Chemisorption of H2 and H2S on the (100) surface of RuS2: an ab initio theoretical study

surface s c i e n c e ELSEVIER

Surface Science 389 (1997) 131-146

Chemisorption of H 2 and H2S on the(100) surface of RuS2: an ab initio theoretical study F. Frechard a,b, p. Sautet a,b,, a Institut de _Recherches sur la Catalyse, CNRS, 2 Av A. Einstein, 69626 Villeurbanne, Cedex, France b Ecole Normale SupOrieure de Lyon, 43 AllOe d'Italie, 69364 Lyon, Cedex 07, France

Received 17 December 1996; accepted for publication 12 May 1997

Abstract

The chemisorption of H 2 and HzS molecules on the (100) surface of RuS 2 is studied with a periodic Hartree-Fock approach

completed by density functional correlation energy corrections. The stable (100) surface is modeled by a slab for which both molecular and dissociative chemisorptions are considered. For H2, as well as for H2S, the best optimized situations are for the molecular adsorptions, even if dissociated forms are found to be more favorable than the free gas molecules. The H 2 molecule is chemisorbed on the surface Ru atom in an ~2 lateral coordination with a strongly weakened H-H bond, whereas the H~S molecule is interacting by the S lone pairs with the Ru completing around it the bulk-like environment. The electronic structures for both molecular and dissociated optima are discussed in terms of orbital analysis with the help of projected density-of-states and crystal orbitals overlap populations. The crucial role of the Sz pairs at the surface on the chemisorption properties is underlined, © 1997 Elsevier Science B.V. Keywords: Ab initio quantum chemical methods and calculations; Chemisorption; Hydrogen; Hydrogen sulphide; Ruthenium;

Sulphides; Surface chemical reaction

1. Introduction The interaction of hydrogen- and sulfur-containing m o l e c u l e s with t r a n s i t i o n m e t a l sulfides is a subject o f b o t h f u n d a m e n t a l a n d a p p l i e d interest. U n d e r s t a n d i n g these e l e m e n t a r y r e a c t i o n steps is i m p o r t a n t b e c a u s e o f the large n u m b e r o f i n d u strial processes r e l a t e d to them. T r a n s i t i o n m e t a l sulfides a r e m a i n l y i n v o l v e d in c a t a l y t i c h y d r o t r e a t m e n t r e a c t i o n s for sulfur o r o t h e r h e t e r o a t o m r e m o v a l f r o m h y d r o c a r b o n s p r o d u c e d f r o m oil. H y d r o g e n is a n especially i m p o r t a n t p a r t n e r for * Corresponding author. Fax: (+ 33)4 72445399; e-mail: [email protected] .fi0039-6028/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. P I I S0039-6028 ( 9 7 ) 00405-6

these h y d r o t r e a t i n g r e a c t i o n s involving C - S (or C - N , etc.) b o n d cleavage, a n d a high p r e s s u r e a n d t e m p e r a t u r e are necessary. R u t h e n i u m disulfide R u S 2 is one o f the m o s t active chalcogenides: C o m p a r e d with m o l y b d e n u m sulfide, w h i c h is a reference catalyst, it is 13 times m o r e active for the h y d r o d e s u l f u r i z a t i o n o f thiop h e n e a n d ten times m o r e active for the h y d r o g e n a t i o n o f b i p h e n y l [1,2]. O n e specific feature o f RuS2, c o m p a r e d for e x a m p l e to MoS2, is t h a t in the structure the S a t o m s all c o m e in $2 pairs. T h e solid can t h e n be seen as a t h r e e - d i m e n s i o n a l c o m p l e x o f R u a t o m s w i t h S z molecules. H - H , C - S a n d S - H b o n d a c t i v a t i o n s are i m p o r t a n t e l e m e n t a r y steps for the h y d r o g e n a t i o n a n d

132

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F Frechard, P. Sautet / Surface Science 389 (1997) 131-146

hydrodesulfuration reactions. In contrast to metal surfaces, very little is known about the chemisorption of H2 or other molecules on transition metal sulfides, and especially on RuS2, since information from Surface Science techniques is very scarce. However the (100) bare surface of RuS/ has recently been characterized by scanning tunnelling microscopy (STM) experiments [3] that showed that this cleavage surface is stable and shows no tendency to reconstructi Recently, the interest in H2 chemisorption on RuS2 was renewed by the identification of hydride Ru-H species [4,5]. It was shown that the capability for ruthenium sulfide to chemisorb H2 is dependent on the sulfur to metal ratio, during the reduction process, and two types of hydrogen species have been characterized on the surface by thermodesorption, 1H NMR [4] and inelastic neutron scattering (INS) [5]. One was assigned to H adsorbed on surface sulfur atoms, whereas the second was ascribed to hydridetype Ru-H adsorbed species. The ratio between S-H and R u - H species is found to be between 0.8 and 1.0 by INS in the case of a sample with a composition close to stoiChiometric [5]. More0ver, a relation has been established between the concentration of hydride on the surface a n d the catalytic activity in the hydrogenation reaction [6]. However, nothing is known about the nature of the active site, nor about the active surface plane. In order to gain some insight on the stable species and structures on RuS2 upon adsorption, we have studied the chemisorption of H2 and H2S molecules on a model of the (100) surface by means of ab initio periodic Hartree-Fock (HF) calculations. The focus of the study is on the nature of the bonds being made between the surface and the molecule, on the comparison between molecular and dissociated forms, and on the influence of the chemisorption on the bonds within the surface. The geometric and electronic structures of the bare (100) RuS2 surface have been described in detail in a previous paper [7], so only the major results will be recalled in Section 3 for comparison with the chemisorbed systems: The choice of the (100) face was mainly driven by the fact that it is a stable cleavage surface, as observed with STM, and that ,there is no ambiguity on the bare surface termination; in the case of the (100) surface no S-S bond is

broken at the interface and all surface S atoms belong to a n S 2 pair. Moreover, it is important to understand the chemical activity of this ( t 0 0 ) surface. The bulk properties of transition metal sulfdes have been the subject of several theoretical studies, owing to their specific electronic and magnetic properties. Several studies have attempted to correlate the bulk properties with the catalytic activity [8-11]. The chemisorpfions of H2 and other molecules have been studied on MoS2 clusters with the ASED-MO technique [ 12-15]. The homolytic adsorption on surface S atoms is found to be stable, but the most stable situation corresponds to the heterolytic chemisorption at the lower coordination edge atoms. The adsorption of H and the removal of S atoms from MoS2 clusters was also studied with extended-Hfickel calculations [1618]. Recently, hydrodesulfuration pathways have been characterized on small NixSy clusters by density functional theory (DFT) calculations [19]. Chemisorption of H~2was found to depend on the Ni/S ratio in the cluster. Ni3S2 substrate favors a molecular chemisorption without H - H bond cleavage, whereas the Ni3S cluster yields the dissociative chemisorption. On the small NiS clusters, d~ssociative adsorption of H2S occurs. The chemisorption o f sulfur-containing molecules has also been studied on MoS2 surfaces with periodic extendedHt~ckel calculations [20], and the activation of the S-C bond was discussed as a function of the binding mode. SCF calculations using INDO [21], CNDO [22,23], or Xe [24] have been performed for thiophene and other molecules on MoS2 clusters. In contrast, for RuS2, only a few calculationS have been devoted to the surface electronic structure and chemisorption properties with DFT [25] and Xc~ [26] techniques on clusters. I n order to model RuS2, different sizes Of cluster have been considered: RuS 6- [25], where all;I s atoms are isolated and do not belong t o $2 pairs, RuzS~and Ru4S[-1 [26 ], with significantls)' different results as a function of cluster size. However, no information on molecular binding energies at the surface is provided by these calculations. The method of calculation and the computational conditions will be introduced briefly in Section 2, and the electronic structure of the (100) face will be recalled in Section 3. The results on

F. Frechard, P. Sautet / Surface Science 389 (1997) 131-146

the chemisorption of H2 and H2S will be detailed respectively in Sections 4 and 5.

2. Method of calculation and computational conditions In the calculations, the surface is described by a periodic slab, parallel to the surface in the x and y directions and finite in the orthogonal z direction. The square unit cell for the (100) face, with six atomic layers, contains four R u and eight S atoms. Molecules are chemisorbed symmetrically on both sides of the slab. The CRYSTAL95 program [27] has been used to perform the periodic H F calculations. The details and implementation of this m e t h o d h a v e been p u b l i s h e d elsewhere [28]. CRYSTAL95 allows to solve self-consistently the H F equations for a periodic system. The H F energy is corrected for correlation by a density functional DF-type [29] calculation [30]. This type of correction has proved to be very efficient in improving bond energies. On a family of semiconducting solids the binding energy error compared w i t h experimental values is + 1 to - 3 % instead of - - 2 5 ' t o = 4 0 % with plain HF. This inclusion of the influence o f the electronic correlation tends, in our case, to increase the binding energy between atoms c o m p a r e d with the H F :solution. For the RuSz bulk the optimum geometry is, therefore, more compact, with reduced cell parameter a and crystallographic coordinate u compared with the previous H F results (5~55 A for a with 5.61 experimentally and 5,72A as H F optimum, and 0.393 for u with 0.388 experimentally and 0.395 as H F optimum), T h e resulting° bond lengths are 2.06 A f o r S=S b o n d : (2.17 A experimental and 2.08 ]k H F ) a n d 2.34A for the R u - S bond (2.35 experimental and 2 . 4 t A : H F ) , whereas the different angles ate within :1 or 2 ° f r o m experimental values. As one could expect, the D F correction does not change qualitatively the H F results but allows ~one to obtain an improved quantitative description o f the relative energies [30]. All the values for the s l a b will refer to the optimized geometry. The chemisorption or bond energy is calculated as the difference between the energy of the separated entities and the energy of the

133

adsorbed system, for 1 tool of molecules, even for the dissociated case. Thus, a bonding situation corresponds to a positive energy. Gaussian-type orbitals (GTO) are used for the basis set and core electrons have been modeled by well-assessed effective-core-potential (ECP) techniques [31 ]. The basis set used for describing valence electrons is similar to the one used in Ref. [7]. Split-valence functions have been used for all atoms. The sulfur atoms are described by a 31G* set. A 31G set also describes the d orbitals of ruthenium, and an 11G set is used for the s and p functions. For the H atom a 21G basis set has been used with, in some cases, as indicated, additional 2p polarization orbitals [32]. A good accuracy is used in the calculation for the cut-off o f the integrals [27]. In order to determine the chemisorption geometry, manual geometry optimization has been undertaken. Except in the cases explicitly mentioned in the text, the surface structure was not optimized upon chemisorption. The position of the center of mass and the internal coordinates of the adsorbates were..completely optimized in a sequential and stepwise approach. After a rough search, t h e step sizes around the energy minimum were 0.02 A for distances and 5 ° for angles, with a final parabolic interpolation. The analysis of the electronic structure is based o n projected density-of-states (DOS) and the bonds are characterized by crystal overlap orbital population (COOP) curves, whose calculations have been implemented in the CRYSTAL code. A Monkhorst-Pack mesh of 9 k points was used [28], which provides a good accuracy for the k space integration, since no significant energy difference appears between a set o f 9 k and 25 k points (less t h a n 0.5 keal mol - 1).

3. The (100) surface of RuS2: electronic structure The (100) surface of RuS2 has been studied in detail in Ref. [7], and only the major points necessary for comparison with the chemisorption systems are recalled here. This (100) surface, shown in Fig. 1, is just parallel to a face of the facecentered cube of bulkRuS2, yielding a square unit cell, The stacking of layers, perpendicular to the surface (Fig. lb), can be described as a repetition

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F. Freehard, P. Sautet / Surface Science 389 (1997) 131-146

(b) Fig. 1. A 3D representation of the bare (100) six-layerslab of RuS2: (a) top view; (b) side view. Dark gray balls represent the Ru atoms and light gray balls the S atoms. of S - R u - S units, where S atoms on different sides of the R u plane are bonded in inclined $2 pairs. The most stable surface termination, as suggested by STM, is ending with such an S - R u - S triple layer, with two S atoms in the topmost layer and two Ru atoms in the layer below for each surface unit cell. No $2 pair is broken at the surface where the first-layer S atoms are bonded to two Ru atoms in the second layer (compared with three S - R u bonds in the bulk) and to one S a t o m in the third layer. The Ru atoms at the surface are easily accessible, since they are interacting with five S2 pairs instead of six in the bulk. This surface is stable and does not show any

marked tendency to relax its geometry compared with bulk termination coordinates. The inclusion of D F corrections for correlation energy produces a larger surface energy of 2 . 0 J m -2 compared with 1.1 J m - a for the H F calculation. The surface electronic structure, which is close to the bulk one, is given in Fig. 2 with DOS projected on the surface S and Ru atoms. The Fermi level is indicated by the horizontal line. RuSz is a semiconductor with a band gap of ~ 1 eV. It is well known that H F calculations yield a poor description of vacant states, with a strongly overestimated gap. Hence, these vacant states will not be considered in the DOS. If we start with the S atom in Fig. 2a, the DOS projected on both external (surface) and internal (bulk-like) S are shown. The position of the bands is identical (no surface states) but the DOS amplitude is modified. The two peaks in the low part of the spectrum are originating from the atomic S 3s levels, which are split because of the S-S bond. Above these two peaks we find a broad band, which comes from the interaction of the S 3p levels with 4d and 5sp states on Ru, since this band also has a large weight on the Ru atoms (Fig. 2b). In this band, the DOS is larger at the bottom of the band for the bulk-like S atoms, whereas the DOS projected on the external S is enhanced in the top part, near the Fermi level. The surface S atom, owing to its unsaturation, is then less stable and a better electron donor than in the bulk case. The S-S bond in the $2 pairs can be analyzed by means o f the S-S COOP, curve which corresponds to DOS(E) weighted by the S-S overlap population for the state at energy E. A positive (or negative) value corresponds to a bonding (or antibonding) situation. This COOP curve is shown in Fig. 2c. As indicated previously, the low peaks correspond mainly to bonding and antibonding combinations of 3s orbitals in the Sz pair. The molecular orbitals (MOs) of $2 can be simply divided into a and rc orbitals, with respect to the S-S direction in the given pair. For the neutral molecule, the n bonding orbitals are occupied and two electrons are also positioned in the antibonding n* orbital. The broad 3p band shows contributions from S-S o- and n states. The bottom part (positive) corresponds to contributions of the o- lone pairs and the bonding n orbitals, whereas

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the negative upper part is associated to the antibonding re* S-S levels. The electronic states close to the Fermi level therefore correspond to an antibonding interaction between S atoms. Compared with the isolated S2 molecule, these n* levels are stabilized and receive an additional fraction of electronic charge by the interaction with the R u atom. This charge transfer toward $2 is larger at the surface (0.25 e - ) than in the bulk (0.16 e - ) but, however, remains moderate. The net charges for the various atoms at the surface are indicated in Scheme 1. As expected for its increased donor character, the charge on the external S atom is somewhat more negative: the R u atom is more positive and the internal S atom charges are similar to the bulk case. Notice that the small charge separation between metal and sulfur and their mixed levels indicates a rather covalent nature of the bonds at the surface and within the solid. For the following study of the adsorption of molecules on this surface, the specific coordinate axes depicted in Scheme 1 have been chosen in order to simplify the discussion. The Ru atom at the surface is located in a pseudo-octahedral situa-

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tion, with one ligand missing ow!ng to the interface. The position of the adsorbate on tN¢ surface will be compared with that of the S or R u atom that would be present if the bulk strudture was continuing (this "'broken" bond)is indicated by dashed lines and the "missing" a~Om is indicated in italics). In order to limit the size of the surface unit cell, high coverage situations have been considered for chemisorption. The adsorbate-adsorbate interactions are, however, only moderate. A test for the H2S molecule with a coverage divided by two gave an adsorption energy increase of 4 kcal mol - 1

136

F. Frechard, P. Sautet / Surface Science389 (1997) 131-146

4. Molecular and dissociative adsorption of H2 4.1. Molecular adsorption of H2 The molecular adsorption of H2 has been tested above the surface Ru and S atoms, but the only favorable interaction is found with the Ru atom. The minimum for the optimized geometry gives a binding energy of 31 kcal tool -1 and if p atomic orbitals (AOs) are added to the H basis set this value increases to 35kcalmo1-1. The basis set

superposition error (BSSE) has been checked with a molecular calculation and it appears to be negligible (less than 0.5 kcal mo1-1). The best adsorption geometry is depicted in Fig. 3a and b, which display the top and side views respectively of a portion of the six-layer slab with the adsorbed H 2. Scheme 2 gives details about this geometry: this is an t/z adsorption mode with the two H - R u bonds having the same length of 1.63 A. The H - H bond is stretched from 0.74 to 0.84A and the H atoms are symmetrically positioned with respect to the local z axis (broken R u - S bond, dotted line on Scheme 2). A small rotation occurs around this z axis (10 ° with respect to the xz plane). This rotation comes from the small repulsions between H2 and the S atoms coordinated to the Ru atom (average H - S distance is ~2.55 A). This adsorption geometry is quite similar to the ones observed for Ru complexes of H 2 [331. (+°-11)~. . ~ 4 A

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Fig. 3. A 3D representation of the (100) six-layer slab with molecular adsorption of Hz: (a) top view; (b) side view. Dark gray balls represent the Ru atoms, light gray balls the S atoms and white balls the H atoms,

Now that the optimal geometry is described we are going to focus on the electronic structure of the adsorbate-surface system, on the modifications induced on each partner a n d to compare with the separated molecule a n d surface: Charge transfer happens from the H2 molecule to the Ru and, to a smaller extent, from the surface S to the Ru (see Scheme 2). For the Ru atom these changes in the electronic charge are located mainly on the p= ( + 0.10 e - ) and the d=2 orbitals (+ 0.21 e - ) , which are the orbitals that have the best interaction with the adsorbate. This bond analysis can be completed with the help of the projected DOS on both H atoms and on the Ru atom (Fig. 4a and b). The principal feature is the narrow peak which appears at - 1 9 eV in the two DOS curves. It is shared by the two H (72%) with the Ru (21%) and it can be

F. Frechard, P. Sautet / Surface Science 389 (1997) 131-146

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contribution ( - 0 . 1 0 ) . Therefore, in the molecular chemisorption form, the H2 molecule is strongly activated, with an important decrease of the H - I t bond strength; this results in a very significant increase of the bond length (+0:1 * or +12%), which is comparable with that observed in some transition metal complexesof H2 [33]. The reactivity of such a perturbed H2 molecule is expected to be different from that of gas-phase H2. The total overlap population between Ru and H2 (0.34 compared with 0.24 for the R u - S bond in the bulk) is much stronger than the H - H one in the chemisorbed structure, and the interaction of the o-(H2) orbital is responsible for 75% of this adsorbate surface overlap population. On the Ru side, the decomposition Of the R u - H 2 bond points out the main participation of the d orbitals (60%, d=~ 30% and dx= 27%) compared with the p (25%) and s (15%) AOsL The two H atoms are almost equivalent, with only small differences that result from the weak repulsions with surface atoms as described before.

4.2. Dissociative chemisorption of Hz Several possible geometries for dissociative chemisorption of H2 on the surface have been consid-

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F. Frechard, P. Sautet / Surface Science 389 (1997) 131-146

ered, but only the heterolytic case with one H atom on the surface Ru and S atoms was found to be stable, The associated geometry is depicted in Fig. 5a and b and it simply corresponds to the replacement of the cleaved S - R u bonds by an S - H and an R u - H bond (see Scheme 3), the bond lengths being 1.34 A for S - H and 1.57 A for R u - H . The binding energy of this optimized geometry is 8 kcal mol -1; therefore, dissociative chemisorption is stable compared with the H2 molecule far from the surface, but it is much less favored than the previous molecular chemisorption case. The addi-

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Fig. 5. A 3D representation of the (100) six-layer slab with dissociated H/: (a) top view; (b) side view. Dark gray balls represent the Ru atoms, light gray balls the S atoms and white balls the H atoms.

tion of p AOs to the H basis set only increases the binding energy by 1 kcal mo1-1. The homolytic chemisorption cases with two H atoms linked to either the Ru or the surface S atoms are not favorable. However, the most unexpected outcomes were when we tried to link a single H atom to only the S or the Ru surface atom. After an extensive optimization of H in the two configurations we found a weak R u - H interaction just able to stabilize the H" species, but, above all, the surface S - H system was higher in energy than the separated H" and the (100) slab. The electronic population analysis reveals that the H atom is botmd to the S atom, but that at the same time the S-S bond is weakened strongly (the S-S overlap population is 0.18 without and 0.05 with the H atom). Therefore, bonds within the surface are destabilized strongly by chemisorption. A displacement of the SH group along the S-S bond axis was tried in order to obtain some surface relaxation, but with no success. Without a drastic reconstruction of the substrate, which is out of reach of the present approach where the forces are not calculated, it does not seem possible to obtain a stable adsorption for the H atom on the surface S atoms only. The interaction between S and H reorganizes the S - R u bonds, and the electron transfer from Ru levels towards the high-lying antibonding S2 o-* MO is increased sufficiently to nearly break the $2 pair. This important perturbation of the S-S bond at the interface by H atoms prevents a stable H chemisorption on the S atom alone. This effect underlines the difference between RuS2(100), where the S atoms at the surface belong to $2 pairs, and surfaces of other sulfide compounds, like MoS2 [12-18] and nickel sulfide [ 19], that show individual S atoms. The qualitative discussion on this point is usually based on the difference in the formal S oxidation number: - 1 in $2 compared with - 2 in S. However, the small charge in the S atom ( - 0 . 0 8 in RuS2) suggests a more covalent bonding mechanism [7]. The S atom in an S2 pair has a much more saturated nature than an isolated S atom, and the binding of the H atom requires a significant electronic rearrangement of the S2 fragment, at the expense of the S-S bond strength. Let us now return to the stable configuration of

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Scheme3. Schematics of the dissociated chemisorption geometry for H2 on the Ru (left) and S (right) surface atoms. Atomic charges are given between parentheses. dissociated H 2 with one R u - H and one S - H bond formed. In that case the perturbation of the S-S bond by H is balanced by the interaction of the H atom with the R u center, and, as a consequence, the S-S surface bond is only slightly modified (0.18 for the bare surface and 0.17 for the surface with H2 dissociated). This situation is certainly the simplest and most expected one, similar to the case of metal oxides; however, the binding energy here is small, and this should prevent dissociation on the (100) surface. The important charge transfers appear at two levels, as can be seen from Scheme 3. As expected, the first one concerns the S - H and R u - H groups (from H to the surface S and from Ru to H ) , but an additional transfer from the S - H to R u - H group is also present. If the H atom o f the S - H groups has a +0.26 charge, the electronic population of the S atom is also reduced by 0.19 e - , compared with the bare surface, resulting in an overall small positive charge for t h e S - S - H fragment. On the contrary, the hydride gains 0 . 3 2 e - , but the electronic population also increases for the R u atom (+0.15 e - ) . From their charges, it can be seen that the H on the Ru atom has a hydride character and that the H on S atom is a proton-like species. The projected DOS on the two H atoms, displayed in Fig. 6a, emphasize these features. For the hydride, the increased average electron-electron interaction pushes up the levels toward the Fermi energy and nearly completely above the H atomic level at - 1 3 . 6 eV. The two principal peaks share the - 1 1 to - 8 eV range with the main peak of the Ru projected DOS (Fig. 6b). On the contrary, the proton-like hydrogen levels are stabilized by the positive charge (Fig. 6a) mixing with the a,o-* ( - 2 5 , - 3 0 eV) and the rt levels ( - 1 8 eV) of the S, as can be seen in the S projected DOS (Fig. 6b).

139

The S H and S-S COOP curves allow us to attribute the three principal groups of bands to bonding states. The two lower peaks ( - 3 0 and - 2 5 eV) are centered on the $2 o levels, and the most important one ( - 1 8 to - 1 6 eV ) lies in the $2 7t zone. Besides some contribution of low-lying S 3s states, most of the S - H bond is coming from the $2 n orbitals, the cr lone pair having a repulsive influence. All this information allows a better understanding of the charge transfers. The proton-like H gives electrons to the S which has its Sz pair bonding levels filled, as well as a part of its antibonding 7z* ones. However, the interaction with the low lying H ( l s ) orbital destabilizes the S 3p orbitals so that a fraction of these states are pushed above the Fermi level, and hence depopulated. Notice the smaller S projected DOS for the filled states near the Fermi level in Fig. 6b, compared with the bare surface of Fig. 2; this illustrates the fact that part of the associated 3p states are pushed to vacant levels by chemisorption. As a consequence, the electronic charge does not stay on the high-lying S atom levels, but is donated into lower-lying available R u orbitals (p= and d~2). This is why the electronic population on Ru increases, compared with the bare surface, even if it is donating electrons to the hydride H atom. The electron transfer between the two H atoms in that case is, therefore, mediated by the interactions within the surface. An equivalent, but simpler, approach is to consider a full charge separation and to envisage an H ~ on the S and an H - on the Ru as a fictitious starting point. Then the S S pair gives electrons to the proton to yield a smaller positive charge ( + 0.26 e - ) and the H - is stabilized by donation to Ru, which explains the final charge distribution:

5. Molecular and dissociative adsorption of H2S 5.1. Molecular adsorption o f H z S

The HzS molecule binds only on the Ru site, the approach of HzS above the surface S being repulsive. The optimized geometry of the adsorption is displayed in Fig. 7a and b and in Scheme 4,

140

F. Frechard, P. Sautet / Surface Science 389 (1997) 131-146 0

0,2 0,4 0,6 0,8

1

1,2 1,4 1,6 1,8

1

2

2

• I.I,

-5.

I

E

d I

I

-15 E(eV)

4 I.

/1

5

6

7

8

I i l . l , l , ~ . l i i i l i l

]

i

-I0-

--

t f,-"

H(S) - - -H(S) integ .....

-15.

I

E(eV)

H(Ru)

i i i

-25

--

1

t -20

],l,

/

~;

~.~..j,

3

I,

E

-~.~_: :: - - " "":-'-/"" ..1........ -1 O- ~" ~ . , : ~ - - ? r ~ ::-"~::' '.44

I.

-S ~

I

-20.

I

......... H(Ru) integ

[~===~

:

Ru - - "Ru integ

I -25.

~..-~:-::::~!-,,-

I I

-......

s

.... s Irlteg

I

-30

~=-

-35

-30.

~::--.~............

-35

(a) Fig. 6. DOS of the (100) six-layerslab, with dissociated H2~ projected on the two types of H atom, bonded to S or to Ru (a), and on the Ru and surface S atoms (b). Integrated DOS or COOP curves are also given. which gives the related characteristic values. The two parameters which control the H2S internal geometry ( S - H bond length of 1.33 A and H - S - H angle o f 92 ° ) are not very sensitive to the adsorption, and their values remain very close to that of the gas-phase molecule. Compared with the missing bulk S atom, the R u - S ( H 2 ) bond makes an angle o f 75 ° with the surface plane instead o f 70 ° due to the repulsion between HzS and subsurface S atoms. The angle e in Scheme 4 corresponds to the angle formed by the median of the H - H segment in the H - S = H triangle and the R u - S ( H 2 ) bond. The optimum binding energy is 35 kcal mo1-1 with c~= 110 °, with a second relative optimum for 0d--250 ° (31 kcal mo1-1) which is the symmetric situation with the H a t o m s slightly pointing toward the surface (e--180 ° corresponds to the transition state with 15 kcal tool-1). The optimum geometry is analogous to configurations found for Ru complexes of thioethers [34]. The BSSE, which has been tested with the Ru basis orbitals in association with HzS in a molecular calculation, is negligible. Before the adsorption discussion, it is important to recall the electronic structure of HzS: the orbital representation of its molecular levels is ,depicted in

Fig. 8a, together with the projected DOS for the S of the adsorbed H2S. The local reference axes for both the adsorbed and the gas-phase HzS molecule are oriented as shown in Scheme4. Starting from the low-lying orbitals of HzS , we find two bonding orbitals: the lal level, composed mainly of the S 3s AO and the H s A O s , and the b 2 MO,0z UzS ) evenly distributed on the Spy and H s AOs. The third MO is the 2al level, a nearly non-bonding lone pair on S Pz and s AOs, whereas the fourth and last occupied level is the n o n bonding lone pair bl (S Px AO). Scheme 4 shows the charge distributions for the atoms of the surface and for: both adsorbed and gas-phase HzS. One c a n first n o t i c e the small donation of 0 . 1 4 e - from H z S t o the Ru atom. The charge transfer occurs mainly from the highlying b, lone pair, but the b2 and at MOs also participate. For the Ru AOs the increase of electronic charge is mainly into the p~ (+0.04 e - ) and d~2 ( + 0.14 e - ) orbitals. These AOs are respon, sible for the main part of the overlap population of 0.21 for the R u - S ( H 2 ) bond. From Fig: 8a, the lowest group of peaks around --27 eV can be associated with the lal MO and the narrow peak at - 1 6 . 5 eV corresponds to the

F. Frechard, P. Sautet / Surface Science 389 (1997) 131-146

(a)

(b) Fig. 7. A 3D representation of the (100) six-layer slab with molecular adsorption o f H2S: (a) top view; (b) side view. Dark gray balls represent the Ru atoms, light gray balls the S atoms. white balls the H atoms and medium gray balls the S atoms of the H2S molecules.

(-021

(+0.15)H (+0"I8)H~. s (-0.19)

(+0.I5) M

~et=110°( 1.2.28,~

920(--~:I~,,f~

(+0.13/ ~

[

Y

Z

(-0.06)

S Scheme 4. Schematics of the molecular adsorption geometry of H~S on the Ru atom (left). Atomic charges are given between parentheses. The local~coordinate axes for the HzS molecule are indicated (right). ,

141

b2 MO, its weak interaction with Ru explaining the absence of dispersion. The bottom, - 1 5 to - 1 3 . 5 eV, of the wide group of peaks between - 1 5 and - 6 eV is principally the 2at level, and the rest of this group is related to the bl level. This chemisorption of H2S restores the bulk coordination only on the Ru a t o m , and the unsaturation of the surface S atom is kept, with a similar charge difference between surface and internal S atoms as for the bare surface. As can be seen from the COOP curve between Ru and S(Hz) depicted in Fig. 8b, the R u - S interaction is bonding up to - 11 eV with an antibonding region above, where the 2al and bl lone-pairs are involved. In Fig. 8b the COOP curve between Ru and S(H2) is decomposed in the 1al + 2a~ (s + Pz) and the b 1 + bz (Px + Pr) contributions. The principal part of the (H2)S-Ru bond overlap population of 0.21 can be attributed to the al levels with 0.11 and, mainly, to the s AO with 0.09, whereas the ba lone pair and the b2 orbital have a smaller contribution. This contrasts with the importance of the bl lone-pair for the electron donation: The interactions between bx and Ru are weaker than the a t ones for overlap reasons; therefore, the splitting of this bl MO is less important. More antibonding levels stay below the Fermi level and are occupied, thereby reducing the bonding character o f , t h e b~-Ru bond; this effect occurs to a smaller extent for the 2al orbital because of the better interaction. Hence, the H2S-Ru bond is not only a donor acceptor interaction between the b~ a n d 2al lone pairs of H2S and empty leVels on Ru, but there is also a significant contribution f r o m the low-lying la~ orbital. However, the tilt of the molecule on the surface is explained by an optimal interaction of the lone pairs with the Ru atom.

5.2. Dissociative chemisorption of HzS Several cases for dissociative chemisorption of H2S have been envisaged (homolytic on Ru, SH group bound to the surface S, and H to the Ru) but only the heterolytic situation with the sulfhydril on Ru and H on the surface S atom is stable compared with gas-phase H2S. The optimized dissociated geometry, shown in Figs. 9a and b and

F. Frechard, P. Sautet / Surface Science 389 (1997) 131-146

142

I 2 3 .,.~.~.~.,.,.,.,.,,,.

4

5

6 7 f.~.,.,.~

i

-5" Ef

8

-0.24

-016

-0,08

0

008

-5, gf

I

J

016

024

;

!

-10-

2a -15

..__ ,"

S (aQ-Ru

~ _ ~

-15. ........ S (a 1)-Ru

b2

E(eV)'

E(eV)

integ

------ S (bl+bz}-Ru

-20.

-20 -

- - "S (bl+b~)'Ru t • integ -25

~____Z.~"

la~

-25.

-30

-30,

-35

-35

(a)

i

..............

' .......

(b)

Fig. 8. DOS of the (100) six-layer slab with molecular chemisorption of HzS , projected on the S atom of the HaS adsorbate (a), and COOP curve between the Ru atom and the S atom of HaS decomposed in the lal +2al and ba + b : symmetry contributions (b).

detailed in Scheme 5, has a binding energy of 18 kcal mol - 1 and is, therefore, a metastable situation significantly less stable than the molecular adsorption. For clarity, the sulfur atom coming from the H2S decomposition will be denoted S', with the surface S atom denoted S. The H atom bound to the surface S atom lies along the cleaved S - ( R u ) bond with an S - H length of 1.34 A. The sulfhydril group binds to the Ru atom still along the R u - ( S ) cleaved b o n d , the H - S ' - R u angle (102 °) being larger than the H - S - H one (92°). The R u - S ' H bond is longer than that of Ru-SH2 (2.37 A compared with 2.28 A), and has a different nature, Some small displacements compared with the bulk case are induced by the repulsions between the S' atom of the sulfhydril group and the subsurface S. Another influence comes from the hydrogen-bond-type interaction which occurs between the H on the surface S and the S' of the sulfhydril with a distance of 2.41 A and an overlap population of 0.05. If we consider the global atomic charges, we note that, similar to the H2 dissociation case, an electronic transfer takes place from the surface S and H atoms to the Ru-S'I-I group. The two types o f H atom on the surface have a positive charge,

but those charges are not equivalent, with a lower value for the H atom of the S'H group (+0.13 compared with + 0 . 3 2 ) . The projected DOS (Fig. 10) shows that, in contrast with the nondissociated case, the Fermi level is lower than that of the bare slab by about 1 eV, the lower S 3s levels being shifted down by around 4 eV. All the electronic levels are pushed down, a n d if the Fermi level is shifted less it is only because some highlying filled bands are added by the SH adsorbate. For the Ru atom, besides the 4 eV down-shift already mentioned, an increase o f the DOS near the Fermi level can be noticed. This change for the Ru in the interval - 1 3 to - g e V can be attributed to the R u - S ' H bond, as shown by the COOP curve of Fig. 10c with a bonding interval up to - 10 eV and an antibonding peak thereafter. This narrow peak just below the Fermi level is centered on the S' atom and is related to a surface state, corresponding to an antibonding ~-like interaction between the S' py AO (perpendicular to the H - S - R u plane) and the Ru dyz AO. The total overlap population between Ru and S'(H) is 0.28. This is more important than the value of 0.21 for the molecular adsorption of HzS, and it seems in contradiction with the facts that the R u - S ' H bond

F. Frechard, P. Sautet / Surface Science 389 (1997) 131 146

Fig. 9. A 3D representation o f the (100) six-layer slab with dissociated HzS: (a) top view; (b) side view. Dark gray balls represent the Ru atoms, light gray balls the S atoms, white balls the H atoms and medium gray balls the S' atoms of the STI group. (+0.13) H.,~'1'34 h f ~ s ' (-0.48)

,,,,,o,'°'° (+0.0

~

)2,"

7 .................

$

(+0.32)

H

(4)09)

-- ~ "

143

is longer (2.37 ,~) than the R u - S H 2 one (2.28 A). This longer R u - S ' distance could be explained by the electrostatic effect (repulsion) induced by the two positive H species at the surface. F r o m the Ru, S' and H DOS (Figs. 10a and 10b) and the C O O P curves for the R u - S ' ( H ) and ( R u ) S ' H bond (Fig. 10c) an analogy appears between the R u - S ' and the H S ' bonds for energies up to - 1 3 eV. The situation is, however, different for the two groups of bands above - 13 eV, where a weak n-type interaction appears between the dyz A O of R u and the Pr AO of S, a part of the antibonding contribution being filled. For the H a t o m bonded to the surface S atom the situation is similar to that of dissociated H2, with a marked proton character and low-lying levels. Owing to the markedly non-linear S - S - H configuration, the zt orbitals of S 2 have a great importance for the S - H bond with 88% of the overlap population (0.25). It can be noticed that, in addition to the charge differences between the types of H, their projected DOS are characteristic of the group of atoms to which they are bonded ($2 pair or S' atom). The H S ' group binds more strongly to the metal and stabilizes more efficiently the R u levels than the H a t o m does. Similar to the H2 dissociation, the donation-backbonding scheme between the R u a t o m and the $2 group is modified. There is less backbonding f r o m the metal to the $2 pair, as the levels, from both atoms, involved in that backbonding interaction are n o w taking part in the S - H or R u - S H bonds, but the donation from the $2 pair is not strongly modified. The charge transfer from S - H to R u - S ' H is more important t h a n f o r the H2 dissociation as the Ru acceptor levels are more stabilized.

6. Conclusion tRu

Fs /x : °

Scheme 5. Schematics of the dissociated adsorption geometry of HzS, with an H atom on the surface S atom (right) and an S'H group on the Ru atom (left). Atomic charges are given between parentheses.

On the chosen stoichiometric RuS2(100) surface, molecular chemisorption is favored by the calculation, even if the dissociated situations correspond to stable cases. For H2, this m o l e c u l a r adsorption structure, proposed for the first time on an RuS2 extended surface, is similar to coordination transition metal

F. Frechard, P. Sautet / Surface Science 389 (1997) 131-146

144 1

0

2

21

, I . l . I . I . I ,

4

5

I . [ , I . r . l , I

6 iI.

7 I.

8

I.

0

I,I

0,2 0,4 0,6 0,8

I.I.

[ . I . l . l t l

1

i 1.1.1.1.

1,2 1.4 1,6 1,8

2

1 . 1 . 1 . 1 . 1 . 1 . 1 . I , I .

I

-0,4

-0,2

0

0.2

0,4

-5

-5-

1

' .i

,, E

-10, ~

.

"':~";~ . . . .

-

. ........... . _ ~

~-~

,-.~--'--:-~:-

--'-~'~

-10

~

/

RU

]

. . . . . Ru integ

~

.... s' •

"

.~-~v.'::2:,'-'2222.~

,'

,... ,, -

/

i

-

~

'-15,

E(eV)I ~ , " -20- "~,"

.....

-10

5

-]5-

.--~.

.

H(S') E(eV

....

- 20 J ~ . : L _

H(S') integ

....

E(eV) -20

I i

---- H

......... S' integ

H-S'(Ru) integ . . . . . . Ru-S'(H) ....... Ru-S'(H) integ

i

-25

-25 -

-30

-30

%

i~

-35

-35 ®

......... H integ

-25

-30"-

-35

tc)

Fig. 10. DOS of the (100) six-layer slab with dissociated H2S, projected on the Ru atom and S' atom of the HS' adsorbate (a), on the two types of H. atom (b); COOP curves between the S' atom of S'H and the Ru and H atoms (c).

complexes of H 2 with an q2 coordination on the Ru atom. The H - H bond is still present but strongly weakened, with a 0.1 A elongation comp a r e d with the gas-phase structure. The binding energy is 35 kcal mo1-1. The o" H - H bond introduces a split-off state in the surface DOS, and, therefore, appears as a highly localized level mixing only w i t h the R u atom. The electron donation from this o- H - H orbital' towards the R u a t o m is 0.48 e - , whereas 0.28 e - are transferred back from the metal to H 2 by interaction with the v a c a n t 0-* orbital. The o-* contributions to the DOS below the Fermi level are delocalized over the RuS2 valence band. However, m o s t o f the R u - H 2 bonding character is given by the o H - H split-off state, A m o n g all the dissociated structures for H 2 , only the heterolytic case, with one H on the R u atoms and one H on the surface S atoms, is found to be stable, but significantly less favored than the molecular H2 chemisorption. Surprisingly, a homolytic dissociation of H2 on t h e s u r f a c e S atoms only is found to be unstable; this is explained by a strong perturbation of the S-S surface b o n d by the H adsorbate. This situation contrasts significantly w i t h other sulfide materials where the S atoms are isolated, not present in pairs, and where

simple chemisorption of H on the surface S atoms is possible. The system becomes stable once a second H atom is positioned on Ru. In that stable case the R u - H and S - H bonds are oriented in a bulk-continuation-like structure, tending to saturate, the R u - S a n d S - R u bonds broken at the interface. The hydrogen atom on the Ru has a hydride character, whereas the H on the S atom is proton-like with a significant positive charge. The projected DOS allows one to differentiate clearly between the two H atoms. The electron transfer from the protonic towards the hydride H a t o m is mediated by the interactions within the surface. For the H2S molecule the binding energy of the molecular chemisorption is 35 kcal mol -1, with an interaction of the S a t o m with the surface Ru atom. The molecule is more-or-less lying flat on the surface and the H2S adsorbate restores the Ru pseudo-octahedral bulk environment. There is a small electron donation from H z S t o R u , mostly coming from the bl lone-pair orbital of H 2 S. However, an important part of the R u - S bond comes from the al orbitals of HzS, because some antibonding contributions involving the bl orbital are filled in the top part of the occupied DOS. The

F. Frechard, P. Sautet / Surface Science 389 (1997) 131-146

S - H b o n d s o f the a d s o r b a t e s are w e a k e n e d slightly b y the c h e m i s o r p t i o n , b u t this p e r t u r b a t i o n o f the b o n d s is s m a l l e r t h a n in t h e case o f H2, as the o r b i t a l s i n v o l v e d in the c h e m i s o r p t i o n are m a i n l y lone pairs. T h e dissociative c h e m i s o r p t i o n o f H2S is also a m e t a s t a b l e s i t u a t i o n w i t h a b i n d i n g energy o f 18 k c a l m o l - 1 . This s i t u a t i o n is r a t h e r similar to the H2 d i s s o c i a t i o n structure with S H i n s t e a d o f H b i n d i n g to the R u a t o m . B o t h H a t o m s are p o s i t i v e l y c h a r g e d a n d the w o r k f u n c t i o n increase is stronger. T h e R u - S - H u n i t has similarities with the H2S molecule, w i t h s o m e a d d i t i o n a l 7r-type i n t e r a c t i o n s b e t w e e n S a n d R u . T h e stabilization, a n d t h e r e f o r e the filling o f the d o r b i t a l s o f R u , is s o m e w h a t s t r o n g e r in t h a t case c o m p a r e d w i t h the dissociated hydrogen. I t is clear t h a t the b a l a n c e d e s c r i b e d b e t w e e n m o l e c u l a r a n d dissociative states, here in f a v o r o f the m o l e c u l a r structure, w o u l d d e p e n d o n the R u / S r a t i o at the surface. R e d u c e d surfaces w i t h S vacancies c o u l d f a v o r the d i s s o c i a t e d structures because they would have an enhanced unsaturated c h a r a c t e r at t h e metal. S u c h a t r e n d was i n d e e d f o u n d o n nickel sulfide clusters [19]: the Ni3Sa cluster gives a m o l e c u l a r c h e m i s o r p t i o n for H2 a n d H2S, similar to t h o s e f o u n d here, w h e r e a s a r e d u c e d Ni3S cluster f a v o r s d i s s o c i a t e d structures. T h e (100) surface o f R u S 2 is a stable surface, w h e r e n o S - S b o n d is b r o k e n , w h i c h m a i n l y shows a Lewis acidic c h a r a c t e r a n d w h i c h associatively c h e m i s o r b s H2 a n d H2S w i t h o u t dissociation. Its e x p e c t e d c h e m i c a l activity w o u l d , therefore, be low, this surface b e i n g s o m e w h a t c o m p a r a b l e with the b a s a l p l a n e o f MoS2. T h e specific activity o f RuS2 s h o u l d t h e n be a s s o c i a t e d w i t h surface defects o r w i t h o t h e r c r y s t a l p l a n e s o n the particles like t h e (111 ) surface p l a n e , w h e r e the S - S b o n d s can be broken.

Acknowledgements T h e a u t h o r s wish to t h a n k R. Dovesi, S a u n d e r s a n d C. R o e t t i f o r the access C R Y S T A L 9 5 b e f o r e its d i s t r i b u t i o n .

V. to

145

References [1] M. Lacroix, N. Boutarfa, C. Guillard, M. Vrinat, M. Breysse, J. Catal. 120 (1989) 473. [2] M. Lacroix, H. Marrakchi, C. Calais, M. Breysse, C. Forquy, in: M. Guisnet et al. (Eds.), Heterogenous Catalysis and Fine Chemicals II, Elsevier, Amsterdam, 1991, p. 277. [3] H. CoM1, M. Bronold, S. Fiechter, HI Tributsch, Surf. Sci. 303 (1994) L361. [4] M. Lacroix, C. Mirodatos, M. Breysse, T. Decamp, S. Yuan, Proc. 10th Int. Congress on Catalysis, Budapest, July 19-24, 1993, Elsevier, Amsterdam, p. 597. [5] M. Lacroix, S: Yuan, M. Breysse, C. Dorrmieux-Morin, J. Fraissard, J. Catal. 138 (1992) 409. [6] H. Jobic, G. Clugnet, M. Lacroix, S. Yuan, C. Mirodatos, M. Breysse, J. Am. Chem. Soc. 115 (1993) 3654. [7] F. Frechard, P. Sautet, Surf. Sci. 336 (1995) 149. [8] S. Harris, R.R. Chianelli, J. Catal. 98 (1986) 17. [9] S. Harris, R.R. Chianelli, in: J.B. Moffat (Ed.), Theoretical Aspects of Heterogeneous Catalysis, Van Nostrand Reinhold, New York, 1990, p. 206. [10] J.K. Norskov, B.S. Clausen, H. Topsoe, Catal. Lett. 13 (1992) 1. [11] T.S. Smit, K.H. Johnson, Catal. Lett. 28 (1994) 361. [12] A.B. Anderson, Z.Y. AI-Saigh, W.K. Hall, J. Phys. Chem. 92 (1988) 803. [13] A.B. Anderson, J.J. Malhoney, J. Yu, J. Catal. 112 (1988) 392. [14] J. Yu, A.B. Anderson, J. Mol. Catal. 62 (1990) 223. [15] A.B. Anderson, J. Yu, J. Catal. 119 (1989) 135. [16] J. Joffre, P. Geneste, D.A. Lerner, J. Catal. 97 (1986) 543. [17] R. Pis Diez, A.H. Jubert, J. Mol. Catal. 73 (1992) 65. [18] R. Pis Diez, A.H. Jubert, J. Mol. Catal. 83 (1993) 219. [19] M. Neurock, R.A. van Santen, J. Am. Chem. Soc. 116 (1994) 4427. [20] M.G. Zonnevylle, R. Hoffmann, S. Harris, Surf. Sci. 199 (1988) 320. [21] J. Rodriguez, Surf. Sci. 278 (1992) 326. [22] F. Ruette, N. Valencia, R.J. Sanchez-Delgado, J. Am. Chem. Soc. 111 (1989) 40. [23] A.E. Gainza, E.N. Rodriguez-Arias, F. Ruette, J. Mol. Catal. 85 (1993) 345. [24] C. Rong, X. Qin, J. Mol. Catal. 64 (1991) 321. [25] T.S. Smit, K.H. Johnson, Chem. Phys. Lett. 212 (1993) 525. [26] C. Rong, X. Qin, H. Jinglong, J. Mol. Catal. 75 (1992) 253. [27] R. Dovesi, V.R. Saunders, C. Roetti, CRYSTAL92 and CRYSTAL95, User manual, University of Turin (Italy), SERC Daresbury Laboratory (UK). R. Dovesi, C. Pisani, C. Roetti, V.R. Satmders, CRYSTAL88 in Quantum Chemistry Exchange, Indiana University, Department of Chemistry, Bloomington, IN, 1988. [28] C. Pisani, R. Dovesi, C. Roetti, Hartree-Fock Ab Initio Treatment of Crystalline Systems, Lecture Notes in Chemistry, vol. 48 Springer, Heidelberg, 1988.

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