reduction of cuo-containing catalysts: cuo/zno

React. Kinet. Catal. Let t . , Vol. 44, No. 2, 309-319 (1991)

REDUCTION OF CuO-CONTAINING CATALYSTS: CuO/ZnO

A.L. Boyce, P.A. Sermon x, M.S.W. Vong and M.A. Yates

Department of Chemistry, Brunel University

Uxbridge, Middlesex, UB8 3PH

Received November 23, 1990 Accepted January 15, 1991

CO reduction of CuO in CuO/ZnO samples at 423 K is not

retarded by the support and occurs with no evidence of

Cu20 formation as reported (Porta et al. 1989). The po-

sitive order with regard to CO concentration varies

with the precise range of CO partial pressure.

Ha BOCCTaHOBneHHe CuO C nOMO~bm CO B o@pa3~ax CuO/ZnO

npH 423 K HOCMTenb He OKaBMBaeT 3aMe~nHD~eFo BnH~HHH H

HHKaKHx ~OKagaTe~SCTB O6pH3OBaHHH CugO He Ha6nD~a~ocs,

O qeM coo6~anH (Porta etal Solid State Ionics 32-33,1019,(1989)).

HO~OKHTe~bHbI~ HopH~OK KOH~eHTpa~HH CO H3MeHHeTCH B BaBM-

CMMOCTH OT HpHMeHHeMO~ o6~aCTH HapHHa~bHOFO ~aBneHM~ CO.

INTRODUCTION

TWO views have been taken of the mode of operation of

Cu/ZnO/AI203 catalysts in converting C0(I0%)/C02(I0%)/H2(80% ) at 50-100 atm and 523-573 K to methanol; these are that:

(i) CO is the component converted to CH3OH via sequential hy-

drogenation at a Cu + ion substituted into the ZnO lattice,

while the CO 2 maintains the copper in a positive oxidation

state; not Surprisingly this model invokes a synergy between

the copper and zinc oxide components [1].

(ii) CO 2 is more readily hydrogenated (with CO [2] scavenging

from the copper surface liberated by CO 2) on the active sur-

Akad~miai Kiad~, Budapest

BOYCE et al.: CuO/ZnO

face which is copper alone; this has no need to invoke copper/

ZnO synergy and CO 2 and H 2 form formate species whose hydroge-

nation/hydrogenolysis is the rate-determining step; H 2 is also

possibly held by ZnO [3] in a subsurface form.

In this context there is thus disagreement about the oxid-

dation state of Cu [5], with some emphasizing zero-valent com-

ponents [6] and others the positive oxidation state [7]. There-

fore, although supported copper is used to catalyze the synthe-

sis of methanol (e.g. Cu/ZnO/AI203 or Cu/ZnO/Cr203 [4]), there

is uncertainty and current interest in the chemical state of

this copper in contact with ZnO under reaction conditions.

CuO/ZnO is a model methanol synthesis catalyst [8] and has

been studied here using in situ X-ray diffraction in a flowing

reductant atmosphere at moderate temperatures. Previously, some

of the present authors have followed the isothermal reduction

of 15% CuO/ZnO by hydrogen at 477 K [9] by in situ X-ray dif-

fraction; this has revealed replacement of monoclinic CuO by

zero-valent Cu ~ The kinetic data derived from such diffraction

results [9] obey the Avrami relationship, suggesting that this

catalyst transformation is nucleation-controlled. Now it has

been shown [10] for CuO that CO is a better reductant for CuO

than H 2. Therefore, it seemed important to investigate the cor-

responding isothermal reduction of model CuO/ZnO catalysts [9],

for which even H 2 reduction is complicated by whether the cop-

per passes directly from Cu 2+ to Cu ~ or whether it goes through

intermediate Cu + prior to total reduction [ii]. However, CO re-

duction of these catalysts has been even more poorly considered

and understood and, therefore, bearing in mind the use of such

catalysts in CO/H 2 reactions, this seemed to require attention

in terms of the reduction processes, mechanism and kinetics. In

situ X-ray diffraction (XRD) has been utilized here to attempt

to consider the reducibility of such CuO/ZnO catalysts by CO

principally.

310

BOYCE et al.: CuO/ZnO

EXPERIMENTAL

Materials. The following gases (BOC) were used:

2.5% CO/N 2

6.0% CO/N 2 6% H2/N 2

12.0% CO/N 2

100% CO

CuO (Ventron, Alfa Produkte} and Cu ~ powder (Johnson Matthey

99.8%) were used for calibration purposes. Catalysts are de-

scribed by their composition in the reduced state even though

as used here, they have been calcined. 3-15% Cu/ZnO catalysts

were prepared by co-precipitation from aqueous solution of

cupric nitrate: Cu(NO3)2.3H20 (Johnson Matthey; Puratronic)

zinc acetate: Zn(CH3COO) 2-2H20 (BDH; AnalaR, 99.5%)

with NaHCO3/NaOH. After allowing this mixture to stand for 0.5

h at 298 K, it was filtered, and the precipitate washed well

with deionized water until the conductivity of the filtrate was

constant and low (<10 S cm2). It was then dried at 403 K over-

night (16 h) and finally calcined (see Fig. i).

In situ XRD. A Philips PWI710 with CuK~ was utilized with an in

situ cell [123 held at constant temperature (423 K). Through

this the reactant gas was admitted and analysis indicated as a

function of time under flowing conditions using the following

underlined reflections:

CuO o2@ 38.8

(100%)

Cu20 o20 42.2

(4o%)

Cu ~ ~ 43.2

(100%) ZnO 020 36.8

(100%) 31.7 34.5

(70%) (6O%)

Unfortunately, the observation of the emergence of Cu20 was

only possible in terms of a rather weak peak. Figure 1 shows X-

311

BOYCE et a l . : CuO/ZnO

65.2 ~

Fig. i. XRD traces of 8% CuO/ZnO (precipitate dried at

403 K for 16 h (a), calcined for 3 h at 523 K (b),

calcined for 3 h at 623 K (c) and calcined for

3 h at 723 K (d)

ray analyses an 8% CuO/ZnO sample as a dried precipitate and

during subsequent calcination. Clearly the dominant peaks are

those of essentially ZnO (which is not surprising for a ZnO-

rich sample). The dried precipitate was copper-containing hy-

drozincite, which is different from that in more Cu-rich sam-

ples [13]. Since the CuO peak at d=0.2315_ 8 nm was well re-

solved in 6% and 15% CuO/ZnO, these samples were considered for

in situ analysis in greater detail, although it is appreciated

that the X-ray analysis is then only of the most crystalline

component of the samples.

TPR of samples (200 mg) was carried out with flowing 6% H2/N 2

reductant at 4 K/min. Some measurements were also carried out

with CuO/ZnO at heating rates B of 6-16 K/min.

XPS. For in situ work the chamber associated with a KRATOS ES

300 spectrometer was raised to 423 K in vacuum and 6% CO/N 2 ad-

mitted to the sample. After re-evacuation to 13.3 mPa XPS anal-

312


Fig. 2.

il r

I t i I I J i

373 &73 573 T(K)

TPR profiles for the reduction of CuO/ZnO in 6%

H2/N 2 (% Cu loadings are indicated)

yses [113 with a monochromatic X-ray source (AIKa:I486.6 eV)

were carried out.

RESULTS

Since in XPS of 15% Cu/ZnO heated in situ at 473 K in 6

kPa CO the CU2p peaks fell to 933.3 and 952.8 eV the sample

was reduced. TPR profiles in 6% H2/N 2 (see Fig. 2) for Cu/ZnO

samples showed that the maximum rate of reduction was independ-

ent of Cu loading. This was also true for the percent CuO re-

duction (i.e. for 3% CuO/ZnO, 6% CuO/ZnO, 9% CuO/ZnO; 12% CuO/

ZnO and 15% CuO/ZnO the percent reductions of CuO seen by TPR

were 70%, 76%, 76%, 74% and 74%). TPR data indicated that the

relevant activation energies for reduction by H 2 (27 kJ/mol)

were much lower than seen for CuO alone [103. However, tempera-

tures of the maximum rates of reduction by H2(T m) are similar

313


200

A

E 160 E

e.-

120 r

r

Q n - 80 x

40

\ %

# ##

\

' ' ' / o 5'0 0 10 20 30 t(min)

Fig. 3. Reduction profiles of 15% CuO/ZnO by 2.5% CO

(20 cm3/min) at 423 K indicated in terms of CuO

loss {o) and Cu ~ formed (e)

to those seen for CuO alone [I0].

XRD

Effect of ZnO support. Using 15% Cu/ZnO no reduction was ob-

served at 423 K with 6% H 2 over a period of two hours, from

which it may be inferred that CuO in contact with ZnO i_~s less

readily reduced than CuO alone [I0] by hydrogen. However, pre-

viously [9 ] ZnO has been found not to retard CuO reduction by

H 2 at 419-427 K and this is also suggested by the above TPR

results, although maximum extents of CuO reduction in CuO/ZnO

are only 74%. However, with only 2.5% or 6% CO this 15% CuO/ZnO

314

E E

v

80-

64-

x

~ 48- o4 N

~ 3)+- O

Fig. 4.

BOYCE eta].. : CuO/ZnO

0

./-" I I I

2o 8; t(min)

Production of Cu ~ (e) from 6% CuO/ZnO at 423 K in

6% CO (20 cm3/min)

(and also 6% CuO/ZnO) were seen to reduce at 423 K (see Figs

3,4). The process in the latter catalyst may show some oscil-

latory behavior in the rate of Cu ~ formation, but at this lower

Cu loading the CuO phase was not observable. The effect of the

ZnO support on CuO reduction in CuO/ZnO catalysts will be con-

sidered further.

Effect of Partial Pressure of CO. Using 15% CuO/ZnO the effect

of Pco on reduction rate was determined and the results are

shown in Fig. 5; clearly as the partial pressure of CO increases

so the rate of reduction increases and the extent of reduction

also increases. At lower partial pressures of CO the order with

respect to CO appears to approach unity, but decreases at the

highest CO partial pressures considered.

DISCUSSION AND CONCLUSIONS

Comparing in Fig. 6 the rates of conversion of pure CuO to

that of the supported 15% CuO on ZnO, it can be seen that there

is little difference between the two. Specifically, here it has

been noted that (although most Cu in CuO/ZnO is not X-ray de-

315


500

400 0

E ~ 3O0

e-.

" - 200 4 r

Q n." 100 X

}S Fig. 5.

I I I

o 2h 30 so t(min)

CO reduction of 15% Cu/ZnO at 423 K with 2.5% (o),

6% (o), 12% (m) and 100% (e) CO

tectable below 10% Cu/CuO) CuO is reduced by CO at 423 K at

about the same speed irrespective of whether the CuO is in con-

tact with ZnO or not. This is quite surprising in the light of

the possible stabilizing role of ZnO in CuO/ZnO catalysts [13.

Thus, both have an induction period of approximately 25 min and

both react to their final equilibrium extents of reduction in

a similar time. However, this may be for more captalline CuO

not in close contact with ZnO.

Increasing the CO partial pressure does decrease the in-

duction period and increases the isothermal reduction rate. It

would seem that 6% CO would prove to be the best compromise

between having good rates and extents of reduction, coupled

with a modest danger of sintering in the exothermic reaction.

Shorter induction periods (approximately i0 min) at low Cu

316

300 %

r "

c_

OJ C

- - 100 Q n~ x

A v

..... t12o ~ -moo


0 I l I I I I L

80 "l:(rnin)

BOO

600

ZOO

200

0

Fig. 6. Comparison of rates of reduction at 423 K of

Ultrapure CuO (o) and 15% CuO/ZnO (e)

loadings could be due to higher surface areas. There is no evi-

dence of the formation of Cu20 on reduction of CuO/ZnO as noted

by others [113, or at least its concentration appears to be

below the X-ray threshold, but here direct observation of Cu20

is preferred to indirect inference [113 of up to 60% Cu20 and

this is difficult by direct XRD of CuO/ZnO since its (200) and

(Iii) peaks are too close to the (iii) peak for Cu ~ and the

(I01) peak of ZnO.

It could be that a significant fraction of copper is not

analyzed by X-ray diffraction in Cu/ZnO and this may have its

reduction characteristics modified by ZnO. Indeed this fraction

may be within the ZnO matrix itself, as suggested by high re-

solution electron microscopy (unpublished results).

Acknowledgements. The support of SERC for MSWV and MAY is

gratefully acknowledged.

317


REFERENCES

i. K. Klier: Adv. Catal., 31, 243 (1982); E. Giamello, B. Fu-

bini: J. Chem. Soc., Farad. Trans. I., 79, 1995 (1983)

2. Yu.B. Kagan, L.G. Liberov, E.V. Slivinskii, S.M. Loklev,

G.I. Lin, A.Ya. Rozovskii, A.N. Bashkirov: Dokl. Akad. Nauk

SSSR, 221, 1093 (1975); M. Bowker, H. Houghton, K.C. Waugh:

J. Chem. Soc. Farad. Trans. I., 77, 3023 (1981); M. Bowker,

J.N.K. Hyland, H.D. Vauderve!l, K.C. Waugh: Proc. 8th Int.

Congr. Catal., I I, 35 (1984); Verlag Chemie; G.C. Chinchen,

P.J. Denny, P.G. Parker, D. Short, M.S. Spencer, K.C. Waugh,

D.A. Whan: Prep. Am. Chem. Soc., Div. Fuel Chem., 2-9, 178

(1984); G.C. Chinchen, P.J. Denny, D.G. Parker, M.S. Spen-

cer: Appl. Catal., 30, 333 (1987); G.C. Chinchen, K.C.

Waugh, D.A. Whan: Appd. Catal., 25, i01 (1986); B. Denise,

R.P.A. Sneeden, B. Beguin, O. Cherifi: AppI. Catal., 30,

353 (1987); M. Bowker, R.A. Hadden, H. Houghton, H.N.K. Hy-

land, K.C. Waugh: J. Catal., 263 (1988)

3. G.C. Chinchen, C.M. Hay, H.D. Vaudervell, K.C. Waugh: J.

Catal., 79, 103 (1987)

4. R.G. Herman, K. Klier, G.W. Simmons, G.W. Finn, B.P. Bulko,

J.B. Kobylinski: J. Catal., 56, 407 (1979)

5. S.P.S. Andrew': 7th Int. Congr. Catal. Symp., Osaka, paper

12, 1980.

6. G.C. Chinchen, K.C. Waugh, D.A. Whan: Appl. Catal., 2_~5,

i01 (1986)

7. U.S. Patent 4,332,650 (1982)

8. V.E. Ostroskii, A.A. Dyatlov, N.N. Pobrovolskii: Kinet.

Katal., i_~7, 405 (1976)

9. P.A. Sermon, M.S.W. Vong, K. Grant: 'Structure and Reacti-

ity of Surfaces' ed. Morterra, C., Zecchina, A., Costa G.

p. 55. Elsevier 1989; M.S.W. Vong, P.A. Sermon, K. Grant:

Catal. Lett (in press).

i0. A.L. Boyce, S.R. Graville, P.A. Sermon, M.S.W. Vong: React.

Kinet. Catal. Lett., 44, I (1991); ibid., 44, 13 (1991).

ii. P. Porta, G. Fierro, M. Lo Jacono, G. Morets Catal.

Today, 2, 675 (1988); P. Porta, R. Dragone, M.L. Jacono,

318


G. Minelli, G. Moretti: Reactivity of Solids, 2, 1019

(1988), ed. M.S. Whittingham, S. Bernasek, A.J. Jacobson,

A. Navrotsky, North Holland} C. Fragale, M. Gargano and M.

Rossi: Cong. Naz. Chim. Inorg., 15, 185 (1982); P. Porta,

R. Dragone, L.M. Jacono, G. Minelli, G. Moretti: Solid

State Ionics, 32-33, 1019 (1989).

12. M.S.W. Vong, P.A. Sermon, V.A. Self, K. Grant, A.J. Black-

burn: J. Phys. E, 21, 495 (1988)

13. D. Waller, D. Stirling, F.S. Stone, M.S. Spencer: J. Chem.

Soc. Faraday Trans. Disc., paper 231 (1989)

319

reduction of cuo-containing catalysts: cuo/zno

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