Preparation of RhO_x/TiO₂ with Pt cocatalyst effective for photocatalytic alcohol dehydrogenation under irradiation of visible light

Abstract

Photocatalytic alcohol dehydrogenation is a promising technique for the simultaneous production of hydrogen (H₂) and carbonyl compounds. In this study, titanium(IV) oxide (TiO₂) photocatalysts co-loaded with rhodium oxide (RhO_x) and platinum (Pt) particles were successfully prepared via a multi-step process involving the equilibrium adsorption of Rh ions, post-calcination, and Pt colloid impregnation. In the resulting system, RhO_x acts as a visible-light sensitizer, while Pt serves as the hydrogen evolution co-catalyst. A series of RhO_x/TiO₂–Pt samples were synthesized by systematically varying the calcination temperature of the RhO_x/TiO₂ precursor prior to Pt loading. These materials were then evaluated for photocatalytic alcohol dehydrogenation under visible light irradiation. The effects of calcination temperature on the electronic states and light absorption characteristics of the Rh species were investigated, along with their influence on photocatalytic activity. The RhO_x/TiO₂–Pt photocatalysts exhibited efficient dehydrogenation of various alcohols. In particular, benzyl alcohol was selectively converted to benzaldehyde and H₂ in a stoichiometric ratio, with no over-oxidation observed. This work demonstrates a novel strategy for coupling oxidative organic synthesis with hydrogen production under visible light, offering new insights into photocatalyst design for sustainable energy and chemical synthesis.

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1. Introduction

Extensive research has been devoted to the development of TiO₂-based photocatalysts that respond to visible light. This sustained interest is driven by several key factors, among which the following three stand out:

1. TiO₂ exhibits exceptional and unrivaled photocatalytic properties;

2. Titanium is an abundant and safe element, making it attractive from both environmental and resource-availability perspectives;

3. There is a strong demand to harness visible light, in addition to ultraviolet (UV) light, for practical photocatalytic applications.

Therefore, narrowing the band gap of TiO₂ or sensitizing it to visible light is a critical challenge. Various approaches have been explored, such as creating oxygen vacancies, elemental doping (both cationic and anionic), and forming heterostructures.¹ Although these methods have successfully improved visible-light absorption, they sometimes introduce charge recombination centers or exhibit thermal instability. In this context, surface modification has emerged as a simple and effective strategy for enabling wide-bandgap TiO₂ to absorb visible light. A variety of surface modifiers have been explored, including transition metals in different chemical forms, halides,² hydroxides,³ metal ions,⁴ and oxides,⁵ as well as metal complexes⁶ and organic compounds.⁷ In these modified TiO₂ systems, the modifying species are not incorporated into the crystal lattice but are instead supported on the surface of the TiO₂ particles. This surface configuration facilitates efficient charge transfer between the modifiers and TiO₂ (Fig. 1). Importantly, the surface-bound species serve dual roles as visible light absorption centres and oxidation sites.

Fig. 1 Proposed energy diagram; (a) band-gap excitation of TiO₂ and (b) charge transfer from a sensitizer to the CB of TiO₂.

When TiO₂-based photocatalysts are employed for degradation or oxidation reactions under aerated conditions, molecular oxygen (O₂) acts as an effective electron scavenger. The conduction band (CB) potential of TiO₂ (∼−0.3 V vs. NHE) is sufficiently negative to drive the reduction of O₂, allowing these reactions to proceed without the need for further modification. For instance, TiO₂ modified with rhodium (Rh) ions has been shown to degrade volatile organic compounds (VOCs) under visible light irradiation.⁸ Moreover, selective oxidation of aromatic alcohols has been achieved using TiO₂ modified with Rh, palladium (Pd), and ruthenium (Ru) ions.⁹ The structural evolution of Rh species on TiO₂ and their influence on photocatalytic performance have also been studied in detail. It was found that calcination-induced changes in RhO_x/TiO₂ can significantly affect activity without altering the intrinsic physical properties of TiO₂.¹⁰ Notably, atomically isolated Rh³⁺ species were identified as the most active form, while aggregation of Rh species led to a marked decline in photocatalytic efficiency.

When TiO₂-based photocatalysts (including both unmodified and modified forms) are used for the reduction of a compound under deaerated conditions in the presence of an electron donor, the compound accepts photogenerated electrons either directly or indirectly. In most cases, the incorporation of a co-catalyst is essential to achieve a practical reaction rate and high selectivity for the desired product. A representative example is the hydrogen (H₂) evolution reaction. Given the relatively low overpotential for hydrogen evolution, platinum (Pt) is widely recognized as the most effective co-catalyst. In other systems, the choice of co-catalyst significantly influences selectivity. For instance, in the selective reduction of nitrates to ammonia using an aqueous suspension of a TiO₂ photocatalyst, copper (Cu) co-catalysts demonstrated superior selectivity.¹¹ Similarly, silver (Ag) co-catalysts exhibited high activity for the deoxygenation of epoxides in alcoholic suspensions of TiO₂ photocatalysts.¹² When a modified TiO₂ photocatalyst is used in conjunction with a co-catalyst, both the visible-light sensitizer (modifier) and the co-catalyst must be introduced onto the TiO₂ surface. To ensure efficient photocatalytic performance, it is critical to prevent interference between these two components. Therefore, specialized preparation methods are required to preserve and optimize the individual functions of the modifier and the co-catalyst.

Many researchers have reported H₂ production from various compounds using TiO₂ photocatalysts under UV irradiation.¹³ This approach represents a promising and environmentally friendly technology for generating clean energy from renewable resources. However, UV light accounts for only about 5% of the total solar spectrum, whereas visible light contributes approximately 50%. Consequently, significant research has focused on developing visible-light-responsive photocatalysts for H₂ production.¹⁴

Photocatalytic alcohol dehydrogenation has emerged as a particularly attractive route for H₂ generation.¹⁵ One advantage is its favourable thermodynamics compared to water splitting. For instance, the standard Gibbs free energy change (Δ_rG⁰) for ethanol dehydrogenation (C₂H₅OH(l) → CH₃CHO(l) + H₂(g)) is 47 kJ mol⁻¹, substantially lower than that for water splitting (Δ_rG⁰ = 237 kJ mol⁻¹). Additionally, alcohol dehydrogenation produces a valuable carbonyl compound as an oxidation product without requiring any additives. In contrast, achieving selective oxidation to carbonyl compounds using conventional oxidants such as oxygen or permanganate is often challenging. In this study, we applied RhO_x/TiO₂ photocatalysts for H₂ evolution via alcohol dehydrogenation under visible light irradiation. To meet the dual requirements of visible-light absorption and efficient hydrogen evolution, a new preparation method was employed. Specifically, Pt colloids were loaded onto RhO_x/TiO₂ to introduce small Pt co-catalyst particles. As anticipated, the resulting RhO_x/TiO₂–Pt samples exhibited significantly higher reaction rates compared to co-catalyst-free RhO_x/TiO₂ under visible light irradiation. Here, we report the characterization of photocatalysts prepared under various conditions and evaluate their catalytic performance in the dehydrogenation of alcohols under visible light irradiation.

2. Experimental section

2.1 Synthesis of TiO₂

Nanocrystalline TiO₂ powder was synthesized via hydrothermal crystallization in organic media (HyCOM) at 300 °C.¹⁶ Titanium(IV) butoxide and toluene were used as the titanium precursor and solvent, respectively. The resulting product was calcined at various temperatures for 1 h in a box furnace. Calcination improved the crystallinity of the HyCOM-TiO₂ samples while maintaining a high specific surface area up to 89 m² g⁻¹ even after treatment at 500 °C. The TiO₂ sample calcined at 500 °C was used in most of the subsequent experiments as the support material for Rh species. To investigate the effect of TiO₂ properties, the HyCOM-TiO₂ sample was calcined at different temperatures. The corresponding pre-calcination temperature (T₁) is indicated in parentheses following TiO₂; for example, TiO₂(700) refers to the sample calcined at 700 °C.

2.2 Preparation of RhO_x/TiO₂

The TiO₂ sample was modified with Rh³⁺ ions using an equilibrium adsorption method. Pre-calcined TiO₂ powder was added to an aqueous solution of rhodium(III) chloride (RhCl₃), in an amount corresponding to 1.0 wt% Rh metal. The mixture was stirred and heated in a water bath at approximately 85 °C. After equilibration, the suspension was filtered, and the resulting filter cake was thoroughly washed with distilled water to remove residual chloride ions. The solid was then dried under vacuum for 1 h to obtain the Rh³⁺/TiO₂ sample. The complete adsorption of Rh³⁺ onto the TiO₂ surface was confirmed by analyzing the filtrate using inductively coupled plasma atomic emission spectroscopy (ICP-OES, Thermo Fisher Scientific Inc., iCAP7600D), which detected no remaining Rh³⁺ in the solution. To modify the structure of the Rh³⁺ species on the TiO₂ surface, the Rh³⁺/TiO₂ samples were subjected to post-calcination at various temperatures (T₂) for 1 h. This thermal treatment converted the highly dispersed Rh³⁺ species into RhO_x species on the TiO₂ surface. The resulting photocatalysts are denoted as RhO_x/TiO₂(T₁–T₂), where T₁ and T₂ represent the pre- and post-calcination temperatures, respectively. For example, the sample pre-calcined at 500 °C and post-calcined at 400 °C is referred to as RhO_x/TiO₂(500–400).

2.3 Platinization of RhO_x/TiO₂

Colloidal Pt nanoparticles were prepared according to the method reported by Frens.¹⁷ An aqueous solution of hexachloroplatinic(IV) acid (H₂PtCl₆, 0.0402 g cm⁻³, 750 cm³) was mixed with 100 cm³ of an aqueous sodium citrate solution (39 mmol dm⁻³). The mixture was heated to boiling and maintained at reflux for 1 h. As the reaction proceeded, the solution color changed from yellow to deep gray, indicating the formation of Pt nanoparticles. Boiling was continued for an additional 30 min to ensure complete reduction. After cooling to room temperature, 50 cm³ of Amberlite MB-1 ion exchange resin (ORGANO) was added to the solution to remove excess sodium citrate. After 1 h of treatment, the resin was removed by filtration using a glass filter. Pt loading onto the RhO_x/TiO₂ samples was carried out via the colloid salting-out (CS) method.¹⁸ A typical preparation of RhO_x/TiO₂ loaded with 0.5 wt% Pt is described as follows: RhO_x/TiO₂ powder (150 mg) was suspended in 20 cm³ (0.754 mg) of the prepared colloidal Pt solution and stirred at room temperature for 30 min in a centrifuge tube. Then, approximately 1 g of ammonium chloride was added to the mixture, which was stirred for an additional 30 min to induce deposition of the Pt nanoparticles onto the TiO₂ surface. The resulting solid was collected, washed thoroughly with distilled water to remove residual salts, and air-dried. The final platinized photocatalysts are denoted as RhO_x/TiO₂(T₁–T₂)–Pt(Y), where Y represents the Pt loading (e.g., 0.5 wt%).

2.4 Characterization

Diffuse reflectance spectra of the RhO_x/TiO₂ samples were recorded using a UV-visible spectrophotometer (UV-2600, Shimadzu, Kyoto) equipped with a diffuse reflectance accessory (ISR-2600Plus, Shimadzu). The morphology of the samples was examined by transmission electron microscopy (TEM) using a JEOL JEM-2100F microscope operated at 200 kV at the Joint Research Center (JRC) of Kindai University. Rhodium K-edge X-ray absorption fine structure (XAFS) measurements were conducted in transmission mode at beamline BL01B1 of the SPring-8 synchrotron radiation facility (Hyogo, Japan). Standard data processing procedures, including background subtraction and normalization, were performed using Athena¹⁹ (version 0.9.26). Powder X-ray diffraction of TiO₂ was recorded on a MiniFlex (Rigaku, Tokyo, Japan) in the JRC at Kindai University using CuKα radiation with a monochromator.

2.5 Hydrogen formation by dehydrogenation of 2-propanol

In this study, the dehydrogenation of 2-propanol, i.e., producing hydrogen (H₂) and acetone, was selected as a model reaction under visible light irradiation, as shown in eqn (1):

(CH₃)₂CHOH → (CH₃)₂CO + H₂. (1)

Dried photocatalyst powder (50 mg) was suspended in 5 cm³ of 2-propanol, degassed by bubbling with Ar, and sealed with a rubber septum. The suspension was then irradiated under visible light using a 500 W xenon (Xe) lamp (Eagle Engineering, Tokyo) equipped with a Y-43 cutoff filter (HOYA Corporation, Tokyo), while magnetically stirred in a water bath maintained at 25 °C. The amount of H₂ generated in the gas phase was measured using a Shimadzu GC-8A gas chromatograph equipped with an MS-5A column. Acetone produced in the liquid phase was quantified using a Shimadzu GC-2025 gas chromatograph fitted with a DB-WAX column, with toluene used as an internal standard.

3. Results and discussion

3.1 Characterization of samples

Fig. 2 shows the UV-vis absorption spectra of TiO₂, TiO₂–Pt(0.5), RhO_x/TiO₂(500–400), and RhO_x/TiO₂(500–400)–Pt(0.5) samples. The unmodified TiO₂ exhibited absorption only below 400 nm, consistent with its intrinsic bandgap excitation. Upon deposition of Pt particles, TiO₂–Pt(0.5) showed a baseline increase in photoabsorption, typically accompanied by a visible color change from white to gray. The RhO_x/TiO₂(500–400) sample displayed additional absorption in the 400–600 nm range, attributed to the presence of Rh₂O₃ nanoparticles.¹⁰ In the RhO_x/TiO₂(500–400)–Pt(0.5) sample, both RhO_x and Pt contributed to enhanced photoabsorption, resulting in a broader and more intense absorption profile. Fig. S1 (SI) presents a TEM image and particle size distribution of the colloidal Pt particles, indicating an average particle diameter of 2.9 nm with a relatively narrow size distribution (standard deviation: 1.1 nm). Fig. S2 (SI) shows a TEM image of the RhO_x/TiO₂(500–400)–Pt(0.5) sample prepared via the CS method using a Pt colloidal solution. Pt particles were clearly observed on the TiO₂ surface, with an average diameter of 2.8 nm, closely matching the original size of the colloidal Pt particles prior to loading (Fig. S1, SI).

Fig. 2 UV-vis diffuse reflection spectra of various samples.

3.2 Formation of H₂ by dehydrogenation of 2-propanol

The photocatalytic activity of TiO₂, TiO₂–Pt(0.5), RhO_x/TiO₂(500–400), and RhO_x/TiO₂(500–400)–Pt(0.5) was evaluated for H₂ production via the dehydrogenation of 2-propanol under visible light irradiation. The experiments were conducted at 25 °C using an Xe lamp equipped with a Y-43 cutoff filter. The spectrum of the light from the Xe lamp with the Y-43 cut-off filter used for this reaction is shown in Fig. S3 (SI). The resulting H₂ generation rates for the various photocatalysts are presented in Fig. 3. No H₂ evolution was observed for TiO₂ or TiO₂–Pt(0.5). This result is expected, as the Y-43 cutoff filter blocks UV light, preventing bandgap excitation of TiO₂, and the thermal catalytic activity of Pt is negligible under these conditions. In contrast, the RhO_x/TiO₂(500–400) sample generated H₂ at a rate of 1.3 μmol h⁻¹, demonstrating visible-light-driven photocatalytic activity due to the presence of Rh species. Notably, the RhO_x/TiO₂(500–400)–Pt(0.5) sample showed a significantly enhanced H₂ production rate of 2.8 μmol h⁻¹, indicating that Pt particles effectively function as co-catalysts for H₂ evolution in this system. To confirm that this reaction proceeds via photocatalysis and not thermocatalysis, the RhO_x/TiO₂(500–400)–Pt(0.5) sample was also tested under dark conditions at 25 °C. As shown in Fig. 3, no H₂ was detected, confirming that thermal dehydrogenation of 2-propanol is negligible at this temperature. To further evaluate the roles of Rh species and Pt particles, separate samples of RhO_x/TiO₂(500–400) and TiO₂(500)–Pt(0.5) (25 mg each) were physically mixed and irradiated under the same conditions. The resulting H₂ evolution rate was 0.51 μmol h⁻¹, which is approximately half that of RhO_x/TiO₂(500–400) alone (1.3 μmol h⁻¹). This finding highlights the importance of co-loading both Rh species and Pt particles onto the same TiO₂ support to achieve synergistic enhancement and maximize photocatalytic H₂ production from 2-propanol dehydrogenation.

Fig. 3 Rate of H2 formation from 2-propanol suspension of various photocatalysts.

3.3 Time courses of H₂ and acetone produced by 2-propanol dehydrogenation

Fig. 4 shows the time-dependent production of H₂ and acetone during the photocatalytic dehydrogenation of 2-propanol over the RhO_x/TiO₂(500–400)–Pt(0.5) sample. The reaction was carried out under deaerated conditions and visible light irradiation using an Xe lamp equipped with a Y-43 cutoff filter. Immediately upon light irradiation, both H₂ and acetone were generated from the suspension. As irradiation time increased, the amounts of H₂ and acetone increased linearly. The formation rates were determined to be 2.8 μmol h⁻¹ for H₂ and 2.7 μmol h⁻¹ for acetone. The close agreement between the production rates of H₂ and acetone confirms that the dehydrogenation of 2-propanol proceeded stoichiometrically, as shown in eqn (1). To assess the photocatalyst's stability, the RhO_x/TiO₂–Pt sample was reused in repeated cycles, as shown in Fig. S4 (SI). After each cycle, the reaction mixture was purged with Ar and irradiated again under the same conditions. H₂ evolution resumed without any noticeable decline in activity, even after prolonged irradiation, demonstrating the excellent durability of the catalyst. The total amount of H₂ produced reached approximately 47 μmol, which exceeds the stoichiometric amount of Rh present in the catalyst (4.9 μmol, corresponding to 1.0 wt%). This clearly indicates that H₂ evolution was not due to a chemical reaction between Rh species and 2-propanol, but rather was the result of a true (photo)catalytic process. The results of XPS and UV-vis for the RhO_x/TiO₂–Pt sample before and after the photocatalytic H₂ formation reaction under visible-light irradiation are shown in Fig. S5 and S6 (SI). In both cases, no changes in the spectra caused by the photocatalytic reaction were observed, clearly demonstrating that the catalyst in this study was stably driven.

Fig. 4 Time courses of amounts of H₂ and acetone in 2-propanol suspensions of RhO_x/TiO₂(500–400)–Pt under irradiation of light from Xe lamp with cut filter Y-43.

3.4 Effect of post-calcination temperature (T₂)

The structure of rhodium species was altered by varying the post-calcination temperature (T₂), and its effect on the photocatalytic dehydrogenation of 2-propanol was investigated. Fig. 5a shows the UV-vis diffuse reflectance spectra of RhO_x/TiO₂(500–T₂) samples calcined at different temperatures. All samples exhibited visible-light absorption, with the intensity increasing as the calcination temperature rose. This enhanced photoabsorption was attributed to structural changes in the Rh species immobilized on the TiO₂ surface, rather than modifications in the bulk properties of the TiO₂ support caused by calcination. In the case of Rh³⁺/TiO₂(500) without post-calcination, previous studies have shown that Rh species exist as surface complexes coordinated to TiO₂ surface hydroxyl groups, hydroxide ions, and water molecules. The positive charge of Rh³⁺ is balanced by these surface groups, including terminal oxygen atoms and hydroxyls on the TiO₂ surface.¹⁰ During post-calcination, coordinated hydroxide ions and water molecules are gradually removed, which is believed to enhance the interaction between Rh species and the TiO₂ surface. This improved interaction likely facilitates more effective charge transfer between the Rh species and TiO₂, leading to stronger visible-light absorption. The formation of Rh₂O₃ nanoparticles due to Rh species aggregation during calcination is also considered a plausible contributor to this absorption enhancement. To investigate changes in the electronic structure of Rh species, XAFS measurements were performed. Fig. 5b presents the Rh K-edge XANES spectra of commercial Rh foil, Rh₂O₃, and both uncalcined and calcined RhO_x/TiO₂ samples. The spectra of Rh³⁺/TiO₂ and RhO_x/TiO₂(500–T₂) were similar to that of Rh₂O₃, but clearly different from that of Rh foil. The absorption edge energies of both uncalcined and post-calcined samples closely matched that of Rh₂O₃, indicating that the Rh species predominantly remained in the trivalent oxidation state. Fig. 5c shows H₂ production rates from 2-propanol dehydrogenation using platinized RhO_x/TiO₂(500–T₂) samples. In all cases, H₂ was generated, and the yield increased with increasing T₂ up to 400 °C. The sample calcined at 400 °C exhibited the highest activity, producing 8.4 μmol of H₂ after 3 h. This improvement in photocatalytic activity is attributed to the enhanced visible-light absorption associated with structural changes in Rh species. However, at calcination temperatures above 400 °C, H₂ production decreased, likely due to excessive aggregation of Rh species, which reduces the active surface area and impairs charge transfer efficiency.

Fig. 5 (a) UV-vis diffuse reflection spectra of RhO_x/TiO₂(500–T₂), (b) Rh K-edge XANES spectra of Rh foil, Rh₂O₃, a) Rh³⁺/TiO₂, b) RhO_x/TiO₂(500–200), c) RhO_x/TiO₂(500–300), d) RhO_x/TiO₂(500–400) and e) RhO_x/TiO₂(500–500) and (c) amounts of H₂ in 2-propanol suspensions of RhO_x/TiO₂(500–T₂)–Pt(0.5) under irradiation of light from Xe lamp with cut filter Y-43 for 3 h.

3.5 Action spectrum

The action spectrum has become increasingly important in the study of visible-light-responsive photocatalysis, as it represents the efficiency of light utilization as a function of wavelength. This metric is critical for identifying both the effective wavelength range and the underlying photoactivation mechanisms in a given photocatalytic system. To evaluate the action spectrum of the current system, 2-propanol dehydrogenation over RhO_x/TiO₂(500–400)–Pt(0.5) was conducted at 25 °C under monochromatic visible light irradiation. Light was provided by an Xe lamp equipped with a monochromator (bandwidth ±10 nm). The apparent quantum efficiency (AQE) at each central wavelength was calculated using eqn (2), which relates the rate of H₂ formation to the photon flux:The results are shown in Fig. 6. The AQE values as a function of wavelength for RhO_x/TiO₂–Pt closely matched those for RhO_x/TiO₂, indicating that H₂ evolution was primarily driven by photoabsorption associated with Rh species on the TiO₂ surface. These results confirm that the RhO_x/TiO₂(500–400)–Pt(0.5) photocatalyst is active under visible light irradiation up to 550 nm, consistent with the visible-light absorption properties introduced by the Rh species.

Fig. 6 (a) UV-vis diffuse reflection spectra of RhO_x/TiO₂(500–400), and (b) action spectrum of RhO_x/TiO₂(500–400)–Pt in formation of H₂ from 2-propanol.

3.6 Effect of physical properties of TiO₂ and the amount of Pt co-catalyst

The effect of pre-calcination temperature (T₁) on the catalytic performance of RhO_x/TiO₂(T₁–400)–Pt(0.5) was investigated. Pre-calcination influences the physical properties and crystal structure of the TiO₂ support. The XRD patterns of TiO₂ samples calcined at different temperatures are shown in Fig. S7 (SI). All the diffraction patterns for the samples calcined in the 400–800 °C range were characteristic of the anatase TiO₂ phase (JCPDS No. 21-1272). Table 1 summarizes the H₂ yields after 3 h of visible light irradiation, along with the specific surface area and crystalline phase of TiO₂ prior to Rh³⁺ modification. All samples showed measurable H₂ production, with activity generally correlating with the specific surface area of the TiO₂, regardless of its crystalline phase. Among the tested samples, RhO_x/TiO₂(500–400)–Pt(0.5), which utilized anatase-type TiO₂, exhibited the highest H₂ yield of 8.4 μmol after 3 h.

Table 1 Specific surface area and crystalline phase of TiO₂ before the modification with Rh³⁺ and the amount of H₂ produced from 2-propanol suspensions of RhO_x/TiO₂(T₁–400)–Pt(Y) under irradiation of light from Xe lamp with cut filter Y-43 for 3 h

Entry	Sample	SBET/m^2 g^−1	Crystal structure^a	H2 yield/μmol
a A: anatase, R: rutile.
1	RhOx/TiO2(500–400)–Pt(0.5)	89	A	8.4
2	RhOx/TiO2(600–400)–Pt(0.5)	68	A	7.2
3	RhOx/TiO2(700–400)–Pt(0.5)	40	A	5.3
4	RhOx/TiO2(800–400)–Pt(0.5)	16	A/R	2.4
5	RhOx/TiO2(900–400)–Pt(0.5)	5.6	R	0.56
6	RhOx/TiO2(500–400)–Pt(0.2)	89	A	3.8
7	RhOx/TiO2(500–400)–Pt(0.3)	89	A	5.0
8	RhOx/TiO2(500–400)–Pt(0.4)	89	A	6.1
9	RhOx/TiO2(500–400)–Pt(0.6)	89	A	6.0
10	RhOx/TiO2(500–400)–Pt(0.8)	89	A	5.0

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To evaluate the influence of Pt loading on the H₂ evolution rate, a series of RhO_x/TiO₂(500–400)–Pt(Y) photocatalysts with varying Pt contents (Y) were prepared. The corresponding H₂ yields under visible light irradiation (after 3 h) are also shown in Table 1 (entries 1 and 5–10). The Pt-free RhO_x/TiO₂(500–400) sample produced only a small amount of H₂ (1.8 μmol), consistent with previous results. A low Pt loading of just 0.2 wt% significantly enhanced H₂ evolution (3.8 μmol), indicating that Pt nanoparticles function efficiently as electron sinks (reduction sites). The H₂ yield continued to increase with Pt loading, reaching a maximum of 8.4 μmol at 0.5 wt% Pt. However, further increases in Pt content (Y > 0.5 wt%) resulted in decreased activity. TEM and chemical analysis of the liquid phase confirmed that all Pt particles prepared by the CS method were successfully loaded onto TiO₂ without size variation. Therefore, higher Pt loadings correspond to a greater number of Pt particles, not larger particles. The decline in photocatalytic activity at Pt loadings above 0.5 wt% is attributed to increased light scattering by the excess Pt particles, which reduces the efficiency of light absorption by the active RhO_x/TiO₂ component.

3.7 H₂ production by dehydrogenation of various alcohols

To explore the versatility of the RhO_x/TiO₂–Pt photocatalyst for alcohol dehydrogenation under visible light, a range of alcohols were tested as substrates. As summarized in Table 2, the RhO_x/TiO₂(500–400)–Pt(0.5) photocatalyst was effective in promoting the dehydrogenation of various types of alcohols, including primary aliphatic alcohols (methanol, ethanol, 1-propanol, 1-butanol, and 1-octanol), secondary aliphatic alcohols (2-propanol, 2-butanol, and 2-pentanol), alicyclic alcohol (cyclohexanol), primary aromatic alcohols (benzyl alcohol and 2-phenylethanol), secondary aromatic alcohol (1-phenylethanol). These results demonstrate that the RhO_x/TiO₂–Pt photocatalyst has a broad substrate scope for visible-light-driven alcohol dehydrogenation. For example, in the case of benzyl alcohol (entry 10), 0.54 μmol of H₂ was generated after 3 h of irradiation. After 20 h, 3.5 μmol of H₂ was detected in the gas phase, while 3.6 μmol of benzaldehyde was found in the liquid phase. The close agreement between the amounts of H₂ and benzaldehyde confirms the stoichiometric dehydrogenation of benzyl alcohol, as described by eqn (3):

C₆H₅CH₂OH → C₆H₅CHO + H₂. (3)

This result also indicates that benzaldehyde remained stable under the reaction conditions and was not further oxidized to benzoic acid, despite its known susceptibility to oxidation. The sustained high selectivity toward benzaldehyde highlights the mild and controlled nature of this photocatalytic process.

Table 2 Amounts of H₂ produced from various alcohol suspensions of RhO_x/TiO₂(500–400)–Pt(0.5) under irradiation of light from Xe lamp with cut filter Y-43 for 3 h

Entry	Alcohols	H2/μmol
1	Methanol	3.6
2	Ethanol	2.3
3	1-Propanol	3.6
4	1-Butanol	3.0
5	1-Octanol	0.33
6	2-Propanol	8.4
7	2-Butanol	6.5
8	2-Pentanol	4.8
9	Cyclohexanol	2.2
10	Benzyl alcohol	0.54
11	1-Phenylethanol	0.39
12	2-Phenylethanol	0.28

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3.8 Expected reaction mechanism

Electron transfer from Rh species to the TiO₂ substrate under visible light irradiation has been confirmed by electrochemical measurements.⁸ Based on these findings, the proposed mechanism for alcohol dehydrogenation, namely, the production of H₂ and the oxidation of alcohols, over the RhO_x/TiO₂–Pt photocatalyst under visible light is illustrated in Fig. 7.

Fig. 7 Expected reaction mechanism for formation of H₂ and carbonyl compounds from various alcohols over RhO_x/TiO₂–Pt photocatalyst under irradiation of visible light.

The overall reaction proceeds through the following four steps:

1. Photon absorption: incident visible light is absorbed by the Rh species on the TiO₂ surface.

2. Electron injection: the excited Rh species inject electrons into the conduction band of TiO₂.

3. Alcohol oxidation: the resulting electron-deficient Rh species oxidize alcohols to their corresponding carbonyl compounds and are thereby regenerated.

4. H₂ evolution: the injected electrons in the TiO₂ conduction band migrate to the Pt co-catalyst, where protons (H⁺) are reduced to generate molecular hydrogen (H₂).

This mechanism highlights the synergistic role of Rh and Pt species: Rh serves as a visible-light sensitizer and oxidation site, while Pt acts as a reduction site for hydrogen evolution.

Conclusions

TiO₂ was successfully modified through the equilibrium adsorption of Rh ions followed by post-calcination, resulting in the formation of RhO_x/TiO₂. The resulting photocatalyst was employed for alcohol dehydrogenation, leading to the production of H₂ and carbonyl compounds under visible light irradiation from an Xe lamp equipped with a Y-43 cutoff filter. The dehydrogenation rate increased significantly upon further modification of RhO_x/TiO₂ with Pt using a colloid impregnation method. In the RhO_x/TiO₂–Pt photocatalyst system, TiO₂ serves as the support, RhO_x acts as the visible-light sensitizer, and Pt functions as the co-catalyst for H₂ evolution. The overall reaction performance was found to depend on three key factors: the pre-calcination temperature, the post-calcination temperature, and the Pt loading. Optimal conditions were identified for each parameter. Among the tested compositions, RhO_x/TiO₂(500–400)–Pt(0.5), based on anatase-phase TiO₂, exhibited the highest H₂ yield. Furthermore, the RhO_x/TiO₂–Pt photocatalyst demonstrated effective activity for the dehydrogenation of a range of alcohols under visible light irradiation.

Author contributions

Atsuhiro Tanaka: conceptualization, formal analysis, writing – original draft, review & editing, supervision, funding acquisition, project administration, final approval of the article. Masaaki Fukuda: investigation, acquisition of data, analysis of data, final approval of the article. Takumi Nagai: investigation, acquisition of data, analysis of data, final approval of the article. Hiroyuki Asakura: acquisition of data, analysis and interpretation of data, final approval of the article. Hiroshi Kominami: writing – review & editing, supervision, funding acquisition, project administration, final approval of the article.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information: Fig. S1–S3 See DOI: https://doi.org/10.1039/d5cy00963d.

Acknowledgements

This work was supported by KAKENHI 22H00274, 23H01767 and 23K17964 from the Japan Society for the Promotion of Science. A. T. is grateful for financial support from Proterial Materials Science Foundation. H. K. expresses gratitude for the financial support from Nippon Sheet Glass Foundation for Materials Science and Engineering. XAS experiments were performed at BL01B1, SPring-8 with the approval of JASRI (Proposal No. 2023A1533).

References

1. R. Medhi, M. D. Marquez and T. R. Lee, ACS Appl. Nano Mater., 2020, 3, 6156–6185.
2. H. Kisch, L. Zang, C. Lange, W. F. Maier, C. Antonius and D. Meissner, Angew. Chem., Int. Ed., 1998, 37, 3034–3036; H. Kominami, K. Sumida, K. Yamamoto, N. Kondo, K. Hashimoto and Y. Kera, Res. Chem. Intermed., 2008, 34, 587–601; K. Hashimoto, K. Sumida, S. Kitano, K. Yamamoto, N. Kondo, Y. Kera and H. Kominami, Catal. Today, 2009, 144, 37–41.
3. K. Maeda, K. Ishimaki, Y. Tokunaga, D. Lu and M. Eguchi, Angew. Chem., Int. Ed., 2016, 55, 8309–8313.
4. N. Murakami, T. Chiyoya, T. Tsubota and T. Ohno, Appl. Catal., A, 2008, 348, 148–152; H. Irie, S. Miura, K. Kamiya and K. Hashimoto, Chem. Phys. Lett., 2008, 457, 202–205; H. Irie, T. Shibanuma, K. Kamiya, S. Miura, T. Yokoyama and K. Hashimoto, Appl. Catal., B, 2010, 96, 142–147; F. Kuttassery, D. Yamamoto, S. Mathew, S. Remello, A. Thomas, Y. Nabetani, A. Iwase, A. Kudo, H. Tachibana and H. Inoue, J. Photochem. Photobiol., A, 2018, 358, 386–394.
5. H. Yu, H. Irie, Y. Shimodaira, Y. Hosogi, Y. Kuroda, M. Miyauchi and K. Hashimoto, J. Phys. Chem. C, 2010, 114, 16481–16487; Q. Jin, T. Ikeda, M. Fujishima and H. Tada, Chem. Commun., 2011, 47, 8814–8816; H. Tada, Q. Jin, H. Nishijima, H. Yamamoto, M. Fujishima, S. Okuoka, T. Hattori, Y. Sumida and H. Kobayashi, Angew. Chem., Int. Ed., 2011, 50, 3501–3505; K. Maeda, K. Ishimaki, M. Okazaki, T. Kanazawa, D. Lu, S. Nozawa, H. Kato and M. Kakihana, ACS Appl. Mater. Interfaces, 2017, 9, 6114–6122; M. Okazaki, Y. Wang, T. Yokoi and K. Maeda, J. Phys. Chem. C, 2019, 123, 10429–10434.
6. E. Reisner, D. Powell, C. Cavazza, J. Fontecilla-Camps and F. Armstrong, J. Am. Chem. Soc., 2009, 131, 18457–18466; K. Dalle, J. Warnan, J. Leung, B. Reuillard, I. Karmel and E. Reisner, Chem. Rev., 2019, 119, 2752–2875; H. Moon and K. Yong, Appl. Surf. Sci., 2020, 530, 147215; A. Charisiadis, V. Nikolaou, E. Nikoloudakis, K. Ladomenou, G. Charalambidis and A. Coutsolelos, Chem. Commun., 2025, 61, 4630–4646.
7. N. Wang, L. Zhu, Y. Huang, Y. She, Y. Yu and H. Tang, J. Catal., 2009, 266, 199–206; G. Zhang, G. Kim and W. Choi, Energy Environ. Sci., 2014, 7, 954–966; D. Won, J. Lee, J. Ji, W. Jung, H. Son, C. Pac and S. Kang, J. Am. Chem. Soc., 2015, 137, 13679–13690; D. Franchi and Z. Amara, ACS Sustainable Chem. Eng., 2020, 8, 15405–15429; N. O. Balayeva, Z. Mamiyev, R. Dillert, N. Zheng and D. W. Bahnemann, ACS Catal., 2020, 10, 5542–5553.
8. S. Kitano, N. Murakami, T. Ohno, Y. Mitani, Y. Nosaka, H. Asakura, K. Teramura, T. Tanaka, H. Tada, K. Hashimoto and H. Kominami, J. Phys. Chem. C, 2013, 117, 11008–11016.
9. S. Kitano, A. Tanaka, K. Hashimoto and H. Kominami, Phys. Chem. Chem. Phys., 2014, 16, 12554–12559; S. Kitano, A. Tanaka, K. Hashimoto and H. Kominami, Appl. Catal., A, 2016, 521, 202–207.
10. S. Kitano, M. Sadakiyo, K. Kato, M. Yamauchi, H. Asakura, T. Tanaka, K. Hashimoto and H. Kominami, Appl. Catal., B, 2017, 205, 340–346.
11. H. Kominami, A. Furusho, S. Murakami, H. Inoue, Y. Kera and B. Ohtani, Catal. Lett., 2001, 76, 31–34.
12. H. Kominami, S. Yamamoto, K. Imamura, A. Tanaka and K. Hashimoto, Chem. Commun., 2014, 50, 4558–4560.
13. R. Singh and S. Dutta, Fuel, 2018, 220, 607–620; F. Bhom and Y. Isa, Glob. Chall., 2024, 8, 2400134.
14. T. Hisatomi and K. Domen, Nat. Catal., 2019, 2, 387–399.
15. K. Imamura, H. Tsukahara, K. Hamamichi, N. Seto, K. Hashimoto and H. Kominami, Appl. Catal., A, 2013, 450, 28–33; Z. Liu, J. Caner, A. Kudo, H. Naka and S. Saito, Chem. – Eur. J., 2013, 19, 9452–9456; M.-Y. Qi, M. Conte, M. Anpo, Z.-R. Tang and Y.-J. Xu, Chem. Rev., 2021, 121, 13051–13085.
16. H. Kominami, M. Kohno, Y. Takada, M. Inoue, T. Inui and Y. Kera, Ind. Eng. Chem. Res., 1999, 38, 3925–3931.
17. G. Frens, Nat. Phys. Sci., 1973, 241, 20–22.
18. A. Tanaka, A. Ogino, M. Iwaki, K. Hashimoto, A. Ohnuma, F. Amano, B. Ohtani and H. Kominami, Langmuir, 2012, 28, 13105–13111.
19. B. Ravel and M. Newville, J. Synchrotron Radiat., 2005, 12, 537–541.

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