Langmuir 1 9 8 5 , 1 , 2 4 3 - 2 4 5 Table 111. Electrostatic Field Strength and E d Term E d X 10l2, evacuation Fl, F2, statvoltcm-i statvoltcm-i erg molecule-i method temp eq 4-7
slope
600 O C 100, 500
600OC 100 "C
O 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 sof 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-supported Rh 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.
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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
DeCanio et al.
244 Langmuir, Vol. 1, No. 2, 1985
*t
A
dn L
ti
0 0
5 ,
0
O0
0 0J " : I 2
4I
I
6 I
8
Time, h
Figure 2. (A) Population of benzene ( 0 )and cyclohexar (0)
molecules on the catalyst surface (as determined by integration of the NMR signals of Figure 1) during the hydrogenation of benzene; (B) numbers of benzene ( 0 ) and cyclohexane (0) molecules in the gas phase, as determined by gas chromatography; (C) total numbers of benzene (0)and cyclohexane (0) molecules obtained by summation of the data of A and B above.
IO
0 ppm
Figure 1. 'H spectra obtained during the hydrogenation of benzene to cyclohexane catalyzed by Rh/A1203at 25 "C. The numbers to the left of the spectra indicate the times (in minutes) after initiation of the reaction. The first two spectra are multiplied by two and four, as indicated. 120 "C and calcined in air at 500 "C, giving a sample containing The rhodium loading (0.7 wt Rh in the +3 oxidation %) was kept low to avoid a broad distribution of metal particle sizes and the resulting loss of resolution in the NMR spectrum. Prior to the catalytic hydrogenation reaction in a batch reactor (the NMR cell), the catalyst sample (0.10-0.15 g) was gently reduced by exposure to flowing hydrogen at 25 "C for 2 h. This treatment was intended, for the reasons cited above, to give very small aggregates of Rh-even with less than complete reduction. The catalyst was outgassed at 25 OC for 1 h before the start of the catalytic reaction to remove hydrogen that might have spilled over onto the support. A specified amount of a hydrogen-benzene mixture was then introduced into the reactor. Proton NMR spectra were recorded with the catalyst in the working state; the spectrometer has been described previously.1° The spectra were sums of 250 transient accumulations requiring approximately 10 min, with data acquisition set so that no distortion due to differential spin-lattice relaxation of the adsorbed species would affect the spectra." Examination of the sample using the eight-pulse technique of Rhim et al.l2,l3demonstrated ~~
(6) Rice, C. A.; Worley, S. D.; Curtis, C. W.; Guin, J. A.; Tarrer, A. R. J. Chem. Phys. 1981, 74,6487. (7) Alumina-supported rhodium and platinum show a variety of oxidation state8, see, for example: Huizinga, T. Ph.D. Thesis, University of Eindhoven, Netherlands, 1983. (8) DeCanio, S. J.; Apple, T. M.; Dybowski, C. J . Phys. Chem. 1983, R7 - . , 194 - - -. (9) Huizinga, T.; Prins, R. J . Phys. Chem. 1981,85, 2156. (10) Apple, T. M.; Gajardo, P.; Dybowski, C. J. Catal. 1981,68, 103. (11) For S O z and A&03supports used in this study, TIvalues of adsorbed benzene and cyclohexane are between 1 and 3 s. For the supported metal samples, the T,'sare somewhat less, but still greater than 0.5 8.
that the resonances of the benzene and cyclohexane were not broadened by the dipole-dipole interaction. We infer, therefore, that these species were undergoing rapid translational and rotational motions and that our observations are of the physisorbed species. In the experiments reported here, the NMR spectra are the Fourier transforms of the responses to a 90' pulse. Samples (25 pL) of the gas phase in contact with the catalyst were drawn periodically during the experiment for gas chromatographic analysis. The HewletbPackard 5150 gas chromatograph was equipped with a column of 5.16% DEGS and 1.7% Bentone 343 on Chromasorb W connected in series with a column containing 5 % Apiezon L on Chromasorb W. The NMR spectrometer was calibrated quantitatively with a known amount of water in an N M R tube, the measurement being made under conditions identical with those for which the NMR spectra of the surface were measured. The spectra characteristic of adsorbed benzene and of adsorbed cyclohexane were determined in separate experiments. The gas chromatograph was calibrated with known mixtures of cyclohexane and benzene.
Results and Discussion
NMR spectra obtained in an experiment with 0.115 g of the Rh/A1203 catalyst initially in the presence of benzene (40 torr) and H2(360 torr) at 25 "C are shown in Figure 1. The signals for benzene and cyclohexane are clearly distinguished, the resonance of the protons on the benzene being in the downfield region and that of the protons on cyclohexane in the upfield region. The resonances are broad by the standards of liquid-state NMR spectroscopy (fwhh 1.5 ppm), but they are separated well enough to be integrated for a quantitative determination of the number of molecules of each kind on the catalyst surface. In a separate experiment, the catalyst was transferred to another part of the reactor, and the
-
(12) Rhim, W.-K.; Elleman, D. D.; Vaughan, R. W. J. Chem. Phys. 1973,59,3740; J. Chem. Phys. 1973,58,1772; J. Chem. Phys. 1974,60, 4595. (13) Mehring, M. 'Principles of High Resolution NMR in Solids"; Springer: Berlin, 1982.
Langmuir 1985,1, 245-250
NMR signal was no longer detected; this result confirms that the NMR spectra represent adsorbed molecules and not those in the gas phase. Slight shifts of the resonances relative to an external Me4Si standard (7.0 ppm for the adsorbed benzene, compared with 7.2 ppm for neat benzene, and 1.0 ppm for cyclohexane, compared with 1.4 ppm for neat cyclohexane) probably indicate susceptibility differences. The surface concentrations of benzene and cyclohexane determined by the NMR data are plotted as a function of time of reaction in Figure 2A. The gas-phase concentrations determined by gas chromatography in the same experiment are shown in Figure 2B. The sums give the total number of benzene and cyclohexane molecules in the systems, as shown in Figure 2C. The reaction is zero order in benzene partial pressure; the slopes of the lines for benzene and cyclohexane in Figure 2C both give a rate of 2.7 (fO.l) X 10" molecules/min, which corresponds to a rate of 9.6 X lo4 molecules of benzene converted per total rhodium atom per second. The mass balance does not close exactly, as shown by the early data for total detected benzene and cyclohexane (Figure 2C). At times
