Optimization of the experimental conditions of hydrogen production by the Pt–(CdS/ZnS) system under visible light illumination

Abstract

The photocatalytic activity of a Pt–(CdS/ZnS) system towards hydrogen generation from water and in presence of a sacrificial organic is studied. Pt–(CdS/ZnS) was prepared by two different methods. In the first one the composite was synthesized by precipitation of CdS onto the ZnS powder and subsequent loading with Pt. In the second one it was prepared by simply sintering the mixture of CdS and ZnS powders, also followed by Pt loading. The photocatalysts were characterized by X-ray powder diffraction (XRD), SEM and TEM microscopy, UV-vis absorption spectroscopy and nitrogen adsorption–desorption (BET method). The effects of preparation method, calcination temperature, catalyst loading, initial acid formic concentration, and pH on the photocatalytic activity were studied. The reusability of the photocatalyst was also tested and the presence of Cd²⁺ and Zn²⁺ in solution was evaluated to verify the existence of catalyst photocorrosion. The results show a large difference in the hydrogen generation according to the preparation method used.

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1. Introduction

Nowadays, the growing concern about the excessive use of fossil fuels and their negative impact on the environment^1,2 makes necessary the development of new systems based on renewable energy sources that are environmentally friendly.³ In this sense, heterogeneous photocatalysis is a promising technique since it allows, by using an appropriate catalyst and sunlight, the production of hydrogen from water along with the consumption of sacrificial agents (organic pollutants that can be simultaneously removed from water).^4–6 This way of producing hydrogen by using water and sunlight as primary sources is a clear example of a sustainable technology that involves no polluting emissions.

TiO₂, the most studied material in heterogeneous photocatalysis, has an important drawback: it only absorbs photons below 400 nm (UV) and, consequently, visible photons, largely available at the earth surface, cannot be used to activate that catalyst.^7,8 Thus, in order to have a feasible technology the use of catalysts that absorb in the visible range is required. In this sense, CdS has been one of the most investigated catalysts because it has a suitable band gap energy (2.4 eV) and band positions that satisfy the requirements for simultaneous water reduction and sacrificial agent oxidation.^9–11 Also, ZnS has been extensively studied because its conduction band edge is by far more negative than the H⁺/H₂ redox potential, and its mixing with other photocatalysts improves H₂ evolution.^12,13 Although its band gap is in the ultraviolet range, it has been recently reported that coupling of ZnS with CdS to form Cd_1−xZn_xS solid mixtures greatly increases the photocatalytic efficiency of the CdS in the visible range.^14–16

In this work we report for the first time the synthesis of highly photocatalytically active (Zn_0.78Cd_0.22)S phases by precipitation of CdS onto ZnS, and their potential for hydrogen production by using formic acid as sacrificial agent, a molecule that has proven to be an effective electron donor in many photocatalytic processes.¹⁷ Compared with bare CdS this material turns out to be 9 times more efficient in terms of H₂ generation. Also, the effect of several experimental parameters, such as pH, concentration of sacrificial agent and catalyst loading were studied. Additionally, by detecting the presence of Cd and Zn in solution it was evidenced the existence of composite photocorrosion during irradiation, and its increase with higher initial concentrations of formic acid. Another important aspect evaluated was the simultaneous degradation of formic acid (a potential organic pollutant) during the hydrogen generation.

2. Experimental

2.1. Reagents

CdSO₄ (ACS reagent), ZnS (99.99%) and H₂PtCl₆ (99.995%) were provided by Sigma-Aldrich, (NH₄)₂S (pure) and formic acid (98%) were provided by Panreac, and ammonium citrate (≥99.0%) was provided by Fluka. High purity N₂ (99.999%) was used to provide an O₂ free atmosphere. Water used to prepare aqueous solutions was Milli-Q grade.

2.2. Synthesis of the Pt–(CdS/ZnS) system

2.2.1 1^st procedure (precipitation). CdS/ZnS was prepared by precipitation of CdS onto the commercial ZnS powder. In brief, 1.5 g of CdSO₄ were dissolved in 10 mL of water. Afterwards 1 g of ZnS was added to the solution and, finally, 4.9 mL of a 10% w/v (NH₄)₂S solution were added dropwise to the solution. The resulting mixture was kept under agitation during 24 h. Subsequently, the obtained precipitate was filtered, washed several times with Milli-Q water, and sintered in an oven at 700 °C for 30 min under N₂ flow.

2.2.2 2^nd procedure (mixing). 1 g of CdS previously prepared by precipitation and 1 g of ZnS were mixed, and the mixture was calcined in an oven at 700 °C following the same conditions used for the first composite.

The platinization of both CdS/ZnS composites was accomplished by dissolving 30 mg of H₂PtCl₆ in 120 mL of water, followed by addition of 30 mL of 1% ammonium citrate solution and 1 g of the CdS/ZnS material, and taking the mixture to reflux during 4 h. Finally, the platinized slurry was filtered and rinsed several times with Milli-Q water.¹⁸ With this procedure a Pt–(CdS/ZnS) system with a 2.3% Pt load was obtained. Additionally, in order to evaluate the role of ZnS in the photocatalytic activity of the system, another CdS/Pt–ZnS system was prepared by following the same precipitation described above but using a ZnS that was previously platinized.

2.3. Sample characterization

The crystalline structure of the as-synthesized catalysts were determined by X-ray powder diffraction using a Philips X-Pert diffractometer with a Cu Kα radiation source (λ = 1.54056 Å). X-ray diffraction data were collected in the 2θ range of 20–70° using a scan rate of 0.05° by 10 s. The morphologies of the samples were obtained by scanning electron microscopy on a JEOL JEM-6300 microscope equipped with an EDX spectrometer Zeiss evo MA10 and TEM images were taken on a JEOL JEM-1400 instrument. The Brunauer–Emmett–Teller (BET) surface areas were obtained by using a Micromeritics, ASAP 2020 V3H apparatus. The adsorption–desorption isotherms were evaluated at −196 °C after the samples were degassed at 120 °C for 10 h. Optical absorption of the catalysts was characterized by using a UV-Vis diffuse reflectance spectrophotometer (3600 Shimadzu). BaSO₄ was used as a reflectance standard. Determination of Cd²⁺ and Zn²⁺ in solution was carried out by mass spectroscopy with inductively coupled plasma (ICP-MS Agilent, model 7500ce). The point of zero charge (pzc) of the Pt–(CdS/ZnS) catalyst was determined experimentally: 0.1 g of composite were suspended in 50 mL water samples at different pH to determine the pH value that remained nearly constant after powder addition.

2.4. Photocatalytic setup

The main element of the experimental set up used for photocatalytic hydrogen generation was a double wall cylindrical Pyrex reactor of 200 mL volume, fitted with gas inlet and outlet, connected to a thermostatic bath, and placed on a magnetic stirrer. Four lamps (15 W visible compact fluorescent lamps) surrounding the reactor provided visible-light radiation, and a NaNO₂ 1 M solution was used as cutoff filter for wavelengths below 400 nm. As indicated in the next section, for some of the experiments two of the four lamps were replaced by 15 W UV compact fluorescent lamps and the NaNO₂ solution replaced by tap water in order to provide simultaneous UV-Vis light irradiation. In all experiments, 0.1 g of catalyst were added to 50 mL of an aqueous solution of formic acid or glycerol. The aqueous slurry was stirred in the dark for 30 min to establish the adsorption–desorption equilibrium and then the remaining gas phase volume (150 mL) was pumped out and refilled with pure N₂. After that, the lamps were turned on and the solution was then irradiated for 6 h. Hydrogen was analyzed by gas chromatography with a Shimadzu GC-2014 chromatograph equipped with a packed column (Carboxen 1000 stationary phase) and a TCD detector using pure N₂ as carrier gas. To evaluate the photostability of the Pt–(CdS/ZnS) sample after the first run (6 h photochemical reaction), the solid was recovered from the suspension, washed with distilled water and dried at room temperature. The recovered photocatalyst was then used into a second and a third run of the photoreaction under the same conditions and with new formic acid solution.

Reinecke's salt actinometry¹⁹ was performed to quantify the amount of photons entering the reactor when using the four visible lamps (5.3 × 10⁻⁷ Einstein s⁻¹). The incident power in the outer reactor surface could be calculated by taking into account the commercial lamp spectrum and by performing spreadsheet calculations based on the assumption of a hypothetical lamp power output that is converted in a number of photons. The assumed lamp power is changed in the spreadsheet until the calculated number of photons matches the actinometry outcome. In this way, and considering the reactor geometry, it was possible to estimate an incident power of 0.0031 W cm². The actinometry allowed the assessment of the photonic efficiency (φ), estimated by using the following equation:

3. Results and discussion

3.1. Photocatalyst characterization

The XRD patterns of CdS, ZnS and CdS/ZnS systems are illustrated in Fig. 1. The XRD patterns corresponding to commercial ZnS are shown in Fig. 1a, and the diffraction peaks located at 2θ of 28.5°, 33.0°, 47.4° and 56.3° can be indexed to the cubic phase (ICDD card no 05-0566). Fig. 1b displays the XRD pattern of CdS obtained by precipitation. The diffraction lines located at 2θ of 24.9°, 26.5°, 28.2°, 36.2°, 43.7°, 47.8° and 51.9° indicate the formation of the hexagonal phase (ICDD card no. 41-1049), that has been reported as the most active phase in photocatalysis.²⁰ The system prepared by mixing (Fig. 1c) shows diffraction lines that can be identified as the overlapping of the XRD diffractograms from both CdS and ZnS, although it also displays some of the well-resolved XRD peaks of ZnS or CdS. On the other hand, Fig. 1d shows that the diffraction pattern of the material prepared by precipitation is completely different from the XRD patterns of the starting compounds. This fact points towards the formation of a new compound with different phases. According to the database Powder Diffraction File version 2.2, the diffractogram obtained would correspond to a mixture of Zn_0.78Cd_0.22S (ICDD card no. 35-1469) and Zn_0.9Cd_0.1S (ICDD card no. 24-1137). Because of the lack of reliable pure standards it was not possible to assess the contribution of each phase to the final solid. On the other hand, platinization of the composites provided no appreciable changes to the diffractograms (data not shown) indicating that the presence of Pt has no influence on the crystallographic nature of the composites. Finally, an energy dispersive X-ray (EDX) spectrum of the Pt–(CdS/ZnS) clearly showed the presence of cadmium, zinc and sulfur elements in the composite.

Fig. 1 XRD patterns of photocatalyst samples: (a) ZnS, (b) CdS, (c) CdS/ZnS (mixing) and (d) CdS/ZnS (precipitation).

3.2. SEM and TEM analysis

SEM images of the catalysts are shown in Fig. 2. As can be seen in Fig. 2a the ZnS is formed by aggregates of smaller spherical particles with an approximate size of 1 μm, while Fig. 2b shows that CdS consists of an agglomeration of large particles along with smaller ones, average size ranging from 0.5 to 2 μm. However, in Fig. 2c, corresponding to CdS/ZnS, it is not possible to precisely determine the size of particles from SEM images because of the large agglomeration of particles observed. Likewise, in Fig. 2d (TEM image) it can be noticed that ZnS particles are covered with precipitated CdS.

Fig. 2 SEM image of: (a) ZnS, (b) CdS, (c) Pt–(CdS/ZnS) (precipitation) and TEM image of (d) Pt–(CdS/ZnS) (precipitation).

3.3. UV-visible diffuse reflectance spectra

Fig. 3 shows the UV-visible diffuse reflectance spectra of pure CdS and ZnS samples as well as CdS/ZnS systems. As can be seen the absorption edges of the composites are located between those of the ZnS and CdS, lying in the visible range. The band gap energies estimated from the absorption data by using the Tauc relationship (see Fig. S1 in the ESI†) are listed in Table 1. Clearly, gap values of 2.44 eV (CdS/ZnS by precipitation) and 2.56 eV (CdS/ZnS by mixing) confirm that composites with different properties that directly influence the performance towards hydrogen evolution can be obtained depending on the method of preparation.

Fig. 3 UV-vis diffuse reflectance spectra of: CdS, ZnS, CdS/ZnS (mixing), CdS/ZnS (precipitation) and CdS/Pt–ZnS.

Table 1 Band gap energy and surface area of CdS, ZnS, CdS/ZnS (precipitation), CdS/ZnS (mixing) and CdS/Pt–ZnS

Sample	Band gap energy (eV)	Surface area (m^2 g^−1)
CdS	2.23	0.3
ZnS	3.43	9.0
CdS/ZnS (precipitation)	2.44	2.1
CdS/ZnS (mixing)	2.56	—
CdS/Pt–ZnS	2.36	2.5

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3.4. BET surface area

Nitrogen adsorption–desorption isotherms of the starting materials (CdS and ZnS) and the different binary systems can be found in Fig. S2 (ESI†). The BET surface areas estimated from those isotherms are shown in Table 1. A typical III type adsorption isotherm was observed for all the samples, except for CdS. The isotherm of that material did not fit into any of the forms classified by IUPAC. Certainly, the small surface area of CdS seems to be related to the large size particles obtained, as evidenced in the SEM images (Fig. 2b).

3.5. Photocatalytic activity for H₂ production

3.5.1 Effect of preparation method. Fig. 4 shows the influence of the preparation method (precipitation or mixing) in the photocatalytic efficiency. Noticeably, the CdS/ZnS material prepared by precipitation shows a remarkably high production of hydrogen when compared to the material obtained by a simple sinterization of the CdS and ZnS precursors. This improvement is due to the formation of totally new phases with distinct optical and structural properties that are the result of a largest intimate contact between particles during the synthesis, as evidenced above. Based on those results, CdS/ZnS prepared by precipitation was selected for the following tests.

Fig. 4 Rate of hydrogen evolution as a function of irradiation time obtained with the CdS/ZnS photocatalysts. Reaction conditions: 0.1 g of catalyst, 50 mL of 10⁻³ M formic acid aqueous solution at pH 3.5.

The relation between calcination temperature and hydrogen generation is shown in Fig. 5. Clearly, the material obtained by precipitation only exhibits appreciable performance when it is calcined at high temperature. XRD patterns help to detect changes on the structure (see Fig. S3 on ESI†) that can account for the activity differences observed at different calcination temperature. When the composite is calcined at 300 °C only Zn_0.9Cd_0.1S and CdS cubic phases are generated, and they seem to be not very efficient in the generation of hydrogen because with such a mixture the poorest H₂ generation rates are obtained. While for the three temperatures tested a Zn_0.9Cd_0.1S phase was always detected, only when CdS/ZnS was calcined at 700 °C a Zn_0.78Cd_0.22S new phase was also formed. A change in the sample color is also indicative of the formation of a material with new properties. Since the calcination at 700 °C also gives the noticeable increase of photocatalytic activity, this should be related to the presence of the mixture of Zn_0.78Cd_0.22S and Zn_0.9Cd_0.1S phases.

Fig. 5 Rate of hydrogen evolution as a function of irradiation time obtained with the Pt–(CdS/ZnS) photocatalysts prepared at different calcinations temperatures. Reaction conditions: 0.1 g of catalyst, 50 mL of 10⁻³ M formic acid aqueous solution at pH 3.5.

The ZnS : CdS ratio is another parameter of the material preparation that can influence the photocatalytic activity. In agreement with the results reported by other groups,²¹ in the present study an optimal ZnS : CdS ratio of 1 : 1 has been found for hydrogen generation.

As can be seen in Fig. 6, the rate of hydrogen evolution when Pt–CdS is used as photocatalyst is rather low, and it is even worst when Pt–ZnS (irradiated with UV light) is used. However, when Pt–(CdS/ZnS) is illuminated with visible light, the rate of hydrogen evolution is largely enhanced, showing a good synergistic effect between both catalysts. This effect is even better when the catalyst was at the same time irradiated with ultraviolet and visible light. On the other hand, when CdS/Pt–ZnS was used, the observed performance surpasses the one obtained with Pt–CdS and Pt–ZnS, but it is still poor if compared with the Pt–(CdS/ZnS) performance. Based on these results, it seems clear that, although the conduction band of ZnS is more negative than the conduction band of CdS, the latter, being the catalyst that can be activated by visible light, is the responsible for reduction of H⁺ into H₂, while the ZnS probably works as hole scavenger and/or reduces CdS charge recombination.²² In any case the use of Pt deposits, or other noble metal as co-catalyst proves once more to be essential for a dramatic improvement of the photocatalytic generation of H₂. Although practical applications of noble metals at large scale are greatly limited by their high cost, many efforts are being recently made to develop low-cost and noble metal free co-catalysts for (Zn/Cd)S photocatalysts.^23–25

Fig. 6 Comparison of the photocatalytic hydrogen production rate with: (a) Pt–(CdS/ZnS) irradiated with mixed ultraviolet and visible light, (b) Pt–(CdS/ZnS) irradiated with visible light, (c) CdS/Pt–ZnS irradiated with visible light, (d) Pt–CdS irradiated with visible light and (e) Pt–ZnS irradiated with ultraviolet light. Reaction conditions: 0.1 g of catalyst, 50 mL of 10⁻³ M formic acid aqueous solution at pH 3.5. Data corresponding to 6 hours of irradiation.

3.5.2 Effect of catalyst loading. The effect of the amount of catalyst used in the photocatalytic process was investigated in order to find the optimal catalyst dose that provides a large light absorption along with a moderate light scattering effect. Different amounts of catalyst ranging from 0.025 to 0.5 g were added to 50 mL of formic acid (10⁻³ M) solutions and irradiated with visible light for 6 hours. A plot of the hydrogen generated vs. the amount of catalyst is shown in Fig. 7. As can be seen the highest performance was obtained with 0.15 g (3 g L⁻¹) of catalyst, an amount that is only slightly larger than the 0.1 g (2 g L⁻¹) used along the experiments of the present study.

Fig. 7 Hydrogen generated with different amounts of Pt–(CdS/ZnS). Reaction conditions: 50 mL of 10⁻³ M formic acid aqueous solution at pH 3.5. Data corresponding to 6 hours of irradiation.

3.5.3 Effect of initial formic acid concentration. In order to study the influence of the sacrificial agent in the amount of hydrogen generated, different initial concentrations of formic acid were tested (Fig. 8). As can be seen, an increment of H₂ production takes place when formic acid concentration is increased up to 5 × 10⁻² M, but a H₂ production decrease is noticed for higher concentrations (up to 10⁻¹ M). The increase of hydrogen production might be explained by the larger capture of holes that takes place with increasing formic acid concentrations, a fact that reduces charge recombination in the catalyst. The only sound explanation of the behavior observed for the largest formic acid concentrations might be the appearance of a surface blocking effect produced by this chemical, a blockage that would hamper water adsorption on the catalyst surface, and thus, the evolution of hydrogen.

Fig. 8 Rate of hydrogen evolution as a function of irradiation time obtained with the Pt–(CdS/ZnS) photocatalyst, and in presence of different initial formic acid concentrations. Reaction conditions: 0.1 g of catalyst in 50 mL of solution.

3.5.4 Effect of pH. The generation of hydrogen using formic acid and glycerol as sacrificial agents of the heterogeneous photocatalytic process was studied at pH values of 3.50, 7.02, 11.05 and 3.10, 7.00, 11.01, respectively. Fig. 9 shows that the amount of hydrogen generated was clearly influenced by the value of pH. A remarkable production of hydrogen is only detected under acid conditions. Obviously, one of the explanations of such an experimental behavior could be the larger amount of H⁺ present at acid pH that facilitate H₂ production. But it can also be explained by taking into account the values of the zero point charge of Pt–(CdS/ZnS) that was found to be 7.03, and the pK_a of formic acid, which is 3.75. Then, at acid pH (3.50) the catalyst surface is positively charged while a large fraction of HCOOH is still in its anionic form. The interaction between HCOO⁻ and the surface is strong under those conditions facilitating the reaction. At higher pH (7.02 and 11.05) the catalyst surface is neutral or negatively charged and this eliminates the electrostatic interaction, or even creates a strong electrostatic repulsion towards HCOO⁻ that results in a lower hole capture rate and in a low production of hydrogen.

Fig. 9 Rate of hydrogen evolution as a function of irradiation time and pH. Reaction conditions: Pt–(CdS/ZnS) photocatalysts, 0.1 g sample, 50 mL of 10⁻³ M formic acid aqueous solution or 10⁻³ M glycerol aqueous solution. Data corresponding to 6 hours of irradiation.

A hole scavenger with a different pK_a was tested in an attempt to improve the efficiency of hydrogen production at neutral and basic pH. Glycerol (pK_a = 14.15) was chosen for this purpose. As seen in Fig. 9 glycerol and formic acid gave similar results. In the case of glycerol no electrostatic repulsion that could hamper hole capture is expected between the organic and the catalyst surface at neutral pH where the neutral form of the molecule predominates. Thus, the unexpected low hydrogen production observed at large pH values can only be explained by taking into account the low number of H⁺ ions available at those pHs for electron capture.

3.5.5 Reusability and photocorrosion of the photocatalyst. The stability and reusability of the composite in the generation of hydrogen were tested. After each run the solid was allowed to settle and then recovered and dried at room temperature before reuse. Fig. 10 shows the hydrogen production during the recycling of the same Pt–(CdS/ZnS) catalyst sample in consecutive runs carried out with fresh formic acid solutions. As can be seen, the efficiency of the process decreased nearly to 50% after 3 consecutive runs. The presence of Cd²⁺ and Zn²⁺ in the solution after each run (Table 2) indicates that the loss of activity could be possibly caused by photocorrosion of the composite.^26–28

Fig. 10 Activity of a reutilized Pt–(CdS/ZnS) sample. Reaction conditions: 0.1 g sample, 50 mL of 10⁻³ M formic acid aqueous solution at pH 3.5. Data corresponding to 6 hours of irradiation.

Table 2 Cd²⁺ and Zn²⁺ concentrations detected in solution after each experiment. 10⁻³ M formic acid was used as hole scavenger except for: ⁽¹⁾10⁻² M formic acid; ⁽²⁾10⁻³ M glycerol; pH 3.5; irradiation time 6 hours

Sample	Experiment	Zn (mg L^−1)	Cd (mg L^−1)
Pt–CdS		—	3.51
Pt–(CdS/ZnS)	1	3.35	4.27
	2	1.21	1.49
	3	2.95	0.46
Pt–(CdS/ZnS)	1	2.95	5.29
	2	3.76	1.43
	3	—	—
Pt–(CdS/ZnS)^1	1	3.38	6.09
Pt–(CdS/ZnS)^2	1	4.12	7.42

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Furthermore, as shown in Table 2, two facts can be highlighted from the data: (1) the larger photocorrosion is always detected after the first run, clearly decreasing after the second and third runs; (2) photocorrosion is always parallel to hydrogen production. In this sense, a decrease of hydrogen production in runs 2 and 3 is accompanied by a lower photocorrosion. The increase in hydrogen production obtained for larger hole scavenger concentrations also implied larger amounts of soluble Cd or Zn detected. The larger production of hydrogen observed when using glycerol as hole scavenger was also accompanied by a larger photocorrosion. These are rather surprising results because the general idea exists that hole scavengers efficiently trap holes thus improving the efficiency of conduction band electron capture reaction (in the present case the reaction of hydrogen formation) and also decreasing the hole availability for anodic photocorrosion reactions. In other words photocorrosion decrease would have been expected for larger hole scavenger concentrations or when using a more efficient hole scavenger. The experimental results obtained in this work show that photocorrosion is clearly related to hydrogen production, indicating that an improvement of the electron capture rate cannot be accomplished without a parallel increase of hole capture by the catalyst itself that lead to an increasing partial degradation.

Previous photocatalytic studies indicate that the presence of Na₂S and Na₂S + Na₂SO₃ mixtures in solution helps to decrease photocorrosion of CdS.^29,30 Therefore, experiments were also carried out where such a mixture of hole scavengers was tested. Table 3 displays the concentrations of aqueous Cd and Zn detected after reaction, along with the amounts of hydrogen produced. As can be seen, in all the experiments the concentrations of aqueous Cd and Zn detected were lower than the ones obtained when formic acid was used as sacrificial agent (Table 2). However, the hydrogen generated was higher in the experiments that used formic acid (19.5 μmol for the same experimental conditions and after the same reaction time reported in the experiments of Table 3).

Table 3 Cd²⁺ and Zn²⁺ concentrations detected in solution after each experiment and the corresponding generation of hydrogen in the photocatalytic process. Sacrificial agent concentration: 5.0 × 10⁻² M Na₂S, 5.0 × 10⁻² M Na₂SO₃ and 5.0 × 10⁻² M Na₂S/Na₂SO₃ (1 : 1 molar ratio), pH 12.70; irradiation time 6 hours

Sample	Hole scavenger	Zn (mg L^−1)	Cd (mg L^\1)	H2 generated (μmol)
Pt–(CdS/ZnS)	Na2S	1.2	1.7	14.8
Pt–(CdS/ZnS)	Na2SO3	0.93	1.4	1.8
Pt–(CdS/ZnS)	Na2S/Na2SO3 (1 : 1 molar ratio)	0.82	1.2	15.7

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3.5.6 TOC measurements. TOC measurements were performed to verify that mineralization of formic acid occurs simultaneously to the photocatalytic hydrogen production. The initial TOC value of the 5.0 × 10⁻² M formic acid aqueous solution (pH 3.50) was 682 mg L⁻¹; this value decreased to 632.9 mg L⁻¹ after 6 h of irradiation, while the amount of produced hydrogen increased from 0 to 19.5 μmoL, which is in stoichiometric agreement with the value of formic acid degraded. This fact confirms that the photocatalytic hydrogen production and the decomposition of formic acid occurred simultaneously.

Finally, taking into account the fastest rate of hydrogen generation observed around 60 min of visible light irradiation of the Pt–(CdS/ZnS) aqueous slurry with 5.0 × 10⁻² M formic acid (the slope of the corresponding curve in Fig. 8 gives a H₂ generation rate of 130.4 μmol min⁻¹), and the experimental results of the actinometric measurements of the visible lamps (5.3 × 10⁻⁷ Einstein s⁻¹), the calculated photonic efficiency of hydrogen generation was 0.52%.

4. Conclusions

It can be concluded that the preparation method of the Pt–(CdS/ZnS) greatly influences the optical and structural properties of the catalyst and, thus, it determines the existence of a larger contact between ZnS and CdS, a fact that seems to be required for higher hydrogen generation. Also, the structure of the material can be modified with an increase of the calcination temperature. Optimal hydrogen generation was obtained with a Pt–(CdS/ZnS), prepared by precipitation and calcined at 700 °C, a catalyst that was able to give a photonic efficiency of 0.52%. On the other hand, 3 g L⁻¹ of catalyst, 0.05 M of sacrificial organic (formic acid), and pH 3.5, were the best tested experimental conditions for hydrogen generation. The reuse of the catalyst showed that a remarkable efficiency loss took place after the first catalyst recovery, and this was always accompanied by the appearance of soluble Cd and Zn in aqueous solution. Surprisingly, the amount of those cations detected in solution was always related to the hydrogen generated, even if large amounts of sacrificial organic were used. Thus, the expected minimization of catalyst photocorrosion with increasing concentrations of hole scavenger was not observed. Finally, although the photocorrosion could be decreased with the presence of Na₂S and Na₂SO₃, this also led to a decrease in the photocatalytic efficiency of the catalyst.

Acknowledgements

The authors want to thanks financial support from the Spanish “Ministerio de Ciencia e Innovación”, through project HIDROPILSOL (CTQ2013-47103-R).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra03241a

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