J. Phys. Chem. 1991, 95,6341-6346
6341
C02 Hydrogenation over Nickei/Zirconia Catalysts from Amorphous Precursors: On the Mechanism of Methane Formation Cbristopb W l d , Alexander Wokaun,* Physical Chemistry II, University of Bayreuth, P.O. Box 101251, D- W-8580 Bayreuth, Federal Republic of Germany
Rene A. Koeppel, and Alfons Baiker Department of Chemical Engineering and Chemical Technology, Swiss Federal Institute of Technology, ETH Zentrum, CH-8092 Ziirich, Switzerland (Received: January 10, 1991)
An amorphous NiUZrg6alloy is used as a precursor for a carbon dioxide hydrogenation catalyst. Upon exposure to C02 hydrogenation conditions, the glassy metal is partly transformed into crystalline metallic nickel particles and less well-ordered zirconium dioxide. A conventional Ni/Zr02 catalyst, which was synthesized by coprecipitation and calcination of the amorphous precipitate, served as a reference. The as-prepared catalyst exhibits a very similar catalytic behavior to the alloy-derived catalyst. The structural and chemical changes are characterized by gas adsorption, X-ray diffraction, and thermal analysis. In COz hydrogenations over these catalysts, methane is almost exclusively produced, and traces of ethane are formed besides, as evidenced by gas chromatography. To study the origins of this selectivity behavior, in situ diffuse reflectance FHR spactroscopy has been applied. Surface species in carbon dioxide as well as carbon monoxide hydrogenation reactions are correlated with the formation of gas-phase products. C02/Hz mixtures rapidly yield surface formate as the immediate precursor of the methane product on the coprecipitated catalyst. Doubly and singly bound adsorbed CO is detected besides; doubly bound CO is observed to originate from surface formate. In CO/H2 reactions, higher saturated hydrocarbons are produced besides methane. The formation of these products proceeds from singly bound adsorbed CO via a dissociative mechanism. The respective hydrogenation steps are discussed in detail in a corresponding reaction scheme.
Introduction Hydrogenation reactions of carbon monoxide have been extensively studied over supported1+ and unsupported nickel catal y s t ~ .In ~ ref 6 it has been proposed that methane as well as higher hydrocarbons are produced on a common reaction pathway which involves the formation of adsorbed CH species. Addition of hydrogen to these species would then lead to methane, whereas combination reactions of two or more CH(ads) units would result in hydrocarbon formation6 The corresponding reverse reaction, Le., the abstraction of hydrogen from methane to form adsorbed methyl radical species, is commonly viewed as the first step in the oxidative coupling reaction of methane, yielding ethane.7 Compared to carbon monoxide hydrogenation, relatively little attention has been paid so far to the hydrogenation of carbon dioxide.s-12 C02 hydrogenation has been proposed to proceed via a similar mechanism as CO hydrogenation,8JoJ2 with the first step being C02 dissociation prior to methane formation. Methanation of C02 is reported to proceed with a lower activation energy than CO methanation over the same catalyst^.'^ There are comparatively few investigations on hydrogenation reactions over nickel/zirconia catalysts obtained from amorphous alloys.1c17 The oxidation and surface segregation phenomena ( I ) Vannicc, M. A. J . Curd 1976, 44, 152. (2) Rabo, J. A.; Risch, A. P.; Poutsma, M. L. J . Caral. 1978, 53, 295. (3) Sen,B.; Falconer, J. L. J . Curd. 1990, 122, 68. (4) Copper, M. E.; Frost, J. Appl. Caral. 1990,57, L5. ( 5 ) Goodman, D. W.; Kelley, R. D.; Madey, T. E.; Yates, J. T., Jr. J. Caral. 1980, 63, 226. (6) Joyner, R. W. J . C a r d 1977,50, 176. (7) Miro, E.; Santamaria, J.; Wolf, E. E. J . Carol. 1990. 124, 451. fbid. 1990, 121,465. (8) Falconer, J. L.;Zagli, A. E. J . Curd 1980, 62, 280. (9) Vance, C. K.; Bartholomew, C. H. Appl. Carol. 1983, 7, 169. (10) Peebles, D. E.; Goodman, D. W.; White, J. M.J. Phys. Chem. 1983, 67, 4378. ( I I ) Kwter, K. 8.; Zagli, E.; Falconer, J. L. Appl. Curd 1986, 22, 31 1 . (12) Campbell, T. C.; Falconer, J. L. Appl. Caral. 1989, 50, 189. (13) Mills, G. A.; Stcffgen, F. W. Cural. Rev. 1973, 8, 159. (14) &rtolini, J. C.; Brimt, J.; Le Mogne, T.; Montes, H.; Calvayrac, Y.; Bigot, J. Appl. Surf. Scl. lM7, 29, 29. (15) Yamashita, H.; Yoshikawa, M.; Funabiki, T.; Yoahida, S. J . Chem. Soc., Faruday Truns. I 1987.83, 2883.
0022-3654/91/2095-6341$.02.50/0
of amorphous Ni-Zr alloys occurring during activation have recently been studied by electron spectroscopic techniques,18 thermoanalytical methods, and X-ray diffraction.I9 It was shown that the oxygen uptake by the zirconium component of the alloy during activation leads to a pronounced surface enrichment in Zr. In the activated catalyst, metallic nickel particles are embedded in a (predominantly tetragonal) Z r 0 2 matrix. Hydrocarbon synthesis over Ni/zirconia starting from carbon monoxide has been studied in refs 20-22. In the present paper, hydrogenation reactions of carbon dioxide on nickel/zirconia catalysts prepared by different techniques are being analyzed. Two catalysts, derived from amorphous precursors, are compared. The samples are prepared from a glassy metal alloy, NiUZrg6,and by a conventional coprecipitation method. Our aims are to characterize these catalysts with respect to their activity and selectivity behavior in C02 hydrogenation reactions and to obtain some insight on the mechanism of both carbon dioxide and carbon monoxide hydrogenation reactions by in situ diffuse reflectance FTIR spectroscopy.
Experimental Section Catalysts. The amorphous NiMZrg6alloy used as a catalyst precursor was prepared from the premixed melt of the pure metals by rapid quenching according to the melt-spinning technique. For the catalytic tests, the 20-30 bm thick ribbons were ground under liquid nitrogen to a size of 0.5-1 mm. The BET surface area of the resulting flakes, as measured by krypton adsorption at 77 K, (16) Mahmoud, S.S.; Forsyth, D.A,; Gieascn, B. C. Mater. Res. Soc. Symp. Prac. 1986,58, 13 1 . (17) Armbruster, A+;Baikcr, A.; GLLntherodt, H. J.; Schlbgl, R.; Walz, B. Prepamrlon o/Carulysrs fV; Delmon, B., Grange,P., Jacobs, P. A., Poncelet, G., Us.Elsevier: ; Amsterdam, 1987; Stud. Sur/. Scl. Caral. 1987,31, 389. (181 Walz, B.; Oelhafen, P.; GLLntherodt. H. J.: Baiker, A. ADD/. .. Surf Scl. 1989,37,337. (19) Macieiewski. M.:Baiker. A. J. Chem. Soc.. Furuduv Trans. I 1990. 86,'843. (20) Komiyama, H.; Inoue, H. J. Fac. Eng., Uniu. Tokyo 1983, A-2/,62. (21) Shimogaki, Y.; Komiyama, H.; Inoue, H.; Mssumoto, T.; Kimura, H.Chem. f e r r . 1985. 661. (22) Shimogaki, Y . ; Komiyama, H.; Inoue, H.; Masumoto, T.; Kimura, H.J . Chem. Eng. Jpn. 1988,2/, 293.
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6342 The Journal of Physical Chemistry, Vol. 95, No. 16. 1991 was less than 0.1 m2 g-', X-ray diffraction indicated that the ground metallic glass was still completely amorphous. A conventionally synthesized nickel/zirconia catalyst with the same atomic ratio of the constituents was used as a reference. The catalyst was prepared by coprecipitation of the corresponding metal nitrates at constant pH and temperature. Aqueous solutions of NaOH/sodium formate (2M each) and the metal nitrates (0.45 M ZrO(N03),, 0.8 M Ni(N03)2)were slowly added to a Pyrex vessel containing 250 mL of deionized water. The temperature was kept at 363-368 K, and a pH of 7 was maintained. The green precipitate was aged for 30 min a t 363 K under stirring and then filtered. The residue was washed thoroughly with deionized water and methanol. After drying a t 393 K in vacuo for 15 h, the material was calcined in air at 573 K for 3 h and finally crushed to a grain size of 50-150 pm. Catalyst Characterization. The starting materials as well as the final catalysts were characterized by means of gas adsorption (nitrogen and krypton), X-ray diffraction (XRD), and thermal analysis (TG / DTA). Surface areas (SBET) were calculated by using a crass-sectional area of 0.163 nm2 for nitrogen and 0.195 nm2 for the krypton atom.23 Pore size distributions were determined according to the BJH method,24using the equation of H a l ~ e y . ~ ~ Thermal analysis experiments were carried out on a Mettler 2000C thermoanalyzer in air; a heating rate of 10 K m i d was employed. X-ray analysis was performed on a powder diffractometer (Philips PW 1700), using CuKa radiation. Mean crystallite sizes were estimated from the half-width of the Ni( 1 1 1) reflection, using the Scherrer equation. The measured peak width was corrected for instrumental broadening using the function proposed by Warren.26 Catalytic Tests. Catalytic tests were performed in a continuous tubular flow fixed-bed microreactor operated a t 1.7 MPa. Apparative details have been reported in ref 27. Feed and product gas analysis was performed in a gas chromatograph (HP, Model 5890 A) equipped with a thermal conductivity detector. Products in. i.d.) were separated in a stainless steel column (5 m, containing 80-100 mesh Poropak QS. Experiments were carried out by using 1.0 g of catalyst and a reactant flow rate of 90 mL min-' (STP) of C 0 2 / H 2 (1:3) in the temperature range 413-493 K. The reactant gases CO, (99.9%) and H2 (99.999%) were fed from high-pressure cylinders without further purification. Activation of the amorphous Nis4Zr36alloy was achieved by exposure to reaction conditions at 493 K. The catalyst prepared by coprecipitation was prereduced by heating to 573 K at a heating rate of 15 K m i d in 1.25% H2/N2 a t a pressure of lo5 Pa. Subsequently, the H2concentration was increased stepwise to 2.5/5/10/20/50/100%(30min per step). After the temperature had been decreased to 493 K, the hydrogen was replaced by a reactant mixture, and the pressure was increased to 1.7 MPa. Spectroscopic Measurements. The spectra were recorded on an FTIR spectrometer (Perkin-Elmer, Model 1710) using a diffuse reflectance unit equipped with a controlled environmental chamber (Spectra Tech). The temperature in the chamber was controlled within f2 K. Spectrometer and temperature controller were connected to a personal computer (Nixdorf, AT 836) for data handling and processing. The gas flows were adjusted with a specially designed gas dosing system to the desired values by using volume flow controllers (Rota). Carbon dioxide (99.9993%),carbon monoxide (99.997%), and hydrogen (99.999%) were used without further purification. The catalyst samples, which have been stored under atmospheric conditions, are reactivated in hydrogen flow a t elevated tem(23) Gregg, S. J.; Sing, K. S. W. Surf. Colloid Sci. 1976, 9, 254. (24) Barrett, E. P.: Joyner, L. S.; Halenda, P. P. J . Am. Chem. Soc. 1951.
Schild et al. peratures (450-500 K) for several hours before the FTIR measurements. After catalyst reactivation, the background spectrum is recorded at an appropriate temperature in the range which will be studied during the following catalytic reactions (e.g., at 393 K for a range of 363-423 K). Subsequently, three major techniques have been applied to investigate catalytic surface reactions. In the 'dynamic" experiments, the catalysts are exposed to a continuous reactant flow, and the observation of surface species is facilitated by the continuous purging of gaseous products. In the "static" system a reaction mixture is enclosed in the chamber, and the accumulation of the predominant reaction products is monitored as a function of reaction temperature. Further important information is derived from experiments in which the reaction of adsorbed surface species is studied by expasure to pure hydrogen. Typically, 50-250 scans at a resolution of 8 cm-' have been accumulated.
RHults Structunl Properties of Ni/Zr02 Catalysts. Upon exposure to reaction conditions, part of the initially completely amorphous Nis4zr36precursor transformed into crystalline metallic nickel particles and less well-ordered zirconium dioxide. As evidenced by XRD, the crystalline zirconia fraction consisted of monoclinic and tetragonal ZrO,. Estimated from the line broadening of the Ni(ll1) reflection, the nickel particles had a mean size of =10.5 nm. The chemical changes in the amorphous Nis4Zr36occurring during the in situ activation procedure were accompanied by marked changes in its textural properties. The BET surface area of the precursor alloy increased from CO.l to 8.4 m2 g-' for the final catalyst. The shape of the nitrogen adsorption isotherm, which was of type I (BDDT cIassification),2*as well as calculated t plots,29 indicated the presence of a microporous system with a mean pore diameter C2 nm. The calcination procedure of the coprecipitated Ni/Zr02 precursor after drying at 393 K was monitored by thermoanalytical methods. DTA measurements showed an exothermic peak at 550 K and a small endothermic event at 570 K. TG revealed a weight loss of 13.5% at 550 K. The BET surface area of the coprecipitated catalyst after exposure to C02 hydrogenation conditions amounted to 109 m2 g-'. The nitrogen isotherm was of type IV (BDDT classification) with a type H3 shaped hysteresis loop (IUPAC cla~sification),2~ indicating aggregates of platelike particles. Mesopore size distributions calculated from the desorption branch of the isotherm showed a narrow pore size maximum a t 3.6 nm and a broad maximum between 8 and 1 1 nm. Catalytic Behavior in C02 Hydrogenation Reactions. When exposed to reaction conditions at 493 K, the amorphous NiuZr36 did not exhibit any measurable catalytic activity in the beginning; significant CO, conversion was observed after the alloy had been on reactant stream for a5 h. Almost stable activity was reached after =60 h on stream, corresponding to a C02conversion rate of 25%. In the initial period, methane and water were detected as the reaction products, with a higher formation rate of water than expected from the methanation stoichiometry. With longer reaction times, traces of ethane were formed in addition. In the Arrhenius plots presented in Figure 1, the activated catalyst from amorphous Nis4zr36 is compared to the coprecipitated Ni/Zr02 catalyst. Methane production rates, referred to the sample weight ( r w )as well as to the BET surface area ( f a ) , are plotted on a logarithmic scale versus the inverse of the reaction temperatures. The results reflect the steady-state behavior of the catalysts in the CO, conversion range from 0.5% to 8%. The activation energies of the methanation reactions, resulting from the respective slopes, are calculated as 94.9 f 1.9 kJ mol-' for the catalyst derived from the amorphous alloy and 100.6 f 1.6 kJ mol-! for the coprecipitated catalyst. In this respect, it should
73, 373.
(25) Halsey, G. J . Chem. Phys. 1948, 16, 931. (26) Warren, B. E. J . Appl. Phys. 1941, /2* B75. (27) Gasser, D.; Baiker, A. Appl. Carol. 1989, 18, 279.
(28) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.: Rouqueroi, J.; Siemienicswka, T. Pure Appl. Chem. 1985, 57,603. (29)Lippens, B. V.;de Boer, J. H. J . Carol. 1965, I, 319.
The Journal of Physical Chemistry, Vol. 95, No. 16, 1991 6343
COz Hydrogenation over Nickel/Zirconia Catalysts -16
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"i
0.20
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2.25
1
2.30
2.35
1
2.40
2.45
1
T*10-3 K-' Figure 1. Arrhenius plots of methanation rates per unit weight r, (mol of CHI formed &,-I S-I) are shown in the lower panel, and per unit surface area r, (mol of CH, formed m-2 s-l), in the upper panel. Open
squares: Catalyst from amorphous Ni64Z36.Filled squares: Coprccip itated Ni/Zr02 catalyst. TABLE I: hignmcat of obremd Vibrrtiorul Frequcaeies 1, cm-I vibration molecule remarks 3014 2980-2960 2895-2880 2880 2340 2150 2060-2020 1960-1900 1600 1670-1600 1600-1580 1390-1 360 13 10-1 280 1308
Figure 2. Reaction products observed over a catalyst derived from an amorphous NiMZrMalloy. The catalyst is exposed to a static atmosphere of C02 and H2 (1:4 by volume), at T = 483 K and p = 3 bar. Timeobserved at higher coverage~~z~~ dependent changes are monitored by diffuse reflectance FTIR (DRIFT) n> 1 n> 1
spectroscopy. singly bound on Ni(l11)3' doubly bound on Ni( 1 1 multiplet (=i800-1400 cm-I) bidentate surface carbonate two types of formate" two types of formate" bidentate surface carbonate
be emphasized that both catalysts exhibit an identical selectivity behavior; Le., methane is produced almost exclusively, and traces of ethane are formed besides. FTIR Spectroscopic Measurements. As a reference for the interpretation of spectra presented in this section, relevant vibrational frequencies of gaseous and adsorbed species are summarized in Table I. These data originate from the current literature,mOJ' as well as from our previous adsorption experiments performed with other metal/zirconia On the catalyst derived from the Ni64Zr36amorphous alloy, only gaseous products can be observed under COz hydrogenation conditions. Surface species are not detected either during supplementary experiments in which the reaction chamber is purged with hydrogen or nitrogen subsequent to exposure to reaction conditions, to check whether the strong absorptions from the reactant gases might overlap with weak signals from surface species. Spectra obtained during the reaction of a static C02/H2mixture a t 483 K (Figure 2) show that the reactants are completely converted within a few minutes. The hydrogenation product, (30, See, e.&: Herzberg, G . Infrared and Roman Spectra of Polyatomic Molecules; Van Nostrand: New York, 1945. Kiselev, V. F.; Krylov, 0. V. Adsorption and Catalysis on Transition Metals and their Oxides; Springer: Berlin, 1989 and references therein. (31) Erley, W.; Ibach, H.; Lshwald, S.; Wagner, H. Surf, Sci. 1979,83, 585. (32) Schild, C.; Wokaun, A.; Baiker, A. J . Mol. Carol. 1990. 63, 223. (33) Schild, C.; Wokaun, A.; Baiker, A. J . Mol. Catal. 1990, 63, 243. (34) Schild, C.; Wokaun, A.; Kbppel, R. A.; Baiker, A. J. Chem. Soc., Faruday Trans. I , in press.
methane, is characterized by bands at 3014 and 1308 cm-! gaseous water gives rise to a multiplet centered at 1600 cm:l. As the hydrogenation reaction is proceeding, an increase in the absorptions of water with time is expected. Contrary to this expectation, the water absorptions are observed to decrease in the difference spectrum shown in the top trace of Figure 2. Most likely, the oxidation of residual metallic zirconium from the amorphous precursor is taking place in parallel and consumes the water produced in the C02 hydrogenation reaction. As a next step, COz hydrogenation reactions over the coprecipitated Ni/ZrO2 catalyst were monitored as a function of surface temperature under static conditions (Figure 3). At 363 K, a mixture of bidentate surface carbonate and formate is formed, as evidenced by the broad doublet around 1600 and 1350 cm-' (cf. Table I). Methane (3014, 1308 cm-I) and water (multiplet around 1600 cm-I) are produced at temperatures higher than 383 K, at a rate that is strongly increasing with temperature. Methane is the only hydrogenation product detected over the whole temperature range under study; no gaseous CO is observed. These results are in good agreement with the activity and selectivity data discussed in the above section. In a further experiment, the surface reactions were investigated as a function of time while a stoichiometric flow of COz and hydrogen (1:4 v/v) was continuously passed over the catalyst sample (Figure 4). Surface carbonate and formate are detected from the beginning; with longer times (10 and 60 min), singly bound surface CO molecules (2060 cm-') as well as doubly bound CO (=I900cm-I) are observed in addition. Formate is steadily being accumulated on the surface, as evidenced by the characteristic doublet at 1590 and 1380 cm-' in the difference spectrum (Figure 4, top trace). Recalling from Figures 2 and 3 that no indications for the formation of gaseous CO have been obtained, we conclude that the adsorbed carbon monoxide originates from surface formate. To investigate the role of formate in the hydrogenation reaction, the surface temperature was decreased to 383 K after the dynamic
Schild et al.
6344 The Journal of Physical Chemistry, Vol. 95, No. 16, 1991
60 min
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.
5 min
2 min
li,,l ,
l " " l '
3500
3000
2500
2000
,
,
(
1500
1000
ravenumber / cm'l Figwe 3. Temperature dependence of C02hydrogenation reactions over a Ni(64 atom %)/zirconia catalyst prepared by coprecipitation. The surface is exposed to a static C02/H2 (1:4) mixture at p 5 3 bar. DRIFT spectra are recorded while the temperature is being raised from 363 to 423 K.
diff.3-1
3500
3OOO
2500
2000
1500
1000
ravenumber / cm'l Figwe 4. Time dependence of C02hydrogenation reactions over a Ni(64 atom %)/zirconia catalyst prepared by coprecipitation. The surface is first exposed to a flow of pure hydrogen at T = 403 K and p = 3 bar. Time-dependent changes in the DRIFT spectrum are recorded after the flow has been switched from H2 to a C 0 2 / H 2 hydrogenation mixture (1 :4 VIV).
3500
3000
2500
ZOO0
1500
1000
ravenumber / cm-l Figure 5. Reactions of species accumulated on the surface of a Ni(64 atom %)/zirconia catalyst under C 0 2 hydrogenation conditions. The hydrogenation of the surface specie is being studied after the flow has been switched from C 0 2 / H 2 (1:4) to pure H2, at p = 3 bar and T = 383 K.
experiment shown in Figure 4, and the C02/H2atmosphere was replaced by pure hydrogen under static conditions (p = 3 bar). The reactions occurring as a function of time are documented in Figure 5. This set of spectra shows that methane is being formed while the formate signals are decreasing. This behavior is in qualitative agreement with corresponding observations on palladium/zirconia cataly~ts,)~.'~ where surface formate was identified as the immediate precursor of the methane product. The results of a complementary experiment, in which the Ni/Zr02 catalyst has been exposed to a carbon monoxidelhydrogen reaction mixture, are presented in Figure 6. At low temperatures, CO is adsorbed in a singly bound geometry, as seen by the sharp signal at 2060 cm-' in the lowest trace of Figure 6. Upon raising of temperature, the 2060-cm-' peak decreases, and weak signals from gaseous water are observed (Figure 6, second trace from bottom). Apparently, water is produced from CO and H2 by CO dissociation, which should be accompanied by the formation of some carbon surface species. Note that there are no significant signals in the C-H stretching region (2800-3000 cm-') under these conditions, which implies that the relevant carbon species are not hydrogenated prior to C 4 bond breaking. The spectrum taken at a temperature of 403 K corresponding to reaction conditions (Figure 6,second trace from top) reveals that the CO dissociation rate has increased, as evidenced by the strong water signals. The gaseous hydrogenation products of the carbonaceous species formed upon CO dissociation are identified as saturated hydrocarbons, C,Hm2 ( n > I ) , from the doublet observed at 2970 and 2890 cm-'. Methane production occurs only at higher temperatures (423 K). At this temperature, C02is also present (Figure 6,top trace), resulting from the oxidation of the CO reactant in the water gas shift reaction occurring on the surface. This observation is confirmed in an analogous experiment with the catalyst from amorphous NiuZrw Under CO hydrogenation (35) Schild, C.; Wokaun, A.; Baiker, A. J . Mol. Carol., in press.
The Journal of Physical Chemistry. Vol. 95, NO. 16, 1991 6345
COz Hydrogenation over Nickel/Zirconia Catalysts
co
423 Y
403 K
I
1
so
2
0.23
Q
2
:
363 K
d
363 K
3 10
3000
2500
2000
1500
1 IO
ravenumber / cm-l Figure 6. Temperature dependence of CO hydrogenation reactions over a Ni(64 atom %)/zirconia catalyst prepared by coprecipitation. The surface is exposed to a static CO/H2 (1:3) mixture at p = 3 bar. DRIFT' spectra are recorded while the temperature is being raised from 363 to 423 K.
conditions, the alloy-derived catalyst exhibits a similar behavior as the coprecipitated Ni/ZrOz catalyst, with singly bound adsorbed C O as the pivotal surface intermediate. However, over the NiuZrW catalyst the C O oxidation yielding COS is observed to occur already at -363 K,i.e., at markedly lower temperatures than with Ni/Zr02. As a consequence, the CH4/C,Hb;+2 selectivity ratio is significantly shifted toward methane over activated Ni,Zr36, at temperatures corresponding to hydrogenation conditions. Discussion
The catalytic tests showed no measurable activity for the amorphous NiuZr36 alloy due to its low intrinsic surface area, which is usually covered with an inactive zirconia layer.'* Upon exposure to COz hydrogenation conditions, the massive alloy slowly transforms into a microporous solid containing metallic nickel particles embedded in a zirconium dioxide matrix. Crucial reactions occurring during the transformation are the oxidation of zirconium to zirconium dioxide by COz and the crystallization of metallic nickel particles. Note that the amorphous NiuZr36 undergoes crystallization under an inert atmosphere only at a much higher temperature of ca.810 K.I9 The oxidation of the zirconium by COS destabilizes the alloy and leads to phase separation. Similar findings have been reported by Maciejewski and Baiker,I9 who investigated the oxidation behavior of an amorphous NiuZr36 in air. Oxidized samples also contained metallic nickel particles, which were well dispersed in an oxidic matrix consisting o f mainly tetragonal and little monoclinic zirconia. Shimogaki et aLzzinvestigated the activation mechanism of amorphous Ni,Zrlmx alloys during CO/Hz reaction. They showed that the activity increase during the activation process was mainly caused by the formation of a porous structure within the amorphous bulk alloy. It is interesting that the two differently prepared Ni/ZrOz catalysts exhibit an almost identical catalytic behavior; both are good CO2 methanation catalysts and reveal about equal activation energies. The latter observation indicates that the reaction
CnHZn+ 2
Figure 7. Proposed reaction scheme for the hydrogenation of CO and C02Over nickel/zirconia catalysts; for details, see the text. (Dashed lima indicate reactions that are thermodynamically feasible, but are not observed under the present conditions.)
mechanism is probably the same and that the active sites are similar on both catalysts. This result is remarkable in the light of the vastly different structural properties of both systems. The performance of the catalysts for carbon dioxide methanation does not appear to be significantly influenced by the chemical and structural properties of the zirconia matrix. Further support for the relatively weak influence of the support emerges from the fact that the activation energies found in this work agree well with previous studies, which reported 89 & 3 kJ mol-' for Ni/SiOSl2 and 106 kJ mol-' for N i l y - a l ~ m i n a . ~ ~ The observed correlations and conversions between the reactants, products, and surface species during COz and C O hydrogenations over our nickel/zirconia catalysts will be discussed by referring to a proposed reaction scheme, which is presented in Figure 7; the observed sequence of events described above is represented by the solid arrows. Concerning the reactants, COS and CO, there are no apparent signs of an interconversion of the two species according to the reverse water gas shift/water gas shift (RWGS/WGS) equilib rium, as was observed with palladium/Zr02 and copper/zirconia catalyst^.^^,^^ During COS hydrogenations over Ni/ZrOz, only adsorbed CO is detected, which is dissociated rather than being desorbed from the surface. In contrast, on Pd/ZrOz catalysts the carbon monoxide produced by the RWGS reaction remains bound to the surface without dissociation. Over Au/ZrOz, indications have been obtained for a 'basic variant" of the RWGS reaction occurring on the surface; Le., gaseous CO is produced from C o t and hydrogen." [In the experiments reported in this study (Figure 4), the presence of surface carbonate bound in the bidentate geometryMis established unambiguously from the bands observed at 1650 and 1300 cm-' (cf. Table I); however, carbonate appears not to participate significantly in the hydrogenation reactions.)r3s] The reverse reaction from CO to COS does however occur over our nickel/zirconia catalysts at elevated temperatures: C O is reacting with water (most likely formed upon CO dissociation) to yield C 0 2 and hydrogen (WGS reaction, curved arrow in Figure 7). In this context it is interesting to note that Barrault et al.)' have recently suggested an intermediacy of formate in the RWGS/ WGS equilibrium. The authors reported" both an "associative* mechanism for the CO HzO reaction involving surface formate and a "regenerativen mechanism in which CO is oxidized by oxygen from the dissociation of water. The latter reaction pathway is assigned to catalysts containing surface cations capable of changing their oxidation state and is likely to prevail on our nickel/zirconia systems. As for the observed adsorption characteristics of the reactants, Le., formate formation from CO2/H2and dissociation of CO/Hz, there is excellent agreement with the results obtained during in situ activation of amorphous Ni-Zr alloys.3* In a CO/H2 atmosphere, large amounts of surface carbon were formed, whereas almost no surface carbon at all resulted from the activation in C02/H2. A dissociative CO/H2 adsorption mechanism is also generally accepted in the literature.'+ From the present results, CO dissociation proceeds, most likely, from singly bound adsorbed
+
~~~
~
(36) Herwijnen, T. V.; van Dasburg, H.; de Jong, W.A. J. Card. 1973, 28, 39 1 . (37) Barrault, J.; Alouche, A. Appl. Coral. 1990, 58, 255. (38) De Pietro, J. Ph.D. Thesis No. 8936, Zurich, 1989.
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J. Phys. Chem. 1991,95,6346-6351 in C 0 2hydrogenations with nickel/zirconia catalysts is interpreted in terms of a nondissociative mechanism, with formate as the pivotal surface intermediate (Figure 7, left side). In addition, the C 0 2 / H 2 reaction to formate supports our findings with other zirconia-based catalyst^.^^-^^ Recent adsorption experimentsMhave indicated that at least two formate species can be distinguished, which are adsorbed on different sites on the zirconia matrix. When asserting the intermediacy of formate on the route to methane, it is important to discuss the presence or absence of other reaction pathways. The experiments in which a catalyst covered with surface formate species is exposed to hydrogen (Figure 5) are informative in this respect. Note that, in an analogous experiment over Pd/Zr02,32we did not observe intermediate formation of gaseous C 0 2 . In the present case, however, C02(g) is detected (Figure 5); therefore it cannot be excluded that gaseous carbon dioxide, released from surface formate, reacts to methane on a further reaction pathway (Figure 7, curved arrow connecting C 0 2 and CHI). However, we are favoring an analogous interpretation as with the other catalysts mentioned above: formate is suggested to represent the surface intermediate in carbon dioxide hydrogenation to methane. The occurrence of C02(g) (as observed in Figure 5) is due to desorption starting from formate, by the reverse reaction of the adsorption process. For further surface reactions of carbon dioxide and hydrogen, the role of lattice anion vacancies must be taken into account. These vacancies are also involved in the interpretation of current results on palladium/zirconia methanation catalysts, where a similar behavior has been observed. These findings are analyzed in terms of a reaction scheme in which the methanation of formate involving anion vacancies is discussed, as reported in more detail at another place.35
C O (b = 2060 cm-I), which represents the only detected surface species in CO/H2 reactions, prior to hydrogenation of the resulting carbonaceous species. Singly bound CO is observed to be produced in two ways, i.e., from gaseous C O and from doubly bound C O (b = 1900 cm-l), which appears to be equilibrated with surface formate. For comparison, we note that for the respective adsorbates on Ni( 111) surfaces, experimental vibrational frequencies of 2020-2060 and 1900-1 960 cm-' have been reported in ref 3 1. In a recent theoretical study,39 ranges of 2040-2062 and 1842-1856 cm-' have been calculated for singly and doubly bound CO on Ni( 100) surfaces. For the products arising from CO hydrogenations over supported nickel catalysts, hydrocarbons with methane as the main component are generally reported. On our coprecipitated Ni/Zr02 catalyst, mainly higher hydrocarbons are produced when starting from CO/H2 (Figure 7,right-hand side), and methane appears to be a side product. Note, however, that methane is the predominantly formed product if CO, is present in the reaction mixture. In contrast, over the Ni,Zrw-derived catalyst the preferential production of methane is also observed upon exposure to CO/H2; with this system, large amounts of C02 are formed by the WGS reaction. From the correlation between C02 formation and methane production, carbon dioxide may be assigned as the likely precursor of the methane product, although this assignment cannot be made unambiguously from the present results. In the following paragraphs, the origins of the exclusive production of methane when starting from C02 and hydrogen will be discussed. A similar behavior observed by Barrault et al." in C 0 2 / H 2 reactions over Ni/La203 and Ni/Ce203 catalysts was explained by assuming a specific environment for carbon species arising from C 0 2 dissociation, different from those produced during CO hydrogenation. From out findings, the CH4 selectivity
Acknowledgment. Financial support by the Deutsche Forschungsgemeinschaft and the Schweizerische Bundesamt fIir Energiewirtschaft is gratefully acknowledged. One of us (C.S.) is indebted to the Fonds der Chemischen Industrie for providing a graduate research fellowship.
(39) Maruca, R.; Kusuma, T.; Hick, V.; Companion, A. Surf.Sci. 1990, 236,2 10.
X-ray Diffraction, Electron Paramagnetic Resonance, and Electron Spin Echo Moduiatlon Studles of the PbO-PbCi,-CuCi, Ternary Glass System P. Raghumthan* and S. C. Sivasubramanian Department of Chemistry, Indian Institute of Technology, Kanpur 208016, India (Received: August 27, 1990; In Final Form: January 10, 1991) X-ray diffraction, EPR, and ESEM studies are reported for a novel ternary glass 43PMF56PbCl2-1CuClI. The X-ray PDF data lead to a structural model in which octahedral building units of Cu04C12are predominant. EPR spectra of the glass at X and Q bands have been fitted, by line shape simulation, to the distributed spin Hamiltonian parameters gr(mean) = 2.34, uII= 0.03, g,(mean) = 2.06, uL = 0.008, p = 0.95, Aw(mean)= 131.1 X 10-4 cm-l, and A,(mean) = 5.8 X 10" cm-I. These data suggest a distribution of bonding geometries for Cu2+in the glass structure, with the above-mentioned elongated octahedral CuO4CI2dominating. ESEM results suggest that Cuz+may be surrounded by four Pb2+in the second coordination shell at a distance of 3.8 A.
Introduction Lead oxide, PbO, is known to facilitate glass formation, and glasses formed by mixtures of PbO and the more ionic halides PbC12 and PbF2 have lately aroused much structural From the technological viewpoint, not only are lead glasses exploited as radiation shields, but the halide glasses are becoming (1) Rao, B. G.; Rao, K. J. Phys. Chem. Glasses 1984, 25, 1 1 . (2) Rao, K. J.; Wong,J.; Rao, B. G. Phys. Chem. Glasses 1984. 25, 57. (3) Rao, K. J.; Rao, 8. G.; Elliott, S. R. J . Mater. Scl. 1985, 20, 1678. (4) Reo, B.G.; Rao, K. J. Chem. Phys. 1986, 102, 121. ( 5 ) Rao, K. J.; Rao, B. 0.;Wong, J . J . Chem. Soc., Faraday Trans. I 1988.89. 1779.
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increasingly important because of their potential use in infrared optical components and ultra-low-loss optical fibers.6 Also, inorganic glasses containing paramagnetic ions often display further interesting optical and electronic properties. The challenge of structure-property correlations in such glasses necessitates their structural characterization to the finest possible detail. In recent years, much interest has been shown in understanding the structures of inorganic glasses in terms of general models based on a network of close-packed ionic spheres. For example, studies (6) Almeida, R.M. HalIde Glassesf w Infrared and Flberoptics; Martinus Nijhoff Publishers: Boston, 1987.
(33 1991 American Chemical Society
