V. T. Coon, T.
1070
kJ/mol from measurements of adsorption isotherms,6 most of the data values in the range obtained by the present investigation. On the surfaces of ZnO crystal of wurtzite structure, two kinds of planes are observed because of perfect cleavage, i.e., the side plane of hexagonal prism, ( l O i O ) , and the planes perpendicular to the c axis, (0001) and (0007). The water adsorption anomaly on ZnO surfaces has been discovered, that is, the physisorption isotherm of water on crystalline ZnO surfaces reveals a discontinuity at a moderate range of relative p r e ~ s u r e , l the ~ - ~possibility ~ of this phenomenon being ascribed to closed-hydrogen bonding of surface hydroxyls produced on the well-developed (1070) plane.l7 On the other hand, the hydration of the planes, (0001) and (OOOi), on ZnO surfaces forms free and protruded hydroxyls, which results in more hydrophilic sites. Thus, it may be reasonable to consider that these two kinds of planes on ZnO crystals will behave differently but each uniformly also for the chemisorption of C02. Furthermore, it is well known that on crystal surfaces several kinds of surface defects such as kinks, steps, edges, corners, and others are present; they may reasonably constitute heterogeneous and most active sites for chemisorption of C02.
Takeshita, W. E. Wallace, and R. S.Craig
Acknowledgment. The authors wish to express their thanks to Dr. Mahiko Nagao for his help in measuring chemisorbed cos.
References and Notes (1) V. M. Stowe, J. Phys. Chem., 56, 487 (1952). ( 2 ) T. Morimoto and K. Morishige, Bull Chem. SOC.Jpn., 47, 92 (1974). (3) T. Morimoto and K. Morishige, J. Phys. Chem., 79, 1573 (1975). (4) T. Morimoto and H. Muraishi, Chem. Commun., 323 (1976). (5) 0. Levy and M. Steinberg, J. Catal., 7, 159 (1967). (6) P. M. G. Hart and F. Sebba, Trans. Faraday Soc., 56, 551 (1960). (7) T. Morimoto and H. Naono, Bull. Chem. SOC.Jpn., 46, 2000 (1973). (8) M. Nagao, K. Morishige, T. Takeshita, and T. Morimoto, Bull. Chem. SOC. Jpn., 47, 2107 (1974). (9) M. Nagao and T. Morimoto, J. Phys. Chem., 73, 3809 (1969). (IO) T. Morimoto, M. Nagao, and M. Hirata. Kolioid-Z. Z. Polym., 225, 29 (1968). (11) "Kagaku Binran", Maruzen, Tokyo, 1975. (12) K.Atherton. G.Newbold, and J. A. Hockey, Discuss. Faraday Soc., 52, 33 (1971). (13) S. Dana and W. E. Ford, "A Textbook of Mineralogy", Wiley, New York, N.Y., 1960. (14) T. Morimoto, M. Nagao, and F. Tokuda, Bull. Chem. SOC.Jpn., 41, 1533 (1968). (15) T. Morimoto and M. Nagao, Bull. Chem. SOC.Jpn., 43, 3746 (1970). (16) M. Nagao, J. Phys. Chem., 75, 3822 (1971). (17) T. Morimoto and M. Nagao, J. Phys. Chem., 78, 1116 (1974).
Rare Earth Intermetallics as Catalysts for the Production of Hydrocarbons from Carbon Monoxide and Hydrogen' V. T. Coon, T. Takeshita, W. E. Wallace,* and R. S. Craig Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 (Received February 23, 1976) Publication costs assisted by the National Science Foundation
A number of rare earth intermetallics have been shown to be active catalysts for the reaction of CO and H2 to form CH4. For example, LaNi5 when used to catalyze the reaction CO 3H2 at 381 "C shows in a single pass conversion of 95% of the CO. CH4 is the principal product with some C02 produced and also traces of C ~ H G and C2H4. In the range 250-300 "C the catalytic effectiveness of LaNib increases with time over a periad of a few daw, twobablv because of a rise in surface area as the reaction progresses. A turnover number of -100 h-l for LaNib is found at 275 "C.
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In a recent paper results were presented2 concerning the utility of rare earth intermetallics3 as synthetic ammonia catalysts. Some of these materials proved to be very effective as catalysts, with specific activity exceeding that of a typical industrial catalyst by an order of magnitude. In the present work we have found that these materials are also effective catalysts for the reduction of CO with H2. The present note is a preliminary account of work which will be described more fully later. The rare earth systems were prepared by methods4 which have been standard for some years in this laboratory. Stoichiometric proportions of the constituent metals in the best purity attainable commercially (99.9% for the rare earths, exclusive of gaseous impurities, and 99.99% or better for the transition metals) were melted together in a water-cooled The Journal of Physical Chemistry, Vol. 80, No. 17, 1976
copper boat. In some cases, e.g., LaNib, the desired intermetallic is formed on solidification of the melt; in other cases solidification is followed by an appropriate heat treatment, chosen by reference to the phase diagram, to develop the compound desired. Confirmation of the desired structure is obtained by conventional x-ray powder diffraction analysis. The intermetallics, which are all quite brittle, readily grind to a fine powder, which is the form in which the catalytic material is introduced into the reactor. The samples, prepared as indicated in the preceding paragraph, were placed in the reaction chamber and activated by exposure to hydrogen at 1atm and at temperatures of 200-300 "C. Hydrogen dissolve^^-^ in the materials and further reduces particle size; this reduction in particle size is probably the activation process in the present work. The glass system of
Production of Hydrocarbons from CO and H2
T ("C) Figure 1. Conversion of CO over LaNi5 in a CO -t 3H2 gas mixture. Relative amounts of CH4, COP,and C2H6 (X32), as deduced from the GC peak areas, are also given. The solid line and open circles are results obtained in the sequence involving rising temperatures; filled circles and dashed lines give corresponding results for decreasing temperature. At the highest temperature studied (381 "C) 95% of the CO is converted into CH4 (83%), C02 (17%), C2H6 (trace), and C2H4 (trace).
which the reaction chamber formed a part was evacuated to high vacuum and cooled to liquid nitrogen temperature. Surface area was measured by Ar absorption. The catalyst was warmed first to room temperature and to successively higher temperatures, up to 600 "C in some cases, and a gas mixture of composition 3Hz 1 CO was passed over it a t a rate of 30 ml (STP)/min for LaNib and half this for all other catalysts. The effluent gas was monitored by a gas chromatograph arranged to detect HP, CO, CH4, COz, H20, higher saturated hydrocarbons, and olefins. Metallic systems studied were elemental Gd, GdzNi17, LaNiB, ErNi5, ErFez, CeCo5, Er2Fe17, and ErFes. These were chosen to exemplify several of the various stoichiometries observed in compounds between the rare earths and Fe, Co, and Ni. Gd was essentially inactive as a catalyst; it did form
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traces of ethane for T > 250 "C. All the others catalyze the formation of CH4 above some minimal temperatures, Tmin. Tminis -250 "C for the Ni compounds and 420 and 325 "C for ErFez and CeCos, respectively. ErFes begins to form CzHd and CH4 a t 290 and 410 "C, respectively. The extent of reaction was established by noting the decrease in CO content and the increase in CH4 and COPcontent in the effluent gas. Typical results are shown for LaNi5 in Figure 1.Results obtained for LaCuj under 20 atm pressure and in circulating system show alcohols as the primary product. Results for ErNi5 are generally similar to those for LaNi5 except that -0.1% propane is generated in this case. Rough calculations for LaNib give a turnover number of 100 h-l at 275 OC. At 625 "C 85%of CO is converted over ErFe2, of which 58% is CH4. Over CeCo5 a t 450 "C 94% of the CO reacts during a single pass through the reactor. The effluent gas contains 74% CH4,16% CO, 6% CO2, and 0.2% CzHs. An interesting feature of these materials is that in the range 250-300 "C the catalytic effectiveness of LaNib increases with time a t a given temperature. This effect saturates out after 1 day or so. The catalyst is being further activated by the CO-Hz mixture. This is probably due to increase in surface area with time during the early stages of the reaction, a point which is currently under further investigation in our laboratory.
Acknowledgment. The authors wish to acknowledge many useful discussions with Professor Paul Emmett during the course of the work. References and Notes (1) This work was supported in pari by grants from the Pennsylvania Science and Engineering Foundation and from the National Science Foundation. (2) T. Takeshita, W. E. Wallace, and R . S. Craig, J. Catal., in press. (3) These intermetallics are discussed in detail in a monograph by W. E. Wallace, "Rare Earth Intermetallics", Academic Press, New York, N.Y., 1973. (4) See ref 2 for many references to the original literature pertaining to the preparation and characterization of rare earth intermetallics. (5) J. H. N. Van Vucht. F. A. Kuijpers, and H. C. A. M. Bruning, Philips Res. Rep., 25, 133 (1970). (6) F. A. Kuijpers, Ph.D. Thesis, Technische Hogeschool, Delft, 1973. (7) T. Takeshita, W. E. Wallace, and R. S. Craig, Inorg. Chem., 13, 2282, 2283 11R7A\ (8) Bechman, A. Goudy, T. Takeshita, W. E. Wallace, and R. S. Craig, Inorg. Chem., submitted for publication.
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The Journal of Physical Chemistry, Vol. 80, No. 17, 1976
