered. The rate for a given pressure and temperature declined slowly with time. If after several hours the gas preseure was reduced to zero, the physically sorbed gas quirkly desorbed and the magnetic susceptibility fell to the initial value. A slow desorption of the chemisorbed gas followed with no change in magnetic susceptibility. Because of this slow desorption, it was found desirable to warm the sample and remove all the adsorbed nitric (4) Personal communication from Professor .J. H. deBoer.
AAGESOLBAKKEN AND LLOYD H. REYERSOX
1904
Vol. 64
Results Physical Adsorption.-Since the NO was p . physically adsorbed rapidly, the equilibrium for this part of the process was reached in a couple of minutes. The chemisorption which I 2 5.000 co. followed gave a rate curve which was reasonably straight after the conclusion of physical adsorpX G 4.000 7000, tion. It was considered reasonable to extrapolate this curve back to zero time and use the value of this intercept for the amount of physical adsorption. Figure 1 shows the data 4j z.oooi ' \ obtained for two pressures a t 192'K. The c p. \ small circles show the amounts sorbed plotted against time, while the small crosses give the ,192'K 1 .ooo 453mm H g values of the magnetic force us. time. The long, \ , , , , , . fine lines headed by arrows tie the curves for 1 2 3 4 5 6 7 sorption with the magnetic curves for each of Hours. the two pressures. The results show that the Fig. 1. magnetic susceptibility of the adsorbate-adsorbent system rises to a maximum and then 1 NO or Ol76Og&gd remains almost constant or falls slightly during the slow chemisorption. At the points P 5.000 the gas pressure was quickly reduced to zero I O A and the physically adsorbed NO came off the surface rapidly, followed by a slow desorption of the chemisorbed gas. The susceptibility fell 13.000 -$ to its initial zero value during desorption of the physically adsorbed gas and then remained constant. Figure 2 gives the adsorption-rate data as well as the magnetic susceptibility values obtained a t three different temperatures using essentially the same gas pressure. These results clearly show that the rate of chemisorption rises as the temperature is lowered. Space 10 20 30 40 10 20 30 40 50 10 20 30 40 50 does not permit presenting all the data obMinutes. tained. The chemisorption data together with Fig. 2. the data presented in Figs. 1-2 will be discussed in the section on chemisorption which follows. 5.000 . The amounts of physically adsorbed gas, obtained by the indicated extrapolated points at zero time, give the adsorption isotherms presented in Fig. 3. The scattered points a t the upper end of the iso4.000 T therm at 192'K. show the extremes in the accuracy that may be obtained by extrapolation of the straight line plots at higher pressures. i'3.000 . The isotherms for physical adsorption were all s obtained a t temperatures above the critical temX perature for NO and show a progressive increase in 62.000 the amounts adsorbed as the temperatures are lowered. The magnetic data for physical adsorption are presented in Figs. 4 and 5. It will be observed in Fig. 4 that, as reported for silica gel,3 the 1 .ooc on O.I760g&gf% physically sorbed gas at 273' has the same magnetic susceptibility as the gaseous mixture a t that tem273'K, perature. The points give the experimental results, _ _ _ _ -- - - * while the solid line represents the calculated sus200 400 ceptibility of the gas a t 273'K. In the lower part Mm. of Fig. 4 and for Fig. 5 , the susceptibility of the Fig. 3. first gas adsorbed follows the curve calculated for a oxide before starting each new set of determinations. Thus magnetic value of 2 Bohr magnetons in the gaseous the weight of the adsorbent was the same a t the beginning state. The lower straight lines in each case gives of each new isotherm. The first isotherms were determined the susceptibility that should be observed were the a t 192°K. and those a t 181,207 and 273'H. followed. Buoy- gas adsorbed in the same state as in the gas phase. ancy corrections, as well as corrections for the magnetic As was found for silica gel, a break occurs in the character of the gas present, were applied to each measurecurves for the observed data. Above the break ment. NO M 0.1760 q&gd
1
n.
0
;
1
t
[/
,,
point the data gives values which definitely show that the additional adsorbed gas has the same magnetic character as that in the gas phase, while below the break point the gas shows a magnetic behavior of 2 Bohr magnetons. Table I gives the values of the amounts of NO adsorbed per cm.2 a t the point where there is an abrupt change in the curves obtained by plotting susceptibility vs. amounts adsorbed. It can be noted that the numAMOUNT^
OF
1905
SORPTION OF NITRIC OXIDEBY ALUMINAGEL
Dec., 1960
TABLE I KO ADSORBED PER UNITAREA AT THE BREAK POINT OF THE MAGNETIC C~VES
--Silica gelTemp., Molecules/ OOK. G./cm.t cm.2 181 4.23 X lo-'' 0.85 X 1014 ......... ......... 192 193 3.66 X 10-9 0.73 X 101' 207 273 0.00 0.00 293 0.00 0.00
........
.........
-Alumina
G./cm.Z 1.31 X 10-9 1.70 X 10-9
el%Molecules/ cm.2 0.26 X 1014 0.34 X 101'
2.94 X 0.00
0.59 X 101+ 0.00
.........
..
/
..DO dyne force on phys.ods. NO
.200
/
.400
I
ID00dyne force on phys. ads. NO on 0.1760 g AI.-gel. (H%s)= 5.451.IO6
.........
..
ber of molecules per unit area of surface which exhibit the higher magnetic property is greater for silica gel than for alumina gel. Furthermore, the number of these molecules is greater a t lower temperatures. In the case of alumina gel, the largest number exhibiting the higher magnetic character was found a t 207'K. and this number decreased as the temperature was lowered. It thus appears that the two surfaces act somewhat differently in physically adsorbing NO. The heats of adsorption for the physically bound NO as calculated from the isotherms of Fig. 3 by the Clausius-Clapeyron expression are given in Fig. 6. Although these calculations cannot be too accurate, the curves definitely show a change in the heats of adsorption at the same amount adsorbed as at the break point in the magnetic susceptibility curves in Figs. 4 and 5. These results are similar to those previously ~ b s e r v e d . ~ Chemisorption.-The rates of chemisorption are calculated from tangents to the rate curves shown in Figs. 1-2 as well as to curves not shown because of space limitations, and are plotted as a function of the pressure in Fig. 7. The rates are definitely proportional to the pressures a t lower values while the scattering of the points at higher pressures is due to the lack of precision in drawing the tangents to the curves because of the more rapid changes in weight. Arrow points represent two extreme tangents. From the plots it can be seen that chemisorption proceeds faster at lower than a t higher temperatures indicating a negative heat of activation if one uses the usual method for calculating chemisorption processes. By extrapolating the rates to one atmosphere of gas pressure and plotting a usual log K I T us. 1/T one finds an enthalpy of activation AH* = -3375 cal./mole (assuming the change in the entropy of activation with the temperature to be negligible). From free energy considerations, the theoretical rate constants, based on two different assumptions as t o initial and final states, are calculated for a transmission coefficient of unity. These results are given in Table I1 where they are compared to the observed experimental results. It turns out that a transmission coefficient of about
1.000
000
Fig. 4.
.1.500dyne force on physods NO on01760gA1-gelat192°K (HbnA.)=5.466 IO' _Iom
Ads NO in g IO' IO00
2 000
3000
1500dyne force on phys ads NO on 017609 AI-gel at 18PK
1500
Fig. 5.
is required in order that the theoretical and experimental results agree reasonably well. The calculations for Table I1 section A were carried out on the basis of gaseous NO at one atmosphere pressure going to the activated complex of chemisorbed NO on the surface. This is the usual method adopted for the calculation of chemisorption rates. However, the negative enthalpy of activation indicates that the mechanism involves an intermediate state. In this case the intermediate state must be the physically adsorbed NO. The isotherms of Fig. 3 permit the determination of the amounts of physically adsorbed NO at the several temperatures. The results were then used to obtain values for the rates of chemisorption a t the same coverage of the surface at all of the temperatures used in the study. The rates were compared for an adsorption of 0.250 mg. of NO on the total area of the sample, L e . , 64.8 X lo4 cm.2. Using these
AAGESOLBAKKEN AND LLOYD H. REYERSON
1908
Vol. 64
TABLEI1 B
A Calcd. data are based on the assumption that gaseow NO goes to the activated complex of chemisorbed NO. AH = -3,375 od./mole 70
Temp..
x
*
108
(mole/ eec.)
Rtheor.
Kexptl.d
OK.
AS*b
X 10-
X 101
x X 108
181 192 207 273
7.49 4.82 2.40 0.48
-43.95 -44.28 -45.04 -46.98
11.2 5.45 2.03 0.14
6.91 4.42 2.24 0.44
0.6 0.8 1.1 3.1
-
Calcd. data are based on the assumption that the physically 1.020 adsorbed NO goes t o final Chemisorbed NO. AH* cal./mole a t 0.250 mg. NO adsorbed on the sample. 8 = 0.1 for 10" adsorption sites/cm.* re
x
Ktboor.
AS*:'
109
x
IO-:
Kezptl.
x
10'
x
x
108
-34.95 -35.19 -35.50 -36.58
4.6 9.40 0.98 5.1 0.57 1.2 5.8 0.73 1.3 8.1 1.62 2.0 a Rate is extrapolated to 1 atm. pressure. A S * = -(Sotlan, Sorotation) at 1 atm. '&heor. = KT e A s * / R e-AH*/RTsec.-' h TN atm.-l. d KeXpt.= - sec.l atm.-l. r in moles/sec. A = area of sample 64.8 X l o 4 cm.1. s = no. of sites. 1014 sites/ As cm.*. e Rate is calculated a t a coverage of 0.250 mg. NO on total sample. f A S = -(Sotrsnl Sorot) as a two-dimensional gas at a coverage of 0.250 mg. NO on sample (84.8 X lo4cm.*). x = transmission coefficient. 1.45 0.65 0.51 0.35
+
*
14000 NO ads un 01760 g Al-gel
1
1 .ow g.
r
x
2.000
lo-'. Fig. 6.
.OO .IO-8grnol/sec chemisorbed N O on 0.1760 g Al-gel.
/400
I
181°K
/
/
/
2 00
273'K
0.1
0.2
0.3 0.4 0.5 Atm. NO pressure. Fig. 7.
0.6
0.7
rates the log K / T us. 1/T plot gave an enthalpy of activation of AH = 1,030 cal./mole. Assuming that the physically adsorbed NO behaves as a twodimensional gas, which has lost one degree of translational freedom, its entropy was calculated. The entropy of the activated chemisorbed complex
*
+
was considered t o be only vibrational entropy, Section B of Table II gives the results of such calculations. Again a transmission coefficient of about the same order of magnitude, z.e., lo+, is needed to bring the experimental results in line with the theoretical rate constants. In Fig. 8 an attempt is made to show the potential energy of the system as a function of the distance from the adsorption surface. The probable energies for the physically adsorbed state, the activated complex of the chemisorbed state and the chemisorbed state are shown on the curves. The differential enthalpies of physical adsorption, as calculated from isotherm data and shown in Fig. 6, vary between -4,000 and -5,000 cal./mole. If the value of the heat of activation for the process of the physically adsorbed NO going to the chemisorbed state, AH* = 1,030 cal./mole, be added to these values then we obtain values between -3,000 and -4,000 cal. for the potential energy of the activated complex when referred to the initial gaseous state. This is in quite good agreement with the negative heat of activation of -3,375 cal. obtained by using usual methods for calculating the chemisorption rates using the gaseous state and the clean surface as the reacting species. The agreement between the energy results obtained by considering two different mechanisms suggest the possibility that both mechanisms may be operating. The experimental results together with the theory seem to the authors to be better satisfied by the second mechanism. The following equations suggest the possible path of this chemisorption process. Gaseous molecules of NO are first physically adsorbed very rapidly by the A1203. The physically adsorbed complex then undergoes an unpairing of electrons in the A1203 yielding a covalent compound with one unpaired electron. In the final step, a second physically adsorbed NO combines rather rapidly with the intermediate compound giving the final diamagnetic product. NO. + A1203 J_
+
IA1~03 N O . } I A1103 NO. 1 { .Al&a.NO.} { .A120a.N0.} xO:A1203.
+
physical adsorption slow multiplicity change covalent interrncdiate
+
NO:A1203. NO. NO: A1203: NO
final product6
Dec., 1960
SORPTIONOF NITRICOXIDEBY ALUMINAGEL
Discussion It is evident that, in this study, we have an unusual example of physical adsorption and chemisorption taking place at a given solid-gas interface a t the same time. Physical adsorption is rapid at 181, 192, 207 and 273°K. The magnetic susceptibility rises almost immediately to a value which is equivalent to that of NO in the excited state or that of the mixture of the two states. At the same time a slow chemisorption begins and during the whole chemisorption process, as measured in this study, there was no further appreciable change in the magnetic susceptibility of the gel with its adsorbed NO. Thus the odd electron of the chemisorbed NO mcst be bonding with an electron of the alumina gel. If the gas pressure of NO is quickly reduced to nearly zero the physically adsorbed gas comes off a t once and the susceptibility falls to the original value for the gel. The chemisorbed NO comes off very slowly, indicating a sizable energy of activation. There is ample proof that dimerization of NO is not involved. The fact that a t lower temperatures all of the NO, physically adsorbed up to nearly mono layer coverage, shows a magnetic susceptibility equivalent to 2 Bohr magnetons, suggests that the surface-NO interaction involves a total uncoupling of the spin-orbital interaction in the odd electron of NO rather than that the surface specifically adsorbs NO in the 2aa/2 state. Since this does not happen a t room temperature it would seem likely that the NO molecules are closer to the surface at the lower temperatures. If so the uncoupling forces might be perturbation forces which are very dependent upon the distance. Present knowledge of surface states makes it difficult to define actually the kinds of forces but these results suggest that such forces are strongly dependent on the temperature-dependent concentrations of electrons in certain energy levels in the surface of the solid. Several interesting facts become evident from the chemisorption process. The process itself is slow but the rate increases as the temperature is lowered. Chemisorption is about twice as fast a t 181 as a t 192" K. During the long period of chemisorption the magnetic susceptibility of the -4120rNO (,omplex either remains constant or falls slightly. This can only mean that there is a change of electron multiplicity involved as each NO molecule becomes, chemisorbed. The single odd electron of NO, in an uncoupled spin state, probably induces an unto pairing of electrons in the alumina gel (A1203) yield an intermediate covalent compound having one unpaired electron. In the final step a second NO molecule, either physically adsorbed or as a gas molecule colliding with the surface, combines rapidly with this unpaired electron to give the final diamagnetic product. The results given in Table I1 show that essentially the same transmission coefficient of lo-* is obtained on the basis of either assumed mechanism for the chemisorption process. This would indicate that we are dealing with a non-adiabatic reaction. In such a reaction, especially when a change in multiplicity is involved, it seems probable that the reso(6) The authors acknowledge with thanks the helpful suggestione of Professor Rufus Lumry.
19Oi
IC
a E
4 - 2 aJ
Z0
NO
-2
Y V
-4 -6
-8 I
I
a
\
.'
0
; tn 2
chemisorption Fig. 8.
nance energy, between the reaction species on both sides of the activated complex, is low. Thus t,he probability of passing from one state to the other would be low in spite of the fact that the thermodynamic changes appear to be favorable. The Landau-Zener' expressionss were used in these calculations. It must be remembered that the chemisorption rate is directly proportional to the pressure of the gas and the slow desorption of the chemisorbed gas tends to support this point of view. Since there is a change in spin multiplicity, there will be no overlapping of the Hamiltonians of the system. The small resonance energy which remains seems likely to be due to perturbation forces having the character of a spin-spin and spin-orbital interaction. This makes the top of the energy curve (Fig. 8), near the activated state, very sharp and there is considerable probability that the reacting species may continue to the upper surface instead of passing over to the product side. Because of the high potential energy of the reacting complex it will not exist on the upper surface, but will fall back into the physically adsorbed state and thus will not have been chemisorbed. In conclusion it may be said that the results of this study have shown, for the first time, that a transmission coefficient of 10+ is to be found in a heterogeneous system involving the chemisorption of a gas on a solid surface. This value of 10-8 is one of the lowest so far found for systems having low resonance energy between the reacting species. These results should be of real interest to those working in the field of heterogeneous catalysis. Related studies are in progress in these laboratories. L. Landau. Phyr. 2. U.S.S.R.. 1, 88 (1932); 9, 46 (1932). (7) C. Zener, PIOC. Roy. SOC.(London), 1 8 7 8 , 8 9 6 (1932); llOA, 66 (1933). (8) See abo K. J. Laidler, "The Chemical Kinetics of Excited States," Oxford University Preas. London, 1955. (6)
