Application of computer graphics to catalyst design

201

cati~.sipTodoy,1o (1991) 201-211 ElsevierSciencePnblishersB.V.,Amsterdam APPLICATION OF COMPUTER GRAPHICS TO CATALYST DESIGN A. MIYAMOTO and T. INIJI

Department of Hydrocarbon Chemistry, Faculty of Engineering, Kyoto University, Sakyo-ku, Kyoto 606 (Japan) SUMMARY The suitability of computer graphics for the investigationof the structure and function of heterogeneous catalysts has been demonstrated with supported Perovskite oxide catalysts, supported vanadium oxide catalysts, supported gold catalysts, and acid-base cooperative catalysis in the side-chain alkylation of toluene with methanol on alkali-ion exchanged zeolites.

Computer graphics is

especially effective for discussing the geometrical factors relevant to catalysis and catalyst formulation. INTRODUCTION Computer graphics provides a key technology in a variety of fields covering animation, architecture, business, management, mapping, printing, publishing, science, videotechnology,and visual arts and design.

In the field of the

science and technology of catalysts, too, the visualization of catalyst structures by computer graphics is expected to provide an important method for understanding the catalytic function and designing novel catalytic materials [e.g.l].

This paper summarizes

the results of our computer

graphic

investigation on the structure and function of heterogeneous catalysts.

MFITHODS In addition to experimental techniques for the characterization and study of catalytic reactions, calculations were made with a DEC Micro VAX II super minicomputer and Evans & Sutherland PS390three-dimensional color

graphic

terminal coupled with Chem-X (Chemical Design Ltd.) and MOGLI (Evans & Sutherland Ltd.) software packages.

RESULTS AND DISCUSSION Epitaxial growth of Ba

O7_x on SrTiO3, MgO, and ZrOl -Much attention hasrecently been given to thin films of high temperature

superconducting oxides in relation to their application to electronic devices and a possibility for the formation of novel superconducting phases.

The

material has also been found to be active in the catalytic reduction of nitric oxide[2].

Important results have been published on the method of preparation

and the selection of substrate materials and, among a number of substrate materials examined, SrTi03, MgO, and Zr02 have been found to be most effective as substrates of Ba2YCu307_x[e.g.3, 41. Fig. 1 shows three-dimensional computer graphic representations of the (001) and (100) planes of Ba2YCu307_x having an orthorhombic structure[5]. On the basis of the experimental occupation factors at sites lb(0.84) and le(0.05).the occupation factors of oxygen are approximated to unity for site lb and zero for site le.

There are two kinds of Cu ions in crystalline

Ba2YCu307_x: (i) pyramidal Cu05 and (ii) fence-like chains of planar Cu04. Cu05 pyramids align on both sides of the Y3+ plane at the Z=1/2 level, their bases facing the Y3+ plane.

Each Cu05 pyramid is composed of four shorter,

non-planar Cu-0 bonds and one longer Cu-0 bond parallel 'to the [OOl] 2t direction, indicating the Jahn-Teller effect on the coordination around Cu, in the crystal.

Cu atoms on the Z=O plane form fence-like chains of CuO4

planes parallel to the [OOl] direction, and these chains extend linearly along the [loo] direction.

The coordination number of the Ba2+ion is 10, while

the Y3' ion has eight oxygen atoms as its nearest neighbors, forming a tetragonal prism. In the bulk SrTi03

crystal

of cubic

symmetry,

each Ti cation

is

octahedrally coordinated by 6 oxygen anions, while each Sr2+ ion is surrounded by 12 oxygen anions.

Fig. 2 shows computer graphic pictures of the (100)

and (110) planes of the SrTiG3 crystal.

Each surface Ti cation at the (100)

plane is coordinated by four surface oxygen anions and one subsurface anion. Figure 2(b) shows a computer graphic picture of the (110) plane of the SrTiO3 crystal.

Here a surface Ti cation is coordinated by four surface oxygen

anions, while each surface Sr cation is surrounded by 7 oxygen anions. As shown in Figs. 1 and 2, the (001) plane of Ba2YC~307_~ is geometrically

Fig. 2 The (100) plane(a)and (X10) plane of SrTi03.

b

a

Fig. 3 Top view(a)

side view(b

the interface between (001) plane of

Ba2YCu307_xand (100)plane of SrTiOy

Fig. 4

Top view(s)

side view(b

the interfacebetween(110) plane of

BazYCu307_xand (110)plane of SrTiOg

204 similar to the (100) plane of SrTiO3.

Fig. 3 shows the crystallographic fit

of the (001) plane of Ba2YCu307_x with the (100) plane of SrTi03. agreement with the lattice parameters, the atomic arrangement

In

in the (001)

plane of Ba2YCu307_x is almost the same as that in the (100) plane of SrTi03 Since the nearest neighbor interaction is of primary importance, this means that the crystallographic fit between the (001) plane of Ba2YCu30y_x and the (100) plane of SrTi03 is stabilized by the Coulomb interaction between ions at the interface.

These results are consistent with the experimental data on

the epitaxial growth of the(OO1)

plane of Ba2YCu30y_x on the (100)plane of

SrTi03 substrate[3]. Fig. 4 shows

the crystallographic

fit between

Ba2YCu307_x and the (110) plane of SrTi03.

the (110) plane of

As shown, the (110) plane of

Ba2YCu307_x fits well with the (110) plane of SrTi03

The arrangement of Cu

ions on Ba2YCu307_x is similar to that of Ti ions on SrTi03, while that of Ba ions on Ba2YCu307_x is close to that of Sr ion on SrTi03

These results are

also consistent with experimental data on the epitaxial growth of the (110) plane o*fBa2YCu307_x on the (110) plane of SrTi03[3].

It has also been

demonstrated that MgO and Zr02 surfaces are effective for the crystallographic fit with the B~~YCU~O,_~ surface.

Supported Vanadium Oxide Catalysts Supported vanadium oxides are industrially very important as catalysts for such reactions

as the selective

oxidation

of hydrocarbons,

selective

ammoxidation of hydrocarbons,and selective reduction of NO with NH3. been found that kind

of support.

performance measurements

It has

the activity and selectivity change significantly with the Ti02

(anatase)

than SiO2 or A1203

support

support,

usually exhibits a better

and

various

physicochemical

have been made in order to understand the difference[6-8).

Although several models have been proposed for the intimate interaction between V205 and Ti02(anatase)[8], a computer graphic

investigation of the

system has not yet been performed[9]. The V205 crystal forms a layer structure, and the V=O species, which are active in various catalytic reactions, are located on the (010) plane. Vk

cation is coordinated by five O*- anions.

Each

The double-bonded oxygen (Ot)

is located at the top of a square pyramidal VO5 unit, and the other four oxygen anions (Ob) form the base plane of a square pyramid. Fig.,5 shows top and side views of the V05 unit fixed on the (100) plane of Si02 (cristobalite). Since each Si ion is tetrahedrally coordinated by four oxygen anions in the Si02 crystal, the surface of Si02 is not smooth but forms an open structure.

The V05 square pyramidal unit cannot be well fixed on

such a rough surface.

Although two Ob anions of a VO 5 unit can fit two

Top view(a) and side view0 > (100)planeof Si02(cristobalite !I.

Fig. 5

of

the V05 unit of V205 fixed on the

b

a

Fig. 6 Top view(a) and side view(b)of the VO5 and V209 units of V205 fixed on the (010)plane of TiO$anatas se).

b

a

Fig. 7 Top view(a)

side view(b)of the multinuclearV205 layer fixed on

the monolayerV205/Ti02(anatase).

oxygen anions on the (100) plane of SiO2 (cristobalite), the other two Ob atoms of the VQ5 unit canhot find their partners on the SiO2 surface. indicates

that

even

a VO5

unit

cannot

fit the (100) plane

This

of SiO2

Although these are results for the (100) plane of SiO2

(cristobalite).

(cristobalite),similar results were also obtained for other planes of SiO2 These

(cristobalite)and for various planes of other modifications of SiO2.

results suggest that epitaxial growth of crystalline V205 cannot take place on a SiO2 support, a conclusion consistent with the experimental results[6]. Fig. 6 shows the interaction of VO5 and V209 units with the (100) plane of TiO2 (anatase), as a model of the monolayer V205/Ti02 catalyst.

In

crystalline Ti02 (anatase). each Ti4+ cation is octahedrally coordinated by six oxygen anions.

At the (100) plane of Ti02(anatase), a Ti4+ cation is

surrounded by four oxygen anions within the surface, as well as a subsurface The VO5 and V209 units can be fixed on the surface, because

oxygen anion.

four Ob anions of the base plane of VO5 unit well fit the surface Tications on&he

Ti02 (lw)

plane.

Similarly, V209 unit can fit the (100) plane of

,It should, ,however, be noted that the crystallographic fit

Ti02(enarase), hefween the V#5

with the (100) plane of TiO*(anatase) is not complete,

suggesting thak ia.considerable.deformationof the V205 units takes place at the interface with Ti02(anatase),

a conclusion

consistent

with

the

observations for the*.monolayerV205/Ti02 catalpst[e.g.71. Fig. 7 shows the interaction of multinuclear V205 layer with the monolayer V205/Ti02(anetase). As shown, the Ot anion of the monolayer VO5 or V209 unit can'coordinate to the Vk

cation of the multinuclear V205 cluster.

This is

consistent with experimental data that the (010) plane of V205 is selectively exposed to the catalyst surface when 5-8 V205 layers are formed to cover the TiO2 surface[6].

Supported w

Catalysts

Gold supported on Fe&,

CoS04, and NiO has been found to be highly active

for the l~w'temperature oxidation of CO, and the role of the support

in the

gtaiyst provi:dem,aninteresting topic of investigation[lO]. According to observations made by using a high resolution transmission electron microscope[lO], the (111) plane of Au is in contact withthe(111) plane of CoS04.

As

shown in pictures of both crystal planes (Fig. 8). the

arrangement of atoms inthe(lll) (111) plane of Cog04

plane of Auis very similar to that in the

Fig. 9 shows top and side views of the Au(ll1) plane-

CoS04(lll) plane interface.

Each Au atom at the interface is in contact

with Co cation of Cog04. and eachCo cation is locatedin

the neighbor of Au

atom, ind%cating a crystallographic fit at the interface.

Similar results

were also observed for the interface between Au(ll1) plane and NiO(ll1) plane

a

Fig. 8

a

plane of Co304(b).

The (111) plane of Au(a) and

b

Fig. 9

Top view(a)

aide view(b

the interface between (111) plane of

Au and (111) plane of Co304.

a

Fig. 10

Top view(a) and side view(b) of

Au and (100) plane of MgO.

and that between Au(100) plane and MgO(100) plane (Fig. lo), in accordance with experimental results obtained by electron microscope(ref. 11).

It

should be noted that such a crystallographicfit between metal and metal oxide has also been observed for Fe/Fe304 interface, which is important in magnetic materials.

Consequently,geometrical fit at the interface plays an important

role not only in supported metal oxide catalysts but also in supported metal catalysts[l2].

Acid-Base Cooperative Catalysis Acid-base cooperative catalysis is a key concept which may lead to the design of highly active and selective catalysis[l3].

Previous investigations

have suggested that the side-chain alkylation of toluene with methanol[l4] provides an example of acid-base cooperative catalysis (Fig. 11)[15,16]. The objective of the present study is to demonstrate and visualize the idea by the use of computer graphics. Fig. 12(a) illustrates three-dimensional picture of NaY zeolite, as an example of zeolites in general.

Here, small spheres are T atoms (X4+

or

A13+ cations), while large white and dark spheres represent oxygen anions and Na cations, respectively. Ion-exchange of NaY with various alkali cations leads to the formation of acidic or basic site[lk,lS].

For example, Fig.

12(b) shows a picture of Rb and Li ion-exchanged NaY zeolite; part of the Na ion in NaY is graphically replaced by Rb or Li ion.

A basic site is

considered to be formed on the oxygen anion adjacent to Rb cation, while acidic site is formed on Li cation. It has been suggested that the side-chain alkylation of toluene with methanol proceeds on acid-base bifunctional site; the acid site interacts with the benzene ring of toluene to stabilize the adsorbed state, while the basic site interacts

with

'the methyl

group

of toluene

to facilitate

electrophilic attack of formaldehyde (or methanol)(Fig.11)[15].

the

The ide?

has been understood by visualizing HOMO orbital of toluene interacting with an acidic site and a basic site

On an acid site the frontier electron density

for.'theelectrophilic reaction on benzene ring is higher than that on the

acid Fig. 11

base

Interaction of toluene with the acid-base cooperative site.

Fig.

12

NaY (a) and RbLiNaY (b) zeolites.

13

Interaction

B

Fig.

methyl

group,

of toluene

while

that

beozsne ring OR the basic that

the side-chain

effectively

cooperative

variety

of

of toluene pair site;

sites

density

sites

methanol

than that

on the

have indicated

is catalyzed

and basic

sites

most

interact

Formation of such

respectively.

the computer

site

interaction for

this

of toluene

material

or the high %/Al

the cooperative

interaction

for a

graphics

the adsorption

of

ratio.

and

of the low

On the basis with acidic

of and

ion exchaaged mordenites.

toluene

(RbLiNaY) and Rb and Li ioa exchanged

with acidic

because

of taluene

was found not to be aaay for alkali

Fig. 13 illustrates zeolite

vith

was exarrrined using

was found to be difficult

of the ion-exchange sites

higher

2eolites.

the seats reasoning basic

zeolitear

calculations

the acidic

and the methyl group,

In regard to ZSM-5, the simultaneous basic

group is

Quautua chemical

alkylation

with the benzene-ring

and Ran

Riley

on the @ethyl site.

at the eeid-base

acid-base

with

on Rb and Li ion exchanged Y

X zeolite

(RbLiNaX).

A basic

210 site is considered to be formed on the oxygen anion adjacent

to Rb cation

while acidicsite is formed on Li cation. On these xeolites,Li ion (acidic site) is located close to the basic oxygen anion, and therefore the cooperativeinteractionof toluenewith acidicand basic sites is much easier than that on ZSM-5 'OFmordenite. This explainsthe higheractivityof X and Y xeolites than ES+!-5 or mordenite for the side-chain alkylation of toluene[14,15]. Furthermore,a closer inspectionof Fig. 13.indicatesthe followingresults. On RbLiNaY catalyst,even if the benzene ring interacts with a Li cation,the methyl group cannot fit the basic oxygenanion well due to:thedistance between the Li cation and the basic oxygen anion.

On the

other hand, simultaneousinteractionof atoluene molecule with acidic and basic sites is possible for RbLiNaX zeolite. This is also consistent with the experimental data that the activity of xeolite Xis higher than that of zeoliteY[14,15]. CONCLUDINGREMARKS As described ahove,the geometrical fit at theactive

component-support

interfaceis importantfor designingboth supportedmetal oxide catalystsand supportedmetal catalysts.

The geometricalfit has also been demonstrated

to be essentialfor the highly active catalyticreactions,such as the sidechain alkylation of toluene with methanol on Rb and Li exchanged X-type xeolite.

Computer graphics is useful for investigatingsuch geometrical

factorsin catalysisand catalystformulation.

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