Characterization and Assessment of the Photo-catalytic Efficiency of Palladium/Silver Doped TiO2Nanoparticles

The effect of modified TiO2with palladium, silver and co-dopant of palladium and silver on TiO2with its photocatalytic efficiency were studied using X-ray diffraction (XRD) and Brunauer-Emmet –Teller (BET). The photocatalysts were prepared by sol immobilization method and characterized using XRD and BET techniques. The X-ray diffraction patterns of the dopants were found to be uniform with the standard P25 degussa TiO2. From the peak formation of anatase and rutile phases, it was found that the Pd/TiO2formed uniform matrix of anatase and rutile indicating that palladium ion disperses evenly on P25 degussa TiO2. The peak formation on Ag/TiO2 and Pd/AgTiO2shows the same uniform distribution of silver and palladium ion, only that foreign peak were observed on the formation of anatase and rutile because of impurity on silver. The crystalline size of the catalyst and full width at half maximum (FWHM) were also calculated at different angles of diffraction. The BET shows that the photocatalysts were mesoporous and is type IV isotherm. The high mesopore of the catalyst increases its photocatalytic activity, so also type IV isotherm. The BET analysis shows that the pore size distribution of the catalyst is between 2nm and 50nm which shows that the catalyst is mesoporous. It also gives high surface area with high volume and low pore size (crystalline size) which increases the photocatalytic efficiency. So co-doping of palladium and silver on TiO2 can serve as a strategy for design of high performance photocalysts for environmental benefits. DOI: https://dx.doi.org/10.4314/jasem.v22i9.01 Copyright: Copyright © 2018 Aroh et al. This is an open access article distributed under the Creative Commons Attribution License (CCL), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Dates: Received: 07 July 2018; Revised: 25August: 2018; Accepted: 23September 2018

A photo-catalyst is defined as a substance that is activated by the absorption of photon and helps to accelerate a reaction without being consumed (Abhang et al., 2011).Titanium (IV) oxide photo-catalyst is one of the revolutionized technologies in the field of environmental purification and energy generation, and has found extensive application in heterogeneous photo-catalysis for removing organic pollutants from air, water, soil and also in hydrogen production from photocatalytic water-splitting. Its use is popular because of its low cost, low toxicity, high chemical and thermal stability (Shon et al., 2007). The standard P25 degussa (Evonik) Aeroxide is a widely used titaniaphotocatalyst because of its relatively high levels of activity in many photo-catalytic reaction systems. It is not easy to find a photocatalyst showing activity higher than that of P25degusaTiO2 (Ohtani et al., 2010).
In recent years, photocatalytic degradation of organic compounds has been widely studied. One of the most important photocatalyst being frequently employed in this process is titania.TiO2 occurs in three polymorphous states: anatase, rutile and brookite (Jakub et al., 2012). Unfortunately, only the anatase and to a lesser extent, rutile TiO2 exhibit noticeable photo-activity under UV light. Anatase-titania is usually considered to be more active that rutile. Though, the latter is a thermodynamically stable phase, and reveals a lower band gap than the anatase (Silva et al., 2009).
In addition, research on TiO2 has attracted extensive interest because of its potential applications to photocatalysis, chemical sensors, solar cell electrodes and hydrogen storage materials (Kunst et al., 2006). Notwithstanding, TiO2photocatalyst is known to have some shortcomings in practical applications. One of these is that TiO2 has activity when it is only under light of wavelength shorter than 388nm, because of its wide band gap (3.2eV) (Ge and Xu, 2006). The wide band gap limits the use of sunlight as excitation energy and the high rate of recombination of photo-generated electron hole pairs inTiO2 results in low photocatalytic efficiency (Ahmed et al., 2013). Therefore, in order to surmount these two problems, many efforts have been made to modify TiO2 1370 AROH, AO; GIMBA, CE; OMONIYI, KI; ABBA, H; YILLENG, MT nanoparticles. One of the promising approaches is based on the metal loading. Various metals such as Pt, Au, Pd, Rb and Ag have been used as electron acceptors to separate the photo-induced hole/electron pair and promote interfacial charge-transfer process (Wu and Chen, 2004).
Moreover, to achieve high photocatalytic degradation efficiency, nano-TiO2 should be mesoporous and should exhibit high crystallinity and high specific area (Young et al., 2018). The formation of a high percentage of the anatase phase, small crystallite size and high specific surface area of nano-doped-TiO2 increases the photo catalytic efficiency (Yu et al., 2007). Shon et al. (2007) reported that doping of Ag with TiO2 (600 o C) led to decreased photocatalytic size from 37nm to 19nm, while the specific surface area increased from 45m 2 /g to 63m 2 /g and the photocatalytic activity increased by 18%. In addition, optical characterization of Au doped TiO2using UVvisible spectrophotometry showed a shift in optical absorption wavelength to visible region which may be attributed to the incorporation of gold nanoparticles (1-2%) into TiO2 structure (Shon et al., 2007). The kinetic study indicated that the rate of decomposition of phenol by Au/TiO2 photocatalyst was improved by 2-2.3 times compared to the undoped TiO2.
Au doped TiO2 showed higher activity for the removal of dibenzothiophene (DBT) compared to pure titania, with the optimum Au loading being 1.5 wt % Au. The Au nanoparticles act as electron sink to enhance e/h + charge separation and produce number of oxidizing species, thereby increasing the reaction rate (Suzan and Selva, 2008). Therefore, the aim of this work is to assess the effect of palladium and silver dopants on TiO2, and to compare the effects of mono-and bi-metal dopant on the photocatalytic efficiency of TiO2 P25 Degussa.

MATERIALS AND METHOD
Preparation of Ag/TiO2 and Pd/TiO2catalyst: The catalysts used were prepared using sol immobilization method as reported by Moses (2014), with a few modifications. Standard sol-immobilization method was utilized to prepare the Ag, Pd, Ag-Pd nano particles supported on TiO2.The supported silver and silver-palladium colloids were prepared by using Poly vinyl alcohol(PVA) as protective ligand, aqueous solutions of 0.005mol/dm 3 PdCl2 and 0.006mol/dm 3 AgNO3were prepared. A 1wt % aqueous PVA, (Aldrich, MW = 10000, 80% hydrolyzed) solution was freshly prepared just prior to synthesis of the metal colloid. A representative protocol for preparing a catalyst comprising Ag-Pd nano-particles with 1 wt% total metal loading on a TiO2was carried out as follows: To an aqueous 0.005mol/dm 3 PdCl2 and 0.006mol/dm 3 AgNO3 solution, 100 cm 3 of PVA solution (1 wt %) was added (PVA/ (Ag and Pd)(w/w) = 1.2); a freshly prepared solution of NaBH4 (0.1 M, NaBH4/ (Ag andPd)(mol/mol) = 5) was then added to form a dark brown sol.
After 30 min of sol generation, the colloid was immobilized by adding TiO2 (acidified to pH 1-2 by using tetraoxosulphate (VI) acid with vigorous stirring with a glass rod. The amount of support material required was calculated to have a total final metal loading of 1 wt %.After 2 h, the slurry was filtered and the catalyst washed thoroughly with distilled water (neutral mother liquors) and dried at 120ºC overnight in an oven. Sol immobilized mono-metallic silver and palladium catalysts were prepared using similar procedure.
Characterization of Catalyst (Ge and Xu (2006): Xray diffraction measurement of Ag/TiO2, Pd/TiO2 and Ag/Pd-TiO2 was performed at room temperature using a Rigakuutima IV X-ray diffraction meter with CU-Kα& radiation (Philips) in England. The diffraction meter was operated at 40kn and 44 MA, scanned with a step size of 0.02 o and a count time of 1 o /min in the range of 2θ from 10 o to 80 o .
The textural properties such as surface area pore size distribution of Ag/TiO2, Pd/TiO2 and Ag/Pd-TiO2 were analyzed by using N2physisorption using a NOVA 2200e (Quantachrome instrument, England) surface area and pore size analyzer. After the Ag/TiO2, Pd/TiO2 and Ag/Pd-TiO2 were dried, they were degassed extensively at 100 o C prior to the adsorption measurements, then the N2 isotherm were obtained at 196 o C.
The surface area of the synthesized materials was calculated by using the Brunaer-Emmett-Teller (BET) equation with a relative pressure P /P o range of 0.05-0.30. The pore volume was determined from the amount of N2 adsorbed at the highest relative pressure P /P o = 0.99, then the diameter and pore size distribution plots were designed by applying the Barrett-Joyner Halenda (BJH) model.

RESULTS AND DISCUSSION
X-Ray Diffractometer (XRD) spectra of P25 Degussa and 0.5%Pd/TiO2 catalyst: From the result of the XRD spectra of the catalyst prepared using micro-emulsion (Figure 1 and 2). The XRD pattern of P25 degussa TiO2 indicated that Pd loaded on TiO2 surface almost has no influence on the crystalline structure of TiO2when compared to the P25 Degussa TiO2. Pd phase was not detected in the XRD pattern of Pd/TiO2 powders, possibly because the Pd content in the TiO2 surface is not enough to form clearly crystalline structure. The peak distribution of P25 Degussa TiO2 standard and doped Pd/TiO2 showed the same peak formation. The shape of diffraction peaks of the crystal plane of P25 Degussa standard is quiet similar to those of Pd/TiO2, the peak obtained for the two sample is similar to that of predominant formation of anatase reflex at 2θ=25.3 o with uniform distribution of anatase and rutile with more of sharp peak of anatase reported by Ohtani et al. (2010) [3]. Also, 0.5% Pd/TiO2 had sharp peak at 2θ = 25.3 0 and P25 degussa has sharp peak at 2θ = 25.3 0 for anatase and the sharp peaks formation for P25 degussa and Pd/TiO2 is 2θ = 27.4 0 for rutile indicating that there is uniform distribution of Pd ion on the catalyst (TiO2). Amano et al. (2009) used pt deposited on p25 TiO2 they found out that the peak formation of the anatase and rutile was the same with sharp peak formation of 2θ = 25.3 0 and 27.4 0 for anatase and rutile respectively.  X-Ray Diffractometer (XRD) spectra of 0.5%Ag/TiO2 catalyst: From the result of the XRD spectra of catalyst prepared using micro-emulsion, shown in Figure 3, the XRD spectra of Ag/TiO2 was accompanied with some foreign peaks that can be ascribed to the impure nature of the AgNO3 precursor used for Ag. For Ag/TiO2, the peak formation is 2θ = 25.3 0 and 27.4 0 respecively for anatase and rutile phase only that you have to look very closely in order to read out the peak because of the foreign peak formed. The result also reveals that Ag ions are uniformly dispersed in TiO2. The obtained peak is similar result of predominantly formation of anatase reflex at 2θ=25.3 o with uniform distribution of anatase and rutile 27.4 0 with more of sharp peak of anatase reported by Ohtani et al. (2010).This study supports the report that P25 Degussa TiO2 is composed of anatase and rutile crystallites, the ratio being typically 70:30 or 80:20 (Ohtani et al., 2010).  Contamination found in Ag/TiO2 affects the peak formation of Pd/Ag-TiO2, making it to give more foreign peaks. The result obtained is similar to that of predominant formation of anatase reflex at 20=25.3 o with uniform distribution of anatase and rutile, though with more sharp peak of anatase. XRD pattern of Pd/TiO2, Ag/TiO2 and Ag/Pd -TiO2 compared to P25 degussa TiO2 standard indicated that the peak formation of the anatase and rutile were uniformly distributed. Peak formation of the anatase and rutile phase increases the photo-catalytic efficiency of doped Pd/TiO2, Ag/TiO2 and Ag/Pd -TiO2 as more of anatase is formed, which is the reactive phase.    From Tables 1, 2 and 3, the full width at half maximum (FWHM) was calculated using the formula shown in Figure 5. From the calculation, Pd/Ag-TiO2 gives smaller crystalline size, so this supports its high photocatalytic activity.  Brunauer-Emmett-Teller (BET) of 5% Pd-TiO2 catalyst: Table 4 shows that Pd/Ag-TiO2 has the highest surface area (52.85 m 2 g -1 ), higher pore volume (1.749 cm 3 g -1 ) and smallest pore diameter (1.322 nm) which favours high photocatalytic activities. From Figure 6 and 7, the pore size distribution of Pd/TiO2, has the width to be between 2 nm and 50 nm. Therefore, this implies that Pd/TiO2 is mesoporous, so the physisorption isotherm is type IV isotherm which favours high photocatalytic activity (Sing et al., 1984). Pd/TiO2 has surface area of 49.66 m 2 /g pore volume of 17.7m 3 /g and pore size of 13.8 nm signifying high surface area, high pore volume and small pore size implies high photocatalytic efficiency. In conformity with some other work, doping techniques promote the production of smaller crystallite nano-doped TiO2 with resultant larger surface area, so as to prevent the problem of particle agglomeration (Asilturk et al., 2009).
Brunauer-Emmett-Teller (BET) of 5% Pd/Ag-TiO2 catalyst: Furthermore, Figure 8,9,10, and 11 indicated thatAg-TiO2 and Pd/Ag-TiO2 are mesoporous and is type IV isotherm. According to Sing et al. (1984), the pore widths of between 2 nm and 50 nm is called mesopores. From the plot of pore size distribution, the pore size declined between the ranges of 2 nm to 50 nm, whereas cluster in the plot indicated the pore size. According to Wu et al, (2004), transition metals dopants such as palladium, chromium and silver enhance the photo catalytic performance of nanodoped TiO2. Among the transition metals used as dopants, Pd ion showed the strongest interaction with nano-TiO2and improved its morphology most effectively.
From this study, when Ag is co-doped to form Pd/Ag TiO2, this better improved the photocatalytic property to give larger surface area, high pore volume and small pore size which favours photo catalytic efficiency of the prepared catalyst Pd/Ag-TiO2. Shon et al. (2007) reported that doping silver on TiO2 decreases its crystalline size from 37 nm to 19 nm and specify surface area increased from 45 m 2 /g to 63 m 2 /g, thereby increasing its photocatalytic efficiency. The report of this study is similar to that of Kirilov et al.(2006) in which 0.5% Ag/TiO2 has specific surface area of 47m 2 /g and 0.5% Pd/TiO2 has specific surface area of 51m 2 /g showing that doped Pd/TiO2 has more photocatalytic efficiency than doped Ag/TiO2. Conclusion: The doping and codoping of Pd/TiO2, Ag/TiO2 and Ag/Pd/TiO2 improve the photocatalytic activity of P25 degusa TiO2 by production of more active phase (anatase) which favours photocatalysis. The BET analysis indicates that doping also helps to increase surface area of the catalyst, the pore volume and reduces the pore size thereby increasing photo catalytic activity. The XRD of Pd/TiO2, Ag/TiO2 and Ag/Pd/TiO2 shows uniform dispersion of the transition metal dopants on TiO2 matrix. There was high reactive phase of anatase and improvement in surface area, pore volume and crystalline size of doped TiO2 for enhanced photocatalytic.