Journal of lkfolecular Catalysis, 75 (1992) 253-276 M3011

253

Quantum chemical study of hydrodesulfurization catalysts Part II*. DV-Xa calculation on clusters of Co,& and RuS,

Chen Rong, Xin &in and Hu Jinglong Dalian Institute of Chemical Physics, Chinese Academy of Science, Dalian (China]

(Received May 30, 1991; revised April 7, 1992)

Abstract

The electronic and bonding properties of Co&, RuS, and the related thiophene adsorption systems have been studied by DV-Xa calculations on model clusters. The bonding properties of Co,S, and RuS, exhibit significant differences: for RuSa the Ru atoms are held together through bonding with S and the S atoms are held together mainly through the bondings with Ru metal. In addition, the nearest S-S pair of RuSa plays an important role in bonding. For Co&, the S atoms are held together mainly through bonding with Co, the octahedrally coordinated Co(I) atoms are held together through bonding with S, while tetrahedrally coordinated Co(I1) atoms are held together through bonding with S as well as through bonding between the special Co(R)-Co(R) pair. All these metal-sulfur bondings M-S are not strong, and part of the surface S is easily removed through the formation of surface SH groups. Different types of S atoms may be distinguished by their location, coordination and bonding state. The d band of these sulfides is different from that of Group VIII transition metals and markedly influences their adsorption properties. Thiophene molecules adsorbed on these sulfides exhibit greatly different behavior depending on their mode of adsorption. The activation pattern of adsorbed thiophene is determined by the electronic and bonding properties of the adsorbate itself and the adsorption site of the substrate.

Introduction

As hydrotreatment processes become increasingly important in petroleum refining, more effective catalysts need to be developed. Thus basic research on the origin of the catalytic activity and the promotion effect exhibited by the hydrotreating catalysts has attracted the attention of many authors. In these respects many interesting results have been published [2-151 and excellent reviews have appeared [ 16-241. Various models have been proposed to elucidate the fundamental aspects uncovered from the wealth of exper- imental information. However, they are still in the development stage, and more work remains to be done.

In the work of Chianelli [25(a)] on quantum chemical calculations for a series of transition metal sulfide clusters, the electronic factors are related to the catalytic activity in the case of hydrodesulfurization (HDS). But the

*For Part I of this series see [ 11.

0304-5102/92/$5.00 0 1992 - Elsevier Sequoia. All rights reserved

254

detailed aspects of the electronic factors were not elaborated in their work. Zonnevylle et al. [25(b)] have examined the nature of the active site and the mechanism of desulfurization in the case of thiophene on MoS, by using the extended Hiickel tight binding method. They concluded that the q5-bound sites are particularly effective in weakening the S-C bond, while pi-bound sites are less active. The removal of surrounding surface S atoms increases the HDS potential of the q5 geometry but is ineffective for other geometries. Furthermore, metals which replace a surface MO tend to poison HDS, whereas those which ‘pseudointercalate’ between the S-S layers can serve to promote the reaction.

Our first paper of this series [ 1 ] examined the electronic and bonding properties of MoS,, thiophene, tetrahydrothiophene and their related ad- sorption systems. In order to understand more about the catalytic activity and the promotion effect of HDS catalysts, the quantum chemical study was performed on the Co9Ss and RuS2 systems which are known to be promoters as well as active catalysts for the HDS process [2, 13, 22, 351. In the present paper, the electronic and bonding properties of Co9Ss and RuS, and the activation pattern of thiophene on the vacancies of model clusters for these substrates will be discussed. In our next paper [ZS], some properties of the adsorption center and a preliminary view of the promotion effect will be presented.

Method of calculation and models

Computatimal procedure The DV-Xa method has been used by the authors. In this method the

exchange term in the Hartree-Fock equation is approximately calculated as [28]:

VJr) = - 3cz((3/8~r)p(~))l’~ (I)

where p(r) is the electron density at position r, and (Y is the exchange potential parameter, which is chosen to be 0.7. The molecular orbitals are constructed as the linear combinations of symmetry orbit& by group theory, in which the symmetry orbitats are taken to be the linear combinations of numerical atomic orbitals centered on the nuclei.

The molecular orbitals and eigenvalues are determined using the self- consistent charge (SCC) approximation to the potential [ 271. The cluster electron density p required to generate the potential is approximated as

Pscc = “Z {L, &&“J12 (2) I 9

where& is the population for atom Y of the nl atomic orbital. Self-consistency is obtained when the input and output atomic orbital populations are equal.

The basis functions were obtained from numerical solutions of the free atomic Hartree-Fock-Slater equation. In the present procedure the MO 1s . ..4p. Co ls...3p, Ru ls...4p, S Is... 2p and C 1s core orbit& have

255

not been varied, i.e. we have used a ‘frozen core’ approximation. For the valence electrons, the configurations are taken to be: MO 4d55s’5p0, Co 3d84s’4p0, Ru 4d75s’5p0, S 3s23p4, C 2s22p2, H Is’, which are kept fixed in the course of self-consistent procedure. The Hamiltonian matrix elements are evaluated by the DVM numerical integration method [29].

Description of model cluster-s It is known that in CosSg, the S atoms form a close-packed cubic array

and the Co atoms are located at the centers of half of the tetrahedral holes and l/S of the octahedral holes of the array [30]. In this crystal the Co(I) atom, surrounded octahedrally, has six S(R) atoms located 4.32 a.u. apart; the Co(I1) atom, surrounded tetrahedrally, has one S(I) located at 4.01 a.u. and three S(R) located at 4.18 a.u. apart respectively, it also has three Co(R) neighbors located at 4.76 a.u. apart; The S(I) atom has four Co(I1) neighbors disposed tetrahedrally; S(I1) has one Co(I) and four Co(R) neighbors [30].

Figure 1 is a pictorial view of the structure for l/4 of the Co&& crystal unit cell. In this figure, the large circles represent the S atoms and the small circles represent the Co atoms. It can be seen that S4 -S” atoms form an octahedron in which Cog is the octahedrally-surrounded Co(I), while Sg, So, S’ and S13 form a tetrahedron in which Co’ is the tetrahedrally-surrounded Co(I1). Evidently Co2 - Co8 atoms belong to the Co(R) as well. The superscript on the atoms, such as 1 of S’ and 9 of Cog etc., denotes the sequential

0 5 0 co

Fig. 1. Pictorial representation of the structure for l/4 of the Co& crystal unit cell. The large circles represent S atoms and the small circles represent Co atoms. The numbers in the circles represent the sequential number for the same kind of atom. In this figure, S-S8 forms a larger cube within which Co’ -Co’ forms a smaller cube. SQ-S’* are positioned outside the larger cube and over the center of its six faces respectively; Sr4- S” forms an octahedron which is positioned above the larger cube and Co9 is positioned at the center of this octahedron. At the front left lower comer of this figure S’, SQ, S” and Sr3 form a tetrahedron and Co’ is positioned near the center of this tetrahedron. Refer to the text for other details. The model clusters are taken from part of the atoms in this figure.

256

number of the same kind of atoms in the clusters and is the same as that in the circles in Fig. 1. This convention will be used throughout in this paper. Similarly it can be seen that S14 has one Co(I), i.e. Co’, and four Co(R), i.e. Co5-Co8, as its neighbors and it belongs to S(R). Certaiiy all the So-S9 atoms belong to S(I1) also. All the S’ - S8 atoms belong to S(1). Referring to Fig. 1, we infer that S’ -S8 atoms have four Co(R) atoms disposed tetrahedrally as their neighbors, as this figure is l/4 of a crystal unit cell.

In our calculations the models used for CogSg are chosen from part of the atoms shown in Fig. 1 and the details of these models are described in Table 1 (a).

As for RuS,, it possesses a pyrite structure in which the metal atoms form a f.c.c. array and the S atoms are arranged in pairs across the midpoints of the cell edges and the cell center. The metal atom is octahedrally surrounded by six S atoms and the S atom is tetrahedrally surrounded by three metal atoms and one S atom [30]. The crystal parameter is 10.60 a.u. [31], the Ru-S distance is 4.45 a.u. and the S-S pair is 4.04 a.u. apart. We call this S-S pair the nearest S-S pair, in order to differentiate it from any other S atom pairs with longer distances.

TABLE l(a)

Various models for the CogSa surface”

Model Atoms in cluster the cluster

State of coordination

~a&4 Co’ - co8

S-s*

s9 _ s’*

CO8S9 Co’ -Cod, s’ - s4 co5 - co8 s9 u p

Co& co5 - co8

s5 _ p

co*s9 co5 -coa, P-P S4

~G%H, co5 - Co8, s5 - s=, Si4 4H

Corsnl co5 koa cog, s4 s5-s8 95 _ S’9

Co(II), every Co atom is coordinated as in the bulk crystal.

S(I), every S is coordinated with only 1 Co(I1) atom.

S(U), for each of the S atoms there are 4 Co(I1) neighbors.

Identical with that in Co&,. Each is coordinated with 2 S(1). Identical with S14 in Co&,. Each has one Co(H) neighbor less

than in the bulk Identical with Co,&,. Identical with Co&. It is the same as S“’ in Co&III. Identical with Co,Ss and the 4 H

are on top of the S5-S8 Identical with Co&. Identical with the buk Identical with Co8Si4. Each of them is coordinated with

1 Co(I) only.

“See Fig. 1.

257

TABLE l(b)

Various models for the adsorption systems of thiophene on Co&S, surface

Model cluster Substrate cluster described in Table l(a)

Position of the adsorbate

co&&- thiophene (A)

co8sQ The thiophene molecule is positioned in the y-z plane and standing perpendicular to the z-y plane, its S atom is S4 in Pig. 1, the system possesses a reflection symmetry with respect to the z--LG plane.

co8sQ- thiophene (B)

Co&- thiophene

co,s,- thiophene

co8sQ

CGb

Co,&, with S” removed

The thiophene molecule lies parallel to the x-y plane and possesses a reflection symmetry with respect to the x-z plane, its S atom is positioned at an appropriate distance to Co5 and Co’

Identical to thiophene in Co,SQ-thiophene (A)

The thiophene molecule stands on top of Co9 and perpendicular to the z-y plane, its S atom is at the S” position and it possesses a reflection symmetry with respect to the z-.z plane

Fig. 2. Pictorial representation of the Ru,S,, model cluster, a part of the RuSa crystal which is of the pyrite structure. The large circles represent S atoms and the small circles represent Ru atoms. The numbers in the circles represent the sequential number of the same kind of atom. The other model clusters can be represented by removing some of the S atoms from this figure.

Figure 2 is a pictorial view of the RulSzo model cluster for the RuSz crystal surface. As we can see in Fig. 2, the lattice represents the plane of the Ru atoms and the perpendicular lines connecting this plane with S atoms

258

indicate that the S atom lies above or below the plane at the appropriate distance. The S2 and S7 atoms form a nearest S-S pair, and the Sr2 and S17 atoms form another nearest S-S pair etc. All the models used for RuS2 are chosen from part of the atoms in this figure and are described in Table 2(a). The models for the systems of thiophene adsorption on Co& and on RuS2 are described in Tables l(b) and 2(b) respectively. The bond lengths and bond angles of thiophene are taken from reference [32 1 and have been described elsewhere [ 11.

In all the models for the adsorption systems except Co,S,-thiophene (B), the sulfur atom of the thiophene molecule S(T) is positioned at the vacancy of the substrate S atom, i.e. the S(T)-M distance is the same as

TABLE 2(a)

Various models for the RuSz surface (see Fig. 2)

Model cluster

Atoms in the cluster

State of coordination

Ru&o

RuG%

Ru&

Ru&oH,

Ru’ - Ru4

S’ _ S20

Ru’ -Ru4, 16 S

Ru’ -Ru4 12 s

Ru’ - Ru4, S-S”, 4 H

All the Ru atoms are coordinated octahedrally.

S2, Sr; S”, S17; S’, S” are the three nearest S-S pairs.

The S5, S6, S15 and S16 atoms are removed from Ru&,.

The S5 Se Sg S”, S15, S16, S” and Szo &e remdved from Ru&,.

The H atoms are just on top of Sg, S” and below S’, S19 respectively, the S-H distance is taken to be 2.51 a.u.

TABLE 2(b)

Various models for the adsorption systems of thiophene on RuS, surface (see Fig. 2)

Model cluster

Substrate cluster described in Table 2(a)

Position of the adsorbate

Ruz%-

thiophene Half of Ru,S,,, which contains Rue, Rx?, S’, Sr, SS S” $2 $3

,d ,I> ’

The thiophene molecule lies parallel to the surface with its S just at the position of S9; the bisector of the C(a)-S-C(a) angle and the line connecting Ru’ and Ru3 form a plane

Ru,S,,- Ru.,S,~ with S” thiophene removed

The thiophene molecule lies inclined to the surface with its S at the position of S”, the molecular plane of thiophene forms an angle of 45” with the surface and the projection of the bisector of the C(a)-S-C(U) angle is perpendicular to the Line connecting Ru3 and Ru4

259

that in the crystal. Since the adsorption geometries have not been optimized, this choice of adsorption geometry may afford an adequate base for comparison of the data. As indicated in reference [25(b)], the chemisorptive bond length will simply alter the magnitude of the interactions, but will not reverse the general trends or change the nature of the electronic interactions. For Co&G,-thiophene (B), in order that every atom of thiophene may be placed at an appropriate distance from Co5 - Co’ and the molecular plane of thiophene may be parallel to the plane of Co5 - Co*, the S(T)-Co5 and S(T) -Co* distances remain the same as that in the substrate but S(T) is not positioned at a vacancy.

For the adsorption systems on cobalt sulfide, the model of Co,&-thiophene represents adsorption on octahedrally-coordinated Co and the other three models represent adsorption on tetrahedrally coordinated Co with different vacancies and/or at different directions. For the adsorption systems on RuS,, in the model for Ru,S,,-thiophene, the distance between S(T) and S’ of the substrate is the same as that of the nearest S-S pair, while in the model for Ru&-thiophene, there is no such nearest S(T)-S pair.

Strictly speaking, the models may differ greatly from the bulk of the sulfide crystal, but they may represent the state of the surface boundary for the catalysts which are of interest in the catalysis and adsorption study.

We would like to illustrate this with an example using CogSa clusters as models. Large crystals of Co&S, are certainly not catalytically active in HDS, nevertheless the cobalt sulfide on active carbon exhibits high activity for HDS [2]. For highly dispersed cobalt sulfide, a great number of Co atoms in the microcrystalline Co9Ss are located on defects and grain boundaries. This condition may be responsible for the activity, and needs to be studied. The six model clusters used are taken from part of the Co9S8 crystal unit cell and all of them except CO&~, which is not used for the adsorption calculation, possess some vacancies. Some resemblance may exist between the model clusters and the highly dispersed cobalt sulfide. For these model clusters, their ability to bond with thiophene and to activate the thiophene molecule may be revealed in the calculated results. Of course, the adsorption process differs from the catalytic reaction, as the latter is more complex.

Calculated results

The bond orders are calculated as [33]:

(3)

where & and & may be atomic orbit.& centered on different atoms, respectively, or group orbitals obtained by group theory; CPi, Cti are their corresponding coefficients in the occupied molecular orbitals i, andfi is the occupancy of the molecular orbital i.

The bond orders between Co -Co and between S-S for the Co&, model clusters are listed in Tables 3a and 3b respectively. The bond orders between Co-S for the Co&& model clusters and for their related adsorption systems,

260

TABLE 3(a)

Bond orders between Co-Co pairs in Co,S,

In the plane of Co’Co2C03C04 co’(cog-coyco*) Co’-co3, coz-co~

In the plane of Co’Co2CoaCo5 a Co’-co5, co2-co6 Co’-COG, co2-co5

In the plane of Co5C06C07C08 coyco~)-coyco*) co5-co?, cog-co* Co’-COT, coz-co*, co3-co5, cod--co6 COQ-Co6(Co6, COT, COS) i.e. Co(I)-Co(I1) pairs

c0*s,, a3s9 C%SO %310

- 0.423 - 0.386 - 0.264 - 0.257

- 0.423 - 0.258 - 0.264 -0.331

- 0.423 0.313 - 0.305 0.065 - 0.264 - 0.306 -0.219 - 0.207

0.247 0.208

-0.100

“rhe data are the same for the planes Co2C03C07C06, Co3C04C08C07 and Co4Co’Co5C08.

and between Ru-S for the RuS, model clusters and for their related adsorption systems are listed in Tables 312, 3d and 4 respectively. The electronic charges on the metal and S atoms for the Co,S,-thiophene and for RuS,-thiophene systems are listed in Tables 5 and 6 respectively. The HOMO, LUMO, d band top, d band bottom and d band width for CosS8 and RuS, model clusters and their related adsorption systems are listed in Table 7. In order to discuss the adsorption and activation of thiophene on the substrate, the cr and 7~ bond order for C(a)-S(T) and C(a)-C(p) of thiophene before and after adsorption and that for M-S(T) of the adsorption systems are listed in Table 8. The bond orders or bonding-antibonding states for the C(a)-S(T) and C(a)-C(p) of each orbital of the thiophene molecule are listed in Table 9. We will now discuss the electronic and bonding properties of Co&& and RUE& and their related adsorption systems from the results reported in the following paragraphs.

Discussion

General remarks A general remark on the reliability of the results presented in this paper

will be made first. As is known, the Mulliken population analysis is sensitive to the choice of the basis set, therefore bonding properties will be influenced by this choice. Unfortunately there are no available data for the comparison of the bonding of cobalt sulfide and RuSa obtained from calculations with different basis sets, and we can only use the data of thiophene for a preliminary comparison. The atomic populations of thiophene obtained from different basis sets are listed in Table 10, from which it can be seen that our result

261

TABLE 3(b)

Bond orders between S-S pairs in CosSs

CO8Sl4 cosss co4s9 C%SlO

S’ - S2(S4, S5) 0.003 - 0.002 S’ - P(S6, s*> 0.000 0.001 s-s7 0.000 P-@(S8, S’) 0.003 - 0.002 - 0.003 s5--S(S2, S”) 0.000 0.001 0.000 P-S3 0.000 S’3-S’(S2, s3, S”) - 0.035 - 0.009 s’3-sys 10, S”, P) -0.047 - 0.050 P-+qS~, ST, SS) 0.001 S’3_$4 0.004 S-S5(SG s7 SS) S’4_S9(S’b, &, P)

- 0.035 0.022 - 0.023 - 0.047 - 0.036

s’4-S’(S2, s3, S4) 0.001 s’4_p 0.004 S9-S’(S4, P, sy - 0.035 - 0.048 -0.015 s9-sys 13, P, S’4) -0.047 - 0.042 - 0.034 s9-sys3, s6, ST) 0.001 0.000 0.001 S9-S” 0.004 0.006 0.000 s’4-sys I6 , SIT, S’9 - 0.063 P-sys 16 , S’7 p > SyS’7)-S’yS’S)

- 0.048 0.016

S’5_S’7 S’6_S’8

s’9_s14’ -0.011 - 0.001 P-sys6 s7 S8) SLS’5(S’~), &-p(S’7),

0.002

s7-s7(s’*), s*-S8(s’5) 0.046 S5--S’~(S’8), sg-S’yP),

s~-s’ys’~), S8-P(S’~) 0.002

is in good agreement with the ab initio calculations with the Gaussian-type minimum basis set and with the double zeta Slater-type minimum basis set.

As described above, the bond order value is calculated with eqn. (3) and is summed over many terms, of which some are positive and others are negative. The partial sum of positive terms and that of negative terms together form the bond order value. As the basis set is changed, every term in the sum may be influenced and varied in a random fashion, and there may be compensation among the variations of these terms. Thus the variation of the sum, i.e. the change in bond order value, will not be large. As indicated from the data in Table 10, for the single atomic orbital population, different results show a general difference of - 0.050 and reach a maximum of 0.100. It seems that the bond order values for different basis sets will show differences like that of atomic orbital populations. If a bond order value is larger (smaller) than 0.100 ( - O.lOO), even if its difference from the results for the other basis sets reaches a maximum of - 0.100 (0. loo), the bonding properties still will not change. If the bond order value is 0.050 (-0.050), a general

TABLE 3(c)

Bond orders between Co and S in Co& model clustersa

c%s14 co8sQ co,sQ Co*% C",S,H, co,s,o

Co’ -

co* b

co5 -

Co8 b

co9

Co’-s’

Co’--SQ Co’-- so Co’- s3 av.

co5-s5

cos-sQ co5-so co5-s4 av.

coQ-s4 co9-s’5 cog-s6 cog--s7 cog-s’s co9-s’Q av.

0.654 0.792

0.262 0.261 0.262 0.261 0.262 0.261 0.360 0.394

0.654 0.600

0.262 0.248 0.431 0.262 0.248 0.431 0.262 0.247 0.360 0.427

0.700 0.448 0.796

0.387 0.397 0.387 0.397

0.214 0.178 (0.491) 0.364 (0.487)

0.184 0.374 0.374 0.374 0.374 0.254 0.322

“Co’ - Co’ belongs to Co(B); Co9 belongs to Co(I); S’ - S” belongs to S(1); SQ - Si4 belongs to S(I1); see Table l(a). qhe data are the same for Co’ -Co* and for Co5 - Co*.

difference of -0.050 (0.050) will have no effect, but a maximum difference of - 0.100 (0.100) will indeed change its bonding properties. Thus for bond order values between - 0.050 and 0.050, the bonding-antibonding properties may be uncertain due to the influence of the basis set choice. We have termed this case ‘near nonbonding’.

For M-S and M-M, all the absolute bond order values obtained exceed 0.100, while for S-S almost all the absolute values are less than 0.050.

From the above discussion it may be inferred that the choice of basis set will not influence conclusions on the bonding in the studied systems.

InJluence of negative charge We would like to discuss the influence of the negative charge of the

clusters on the results. As indicated in [ 11, the nonstoichiometry of the MO/S ratio, i.e. the negative charge on the cluster, would not produce a notable influence on the electronic properties. The same is true for CogSs and RuS, clusters. For the Co,SI, and Co& clusters, the negative charge is different, the average single bond orders for Co’-S (Co2-S, Co3-S and Co4-S are the same) are 0.360 and 0.394 respectively (Table 3c), the net charges on Co’ - Co4 are 0.20 and 0.22 respectively (Table 5), the differences in HOMO, LUMO and Er are 0.4 eV (Table 7). For Co.& and CoqS9H4, the average single bond order values for Co5-S (Co6-S, Co7-S and Co’--S

263

TABLE 3(d)

Bond orders between Co and S for the Co,S&hiophene model clusters

co,s*- cosss- co&GQ- Co&o- thiophene thiophene (A) thiophene (B) thiophene

co’ -

co4 =

co5 - co8 a

COQ

Co’-S’

Co’-s9 co’-s’” Co’-__s’”

av.

co5-s5 co5-s9 Cob--so co5-s4 CiV.

cog-s4 cog-s5 cog-s6 cog-s7 COQ- P co9-s’g av.

0.768

0.228 0.228 0.280 0.376

0.678 0.435 0.314 0.435 0.314

0.771

0.207 0.207 0.260 0.361

0.781 0.312 0.312

0.193 (0.487)

0.161 0.399 0.399 0.399 0.399

(0.054)b 0.351

“rhe data are the same for Co’ - Co4 and for Co5 - Co’. bBond order between S(T)-Co’.

TABLE 4

Bond orders between Ru and S in RuS, model clusters

Ru&o RGm Ru.3,~ RG%.oH4 Ru&- R&%I- thiophene” thiophene

RLI-S’, Ru3-S” 0.136 R&-S’, RI&S” 0.187 Ru’-S3, Ru3-S13 0.221 Ru’-S4, Ru3-S14 0.323 Ru’-S5, Ru3-S15 0.270 Ru’-SO, Ru3-S16 0.270 av. 0.235 Ru2-S’, Ru4--S1’ 0.114 Ru2-ST, Ru4-S” 0.169 RI?-S8, Ru4-S’* 0.329 RI?--S9, Ru4--S” 0.365 Ru’-S”, Ru*-S2’ 0.365 Ru2-S’*, Ru*-S4 0.198 av. 0.257 Nearest S-S 0.412

0.189 0.164 0.346 0.330 0.443 0.467 0.432 0.454

0.353 0.086 0.121 0.338 0.366 0.366 0.165 0.240 0.243

0.354 0.233 0.241 0.543

0.295 0.328 0.393

0.117 0.025b 0.255 0.361 0.318 0.318 0.232 0.144 0.196 0.357 0.240b 0.411 0.198 0.258 0.324

0.207 0.265 0.266 0.062 0.368 0.348 0.250 0.322

0.273 0.249 0.234 0.116 0.491 0.354 0.400 0.266

0.350 0.275 0.369 0.253

“For this model there are only Ru’ and Ru3. -his is for the SH group.

264

TABLE 5

Net charges on Co and S of the various model clusters

Model cluster Charges on the atoms

Co’-co4 co5-co8 s-9 s5-s8 s9_ s’3 S’3 S4

Co&%4 0.20 Co&k 0.22 co,s,- 0.41

thiophene (A) co,s,- 0.54

thiophene (B) C%S9 0.34 C@Q& 0.39 co,ss- 0.33

thiophene Co&, b 0.42 co,s,- 0.51

thiopheneb

0.20 - 0.06 - 0.06 -0.19 -0.19 -0.19 0.22 -0.11 -0.28 -0.21 0.15 -0.12 - 0.33 -0.16 0.03”

0.18 -0.11

-0.10 -0.16 - 0.32 -0.24 -0.19 - 0.39 - 0.09 -0.17 0.06”

0.23 -0.15 - 0.07 0.18 -0.15 -0.13

- 0.34 -0.20 0.18”

“this is the net charge on the S atom of thiophene. bathe net charge on the first column is the net charge on Co’; the net charges on the S15 - S’s of these two models are -0.32 and -0.28 respectively; the net charge on S” of CO&~ is -0.08 and that on the S atom of thiophene for the Co&-thiophene system is 0.29.

TABLE 6

Net charges on Ru and S atoms of the model clusters

Model clusters

RuG%o Ru&, Ru& Ru&oH, Ru&r- thiophene

Ru&- thiophene

Ru’, Ru3 0.46 0.42 0.40 RI?, Ru4 0.44 0.48 0.44 S’, S” -0.15 -0.17 -0.18

s2, P s3, s3 s4, s4, 9, s7 9, ss SQ, s9 SIO s20

s5, ‘P so, S6 HI-H4

- 0.05 - 0.09 -0.19 - 0.04 -0.07 - 0.08 - 0.08 - 0.08 - 0.08

- 0.04 - 0.08 -0.21 - 0.05 - 0.07 - 0.09 - 0.09

- 0.07 -0.11 -0.24 - 0.08 -0.11

0.50 0.50

-0.17

- 0.08 -0.11 -0.21 -0.06 -0.10 -0.14 -0.14 -0.11 -0.11

0.10

0.66 0.74 0.68 0.51

- 0.36 - 0.03 0.37”

- 0.04 -0.18 0.34” - 0.34 - 0.20b -0.14 - 0.26b -0.18

- 0.22b

“Charges on S(T). bin these cases, -0.22 is the charge on S’a and Sr; -0.20 is the charge on S13, Ss; -0.26 is the charge on S14; see Fig. 2 and Table 2b.

265

TABLE 7

The HOMO, LUMO, Ef and d band for the Co& and RuSa model clusters and their related

adsorption systems (eV)

Model cluster HOMO LUMO Ef d band

top

d band bottom

d band width

%9s,* - 6.66

C%S9 - 6.28

CG9 -6.52

C%% -6.38

Ca&& -6.32

mJs,o -6.16 CosSg-thiophene (A) -5.44 CosSg-thiophene (B) -5.88 Co&-thiophene -6.30 Co,Sg-thiophene -5.57

Ru,Szo -6.37

Ru,S,, - 6.99

RG312 - 6.54

Ru312H4 -6.15 Ru&-thiophene -5.78

- 6.66 - 6.25 - 6.48 - 6.38 - 6.32 -6.16 - 5.42 - 5.87 - 6.30 -5.57 -6.37 -6.99 -6.37 -6.13 -5.73

- 6.66 - 6.26 - 6.50 - 6.38 -6.32 -6.16 - 5.43 - 5.88 - 6.30 -5.57 - 6.37 - 6.99 - 6.46 -6.14 -5.75

-3.79

-3.77 - 5.86 - 5.09 - 5.50 - 3.97 - 2.64 - 3.59 - 2.83 - 3.34 - 1.88 - 0.27 - 2.45 - 1.92 -0.12

- 14.94 11.15 - 14.27 10.50 - 13.29 7.43 -11.97 6.88 - 13.02 7.52 - 13.44 9.47 - 14.24 11.60 - 14.46 10.87 - 14.59 11.76 - 12.96 9.62 - 12.71 10.83 - 13.21 12.94 - 12.40 9.95 - 12.92 11.00 - 14.42 14.30

are the same) are 0.427 and 0.364 respectively (Table 3c), the net charges on Co5 -Co8 are 0.34 and 0.39 respectively (Table 5), the differences in HOMO, LUMO and E, are - -06 eV (Table 7). It thus can be said that the negative charge on clusters of cobalt sulfide does not influence the results significantly.

Similar cases occur for the RuS2 clusters. For the clusters Ru~.‘&~ and RuqSZ,,H4, the negative charges are different, the average single bond order values for Ru’-S and Ru3-S are 0.235 and 0.232 respectively, while for Ru2-S and Ru4-S they are 0.257 and 0.258 respectively. The net charges on Ru’(Ru3) are 0.46 and 0.50 respectively while on Ru2(Ru4) they are 0.44 and 0.50 respectively. The differences in HOMO, LUMO and Er are -0.24 eV. From the above-mentioned comparable data, it may be concluded that the negative charges on the clusters would not produce a notable influence on the bonding and electronic properties.

Bonding and electronic properties of Co&, As shown in Fig. 1 and described above, in the model cluster CO$~~,

the eight Co(I1) atoms form a cube in which each Co(R) is tetrahedrally coordinated with S like that in the Co& crystal, thus the data for co&?+14 are pertinent to the bonding properties between Co(I1) atoms. From Table 3a it can be seen that for CO$S~~, the Co(II)-Co(I1) pairs are antibonding except the four body diagonal pairs Co’ -Co7 etc. For Co8Ss, each of the Co5-Co8 atoms is coordinated with only two S atoms, thus the bond orders differ from that of CO&&~, especially for Co5(Co7)-Co’(Co’) in which the bonding character changes from antibonding for Co8S1, into bonding for

TABLE 8

(T and rr bond orders for C(a)---S(T) and C(a)-C(P) of thiophene before and after adsorption, and those of the M-S(T) of the related adsorption systems

System Bond order

C(a)-S(T) Delta” C(a)-C(P) Delta” M--S(T)

thiophene (T 1.132 1.159 7r 0.294 0.749 Total 1.426 1.908

tetrahydrothiophene (T 1.074 1.011 ?r -0.132 - 0.094 Total 0.942 0.917

Co,Sa-thiophene (r 1.278 to.15 1.213 + 0.05 0.114 ?r 0.292 - 0.00 0.757 +0.01 - 0.060 Total 1.570 +0.14 1.976 + 0.07 0.054

Co&,-thiophene (A) D 1.016 -0.12 1.248 + 0.09 - 0.046 rr 0.155 -0.14 0.764 + 0.02 0.202 Total 1.171 - 0.26 2.012 +0.10 0.156

Co,&thiophene (B) u 1.209 + 0.08 1.428 +0.27 * 0.134 -0.16 0.387 -0.36 Total 1.343 - 0.08 1.815 - 0.09

Ru&-thiophene (+ 1.246 f0.11 1.123 + 0.04 0.015 w 0.009 - 0.29 0.367 - 0.38 0.147 Total 1.255 -0.17 1.580 - 0.33 0.162

Ru,S,,-thiophene (T 1.132 0.00 1.425 + 0.27 r 0.207 - 0.09 0.416 - 0.33 Total 1.339 - 0.09 1.841 - 0.07

aDelta A is the bond order value for the adsorbed thiophene minus the corresponding value for the free thiophene molecule.

Co&& because of the lack of sufficient coordinated S. For Co,& and Co5Si0, there are only four Co(I1) in the model, therefore they cannot represent the bonding properties between Co(I1) atoms. From the data in the last column of Table 3a, it can be seen that the Co(I)-Co(I1) pairs are antibonding. As a whole, for the Co&& crystal, the Co-Co pair is antibonding, except for the body diagonal Co(II)-Co(I1) pair in the cube formed by eight Co(I1). This pair is termed the “special” Co(U)--Co(I1) pair throughout this paper. From Table 3b it can be seen that for every S-S pair of all the Co&l8 cluster models, the bond order value varies from -0.063 to 0.048. It is near nonbonding, as defined in the general remark above.

From Table 3c it can be seen that for tetrahedrally coordinated Co(II), the average single bond order of Co(II)-S takes a value of approximately 0.36-0.43 while for the octahedrally coordinated Co(I), it takes a value of 0.32. For tetrahedrally coordinated Co(II), the bond length of Co(II)-S(I) is different from that of Co(II)-S(H). Their bond order values also differ

TABLE 9

Bond orders for C(U)-S and C(a)-C(p) for each orbital of the thiophene molecule

Orbitals Energy level

(ev)

Bond ordeP

C(a)--s C(a)-C(P)

Al 1 - 22.84 2 - 19.37 3 - 15.50 4 - 13.97 5 -11.41 6 - 9.84 7 1.97 8 2.87 9 6.27

10 10.57

1 2

-7.10 -0.31

1 -19.17 2 - 15.22 3 -11.79 4 -11.36 5 0.33 6 4.66 7 5.79 8 10.99 9 15.49

Ba 1 - 9.97 2 - 7.31 3 - 2.35

0.412 0.372 0.248 0.218

- 0.060 0.110 0.040 0.094

- 0.088 - 0.206 - 0.050 - 0.204 -

0.119 0.205

- 0.010 0.316

-

-

0.344 - 0.050 -

+ +

0.390 -

0.492

0.080 0.217

- 0.022 -

0.304 0.056

-

“For the unoccupied orbitals, +indicates bonding and -antibonding.

greatly. The former takes a higher value of 0.60 L 0.80 corresponding to its shorter bond length, while the latter takes a lower value of 0.22 N 0.43 corresponding to its longer bond length. This difference is more remarkable than that between Co(H)-S and Co(I)-S. If there are vacancies of S, the average single bond order for Co(H)--S will change; it increases when there is a S(I1) vacancy and decreases when there is a S(1) vacancy. When there are SH groups such as those in the model cluster Co,S9H, where four H atoms are positioned on top of the four S(I) of Co.& respectively, the average single bond order decreases from 0.427 for Co& to 0.364.

From Table 3d it can be seen that after adsorption of thiophene, the bond order between Co-S on the average is changed slightly, but it still falls into the range mentioned above if the bond order between Co-S(T) is not considered. Like the case for the MO-S bond in MO&, the Co-S bond in Co&l8 is also not strong and shows small changes under various conditions. In general, in the Co9Ss crystal the Co atoms are held together

268

TABLE 10

Atomic populations on atoms of thiophene obtained by different authors with different basis sets

Present Clark and Palmer Gelius results Armstrong” et aLb et al.’

S 3s

3P,

3Pw Total

C(o) 2s

2P,

C(P)

2P, 1.154 Total 6.429

2s 1.136

2P, 2.030

2P, Total

H(a) H(P)

1.757 2.295

1.647 15.699

1.212 2.063

1.022 6.188

0.743 0.790

1.716 0.872 1.347 1.680

15.616

1.179 1.114 0.997 1.106 6.396

1.096 1.023 1.076 1.054 6.249

0.769 0.778

15.856 15.518

2.039

1.094 6.239 6.568

2.075

1.037 6.165 6.177

0.827 0.733 0.840 0.764

“Ab initio calculation with Gaussian minimum basis set [36]. bAb initio calculation with Gaussian minimum basis set [ 371. “Ab initio calculation with double zeta Slater basis set [38].

through bonding with S atoms as well as through bonding of the special Co(II)-Co(I1) pairs (see Fig. l), while the S atoms are held together mainly through bonding with Co atoms.

As stated above, Co(II)-S(1) possesses an average bond order almost twice as large as that of Co(H) - S(II), yet the single bond orders of Co(I1) -S(I), Co(H) -S(II) and Co(I) - S(I1) themselves change depending on their different coordinating conditions, as can be seen from Table 3c. For Co(II)-S(I), this varies from 0.60 to 0.80, for Co(II)-S(I1) it varies from 0.22 to 0.43, for Co(I)-S(I1) it varies from 0.18 to 0.37. The structure for the Co&+& bulk crystal has been described previously [31]. For the case of the crystal surface, many types of sulfur may be distinguished by their locations on the surface, by their coordination number with respect to the metal atom and by their bond order values. A general analysis may be made in this regard, as has been done for MO& by S. Kasztelan et al. [ 151. But CogSa is more complicated than MO&, e.g. for the Co(II)-S(I) bonding pair, Co(U) may be coordinated with 0, 1, 2 or 3 S(II)s respectively. At the same time, it may also be coordinated with 0, 1, 2 or 3 other Co(II)s respectively and S(I) may be coordinated with another 0, 1, 2 or 3 Co(II)s. Thus a total of 64 different coordination combinations can be found. Similarly, a total of 90 and 24 different coordination combinations can be found for the Co(II)-S(I1) and

269

Co(I)-S(I1) pairs respectively. Except the cases which are not of interest in simulating the Co& surfaces in the HDS catalysts, a great many cases still need to be considered and therefore much data is needed to clarify this aspect. Here we analyze only our proposed models, and the general case may be understood by inference.

Inspecting the bond order values for Co’-S’, Co’-Ss2, Co3-S3, Co4-S4, Co5-S5 etc. in Table 3c, we see that they vary from 0.60 to 0.80 for these Co(II)-S(1) pairs for various models. Except for the case of Co4SSH4 where a H atom is attached to S(I), in all the other models S(1) is coordinated similarly with a single Co(II), while Co(R) is coordinated differently. For the case of Co8Sr4 and Co&, even the Co(I1) is coordinated similarly, with three S(I1) and three other Co(I1). Only one of the three other Co(R), i.e. Co5, is coordinated differently for these two models, which causes a bond order difference of 0.14. For the Co(II)-S(1) of Co&& and Co4S9, there is one more Co(I1) coordinated with the Co(R) in Co& than that in Co,S,, which causes a bond order difference of 0.19. Similarly for Co,& and Co,&, there is one more S(I1) coordinating with the Co(R) of Co(R)-S(I) in Co,&, than in Co4Ss, and the bond order values differ by 0.10.

Similar to the case for MO&,, the presence of surface SH groups on Co,& modifies the bonding between metal and sulfur significantly. It can be seen from Table 3c that for the Co5-S5 pair, the bond order changes from 0.600 for Co4S9 to 0.448 for Co4S9H4. Furthermore, all the bond order values for the bonding pairs of Co5 in Co4SgHq are lower than those in Co4Ss. Thus the presence of SH groups would be favorable to the removal of S from the CosSs surface.

From Table 5 it can be seen that the Co(R) and Co(I) atoms are charged with 0.2 and 0.4 positive charge respectively. After adsorption these values increase and the Co atoms are more positively charged, since there is some charge transfer from the substrate to the adsorbate. As for the S atoms, the net charges and their variations are even smaller. As a whole the charge transfer is small. Since the metal and the S atoms are covalently bonded, small charge transfer would reveal weak bonding. All the models used for CosSs possess C4” symmetry and those for their related adsorption systems possess CBV symmetry.

Bonding and electronic properties of RUS,

From Table 4 it can be seen that for the octahedrally coordinated Ru the Ru-S average single bond order is about 0.24 - 0.26. If there are vacancies for this Ru it would increase to a higher value, e.g. for the Ru4SlG model there are two vacancies for Ru’ and the average single bond order increases to 0.35. Adsorption also causes a small change, but all the values are lower than 0.40. For every Ru-Ru pair, the bond order is negative and antibonding in character. One striking fact is that for various cases the nearest S-S pair possesses a bond order value of 0.24 - 0.41 which is comparable to that of the Ru-S bond, and thus RuSz might behave differently from CosSs and MO& where S-S is near nonbonding. As for all the other S-S pairs in RuS,,

270

they are near nonbonding since their bond order value is small (negative or positive). A general statement may be made that the bonding between Ru-S is not strong, and in RuS, the Ru atoms are held together through octahedral bonding with S, and the S atoms are held together through tetrahedral bonding with the three Ru atoms and the nearest S atom. It is noteworthy that there is only one nearest S-S pair for every S atom in the bulk crystal, and this nearest S-S pair contributes to the bonding of the RuSa crystal just as Ru-S pairs do.

Similar to the cases of MO& and CogS8, for every Ru in RuSa the single Ru-S bond order varies considerably, e.g. for Ru& it varies from 0.19 to 0.44 for Ru’ and from 0.09 to 0.37 for Ru2. Evidently these variations are caused by the difference in their location and coordination states in the surface, as discussed for MoS, and CoaSa.

It can also be seen from Table 4 that for the surface SH groups the bonding between the S of SH and Ru is considerably weakened, e.g. the bond order decreases from 0.187 of RuqS2,, to 0.025 of Ru$~~H+ Thus this S atom is easily removed to form a vacancy.

From Table 6 it can be seen that Ru is positively charged with 0.4 N 0.5 charges. After adsorption Ru is more positively charged, reaching a value of 0.6-0.7. All the S atoms are negatively charged and their values vary with their bonding circumstances (location and coordination).

Common points and diflretmces for the properties of MO&, Cops8 and

RuS, Now we would like to emphasize the common points for the bonding

and electronic properties of the three sulfides MoS2, Co&S, and RuS, from the above discussion and from [ 11.

(1) On the crystal surface the metal-sulfur bonding M-S is not strong, and it is even weaker for the bonding between the metal and the S of the surface SH groups. Therefore under an atmosphere with a partial pressure of hydrogen, the S atom on the surface of these sulfides is easily removed and vacancies may be formed.

(2) For these three sulfides, except the special Co(II)-Co(I1) pairs in Co&& it is antibonding for all the other metal-metal pairs M-M. Except for the very special nearest S-S pairs of RuS2, it is near nonbonding for all the other S-S pairs.

(3) The metal atom is coordinated octahedrally or tetrahedrally with S and the S atom is coordinated tetrahedrally, square-pyramidally (five-coor- dinated) or tripodally (in the bulk of MoS,) with M, therefore the M-S bonding is directional for these sulfides.

(4) If vacancies are present for a metal atom coordinated with S, the remaining individual M-S bondings for this metal atom would be increased.

(5) As can be seen from Table 7, due to the interaction of the metal atom with sulfur, the d band of these sulfides is much wider than that of the corresponding metals, reaching a maximum value of 13 eV. Values vary from 7 to 13 eV according to the degree of interaction. The top of the d

271

band is positioned at the vicinity of the vacuum level (MO&, Ru&) or 4 - 6 eV below it (Co,S8). The difference between LUMO and HOMO is small ( < 0.1 eV). The Fermi level Ef of these sulfides passes through the middle of the d band, which is markedly different from that of the transition metals where Er passes through the top of the d band [34].

In spite of the above common points for these three sulfides, some differences are revealed from the obtained results:

(a) The bonding of every Co(I1) in Co&S8 is different from that of the metal in MO&., RuS, and that of Co(I) in Co&+ It is bonded to one of the other Co(R) in addition to bonding with the coordinated S. The very nearest S-S pairs contribute signillcantly to the bonding for Ru&, while the other S-S pairs for these three sulfides are near nonbonding.

(b) There are differences in the M-S bonding strengths of these three sulfides. It should be recognized that the bond order for different atom pairs is not strictly comparable. But in our case for these three sulfides the Co(II)-S and the Co(I) -S bonding strengths are comparable, and a comparison between them may be used as a reference. Moreover, there is a common S atom in the atom pairs for comparison. Perhaps some tendency may be revealed from the bond order values for these three sulfides. It seems that the sequence for the bonding strength is MoS, > Cog&> RuS,. Since the average bond order of both the single MO-S and Co(II) -S bondings lies between 0.36 - 0.40, and the average single bond order of Co(I)-S is 0.32 and that of Ru-S lies between 0.24 -0.26, it is reasonable to make such an evaluation for the sequence of bonding strength if the bond order of Co-S is obtained from the weighted mean of Co(I)-S and Co(R)-S. One may point out that this sequence of bonding strengths is inconsistent with that for the heat of formation M, in which MO& > RuSz > Co9S8. However, the following argument may be put forward. We are discussing surface properties, while AH, is a bulk property. Although the very nearest S-S pair of RuS, contributes greatly to the bulk bonding, it may contribute negligibly to the surface bonding. Thus the sequence for surface properties may diifer from that for bulk properties.

(c) The sequence for the magnitude of positive charge on the metal of these three sulfides is Mo( + 0.6) > Ru( + 0.5) > Co( + 0.2 [Co(II)] or + 0.5[Co(I)]). This sequence is fully in accord with that for AH. Since the bonding is mainly covalent, the metal transfers electrons to S and then bonds with it, and this electron transfer does not involve the very nearest S-S pairs, so the sequence of positive charges on the metals of these three sulfides is reasonable.

(d) As stated above, the S atom on the surface of these three sulfides is easily removed. In some cases the S of SH on the RuS, surface is the most easily removed, as indicated by its bond order value of 0.03 (see data for Ru.&~H~ in Table 4). Therefore RuS2 exhibit greater facility in activating the hydrogen molecules by forming SH. This may explain the experimental results in which more effective hydrogenation properties are observed for RuSz than for MoS, and Cogs8 [35].

272

About some properties of thiophme and tetrahydrothiophene

Some of the bonding and electronic properties of thiophene and te- trahydrothiophene have been listed and discussed in [ 1 ]. Now we would like to extend this discussion. As is known, thiophene may form (T bonding in the molecular plane of the five-membered ring and form 7 bonding per- pendicular to this plane. The calculated u and n bond orders between C(a)-S(T) and C(a)--C(p) of thiophene and tetrahydrothiophene molecules are listed in Table 8. These bondings are highly directional. At the same time, tetrahydrothiophene is saturated with hydrogen in the five-membered ring, its T bond order value for C(a)--_(T) and C(a)-C(p) is negative and antibonding. It is clear from Table 8 that the u bonding for thiophene is slightly larger than that for tetrahydrothiophene, and the higher bond strength for thiophene is due to the r bonding. Since these bondings are highly directional, it is necessary to have appropriate adsorption centers on the substrate surface to activate the thiophene molecule.

From Table 9 it can be seen that for the r orbit& of the thiophene molecule, one of the valence orbitals is bonding for both C(o)-S(T) and C(a)-C(p), and it is the orbital possessing lower energy and positioned below the HOMO. One of the valence orbitis is antibonding for both C(a) -S(T) and C(a) -C(p); it is the valence orbital further above the HUM0 and possessing higher energy. There are still other valence orbitals which are bonding with respect to C(a)-S(T) and antibonding with respect to C(a)-C(p) or vice versa; these are the valence orbitals in the vicinity of HOMO and possessing intermediate energies.

The adsorption and activation of thiophene is related to those orbitals in the vicinity of the LUMO and HOMO. In the adsorption process it often appears that C(a)-S(T) is weakened while C(a)-C(p) is strengthened, or vice versa. Here by ‘activation of thiophene’ we mean that the C(cu)-S(T) and/or C(a)-C(p) bond is weakened for the adsorbed thiophene and thus a subsequent process such as hydrogenation and/or desulfurization may occur.

Another point to be mentioned is that, for C(a)-S(T) of thiophene, there are four antibonding (+ and one antibonding r orbitals which are occupied, and for C(a)-C(p) there are three antibonding u orbit& which are occupied. In order that the adsorbate may be activated, there must be interaction between unoccupied antibonding orbitals of thiophene and the occupied orbitals of the substrate. The lowest of the unoccupied antibonding orbitals of thiophene is 0.33 eV for u and -2.35 eV for 7r orbitals. The HOMO of these sulfide substrates lies between -5 and - 7 eV. Therefore interaction occurs easily.

Adsorption of thiophem on Co& and RuS2 surfaces From Table 8 it can be seen adsorption of thiophene on Co,& and RuSa

surfaces leads to different behaviors. We illustrate this aspect using typical examples.

273

(1) For the Co,S&-thiophene system, there is almost no change in the rr bondings of C(a) -S(T) and C(a) -C(p) while their u bondings are strength- ened, as manifested by the difference in bond order values before and after adsorption for the u and rr bondings respectively (A[C(cu)-S(T) I and A[C(cu)-C(p)] in Table 8). On the other hand, the total bond order M-S(T) between the Cog of substrate and S(T) of thiophene is 0.054, which indicates weak adsorptive bonding. At the same time the u orbit& of the adsorbate are mainly involved in M-S(T) bonding. This is obvious from the data listed in Table 8, where the u and the r bond orders of M-S(T) are 0.114 and -0.06 respectively, giving a total bond order of 0.054. It can be deduced from this that the thiophene molecule is not activated. Therefore, in general, weak adsorption causes no activation.

(2) For the Co,Sg-thiophene (A) system, both the (+ and r orbitals of thiophene are involved in the M-S(T) bonding, the u bond order of M-S(T) is negative (antibonding) while the r bond order of M-S(T) is positive (bonding). Therefore rr bonding plays an important role in forming the adsorptive bond.

Examining the bond order between C(a)-S for each orbital of the thiophene molecule in Table 9, we can see that the three antibonding orbit& with A1 symmetry contribute a bond order value of - 0.198, while the antibonding orbital with Br symmetry contributes a bond order value of -0.010, and the antibonding orbital with B2 symmetry contributes a bond order value of -0.050. These may be used as the basis for comparison.

It is found that for this system there is an orbital with Br symmetry which contributes an antibonding bond order of - 0.128 to C(U)-S(T) and plays the important role of activating the u bonding. The energy of this orbital is - 13.43 eV. It is also found that there is another orbital with Bz symmetry which contributes an antibonding bond order of -0.066 to C(a)-S(T) and plays an important role in activating the n bonding. The energy level of this orbital is - 9.53 eV. At the same time there are still other u orbitals with Bz or Al symmetry which give negative bond orders and contribute to the activation of bonding. Thus in this case the activation of thiophene in the adsorption process is caused by the transfer of electrons from the substrate metal atom to the antibonding orbit& of both the u and the n orbitals of thiophene.

(3) For the Ru$!$-thiophene system, the r bond order of C(a)-_(T) and of C(cr)-C(p) is greatly decreased, while the u bond order is considerably increased. It is thus clear that the r bonding is highly activated at the expense of strengthening the u bonding. On the other hand, the bond order between Ru2-S(T) is 0.163, indicating strong adsorption, as compared to the bond order -0.25 of Ru-S in the surface of RuS,. Since in the surface of RuS2, Sg is bonded to Ru2 mainly with pr and the thiophene molecule is adsorbed with its molecular plane parallel to the substrate surface, while S(T) is just positioned at the vacancy of Sg, so the pz of S(T) which forms the r orbital for the C(a)-S(T) in the thiophene molecule would strongly interact with Ru’. Therefore strong activation of r bonding is reasonable.

274

But such a simultaneous strong activation of C(a)-S(T) and C(a)-C(p) requires further examination. On the other hand, it is also different from that observed in Co&&hiophene (A) where activation of C(a)-S(T) occurs at the expense of strengthening C(a)-C(p). Further examination indicates that there is a strong interaction between C(a’) and S” which is mainly of rr character. Thus the rr bonding of C(a ‘) -S is further weakened substantially. In the comparison of the bonding properties of thiophene with those of tetrahydrothiophene, as discussed above, it may be inferred that hydrogenation would weaken the rr bonding of thiophene. As discussed in [ 11, thiophene may be first hydrogenated to tetrahydrothiophene and then desulfurized. This is at least one of the most probable mechanisms.

From the examples for RuS,, it can be seen that the interaction between the S of RuSz and the C of thiophene at an appropriate special position may cause considerable weakening of the w bonding in the thiophene molecule. As discussed above, some of the S atoms in RuS, are the most easily removed. At the same time, the 7~ bonding of the thiophene adsorbed on the surface of RuS, may be activated to a certain degree by hydrogenation and other effective interactions. These two factors together may be responsible for the experimental observation that RuS, exhibits higher activity for both the hydrogenation and desulfurization processes [ 35 I.

Conclusions

(1) In Co&& RuS, and MoS2, the metal-sulfur bonding M-S is not strong, M-M is antibonding except for Co(R), which is bonding with one of the other Co(II)s, and S-S is near nonbonding except the very nearest S-S pair in RuS,. For MO&, the metal atoms are held together through bonding with S and the S atoms are held together mainly through bonding with the metal. For RuS,, the metal atoms are held together through bonding with S, and the S atoms are held together through bonding with the metal as well as its partner of the very nearest S-S pairs. For CogSa, the S atoms are held together mainly through bonding with Co, the Co(I) atoms are held together through bonding with S while the Co(I1) atoms are held together through Co(H) -S and the special Co(II)-Co(R) bondings. Various types of S atoms may be distinguished by their locations in the crystal, their coordination numbers with respect to the metal and their bond orders.

(2) For these three sulfides, the average charge on the metal is 0.6, 0.5, 0.2 and 0.5 for MO, Ru, Co(I1) and Co(I) respectively; if the charge on Co(I1) is weight-averaged with that on Co(I), then the sequence of the charges on the metals is in accordance with the sequence of heats of formation of their corresponding sulfides. The average charge on S depends on the M/S ratio, and for the single S atom the charge is determined by its location, coordination number and bond state.

(3) Part of the surface S is easily removed through the formation of surface SH groups which weaken the single M-S bonding considerably and

facilitate the formation of vacancies. This is particularly so for some cases of the SH group on the RuSa surface, in which the single Ru-S bond order may be as low as 0.03; this S is expected to be the most easily removed. This may be responsible for the higher activity of RuSa in hydrogenation as well as in HDS.

(4) The d band width of these three sulfides ranges from 7 to 13 eV, which is wider than that of their corresponding metals. The top of these d bands lies not far below the vacuum level and the Fermi level passes through the middle of the d band. Thus these d bands are half occupied. These properties are different from that of a transition metal and will influence the adsorptive properties of these sulfides.

(5) The adsorptive bonding differs for different adsorption systems. For Co,&-thiophene systems, the u orbitals of thiophene are mainly involved in M-S(T) bonding and the bonding is weak. While for Co8Sg-thiophene (A) and RuS,-thiophene systems, the r orbitals of thiophene are mainly involved in the M-S(T) bonding and the bonding is strong. For the substrate metal atoms, the orbit& mainly involved in the adsorptive bond may be sp orbitals, such as for the Co,&-thiophene (A) system, or d orbitals such as for the Ru&-thiophene system.

(6) Activation of C(a)-S(T) bonding in the adsorbed thiophene occurs mainly through the transfer of electrons from the substrate metal atom to both of the antibonding CT and T orbitals of thiophene. Hydrogenation of thiophene means activation of the rr bonding, thus it is favorable to the activation of thiophene as well as to the HDS process.

Acknowledgement

The authors thank Prof. Wang Hongli for reviewing the entire manuscript. They are also indebted to the referees for valuable remarks and pertinent suggestions.

References

1 Chen Rong and Xin &in, J. Mol. Cutal., 64 (1991) 321. 2 M. J. Ledoux, 0. Michaux, G. Agostini and P. Panissod, J. Catal., 68 (1985) 189. 3 H. Topsoe, B. S. Clausen, R. Candia, C. Wivel and S. Morup, J. Cutal., 68 (1981) 433. 4 J. C. Duchet, E. M. van Oers, V. H. J. de Beer and R. Prins, J. Cat&., 80 (1983) 386. 5 Xin &in, Guo Xiexian, P. Grange and B. Delmon, Proc. 9th Int. Congr, Cutal., Calgary,

Cunada, 1988, The Chemical Institute of Canada, Ottawa, 1988, Vol. 1, p. 66. 6 B. A. Horrel and D. L. Cocke, Cutul. Rev., 29 (1987) 447. 7 K. Y. S. Ng and E. Gulari, J. Catal., 92 (1985) 340. 8 Zhuang Shuxian, W. K. Hall, G. Ertl and H. Knozinger, J. Cutal., 100 (1986) 167. 9 K. Segawa and W. S. Mlllman, J. Catal., 101 (1986) 218.

10 B. Delmon, in H. F. Barry and P. C. H. Mitchel (eds.), Proc. 3rd Intern. COT& on Chemistry and Uses of Molybdenum, Climax Molybdenum Co., Ann Arbor, MI, 1979, p. 73.

11 A. L. Farragher and P. Cossee, in J. W. Hightower (ed.), Proc. 5th Int. Congr. Catal., North Holland, Amsterdam, London, 1973, Vol. 2, pp. 1301-1318.

276

12 H. Topsoe and B. S. Clausen, Appl. Cutah, 25 (1986) 273. 13 M. J. Ledoux, J. Chem. Sot., Faraday pans. 1, 83 (1987) 2173. 14 B. Delmon and J. I. Dallons, Bull. Sot. Chim. Be@, 97 (1988) 473. 15 S. Kasztelan, H. Toulhoat, J. Grimbolt and J. P. Bonnelle, Appl. Cutal., 13 (1984) 27. 16 E. Furimsky, Cutal. Rev., 22 (1980) 371. 17 P. Ratnasamy and S. Sivasanker, Catal. Rev., 22 (1980) 401. 18 0. Weisser and S. Landa, Su@de Catalysts: their Properties and Applications, Pergamon

Press, Oxford, 1973. 19 M. Zdrazil, Catal. Today, 3 (1988) 269. 20 T. C. Ho, Catal. Rev., 30 (1988) 11. 21 F. E. Msssoth, Adv. Cutal., 27 (1978) 265. 22 H. Knozinger, in M. J. Phillips and M. Ternan (eds.), Proc. 9th ICC, Calgary, 1988, The

Chemical Institute of Canada, Ottawa, Vol. 5, pp. 20-53. 23 B. Dehnon, in D. L. Trimm et al. (eds.), Catalysts in Petroleum Re$ning 1989, Elsevier,

Amsterdam, 1990. 24 R. Prins, V. H. J. de Beer and Somorjai, Catal. Rev., 31 (1989) 1. 25 (a) R. Chianelli, Cutal. Rev., 26 (1984) 361; idem, J. Catal., 86 (1986) 400; idem, J.

Cutal., 98 (1986) 17; (b) M. C. Zonnevylle, R. Hoffmann and S. Harris, Sue Sci., 199

(1988) 320. 26 Chen Rong and Kin Qii, to be presented at the l&h I.C.C, Budapest, 1992. 27 A. Rosen, D. E. Ellis, H. Ada&i and W. Averill, J. Cha. Phys., 65 (1976) 3629. 28 J. C. Slater, Quantum Theory of Molecules and Solids, Part V. The SeCf-Consistent Field

far Molecules and Solids, McGraw-Hill, New York, 1974. 29 E. J. Baerends, D. E. Ellis and P. Ros, C&m. Phys., 2 (1973) 41. 30 W. B. Pearson, The Crystal Chemistry and Physics of Metals and Alloys, Wiley, New

York, 1972, pp. 402-404, 410-411. 31 J. C. Slater, Quantum Theory of Molecules and Solids, Vol. 2, McGraw-Hill, New York,

1965, pp. 63, 304 and 319. 32 A. R. Katritzky, Advances in Hetwocyclic Chemistry, Vol. 1, Academic Press, New York,

1963, p. 12. 33 R. S. Mulhken, J. Chern. Phys., 23 (1955) 1833, 8141; 36 (1962) 3428. 34 R. P. Messmer, S. K. Knudson, K. H. Johnson, J. B. Diamod and C. Y. Yang, Phys. Ra.

B, I3 (1976) 1396. 35 Xin &in et al. to be submitted. 36 D. T. Clark and D. R. Armstrong, J. Chem. Sot. @), Chern. Commun., (1970) 319. 37 M. H. Palmer, R. H. Findley and A. T. Gasbell, J. Chem. Sot., Per-kin Trans. 2, (1974)

420. 38 0. Gelius, B. Roos and P. Siegbahn, Theor. Chim. Actu, 27 (1972) 171.