Butane Hydrogenolysis over Single-crystal Rhodium Catalysts

Transcription

1 Faraday Discuss. Chem. SOC., 1989, 87, Butane Hydrogenolysis over Single-crystal Rhodium Catalysts Abhaya K. Datye* and Bernard F. Hegarty Department of Chemical and Nuclear Engineering, University of New Mexico, Albuquerque, New Mexico 87131, U.S.A. D. Wayne Goodman* Department of Chemistry, Texas A&M University, College Station, Texas 77843, U.S. A. Hydrogenolysis of n-butane has been studied over the (1 10) and (1 11) surfaces of Rhodium. On Rh( 1 lo), the products of the hydrogenolysis consist of methane > ethane > propane. The hydrogenolysis reaction exhibits a good fit to Arrhenius behaviour for reaction temperatures up to 500 K. At higher temperatures the reaction rate tends to roll over due to insufficient surface coverage of hydrogen. The roll over affects product distribution, yielding more complete hydrogenolysis to methane. The behaviour is qualitatively similar on Rh( 11 l), with roll over occurring at a lower temperature, namely 475 K. However, the hydrogenolysis is more selective on Rh( 11 l), yielding 50 mol O/O ethane. The high ethane selectivity seen in previous work on Ir( 110) is not seen on the Rh( 110) surface presumably because the Rh surface does not exhibit the (1 x 2) missing-row reconstruction that is stable on Ir( 110) surfaces under reaction conditions. The hydrogenolysis selectivity of the Rh single-crystal surfaces correlates well with supported Rh metal particles subjected to oxidation-reduction cycling. An important lingering question in catalysis is the relationship between the structure and composition of a catalytic surface and the reactivity and selectivity demonstrated by that surface. The use of orientated single crystals has been shown to be particularly informative regarding the assessment of the effects of surface composition and geometry. For example, in our laboratories, the activity for ethane and butane hydrogen~lysis~ to methane on nickel has been shown to depend critically on the particular geometry of the surface, the more open (100) plane being far more reactive than the closed-packed (1 11) plane. The latter surfaces are encountered more prevalently in f.c.c. materials as the particle size is increased via successively higher annealing temperature^.^ These results then are consistent with rate measurements on supported nickel catalysts, which show hydrogenolysis activity to be a strong function of particle size, the larger particles exhibiting the lower rates. In a related study, the selectivity for ethane production from the hydrogenolysis of n-butane over Ir single crystals has been demonstrated to scale with the concentration of low-coordination-number metal surface atoms. The Ir( 110)-(1 x 2) surface, which has a stable missing-row structure, has been found to produce ethane very selectively. This contrasts with the results for the close-packed Ir(ll1) surface, where only the statistical scission of C-C bonds has been observed. The results of this study correlate qualitatively with the observations made previously for selective hydrogenolysis of n-butane to ethane on supported Ir catalysts as a function of Ir particle size. As shown in fig. 1, the results for Ir( 1 lo)-( 1 x 2) model very well the small-particle limit whereas the results for Ir( 11 1) relate more closely to the data for the corresponding large particles (>lo nm). By assuming particle shapes the general behaviour of declining selectivity with larger particle size can be accurately modelled, as illustrated in fig

2 338 Butane Hydrogenolysis side view 20-0 I I I I Fig. (a) Schematic representation of the (110)-(1 x 2) and (111) surfaces of Ir. The z axis is perpendicular to the plane of the metal surface. C,, designates the coordination numbers of the metal surface atoms. (b) Selectivity for C2H, production (mol o/o total products) for n-butane hydrogenolysis on Ir single crystals and supported Ir catalysts at 475 K. The effective particle size for the single-crystal surfaces is based on the specified geometrical shapes. A, lra1203; W, Ir/Si02. The stoichiometry of the surface intermediate leading to high ethane selectivity, based on kinetics and surface carbon coverages subsequent to reaction, is suggested to be a metallocycle pentane.6 Based on analogous chemistry reported in the organometallic literature, the mechanism responsible for ethane is postulated to be the reversible cleavage of the central C-C bond in this metallocycle intermediate. This is a 1,4- diadsorbed hydrocarbon species on the sterically unhindered and coordinatively unsaturated C7 9 sites on the corrugated Ir( 1 lo)-( 1 x 2) surface. These studies have shown that it is now possible using single-crystal surfaces to construct in a systematic fashion uniform surfaces with specific reaction sites. This then enables a rather precise approach to detailing at the atomic level the origins of site specificity in surface-catalysed reactions. In this paper we report a study of n-butane hydrogenolysis on Rh( 110) and Rh( 11 1) single crystals. This work, together with the previous data for Ir, strongly suggest that a key factor in determining the selective catalytic chemistry of Ir and Rh for butane hydrogenolysis to ethane is the availability of an unhindered (coordinatively unsaturated) atom site. Such sites are available only at high metal dispersions on supported metal catalysts or on the Ir( 1 lo)-( 1 x 2) surface. On the Rh( 110) surface, where the (1 x 2) reconstruction does not occur, and for the close-packed Rh(ll1) surfaces, a second mechanism, presumably involving an intermediate coordinated to multiple surface sites, may be operative. Experimental The experiments were performed in a stainless-steel, dual-chambered apparatus which has been described in detail elsewhere. The chambers are linked uia a gate valve and

3 A. K. Datye, B. F. Hegarty and D. W. Goodman lo3 K/ T Fig. 2. Specific reaction rate for the hydrogenolysis of n-butane over Rh( 110) (n, = 9.6 x 10'' sites crn-*). Pbutglne = 10 Torr and PHI = 200 Tom. 0, Methane; +, ethane; A, propane. each can be evacuated to <lo-'' Torr.? Crystals were mounted on a retraction bellows and translated vertically between the analysis chamber and the reaction chamber. The analytical chamber is equipped with a cylindrical mirror analyser (CMA) for Auger spectroscopy (AES) and a quadrupole mass analyser for thermal desorption spectroscopy (TDS). The reaction chamber, which can be pressurized to several atmospheres,$ was operated as a batch microreactor. The crystal temperature was monitored by either a chromel/alumel thermocouple (for Rh) or a W-5% Re/W-26'/0 Re thermocouple (for Ir) spot-welded to the back of the crystal. The temperature of the sample was maintained during reaction by an RHK temperature programmer to *1 K. Rhodium crystals were cleaned in the analysis chamber by annealing at 1100 K for 10 min followed by oxidation at 5 x lo-* Torr O2 at 700 K for 1 min and then a second anneal to 1000 K until no surface impurities were detectable by AES. Reaction products were analysed by gas chromatography. Absolute reaction rates on the Rh crystals were calculated from the reactor volume (600cm'), duration and temperature of reaction, the measured surface area of the crystals and the known atomic density of each Rh surface.'" Butane (Scientific Gas Products, nominally 99.99% ) was degassed repeatedly at 80 K. Impurity ethane and propane were removed by multiple distillations from a liquid-solid hexane bath. Hydrogen (99.99%) was supplied by Matheson and used without further purification. Results The specific turnover frequencies for the production of methane, ethane and propane from 20 : 1 H,-n-butane on an Rh( 110) single crystal in the temperature range K are plotted in fig. 2. At all temperatures the order of the abundance of products is methane > ethane > propane. Below 500 K the activation energy for all three products was 135 * 6 kj mol-'. Above 500 K, the product distribution shifted to reflect more complete hydrogenolysis. t 1 Torr = /760 Pa $ 1 atm = Pa.

4 3 40 Butane Hydrogenolysis o. o o :. ~ ZOO PH Torr Fig. 3. Hydrogen pressure dependence for n-butane hydrogenolysis over Rh( 110). T = 500 K, Pbulane = 10 Torr. 0, Methane; *, ethane; A, propane. I0 Fig. 4. Specific reaction rate for the hydrogenolysis of n-butane over Rh( 111) (n, = 1.6 x 0, Methane; *, ethane; A, propane. Phu,.,ne = 10 Torr. PH> = 200 Torr. The variation in the reaction rate with change in the partial pressure of hydrocarbon and hydrogen was studied at 500 K. A change in the butane partial pressure was observed to produce little variation in activity and selectivity. In contrast, variations in the partial pressure of hydrogen at this temperature (fig. 3) show a volcano curve. Above 350 Torr, the hydrogen dependence is of negative order. Below 350 Torr, the reactivity decreases with decreasing hydrogen pressure, and the selectivity towards ethane and propane is reduced. Fig. 4 and 5 show the results for kinetic studies of the butane-hydrogen reaction on the Rh(ll1) surface. The Arrhenius plot of fig. 4 shows the same trend towards more

5 A. K. Datye, B. F. Hegarty and D. W. Goodman P,,/Torr Fig. 5. Hydrogen pressure dependence for n-butane hydrogenolysis over Rh( 111). T = 475 K, Pbutene = 10 Torr. 0, Methane; +, ethane; A, propane. complete hydrogenolysis at higher temperatures as was observed for the Rh( 110) surface. The onset of roll over is at a lower temperature, 475 K. At temperatures below 475 K the yield of ethane is consistently higher than that of methane by a factor of ca The hydrocarbon partial pressure dependence for the Rh(ll1) surface at 475 K is slightly positive, whereas the selectivity is independent of butane pressure. An increase in the partial pressure of hydrogen (fig. 5) up to 200Torr results in an increase in catalytic activity. Above 200 Torr the reaction is negative order in hydrogen; below 200 Torr the selectivity to ethane and propane increases with increasing hydrogen pressure. Discussion The literature reports of butane hydrogenolysis on supported Rh are less definitive than the results for Ir. Wong et al. reported that at 454 K in a pulse reactor, 1.5 nm Rh particles on silica gave an ethane/propane ratio of 7.8, while 5.7nm particles gave a ratio of 3. Bernard et az. also report high ethane selectivity on Rh/alumina. In related studies, central scission of n-pentane (to ethane and propane) has also been seen on highly dispersed Rh on alumina,13 and on highly-dispersed Rh/Si02.I4 A key issue in the present studies is the suitability of the corrugated Rh(ll0) for modelling highly dispersed, supported Rh catalysts as is the case for Ir( 1 lo)-( 1 x 2) and supported Ir catalysts.6 Likewise, if the correlation between Ir and Rh holds for the (1 10) surfaces, it is expected that Rh( 11 1) will demonstrate the chemistry of a relatively large (> 100 A) supported Rh particle. For both Rh surfaces, the extent to which hydrogenolysis proceeds increases with increasing reaction temperature (fig. 2 and 4). This is in keeping with the general trend for increased cracking at higher temperatures for alkane reactions. The term roll over has been used to describe the fall in overall activity at the high temperatures which leads to a decrease in the selectivity for the production of ethane and propane on Ir crystals.6 For Ir, decreasing the partial pressure of H2 at the temperature of onset of roll over induces the same selectivity change as observed for an increase in reaction temperature. The same correlation was observed for Rh in the present study. The origin

6 342 Butane Hydrogenolysis of this effect is believed to be the same for both Ir and Rh, viz. as the reaction temperature is raised beyond a critical temperature, defined primarily by the hydrogen partial pressure, the hydrogen surface coverage falls below a saturation or critical coverage. The lower hydrogen coverage then reduces the efficiency of the hydrogenation of surface hydrocarbon fragments. The higher temperature of onset of roll over on the more open Rh surface was also observed for Ir. This behaviour correlates with the higher binding energy of hydrogen adatoms on the Ir(ll0)-(1 x2) surface and suggests that the source of the reactive hydrogen is the metal surface rather than, for example, an active carbonaceous overlayer. The possibility of adsorbed hydrocarbon species acting as transfer agents of hydrogen has been suggested for various alkane reactions.15 The absence of high ethane selectivity on Rh(ll0) very likely relates to stability of the Rh( 110) surface towards the (1 x 2) reconstruction. The corresponding Ir( 110) surface undergoes a reconstruction (stable under reaction conditions), described as Ir( 1 lo)-( 1 x 2) or missing-row structure, resulting in rows of the highly coordinatively unsaturated C, sites. These sites can form the metallocyclopentane species which has been proposed as an intermediate in the central scission of butane to ethane. No analogous sites exist on the unreconstructed Rh( 110) surface. Although the absence of C, sites is the most likely explanation for the observed selectivity in hydrogenolysis products, there are noteworthy differences in the organometallic chemistry of Ir and Rh. Specifically, the rhodiacyclopentane analogue of the iridiacyclopentane complex has been shown to be more difficult to synthesize and, once made, decomposes more readily. Butane hydrogenolysis on Rh( 111) appears to operate via the same mechanism as it does on the Ir( 111) surface. First, dissociative chemisorption of butane and hydrogen occurs followed by irreversible cleavage of the terminal carbon-carbon bond of the adsorbed hydrocarbon. Further C-C bond cleavage prior to product desorption leads to the methane and ethane observed as initial products. The reaction kinetics on the Rh( 111) surface (fig. 4 and 5) follow the same trend as those on the Rh(ll0) surface. The roll over in the Arrhenius plots sets in at a lower temperature (ca. 475 K) but has the same overall effect, namely, decreasing ethane and propane selectivity (fig. 4). The reaction is slighly positive order in hydrocarbon partial pressure. A decrease in the partial pressure of hydrogen at 475 K (fig. 5) has the same effect on product selectivity as has an increase in temperature. Likewise for this surface, the rollover in the Arrhenius plots at ca. 475 K can be ascribed to a decrease in the surface concentration of hydrogen. The primary difference then between the Rh(ll0) and Rh(ll1) surfaces is the relatively large ethane selectivity for the close-packed Rh( 111) surface. Although the ethane selectivity is not as distinctive on Rh( 111) as for the Ir( 1 lo)-( 1 x 2) surface, the selectivity towards its production is consistently higher (by a factor of 1.55) at reaction temperatures below 475 K. Braunschweig et al. have used high-resolution transmission electron microscopy (HRTEM) to correlate changes in Rh particle morphology, induced by oxidationreduction cycles, with change in butane hydrogenolysis selectivity. Ethane selectivity was lower for Rh particles that were oxidized and subsequently reduced under mild conditions, leading to a roughened surface structure. When the Rh was reduced at higher temperatures, where the more stable (111) surface facets would be exposed, the ethane selectivity increased. The oxidation-reduction cycling of Rh/Si02 by Gao and Schmidt also shows similar trends. Hence, the dominance of ethane production on the close-packed Rh( 111) surface is in qualitative agreement with the results on supported Rh metal particles that were 5.0 nm in diameter or larger. The selective route to butane hydrogenolysis seen on highly dispersed Ir7 and on the single-crystal Ir( 1 lo)-( 1 x 2) surface is not evident on any of the Rh surfaces studied

7 A. K. Datye, B. F. Hegarty and D. W. Goodman 343 here. Other reports on Rh, t- 4 however, do imply that highly dispersed Rh catalysts are indeed selective towards central bond scission. For instance, Yao et a/. reported that central bond scission of n-pentane was the only hydrogenolysis mode on the highly dispersed so-called delta phase on Rh/AI2O3. This selectivity was lost when the metal loading was increased, presumably due to formation of metallic Rh surfaces. The data imply that the mechanism involving the 1,4-diadsorbed n-butane which may occur only on low-coordinated C, sites is absent on the Rh( 110) and Rh( 11 1) surfaces studied in this work. Conclusions Single-crystal metal surfaces allow us to study in a systematic fashion the role of surface structure on catalytic activity and selectivity. We have found that the selectivity in n-butane hydrogenolysis is markedly affected by surface structure. Ir( 110) surfaces which have a high concentration of C, low-coordination sites show marked propensity to central bond scission. This selective hydrogenolysis route may involve adsorption of the n-butane as a metallocycle pentane and subsequent cleavage at the central carboncarbon bond. Similar surface chemistry is exhibited by highly dispersed Ir and Rh supported metal catalysts. On larger metal particles, and on Ir( 11 1) surfaces, a nonselective hydrogenolysis is observed, yielding equal amounts of ethane, methane and propane. On Rh( 1 lo), which is not stable towards the (1 x 2) reconstruction, we find a rather non-selective hydrogenolysis, but on Rh( 11 1) a slightly higher proportion of ethane is observed in the products. This implies that for the close-packed Rh(ll1) surfaces, a second mechanism, presumably involving an intermediate coordinated to multiple surface sites may be operative. We acknowledge with pleasure the partial support of this work by the Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, and by Sandia National Laboratories, Albuquerque. References W. Goodman, J. Vac. Sci. Techno/., 1982, 20, 522; D. W. Goodman, Ace. Chem. Res., 1984, 17, 194; D. U. Goodman, Annu. Rev. P/ij,s. C/7em., 1986, 37, 425; D. W. Goodman and J. E. Houston, Science, 1987,236, 403; A. G. Sault and D. W. Goodman, in Molecule-Surface Interactions, ed. K. Lawley (John Wiley, Chichester, 1988). 2 D. W. Goodman, Surf Sci., 1982, 123, L D. W. Goodman, Proc. 8th Int. Congr. Catal., Berlin, July, 1984 (Verlag Chemie, Berlin, 1984). 4 J. E. A. Clark and J. J. Rooney, Adc. Caral., 1976, 25, J. T. Carter, J. A. Cusumano and J. H. Sinfelt, J. Phys. Chem., 1966, 70, 2257; D. J. C. Yates and J. H. infelt, J. Catal., 1967, 8, 348; G. A. Martin, J. Catal., 1979, 60, J. R. Engstrom, D. W. Goodman, and W, H. Weinberg, J. Am. Chem. Soc., 1986, 108, 4653; J. R. Engstrom, D. W. Goodman and U. H. Weinberg, J. Am. Chem. Soc., 1988, 110, K. Foger and J. R. Anderson, J. Catal., 1979, 59, For example, R. H. Grubbs and A. Miyashita, J. Am. Chem. Soc., 1978, 100, 1300; R. H. Grubbs, A. Miqashita, M. Liu and P. Burk, J. Am C hem. So(,., 1978, 100, R. \an Hardeveld and F. Hartog, Adc. Caral., , T. W. Root, L. D. Schmidt and G. B. Fisher, Surf: Sci., 1985, 150, T. C. Wong, L. C. Chang, G. L. Haller, J. A. Oliver, N. R. Scaife and C. Kemball, J. Caral., 1984, 87, 389; T. C. Wong, L. F. Brown, G. L. Haller and C. Kemball, J. Chem. Soc., Faradax Trans. 1, 1981, 71, J. R. Bernard, J. Bouquet and P. Turlier, Proc. 7th Int. Congr. Catal., paper A7, Tokko (1980). 13 H. C. Yao, Y. F. Y. Yao and K. Otto, J. Cafal., 1979, 56, J. K. A. Clarke, K. M. G. Rooney and T. Baird, J. Catal., 1988, 111, M. W. Vogelzang and V. Ponec, J. Catal., , 77; S. M. Davis, Zaera and G. A. Somorjai, J. Caral., 1985, 84, 206; 1977, 82, 439.

8 344 Butane Hydrogenolysis 16 A. Cuccuru, P. Diversi, G. Ingrosso and A. Lucherini, J. Organomer. Chem., 1981, 204, E. J. Braunschweig, S. Chakraborti, A. D. Logan and A. K. Datye, Proc. 9fh Int. Congr. CafaL, ed. M. J. Phillips and M. Ternan (The Chemical Institute of Canada, 1988), vol. 111, p S. Chakraborti, A. K. Datye and N. J. Long, J. CafaL, 1987, 108, S. Gao and L. D. Schmidt, J. Cafal., 1988, 111, 210. Paper 8/05055D; Received 19fh December, 1988