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Hydrogenation of carbon dioxide over copper—zirconia catalysts prepared by in-situ activation of amorphous copper—zirconium alloy
Applied Catalysis, 48 (1989) 279-294 Elsevier Science Publishers B.V., Amsterdam -
279
Printed in The Netherlands
Hydrogenation of Carbon Dioxide over CopperZirconia Catalysts Prepared by In-Situ Activation of Amorphous Copper-Zirconium Alloy DANIEL GASSER and ALFONS BAIKER* Department of Industrial and Engineering Chemistry, Swiss Federal Institute of Technology, ETH - Zentrum, CH-8092 Ziirich (Switzerland) (Received 11 July 1988, revised manuscript received 17 November 1988)
ABSTRACT Amorphous Cu,Zr, has been applied as precursor for the preparation of copper on zirconia catalysts used for carbon dioxide hydrogenation. The catalysts were prepared by in situ activation of the precursor in CO,/H, and CO/H, reactant gas mixtures. During this activation, the initially low activity of the amorphous alloy increased largely and reached finally a steady state. Structural and chemical changes of the amorphous precursor during its transition to the active stable catalysts were investigated using gas adsorption, X-ray diffraction, N,O-titration, Auger electron spectroscopy and scanning electron microscopy. The as-prepared catalysts consisted of copper predominantly present as metal particles and stabilized by amorphous zirconium dioxide. XRD indicated that the metallic copper particles existed in two forms, as small, probably disordered particles and as larger crystalline particles. The most important changes the amorphous Cu-Zr alloy underwent during in-situ activation were: (i) oxidation of zirconium to zirconium oxides; (ii) segregation of copper from the bulk onto the surface; and, (iii) partial crystallization. The activity and the selectivity behaviour of the Cu/ZrO, derived from the amorphous Cu7Zr, alloy were found to be similar as those of Cu/ZrO, prepared by coprecipitation of the corresponding metal nitrates. This suggests that the copper sites active for carbon dioxide hydrogenation are in both preparations of similar nature.
INTRODUCTION
Amorphous metal alloys prepared by rapid quenching techniques [l] have gained attention as novel catalyst materials. In the beginning, main attention was paid to the investigation of the intrinsic catalytic properties of these materials [2,3]. However, the small surface area, the low thermal stability, and the formation of stable oxide layers during the preparation were found to be serious limitations for the practical application of as-prepared amorphous ribbons in catalysis. More recently, researchers pointed out the potential of amorphous metal alloys as catalyst precursors [ 4-101. It was shown that the catalvic properties
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0 1989 Elsevier Science Publishers B.V.
280
of amorphous alloys could be improved significantly by pretreatment procedures which modify the chemical and textural properties of the surface of the precursor materials. In the past few years the hydrogenation of carbon monoxide over amorphous alloys has been the subject of numerous studies [ 11-191. Most of these studies were carried out on zirconium containing alloys. Yokoyama et al. [ 161 investigated the carbon monoxide hydrogenation over amorphous Pd,Zr, at atmospheric pressure. They prepared a highly active methanation catalyst by in-situ activation of amorphous Pd,Zr,. Shimogaki et al. [ 171 described the preparation of a highly active methanation catalyst, starting from amorphous NizZr,. Shibata et al. [18] carried out carbon monoxide hydrogenation over various amorphous transition metal-zirconium alloys at 6 bar. Carbon monoxide was found to react hardly over amorphous alloys below 600 K, exceptions were AulZrB and PdlZra. The main product over Au,Zr, was methane, while over Pd,Zr, the formation of methanol was prevalent. The authors found that Au,Zr3 was oxidized to metallic gold and ZrOz, while Pd,Zr, was transformed into a weakly bound Pd-0-Zr type complex oxide and Zr02 under reaction condition. In an other study Shibata et al. [ 191 reported the preparation of a highly active catalyst for methanol synthesis from an amorphous CuTZrsalloy. The selectivity to methanol over the catalyst prepared from the amorphous alloy was higher than over the alloy crystallized by annealing at 800 K in vacuum. The amorphous CqZr, alloy was transformed into metallic copper and a ZrO, phase during reaction. The as-prepared catalyst had a large surface area and contained metallic copper particles dispersed in the ZrOa phase. The use of crystalline intermetallic compounds as catalyst precursors for carbon monoxide hydrogenation has been reported by Owen et al. [ 201. Activation of copper-lanthanide intermetallic precursors in CO/H2 at pressures up to 50 bar and temperatures in the range 373-533 K led to catalysts containing crystallites of metallic copper and the corresponding lanthanide oxide, Among these precursors, cerium-copper compounds were found to exhibit a methanol synthesis activity comparable to the one of presently used commercial catalysts. Similar work with amorphous copper-lanthanide alloys [ 211 has shown that such amorphous alloys are far less effective as catalyst precursors than the corresponding crystalline alloys, which stands in contrast to many reports [l191 where the use of amorphous materials has been found to result in considerable improvements in catalyst performance. In-situ XRD observations and concurrent measurements of methanol activity were performed by Nix et al. [ 221 to study the activation of crystalline Nd/ Cu and Ce/Cu alloy precursors for methanol synthesis catalysts. They found that the formation of intermetallic hydride intermediates is crucial to the eventual production of highly active catalysts and that methanol activity does not correlate with copper crystallite size.
281
Methanol synthesis from carbon dioxide and hydrogen over copper supported on zirconia has recently been studied by Amenomiya [ 231 and Pommier and Teichner [24]. In both studies copper on zirconia was found to exhibit high activity and selectivity for methanol formation. Little attention has been paid so far to the use of amorphous metal alloys and intermetallic compounds for the hydrogenation of carbon dioxide. In the present work we report the catalytic behaviour of copper on zirconia catalysts prepared by in-situ activation of an amorphous Cu,Zr, alloy in the hydrogenation of carbon dioxide. The catalytic behaviour of as prepared catalysts is compared to that of a coprecipitated Cu/ZrO, catalyst. EXPERIMENTAL
Catalyst
The amorphous CuTZr, used as catalyst precursor was prepared from the pre-mixed melts of the pure metals using the technique of melt spinning. For use in catalytic tests, the 5 mm wide and 20-30 pm thick ribbons produced by melt spinning were ground to flakes of 0.5-l mm size under liquid nitrogen. The BET surface area of this material, as measured by krypton adsorption at 77 K, amounted to 0.28 m’/g. X-ray diffraction indicated that the bulk of the ground material was still amorphous after grinding. A conventionally prepared Cu/ZrO, catalyst containing the same amount of copper was used as reference. This catalyst was prepared by coprecipitation of the corresponding metal nitrates. Aqueous ammonium carbonate solution (1 M) was slowly added to the nitrate solutions at 338 K until the pH-value reached eight, Subsequently the solution was stirred at 338 K for one hour. Then the precipitate was allowed to settle overnight, before it was filtered and thoroughly washed with deionized water. Finally, the precipitate was calcined in air for two hours at 623 K and crushed to a powder. The grain size used for the catalytic tests was 50-150 pm. Catalyst characterization
The catalysts were characterized by means of gas adsorption (nitrogen and krypton), nitrous oxide titration, X-ray diffraction (XRD), Auger electron spectroscopy ( AES ) and scanning electron microscopy (SEM ) . The adsorption measurements were performed in an apparatus especially designed for the volumetric determination of adsorption isotherms and equipped with a high precision pressure gauge (Ruska DDR 6000) with an accuracy of 0.2 Pa. BET surface areas of the precursor materials and the active catalysts were measured by krypton or nitrogen adsorption at 77 K, using a cross-sectional area of 19.5 A” [ 251 for the krypton atom and 16.3 A2 for nitrogen.
282
Nitrous oxide titration measurements were carried out to evaluate the number of surface copper atoms. Before nitrous oxide titration, samples were reduced in pure hydrogen at 523 K for 15 h. The copper surface was estimated assuming a cross-section area of 6.8~10Wzom2 [ 261 for the copper atom. X-ray diffraction was used for qualitative phase analysis. Mean crystallite sizes were estimated from the peak width at half maximum using the Scherrer equation. X-ray diffraction patterns were obtained using a powder diffractometer and Cu Kcu radiation. Scanning electron microscopy was used to investigate the grain morphology of the samples prepared. Elemental microanalysis was performed with an EDAX System.
Catalytic tests Activity and selectivity measurements were carried out at a pressure of 15 bar in a continuous tubular fixed-bed reactor. The stainless steel reactor tube was 30 cm long and had an I.D. of 1 cm. The reactor tube was installed in the oven of a gas chromatograph for temperature control. The temperature in the catalyst bed was measured by a thermocouple which was inserted into the centre of the bed. The temperature could be controlled within 2 0.5 K. The reactant gas mixture of carbon dioxide and hydrogen (CO, (99,9% ) : H2 (99,999% ) = 1:3) and of carbon monoxide and hydrogen (CO (99,99% ) : H2 (99,999% ) = 1:2) were taken from commercial gas cylinders without further purification. Nitrogen (99,995% ) was used as reference gas. Feed gases were dosed to the reactor by electronic mass flow controllers (Brooks). The total pressure was regulated by a back pressure regulator. To avoid condensation of reactants and products, the tubing between the reactor outlet and the automatic gas sampling valve was heated to 473 K. The product gas mixtures were analyzed using a gas chromatograph (HP 5890 A), equipped with a column (5 m, l/8 in. i.d.) packed with Poropak QS (80-100 mesh). Samples of the reactants and products were injected into the gas chromatograph via an eight-port gas sampling valve, Calibrations were made for each component and the molar fractions of the components were calculated from the corresponding peak areas. The sum of the molar fractions obtained by the GC analysis agreed within 5% with the expected theoretical value of 1. The experiments were conducted under the following standard conditions: amount of catalyst, 1.2 g for in situ activation, 0.3 g for kinetic measurements; feed rates of reactants: (COz, 2.3 pmol/s; Hz, 7.6 pmol/s; Nz, 1.2 pmol/s); (CO, 3.3 pmol/s; Hz, 6.6 pmol/s; NZ, 1.2 pmol/s); total pressure, 15 bar. Conversion and selectivity were defined as follows:
283
Conversion [ % ] =
moles CO, reacted moles CO, fed to the reactor
Selectivity (i) [ % ] =
. 100
moles of component (i) formed ’100 moles CO, reacted
RESULTS
Thermodynamics Equilibrium carbon dioxide conversion were estimated using the subsequent thermodynamic calculations and data from literature refs. 27 and 28. In previous studies [ 23, 29-311, it has been suggested that methanol is formed directly from carbon dioxide and the reverse water-gas shift reaction takes place in parallel. Thus the reaction CO+2Hz +CH,OH, where carbon monoxide is originating from the reverse water-gas shift was assumed to be negligible and the relevant equilibria considered were: COz+3Ha 3 CO,+H,
”
CH30H + H,O CO+H,O
(1) (2)
with: Y(H,O).Y(CH,OH) K1(Y)=
Y(C02).Y(H2)3
(,)=WW)+‘(W
K
2
(3) (4)
Y(CO,).Y(H,)
where Y(i) represents the molar fraction of species (i). The equilibrium constants Ki of the two reactions calculated for the temperature range 400 K-600 K and 15 bar total pressure are: K, (400 K) =2.91*10W1; Ki (500 K) = 1.01*10-2; K1 (600 K) =9.13s10-4; &(400 K) =6.37.10-4; I&(500 K)=7.16*10-3; K2 (600 K) =3.50.10-2. The equilibrium compositions expected for the hydrogenation of carbon dioxide under the conditions used were calculated using eqns. (3) and (4). Considering the mass balance for each species, Y(i) in the above equations can be expressed in terms of
(5)
(6) Eqns. (5 ) and (6 ) were solved numerically. Fig. 1 shows the calculated equilibrium concentration of methanol, carbon dioxide, carbon monoxide, hydrogen and water as a function of temperature assuming that reactions (1) and (2) are equilibrated under reaction conditions (15 bar, hydrogen-to-carbon dioxide ratio = 3). Note that the equilibrium yield of methanol decreases with increasing temperature due to an increase in the reverse water-gas shift reaction. Changes of amorphous precursors during in situ activation The amorphous Cu7Zr3 flakes used as catalyst precursor exhibited a broad intensity maximum in the XRD (Fig. 2a) typical for materials without any long-range ordering of the constituents. Further support for the amorphous nature of the precursor alloy came from measurement of the heat of crystallization using differential scanning calorimetry. These measurements which were performed in a nitrogen atmosphere with a heating rate of 10 K/min showed
500
temperature
600
[K]
Fig. 1. Temperature dependence of methanol, carbon monoxide, carbon dioxide and water concentration at equilibrium. Total pressure 15 atm, HJCO, = 3.
285
I
20
30
I
L
40
50
I
60
I
70
I
28
Fig. 2. X-ray diffraction patterns (Cu Ka) of (a) the amorphous Cu,Zr, precursor and (b) the active catalyst Cu/Zr02 prepared by in-situ activation of amorphous Cu,Zr:, at 553 K (in CO,/ HZ).
that crystallization occurred at about 770 K, which is in good agreement with earlier investigations [ 32 1. Fig. 3 depicts the changes in the catalytic behaviour of the amorphous precursor during its transformation to the final catalyst at different conditions of in-situ activation. The activation in C02/H2 at 553 K (Fig. 3A) showed high carbon dioxide conversion (80% ) in the initial period. However, thermodynamic calculations indicated that the observed carbon dioxide consumption could not be due to carbon dioxide conversion to methanol. For this reaction the maximum conversion is about 22% (dashed line in Fig. 3A) under the conditions given. Most of the carbon dioxide consumed in the initial period of activation was due to the oxidation of the precursor with carbon dioxide. Note that after the initial period of high carbon dioxide consumption, the carbon dioxide dropped to the calculated equilibrium value. The activity and selectivity reached a steady state after the precursor has been on stream for one hour. After reaching stable activity the major reaction products were carbon monoxide, methanol and water. Besides these products, small amounts of methane, dimethyl ether and ethanol were also found in the product mixing during the initial period of ac-
_---___T_____
c
a-
?
--o--o-o
____=.
0 I
0
1
I
8
B 493 K (C02/Hz) b 3
:
C
8 =-.
80-\
553 K (CO/Hz)
T---o-o-o-c
4
8
12
16
time on stream [h] Fig. 3. Change of activity of the amorphous Cu7Zr, alloy during in-situ activation. Carbon dioxide conversion, methanol and carbon monoxide selectivity is plotted versus time-on-stream. Conditions see Experimental part. Dashed lines indicate the calculated equilibrium conversion to methanol and carbon monoxide. (0 ) Conversion, ( 0 ) methanol, (H ) methane, ( V ) carbon monoxide, ( A ) ethanol, ( 0 ) carbon dioxide, (V ) dimethyl ether.
tivation. It is interesting to note that in the early stage of the in-situ activation no water was detected in the product stream. The water formed was presumably consumed for the oxidation of the precursor, according to Zr+2H,0+Zr0,+2Hz. Activation of t.he precursor under similar gas phase conditions, but lower temperature (Fig. 3B ) showed a significantly different behaviour. The initial carbon dioxide conversion was very low and increased steadily showing a maximum after about 2 h on stream. The steady-state conversion was slightly lower than the calculated equilibrium conversion (dashed line in Fig. 3B). In the initial period the selectivity to methanol passed through a maximum at about 80% and decreased then reaching a steady-state value of about 50%. Again the products formed under steady-state conditions were methanol, carbon mon-
287
oxide and water. Some ethanol was formed during the initial period of activation as a by-product. Fig. 3C shows the catalytic behaviour of the Cu,Zr3 precursor during in-situ activation in the CO/H2 gas mixture. In the initial period, the precursor was only little active and methane formation was prevalent. Methane production is assumed to arise predominantly from the removal of surface carbon which is deposited by carbon monoxide dissociation during zirconium oxidation. Methane formation stopped when the bulk of the zirconium was transformed completely to ZrO, as evidenced by X-ray diffraction. The active catalyst produced methanol, dimethyl ether, carbon dioxide and methane. Fig. 2. depicts the bulk structural changes the amorphous CuTZr, underwent during the in-situ activation procedures illustrated in Fig. 3A. The most striking change seen in the XRD patterns of the activated samples is the occurrence of reflections due to crystalline copper particles, and less well ordered zirconium dioxide. High-resolution scans of the prominent copper reflections indicated that copper was existing in two forms, well-developed crystalline particles and smaller, probably disordered particles giving rise to a significant broadening of the footings of the copper reflections in the XRD patterns. The larger particles had a mean size of about 25 nm as evidenced from the line broadening of the Cu (111) reflection. The fact that ZrO, reflections are only weakly seen in the XRD patterns indicates that this phase was predominantly amorphous. XRD patterns of all activated samples were of similar shape. After in-situ activation, the catalyst showed the colour of metallic copper. Depth concentration profiles of the precursor material measured by AES combined with argon ion sputtering before and after use in the catalytic tests indicated that the surface region (layer of about 250 A) of the amorphous precursor material contained already a substantial amount of oxygen probably in the form of ZrO*. This oxygen contamination originated from exposure to air. Depth profiles of the catalysts measured after steady-state activity had been attained indicated that copper segregated from the bulk to the surface during the in-situ activation. Similar behaviour has recently been shown for amorphous Cu,Zr, exposed to an oxygen containing hydrogen atmosphere at 473 K [33,34]. Electron microscopy (Fig. 4) showed that the bulk structural changes occurring during in situ activation led to a drastic change of the textural properties of the precursor. The initially relatively flat surface of the amorphous precursor was transformed into a very rough surface which was mainly made up of precipitated copper particles, as was evidenced by EDAX measurements. Further support for the roughening of the surface emerges from a comparison of the BET surface areas and copper metal surface areas measured for the precursor and the active catalysts (Table 1). The BET surface area of the precursor alloy was 0.28 m*/g after grinding in liquid nitrogen; during pretreat-
288
Fig. 4. Scanning electron micrographs showing the surface morphology of amorphous Cu,Zr, after in-situ activation in carbon dioxide hydrogenation at 553 K. Corresponding activity behaviour is plotted in Fig. 3A.
ment it increased to 46-79 m*/g depending on the conditions used. Simultaneously the copper metal surface area increased to several m2/g upon in-situ activation. Activation in a CO/H, gas mixtures seems to lead to larger copper metal surface areas. Pore size distributions were determined from the adsorption results using the desorption branch of the measured hysteresis and the method described by Pierce [ 351. These measurements indicated that the Cu/ ZrO, catalysts prepared from the amorphous Cu,Zr, precursor mainly contained micropores of about 4 nm mean diameter. Catalytic behaviour of catalysts
Preliminary experiments with respect to possible influences on the activity and selectivity caused by interparticle and intraparticle mass transfer limitations confirmed that such limitations could be ruled out under the conditions used in the catalytic tests. The results of the carbon dioxide hydrogenation experiments over the differently prepared catalysts are summarized in Table 1. They reflect the steady-state behaviour of the catalysts. We note that the
289 TABLE
1
Carbon dioxide hydrogenation on differently prepared catalysts
Cu/ZrO, catalysts preparedfrom
Surface Copper Temper- Conversion Selectivity Selectivity TOF area area ature to methanol to carbon overall (%) monoxide (ks- ’) W/g) W’/gI (K) (%10) (%)
Cu,Zr,, 493 K (COJHz)
46.8
5.0
493
4.8
483 473 463
3.3 2.4 1.7 1.2
453
Cu,Zr,, (COJHZ)
76.7
14.8
3.9 533
553 K
523 513 503
78.9
8.2
26.0
3.3
50.2 57.7 65.9
49.8 42.3 34.1
1.5
19.8 10.5
553 533
17.5
15.7
84.3
12.5
29.6
70.4
523 513 503
8.9 7.1
36.8 43.7
5.0 3.6
52.0 56.9 66.2
63.2 56.3 48.0
473 463
(Coprecipitated)
76.6 58.4
12.6
26.7
493 483
Cu/ZrOp
23.4 41.6
17.1 13.2
73.3
483 473
Cu-;Zr:> K (CO/H?)
4.1
36.3 28.5
80.2 89.5
493
553
9.1 7.0 5.2
63.7 71.5 83.0 86.8 87.4
3.0 2.1
2.5 1.8 1.2
74.6 79.7
13.5
26.0
523
8.1 6.3
513 503
4.4 3.4
50.7 62.4 72.7
493
2.5 1.7
553 533
483
80.7 87.8 98.0
2.9
2.0 1.4 1.0 0.7 11.9 7.3 5.7 4.2 3.3 2.4 1.7 1.2
33.8 25.4 20.3
7.1 5.0 3.6 2.8 2.0 1.5 1.0 0.7 0.5
73.4 49.3 37.6 27.3 19.3 12.2 2.0
12.7 7.7 6.0 4.1 3.2 2.4 1.6
43.1
activity and selectivity behaviour of the catalysts was similar. This emerges from a comparison of the conversions and product distributions of the catalysts at selected temperatures. The results indicate that carbon containing products were methanol and carbon monoxide only. Methane formation was negligible unless the reaction temperature was higher than 593 K. The selectivity to methanol decreased with increasing temperature due to enhanced formation of carbon monoxide.
290
The activities of the differently prepared Cu/Zr02 catalysts were also compared on the basis of measured turnover frequencies (TOF ). The turnover frequencies were calculated as molecules of product formed per copper surface atom and per second. Fig. 5 compares the kinetic results of the carbon dioxide hydrogenation performed on the different catalysts. Note the coincidence of the Arrhenius lines of the Cu/Zr02 catalysts prepared by in-situ activation of the amorphous Cu,Zr, in CO,/H, and of the coprecipitated Cu/Zr02 catalyst. It should be noted here that pure ZrO, prepared by calcination of zirconium hydroxide in air at 673 K did not exhibit significant activity under the reaction conditions used. As to the reaction scheme, the dependence of the product ratio on the contact time shown in Fig. 6 indicates that methanol was most likely formed directly from carbon dioxide and that the reverse water-gas shift reaction occurred as parallel reaction. This behaviour was observed with all Cu/Zr02 catalysts used in this work. In the case of consecutive reactions we would expect that the
Cu7Zr3
493 K (CO2/H2)
Cu7Zr3
553 K (CO/H2)
Cu/ZrOp
(coprecipitated)
1000/T
[K-l]
Fig. 5. Arrhenius plots of carbon dioxide hydrogenation rates (TOF) of Cu/ZrO, catalysts prepared by in-situ activation of amorphous Cu,Zr, and by coprecipitation.
291
0.01
0.03
0.05
contact time [min g/ml] Fig. 6. Influence of the contact time on the product ratio methanol-to-carbon monoxide Cu/Zr02 catalyst prepared by in-situ activation of amorphous Cu,Zr, at 553 K (in COJH,).
product ratio approaches either zero or infinity with decreasing contact time. Similar behaviour was found in earlier investigations [ 23, 29-311. DISCUSSION
The results presented in Fig. 3 indicate that the amorphous Cu-,Zr, precursor exhibits only little activity for the hydrogenation of carbon dioxide. A major reason for this behaviour is the very low surface area of the precursor material (0.28 m’/g) and its relatively low copper content in the surface and subsurface region as evidenced by AES depth profiling. The lower copper content in the surface region is most likely due to oxygen induced surface segregation of zirconium occurring upon exposure of the precursor to air. Similar behaviour has also been observed with other zirconium alloys [ 6,8,33]. The catalytic activity develops during exposure of the amorphous precursor to reaction conditions, which results in oxidation of the zirconium to zirconium dioxide by carbon dioxide (carbon monoxide), and probably also methanol and water produced in the reaction. This process is accompanied by conversion of massive alloy into porous high-surface-area catalysts containing a lot of highly dispersed copper crystallites. Similar behaviour has been reported by Shibata et al. [ 191 for amorphous Cu-Zr alloys exposed to carbon monoxide hydrogenation conditions. Nix et al, [ 221 pointed out the importance of the formation of intermediate hydrides in the production of highly active methanol synthesis catalysts from rare earth-copper alloys. Although intermediate hydride formation was not evidenced in this work, it is likely to play an important role in the transformation from the amorphous precursor to the active copper-zirconia catalyst. Hydride formation upon exposure of amorphous Cu,Zr, to a hydrogen atmosphere has recently been evidenced by combined use of TPR and XRD [ 341.
292
As to the catalytic behaviour of the catalysts, the measured turnover frequencies (Table 1, Fig. 5) indicate that copper in catalysts prepared by activation of the amorphous Cu-Zr precursor in CO,/H, gas mixture exhibits about the same activity as copper in Cu/ZrO, prepared by coprecipitation. The activity (TOF) of copper in the catalyst prepared by activation in CO/ H, gas mixture seems to be slightly lower. Note that the specific copper surface area of this catalyst (8.2 m2/g) is almost twice as large as the corresponding areas of the catalysts prepared in CO,/H, gas mixtures (4-5 m2/g ) . The reason for this behaviour is presently not understood. It may be due to uncertainties involved in the determination of the copper metal surface area of as-prepared catalysts. It has been pointed out by Nix et al. [ 221 that very small clusters of copper embedded within and coordinated to the oxide matrix might give a form of copper that is unreactive with respect to nitrous oxide decomposition. The fraction of such sites is likely to depend on the gas atmosphere to which the amorphous Cu,Zr, alloy is exposed. Different solid-state reactions contribute to the oxidation of zirconium to ZrOz, depending on whether CO/H2 or C02/ H, gas mixtures are used. It should be stressed that all catalysts exhibit similar selectivity behaviour which further supports that the nature of the active copper sites is similar in all catalyst preparations. Whether these sites are metallic or electron deficient copper cannot be answered by our investigation, since so far no efforts were undertaken to measure the surface composition of the catalysts in the working state. Recently, Pommier and Teichner [ 241 have proposed that copper is rather electron deficient in Cu/ZrO:! catalysts under carbon dioxide hydrogenation conditions.
CONCLUSIONS
Copper-on-zirconia catalysts were prepared by exposing amorphous Cu,Zr, to carbon dioxide and carbon monoxide hydrogenation conditions, respectively. Zirconium was oxidized to zirconium dioxide and copper segregated onto the surface under these conditions. As prepared catalysts contained two types of copper particles; small, probably disordered particles and larger crystalline particles. The carbon dioxide-hydrogenation activity of copper in such catalysts is comparable to that of copper in a Cu/ZrO, catalyst prepared by coprecipitation. The same holds for the selectivity behaviours of these catalysts. This indicates that the copper sites contributing to carbon dioxide hydrogenation activity are of similar nature in both catalyst preparations. The transformation of the amorphous Cu-Zr precursors to the active Cu/Zr02 catalysts was accompanied by a large increase (more than two orders of magnitude) in both the copper metal surface area an the BET surface area.
293
ACKNOWLEDGEMENTS
The authors thank Dr. D. Monti for his help with the nitrous oxide titration measurements and P. W8gli for the SEM investigations. Financial support by the “Swiss National Science Foundation” (Project Nr. 2000 - 5.002), “Schweizerischer Schulrat” and Lonza AG is kindly acknowledged.
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24 25 26 27 28 29 30 31 32 33 34 35
B. Pommier and S.J. Teichner, in M.J. Phillips and M. Ternan (Eds.), Proc. 9th. Int. Congr. Catal., Calgary, 1988, Vol. 2, Chem. Inst. Can., 1988, p. 610. S.J. Gregg and K.S.W. Sing, Surf. Colloid. Sci., 9 (1976) 254. J.R. Anderson and K.C. Pratt (Eds.), Introduction to Characterization and Testing of Catalysts, Academic Press, London, 1985, p. 268. D.R. Stull and H. Prophet (Eds.), JANAF Thermochemical Tables, Washington, DC., 1971. D.R. Stull, E.F. Westrum and G.C. Sinke (Eds.), The Chemical Thermodynamic of Organic Compounds, Wiley, New York, 1969, p. 422. A.Ya. Rozovskii, G.I. Lin, L.G. Liberov, E.V. Slivinskii, S.M. Loktev, Yu. B. Kakgan and A.N. Bashkirov, Kinet. Katal., 18 (1977) 691. G.C. Chinchen, P.J. Denny, D.G. Parker, G.D. Short, MS. Spencer, K.C. Waugh and D.A. Whan, ACS National Meeting, Prepr., Div. Fuel Chem., 29 (1984) 178. G.C. Chinchen, P.J. Denny, D.G. Parker, MS Spencer and D.A. Whan, Appl. Catal., 30 (1987) 333. E. Kneller, Y. Khan and U. Gorres, Z. Metallkde, 77 (1986) 152. F. Vanini, S. Biichler, Xin-nan Yu, M. Erbudak, L. Schlapbach and A. Baiker, Surf. Sci., 189/190 (1987)1117. A. Baiker, H. Baris, F. Vanini and M. Erbudak, in M.J. Phillips and M. Ternan (Eds.), Proc. 9th Int. Congr. Catal., Calgary, 1988, Vol. 4, Chem. Inst. Can., 1988, p. 1928. G. Pierce, J. Phys. Chem., 57 (1953) 149.
279
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Hydrogenation of Carbon Dioxide over CopperZirconia Catalysts Prepared by In-Situ Activation of Amorphous Copper-Zirconium Alloy DANIEL GASSER and ALFONS BAIKER* Department of Industrial and Engineering Chemistry, Swiss Federal Institute of Technology, ETH - Zentrum, CH-8092 Ziirich (Switzerland) (Received 11 July 1988, revised manuscript received 17 November 1988)
ABSTRACT Amorphous Cu,Zr, has been applied as precursor for the preparation of copper on zirconia catalysts used for carbon dioxide hydrogenation. The catalysts were prepared by in situ activation of the precursor in CO,/H, and CO/H, reactant gas mixtures. During this activation, the initially low activity of the amorphous alloy increased largely and reached finally a steady state. Structural and chemical changes of the amorphous precursor during its transition to the active stable catalysts were investigated using gas adsorption, X-ray diffraction, N,O-titration, Auger electron spectroscopy and scanning electron microscopy. The as-prepared catalysts consisted of copper predominantly present as metal particles and stabilized by amorphous zirconium dioxide. XRD indicated that the metallic copper particles existed in two forms, as small, probably disordered particles and as larger crystalline particles. The most important changes the amorphous Cu-Zr alloy underwent during in-situ activation were: (i) oxidation of zirconium to zirconium oxides; (ii) segregation of copper from the bulk onto the surface; and, (iii) partial crystallization. The activity and the selectivity behaviour of the Cu/ZrO, derived from the amorphous Cu7Zr, alloy were found to be similar as those of Cu/ZrO, prepared by coprecipitation of the corresponding metal nitrates. This suggests that the copper sites active for carbon dioxide hydrogenation are in both preparations of similar nature.
INTRODUCTION
Amorphous metal alloys prepared by rapid quenching techniques [l] have gained attention as novel catalyst materials. In the beginning, main attention was paid to the investigation of the intrinsic catalytic properties of these materials [2,3]. However, the small surface area, the low thermal stability, and the formation of stable oxide layers during the preparation were found to be serious limitations for the practical application of as-prepared amorphous ribbons in catalysis. More recently, researchers pointed out the potential of amorphous metal alloys as catalyst precursors [ 4-101. It was shown that the catalvic properties
0166-9834/89/$03.50
0 1989 Elsevier Science Publishers B.V.
280
of amorphous alloys could be improved significantly by pretreatment procedures which modify the chemical and textural properties of the surface of the precursor materials. In the past few years the hydrogenation of carbon monoxide over amorphous alloys has been the subject of numerous studies [ 11-191. Most of these studies were carried out on zirconium containing alloys. Yokoyama et al. [ 161 investigated the carbon monoxide hydrogenation over amorphous Pd,Zr, at atmospheric pressure. They prepared a highly active methanation catalyst by in-situ activation of amorphous Pd,Zr,. Shimogaki et al. [ 171 described the preparation of a highly active methanation catalyst, starting from amorphous NizZr,. Shibata et al. [18] carried out carbon monoxide hydrogenation over various amorphous transition metal-zirconium alloys at 6 bar. Carbon monoxide was found to react hardly over amorphous alloys below 600 K, exceptions were AulZrB and PdlZra. The main product over Au,Zr, was methane, while over Pd,Zr, the formation of methanol was prevalent. The authors found that Au,Zr3 was oxidized to metallic gold and ZrOz, while Pd,Zr, was transformed into a weakly bound Pd-0-Zr type complex oxide and Zr02 under reaction condition. In an other study Shibata et al. [ 191 reported the preparation of a highly active catalyst for methanol synthesis from an amorphous CuTZrsalloy. The selectivity to methanol over the catalyst prepared from the amorphous alloy was higher than over the alloy crystallized by annealing at 800 K in vacuum. The amorphous CqZr, alloy was transformed into metallic copper and a ZrO, phase during reaction. The as-prepared catalyst had a large surface area and contained metallic copper particles dispersed in the ZrOa phase. The use of crystalline intermetallic compounds as catalyst precursors for carbon monoxide hydrogenation has been reported by Owen et al. [ 201. Activation of copper-lanthanide intermetallic precursors in CO/H2 at pressures up to 50 bar and temperatures in the range 373-533 K led to catalysts containing crystallites of metallic copper and the corresponding lanthanide oxide, Among these precursors, cerium-copper compounds were found to exhibit a methanol synthesis activity comparable to the one of presently used commercial catalysts. Similar work with amorphous copper-lanthanide alloys [ 211 has shown that such amorphous alloys are far less effective as catalyst precursors than the corresponding crystalline alloys, which stands in contrast to many reports [l191 where the use of amorphous materials has been found to result in considerable improvements in catalyst performance. In-situ XRD observations and concurrent measurements of methanol activity were performed by Nix et al. [ 221 to study the activation of crystalline Nd/ Cu and Ce/Cu alloy precursors for methanol synthesis catalysts. They found that the formation of intermetallic hydride intermediates is crucial to the eventual production of highly active catalysts and that methanol activity does not correlate with copper crystallite size.
281
Methanol synthesis from carbon dioxide and hydrogen over copper supported on zirconia has recently been studied by Amenomiya [ 231 and Pommier and Teichner [24]. In both studies copper on zirconia was found to exhibit high activity and selectivity for methanol formation. Little attention has been paid so far to the use of amorphous metal alloys and intermetallic compounds for the hydrogenation of carbon dioxide. In the present work we report the catalytic behaviour of copper on zirconia catalysts prepared by in-situ activation of an amorphous Cu,Zr, alloy in the hydrogenation of carbon dioxide. The catalytic behaviour of as prepared catalysts is compared to that of a coprecipitated Cu/ZrO, catalyst. EXPERIMENTAL
Catalyst
The amorphous CuTZr, used as catalyst precursor was prepared from the pre-mixed melts of the pure metals using the technique of melt spinning. For use in catalytic tests, the 5 mm wide and 20-30 pm thick ribbons produced by melt spinning were ground to flakes of 0.5-l mm size under liquid nitrogen. The BET surface area of this material, as measured by krypton adsorption at 77 K, amounted to 0.28 m’/g. X-ray diffraction indicated that the bulk of the ground material was still amorphous after grinding. A conventionally prepared Cu/ZrO, catalyst containing the same amount of copper was used as reference. This catalyst was prepared by coprecipitation of the corresponding metal nitrates. Aqueous ammonium carbonate solution (1 M) was slowly added to the nitrate solutions at 338 K until the pH-value reached eight, Subsequently the solution was stirred at 338 K for one hour. Then the precipitate was allowed to settle overnight, before it was filtered and thoroughly washed with deionized water. Finally, the precipitate was calcined in air for two hours at 623 K and crushed to a powder. The grain size used for the catalytic tests was 50-150 pm. Catalyst characterization
The catalysts were characterized by means of gas adsorption (nitrogen and krypton), nitrous oxide titration, X-ray diffraction (XRD), Auger electron spectroscopy ( AES ) and scanning electron microscopy (SEM ) . The adsorption measurements were performed in an apparatus especially designed for the volumetric determination of adsorption isotherms and equipped with a high precision pressure gauge (Ruska DDR 6000) with an accuracy of 0.2 Pa. BET surface areas of the precursor materials and the active catalysts were measured by krypton or nitrogen adsorption at 77 K, using a cross-sectional area of 19.5 A” [ 251 for the krypton atom and 16.3 A2 for nitrogen.
282
Nitrous oxide titration measurements were carried out to evaluate the number of surface copper atoms. Before nitrous oxide titration, samples were reduced in pure hydrogen at 523 K for 15 h. The copper surface was estimated assuming a cross-section area of 6.8~10Wzom2 [ 261 for the copper atom. X-ray diffraction was used for qualitative phase analysis. Mean crystallite sizes were estimated from the peak width at half maximum using the Scherrer equation. X-ray diffraction patterns were obtained using a powder diffractometer and Cu Kcu radiation. Scanning electron microscopy was used to investigate the grain morphology of the samples prepared. Elemental microanalysis was performed with an EDAX System.
Catalytic tests Activity and selectivity measurements were carried out at a pressure of 15 bar in a continuous tubular fixed-bed reactor. The stainless steel reactor tube was 30 cm long and had an I.D. of 1 cm. The reactor tube was installed in the oven of a gas chromatograph for temperature control. The temperature in the catalyst bed was measured by a thermocouple which was inserted into the centre of the bed. The temperature could be controlled within 2 0.5 K. The reactant gas mixture of carbon dioxide and hydrogen (CO, (99,9% ) : H2 (99,999% ) = 1:3) and of carbon monoxide and hydrogen (CO (99,99% ) : H2 (99,999% ) = 1:2) were taken from commercial gas cylinders without further purification. Nitrogen (99,995% ) was used as reference gas. Feed gases were dosed to the reactor by electronic mass flow controllers (Brooks). The total pressure was regulated by a back pressure regulator. To avoid condensation of reactants and products, the tubing between the reactor outlet and the automatic gas sampling valve was heated to 473 K. The product gas mixtures were analyzed using a gas chromatograph (HP 5890 A), equipped with a column (5 m, l/8 in. i.d.) packed with Poropak QS (80-100 mesh). Samples of the reactants and products were injected into the gas chromatograph via an eight-port gas sampling valve, Calibrations were made for each component and the molar fractions of the components were calculated from the corresponding peak areas. The sum of the molar fractions obtained by the GC analysis agreed within 5% with the expected theoretical value of 1. The experiments were conducted under the following standard conditions: amount of catalyst, 1.2 g for in situ activation, 0.3 g for kinetic measurements; feed rates of reactants: (COz, 2.3 pmol/s; Hz, 7.6 pmol/s; Nz, 1.2 pmol/s); (CO, 3.3 pmol/s; Hz, 6.6 pmol/s; NZ, 1.2 pmol/s); total pressure, 15 bar. Conversion and selectivity were defined as follows:
283
Conversion [ % ] =
moles CO, reacted moles CO, fed to the reactor
Selectivity (i) [ % ] =
. 100
moles of component (i) formed ’100 moles CO, reacted
RESULTS
Thermodynamics Equilibrium carbon dioxide conversion were estimated using the subsequent thermodynamic calculations and data from literature refs. 27 and 28. In previous studies [ 23, 29-311, it has been suggested that methanol is formed directly from carbon dioxide and the reverse water-gas shift reaction takes place in parallel. Thus the reaction CO+2Hz +CH,OH, where carbon monoxide is originating from the reverse water-gas shift was assumed to be negligible and the relevant equilibria considered were: COz+3Ha 3 CO,+H,
”
CH30H + H,O CO+H,O
(1) (2)
with: Y(H,O).Y(CH,OH) K1(Y)=
Y(C02).Y(H2)3
(,)=WW)+‘(W
K
2
(3) (4)
Y(CO,).Y(H,)
where Y(i) represents the molar fraction of species (i). The equilibrium constants Ki of the two reactions calculated for the temperature range 400 K-600 K and 15 bar total pressure are: K, (400 K) =2.91*10W1; Ki (500 K) = 1.01*10-2; K1 (600 K) =9.13s10-4; &(400 K) =6.37.10-4; I&(500 K)=7.16*10-3; K2 (600 K) =3.50.10-2. The equilibrium compositions expected for the hydrogenation of carbon dioxide under the conditions used were calculated using eqns. (3) and (4). Considering the mass balance for each species, Y(i) in the above equations can be expressed in terms of
(5)
(6) Eqns. (5 ) and (6 ) were solved numerically. Fig. 1 shows the calculated equilibrium concentration of methanol, carbon dioxide, carbon monoxide, hydrogen and water as a function of temperature assuming that reactions (1) and (2) are equilibrated under reaction conditions (15 bar, hydrogen-to-carbon dioxide ratio = 3). Note that the equilibrium yield of methanol decreases with increasing temperature due to an increase in the reverse water-gas shift reaction. Changes of amorphous precursors during in situ activation The amorphous Cu7Zr3 flakes used as catalyst precursor exhibited a broad intensity maximum in the XRD (Fig. 2a) typical for materials without any long-range ordering of the constituents. Further support for the amorphous nature of the precursor alloy came from measurement of the heat of crystallization using differential scanning calorimetry. These measurements which were performed in a nitrogen atmosphere with a heating rate of 10 K/min showed
500
temperature
600
[K]
Fig. 1. Temperature dependence of methanol, carbon monoxide, carbon dioxide and water concentration at equilibrium. Total pressure 15 atm, HJCO, = 3.
285
I
20
30
I
L
40
50
I
60
I
70
I
28
Fig. 2. X-ray diffraction patterns (Cu Ka) of (a) the amorphous Cu,Zr, precursor and (b) the active catalyst Cu/Zr02 prepared by in-situ activation of amorphous Cu,Zr:, at 553 K (in CO,/ HZ).
that crystallization occurred at about 770 K, which is in good agreement with earlier investigations [ 32 1. Fig. 3 depicts the changes in the catalytic behaviour of the amorphous precursor during its transformation to the final catalyst at different conditions of in-situ activation. The activation in C02/H2 at 553 K (Fig. 3A) showed high carbon dioxide conversion (80% ) in the initial period. However, thermodynamic calculations indicated that the observed carbon dioxide consumption could not be due to carbon dioxide conversion to methanol. For this reaction the maximum conversion is about 22% (dashed line in Fig. 3A) under the conditions given. Most of the carbon dioxide consumed in the initial period of activation was due to the oxidation of the precursor with carbon dioxide. Note that after the initial period of high carbon dioxide consumption, the carbon dioxide dropped to the calculated equilibrium value. The activity and selectivity reached a steady state after the precursor has been on stream for one hour. After reaching stable activity the major reaction products were carbon monoxide, methanol and water. Besides these products, small amounts of methane, dimethyl ether and ethanol were also found in the product mixing during the initial period of ac-
_---___T_____
c
a-
?
--o--o-o
____=.
0 I
0
1
I
8
B 493 K (C02/Hz) b 3
:
C
8 =-.
80-\
553 K (CO/Hz)
T---o-o-o-c
4
8
12
16
time on stream [h] Fig. 3. Change of activity of the amorphous Cu7Zr, alloy during in-situ activation. Carbon dioxide conversion, methanol and carbon monoxide selectivity is plotted versus time-on-stream. Conditions see Experimental part. Dashed lines indicate the calculated equilibrium conversion to methanol and carbon monoxide. (0 ) Conversion, ( 0 ) methanol, (H ) methane, ( V ) carbon monoxide, ( A ) ethanol, ( 0 ) carbon dioxide, (V ) dimethyl ether.
tivation. It is interesting to note that in the early stage of the in-situ activation no water was detected in the product stream. The water formed was presumably consumed for the oxidation of the precursor, according to Zr+2H,0+Zr0,+2Hz. Activation of t.he precursor under similar gas phase conditions, but lower temperature (Fig. 3B ) showed a significantly different behaviour. The initial carbon dioxide conversion was very low and increased steadily showing a maximum after about 2 h on stream. The steady-state conversion was slightly lower than the calculated equilibrium conversion (dashed line in Fig. 3B). In the initial period the selectivity to methanol passed through a maximum at about 80% and decreased then reaching a steady-state value of about 50%. Again the products formed under steady-state conditions were methanol, carbon mon-
287
oxide and water. Some ethanol was formed during the initial period of activation as a by-product. Fig. 3C shows the catalytic behaviour of the Cu,Zr3 precursor during in-situ activation in the CO/H2 gas mixture. In the initial period, the precursor was only little active and methane formation was prevalent. Methane production is assumed to arise predominantly from the removal of surface carbon which is deposited by carbon monoxide dissociation during zirconium oxidation. Methane formation stopped when the bulk of the zirconium was transformed completely to ZrO, as evidenced by X-ray diffraction. The active catalyst produced methanol, dimethyl ether, carbon dioxide and methane. Fig. 2. depicts the bulk structural changes the amorphous CuTZr, underwent during the in-situ activation procedures illustrated in Fig. 3A. The most striking change seen in the XRD patterns of the activated samples is the occurrence of reflections due to crystalline copper particles, and less well ordered zirconium dioxide. High-resolution scans of the prominent copper reflections indicated that copper was existing in two forms, well-developed crystalline particles and smaller, probably disordered particles giving rise to a significant broadening of the footings of the copper reflections in the XRD patterns. The larger particles had a mean size of about 25 nm as evidenced from the line broadening of the Cu (111) reflection. The fact that ZrO, reflections are only weakly seen in the XRD patterns indicates that this phase was predominantly amorphous. XRD patterns of all activated samples were of similar shape. After in-situ activation, the catalyst showed the colour of metallic copper. Depth concentration profiles of the precursor material measured by AES combined with argon ion sputtering before and after use in the catalytic tests indicated that the surface region (layer of about 250 A) of the amorphous precursor material contained already a substantial amount of oxygen probably in the form of ZrO*. This oxygen contamination originated from exposure to air. Depth profiles of the catalysts measured after steady-state activity had been attained indicated that copper segregated from the bulk to the surface during the in-situ activation. Similar behaviour has recently been shown for amorphous Cu,Zr, exposed to an oxygen containing hydrogen atmosphere at 473 K [33,34]. Electron microscopy (Fig. 4) showed that the bulk structural changes occurring during in situ activation led to a drastic change of the textural properties of the precursor. The initially relatively flat surface of the amorphous precursor was transformed into a very rough surface which was mainly made up of precipitated copper particles, as was evidenced by EDAX measurements. Further support for the roughening of the surface emerges from a comparison of the BET surface areas and copper metal surface areas measured for the precursor and the active catalysts (Table 1). The BET surface area of the precursor alloy was 0.28 m*/g after grinding in liquid nitrogen; during pretreat-
288
Fig. 4. Scanning electron micrographs showing the surface morphology of amorphous Cu,Zr, after in-situ activation in carbon dioxide hydrogenation at 553 K. Corresponding activity behaviour is plotted in Fig. 3A.
ment it increased to 46-79 m*/g depending on the conditions used. Simultaneously the copper metal surface area increased to several m2/g upon in-situ activation. Activation in a CO/H, gas mixtures seems to lead to larger copper metal surface areas. Pore size distributions were determined from the adsorption results using the desorption branch of the measured hysteresis and the method described by Pierce [ 351. These measurements indicated that the Cu/ ZrO, catalysts prepared from the amorphous Cu,Zr, precursor mainly contained micropores of about 4 nm mean diameter. Catalytic behaviour of catalysts
Preliminary experiments with respect to possible influences on the activity and selectivity caused by interparticle and intraparticle mass transfer limitations confirmed that such limitations could be ruled out under the conditions used in the catalytic tests. The results of the carbon dioxide hydrogenation experiments over the differently prepared catalysts are summarized in Table 1. They reflect the steady-state behaviour of the catalysts. We note that the
289 TABLE
1
Carbon dioxide hydrogenation on differently prepared catalysts
Cu/ZrO, catalysts preparedfrom
Surface Copper Temper- Conversion Selectivity Selectivity TOF area area ature to methanol to carbon overall (%) monoxide (ks- ’) W/g) W’/gI (K) (%10) (%)
Cu,Zr,, 493 K (COJHz)
46.8
5.0
493
4.8
483 473 463
3.3 2.4 1.7 1.2
453
Cu,Zr,, (COJHZ)
76.7
14.8
3.9 533
553 K
523 513 503
78.9
8.2
26.0
3.3
50.2 57.7 65.9
49.8 42.3 34.1
1.5
19.8 10.5
553 533
17.5
15.7
84.3
12.5
29.6
70.4
523 513 503
8.9 7.1
36.8 43.7
5.0 3.6
52.0 56.9 66.2
63.2 56.3 48.0
473 463
(Coprecipitated)
76.6 58.4
12.6
26.7
493 483
Cu/ZrOp
23.4 41.6
17.1 13.2
73.3
483 473
Cu-;Zr:> K (CO/H?)
4.1
36.3 28.5
80.2 89.5
493
553
9.1 7.0 5.2
63.7 71.5 83.0 86.8 87.4
3.0 2.1
2.5 1.8 1.2
74.6 79.7
13.5
26.0
523
8.1 6.3
513 503
4.4 3.4
50.7 62.4 72.7
493
2.5 1.7
553 533
483
80.7 87.8 98.0
2.9
2.0 1.4 1.0 0.7 11.9 7.3 5.7 4.2 3.3 2.4 1.7 1.2
33.8 25.4 20.3
7.1 5.0 3.6 2.8 2.0 1.5 1.0 0.7 0.5
73.4 49.3 37.6 27.3 19.3 12.2 2.0
12.7 7.7 6.0 4.1 3.2 2.4 1.6
43.1
activity and selectivity behaviour of the catalysts was similar. This emerges from a comparison of the conversions and product distributions of the catalysts at selected temperatures. The results indicate that carbon containing products were methanol and carbon monoxide only. Methane formation was negligible unless the reaction temperature was higher than 593 K. The selectivity to methanol decreased with increasing temperature due to enhanced formation of carbon monoxide.
290
The activities of the differently prepared Cu/Zr02 catalysts were also compared on the basis of measured turnover frequencies (TOF ). The turnover frequencies were calculated as molecules of product formed per copper surface atom and per second. Fig. 5 compares the kinetic results of the carbon dioxide hydrogenation performed on the different catalysts. Note the coincidence of the Arrhenius lines of the Cu/Zr02 catalysts prepared by in-situ activation of the amorphous Cu,Zr, in CO,/H, and of the coprecipitated Cu/Zr02 catalyst. It should be noted here that pure ZrO, prepared by calcination of zirconium hydroxide in air at 673 K did not exhibit significant activity under the reaction conditions used. As to the reaction scheme, the dependence of the product ratio on the contact time shown in Fig. 6 indicates that methanol was most likely formed directly from carbon dioxide and that the reverse water-gas shift reaction occurred as parallel reaction. This behaviour was observed with all Cu/Zr02 catalysts used in this work. In the case of consecutive reactions we would expect that the
Cu7Zr3
493 K (CO2/H2)
Cu7Zr3
553 K (CO/H2)
Cu/ZrOp
(coprecipitated)
1000/T
[K-l]
Fig. 5. Arrhenius plots of carbon dioxide hydrogenation rates (TOF) of Cu/ZrO, catalysts prepared by in-situ activation of amorphous Cu,Zr, and by coprecipitation.
291
0.01
0.03
0.05
contact time [min g/ml] Fig. 6. Influence of the contact time on the product ratio methanol-to-carbon monoxide Cu/Zr02 catalyst prepared by in-situ activation of amorphous Cu,Zr, at 553 K (in COJH,).
product ratio approaches either zero or infinity with decreasing contact time. Similar behaviour was found in earlier investigations [ 23, 29-311. DISCUSSION
The results presented in Fig. 3 indicate that the amorphous Cu-,Zr, precursor exhibits only little activity for the hydrogenation of carbon dioxide. A major reason for this behaviour is the very low surface area of the precursor material (0.28 m’/g) and its relatively low copper content in the surface and subsurface region as evidenced by AES depth profiling. The lower copper content in the surface region is most likely due to oxygen induced surface segregation of zirconium occurring upon exposure of the precursor to air. Similar behaviour has also been observed with other zirconium alloys [ 6,8,33]. The catalytic activity develops during exposure of the amorphous precursor to reaction conditions, which results in oxidation of the zirconium to zirconium dioxide by carbon dioxide (carbon monoxide), and probably also methanol and water produced in the reaction. This process is accompanied by conversion of massive alloy into porous high-surface-area catalysts containing a lot of highly dispersed copper crystallites. Similar behaviour has been reported by Shibata et al. [ 191 for amorphous Cu-Zr alloys exposed to carbon monoxide hydrogenation conditions. Nix et al, [ 221 pointed out the importance of the formation of intermediate hydrides in the production of highly active methanol synthesis catalysts from rare earth-copper alloys. Although intermediate hydride formation was not evidenced in this work, it is likely to play an important role in the transformation from the amorphous precursor to the active copper-zirconia catalyst. Hydride formation upon exposure of amorphous Cu,Zr, to a hydrogen atmosphere has recently been evidenced by combined use of TPR and XRD [ 341.
292
As to the catalytic behaviour of the catalysts, the measured turnover frequencies (Table 1, Fig. 5) indicate that copper in catalysts prepared by activation of the amorphous Cu-Zr precursor in CO,/H, gas mixture exhibits about the same activity as copper in Cu/ZrO, prepared by coprecipitation. The activity (TOF) of copper in the catalyst prepared by activation in CO/ H, gas mixture seems to be slightly lower. Note that the specific copper surface area of this catalyst (8.2 m2/g) is almost twice as large as the corresponding areas of the catalysts prepared in CO,/H, gas mixtures (4-5 m2/g ) . The reason for this behaviour is presently not understood. It may be due to uncertainties involved in the determination of the copper metal surface area of as-prepared catalysts. It has been pointed out by Nix et al. [ 221 that very small clusters of copper embedded within and coordinated to the oxide matrix might give a form of copper that is unreactive with respect to nitrous oxide decomposition. The fraction of such sites is likely to depend on the gas atmosphere to which the amorphous Cu,Zr, alloy is exposed. Different solid-state reactions contribute to the oxidation of zirconium to ZrOz, depending on whether CO/H2 or C02/ H, gas mixtures are used. It should be stressed that all catalysts exhibit similar selectivity behaviour which further supports that the nature of the active copper sites is similar in all catalyst preparations. Whether these sites are metallic or electron deficient copper cannot be answered by our investigation, since so far no efforts were undertaken to measure the surface composition of the catalysts in the working state. Recently, Pommier and Teichner [ 241 have proposed that copper is rather electron deficient in Cu/ZrO:! catalysts under carbon dioxide hydrogenation conditions.
CONCLUSIONS
Copper-on-zirconia catalysts were prepared by exposing amorphous Cu,Zr, to carbon dioxide and carbon monoxide hydrogenation conditions, respectively. Zirconium was oxidized to zirconium dioxide and copper segregated onto the surface under these conditions. As prepared catalysts contained two types of copper particles; small, probably disordered particles and larger crystalline particles. The carbon dioxide-hydrogenation activity of copper in such catalysts is comparable to that of copper in a Cu/ZrO, catalyst prepared by coprecipitation. The same holds for the selectivity behaviours of these catalysts. This indicates that the copper sites contributing to carbon dioxide hydrogenation activity are of similar nature in both catalyst preparations. The transformation of the amorphous Cu-Zr precursors to the active Cu/Zr02 catalysts was accompanied by a large increase (more than two orders of magnitude) in both the copper metal surface area an the BET surface area.
293
ACKNOWLEDGEMENTS
The authors thank Dr. D. Monti for his help with the nitrous oxide titration measurements and P. W8gli for the SEM investigations. Financial support by the “Swiss National Science Foundation” (Project Nr. 2000 - 5.002), “Schweizerischer Schulrat” and Lonza AG is kindly acknowledged.
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