CO Mechanism

In situ IR studies on the mechanism of methanol synthesis overan ultra®ne Cu/ZnO/Al2O3 catalyst

Qi Suna, Chong-Wei Liub, Wei Panb, Qi-Ming Zhub, Jing-Fa Denga,*

aDepartment of Chemistry, Fudan University, Shanghai 200433, ChinabState Key Laboratory of C1 Chemistry and Technology, Tsinghua University, Beijing 100084, China

Received 22 October 1997; received in revised form 4 February 1998; accepted 26 February 1998

Abstract

Methanol synthesis from CO2 and CO/CO2 hydrogenation was carried out under real reaction conditions over an ultra®ne

Cu/ZnO/Al2O3 (Cu/Zn/Al�60/30/10, molar ratio) catalyst. The formation and variation of surface species were recorded by in

situ FT-IR spectroscopy. The mechanisms of methanol synthesis and RWGS reaction were discussed. The result revealed that

methanol was formed directly from CO2 hydrogenation for CO2/H2 or CO/CO2/H2 reaction systems. b-HCOOÿs was the

necessary intermediate for methanol synthesis. A scheme of methanol and RWGS reaction was proposed. # 1998 Elsevier

Science B.V. All rights reserved.

Keywords: Methanol synthesis; Mechanism; In situ; FT-IR

1. Introduction

In industrial processes, methanol is synthesized

from hydrogenation of mixtures of CO and CO2 over

Cu-based catalyst. Addition of a small amount of CO2

to the mixture of CO and H2 can promote methanol

yield remarkably. Because of this attractive promoting

effect, methanol synthesis from CO2 and H2 has

caused considerable attention in recent years [1±7].

However, there is still controversy over some impor-

tant questions, such as:

1. the role of carbon dioxide in the process of

methanol synthesis;

2. whether CO or CO2 serves as the primary carbon

source for methanol synthesis;

3. whether the inter-conversion between CO and CO2

via the water-gas shift (WGS)/or reverse water-gas

shift (RWGS) is an indispensable process or not;

and

4. what are the reaction intermediates involved in

methanol synthesis.

By the use of the temperature-programmed-desorption

(TPD) [8±10], IR spectroscopy [11±13], chemical

trapping methods [14] and 14C or 13C isotopic tracer

[15,16], various surface species were found on Cu-

based catalyst in the course of methanol synthesis and

some reasonable mechanisms were suggested, but

most of these experiments were carried out at high-

vacuum or atmospheric pressure conditions using

clean Cu single crystal, supported Cu/SiO2 catalyst

Applied Catalysis A: General 171 (1998) 301±308

*Corresponding author. Fax: +86 2165 641740; e-mail:

[email protected]

0926-860X/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved.

P I I S 0 9 2 6 - 8 6 0 X ( 9 8 ) 0 0 0 9 6 - 9

or Cu-based catalyst with low copper content. The

results obtained under these conditions diverge from

the results obtained under real reaction processes. In

this work, a high activity ultrafine Cu/ZnO/Al2O3

catalyst (Cu/Zn/Al�60/30/10, molar ratio) prepared

by `̀ oxalate gel co-precipitation method'' [17±19] was

used as working catalyst to study the mechanism of

methanol synthesis from CO2 and CO/CO2 hydroge-

nation by means of FT-IR spectroscopy under real

reaction conditions (high pressure, high temperature

and feed gas continuously flowing over the catalyst).

A scheme of methanol synthesis formation was

suggested.

2. Experimental

Cu/ZnO/Al2O3 catalyst (Cu/Zn/Al�60/30/10,

molar ratio) used in this study was prepared by

`̀ the oxalate gel co-precipitation method'' similar to

that described previously [17±19]. The BET surface

area of the Cu/ZnO/Al2O3 catalyst is 49.7 m2/g, Cu

surface area measured by N2O titration is 36.3 m2/g

and the metallic copper particle size evaluated by

XRD and TEM is 10.7 nm. Infrared experiments were

conducted on a 30 mg wafer of the mixture of the

catalyst and g-Al2O3 (catalyst/g-Al2O3�1/6, weight

ratio) crushed to a powder smaller than 500 mesh

placed inside an in situ reaction cell (shown as in

Fig. 1). Infrared spectra were recorded with a Perkin-

Elmer system 2000 FT-IR spectrometer. A resolution

of 4.0 cmÿ1 was used throughout the investigation and

50 scans taken over a 20 s interval were averaged to

achieve a satisfactory signal-to-noise ratio. The ultra-

high purity (>99.999%) gases of H2, CO, N2 and

chemical purity CO2 (>99.9%) were used. The gases

in the in¯ow were puri®ed by passing through a 5 AÊ

molecular sieve-trap. After reduction by H2 at 513 K

for 4 h, the reaction cell packed with the catalyst was

rapidly cooled to room temperature and ¯ushed with a

high-purity N2 for 2 h. After switching N2 with reac-

tion gas (CO2/H2�1/3 or CO/CO2/H2�20/5/75) and

pressurizing slowly to 2.0 MPa, temperature-

programmed-reaction (TPR) experiments (the ¯ow

rate of reaction gas was 30 ml/min and the rate of

temperature increase was 2 K/min) were performed

and IR spectra were recorded synchronously.

3. Results and discussion

Fig. 2 shows part of the IR spectra obtained in

¯owing CO2/H2 reaction gas at 2.0 MPa. It is found

that a few adsorbed CO can be formed easily at room

temperatures. The band at 2079 cmÿ1 is resolved only

at a low temperature range and is attributed to CO

adsorbed on a low Miller-index plane of reduced Cu

[20]. The peaks at 2171 and 2118 cmÿ1 are character-

istic of gaseous CO. With increase in the temperature,

exceeding the level of 313 K, it is found that the

intensity of bands of surface adsorbed CO

(2079 cmÿ1) and gaseous CO (2118 and

2171 cmÿ1) decrease gradually, indicating the

decrease of CO concentration over catalyst surface

Fig. 1. Schematic diagram of in situ IR reaction cell: (1) cell body; (2) cell core; (3) window frame; (4) NaCl crystal window; (5) O-ring; (6)

sample fixing ring; (7) sample wafer.

302 Q. Sun et al. / Applied Catalysis A: General 171 (1998) 301±308

and gas phase, and the rato-vibrational spectrum of

H2O (1600±1800 cmÿ1) begins to arise and increase.

Meanwhile, monodentate formate species (m-

HCOOÿ) are observed at 1585 cmÿ1. The results

indicate that CO species are formed from dissociative

adsorption of CO2 over catalyst surface. With the

temperature increasing further (higher than 323 K),

during the increase of the intensity of peak at

1585 cmÿ1, the intensity of another peak at

1593 cmÿ1 due to bidentate carbonate species

�b-CO2ÿ3 � also increases more rapidly. Up to about

493 K, it is found that these two peaks weaken again.

Fig. 3 shows the process of formation and change for

CH3O and CH3OH species at various temperatures. It

is found that several peaks at 2857 and 2940 cmÿ1,

which are characteristic of b-HCOOÿs , are present at

lower temperatures, e.g. 313 K. As the temperature is

raised further, the intensity of the peaks at 2857 and

2940 cmÿ1 is also increased and reaches a steady-state

concentration above 413 K. However, the peaks at

2918 and 2965 cmÿ1, assigned to CH3Os and CH3OH

species, respectively, occur and are ampli®ed gradu-

ally until the temperature is raised above 433 K. Such

a result reveals that, although some adsorbed and

gaseous CO are present at low temperature, no CH3Os

(adsorbed methoxy species) or CH3OH were formed,

and b-HCOOÿs species also could be ignored. While

the temperature is higher than 393 K, the amount of

CO species (COs and gaseous CO) and that of

m-HCOOÿs decrease, and CH3Os as well as product

CH3OH increase due to the increase of b-HCOOÿs .

From these results, it is concluded that methanol is

formed directly from hydrogenation of CO2, and that

methanol formation from hydrogenation of carbon

monoxide formed via RWGS is negligible. The key

Fig. 2. IR spectra for methanol synthesis from CO2/H2 recorded during temperature-programmed-reaction (TPR) process at 2.0 MPa.

Fig. 3. Formation and variation of methoxy group and methanol

during temperature programmed reaction (TPR) process at 2.0 MPa

for methanol synthesis from CO2�H2.

Q. Sun et al. / Applied Catalysis A: General 171 (1998) 301±308 303

intermediate species for methanol synthesis from CO2

hydrogenation is bidentate formate �b-HCOOÿs �instead of monodentate formate �m-HCOOÿs �.Further, when we switch feed gas CO2/H2 with pure

H2, all the band intensities of CO, CO2, m-CO2ÿ3 , m-

HCOOÿ become weaker slowly, but CH3Os and

CH3OH could still be clearly detected due to the

existence of surface formate b-HCOOÿ at 2940

cmÿ1. When b-HCOOÿ (2940 cmÿ1) fades away,

no methanol species could be observed by IR. The

above experimental observation and analysis strongly

indicate that the process of the hydrogenation of

bidentate formate �b-HCOOÿs � is the rate-limiting step

for methanol synthesis from CO2 hydrogenation. This

result is in agreement with previous reports [11,21].

TPR-IR experiments were also conducted with H2/

CO/CO2 feed gas. Fig. 4 shows the spectra obtained

when the temperature was increased from 293 K with

the rate of 2 K/min at a pressure of 2.0 MPa and gas

¯ow rate of 30 ml/min. It is found that a sharp peak at

2006 cmÿ1 ascribed to absorbed CO (carbonyl band)

varied from weak to intense in the temperature range

293±343 K and then to weak again with the increase of

temperature. It is coincident with the variation of the

peak at 1634 cmÿ1 due to the bending mode of H2O

physisorbed on Cu. However, no formate or carbonate

species appear under these conditions. As the tem-

perature increases to above 353 K, the peak at

2006 cmÿ1 disappears and a broader peak at

1972 cmÿ1 grows rapidly and then decreases gradu-

ally. Moreover, the bands at 1591 and 1601 cmÿ1,

which are characteristic of m-HCOOÿ and b-CO2ÿ3

species, respectively, increase gradually and reach the

maximum at 493 K. Furthermore, when the tempera-

ture is increased again, two bands of 1591 and

1601 cmÿ1 diminish. This variation is similar to the

change of CO2�H2 in the TPR-IR experiment. How-

ever, it is also found that, besides the bands of 2855

and 2940 cmÿ1 (ascribed to bidentate formate

b-HCOOÿs and CH3Os, respectively), product CH3OH

is also detected upon the temperature of 443 K. This

indicates that the necessary intermediate is still

Fig. 4. IR spectra for methanol synthesis from CO/CO2/H2 recorded during temperature-programmed-reaction (TPR) process at 2.0 MPa.


b-HCOOÿs species instead of m-HCOOÿs or other

species. It is noteworthy that the intensity of bands

of the rato-vibrational spectrum of H2O at 1600±

1800 cmÿ1 is obviously weaker for CO/CO2�H2 sys-

tem than that for CO2/H2. So the addition of CO could

inhibit the reaction of RWGS. Namely, the formation

of water species could be suppressed and the forma-

tion rate of methanol could be enhanced by the addi-

tion of CO into CO2/H2 feed gas. Fig. 5 shows the

methanol formation rates for feed gases containing

only CO2 and H2, containing only CO and H2 as well

as containing CO/CO2 mixture and H2. It is found that

both the methanol formation rates for the individual

CO and CO2 hydrogenation are much lower than that

for CO/CO2 mixture hydrogenation, and the methanol

formation rate for CO hydrogenation is lower than for

CO2 at 499 K. It is clear that the addition of CO into

CO2/H2 feed gas promotes the formation rate of

methanol signi®cantly. Especially, we found that the

variation of the amount of water is very signi®cant for

CO2/H2 and CO/CO2/H2 reaction systems. After addi-

tion of CO into CO2/H2, only a trace of water was

detected and the amount is three orders of magnitude

lower than that of CO2/H2 reaction system. This result

was also found in this in situ IR±TPRS experiment and

in previous kinetic and catalytic activity testing [19].

According to the Arrhenius equation, ln(TOF) of

methanol formation is proportional to the inverse

temperature. By plotting the formation rate from

kinetic and activity testing versus 1/T (Fig. 6), an

apparent activation energy could be determined from

the slope. The slope is measured at the temperature

ranging from 433 to 453 K, where the rates of both

methanol synthesis and RWGS start to increase

rapidly before the conversion signi®cantly alters the

gas-phase composition. The activation energy for

methanol synthesis is 20.7 kcal/mol and that for

RWGS reaction is 22.73 kcal/mol. After the addition

of CO into CO2/H2 feed gas, however, the activation

energy for methanol synthesis is decreased to

6.52 kcal/mol, although there is no signi®cant change

in the activation energy for RWGS reaction. It is clear

that the addition of CO into CO2/H2 feed gas could

lead to the decrease of the activation energy for

methanol synthesis. As a result, the rate of methanol

formation is increased and the methanol selectivity is

promoted to a large extent. This is in agreement with

the results from this in situ IR experiment.

According to the above analysis, some suggestions

for methanol synthesis from CO/CO2/H2 could be

proposed:

1. The procedures of dissociative adsorption and

hydrogenation of CO2 are two parallel competitive

reactions. At low temperatures, the formation of

Fig. 5. Methanol synthesis rate for CO/H2, CO2/H2 and CO/CO2/

H2 feeds over the ultrafine Cu/ZnO/Al2O3 catalyst. P�2.0 MPa,

space velocity�4500 hÿ1.

Fig. 6. Arrhenius plot ln(TOF) for methanol formation and RWGS

reaction versus the inverse reaction temperature (1/T) over Cu/

ZnO/Al2O3 catalyst for CO2�H2 (&,*) and CO/CO2�H2 (~).


CO (corresponding to the bands of 2007 cmÿ1)

along with the adsorbed Os by dissociative

adsorption is a prior process. These adsorbed

oxygen species (Os) combine with nascent hydro-

gen (Hs) to form hydroxy species (OHs) and then

to H2O, so the peak at 1634 cmÿ1 is observed. As

temperature increases, the rate of CO2 adsorption/

hydrogenation increases and the process of

dissociative adsorption of CO2 is suppressed, so

the bands of 1634 and 2007 cmÿ1 diminish

gradually and then disappear.

2. With the increase of temperature, the bands at 1591

and 1601 cmÿ1 (ascribed to m-HCOOÿs and

m-CO2ÿ3 species, respectively) increase gradually,

and then diminish after reaching the maximum,

while the bands at 2857 �b-HCOOÿs �, 2918 and

2940 cmÿ1 (CH3Os) increase gradually along with

the variation of the intensity of the band at

2965 cmÿ1 (CH3OH). This indicates that the for-

mation of CH3Os and CH3OH species are accom-

panied by the formation of bidentate formate

�b-HCOOÿs �, and b-HCOOÿs species is a key inter-

mediate, which undergoes stepwise hydrogenation

to form CH3Os then to CH3OH. The rate-limiting

step for methanol synthesis over the ultrafine Cu/

ZnO/Al2O3 catalyst is the hydrogenation of

b-HCOOÿs .

3. The effects of addition of CO to the CO2/H2 feed

gas are ascribed to the inhibition of the RWGS

reaction (or enhancement of the WGS reaction).

Water produced via methanol synthesis from CO2

hydrogenation is consumed by the fast water-gas

shift reaction that simultaneously provides CO2,

which is the feed gas for methanol synthesis. As a

result, the limitation of thermodynamic equili-

brium for CO2 hydrogenation to CH3OH is

removed and leads to a higher methanol yield

(methanol synthesis formation rate) and selectivity.

This result has been discussed in kinetic terms

previously [19].

From Fig. 6, it is also found that with the addition of

CO into CO2/H2 feed gas, apparent activation energy

for methanol synthesis is decreased. It led to the

promotion of methanol formation rate and selectivity.

One explanation is the following: in the absence of

CO, H2O is formed earlier via RWGS reaction and it

prohibits the reaction of methanol synthesis; when CO

is introduced, relative high coverage of CO leads to the

reduction of dissociative adsorption of CO2 on the

catalyst surface and the inhibition of RWGS reaction.

Therefore, the reaction of methanol synthesis can take

place quickly. Moreover, the Os species formed via

b-HCOOÿs hydrogenation to H2COs is consumed by

the adsorbed COs to form [CO2]s rapidly. Thus, a

direct result is that methanol formation rate and

selectivity is increased. From such an analysis, it is

easy to illustrate the pathway of methanol synthesis

and RWGS reaction by the scheme shown in Fig. 7: In

the absence of CO, the process of dissociative adsorp-

tion for CO2 could take place easily over the catalyst

surface [22] and there is a strong tendency for pathway

(I) to happen; even if the reaction occurs via pathway

(II), this surface (O=C±OH)s could dissociate to form

COs and OHs easily due to the absence of CO in gas

phase. However, when CO is introduced into CO2/H2

feed gas, the process of dissociative adsorption for

CO2 is inhibited due to the high coverage of CO. The

reaction tends to take place via pathway (II) to form

O=C±OHs, which then isomerizes to form other inter-

mediate species b-HCOOÿs , but could not dissociate to

form COs and OH easily. The direct result is that IR

band absorption intensity at 1600±1800 cmÿ1

ascribed to the rato-vibrational spectrum of H2O in

CO/CO2/H2 system is much weaker than that in

CO2�H2 system, and methanol formation rate and

selectivity are promoted significantly.

On the other hand, if the conclusions that b-HCOOÿsspecies are a key intermediate and hydrogenation of

b-HCOOÿs is the rate-limiting step for methanol synth-

esis are true, it seems to be ambiguous to explain the

fact of the lowering of apparent activation energy after

introducing CO gas into CO2/H2 mixture. Although

we did not make further study on the adsorption/

desorption species and hydrogenation activity of those

adsorbed species on this ultra®ne Cu/ZnO/Al2O3 cat-

alyst, some previous research works have given some

powerful hints which could be helpful to explain the

above divergency. Fujita et al. [23], found that two

types of adsorbed formate species, HCOO±Cu and

HCOO±Zn, can be formed and hydrogenate to form

CH3O±Zn and then methanol, but the rate constant of

the hydrogenation of HCOO±Cu is about 10 times

greater than that of the hydrogenation of HCOO±Zn

and the activation energy of the former is lower than

that of the latter. Based on this result, we can easily

explain the mechanism of methanol synthesis and the


fact of the lowering of apparent activation energy after

introducing CO into CO2/H2 mixture. For CO/H2

reaction system, a mount of Os or OHs is formed with

CH3O±Zn being formed from HCOO±Cu hydrogena-

tion, as shown in Fig. 7. The adsorbed Os or OHs could

inhibit the hydrogenation of HCOO±Cu, so, under

steady-state condition, methanol formation rate is

low and apparent activation energy is high. However,

after introducing CO into CO2/H2 system, CO reacts

quickly with Os to form CO2 and remove the adsorbed

Os from the equilibrium system. As a direct result, the

hydrogenation of HCOO±Cu is promoted and metha-

nol formation rate is accelerated as well as the appar-

ent activation energy of methanol synthesis is lowered.

4. Conclusion

The mechanism of methanol synthesis and RGWS

reaction was studied by using in situ IR technique. The

experimental results indicate that methanol was

formed directly from CO2 hydrogenation both for

CO2 and CO/CO2 hydrogenation. b-HCOOÿs is the

key intermediate for methanol synthesis and hydro-

genation of b-HCOOÿs is the rate-limiting step for

methanol synthesis. The addition of CO to CO2/H2

system not only affects the reaction pathway but also

lowers the apparent activation energy of methanol

synthesis.

References

[1] B. Denise, R.P.A. Sneeden, J. Mol. Catal. 17 (1982) 359.

[2] B. Denise, O. Cherifi, M.M. Bettahar, R.P.A. Sneeden, Appl.

Catal. 48 (1989) 365.

[3] Y. Amenomiya, Appl. Catal. 30 (1987) 57.

[4] J.A.B. Bourzutschky, N. Homs, A.T. Bell, J. Catal. 124 (1990)

73.

[5] Y. Nitta, O. Suwata, Y. Ikeda, Y. Okamoto, Catal. Lett. 26

(1994) 345.

[6] R.A. Koeppel, A. Baiker, Appl. Catal. 84 (1992) 77.

[7] J.S. Lee, K.H. Lee, S.Y. Lee, Y.G. Kim, J. Catal. 144 (1993)

414.

Fig. 7. Scheme for mechanism of methanol synthesis and RGWS/GWS reaction from CO2/H2 and CO/CO2/H2 over ultrafine Cu/ZnO/Al2O3

catalyst.


[8] D.L. Roberts, G.L. Griffin, J. Catal. 110 (1988) 117.

[9] D.L. Roberts, G.L. Griffin, Appl. Surf. Sci. 19 (1984)

298.

[10] D. Bianchi, J.L. Gass, M. Khalfauah, S.J. Teicher, Appl.

Catal. A 123 (1995) 89.

[11] D.B. Clarke, A.T. Bell, J. Catal. 154 (1995) 314.

[12] G.N. Stelios, A.J. Marchi, G.F. Froment, Appl. Catal. A 86

(1992) 45.

[13] J.F. Edwards, G.L. Schrader, J. Catal. 94 (1985) 175.

[14] G.A. Vedage, R. Pitchai, R.G. Herman, K. Klier, Amer.

Chem. Soc. Div. Fuel Chem. Prepr. Paper 29 (1984)

196.

[15] G.C. Chinchen, K. Mansfield, M.S. Spencer, CHEMTECH 20

(1990) 692.

[16] K.C. Waugh, Catal. Today 15 (1992) 51.

[17] J.F. Deng, Q. Sun, Y.L. Zhang, S.Y. Chen, D. Wu, Appl.

Catal. A 139 (1996) 75.

[18] Q. Sun, Y.L. Zhang, H.Y. Chen, J.F. Deng, D. Wu, S.Y. Chen,

J. Catal. 167 (1997) 92.

[19] Y.L. Zhang, Q. Sun, J.F. Deng, D. Wu, S.Y. Chen, Appl.

Catal. A 158 (1997) 105.

[20] N. Sheppard, T.T. Nguyen, in: R.J.H. Clark, R.E. Hester

(Eds.), Advances in Infrared and Raman Spectroscopy,

Heydon, London, 1978.

[21] M. Bowker, R.A. Hadden, H. Houghton, J.N.K. Hyland, K.C.

Waugh, J. Catal. 109 (1988) 263.

[22] R.A. Hadden, H.D. Vandervell, K.C. Waugh, G. Webb, Catal.

Lett. 1 (1988) 27.

[23] S. Fujita, M. Usui, H. Ito, N. Takezawa, J. Catal. 157 (1995)

403.


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