Langmuir 1985,1,239-243
239
reducibility of the films was detected. Amorphous, hydrated films of similar composition have been reported on polycrystalline Fe and stainless steels.lB Annealing at 700-900 "C led to Cr oxidation with concomitant reduction of Fe to the metallic state. LEED patterns showed that the Cr-enriched films contained principally crystalline Cr203(001),oriented on the alloy substrate with its interatomic 0-0vectors parallel to the interatomic vector of the alloy (Figure 4A). Annealed films formed in borate contained only this chromium oxide. Annealed films formed in acidic media contained also thin regions of a square-Cr0 structure (Figure 4B),which gave rise to integral index and a square mesh of LEED beams, as well as high Cr:O ratios (0.6:l to 1.5:l) in the Auger spectra. Since stainless steels are produced at high temperatures, these annealed chromium oxide film are probably representative of the chemical composition of the passive films formed in practical situations. Whether ordered oxides of this type are formed also on polycrystalline stainless steel is not known at the present time. The annealed oxide surfaces underwent further film formation upon immersion or electrolysis in electrolyte, to give a superficial amorp-
hous, hydrated iron oxide layer. The amount of additional film formation was substantial for borate-formed annealed oxide in borate solution at positive potentials but much less at open-circuit potential or for HC104-formedoxides reimmersed into HCIOl or HC1 solutions. The voltammograms in 0.1 M KC1 and 0.1 M HC1 were similar, indicating that pitting and dissolution in chloride media are pH independent, as reported by others.I7 The role of C1adsorption in the pitting process is not well understood, and many theories have been p r o p ~ s e d . ' ~ J ~Our J ~ results show that although significant amounts of Cl(2-8%) were incorporated into the film formed on a clean Fe-Cr-Ni(111)surface in HC1, only traces of C1 (about 1% ) were detected by Auger spectroscopy when breakdown of an annealed oxide film occurred in HC1. Evidently C1 adsorption occurs only at localized sites and at total concentrations too small to be detected by Auger spectroscopy.
(16)(a) Pou, T.E.; Murphy, 0. J.; Young, V.; Bockris, J. OM. J. Electrochem. SOC.1984,131, 1243. (b) O'Grady, W.;Bockris, J. OM. Surf. Sci. 1973,38,249.(c) Okamoto, G.; Shibata, T. Corros. Sci. 1979, 10, 371. (d) Okamoto, G.;C o m e . Sci. 1973, 13, 471. (e) Saito, H.; Shibata, T.; Okamoto, G. Corros. Sci. 1970,19,693. (0 Murphy, 0.J.; Bockris, J. O'M.; Pou, T. E. JElectrochem. SOC.1982,129,2150.
94944-54-4.
Acknowledgment. We are grateful to Dr. Jesse B. Lumsden for helpful discussions. Acknowledgement is made to the National Science Foundation (Grants DMR 8213521 and 8205799) for support of this research. Registry No. KH2B03,15119-96-7;H2S04,7664-93-9; HC104, 7601-90-3; KC1, 7447-40-7; HC1, 7647-01-0; Fe-Cr-Ni alloy,
(17)Szklarska-Smialowska, Z. Corrosion (Houston) 1971,27,223. (18)Janik-Czachor, M.J. Electrochem. SOC.1981,128,513C.
Heat of Immersion of ZnO in Organic Liquids. 1. Effect of Surface Hydroxyls on the Electrostatic Field Strength Tetsuo Morimoto* and Yasuharu Suda Department of Chemistry, Faculty of Science, Okayama University, Okayama 700, Japan Received September 28, 1984. In Final Form: December 13, 1984 The influence of surface hydroxyls on the electrostatic field strength of ZnO was investigated by measuring the heat of immersion into linear aliphatic organic liquids having different dipole moments. The amount of surface hydroxyls was controlled by evacuating the fully hydroxylated surface at various temperatures from room temperature to 600 "C. The results showed that the surface hydroxyls strongly affect the field strength of ZnO that is, on the hydroxylated surface the field strength is negligibly small, 6.03 X lo2statvolt cm-', compared to that on the surface dehydroxylated at 600 "C, 3.17 X lo5 statvolt cm-'. Further, there was a linear relationship between the field strength and the concentration of surface hydroxyls. The effect of the occupied area of the adsorbed molecules on the calculation of the field strength was also tested by introducing the experimental molecular areas obtained from the adsorption isotherm, and it was found that the use of the real areas brings about reasonable results.
Introduction I t has been clarified that hydroxyl groups are present on the surface of metal oxides in the atmosphere and have large influences on the surface properties of the solid.'-' Very recently, quantitative investigations were carried out on the effect of surface hydroxyls on the adsorbability of organic molecule^.^^^ The results obtained indicate that (1)Morimoto, T.; Nagao, M.; Tokuda, F. Bull. Chem. SOC.Jpn. 1968, 41,1533. (2)Morimoto, T.;Nagao, M.; Tokuda, F. J. Phys. Chem. 1969,73,243. (3)Morimoto, T.;Nagao, M. J. Phys. Chem. 1974,78,1116. (4)Morimoto, T.;Morishige, K. J. Phys. Chem. 1975,79, 1573. (5)Morimoto, T.;Yanai, H.; Nagao, M. J.Phys. Chem. 1976,80,471. (6)Nagao, M.; Yunoki,K.; Muraishi, H.; Morimoto, T. J. Phys. Chem. 1978,82,1032. (7)Nagao, M.; Morimoto, T. J.Phys. Chem. 1980,84,2054.
0743-7463/85/2401-0239$01.50 f 0
the number of surface hydroxyls has a decisive effect on the surface properties of real solids. Measurement of the immersional heat of a solid into organic liquids having different dipole moments permits the evaluation of the electrostatic field strength of the surfaceP15 So far none of the data reported on the field (8) Nagao, M.; Matsuoka, K.; Hirai, H.; Morimoto, T. J. Phys. Chem. 1982,86,4188. (9)Healey, F. H.; Chessick, J. J.; Zettlemoyer, A. C.;Young, G.J. J. Phys. Chem. 1954,58,887. (10)Chessick, J. J.; Zettlemoyer, A. C.; Healey, F. H.; Young, G. J. Can. J . Chem. 1955,33,251. (11)Zettlemoyer, A. C.; Chessick, J. J.; Hollabaugh, C.M. J. Phys. Chem. 1958,62,489. (12)Romo, L.A. J . Colloid Sci. 1961,16,139. (13)Dear, D.J. A.; Eley, D. D.; Johnson, B. C. Trans. Faraday SOC. 1963,59,713.
1985 American Chemical Society
240 Langmuir, Vol. I , No. 2, 1985
Morimoto and Suda
' I
N
E
m
1 0
1 100
200
400 Temperature, "C 300
500
600
Figure 1. Dependence of surface hydroxyl content of ZnO on degassing temperature. strength have been discussed on the basis of the amount of surface hydroxyls. Moreover, during the course of calculation of the field strength, the occupied areas of various organic molecules were assumed to be equal to each other. The purpose of the present work is to investigate the effect of surface hydroxyls on the surface field strength of ZnO by measuring the heat of immersion of ZnO into various organic liquids and by taking account of the molecular areas obtained by the adsorption measurement.
Experimental Section ZnO used in this experiment, supplied from Sakai Chemical Co., was prepared by burning zinc metal of 99.99% purity in the air. The sample is composed of fiie crystals of wurtzite type, on which the (1010) plane is well de~eloped.~ The amount of surface hydroxyls of the sample was controlled in the following way. First, the sample was treated at 600 "C for 4 h in a vacuum of 1.33 X N m-2,in order to remove all the physisorbed species and most chemisorbed H20and C02,both of which had been adsorbed from the atmosphere. Next, the sample was exposed to saturated water vapor for 5 h to ensure maximum surface hydration. Finally, the sample was evacuated at various temperatures from room temperature to 600 "C for 4 h, which left the surfaces with different amounts of chemisorbed water, i.e., surface hydroxyls. The content of surface hydroxyls remaining on the surface was measured by the successive ignition loss method.I6 The surface area of the sample pretreated at 600 "C in vacuo was measured by applying the BET method to the N2adsorption data and found to be 3.30 m2g-l, being unchanged by further treatment at temperatures lower than 600 "C. Immersional liquids used were n-C7HI6,1-C4HgOH,1-C4HgCl, and CHBN02.The liquids were dried by the use of a suitable desiccant, distilled, and stored in the presence of activated molecular sieve 4X, only CH3N02being stored in the same way without distillation. The heat of immersion was measured at 28 f 0.1 "C in an adiabatic calorimeter17equipped with a thermistor of 10-kQresistance as a temperature-sensing element. Results and Discussion Effect of Surface Hydroxyls on Surface Field Strength of ZnO. The content of surface hydroxyls on ZnO measured by the successive ignition loss method is shown in Figure 1as a function of the degassing temperature. From the crystallographic data, the number of Zn and of 0 atoms can be computed to be 5.935 nm-2 on the (14) Cochrane, H.; Rudham, R.Tram. Faraday SOC.1965,61,2246. (15) Lavelle, J. A,; Zettlemoyer, A. C. J. Phys. Chem. 1967, 71, 414. (16) Morimoto, T.; Shiomi, K.; Tanaka, H. Bull. Chem. SOC.Jpn. 1964, 37, 392. (17) Nagao, M.; Morimoto, T.J. Phys. Chem. 1969, 73, 3809.
1000
1
I 100
0
200
300
400
500
600
Temperature, "C Figure 2. Dependence of heat of immersion of ZnO in organic liquids on degassing temperature. (0)n-C7H16,(m) 1-C4HgCl,(0) l-CIHgOH, (e) CH3N02.
r\E10O21000 l
0
2
4
6
8
1
0
Surface hydroxyls, nm-2
Figure 3. Relation between net heat of adsorption of organic n-C7H16,(w) molecules and surface hydroxyl content of ZnO. (0) 1-C4HgCl, (0) l-CdHgOH, ( 0 )CH3N02. (1010) plane of ZnO, and therefore the surface hydroxylation of this surface will form 11.87 OH's/nm2, which is very similar to the experimental value in Figure 1. The data in Figure 1 also show that the surface hydroxyl content decreases with rising temperature, especially sharply around 300 "C. The heat of immersion of ZnO in organic liquids is plotted against the degassing temperature of the fully hydroxylated ZnO sample, as illustrated in Figure 2. The data in Figure 2 reveal that the heat value remains almost constant for each liquid in the temperature range up to 100 "C, where the surface is fully covered with hydroxyls, increases sharply by the treatment between 200 and 400 "C, correspinding to a sharp decrease in the amount of surface hydroxyls (Figure l),and after that increases very slowly by the treatment up to 600 "C. The heat values on a surface treated a t a given temperature depend on the nature of organic molecules used; the larger the dipole moment of an organic molecule, the larger the immersional heat evolved. The net heat of adsorption can be obtained by subtracting the surface enthalpy of each liquid from the measured heat value and plotted as a function of the amount of surface hydroxyls as shown in Figure 3. It is clear from Figure 3 that the heat of adsorption of 1C4HgOHand CH3N02decreases linearly with increasing
Heat of Immersion of ZnO in Organic Liquids
Langmuir, Vol. 1, No. 2, 1985 241
Table I. Number of Adsorbed Molecules, Molecules nm-2 n-CIHl6 1-CdHgOH 1-CIHBCl CHBNOZ
25 OC
100 OC
200 "C
1.32 2.80 2.08 2.71
1.33 2.83 2.10 2.75
1.37 2.89 2.18 2.88
amount of surface hydroxyls and more sharply at the higher concentration range of surface hydroxyls, while that of n-C,Hl, and 1-C4HgC1decreases linearly over the whole range of coverage of surface hydroxyls. Recent investigation has revealed that 1-C4HgOHand CH3NO2are dissociatively chemisorbed on the dehydroxylated surface of ZnO, while n-C,Hle and 1-C4HgC1are only physisorbed on it? The difference in the shape of adsorption heat curves in Figure 3 may depend on that of the adsorption mechanisms. Usually, we plot the net heat of adsorption against the dipole moment of organic molecules used as immersional liquids for the calculation of the electrostatic field strength of the solid surface. In this calculation, it is necessary to evaluate the area of adsorbed molecules. Because of the lack of available data, most authors have assumed that molecules of n-CCH14, 1-C4HgOH,1-C4HgC1,and CH3N02 occupy the same area, e.g., 0.22 nm2,12when adsorbed. However, recent experiments on adsorption in the present systems has given precise information about the occupied areas of these molecules. Table I shows the data for the number of organic molecules adsorbed on ZnO, as calculated from the monolayer capacity in the first adsorption isotherm of each molecule.* These data demonstrate that the number of adsorbed molecules increases with increasing number of dehydroxylated sites. Furthermore, the molecular area varies widely depending upon the nature of adsorbates: the occupied area is greatest for n-C7H16,while it is rather small for 1-C4HgOHand CH3N02,which give rise to dissociative adsorption on the dehydroxylated surface of ZnO. However, the area occupied by l-C4HgOHand CH3N02on a drastically dehydroxylated sample, e.g., on the 600 "C treated one, is still larger than the mean area of a neighboring pair of Zn and 0 atoms, 115.935 nm2, on which dissociative adsorption occurs. Taking into account the number of adsorbed molecules in Table I, we can calculate the mean net heat of adsorption per molecule and plot it as a function of the dipole moment of organic molecules as shown in Figure 4. It is not surprising to see that the heat values for four organic molecules do not fall on a straight line, because 1-C4HgOH and CH3N02are adsorbed dissociatively. However, it is rather surprising that the net heats of adsorption of these two molecules are considerably smaller than those expected from the straight line connecting the heat values of physisorbed molecules, n-CyH16 and 1-C4H&1. It is clear from Figure 4 that the slope of the straight line connecting the heat values for n-C7& and 1-C4HgC1 increases with rising evacuation temperature of the hydroxylated sample. From this slope, we can calculate the electrostatic field strength of the surface according to the traditional method and represent it as a function of the amount of surface hydroxyls as given in Figure 5. For comparison, another calculation was carried out by assuming the molecular area of both physisorbed molecules to be the same, e.g., 0.22 nmz, as done by Romo,12and the calculated values are also plotted in Figure 5. Here, we can see the indisputable result that the field strength of the surface of ZnO depends strongly upon the number of surface hydroxyls, decreasing linearly with increasing surface concentration, though the assumption of equal
300 O C 1.45 3.03 2.33 3.13
500 OC 1.61 3.30 2.62 4.00
400 "C 1.59 3.27 2.58 3.83
600 OC 1.63 3.34 2.66 4.21
U
L
P,
RH
2
ROH RCI
RNO2
I I,
oL 0
1
2
I
3
Dipole moment, Debye
Figure 4. Relation between net heat of adsorption of organic molecules on ZnO and dipole moment. Degassing temperature: ( 0 )room temperature, (0)100, (m) 200, (0) 300,(A) 400, (A) 500, ( 0 )600
"C.
0
2 b 6 8 1 0 Surface hydroxyls, n r f 2
Figure 5. Relation between electrostatic field strength of ZnO and surface hydroxyl content, the former being calculated by using experimental molecular areas (0) and by assuming the same area, 0.22 nm2 ( 0 ) .
molecular area brings about a large increment in the field strength. Many authors have dealt with the field strength of solids having surface hydroxyls, but no one has reported their direct influence on it. Here, we have established that surface hydroxyls have a surprisingly large effect on the field strength it amounts to 3.17 X los statvolt cm-l on the surface dehydroxylated at 600 "C, but it is very small on the fully hydroxylated surface. The total interaction energy of a solid surface with a molecule can be expressed by the following relationship: hi - hL = Ed + E, + E , (1) where hI is the heat of immersion, hL the surface enthalpy of the immersional liquid, and therefore hI - hL the net heat of adsorption, which is calculated in Figure 3. Ed is
242 Langmuir, Vol. 1,No. 2, 1985
Morimoto and Suda
Table 11. Calculation of Interaction Energies (xlOlJ,erg molecule-')
n-C7H16 0 1-C4HgC1 0.08
11.48 0 11.48 2.53
1.10 X 8.12 X
0 5.81
6.41 4.75
19.72 20.02
600 "C
500 "C n-C,H,B 1-CIHgCl
1.21 0.90
21.65 23.31
0 6.42
7.83 5.80
19.27 21.31
the energy term due to the dispersion force, E,, is that due to the interaction between the electrostatic field of solid surface and the dipole moment of adsorbed molecule, and E , is that due to the contribution of induced dipole. Furthermore, E,, and E , can be expressed as follows:
(C 1 (d) Figure 6. Adsorption models of 1-C4H9C1 and n-C7H16on deh-
E , = -pF
(2)
ydroxylated (a, c ) and hydroxylated (b, d) ZnO (1010) planes.
E, = - 7 4 3
(3)
1,the net heat of adsorption per molecule of 1-C4HgC1on a ZnO surface treated at a given temperature is expressed by eq 4. Here, (hr - hL)CIHgCI is the net heat of adsorption
Here, F is the electrostatic field of the surface as stated above, and p and a are the dipole moment and the molecular polarizability. Thus, we can estimate each term from these three relationships when the heat of immersion value is available. The linear part of the curves in Figure 4 implies the relationship of eq 2. By using the F values in Figure 5, we can compute each energy term as listed in Table 11, except for the molecules of 1-C4HgOHand CH3N02,which give rise to dissociative adsorption. Another calculation, carried out on the basis of the F values obtained from the assumption of equal molecular area (0.22 nm2),shows that the Ed values on n-C7H16decrease with increasing evacuation temperature of ZnO and may even give negative values for the 500 and 600 "C treated samples, though the calculated values are not tabulated here. This fact supports the argument that the experimental cross-sectional area should be used for the calculation of the F value. Analysis of Field Strength on Surfaces Having Two Kinds of Sites. The above discussion on field strength is based on the presumption that a surface treated at a given temperature has a uniform field. However, the real surface of the ZnO samples treated at any temperature is composed of two parts, i.e., dehydroxylated and hydroxylated ones, both of which are considered to have individual field strengths F1 and F2, respectively. Thus, the true heat of adsorption must be the sum of heats expelled from both surfaces when the adsorption occurs. Figure 6 shows how we model the adsorption of 1-C4HgC1 and n-C7H16on the (1010) plane of ZnO. Here, we assume that on the dehydroxylated surface 1-C4HgC1is adsorbed by directing the negative pole, or the chlorine atom, to a surface zinc atom, keeping the hydrocarbon chain perpendicular to the surface. Therefore, the interaction energy of 1-C4HgC1with the bare surface can be written as the expression pFl + l/pl2aCl+ Edl,where p is the dipole moment of the molecule, acl the polarizability of chlorine atom, and Edl the energy term due to the dispersion force. On the hydroxylated surface, we assume that the molecule is adsorbed by directing the chlorine atom to the hydrogen atom of a surface hydroxyl, keeping the hydrocarbon chain parallel to the surface. Thus, the interaction energy of the 1-C4H9Clmolecule with the hydroxylated surface of ZnO + is expressed by the sum of three terms, pF2 + 1/2F22acI Ed2, where Ed2 is the energy term due to the dispersion force. Here, we neglect the polarization and dispersion terms for the hydrocarbon chain. Since the ratio of the dehydroxylated part of ZnO to the hydroxylated part depends on the evacuation temperature as shown in Figure
of a molecule of 1-C4HgC1,and 3c and y are the numbers of adsorbed molecules per nm2 on the dehydroxylated and hydroxylated surfaces, respectively, being obtained by solving the following two equations: x+y=v, ax by = 1
+
(5)
(6) where V, is the monolayer capacity (molecules nm-2) and a and b are the areas occupied by an adsorbed molecule of 1-C4HgC1on the dehydroxylated and hydroxylated surfaces, respectively. The values of a and b can be calculated from the adsorption models of l-C4HgCltogether with the molecular structure and were found to be 0.337 and 0.500 nm2, respectively. These values can also be estimated from the experimental V , values obtained on the 600 "C treated and the fully hydroxylated surfaces and were found to be 0.376 and 0.481 nm2,respectively, values which are in good agreement with the calculated ones. For the adsorption of n-C7H16,we assume that the molecule having no permanent dipole moment is adsorbed parallel to the surface (both dehydroxylated as well as hydroxylated) and that one molecule interacts with one site, though it can cover two sites (Figure 6). Therefore, the interaction energy of the molecule can be written as 1/2F12aCH2 + Ed3,for the bare surface, and as 1/2F22acH2 + Ed4,for the hydroxylated surface. Here, acH2is the polarizability of CH2group, and Ed3 and Ed4 are the energy terms due to the dispersion force. Taking into account the ratio of the dehydroxylated surface to the hydroxylated one, the net heat of adsorption of a molecule of n-C7H16 can be expressed in the following equation:
(7) where r T is the number of surface hydroxyls per nm2 at a given evacuation temperature T and rRT is the one at room temperature, the latter being the number of surface hydroxyls on the fully hydroxylated surface.
Langmuir 1985,1,243-245 Table 111. Electrostatic Field Strength and E d Term E d X 10l2, evacuation Fl, F2, method temp statvoltcm-i statvoltcm-i erg molecule-i eq 4-7
600 O C 100, 500 O C
slope
600OC 100 "C
3.50 X lo6 3.17 X lo6 3.38 X lo6
6.03
X
lo2
3.68 x 103
2.47 2.63 2.03 1.15
Assuming all four dispersion interaction terms, Edl, E a , and Ed4, are equal (Ed) and substituting the data Of the net heat of adsorption, the n ~ d X r of s surface hydroxyls and adsorbed molecules into the eq 4-79 we can calculate the values Of Fl, F2, and Ed. The results Obtained are listed in 'I1. In 'I1, we add the values F1 and F2 read from the 'lope On the 6oo and loo treated samples in Figure 5, respectively. As is seen from Table 111, each of the F1and F2values obtained by the two different methods is very close-to each other. This will support the use of the experimental Ed39
"'
243
number of adsorbed molecules for the calculation of field strength and the estimation of the field strength by the use of adsorption models in Figure 6. Furthermore, the linear relationship between the field strength and density of surface hydroxyls substantiates that the additivity of the two values of field strength, F1and F,, of the dehydroxylated and hydroxylated parts holds on a composite surface. When the C4H9C1molecule is adsorbed on the hydroxylated surface, two effects other than those considered above should be involved: the repulsive interaction betweenthe positive end of the dipole and the field caused by Zn2+ ion and the effect of polarhation of CH2group due to the field. However, the result that the above calculation leads to a reasonable F value might be due to the fact that the effects of these two terms are canceled. Registry No. ZnO, 1314-13-2; 1-C4HgC1,109-69-3; n-C,H,,, 142-82-5; 1-C4H90H, 71-36-3; CH3N02,75-52-5.
lH NMR Investigation of a Working Catalyst: Benzene Hydrogenation on Alumina-Supported Rhodium S. J. DeCanio,? P. S. Kidin,$ H. C. Foley,* C. Dybowski,*t and B. C. Gates*t Center for Catalytic Science and Technology, Departments of Chemistry and Chemical Engineering, University of Delaware, Newark, Delaware 19716 Received November 28, 1984 Proton NMR spectroscopy was used to monitor surface concentrations of physisorbed benzene and cyclohexane on an alumina-supportedRh catalyst during benzene hydrogenation in a batch reactor at 25 "C and 400 torr. Analyses of the gas by gas chromatography combined with analyses of the adsorbate by NMR indicate that the reaction was zero order in total benzene. The results demonstrate the value of NMR a a method for quantitative determination of adsorbate concentrations on surfaces in the presence of reactive atmospheres.
Introduction Proton NMR spectroscopy, a powerful technique for elucidating structures of organics and organometallics in the liquid state, has often been used to determine kinetics of solution reactions. Molecular dynamics of physisorbed and chemisorbed species can also be probed with NMR, most effectively by measurement of the spin-lattice and spin-spin relaxation times of the nuclei attached to the surface or in the adsorbed molecules.' In this paper, we report the application of proton NMR spectroscopy to the characterization of organics physisorbed on a supported rhodium catalyst; the results provide the first demonstration of the application of this technique for determination of catalytic reaction kinetics by direct measurement of surface concentrations. One of the major difficulties of NMR characterization of catalytic surfaces is associated with the heterogeneity of the surfaces. Because NMR spectroscopic measurementa are extremely sensitive to minor variations in the electronic surroundings, the resonances of species on typical catalyst surfaces are broadened, with a consequent loas of resolution. With careful preparation of a supported metal catalyst to give a narrow distribution of metal t
Department of Chemistry. Department of Chemical Engineering.
0743-7463/85/2401-0243$01.50/0
particle sizes, effects of heterogeneity can be minimized sufficiently to allow distinction of resonances of adsorbed benzene and cyclohexane.2-s Techniques analogous to those of liquid-state NMR spectroscopy can then be used for adsorbed species; here we demonstrate the use of NMR spectroscopy to monitor the kinetics of benzene hydrogenation on alumina-supported rhodium. The results, combined with gas-phase concentrations of reactants and products in contact with the catalyst, provide a more detailed quantitative characterization of the catalysis than is possible by analysis of the gas alone.
Experimental Methods The catalyst was prepared by the ion-exchange technique by mixing Rh(N03)3with yA1203 (Conoco, surface area = 200 m2 g-') in deionized water.6 The material was filtered and dried at (1) (a) Pfeifer, H. NMR: Basic Princ. Prog. 1972, 7, 53. (b) Pfeifer, H.; Meiler, W.; Deininger D. Annu. Rep. NMR Spectrosc. 1981,15,291. (c) Duncan, T. M.; Dybowski, C. Surf. Sci. Rep. 1981, I, 157. (2) DeCanio, S. J.; Foley, H. C.; Dybowski, C.; Gates, B. C. J. Chem. SOC.,Chem. Commun. 1982, 1372. (3) Foley, H. C.; DeCanio, S. J.; Tau, K. D.; Chao, K. J.; Onuferko, J. H.; Dybowski, C.; Gates, B. C. J. Am. Chem. SOC.1983, 105, 3077. (4) DeCanio, S. J. Ph.D. Thais,University of Delaware, Newark, 1983. (5) DeCanio, S. J.; Onuferko, J. H.; Foley, H. C.; Gates, B. C.; Dybowski, C. Surf. Sci. 1984, 136, L67.
0 1985 American Chemical Society
