Migration and reduction of formate to form methanol on CuZnO catalysts

~

A PT PA LL E IY DSS CA I A: GENERAL

ELSEVIER

Applied Catalysis A: General 135 (1996) 273-286

Migration and reduction of formate to form methanol on Cu/ZnO catalysts Oh-Shim Joo a, Kwang-Doeg JunK a, Sung-Hwan Han a, Sung-Jin Uhm a,*, Dong-Keun Lee b, Son-Ki Ihm c a Catalysis Laboratory, Korea Institute of Science and Technology, Cheongryang P.O. Box 131, Seoul, South Korea b Department of Chemical Engineering, Research Institute of Industrial Technology, Gyeongsang National University, Chinju, Gyeongnam, South Korea c Department of Chemical Engineering, Korea Advanced Institute of Science and Technology, Kusung-dong, Yusung-ku, Taejeon, South Korea

Received 27 April 1995; accepted 20 September 1995

Abstract

Temperature-programmed decomposition (TPD) and temperature-programmed hydrogenation (TPH) experiments were performed on copper and zinc formates. Formic acid was found to be the only organic product from the copper formate reactions. Methanol and methyl formate were found to be the major organic products from the zinc formate reactions. When ZnO was mixed with copper formate, the yields of methanol and methyl formate increased. Temperature-programmed IR (TPIR) spectroscopy of the copper f o r m a t e / Z n O indicated the migration of the formate from copper to ZnO in the temperature range below 430 K at which the decomposition of the copper formate began. The experimental results elucidated the synergistic effect between copper and ZnO; formate migration onto ZnO and its hydrogenation to methanol on ZnO. Based on the synergistic effect, a reaction mechanism for methanol synthesis from the hydrogenation of carbon oxides in C u / Z n O catalysts was proposed. Keywords: Synergistic effect; Formate; Copper; Zinc oxide; Migration; Carbon dioxide; Carbon monoxide; Methanol; Methyl formate; TPD; TPH; TPIR

1. Introduction The reaction mechanism of methanol synthesis from the hydrogenation of carbon oxides has been a subject of both fundamental science and technology * Corresponding author. Tel. (+ 82-2) 9585211, fax. (+ 82-2) 9585219. 0926-860X/96/$15.00 © 1996 Elsevier Science B,V. All rights reserved SSDI 0926-860X(95)00256- 1

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[1]. Despite a number of mechanistic studies on the methanol synthesis over C u / Z n O catalysts for the last half century, there have been many controversies on the reaction pathway, particularly on the origin of the synergistic effects between copper and ZnO. It was reported that the activities of copper containing catalysts were proportional to the surface area of copper independent of the nature of the oxide support [2,3]. It was also proposed that alumina as a third component had no actual activity for the catalytic reactions but prevented copper from sintering and increased the surface areas of the catalysts [4,5]. On the other hand there have been many experimental evidences for the synergistic effects between copper and ZnO. Klier [6] and Herman et al. [7] proposed that the Cu + dissolved in ZnO was the active species for the hydrogenation reaction of carbon monoxide. Frost [8] suggested that the active site of the catalyst was the oxygen defect in ZnO, electronically modified by copper. Burch et al. [9,10] found that the rate of methanol synthesis was proportional to the surface area of copper and was dependent on the nature of oxides added to the copper containing catalysts. The authors proposed that hydrogen was spilled over and stored on the oxide supports during the reaction, which promoted the hydrogenation process [ 11 - 13]. The formate on the catalyst surface has been suggested as an intermediate of methanol synthesis [14-22] and water-gas shift reaction [23-25]. Bowker et al. [14] reported that the formate could be easily formed on the surface of a polycrystalline copper metal at 32°C from co-dosing of CO2/H 2 and be decomposed at 440 K. The formate could be generated on ZnO at elevated temperatures of 500 K and be decomposed to C O / H 2 at about 570 K healing the anion defects introduced by the prereduction [26-28]. They also suggested that the hydrogenation of a formate to methoxy was the rate determining step of the methanol synthesis reaction from carbon oxides. Recently, Neophytides et al. [29] observed that the formate species, formed by the hydrogenation of carbon dioxide and adsorbed on copper, was the pivotal intermediate leading to methanol. It has been proposed that the synergistic effect between copper and ZnO has been attributed to the formation of formates at the interface of copper and ZnO [I 1,18,22,30]. Herein we report a mechanistic study on the hydrogenation of carbon oxides focusing on the origin of the synergistic effect. The reactivities of copper and zinc formates were monitored by temperature-programmed decomposition (TPD) and temperature-programmed hydrogenation (TPH) methods. The mixing effect of ZnO into a copper formate was also investigated. In addition, temperatureprogrammed IR (TPIR) spectroscopy was used to demonstrate the migration of formate from copper to ZnO. The nature of the synergistic effect between copper and ZnO for the hydrogenation of carbon oxides was attributed to the migration of the formate onto ZnO and the hydrogenation of the formate on the ZnO to form methanol. Based on these findings, a mechanistic route is proposed for the hydrogenation of carbon oxides to form methanol.

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275

2. Experimental

2.1. Preparation of copper and zinc formates and copper containing catalysts The formates of copper and zinc were prepared by the addition of corresponding metal carbonates into formic acid solutions. The precipitates were filtered, washed with ether 4 times to remove excess formic acid and dried in vacuum for 3 days following the procedures described in the literature [31,32]. The products were characterized by the CHN analysis, atomic absorption, and IR spectroscopy. Samples of C u / Z n O , Cu/A1203 and C u / Z n O / A 1 2 0 3 were prepared by coprecipitation of the corresponding metal nitrates. Samples of ZnO, ZnO/A1203 and A1203 were prepared by the same method described above. A solution of 1 M ammonium carbonate was added dropwise into the metal nitrate solution to form precipitates. The precipitates were filtered and subsequently washed, dried and calcined in a furnace at 723 K for 16 h. The composition of the resulting catalysts was CuO:ZnO = 3: 1, 2:1, 1:1, 1:2; CuO:A1203 = 3:1, 2:1, 1:1, 1:2; CuO:ZnO:A1203 = 6:3:1, 3:6:1, 3:3:1; and ZnO:AlzO 3 = 6:1 in molar ratios. EP grade chemicals were used, and the gas purities were minimum 99.9%.

2.2. Temperature-programmed decomposition (TPD) and temperature-programmed hydrogenation (TPH) of metal formates TPD and TPH experiments were carried out in flows of helium and hydrogen, respectively, with programmed heating up to 570 K at a rate of 10 K / m i n . The gas flows were regulated by mass flow controllers at 50 ml/min. Amounts of 10 mg each of copper and zinc formates was charged in a quartz reactor (6 mm O.D., 45 cm length). For the TPD and TPH of copper formate/ZnO mixture, 5 mg of ZnO was added to 10 mg of copper formate and was charged in the same quartz reactor. A quadrupole mass spectrometer (Balzers, MS-Cube 200) was used to analyze the product components of the reaction stream. For the detection of formic acid, formaldehyde, methanol, and methyl formate, the peaks at m/e = 46, 30, 31, 60 were monitored respectively (HCOOH: 29(100), 46(62), 45(48); H2CO: 29(100), 30(88), 28(30); CH3OH: 31(100), 32(67), 29(65); HCOOCH3: 31(100), 29(78), 32(34), 60(32). from [33]). Since the parent peak of methanol (m/e = 31) overlapped with the fragment peak of methyl formate, the liquid products were condensed at liquid nitrogen temperature and analyzed by gas chromatography (TCD, Porapak Q column). High pressure TPH of copper formate, zinc formate and copper formate mixed with oxides (ZnO, A1203 and ZnO/A1203) was also carried out in the stream of H 2 at 27.2 bar as described above. The samples for the high pressure TPH were charged in a stainless steel reactor (9 mm O.D., 20 cm length).

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276

2.3. IR spectroscopic investigation of copper formate / ZnO For TPIR, copper formate/ZnO (1:2 weight ratio) was pressed to prepare a 2 cm wafer. The wafer was then placed in the infrared cell designed by the method described by Hicks et al. [34]. The sample was heated up to 450 K in helium atmosphere, 50 m l / m i n and heating rate of 10 K / m i n . The spectra were recorded by the Brucker IFS 66 FTIR spectrometer with a resolution of 2 - 4 cm

-1

2.4. Carbon dioxide hydrogenation to form methanol The carbon dioxide hydrogenation to form methanol was carried out in a stainless steel fixed reactor (9 mm O.D., 20 cm length) heated by an electrical furnace. Catalysts were reduced in situ by 5% hydrogen in helium at 523 K for 1 h before the reaction. The mixed gas of C O 2 / H 2 / A r ( 2 3 / 6 7 / 1 0 in volume ratio, argon: internal standard) was introduced into the reactor at 523 K and 27.2 bar. The gas flow was regulated by a mass flow controller, and the products were analyzed by an on-line gas chromatography (Shimadzu GC-8A, TCD, Porapak Q and Silica column, 1/8 in. × 4 m). The line between the reactor and the GC was heated to avoid condensation of liquid product. The surface areas of the catalysts were measured by a Micromeritics ASAP 2000. The copper surface areas (Scu) of catalysts were determined by nitrous oxide titration following the procedure described in the literature [35].

3. Results

3.1. TPD of metal formates The formates of copper and zinc were decomposed in helium carrier gas with programmed heating up to 570 K at a heating rate of 10 K / r a i n (Fig. 1, Table 1). The maximum decomposition temperature of copper formate was 450 K. Formic acid was a unique organic product in the 11.8% yield with carbon dioxide, carbon monoxide, hydrogen and water as other major decomposition products. On the other hand, the zinc formate, decomposing at 525 K, showed a different decomposition pattern from that of copper formate. The TPD of zinc formate produced a group of organic products such as methyl formate (15.2%), formaldehyde (3.1%), formic acid (0.7%) and methanol (0.2%). Formic acid, a major organic product in TPD of copper formate, became a minor product in the 0.7% yield. It is interesting to note that methyl formate and methanol, which were further hydrogenated products than formic acid, were the major organic products even without the addition of external hydrogen. The zinc formate had two crystal water per molecule. The crystal water was desorbed below 480 K,

O.-S. Joo et al. /Applied Catalysis A: General 135 (1996) 273-286

E2

420

450

480

510

540

0

570

Temperature/K

,"

,

,

420

450

420

450

,

,

,

,

,

277

,

,

,

,

,

,

,

480 510 Temperature/K

540

570

480

540

570

g

rr

420

450

480

510

540

570

Ternperature/K

5;10

Temperature/K

Fig. 1. TPD spectra of metal formates. (a), (b); copper formate and (c), (d); zinc formate, i: m / e = 44 (CO2) , ii: m / e = 2 8 (CO), iii: m / e = 2 (H2), iv: m / e = 1 8 (H20), v: m / e = 4 6 (HCOOH), vi: m / e = 3 0 (HCHO), vii: m / e = 31 (CH3OH and HCOOCH3), viii: m / e = 6 0 (HCOOCH3). He flow rate = 50 ml/min, heating rate = 10 K/min.

and a trace of water was formed in the decomposition temperature range in the TPD of the zinc formate. The zinc formate was thermally more stable and produced higher yields of methanol and methyl formate than the copper formate.

3.2. TPH of metal formates Copper and zinc formates were hydrogenated at the hydrogen pressure of 1 (Table 2) and 27.2 bar (Table 3, nos. 3 and 6), respectively, by the programmed heating up to 570 K. The TPH of copper formate at atmospheric pressure showed a product distribution similar to that of TPD, and no detectable amounts of methanol and methyl formate were produced. Meanwhile, the high pressure TPH of the copper formate showed the formation of methanol and methyl formate, new products in traces, and a decrease in yield of formic acid. The TPH

Table 1 Product distributions from TPD of the formates of copper and zinc. He flow rate = 50 ml/min, heating rate = 10 K / m i n Sample

Product yield (%) a CO 2

CO b

H2CO

CH30 H

HCOOCH 3

HCOOH

1. copper formate 2. zinc formate

67.1 52.7

21.5 27.0

3.1

0.2

15.2

11.8 0.7

a Mole percent based on the carbon of the metal formates. b The data were obtained by mass spectrometry.

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Table 2 Product distributions from T P H o f the formates of c o p p e r and zinc at atmospheric pressure. H 2 flow rate = 50 m l / m i n , heating rate = 10 K / r a i n Sample

1. c o p p e r formate 2. zinc formate

Product yield (%) a CO 2

CO b

H2CO

CH3OH

HCOOCH 3

66.1 47.6

21.0 25.2

0.3

0.8

21.4

HCOOH 12.4

a Mole percent based on the carbon o f the metal formates. b The data were obtained by a mass spectrometer.

of zinc formate, however, provided higher yield in methyl formate (21.4%) compared to the TPD. Furthermore, the selectivities to hydrogenated products such as methanol and methyl formate changed dramatically at high hydrogen pressure. The methanol yield increased from 0.8% to 18.8% while the methyl formate yield decreased from 21.4% to 0.5%; neither formaldehyde nor formic acid was observed. Methanol became a major hydrogenated product as a result of the hydrogen pressure. 3.3. Addition of oxides to copper formate The mixing of ZnO into copper formate altered the decomposition temperature and product distribution of the copper formate remarkably. Two new stable formate species were found from the mixing of ZnO with copper formate, which were decomposed at 478 K and 505 K along with the decomposition peak of copper formate only at 455 K (Fig. 2). The addition of the ZnO changed not only the decomposition temperature of the copper formate but also the yields of

Table 3

Product distributions from T P D and T P H of c o p p e r formates mixed with oxides at 1 and 27.2 bar. Flow rate o f H 2 and He = 50 m l / m i n , heating rate = 10 K / m i n Product yield (%) a

Sample

1. 2. 3. 4. 5. 6. 7.

copper formate/ZnO c copper formate/ZnO d c o p p e r formate e copper formate/ZnO e copper formate/ZnO/AI203 zinc formate e copper formate/A1203 ~

e

CO 2

CO b

H2CO

CH30 H

HCOOCH 3

HCOOH

66.4 69.8 41.2 47.2 47.8 42.0 40.6

24.6 16.3 52.8 34.2 32.3 38.7 59.2

1. I 0.2

trace trace 0.5 12.3 16.7 18.8 0.2

0.7 5.5 0.8 6.3 3.2 0.5

4.1 3. ! 4.7

a Mole percent based on the carbon o f the metal formate. b The data were obtained b y a mass spectrometer. ¢ T P D in helium o f 1 bar. d T P H in h y d r o g e n o f 1 bar. e T P H in h y d r o g e n of 27.2 bar.

O.-S. Joo et al./ Applied Catalysis A: General 135 (1996) 273-286

420

450

480

510

279

570

540

Temperature/K

J

420

i

i

450

t

i

J

480

i

i

i

510

J

J

i

540

i

i

570

Temperature/K

Fig. 2. TPD spectra of copper formate and ZnO mixture, i - v i i i as in Fig. I. He flow rate = 50 m l / m i n , heating rate = 10 K / m i n .

organic products. As shown in Table 3, the TPD and TPH of the copper f o r m a t e / Z n O physical mixture produced further hydrogenated products such as methanol, methyl formate and formaldehyde (Table 3, nos. 1 and 2). The first decomposition peak of copper f o r m a t e / Z n O mixture at 455 K (Fig. 2) was similar to that of copper formate only (Fig. 1a). Formic acid, the unique organic product in the TPD and the TPH of the copper formate, was not detected in the first decomposition reaction of the copper formate/ZnO mixture at 455 K. Instead, the formic acid was detected over a broad temperature range with maximum at 478 K, the second decomposition temperature. At the second decomposition temperature, carbon dioxide was the major product with water and formic acid as minor products (Fig. 2). Both carbon monoxide and hydrogen increased at the third decomposition temperature of 505 K with a trace of water. The decomposition pattern at 505 K was similar to that of zinc formate (Fig. lc) although the decomposition temperature was shifted to lower temperature. However, the introduction of hydrogen to the copper formate/ZnO mixture increased the formation of methyl formate (Table 3, no. 2). The hydrogenation reaction of the sample was further promoted at the elevated hydrogen pressure of 27.2 bar. The yields of some organic products, methanol and methyl formate, improved drastically (Table 3, no. 4). Especially for the formation of methanol, the yield reached 12.3% without any formation of formic acid. The ZnO mixing effect was improved with Z n O / A I 2 0 3 which is a traditional support for the industrial catalysts of methanol synthesis. The TPH experiment of a copper f o r m a t e / Z n O / A 1 2 0 3 gave an increase in yield of methanol to 16.7% and a decrease in yield of methyl formate to 3.2% (Table 3, no. 5). Methyl formate was a major product in low pressure TPH, whereas methanol became a major

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O.-S. Joo et al./Applied Catalysis A: General 135 (1996) 273-286

one as the hydrogen pressure increased. The presence of ZnO was an essential part of the promotion of the methanol formation in the formate reduction. It is also important to note that the mixing of ZnO to a copper formate showed a similar product distribution to that of TPH of zinc formate at high hydrogen pressure. The behavior of the formates in both copper f o r m a t e / Z n O mixture and zinc formate seemed to be similar. This phenomenon suggested two possibilities: the copper formate was hydrogenated either (i) on the ZnO after its migration to the ZnO support, or (ii) on the interracial site between copper and ZnO. These suggestions were further investigated in detail by in situ IR spectroscopy in the next section. The mixing of alumina to copper formate instead of ZnO hardly yielded any organic product except a trace of methanol (Table 3, no. 7). The result assured the promotional effect of ZnO for the formation of methanol from formate.

3.4. Migration of formate from copper to ZnO: In situ IR spectroscopic investigations The temperature-programmed IR (TPIR) spectroscopic investigations were performed on copper formate and ZnO mixture in a helium atmosphere. The temperature range of this experiment was below the decomposition temperature of copper formate. As the temperature increased to 353 K, two peaks of 1352.1 and 1552.1 cm -~, characteristic peaks of the formate coordinated to a copper [19,20], decreased in intensity and disappeared completely above 433 K (Fig. 3). At the same time two new peaks of 1382.6 and 1598.8 cm - l began to appear from 353 K and gradually increased as the temperature reached 433 K. Two new peaks of 1382.6 and 1598.8 cm - l were identical to those of a zinc formate [19,20]. At the end of TPIR, the final spectrum was almost the same as the one of zinc formate. The absorbance of the final spectrum of the formate was 85%

1.51.2-

s,~tz i ~

0.9

0.6

0.3017o~

16bo

lsbo

1,~

1~

Wavenumber/cm" '

Fig. 3. TPIR spectra of copper f o r m a t e / Z n O mixture at i 313 K; ii 353 K; iii 373 K; iv 393 K; v 413 K; vi 433 K. Heating rate = 10 K / m i n .

O.-S. Joo et al./Applied Catalysis A: General 135 (1996) 273-286

281

12 ¸

c~z,,o

.c

Cu:M203 o) m

-5

Cu:ZnO:A/203

E E

v

"o o

>"

"5

4

o

[]

o

15 Surface ar~. of copper ( m2/g )

Fig. 4. D e p e n d e n c e o f methanol yield on copper surface area from C O 2 hydrogenation over Cu-based catalysts, feed composition; C O 2 / H 2 / A r = 2 3 / 6 7 / 1 9 , feed rate = 100 m l / m i n , total pressure = 27.2 bar, reaction temperature = 523 K, catalyst = 0.5 g.

of the initial absorbance of the copper formate. These observations confirmed that the formate of copper migrated to ZnO without a significant decomposition.

3.5. Hydrogenation of carbon dioxide to form methanol Hydrogenation of carbon dioxide was performed in order to understand the synergistic effects between copper and ZnO. The reaction was conducted at 523 K, 27.2 bar and feed gas ratio of 23:67:10 (CO2:H2:Ar). Fig. 4 and Table 4 show a linear dependence of methanol yield on the surface area of metallic copper. Despite the linearity, however, there were large productivity differences depending on oxide supports. The conversion of carbon dioxide and the selectivity to methanol on C u / Z n O catalysts were higher than on C u / A l z O 3 catalysts at the same copper surface area. As was expected from Table 3, the

Table 4 Product distributions from C O z hydrogenation. Feed composition; C O 2 / H 2 / A r = 2 3 / 6 7 / 1 0 , feed rate = 100 m l / m i n , pressure = 27.2 bar, reaction temperature = 523 K, catalyst = 0.5 g Catalyst

Scu ( m 2 / g )

C u O : Z n O = 3:1 C u O : Z n O = 2:1 CuO:ZnO = l :1 C u O : Z n O = 1:2 C u O : A 1 2 0 3 = 3:1 C u O : A 1 2 0 3 = 2:1 C u O : A 1 2 0 3 = l:1 C u O : A I 2 0 3 = 1:2 C u O : Z n O : A 1 2 0 3 = 6:3:1 C u O : Z n O : A 1 2 0 3 = 3:6:1 C u O : Z n O : A 1 2 0 3 = 3:3:1

9.0 7.4 6.6 5.9 8.9 6. l 4.3 5.2 13.2 11.9 12.6

Scat. ( m 2 / g )

27.0 28.0 20.0 24.0 71.0 52.0 72.0

110.0 47.0 58.0 79.0

C o n v e r s i o n ( m m o l / g c a t h)

16.8 16.3 14.2 15.1 10.5 7.9 6.0 8.6 21.9 19.2 21.2

Yield ( m m o l / g c a t h) CH3OH

CO

8.8 8.0 6.7 7.7 4.6 3.4 3.1 3.4 I0.0 8.9 8.5

8.0 8.3 7.5 6.4 5.9 4.5 2.9 5.2 11.9 10.3 12.7

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C u / Z n O / A l 2 0 3 catalyst showed the highest yield of methanol because of the high surface area of the catalyst which maximized the mixing effect. The kinetic data also indicated the active participation of ZnO during the hydrogenation of carbon dioxide. Compared with the TPH experiments in Table 3, Cu/AI203 catalysts showed reasonable activities in catalytic reactions at high pressure. These phenomena inferred that the total activity in an actual catalytic reaction was the sum of the activities of copper and ZnO. These results thus indicated the existence of a synergistic effect between copper and ZnO. The catalytic reactions were extensions of the TPH experiments of a copper formate/ZnO. The reactivity pattern of the catalytic reactions and the TPH of a copper formate/ZnO were identical.

4. Discussion The synergistic effect of C u / Z n O catalysts can be explained by the experimental results described above. A formate has been suggested as an intermediate in the hydrogenation of carbon oxides to methanol. It has also been known that the formate can be easily formed on copper even at room temperature, and that the hydrogenation of the formate was the rate determining step [14]. The formation of formate on ZnO was rather difficult below 450 K [26,27]. Even above 450 K, the concentration of the formate on copper was higher than on the ZnO by an order of magnitude [26,36], but the fate of the formate formed on copper has not been clearly understood. When copper formate was decomposed under helium atmosphere, the only organic product was formic acid at 450 K (Fig. 1, Table 1). Meanwhile the major organic products from the TPD of a zinc formate were methanol and methyl formate at higher decomposition temperature. The difference of decomposition temperatures of two metal formates might be attributed to the reduction potentials of copper and zinc [37]. Cu2++ 2e-

~

Cu

E 0 = 0.346Vvs. NHE

C u + + e-

--+

Cu

E 0 = 0.170V

Zn 2+ + 2e-

~

Zn

E 0 = - 0.763 V

Copper has a higher reduction potential than zinc and can be easily reduced to form copper metal even at low temperature. During the course of TPD of copper formate, the copper was reduced at low temperature to liberate formate as a form of formic acid by the abstraction of hydrogen from its decomposition products. As zinc has a very low reduction potential, it is difficult to reduce even above 600 K. As a result, the formate coordinated to zinc became more stable than that of copper. The zinc formate could then reach high temperatures above 500 K without decomposition and be reduced to methanol or methyl formate in that temperature range.

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Addition of hydrogen instead of helium improved the selectivity towards methanol and methyl formate (Tables 2 and 3). In the high pressure TPH of copper formate, traces of methanol and methyl formate were formed. In the high pressure TPH of zinc formate, however, methanol was found to be the major product with high selectivity. These observations indicated that zinc was an essential component to reduce formate to methanol and methyl formate under hydrogen atmosphere. The mixing of ZnO with copper formate changed the decomposition pattern of the copper formate. There were three decompositions at 455 K, 478 K and 505 K (Fig. 2). As was indicated above, the copper formate seemed to be easily reduced to generate free formate. At the second decomposition temperature, part of the free formate desorbed in the form of formic acid. At the third decomposition temperature, the free formate reached a temperature high enough to be reduced to formaldehyde and methyl formate. In the TPH of that sample, formaldehyde and methyl formate were formed at 478 K which was the same temperature as the second decomposition temperature of the TPD experiment (Fig. 2) [38]. In the elevated hydrogen pressure TPH of copper f o r m a t e / Z n O mixture (Table 3, no. 4), methanol became the major hydrogenation product. It is worthwhile to note that the mixing of ZnO to copper formate showed an almost similar product distribution to that of hydrogenation of zinc formate under high hydrogen pressure. The behavior of formate in the copper formate/ZnO mixture and zinc formate seemed to be similar except for their decomposition temperature. This similarity meant that the active site for the hydrogenation of formate was ZnO. However, there were two possibilities that the hydrogenation reaction took place either on ZnO or on the interface of copper and ZnO. The TPIR investigation of the copper formate/ZnO mixture supported the complete migration of the formate from copper to ZnO (Fig. 3). The physical mixture of copper formate and ZnO had the characteristic peaks of copper formate only at 1552.1 and 1352.1 cm -1 at 313 K. Two new peaks at 1598.8 and 1382.6 cm -~ appear,,d as the temperature increased above 353 K, which were characteristic peaks of a zinc formate. It is important to find that the final spectrum of TPIR was almost identical to the zinc formate spectrum. These experimental results of the copper formate/ZnO mixture strongly suggested the migration of formate on copper to ZnO. Since the TPIR experiment was performed below the decomposition temperature of copper formate, the formate on copper might have transferred to ZnO without a significant decomposition. Chauvin et al. [36] adsorbed formic acid on Cu/ZnA1204 to observe the mixture of formates in the IR spectrum. As the temperature increased to 403 K, the peak of the formate coordinated to the ZnA1204 increased. It was concluded that copper formate was not thermally stable and transformed into CO~ and CO. Then the CO 2 and CO produced the formate on the ZnAI204 at higher temperature, but the TPIR and TPD results from this work demonstrated the migration of formate from copper

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to ZnO without decomposition. Especially, the presence of two isosbestic points in TPIR at 1570 and 1360 cm-1 strongly support the migration of formate onto ZnO. There might be two other possible routes to form zinc formate from a copper formate/ZnO sample. First, formic acid produced from decomposition of copper formate could be readsorbed on ZnO [19]. Second, zinc formate could be generated from the reaction of carbon oxides/hydrogen which were produced by the decomposition of copper formate [26,27]. The temperature for the TPIR experiment, however, was far below the decomposition temperature of copper formate. Therefore, we could conclude that formate migration might be the plausible pathway for the reaction excluding the two other possibilities. The kinetic studies on the hydrogenation of carbon dioxide to form methanol could be reviewed on the results discussed above; the migration of formate onto ZnO and the hydrogenation of formate to form methanol on ZnO. The productivities of three groups of catalysts to form methanol were proportional to the surface area of metallic copper (Fig. 4, Table 4) consistent with the work previously reported [12,13]. However, when the copper surface areas were the same, their productivities for methanol were different depending on the supports. As shown in Fig. 4, the addition of ZnO to the catalytic system doubled the productivity for methanol. Compared to the TPH experiments in Table 3, Cu/A1203 catalysts also showed reasonable activities in high pressure catalytic reactions. The catalytic activity of Cu/AI203 indicated that formate could also be reduced to form methanol on copper at the rigorous reaction conditions as a minor pathway. This result suggested that the total activity of the actual catalytic reaction was the sum of the activities of copper and ZnO. The reactivity differences between C u / Z n O and Cu/AI203 could be explained with the two observations; the migration of formate onto ZnO and the hydrogenation of /H o~C _=

z. -oAo,,Z-

co, c°

7.(

O--C Cu 0 = ', Zn -7- Z n / Z n

Me0H + H20

"~/\o

CHa0H * H ~ 0 ~

/

tI

O~c-/H

. %)o z\~/z% /Zn 0

0

\

Cu

0

=

/

z\-~/z%/z~ 0

0

Scheme 1. A reaction route for the h y d r o g e n a t i o n of c a r b o n oxides to form methanol.

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formate to methanol on ZnO. Also these two observations could elucidate the nature of the synergistic effect between copper and ZnO. On the basis of understanding the synergistic effects and the role of ZnO, a reaction mechanism for the hydrogenation of carbon oxides to form methanol is proposed in Scheme 1. The first step of the reaction scheme is the generation of a formate on the copper surface [14]. The formate then migrates onto the surface of ZnO. The reduced copper can activate hydrogen and transfer it to ZnO and consequently enhance the reactivity of the catalyst system [11]. The formate on the ZnO is also hydrogenated to form methanol to regenerate the catalytic system. The proposed reaction scheme to form methanol via zinc formate might be a major pathway, but as was pointed out in Fig. 4, the copper catalysts without ZnO also showed reasonable methanol yield. This result points to the presence of a minor reaction pathway to form methanol via the hydrogenation of formate on copper.

5. Conclusions In this study we have reached two conclusions: (i) the migration of formate from copper onto ZnO, and (ii) the hydrogenation of the formate to form methanol on ZnO. Copper formate migrated to ZnO without significant decomposition as the temperature increased. The formate on ZnO was hydrogenated to form methanol and methyl formate, whereas a copper formate was hydrogenated to form decomposition products and small amounts of formic acid. It is evident that the origins of the synergistic effects in C u / Z n O catalysts were the migration of formate from copper onto ZnO and the hydrogenation of formate on ZnO to yield methanol. Based on these observations, a new model for the methanol synthesis from carbon oxides/hydrogen mixture was proposed.

References [1] K.C. Waugh, Catal. Today, 15 (1992) 51. [2] G.C. Chinchen, K.C. Waugh and D.A. Whan, Appl. Catal., 25 (1986) 101. [3] G.C. Chinchen, P.J. Denny, D.G. Parker, G.D. Short, M.S. Spencer, K.C. Waugh and D.A. Whan, Prepr. Am. Chem. Soc. Div. Fuel Chem., 29 (1984) 178. [4] G.C. Chinchen, P.J. Denny, J.R. Jennings, M.S. Spencer and K.C. Waugh, Appl. Catal., 36 (1988) 1. [5] K. Shimomura, K. Ogawa, M. Oba and Y. Kotera, J. Catal., 52 (1978) 191. [6] K. Klier, Adv. Catal., 31 (1982) 243. [7] R.G. Herman, K. Klier, G.W. Simmon, B.P. Finn, J.W. Bulko and T.P. Kobylinka, J. Catal., 56 (1979) 407. [8] J.C. Frost, Nature (London), 334 (1988) 557. [9] R. Bureh and R.J. Chappell, Appl. Catal., 45 (1988) 131. [10] R. Burch, S.E. Golunski and M.S. Spencer, J. Chem. Soc., Faraday Trans., 86 (1990) 2683. [11] R. Burch, R.J. Chappell and S.E. Golunski, J. Chem. Soc., Faraday Trans., 85 (1989) 3569. [12] G.J.J. Bartley and R. Burch, Appl. Catal., 43 (1988) 141.

286 [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38]

O.-S. Joo et al./Applied Catalysis A: General 135 (1996) 273-286

R. Burch, S.E. Golunski and M.S. Spencer, Catal. Lett., 5 (1990) 55. M. Bowker, R.A. Hadden, H. Houghton, J.N.K. Hyland and K.C. Waugh, J. Catal., 109 (1988) 263. E. Ramaroson, R. Kieffer and A. Kiennemann, Appl. Catal., 4 (1982) 281. J.F. Edwards and G.L. Schrader, J. Catal., 94 (1985) 175. G.J. Miller and C.H. Rochester, J. Chem. Soc., Faraday Trans., 88 (1992) 2085. G.J. Miller and C.H. Rochester, J. Chem. Soc., Faraday Trans., 89 (1992) 1109. G.J. Miller, C.H. Rochester, S. Bailey and K.C. Waugh, J. Chem. Soc., Faraday Trans., 88 (1992) 1033. G.J. Miller, C.H. Rochester, S. Bailey and K.C. Waugh, J. Chem. Soc., Faraday Trans., 88 (1992) 3497. R.J. Klinger and J.W. Rathke, J. Am. Chem. Soc., 106 (1984) 7650. A. Kiennemann, H. Idriss, J. Hindermann, J. Lavalley, A. Vallet, P. Chaumette and P. Courty, Appl. Calal., 59 (1990) 165. J.F. Edwards and G.L. Schrader, J. Phys. Chem., 88 (1984) 5620. A. Ueno, T. Onishi and K. Tamaru, J. Catal., 66 (1970) 756. P.C. Ford, Acc. Chem. Res., 14 (1981) 3 I. M. Bowker, H. Houghton and K.C. Waugh, J. Chem. Soc., Faraday Trans. 1, 77 (1981) 3023. M. Bowker, H. Houghton, K.C. Waugh, T. Giddings and M. Green, J. Catal., 84 (1983) 252. M. Bowker, Vacuum, 33 (1983) 669. S.G. Neophytides, A.J. Marchi and G.F. Froment, Appl. Catal. A, 86 (1992) 45. K.M. Vanden Bussche and G.F. Froment, Appl. Catal. A, 112 (1994) 37. R.L. Martin and H. Waterman, J. Chem. Soc., (1959) 1359. J. Kendal and H. Adler, J. Am. Chem. Soc., 43 (1921) 1470. S. Einar, A. Sixten and W. M. Fred, Registry of Mass Spectral Data, Vol. 1, Wiley, New York, 1974. R.F. Hicks, C.S. Kellner, B.J. Savatsky, W.C. Hecker and A.T. Bell, J. Catal., 71 (1981) 216. J. W. Evans, M. S. Wainwright, A. J. Bridgewater and D. J. Young, Appl. Catal., 7 (1983) 75. C. Chauvin, S. Jacques, J. C. Lavalley, H. Idriss, J. P. Hindermann, A. Kiennemann, P. Chumette and P. Courty, J. Catal., 121 (1990) 56. CRC Handbook of Chemistry and Physics, 67th Edition, CRC Press, Florida, 1986-1987. O.-S. Joo, K.-D. Jung, S.-H. Han, S.-J. Uhm, J. Catal., 157 (1995) 259