H2S Adsorption on γ-Al2O3 Surfaces: A Density ... - ACS Publications

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H2S Adsorption on γ-Al2O3 Surfaces: A Density Functional Theory Study John M. H. Lo,*,†,‡ Tom Ziegler,‡ and Peter D. Clark†,‡ †

Alberta Sulphur Research Ltd., University of Calgary, University Research Center, Unit 6-3535 Research Road Northwest, Calgary, Alberta, Canada T2L 2K8 ‡ Department of Chemistry, University of Calgary, 2500 University Drive Northwest, Calgary, Alberta, Canada T2N 1N4 ABSTRACT: The structures and energetics of various adsorption modes of H2S on the surfaces of γ-Al2O3 have been determined using periodic density functional theory. The calculations located in total 10 stable adsorption configurations of H2S on the dehydrated (100)E and (110)C as well as the hydrated (110)C surfaces of γ-Al2O3, and their binding energies fall in the range of 12-35 kcal/mol, which is in good agreement with experiments. It was noticed that H2S prefers chemisorption on clean surfaces free of water, while physisorption becomes important in the presence of surface hydroxyl groups. The computed vibrational frequencies for the adsorbed H2S are consistent with the reported values in the literature although slightly overestimated. The variations of the OH stretching modes observed in this study can be properly accounted for in terms of the geometric and electronic structures of the OH groups, the presence of hydrogen bonds, and the surface reconstruction due to H2S adsorption.

1. INTRODUCTION As one of the major sulfur-containing contaminants, H2S is found in natural gas and petroleum products generated in oil refinery. Due to the environmental concerns about SO2 emission resulting from fuel combustion, hydrodesulfurization is usually performed to remove sulfur from fuel oils. Two major strategies are commonly employed to accomplish this task. The first approach is the noncatalytic thermal cracking of H2S under Claus furnace conditions,1 taking advantage of the high temperatures as a result of the oxygen enrichment techniques. This method produces H2, which can be utilized in the refinery but, unfortunately, suffers some serious drawbacks such as the endothermicity of the reaction 2H2 S f 2H2 þ S2 ð1Þ and the rapid recombination of H2 and S2 during the quenching process.2,3 An alternative approach is the catalytic conversion of H2S through either the modified Claus reaction or selective oxidation of H2S. In these processes, H2S is oxidized to elemental sulfur and water by reacting with SO2 (the modified Claus process) generated in the combustion of H2S in the reaction furnace and O2 (selective oxidation), respectively, in the presence of catalysts such as activated carbon, zeolites, or metal oxides.4 Overoxidation is often observed in the latter one H2 S þ 3 = 2 O2 f H2 O þ SO2

ð2Þ

producing SO2 instead and lowering the selectivity toward sulfur. It has been reported that γ-Al2O3, which is an active catalyst for the modified Claus reaction, affords a high selectivity toward H2 and sulfur in the partial oxidation of H 2S at 400
C in a r 2011 American Chemical Society

short-contact-time reactor5 2H2 S þ 1 = 2 O2 f H2 þ H2 O þ S2

ð3Þ

and minimizes the production of SO2. Owing to its relevance to the H2 and sulfur recovery in oil refineries, the adsorption and reactions of H2S on γ-Al2O3 catalyst have attracted much attention. While spectroscopic6-10 and kinetic1,5,11,12 measurements have been extensively performed to acquire insight into the details regarding the reaction mechanisms, kinetics, and influences of the acid-base properties of γ-Al2O3 on H2S adsorption, only a few theoretical studies have been devoted to this issue.13-15 It is therefore the aim of this work to better understand, at the molecular level, the mechanisms of adsorption of H2S on various active sites of γ-Al2O3 and the factors that affect this process.

2. COMPUTATIONAL DETAILS Total energy minimizations were performed using periodic density functional theory within the generalized gradient approximation of Perdew, Burke, and Ernzerhof16 as implemented in the Vienna Ab-initio Simulation Package (VASP).17-20 The oneelectron orbitals were expanded on plane-wave basis sets with an energy cutoff of 400 eV, while the ion-electron interaction was described by the projector-augmented wave (PAW) method.21 Electronic relaxations were done with a convergence criterion of 0.0001 eV. To facilitate the convergence of the total energy, a Gaussian smearing function of 0.1 eV was employed.22 The full Received: July 2, 2010 Revised: December 8, 2010 Published: January 11, 2011 1899

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optimization of atomic positions was carried out with the conjugate-gradient algorithm23 until all force components were smaller than 0.03 eV/Å. Due to the large size of the unit cells, the sampling of the irreducible Brillouin zone was done merely at the Γ-point. Spin polarization was used in all calculations. In all calculations, molecules were allowed to adsorb on only one side of the slab. Therefore, between two repeating slabs a vacuum layer of ∼11 Å was inserted to eliminate the dipole-dipole interaction caused by the presence of adsorbates. In addition, a dipole correction along the surface normal was also included in all geometry optimization calculations. The adsorption energy of a species, Ead, was computed using the following equation: Ead ¼ Esurf þ EH2 S - Esurf þ H2 S ð4Þ where Esurf and EH2S are the energies associated with a clean surface and a H2S molecule in the gas phase, respectively, and EsurfþH2S is the energy for an adsorbed H2S molecule. In this convention, a positive value of Ead represents an exothermic adsorption of H2S. Normal-mode analysis was utilized to confirm the identities of various species on the potential energy hypersurface. The vibrational frequencies were obtained from numerical Hessian calculations on the basis of the double-finite difference scheme with a step size of 0.015 Å. The geometries of the transition states and the associated energy profiles were computed using the climbing-image nudgedelastic band (ciNEB) method;24-26 seven images were created by interpolation between the reactant and product species. The rms-DIIS algorithm27 was used to relax the ion positions in all ciNEB calculations. Two surfaces of γ-Al2O3 were considered in this work, namely, the (100) and (110) planes. These planes have been previously shown, using the Gibbs-Curie-Wulff law,28 to be most abundant on γ-Al2O3 which is produced from the calcination of boehmite.29 The lattice constants of γ-Al2O3 were determined utilizing the approach as described by Vijay et al.;30 the computed value of 7.9725 Å employing an ultrafine 12  12  12 k-point sampling grid agrees fairly well with the experimental value of 7.90 Å.31 On the basis of the optimized lattice structure of γ-Al2O3, two surface models were constructed which contain 12 layers for the (100) surface and 6 layers for the (110) surface. Due to the spine-based configuration of γ-Al2O3, two types of facets are possible for each of the (100) and (110) surfaces; the present study only explored the most thermodynamically stable surfaces, i.e., (100)E and (110)C, as justified by surface energy calculations.32 The (100)E surface contains only the 5-foldcoordinated Al atoms, while the (110)C surface possesses both the 3-fold- and 4-fold-coordinated Al atoms. It is well-known that the acid-base properties of γ-Al2O3 are attributed significantly to the presence of surface hydroxyl groups which serve as both a Brønsted acid and a Lewis base.33,34 Therefore, it is anticipated that the reaction between H2S and γ-Al2O3 depends heavily upon the degree of hydroxylation of the γ-Al2O3 surfaces. The DFT-based Gibbs free energy calculations of Digne et al.35 revealed that, at Claus reaction conditions (∼600 K), the (100)E surface is completely dehydrated while the (110)C surface is covered by water at a concentration of about 8.9 OH groups/nm 2. Accordingly, apart from the clean (100)E and (110)C model surfaces, an additional model of hydroxylated (110)C surface as described by Digne et al.36 was also investigated in this work. The three types of surfaces discussed in this work are illustrated in Figure 1.

Figure 1. The three surface models of γ-Al2O3 considered in this work (blue, Al; red, O; pink, H).

3. RESULTS AND DISCUSSION 3.1. H2S Adsorption on a Dehydrated γ-Al2O3 (100)E Surface. According to the early calorimetric studies of Glass

and Ross, H2S can adsorb to γ-Al2O3 through two different channels; it can adsorb either irreversibly (dissociatively) on a Lewis acid site or reversibly (nondissociatively) on a Lewis basic site through a hydrogen bond.37 These suggestions have been supported by infrared studies due to Deo et al.,6 Datta et al.,8 Okamoto et al.,38 and Desyatov et al.9,39 In addition to these two adsorption modes, Datta and Cavell in their work also proposed a nondissociative adsorption of H2S on an isolated Al3þ or Al cluster, accounting for the presence of both the SH stretching and HSH bending modes in the IR spectrum for adsorbed H2S on the γ-Al2O3 activated at 700
C.8 Due to the fact that the (100)E surface is totally dehydrated under Claus reaction conditions,35,36 the physisorption of H2S on hydroxyl groups is not possible; only the adsorption modes of H2S involving its coordination to surface Al sites are feasible. The molecular adsorption of H2S could take place at either a pentacoordinated Al site or a tetracoordinated Al site, and the structures are shown in Figure 2. In the first configuration (EM1), a H2S molecule adsorbs to an exposed pentracoordinated Al site with a calculated Al-S distance of 2.626 Å, inferring 1900

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Figure 2. Optimized geometries of EM1-EM4 on the dehydrated (100)E γ-Al2O3 surface (blue, Al; red, O; yellow, S; pink, H).

physisorption of H2S on the surface. The molecular plane containing H2S is almost parallel to the (100)E surface, and the structural parameters (r(H-S) = 1.352 Å and — HSH = 92.8
) are close to those computed for a gas-phase H2S molecule (r(H-S) = 1.349 Å and — HSH = 91.7
). This stationary state has also been observed in other studies, and the calculated adsorption energy of 12.26 kcal/mol is in excellent agreement with the theoretical values reported previously by Maresca et al.,13 Ionescu et al.,14 and Arrouvel et al.15 and the experimental quantity (12.91 kcal/mol) deduced from TPD measurements.38 The other proposed molecular adsorption mode involves the adsorption of H2S over the grooves in the [110] direction made of the subsurface tetracoordinated Al atoms. The Al sites are coordinatively saturated; therefore, it is expected that only a weak interaction is present between these sites and H2S. The computed adsorption energy of H2S at this site is merely 0.1 kcal/ mol, which is far below the accuracy limit of the current DFT implementation without correction to the dispersion interactions. Accordingly, the existence of this adsorption mode is highly questionable. Normal-mode analysis also revealed that this state is not a minimum of the potential energy surface. There are two possible reaction routes with regard to the dissociative chemisorption of H2S. While mercaptan species preferentially adsorb to a pentacoordinated Al site, the dissociated H can be transferred to a neighboring O atom which is bonded to either a subsurface hexacoordinated Al atom or a tetracoordinated Al atom on the [110] groove. The present study found that only the first configuration is favorable. This species (EM2) possesses a rather short Al-S bond (2.300 Å) and induces a slight upward relaxation of the Al atom directly bonded to SH. The newly formed OH bond is about 0.992 Å, and the Al-O bond to the subsurface Al atom is stretched remarkably by 0.22 Å. The associated binding energy is 20.39 kcal/mol, which is much smaller than the experimental dissociative adsorption heat of H2S (∼35 kcal/mol) but rather close to that for H2S adsorbed on a tricoordinated Al site on the (110)C surface (∼26 kcal/mol).13

It is well-known that γ-Al2O3 adopts a defective spinel structure where holes are present depending on the exact stoichiometry.40 To examine the influences of structural defects on the adsorption of H2S, a special configuration was considered where SH is bonded to a surface pentracoordinated Al site while the dissociated H is transferred to the O atom that lies right above an octahedral Al defect. Calculations showed that this configuration (EM3) is thermodynamically stable with an adsorption energy of 25.15 kcal/mol. The OH group is almost parallel to the (100)E surface and points toward a neighboring tricoordinated O atom, forming a weak hydrogen bond with a distance of ∼2.16 Å. It is worth noting that the Al-S distance is long, being 2.518 Å, which may suggest a dative coordination of the resulting mercaptan (or mercaptide) to the Al site. Owing to the low probability of possessing a subsurface octahedral Al defect,32 a negligible population of adsorbed H2S for the EM3 configuration on the (100)E surface is expected. Unlike the predictions based on Lewis acid-base theory, the present study identified a stationary state which has the SH group bonded to an O atom located above an octahedral Al defect. This configuration is the most stable among all adsorption modes on the (100)E surface, having an adsorption enthalpy of 34.13 kcal/mol, which is similar to the experimental value (34.89 kcal/mol) corresonding to the dissociative adsorption of H2S.38 The S-O bond is rather long (1.703 Å), but the SH group orients toward a nearby tricoordinated O atom, forming a noticeable hydrogen bond (1.766 Å) that stabilizes the configuration. It is speculated by comparing with EM2 that the presence of Al defects favors the formation of an O-SH bond. A rationale for this observation may come from the density of states calculations, which showed that the O atom above the Al defect is more electron deficient than other surface O atoms, while the upper valence band of the O atoms surrounding the vacancy is more populated. The charge transfer to the bulk near the vacancy results in a slightly enhanced acidity of the O atom and its stronger interaction with SH. 3.2. H2S Adsorption on a Hydrated γ-Al2O3 (110)C Surface. According to the thermodynamic calculations of Digne et al.,35,36 1901

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Figure 3. Optimized geometries of HM1-HM2 on the hydrated (100)C γ-Al2O3 surface (blue, Al; red, O; yellow, S; pink, H).

the (110)C surface of γ-Al2O3 is covered with water up to a concentration of 8.9 OH groups/nm2 at about 300
C. Therefore, in addition to the direct adsorption on the Lewis acidic Al sites, there exists another reversible adsorption mode of H2S associated with the surface hydroxyl groups.4 With regard to the dissociative chemisorption of H2S, there are two possible Al sites where this process can take place. H2S can decompose on either the tricoordinated Al site or the tetracoordinated Al site; the former one has been found to be more acidic on the basis of density of states calculations36 and is thus more reactive toward H2S adsorption. However, at about 300
C all the tricoordinated Al sites are occupied by OH groups; consequently, H2S can only adsorb dissociatively on the tetracoordinated Al sites. A stable configuration, HM1, was found where SH is μ2-bonded to two adjacent tetracoordinated Al sites. As illustrated in Figure 3, the two Al-S bonds are not symmetrical (2.440 and 2.478 Å) because of the surrounding hydroxyl groups. The estimated adsorption energy for this configuration is 21.30 kcal/mol and is almost 40% lower than the experimental dissociative adsorption energy. This remarkable discrepancy can be attributed to the fact that the experimental quantity was possibly measured for H2S dissociatively adsorbed on the strongly acidic tricoordinated Al sites which are not accessible in the present study. The physisorption of H2S on a hydrated (110)C surface is versatile because of a variety of μ1-, μ2-, and μ3-OH groups available on the surface, each of which offers a unique coordination environment. In total three different types of physical interactions have been investigated, namely, (i) OH 3 3 3 SH2, (ii) Al 3 3 3 SH2, and (iii) OH 3 3 3 SH2 3 3 3 HO. The first configuration has been proposed by several groups on the basis of the experimental observations of an intense band and a broad band at about 1335 and 3500 cm-1, respectively.6,8,37 They suggested that the broadening of the OH band results from the hydrogen-bonding interaction with H2S and the sharp band at 1335 cm-1 arises from the HSH bending. The present study predicted, however, that this structure is not a minimum, although it is thermodynamically stable with respect to gas-phase H2S. The binding energy is merely 1.16 kcal/mol, and the OH 3 3 3 SH2 hydrogen bond is 2.420 Å. The weak hydrogen bond is likely a consequence of the second hydrogen bond between the O atom of the OH group with a neighboring hydroxyl that makes the H atom less electropositive and weakens the H 3 3 3 S interaction. The second configuration involves the direct molecular adsorption of H2S on a surface Al site. Contrary to EM1, which has a moderate thermal stability, this structure corresponds to a highorder saddle point of the potential energy surface and has a

computed binding energy of 0.72 kcal/mol. The Al 3 3 3 S distance is extremely long (∼3.6 Å), implying no attractive interaction at all between H2S and the surface. The third structure (HM2, depicted in Figure 3) is derived from the first configuration by the addition of a hydrogen bond with a neighboring OH group. This configuration corresponds to a stationary state and is thermodynamically stable, with an adsorption enthalpy of 13.56 kcal/mol, which is in very good agreement with the value deduced from the TPD experiments (12.91 kcal/mol).38 In this mode, H2S is coordinated simultaneously to a μ1-OH and a μ2-OH in close proximity, forming two short hydrogen bonds (1.844 and 2.317 Å, respectively). The S-H bond involved in the hydrogen bond formation is significantly stretched from 1.351 to 1.382 Å, while the other S-H bond is only elongated slightly to 1.358 Å. A similar multiple-site assisted adsorption of H2S has also been proposed by Datta and Cavell to account for the fact that the measured adsorption heat corresponding to the dissociative adsorption of H2S is close to the bond energy of Al-S bond.8 3.3. H2S Adsorption on a Dehydrated γ-Al2O3 (110)C Surface. The IR spectra for adsorbed H2S on γ-Al2O3 showed distinct features when γ-Al2O3 was activated at different tempeartures.8 The changes were attributed to the removal of surface hydroxyl groups, which makes more Al and O sites available for different adsorption modes of H2S on the surface. To investigate how OH groups affect the physisorption and chemisorption of H2S, stable configurations for H2S adsorbed on the dehydroxylated (110)C surface were computed. In total four adsorption modes have been located; interestingly, all configurations resulted from dissociative adsorption of H2S, and all were associated with a large adsorption energy. Their structures are given in Figure 4. In general, these four adsorption modes can be classified into two categories depending on whether the SH coordination is monodentate or bidentate. As mentioned above, two types of Al sites are available on the dehydrated (110)C surface for H2S adsorption, and their acidities follow the order tricoodinated Al > tetracoordinated Al.36 It is therefore expected that H2S adsorbs more preferentially on the tricoordinated Al sites than on the tetracoordinated Al sites. The present study showed that the dissociative adsorption of H2S on a tricoordinated Al site results in a strongly bound SH species (DM1) with a calculated adsorption enthalpy of 76.47 kcal/mol. The Al-S bond distance is 2.193 Å, which is longer than that for a gas-phase Al-S molecule (2.029 Å),41 and the Al atom relaxes outward by 0.54 Å, attaining a tetrahedral geometry. The dissociated H atom is transferred to an adjacent tricoordinated O atom and forms a hydrogen bond of 1.992 Å with a neighboring O atom. 1902

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Figure 4. Optimized geometries of DM1-DM4 on the dehydrated (100)C γ-Al2O3 surface (blue, Al; red, O; yellow, S; pink, H).

On the other hand, the dissociative adsorption of H2S on a single, tetracoordinated Al site leads to two possible stable configurations, differing in the position the dissociated H atom resides on. The first structure (DM2) involves the H atom bonded to a tricoordinated oxygen that bridges the subsurface hexacoordinated Al atoms. The protonation of this oxygen induces a remarkable reconstruction of a surface Al atom directly linked to it, relaxing inward from a surface octahedral site to a bulk tetrahedral site. This relaxation, in conjunction with the hydrogen bond between the SH and OH groups (∼2.167 Å), gives rise to a large adsorption energy (67.49 kcal/mol) comparable with that for DM1. The second structure (DM3) emerges when the dissociated H atom adsorbs to the adjacent, less basic, tricoordinated O atom which is bonded to a very acidic tricoordinated Al atom. This also causes the rupture of the Al-O bond and a small degree of relaxation of the Al atom to which SH is bonded. However, this reconstruction is not favorable, and the hydrogen bonds thus formed are rather weak (2.3-2.4 Å). Consequently, this configuration is much less stable (ΔEad = 37.20 kcal/mol) compared to the other μ1-SH counterparts (DM1 and DM2). In both DM2 and DM3, the Al-S bond is slightly stretched by 0.1 Å relative to that of DM1. This observation is consistent with the lower acidity of tetracoordinated Al atoms, which infers weaker bonds formed between these sites and SH. H2S can also dissociate on a bridge position where it forms surface bonds with two tetracoordinated Al atoms simultaneously. The resulting structure may have the dissociated H atom residing on either the O atom bonded to subsurface hexacoordinated Al atoms or the O atom bonded to the surface tricoordinated Al atom. It was found that the former one is more favorable because of the higher basicity of the O atom.42,43This state, denoted by DM4, is made up of SH bridged asymmetrically over two surface tetracoordinated Al atoms, with the Al-S bond distances being 2.377 and 2.450 Å. It is noticed that these bonds are much longer

than the usual Al-S single bond (∼2.1 Å, from ref 41), indicating that this configuration should have a lower stability compared to DM1 and DM2. The same trend of relative stability can be obtained from the comparison of their adsorption enthalpies; the binding energy of DM4 is only 56.78 kcal/mol, which is from 10 to 20 kcal/mol smaller than those for DM2 and DM1. It is worth mentioning that, unlike the other adsorption configurations on the same surface, DM4 possesses no hydrogen bonds between SH, OH, and surface O atoms; the closest O 3 3 3 H and S 3 3 3 H distances are 2.699 and 2.952 Å, respectively. Apparently, the computed adsorption energies associated with the dissociative adsorption of H2S on the dehydrated (110)C surface are not in agreement with the experimental observation that the upper limit of the adsorption heat is only 35 kcal/mol.38 It has to be emphasized that, under the normal Claus reactor conditions, the (110)C surface is almost fully hydrated.36 Activation of γ-Al2O3 at 700
C can effectively remove all hydroxyl groups on the surface; nevertheless, water is generated during the dissociative adsorption of H2S, which subsequently rehydrates the surface and reduces the number of Al sites available for H2S adsorption. Consequently, the fully dehydrated (110)C surface represents merely an extreme example of the surfaces of γ-Al2O3 which are possibly exposed in reality; its contribution to the total surface area of γ-Al2O3 is likely small, and the contributions of DM1-DM4 to the experimental adsorption heat of H2S are thus nearly negligible. 3.4. Density of States Analysis for Adsorbed H2S. It has been discussed in the previous sections that the strength of interaction between the adsorbed H2S and γ-Al2O3 surfaces is highly dependent upon the nature of the surfaces and the presence of structural characteristics such as unsaturated Al sites, O defects, and hydroxyl groups. For a better understanding, at the molecular level, of the observed higher adsorption enthalpies of H2S on the (110)C surface than on the (100)E surface, a detailed density of states analysis was performed at the optimized geometries 1903

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Figure 5. p-band PDOSs of free SH and adsorbed SH in EM2-EM4 and DM1-DM4 (red, p0-band; green, p(1-bands). Energy values are scaled with respect to the Fermi level.

of EM2-EM4 and DM1-DM4. Note that the physisorbed species (i.e., EM1, HM2) were excluded as the employed DFT method was not able to describe this interaction very accurately. Moreover, HM1 was not considered due to the extensive H-bond network. The computed local projected densities of states (PDOSs) of the Al and O adsorption sites of EM2-EM4 and DM1-DM4 as well as the adsorbed SH fragments are illustrated in Figures 5-10. 3.4.1. Dehydrated (100)E Surface. The S p-band DOS of gasphase SH radical is presented in the top panel of Figure 5. The doublet at -4 eV represents the SH σ bond, while the spikes at -1.1 eV (p0) and the Fermi level (p(1) are attributed to the unpaired electron and a lone pair on S, respectively. The empty SH σ* bonding appears at 5.3 eV above the Fermi level. Remarkable differences in the SH DOS are noticed upon SH adsorption on the (100)E surface in different configurations. The S local DOS for adsorbed SH in the EM3 configuration resembles closely the gas-phase SH DOS except the p0-band, which spreads over -5 to -1 eV and shows a sharp band near the Fermi level. These changes suggest that the unpaired electron on S is involved in the formation of a relatively weak surface Al-S bond. This is supported by the local p0-band DOS of the surface Al atom bonded to SH in the EM3 configuration (see Figure 6); the

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Figure 6. p-band PDOSs of Al sites on the (100)E surface (red, p0-band; green, p(1-bands). Energy values are scaled with respect to the Fermi level.

depletion of the antibonding orbital at 7 eV and the appearance of an isolated band at -0.3 eV indicate that the Al-S bond likely results from the interaction of the unpaired electron of S and the vacant 3pz orbital of Al. The formation of the Al-S bond also leads to a weakened surface bonding between the Al atom and the bulk as indicated by the decreased Al valence p0-band occupancy (Figure 6). On the other hand, the adsorption of SH in the EM2 configuration causes a significant downward shift of both the valence and conduction p-bands of S. The valence bands smear over the range of -3 to -10 eV, showing a bonding interaction of the px and py orbitals of S and the surface atoms. The lower p0-band centroid of EM2 (∼-5 eV) compared to EM3 (∼-3 eV) explains its shorter Al-S bond distance (2.300 Å versus 2.518 Å). Meanwhile, the splitting of the conduction p0 and p(1-bands of Al, as shown in Figure 6, results from the outward relaxation of the Al atom upon SH adsorption, which reduces the interaction between the Al atom and the sublayer O atom. Another noticeable difference between the Al local PDOSs for EM2 and EM3 is the apperance of a strong p(1-band at -7 eV for EM3 corresponding to the bonding interaction with the surface OH group on a neighboring O defect site. The same band, arising from the OH σ bond, is also observed in the local PDOS of the O atom at the defected site of EM3 in Figure 7. This unexpected bonding interaction can be attributed to the coordinative unsaturation of the 1904

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Figure 7. p-band PDOSs of the normal and defective O sites on the (100)E surface (red, p0-band; green, p(1-bands). Energy values are scaled with respect to the Fermi level. O(II) and O(III) denote an O defect site and a normal tricoordinated O site, respectively.

defected O site, which enhances the surface bond with the neighboring Al atom, and as a consequence gives rise to a slightly higher adsorption enthalpy of EM3 despite a longer Al-S bond. The computed SH, Al, and O local PDOSs for EM4 are markedly different from those for EM2 and EM3 mainly due to the fact that SH adsorbs on a defected O site instead of a pentacoordinated Al site. The S p0- and p(1-bands for EM4 and EM2 look alike except the two p(1 sharp bands which are found at -3 and 1 eV for EM4. These bands are ascribed to the surface S-O σ and σ* orbitals, as supported by the same features in the O local PDOS for EM4 in Figure 7. The S p0-band for EM4 is further shifted downward to -7 eV because of the hydrogen bonding with surface O atoms (see Figure 2). Owing to a great stabilization of the defected O site by SH adsorption, as reflected by the shift of the p-band centroids from -3 to -8 eV, EM4 is found to be the most favorable configuration for dissociatively adsorbed H2S on the (100)E surface. 3.4.2. Dehydrated (110)C Surface. Figure 8 depicts the local PDOS of a tricoordinated Al atom and a bridging O atom of the dehydrated (110)C surface before and after the dissociative adsorption of H2S. It is observed that the intense conduction p0-band of Al at 4 eV, associated with the high Lewis acidity of the tricoordinated Al site, is shifted to -4 eV, indicating a strong bonding interaction of the SH fragment with the empty 3pz

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Figure 8. p-band PDOSs of Al and O sites of DM1 (red, p0-band; green, p(1-bands). Energy values are scaled with respect to the Fermi level. Al(III) refers to a tricoordinated Al site.

orbital of the Al atom. This fact is supported by the S PDOS for DM1 in Figure 5, where a sharp p0-band is noticed at -4 eV. The Al conduction p(1-band is shifted slightly upward from 4 to 7 eV; this is possibly caused by the antibonding interaction with the 3p orbitals of S. On the other hand, the adsorption of H on a briding O atom splits both of its p0- and p(1-bands, and several sharp bands emerge. These localized bands arise from the adsorptioninduced reconstruction of the bridging O site, which, to a certain extent, reduces the participation of the O p-bands in the surface bondings. Figure 9 illustrates the local PDOSs of tetracoordinated Al sites where SH fragments are adsorbed in the DM2, DM3, and DM4 configurations. The formation of an Al-S bond does not alter very much the resulting Al valence p-bands, while the conduction p0-band at 3 eV vanishes for DM2 and DM3, suggesting a bonding interaction between the vacant 3pz orbital of Al and the SH fragment. In the meantime, the p(1-band for DM3 splits due to the relaxation of the Al site during adsorption where a surface Al-O bond is cleaved, rendering the Al site more coordinatively unsaturated. The surface Al-O bond cleavage also enhances the electron density around the relaxed O site where H adsorbs and leads to the appearance of an isolated band at about -1.5 eV of the O local PDOS for DM3 as shown in Figure 10. Such reconstruction around the tetracoordinated Al 1905

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Figure 9. p-band PDOSs of Al sites on the (110)C surface (red, p0-band; green, p(1-bands). Energy values are scaled with respect to the Fermi level. Al(IV) designates a surface tetracoordinated Al site.

adsorption site does not occur for DM2, and thus, its conduction p(1-band remains approximately unchanged after H2S adsorption. Similar to DM3, high-energy valence p-bands appear at -1.1 and -1.9 eV in the O PDOS for DM2 (see Figure 10). These bands are formed as a consequence of the surface reconstruction around the O adsorption site; its bond to the neighboring tetracoordinated Al atom is ruptured heterolytically, and the bonding electrons reside mainly on the p(1-band and to a small extent on the p0-band of the O atom. It is apparent from Figure 5 that these surface orbitals may have an effective overlap with the adsorbed SH fragment. Unlike DM2 and DM3, the Al local PDOS for DM4 resembles more closely the corresponding Al PDOS on a clean dehydrated (110)C surface because no surface reconstruction is induced by H2S adsorption. The centroids of the valence band and conduction band are shifted upward by about 1 eV. A notable difference is the overlap of the p0 and p(1 conduction bands at 4 eV due to the fact that both the 3pz and 3px, 3py orbitals of Al are involved in the μ2-type adsorption of the SH fragment (see Figure 5 for comparison). The corresponding Al-S-Al bonding band is located at -4 eV. 3.5. Vibrational Frequency Analysis for Adsorbed H2S. IR spectroscopy has been extensively used to probe the structures of adsorbed H2S and its reactions on γ-Al2O3. The study by

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Figure 10. p-band PDOSs of O sites on the (110)C surface (red, p0-band; green, p(1-bands). Energy values are scaled with respect to the Fermi level. O(I) and O(II) represent type I and type II O sites whose descriptions are given in section 3.3.

Deo et al. showed that there are two major absorption bands at 2560 and 1335 cm-1 corresponding to the SH stretching and HSH bending modes of H2S, respectively, accompanied by the disappearance of a sharp band at 3700 cm-1 and the formation of a broad band at about 3500 cm-1 which are due to the hydrogen bonding of H2S with surface OH groups.6 The same results have also been obtained by Liu et al.44 and Slager et al.45 except that an additional band at 1568 cm-1 was detected which was assigned to the AldO bond stretching of γ-Al2O3. The effects of the activation temperatures of γ-Al2O3 and dosage of H2S on the resulting IR spectra for adsorbed H2S have been thoroughly investigated by Datta and Cavell.8 They discovered that H2S on Al sites can either adsorb molecularly or dissociate to produce Al-SH, Al-S, and Al-OH2 , each of which can be distinctly identified in the IR spectra. Regardless of the activation temperature of γ-Al2O3, H2S first decomposed to give rise to the S-H stretching band in the 2558-2578 cm-1 region. The 1331 cm-1 band corresponding to the scissoring mode of H2S only appeared when the surface was exposed to high-dose H2S. On the other hand, the spectra recorded for adsorbed H2S on γ-Al2O3 activated at 700
C showed a less intense 1620 cm-1 band compared to that obtained for the system where γ-Al2O3 was only activated at 400
C, indicating that a smaller amount of water is produced when surface OH groups are absent. 1906

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Table 1. Calculated Vibratioanl Frequencies of Various Adsorption Modes of H2S on γ-Al2O3 modea

dehydrated (100)E

EM1

2627 (asym SH str), 2614 (sym SH str), 1196 (HSH bend)

EM2

3255 (OH str), 2588 (SH str)

EM3

3440 (OH str), 2593 (SH str)

EM4

3318 (OH str), 2196 (SH str)

HM1

3322 (OH str), 2658 (SH str)

HM2

2547 (asym SH str), 2247 (sym SH str), 1235 (HSH bend)

DM1

3338 (OH str), 2592 (SH str)

DM2 DM3

2941 (OH str), 2621 (SH str) 3217 (OH str), 2594 (SH str)

DM4

3648 (OH str), 2620 (SH str)

hydrated (110)C dehydrated (110)C

exptl6,844,45 a

scaled frequencies46 (cm-1)

surface

3200-3600 (H-bonded OH str), 2558-2578 (asym/sym SH str), 1331 (HSH bend)

Definitions from Figures 2-4.

The vibrational frequencies of the species considered in the present work are summarized in Table 1. As can be seen, the computed frequencies are in acceptable agreement with the reported experimental values, although the SH stretching modes were always overestimated by about 40-50 cm-1. This deviation can be attributed to the choice of the scaling factor of vibrational frequency calculations. Note that the value of 0.99346 was obtained from the reference set at PBE/cc-pVTZ level while the PAW method with plane-wave basis sets was employed instead in the present study. Apparently, the scaling factor has to be decreased to bring the calculated frequencies to a closer agreement with the experiments. Please note that, in this work, no frequencies as a result of water formation have been observed. The reason is that only the initial decomposition step of H2S to SH and OH was considered for the three surfaces. The study regarding the formation of water from H2S dissociation through either concerted mechanisms45 or stepwise hydrogen transfer47 is in progress. 3.5.1. Hydrated (110)C Surface. In spite of the noticeable absolute discrepancies, the experimental trends of variations in the vibrational modes of adsorbed H2S are still observable. The calculated S-H stretching and HSH bending modes of a gasphase H2S molecule are 2669, 2650, and 1173 cm-1 respectively. Upon molecular adsorption on the hydrated (110)C surface (i.e., HM2), the stretching frequecies are reduced to 2547 and 2247 cm-1 while the bending frequency is increased to 1235 cm-1. These changes are consistent with the observations by Slager and Amberg where shifts of -116 and þ51 cm-1 were found for the asymmetric stretch and bending of H2S.45 The significant red shift of the symmetric stretch of HM5 is likely due to the strong hydrogen bond (1.844 Å) formed with a neighboring μ1-OH group that elongates the S-H bond by 0.04 Å (see Figure 3) and weakens the bond. The adsorption of H2S also affects the stretching modes of surface hydroxyl groups directly coordinated to it. The OH stretching modes of the four hydroxyls (two μ1-OH and two μ2-OH groups) hydrogen-bonded to H2S are all lowered to an extent depending on the nature and length of the hydrogen bonds. Since H2S is hydrogen-bonded via its H atoms to the two μ1-OH groups, the influence on the OH stretching of these hydroxyl groups is minimal. The shift varies from 13 cm-1 where the hydrogen bond is 1.84 Å to only 2 cm-1 when the hydrogen bond is 2.34 Å. Contrary to the μ1-OH groups, the OH stretching modes of the two μ2-OH groups are strongly damped because hydrogen bonds are formed between their H atoms and the S

atom of H2S. The reduction in the OH stretching frequency is inversely proportional to the S 3 3 3 H bond length (382 cm-1 for r(S 3 3 3 H) = 2.32 Å and 94 cm-1 for r(S 3 3 3 H) = 2.58 Å). On the other hand, when H2S undergoes a dissociative adsorption on the same surface (HM1), SH is produced with a S-H stretching mode that is decreased slightly compared to that of free-space H2S due to the fact that the inappropriate orientations of SH and OH groups do not allow the formation of any effective hydrogen bonds. In the meantime, variations of the OH stretching modes for the hydroxyl groups in close vicinity are noticeable. Since no well-defined hydrogen bonds are formed between SH and OH groups, these changes should arise from indirect electronic or steric factors induced by the adsorption of H2S. As illustrated in Figure 11, there exist three types of OH groups that share the same Al sites with SH. The bridging OH over two tetracoordinated Al atoms is subjected to the smallest increase in OH stretching frequency. This increment is the consequence of the electron transfer from SH to electron-deficient Al sites, which weakens the geminal Al-O bond and in turn strengthens the OH bond, leading to its higher stretching frequency. The same enhancement effect is also observed for the μ2-OH that connects simultaneously to a tetracoordinated Al atom and a subsurface hexacoordinated Al atom, yet the situation is more complex. The blue shift of the corresponding OH stretching mode is more pronounced (∼250 cm-1), and the OH bond distance is reduced by 0.02 Å. The strong positive influence is likely ascribed to the lower acidity of the subsurface Al atom, which facilitates the weakening of the Al-O bond when SH adsorbs. Meanwhile, this OH group forms a very short hydrogen bond (1.660 Å) with a neighboring μ1-OH group, which, supposedly, reduces the OH stretching frequency significantly. It is therefore believed that the resulting shift of the OH stretching mode is the outcome of two adverse electronic interactions. Unlike those previously described, the μ2-OH group bonded to a tricoordinated Al atom experiences a red shift of 43 cm-1 in its OH vibrational frequency. This can be accounted for by two factors. First, tricoordinated Al atoms are the most Lewis acidic on the (110)C surface;36 therefore, the electron-donating effect due to the SH adsorption is counteracted by the electron-withdrawing effect of the tricoordinated Al atom, resulting in a negligible influence on the μ2-OH bond strength. Second, upon the adsorption of SH, this OH group has drifted toward a neighboring μ1-OH and the hydrogen bonding between these OH groups is enhanced, thus reducing 1907

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Figure 11. Hydrogen bond configurations in HM1 and HM2 (blue, Al; red, O; yellow, S; pink, H).

Table 2. Structural and Spectroscopic Parameters for OH Groups of DM1-DM4 mode

r(OH) (Å)

ν(OH) (cm-1)

designation43

DM1

0.990

3338

346 f 34

DM2

1.007

2950

466 f 66

DM3

0.993

3227

346 f 36

DM4

0.973

3659

466

the bond strength and stretching frequency of the μ2-OH group. 3.5.2. Dehydrated (110)C Surface. When the (110)C surface of γ-Al2O3 is fully dehydrated, the adsorption of H2S only leads to dissociative products, SH and OH, which yield much simplified IR spectra compared to the hydrated (110)C surface. For the species DM1-DM4, the surface SH group gives an absorption band in the 2592-2621 cm-1 region arising from the S-H stretch. These values agree with the experimental band at 25602580 cm-1 attributed to the S-H stretching mode of adsorbed H2S.6,8,44,45 It has to be emphasized, however, that these bands from DM1-DM4 are not anticipated to have significant contributions to the overall IR spectrum for adsorbed H2S on γ-Al2O3 because of their low concentrations on the surfaces. Though lying at the lower end of the 3200-3700 cm-1 region where a broad band corresponding to hydrogen-bonded OH groups appears and masks all other absorption peaks, the analysis of the OH stretching modes of DM1-DM4 is still able to provide insight into the relationship between the structure and electronic and spectroscopic properties of surface hydroxyl groups. On the basis of the classifications by Tsyganenko and Mardilovich, hydroxyl groups on the surfaces of γ-Al2O3 can be distinguished according to the types of Al atoms to which OH groups are coordinated,43 and the acidity of the Al atoms directly affects the resulting bond strength and vibrational frequency of the attached OH group. Selected structural data and stretching modes of the OH groups of DM1-DM4 are listed in Table 2. The OH stretching mode of DM4, which is designated as 466 since it is coordinated simultaneously to two subsurface hexacoordinated Al atoms and a tetracoordinated Al atom on the surface, was found to be 3659 cm-1, which is in perfect agreement with the observations reported by Kn€ozinger and Ratnasamy42 and Tsyganenko and Mardilovich43 for μ3-OH. Since the Lewis acidity of Al is inversely proportional to the number of coordination to O atoms and is also negatively correlated to the bond strength of the attached OH

group, it is expected that the OH stretching frequency follows the order 466 > 346 for μ3-OH and 66 > 36 > 34 for μ2-OH. Nevertheless, an opposite trend was noticed in the present study (see Table 2), where the type 34 μ2-OH group shows a stretching frequency of 3338 cm-1 while the type 36 μ2-OH has a stretching mode of 3227 cm-1. The computed frequency for the type 66 μ2-OH is even lowered to 2950 cm-1. This unexpected trend can be explained in terms of the hydrogen bonding and surface reconstruction. Note from Figure 4 that the surface in close proximity to the adsorption sites of SH and OH in DM1-DM3 is under a remarkable reformation, which as a consequence leads to a severe distortion to the electron density on the surface. Therefore, the OH stretching modes of these species are significantly reduced relative to that of DM4. It can be seen by comparing DM1 and DM3 that the adsorptioninduced surface reconstruction is less substantial for DM1, and this causes a smaller reduction of its OH stretching frequency. Moreover, the adsorption of SH on the tricoordinated Al atom induces an electron-donating effect that strengthens the OH bond at the adjacent position. Although these positive effects are partly offset by the formation of a hydrogen bond (1.992 Å), the OH stretching mode of DM1 is still larger than that of DM3 by about 100 cm-1 . On the other hand, the low-frequency OH stretching mode of DM2 is caused by the surface reconstruction where one of the Al atoms bonded to μ2-OH turns out to be effectively pentacoordinated and more acidic, withdrawing the electron density from the O atom and weakening the OH bond. In the meantime, the μ2-OH group forms a hydrogen bond (2.167 Å) with the adsorbed SH, which further lowers the OH stretching frequency. It is worth mentioning that the computed OH vibrational frequencies for DM1-DM4 follow a linear relationship with their bond distances. The results are depicted in Figure 12. 3.5.3. Dehydrated (100)E Surface. Figure 12 also illutrates the linear relationship between the stretching frequencies and the bond lengths of the OH groups on the dehydrated (100)E surface. It is noted that the trends observed for these surfaces are not identical; on the dehydrated (100)E surfaces, the OH vibrational frequencies are slightly less dependent on the OH bond distances. This observation could be attributed to the fact that all OH groups formed on this surface originate from the same type of surface O atoms (type 655 in Tsyganenko’s notation;43 the other one is in type 455) so that their electronic structures are not altered much due to H2S adsorption. Despite this outcome, the variation of the OH stretching frequencies can be correlated to the strength of hydrogen 1908

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Figure 12. Correlation plot of vibrational frequencies and bond distances of OH groups on the dehydrated (100)E and dehydrated (110)C surfaces.

bonding between the OH group and surface O atoms. In both EM2 and EM4, the H atom from H2S is bonded to a tricoordinated O atom; however, the former species forms a stronger hydrogen bond (2.089 Å cf. 2.163 Å) with the neighboring type 655 O atoms, and therefore, its OH bond is more elongated (0.992 Å cf. 0.989 Å), and the stretching frequency is more reduced (3255 cm-1 cf. 3318 cm-1; see Table 1). A direct comparison of EM2 and EM3 reveals that H transfer to the oxygen next to the vacancy dramatically increases the OH vibrational frequency. Note that in EM3 the dissociated H atom is attached to a bicoordinated O atom while in EM2 the H atom is bonded to a tricoordinated O atom. The decrease in the degree of coordination renders the O atom in EM3 more electron-rich, therefore enhancing the OH bond strength as reflected by a much shorter O-H bond distance (0.980 Å) and a higher stretching frequency compared to that of EM2 (0.992 Å). The calculated SH vibrational modes for EM2 and EM3 are in good agreement with experiments (Table 1). Since no hydrogen bond exists between the SH group and surface O atoms in these species, it is speculated that the experimental band at 2580 cm-1 likely arises from either H2S molecularly adsorbed via hydrogen bonds with OH groups or the adsorbed SH groups which form no hydrogen bonds with the surface. Similar to the OH group in DM2, the stretching frequency of the SH group in EM4 is significantly shifted to a lower value because of the strong hydrogen bond (1.766 Å; see Figure 1) it forms with a surface oxygen atom. The molecular adsorption of H2S on the dehydrated (100)E surface (i.e., EM1) does not result in changes in both the stretching and bending modes of H2S as much as in the case of the hydrated (110)C surface (i.e., HM2). The absence of surface hydroxyl groups on the dehydrated (100)E surface eliminates all possible hydrogen bonding, which impedes the vibrational motion of H2S. Accordingly, the adosrbed H2S shows stretching and bending modes resembling those of a gas-phase H2S molecule.

4. CONCLUSIONS The adsorption process of H2S on γ-Al2O3, which is an active catalyst in the Claus reaction, has been investigated by density functional theory. In particular, the chemisorption of H2S was studied for the two most exposed surfaces of γ-Al2O3, namely, the (100)E and (110)C planes. To acquire a better understanding of

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the effects of the surface water content on the adsorption and decomposition of H2S, a hydrated (110)C surface was also taken into consideration. According to the present study, the adsorption enthalpies of H2S on these surfaces vary from 12.26 kcal/mol for physisorption to 34.13 kcal/mol for chemisorption. This range agrees very well with the experimental values determined by the TPD technique.38 It has been observed that the H2S adsorption occurs preferentially on the dehydrated (100)E surface from a thermodynamic viewpoint. More specifically, the H2S adsorption shows a high selectivity toward the O atom near the structure defect (Al vacancy); the calculated adsorption energy when H is attached to the O atom next to the Al defect is about 5 kcal/mol larger than when the H atom is attached to other tricoordinated O atoms. Surprisingly, an additional, strongly bound state of chemisorbed H2S was found at this site, where SH is bonded directly to the O atom. The true bonding mechanism regarding its formation is not known, but apparently the presence of hydrogen bonds and the increased basicity of the O atom due to defects play a role. The effects of hydration on the H2S adsorption can be seen by comparing the results for the dehydrated and hydrated (110) surfaces. First, H2S adsorption on the dehydrated surface is more energetically favorable; the adsorption energies for DM1-DM4 are in the range of 37-76 kcal/mol, which are 2-5 times higher than those for HM1 and HM2. Second, no physisorption modes of H2S could be found on the dehydrated (110)C surface, suggesting that OH groups are key to the molecular adsorption of H2S. Though highly favorable from a thermodynamic perspective, DM1-DM4 are not expected to be observed in noticeable quantity in the Claus reaction since under such conditions the (110)C surface is almost fully hydrated.36 The calculated vibrational modes for SH and OH stretching as well as HSH bending are in good agreement with the values tabulated in the literature, although they are slightly overestimated. The variations of the SH stretching modes can be accounted for by the formation of hydrogen bonds with surface hydroxyl groups. On the other hand, the vibrational frequencies of the OH groups formed from the dissociative adsorption of H2S on the dehydrated surfaces were found to be correlated to the local electronic structure and geometry of the O atoms. Consistent with the propositions by Tsyganenko et al.,43 the OH group bonded to the Al atoms which are more coordinatively unsaturated is subjected to a larger red shift in its OH stretching mode. For instance, the OH vibrational frequency of DM4 (classified as type 466 in Tsyganenko’s notation) is higher than that of DM1 (type 346) by about 200 cm-1. Meanwhile, the surface reconstruction upon H2S adsorption and formation of hydrogen bonds also contribute to the shift of OH stretching frequencies, resulting in a rather complex correlation (e.g., DM1-DM3 in Table 2).

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT J.M.H.L. acknowledges an Industrial Research and Development Fellowship from the Natural Sciences and Engineering Research Council of Canada (NSERC) and Alberta Sulphur Research Ltd. The computing facility was provided by the Western Canada Research Grid. 1909

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