AT

Ra

Pb

a

ARRAA

KHCADD

1

c[mkacAwciio

R

(

0h

Applied Surface Science 258 (2012) 9932– 9943

Contents lists available at SciVerse ScienceDirect

Applied Surface Science

jou rn al h om epa g e: www.elsev ier .com/ locate /apsusc

mechanistic study of H2S adsorption and dissociation on Cu2O(1 1 1) surfaces:hermochemistry, reaction barrier

iguang Zhanga, Hongyan Liua,b, Jingrui Lia, Lixia Linga, Baojun Wanga,∗

Key Laboratory of Coal Science and Technology of Ministry of Education and Shanxi Province, Taiyuan University of Technology, Taiyuan 030024, Shanxi,eople’s Republic of ChinaCollege of Chemistry and Chemical Engineering, Shanxi Datong University, Datong 037009, Shanxi, People’s Republic of China

r t i c l e i n f o

rticle history:eceived 18 April 2012eceived in revised form 15 June 2012ccepted 15 June 2012vailable online 23 June 2012

eywords:2Su2O(1 1 1)dsorptionissociationensity functional theory

a b s t r a c t

The interaction mechanism of H2S with different Cu2O(1 1 1) surfaces, including perfect, oxygen-vacancyand sulfur-containing surfaces, have been systematically studied using periodic density functional cal-culations. Different kinds of possible modes of H2S, as well as the resultant SH and S species adsorbed onthese surfaces are identified. Two types of pathways via molecular and dissociative adsorption processesare mapped out. Our results show that sulfur species (H2S, SH and S) interact with surface Cu centers;H2S exists in the form of molecular adsorption on perfect and sulfur-containing surfaces; the dissociativeadsorption of H2S occurs predominantly on oxygen-vacancy surface, suggesting that oxygen-vacancyexhibits a strong catalytic activity toward the dissociation of H2S. On the other hand, the dissociationprocesses of the molecular and dissociative adsorption H2S, leading to final product S species on theseCu2O(1 1 1) surfaces, show that the overall dissociation process is exothermic. Meanwhile, with respect tomolecular adsorption H2S, the activation barrier and reaction energy of the overall dissociation process

on perfect and oxygen-vacancy surfaces indicate that H2S can easily dissociate into S species. Impor-tantly, in the case of dissociative adsorption of H2S, the dissociation of H2S into S species is a spontaneousprocess with respect to molecular adsorption H2S. However, on sulfur-containing surface, the presenceof surface S atom goes against the H S bond-breaking process both thermodynamically and kinetically.Finally, the vibrational frequencies for the adsorbed H2S, SH and S species on these surfaces have beenobtained, which can be applied to guide surface vibrational spectroscopy in experiment.

. Introduction

Hydrogen sulfide (H2S) is the most common sulfur-containingompound found in fossil derived fuels and chemical feedstock1]. It leads to environmental pollution, as well as pipe and equip-

ent corrosion. Apart from above problems, H2S is also commonlynown for rapidly deactivating or poisoning most metal/oxide cat-lysts used in the chemical or petrochemical industries and in theontrol of CO and NOx emissions from automobile exhaust [2–9].

fundamental understanding of the interaction of H2S moleculesith oxide surfaces is important for two reasons [1]. First, most

ommercial catalysts poisoned by sulfur involve oxides, and there

s a clear need to improve their lifetime [3,4]. And, second, in manyndustrial operations, oxides are used as sorbents for the removalr destruction of sulfur-containing molecules [10–12].

∗ Corresponding author at: No. 79 Yingze West Street, Taiyuan 030024, People’sepublic of China. Tel.: +86 351 6018539; fax: +86 351 6041237.

E-mail addresses: wangbaojun@tyut.edu.cn, wangbaojuntyut@163.comB. Wang).

169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.apsusc.2012.06.053

© 2012 Elsevier B.V. All rights reserved.

On the surface of a metal oxide, sulfur can interact with a metalor an oxygen atom, producing species with very different electronicproperties (“sulfide” versus “sulfate” formation). Experiments forthe adsorption of H2S on a series of oxides [13–21], for example,UO2, CuO, Cu2O, ZnO, Al2O3, Fe2O3, V2O5, and so on, have revealedthat sulfur species produced by the dissociation of H2S moleculemainly interact with the metal centers of the surface. Among abovemetal oxides, copper (I) oxide (Cu2O) has been extensively studieddue to its electronic structure characterized by LEED, XPS, TDS, andUPS, as well as the lower equilibrium H2S level [22–28]. Mean-while, Cu2O has been identified as a thermodynamically stablephase of copper-based catalysts at relevant catalytic conditions[29]. Furthermore, Cu(I) ions are known to be an active componentin the Cu/ZnO catalyst, which is found to be more sensitive to H2Sespecially for methanol synthesis from syngas [6,7], the water gasshift reaction and methanol partial oxidation to formaldehyde [8,9].Therefore, it is necessary to understand the interaction mechanism

of H2S with Cu2O, a reliable mechanistic study for H2S adsorp-tion and dissociation on Cu2O surface at the molecular level mayincrease the understanding of the position and behavior of H2S, theresultant SH and S species on Cu2O(1 1 1) surfaces, which may be of

ce Sci

ima

idctotieSfinctpooliugdtoAfmpotHttssafsafca

[vstsSocrqsi

2

2

i

R. Zhang et al. / Applied Surfa

nterest to researchers attempting to develop new sulfur-tolerantaterials. However, up to now, information about the interaction

nd reaction of H2S with Cu2O surface is little reported.In the case of H2S interaction with Cu2O surface, only the exper-

mental studies by Galtayries and Bonnelle [19] have shown thatissociative adsorption of H2S on Cu2O, is highly dependent on thehemical nature of the exposed surface rather than on tempera-ure. Meanwhile, Lin et al. [15] investigated the adsorption of H2Sn Cu2O(1 1 1) surface using variable-energy photoelectron spec-roscopy, suggesting that H2S chemisorbed on Cu2O(1 1 1) surfaces found to be dissociated, forming sulfide, HS and hydroxide. How-ver, the position and behavior of H2S, as well as the resultantH and S species on Cu2O(1 1 1) surface are less well understoodrom above two experiments. Therefore, for a detailed understand-ng of the surface process, experimental information is howeverot always sufficient and accompanying theoretical calculationsan be helpful to clarify some questions. Recently, density func-ional theory (DFT) method has already been extensively used torovide qualitative and quantitative insights into the structuresf active surfaces and surface reaction [30–36]. Unfortunately, tour knowledge, the interaction of H2S with Cu2O surfaces is stillacking according to current literatures, only Casarin et al. [37]nvestigated the adsorption of H2S on perfect Cu2O(1 1 1) surfacesing DFT method coupled with the cluster model, however, it isenerally accepted that cluster model has boundary effect due toangling bonds, and more importantly, they have not mentionedhe process of H2S dissociation, as well as the effect of vacanciesver Cu2O(1 1 1) surface on the adsorption and dissociation of H2S.s a matter of fact, under a realistic condition, metal oxide sur-

aces are not always perfect and the important properties of mostetal oxides, including surface reactivity, are closely related to the

resence of surface vacancies [38]. For example, the presence ofxygen-vacancies on the NiO(1 0 0) surface leads to an increase inhe adsorption energy of H2 and reduces the energy barrier of the

H bond cleavage. In contrast, a perfect NiO(1 0 0) surface exhibitshe negligible reactivity toward H2 [39]. Further, the studies abouthe interaction of O2 and H2 with the oxygen-vacancy Cu2O(1 1 1)urface by Zhang et al. [40,41] show that oxygen vacancy exhibits atrong chemical reactivity toward the dissociative adsorption of O2nd H2. On the other hand, for H2S adsorption on metal oxide sur-ace, it has been generally realized that O-sites and vacancies on theurface play an important role in the initial replacement betweendsorbed S and surface O, which leads to the sulfur-containing sur-ace [13]. Thus, on sulfur-containing surface, whether the S atoman affect the dissociation process of H2S, up to now, few studiesbout this question have been reported.

Cu2O(1 1 1) surface is shown to be the main surface of Cu2O23,42], in this study, taking into consideration the perfect, oxygen-acancy and sulfur-containing Cu2O(1 1 1) surfaces, we report aystematic DFT together with periodic model investigation abouthe mechanisms of interaction between H2S and these Cu2O(1 1 1)urfaces, involving the position and behavior of H2S, the resultantH and S species on these Cu2O(1 1 1) surfaces, as well as the rolef surface oxygen-vacancy and sulfur, together with the thermo-hemistry, reaction barrier of each elementary step for all low-lyingeaction pathways from H2S to S. In addition, the vibrational fre-uencies for various optimized structures of adsorbed H2S and HSpecies are also calculated to guide surface vibrational spectroscopyn experiment.

. Computational models and methods

.1. Surface models

Previous studies [43,44] have shown that the non-polar sto-chiometric O-terminated Cu2O(1 1 1) surface has notably lower

ence 258 (2012) 9932– 9943 9933

surface free energy, and the low energy surface structures are moststable under realistic catalytic conditions. Moreover, the non-polarstoichiometric O-terminated Cu2O(1 1 1) surface has often beenchosen as ideal model system to investigate the stability, structureand adsorption properties in theory [42,45–47] and experiment[15,23,29].

On the basis of above studies, the ideal Cu2O(1 1 1) surface willbe terminated by an outer atomic layer of oxygen anions, with asecond atomic layer of Cu+ cations, and a third atomic layer ofoxygen anions. The perfect Cu2O(1 1 1) surface includes four chem-ically different types of surface atoms, which are denoted as CuCUS,CuCSA, OSUF and OSUB (see Fig. 1(a)). Then, the removal of the oxy-gen of top atomic layer from the perfect surface results in what iscalled the oxygen-vacancy Cu2O(1 1 1) surface [45], as presentedin Fig. 1(b). Each oxygen vacancy gives rise to a threefold site ofsingly-coordinate Cu+ cation (i.e., Cu2, Cu3 and Cu4 atoms as labeledin Fig. 1(b)). Finally, the oxygen-vacancy site replaced by S atomleads to what is called sulfur-containing Cu2O(1 1 1) surface, asdisplayed in Fig. 2(d). Therefore, our studies on these Cu2O(1 1 1)surfaces, including perfect, oxygen-vacancy and sulfur-containingsurfaces have been done by using a p(2 × 2) supercell slab modelsof six atomic layers. The vacuum region separating the slabs in thedirection perpendicular to the surface direction is set to 10 A. In allcalculations, adsorbate and the three outermost atomic layers ofthe substrate are allowed to relax, whereas the three bottom-mostatomic layers of the substrate are frozen in their bulk positions.

2.2. Computational methods

In this study, DFT has been employed to perform for all cal-culations. The main calculations present here are based on theGGA of Becke-Lee-Yang-Parr (BLYP) exchange-correlation func-tional [48,49]. In the computation, the inner electrons of copperatoms are kept frozen and replaced by an effective core potential(ECP) [50,51], and oxygen atoms are treated with an all-electronbasis set. The valence electrons functions are expanded intoa set of numerical atomic orbital by a double-numerical basiswith polarization functions (DNP) [52]. Brillouin-zone integrationshave been performed using 4 × 4 × 1 Monkhorst-Pack grid and aMethfessel–Paxton smearing of 0.005 Ha. All calculations were car-ried out with the Dmol3 program package in Materials Studio 4.4.

3. Results and discussion

3.1. Evaluation of the method and model

Firstly, to validate the reliability of calculation methods, ourcalculated bulk lattice constants for Cu2O are 4.430 A, which isin agreement with the experimental values of 4.270 A [53,54].Meanwhile, as summarized in Table 1, our calculated geometricalparameters, dissociation energy of H S bond (Edis), and vibrationalfrequencies of gas-phase H2S and HS are in line with availableexperimental and theoretical data. Then, to confirm the sufficiencyof the chosen model (a p(2 × 2) supercell of six atomic layers), six-layered model for H2S adsorbed at CuCUS site on perfect surfaceis validated by using nine-layered model with top six layers relax,the differences of the structure and energetic parameters betweenthese two models are negligible. Moreover, a p(2 × 3) surface super-cell with six-layered has also been considered to obtain the sizeeffect of surface on calculation results, suggesting that a p(2 × 2)

supercell is large enough to neglect the lateral interactions of theadsorbate.

According to above evaluations, we are confident in the ability ofthe chosen method and model to describe the adsorption properties

9934 R. Zhang et al. / Applied Surface Science 258 (2012) 9932– 9943

F b) oxys of theo

os

3s

sE

E

wiips

CstI

FWi

ig. 1. The slab model of Cu2O(1 1 1)-2 × 2, (a) perfect Cu2O(1 1 1)-(2 × 2) surface; (tand for O atoms, and Black ball stand for oxygen-vacancy site. (For interpretationf the article.)

f H2S and HS species, as well as the features of the potential energyurface of H2S and HS dissociation on Cu2O(1 1 1) surface.

.2. Adsorptions of SH and atomic S, H species on Cu2O(1 1 1)urface

The adsorption energy is always regarded as a measure of thetrength of adsorbate–substrate adsorption. The adsorption energy,ads, is defined as follows:

ads = Esub + Emol − Emol/sub

here Emol/sub is the total energy of adsorbate–substrate system ints equilibrium state, Esub is the energy of the substrate, and Emols the energy of the isolated adsorbate. With this definition, moreositive values reflect strong interaction of adsorbed species withurface atoms.

The adsorption of SH and atomic S, H species separately on

u2O(1 1 1) surface, at a coverage of 0.25 ML with one adsorbedpecies in every (2 × 2) unit cell, have been investigated in this sec-ion. Five distinct adsorption sites presented in Fig. 1 are examined.n the case of CuCUS, CuCSA, OSUB and OSUF sites, the perfect surface

ig. 2. Optimized adsorption configurations of (a) SH at CuCUSsite, (b) SH at Ovacancy site, (hite and yellow balls stand for H and S atoms, respectively, and others are the same as in

s referred to the web version of the article.)

gen-vacancy Cu2O(1 1 1)-(2 × 2) surface. Orange balls stand for Cu atoms, red balls references to color in this figure legend, the reader is referred to the web version

is employed to calculations, while in the case of Ovacancy site, theoxygen-vacancy surface is employed to calculations.

3.2.1. SH adsorptionTwo molecular orientations, S-down and H-down, over five

adsorption sites of the Cu2O(1 1 1) surface are considered. The S Haxis is perpendicular to the surface in all the optimized models. Inthe case of S-down molecular orientation, the optimized structureof SH adsorbed at CuCUS site is first shown in Fig. 2(a), the bondlength of CuCUS SH is 2.189 A, the S H bond (1.366 A) is nearlyparallel to the surface with the CuCUS S H angle of 96.8◦, the cor-responding adsorption energy is 213.6 kJ mol−1. Meanwhile, ourcalculated bands at 2508 and 650 cm−1 are attributable to the S Hstretching and S H bending modes of adsorbed SH species, respec-tively. However, SH adsorbed at CuCSA and OSUB sites are shownto migrate to CuCUS site after optimization, as shown in Fig. 2(a),meanwhile, SH adsorbed at OSUF site is inclined to keep away fromthe surface. In the case of Ovacancy site, the vacancy site is filled

with SH, with S H bond length of 1.363 A, as presented in Fig. 2(b),the adsorption energy is 262.5 kJ mol−1. The bands at 2498 and646 cm−1 are observed, which correspond to the S H stretchingand S H bending modes of adsorbed SH species. In addition, our

c) atomic S at 3Cu site, (d) atomic S at Ovacancy site, and (e) atomic H at Ovacancy site. Fig. 1. (For interpretation of the references to color in this figure legend, the reader

R. Zhang et al. / Applied Surface Science 258 (2012) 9932– 9943 9935

Table 1Geometrical parameters, dissociation energy of H S bond (Edis), and vibrational frequencies of gas-phase H2S and HS calculated at the GGA-BLYP method.

Properties H2S HS

Cal.a Exp.b Cal.a Exp.c

r (S H) (A) 1.356 [1.350] 1.328 1.362 [1.355] 1.345� (H S H) (◦) 91.4 [92.1] 91.6�asym (cm−1) 2624 [2669] 2628 2576 [2612] 2660�sym (cm−1) 2607 [2658] 2615�bend (cm−1) 1206 [1226] 1183Edis (kJ mol−1) 384.1 381.4d 370.2 353.1e

a Values in brackets are predicted at the GGA-PW91 level of theory [55].b From Ref. [56].

c2m

dtStft

3

uailtanoiafisa

3

atcaaf(HCs

3

3

eatfFt

c From Ref. [57].d From Ref. [58].e From Ref. [59].

alculated results show that for SH free radical, only the band at576 cm−1 corresponds to the S H stretching, and no S H bendingode is observed.In the case of H-down modes, it is interesting to find that the H-

own models at five different adsorption sites are all converted tohe corresponding S-down models after optimization. Therefore,H with S-down mode adsorbed at CuCUS and Ovacancy sites arehe most stable configurations on perfect and oxygen-vacancy sur-aces, respectively, which means that CuCUS and Ovacancy sites arehe active centers for SH adsorbed on Cu2O(1 1 1) surfaces.

.2.2. Atomic S adsorptionIn the case of atomic S adsorbed on perfect surface, the config-

rations of atomic S adsorbed at OSUF, CuCUS, CuCSA and OSUB sitesre optimized but atomic S adsorbed all bounds to three Cu atoms,.e., one CuCUS and two CuCSA, as seen in Fig. 2(c), one CuCUS S bondength is 2.155 A, other two CuCSA S bond lengths are 2.408 A. Inhis study, the site consisting of one CuCUS and two CuCSA is defineds “3Cu” site, which reflects the important contribution from theearby cations. The adsorption energy is 286.6 kJ mol−1. In the casef Ovacancy site, the vacancy site is filled with atomic S, as shownn Fig. 2(d), the adsorption energy is 481.8 kJ mol−1. As a result,tomic S adsorbed at 3Cu and Ovacancy sites are the most stable con-gurations on perfect and oxygen-vacancy surfaces, respectively,uggesting that 3Cu and Ovacancy sites are the active centers fortomic S adsorbed on Cu2O(1 1 1) surfaces.

.2.3. Atomic H adsorptionIn the case of atomic H adsorbed on perfect surface, the

tomic H initially lies above CuCSA site, the configuration ishen optimized but the H still bounds to CuCUS site. In thease of Ovacancy site, the vacancy site is filled with atomic H,s presented in Fig. 2(e). The strengths of atomic H adsorbedt five types of adsorption sites could be assigned in theollowing order, Hads Ovacancy (553.1 kJ mol−1) > Hads OSUF462.4 kJ mol−1) > Hads CuCUS Hads CuCSA (454.0 kJ mol−1) >ads OSUB (426.2 kJ mol−1), which indicates that Ovacancy, OSUF anduCUS sites are the active centers for H adsorption on Cu2O(1 1 1)urfaces [41].

.3. Adsorption and dissociation of H2S on perfect surface

.3.1. H2S adsorptionFor H2S adsorption, we consider two modes with different ori-

ntations of H2S, S-down and H-down; S-down mode is that Htom of H2S against the surface, and S atom of H2S interacts with

he surface; H-down mode is that H atom of H2S toward the sur-ace. Four adsorption sites over perfect surface are still considered.ig. 3 presents the geometrical parameters describing the adsorp-ion structures.

For the S-down modes, the optimized structure of H2S adsorbedat CuCUS site is shown in Fig. 3(a), the plane of H2S molecule is nearlyparallel to the surface, two H S bonds of H2S (1.364 and 1.366 A)are stretched compared to that in free H2S molecule (1.356 A),which can also contribute to the dissociation of H2S. One CuCUS Sbond (2.298 A) is formed; this chemisorption of H2S has an adsorp-tion energy of 86.4 kJ mol−1. Our vibration frequencies show thatthe bands appear at 2514 and 2494 cm−1 for adsorbed H2S, dueto the anti-symmetric and symmetric S H stretching vibrationsof H2S, and 1177 cm−1 is contributed to the S H bending modes.Meanwhile, our calculated bands in free H2S molecule are seenat 2624 and 2607 cm−1 due to the anti-symmetric and symmet-ric S H stretching vibrations, as well as that at 1206 cm−1 forS H bending modes. Thus, the anti-symmetric and symmetric S Hstretching frequencies of H2S are softened noticeably upon adsorp-tion. Interestingly, the bending frequency is essentially unchangedupon adsorption. The studies by Jiang and Carter [60] show thesame results about the trend of S H frequency change for H2Sadsorption on Fe(1 0 0) surface. In addition, the red-shift of theS H stretching frequencies indicates that the intensity of S H bonddecreases, these results are in line with the results obtained fromthe S H bond length. In the case of H2S adsorbed at OSUB and CuCSAsites, H2S is inclined to keep away from the surface after optimiza-tion, as shown in Fig. 3(b) and (c), the geometrical structures of H2Sare almost unchanged upon adsorption. However, H2S adsorbed atOSUF site is converted to that at CuCUS site, as presented in Fig. 3(a).

In the case of H-down modes, H2S adsorbed at CuCUS site isconverted to the corresponding S-down model after optimiza-tion. Then, the optimized results show that H2S adsorbed at CuCSAand OSUB sites is inclined to keep away from the surface. For H2Sadsorbed at OSUF site, there is a hydrogen bond interaction betweenHa atom of H2S and lattice OSUF, the mutual distance between OSUFand Ha of H2S is 2.162 A, the angle of SHaO is 177.8◦, as presentedin Fig. 3(d), the adsorption energy of H2S is 15.0 kJ mol−1, whichis typical of physisorption. The bands at 2586 and 2467 cm−1 areattributable to the anti-symmetric and symmetric S H stretchingvibrations, as well as that at 1215 cm−1 for S H bending modes.

As a result, two kinds of interactions for H2S on perfect surfaceexist, one is chemisorption of H2S at CuCUS site via the lone pairelectrons of S atom to the surface Cu atom, the other is physisorp-tion of H2S at OSUF site via a weak hydrogen bond between H andsurface O atom. The chemisorption of H2S at CuCUS site is the moststable configuration, CuCUS is the favorable site for H2S adsorption.

3.3.2. H2S dissociationIn order to locate the minimum energy path and determine

accurate activation barrier of H2S dissociation, complete LST/QSTmethod is chosen to search for the transition states of reactions [61].The LST/QST method requires that the initial and final states of thetransition be known beforehand. In addition, frequency analysis has

9936 R. Zhang et al. / Applied Surface Science 258 (2012) 9932– 9943

F 2O(1 1a lor in

bTc

tca[fSrs

sftawcptTbHtcHbtTtHwgw9

3

eodesatH

atrm

ig. 3. Optimized geometries of adsorbed H2S species at various sites on perfect Cure in A. See Figs. 1 and 2 for color coding. (For interpretation of the references to co

een used to validate the optimized transition state structures, andS confirmation is performed on every transition state structure toonfirm that they lead to the desired reactants and products.

In this section, we will focus on the characteristics ofhe minimum energy path and the dissociation mechanism ofhemisorption H2S on perfect surface. The dissociation mech-nism studied here involves sequential dissociation of H2S60,62–65]. This sequential S H dissociation gives rise to sur-ace SH and S species on surface (i.e. H2S(ads) → H(ads) + SH(ads) andH(ads) → H(ads) + S(ads)). Then, the thermochemistry, activation bar-ier for each elementary step, and its dissociation products areystematically investigated.

Structures of initial states (ISs), transition states (TSs), finaltates (FSs), and the potential energy profiles are illustrated in Fig. 4or H2S dissociation. In the first dissociation step of H2S, we choosehe most stable site as the initial state (IS1), the CuCUS site of H2Sdsorbed on perfect surface shown in Fig. 3(a). For the final state,e select a configuration FS1 presented in Fig. 4 for the first disso-

iation step of H2S, because we know that both HS and H speciesrefer the CuCUS and OSUF sites. We explore this path from the ini-ial state IS1 to the final state FS1, and obtain a transition stateS1 along this path. One can see that H2S breaks one Ha S bondy tilting one Ha atom toward a nearby surface O atom and thea S bond dissociates over CuCUS atom, the main component of

he reaction coordinate is the elongation of Ha S bond; the disso-iation barrier is 52.4 kJ mol−1, indicating the ease of breaking thea S bond on Cu2O(1 1 1) surface. In TS1, the dissociating Ha Sond distance is about 1.623 A, which is only 0.257 A longer thanhat in IS1 (1.366 A). The S CuCUS bond length is about 2.283 A. ForS1, only an imaginary frequency of 1241i cm−1 corresponding tohe Ha S stretching vibration is observed. After TS1, the dissociateda atom breaks away and moves to an adjacent surface OSUF siteith an energy drop of 74.8 kJ mol−1. In FS1, the Ha S axis of HaS

roup is nearly parallel to the surface, which adsorbed at CuCUS siteith the S CuCUS distance of 2.196 A and the CuCUS S Ha angle of

7.3◦, the distance between S and Ha atoms is 2.852 A.

.3.3. HS dissociationIn this investigation of how HS dissociates on perfect surface, the

ffect of the neighboring H atom (produced from H2S dissociation)n the dissociation of HS is considered, because we noted that theiffusion barrier for atomic H on Cu2O(1 1 1) surface is very high, forxample, the diffusion barrier for atomic H from CuCUS site to OSUFite is calculated to be 244.3 kJ mol−1, suggesting that adsorbed Htom cannot diffuse away promptly. Therefore, FS1 is selected ashe initial state of HS dissociation in the second dissociation step of2S.

Starting from FS1, a final state (FS2) with S and Hb atoms

dsorbed at neighboring 3Cu and OSUF sites is chosen accordingo the facts that both S and H atoms prefer 3Cu and OSUF sites,espectively. As shown in Fig. 4, like H2S dissociation, HbS frag-ent in FS1 also breaks its Hb S bond by tilting the Hb atom toward

1) surface, (a) CuCUS site, (b) OSUB site, (c) CuCSA site, and (d) OSUF site. Bond lengths this figure legend, the reader is referred to the web version of the article.)

another surface O atom. This step goes through a direct Hb S bondcleavage of HbS via the transition state TS2. At TS2, only an imagi-nary frequency of 1452i cm−1 corresponding to the Hb S stretchingvibration exists. The Hb S bond is elongated from 1.362 A in FS1 to1.700 A in TS2, and 3.324 A in FS2. The activation barrier for thiselementary reaction is 82.7 kJ mol−1, similar to the magnitude ofthe barrier for the first dissociation step of H2S. Meanwhile, thisreaction is found to be slightly endothermic by 13.8 kJ mol−1.

On the other hand, to gain more insight into the H S bond-breaking process, we also examine the dissociation of single HSgroup adsorbed on perfect surface, as shown in Fig. 2(a), which ischosen as the initial state IS2. Starting from IS2, through the tran-sition states TS3, the H S bond cleavage of HS group can lead tothe final state FS3, as presented in Fig. 4. In TS3, the activated H Sbonds are elongated to 1.992 A from 1.366 A in IS2. TS3 has onlyan imaginary frequency of 280i cm−1 corresponding to the H Sstretching vibration. After TS3, the dissociated S atom moves to anearby 3Cu site, and H atom moves to its adjacent surface OSUF site.This elementary step has an activation barrier of 106.9 kJ mol−1,and is found to be largely endothermic by 35.9 kJ mol−1.

3.3.4. Brief summaryFollowing the potential energy profiles of the pathways from

H2S or single SH species to final products H and S atoms, as pre-sented in Fig. 4, we can clearly see that the dissociation step of SHgroup occurs at a relatively high activation barrier in comparisonwith the first dissociation step of H2S leading to SH species; how-ever, with respect to molecular adsorption H2S (IS1), the highestbarrier and reaction energy are only 60.3 and −8.6 kJ mol−1 for H2Sdissociation into S and H species, indicating the ease of breakingthe H S bond of H2S to form S species on perfect surface. Mean-while, the dissociation step of HS produced from H2S dissociation inthe presence of neighboring H atom is more preferable both kinet-ically and thermodynamically than the dissociation step of singleSH group adsorbed on perfect surface, suggesting that the presenceof H atom (produced from H2S dissociation) on perfect surface isin favor of the HS dissociation. Finally, the overall dissociation pro-cess of molecular adsorption H2S on perfect surface is found to beexothermic.

3.4. Adsorption and dissociation of H2S on oxygen-vacancysurfaces

Defects play an important role in catalyzing various surfacereactions [66–69]. We used oxygen-vacancy surface to model H2Sadsorption and dissociation on surface defects. The adsorption ofH2S on the oxygen-vacancy Cu2O(1 1 1) surface is at a coverage of1/4 ML with one molecule in every (2 × 2) unit cell. Admittedly,

the geometric structures of Cu2O(1 1 1)-(2 × 2) with 25% oxygen-vacancies (1/4 of OSUF atoms are missing) are a large number ofvacancies and may not depict reality, but the intention here isto obtain only a qualitative understanding of the oxygen-vacancy

R. Zhang et al. / Applied Surface Science 258 (2012) 9932– 9943 9937

F SH groi e Figsl

sbavtCt

3

cvwtoLt

fhaHH

ig. 4. Potential energy profiles for the dissociations of H2S (black line) and single

nitial states (IS), transition states (TS) and final states (FS). Bond lengths are in A. Seegend, the reader is referred to the web version of the article.)

urface. Meanwhile, above results show that CuCUS is the most sta-le adsorption site. Herein, only two different kinds of catalyticctive sites subjected to the adsorption of H2S on the oxygen-acancy surface, CuCUS and Ovacancy sites, are investigated. Owingo the fact that the Cu atoms nearby the Ovacancy site (i.e., Cu1, Cu2,u3 and Cu4 atoms remarked in Fig. 1(b)) are all singly coordination,hese Cu atoms belong to the same kind.

.4.1. H2S adsorptionAs shown in Fig. 5, the adsorption of the H2S has two possible

onfigurations: one is that H2S species bind to the surface eitheria the S or H atom, where H2S moiety forms a V-shape structureith S atom close to the surface; the other is that H2S species bind

o the surface via both H and S atoms in �2 S �1 H configurationr S atom interacting with the surface, where H2S moiety forms a-shape structure, and one H S bond of H2S is almost parallel tohe surface.

As a result, for the adsorption of H2S on oxygen-vacancy sur-ace, as presented in Fig. 5, nine types of initial adsorption modes

ave been examined: (1) H2S lies over CuCUS site with S atom in

V-shape structure; (2) H2S lies over CuCUS site with H atom; (3)2S lies over Ovacancy site with S atom in a V-shape structure; (4)2S lies over Ovacancy site with H atom; (5) H2S lies flatly over

up (red line) at CuCUS site on perfect surface in together with the structures of the. 1 and 2 for color coding. (For interpretation of the references to color in this figure

Cu2 Cu4 bridge with S atom in a V-shape structure. (6) H2S liesover Ovacancy site in �2 S �1 H configuration of L-shape; (7) H2Slies over Ovacancy site with S atom in a L-shape structure; (8) H2Slies flatly over Cu1 Ovacancy bridge in �2 S �1 H configuration ofL-shape; (9) H2S lies flatly over Cu1 Ovacancy bridge with S atomin a L-shape structure; the optimized adsorption configurations forabove nine types of initial adsorption modes are shown in Fig. 6.

Our results show that the initial adsorption modes of M(1),M(2) and M(3) in Fig. 5 are all converted to the optimized M(a)structure after optimization, as shown in Fig. 6(a). In M(a), H2Sadsorbed at CuCUS site with one CuCUS S bond (2.311 A) formed;the plane of H2S molecule is nearly parallel to the surface, and theadsorption energy is 83.8 kJ mol−1, which is typical of chemisorp-tion. Both H S bonds of H2S (1.363 and 1.368 A) are stretchedcompared to that in free H2S molecule (1.356 A), which can con-tribute to the dissociation of H2S. Our calculated frequencies forM(a) show that bands for molecularly adsorbed H2S are seen at2537 and 2477 cm−1 due to the anti-symmetric and symmetric S Hstretching vibrations of H2S, at 1178 cm−1 for S H bending modes.

Similar to perfect surface, the anti-symmetric and symmetric S Hstretching frequencies of H2S are softened noticeably upon adsorp-tion. Interestingly, the bending frequency is essentially unchangedupon adsorption.

9938 R. Zhang et al. / Applied Surface Sci

Fig. 5. Initial possible adsorption modes of H2S adsorbed on oxygen-vacancyCu O(1 1 1) surface. See Figs. 1 and 2 for color coding. (For interpretation of thert

mabvHrF

Fr

2

eferences to color in this figure legend, the reader is referred to the web version ofhe article.)

Then, the initial adsorption modes of M(4) and M(6) are opti-ized to M(b) structure in Fig. 6(b), in M(b), both SHb group and Ha

tom are adsorbed at Cu2 Cu4 bridge with S atom and Cu3 CuCUSridge site, respectively; after optimization, M(5) mode is con-

erted to M(c) structure in Fig. 6(c), in M(c), both SHb group anda atom are adsorbed at Cu3 Cu4 bridge with S atom and 3Cu site,

espectively; for M(7) mode, it is optimized to M(d) structure inig. 6(d), in M(d), both SHb group and Ha atom are adsorbed at

ig. 6. Optimized geometries of H2S adsorbed on oxygen-vacancy Cu2O(1 1 1) surface. Beferences to color in this figure legend, the reader is referred to the web version of the a

ence 258 (2012) 9932– 9943

Ovacancy site with S atom and Cu1 Cu4 bridge site, respectively;The initial M(8) mode is optimized to M(e) structure in Fig. 6(e), inM(e), both SHb group and Ha atom are adsorbed at Ovacancy site withS atom and Cu1 site, respectively; for M(9) mode, it is optimized toM(f) structure in Fig. 6(f), in M(f), both SHb group and Ha atom areadsorbed at Cu1 Cu4 bridge with S atom and Ovacancy site, respec-tively. As a result, we can clearly see from the optimized M(b)–M(f)structures that the rather long distances between Ha and S atom(2.836, 3.050, 3.113, 3.507 and 2.970 A, respectively) indicate thatthe Ha S bond of H2S has been completely broken, suggesting thatthe dissociation of H2S for the initial M(4)–M(9) modes is sponta-neous, which are typical of dissociative adsorption modes. In otherwords, no activation barrier is required for the dissociation of H2Sinto H atom and HS group on oxygen-vacancy surface. In fact, theoxygen-vacancy of Cu2O(1 1 1) surface will inevitably be formedunder a realistic reaction environment, which means that the dis-sociative adsorption of H2S is the main dissociation pathway of H2Son Cu2O(1 1 1) surface in experiment.

3.4.2. H2S dissociationAs mentioned above, when the dissociative adsorption of H2S

for M(4)–M(9) modes exist as a main pathway of H2S dissociationwithout any activation barrier required, a small quantity of molec-ular adsorption H2S dissociation at CuCUS site also occurs due to theobvious elongation of the H S bond. However, the H S bond hasnot been broken, the dissociation processes of molecular adsorp-tion H2S virtually need to overcome an activation barrier, thus, wefurther investigate the dissociation of molecular adsorption H2S atCuCUS site, M(a), as shown in Fig. 6(a), which is employed as theinitial state (IS3) for the first dissociation step of H2S leading to SHand H species. The potential energy profiles of H2S dissociation intogether with the corresponding structures are presented in Fig. 7.

Starting from IS3, the Ha S bond cleavage of H2S can lead tothe formation of the co-adsorbed SHb and Ha (FS4) through a tran-

sition state TS4. In FS4, Ha is located at Ovacancy site; SHb group isadsorbed at Cu1 Cu4 bridge site with S atom through two S Cubonds (2.222 and 2.425 A). In this step, the distances of Ha S isextended to 1.746 A in TS4, 3.009 A in FS4 from 1.368 A in IS3. TS4

ond lengths are in A. See Figs. 1 and 2 for color coding. (For interpretation of therticle.)

R. Zhang et al. / Applied Surface Science 258 (2012) 9932– 9943 9939

F e) andt statest n of th

hHtHf7

3

sfiHocn

3iosa

bwl

ig. 7. Potential energy profiles for the dissociations of H2S at CuCUS site (black linogether with the structures of the initial states (IS), transition states (TS) and final

he references to color in this figure legend, the reader is referred to the web versio

as only an imaginary frequency of 751i cm−1 corresponding to thea S stretching vibration. The activation barrier for this elemen-

ary step is only 53.4 kJ mol−1, indicating the ease of breaking thea S bond of molecular adsorption H2S on oxygen-vacancy sur-

ace. Moreover, this reaction is found to be highly exothermic by7.7 kJ mol−1.

.4.3. HS dissociationIn this investigation of how HS dissociates on oxygen-vacancy

urface, three types of HS dissociation are discussed in detail. Werst consider the effect of the neighboring H atom (produced from2S dissociation) on the dissociation of HS; then, the dissociationf single adsorbed SH group is also compared; finally, the disso-iation of SH from H2S dissociative adsorption in the presence ofeighboring H atom is further investigated.

.4.3.1. SH from H2S dissociation. Similar to perfect surface, follow-ng the products from the first dissociation step of H2S dissociationn oxygen-vacancy surface, FS4 in Fig. 7 is selected as the initialtate of HS dissociation to consider the effect of the neighboring Htom on the second dissociation step of H2S.

Beginning with FS4, like H2S dissociation, SHb breaks its Hb Sond also by tilting the Hb atom toward a surface oxygen atom,hich can directly form FS5, as presented in Fig. 7. In FS5, Hb is

ocated at the surface OSUF site, S atom is adsorbed at 3Cu site, as

single adsorbed SH group at Ovacancy site (red line) on oxygen-vacancy surface in (FS). Bond lengths are in A. See Figs. 1 and 2 for color coding. (For interpretation ofe article.)

already mentioned in Section 3.2, they are the most stable configu-rations of H and O atoms on Cu2O(1 1 1) surface, respectively. Thisstep goes through a direct Hb S bond cleavage via the transitionstate TS5. The Hb S bond is elongated from 1.362 A in FS4 to 1.635 Ain TS5, and 3.299 A in FS5. For TS5, only an imaginary frequencyof 1268i cm−1 corresponding to the Hb S stretching vibration isobserved. This elementary reaction has a high activation barrier of92.0 kJ mol−1, and is endothermic by 13.1 kJ mol−1. The magnitudeof activation barrier for this step is similar to the first dissociationstep of H2S.

3.4.3.2. SH of single adsorption. As shown in Fig. 2(b), single SHgroup with S-down mode adsorbed at Ovacancy site is the most stableconfiguration on oxygen-vacancy surface, which is chosen as theinitial state IS2 to obtain more insight into the H S bond-breakingprocess. Starting from IS4, through the transition states TS6, theH S bond cleavage of HS group can lead to the final state FS6, aspresented in Fig. 7. In TS6, the activated H S bond is elongated to2.350 A from 1.363 A in IS4. TS6 has only an imaginary frequencyof 576i cm−1 corresponding to the H S stretching vibration. AfterTS6, the dissociated S atom is still adsorbed at Ovacancy site, and H

atom moves to its adjacent CuCUS site. This elementary reaction hasa high activation barrier of 176.3 kJ mol−1, and is highly exothermicby 77.9 kJ mol−1. Then, as already mentioned before, H at OSUF siteis the most stable configuration on Cu2O(1 1 1) surface, so following

9 ce Sci

FtaHtttn

3tlrcnSntwsH

bbpaMisBtCtarooaaSrspa

3

swscatsfStraCdsOiar

940 R. Zhang et al. / Applied Surfa

S6, in the presence of S atom, H atom is transferred from CuCUS siteo surface OSUF site via a transition state TS7, leading to FS7. Onlyn imaginary frequency of 556i cm−1 for TS7 corresponding to the

CuCUS stretching vibration is obtained. The activation barrier forhis elementary reaction is 244.3 kJ mol−1, and the reaction is foundo be slightly endothermic by 5.0 kJ mol−1. Above results mean thathe hydrogen atom diffusion on surface is difficult to occur, thiseeds to overcome a significantly high activation barrier.

.4.3.3. SH of dissociative adsorption. As mentioned in Section 3.4.1,he dissociative adsorptions of H2S for M(4)–M(9) modes canead to the formation of M(b)–M(f) without any activation bar-ier required, subsequently, adsorbed SHb in M(b)–M(f) structuresan also further dissociate into S and H atoms in the presence ofeighboring H atom. As a result, we investigate the dissociation ofH (produced from H2S dissociative adsorption) in the presence ofeighboring H atom. Fig. 8 presents the potential energy profile ofhe pathways from H2S to final products H and S atoms in togetherith the corresponding transition states and products. Every tran-

ition state has only an imaginary frequency corresponding to theb S stretching vibration.

As shown in this figure, for SH dissociations of M(b)–M(e), SHbreaks its Hb S bond also by tilting the Hb atom toward a neigh-oring CuCUS atom; they can directly form the final state, FS8, asresented in Fig. 8. In FS8, Hb is located at the surface CuCUS site, Stom is adsorbed at Ovacancy site. In the case of SH dissociations for(b) and M(c), SHb is adsorbed at Cu Cu bridge site, with respect to

ts initial state, the activation barriers of SHb dissociations via tran-ition states TSb and TSc are 100.3 and 94.5 kJ mol−1, respectively.oth reactions are exothermic by 58.3 and 54.8 kJ mol−1, respec-ively. Meanwhile, the dissociation of SHb for M(f), SHb adsorbed atu Cu bridge site, can form the final state FS5 by tilting the Hb atomoward a neighboring surface oxygen atom; the correspondingctivation barrier and reaction energy is 109.9 and 12.6 kJ mol−1,espectively. However, it is interesting to note that the dissociationsf SHb adsorbed at Ovacancy site in M(d) and M(e), the dissociationsf SH b via transition states TSd and TSe have significantly highctivation barriers of 183.8 and 201.2 kJ mol−1, respectively, whichre found to be exothermic by 65.9 and 53.3 kJ mol−1, respectively.ubsequently, as mentioned already before, a very high barrier cor-esponds to the diffusion of H from the CuCUS site in FS8 to theurface oxygen site in the presence of S atom, producing similarroducts as in the case of single SH group adsorbed at Ovacancy sites,s illustrated in Fig. 7.

.4.4. Brief summaryFor the adsorption and dissociation of H2S on oxygen-vacancy

urface, the dissociative adsorption is the main dissociation path-ay of H2S, which suggests that the oxygen vacancy on Cu2O(1 1 1)

urface exhibits a strong chemical reactivity toward the disso-iation of H2S. Meanwhile, only a small quantity of moleculardsorption H2S exists at CuCUS site. On the other hand, followinghe potential energy profile of the pathways from H2S or single SHpecies to final products H and S atoms on oxygen-vacancy sur-ace, as presented in Figs. 7 and 8, we can clearly see that whenHb is adsorbed at Ovacancy site in M(d), M(e) and IS4 configura-ions, the dissociation of SHb occurs at a significantly high barrierelative to that of SHb in M(b), M(c), M(f) and FS4, where SHb isdsorbed at Cu Cu bridge site. This means that SH adsorbed atu Cu bridge site has a more weakened S H bond already beforeissociation. Thus, it requires less energy to reach the transitiontate, leading to a lower barrier, suggesting that SHb adsorbed at

vacancy site cannot easily dissociate into S and H atoms in compar-

son with that of SHb adsorbed at Cu Cu bridge site. As a result, SHbdsorbed at Cu Cu bridge site in M(b), M(c), M(f) and FS4 configu-ations is dominantly responsible for the dissociation of SH to form

ence 258 (2012) 9932– 9943

S atom on oxygen-vacancy surface, and the corresponding activa-tion barrier (100.3 and 94.5 kJ mol−1) seems to be readily overcomeunder the experimental conditions associated with pressures andtemperatures according to the experiments [15]. In addition, thedissociative adsorption is exothermic, which can also contributeto the reactive energy compensation. Therefore, it is reasonable toconclude that the formation of S species on Cu2O(1 1 1) surface isfacile.

3.5. Adsorption and dissociation of H2S on S Cu2O(1 1 1) surface

Under a realistic reaction condition, the oxygen-vacancy ofCu2O(1 1 1) surface will inevitably be formed, meanwhile, as men-tioned above, if the dissociation of H2S on this surface producesS atom, subsequently, the S atom will preferably adsorbed at theOvacancy site leading to the sulfur-containing Cu2O(1 1 1) surfacewith lattice O of Cu2O replaced by S, as shown in Fig. 2(d), similar toperfect surface, we denoted this kind of sulfur-containing surface as“S Cu2O(1 1 1)” surface. Therefore, to probe into the effect of S atomover Cu2O(1 1 1) surface on H S bond-breaking process, we furtherexamine the adsorption and dissociation of H2S on S Cu2O(1 1 1)surface.

3.5.1. H2S adsorptionFor H2S adsorption, we still consider two modes with differ-

ent orientations of H2S, S-down and H-down; different sites onS Cu2O(1 1 1) surface have been considered. As shown in Fig. 9,IS5, the chemisorption of H2S at CuCUS site, is the most stable con-figuration with an adsorption energy of 80.5 kJ mol−1. The planeof H2S molecule is nearly parallel to the surface, the bands appearat 2492 and 2476 cm−1 due to the anti-symmetric and symmet-ric S H stretching vibrations of H2S, and the band at 1173 cm−1

is contributed to the S H bending mode. Then, this structure isselected as the initial state (IS5) for our study of H2S adsorptionand dissociation on S Cu2O(1 1 1) surface.

3.5.2. HS adsorptionFor HS adsorption, we still consider two modes with dif-

ferent orientations of HS, S-down and H-down; different siteson S Cu2O(1 1 1) surface have been considered. Our calculatedresults show that SH with S-down adsorbed at CuCUS Cu4 bridgesite is the most stable configuration, two Cu S bonds (2.209and 2.556 A) are formed, the corresponding adsorption energyis 312.5 kJ mol−1. Meanwhile, our calculated bands at 2539 and635 cm−1 are attributable to the S H stretching and S H bendingmodes of adsorbed SH species, respectively.

3.5.3. H2S dissociationFor H2S dissociation on S Cu2O(1 1 1) surface, we consider two

types of dissociations to illustrate the effect of surface S atom onH2S dissociation, one is the S Ha bond cleavage of H2S with theinteraction of surface O, the other is the S Hb bond cleavage of H2Swith the interaction of surface S, Fig. 9 shows a potential energydiagram of the first dissociation step for molecular adsorption H2S,which leads to the product SH species on S Cu2O(1 1 1) surfaces.

Starting from IS5, we first investigate the S Ha bond cleavageof H2S on S Cu2O(1 1 1) surface, the S Ha bond cleavage can formthe coadsorbed SH and H species (FS9) through a transition stateTS8. In FS9, Ha is located at surface oxygen atom, SHb binds thesubstrate through one Cu S (2.206 A) bond, as shown in Fig. 9. Asalready mentioned above, they are the most stable configurations ofH atom and SH species on Cu2O(1 1 1) surface, respectively. In this

step, the main component of the reaction coordinate is elongationof the S Ha bond; this bond is extended to 1.585 A in TS8, 2.900 Ain FS9 from 1.366 A in IS5. The activation barrier for this elementaryreaction is 42.0 kJ mol−1, and the reaction is found to be exothermic

R. Zhang et al. / Applied Surface Science 258 (2012) 9932– 9943 9941

F ve adsa he iniS is figu

bos

atbaSfac

3

oShcfacac

ig. 8. Potential energy profiles for the dissociations of SH produced from dissociatind M(f) (blue line) on oxygen-vacancy surface in together with the structures of tee Figs. 1 and 2 for color coding. (For interpretation of the references to color in th

y 19.2 kJ mol−1. Above results show that the S Ha bond cleavagef H2S with the interaction of surface O on S Cu2O(1 1 1) surface isimilar to that on perfect surface.

On the other hand, beginning with IS5, the S Hb bond cleav-ge of H2S on S Cu2O(1 1 1) surface can lead to the formation ofwo SH groups (FS10) through a transition state TS9. The distanceetween S and Hb is elongated from 1.367 A in IS5 to 2.116 A in TS9,nd 3.766 A in FS10. In FS10, SHa binds at Cu Cu bridge site, andpHb binds upright at Ovacancy site. With respect to IS5, this step isound to be endothermic by 40.3 kJ mol−1, with a significantly highctivation barrier of 130.5 kJ mol−1 relative to that of S Ha bondleavage.

.5.4. Brief summaryAs shown in Fig. 9, following the potential energy diagram

f the first dissociation step for molecular adsorption H2S on Cu2O(1 1 1) surface, with respect to IS5 in a black solid line, theighest barrier and reaction energy of the initial step for S Ha bondleavage are 42.0 and −19.2 kJ mol−1, respectively, whereas, thoseor the initial step for S Hb bond cleavage in a red line are 130.5

nd 40.3 kJ mol−1, respectively, which suggests that the S Ha bondleavage of H2S with the interaction of surface O is more prefer-ble both kinetically and thermodynamically than the S Hb bondleavage of H2S with the interaction of surface S. Our results mean

orption in M(b) (magenta line), M(c) (black line), M(d) (red line), M(e) (white line),tial states (M(n)), transition states (TS) and final states (FS). Bond lengths are in A.re legend, the reader is referred to the web version of the article.)

that when surface O atom of Cu2O(1 1 1) is replaced by S atom, thenumber of surface oxygen, namely, the preferable adsorption site ofH atom, decreases, which goes against the H S bond-breaking pro-cess of H2S. Therefore, we conclude that with the S atom increasingon surface along with the dissociation reaction, Cu2O may lose thecatalytic activity for H2S dissociation.

3.6. General discussion

Minimum energy pathways and energy profiles for the (i)decomposition of H2S into adsorbed SH and H species, and (ii) thedecomposition of SH into adsorbed S and H species have been deter-mined. The predicted pathway for H2S dissociation process canbe approximately divided into two types. One is the dissociationof molecular adsorption H2S on both perfect and oxygen-vacancysurfaces, the other is that of dissociative adsorption H2S on oxygen-vacancy surface. Fig. 10 only shows a simplified potential energydiagram relative to molecular adsorption H2S by only includingthe highest barrier of H2S dissociation into S and H species onperfect and oxygen-vacancy Cu2O(1 1 1) surfaces. As shown in the

figure, in the view of thermodynamics, the overall dissociationprocess from the adsorption of H2S to final product S species onCu2O(1 1 1) surfaces is found to be exothermic. In addition, ther-modynamics in previous studies [70] have shown that at room

9942 R. Zhang et al. / Applied Surface Science 258 (2012) 9932– 9943

F e) andi al stato sion o

tsnr6vcisticfqo

Fosapr

ig. 9. Schematic potential energy diagrams for the initial Sa H cleavage (black linn together with the structures of the initial states (IS), transition states (TS) and finf the references to color in this figure legend, the reader is referred to the web ver

emperature, the reactions of Cu2O with H2S to form the metalulfide have negative free-energy changes and should be sponta-eous. Kinetically, on perfect surface, the activation barrier andeaction energy, with respect to molecular adsorption H2S, are0.3 and −8.6 kJ mol−1, respectively, whereas those on oxygen-acancy are 53.4 and −64.6 kJ mol−1, respectively, indicating H2San easily dissociate into S species on these two surfaces. Moremportantly, in the case of dissociative adsorption of H2S, the dis-ociation of H2S into S species is a spontaneous process with respecto molecular adsorption H2S, because the dissociative adsorptions exothermic, which can also contribute to the reactive energyompensation. Consequently, S species can be easily formed on per-

ect and oxygen-vacancy Cu2O(1 1 1) surfaces. Such result agreesuite well with experimental findings of Lin et al. [15] for H2Sn Cu2O(1 1 1) surface, which found by means of variable-energy

ig. 10. Schematic potential energy diagrams for H2S dissociation to S and H speciesn perfect surface (blue line), oxygen-vacancy surface (black line denotes the dis-ociation of molecular adsorption H2S, and red line represents the dissociativedsorption, respectively). Only transition state (TS) with the highest barrier for eachath is shown. (For interpretation of the references to color in this figure legend, theeader is referred to the web version of the article.)

Sb H cleavage (red line) of H2S on S Cu2O(1 1 1) surface with S atom at Ovacancy

es (FS). Bond lengths are in A. See Figs. 1 and 2 for color coding. (For interpretationf the article.)

photoelectron spectroscopy that H2S, at low temperature and lowcoverages (0–0.3 ML), completely deprotonated into S species.

In addition to above mechanistic studies of H2S adsorption anddissociation, we also obtain the vibrational frequencies for theadsorbed H2S and SH species on Cu2O(1 1 1) surfaces, as summa-rized in Table 2, suggesting that the anti-symmetric and symmetricS H stretching frequencies of H2S are softened noticeably uponadsorption. Interestingly, the bending frequency of H2S is essen-tially unchanged upon adsorption. Meanwhile, S H stretchingfrequencies of HS also decrease obviously upon adsorption, and thebending frequency of adsorbed HS species relative to gaseous SHradical is observed. Since the adsorbed sulfur atoms are stronglybound to the three surface Cu atom, predicted vibrational frequen-cies (341 cm−1) for the surface-S stretching mode may be closeto that of a sulfur atom solely adsorbed at the three-fold site ofCu(1 1 1) (322 cm−1) [71]. As a result, our calculated vibration fre-

quencies can be applied to guide surface vibrational spectroscopyin experiment. However, up to now, it is a pity that no experi-mental results on Cu2O(1 1 1) surfaces are available for comparison

Table 2GGA-BLYP results for gaseous H2S: asymmetric stretching frequency (�asym), sym-metric stretching frequency (�sym), bending frequency (�bend), together with thosefor gaseous HS: stretching frequency (�asym), bending frequency (�bend).

Species Vibrational frequencies (cm−1)

�asym �sym �bend

H2S Perfect-CuCUS 2514 2494 1177Perfect-OSUF 2586 2467 1215Oxygen-vacancy 2537 2477 1178Sulfur-containing 2492 2476 1173Gaseous 2624 2607 1206

HS Perfect 2508 650Oxygen-vacancy 2498 646Sulfur-containingGaseous 2576

ce Sci

wosHc

4

feopb

mofotcaSmowfapbfa

A

dNa

R

[

[

[

[[

[

[[

[

[[

[

[[[[[[[[[[[

[[[

[

[

[[

[

[

[

[

[[

[

[

[[[

[

[[

[

[

[

[

[[

[[[[[[[[[

[Clausen, J.K. Norskov, F. Besenbacher, Nature Materials 4 (2005) 160–162.

R. Zhang et al. / Applied Surfa

ith our calculated S H vibration frequencies to confirm the typesf H2S adsorption and the resultant SH and S species. We expectome experiments should be performed to determine the types of2S adsorption and its SH and S products for confirmation of ouralculation in our future work.

. Conclusions

The interactions of H2S with Cu2O(1 1 1) surfaces, including per-ect, oxygen-vacancy and sulfur-containing surfaces, have beenxamined by periodic DFT calculations to elucidate the mechanismf H2S adsorption and dissociation on these surfaces. Two reactionathways via molecular and dissociative adsorption are studiedased on energetically favorable species.

Our results show that sulfur-containing species (H2S, SH and S)ainly interact with surface Cu centers; H2S can exist in the form

f molecular adsorption on both perfect and sulfur-containing sur-aces; the dissociative adsorption of H2S occurs predominantly onxygen-vacancy surface, which exhibits a strong catalytic activityoward the dissociation of H2S. Meanwhile, the dissociation pro-esses of the molecular and dissociative adsorption H2S on perfectnd oxygen-vacancy Cu2O(1 1 1) surfaces, leading to final product

species, show that the overall dissociation process is exother-ic. Kinetically, S species can be easily formed on perfect and

xygen-vacancy Cu2O(1 1 1) surfaces, such result agrees quite wellith previous experimental findings for H2S on Cu2O(1 1 1) sur-

ace. However, on sulfur-containing surface, namely, the surface Otom of Cu2O(1 1 1) replaced by S atom decreases the number ofreferable adsorption site of H atom, which goes against the H Sond-breaking process of H2S. Finally, the vibrational frequenciesor the adsorbed H2S, SH and S species on these surfaces can bepplied to guide surface vibrational spectroscopy in experiment.

cknowledgment

This works is supported by the National Natural Science Foun-ation of China (20906066, 20976115 and 21103120), the Nationalatural Science Foundation of Shanxi Province (no. 2009021015),nd Doctoral Fund of Ministry of Education (no. 20091402110013).

eferences

[1] J.A. Rodriguez, A. Maiti, Journal of Physical Chemistry B 104 (2000) 3630–3638.[2] J.A. Rodriguez, J. Hrbek, Accounts of Chemical Research 32 (1999) 719–728.[3] J.M. Thomas, W. Thomas, J. Principles, Practice of Heterogeneous Catalysis, VCH,

New York, 1997.[4] J. Oudar, H. Wise (Eds.), Deactivation and Poisoning of Catalysts, Dekker, New

York, 1985.[5] H. Topsøe, B.S. Clausen, F.E. Massoth, Hydrotreating Catalysis, Springer-Verlag,

New York, 1996.[6] G.P. Vissokov, Catalysis Today 89 (2004) 213–221.[7] G.P. Vissokov, Catalysis Today 89 (2004) 223–231.[8] T. Schedel-Niedrig, T. Neisius, I. Böttger, E. Kitzelmann, G. Weinberg, D. Demuth,

R. Schlögl, Physical Chemistry Chemical Physics 2 (2000) 2407–2417.[9] T. Schedel-Niedrig, M. Hävecker, A. Knop-Gericke, R. Schlögl, Physical Chem-

istry Chemical Physics 2 (2000) 3473–3481.10] T. Jirsak, J. Dvorak, J.A. Rodriguez, Journal of Physical Chemistry B 103 (1999)

5550–5559.11] J.A. Rodriguez, T. Jirsak, S. Chaturvedi, M. Kuhn, Surface Science 442 (1999)

400–412.12] S. Chaturvedi, J.A. Rodriguez, T. Jirsak, J. Hrbek, Journal of Physical Chemistry B

102 (1998) 7033–7043.13] Q. Wu, B.V. Yakshinskiy, T.E. Madey, Surface Science 523 (2003) 1–11.14] T. Kyotani, H. Kawashima, A. Tomita, Environmental Science and Technology

23 (1989) 218–223.

15] J. Lin, J.A. May, S.V. Didziulis, E.I. Solomon, Journal of the American Chemical

Society 114 (1992) 4718–4727.16] N. Haimour, R. EI-Bishtawi, A. Ail-Wahbi, Desalination 181 (2005) 145–152.17] J. Abbasian, R.B. Slimane, Industrial and Engineering Chemistry Research 37

(1998) 2775–2782.

[

[

ence 258 (2012) 9932– 9943 9943

18] T.V. Reshetenko, S.R. Khairulin, Z.R. Ismagilov, V.V. Kuznetsov, InternationalJournal of Hydrogen Energy 27 (2002) 387–394.

19] A. Galtayries, J.P. Bonnelle, Surface and Interface Analysis 23 (1995) 171–179.20] J.A. Rodriguez, S. Chaturvedi, M. Kuhn, J. Hrbek, Journal of Physical Chemistry

B 102 (1998) 5511–5519.21] J.A. Rodriguez, T. Jirsak, S. Chaturvedi, Journal of Chemical Physics 111 (1999)

8077–8087.22] D.F. Cox, K.H. Schulz, Surface Science 256 (1991) 67–76.23] K.H. Schulz, D.F. Cox, Physical Review B 43 (1991) 1610–1621.24] K.H. Schulz, D.F. Cox, Surface Science 278 (1992) 9–18.25] K.H. Schulz, D.F. Cox, Surface Science 262 (1992) 318–334.26] K.H. Schulz, D.F. Cox, Journal of Physical Chemistry 97 (1993) 647–655.27] K.H. Schulz, D.F. Cox, Journal of Physical Chemistry 96 (1992) 7394–7698.28] A. Martinez-Ruiz, M. Moreno, N. Takeuchi, Solid State Science 5 (2003) 291–295.29] A. Önsten, M. Göthelid, U.O. Karlsson, Surface Science 603 (2009) 257–264.30] Y.X. Pan, C.J. Liu, Q.F. Ge, Journal of Catalysis 272 (2010) 227–234.31] R.T. Carr, M. Neurock, E. Iglesia, Journal of Catalysis 278 (2011) 78–93.32] Z.P. Liu, P.J. Hu, Journal of the American Chemical Society 125 (2003)

1958–1967.33] D.H. Mei, M. Neurock, C.M. Smith, Journal of Catalysis 268 (2009) 181–195.34] Y.H. Chin, C. Buda, M. Neurock, E. Iglesia, Journal of Catalysis 283 (2011) 10–24.35] C.F. Huo, Y.W. Li, J.G. Wang, H.J. Jiao, Journal of the American Chemical Society

131 (2009) 14713–14721.36] M. Pozzo, D. Alfè, International Journal of Hydrogen Energy 34 (2009)

1922–1930.37] M. Casarin, C. Maccato, N. Vigato, A. Vittadini, Applied Surface Science 142

(1999) 164–168.38] S. Yin, X.Y. Ma, D.E. Ellis, Surface Science 601 (2007) 2426–2437.39] J.A. Rodriguez, J.C. Hanson, A.I. Frenkel, J.Y. Kim, M. Pérez, Journal of the Amer-

ican Chemical Society 124 (2002) 346–354.40] R.G. Zhang, H.Y. Liu, H.Y. Zheng, L.X. Ling, Z. Li, B.J. Wang, Applied Surface Science

257 (2011) 4787–4794.41] R.G. Zhang, B.J. Wang, L.X. Ling, H.Y. Liu, W. Huang, Applied Surface Science 257

(2010) 1175–1180.42] M.M. Islam, B. Diawara, V. Maurice, P. Marcus, Journal of Molecular Structure

(THEOCHEM) 903 (2009) 41–48.43] A. Soon, M. Todorova, B. Delley, C. Stampfl, Physical Review B 75 (2007) 125420-

1–125420-9.44] C. Li, F. Wang, S.F. Li, Q. Sun, Y. Jia, Physics Letters A 374 (2010) 2994–2998.45] B.Z. Sun, W.K. Chen, J.D. Zheng, C.H. Lu, Applied Surface Science 255 (2008)

3141–3148.46] W.K. Chen, B.Z. Sun, X. Wang, C.H. Lu, Journal of Theoretical and Computational

Chemistry 7 (2008) 263–276.47] B.Z. Sun, W.K. Chen, X. Wang, C.H. Lu, Applied Surface Science 253 (2007)

7501–7505.48] A.D. Becke, Journal of Chemical Physics 88 (1988) 2547–2553.49] C. Lee, W. Yang, R.G. Parr, Physical Review B 37 (1988) 785–789.50] M. Dolg, U. Wedig, H. Stoll, H. Preuss, Journal of Chemical Physics 86 (1987)

866–872.51] A. Bergner, M. Dolg, W. Kuechle, H. Stoll, H. Preuss, Molecular Physics 80 (1993)

1431–1441.52] Y. Inada, H. Orita, Journal of Computational Chemistry 29 (2008) 225–232.53] R. Restori, D. Schwarzenbach, Acta Crystallographica. Section B, Structural Sci-

ence 42 (1986) 201–208.54] M.A. Nygren, L.G.M. Pettersson, Journal of Physical Chemistry 100 (1996)

1874–1878.55] Y.M. Choi, C. Compson, M.C. Lin, M. Liu, Chemical Physics Letters 421 (2006)

179–183.56] G. Herzberg, Molecular Spectra and Molecular Structure Electronic Spectra and

Electronic Structure of Polyatomic Molecules, vol. III, Krieger, Malabar, FL, 1966.57] K.P. Huber, G. Herzberg, Molecular Spectra and Molecular Structure Constants

of Diatomic Molecules, vol. IV, Van Nostrand, New York, 1979.58] S.H.S. Wilson, J.D. Howe, M.N.R. Ashfold, Molecular Physics 88 (1996) 841–858.59] R.E. Continetti, B.A. Balko, Y.T. Lee, Chemical Physics Letters 182 (1991)

400–405.60] D.E. Jiang, E.A. Carter, Journal of Physical Chemistry B 108 (2004) 19140–19145.61] T.A. Halgren, W.N. Lipscomb, Chemical Physics Letters 49 (1977) 225–232.62] D.R. Alfonso, A.V. Cugini, D.C. Sorescu, Catalysis Today 99 (2005) 315–322.63] R. Dominic, R. Alfonso, Surface Science 602 (2008) 2758–2768.64] D.E. Jiang, E.A. Carter, Surface Science 583 (2005) 60–68.65] E.J. Albenze, A. Shamsi, Surface Science 600 (2006) 3202–3216.66] T. Zambelli, J. Wintterlin, J. Trost, G. Ertl, Science 273 (1996) 1688–1690.67] Y. Huang, Z.X. Chen, Journal of Physical Chemistry C 115 (2011) 18752–18760.68] B.L.M. Hendriksen, M.D. Ackermann, R. van Rijn, D. Stoltz, I. Popa, O. Balmes,

A. Resta, D. Wermeille, R. Felici, S. Ferrer, J.W.M. Frenken, Nature Chemistry 2(2010) 730–734.

69] R.T. Vang, K. Honkala, S. Dahl, E.K. Vestergaard, J. Schnadt, E. Laegsgaard, B.S.

70] D.D. Wagman, W.H. Evans, V.B. Parker, R.H. Schumm, I. Halow, S.M. Bailey,K.L. Churney, R.L. Nuttall, Journal of Physical and Chemical Reference Data 11(Suppl. 2) (1982).

71] M. May, S. Gonzalez, F. Illas, Surface Science 602 (2008) 906–913.