J. Phys. Chem. 1985, 89, 4440-4443
4440
values for free Tempol at 303 and 294 K, respectively. The equilibrium constant K obtained in the previous study4 was used for this plot at both temperatures. As shown in the previous report, the N O group of Tempol and DTBN are included in the cavity of @-CDX;thus the cavity wall may protect the N O group in a reactive collision with an ascorbate ion. On the contrary, the N O group of Tempo in the inclusion complex is exposed outside the cavity. Thus, the ascorbate ion collides to react with the N O group in the bulk phase outside the cavity of /3-CDX. Kinetic Parameters. The activation enthalpies of the reaction in the mixed solvents are considerably smaller than those in the aqueous buffer solution. It is well-known that some water molecules are bound to the hydrophobic solute molecule in aqueous solution. The substituent groups of the aminoxyl radicals employed in the present study are hydrophobic; thus the NO group is blocked by these bulky substituent groups and also by the water molecules bound to these groups. In the activated complex, some of the water molecules may be replaced by the ascorbate ion. If we assume the same structure for the activated complexes in both systems, in the buffer solution and in the mixed solvent, a decrease in AH* may indicate that the hydrophobic interaction is weakened in the mixed solvents. The decrease in the activation entropy AS* in the mixed solvents can be explained by the increase of the entropy of the starting state, because the hydrophobic interaction is weakened. The AH*for the inclusion complex of DTBN is almost the same as that for free DTBN in the aqueous buffer solution. This may indicate that, after the collision with an ascorbate ion, the included DTBN molecule reacts with the ascorbate ion outside the cavity of P-CDX. The frequency of the reactive collision may be smaller
for the inclusion complex than for the free DTBN. Thus, we propose the elementary steps of the reaction as follows: (1) An ascorbate ion collides with the inclusion complex. (2) Some of the collisions may result in the formation of an activated complex. This process depends on the particular location at the instant of the collision of the guest DTBN molecule which migrates between the inside and outside of the cavity of P-CDX. (3) The activated complex is formed outside the cavity and the structure of this complex is the same as that of free DTBN. The activation enthalpies of the reactions of Tempo and Tempol with ascorbate ion in the aqueous buffer solution are considerably smaller than that of DTBN. This difference may be related to the angle between the two C N bonds. The two tert-butyl groups in DTBN may cover the reactive center, Le., the N O group, more extensively than two other cases. Thus, the access of the reagent to the NO group in DTBN is more difficult than in the other cases.
Conclusion The mechanism for the reduction of DTBN with ascorbic acid in aqueous solution in the presence of p-CDX has been discussed. The decrease in the reaction rate upon addition of p-CDX in these systems could be explained by the “protection by the cavity wall of P-cyclodextrin”. By analysis of the reaction rates, it is suggested that the activation complex is formed outside of the cavity of 0-CDX when the ascorbate ion attacks the guest radical of the inclusion complex. If “protection” by the p-CDX cavity is assumed, changes in the reactivity of the included aminoxyl radicals are consistent with the molecular dispositions. Registry No. DTBN, 2406-25-9; p-CDX, 7585-39-9;Tempo, 256483-2; Tempol, 2226-96-2;ascorbic acid, 50-8 1-7.
I n Situ High-pressure FT-IR Studies on the Surface Species Formed in CO Hydrogenation on Si0,-Supported Rh-Fe Catalysts Takakazu Fukushima,+Hironori Arakawa,t and Masaru Ichikawat§* Sagami Chemical Research Center, 4 - 4 - 1 Nishi-Onuma, Sagamihara, Kanagawa. 229, Japan, and National Chemical Laboratory f o r Industry, Yatabe, Ibaragi. 305, Japan (Received: January 22, 1985; In Final Form: June 24, 1985) High-pressure in situ FT-IR spectroscopic techniques were applied to study the surface species on Si02-supported Rh-Fe catalysts which efficiently catalyze the formation of methanol and ethanol in the prevailing pressured CO-H2 reaction. Three different related oxygenate species, e.g., methoxy, ethoxy, and acyl, which are bonded to Rh and/or Fe, have been proposed as the surface intermediates. The interpretation of the surface structures has been discussed in terms of the band shifts corresponding to the deuterated species formed in the CO-D2 reaction on the catalyst.
Introduction It is frequently proposed that CH,O surface species are possible intermediates in the methanation reaction and methanol synthesis.’ Concerning the formation of C2 oxygenates such as ethanol and CH,CHO in C O hydrogenation on Rh catalysts, recent studies have ~ u g g e s t e dthat ~ ~ an ~ ~acyl species CH,CO is a possible intermediate, which is formed by C O insertion with CH,/CH2 surface hydrocarbons on Rh metal. Nevertheless, the isolation and identification of such surface species are very difficult. There has been a limited effort to reasonably identify surface species, e.g., methoxy on clean Ni( 1 11) surface by using high resolution electron energy loss spectro~copy~~ and a formate on Pd-Na/Si02 by FT-IR o b ~ e r v a t i o n . ~ ~ Some early transition metals, e.g., Mn,” Ti, Zr,5 and Fe,6 exhibit significant effects which modify activity and selectivity for oxy-
’Sagami Chemical Research Center.
‘National Chemical Laboratory for Industry. Present address: Ipatieff Laboratory, Department of Chemistry, Northwestern University, Evanston IL 60201.
0022-3654/85/2089-4440$01.50/0
genates in CO hydrogenation on supported Rh catalysts. As shown in Table I, the effect of Fe added to Rh/Si02 is unexpectedly large and specific, basically changing the product distribution from a (1) (a) Vannice, M. A. Catal. Rev. Sci. 1976, 14, 153. (b) Blyholder, G.; Neff, L. D. J. Phys. Chem. 1966, 70, 1738. (c) Deluzarche, A,; Cresseley, J.; Kieffer, R. J. Chem. Res. 1979, 1657. (d) Rabo, J. A,; Risch, A. P.; Poutsma, M. L. J . Card. 1978, 53, 295. (2) (a) Ichikawa, M.; Shikakura, K. Stud. Surf: Sci. Catal. 1981, 7,925. (b) Takeuchi, A,; Katzer, J. R. J . Catal. 1983.82, 351; J . Phys. Chem. 1981, 85,937. (c) Orita, H.; Naito, S.; Tamaru, K. J . Chem. Soc., Chem. Commun. 1984, 150. (d) Ichikawa, M.; Fukushima, T. J . Chem. Soc., Chem. Commun. 1985, 321. (3) (a) Demuth, J. E.; Ibach, H. Chem. Phys. Lett. 1979, 60, 395. (b) Kikuzono, Y.;Kagami, S.; Naito, S.; Onishi, T.; Tamaru, K. Faraday Discuss. Chem. SOC.1982, 72, 135. (4) (a) Ellgen, P. C.; Bartley, W. J.; Bhasin, M. M.; Wilson, T. P. Adv. Chem. 1979, 178, 147. (b) Wilson, T. P.; Kasai, P. H.; Ellgen, P. E. J. Catal. 1981, 69, 193. (5) (a) Ichikawa, M. Bull Chem. SOC.Jpn. 1978, 51, 2273. (b) Ichikawa, M.; Sekizawa, K.; Shikakura, K.; Kawai, M. J . Mol. C a u l . 1981, 11, 167. (6) Bhasin, M. M.; Bartley, W. J.; Ellgen, P. C.; Wilson, T. P. J . Catal. 1978, 54, 120.
0 1985 American Chemical Society
The Journal of Physical Chemistry, Vol. 89, No. 21, 1985
High-pressure IR Studies on Si02-Supported Rh-Fe
4441
TABLE I: Product Distribution on Rh-Fe/Si02 in Varying the Fe ContenP STY, g/ Fe/Rh atomic ratio 4% Rh 0.05 0.1 0.3 0.5
(L of cat h )
selectivity, % CEb CO conv, 7% 1.12 0.81 0.89 2.27 3.48
MeOH 1.7 11.1 17.1 26.6 27.4
AcH 29.4 2.7 1.3 0.5 0.3
EtOH 8.6 34.6 31.6 31.3 28.3
AcOH 10.7 6.3 4.0 1.4 0.7
C2-0 48.8 43.6 36.8 33.2 29.3
CH4 42.2 42.8 45.3 37.0 37.8
MeOH 0.5 2.5 4.5 16.2 25.8
EtOH 1.9 5.6 6.0 13.7 19.2
"Temperature = 250 OC, pressure = 20 kg/cm%, CO/H2 = 0.5 volumetric ratio, catatlyst Rh = 0.912 mmol, 5 mL; SV = 6000 h-'. 18-mminner diameter X 500 mm long Ti tubing reactor was used. AcH = CH3CH0, AcOH = CH3COOH, C2-0 = total oxygenates; AcOH was produced in the form of acetates. The catalysts were prepared by coimpregnation of RhCI, and FeC& on S i 0 2 gel (10 mesh pellet; Davison grade 57, surface area = 280 cm2), followed with H2 reduction at 400 "C for 2 h. b % CE = iCJ/xJiCJX 100, where i = number of carbon atoms in a product molecule and Cj = concentration of j the product molecule.
mixture of C H 3 C H 0 and CH3COOHon Rh/Si02 to a mixture of C 2 H 5 0 Hand C H 3 0 H on Rh-Fe/Si02. Fe/Rh ratios were below 0.3, the addition of Fe to Rh/Si02 did not influence the CO conversion and the selectivity to total C2-oxygenates, while ethanol selectivity dramatically increases at the expense of C H 3 C H 0 and CH,COOH. With the large content of Fe above F e / a = 0.3, the yield and selectivity of methanol are substantially enhanced, even compared with the increase of ethanol. Our results from a 21-atm CO-H2 reaction on Rh-Fe/Si02 catalysts were basically consistent with those previously obtained by Wilson et aL6 under a syngas pressure of 70 atm, CO/H2 = 1.0, and at 300 "C. These results suggest that such specific metal additives as Fe exert a modifying role due to their interfacial interaction with Rh, which is possibly reflected in the change of the surface species related to oxygenate formation in GO hydrogenation. Consequently, we have conducted in situ high-pressure IR observation to study the surface intermediates formed on the SO2-supported Rh-Fe catalysts under high-pressure reaction conditions. From the characteristic IR vibrations observed, we suggest that methoxy, ethoxy, and acyl species are formed on the Rh-Fe catalyst surface. We briefly relate these findings to the proposed mechanism of alcohol formation and the role of Fe in enhancing the yields of methanol and ethanol from CO hydrogenation on the Rh-Fe catalysts.
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