960
Langmuir 1989, 5, 960-965
Enhancement of NO Dimerization by Micropore Fields of Activated Carbon Fibers Katsumi Kaneko," Naomi Fukuzaki, Kazunori Kakei, Takaomi Suzuki, and Sumio Ozeki Department of Chemistry, Faculty of Science, Chiba Uniuersity, Yayoi 1-33, Chiba-shi 260, Japan Received August 23, 1988. I n Final Form: February 15, 1989 The magnetic susceptibility x of the NO-adsorbed activated carbon fibers (ACF) at 273-423 K was measured in order to elucidate the adsorbate-adsorbate interaction in micropores. The micropore structure of the ACF was characterized both by N2 adsorption at 77 K and benzene adsorption at 303 K. The slit width of the ACF's micropores obtained from the t plot analysis for N2 adsorption is 0.8-0.9 nm. The adsorbed species is only NO as determined by temperature-programmed desorption examinations. The x of the adsorbed NO at 298 K is negative and increases with measuring temperature due to release of the adsorbed NO to the gas phase. This magnetic result indicates that more than 98% of the adsorbed NO molecules at 298 K form the dimers and that their dissociation energy is 22-25 kJ/mol, which is greater than that of the NO dimers of the condensed phase at low temperature. The model of the NO dimers in the slit-shaped micropores is shown.
Introduction The Kelvin equation cannot be used for the analysis of adsorption in pores whose width is less than 2 nm, because a liquidlike meniscus is not Dubinin3 proposed a concept of micropore filling that is different from the capillary condensation expressed by the Kelvin equation. In the micropore filling, the adsorption is enhanced by the overlap of the adsorption fields from the opposite walls of the pores. The enhancement of adsorption fields in micropores is confirmed by experimental evidence, such as higher values of isosteric heat.3p4 Also, theoretical calculations by Everett and P0w15showed an enhancement of adsorption potentials in slit-shaped micropores whose width is less than two molecular diameters; appreciable enhancement is predicted for cylindrical pores of six molecular diameters. Gregg and Sing6 indicated that the enhancement effect can be observed in larger micropores than the pore size predicted by Everett and Powl. It is expected that not only adsorbate-adsorbent interactions, but also adsorbate-adsorbate interactions, are enhanced by the micropore fields; cooperative effects in the microthere are very pore filling have been p r o p ~ s e d .However, ~~~ few quantitative data on adsorbate-adsorbate interactions in micropores. If we use an appropriate probe molecule as adsorbate, a strong enhancement of adsorbate-adsorbate interaction by the micropore fields should be shown. The NO molecule shows paramagnetism because of the presence of an unpaired e l e ~ t r o n . ~NO molecules form the dimer (NO)2in the condensed phase a t low temperature, showing diamagnetism.lOJ1 The NO molecule has (1) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity, 2nd ed.; Academic Press: London, 1982; p 153. (2) Fisher, L. R.; Israelachvili, J. N. J. Colloid Interface Sci. 1981,80, 528. (3) Dubinin, M. M. Chem. Reu. 1960, 60, 235. (4) Bhambhani, M. R.; Cutting, P. A,; Sing, K. S. W.; Turk, D. H. J . Colloid Interface Scz. 1972, 38, 102. (5) Everett, D. H.; Powl, J. C. J . Chem. SOC., Faraday Trans. I 1976, 72, 619. (6) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity, 2nd ed.; Academic Press: London, 1982; p 207. (7) Marsh, H. Carbon 1987, 25, 49. (8) Carrott, P. J. M.; Roberts, R. A.; Sing, K. S. W. Carbon 1987, 25, 59
I-.
(9) Smith, A. L.; Johnston, H. L. J . A m . Chem. SOC.1952, 74,4696. (10) Dulmage, W. J.; Meyers, E. A,; Lipscomp, W. N. J . A m . Chem. SOC.1952, 74, 4696.
0743-7463/89/2405-0960$01.50/0
been of considerable interest in connection with a van der Waals m01ecule;'~J~ the dimer of low concentration in the gas phase is found spectroscopically. The NO adsorbed on a flat surface forms a dimer below 90 K according to Enault and Larher.14 However, a small quantity of dimerized NO molecules on a solid surface was detected above room temperature by IR spectroscopy.15J6 At room temperature, the usual microporous adsorbents cannot adsorb NO molecules, whose critical temperature is 180 K.17J8 Activated carbon fibers (ACFs) have uniform micropores of slit shape. We found that a-FeOOH- or a-Fe203-dispersedACF can adsorb abundant NO a t 303 K by a chemisorption-assisted micropore filling mechanism;lS2l the data on the isosteric heat of adsorption suggest the dimerization of NO molecules within the micropores. In a preceding paper,22we reported that almost all adsorbed NO molecules are dimerized in the micropores of the unmodified ACF. We could not use the iron oxide dispersed ACF due to the large magnetic susceptibility of iron oxides. In this paper, we describe NO dimerization in the micropores of ACF as a marked adsorbate-adsorbate interaction enhanced by the micropore fields and discuss the relationship between the dimerization and the micropore structures.
Experimental Section Characterization of ACF and NO Adsorption. Three kinds of commercial ACFs were used in this study: cellulose- (Toyobo), poly(acrylonitry1e)-(Toho Rayon), and pitch-based (Osaka Gas) ACF samples are designated CEL, PAN, and PIT, respectively. (11) Wang, F. E.; May, W. R.; Lippert, E. L. Acta Crystallogr. 1961, 14, 1100.
(12) Billingsley, J.; Callerar, D. B. J. Chem. SOC. 1971, 67, 589. (13) Kajimoto, 0.;Honma, K.; Kobayashi, T. J. Phys. Chem. 1985,89, 2725. (14) Enault, A.; Larher, Y. Surf. Sci. 1977, 62, 233. (15) Hanoch, D.; Folman, M. J . Chem. SOC.,Faraday Trans. I 1979, ~~
-
7.5 . - , 1788 - .- .
(16) Busca, G.; Lorenzelli, V. J. Catal. 1981, 72, 303. (17) Handbook of Chemistry and Physics, 54th ed.; CRC Press: New York, 1973-1974; p F-77. (18) Kaneko, K.; Inouye, K. Adsorption Sci. Tech., in press. (19) Kaneko, K. Langmuir 1987, 3, 357. (20) Kaneko, K.; Inouye, K. Carbon 1986,24, 772. (21) Kaneko, K. Characterization of Porous Solids; Unger, K. K., et al., Eds.; Elsevier: Amsterdam, 1988; p 183. (22) Kaneko, K.; Fukuzaki, N.; Ozeki, S. J . Chem. Phys. 1987,87, 776.
'0 1989 American Chemical Society
Enhancement
of NO
Dimerization
The adsorption isotherms of nitrogen on the ACF at 77 K were measured by using an automatic gravimetric apparatus developed in our laboratory over a relative pressure range of 0.00005-1.0 (80 measurements) with a sensitivity of 5 Pa and 0.1 mg, in order to obtain detailed information on the micropore structure. Adsorption isotherms of NO and benzene at 303 K were measured gravimetrically by use of a quartz spring with a sensitivity of 5 mg mm-' and a cathetometer. The whole adsorption apparatus used to measure the benzene isotherm was kept at 303 + 0.5 K. The ACF was preheated at 383 K under Pa for 15 h prior to the adsorption experiments. NO gas (Takachiho Kagaku) of 99.0% purity was used after purification by vacuum distillation. Density Measurement of NO Filled in Micropores. We determined an apparent density pNOof NO filled in micropores by use of the adsorption of N2 onto NO-preadsorbed ACF. p N 0 was calculated by M(N0) (1) = W0(N2)*- W0(N2/NO) Here, M(N0) is the weight in mg/g of NO adsorbed per unit weight of an adsorbent at 303 K and 80 kPa; Wo(Nz)*and WO(N2/NO)are the pore volume of the ACF without adsorbed NO and that of the NO-preadsorbed ACF, respectively. A detailed explanation on these volumes will appear in the Results and Discussion. The adsorption isotherms of nitrogen on NOpreadsorbed ACF at 77 K were measured with the same adsorption apparatus as used in NO adsorption. Temperature-Programmed Desorption of NO. We measured temperature-programmed desorption (TPD) spectra of NO adsorbed on the CEL samples at 303 K and 13 kPa NO with the use of a mass filter (ULVAC, MSQ-150A) and temperature controller (Chino NP161) set at a constant heating speed of 10 K/min. NO gas in the gas phase was removed by evacuation at 303 K for 10 min prior to the TPD measurements. Magnetic Susceptibility. The magnetic susceptibility was measured by a Faraday method with the aid of a Cahn balance at 273-423 K. The gradient of the magnetic field at the specimen was determined by both a Gauss meter and standard samples (NaC1,23KMnF3,24iron alum,23and Mohr's salt2% The ACF packed in a glass ampule of 1 mL was evacuated at 383 K and Pa, and then the ampule was sealed after adsorption of NO on the ACF at 80 kPa and 303 K. The weight of ACF samples for magnetic measurements is 100 mg for CEL and PAN and 80 mg for PIT. The diamagnetic contribution of the ampule was subtracted before calculation of the magnetic susceptibilities of the ACFs with and without adsorbed NO.
Results and Discussion Micropore Structure. The adsorption isotherms of Nz on all ACF samples at 77 K are type I, which suggests the presence of micropores. We have no definitive analytical method for the N2 adsorption isotherms in order to elucidate the microporosity of carbons. However, t and Dubinin-Radushkevish (DR) plots give us the approximate structure of the micropores. The t plots were constructed from the N2 adsorption isotherms. We used the value of the thickness of the adsorbed nitrogen layer, t , on the nonporous becawe IUPAC26recommends use of the standard thickness value of N2 on the same substance rather than on the substance having the same c value as the BET equation. Each t plot passes through the origin in the lower t region and then bends near 0.4 nm. The specific surface area S, was obtained from the slope of each t plot near the origin. Also, the external specific surface area Sext, which does not contain the contribution of the micropores, was determined from the slope of the t plot
-
I
I
I
I
'. .
€I@--.
5 O1
I
10
0
I
I
603.
I
30
20
L0
I d PdP
Figure 1. DR plots for N2 adsorption. Table I. Adsomtion Data on Microoore Structures sample CEL PAN PIT N2 Adsorption S, (m2/d 1310 830 1230
si,, (mT'jg, Dore
width (nm)
WAN,) (mL/g)
Wo(N2)(mL/g) Wo(N2)* (mL/g) Eo(%) (kJ/mol) "qst(N2)(kJ/mol)
12 0.8 f 0.1
2
0.57 0.50 0.58 16.4
0.33 0.32 0.34 18.2 11.7
10.9
0.8 f 0.1
5 0.8 f 0.1
Benzene Adsorption W&benzene)(mL/g) 0.55 0.34 Eo(benzene)(kJ/mol) 15.0 25.8 'q,,(benzene) (kJ/mol) 49.0 60.0 p(benzene) (mL/g) 0.96 0.91 p*(benzene)(mL/g) 0.83 0.85
'Q s t
0.49 0.47
0.49 17.4 11.4 0.46
24.6 58.7 0.79
0.76
is qst,d-l/e.
in the higher t region. The micropore volume Wt(N2)was determined from the intercept of the line extrapolated from the higher t region. The width w of the slit-shaped micropores obtained from the t plots by the MP method2' is 0.8-0.9 nm. The micropore filling is well described by the DubininRadushkevich (DR) equation:
W = Wo exp(-t2/E2); E = @Eo
(2)
where W is the amount of adsorption, Wo the micropore volume, E = RT In (Po/P)the adsorption potential, Po the saturated vapor pressure, Eo the characteristic adsorption energy, and /3 an affinity coefficient. Figure 1shows the DR plots for N2 adsorption. The DR plots deviate upward at higher values of PIPo. T h e micropore volume W0(N2) and E0(N2)were determined by back-extrapolation of the good linear section of the DR plots in the 10-40 ln2 (Po/P) region; we used /3 = 0.33 for Nz after Dubinin28to obtain E0(N2). The total volume Wo(Nz)*was obtained by extrapolation of the deviated linear section. These values are collected in Table I. The micropore volume from the t plot, W,, is equal to W0(N2)*rather than W0(N2). The isosteric heat of adsorption, qst,e=l/e,at a coverage of l / e (about 0.368) was calculated fromlg qst,e=l/e= Hv + E
(23) Kaguku Binran II; Maruzen: Tokyo, 1975; pp 1235, 1238. (24) Hirakawa, K.; Hashimoto, T. J . Phys. SOC.Jpn. 1960, 15, 2063. (25) Rodriguez-Reinoso, F.; Martin-Martinez, J. M.; Prado-Burgete, C.; MacEnaney, B. J. Phys. Chem. 1987,91, 515. (26) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouquerol, J.; Siemieniewska, T. Pure Appl. Chem. 1985,57,
I
I
(3)
(27) Mikhail, R. S.; Brunauer, S.; Boder, E. E. J . Colloid Interface Sci. 1968, 26, 45. (28) Kagaku Binran II; Maruzen: Tokyo, 1975; p 913. (29) Dubinin, M. M. Chemistry and Physics of Carbon; Walker, P. L. J., Ed.; Marcel Dekker: New York, 1966; Vol. 2, p 51.
962 Langmuir, Vol. 5, No. 4 , 1989
Kaneko et al. 150 PA N
cn . r. 100
n w
m
LL
51n 4
0
z
50 I
5.0
L 0
IO
20
0
I
I
NO
Figure 2. DR plots for benzene adsorption. Here, H , is the heat of vaporization (5.6 kJ/mol at 77 K28 for N2). Figure 2 shows the DR plots for benzene adsorption. All plots are linear in the whole region; each plot has a more different slope than that for Nz. The micropore volume from benzene, Wo(benzene), and Eo(benzene) were determined by the back-extrapolation of the DR plot. Here, p = 1 for benzene is used,28 Also, the qat,o=llevalues are obtained by use of H , = 34.1 kJ/mo128 in a similar way. The adsorption parameters for benzene are collected in Table I. The density p or p* of benzene filled in the micropores was estimated by using both Wo(benzene) in mg/g and W0(N2)or W0(N2)*in mL/g, respectively. Table I shows the densities; p* is smaller than p , being near to the density of liquid benzene at 303 K (0.88 g/mLIe3O As the state of molecular aggregates in micropores is said to be quasi-liquid, W0(N2)*rather than W0(N2)is regarded as the true micropore volume. The upward deviation of the DR plots for Nz is not caused by the presence of mesoporosity or multilayer adsorption of an external surface, because the DR plots for benzene are straight lines. Although ACF may have slight heterogeneity in micropores, we consider that the deviation of the DR plot for N2 arises from the two-step mechanism of micropore filling, that is, the monolayer adsorption on bare surfaces and the filling between monolayer-covered surfaces. In the case of benzene adsorption, the DR plot for benzene is assumed not to give rise to such deviation as observed in the DR plots for N2,since the difference in heats of adsorption on bare carbon surfaces and filling in the benzene-coated pores is so small. Thus, the conclusion from the DR plot does not necessarily contradict the result from the t plot analysis. The ACF probably has considerably uniform micropores, pending a further examination of the micropore structure by other techniques. NO Adsorption. The amount of NO adsorption swiftly increases within the initial 2 h, but the adsorption hardly changes during 30 min after 5-8 h. The adsorption reaches equilibrium after 10 days. The amount of NO adsorption after 5 h is about 80% of the equilibrium value after 10 days. We determined the adsorption after 5-7 h; the obtained isotherm is therefore of pseudoequilibrium. The adsorption isotherms of NO on the ACF are of Langmuir type and have hystereses as shown in Figure 3; we ap(30) Riddick, J. A.; Bunger, W. B. Organic Soluents;Wiley-Interscience: New York, 1970; p 107.
1
I
80
40
I"*po/P
PRESSURE, kPa
Figure 3. Adsorption isotherms of NO on ACF samples at 303 K. Solid line, adsorption; broken line, desorption.
350
450
550
TEMPERATURE,
750
650
K
Figure 4. TPD spectra of an NO-adsorbed CEL. Table 11. Thermodynamic Quantities for NO Adsorption sample
CEL PAN PIT
WL(NO)," WLO")/ mL/g Wo(N2)* 0.054 0.10 0.062
0.09 0.28 0.13
Eo(NO),
Po(NO),
qst,.wle,
12 15
360 340 310
30 24
kJ/mol 9
kPa
kJ/mol 27
"The volume is obtained by use of the density of liquid NO at 1 2 1 K.
proximated the saturated amount of NO adsorption, WL(NO),by the Langmuir plot. The amount of NO adsorption on PAN is about 2 times greater than NO adsorption on CEL and PIT. The degree of micropore filling for NO is expressed by the ratio of WL(NO) to Wo(N,)*; it is in the range 0.09-0.28. We cannot apply the usual DR plot to the adsorption data on gases above Tc, because it is impossible to determine the relative pressure. In earlier paper^,^^,^^ we estimated the saturated vapor pressure of NO, Po(NO),in the micropores and the characteristic adsorption energy for NO, Eo(NO), by use of a modified DR plot expressed by (In WL(NO)/W)l/z= (RT/PEo)[ln Po(NO) - In PI
(4)
qst,s=lje(NO) was determined from the PEo(NO) and H , value (13.8 kJ/mo1)28for bulk liquid NO a t 121 K by use of eq 3. Table I1 summarizes the data on NO adsorption. (31) Kaneko, K.;-Ozeki, S.; Inouye, K. Colloid Polym. Sci. 1987,265, 1018.
Langmuir, Vol. 5, No. 4 , 1989 963
Enhancement of NO Dimerization r
LOO
I
1
7
It
-0.2
z oL
-
d a,
e 8
73
3
I
0
c
I
I
PAN
I
"
-0.6I
0
0.6 0.8 1.0 P/ Po Figure 5. Adsorption isotherms of N2 at 77 K on ACF samples before and after preadsorption on NO at 303 K: 0 , before NO preadsorption;m, after NO preadsorption. 0.2
.O
04
Table 111. Apparent Density of Adsorbed NO and Dissociation Enthalpy of NO Dimer CEL PAN PIT lit values PNO 0.16 0.58 0.41 solid 1.0430 (g/mL) liquid: 1.27% 24 25 22 9.4 (100-140 K)" H d (kJ/mol) 6.4 (100-280 K)33 11.6 (122-273 K)% 10.2 (77-150 K)35
- 0.2 -0.4
1
275
325
42:
375
TEMPERATURE,
K
Figure 6. Temperature dependences of the magnetic susceptibilities of the ACF samples (open symbols) and NO-adsorbed ACF samples (solid symbols).
TPD Spectra and Apparent Density. Figure 4 shows T P D spectra of the NO-adsorbed CEL. Adsorbed NO starts to desorb around 325 K; there is a single NO peak a t 445 K. NOz is not detected. Nz and COz are evolved above 375 K due to the reaction expressed by eq 5;32the amounts of Nz and C02 evolved increase with temperature up to 550 K.
C
+ 2 N 0 = N2 + C02
(5)
The adsorption isotherms of Nz at 77 K on CEL and PAN before and after adsorption of NO at 303 K are shown in Figure 5. On preadsorbtion of NO at 303 K by the amount indicated by the line with arrows in Figure 5, the Nz adsorption remarkably decreases to give isotherms lying much lower. The difference between amounts of N2 adsorption before and after the NO preadsorption does not agree with the amount of preadsorbed NO but is much greater than that. The greater such a difference, the more NO molecules are preadsorbed near the entrance of micropores and block the micropores to decrease the amount of Nz adsorption. The apparent density pNO of the adsorbed NO layers in the micropores was obtained by eq 1. The pNO values are compared with those of solid and liquid NO33 in Table 111. The pNO probably underestimates the true density of the NO filled in micropores because of the blocking of the entrance of micropores. The percentages of the pNO against the solid density are in the range 13-72%, so the adsorbed NO seems to build a rather compact layer in the micropores. (32) Shah, M. S. J. Chem. SOC.1929, 2661, 2676. (33) Osa, T.; Sato, S.;Koda, S.; Yoshida, T.; Takahashi,H.; Tominaga, H. Chemistry of NO,; Kyoritu: Tokyo, 1978; p 48.
-0.8 275
325
375
42 5
TEMPERATURE, K
Figure 7. Temperaturedependence of the magnetic susceptibility x of gaseous NO, CEL, and NO-adsorbed CEL and the temperature dependence of the calculated x value from the values of both gaseous NO of the same amount as the adsorbed NO and CEL.
Temperature Dependence of Magnetic Susceptibility. Figure 6 shows the temperature dependence of magnetic susceptibility of each ACF with and without the adsorbed NO. All ACF samples show the diamagnetism that arises from the two-dimensional electron
964 Langmuir, Vol. 5, No. 4 , 1989 and their susceptibilities are independent of temperature. On the other hand, the magnetic susceptibilities of the ACF samples with the adsorbed NO at 298 K are negative, and they increase up to 380 K, accompanying a steep decrease with temperature above 380 K. The features in three x-T relationships are similar, but the absolute values are different from each other. Figure 7 shows the x-T relationships of the gaseous No and CEL with and without the adsorbed NO. The x of the gaseous NO is positive and large, and it decreases with temperature. x vs 1 / T is briefly described by the Curie law. The x-T relationship for the NO gas almost agrees with the data in the literat ~ r e .The ~ ~broken line of Figure 7 shows the x-T relationship calculated from the x values of both the gaseous NO of the same amount as the adsorbed NO and 100 mg of CEL. The calculated x is positive and greater than the absolute x value of the NO-adsorbed CEL. Also, other NO-adsorbed ACF samples have a similar discrepancy between the observed and calculated x values. The adsorbed NO is, therefore, not paramagnetic but diamagnetic, indicating that the adsorbed NO molecules are dimerized in the micropores even around room temperature; the micropore fields must assist the dimerization of NO molecules. The NO molecule can be more sensitively affected by the micropore field than the Nz molecule. NO Dimer in Micropores. The fraction of the dimers can be determined from the amount of NO adsorption, observed x values of the NO-adsorbed ACF, and the x values of gaseous and dimerized NO. Here, we used the x value of the NO dimer in the condensed phase a t low t e m p e r a t ~ r e .The ~ fraction of the dimer thus determined is 0.99 at 298 K, gradually decreasing to 0.95-0.96 at 373 K. As NO is a supercritical gas, NO cannot be filled in the micropores in the form of the monomer. The NO dimer should be a vapor at room temperature, although there are not data on the critical point of the NO dimer. The Po(NO)values in Table I1 are presumed to arise from the dimer in the micropores. Hence we can assume that the amount of adsorption is proportional to the amount of the dimerized NO molecules and that NO molecules in the micropores desorb after dissociation of the dimer. The TPD spectrum of NO is deeply associated with the changes in the x-T relationship of the NO-adsorbed ACF. The adsorbed NO starts to desorb above 325 K, and diamagnetic Nz and C 0 2 are produced above 375 K as shown by the T P D results. The increase in x of the NO-adsorbed ACF below 375 K is caused by the increase in the paramagnetic NO monomers dissociated from the dimers. The reduction of x above 375 K is due to the increase in the diamagnetic Nz and COz gases, as shown by the T P D results; we discuss the data on the temperature dependence of the x below 380 K. We may assume the following dissociation equilibrium for the NO dimer below 380 K: (NO):! = 2 N 0 (6) We have an equilibrium constant K
K = [NOJ2/[(NO)21 (7) The K values in mole fractions as a function of the measuring temperature for the x-T relations are obtained. Figure 8 shows the van’t Hoff plots; the slopes of these good straight lines give us the dissociation enthalpy A H d . The A H d values obtained here are compared with those of the NO dimer at low temperature in the literature.lZ*% (34) Matsubara, K.; Kawamura, K.; Tsuzuku, T. Jpn. J. Appl. Phys. 1986,25, 1016. (35) Lips, E. Helu. Phys. Acta 1935, 8, 247.
Kaneko et al.
Y
-C
2.5
3.5
3.0
,
I/K Figure 8. van’t Hoff plots of the dissociation constants of the NO dimer on ACF samples. 103/ T
Figure 9. Schematic model of the NO dimer in the slit-shaped micropores. The AH,values for the NO dimer in the condensed phase are almost equal to that of the gas phase according to the literature. However, the A H d values for the NO dimer in micropores are much greater than those in the literature. The micropore fields enhance the inter-NO molecular interaction to produce the NO dimer and stabilize them by 10 kJ/mol. The fact that the AH,values of 22-24 kJ/mol are near to the difference (13-19 kJ/mol) between q,t,o=l/,(NO)and qst,,kl &NZ)suggests the importance of the dimerization in N d micropore filling. As pNO values of PAN and PIT are near or over one-half of the solid NO density, we assume that the NO dimers orientate to the slit-shaped carbon wall, as illustrated in Figure 9. The dimer in the condensed phase a t low temperature takes a symmetric cis form. The cis-NO dimer has a molecular area of 0.22 nm2 and a thickness of 0.3 nm,14 so a triple layer can form between the slits. Almost all dimers are directly affected by the adsorption field of the walls on both sides. Ohlsen and Laane39found not only the symmetric dimer (O=N-N=O) but also the asymmetric form (O=NO=N) in the gas phase by Raman and IR spectroscopies. Kasal and Gauraa reported the dimer in triplet on Na-A zeolites in their ESR study. There is a possibility that an (36) (37) (38) (39) (40)
Scott, R. L. Mol. Phys. 1966, 11, 399. Guggenheim, E. A. Mol. Phys. 1966, I O , 401. Dinerman, C. E.; Ewing, G. E. J. Chem. Soc. 1970,53, 626. Ohlsen, J. R.; Laane, J. J . Am. Chem. Soc. 1978,100,6948. Kasal, P. H.; Gaurs, R. M. J. Phys. Chem. 1982,86, 4257.
Langmuir 1989, 5, 965-972 unusual diamagnetic dimer form can be produced under the micropore fields of the ACF. The investigation on the enhancement of the NO dimerization by micropores that can adsorb much more NO molecules is in progress. An FT-IR study will also be necessary for clarifying the molecular structure of the NO dimer in the micropores.
965
Acknowledgment. This work was supported by both a Grant in Aid for Fundamental Scientific Research from the Japanese Government and a grant from Osaka Gas Corp. Registry No. NO, 10102-43-9.
Formation of Stable Alkyl and Carboxylate Intermediates in the Reactions of Aldehydes on the ZnO(0001) Surface J. M. Vohs and M. A. Barteau* Center for Catalytic Science and Technology, Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716 Received November 21, 1988. I n Final Form: March 17, 1989 Stable alkyl and carboxylate intermediates were formed by oxidation of higher aldehydes on the zinc polar surface of zinc oxide at low temperatures. Adsorbed acetaldehyde and propionaldehyde underwent nucleophilic attack by lattice oxygen to form dioxyalkylidene species (RCH2CH02). Decomposition proceeded via two separate pathways: hydride elimination to form the corresponding surface carboxylate and alkyl elimination to form surface formate. The alkyl elimination pathway also resulted in the formation of stable surface alkyl species. The selectivity of these two competing elimination reactions was found to be a strong function of the surface temperature, with low temperatures favoring alkyl elimination. TPD and XPS provided the clearest evidence for alkyl elimination and the corresponding formation of surface formates; UPS and XPS provided spectroscopic evidence for the existence of stable surface alkyls. The observation of alkyls as stable surface intermediates in the decomposition of aldehydes on zinc oxide has implications for a variety of catalytic processes, including the synthesis of higher alcohols and oxygenates on oxide surfaces.
Introduction Lattice oxygen ions a t the (0001)-Znpolar surface of zinc oxide react as nucleophiles with adsorbed carbonyl compounds.l+ For simple reactants such as formaldehyde and methyl formate, these nucleophilic reactions are rather straightforward: nucleophilic attack by lattice oxygen followed by elimination results in the formation of surface formate species.lp2 By analogy with these reactions, one might expect higher aldehydes (RCHO) to react similarly on this surface to produce the corresponding higher carboxylates. However, as we have recently reported: reactions of higher aldehydes on the (0001)-Zn polar surface result in the formation of not only the corresponding higher carboxylate (RCOO) species but surface formate (HCOO) species as well. This result suggests that nucleophilic attack by lattice oxygen a t the carbonyl carbon of aldehydes can be followed by either hydride elimination to produce the higher carboxylates or alkyl elimination to form formates. Alkyl elimination in the course of aldehyde oxidation is quite surprising: this reaction would not be expected on the basis of analogous chemistry in basic solution, where hydride elimination from aliphatic aldehydes is always preferred over alkyl eliminati~n.~ It appears that this alkyl elimination pathway is a novel property of the surface chemistry of metal oxides,6 perhaps reflecting the dual acid-base functions provided by the cation-anion site pairs of these materials. Several important questions regarding alkyl and hydride elimination pathways in the reaction of aldehydes on zinc oxide remain unanswered. The factors which control the selectivity of these competing pathways are not understood. The fate of the eliminated alkyl groups has also not
* Author to whom correspondence
should be addressed.
0743-7463 I89 12405-0965$01.50,IO ,
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been elucidated. It was not determined previously whether the eliminated alkyl groups form gaseous products, adsorb as stable complexes, or rapidly decompose to deposit surface carbon. If alkyls are stable intermediates in the decomposition of aldehydes, the reactions of alkyls with C1 species such as CO, COz, or surface formates may provide a route for the synthesis of higher oxygenates on metal oxides. Therefore, in this investigation we have extended our previous studies6 of the reactivity of higher aldehydes on the (0001)-Zn polar surface of zinc oxide in an effort to examine the mechanism of alkyl elimination from aldehydes and to determine the identity of the surface intermediates formed via this reaction. As with other aliphatic oxygenates,1v2,6the (0001)-0 polar surface is inactive for the decomposition of aldehydes.6
Experimental Section All experiments were conducted in a stainless steel ultrahigh-vacuum chamber equipped with a quadrupole mass spectrometer, a double-pass cylindrical mirror analyzer with integral electron gun, and X-ray and ultraviolet photon sources, allowing the collection of TPD, AES, XPS, and UPS spectra. The chamber has previously been described in detail.' The zinc oxide single crystal was obtained from Litton Airtron and was approximately 6 mm X 6 mm X 2 mm. The crystal was aligned to within h O . 5 O of the normal to the c-axis by using the Laue method, and the oxygen and zinc polar surfaces were identified by etching the (1)Vohs, J. M.;Barteau, M. A. Surf. Sci. 1986, 176, 91. (2) Vohs, J. M.; Barteau, M. A. Surf. Sci. 1988, 197, 109. (3) Akhter, S.;Lui, K.; Kung, H. H. J . Phys. Chem. 1984, 89, 1958. (4) Cheng, W.H.; Akhter, S.; Kung, H. H. J . Catal. 1983,82, 341. (5)Akhter, S.; Cheng, W. H.; Lui, K.; Kung, H. H. J. Catal. 1984,85,
437. (6) Vohs, J. M.;Barteau, M. A. J . Catal. 1988, 113, 497. (7) Schubert, W.M.; Kinter, R. R. In The Chemistry of the Carbonyl Group; Patai, S., Ed.; Wiley Interscience: London, 1966; p 695.
0 1989 American Chemical Society
