Photocatalytic Hydrogen Production from Formic Acid Solution with Titanium Dioxide with the Aid of SimultaneousRh Deposition

Photocatalytic hydrogen production was studied with a formic acid solution with titanium dioxide (TiO2) with the aid of simultaneous Rh deposition. The optimum conditions were as follows: Rh loading, 0.1 wt%; formic acid concentration, 1.0%; solution, pH 2.2; temperature, 50 ◦C. Under the optimum conditions, the photocatalytic hydrogen production with TiO2 by the simultaneous deposition of Rh was 5.0 mmol g−1, 12.2 mmol g−1 and 16.0 mmol g−1 after 1 h, 3 h and 5 h of irradiation time for black light, respectively. Rh/TiO2 photocatalysts were characterized by XRD, SEM, photoluminescence spectra, diffuse reflectance spectra and the BET surface area. The reaction mechanism of photocatalytic hydrogen production from formic acid by Rh/TiO2 was also proposed.


Introduction
In recent years, the excessive depletion of fossil fuel resources, global warming, environmental pollution and high energy demand have become serious concerns in the world [1,2]. Hence, environmentally friendly renewable energy resources, such as solar power, wind, tide, heat, biomass, geothermal, ocean, hydropower, nuclear and hydrogen energy, are needed to replace fossil fuels [3]. Hydrogen plays an important role as a renewable energy resource as a result of its unique energy storage, cleanliness, longevity, sustainability and renovation properties [3,4]. Various methods such as steam reforming, partial oxidation and the self-thermal reforming of hydrocarbons as well as fossil resources, the electrolysis of water (alkaline water electrolysis, high temperature steam electrolysis, electrolysis with a steam polymer), the pyrolysis of water, the gasification of biomass and photocatalytic water splitting have been used to produce hydrogen [3,5,6]. Steam reforming, the pyrolysis of water, the electrolysis of water and the gasification of biomass are expensive and produce carbon dioxide (CO 2 ) gas. Furthermore, high thermal energy is also required for these reactions. Therefore, these reactions are not appropriate for sustainable hydrogen production [3]. In contrast, only sunlight and photocatalysts are required for photocatalytic reaction. Furthermore, it can occur under ambient conditions. Hence, photocatalytic water splitting for hydrogen production has recently been the most attractive option due to its cost effective, environmentally friendly and pollution-free nature [3,6]. Several types of semiconductors as photocatalysts have been used in photocatalytic hydrogen production reaction under the irradiation of ultraviolet and visible light [3,7]. For instance, metal oxides (TiO 2 , ZnO, CuO, ZrO 2 , Fe 2 O 3 , VO 2 , WO 3 ), chlacogenides (ZnS, CdS, CdSe), halides (AgX), carbides (SiC) and carbonaceous materials (g-C 3 N 4 ) have been widely used for hydrogen

Photocalytic Hydrogen Production
Hydrogen generation experiments with TiO 2 powder were carried out by using simultaneous Rh deposition. The pyrex column vessel reactor (inner volume, 123 mL) was used for the photocatalytic hydrogen production from formic acid. Normally, 50 mg of the TiO 2 photocatalysts was added to 40 mL of the formic acid solution. Then, the solution containing Rh 3+ was added to the reactor, and the concentration of Rh 3+ was 1.25 ppm. A 15 W black lamp with an emission of about 352 nm (Toshiba Lighting & Technology Corp., Tokyo, Japan) was placed to the side of the pyrex vessel reactor as a light source. The light intensity was measured by a UV radio meter (UIT-201, Ushio Inc., Tokyo, Japan), and the value was 0.25 mW/cm 2 . The TiO 2 photocatalyst was continuously stirred in the formic acid solution by a magnetic stirrer during the irradiation of light. Using a hot stirrer, the reactor temperature was kept constant at 50 • C. The reactor was sealed with a silicon septum. The irradiation time was 3 h. The generated gas was extracted from the upper part of the reactor with a microsyringe (ITO, Co., Ltd., Tokyo, Japan) and measured by gas chromatography (GL Sciences, GC-3200, Japan) with a thermal conductivity detector. The stainless column (4 m long, 2.17 mm i.d.) packed with a Molecular Sieve 5A (mesh, 60-80) was used for the separation. The carrier gas was 99.9% argon gas (Kawase Sangyo Co., Ltd., Mie, Japan). The temperature conditions of the GC were 50 • C for the injection, column and detector. The flow rate of the carrier gas was 7.0 mL/min. The analysis time and analysis sample amount were 10 min and 250 µL, respectively. The reproducibility of the photocatalytic H 2 production test (relative standard deviation) was within an RSD of 10% for more than three of the run numbers.

Characterization of Photocatalysts
After the photocatalytic hydrogen generation experiment, the TiO 2 solution was centrifuged. Then, the supernatant and the precipitate of the Rh-deposited TiO 2 were separated, and the precipitate (Rh/TiO 2 ) was dried. The dried photocatalyst was crushed in an agate mortar for 15 min to obtain a photocatalyst powder. Finally, the obtained Rh-deposited TiO 2 and the pure TiO 2 were analyzed by SEM imaging, BET surface area measurement, X-ray diffraction, photoluminescence spectrum measurement and diffuse reflection spectrum measurement. X-ray powder diffraction (XRD) measurements were performed using a Rigaku RINT Ultima-IV diffractometer by Cu radiation at a scan rate of 0.04 • /s in a scan range of 10-80 • . The nitrogen adsorption and desorption isotherm and the Brunaure Emmett Teller (BET) specific surface area were measured by using a BEL PREP-vacIIBET surface area measuring device (MicrotracBEL Corp., Osaka, Japan). To determine the particle size of the photocatalysts, scanning electron microscope (SEM) observations were performed using a Hitachi S-4000 SEM with an accelerating voltage of 25 kV. The photoluminescence (PL) spectra of photocatalysts were observed using an RF-5300PC spectrofluorophotometer (SHIMADZU, Kyoto, Japan). The diffuse reflectance spectra of the photocatalysts were measured with a UV2450 UV-vis system (SHIMADZU, Kyoto, Japan). BaSO 4 was kept as a reference material in the diffuse reflectance spectra measurement.

Effect of Rh Ion Concentration
The effect of Rh ion concentration using TiO 2 with simultaneous deposition on the photocatalytic hydrogen production was investigated. The results are shown in Figure 1. It was observed that the amount of hydrogen production increased sharply with the increase in Rh 3+ ion concentration. However, there is no dramatic change in the increase in hydrogen production after the addition of an Rh 3+ ion concentration of 1.25 ppm. If we assume that all Rh 3+ ions of a 1.25 ppm solution were deposited after the reaction, the Rh content on the TiO 2 photocatalyst would be 0.1 wt%. Since the trace amount of hydrogen was produced in the absence of the Rh 3+ ions, the amount of hydrogen production was increased by about 250 times in addition to the 0.1 wt% Rh in TiO 2 with the aid of simultaneous deposition. The light filtration by the photo-deposited metal on the TiO 2 surface, the fractional blockage of the surface active site for TiO 2 in the oxidative branch at the photoreaction period and the decline in catalytic activity of the Rh/TiO 2 nanoparticle by its enlargement could be responsible for the almost constant amount of hydrogen generation at higher concentrations of Rh [20,21].

Effect of the Simultaneous Deposition of Rh in TiO 2
The effect of the simultaneous deposition of Rh in TiO 2 was investigated by using previously prepared Rh/TiO 2 and by simultaneously photo-depositing Rh 3+ on TiO 2 on the photocatalytic hydrogen production from the formic acid solution. The results are shown in Figure 2. It was observed in every case that the hydrogen production from the formic acid solution by the simultaneous addition of Rh 3+ in TiO 2 was significantly larger compared with that obtained by the prepared Rh/TiO 2 photocatalyst. The freshly deposited Rh metal on the TiO 2 surface enhanced the photocatalytic hydrogen production activity.

Effect of the Simultaneous Deposition of Rh in TiO2
The effect of the simultaneous deposition of Rh in TiO2 was investigated by using previously prepared Rh/TiO2 and by simultaneously photo-depositing Rh 3+ on TiO2 on the photocatalytic hydrogen production from the formic acid solution. The results are shown in Figure 2. It was observed in every case that the hydrogen production from the formic acid solution by the simultaneous addition of Rh 3+ in TiO2 was significantly larger compared with that obtained by the prepared Rh/TiO2 photocatalyst. The freshly deposited Rh metal on the TiO2 surface enhanced the photocatalytic hydrogen production activity.

Effect of the Simultaneous Deposition of Rh in TiO2
The effect of the simultaneous deposition of Rh in TiO2 was investigated by using previously prepared Rh/TiO2 and by simultaneously photo-depositing Rh 3+ on TiO2 on the photocatalytic hydrogen production from the formic acid solution. The results are shown in Figure 2. It was observed in every case that the hydrogen production from the formic acid solution by the simultaneous addition of Rh 3+ in TiO2 was significantly larger compared with that obtained by the prepared Rh/TiO2 photocatalyst. The freshly deposited Rh metal on the TiO2 surface enhanced the photocatalytic hydrogen production activity. Rh (wt%) Figure 2. Comparison of the activity for the simultaneous photo-deposition of Rh in TiO 2 (blue) with the prepared Rh/TiO 2 (red). TiO 2 , 50 mg; Rh, 1.25 ppm; reaction time, 3 h; reaction temperature, 50 • C; formic acid concentration, 1 wt%.

Effect of Formic Acid Concentration
The effect of formic acid concentration on hydrogen generation using TiO 2 with the aid of simultaneous Rh photo-deposition was investigated. The results are shown in Figure 3. It was observed that little hydrogen production occurred from the pure water. However, the photocatalytic hydrogen production increased with the increasing formic acid concentration, and the amount of hydrogen production remained almost constant in the case of using more than 1.0 wt% of formic acid. The active sites of the TiO 2 surface were saturated with increasing formic acid concentrations, which might result in a decrease in hydrogen generation. Similar results were reported for photocatalytic hydrogen production by CuO@NiO from glycerol [27].

Effect of Formic Acid Concentration
The effect of formic acid concentration on hydrogen generation using TiO2 with the aid of simultaneous Rh photo-deposition was investigated. The results are shown in Figure 3. It was observed that little hydrogen production occurred from the pure water. However, the photocatalytic hydrogen production increased with the increasing formic acid concentration, and the amount of hydrogen production remained almost constant in the case of using more than 1.0 wt% of formic acid. The active sites of the TiO2 surface were saturated with increasing formic acid concentrations, which might result in a decrease in hydrogen generation. Similar results were reported for photocatalytic hydrogen production by CuO@NiO from glycerol [27].

Effect of the pH of the Reaction Solution
The effect of pH on hydrogen generation using TiO2 with the aid of simultaneous Rh photo-deposition from a formic acid solution was investigated. The results are shown in Figure 4. It was observed that the maximum amount of hydrogen was produced (12.2 mmol g −1 ) at pH 2.2. Moreover, the initial pH of the reaction solution containing 1.0 wt% formic acid solution was 2.2. Hence, the adjustment of the pH of the reaction solution in the subsequent experiments was unnecessary. This is because at lower pH values, more H + ions would be adsorbed on the surface of the TiO2 photocatalyst, and the results were reasonable. Therefore, the reduction of the H + ion to H2 was also preferred at a lower pH [28]. However, it may be partially difficult to photocatalytically deposit the Rh 3+ ion on the TiO2 surface at pH 1, although the chemical stability of TiO2 could remain at pH 1. [HCOOH] (%)

Effect of the pH of the Reaction Solution
The effect of pH on hydrogen generation using TiO 2 with the aid of simultaneous Rh photo-deposition from a formic acid solution was investigated. The results are shown in Figure 4. It was observed that the maximum amount of hydrogen was produced (12.2 mmol g −1 ) at pH 2.2. Moreover, the initial pH of the reaction solution containing 1.0 wt% formic acid solution was 2.2. Hence, the adjustment of the pH of the reaction solution in the subsequent experiments was unnecessary. This is because at lower pH values, more H + ions would be adsorbed on the surface of the TiO 2 photocatalyst, and the results were reasonable. Therefore, the reduction of the H + ion to H 2 was also preferred at a lower pH [28]. However, it may be partially difficult to photocatalytically deposit the Rh 3+ ion on the TiO 2 surface at pH 1, although the chemical stability of TiO 2 could remain at pH 1.

Effect of Temperature
The effect of temperature on hydrogen generation using TiO 2 with the aid of simultaneous Rh photo-deposition from a formic acid solution was investigated. The results are shown in Figure 5. It was observed that the amount of hydrogen production increased with the increase in the reaction temperature. Similar results were reported for photocatalytic hydrogen production using Pt/TiO 2 at different temperatures [29]. However, 50 • C was selected as the optimum temperature, since this temperature was possible owing to the waste heat.

Effect of NaCl Concentration
Seawater contains sodium chloride of about 3.0 wt%. Therefore, the effect of NaCl concentration on photocatalaytic hydrogen generation using TiO 2 with the aid of simultaneous Rh deposition from a formic acid solution was inspected. It was observed that the production of hydrogen decreased with the addition of sodium chloride ( Figure 6). The dissolved chloride ion in the aqueous formic acid solution was adsorbed on the surface of the TiO 2 . The chloride on the surface hindered the adsorption of formic acid on the photocatalyst [30]. Thus, the generation of hydrogen gas was disturbed by the addition of NaCl.

Effect of Temperature
The effect of temperature on hydrogen generation using TiO2 with the aid of simultaneous Rh photo-deposition from a formic acid solution was investigated. The results are shown in Figure 5. It was observed that the amount of hydrogen production increased with the increase in the reaction temperature. Similar results were reported for photocatalytic hydrogen production using Pt/TiO2 at different temperatures [29]. However, 50 °C was selected as the optimum temperature, since this temperature was possible owing to the waste heat.

Effect of Temperature
The effect of temperature on hydrogen generation using TiO2 with the aid of simultaneous Rh photo-deposition from a formic acid solution was investigated. The results are shown in Figure 5. It was observed that the amount of hydrogen production increased with the increase in the reaction temperature. Similar results were reported for photocatalytic hydrogen production using Pt/TiO2 at different temperatures [29]. However, 50 °C was selected as the optimum temperature, since this temperature was possible owing to the waste heat. concentration on photocatalaytic hydrogen generation using TiO2 with the aid of simultaneous Rh deposition from a formic acid solution was inspected. It was observed that the production of hydrogen decreased with the addition of sodium chloride ( Figure 6). The dissolved chloride ion in the aqueous formic acid solution was adsorbed on the surface of the TiO2. The chloride on the surface hindered the adsorption of formic acid on the photocatalyst [30]. Thus, the generation of hydrogen gas was disturbed by the addition of NaCl.

Effect of Formate Type
Various formate solutions were tested for photocatalytic hydrogen using TiO2 with the aid of simultaneous Rh photo-deposition. Among these formates, a significant amount of hydrogen evolved from the formic acid solution (Figure 7). This result indicates that negligible amounts of hydrogen were produced from the direct hydrolysis of the formate ions (HCOO − + H2O → H2 + HCO3 − ) [31]. Therefore, formic acid can act as a better scavenging agent compared to ammonium formate, lithium formate, sodium formate and potassium formate.

XRD Analysis
The XRD of the TiO2 and collected Rh/TiO2 photocatalysts were analyzed. The results are shown in Figure 8

Effect of Formate Type
Various formate solutions were tested for photocatalytic hydrogen using TiO 2 with the aid of simultaneous Rh photo-deposition. Among these formates, a significant amount of hydrogen evolved from the formic acid solution (Figure 7). This result indicates that negligible amounts of hydrogen were produced from the direct hydrolysis of the formate ions (HCOO − + H 2 O → H 2 + HCO 3 − ) [31]. Therefore, formic acid can act as a better scavenging agent compared to ammonium formate, lithium formate, sodium formate and potassium formate. TiO2 [32]. The TiO2 photocatalyst also exhibits peaks at 2θ = 27.42°, 36.14° and 41.34°. These three peaks could be attributed to the rutile phase of TiO2. Similar peaks could be observed for the Rh/TiO2 photocatalyst. Furthermore, any additional peak for the Rh/TiO2 photocatalyst could hardly be observed. These facts indicate that the Rh metal was well dispersed on the TiO2 crystal [33]. Hence, the crystal phase of the TiO2 may scarcely change after the photo-deposition of the Rh metal.   (215) planes, respectively. These peaks were observed for anatase phase of TiO 2 . These results are in agreement with the previously reported work on pure TiO 2 [32]. The TiO 2 photocatalyst also exhibits peaks at 2θ = 27.42 • , 36.14 • and 41.34 • . These three peaks could be attributed to the rutile phase of TiO 2 . Similar peaks could be observed for the Rh/TiO 2 photocatalyst. Furthermore, any additional peak for the Rh/TiO 2 photocatalyst could hardly be observed. These facts indicate that the Rh metal was well dispersed on the TiO 2 crystal [33]. Hence, the crystal phase of the TiO 2 may scarcely change after the photo-deposition of the Rh metal.

SEM Analysis
The SEM images of both the TiO 2 and Rh/TiO 2 photocatalysts are shown in Figure 9. Figure 9A shows the spherical morphology of the TiO 2 with a particle size of 30 nm. A similar particle size and shape are also observed for the Rh/TiO 2 ( Figure 9B). In the SEM image of the Rh/TiO 2 photocatalyst, the Rh particle could be hardly observed as a discrete particle, which indicated that very fine Rh particles were uniformly dispersed on the TiO 2 [4].

SEM Analysis
The SEM images of both the TiO2 and Rh/TiO2 photocatalysts are shown in Figure 9. Figure 9A shows the spherical morphology of the TiO2 with a particle size of 30 nm. A similar particle size and shape are also observed for the Rh/TiO2 ( Figure 9B). In the SEM image of the Rh/TiO2 photocatalyst, the Rh particle could be hardly observed as a discrete particle, which indicated that very fine Rh particles were uniformly dispersed on the TiO2 [4].

PL Analysis
Generally, the weak fluorescence intensity is responsible for the lower recombination of the photogenerated electron hole pair. From Figure 10, it was seen that the peak intensity of the PL spectra for Rh/TiO2 was lower than that of TiO2, and it was observed

PL Analysis
Generally, the weak fluorescence intensity is responsible for the lower recombination of the photogenerated electron hole pair. From Figure 10, it was seen that the peak intensity of the PL spectra for Rh/TiO 2 was lower than that of TiO 2 , and it was observed that the photocatalytic activity of Rh/TiO 2 is greater than that of TiO 2 . Thus, photogenerated carrier recombination reduction in the photo-deposition of Rh on TiO 2 was confirmed by the PL spectra [34].

PL Analysis
Generally, the weak fluorescence intensity is responsible for the lower recombination of the photogenerated electron hole pair. From Figure 10, it was seen that the peak intensity of the PL spectra for Rh/TiO2 was lower than that of TiO2, and it was observed that the photocatalytic activity of Rh/TiO2 is greater than that of TiO2. Thus, photogenerated carrier recombination reduction in the photo-deposition of Rh on TiO2 was confirmed by the PL spectra [34].

UV-Vis Diffuse Reflection Spectrum Analysis
UV-Vis diffuse reflection spectroscopy was used to characterize the absorption edge and band gap shift of the TiO 2 before and after the photo-deposition of the Rh metal. In general, the band gap in a semiconductor is related with the absorbed wavelength, where the band gap decreases with the increase in the absorption edges [35]. The reflectance data was converted to the absorption coefficient F(R) values according to the Kubelka-Munk equation; then, the corresponding Tauc plots (plotting αhν vs. hν) were determined for the band gap energy of the photocatalysts ( Figure 11) [36]. It was observed that the absorption edges of the spectra were slightly red shifted to a higher wavelength from 389 nm to 396 nm after the Rh was photo-deposited on the TiO 2 . The metal doping on the TiO 2 could increase the absorption edge and decrease the band gap [4].

BET Surface Area
The N 2 adsorption and desorption isotherms of both photocatalysts at 77 K were measured. The results are shown in Figure 12. The isotherm of TiO 2 and Rh/TiO 2 exhibits a typical type IV isotherm, according to the classification of the adsorption and desorption isotherms by IUPAC. The results indicate that both photocatalysts were porous materials. The BET surface area, total pore volume and average pore diameter of the photocatalysts were determined from the isotherm and are presented in Table 1. It was observed that the BET surface area, total pore volume and average pore diameter of the Rh/TiO 2 increased after the photo-deposition of Rh. More photocatalytic activity for hydrogen production would be correlated with a greater BET surface area, total pore volume and average pore diameter of the photocatalyst. The photocatalytic hydrogen generation activity increased with the increase in the BET surface area and pore volume, and a decreasing pore diameter for La/TiO 2 was reported [37]. was converted to the absorption coefficient F(R) values according to the Kubelka-Munk equation; then, the corresponding Tauc plots (plotting αhν vs. hν) were determined for the band gap energy of the photocatalysts (Figure 11) [36]. It was observed that the absorption edges of the spectra were slightly red shifted to a higher wavelength from 389 nm to 396 nm after the Rh was photo-deposited on the TiO2. The metal doping on the TiO2 could increase the absorption edge and decrease the band gap [4].

Reaction Mechanism
In the present work, the photocatalytic hydrogen generation from a formic acid solution using the TiO 2 photocatalyst with the simultaneous photo-deposition of Rh metal was better relative to the hydrogen generation with bare TiO 2 . On the basis of the characterization of the photocatalyst, the reasons for this may be as follows: (1) the progress of the electron-hole separation, (2) the reduction of the recombination of the electron-hole separation, (3) the metallic catalyst and (4) the slightly red shift of the absorption edge. On the basis of the experimental study in the present work and a few literature reviews of photocatalytical hydrogen generation by modified TiO 2, a possible mechanism is proposed in Figure 13 [1,5,20,21,24,26,[38][39][40]. The pairs of the electron-hole are generated when the TiO 2 is irradiated with UV light with a wavelength of 380 nm or less. The Rh 3+ ion is reduced to Rh metal on the TiO 2 by accepting the electrons. Furthermore, the electrons photogenerated by the TiO 2 move on the Rh. Thus, the electron-hole recombination is reduced and stimulates the hydrogen generation reaction. On the other hand, the proton and CO 2 are generated by the oxidation of water and formate ions with photogenerated holes. Afterwards, the proton is reduced by accepting the electrons on the surface of the Rh to form hydrogen. There are two possible effects on promoting hydrogen generation in this work. Firstly, the promotion of hydrogen production on the surface of Rh metal can occur. The photogenerated electron moves from the conduction band of the TiO 2 onto the surface of the Rh metal and improves the hydrogen generation by promoting the reduction reaction of the proton. Secondly, the oxidation reaction enhances the promotion of hydrogen production. Formic acid is adsorbed on the surface of the Rh metal to promote the oxidation reaction into formaldehyde. The first effect may be considered a leading one for the increase in hydrogen generation. The N2 adsorption and desorption isotherms of both photocatalysts at 77 K w measured. The results are shown in Figure 12. The isotherm of TiO2 and Rh/TiO2 exhi a typical type IV isotherm, according to the classification of the adsorption and desorp isotherms by IUPAC. The results indicate that both photocatalysts were porous mater The BET surface area, total pore volume and average pore diameter of the photocatal were determined from the isotherm and are presented in Table 1. It was observed that BET surface area, total pore volume and average pore diameter of the Rh/TiO2 increa after the photo-deposition of Rh. More photocatalytic activity for hydrogen produc would be correlated with a greater BET surface area, total pore volume and average p diameter of the photocatalyst. The photocatalytic hydrogen generation activity increa with the increase in the BET surface area and pore volume, and a decreasing pore diam for La/TiO2 was reported [37].  when the TiO2 is irradiated with UV light with a wavelength of 380 nm or less. The Rh ion is reduced to Rh metal on the TiO2 by accepting the electrons. Furthermore, the electrons photogenerated by the TiO2 move on the Rh. Thus, the electron-hole recombination is reduced and stimulates the hydrogen generation reaction. On the other hand, the proton and CO2 are generated by the oxidation of water and formate ions with photogenerated holes. Afterwards, the proton is reduced by accepting the electrons on the surface of the Rh to form hydrogen. There are two possible effects on promoting hydrogen generation in this work. Firstly, the promotion of hydrogen production on the surface of Rh metal can occur. The photogenerated electron moves from the conduction band of the TiO2 onto the surface of the Rh metal and improves the hydrogen generation by promoting the reduction reaction of the proton. Secondly, the oxidation reaction enhances the promotion of hydrogen production. Formic acid is adsorbed on the surface of the Rh metal to promote the oxidation reaction into formaldehyde. The first effect may be considered a leading one for the increase in hydrogen generation.

Conclusions
In summary, it was found that the simultaneous photo-deposition of Rh metal on TiO 2 increased the photocatalytic hydrogen production from formic acid by TiO 2 . Under optimal conditions, the photocatalytic hydrogen generation with the aid of the simultaneous photodeposition of Rh metal on TiO 2 was about 250 times better than that obtained with the bare TiO 2 .