Chemical Physics Letters 557 (2013) 106–109

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Chemical Physics Letters

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Computational prediction of hydrogen sulfide and methane separationat room temperature by anatase titanium dioxide

Chenghua Sun ⇑ARC Centre of Excellence for Functional Nanomaterials, Australia Institute for Bioengineering and Nanotechnology, The University of Queensland, QLD 4072, AustraliaCentre for Computational Molecular Science, Australia Institute for Bioengineering and Nanotechnology, The University of Queensland, QLD 4072, Australia

a r t i c l e i n f o a b s t r a c t

Article history:Received 30 October 2012In final form 7 December 2012Available online 19 December 2012

0009-2614/$ - see front matter � 2012 Elsevier B.V. Ahttp://dx.doi.org/10.1016/j.cplett.2012.12.010

⇑ Address: ARC Centre of Excellence for FunctioInstitute for Bioengineering and Nanotechnology, ThQLD 4072, Australia.

E-mail address: c.sun1@uq.edu.au

Removal of hydrogen sulfide (H2S) is a key step for biogas purification. Herein, the adsorption of H2S andmethane (CH4) on anatase titanium dioxide (TiO2) has been studied by first principle calculations. It isfound that TiO2 offers excellent capacity for the H2S/CH4 separation. Using force-field molecular dynamics,this high separation capacity has been examined at room temperature.

� 2012 Elsevier B.V. All rights reserved.

1. Introduction

Biogas, generated from the anaerobic digestion of biologicalwastes, has been viewed as a promising renewable energy source[1,2]. Generally the major compositions of biogas are methane(CH4) and carbon dioxide (CO2), together with small amount ofimpurities, such as hydrogen sulfide (H2S), moisture, nitrogen oxi-des, volatile organic compounds, etc. [1]. Among those impurities,H2S is the most problematic contaminant due to its high toxicityand corrosivity. Therefore, efficient removal of H2S is critical forthe large-scaled applications of biogas.

In the industry, the technology most commonly employed forH2S removal involves the use of active media, such as iron oxides(Fe2O3 and Fe3O4), zinc oxide (ZnO), alkali solution, etc. [3,4].Although such treatment is simple and effective, the removal isactually based on the consumption of the media. Using Fe2O3 asan example, Fe2O3 + 3H2S?Fe2S3 + 3H2O; as a result, the used med-ia needs to be renewed and safely disposed from time to time, whichgets rise to additional cost and environmental concerns [5]. Anotherwell-established technique is based on the Claus process, H2S + 1/2O + 2?S + H2O [6]. Apparently, this process recovers elementarysulfur and no additional solid media has to be sacrificed, while theenvironmental issues associated with its by-products, like SOx, areseriously concerned [7]. To address the environmental problems,it is desirable to develop clean technologies for H2S-removal.

Photo-induced H2S splitting, H2S?S + H2, is attractive, becauseit not only recovers elementary sulfur as Claus process does, butalso generates hydrogen fuel with sunlight as the only energy input.Therefore, photocatalytic decomposition H2S over semiconductors

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nal Nanomaterials, Australiae University of Queensland,

has been proposed as an option to produce solar-hydrogen [8].Using CdS-based photocatalysts, H2S-splitting has been realizedeven under visible light [9], which can harvest more sunlight andthus reduce the purification cost. To make this feasible for biogaspurification, it is essential to ensure that H2S can be efficiently cap-tured by photocatalysts, which underlines the importance of theseparation of H2S. Under this context, we investigated the adsorp-tion of H2S on anatase titanium dioxide (TiO2), one of the mostlywidely employed photocatalysts. The initial motivation is to inves-tigate whether H2S can be efficiently separately from CH4 andstrongly captured by TiO2, which is the basis for photocatalyticH2S-splitting. As shown below, anatase TiO2 is an excellent mediafor H2S/CH4 separation, especially when they are dominated byminority surface (001). The photocatalytic splitting of H2S overTiO2 photocatalysts is out of the scope of this letter, but will bestudied experimentally in the future. In nature, anatase TiO2 crys-tals are dominated by the majority surface (101) [10], but if specialcontrolling agents (e.g., fluorine acid) are employed, well-definedcrystals with high percentage of minority surface (001) can be syn-thesized readily [11–14].

2. Computational methods

In nature, anatase TiO2 crystals are dominated by the majoritysurface (101) [10], but if special controlling agents (e.g., fluorineacid) are employed, well-defined crystals with high percentage ofminority surface (001) can be synthesized readily [11–14]. In thisletter, both (101) and (001) have been investigated and modeledby slab models, as shown in Figure 1.

The adsorption of H2S and CH4 on (001) and (101) surfaces hasbeen studied by density functional theory (DFT) [15]. The calcula-tions were carried out using DMol3 package [16,17]. The exchangeand correlation terms were treated under the generalized gradient

Figure 1. Optimized geometries for (a) CH4 and (b) H2S on TiO2(101), (c) CH4 and(d) H2S on TiO2(001).

C. Sun / Chemical Physics Letters 557 (2013) 106–109 107

approximation (GGA) functional by Perdew et al. [18]. A doublenumerical quality basis set with polarization function (DNP) wereutilized for all geometric optimization and total energy calcula-tions [19]. K-space was sampled by the gamma point due to thelarge size of the supercells. Along the z-direction, a vacuum spaceof 20 Å was employed to avoid the interaction between neighbor-ing images. The gas–TiO2 interaction was described by the aver-aged adsorption energy, Eads, which is defined by

Eads ¼ EðgasÞ þ EðTiO2Þ � Eðgas—TiO2Þ

where E(gas), E(TiO2) and E(gas–TiO2) are total energies of singlegas molecule, clean TiO2 slab and the interacting gas–TiO2 system.By this definition, positive Eads indicates that the adsorption isstable.

Force-field molecular dynamics (FFMD) has been further em-ployed to simulate the separation of H2S from CH4 based on peri-odical boundary models, as shown in the Support information(Figure S1). FFMD was performed in the NVT ensemble at298.0 K, with a timestep of 1.0 fs. The overall simulation time isup to 400 ps (1 ps = 10�12 s), which is long enough to show the sep-aration of H2S as shown below. The random velocities were gener-ated from the Boltzmann distribution and the temperature wasmaintained using the Andersen method [20] with a collision ratioof 1.00. The gas–gas and gas–TiO2 interactions were described bythe COMPASS force field [21].

3. Results and discussion

Figure 1 shows the optimized geometries of H2S and CH4 over(001) and (101) surfaces. As indicated by the adsorption energies(Eads), CH4 can only weakly adsorb on both (001) and (101) withEads = 0.02 and 0.03 eV, respectively, which is typical physisorp-tion, being line with previous report [22]; however, H2S can bestrongly captured by anatase TiO2, with Eads = 0.56 eV via molecu-lar adsorption on (101), agreeing well with the report by Lin and co-workers (Eads = 0.49 eV) [23]. With respect to TiO2(101), (001) ismore reactive and Eads is up to 1.43 eV. H2S actually spontaneously

dissociates on TiO2(001). On (101), there is a barrier of 0.37 eV forthe dissociation of H2S [23]. In fact, similar difference of the adsorp-tion capacity between (101) and (001) has been well known forwater on TiO2 [10,24–26]. From the optimized geometries (seeFigure S2 in the Support information), it is found that both five-coor-dinated titanium (Ti5c) and two-coordinated oxygen (O2c) are in-volved in H2S dissociation on TiO2(001); moreover, short Ti5c–O2c

distance plays a critical role for the formation of H–O2c hydrogenbonds (HBs). Based on the above calculated data, it appears thatTiO2 surfaces, especially minority surface (001), can offer strongcapacity to separate H2S from CH4 effectively.

Experimentally, the adsorption of H2S on TiO2 has been studiedfor many years [27–29]. Using temperature-programmed desorp-tion, both molecular and dissociative adsorptions have been re-ported over rutile surfaces [30,31], and the dissociation isbelieved to be resulted by defects, like oxygen vacancies [30]. Overanatase TiO2, however, only molecular adsorption has been identi-fied [30]. It should be clear that typical anatase TiO2 crystals aredominated by the majority surface (101). As shown in Figure 1b,H2S does adsorb molecularly on (101). While in the case of anataseTiO2(001), which is the minority surface with a low percentage inmost TiO2 samples and has been rarely studied before, spontane-ous dissociation of H2S is obtainable even there is no defect, as pre-dicted in Figure 1d. This may introduce new reaction routes for thedegradation of H2S over anatase TiO2. For instance, HSads generatedby H2S dissociation may be attacked by photo-induced holes, andform S� radicals via HSads + h+?HS�+ads ?S�ads + H+, with an analogyto the general reaction pathways proposed by Portela et al. [28]. S�

radicals may further react with other S� radicals to form elementarysulfur or be directly oxidized by adsorbed O2 or lattice oxygen toform SO2. The above speculations is beyond the scope of this work,but will be investigated in our following work.

Giving that gas adsorption and desorption on surfaces arestrongly affected by gas–gas and gas–TiO2 collisions, it is essentialto take more gas molecules involved in the simulation, which canbe explored by large-scaled FFMD simulation. At the initial state,72 H2S molecules are homogeneously mixed with 288 CH4 mole-cules and 6 H2S molecules adsorb on TiO2 based on energy minimi-zation, as shown in Figure 2a. Only after 50 ps, half of the H2Smolecules are captured by TiO2 (see Figure S3 in the Support infor-mation) and 220 ps later all H2S molecules are separated from CH4

and strongly adsorb on TiO2, as shown in Figure 2b. In the follow-ing 180 ps (t = 220–400 ps), no H2S desorption has been observed,indicating H2S molecules are strongly captured by TiO2 surface,being consistent with the prediction by DFT calculations.

To minimize the cost for renewing the TiO2 media, it is expectedthat TiO2 surfaces can keep the high working efficiency even whenthey are partially covered by H2S molecules. As observed in FFMD,the adsorption rate of H2S has no notable change when more andmore H2S molecules adsorb on TiO2 surfaces. For instance,there are 162 Ti5c-sites on the two surfaces of the slab shown inFigure 2, and at t = 210 ps, 70 sites have been occupied by H2Sand all the left by CH4; however, the adsorption of the last twoH2S molecules only takes 10 ps. Such stable performance has beencontributed by two factors: (i) CH4 desorption occurs almost at thesame rate with its adsorption, obviously due to its weak adsorption(Eads = 0.03 eV, comparable to the thermal energy at room temper-ature 0.026 eV), which is critically important since the adsorptionsites occupied by CH4 can be quickly released for H2S adsorption;and (ii) H2S molecules adsorbed on TiO2 surface are linked withH2S molecules in the H2S/CH4 stream by HBs (similar with theHB network in water) and thus promote the separation of H2S fromCH4. In fact, almost all H2S molecules can be captured by TiO2 sur-faces immediately when the H2S–TiO2 distance is smaller than3.50 Å (Ti–S bond: 2.46 Å) based on the snapshots collected in thissimulation.

Figure 2. FFMD snapshots at (a) t = 0 ps and (b) t = 220 ps. Ti and O are presented as big spheres, and S (yellow), C (gray) and H (white) are shown as small balls.

Figure 3. Concentration of H2S versus simulation time.

108 C. Sun / Chemical Physics Letters 557 (2013) 106–109

In the above simulations, H2S dissociation has not been consid-ered due to the limitation of FFMD, which in principle can be im-proved by first principle MD (FPMD), but FPMD simulations aretoo expensive for large systems. Given H2S dissociation offersstronger adsorption than physisorption described by FFMD, theseparation efficiency from FPMD should be better than that pre-sented above. In addition, H2S keeps intact, the adsorption energyon (001) is close to that on (101); thus, the above separationcapacity predicted from (001) can be expected from (101) as wellbecause (101) also shows strong capacity to capture H2S as indi-cated by Eads = 0.56 eV, although (001)-dominated TiO2 is better(Eads = 1.43 eV).

For raw biogas, the content of H2S varies from 100 to10000 ppm (10000 ppm = 1%) [1,2], depending on the feedstock,which is much smaller than the initial concentration employedin the above simulation (200000 ppm). For lower concentrationsof H2S, the probabilities for H2S colliding with TiO2 are less. In prin-ciple, the rate of adsorption (per unit area of surface, indicated asRads) can be expressed as a product of the incident molecular fluxF and the sticking probability S, as Rads = S � F, in unit of m�2

s�1[32]. Considering there is no barrier for H2S adsorption, andonce H2S adsorbs the desorption is very difficult at room tempera-ture given the barrier is up to 1.43 eV for H2S desorption under theDFT scheme. In fact, among the 400000 FFMD images collected inthis simulation, only one image shows H2S desorption, which isobtained with a desorption barrier smaller than real case consider-ing the dissociative adsorption is not taking into account in FFMDsimulations. Therefore, the sticking probability is assumed to beclose to 1, meaning that once H2S reaches TiO2 surface, it will beundoubtedly captured; as a result, Rads is mainly determined bythe incident molecular flux F, and F can be expressed by [30],

F ¼ p=ð2pm0kbTÞ1=2 ð1Þ

where p is the partial pressure of the objective molecule in the gasstream, m0 is the molecular mass, T is the temperature and kb is theBoltzmann constant. In Eq. (1), the effect of initial H2S concentrationis reflected by the partial pressure p(H2S), and thus F is a linear func-tion of p(H2S) for fixed T. In other words, when the exposed surfacearea and the operation temperature are fixed, the halftime for theH2S separation from H2S/CH4 stream is a constant, which is the typ-ical feature of first-order reactions. Based on FFMD snapshots, theconcentration of H2S versus time has been shown in Figure 3, inwhich the first-order kinetics has been vividly presented with half-time t1/2 = 54 ps. A linear fitting to generate the halftime has beenshown in Figure S4 in the Support information. From Figure 3, it isclear that the separation capacity achieved at high concentration

has no obvious decrement even when the concentration is reducedfrom 200000 ppm to 5000 ppm.

In the above FFMD simulation, H2S and CH4 molecules are con-fined in an ultra narrow space (around 1 nm), corresponding to aseparation environment with huge surface areas. For real separa-tion devices, however, the exposed surface areas may be muchsmaller, and thus the possibility for H2S to collide with TiO2 will re-duce dramatically. Given F in Eq. (1) defines how many moleculescan be captured by unit area of surface per unit time, the overalltime for H2S removal linearly increases with the decrement of sur-face areas exposed to TiO2 media. But even when the effectivesurface areas decrease by an order of 1012 (for instance, the sizeof cylindrical pores increases from 1 nm to 1 mm), the halftime isstill as small as seconds and it only takes a couple of minutes toreduce the concentration of H2S from 10000 ppm to 10 ppm.Certainly, to achieve ultrahigh separation efficiency, devices withmicrometer- or even nanometer-sized pores are preferred. For in-stance, for a separation device with 1.5 meter long, the pore sizedown to micrometers will be good enough to separate H2S mole-cules from a biogas stream with a velocity of 1.5 m/s.

4. Conclusions

In summary, the adsorption of H2S and CH4 on anatase TiO2 sur-faces has been investigated by DFT calculations. It is found that H2Scan be strongly captured by TiO2 while CH4 can only weakly ad-sorb. This adsorption difference allows the separation of H2S fromCH4 feasible, as confirmed by FFMD. It is expected that the pre-dicted separation capacity offered by TiO2 photocatalysts can beutilized in the H2S removal from biogas.

C. Sun / Chemical Physics Letters 557 (2013) 106–109 109

Acknowledgements

This project was supported by Australian Research Council(through its Centres of Excellence grant) and Queensland StateGovernment (Smart Future Fellowship for CS). CS also appreciatesthe generous grants of CPU time from both the University ofQueensland and the Australian National Computational Infrastruc-ture Facility.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.cplett.2012.12.010.

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