On the Surface Sites Distribution in MoO3 Test Catalysts

J.W. Ward (Editor), Catalysis 1987 © 1988 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

567

ON THE SURFACE SITES DISTRIBUTION IN Mo0 3 TEST CATALYSTS

J. M. DOMINGUEZ, 0.

GUZ~~N

and A. GARCIA

Instituto Mexicano del Petr6leo, Apdo. Postal 14-805, C.P. 07730 Mexico, D.F., (Mexico) ABSTRACT The more probable surface sites in MoO l test catalysts were determined by means of microdiffraction analysis. The intensity ratios of spatial-group-forbidden reflections, (100) and (001), indi cate the step heigh~ and configurat~on, at atomic scale, of ~he uppermost crystal layer in contact with the gas reactants. INTRODUCTION During the past, several correlations have been sought between catalytic activity and the surface configuration of some catalysts. More

recently, the partial oxidation reactions of ole fins and low

alcohols were found sensitive to the Mo0 3 crystallographic faces

exposed (1,3). The (020) oriented crystals seem to favour the methanol oxidation reaction towards formaldehyde formation, while in the propylene oxidation the acrolein production is rather suppreThus, it has been concluded that (100) Mo0 3 type faces are more specific for allylic oxidation, while (010) ssed on those faces.

type faces are more active for the complete oxidation reaction

(1-3).

However, those correlations assume that the catalysts sur-

face is rather smooth but, as reported recently (4,5), there are evidences for surface roughness and short-range order (sro) on the Mo0 3 crystallites, used as oxidation test catalysts, that would modify the active surface area by a factor of two or more. Others authors (6,7) have pointed out that small, but real changes in the surface of Mo0 3 single crystals are observed upon The O/Mo ratio presents a minimum at treat-

sequential heating.

ment tempezatures near 600 K.

In fact, the Mo0 3 based catalysts are sensitive to irradiation and thermal treatments (5,6 and 8).

568

1'hus, from a basic viewpoint, :nore understanding on the surface crystallography and stoichiometry is needed to obtain more precise correlations with catalytic activity. In this view, electron microdiffraction and high resolution microscopy techniques are potentially i.:nportant because they give a local information on the real crystal structure. The former method has proven powerful for mappin~ the entire crystal surface of Mo03 crystallites (4,5), and this may lead to determination of the more probable surface site distribution; thus, the aim of the present work was to extend previous results (4,5) with new information on the planar defects and on the crystal surface arrays, which could be relevant for the catalytic hydrocarbon oxidation reactions on Mo0 3test catalysts. EXPERIMENTAL PROCEDURES Microdiffraction The electron microdiffraction techniques are based in the use of a parallel narrow beam, for obtaining the diffraction patterns from very small areas, of about 200 A diameter. As the typical M00 3 crystallites have dimensions of 1 pm (lenght) x 0.2 pm (widthl x 0.03 pm (thickness), it is po,:;slt>le to make the diffraction from crystalline areas under the c rye t.a I d Ims.ns Lons , If a convergent microbeam is used instead, smaller areas, of about 50 A diameter, can be investigated by this method; in this case one obtains a disc pattern instead of a spot pattern. The microdiffraction methods have proved powerful to asses the surface crystallography of thin crystals (4,5). Essentially, the method makes use of the alternate appearance of forbidden reflections, i.e. the (100) and (001) spatial-group forbidden refle£ tions for the orthorhombic Mo03 unit cell. As pointed out earlier (5), the appearance of the (100) and (001) spots in the diffraction patterns is closely related to the incomplete filling of the underor-uppermost unit cell of the crystal. In this way, a systematic mapping of the crystals surface can be made; thus, for a certain number of crystallites in the sample, one obtains a statistical distribution of the more probable surface steps and, consequently, of the real surface sites. However, it was found that continuous observation of thin Mo03 crystals in the electron microscope, lead to some radiation damage. This is not a random process, but it seems to follow a specific

569

pattern, which gives rise to singular features in the diffraction patterns; in turn, those features arise from the generation of surface and bulk defects, which are commonly found also in the Mo0 3 Furthermore, it will be demostra-

crystallites treated thermally.

ted that the appearance of a surface lattice distortion occurs first than other structural features

(5,7).

As those radiation-or-

thermally-induced defects may be relevant for catalysis, the subject will be discussed further. High Resolution Microscopy Experimentally, the high resolution observation of thin enough Mo0 3 crystallites is made at 100 kv, using standard 100 apertures.

The lattice periodicities are

~m

objetive

resolved directly at a

magnification of 3xI0 5, for crystals laying on the (010) faces, that is, with the more extended face perpendicular to the beam direction.

Additionally, the interpretation of the HREM images is made by means of dynamical theory of diffraction and multislice programs (9); this

procedu~e

allow a direct comparison with the experimen-

tal high resolution pictures. Structure Factor Calculations The normal structure factor, F h k l, for the Mo0 3 orthorombic = 2n+l for (hOO) and (001) type reflec-

unit cell, is null for k,l tions.

There is however, strong experimental evidence (4,5) for

the violation of this condition.

In particular,

(100) and (001)

type reflections are usually present in the electron diffraction patterns.

Those reflections remain unchanged upon tilting the

crystal up to

±

30 degrees, which is a clear evidence that those

reflections are not due to multiple scattering, but they are the reciprocal space rods which are expected from a corresponding very thin layer in real space, that is the incomplete unit cell surface layer. In this case, a new structure factor, F h k l, is defined, which takes account of the summation of N unit cells, which are complete (i.e. F'hkl)' plus the additional contribution due to the incomplete unit cell of the crystal (i.e. F h k l); thus the extra term makes it that F h k l is not null for the (100) and (001) type reflections. These calculations were made at intervals of about one tenth of unit cell along b axis and they correspond to specific conditions for the alternate appearance of the forbidden

570 reflections. In summary, the comparison of structure factor calculations (F h k l = F'hkl + F h k l) with experimental diffraction patterns, will allow the determination fo the crystal surface step heights, at atomic level, along [010) axis, from one

~e~th

to a complete unit

cell. RESULTS Surface Reflections A typical Mo0 3 crystallite is shown in the fig la, with its corresponding [010) convergent beam diffraction pattern~fig lb. The normal lattice resolution image of that crystal is shown in fig 2a. This is the projected-potential-image of the Mo0 3 rectangular lattice, seen along [010] axis; this image corresponds, oneto-one, to the ideal structure (i.e. fig 2b), projected on the a-c plane; then one can measure the cell parameters directly, a

=

3.9

Aand

c

=

3.6

A.

b

a

Fig. 1. a) Typical Mo0 3 crystallite in a-c plane, scale: 1 cm = 0.06 pm. (b) Convergent beam pattern under the exact Bragg orientation,

[010) note the faint (100) and (001) forbidden

reflections. Both (100) and (001) type reflections are present in the diffraction pattern shown in fig 1.

This is a pattern arising

from a small region in the crystallite; if a parallel narrow beam is used instead, it is possible to choose distinct areas of about

571

a

b

Fig 2.

(a) Projected - potential - image of Mo0 3 seen along E axis

Scale: 1 cm = 14

A.

(b) Ideal structure of Mo0 3 projected along

E axis. o

200 A diameter, and make them to diffract.

Consequently, a series

of microdiffraction patterns is obtained as illustrated by figs

572 3a to 3d.

It is observed that the normal lattice reflections,

for which F h k l # 0, appear always; but the faint (100) and (001) type reflections show distinct intensity variations from one pattern to another.

a

b

c

Fig 3.

(a)-(G) Series of

tal regions of about 200

d

microdi~fraction

Adiameter.

patterns from the crys-

573

The alternate appearance of both (100) and (001) type reflections, which were identified as the crystal surface reflections (4,5), is related to the incomplete filling of the uppermost unit cell; as shown in table I, the intensity ratious, 1001/1100' vary with the atom positions in the unit cell for both oxygen and molybdenum.

These calculations were performed in an atom-by-atom

basis, in order to obtain the extra terms FOOl and F l OO which make it that the total structure factor, F h k l, is not null for (100)

and (001) reflections.

The 1001/1100 ratio values are grouped into four categories; those are figures close to 0.0,1.0,2.5 and the non-determined (i.e. marked with bars, corresponding to 1 0 0 1 = 1 1 0 0 = 0). These cases are represented in the series of figs. 3a to 3d, which match the configuration noted in tables I and II. grouping of

Obviously, the

figures has an effect of masking the real step

height, but this can be discriminated through the ratios 1001/1200 and 1100/1200' respectively. Provided that the true area thickness is known, i.e. by means of convergent beam thickness fringes distribution will be determined.

(10), the more probable sites

In the present case, the four

configurations, noted in table II, should be potentially the more probable surface configurations for the uppermost unit cell of the crystal; that is the surface layer, which interacts with the gas reactants. Additional exposure to the electron beam, or thermal heating, causes a splitting fo the surface type reflections, i.e.

(100) and

(001), as shown in fig 4a, which has been explained in terms of the surface layer distortion (5,8).

Spike Reflections After further exposure to the electron beam the diffraction patterns, i.e. fig. 4a, show up satellites and spikes around the (hOl) type reflections, where h = 1 = 2n, n = 1,2,3, .. ; these spikes extend along {101) directions and seem associated only to bulk-type-reflections, in opposition to surface-type-reflections. The typical crystal region, giving rise to those diffraction features, is shown in fig 4 b.

A family of microdomains, which

are elongated in the (101) directions, indicate the formation of

574

Fig. 4.

(a) Diffraction pattern obtained after longer exposure to

the electron beam.

Note the splitting of the surface reflections

and the "spikes" in the "bulk" reflections.

(b) Bright field ima-

ge of a crystal region giving rise to "spikes" in the ED pattern, note the microdomains elongated in {lOl} directions. planar defects, similarly to those reported before (11).

The

main point here is that, the features on the diffraction pattern, associated to those microdomains, appear well after the splitting of forbidden reflections occurs and they seem associated only to bulk-type reflections. A further analysis of the spike-reflections, using dynamical theory of diffraction (12), indicates that they are in fact crys-

575

tal planes which are stretched in one end and open in the other end, making angles of 84.8° and 86.7° with respect to the surface plane. TABLE 1 Unit cell positions in Mo0 3 and intensity ratios calculated from structure factors.

,

b, axis

UP

TO

1001 / 1100

°1 MOl 12 11 10 x

4 9 8

x

3

·

6

·

x

7 5 4 2 3 2

x

x

1

·

1

°2 °3 M02 °4 °5

1. 001

2.550 2.186 2.550 1. 001 --

1. 001

°6

0.0

°7 M03

1.067

°8 °9 M04 °10 °11

0.963 1.060 1. 067

0.963 0.0 1.055

°12

OXIGEN

NON DETERMINED

MOLYBDENUM

(1 1 0 0 = 1 0 01 = 0)

Sequence:

--

576

TABLE II Experimental intensity ratios for the more probable cases.

A

B

C

1 001 / 1100 (DENSITOMETRIC)

2.6001

0.0

0.9827

SITES

03 or MOl

°6

or

°10

08 or M0 4

0 --

012 or 04

REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12

J.C. Volta and B Moraweck, J.C.S. Chem.Comm., (1980)338; Volta and M. Forissier, Faraday Disscuss. Chern. Soc., 72(1981)225. JM Tatibouet and JE Germain, J.Catal, 72(1981)375. J.C. Volta and J.M. Tatibouet, J. Catal., 93(1985)467. J.M. Dominguez, S. Fuentes, G. Diaz and A. Vazquez, Surf. Sci., 175(1986)L701. J.M. Dominguez, 0. Guzman and A. Garcia, J. Catal., 103(1987)200. L.E. Firment and A. Ferretti, Surf. Sci., 129(1983)155. L.C. Dufour, 0. Bertrand and N.Floquet, Surf. Sci., 147(1984)396. S. Fuentes, G. Diaz, A. Vazquez and J. M. Dominguez, J. Applied Catalysis, In Press, (1987). D.S. MacLagan, L. A. Bursill and AEC Spargo, Phil. Mag., 35 (3) (1977)757. J.M. Dominguez, A. Garcia and 0. Guzman, in D.B. Willians and D.C. Joy (eds.), Analytical Electron Microscopy, Sn Francisco Press, San Francisco 1984,p.140. L. A. Bursill, W.C. Dowell, P. Goodman and N. Tate, Acta Crystallogr., A34(1974)296. R. Gevers, J. van Landuyt and S. Amelinckx, Phys.Stat.Sol., 18(1966)343.