X-RAY ABSORPTION SPECTROSCOPY STUDY OF Cu-BASED METHANOL CATALYSTS. 2. REDUCED STATE

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X-RAY ABSORPTION SPECTROSCOPY STUDY OF Cu-BASED METHANOL CATALYSTS. 2. REDUCED

STATE

B. Clausen, B. Lengeler, B. Rasmussen, W. Niemann, H. Topsøe

To cite this version:

B. Clausen, B. Lengeler, B. Rasmussen, W. Niemann, H. Topsøe. X-RAY ABSORPTION SPEC- TROSCOPY STUDY OF Cu-BASED METHANOL CATALYSTS. 2. REDUCED STATE. Journal de Physique Colloques, 1986, 47 (C8), pp.C8-237-C8-242. �10.1051/jphyscol:1986844�. �jpa-00226166�

JOURNAL DE PHYSIQUE

Colloque C8, suppl6ment au n o 12, Tome 47, decembre 1986

X-RAY ABSORPTION SPECTROSCOPY STUDY OF Cu-BASED METHANOL CATALYSTS.

2. REDUCED STATE

B.S. CLAUSEN, B. LENGELER', B.S. RASMUSSEN, W. NIEMANN and H. T O P S ~ E

Haldor Topsge Research Laboratories, DK-2800 Lyngby, Denmark

" ~ n s t i t u t fiir Festkorperforschung, Kernforschungsanlage Jiilich, D-5170 Jiilich, F.R.G.

RESUME La spectroscopic d'absorption aux rayons X (XAS) faite &

situ a EtE utilisee pour eclaircir la constitution de catalyseurs

=ethanol binaires Cu-Zn et ternaires Cu-Zn-A1 dans 116tat r6duit.

Les rgsultats de la XAS des catalyseurs binaires montrent des pro- priet6s d'une phasg de cuivre metallique aprss rsduction 2 une temperature de 220 C . I1 a etE constat6 que la constitution des catalyseurs ternaires rgduits depend d'une manikre cruciale et de la concentration de cuivre et de la temperature de redgction. Au contraire des catalyseurs binaires, la reduction ?I 220 C ne mene &

aucun changement significatif cornparse h l'etat calcin6. A 2 6 0 ~ ~ la reduction de la phase de cuivre l'stat metallique se fait, et il a bt6 remarque que la grandeur des crystallites ou le degre d'ordre du cuivre metallique augmente avec la teneur en cuivre.

ABSTRACT In situ X-ray absorption spectroscopy (XAS) has been used to elucidate the structure of binary Cu-Zn and ternary Cu-Zn-A1 meth- anol catalysts in the reduced state. The XAS results of the binary catalysts show the features of a metallic Cu phase after reduction at 2 2 0 ~ ~ . The structure of the reduced terdary catalysts is found to depend critically on both the copper concentration and the re- d u c t i o ~ temperature. In contrast to the binary catalysts, reduction at 220 C does not result in any significant change as compared with the calcined state. At 260°c, reduction of the copper phase to the metallic state is accomplished, and the crystallite size or degree of order of the metallic Cu is found to increase with the copper content.

INTRODUCTION

In a recent study (1) we have investigated the calcined state of binary Cu-Zn and ternary Cu-Zn-A1 methanol catalysts by use of X-ray absorption spectroscopy (XAS), X-ray diffraction (XRD) and electron microscopy (TEM and AEM). These results showed that copper in both

the binary and ternary catalysts is present as cu2+, however, with a local environment different from that of copper in well-crystal- lized CuO. While the binary catalysts showed the presence of small particles of well-defined ZnO, the ternary catalysts were found to contain zinc in microcrystalline or amorphous-like structures.

Furthermore, the results suggested that Zn2+forms a mixed oxide phase with cu2*and possibly aluminum.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986844

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In the present study, XAS results on the reduced state of the cata- lysts will be reported in order to get further chemical and struc- tural information on this state.

Previously, a large number of investigations on the reduced state of Cu-Zn methanol catalysts (prepared by different methods) have been reported in the literature. Klier and coworkers (2-7) have claimed that in reduced, binary Cu-Zn catalysts, Cultspecies are dissolved in the ZnO lattice, and the authors have proposed that these .copper species are catalytically active in the methanol syn- thesis. The presence of these cult species was evidenced by com- bining results from diffuse reflectance spectroscopy, scanning transmission electron microscopy (STEM) and by X-ray diffraction

(2,3,5). It was furthermore claimed that about half of the Cu in typical binary catalysts (30% Cu(Zn0) was present as cult.

More recently, Okamoto et al. (8) have used X-ray photoelectron spectroscopy (XPS) to study similar catalysts prepared by either coprecipitation or impregnation. They found that a monovalent cop- per species was present in the surface of the catalysts and that Cu metal in high dispersion was the predominant species at high cop- per concentration, whereas at low copper concentration the major copper species was most successfully described by a two-dimensional epitaxial meta&lic copper layer over ZnO. They suggested that two- dimensional Cu -~u"species are catalytically active. This model has also been advocated by Tohji et al. ( 9 ) in an XAS study of binary Cu-Zn catal sts. A proposed mechanism for methanol synthesis in- volving CulYsites in the zinc oxide has recently been given by

Edwards and Schrader (lo), who used Fourier transform infrared spec- troscopy to study Cu-Zn and Cu-Zn-Cr catalysts.

EXPERIMENTAL

The preparation of model compounds and catalysts has been described in detail in (1). After coprecipitation and calcination, the cata- lyst powder was pressed into self-supporting wafers (1.125" dia- meter) and placed in specially designed in situ cells equipped with X-ray transparent windows (11). The activation of the catalysts was carried out in these cells by letting a gas mixture consisting of 3% Hz and 3% H20 in Np flow over the catalyst at various temper- atures. After reduction, some of the catalysts were also subjected to the synthesis gas (5% CO, 5% CO2, 87.2% Hp, 2.8% Ar) for dif- ferent time spans. Following the different treatments, the cata- lysts were cooled to room temperature in the flowing gas, and the cell was subsequently sealed off.

The absorber thickness, x , of all the samples was chosen so that yx

-

The extended X-ray absorption fine structure (EXAFS) oscillations were analysed according to the procedure described in detail in

(12,13). This procedure involves a background subtraction by means of cubic spline functions, multiplication of the EXAFS by a factor

k 2 , and normalization by-the jump height at the K-edge. All the data reported in this paper were obtained at room temperature.

The chemical analyses obtained before reduction for Cu and Zn as well as for Cu, Zn and A1 in the binary and the ternary catalysts, respec- tively, are given in Table 1.

TABLE 1: Chemical ~ n a l ~ s e s l ' of the Binary and Ternary Catalysts Metal content, wt%

Sample Cu Zn A1 ZnO crystallite diameter2', A

B1 0.49 75.0 91

B2 1.0 73.7 8 4

B 3 4.9 69.4 7 8

B4 9.9 63.6 5 6

T 1 0 53.2 12.1

T2 4.9 47.3 12.0

T3 9.1 42.0 11.7

T 4 13.8 37.5 11.7 T 5 18.0 33.1 11.7 T 6 26.6 24.6 11.6

1) Besides oxygen, the mixed metal oxides only contain volatiles like H20 and COT. The residual amount of alkali and nitrates is less than 500 ppm.

2) Estimated from the (102)-diffraction line of ZnO. This line could not be resolved for the ternary catalysts.

Figure 1 shows the Fourier transform of the EXAFS above the Cu K- edgg for Cu metal and the binary B3 catalyst after reduction at 220 C. The Fourier transform for the catalyst displays essentially the same peaks as Cu metal, but their intensity is somewhat reduced.

-KH Cu _metal

- ₀₃ catalyst

1 red. at 220 O C

FIGURE 1 Absolute magnitude of the Fourier transforms of EXAFS k2 above the Cu K-edge for metallic Cu and the B3 catalyst.

Furthermore, it was found that the edge position of the binary catalyst is identical with that of Cu metal. Only minor dif- ferences exist between the XAS spectra at the Cu K-edge of the different binary catalysts.

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The Fourier transforms of the EXAFS above the Cu edge for the ternary T2 catalyst in the calcined state and after reduction at 2 2 0 ~ ~ are shown in Figure 2. Essentially, no difference is ob- served for the two transforms. All the ternarg catalysts show al- most identical spectra after reduction at 220 C, irrespective of Cu content.

T2 catalyst

30 ^+H+ - ^calc red at 220°C

- "Y 20

X

-

+

LL

10

0

0 2 6

FIGURE 2 Comparison of the Fourier transformed Cu EXAFS of the T2 catalyst in the calcined state and after reduction at 220°c.

Figure 3 shows the Fourier transform for the same catalyst after re- duction at 2 6 0 ~ ~ . The Fourier transform for Cu metal is also included

in the figure. It is evident that the increased reduction tempera- ture results in significant changes in the EXAFS of the catalyst.

Only backscattering from one or two.shells can be identified above the noise level in the spectra.

In Figure 4 are shown the Fourier transforms of the EXAFS for a high Cu-content catalyst (T6) after reduction at 2 6 0 ~ ~ and after a typi- cal run at methanol synthesis conditions. It is clear that back- scattering from the five nearest neighbours in the FCC structure of metallic Cu can at least be identified in the transforms of the catalysts. However, the peak intensities are lower than those of

the Cu metal.

DISCUSSION

The XAS results show that several preparation and activation para- meters influence the structure and chemical state of Cu-based meth-

anol catalysts. Figure 1 reveals that the binary Cu-Zn catalysts are reduced to copper metal at relatively low temperatures (- 2 2 0 ~ ~ )

.

Furthermore, we find from a study of the position of the Cu K-edge that it is identical to that of metallic copper. The near edge structure (XANES) also displays the typical features of metallic copper. Thus, in contrast to many previous proposals (see e.g. 2,8- l O k the XAS results of the present binary catalysts do not reveal Cu species in any significant amount. The lower intensity of the peaks in the Fourier transform of the catalyst as compared with that of Cu metal suggests that the metallic Cu particles in the catalyst are either very small or have a rather disordered structure.

+w Cu metal

- T 2 catalyst red. a t 260°C

FIGURE 3 Fourier transforms of the Cu EXAFS for the T2 catalyst after reduc- tion at 2 6 0 ~ ~ . The transform for Cu met- al is also shown.

+I+ Cu metal

- T6 catalyst small - ^{red at} 260°C

large - after methanol run FIGURE 4 Fourier transforms for metal- lic copper and the T6 catalyst afger re- duction at 260 C and after run at methanol synthesis conditions.

For the ternary catalysts, reduction at 2 2 0 ~ ~ is not sufficient to reduce the copper atoms beyond the divalent state. Upon increasing the reduction temperature to 260°c, the XAS results show that very different structures are obtained, depending on the copper concen- tration in the catalysts. While the Fourier transform for the catalyst with low Cu content (T2,

-

-

A more detailed study of the Fourier transform for the T2 catalyst displayed in Figure 3 shows that a small peak at about 1.5 A is present after reduction at 2 6 0 ~ ~ . This peak is located at the same position as the dominating oxygen peak in Figure 2. Therefore, it is likely that only part of the copper atoms have Cu as nearest neighbours in the low loading T2 catalyst, and part of the copper still has residual oxygen in the coordination shell.

JOURNAL DE PHYSIQUE

In (1) it was proposed that in ternary Cu-Zn-A1 catalysts in the calcined state, cu2'and zn2+may form a mixed oxide phase which is basically amorphous. This phase may also contain aluminum. The pre- sence of the aluminum clearly has a profound influence on the reduc- tion behaviour of the ternary catalysts, since much higher reduction temperature is needed before these catalysts reduce as compared with the binary ones. Strong interactions between the copper and aluminum species may also be the reason why low loading catalysts apparently form much smaller Cu crystallites than the catalysts with high Cu loading.

In this respect it is interesting that although metallic Cu par- ticles seem to segregate out of the mixed oxide phase upon reduc- tion of the ternary catalysts, essentially no change is observed for the Zn phase. It stays in the highly amorphous state which again indicates the strong interaction with the A1 species. More detailed XAS results at the Zn K-edge will be reported in a forthcoming publi- cation.

Extended use of the ternary catalysts under methanol synthesis con- ditions results in larger crystallites or in a higher degree of order in the metallic copper phase (Figure 4). These changes are probably indicative of some structural changes occurring during ageing of the catalysts, and further studies are in progress to re- late these structural phenomena with catalytic activity.

ACKNOWLEDGEMENTS

We are grateful to HASYLAB for offering beam time on the synchrotron radiation facility and for access to the R6mo spectrometer. We are also grateful to J.W. 0rnbo for technical assistance.

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