**3. Photocatalytic hydrogen sulfide decomposition by gas phase reaction**

Jardim et al. studied the gas phase destruction of H2S with a low concentration range of hundreds of ppm using TiO2 as the catalyst and black light lamp as the light source [19]. In the existence of oxygen and water vapor, H2S could be effectively decomposed (about 99% efficiency) and the main product is determined as SO4 2–. The deactivation of TiO2 would happen with a H2S concentration larger than 600 ppm, and it was mainly caused by the adsorption of by-product on its surface. No elemental S was detected by the color change of TiO2 from white to yellow, and hydrogen evolution was not considered in this study. Notably, if oxygen is absent in the system, H2S could be barely removed.

In a similar experiment, with the assistance of in situ FT-IR, Anderson et al. confirmed that no other gaseous products like SO2 or SO, and SO2− adsorbed on TiO2 may be one intermediate during the "eight electron transfer" process [20]. Furthermore, Sano et al. have found that the photodeposition of Ag on TiO2 would promote the adsorption of H2S on the sample, possibly due to the partially oxidized silver surface, and the deposited Ag could act as a cocatalyst for removal of H2S. Both factors made Ag-deposited TiO2 more efficient for H2S degradation [21].

In addition, Sánchez et al. have tried glass "Raschig" ring, poly(ethylene terephthalate) (PET), and cellulose acetate (CA) as the supports to load TiO2 for photocatalytic treatment of H2S gas [22]. Glass rings supported TiO2 (which has underwent fire treatment) outperforms PET and CA supported TiO2. For PET and CA supports with low temperature treatment, PET supports displayed the higher photocatalytic activity, and TiO2 caused the degradation of CA supports under illumination. Different from reports before, although SO4 2– is one main product of H2S removal, SO2 was detected from these systems.

The interaction of H2S with the semiconductor surfaces has also been investigated. Two adsorption modes of H2S with high defect density rutile TiO2 (110) surfaces were suggested: dissociative adsorption with both H and S atom attached to the Ti atom at low H2S concen‐ tration and molecular adsorption at high H2S concentration [23]. Moreover, the preadsorption of H2S would significantly block O2 adsorption on TiO2 surfaces even in the presence of large Ti3+ cations. Using Langmuir isotherm, Sopyan further discovered that H2S adsorbed more strongly on rutile (0.7 molecules / nm2 ) rather than anatase (0.4 molecules/nm2 ). This is in sharp contrast with other molecules like acetaldehyde and ammonia [24]. Consequently, photoca‐ talytic activity of anatase film is only 1.5 times higher than that of rutile for degradation of H2S.

evolution to oxidation of S2−. Therefore, for a semiconductor qualified for H2S decomposition,

the valance band edge only needs to be more positive than the redox potential of H2S/S2− (0.14 V vs NHE at pH 0). This means that for semiconductors that are capable of water splitting are all qualified for H2S decomposition. Besides, for some semiconductor, even if they may be not proper for water splitting due to the less positive valance band edge, they still have the potential for H2S decomposition. One example is silicon. As seen from Figure 2, the valence band edge of silicon is far more negative than the redox potential of H2O/O2, which determines its inability for oxygen evolution. Nevertheless, it could be used in the system of H2S decom‐

Like water splitting could occur in both gas phase (water vapor) and liquid phase, H2S, as an acid gas, could be decomposed in gas phase directly and disposed in liquid phase indirectly after being absorbed by solution. Moreover, here we will have a review of these two cases,

**3. Photocatalytic hydrogen sulfide decomposition by gas phase reaction**

Jardim et al. studied the gas phase destruction of H2S with a low concentration range of hundreds of ppm using TiO2 as the catalyst and black light lamp as the light source [19]. In the existence of oxygen and water vapor, H2S could be effectively decomposed (about 99%

happen with a H2S concentration larger than 600 ppm, and it was mainly caused by the adsorption of by-product on its surface. No elemental S was detected by the color change of TiO2 from white to yellow, and hydrogen evolution was not considered in this study. Notably,

In a similar experiment, with the assistance of in situ FT-IR, Anderson et al. confirmed that no other gaseous products like SO2 or SO, and SO2− adsorbed on TiO2 may be one intermediate during the "eight electron transfer" process [20]. Furthermore, Sano et al. have found that the photodeposition of Ag on TiO2 would promote the adsorption of H2S on the sample, possibly due to the partially oxidized silver surface, and the deposited Ag could act as a cocatalyst for removal of H2S. Both factors made Ag-deposited TiO2 more efficient for H2S degradation [21]. In addition, Sánchez et al. have tried glass "Raschig" ring, poly(ethylene terephthalate) (PET), and cellulose acetate (CA) as the supports to load TiO2 for photocatalytic treatment of H2S gas [22]. Glass rings supported TiO2 (which has underwent fire treatment) outperforms PET and CA supported TiO2. For PET and CA supports with low temperature treatment, PET supports displayed the higher photocatalytic activity, and TiO2 caused the degradation of CA supports

The interaction of H2S with the semiconductor surfaces has also been investigated. Two adsorption modes of H2S with high defect density rutile TiO2 (110) surfaces were suggested:

efficiency) and the main product is determined as SO4

274 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

if oxygen is absent in the system, H2S could be barely removed.

under illumination. Different from reports before, although SO4

removal, SO2 was detected from these systems.

/H2, but

2–. The deactivation of TiO2 would

2– is one main product of H2S

the conduction band edge should still be more negative than the redox potential of H+

position (see below).

respectively.

In all the above systems, H2S in gas phase is studied within low concentration (tens to hundreds of ppm) and people mainly concerns with the oxidation product of H2S. Little attention is paid to the reductive reaction of H2S. Nevertheless, the reduction of H2S (which is often the conversion of H2S into H2) is more attractive from an energy point of view.

Early in 1990s, Naman has combined thermal and photocalytic decomposition of H2S together and studied the influence of light influx on the thermal decomposition of H2S by VxSy on different substrates (TiO2, Al2O3, and ZnO) [25]. Under light irradiation, the conversion of H2S to H2 was increased by 27.6%, 44.6%, and 16.5% at 500°C, respectively. The Arrhenius activa‐ tion energy for H2S decomposition has also calculated to be 50% of that in darkness. The author tentatively attributes this photoactivation effect on thermal decomposition to the photoexci‐ tation of semiconductors (including VxSy) and the subsequent generated charge carriers.

In 2008, Li et al. have compared the activity of five typical semiconductors TiO2, CdS, ZnS, ZnO, and ZnIn2S4 for the direct decomposition of H2S in gaseous phase [26]. With illumination of Xe lamp and Pt loading (0.2 wt%), the efficiency of the decomposition of 5% H2S in argon decreases as a sequence of ZnS > TiO2 > ZnIn2S4 > ZnS > CdS under the gas flow rate of 6 ± 0.5 mL/min. Various noble metal loadings on ZnS have been compared, and it turns out that Ir is superior than others (Pd, Pt, Ru, Rh, and Au), which improves the hydrogen evolution efficiency from 1.2 to 4.5 μmol/h. Doping ZnS was also carried out, and transition metal Cu2+ doping (0.5% mol) could greatly promote the decomposition process and improve efficiency of the hydrogen evolution by about 20 times in contrast to blank ZnS. In addition, the absorp‐ tion edge of ZnS shift from 400 to 450 nm after Cu doping, and this contributes to a photoca‐ talytic H2 production rate of 17 μmol/h under visible light irradiation (λ > 420 nm). Similarly, one limitation of this research is that only the reduction product, H2, is detected in the system and the oxidative products are ignored.

Although systematic experimental studies of the photocatalytic decomposition of H2S in gaseous phase are scarce, thermodynamic analysis of solar-based photocatalytic H2S decom‐ position has recently been reported, which may be instructive for further studies on experi‐ ments [27]. Analysis indicates that energy efficiency of this process is not significantly affected by the intensity of solar irradiation. Exergy efficiency (the second law efficiency) will decrease with the increase of solar intensity, while the hydrogen yield will increase. Although the exergy efficiency value of current catalyst is calculated to be less than 1%, the author envisioned that an exergy efficiency of 10% could be achieved in the near future, and a maximum exergy efficiency of 27% may be obtained for a chemical conversion ratio of 0.6 if close to optimum cases of the quantum efficiency and the catalyst band gap can be obtained.
