study structure of first and second row transition metal sulfides using scf-sw-xα cluster...

Chemical Physics 67 (1982) 229-237 North-Hotid Pub%hing Company

STUDY OF THE ELECTRONIC STRUCTURE OF FIRST AND SECOND ROW

TRANSITION METAL SULFIDES USING SCF-SW-Xe CLUSTER CALCULATIONS

Suzanne HARRIS

Received 4 September 1981

Energy levels and charge distributions for hlS?- clusters (M = Ti-Ni and Zr-Pd) have been calculated Using the SCFSW-L method C&&ted bir.din-, exergics for the clusters are compared with corresponding features in the XPS spectra of the metal sulfides Trends in the cncrgy lcvcls and charge distniutions indicate that the bonding in the second TOW transition metal sufides is more cowdent thnn in the fist IO% sulfides This rcsulfs k?@y from the increased mctti- ~Aliur d-p z interactions which occur for the second IOW transition met&_ Trends in the bonding in the metal sulfides are discussed in light of the activity of thcx materials as hydrodesulfurization wtdysts

1. introduction

The transition metal sulfides form a group of materials which exhibit 3 number of interesting and varied

properties. One particularly important chemical prop- erty of several of these sillfldes is their ability, in the presence of hydrogen, to catalyze sulfur removal from heterocyclic organic molecules such as thiophene, benzothiophene, and dibenzothiophene. Consequently, transition metd sulfides have been widely used for years in the petroleum industry as hydrodesulfuriia- tion (HDS) catalysts [I]. In spite of their importance ti this application, however, the ori& of the cntalytic activity of these materials is not well understood. (For a recent review see ref. [2] .)

Recently, a systematic study of the ability of bulk

transition metal sulfides to catalyze the HDS of di-

benzothiophene showed that this ability varies with the position of the trarxition metal in the periodic

table [3]. A first step toward understanding the basis of such periodic trends in catalytic activity is clearly

a better understanding of the electronic structure of

the transition metal sulfides themselves. Although a number of the individual sulfides have been the subject of band stkture [4,Sj and cluster [6-l I] calculations, no systematic study based on a consistent set of ca!culations has previously been reported for

this large group of sulfides. We have undertaken such a study, and we report

here results from the first stage of r&is study. We de- scribe results of self-consistent field scattered wave Xa (SCF-SW-Xo) calculations for a series of transition metal sulfur chrsters representing sulfides of four- teen first and second row transition metals. These results, which allow us to make systematic comparisons of energy levels and charge distributions in the model clusters, provide us with infomlation about similari- ties and differences in the metal-sulfur bonding in the first and second row transition metal suliides. This information makes it possible for us to begin to understand trends in the catalytic activity of these materials in terms of their electronic structure.

Section 2 briefly describes the clusters and the calculational method and parameters. Section 3 begins wirh a discussion of the general features in the energy Ievels of the clusters, using RuSi’- as an example. ?his is fcliowed by a series of comparisons between the calculated binding energies of the clusters and the experimental photoelectron spectra of some of the sulfides. In section 4 the differences in the charge distributions in the various clusters are discussed. Finally, section 5 begins with a suinmary of the results as they relate to differences in metal-sulfur bonding in the first and second row sulfides and concludes with

230 S Harri.r/Electronic structure of transition meral mifu’es

a brief discussion of how trends in the bonding may re!ate to the relative catalytic activity of the different

s&ides.

2. Calculational method and parameters

Calculations were carried out for LISP clusters containing the transition metals Ti through Ni and Zr through Pci as the central metal (M). The clusters carry a negative charge (n-) because in each cluster enough electrons have been included so that each sulfur is formally S’- and each metal has the formal oxidation state for the appropriate sulfide. This excess negative charge was compensated in the calculations by surrounding the cluster with a positively charged Watson sphere [ 121. The clusters used for most of the comparisons made here have octahedral coordination. This is the correct idcaked geometry for all

the sulfides considered except MO% and NbS2,

which have trigonal prismatic coordination of the sulfurs around the metal, and PdS where the coordination is square planar. For these sulfides calculations were also carried out for clusters having thcz correct

geometry: and these clusters are used for comparisons

with photoelectron spectra. Since the same general trends are apparent in clusters of either geometry, however, the other results discussed here all refer to the octahedral clusters.

Au rhe ca!culations were carried out using the SCF-Xa scatte:ed wave method [ 131 with tangent spheres. The metal-sulfur Ginnces are the experi- menral bond distances in the corresponding sulfides. The atomic sphere radii were chosen using Norman’s

criteria [ 141, with the outer sphere made tangent to the sulfur spheres. The clusters, metal-sulfur distances, and metal and sulfur sphere radii are listed in table 1. The atomic exchange parameter 0: values for the regions within the metal and sulfur spheres are those of Schwsrz [ 151, and a weighted average of the atomic values was used in the intersphere and outersphere regions. The results reported here were all obtained from spin-restricted SCF-Xo calcuiations. Unrestricted calculations were also carried out where appropriate, but the genera! trends were much the same. Therefore, for ease of comparison, orrry the spin-restricted results are discussed here.

TabIe 1 ;5let+xlfur disttmes and sphere radii for MS~- clusters

Cluster

Ti$

VS:-

c&- Mns~O-

IO- Fe&

cos:“- N&o-

zrs:-

xi&-

hIoS:-

T&-

RuS:‘- 9-

t&_s @I

242

237

242 2.59

Sphere radii (&J

metal sulfur

1.23 1.19

1.19 1.18 1.23 1.19 1.33 1.26

Ra

PdS;‘-

226 1.09 1.17

2.32 1.13 1.19

240 1.18 1.22

2.56 1.35 1.21

247 1.28 1.19

2.42 1.24 1.18

238 1.21 1.17

235 1.15 1.19

237 1.16 1.20

230 1.15 1.15

3. Eneqy levels and photoelectron spectra

We first consider in detail, the energy levels of

RuSi’-, the cluster model for RuS,. This example provides a description of the general features observed in the ener,v !eve!s of all the octahedra! clusters and thus provides a basis for comparing these features throughout the whole group of clusters. The Ru@- cluster is also an informative example in its own right, however, since the electronic structure of RuS2 has not previously been the subject of a theo- retical investigation.

The valence level orbital energies and char c dis- %- tributions in the different regions of the RuS6

cluster are shown in table 2. The energies of the levels have been adjusted so that the 1 tlg level, a nonbonding combination of sulfur p orbit&, has the same energy as the sulfur 3p level in an atomic sulfur calculation. The !alrr and ltlu levels correspond to the Ru 4s and 4p orbit&. The 2alp-le, levels rep- resent the sulfur 3s band and show veryfttle interaction with metal levels. The I t2E-ltlS are the Ievels which in the solid widen to for?m the sulfur 3p band. In the lower half of this group lie the bonding orbit& resulting from the mixing of the sulfur 3p with the Ru G(3a1,), jp{3 tIu) and 4d( 1 t2g and 2eg) orbit&.

Table 2 Ground State valence enera leveis and charge distributions in RL&‘-

Orbital OCCUFatiOn

Energy -E (CV)

Charge distribution (55) --

Ru s Ru P Rud ss SP inter- outcr- SphCrC SphCrC

3cg (0) 2.31 0 0 27.58 S.29 4.17

2tq (6) 5.05 0 0 68.98 0 4.55

‘ItIp (6) 6.53 0 0 0 0 66.59

4t1 U (6) 6.8G 0 l.S6 0 1.11 57.63 ItZU (6) 7.47 0 0 0 0 58.20 3*1 u (6) 8.51 0 1.06 0 1.23 51.84 24 (4) 8.67 0 0 30.88 0.24 45.00 3n1gw 8.75 5.64 0 0 1.32 55.08 1ttg (6) 9.35 0 G 10.36 0 42.41

8.53 18.44

54.4d a.03

Z6.70 7.21 13.86 15.84 34.49 7.31 38.07 7.8 1 15.80 S.Oi 26.02 11.93 4:.38 5.85

leg (4) 17.13 0 0 2.23 79.99 0 14.97 2.76

2t1, is) 17.21 0 0.66 0 78.77 0.18 17.65 5.74 2aq (2) 17.96 1.48 0 0 78.53 0.28 21.99 2.40

ltlu (6) 45.73 0 97.88 0 0.09 0.14 kg (2) 72.70 99.28 0 0 0.02 0.03

--

1.ss 0 0.67 0

Interactions of the metal 5s or Sp orbit& with the sulfur Zp orbit& appear to be ne&$Ae compared to d-p interactions. The strongest mixing occurs in the

2eg level through the o interaction between the Ru 4d,z or 4d_Ya_Yz and sulfur 3p u orbit& The metal 4d-sulfur 3p n mixing which is observed in the It,, level is smaller than the u mixing. The ltzu, 4tlu, and It,, levels in the upper part of the sulfur 3p band are locaked around the su!fur atoms and correspond to nonbonding combinations of sulfur orbit&. Lykg

above and separated from the sulfur 3p band are the 2tzg and 3e, levels. These are the antibonding partners

of the bonding 1 tzK and 2es levels, and they are the levels which correspond to the metal 4d bands in the solid. The highest occupied level in the cluster is the filled 2tzE level, and as table 2 shows, it is localized primarily on the Ru center. The unoccupied 3e, level appears to be quite diffuse, with a large propozon of

the wavefunction lying in the outer sphere region. .4Bhough the pattern of,energy !eveis is similar for

all the metal sulfur chrsters, as it must be because of the imposed octahedrai symmetry, there are consider- able differences in both the relative spacings of the energy levels and the charge distributions in the various clusters. The degree of mix&g of the metal d and sulfur p orbitals is influenced by the relative energies

of these orbit& in the sulfides. In the clusters, the energy separation between the 2tz5 level, localized mainly in the metal sphere, and the 1 tlg level, a

nonbonding combination of sulfur p orbit& which is always the highest in energy of the sulfur levels, re- flects these relative orbital energies. This enemy separation also givss us an indication of the relative posi-

tions of the metal d and sulfur p bands in the solid, so it is instructive to compare how this energy difference varies throughout the group of closters. To facilitnte

the comparison, the energy separation between the 2t2” and 1 tl_ levels in each cluster has been plotted vers’us the cekral metal in the cluster. This plot is shown in fig. 1, and we see from this figure that for the 4d metals there is a steady decrease in th? 2t2_g-1 tlg separation from the left to right side of the series. The only break in this trend occurs between Tc and RLI, where there is also an abrupt change in oxidation state. Overall, the shift from left to right is large enough that for Rh and Pd the 2tTe level ac- tually falls below the itlg level. This trend is less reg- ular in the 3d series, primarily because of the chang- ing otidation states of ihe metals on the iefr side of the series. On the right side of the series, however, a decrease in the 2tzg--ltIg separation is apparent.

A comparison ot the results of our calculations

232

i

3 -La ’ - c

-_ 2< 22

(

-1

r

'.C [

I

.r

).C--

I.?

i

Fig. 1. Plot of the energy difference between the 2tzg and ItIg levels ia csch octahedml \!a6 “II- cluster versus the cantrzl

metal Al.

with available ;i-ray photoelectron spectra (XI’S) of the sulfides shows that the trends observed in the calculated energies correlate well with the trends ob-

served in the spectra. The XPS spectra of FeS2. CoS,, and NiSa [6, 16- lS] , for example, show a decrease in rhe separation between the metal d leve!s and sulfur p levels from FeS2, where the d and p peaks are well separated. to NiSZ where these peaks overlap. The XPS spectra of these three sulfides [ 161 are shown in figs. 3-> 3, and 4 alon? with the relative

binding energies calculated I‘or the corresponding trlq- clusters. AU the calculated binding energies shown here were obtained using Slat&s transition state procedure [ 191. Becxse of the charges on the clusters and the Watson sphere used in the calculations, only relative bindkg energies could be ob- t&cd through this procedure. !t is just these relative energies which are important to our discussion, however, so in each figure the calculated values are lined up xvith the esperimental spectrum to give the best ove;zll spreemcnt between the wo. Although t??ere are suecific features in the wectra of Fe%. Co%. and

X-RAY PHOTOELECTRON SPECTRUM OF Co-!52

Fe. 3. Compuisan of the calculated binding enqies in zn octahedrai COSTS- cluster with the X-ray photoelectron spec

-A, ;, trum [16] ofCoS2.

CALCULATED BINDING ENERGIES FO4 OCTAHEDRAL FeS~‘O-

11. b-

I SPECTRUM OF FeS 2

Fig. 2. Comparison of the ulculated binding energies in an

octnhcdrd FeSp- cluster with the X-ray photoelectron spcc- trum [ 161 oi’F&.

NB,, which do not have counterparts in the calculated energies of these simple clusters (we will return to this point), it is apparent from figs. 2,3, and 4 that the general features are comparable. The most obvi- ous of these is the decrease in the separation between the metal 3d levels and the top of sulfur 3p levels. This decrease is obsen;ed in both the experimental

CALCULATED BINDING ENERGIES FOR OCTAHEDP?L C&G’@

,l,,,7y

: 1

S HarrisjElectTonic structure of transition metui sulfides 233

CALCULATED BINDING ENERGIES FOR OCTAHEDRAL FfiSsl@

Fig, 4. Comp~+on ot- the calculated binding cner$es in an octahedral N&O- cluster i\rivith the X-ray photoelectron spectrum [16] oiN&.

spectra and the calculated binding energies. Both show the metal 3d level separated off from the sulfur 3~

band for Fe, a decrease in this separation for Co, and finally an overlap of the two levels for Ni.

This overlap in energy of the metal and sulfur levels is still not as great in NiS?, however, as in the right

hand side of the second transition series. in PdS, for example, both our calculations and the XPS and X-ray emission spectra [20] suggest that the metal d levels fall wzll within the sulfur p band. The calculated relative binding energies for the square planar ?dSz- cluster and the XPS and X-ray emission spectra of PdS are shown together in fig. 5. In the Ddh point group of the cluster, the occupied levels which have

Ngh palladium 4d character are the bTo, e,, and alg levels labeiled in fig. 5. These levels l&&n the sulfur levels and correlate we11 with the spectral data and assignments of Mikhailova et al. [20] .

Where XFS spectra are available for other second row transition meral sulfides, they also confirm the general trends observed in fig. .I. For example, the XPS spectrum [21] of MoS2 is shown in fig. 6 along

with the calculated binding energies for a trigonal prismatic MoSE- cluster. Under the D,, point group of this cluster, the highest occupied level is an ai level !ocalized primarily on the molybdenum atom. This a; level is labelled in fig. 6 along with the e level

CALCULATED BINDING ENERGIES FOR SOUARE PLANAR PdS$-

Fig. 5. Comparison of the calculated binding energies in :! square planar PdS$- cluster with the ;Y-ray photoelectron and sulfur KD- and LII, III- emission spectra 1201 of PdS.

lying highest in ener,y of the sulfur levels. The separation berween these two levels correlates well with the splitting between the metal and sulfur peaks in

the ?(pS spectrum of MoS2. Another i&resting comparison can be made if we

CALCULATED BINDING ENERGIES FOR TRIGONAL MoSSa

X-RAY PHOTOELECTRON

SPECTRUM OF

,,.. .I.,

I> IG 3,;:,l.,G?.:I,.,.<*,. : c Fis 6. Compaison of the calculated binding energies in a triganal prismatic !vloS~- duster and the X-ray photoelecrrcn spi‘ctrum [21] of bloS2.

234 S Hnrnr/Electn:lic stnrcfzue o,Ftransitfon metai SuiJides

return to RuSq- Fig. 1 shows that in the cIusters containing second-row transition metals the steady shift ia the 2t?, level relative to the sulkr leveis is broken only in g&g from tec*hni+ium to ruthenium: where the formal oxidation state also changes from T@ to RuZ+. From these c!ltster results we would therefore expect that the XPS spcctmm of RuS2 should exhibit a Ru 4d pe& well separated from the S 3p

band. A preliminary ,X’S spectrum [22] for RuS2 indeed shows just such a separation between the metal 4d and sulfur 3p Ievels, and this separation is comparsble in size to that predicted from the cluster results.

Thus we see that the cluster results give a good indication of the relative cner$es of the metal and sulfur levels in Fe&. CoS2, NiS,, MoS,, and PdS The clusterst of course, cannot yield det&ed correlations Gth all the features of the XPS spectra, because they do not include all the interactions which are present in the sulfides. The most severe limitation of the clusters is their failure to mode1 the true environment of the suifur atoms. Consequently, we cannot expect to reproduce the details of the sulfur regions of the XPS spectra. In hIoS2, for esxmple, the sulfur 3p region of the XPS spectrum exhibits a three-peaked structure which appears io be chsracteristic of the layered sulfides. To effectiveiy model this region of the spectrum, we need tc consider larger clusters which include interactions bstween the sulfurs and other metal and sulfur atoms. The sulfur 3s and 3p regions of the XPS spectra of FcS,, Co&, NiS? and RuS2 also display B very distinctive pattern. All of these sulfides occur in the pyrite structure and therc- fore contain discrete Sz- uniis. The sulfur regions of 11~ XPS spectra of several of these sulfides have been interpreted in terms of the energy levels of the disulfide units [ 171, but no calculations other than a band structure [j] of FeS2 have included them. Pre- limiaary calculations in our laboratory for kl(S,)AG clusters indicate that includkg the disulfide unzs

yields much better agreement with the sulfur regions of the spectra.

These simple clusters do provide the transition meial \\,ith 3. close approximation to its real environment in the corresponding sulfide, hoxever, and we have ‘<een that the calculations yiaId 3 reliable estimate of the relative energies of ihc metal and sulfur levels in tilt: sulfides. They shouId rllso provide us with a

reasonable picture of the metal-sulfur interactions which occur in the sulfides, and these interactions are the subject of the following section.

4. Charge distributions

The trends observed in the energy levels of the

clusters suggest that we should see zn increase in metal d-sulfur p mhing from left to right in both the 3d and 4d series, with a more si_tificant increase occurring in the 4d series. This d-p mixing, character- izing increased covalency, is evident in several quanti- ties resulting from the calculations. The first of these is the energy separation between the highest and lowest occupied levels having primarily sulfur 3p character. In the solids this would correspond to the width of the sulfur 3p band. In the RuSi”- cluster this energy corresPonds to E(ltlg) - E(ltZg) = 2.82 eV. This separation increases across both transition series, but the change is considerably larger for the second row- metals (an increase of 1.46 eV from Zr to Pd compared to an increase of 0.41 from Ti to Ni). The widening of this group of levels in the second transition series results from the stabilization of the 1 t,e and 2e, levels relative to the 3alc and 3tlU levels_- T‘his stabilizarion comes about because of the increased bonding character of these levels in the 4d series, i.e. the greater d-p miuing.

It is the degree of this d-p mixing which is responsible for the differences in bonding and charge disrri- butions in these clusters. In all the clusters, interactions between the sulfur 3s orbit& and any of the metal orbitals are negligible, and mixing of the sulfur 3p and metal s or p orbit& is small and nearly con- stant. Thus, we focus our attention on variations in d-p mixing. Breaking this down into a u and 7; component, consider fust the cr interactions in the levels. The charge in the 2e, orbital. a bonding combination of metal d,z or d,z_yz and sulfur 3p u orbit&, is centered largely on the sulfur atoms. The amount of metal character in this orbital is, therefore, indicative of u d-p mixing, and in fig. 7 the percentage of charge contained in the metal sphere for the 2es orbital is plotted versus the central metal in each cluster. Fig. 7 shows that in general, the participation of the 4d metals in this orbital is greater than that of the 3d metals. It can also be seen from fig. 7 that ex-

S. Ham~s/Ekctror;ic srrucNre of transition nieral srtffides 235

Fig. 7. Plot of the metal contribution to the ‘es level in esch octahedral .MSa- ciustcr versus the central meid hf.

cept for the break from Tc to Ru, there is a steady increase in d-p mixing from Zr to Pd. This trend is not so apparent &I the 3d series where there are more changes in oxidation state, but it can be seen from Mn throug!n Ni when the metals are all 2+. A similar comparison for the 3e, orbital is less straightforward since, in most of the clusters this level is unoccupied and the wavefunction tends to be very diffuse, particularly for the 4d metals. Charge is shifted out of the metal sphere, making it difficult to identify the metal and sulfur contributions to this orbital.

Considering next the rr interactions in the tzg orbitals, a comparison of the charge distributions in the I$ and 2t2g orbit& reveals significant variations in the rr distributions in ihe clusters. The charge in the 1 tzg orbital, a bonding combination of metal dxY, d,, or d,, and sulfur 3n orbitals, is centered largely on the sulfur atoins, while the charge in the antibonding counterpart, the 2tZg orbital, is centered largely on the metal atom. The amount of metal character in these orbitah is therefore indicative of the degree of r d-p mixing The percentage of charge

contained withi? &metal sphere for both the It,, and 2tzs levels is plotted versus the central metaI $ each cluster in fig. S. it is immediately apparent from fig. 8 that the metal d contributions to both the 1 tzg and 3-tze orbitals are considerably different for the 3d and 4d series. In the 3d series the tzg set of metal 3d orbitals is nearly nonbonding. There is only a

CENiRAL METAL

Fig 8. Plo? of the metal contribution to the ltzS and 2tsg levels in each octahedral MSa- cluster versus the central

metal hi.

small metal contribution to the 1 tzg level and a corre- spondingly smah sulfur 3p contribution to the 2tzg level. The metal t?, orbitah retain their nonbonding character through?ut the entire 3d series of clusters.

In contrast, consider the charge in the meta sphere for the 4d series. Not only is the d-p mixing much stronger, as reflected by the larger metal character of the 1 tza Ieve and smaller metal character of the 2t,, level, but also the mixing increases substan- tially g&g across the series. As before, a break in this trend is seen only at Ru. In the pi levels, therefore, a stronger interaction between the more diffuse metal 4d and sulfur 3p orbitals is clearly apparent in these clusters.

Thus, as we might expect, the charge distributions in the various clusters indicate that the bonding in the 4d transition metal sulfides is considerably more covalent than in the 3d sulfides. This increased covalency is reflected in both the u and x d-p miuiq, but it is the rr contribution which really distinguishes the Id from the 3d series because n-type d-p interactions are considerably more important for the 4d series.

5. Discussion

These calculations have utilized the simplest pos-

336 S. Hati/Elecrronic smcircre of rtmsition meral mlfides

rib!e clusters to model the transition metal sulfides. In spite of the f2ct that the clusters neglect interactions which are important in a number of the sulfides, the eneq’ levels resulting from these calculations corrcls~ well with available experimental data. In Fnrticular, the relative energies of the meta! d 2nd sul;ilr p leveis ca!culated for the clusters compare tit-orably with corresponding features in the XPS spectra of the sulfides.

Trends which can be seer. in the calculated energy levels 2nd charge distributions of the clusters provide us with a clearer understanding of changes in the met- z!-sulfur interactions in the different sulfides. Shifts in rhe energies of the meta! Ievels relative to t!le sul- !ilr levels suggest thnt we should see variations in the amount of meial d-sulfur p mixing in the different sulfides. Such variations are indeed observed in the clusters when we compare the sulfur and metal char- 2ictcr of the individua! co 2nd tzv orbitrrls. The d-p

miving is seen to hxJe bzth 2 u &d IT component, but it is rhc x component which differentiates the 4d and 3d series. In both the 3d 2nd 4d series 2n increase in d-p mixing is observed from the left to right side of the scrics. but in the 3d series this mi.xing has only 2 u component. In Ihe 4d series, on the other hand, borh the u and 7 contributions are important.

This ;r contribution to the metal-sulfur bonding for the dd meta!s takes on particular significmce if we consider the transition met21 sulfides in their role 2s HDS cstalysts. Pecor2ro 2nd Chianelli’s [3] study of the cat2!ytic sctivity of these materials showed not only that the second and third row trensition metal sulfides ore more active than the first row sulfides, bur. also that the acrivity in the second and third rows varies significantly going across the transition series. It is &ar from the results presented here that the metal-sulfur bonding in the catalytically acti~ second row sulEdes is considerably more covalent than the bonding in the inactive first row transition metal sulfides. More importantly, jlerhaps, rhe availability of metal 4d z-type orbita!s is in large par? responsible for this greater covalency. This suggests a correlation between some optimum mhture of u and 7 metal d-sulfur p interactions 2nd 2 maximum in the catn- lytic activity of the sulfide. Also1 the overall trends in our results suggest 2 correlation fcr the second row sulfides between the proximity in ener,v of the metal 2nd sulfur levels 2nd the catalytic 2ctivity of the sulfide.

Correlations such 2s these are considered in much greater detail elsewhere [23], but it is aIso infonna- tive to note here that proposed mechtisms for the HDS process [24] often include a sult%r vacancy on the transition metal sulfide as the active catalytic site. In these mechanismsl a sulfur-containing molecule such as thiophene sari bind to the catalyst surface throc& the interaction of the sulfur atom in the molecule with the exposed metal atom at the active site. Our results suggest that in the more active sulfide catalysts this interaction may have greater 5~ character,

since the metal d orbitals in these active metal sulfides are more available for x-type bonding with sulfur 3p orbitals. Ultimately in the catalytic process the C-S bonds in the molecule are broken. It is likeiy that the rr-type metal-sulfur interactions just de- scribed implement this C-S bond cleavage and thus play an important role in the HDS process. Further calculations which model this type of interaction will be very informative.

It is clear from the results p;.esented here that a systematic series of calculations usifig even these sim-

ple metal-sulfur clusters can provide us with valu2ble information about the electronic structure of the corresponding sulfides. This information has proved irn- portam because it allows us to begin to view trends in catalytic acrivity in terms of the electronic structure of the HDS catalysts. Further calculations now underway on inrger model systems will aLlow us to better understsnd both the electronic structure of the catalysts themselves and the mechanism of the catalytic process.

Acknowledgement

The author would like to thank Dr. K.H. Johnson for making av2ilable his computer programs 2nd Dr. RR Chianelli for many useful discussions.

References

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thcrciii

S. Herris/Electronic simcture of mmsirion nwral sdjides 237

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[lOI

r111

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[IS] G. KriIl and A. Amanou, J. Phys. Chcm Solids 41 (1980) 531.

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study structure of first and second row transition metal sulfides using scf-sw-xα cluster...

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