Methane conversion and hydrogen production over TiO₂/WO₃/Pt heterojunction photocatalysts

Abstract

Along with the advantages of mild reaction conditions, simple operation, and low energy consumption, the photocatalytic conversion of methane in the presence of water presents great potential in facilitating direct methane conversion into value-added chemicals and H₂ generation. In this work, TiO₂/WO₃ heterojunction photocatalysts modified with Pt nanoparticles were synthesized and their performances towards methane conversion into ethane (C₂H₆) and hydrogen (H₂) in the presence of water were evaluated. The ternary photocatalysts were characterized by X-ray diffraction, UV-vis, scanning and transmission electron microscopy and X-ray photoelectron spectroscopy. The highly active TiO₂/WO₃/Pt photocatalyst achieved C₂H₆ and H₂ production rates of 1.18 mmol g⁻¹ h⁻¹ and 57 mmol g⁻¹ h⁻¹, respectively. These values were 37% (for C₂H₆) and 34% (for H₂) higher than those produced by a TiO₂/Pt photocatalyst. The results show that the presence of WO₃ in a very small concentration on TiO₂ with the introduction of Pt as a co-catalyst contributes to achieving higher activities towards both C₂H₆ and H₂ evolution.

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Introduction

It is common sense that the rise in global population increases the demand for energy, which is mainly met by fossil resources, like coal, natural gas or oil.¹ The consumption of fossil fuels and the emission of CO₂ and other greenhouse gases have increased since the pre-industrial era, presenting one of the most pressing world challenges. The scientific and industrial community efforts over the last few decades have been spurred towards developing CO₂-free emission technologies.^2,3

Methane (CH₄), a potent greenhouse gas and a ubiquitous natural carbon resource, has been mainly used as an energy supply. It has the most robust C–H bonds (439 kJ mol⁻¹) with the highest activation barrier among hydrocarbon molecules,⁴ requiring high temperatures and pressures to achieve high conversion rates, thereby making its conversion more challenging.⁵ Only about 10% of methane is utilized for chemical production to synthesize a variety of high-value-added chemicals.⁶ In this sense, the catalytic conversion of CH₄ into multicarbon (C₂₊) products under mild conditions has received worldwide attention in the last few years.⁷ While many approaches have been followed over the years,⁸ photocatalysis is a promising game-changer. Employing solar energy would be one of the possible alternatives to overcome drawbacks associated with the thermodynamic barrier for the direct conversion of methane under mild conditions.^9,10

Photocatalytic dehydrogenative coupling of methane in the presence of water has been recently reported in gas, liquid and gas–liquid–solid systems.¹¹ Here, photocatalytic water splitting into H₂ and methane conversion to the more valuable product C₂H₆ (ethane) are involved in the same reaction system, where CH₄ is the sacrificial agent for holes.¹² Ishimaru and co-workers¹³ investigated the effect of adding water vapor on the C₂H₆ formation rate over a Pd/Ga₂O₃ photocatalyst during CH₄ conversion and the results showed that water promoted a substantial improvement in the C₂H₆ (selectivity > 80%) and H₂ production (AQE = 14.4%). Despite progress, additional efforts must be done to bridge the gap between experimental studies and industrial implementation, which remains extremely challenging.¹⁴

Many kinds of semiconductor-based photocatalysts, such as TiO₂,¹⁵ GaN,^16,17 Ga₂O₃,^18,19 BiVO₄²⁰ and ZnO,²¹ have been developed for various types of methane conversion, including direct and indirect routes, tuning the product selectivity. The CH₄ conversion and the selectivity of C₂H₆ are both low on pristine TiO₂ due to poor separation efficiency of the photogenerated carriers, resulting in the production of undesired but thermodynamically favorable overoxidized products (i.e., CO and CO₂).¹⁴

Among many semiconductors, it is well known that commercial TiO₂ (P25) is taken as a proof-of-concept model for direct conversion of methane (CH₄) toward value-added multicarbon (C2 +) compounds,²² either through non-oxidative²³ or oxidative routes.²⁴ To overcome the poor performance of TiO₂ under visible light and high recombination rate of the charge carriers, strategies like ion doping and heterojunction construction have shown great results in photocatalysis.²⁵ In particular, heterojunction systems have been attempted as an effective strategy for enhancing the overall photocatalytic efficiency of TiO₂-based photocatalysts.²⁶ For heterojunctions, the incorporation of alternative semiconductors such as WO₃ may synergistically improve the photocatalytic efficiency of TiO₂ under visible light irradiation, resulting from optical and electronic property modification.^27,28 Additionally, the use of various co-catalysts is proven to be an effective approach to augment charge carrier separation.²⁹

In this work, we investigated the photocatalytic properties of TiO₂/WO₃ heterojunction photocatalysts towards methane conversion into ethane (C₂H₆) and hydrogen in the presence of water through a gas–solid–liquid photocatalytic reaction system. To either promote better charge separation and more active sites for H₂ production, Pt nanoparticles were deposited over the heterojunction photocatalyst surface. This work takes a step forward and presents for the first time the study of C₂H₆ photogeneration from CH₄ conversion concomitantly with hydrogen evolution over photocatalysts based on TiO₂/WO₃ modified with Pt to increase the charge carrier separation under mild conditions. Different concentrations of WO₃ were utilized and the photocatalytic activities were evaluated.

Experimental and discussion

Materials and methods

Titanium(IV) dioxide P25 (Degussa) was used as the support material. Ammonium tungstate hydrate ((NH₄)₁₀(H₂W₁₂O₄₂)·4H₂O), ethanol (C₂H₆O), isopropyl alcohol (C₃H₈O), nitric acid (HNO₃), ethylene glycol (C₂H₆O₂), and chloroplatinic acid (H₂PtCl₆·6H₂O) were used. Deionized water used in the experiments was purified to 18.2 MΩ cm resistivity using an ultra-pure Milli-Q Millipore system.

Sample preparation

Synthesis of TiO₂/WO₃/Pt photocatalysts. Firstly, the monoclinic phase of WO₃ was obtained by dissolving 2 g of ammonium tungstate hydrate ((NH₄)₁₀(H₂W₁₂O₄₂)·4H₂O) in 85 mL of deionized water at 80 °C. Subsequently, 15 mL of concentrated HNO₃ was added dropwise, and the suspension was kept stirring for 30 min under constant reflux. The suspension was then transferred to an ultrasonic bath for 30 min. The resulting material was filtered, washed with deionized water, and dried at 80 °C for 24 h. The obtained powder was ground and calcined at 500 °C (ramp rate of 10 °C min⁻¹) for 4 h in air.

For the synthesis of TiO₂/WO₃/Pt, a proper amount of TiO₂ (P25 Degussa) was added into a 250 mL round-bottom flask containing 250 mL of ethylene glycol/water 3 : 1 v/v solution. The suspension was stirred for 10 min and then sonicated for another 10 min. A desired aliquot of H₂PtCl₆·6H₂O aqueous solution and a proper amount of the as-synthesized WO₃ were added dropwise with continuous stirring until complete homogenization, to obtain 500 mg of material varying the WO₃ concentration (0.1, 1, 3, 6, 12 and 25%). The Pt concentration was set to 0.5%, as it was the best amount of Pt previously tested by the group. The system was stirred under reflux at 180 °C for 1 h and then, the material was washed, centrifuged and dried at 80 °C for 2 h.

Photocatalytic activity. The photocatalytic activity of the photocatalysts was evaluated in a gas–liquid–solid system, described in our previous works.^19,21 In a typical procedure, 75 mg of the photocatalyst was dispersed in 250 mL of ultrapure water (18.2 MΩ cm @ 25 °C) in a commercial 250 mL Ace reactor. A steady stream of CH₄ (under 25 mL min⁻¹ of flow rate, controlled with a mass flow controller) was constantly circulated through the suspension during the reaction, whereas a 450 W Hg lamp (UVA/B/C) was used as a light source. In order to maintain the reaction (∼60 °C) and the Hg lamp (∼40 °C) temperatures, a water circulation system was used. The amount of the main evolved products (CO₂, C₂H₆, C₃H₈, CO and H₂) was quantified with gas chromatography-mass spectrometry (GC-MS) equipment (Agilent 7890B coupled to MSD 5977B). The equipment has a thermal conductivity detector (TCD), methanizer (MET), and flame ionization detector (FID), as well as a quadrupole mass spectrometer detector (MSD) and two capillary columns (plot U and a molecular sieve 5 Å column). The product selectivity was calculated by the following equation:

Characterization of the materials

The crystalline structure of the materials was analyzed by X-ray diffraction (XRD) using Cu Kα radiation (λ = 0.15418 nm) and the XRD patterns were recorded on a Rigaku Miniflex II apparatus, with scanning at 2θ from 20° to 90° with 0.05 step and 2 s count. The UV-Vis diffuse reflectance spectra were recorded using a UV-Visible spectrophotometer with a wavelength range of 200–800 nm. The Pt content (wt%) was determined by wavelength-dispersive X-ray fluorescence (WD-XRF) spectroscopy on Rigaku Supermini equipment (Pd source, 50 kV–4 mA) using a calibration curve. The morphology of the photocatalysts was evaluated by transmission electron microscopy (TEM) at an acceleration voltage of 200 kV and the images were collected on a JEOL equipment model JEM 2100F. A scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectroscopy (EDX) unit was used for elemental mapping. The valence states of each element were studied by X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific) with an Al Kα X ray source (hν = 1486.6 eV). The photoluminescence (PL) spectra of the solid photocatalysts were taken with an Ocean Optics 2000 luminescence spectrometer + USB spectrometer with a CCD camera. The excitation wavelength was 265 nm and the spectra were recorded at room temperature over the range of 200–1000 nm, with the scanning speed at 1000 nm min⁻¹, and the PMT voltage was 650 V.

Results and discussion

Commercial TiO₂ (P25) comprises both anatase and rutile phases with an estimated composition of 84.2% anatase and 15.8% rutile.¹⁰ The XRD patterns of TiO₂, WO₃ and TiO₂/WO₃/Pt with 0.5 wt% of Pt varying the amount of WO₃ are displayed in Fig. 1a. Given the low Pt content of the samples, no peaks different from those found in the TiO₂ or TiO₂/WO₃ matrix were observed in the XRD patterns of the as-prepared photocatalysts. It can be observed that the anatase phase (JCPDS 00-021-1272, labelled •) predominates in the crystalline structure of TiO₂ in all materials with a small rutile increment (labelled ).¹² Upon WO₃ incorporation (labelled ), the peaks of the monoclinic phase type (JCPDS 00-043-1035) of WO₃ could be clearly seen only for TiO₂/WO₃ samples with 12% and 25%.³⁰ Since the materials were not further calcined, the anatase/rutile proportion remained the same, as indicated by the XRD patterns. The synthesized photocatalysts were also characterized by UV-vis diffuse reflectance spectroscopy (DRS), and the sample spectra are shown in Fig. 1b. It is possible to notice that the TiO₂ photocatalyst activation occurs around 400 nm of UV light. Upon WO₃ incorporation, by increasing the concentration of WO₃ in the composite, the reflectance tends to show a red shift, resulting from the combination of the two materials absorption with further contribution in the visible range of the small amount of Pt incorporated.

Fig. 1 (a) XRD patterns of TiO₂, WO₃ and TiO₂/WO₃/Pt heterojunction photocatalysts varying their composition and (b) diffuse reflectance spectra of TiO₂, WO₃ and TiO₂/WO₃/Pt photocatalysts varying the concentration of WO₃.

The optical band gap (E_g) of the as-prepared photocatalysts was determined by the Kubelka–Munk plots (ESI† (Fig. S1 and Table S1)). The E_g values of 3.10 and 2.68 eV agree with previous reports for TiO₂ (P25)^31,32 and WO₃,^33,34 respectively. The photocatalysts with a lower amount of WO₃ (1% and 3%) did not change the E_g when compared with pristine TiO₂. The TiO₂/WO₃/Pt photocatalysts with 6, 12 and 25% showed E_g values of 3.03, 3.02 and 2.93 eV, respectively, proving their capacity to additionally absorb parts of the visible light.

Fig. 2 shows the morphology of the as-prepared photocatalysts using TiO₂ (P25 Degussa) as the support material, characterized by TEM. In Fig. 2a, it can be seen that Pt nanoparticles have a spherical shape, with nanoparticle sizes in the range of 2–5 nm. Fig. 2b shows the TEM images of the TiO₂/WO₃ heterojunction photocatalyst, with 6% of WO₃. WO₃ exhibits an irregular shape with sizes in the range of 5–15 nm. Additionally, the WO₃ did not cover the entire surface of TiO₂, but was deposited randomly around its surface.

Fig. 2 TEM images of (a) TiO₂/Pt and (b) TiO₂/WO₃ materials; (c) TiO₂/WO₃/Pt (EDX scan image) with its (d) W, (e) Ti and (f) Pt distribution; TEM images of (g)–(i) the TiO₂/WO₃/Pt heterojunction photocatalyst.

Fig. 2c–f depicts the TiO₂/WO₃/Pt ternary composite morphology with the EDS mapping image, where WO₃ (0.1 wt%) and Pt nanoparticles seem to be more agglomerated. The element distributions confirm the presence of Ti, O, W and Pt under high magnification. The EDX spectrum of the TiO₂/WO₃/Pt photocatalyst is shown in Fig. S2. The crystal structure information was further investigated by HRTEM. Fig. 2g–i show the morphology of the as-prepared TiO₂/WO₃/Pt ternary composite.

It can be observed that Pt nanoparticles are well distributed throughout the material. The clear lattice fringes with the spacings of 0.35 nm, 0.23 nm and 0.39 nm in a high-resolution TEM (HRTEM) image can be assigned to the (101), (111) and (020) crystal planes of TiO₂, Pt and WO₃ nanoparticles, respectively.^35,36

The XPS analysis was used to determine the surface composition and the chemical valence state of the elements in the as-prepared samples, as shown in Fig. 3a–d. The XPS survey spectrum of the ternary composite is shown in Fig. 3a, indicating the presence of Ti, O, W and Pt elements. The survey spectrum of pristine TiO₂ can be found in the ESI† (Fig. S3). In the Ti high-resolution spectrum (Fig. 3b), the peaks at 465.29 eV correspond to Ti2p_1/2 and 459.57 eV to Ti2p_3/2, respectively. Furthermore, the distance of the two peaks indicates the presence of Ti⁴⁺.³⁷ The W4f (Fig. 3c) peaks split into two peaks at 36.26 eV (W4f_5/2) and 38.17 eV (W4f_7/2), indicating the presence of W⁶⁺. The peaks at 74.78 eV and 71.42 eV (Fig. 3d) correspond to Pt 4f5/2 and Pt 4f7/2, respectively.³⁸ The peak shapes can be attributed to nanoparticles with Pt⁰ chemical state. No different oxidation states of Pt were observed, indicating the total reduction of Pt⁴⁺ to Pt⁰.

Fig. 3 XPS spectra of the TiO₂/WO₃/Pt photocatalyst: (a) typical survey, (b) Ti 2p, (c) W4f and (d) Pt 4f.

The effect of the improved charge separation by loading Pt nanoparticles on TiO₂/WO₃ in a heterojunction configuration could be evaluated by the photocatalytic methane conversion with simultaneous hydrogen evolution under UV illumination (75 mg of photocatalyst + 250 mL of ultrapure water, methane flow 25 mL min⁻¹). Fig. 4 shows the CO₂, C₂H₆ and H₂ evolution rates (μmol g⁻¹ h⁻¹) over 7 injections into the GC-MS system under illumination (total irradiation time = 4 h). Prior to Pt deposition on TiO₂/WO₃, the optimum amount of WO₃ was previously evaluated by the photocatalytic experiments. The results have shown that the CO₂ evolution rate was more affected by the materials with higher amounts of WO₃ (12 and 25%), where CO₂ production was decreased by 69% by TiO₂/WO₃(12%) and 49% by TiO₂/WO₃(25%), compared with pristine TiO₂ (P25), as shown in Fig. 4a. The C₂H₆ production was even more affected by higher amounts of WO₃ and it is clearly observed that the production rate decreases constantly from the first to the last injection, reaching values lower than those by pristine TiO₂, suggesting that the WO₃ concentration should be lower. The TiO₂/WO₃ with 1, 3 and 6% of WO₃ generated higher amounts of C₂H₆ with comparable values for CO₂ with pristine P25. As depicted in Fig. 4c, no H₂ evolution was achieved by the pristine TiO₂ and TiO₂/WO₃ photocatalysts.

Fig. 4 CH₄ conversion into products over TiO₂, TiO₂/WO₃ and TiO₂/WO₃/Pt photocatalysts towards (a) C₂H₆, (b) CO₂ and (c) H₂ generation. The TiO₂ used as a support was P25 Degussa and the Pt concentration of Pt was set to 0.5%. The time difference between each injection is 33 minutes, corresponding to 4 hours of continuous irradiation.

Observing the photocatalytic activity of the materials after Pt loading, the beneficial effect of combining the TiO₂/WO₃ heterostructure with the addition of Pt as a co-catalyst was clearly evidenced. The Pt loading on TiO₂/WO₃(25%) significantly decreased the CO₂ and enhanced the C₂H₆ production, accompanied by H₂ generation. The decrease in CO₂ production rate was evidence that the undesired overoxidation to CO₂ was inhibited after Pt loading. However, both C₂H₆ and H₂ production did not show stability throughout the reaction, with a rapid decrease in their production within the first three injections, suggesting that the Pt nanoparticles should be incorporated in TiO₂/WO₃ with lower concentration of WO₃.

The rapid decrease in the C₂H₆ and H₂ evolution rate produced by the TiO₂/WO₃(25%)/Pt photocatalyst within the first three injections (Fig. 4b and c) may be assigned to the possible W⁶⁺ reduction to W⁵⁺ by receiving electrons from the conduction band (CB) of TiO₂ upon illumination,^39–41 decreasing their availability to drive the desired chemical reactions. Therefore, the electron transfer from the CB of TiO₂ to the CB of WO₃ with the subsequent reduction process of W⁶⁺ was competitive with the coupling reactions. This led to a rapid decrease in the C₂H₆ evolution rate accompanied by a photocatalyst colour change from light yellow to dark blue.

After the deposition of Pt nanoparticles on the TiO₂/WO₃(1%) material, a substantial increase in all products was observed, with better stability over the injections. The increased C₂H₆ production was highly different compared with TiO₂/WO₃(25%)/Pt, with a continuous rise during irradiation. The same behaviour is observed for CO₂ and H₂ production, with a boosted production of both products from the first to the second injection, accompanied by a gradual increase in their production. Considering that the photocatalytic activity of TiO₂/WO₃/Pt heterojunction photocatalysts showed a better performance when the WO₃ concentration is very low, Pt loading was carried out on TiO₂/WO₃ with 0.1% of WO₃ and the results are presented in Fig. 5.

Fig. 5 CH₄ conversion towards the products over TiO₂, TiO₂/WO₃ and TiO₂/WO₃/Pt photocatalysts towards (a) C₂H₆, (b) CO₂ and (c) H₂ generation. The TiO₂ used as support was commercial P25 Degussa.

Fig. 5 shows the photocatalytic activity over pristine TiO₂ (P25), TiO₂/Pt and TiO₂/WO₃(0.1%)/Pt photocatalysts. Here, both TiO₂/Pt and TiO₂/WO₃(0.1%)/Pt photocatalysts presented a boosted photoactivity towards CO₂, C₂H₆ and H₂ evolution rate compared with pristine TiO₂ (P25). In Fig. 5a, it is observed that the CO₂ produced by Pt-based photocatalysts was very close, but the C₂H₆ and H₂ production showed a substantial difference. For both the TiO₂/Pt and TiO₂/WO₃(0.1%)/Pt photocatalysts, a continuous increase in C₂H₆ production is observed, but it is more pronounced for TiO₂/WO₃(0.1%)/Pt, reaching values 2.4 times higher from the first (510 μmol g⁻¹ h⁻¹) to the third injection (1196 μmol g⁻¹ h⁻¹).

The H₂ evolution rate was dramatically improved by Pt loading on TiO₂/WO₃ with 0.1% of WO₃. From the first to the second injection, the H₂ production rate was increased by 115% and 119% by the TiO₂/Pt (from 20 175 μmol g⁻¹ h⁻¹ to 43 469 μmol g⁻¹ h⁻¹) and TiO₂/WO₃(0.1%)/Pt (from 29 230 μmol g⁻¹ h⁻¹ to 64 069 μmol g⁻¹ h⁻¹) photocatalysts, respectively. The results suggest that the WO₃ concentration in the ternary composite in a very small content is a key point for boosting both C₂H₆ and H₂ production, while maintaining the same amount of CO₂ and CO evolution. These results indicate that, by decreasing the amount of WO₃, charge carrier recombination is inhibited, and hence the photogenerated electrons become more available to drive the coupling reactions, instead of promoting W⁶⁺ reduction, with the additional Pt effect on collecting them. In contrast, the over-loading of WO₃ domains clearly produced a decrease of the photocatalytic efficiency.

Different works have also shown a significant improvement in charge transfer between TiO₂ and WO₃, reaching better photocatalytic performances when the WO₃ concentration is less than 1%.^42,43 It has been demonstrated that higher WO₃ content increases the concentration of recombination centers for electron/hole pairs, contributing to lowering the photoactivity of TiO₂–WO₃-based photocatalysts.⁴⁴

Karácsonyi and co-workers⁴⁵ showed that significant differences in the photocatalytic performance of the TiO₂/WO₃/(Pt or Au) photocatalysts towards oxalic acid degradation were observed only through the position of the noble metal, if the co-catalyst is deposited on TiO₂ or on WO₃. It was demonstrated that, by changing the WO₃ concentration from 1 to 33%, none of the ternary composites reached H₂ values higher than TiO₂/Pt (without WO₃). In our work, our results indicate that an optimum WO₃ content should be lower than 1% to achieve a beneficial effect towards C₂H₆ and H₂ from CH₄ conversion in the presence of water.

The production rates of the main products (C₂H₆, CO, CO₂ and H₂) generated during the photocatalytic tests are summarized in Table 1. The table also depicts the generation of propane (C₃H₈), but in minor quantity and the selectivity of the products. Comparing TiO₂/Pt with the TiO₂/WO₃(0.1%)/Pt photocatalyst, it can be observed that similar amounts of CO and CO₂ were achieved. However, the incorporation of 0.1% WO₃ to form the heterojunction system increased the C₂H₆ and H₂ production by 37 and 34%, respectively. This behaviour may be associated with the better charge carrier separation promoted by the interaction between the two semiconductors.^46,47 By receiving electrons from the CB of TiO₂ and injecting holes into its valence band (VB), the presence of WO₃ in a small content may significantly decrease the charge carrier recombination in both semiconductors,⁴⁸ giving holes and electrons more probability to drive the coupling reactions. Additionally, the boosted hydrogen production could only be possible after the presence of Pt nanoparticles. From the viewpoint of selectivity, CO₂ is the dominant product generated, reaching 93.2 and 91.2% among the main products produced by TiO₂/Pt and TiO₂/WO₃(0.1%)/Pt photocatalysts, respectively. The higher C₂H₆ selectivity for TiO₂/WO₃(0.1%)/Pt (8.5%) compared with TiO₂/Pt (6.5%) implies the more efficient formation of ˙CH₃ by the photogenerated holes of WO₃.

Table 1 Product formation (average values) using commercial TiO₂(P25) as a support and TiO₂/WO₃ and TiO₂/WO₃/Pt photocatalysts (75 mg of photocatalyst, 250 mL H₂O, 25 mL min⁻¹ CH₄ (flow rate), 450 W Hg lamp) during methane coupling in the presence of water

Photocatalyst	Product formation rates (μmol g^−1 h^−1)					Product selectivity (%)
	C2H6	C3H8	CO	CO2	H2	C2H6	C3H8	CO	CO2
a The Pt concentration in all photocatalysts was set at 0.5% as the best condition previously tested.
TiO2 (P25)	45	4.0	46	177	—	16.5	1.5	16.9	65.1
TiO2/WO3 (1%)	60	8.1	30	134	—	25.9	3.5	12.9	57.7
TiO2/WO3 (3%)	66	9.2	36	133	—	27.0	3.8	14.7	54.5
TiO2/WO3 (6%)	69	7.1	29	234	—	20.3	2.1	8.6	69.0
TiO2/WO3 (12%)	11	0.7	40	189	—	4.6	0.3	16.6	78.5
TiO2/WO3 (25%)	23	1.3	6.7	29	—	38.3	2.2	11.2	48.3
TiO2/Pt^a	866	25	20	12 425	42 500	6.5	0.2	0.1	93.2
WO3/Pt^a	5.8	0.6	6.4	29	—	13.9	1.4	15.3	69.4
TiO2/WO3 (0.1%)/Pt^a	1183	33	20	12 745	57 014	8.5	0.2	0.1	91.2
TiO2/WO3 (1%)/Pt^a	158	8.0	12	1446	4814	16.5	1.5	16.9	65.1
TiO2/WO3 (25%)/Pt^a	109	17	19	96	1847	25.9	3.5	12.9	57.7

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To further investigate the possible reasons for the enhanced photocatalytic activity of TiO₂/WO₃(0.1%)/Pt towards C₂H₆ and H₂ evolution, the PL spectra of pristine TiO₂ (P25), TiO₂/Pt and TiO₂/WO₃(0.1%)/Pt were evaluated, as shown in Fig. 6. The peak located at 440 nm is mainly originated from the recombination of photogenerated electrons in oxygen vacancies or crystal defects in TiO₂.⁴⁹ Among the samples, pristine TiO₂ (P25) displays the highest PL signal. The TiO₂/Pt and TiO₂/WO₃(0.1%)/Pt reduced the PL intensity by 66 and 83%, respectively, compared to pristine TiO₂. The better charge transfer between TiO₂ and WO₃ can significantly accelerate the carrier separation and suppress the fast recombination of the photoexcited electron–hole pair, which is a key factor in the photocatalytic reaction.^50,51 This evidence together with the results shown in Table 1 clearly demonstrates the synergistic effect of charge separation promoted by WO₃ together with the charge transfer promoted by Pt.

Fig. 6 Room-temperature PL emission spectra of pristine TiO₂ (P25), TiO₂/Pt and TiO₂/WO₃/Pt photocatalysts (excitation wavelength at 265 nm).

More recently, Sato and co-workers⁵² have shown that, combining real-time mass spectrometry and operando infrared absorption spectroscopy with ab initio molecular dynamics simulations, the C–H bond breaking of CH₄ can be effectively enhanced in the presence of water, preventing the overstabilization of the intermediates. In addition, the water-assisted⁴ effects contribute to improving the photocatalytic conversion rates at ambient temperatures and pressures. In our work, combining methane conversion in the presence of water using a TiO₂/WO₃/Pt heterojunction under mild conditions collected all the advantages to boost the conversion of CH₄ to C₂H₆ concomitantly with an increase in H₂ evolution, although CO₂ is still the main product coming from CH₄ conversion.

The proper alignment of the energy levels of the VB and CB in TiO₂ and WO₃ promoting the charge transfer between them is displayed in Fig. 7. Upon illumination, both TiO₂ and WO₃ semiconductors absorb energy higher than their bandgap values, generating electron/hole pairs. The formation of a TiO₂/WO₃ heterojunction provides the thermodynamic driving force for the direct electron transfer from the CB of TiO₂ into the CB of WO₃, whereas holes in the VB of WO₃ move to the VB of TiO₂, thereby inhibiting deleterious charge carrier recombination.⁵³ By introducing Pt nanoparticles, efficient cooperation between the charge separation through the heterojunction system and the collection of electrons over Pt sites is achieved.⁵⁴

Fig. 7 Energy diagram depicting the charge carrier flux in the TiO₂/WO₃/Pt ternary photocatalyst under illumination, inhibiting charge carrier recombination towards CH₄ conversion and H₂ production.

Fig. 7 also shows the photocatalytic processes of the main products achieved in our system. Firstly, ˙CH₃ radicals are formed from water reacted with h⁺. Afterwards, the following coupling of ˙CH₃ is responsible for C₂H₆ production, while electrons (e⁻) at the Pt sites reduce H⁺ species to form H₂. The overoxidation of CH₄ towards CO₂ is carried out by holes, being the main product of the overall reaction.

Yu and co-workers⁵⁵ also describe the role of Pt in promoting the activation of CH₄ molecules to form ˙CH₃ radicals over TiO₂/Pt photocatalysts during direct methane conversion and hydrogen evolution in a photocatalytic system. They show that the subsequent coupling of ˙CH₃ brings up the C₂H₆ product, while H₂ is generated through H⁺ reaction with electrons.

In our work, efficient charge carrier separation was obtained through the combination of a TiO₂/WO₃ heterojunction system in cooperation with Pt co-catalysts to promote higher C₂H₆ and H₂ generation. Here, TiO₂/Pt and TiO₂/WO₃(0.1%)/Pt photocatalysts generated equal values of CO₂, while C₂H₆ and H₂ production presented a substantial improvement, demonstrating the beneficial effect of combining charge carrier transfer through the heterojunction of TiO₂/WO₃ with the additional role of Pt on promoting boosted CH₄ conversion.

Conclusions

Owing to the introduction of WO₃ and Pt, the TiO₂/WO₃/Pt heterojunction photocatalyst exhibited better performance over CH₄ conversion into C₂H₆ and H₂ compared to the pristine materials. Pristine TiO₂ (P25) nanoparticles were proved to be ineffective in driving CH₄ conversion with no H₂ production. This study showed that the presence of WO₃ in a low concentration is an efficient way to benefit the C₂H₆ formation rate due to better charge separation and by introducing Pt nanoparticles, H₂ generation was achieved. The synergistic effect between the materials contributed to the selective coupling of CH₄ towards C₂H₆ and H₂ production. Herein, we described the achievement of a new promising candidate material towards methane conversion with simultaneous generation of hydrogen through a non-thermal heterogeneous catalysis of methane under mild conditions. Notwithstanding this, CO₂ remains the primary product generated from methane. The pursuit of enhancing photocatalyst activity and refining selectivity toward the desired products remains an ongoing imperative. This work may be helpful for extending the possibilities towards the design of future nanomaterials for methane coupling with hydrogen generation in the presence of water simultaneously in one system.

Author contributions

S. A. C. performed the synthesis and characterization of the materials with further photocatalytic tests, and writing. E. R. J. collaborated in analyses, and discussion of results. P. S. F. participated in the catalyst preparation and photocatalytic tests. A. P. M. contributed with PL measurements and discussion of the results. J. M. V. and E. V. S. designed the study and reviewed the paper. All authors have read and agreed to the published version of the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors gratefully acknowledge São Paulo Research Foundation (FAPESP – grant numbers 2017/11937-4, 2018/04596-9, 2018/04595-2, 2021/01896-4 and 2023/01980-0) for the financial support and fellowship, Brazilian National Council for Scientific Development (CNPq – Grant numbers 305622/2020-0 and 407967/2022-2), Shell and ANP (Brazilian National Oil, Natural Gas and Biofuels Agency) through R&D levy regulation. We thank LNNano for XPS measurements (proposal no. 20221012) and TEM (proposal no. 20210567). We also thank Prof. Ana Flavia Nogueira from LNES (Laboratório de Nanotecnologia e Energia Solar) at the University of Campinas (UNICAMP) for providing access to the PL spectrometer.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3ma00844d

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