1876
Tetsuo Morimoto and Haruto Muraishi
Determination of Heat of Chemisorption of Carbon Dioxide on Zinc Oxide by Means of Surface Substitution Reaction Calorimetry Tetsuo Morimoto" and Haruto Muraishi Deparlment of Chemistry, Faculty of Science, Okayama University, Okayama, Japan (Received March 29, 1976)
A new method was applied for the measurement of heat of chemisorption of CO2 on ZnO, that is, the heat of immersion of ZnO samples with and without prechemisorbed COZwas measured in water, where the substitution chemisorption of water for prechemisorbed COz occurs immediately after immersion of the sample in water. By taking the heat of solution of C02 in water into account, the immersion data make it possible to calculate the heat of chemisorption of COz. The results obtained as a function of the amount of chemisorbed COz show a particular surface heterogeneity characteristic of the crystalline ZnO sample used.
Introduction In 1952, Stowel measured the heat of immersion of heptane-covered A1203 in water, and obtained data which indicate the substitution adsorption of water for heptane. This technique may reasonably be applied to any substitution chemisorption system in which two adsorbates employed can be chemisorbed through different adsorption forces on a solid surface. Very recently, it has been found that COz molecules chemisorbed on ZnO can be replaced by water molecules a t a measurable rate on exposing the surface to water vapor at room temperature, which results in the formation of surface h y d r o x ~ l s . This ~ , ~ substitution reaction may be considered to occur more rapidly in liquid water than in the vapor. Accordingly, we can expect to determine the heat of chemisorption of COSon ZnO by means of immersion calorimetry in water, i.e., from the measurement of (1)heat of immersion of bare ZnO ( H I )and (2) heat of immersion of COZ-covered ZnO (HI').Then, the heat of chemisorption Qa(r)of C02 on ZnO will be expressed by the equation4 Q a m
= & d ( r ) + HI - H i'
(1)
Here, Q d ( r ) is the heat of solution of COz in water, r being the number of molecules concerned. In this process, the assumption is made that all the C02 molecules desorbed by exchange reaction with water are dissolved in water. The heat of chemisorption of COz on ZnO have been measured by different techniques, i.e., by gas chromatography5 or from adsorption isotherms combined with the ClausiusClapeyron equatione6The present studies were undertaken for measuring the heat of chemisorption of COz on ZnO by a new method, i.e., surface substitution reaction calorimetry as mentioned above, and for comparing the results with those obtained by different methods. Experimental Section Materials. The ZnO sample used in the present studies was produced by burning in the atmosphere metallic zinc from the Sakai Chemical Co. The specific surface area of the sample was measured by the BET method using nitrogen adsorption isotherm at -196 "C, where the molecular area of nitrogen was assumed to be 16.2 A, and the area was found to be 3.00 m2/g after evacuating a t 600 "C for 4 h. COz gas used as the adsorbate was supplied from dry ice; it was purified by repeating sublimation a t the dry ice-ethanol temperature. Water used The Journal of Physical Chemistry, Vol. 80, No. 17, 1976
as the immersion liquid was purified by deionization and double distillation. Preparation of COz-Chemisorbed ZnO Samples. The ZnO sample was pretreated at 600 "C under a vacuum of Torr for 4 h in order to remove adsorbed impurities, kept in an atmosphere of 50 Torr of C02 for 2 h a t room temperature, and evacuated a t room temperature for 2 h. This sample was reevacuated a t various temperatures from 100 to 600 OC, which resulted in surfaces having different amounts of chemisorbed COz. The amount of chemisorbed C02 remaining after the treatment was determined by means of successive ignition loss method in the same way as the determination of chemisorbed ~ a t e rIn. other ~ ~ ~words, the gas evolved by heating the sample a t a given temperature was condensed in a trap at the dry ice-ethanol temperature, and determined volumetrically after evaporation at room temperature; this procedure was repeated by raising the heating temperature step by step from room temperature to 600 OC. Since it was found that the amount of COS expelled by heating the sample at 600 "C was nil, the amount of chemisorbed CO2 remaining on the surface after treatment at a given temperature was calculated by summing the quantity of COZ evolved above the temperature. Determination of C02 Evolved by the Substitutional Reaction of Water. In order to determine the amount of COZ evolved when ZnO samples covered with different amounts of chemisorbed COZ were immersed in water, the supernatant liquid was distilled with sulfuric acid, the liberated gas was absorbed in a standard Ba(0H)z solution, and the solution was titrated with a standard HC1 solution. Immersion Calorimetry. The heat of immersion of ZnO samples with and without chemisorbed COz was measured at 28 OC using an adiabatic calorimeter equipped with a quartz thermometer as a temperature sensing element. The main procedure of the immersion calorimetry was almost the same as described in the previous paperagThe heat of breaking of ampoules was found to be on the average 0.45 J from the measurement of vacant ampoules. Results a n d Discussion The amount of COz chemisorbed on ZnO, which was measured by the two different methods, is plotted against pretreatment temperature in Figure 1. It should be noticed that the two curves in Figure 1 are in good agreement with each other. This manifests that the substitution chemisorption of water for prechemisorbed COz was accomplished in a short
Chemisorption of Carbon Dioxide on Zinc Oxide
t
4t
0
200
400
600
Treatment temperature ("C)
0
Figure 1. Relation between amount of chemisorbed C 0 2 on ZnO and treatment temperature: ( 0 )successive ignition loss method, (0)titration
method. time after immersing the sample in water, and that all the COz molecules expelled from the surface was dissolved in water. The heat of immersion of ZnO in water is given in Figure 2 as a function of pretreatment temperature. For comparison, another series of samples were prepared by degassing the original sample under a vacuum of 10-5 Torr for 2 h at increasingly elevated temperatures, and used for the immersional wetting measurement; the results obtained are represented also in Figure 2. Every measured point in this figure is a mean value of at least five measurements with a probable error within f0.02 J. The heat of immersion of simply degassed ZnO samples in water increases with increasing treatment temperature and attains a maximum value after 500-600 "C treatment as shown in Figure 2, similar to the results reported in the previous paper.1° This phenomenon was elucidated in terms of rehydroxylation of the dehydroxylated surfaces of ZnO samples, i.e., by the fact that a ZnO sample which has lost more hydroxyls by treatment at a higher temperature yields a higher rehydroxylation heat on immersion in water. Also, it is seen from Figure 2 that the immersional heat of COz-covered ZnO samples in water is smaller than that of the bare sample, and increases with decreasing amount of chemisorbed COZ, and that the heats on COz-covered samples are larger compared to those of simply degassed samples at the same temperature. As may be expected, the immersional heat recovers to the original value when precovered COZis completely removed from the surface. Admittedly, the difference between the immersional heat of the 600 "C treated bare surface and those of COz-chemisorbed surfaces should be correlated with the heat of chemisorption of CO2 and that of solution of COz in water as discussed above. Therefore, we can calculate the heat of chemisorption of COz on ZnO from the data in Figures 1 and 2 combined with the eq 1;the results obtained are plotted in two ways in Figure 3, i.e., against pretreatment temperature as well as against amount of chemisorbed COz, where 20.3 kJ/mol is used for the heat of solution of COZ in water.ll Infrared spectroscopic studies show that COz can be chemisorbed on ZnO to form carbonate ions.12 On the other hand, i t has been found that COz is chemisorbed on ZnO up to a t most 3.6 molecules/100 Az as shown in Figure 1,that is, about a half of surface oxide ions on ZnO can be converted to
200
600
400
Treatment temperature
("C)
Figure 2. Relation between heat of immersion of ZnO and treatment temperature: ( 0 )simply degassed sample, (0)COe-covered sam-
ple. Treatment temperature ("CI
Chemisorbed COl's/
100i2
Figure 3. Heat of chemisorption of COP on ZnO plotted (0)against treatment temperature and ( 0 )against amount of chemisorbed COP.
carbonate ions by the chemisorption of COz, probably because of the steric effect of the carbonate ions produced; the surface carbonate ions thus formed can be perfectly displaced by water to form exactly twice as much surface hydroxyls as desorbed COZ molecules in numbew3 The results shown in Figure 3 indicate that the heat of chemisorption of COz on ZnO varies extensively, suggesting surface heterogeneity in the sense that the chemisorption energy is larger on the sites from which chemisorbed COz is desorbable by evacuation at higher temperature. Furthermore, it should be noted that the curves in Figure 3 appear to give a particular shape with two steps, reflecting a surface heterogeneity characteristic of the sample used. The heat of chemisorption plotted in Figure 3 against the amount of chemisorbed COz indicates that there exist a much greater number of chemisorption sites with lower energy values around 77.5 kJ/mol corresponding to COz desorbable by 200 "C degassing, compared to those with higher energy values around 93 kJ/mol corresponding to COz desorbable by 300-400 "C degassing. Several authors have thus far reported the heat of chemisorption of COz on ZnO, i.e., 87.9-125.6 kJ/mol by means of gas chromatography5 and 71.7-87.9 The Journal of Physical Chemistry, Vol. 80, No. 17, 1976
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.
+
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
