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Hydrogenation of CO2 over gold supported on metal oxides
Applied
Catalysis A: General,
Elsevier Science Publishers APCAT
102 (1993)
125
125-136
B.V., Amsterdam
A2566
Hydrogenation of CO, over gold supported on metal oxides Hiroaki Sakurai, Susumu Tsubota and Masatake Haruta Government
Zndustrial
Research
Institute
of Osaka, l-8-31
Midorigaoka,
Zkeda, 563 (Japan)
Received 18 January 1993, revised manuscript received 27 April 1993
Abstract Gold highly dispersed on a variety of metal oxides were prepared by coprecipitation and depositionprecipitation methods. The hydrogenation of CO2 on supported gold catalysts was investigated at temperatures between 150 and 400°C and a pressure of 8 atm. The methanol yields reached a maximum at temperatures between 200 and 300 ’ C, depending on the support oxides. The highest yield and selectivity towards methanol was obtained on Au/ZnO at 250°C. At 200°C Au/Fe,O, was the most active for methanol synthesis, exhibiting activity almost comparable to that of the conventional Cu/ZnO catalyst with the same metal content. Gold supported on TiO, was so active in reducing CO* to CO that the conversion was close to equilibrium. Over Au/ZnO as well as over Cu/ZnO, CO, could be hydrogenated to methanol at lower temperatures than CO. Key words: CO, hydrogenation;
methanol synthesis; support effect; supported gold catalysts
INTRODUCTION
Hydrogenation of CO, can be regarded as one of the options that can be taken through a chemical approach for the suppression of global warming [ 11. Since methanol is the most desirable product from the reaction, most of the previous work has been concerned with Cu/ZnO-based catalysts which are commercially used for methanol production from synthetic gases (CO + 2H2) containing CO, [l-12]. Supported precious metals, such as Pt, Re, Pd, and Rh have also been tested for the reaction [ 13-161. Supported gold catalysts, Au/ ThO, [17], Au/CeO, [ 181, Au/TiO, and Au/SiO, [19], have been investigated for methanol synthesis from CO. However, they have been reported to be relatively inactive compared with Cu-based catalysts. Recently, Baiker and coworkers [ 20,211 prepared Au/ZrO, from two types of amorphous precursors, i.e., Au-Zr alloy and a coprecipitate, and examined them for CO, hydrogenation. Although gold is a little less active than Cu and Correspondence to: Dr. H. Sakurai, Government Industrial Research Institute of Osaka, 1-8-31 Midorigaoka, Ikeda, 563 Japan. Tel. (+81-727)519656, fax. (+81-727)519630.
0926-860X/93/$06.00
0 1993 Elsevier Science Publishers B.V.
All rights reserved.
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Ag in methanol synthesis over ZrO,-supported catalysts, it has been proved that even gold can exhibit appreciable activity when properly deposited on ZrO,. We have previously reported that gold can be deposited with high dispersion on a variety of metal oxides by coprecipitation or by the deposition-precipitation method. Highly dispersed supported gold is extraordinarily active in CO oxidation at temperatures as low as - 70°C [ 22-241, and is markedly active for the CO-NO reaction and hydrocarbon combustion [25,26]. The catalytic properties were found to depend on the type of support oxides. The most suitable supports should be exploited for each different reaction. In this paper, we have studied the catalytic properties of gold supported on various oxides for CO, hydrogenation over a wide range of temperatures from 150 to 4OO”C, paying specific attention to activities of methanol synthesis and product selectivities. Other than stable oxides under hydrogen containing conditions like ZnO, reducible oxides such as Fe203, NiO, and Co,O, were also examined as supports. It was expected that these oxides would remain in a partially reduced state in the presence of C02, thereby providing effective supports. In addition, highly dispersed gold particles have been revealed to be relatively stable even after a partial reduction treatment of the oxide in the case of Au/Fe,O, [ 231. EXPERIMENTAL
Gold supported on ZnO, Fe203, ZrO,, La(OH),, NiO, and Co,O, was prepared by coprecipitation from a mixed aqueous solution of HAuCl, and metal nitrate (Au/ (support metal) = l/19 in atomic ratio) according to the procedure described elsewhere [ 22,231. This method provided highly dispersed gold particles with diameters smaller than 8 nm [ 22,231. Gold on TiO, (Au/Ti = l/ 99, the average particle size of gold is smaller than 5 nm) was prepared by the deposition-precipitation method by using JRC-TIO-4 (reference catalyst of Catalysis Society of Japan) as TiOz support [ 241. Gold on CeO, (Au/Cc = l/ 19) was also prepared by deposition-precipitation. The support was prepared from an aqueous solution of Ce ( NOs)3 by precipitation followed by calcination in air at 400°C for 4 h. For comparison, two samples of Cu/ZnO (Cu/Zn = l/ 19 and 3/7) were prepared by coprecipitation from mixed aqueous solutions of Cu ( NO3 ) 2 and Zn ( NO3 ) 2. The above catalyst precursors were finally calcined in air at 400°C for 4-5 h. In the case of Au/La(OH),, it was calcined at both 200°C and 400” C for 4 h in order to investigate the effect of hydroxide and oxide support. Specific surface areas of the prepared catalysts were determined by the BET single point method using a Quantasorb surface area analyzer. X-ray diffraction of each sample before and after the reaction was measured by a Rigaku Xray powder diffractometer.
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Catal. A 102 (1993) 125-136
127
The hydrogenation of CO2 was carried out by using a fixed-bed flow reactor using 1 ml of catalyst powder sieved between 16-40 mesh. After passing the pretreatment gas (1% Hz balanced with N2) at 250°C for 12 h at atmospheric pressure, the reactant gas mixture ( CO2 23.4%, H, 66.2%, Ar 10.4% ) was passed under a total gas pressure of 8 atm, SV of 3000 h-’ ml/g-cat., and at temperatures of 150, 200, 250, 300, 350 and 400°C. The temperature was raised between these temperatures at a rate of 3.33”C/min, and maintained constant for 45 min. Product analyses were carried out three times at 7,22 and 37 min after reaching each constant temperature. For CO hydrogenation, a CO+H2 gas mixture (CO/H:! = 67/33) was employed instead of CO, + HB. Reaction products were analyzed by two gas chromatographies. A Porapak Q (2 m) column was used for the detection of CO2 and CH,OH, and an activated carbon (2 m) column for Ha, Ar, CO, CH*, and CO,. RESULTS
Hydrogenation activities of gold catalysts Table 1 shows the hydrogenation activities for various supported gold and oxide catalysts at 250” C. Most of samples other than Au/NiO and Au/Co304 showed constant activity for 45 min. Over all the gold-deposited catalysts tested, CO, was hydrogenated with conversions above 2% to produce methanol, CO, and methane. Support oxides were found to have a large influence on both the activity and selectivity. At 250’ C, only Au/ZnO is selective to methanol synthesis giving the highest productivity (382 pm01 h-’ g-cat-‘) and selectivity (50% ). The methanol productivity and selectivity exceeded the corresponding values of Cu/ZnO catalysts. A group of Au/Fe203, Au/ZrO,, Au/CeO,, Au/TiOz, and Au/La (OH), produce CO with a selectivity over 90%, which means that reverse water-gas shift is the main reaction over these catalysts. In the case of the Au/La (OH), catalyst, the calcination temperature was found to be an important factor. Calcination at 400” C instead of 200°C resulted in a complete loss of methanol synthesis ability. Methanol was not produced and the main product was methane over Au/ NiO and Au/Co,O, (Table l), which had good Au dispersion and low-temperature CO oxidation activities [ 22,231. Activity and selectivity (especially methane production) appreciably changed with increasing time over these catalysts. The yield of methane in three sequential runs at 250’ C decreased from 28.2 to 4.2 and to 0.6% over Au/NiO. In the case of Au/Co,O,, it decreased from 4.1 to 1.4 and to 1.1%. Such instability is probably due to the reduction of support oxides during the hydrogenation reaction. From the X-ray diffraction analysis after reaction, only metallic Ni and Au phases were observed in
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Catal. A 102 (1993) 125-136
TABLE 1 Activity of supported No. Catalyst
1 2 3 4 5 6 7 8 9 10 11 12 13 14
Au/ZnO Au/FezO, Au/ZrO, Au/CeO, Au/TiOz Au/La(OH),b Au/NiO Au/CosO, Cu/ZnO(l:19)’ Cu/Zn0(3:7)’ ZnO Fe&s CeO, TiOz
gold and metal oxide supports
for CO* hydrogenation
at 250°C” MeOH production (pm01 h-r g-cat-‘)
Surface area
CO, conv.
Selectivity
(m2g-‘)
(%)
MeOH (%)
CO (%)
CH, (%)
36.0 32.8 16.5 126.5 53.9 11.5 45.2 c.a. 60 49.4 44.2 26.2 33.9 101.7 48.0
2.4 14.7 9.2 3.4 16.1 2.3 2.7 11.5 11.7 16.0 1.2 0.2 0.0 0.0
50.0 4.1 3.3 5.2 1.2 4.3 0.0 0.0 6.5 4.6 0.0 0.0
50.0 95.2 96.7 94.8 93.2 95.7 18.5 3.5 93.5 95.4 100.0 100.0
0.0 0.0 0.0 0.0 5.6 0.0 81.5 95.7 0.0 0.0 0.0 0.0
382 202 94 56 48 46 0 0 236 229 0 0 0 0
a Reaction conditions: COx/Hx = l/3, total pressure = 8 atm, SV = 3000 h-’ ml/g-cat., activity for the three runs in 45 min is given in the table. b Calcined at 200°C. ’ Cu/ZnO (1: 19) and Cu/ZnO (3: 7) catalyst contains 5 and 30 atm-% Cu, respectively.
average
Au/NiO, and the co-existence of Co,O, and metallic Co phase was confirmed in the case of Au/CogO,. Thermodynamic
limitation
Temperature dependency of CO, conversion and the yield of each product over the representative gold catalysts are shown in Figs. l-4. With regards to methanol production shown in Fig. 2, all the catalysts examined increased the yield with rising temperature, then decreased appreciably after a maximum, showing a peak temperature depending on the kind of catalysts used. It is not possible to explain these characteristic parabolic-like curves by thermodynamic constraint if one only considers the following equilibrium: CO,+3H,=CH,OH+H,O
(1)
The calculated equilibrium yields of methanol are 4.39, 2.49, 1.51 and 0.97% for 250, 300, 350 and 400” C, respectively, which are high enough compared with the observed methanol yields. In this reaction system, however, the water produced by eqn. (2) should affect the equilibrium of eqn. ( 1) .
H. Sakurai et al. / Appl. Catal. A 102 (1993) 125-136
129
40
gi-j 30 C 0 ._
e g
20
5 0 0” 0
10
0 100
150
200
250
300
Temperature
350
400
450
(%)
Fig. 1. COz conversion as a function of reaction temperature: ( 0 ) Au/ZnO; ( 0 ) Au/Fe,03; (0) Cu/ZnO (Cu/Zn=3/7); (+) Cu/ZnO (Cu/Zn=1/19); (A) Au/TiOp; (U) Fez03; (0) ZnO. 2.0
I
I
I
200
250
300
I
I
I
350
400
0
100
150
Temperature
450
(‘33
Fig. 2. Methanol yield as a function of reaction temperature: (0 ) Au/ZnO; (0 ) Au/FezOs; (0) Cu/ZnO (Cu/Zn=3/7); (+) Cu/ZnO (Cu/Zn=1/19); (A) Au/TiO*; (m) Fe,Os; (0) ZnO.
CO,+H,=CO+H,O
(2)
The methanol equilibrium yield calculated by considering two equations of eqns. (1) and (2) are 0.95,0.18,0.04 and 0.01% for 250,300,350 and 4OO”C, respectively, which almost agrees with the activity data of Cu/ZnO (Cu/Zn = l/
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100
150
200
250
300
Temperature
350
400
450
(“C)
Fig. 3. CO yield as a function of reaction temperature: (0 ) Au/ZnO; (0 ) Au/FezO,; (0) ZnO (Cu/Zn=3/7); (+) Cu/ZnO (Cu/Zn=1/19); (A) Au/TiOz; (m) Fez03; (0) ZnO.
Cu/
A
1
0 100
150
200
250
300
Temperature
350
400
450
PC)
Fig. 4. Methane yield as a function of reaction temperature: ( 0 ) Au/FezOa; ( A ) Au/TiOz.
19 and 3/7) and Au/FezOB. The observed methanol yield for Au/ZnO and ZnO, however, exceeded this level because CO and water production due to eqn. (2) did not reach equilibrium over these catalysts below 400°C. The equilibrium was checked for these catalysts from the experimentally obtained gas composition. The concentration of COa, HP, and H,O was calculated from the analytical data of CH30H and CO by the material balance equations, and the
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equilibrium of eqn. (1) was checked using obtained Cog, Hz, MeOH, and Hz0 concentrations. The methanol synthesis reaction of eqn. (1) was confirmed to reach an equilibrium at temperatures above 300°C for Au/ZnO, and above 350°C for ZnO. In this temperature range, the observed methanol yield decreased with temperature. Temperature dependence of methanol, CO and methane production The Au/ZnO (Au/Zn=l/19) catalyst gave a methanol yield 1.5 times as high as that for the Cu/ZnO (Cu/Zn = l/19) catalyst at 250” C. In contrast to Cu/ZnO, the Au/ZnO catalyst was inactive at 200°C. It is known that the optimum efficiency in methanol synthesis is obtained for the composition Cu/ Zn = 3/7 in the binary Cu/ZnO system [ 41, to which the Au/ZnO catalyst was inferior in both maximum yield and reaction temperature. While Au/ZnO is poorly active at low temperatures, Au/FezOs can produce methanol at 200’ C, exhibiting almost identical activity with that of Cu/ZnO (Cu/Zn = l/19). As for the reduction of CO, to CO shown in Fig. 3, Au/Fe203 and Au/TiOz are much more active than Au/ZnO over the whole investigated temperature range. At temperatures above 250°C the CO yield reaches the equilibrium value calculated for the reverse water-gas shift reaction (eqn. 2)) which is also observed for Cu/ZnO. Over Au/ZnO the equilibrium yield was not obtained at temperatures below 400°C. Especially, the low-temperature activity of Au/ TiO, is so high that the CO yield is close to equilibrium even at a temperature between 150 and 200°C. This result is in marked contrast to the results reported for CO hydrogenation that the Au/TiOz catalyst was completely inactive [19]. As shown in Fig. 4, in addition to methanol and CO, methane is also produced over Au/TiOz (250°C) and Au/FezOB (300°C). In Au/Fe,O,, a small amount of ethane was also detected at 400” C. X-ray diffraction analysis showed that ferric oxide in Au/Fe203 has changed from cy-Fe203in the original catalyst to Fe304 after the reaction. Such a reduced phase of ferric oxide may be responsible for the production of methane and ethane. Deposited gold was still highly dispersed after the reaction, though the mean particle diameter of gold increased from the original value of 33 to 60 A, which was estimated by calculations made according to Scherrer’s equation from the X-ray diffraction peak for Au (111) . In the case of Au/TiO,, no change of the TiOB support was observed by X-ray diffraction, although the surface of the TiO, was partially reduced after hydrogen reduction treatment and under methanol synthesis as discussed by Frost [ 171 and Lin and Vannice [ 191. Through comparison of the hydrogenation activities of supported gold catalysts with those of simple oxides without gold, it becomes clear that the deposition of gold appreciably enhances CO, conversion (Table 1 and Fig. 1) . The difference in activities between Au/ZnO and ZnO (ca. a factor of 2 at 250-
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Catal. A 102 (1993) 125-136
300°C) is greater than the value expected from the difference in specific surface area (a factor of 1.4). The activity difference in Fe,O,, CeO, and TiOz support oxides was much more pronounced, though the specific surface areas were not greatly different between oxides alone and gold-deposited ones. The production of methanol was never observed over simple oxide catalysts (ZnO, Fe203, CeO, and TiOz) at 250°C. Over ZnO and Fe,O,, small amount of methanol was produced in a higher temperature range (O.l-0.2% at 300350” C ) . By the deposition of gold, not only the methanol yield is increased by a factor of 4-6 but also the reaction temperature is lowered by ca. 100’ C. Comparison between CO, and CO hydrogenation Figure 5 shows a comparison between CO, hydrogenation and CO hydrogenation over Au/ZnO and Cu/ZnO (Cu/Zn = 3/7) catalysts. The hydrogenation of CO, occurred at lower temperatures than CO hydrogenation over both Au/ ZnO and Cu/ZnO. Considering the difference in hydrogen content in the reactant gas mixture (HJCO =2 and H2/C02=3), another experiment was carried out over Au/ZnO for a substoichiometric CO, and Hz mixture (Hz/ CO, = 2). The methanol yield at 250” C decreased to just 2/3 of the yield for the stoichiometric COz and Hz mixture, which is, however, still higher than that for the CO and H2 mixture. Such differences between CO and CO, hydro2
100
150 200
250
300
Temperature
350
400
450
(“C)
Fig. 5. Comparison of methanol yield between CO2 hydrogenation and CO hydrogenation over Au/ZnO and Cu/ZnO (Cu/Zn = 3/7) catalyst: ( 0 ) CO, hydrogenation (H&O, = 3) over Au/ ZnO; ( A ) CO2 hydrogenation (H&O, = 2 ) over Au/ZnO; (0 ) CO, hydrogenation (H,/C02 = 3) over Cu/ZnO; (0 ) CO hydrogenation (H&O = 2) over Au/ZnO; (H) CO hydrogenation (HJ CO= 2) over Cu/ZnO.
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genation are in accordance with the data of Bardet et al. [ 271 reported for the Cu/ZnO system. The difference in the reaction temperature range between the reactant gases was, however, smaller over Au/ZnO than over Cu/ZnO. For CO hydrogenation, Au/ZnO is as active as Cu/ZnO. At 300°C Au/ZnO is a little more active. However, at 400 oC it loses methanol selectivity. The above results suggest that Au/ZnO might be advantageous over Cu/ZnO in the methanol synthesis from the mixture of CO, and CO. DISCUSSION
The above results have clearly demonstrated that supported gold catalysts act as active catalysts for CO, hydrogenation to produce CH,OH, CO, and CH,. Among them, Au/ZnO provides the highest yield and selectivity to methanol, probably due to the basic nature of ZnO. Gold supported on TiOa, which is slightly acidic in nature, is the most active for the reverse water-gas shift reaction yielding the highest total conversion of CO,. It should be noted that although ferric oxide is also slightly acidic in nature and unstable under the reaction conditions, it has potentially the best catalytic nature to act as a support for gold to exhibit low-temperature (200°C) activity for methanol synthesis. The total conversion of CO, can be related to acid-base properties of metal oxides supports. Figure 6 shows CO, conversion at 250’ C over supported gold as a function of the acidity of the support oxides. At this temperature, the maximum CO, conversion over metal oxides without gold was 1.2%, which was observed over ZnO. Any hydrogenation activity exceeding this level should be 20
0’ 7
I
I
I
I
1
I
8
9
IO
11
12
13
14
Electronegativity of Mx* in MxOv Fig. 6. Relationship between the electronegativity of cation composing support oxide and CO, hydrogenation activities of supported gold catalysts.
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considered as the effect of support oxide in gold oxide combined systems. The electronegativity of the metal cations in the metal oxide supports was employed as a measure of acidity. Since a high value of electronegativity corresponds to a high acidity [ 281, it can be concluded that metal oxide supports with stronger acidity lead to a higher CO, conversion. Acidic supports like FezO, and TiOz can also produce CH, at temperatures above 250’ C (Fig. 4 ) . On the other hand, basic supports lead to low CO, conversion. However, as typically shown by ZnO, they are very selective in producing methanol. It has already been reported for CO, hydrogenation over supported Re and Pt catalysts that an acidic support such as SiO, (the electronegativity of Si4+ is 16.2 ) favours the formation of CH, instead of methanol, and a strong basic support such as MgO (the electronegativity of Mg2+ is 6.0) only shows minute reactivity in spite of high selectivity for methanol [ 14,151. The above differences in conversion can be attributed to the difference in interaction of CO, with metal oxide supports. Too strong an interaction of CO, with the basic support is considered to cause low conversion of CO,. However, it may enhance the formation of surface carbonate intermediates. For CO, hydrogenation over the Cu/ZnO catalyst, surface carbonate is proposed to be an intermediate species which is further hydrogenated to formate, methoxy, and finally desorbs as methanol [lo]. Also, in the case of Au/ZrO, catalyst, Baiker and coworkers have reported that surface carbonate and formate species are observed under methanol synthesis conditions from CO, and hydrogen when using a diffuse reflectance FT-IR technique [20,21]. They considered that the observed formate is not the intermediate precursor to methanol, and methanol is produced via x-bonded formaldehyde from CO which is formed by the reverse water-gas shift reaction. In our Au/ZnO system, however, CO, showed higher reactivity than CO for methanol synthesis (Fig. 5)) which seems to indicate methanol is produced directly from CO, not via CO. CO, hydrogenation activity is definitely enhanced by depositing gold on metal oxides, and the enhancing effect differed according to the type of oxide, as described above. This effect can be ascribed, at least, partly to the creation of a CO2 activation site at the gold oxide interface on the oxide surface. Though the activation site of hydrogen is not clear at the present stage, we may recall the following facts. Bond and Sermon found that treatment of Au/ Si& which contains 5 wt.-% gold at 95°C with Dz produces HD and H2 [ 291. They also reported that alkenes, such as 1,3-butadiene and 2-butyne, can be hydrogenated over gold catalysts supported on SiOz, y-A1203, and boehmite even at 100°C [ 291. These results indicate that hydrogen molecules can be activated on a gold surface in the temperature range investigated for CO, hydrogenation. In the case of the Au/ZnO catalyst, some of the CO, and/or H, activation sites may exist on the ZnO surface, because the activity difference between ZnO and Au/ZnO is relatively smaller than other catalysts, and ZnO itself has a CO2 reducing activity to CO of about one half that of Au/ZnO (Fig.
H. Sakurai et al. / Appl. Catal. A 102 (1993) 125-136
135
3). For methanol production, however, Au/ZnO showed much higher activity (5.5 times) at a temperature lower by ca. 100” C than ZnO (Fig. 2), which suggests that the intermediate species to give methanol should be created at the interfacial perimeter around gold particles. In this sense, methanol productivity is expected to change according to the amount of gold oxide interfacial site, which is abundant in the catalysts having a smaller gold diameter. A study to clarify the effect of the diameter of gold is now under way using catalysts whose gold diameter is systematically changed. CONCLUSION
Methanol, CO and methane were obtained as the products of CO, hydrogenation over supported gold catalysts. Methanol and CO yields were largely increased by the deposition of gold compared with oxide alone. Activities and selectivities are greatly influenced by the nature of the support oxide. An acidic support gave a high CO yield due to the reverse water-gas shift reaction, and a basic support, like ZnO, gave a high selectivity to methanol. A large quantity of methane is produced over Au/NiO and Au/Co,O, and a smaller quantity is produced over Au/Fe203 and Au/TiOz in a higher temperature range, which may be associated with the reducibility of oxides. For low-temperature methanol synthesis between 200-250’ C, not only is the deposition of gold with high dispersion indispensable but also the support oxides should be chosen properly, which seems t,o suggest that the active site is a gold oxide interface.
REFERENCES 1 2 3 4 5 6 I 8 9 10
11
12 13 14 15
T. Inui and T. Takeguchi, Catal. Today, 10 (1991) 95. G.C. Chinchen, P.J. Denny, J.R. Jennings, M.S. Spencer and K.C. Waugh, Appl. Catal., 36 (1988) 1. J.C.J. Bart and R.P.A. Sneeden, Catal. Today, 2 (1987) 1. K. Klier, Adv. Catal., 31 (1982) 242. E. Ramaroson, R. Kieffer and A. Kiennemann, Appl. Catal., 4 (1982) 281. B. Beguin, B. Denise and R.P.A. Sneeden, React. Kinet. Catal. Lett., 14 (1980) 9. B. Denise, R.P.A. Sneeden and C. Hamon, J. Mol. Catal., 17 (1982) 359. B. Denise and R.P.A. Sneeden, CHEMTECH, (1982) 110. B. Denise, R.P.A. Sneeden, Appl. Catal., 28 (1986) 235. P. Chumette, Ph. Courty, J. Barbier, T. Fortin, J.C. Lavalley, C. Chauvin, A. Kiennemann, H. Idriss, R.P.A. Sneeden, D. Denise, in M.J. Phillips and M. Ternan (Editors), Proc. 9th International Congress on Catalysis, Vol. 2, The Chemical Society of Canada, Ottawa, Canada, 1988, p. 585. G.C. Chinchen, K. Mansfield and M.S. Spencer, CHEMTECH, (1990) 692. G.C. Chinchen and M.S. Spencer, Catal. Today, 10 (1991) 293. E. Ramaroson, R. Kieffer and A. Kiennemann, J. Chem. Sot., Chem. Commun., (1982) 645. T. Iizuka, M. Kojima and K. Tanabe, J. Chem. Sot., Chem. Commun., (1983) 638. T. Inoue and T. Iizuka, J. Chem. Sot., Faraday Trans. I, 82 (1986) 1681.
136 16 17 18 19 20 21
22 23
24
25
26
27 28 29
H. Sakurai et al. / Appl. Catal. A 102 (1993) 125-136 T. Inoue, T. Iizuka and K. Tanabe, Appl. Catal., 46 (1989) 1. J.C. Frost, Nature, 334 (1988) 577. E.A. Shaw, A.P. Walker, T. Rayment and R.M. Lambert, J. Catal., 134 (1992) 747. S. Lin and M.A. Vannice, Catal. Lett., 10 (1991) 47. R.A. Koeppel, A. Baiker, C. Schild and A. Wokaun, J. Chem. Sot., Faraday Trans., 87 (1991) 2821. A. Baiker, M. Kilo, M. Maciejewski, S. Menzi and A. Wokaun, in L. Guczi, F. Solymosi and P. Tetenyi (Editors), Proc. 10th Intern. Congr. Catal., Budapest, Elsevier, Amsterdam, 1992, p. 208. M. Haruta, N. Yamada, T. Kobayashi and S. Iijima, J. Catal., 115 (1989) 301. M. Haruta, H. Kageyama, N. Kamijo, T. Kobayashi and F. Delanny, in T. Inui (Editor), Successful Design of Catalysts (Studies in Surface Science and Catalysis, Vol. 44)) Elsevier, Amsterdam, 1988, p. 33. S. Tsubota, M. Haruta, T. Kobayashi, A. Ueda and Y. Nakahara, in G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V (Studies in Surface Science and Catalysis, Vol. 63)) Elsevier, Amsterdam, 1991, p. 695. M. Haruta, A. Ueda, S. Tsubota, T. Kobayashi, H. Sakurai and M. Ando, in Proc. 1st JapanEC Joint Workshop on the Frontiers of Catal. Sci. Techn., December 1991, Tokyo (1991) p. 173. A. Ueda, T. Kobayashi, S. Tsubota, H. Sakurai, M. Ando, M. Haruta and Y. Nakahara, in Proc. 1st Japan-EC Joint Workshop on the Frontiers of Catal. Sci. Techn., December 1991, Tokyo, (1991) p. 272. R. Bardet, J. Cazat, and Y. Trambouze, J. Chim. Phys., 78 (1981) 135. K. Tanabe, J.A. Anderson and M. Boudart,in Catalysis Science and Technology, SpringerVerlag, Vol. 2, 1981, p. 231. G.C. Bond and P.A. Sermon, J. Chem. Sot., Chem. Commun., (1973) 444.
Catalysis A: General,
Elsevier Science Publishers APCAT
102 (1993)
125
125-136
B.V., Amsterdam
A2566
Hydrogenation of CO, over gold supported on metal oxides Hiroaki Sakurai, Susumu Tsubota and Masatake Haruta Government
Zndustrial
Research
Institute
of Osaka, l-8-31
Midorigaoka,
Zkeda, 563 (Japan)
Received 18 January 1993, revised manuscript received 27 April 1993
Abstract Gold highly dispersed on a variety of metal oxides were prepared by coprecipitation and depositionprecipitation methods. The hydrogenation of CO2 on supported gold catalysts was investigated at temperatures between 150 and 400°C and a pressure of 8 atm. The methanol yields reached a maximum at temperatures between 200 and 300 ’ C, depending on the support oxides. The highest yield and selectivity towards methanol was obtained on Au/ZnO at 250°C. At 200°C Au/Fe,O, was the most active for methanol synthesis, exhibiting activity almost comparable to that of the conventional Cu/ZnO catalyst with the same metal content. Gold supported on TiO, was so active in reducing CO* to CO that the conversion was close to equilibrium. Over Au/ZnO as well as over Cu/ZnO, CO, could be hydrogenated to methanol at lower temperatures than CO. Key words: CO, hydrogenation;
methanol synthesis; support effect; supported gold catalysts
INTRODUCTION
Hydrogenation of CO, can be regarded as one of the options that can be taken through a chemical approach for the suppression of global warming [ 11. Since methanol is the most desirable product from the reaction, most of the previous work has been concerned with Cu/ZnO-based catalysts which are commercially used for methanol production from synthetic gases (CO + 2H2) containing CO, [l-12]. Supported precious metals, such as Pt, Re, Pd, and Rh have also been tested for the reaction [ 13-161. Supported gold catalysts, Au/ ThO, [17], Au/CeO, [ 181, Au/TiO, and Au/SiO, [19], have been investigated for methanol synthesis from CO. However, they have been reported to be relatively inactive compared with Cu-based catalysts. Recently, Baiker and coworkers [ 20,211 prepared Au/ZrO, from two types of amorphous precursors, i.e., Au-Zr alloy and a coprecipitate, and examined them for CO, hydrogenation. Although gold is a little less active than Cu and Correspondence to: Dr. H. Sakurai, Government Industrial Research Institute of Osaka, 1-8-31 Midorigaoka, Ikeda, 563 Japan. Tel. (+81-727)519656, fax. (+81-727)519630.
0926-860X/93/$06.00
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Ag in methanol synthesis over ZrO,-supported catalysts, it has been proved that even gold can exhibit appreciable activity when properly deposited on ZrO,. We have previously reported that gold can be deposited with high dispersion on a variety of metal oxides by coprecipitation or by the deposition-precipitation method. Highly dispersed supported gold is extraordinarily active in CO oxidation at temperatures as low as - 70°C [ 22-241, and is markedly active for the CO-NO reaction and hydrocarbon combustion [25,26]. The catalytic properties were found to depend on the type of support oxides. The most suitable supports should be exploited for each different reaction. In this paper, we have studied the catalytic properties of gold supported on various oxides for CO, hydrogenation over a wide range of temperatures from 150 to 4OO”C, paying specific attention to activities of methanol synthesis and product selectivities. Other than stable oxides under hydrogen containing conditions like ZnO, reducible oxides such as Fe203, NiO, and Co,O, were also examined as supports. It was expected that these oxides would remain in a partially reduced state in the presence of C02, thereby providing effective supports. In addition, highly dispersed gold particles have been revealed to be relatively stable even after a partial reduction treatment of the oxide in the case of Au/Fe,O, [ 231. EXPERIMENTAL
Gold supported on ZnO, Fe203, ZrO,, La(OH),, NiO, and Co,O, was prepared by coprecipitation from a mixed aqueous solution of HAuCl, and metal nitrate (Au/ (support metal) = l/19 in atomic ratio) according to the procedure described elsewhere [ 22,231. This method provided highly dispersed gold particles with diameters smaller than 8 nm [ 22,231. Gold on TiO, (Au/Ti = l/ 99, the average particle size of gold is smaller than 5 nm) was prepared by the deposition-precipitation method by using JRC-TIO-4 (reference catalyst of Catalysis Society of Japan) as TiOz support [ 241. Gold on CeO, (Au/Cc = l/ 19) was also prepared by deposition-precipitation. The support was prepared from an aqueous solution of Ce ( NOs)3 by precipitation followed by calcination in air at 400°C for 4 h. For comparison, two samples of Cu/ZnO (Cu/Zn = l/ 19 and 3/7) were prepared by coprecipitation from mixed aqueous solutions of Cu ( NO3 ) 2 and Zn ( NO3 ) 2. The above catalyst precursors were finally calcined in air at 400°C for 4-5 h. In the case of Au/La(OH),, it was calcined at both 200°C and 400” C for 4 h in order to investigate the effect of hydroxide and oxide support. Specific surface areas of the prepared catalysts were determined by the BET single point method using a Quantasorb surface area analyzer. X-ray diffraction of each sample before and after the reaction was measured by a Rigaku Xray powder diffractometer.
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The hydrogenation of CO2 was carried out by using a fixed-bed flow reactor using 1 ml of catalyst powder sieved between 16-40 mesh. After passing the pretreatment gas (1% Hz balanced with N2) at 250°C for 12 h at atmospheric pressure, the reactant gas mixture ( CO2 23.4%, H, 66.2%, Ar 10.4% ) was passed under a total gas pressure of 8 atm, SV of 3000 h-’ ml/g-cat., and at temperatures of 150, 200, 250, 300, 350 and 400°C. The temperature was raised between these temperatures at a rate of 3.33”C/min, and maintained constant for 45 min. Product analyses were carried out three times at 7,22 and 37 min after reaching each constant temperature. For CO hydrogenation, a CO+H2 gas mixture (CO/H:! = 67/33) was employed instead of CO, + HB. Reaction products were analyzed by two gas chromatographies. A Porapak Q (2 m) column was used for the detection of CO2 and CH,OH, and an activated carbon (2 m) column for Ha, Ar, CO, CH*, and CO,. RESULTS
Hydrogenation activities of gold catalysts Table 1 shows the hydrogenation activities for various supported gold and oxide catalysts at 250” C. Most of samples other than Au/NiO and Au/Co304 showed constant activity for 45 min. Over all the gold-deposited catalysts tested, CO, was hydrogenated with conversions above 2% to produce methanol, CO, and methane. Support oxides were found to have a large influence on both the activity and selectivity. At 250’ C, only Au/ZnO is selective to methanol synthesis giving the highest productivity (382 pm01 h-’ g-cat-‘) and selectivity (50% ). The methanol productivity and selectivity exceeded the corresponding values of Cu/ZnO catalysts. A group of Au/Fe203, Au/ZrO,, Au/CeO,, Au/TiOz, and Au/La (OH), produce CO with a selectivity over 90%, which means that reverse water-gas shift is the main reaction over these catalysts. In the case of the Au/La (OH), catalyst, the calcination temperature was found to be an important factor. Calcination at 400” C instead of 200°C resulted in a complete loss of methanol synthesis ability. Methanol was not produced and the main product was methane over Au/ NiO and Au/Co,O, (Table l), which had good Au dispersion and low-temperature CO oxidation activities [ 22,231. Activity and selectivity (especially methane production) appreciably changed with increasing time over these catalysts. The yield of methane in three sequential runs at 250’ C decreased from 28.2 to 4.2 and to 0.6% over Au/NiO. In the case of Au/Co,O,, it decreased from 4.1 to 1.4 and to 1.1%. Such instability is probably due to the reduction of support oxides during the hydrogenation reaction. From the X-ray diffraction analysis after reaction, only metallic Ni and Au phases were observed in
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Catal. A 102 (1993) 125-136
TABLE 1 Activity of supported No. Catalyst
1 2 3 4 5 6 7 8 9 10 11 12 13 14
Au/ZnO Au/FezO, Au/ZrO, Au/CeO, Au/TiOz Au/La(OH),b Au/NiO Au/CosO, Cu/ZnO(l:19)’ Cu/Zn0(3:7)’ ZnO Fe&s CeO, TiOz
gold and metal oxide supports
for CO* hydrogenation
at 250°C” MeOH production (pm01 h-r g-cat-‘)
Surface area
CO, conv.
Selectivity
(m2g-‘)
(%)
MeOH (%)
CO (%)
CH, (%)
36.0 32.8 16.5 126.5 53.9 11.5 45.2 c.a. 60 49.4 44.2 26.2 33.9 101.7 48.0
2.4 14.7 9.2 3.4 16.1 2.3 2.7 11.5 11.7 16.0 1.2 0.2 0.0 0.0
50.0 4.1 3.3 5.2 1.2 4.3 0.0 0.0 6.5 4.6 0.0 0.0
50.0 95.2 96.7 94.8 93.2 95.7 18.5 3.5 93.5 95.4 100.0 100.0
0.0 0.0 0.0 0.0 5.6 0.0 81.5 95.7 0.0 0.0 0.0 0.0
382 202 94 56 48 46 0 0 236 229 0 0 0 0
a Reaction conditions: COx/Hx = l/3, total pressure = 8 atm, SV = 3000 h-’ ml/g-cat., activity for the three runs in 45 min is given in the table. b Calcined at 200°C. ’ Cu/ZnO (1: 19) and Cu/ZnO (3: 7) catalyst contains 5 and 30 atm-% Cu, respectively.
average
Au/NiO, and the co-existence of Co,O, and metallic Co phase was confirmed in the case of Au/CogO,. Thermodynamic
limitation
Temperature dependency of CO, conversion and the yield of each product over the representative gold catalysts are shown in Figs. l-4. With regards to methanol production shown in Fig. 2, all the catalysts examined increased the yield with rising temperature, then decreased appreciably after a maximum, showing a peak temperature depending on the kind of catalysts used. It is not possible to explain these characteristic parabolic-like curves by thermodynamic constraint if one only considers the following equilibrium: CO,+3H,=CH,OH+H,O
(1)
The calculated equilibrium yields of methanol are 4.39, 2.49, 1.51 and 0.97% for 250, 300, 350 and 400” C, respectively, which are high enough compared with the observed methanol yields. In this reaction system, however, the water produced by eqn. (2) should affect the equilibrium of eqn. ( 1) .
H. Sakurai et al. / Appl. Catal. A 102 (1993) 125-136
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40
gi-j 30 C 0 ._
e g
20
5 0 0” 0
10
0 100
150
200
250
300
Temperature
350
400
450
(%)
Fig. 1. COz conversion as a function of reaction temperature: ( 0 ) Au/ZnO; ( 0 ) Au/Fe,03; (0) Cu/ZnO (Cu/Zn=3/7); (+) Cu/ZnO (Cu/Zn=1/19); (A) Au/TiOp; (U) Fez03; (0) ZnO. 2.0
I
I
I
200
250
300
I
I
I
350
400
0
100
150
Temperature
450
(‘33
Fig. 2. Methanol yield as a function of reaction temperature: (0 ) Au/ZnO; (0 ) Au/FezOs; (0) Cu/ZnO (Cu/Zn=3/7); (+) Cu/ZnO (Cu/Zn=1/19); (A) Au/TiO*; (m) Fe,Os; (0) ZnO.
CO,+H,=CO+H,O
(2)
The methanol equilibrium yield calculated by considering two equations of eqns. (1) and (2) are 0.95,0.18,0.04 and 0.01% for 250,300,350 and 4OO”C, respectively, which almost agrees with the activity data of Cu/ZnO (Cu/Zn = l/
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100
150
200
250
300
Temperature
350
400
450
(“C)
Fig. 3. CO yield as a function of reaction temperature: (0 ) Au/ZnO; (0 ) Au/FezO,; (0) ZnO (Cu/Zn=3/7); (+) Cu/ZnO (Cu/Zn=1/19); (A) Au/TiOz; (m) Fez03; (0) ZnO.
Cu/
A
1
0 100
150
200
250
300
Temperature
350
400
450
PC)
Fig. 4. Methane yield as a function of reaction temperature: ( 0 ) Au/FezOa; ( A ) Au/TiOz.
19 and 3/7) and Au/FezOB. The observed methanol yield for Au/ZnO and ZnO, however, exceeded this level because CO and water production due to eqn. (2) did not reach equilibrium over these catalysts below 400°C. The equilibrium was checked for these catalysts from the experimentally obtained gas composition. The concentration of COa, HP, and H,O was calculated from the analytical data of CH30H and CO by the material balance equations, and the
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131
equilibrium of eqn. (1) was checked using obtained Cog, Hz, MeOH, and Hz0 concentrations. The methanol synthesis reaction of eqn. (1) was confirmed to reach an equilibrium at temperatures above 300°C for Au/ZnO, and above 350°C for ZnO. In this temperature range, the observed methanol yield decreased with temperature. Temperature dependence of methanol, CO and methane production The Au/ZnO (Au/Zn=l/19) catalyst gave a methanol yield 1.5 times as high as that for the Cu/ZnO (Cu/Zn = l/19) catalyst at 250” C. In contrast to Cu/ZnO, the Au/ZnO catalyst was inactive at 200°C. It is known that the optimum efficiency in methanol synthesis is obtained for the composition Cu/ Zn = 3/7 in the binary Cu/ZnO system [ 41, to which the Au/ZnO catalyst was inferior in both maximum yield and reaction temperature. While Au/ZnO is poorly active at low temperatures, Au/FezOs can produce methanol at 200’ C, exhibiting almost identical activity with that of Cu/ZnO (Cu/Zn = l/19). As for the reduction of CO, to CO shown in Fig. 3, Au/Fe203 and Au/TiOz are much more active than Au/ZnO over the whole investigated temperature range. At temperatures above 250°C the CO yield reaches the equilibrium value calculated for the reverse water-gas shift reaction (eqn. 2)) which is also observed for Cu/ZnO. Over Au/ZnO the equilibrium yield was not obtained at temperatures below 400°C. Especially, the low-temperature activity of Au/ TiO, is so high that the CO yield is close to equilibrium even at a temperature between 150 and 200°C. This result is in marked contrast to the results reported for CO hydrogenation that the Au/TiOz catalyst was completely inactive [19]. As shown in Fig. 4, in addition to methanol and CO, methane is also produced over Au/TiOz (250°C) and Au/FezOB (300°C). In Au/Fe,O,, a small amount of ethane was also detected at 400” C. X-ray diffraction analysis showed that ferric oxide in Au/Fe203 has changed from cy-Fe203in the original catalyst to Fe304 after the reaction. Such a reduced phase of ferric oxide may be responsible for the production of methane and ethane. Deposited gold was still highly dispersed after the reaction, though the mean particle diameter of gold increased from the original value of 33 to 60 A, which was estimated by calculations made according to Scherrer’s equation from the X-ray diffraction peak for Au (111) . In the case of Au/TiO,, no change of the TiOB support was observed by X-ray diffraction, although the surface of the TiO, was partially reduced after hydrogen reduction treatment and under methanol synthesis as discussed by Frost [ 171 and Lin and Vannice [ 191. Through comparison of the hydrogenation activities of supported gold catalysts with those of simple oxides without gold, it becomes clear that the deposition of gold appreciably enhances CO, conversion (Table 1 and Fig. 1) . The difference in activities between Au/ZnO and ZnO (ca. a factor of 2 at 250-
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Catal. A 102 (1993) 125-136
300°C) is greater than the value expected from the difference in specific surface area (a factor of 1.4). The activity difference in Fe,O,, CeO, and TiOz support oxides was much more pronounced, though the specific surface areas were not greatly different between oxides alone and gold-deposited ones. The production of methanol was never observed over simple oxide catalysts (ZnO, Fe203, CeO, and TiOz) at 250°C. Over ZnO and Fe,O,, small amount of methanol was produced in a higher temperature range (O.l-0.2% at 300350” C ) . By the deposition of gold, not only the methanol yield is increased by a factor of 4-6 but also the reaction temperature is lowered by ca. 100’ C. Comparison between CO, and CO hydrogenation Figure 5 shows a comparison between CO, hydrogenation and CO hydrogenation over Au/ZnO and Cu/ZnO (Cu/Zn = 3/7) catalysts. The hydrogenation of CO, occurred at lower temperatures than CO hydrogenation over both Au/ ZnO and Cu/ZnO. Considering the difference in hydrogen content in the reactant gas mixture (HJCO =2 and H2/C02=3), another experiment was carried out over Au/ZnO for a substoichiometric CO, and Hz mixture (Hz/ CO, = 2). The methanol yield at 250” C decreased to just 2/3 of the yield for the stoichiometric COz and Hz mixture, which is, however, still higher than that for the CO and H2 mixture. Such differences between CO and CO, hydro2
100
150 200
250
300
Temperature
350
400
450
(“C)
Fig. 5. Comparison of methanol yield between CO2 hydrogenation and CO hydrogenation over Au/ZnO and Cu/ZnO (Cu/Zn = 3/7) catalyst: ( 0 ) CO, hydrogenation (H&O, = 3) over Au/ ZnO; ( A ) CO2 hydrogenation (H&O, = 2 ) over Au/ZnO; (0 ) CO, hydrogenation (H,/C02 = 3) over Cu/ZnO; (0 ) CO hydrogenation (H&O = 2) over Au/ZnO; (H) CO hydrogenation (HJ CO= 2) over Cu/ZnO.
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genation are in accordance with the data of Bardet et al. [ 271 reported for the Cu/ZnO system. The difference in the reaction temperature range between the reactant gases was, however, smaller over Au/ZnO than over Cu/ZnO. For CO hydrogenation, Au/ZnO is as active as Cu/ZnO. At 300°C Au/ZnO is a little more active. However, at 400 oC it loses methanol selectivity. The above results suggest that Au/ZnO might be advantageous over Cu/ZnO in the methanol synthesis from the mixture of CO, and CO. DISCUSSION
The above results have clearly demonstrated that supported gold catalysts act as active catalysts for CO, hydrogenation to produce CH,OH, CO, and CH,. Among them, Au/ZnO provides the highest yield and selectivity to methanol, probably due to the basic nature of ZnO. Gold supported on TiOa, which is slightly acidic in nature, is the most active for the reverse water-gas shift reaction yielding the highest total conversion of CO,. It should be noted that although ferric oxide is also slightly acidic in nature and unstable under the reaction conditions, it has potentially the best catalytic nature to act as a support for gold to exhibit low-temperature (200°C) activity for methanol synthesis. The total conversion of CO, can be related to acid-base properties of metal oxides supports. Figure 6 shows CO, conversion at 250’ C over supported gold as a function of the acidity of the support oxides. At this temperature, the maximum CO, conversion over metal oxides without gold was 1.2%, which was observed over ZnO. Any hydrogenation activity exceeding this level should be 20
0’ 7
I
I
I
I
1
I
8
9
IO
11
12
13
14
Electronegativity of Mx* in MxOv Fig. 6. Relationship between the electronegativity of cation composing support oxide and CO, hydrogenation activities of supported gold catalysts.
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considered as the effect of support oxide in gold oxide combined systems. The electronegativity of the metal cations in the metal oxide supports was employed as a measure of acidity. Since a high value of electronegativity corresponds to a high acidity [ 281, it can be concluded that metal oxide supports with stronger acidity lead to a higher CO, conversion. Acidic supports like FezO, and TiOz can also produce CH, at temperatures above 250’ C (Fig. 4 ) . On the other hand, basic supports lead to low CO, conversion. However, as typically shown by ZnO, they are very selective in producing methanol. It has already been reported for CO, hydrogenation over supported Re and Pt catalysts that an acidic support such as SiO, (the electronegativity of Si4+ is 16.2 ) favours the formation of CH, instead of methanol, and a strong basic support such as MgO (the electronegativity of Mg2+ is 6.0) only shows minute reactivity in spite of high selectivity for methanol [ 14,151. The above differences in conversion can be attributed to the difference in interaction of CO, with metal oxide supports. Too strong an interaction of CO, with the basic support is considered to cause low conversion of CO,. However, it may enhance the formation of surface carbonate intermediates. For CO, hydrogenation over the Cu/ZnO catalyst, surface carbonate is proposed to be an intermediate species which is further hydrogenated to formate, methoxy, and finally desorbs as methanol [lo]. Also, in the case of Au/ZrO, catalyst, Baiker and coworkers have reported that surface carbonate and formate species are observed under methanol synthesis conditions from CO, and hydrogen when using a diffuse reflectance FT-IR technique [20,21]. They considered that the observed formate is not the intermediate precursor to methanol, and methanol is produced via x-bonded formaldehyde from CO which is formed by the reverse water-gas shift reaction. In our Au/ZnO system, however, CO, showed higher reactivity than CO for methanol synthesis (Fig. 5)) which seems to indicate methanol is produced directly from CO, not via CO. CO, hydrogenation activity is definitely enhanced by depositing gold on metal oxides, and the enhancing effect differed according to the type of oxide, as described above. This effect can be ascribed, at least, partly to the creation of a CO2 activation site at the gold oxide interface on the oxide surface. Though the activation site of hydrogen is not clear at the present stage, we may recall the following facts. Bond and Sermon found that treatment of Au/ Si& which contains 5 wt.-% gold at 95°C with Dz produces HD and H2 [ 291. They also reported that alkenes, such as 1,3-butadiene and 2-butyne, can be hydrogenated over gold catalysts supported on SiOz, y-A1203, and boehmite even at 100°C [ 291. These results indicate that hydrogen molecules can be activated on a gold surface in the temperature range investigated for CO, hydrogenation. In the case of the Au/ZnO catalyst, some of the CO, and/or H, activation sites may exist on the ZnO surface, because the activity difference between ZnO and Au/ZnO is relatively smaller than other catalysts, and ZnO itself has a CO2 reducing activity to CO of about one half that of Au/ZnO (Fig.
H. Sakurai et al. / Appl. Catal. A 102 (1993) 125-136
135
3). For methanol production, however, Au/ZnO showed much higher activity (5.5 times) at a temperature lower by ca. 100” C than ZnO (Fig. 2), which suggests that the intermediate species to give methanol should be created at the interfacial perimeter around gold particles. In this sense, methanol productivity is expected to change according to the amount of gold oxide interfacial site, which is abundant in the catalysts having a smaller gold diameter. A study to clarify the effect of the diameter of gold is now under way using catalysts whose gold diameter is systematically changed. CONCLUSION
Methanol, CO and methane were obtained as the products of CO, hydrogenation over supported gold catalysts. Methanol and CO yields were largely increased by the deposition of gold compared with oxide alone. Activities and selectivities are greatly influenced by the nature of the support oxide. An acidic support gave a high CO yield due to the reverse water-gas shift reaction, and a basic support, like ZnO, gave a high selectivity to methanol. A large quantity of methane is produced over Au/NiO and Au/Co,O, and a smaller quantity is produced over Au/Fe203 and Au/TiOz in a higher temperature range, which may be associated with the reducibility of oxides. For low-temperature methanol synthesis between 200-250’ C, not only is the deposition of gold with high dispersion indispensable but also the support oxides should be chosen properly, which seems t,o suggest that the active site is a gold oxide interface.
REFERENCES 1 2 3 4 5 6 I 8 9 10
11
12 13 14 15
T. Inui and T. Takeguchi, Catal. Today, 10 (1991) 95. G.C. Chinchen, P.J. Denny, J.R. Jennings, M.S. Spencer and K.C. Waugh, Appl. Catal., 36 (1988) 1. J.C.J. Bart and R.P.A. Sneeden, Catal. Today, 2 (1987) 1. K. Klier, Adv. Catal., 31 (1982) 242. E. Ramaroson, R. Kieffer and A. Kiennemann, Appl. Catal., 4 (1982) 281. B. Beguin, B. Denise and R.P.A. Sneeden, React. Kinet. Catal. Lett., 14 (1980) 9. B. Denise, R.P.A. Sneeden and C. Hamon, J. Mol. Catal., 17 (1982) 359. B. Denise and R.P.A. Sneeden, CHEMTECH, (1982) 110. B. Denise, R.P.A. Sneeden, Appl. Catal., 28 (1986) 235. P. Chumette, Ph. Courty, J. Barbier, T. Fortin, J.C. Lavalley, C. Chauvin, A. Kiennemann, H. Idriss, R.P.A. Sneeden, D. Denise, in M.J. Phillips and M. Ternan (Editors), Proc. 9th International Congress on Catalysis, Vol. 2, The Chemical Society of Canada, Ottawa, Canada, 1988, p. 585. G.C. Chinchen, K. Mansfield and M.S. Spencer, CHEMTECH, (1990) 692. G.C. Chinchen and M.S. Spencer, Catal. Today, 10 (1991) 293. E. Ramaroson, R. Kieffer and A. Kiennemann, J. Chem. Sot., Chem. Commun., (1982) 645. T. Iizuka, M. Kojima and K. Tanabe, J. Chem. Sot., Chem. Commun., (1983) 638. T. Inoue and T. Iizuka, J. Chem. Sot., Faraday Trans. I, 82 (1986) 1681.
136 16 17 18 19 20 21
22 23
24
25
26
27 28 29
H. Sakurai et al. / Appl. Catal. A 102 (1993) 125-136 T. Inoue, T. Iizuka and K. Tanabe, Appl. Catal., 46 (1989) 1. J.C. Frost, Nature, 334 (1988) 577. E.A. Shaw, A.P. Walker, T. Rayment and R.M. Lambert, J. Catal., 134 (1992) 747. S. Lin and M.A. Vannice, Catal. Lett., 10 (1991) 47. R.A. Koeppel, A. Baiker, C. Schild and A. Wokaun, J. Chem. Sot., Faraday Trans., 87 (1991) 2821. A. Baiker, M. Kilo, M. Maciejewski, S. Menzi and A. Wokaun, in L. Guczi, F. Solymosi and P. Tetenyi (Editors), Proc. 10th Intern. Congr. Catal., Budapest, Elsevier, Amsterdam, 1992, p. 208. M. Haruta, N. Yamada, T. Kobayashi and S. Iijima, J. Catal., 115 (1989) 301. M. Haruta, H. Kageyama, N. Kamijo, T. Kobayashi and F. Delanny, in T. Inui (Editor), Successful Design of Catalysts (Studies in Surface Science and Catalysis, Vol. 44)) Elsevier, Amsterdam, 1988, p. 33. S. Tsubota, M. Haruta, T. Kobayashi, A. Ueda and Y. Nakahara, in G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V (Studies in Surface Science and Catalysis, Vol. 63)) Elsevier, Amsterdam, 1991, p. 695. M. Haruta, A. Ueda, S. Tsubota, T. Kobayashi, H. Sakurai and M. Ando, in Proc. 1st JapanEC Joint Workshop on the Frontiers of Catal. Sci. Techn., December 1991, Tokyo (1991) p. 173. A. Ueda, T. Kobayashi, S. Tsubota, H. Sakurai, M. Ando, M. Haruta and Y. Nakahara, in Proc. 1st Japan-EC Joint Workshop on the Frontiers of Catal. Sci. Techn., December 1991, Tokyo, (1991) p. 272. R. Bardet, J. Cazat, and Y. Trambouze, J. Chim. Phys., 78 (1981) 135. K. Tanabe, J.A. Anderson and M. Boudart,in Catalysis Science and Technology, SpringerVerlag, Vol. 2, 1981, p. 231. G.C. Bond and P.A. Sermon, J. Chem. Sot., Chem. Commun., (1973) 444.