5 November 1999

Chemical Physics Letters 313 Ž1999. 14–18
www.elsevier.nlrlocatercplett

Active sites for methanol synthesis on a ZnrCu ž100 / catalyst
Zhen-Ming Hu 1, Hiroshi Nakatsuji )Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto UniÕersity, Sakyo-ku, Kyoto,606-8501, Japan
Received 16 July 1999; in final form 30 August 1999

Abstract
The active sites for methanol synthesis on a ZnrCuŽ100. catalyst are clarified using the dipped adcluster model ŽDAM.
combined with ab initio HF and MP2 calculations. In comparison with that on a CuŽ100. catalyst studied previously, it isshown that the Cu–Zn site provides an easier reaction pathway for the rate-limiting step; the energy barriers are all lower
and the dioxomethylene intermediate is more stable than that at the Cu–Cu site on a CuŽ100. catalyst. The present results
prove that the Cu–Zn site is the active site which cooperates with the Cu–Cu site to catalyze methanol synthesis. Electrontransfer is the important factor in affecting the reactivity of the catalysts. q 1999 Elsevier Science B.V. All rights reserved.

1. Introduction
The hydrogenation of CO 2 to methanol on a
Cu-based catalyst has been the subject of numerous
investigations w1x. Currently, most attention has been
directed at clarifying the reaction mechanism and thenature of the active site on the catalyst w2–17x. In
order to answer some of these questions, a series ofinvestigations has been performed for methanol synthesis using model catalysts such as CuŽ100. w5–8x,
CuŽ110. w10x, CuŽ111. and Zn vapor-deposited copper surfaces w11–13x. The advantage of the model
system is that it can give fundamental insight inmethanol synthesis as compared with the behavior ofreal CurZnO catalyst.
)
Corresponding author. Fax: q81-75-753-5910; e-mail:hiroshi@sbchem.kyoto-u.ac.jp
1
Present address: Department of Chemistry, Dalhousie University, Halifax, Canada B3H 4J3.

Chorkendorff and co-workers have shown thatmethanol can be synthesized from H 2 and CO 2 on aclean CuŽ100. surface w6–8x. Methanol is also synthesized from CO 2rH 2 mixtures on a clean polycrystalline Cu foil and CuŽ110. single-crystal surface
w9,10x. These authors suggested that the active site
for methanol synthesis on real CurZnO catalysts is
metallic Cu. Recently, Nakamura and co-workers
carried out a series of studies for methanol synthesis
on Zn-deposited copper single-crystal samples w11–13x. They found that, when the Zn coverage is; 0.2, the Zn-deposited CuŽ111. surface is highly
reactive and promotes methanol synthesis by an order of magnitude. The turnover frequency of
methanol with the model catalyst agreed with thatmeasured on real CurZnO catalyst under the same
reaction conditions w12x. The Cu–Zn is supposed tobe the active site for methanol synthesis w12,13x.
Very recently, Morikawa et al. w18x reported a DFT
periodic slab model study, in which an adsorbed Zn

0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.
PII: S 0 0 0 9 - 2 6 1 4 Ž 9 9 . 0 1 0 4 7 - 7

Z.-M. Hu, H. Nakatsujir Chemical Physics Letters 313 (1999) 14–18

state on a model ZnrCuŽ111. surface was suggested
as a step towards understanding the role of Zn.
In a previous study w19x, we reported a theoretical
study on the reaction mechanism of the hydrogenation of CO 2 to methanol on a CuŽ100. catalyst. It
was shown that formate, dioxomethylene, formaldehyde, and methoxy are the main intermediates, andthat hydrogenation of adsorbed formate to formaldehyde is the rate-limiting step in methanol synthesis. In this Letter, we report our studies for the active
site in methanol synthesis by examining the reaction
mechanism of the rate-limiting step on a ZnrCuŽ100.
catalyst.
The calculational methods are the same as thatused previously w19x, i.e., the dipped adcluster model

15

ŽDAM. w20–22x combined with ab initio Hartree–
Fock ŽHF. and second-order Møller–Plesset ŽMP2.
methods. The former Cu 8 Ž6, 2. DAM cluster ŽFig.
1A. is replaced by a Cu 7 Zn 1 DAM cluster ŽFig. 1B.
to model the ZnrCuŽ100. alloy catalyst. This cluster
can model the Cu–Zn bridge site reasonably and has
almost the same size as the Cu 8 Ž6, 2. cluster; therefore, the results on a ZnrCuŽ100. catalyst can becompared reasonably with those on a CuŽ100. catalyst. The surface coverage of Zn is 0.167 in the
Cu 7 Zn 1 cluster model, which may correspond to theexperimental value of 0.20 for the highly reactive
ZnrCuŽ111. surface w12x. Other details of the calculations are described elsewhere w19,23x.

2. Results and discussion

Fig. 1. Two-layer model clusters used in this study: ŽA. Cu 8 Ž6, 2.
cluster with the atoms 1–6 in the first layer, and 7 and 8 in thesecond layer; and ŽB. Cu 7 Zn 1Ž6, 2. cluster with Zn at the bridge
position of the first layer.

In the previous study w19x, the rate-limiting step inmethanol synthesis from CO 2 and H 2 on CuŽ100.
surface is identified to be the hydrogenation of adsorbed formate to formaldehyde. Fig. 2 shows theenergy diagrams of this step on a ZnrCuŽ100. alloy
catalyst in comparison with that on a clean CuŽ100.
catalyst. The energy diagrams are composed of twoelementary steps: the hydrogenation of formate todioxomethylene Žstep 1. and the hydrogenation of
dioxomethylene to formaldehyde Žstep 2.. The Cu–
Cu site of the CuŽ100. catalyst and the Cu–Zn site of
the ZnrCuŽ100. catalyst are responsible for thesereaction steps, and the reaction mechanisms are essentially the same on both catalysts.
The differences between the two energy diagrams
are evident. On a CuŽ100. catalyst, the activationenergy of step 1 is calculated to be 23 kcalrmol, andthe formation of dioxomethylene is endothermic by
17.1 kcalrmol. The next step Žstep 2. has an activation energy of 17 kcalrmol. The experimental activation energy for the hydrogenation of formate on aCuŽ100. surface was reported to be 19.6 kcalrmol
w8x. The relatively large energy barriers in steps 1
and 2 and an unstable dioxomethylene intermediateat the Cu–Cu site indicate a slow reaction path, andexplain the low activity of the clean CuŽ100. catalyst. On the other hand, on a ZnrCuŽ100. catalyst,
the activation energy of step 1 is calculated to be 8.5
kcalrmol, and it is exothermic by 2.7 kcalrmol. The

16

Z.-M. Hu, H. Nakatsujir Chemical Physics Letters 313 (1999) 14–18

Fig. 2. Energy diagram of the hydrogenation of formate to formaldehyde on a model ZnrCuŽ100. alloy catalyst Žsolid line. incomparison with that on a model CuŽ100. clean catalyst Ždashedline.. Energies are in kcalrmol.

dioxomethylene intermediate formed at the Cu–Znsite is 7.4 kcalrmol more stable than that at theCu–Cu site. It can be supposed that if formate
adsorbs at the Cu–Zn site, the reaction leading to
dioxomethylene would be much faster than that at
the Cu–Cu site. The activation energy of step 2 at
the Cu–Zn site is 11.5 kcalrmol, which is again
lower than the 17 kcalrmol for the same reaction at
the Cu–Cu site. All these results indicate that the
Cu–Zn site on a ZnrCuŽ100. catalyst cooperates
with the Cu–Cu site to make the reaction easier andworks as the active site for the hydrogenation offormate to formaldehyde, the rate-limiting step inmethanol synthesis.
Another interesting feature of the results is thatthe formate and the formaldehyde species adsorbed
at the Cu–Zn site are calculated to be less stable thanthat at the Cu–Cu site. Since the adsorbates prefer tooccupy the most stable site on the surface, most ofthem will be adsorbed at the Cu–Cu site. The Cu–Cu
site should play an important role in formate formation and in the reaction process of formaldehyde tomethanol product. Indeed, these reaction steps arepreviously calculated to proceed much more easily
than the rate-limiting step on a clean CuŽ100. catalyst w19x. Formate can be easily formed at the Cu–Cu
site and has been confirmed to be an intermediate inmethanol synthesis by various studies w2–13x. It wasalso shown that the Cu–Cu site is active for hydrogen dissociation on the catalyst surface w1,3x. Therefore, the Cu–Cu site is very important for methanol
synthesis w5–10x.
The above results provide a clearly understandable reaction mechanism for methanol synthesis on

Zn-deposited copper catalysts. CO 2 reacts with hydrogen on the catalyst to produce the adsorbed formate intermediate. Formate adsorbed at the Cu–Cu
site can be hydrogenated into dioxomethylene with arelatively higher activation energy. On the Zn-deposited copper catalyst, the Cu–Zn alloy sites areformed by substitution of Zn atoms with the Cu
surface atoms, as confirmed by the experimental
STM images w24x. Since the dioxomethylene intermediate at the Cu–Zn site is more stable and reactive
than that on the Cu–Cu site, formate on the Cu–Cu
site will migrate onto the Cu–Zn site and is hydrogenated into dioxomethylene via a lower transition
state. The hydrogenation of dioxomethylene to formaldehyde will proceed mainly at the Cu–Zn site
because of the existence of a low-energy path. The
formaldehyde product at the Cu–Zn site will thenmigrate back to the Cu–Cu site due to its higher
stability. As a result, both the Cu–Zn and Cu–Cu
sites are important and they cooperate with eachother to realize high reactivity in methanol synthesis.
The Cu–Zn site provides an active site for the
rate-limiting step and hence enhances the activity of
the catalyst. As pointed out previously w19x, a key toenhance the overall reactivity in methanol synthesis
is to design the catalyst which stabilizes the dioxomethylene intermediate and works to lower theenergy barrier in the hydrogenation of formate. The
Cu–Zn site on the Zn-deposited copper catalyst plays
such a role and explains the high reactivity reported
experimentally w11–13x.
The electronic origin of the higher reactivity of
ZnrCuŽ100. catalyst as compared to CuŽ100. catalyst is explained from the difference in the electronic
properties of Zn and Cu. Since all the adsorbates areelectron-withdrawing species, the charge transfer
ability of the catalyst appears to be the main factor in
affecting the reactivity. The Zn atom of the
ZnrCuŽ100. catalyst shows a different electronic
state from the Cu atoms: though Cu is in a neutral
metallic state, the Zn atom is almost in the ionic Znq
state. Therefore, Zn acts as an electron source andmodifies the electronic properties of the catalyst to
realize a larger charge-transfer ability. Table 1 showsthat in the rate-limiting step, the adsorbates havelarger negative charges on the Zn–Cu site than on
the Cu–Cu site. This confirms that the Zn-deposited
copper catalyst has larger charge-transfer ability.

Z.-M. Hu, H. Nakatsujir Chemical Physics Letters 313 (1999) 14–18

17

Table 1
Mulliken charges of the adsorbates on the Cu–Cu and Zn–Cu sites of the CuŽ100. and ZnrCuŽ100. DAM clusters
State

Cu–Cu site
Cu 1

Cu 2

adsorbates

Zn

Cu 2

y0.53
y0.84
y1.00
y0.78
y1.16

q0.02
q0.12
q0.14
q0.29
q0.48
q0.34

q0.02
q0.12
q0.14
q0.11
q0.18
q0.19

y0.53
y0.94
y1.32
y1.04
y1.16

q0.31
q0.61
q0.66
q1.12
q1.12
q0.81

y0.11
q0.04
q0.06
q0.20
q0.19
q0.20

a

ClusterFormateTS1
Dioxomethylene
TS2
FormaldehydeaZn–Cu siteadsorbates

Cu 8 Ž100. and ZnrCu 7 Ž100. neutral clusters.

Such an effect becomes significant in the reaction
steps and affects the reactivity of the catalysts towards methanol synthesis. The fact that the promoter
modifies the electronic properties of the metal surface and hence affects the reactivity has been reported for the reactions of S 2 and O 2 with metallic
Cu and CurZnO by Rodriguez and co-workers
w25,26x. It is also noteworthy that, in the rate-limiting
reaction step, the O–Zn bond distance is calculated
to be shorter than the O–Cu bond distance. The
O–Zn interaction is then stronger than the O–Cu
interaction, another difference of the ZnrCuŽ100.
catalyst in comparison with the CuŽ100. catalyst.
More details about the geometries and the electronic
properties of the reaction species will be described
elsewhere w23x.
The present theoretical results support the proposal of Nakamura and co-workers w11–13x that the
Cu–Zn site is an active site which cooperates with
the Cu–Cu site to catalyze methanol synthesis. However, they attributed the role of Zn to be stabilizationof the formate on the Cu–Zn site w12,27x. This is incontrast with our theoretical results which indicatethat the formate is less stable at the Cu–Zn site. The
reason for this difference is not clear at present.
From the calculated energy diagrams shown in Fig.
2, two significant roles of Zn are apparent: one is tostabilize the dioxomethylene intermediate on the
Cu–Zn site, and the other is to lower the activationenergies both in the hydrogenation of formate and in
the hydrogenation of dioxomethylene. The latter effect is more important in determining the reactivityof the catalyst surface.
The promotive effect of Zn was reported to besmall on the ZnrCuŽ100. and ZnrCuŽ110. catalysts

w13x, but significant on the ZnrCuŽ111. catalyst, andthe ZnrCuŽ111. catalyst has the largest reaction rate
w13x. The results on the ZnrCuŽ111. catalyst can beeasily understood: since CuŽ111. is the most inactive
surface and a clean CuŽ111. surface alone cannot
catalyze methanol synthesis, the promotive effect on
this surface should be most significant. From theview point of the surface structure, CuŽ100. is moresimilar to CuŽ111. than to CuŽ110., so that thepromotive effect of Zn for CuŽ100. may also be
evident but smaller than that for CuŽ111..
Finally, the above results may provide a clue forcatalyst design. Since both Cu–Zn and Cu–Cu sites
are very important in methanol synthesis, and the
Cu–Zn site mainly works to enhance the reaction of
the rate-limiting step and the Cu–Cu site mainly
responds for other reaction steps, the key point in
catalyst design is to realize highly dispersed Cu–Zn
and Cu–Cu sites, the latter site should be moreabundant than the former one. It has been shown that
the Zn–Zn site, which may be formed at high Zn
coverage on the surface, plays no role in methanol
synthesis w23x. Therefore, an active catalyst shouldhave Zn atoms in a highly dispersed state. The
reactivity of the catalysts may be very sensitive tothe coverage of Zn on the surface w12,13x, andfurther work is necessary for a deeper understanding.

3. Conclusions
In this study, we have clarified the role of theactive site of ZnrCu catalyst for methanol synthesis
by studying the reaction mechanism of the hydro-

18Z.-M. Hu, H. Nakatsujir Chemical Physics Letters 313 (1999) 14–18

genation of formate to formaldehyde on a model
ZnrCuŽ100. catalyst using the dipped adclustermodel combined with ab initio theoretical methods,
and compared this with previous results on a clean
CuŽ100. surface.
The effect of Zn on a ZnrCuŽ100. catalyst is tomodify the rate-limiting step of the reaction and theroles are two-fold: one is to lower the activationenergies of the hydrogenation reactions of formate
and dioxomethylene, and the other is to stabilize thedioxomethylene intermediate at the Cu–Zn site. The
role of the Cu–Zn site is then to enhance the reactivity of the adsorbed formate and dioxomethylene
species, and so act as an active site in methanol
synthesis.
The formate and formaldehyde species at theCu–Cu site are more stable than that at the Cu–Znsite. Except for the rate-limiting step, other reaction
steps relevant to methanol synthesis may proceed
mainly on the Cu–Cu site. Therefore, for overall
methanol synthesis both Cu–Zn and Cu–Cu sites areimportant and cooperate for smooth overall progressof the series of the reactions.
Electron transfer is an important key feature inthis catalytic reaction process. All the adsorbates areanionic on the surface, and the role of Zn is tomodify the electronic properties of the ZnrCuŽ100.
catalyst so as to realize larger electron transfer. Thisis the main factor which is responsible for the highreactivity of the formate and dioxomethylene on a
ZnrCuŽ100. catalyst.

Acknowledgements
The authors thank Mr. K. Takahashi and Mr. Y.
Maekawa for the assistance of some calculations.
Some calculations were performed using the computers at the Institute for Molecular Science. Part of thisstudy was supported by a Grant-in-Aid for Scientific
Research from the Japanese Ministry of Education,
Science, and Culture, and by a grant from the Kyoto
University VBL project.

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