Characterization and Photocatalytic Efficiency of Palladium Doped-TiO 2 Nanoparticles

The effect of modification of TiO2 with different palladium concentrations on its characteristics and photocatalytic efficiency was studied. Photo catalysts were prepared by the sol-gel method and were characterized by different techniques. A uniform distribution of palladium through the TiO2 matrix was observed. The X-ray diffraction patterns of the pure and palladium doped TiO2 were found to be quiet similar and the average particle size was not significantly changed. As a result of palladium doping, the UV-Vis analysis showed a red shift in the onset of wavelength of absorbance and the band gap was changed from 3.39 to 3.06 eV for the 0.3 wt% Pd/TiO2 sample. Photo catalytic removal study of formic acid showed that the 0.3 wt% palladium doped photocatalyst exhibits the highest efficiency among the different palladium doped photocatalysts using sun light as the radiation source.


Introduction
Titanium dioxide (TiO 2 ) has been widely used as a photocatalyst for degrading a wide range of organic compounds [1].In addition, TiO 2 has attracted extensive interests because of its potential applications to photocatalysis [2], chemical sensors [3], solar cell electrodes [4], and hydrogen storage materials [5].However, the TiO 2 photocatalyst is known to have limitations for practical applications.One of these limitations is that the TiO 2 has activity only under light of wavelength shorter than 388 nm because of its wide band gap (Eg = 3.2 eV) [6][7][8].The wide band gap limits the use of sunlight as excitation energy and the high rate of recombination of photo-generated electron-hole pairs in TiO 2 results in low photocatalytic efficiency [6][7][8].To overcome these two difficulties, many efforts have been made to modify TiO 2 nanoparticles [8][9][10].One of the promising approaches is based on the metal loading.Various metals, such as Pt, Au, Pd, Rh and Ag, have been used as electron acceptors to separate the photo-induced hole/electron pair and promote interfacial charge-transfer processes [11][12][13][14][15][16].
Therefore, the aim of the present work is to study the effect of palladium on the properties and activity of the TiO 2 photocatalyst prepared by the sol-gel method.To investigate the photocatalytic efficiency of the pure and doped TiO 2 , formic acid was used as a model pollutant.Formic acid is a very simple molecule which can be decomposed in simple steps leading to the increase of the pH of the treated solution.

Synthesis details
Titanium tetrachloride (Fluka 98%) was used as a starting material.3 gm of TiCl 4 was added dropwisely to 15 ml absolute ethanol under stirring.The resulting solution was stirred at room temperature to form a gel.Then, the gel was heated on the hotplate at about 80˚C to form a white powder.The powder was then dried at 110˚C for 45 minute in furnace.The dried powders were ground in an agate mortar and calcined, in air, at 350˚C, 400˚C, 480˚C, and 600˚C for 2 h in a muffle furnace.A portion of the dried precipitate was characterized by XRD and used for thermal analysis.Preparation of the Pd doped TiO 2 nanoparticles (Pd/TiO 2 ) was carried out by similar procedures used for the preparation of the undoped TiO 2 except that a calculated amount palladium chloride (required to obtain 0.05, 0.1 and 0.3 wt% of the final catalyst) was dissolved in ethanol before the addition of titanium chloride.The Pd/TiO 2 was obtained by calcinations of the obtained powder at 400˚C for under similar conditions.

Characterization
Thermogravimetric analysis (TG) and Differential Scanning Calorimetry (DSC) were performed on a Netzch STA-409EP apparatus.Thermal analyses were carried out in the range 20˚C -1000˚C, with a heating rate of 10 K•min −1 .Powdered samples (24 mg) were analyzed in alumina crucible by using α-Al 2 O 3 as a reference.
X-ray diffraction spectra were recorded at room temperature using a powder diffractometer Bruker axs D8 Advance, Germany with Cu-Kα radiation source, λ = 1.5406Å and 2Ө in the rang 10˚ -80˚.The average crystallite size of anatase phase was determined according to the Scherrer equation.Particle size determination was carried out with a transmission electron microscope (TEM), Jeol Jem-1230.Visible-Ultraviolet spectrum was performed with a JASCO Corp., V-570 UV-V is spectrophotometer.Analysis of TiO 2 was carried out between 200 and 800 nm.

Photocatalytic Efficiency Experiments
The photocatalytic efficiency of the catalysts was investigated using a 500 ml beaker.150 mg of pure TiO 2 or Pd/TiO 2 photocatalysts were mixed with 500 ml of formic acid solution (initial concentration of about 5 × 10 −3 M).The resulting suspension was stirred to obtain the maximum adsorption of organic pollutant molecules on the photocatalyst surface and to make oxygen available for the reaction.After 6 h under the UV lamp and 4 h under sun light irradiation, 20 ml sample was taken for analysis.Samples were centrifuged before analysis to separate the solid particles.TOC (Phoenix 8000 Laboratory Analyzer uses sodium per-sulfate in combination with UV light to oxidize organic material) was used for the analysis of formic acid.

Results and Discussion
Figure 1 shows the TG and DTG curve of the undoped TiO 2 .The figure presents two weight loss steps.The first step appeared between 50˚C -380˚C.This step shows a decrease in the mass of about 14.19%.This step may be attributed to the evaporation of water and the loss of organic component and transformation of amorphous to anatase form.The second step appeared between 380˚C -950˚C showed a decrease in mass of about 2.0%.This step may be attributed to the dehydroxylation of TiO surface.The total weight loss is 16.19%.It can be concluded that a photocatalyst with a stable weight can be obtained by calcinations at about 400˚C.
Figure 2 shows the corresponding DSC curve of TiO sample.There are two DSC peaks.The first peak, at around 100˚C, can be attributed to the vaporization of water and the subsequent loss of organic impurities.The second adsorption peak at ~580˚C may be attributed to the transformation of TiO 2 from anatase to rutile form [17,18].
The XRD analysis of the dried powder (that used for the preparation of the undoped TiO 2 photocatalysts before calcinations) showed amorphous material with starting of formation of the anatase phase.and rutile.Table 1 lists the average crystallite sizes of TiO 2 (calculated from the XRD peak, according to Scherrer equation).
It can be concluded that from XRD study, anatase phase in the nano scale may be obtained by calcination of the dried powder at 380˚C and less than 480˚C.
Figure 4 shows the X-ray diffraction patterns of the undoped and 0.05%, 0.1%, and 0.3% palladium doped TiO 2 calcined at 400˚C.The XRD patterns didn't show any Pd phase (even for the 0.3% Pd doped TiO 2 ).This may reveal that Pd ions are uniformly dispersed in TiO 2 matrix.In the region of 2θ˚ = 10˚ -80˚, the shape of diffraction peaks of the crystal planes of pure TiO 2 is quite similar to those of Pd/TiO 2 of different Pd concentrations.
The average crystal sizs of TiO 2 and Pd doped TiO 2 nanoparticles were calculated and also, were presented in  Table 1.The average crystal size was not significantly changed due to the addition of the Pd +2 .
Figure 5 shows the TEM result of the undoped TiO 2 nanopaeticles calcined at 480˚C.The TEM image of the undoped TiO 2 nanoparticles has a narrow size distribution (17 -28 nm).The result of the TEM agrees with the XRD results concerning the particle size range.
The EDX (energy dispersive X-ray microanalysis) was recorded in the binding energy region of 0 -11 keV.The result is shown in Figure 6.The peak from the spectrum reveals the presence of two peaks around 4.508 and 0.525 keV, respectively.The intense peak is assigned to the bulk TiO 2 and the less intense one to the surface TiO 2 .The peaks of Pd are distinct in Figure 7 at 2.8 and 3.6 keV.This result confirms the existence of Pd atoms in the TiO 2 matrix.
The UV-visible spectra of the undoped TiO 2 and Pd doped TiO 2 samples prepared by calcinations at 400˚C are shown in Figure 7.The onset wavelength of absorption used to calculate the optical band gap was determined by extrapolation of the base line and the absorption edge.Table 2 shows the calculated absorption onset (λ) and the corresponding band gap (Eg) for doped     and undoped TiO 2 .
The absorption spectrum of Pd doped TiO 2 consists of a single broad intense absorption at the range 365.85 -404.93 nm can be attributed to the charge-transfer from the valence band to the conduction band [11].The undoped TiO 2 showed absorbance in the shorter wavelength region.The UV-Vis absorption results showed a red shift of the absorption onset value due to modification of TiO 2 with Pd of different concentrations as shown in Figure 7.It is known that doping of various transitional metal ions into TiO 2 could shift its optical absorption edge from UV into visible light range [19].

Photocatalytic Efficiency (Removal of Formic Acid)
Formic acid is a simple molecule that can be mineralized in simple steps leading to the increase of the pH of the treated solution.One possible route for formic acid removal may be initiated through the direct transfer of an electron from the adsorbed formic acid to the surface positive hole of the photocatalyst [20].Also, it is well known that hydroxyl radicals are produced in photocatalytic reactions illuminated by radiation of suitable wave length.These hydroxyl radical may react with the HCOO-molecule to form water and •COO-, which can be further decomposed through the reaction with oxygen [20].Presence of palladium can modify the photocatalytic effect through increasing the life time of charge separation and shifting the absorbance to longer wave length.
Formic acid concentration was measured by the Total Organic Carbon (TOC).TOC was decreased from 52.2 mg/l to 35 mg/l using the 0.05% Pd doped TiO 2 under UV irradiation, Figure 8.For the undoped TiO 2 photocatalyst, the TOC was decreased to 23.6 mg/L.The pH of the solution also was changed from 3.06 to 3.17 and 3.3 for doped and undoped TiO 2 photocatalysts, respectively within the same time (see Table 3).The change of the pH was taken as a signal for the removal of formic acid.It can be seen that under UV irradiation, the undoped TiO 2 exhibits better efficiency than the Pd/TiO 2 photocatalyst.
Removal of formic acid by pure TiO 2 and Pd/TiO 2 were examined using sun light as a radiation source Figure 9.It can be seen that Pd/TiO 2 shows higher efficiency than the pure TiO 2 .Also, it can be seen that there is a gradual increase in the efficiency of the Pd/TiO 2 with increasing palladium content in the catalyst.TOC was decreased from 61 mg/L to 49.6, 34.2, 2.

Conclusion
The pure and palladium doped TiO 2 (Pd/TiO 2 ) nanoparticles were prepared by the sol gel method.Samples prepared by calcinations at 380˚C contain anatase phase only.A mixture of anatase and rutile was obtained at higher calcination temperatures.Doping TiO 2 with palladium in the concentration range of 0.05 to 0.3 has no significant effect on the particle sizes and did not result in the formation of a new crystalline phase.It was confirmed that the incorporation of Pd in TiO 2 matrix shifts the onset wave length of absorption to higher values (red shift).Under UV irradiation, the pure TiO 2 exhibited higher efficiency than the palladium doped TiO 2 for formic acid removal from water.However, when sun light was used as the radiation source, the palladium doped photocatalyst exhibited higher efficiency than the pure TiO 2 and the photocatalytic efficiency increases with increasing palladium content up to a concentration of 0.3% (0.3% Pd/TiO 2 ).

Figure 1 .
Figure 1.TG and DTG of the dried powder used for the preparation of the undoped TiO 2 nanoparticles, heating rate of 10 K•min −1 under O 2 flow.

Figure 2 .
Figure 2. DSC of the dried powder used for the preparation of the undoped TiO 2 sample, heating rate of 10 K•min −1 under O 2 flow.
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Figure 4 .
Figure 4. XRD patterns of the doped and undoped TiO 2 nanoparticles calcined at 400˚C.

Figure 5 .
Figure 5. TEM image of TiO 2 nanoparticles prepared by calcinations at 480˚C.

Table 4 . TOC results for doped and undoped TiO 2 nano- particles annealed at 400˚C under sun light irradiation for 4 hrs.
3% Pd doped TiO 2 and undoped TiO 2 , respectively under sun light irradiation within the same time (see Table4).The pH of the solution also was changed from 2.98 to 3.08, 3.02, 3.58 and 2.98 for 0.05%, 0.1%, and 0.3% Pd doped TiO 2 and undoped TiO 2 photocatalysts, respectively, within the same time.