Effects of Reaction Temperature on the Photocatalytic Activity of TiO 2 with Pd and Cu Cocatalysts

: The aim of this study was to investigate the effects of reaction temperature on the photocatalytic activity of TiO 2 with Pd and Cu cocatalysts. N 2 sorption, transmission electron microscopy and high-resolution transmission electron microscopy were used to characterize the speciﬁc surface area, pore volume, pore size, morphology and metal distribution of the catalysts. The photocatalytic destruction of methylene blue under UV light irradiation was used to test its activity. The concentration of methylene blue in water was determined by UV-vis spectrophotometer. Pd/TiO 2 catalyst was more active than Cu/TiO 2 and TiO 2 . At 0–50 ◦ C reaction temperature, the activity of TiO 2 and Pd/TiO 2 increased with an increase of reaction temperature. When the temperature was as high as 70 ◦ C, the reaction rate of TiO 2 drop slightly and Pd/TiO 2 became less effective. In contrast, Cu/TiO 2 was more active at room temperature than the other temperatures. The results indicate that the photocatalytic activity of the catalyst is inﬂuenced by the reaction temperature and the type of cocatalyst. When the reaction temperature is higher than 70 ◦ C, the recombination of charge carriers will increase. The temperature range of 50–80 ◦ C is regarded as the ideal temperature for effective photolysis of organic matter. The effects of reaction temperature mainly inﬂuence quantum effect, i.e., electron-hole separation and recombination.

Barakat et al. [38] reported the effects of the temperature on the photodegradation process using Ag-doped TiO 2 nanostructures. They reported that the increase in the temperature has positive impact on the photocatalytic activity of Ag/TiO 2 and the highest degradation rate was at 55 • C. In contrast, in the case of the nanofibrous morphology, the temperature has negative effect and the optimum temperature was 25 • C. It shows the increase of temperature resulted in increase the kinetic energy of the dye molecules, so the molecules escape from the active thin film surrounding the photocatalyst. Kim et al. [39] investigated the effect of reaction temperature (298-353 K) on photocatalytic H 2 production in bare and Pt/TiO 2 suspensions containing Ethylenediaminetetraacetic acid (EDTA). H 2 production rates increased by~7.8-and~2.5-fold in TiO 2 and Pt/TiO 2 , respectively, from 298 K to 323 K. It shows the positive relationship between reaction temperature 298 and 323 K. The charge carrier mobility and interfacial charge transfer improves at higher temperatures. The photocatalytic degradation of organic components over the TiO 2 /sepiolite composites at different temperatures were investigated [40]. It shows that, the reaction temperature can influence the photocatalytic activity of the catalyst and the optimum temperature was 50 • C. Mateo et al. [41] gave an excellent review on photo-thermal catalysis. Through the synergistic combination of photo-and thermochemical contributions of sunlight, photo-thermal catalysis can enhance reaction rates. The temperature effect is mainly due to the localized surface plasmon resonance. However, the mechanism of reaction temperature on the photocatalytic activity is complicate and remains unclear.
The aim of this study was to investigate the effects of reaction temperature on the photocatalytic activity of TiO 2 with Pd and Cu cocatalysts in the destruction of organic dye. It is believed that the reaction temperature would influence the photocatalytic properties of TiO 2 with Pd and Cu cocatalysts, since electron-hole separation and recombination are influenced by the reaction temperature, and different cocatalysts would have different effects.

X-ray Diffraction (XRD)
The XRD patterns of the samples are shown in Figure 1. Only anatase and rutile phases were detected. The peak positions are the same among all samples, indicating that adding metals did not change the crystalline phases of TiO 2 . No XRD peaks for Pd and Cu were detected, indicating that Pd and Cu metal particles were very less than 4 nm.   Table 1 shows the surface area and pore structure of the samples. Since the commercial TiO 2 was used as the catalyst, adding small amount of metal did not change the pore structure significantly, as expected.

Transmission Electron Microscopy (TEM
The TEM images of the samples are shown in Figure 2. TiO 2 did not change its morphology after metal deposition. However, due to the low resolution of TEM, we were not able to see the clear picture of the metals. It infers that the metal particles were very small. HR-TEM was used in the further study.

High-Resolution Transmission Electron Microscopy (HRTEM) and Energy-Dispersive X-ray Spectroscopy (EDS)
HRTEM images of the Pd/TiO 2 and Cu/TiO 2 are shown in Figure 3. Very small nano metal particles were observed. The particle sizes of Pd and Cu metals were about 2 nm and were homogeneously distributed on the surface of TiO 2 . The results are consistent with the XRD results. The EDS mapping of each element on Pd/TiO 2 are shown in Figure 4. It clearly shows that Pd metal particles were homogeneously distributed on the surface of TiO 2 , in consistent with HR-TEM result. Figure 5 shows the EDS mapping of each element on Cu/TiO 2 . Cu metal particles were also homogeneously distributed on the surface of TiO 2 . The EDS results also show that the Pd and Cu metal loadings are 0.498 wt.% and 0.506 wt.%, respectively. The real metal loadings are the same as the nominal loadings within experimental error. Since the catalysts were prepared by incipient-wetness impregnation, it is very easy to have the same metal loading as that in the precursor.   Figure 6 shows the concentration of methylene blue vs. time on TiO 2 at various reaction temperatures. Some of methylene blue was adsorped on TiO 2 . Figure 7a shows the results of the C/C 0 of bare TiO 2 vs. reaction temperature, where C is the concentration of methylene blue and C 0 is the initial concentration of methylene blue. It shows that the activity at 50 • C was higher than those at other temperature. It can be noted that when the reaction temperature was 0 • C, the reaction rate was very low. It has been known from the literature that when the temperature is lower, the solubility of methylene blue in the water is lower [42]. This may cause partial condensation of methylene blue in water.

Photocatalytic Reaction
Kumar et al. [43] reported that when the reaction temperature is higher than 80 • C, the recombination of charge carriers will increase. The temperature range of 20-80 • C is regarded as the ideal temperature for effective photolysis of organic matter. In their study, four reaction temperatures were tested, i.e., 0 • C, 25 • C, 50 • C and 70 • C. They reported that the higher the temperature is, the higher the reaction rate is of TiO 2 . When the temperature reaches 70 • C, the reaction rate drops slightly. Our results are in accord. It has been reported that the photocatalytic activity of TiO 2 increased as the reaction temperature increased. In the study of Parra et al. [44], the TiO 2 (P25) was tested at 20, 40, 55 and 70 • C, respectively. Their results are in line with this study. Figure 7b shows that the kinetic studies of the degradation of methylene blue over the TiO 2 obtained at different temperatures. The summary of the pseudo-first-order kinetics of the as-prepared samples under UV light irradiation is shown in Table 2. From Figure 7b, it is plausible to suggest that the photocatalytic degradation reactions of methylene blue on the catalyst followed the pseudo-first-order reaction according to the Langmuir-Hinshelwood (LH) model and may be expressed as [17,19,26,28,44,45]: where k is the apparent reaction rate constant, C 0 is the initial concentration of methylene blue and C is the concentration of methylene blue at the reaction time t. The results of rate constants of TiO 2 at different reaction temperature are tabulated in Table 2.    Figure 8 shows the concentration of methylene blue vs. irradiation time on Pd/TiO 2 . It shows that higher amount of methylene blue was adsorbed on Pd/TiO 2 , compared with TiO 2 . It infers that Pd also can adsorb methylene blue in dark. Figure 9a shows the activities of Pd/TiO 2 at various temperatures. At 0-50 • C, the higher reaction temperature is, the higher the reaction rate is. As the reaction temperature increases, the movement of photoelectron-hole pairs becomes more active, electrons can combine with adsorbed oxygen faster and holes can generate OH radicals together with -OH faster [46][47][48], thereby improving the degradation efficiency of methylene blue. However, when the reaction temperature raised to 70 • C, the activity became low. This is because that adsorption is an exothermic reaction, while desorption is an endothermic reaction. Figures 6 and 8 show that the adsorption capacity of Pd/TiO 2 is greater than that of TiO 2 . When the temperature reached 70 • C, methylene blue desorbs faster from Pd/TiO 2 than that from TiO 2 . After 35 min, the degradation rate of TiO 2 was as high as 98%, but it is obvious that the original white TiO 2 was blue-violet color, which proves that most of the methylene blue was adsorbed on TiO 2 . Comparing with TiO 2 , the color of Pd/TiO 2 after degradation was the same as that of the original catalyst. It shows that Pd/TiO 2 was very active at 50 • C. The rate constant of methylene blue degradation on Pd/TiO 2 can be extracted from Figure 9b and listed in Table 2. It shows that Pd/TiO 2 was much active than TiO 2 . By elevating the reaction temperature from 25 to 50 • C, the photocatalytic activity increased, because charge carrier mobility and interfacial charge transfer improves at higher temperatures. Further increase the temperature to 70 • C, the activity decreased since the electron-hole recombination rate increased. Various cocatalysts, such as Pd and Cu, have been sued as the cocatalyst of TiO 2 [49][50][51][52][53][54][55]. The cocatalyst can suppress electron-hole recombination rate and resulted in longer life time of electron and hole and higher photocatalytic activity. Different cocatalyst has different influence on the electron-hole recombination rate. Reaction temperature also plays a crucial role on the recombination rate. Ghasemi et al. [56] accessed the thermodynamic parameters of activation. However, they only calculate the activation energy in the range of positive relationship between activity and reaction temperature. A blank test of reaction was carried out at various temperatures without catalyst. It did not have any degradation of methylene blue. Therefore, the effects of reaction temperature mainly influence quantum effect, i.e., electron-hole separation and recombination. Further research is needed to elucidate this quantum effect.    Figure 11 shows the C/C 0 of methylene blue on Cu/TiO 2 at different reaction temperatures. The activity of Cu/TiO 2 is different from those of bare TiO 2 and Pd/TiO 2 . Cu/TiO 2 has the highest reaction rate at room temperature than at the other temperatures. In the study of Meng et al. [46], a copper-containing photocatalytic material was prepared, and tested for photodegradation of Rhodamin B. At the reaction temperatures of 15, 25, 35 and 45 • C, it had the highest activity at 45 • C. In particular, it also shows that the activity at 25 • C is higher than that at 35 • C. Meng et al. [46] reported that the increase of the reaction temperature more strongly promotes the movement of the dye molecules so that the dye molecules can more easily penetrate the micropores of the material. However, when the reaction temperature continues to rise, the degradation efficiency of dye reached equilibrium. It can be seen that when the reaction temperature rises to a certain value, the effect of reaction temperature on the photocatalytic activity is small [46]. Our results show that Cu/TiO 2 had low photocatalytic activity at 0 • C. The activity increased at 25 • C and 50 • C. There is a little difference in activity between 0 • C and 50 • C.  Comparing the reaction rates of Pd/TiO 2 and Cu/TiO 2 under the most suitable conditions (Table 2), the photocatalytic activity of Pd/TiO 2 was much greater than that of Cu/TiO 2 and that of Cu/TiO 2 was slightly greater than that of TiO 2 at 25 • C. The effects of reaction temperature on the catalyst are varied by the type of cocatalyst.

Materials
Titanium dioxide (P25) was purchased from Evonik Degussa, Essen, Germany. Copper nitrate and palladium nitrate were purchased from Sigma-Aldrich, Merck KGaA, Darmstadt, Germany. Methylene blue was purchased from Alfa Aesar, Lancashire, United Kingdom.

Catalysts Preparation
Pd/TiO 2 and Cu/TiO 2 were prepared by incipient-wetness impregnation method according to the following procedure. First, metal nitrate was dissolved in 1 mL distilled water to form an aqueous solution. It was slowly added in TiO 2 powder. The impregnated sample was dried under vacuum for 3 h. The sample was illuminated under UV light for 1 day. The metal loading of the sample was 0.5 wt.%

Catalysts Characterization
3.3.1. Pore Structure of the Catalyst N 2 sorption was used to measure the specific surface area and pore structure of the sample. The BET (Brunauer-Emmet-Teller) method was used to measure specific surface area and pore volume. The average pore size and its distribution were estimated from the desorption isotherm by Barrette-Joynere-Halenda (BJH) analysis.

Transmission Electron Microscopy (TEM) and High-Resolution Transmission Electron Microscopy (HRTEM)
The morphologies, dispersive effect and particle sizes of the samples were determined by the TEM, JEOL, Tokyo, Japan, JEM-2000 FX II. HRTEM was operated on a JEOL JEM-2010 operated at 200 kV. A small amount of sample was put into a sample bottle filled with distilled water and agitated under an ultrasonic environment for 1 h. One drop of the dispersed slurry was dipped onto a carbon-coated nickel mesh (200 #; Ted Pella Inc., Altadena, CA, USA), and dried in the vacuum oven at 65 • C overnight to remove the water entirely. The chemical composition of the samples and the interaction of the metals were determined by Energy-dispersive X-ray spectroscopy (EDS) attached on the HRTEM. The HRTEM (JEOL JEM-F200, Tokyo, Japan) images were recorded digitally by a Gatan slow-scan camera (GIF) (AMETEK, Berwyn, PA, USA).

Photocatalytic Reaction
For the photocatalytic activity reaction, the catalyst was activated for 12 h by two UVC lamps (Philips, Eindhoven Holland, wavelength = 254 nm, TUV PL-L 18W/4P 1CT/25). 20 mL of methylene blue aqueous solution with a concentration of 10 mg/L was loaded in a beaker. Before performing the photocatalytic activity reaction, the reactor was kept in the dark for 1 h to reach saturated adsorption of methylene blue. The reaction temperature was varied by a heating plate through the separate water bath, using two UVC lamps from top to bottom illumination. The concentration of methylene blue in aqueous solution was determined every 5 min by a UV/Visible/NIR Spectrophotometer (Hitachi, Tokyo, Japan, U-4100). The wavelength for the measurement was 663 nm, which is the maximum characterized adsorption wavelength of methylene blue.

Conclusions
The effects of reaction temperature on the photocatalytic activity of TiO 2 with Pd and Cu cocatalysts were investigated in this study. At 0-50 • C reaction temperature, the activity of TiO 2 and Pd/TiO 2 increased with an increase of reaction temperature. When the temperature was as high as 70 • C, the reaction rate of TiO 2 drop slightly, and Pd/TiO 2 became less effective. In contrast, Cu/TiO 2 was more active at room temperature than the other temperatures.
Comparing the reaction rates of Pd/TiO 2 and Cu/TiO 2 under the most suitable conditions, the catalytic activity of Pd/TiO 2 is much greater than that of Cu/TiO 2 , and that of Cu/TiO 2 is slightly greater than that of TiO 2 . The effects of reaction temperature on the catalyst are varied by the type of cocatalyst. When the reaction temperature is higher than 70 • C, the recombination of charge carriers will increase. The temperature range of 50-80 • C is regarded as the ideal temperature for effective photolysis of organic matter. The effects of reaction temperature mainly influence quantum effect, i.e., electron-hole separation and recombination.