Magnesium doped tio2 photocatalyst on degradation of phenol under uv-Visible light

An Giang University Journal of Science – 2017, Vol. 5 (2), 59 – 65 59 MAGNESIUM DOPED TiO2 PHOTOCATALYST ON DEGRADATION OF PHENOL UNDER UV-VISIBLE LIGHT Pham Phat Tan1 1An Giang University Information: Received: 16/03/2017 Accepted: 01/04/2017 Published: 06/2017 Keywords: Photocatalyst, Mg-doped TiO2, phenol, degradation ABSTRACT In this study, MgO loaded TiO2 Degussa P25 powders were prepared by wet impregnation with difference in the amount of Mg content (i.e 0.5-10%). X-ray patterns studies have not proven the new phases in the MgO loaded TiO2 catalysts as well as BET and SEM studies have not shown the change of its particle size and specific surface area in compared with initial TiO2 . The PZC values of TiO2-Mg samples were higher than that of TiO2-P25 and increased with the additional amounts of Mg. The higher the PZC values were, the lower the saturated adsorption of TiO2 surface to phenol was, because of the basicity of MgO and the negatively charged catalyst surface. The photocatalytic degradation of phenol by using MgO loaded ( 0.5-10% w/w in Mg) TiO2 catalysts has been studied. A catalyst with ~1% w/w Mg content shows a better catalytic behavior than non-loaded TiO2. If the Mg amount was increased by more than 1%, the activity of Mg coated TiO2 decreased. The MgO enhancement of photocatalytic activity of TiO2 with optimal content ~1% Mg under UV-Visible light may be due to the ability of MgO to trap photogenerated electrons. 1. INTRODUCTION Recently, scientists have focused on research to enhance the activity of TiO2 in combination with solar energy. It can be considered as one of the “Advanced Oxidation Processes” (AOPs) (Herrmann, 1994; Trần Mạnh Trí & Trần Mạnh Trung, 2005). For this to happen, TiO2 must be doped in order to reduce band gap energy to expand the absorption in the visible light range or to decelerate recombination of pairs of photo- generated hole on the valence band and photo- generated electron on conductor band (h+CB-e-VB) to create more favorable conditions for the process of creating free radicals such as OH*, an extremely strong oxidizing agent plays a major role in the oxidation of organic contaminants (Colmenares, Aramedía, Marinas, Marinas, Urbano, 2006). The TiO2 catalysts, that are doped by transition metal ions, have been mentioned by many authors and some studies have been published: TiO2 catalysts are doped by the elements Cu, Ag, Fe, Ni , Pt, Pd, Zn, Zr, Cr, W, Ru ... have brought a certain efficiency in the reactions of decomposition of organic compounds (Iliev, 2006; Barakat, 2005; Xu, 2004; Zang, 2004; Wang, 2004; Vaidya, 2004). Especially, J. Bandara and his colleagues studied to prepare MgO/TiO2 by mixing TiO2-P25 powder with MgO and performed reactions of An Giang University Journal of Science – 2017, Vol. 5 (2), 59 – 65 60 decomposition of 2,4-DCP and 4-aminobenzoic acid with UV light (Bandara, Hadapangoda & Jayasekera, 2004). Although more research of TiO2 was doped by different elements, there are not many projects adequately mentioned the Mg-TiO2 catalyst. Therefore in this research, Mg doped TiO2 photocatalyst is prepared by the impregnation of TiO2-P25 Degussa with a solution of Mg(NO3)2. Research is then conducted of their structural characteristics, as well as a photoactivity survey on degradation reaction of phenol with UV-VIS light. 2. EXPERIMENTS 2.1 TiO2 modification with Mg TiO2-P25 Degussa powder (Anatase  80%, rutile  20%, BET surface area  50 m2/g, particle size  30nm) was impregnated by Mg(NO3)2 solution at given concentration in such a way as to Mg content in the catalyst reached 0.5; 1; 3; 5 and 10%. The mixture was stirred for 2 hours, then stabilized for 24 hours. Drying sample at 110oC for 3 hours and heated at 450oC for 3 hours. The catalysts is finely pulverized before surveying physical-chemical characteristics and their activity. Compared sample was carried out similarly but without the Mg(NO3)2. 2.2 Reactor and light source The activity of doped TiO2 catalysts were surveyed on reaction system discontinuously pyrex glass, with 150ml volume, 160mm high, 42mm of diameter. A 150W halogen light (OSRAM HLX) possesses wavelengths from 360nm to 830nm was placed in cylinder of quartz and was cooled by the surrounding water (Pham Phat Tan, 2015). Reactant is phenol concentration of 50mg/l 2.3 Process and analytical methods The catalyst samples were analyzed on their structure and physical-chemical characteristics by methods such as X-ray diffraction (XRD), the sample was measured on XRD instrument (SIEMENS - Germany) with CuK anode electrode (1,5406A0), 2 scanning angle from 15o to 70o; method of scanning electron microscopy (SEM) was done on SEM instrument (JOEL-JSM- 5500-Japan). The BET surface area measurement was performed on the PZC CHEMBET 3000. PZC values are determined by the pH titration method (Preocanin & Kallay, 1998). The activity of catalysts was assessed by the conversion and mineralization of phenol. Concentration of phenol in the reaction time was determined in the characteristic absorption peaks: 211 and 270nm (measured on UV-VIS Jasco V530 machine, Japan, mineralization was determined on ANATOC II, Australia). The calculation formulas are as follows:  % .100o tphenol o C C C    In there, C0: initial concentration of phenol, Ct: concentration of phenol at time t, : the conversion of phenol at time t.  % .100o tTOC o TOC TOC TOC    In there, TOC0, TOCt: total initial organic carbon and sample of corresponding reactants at time t, : the mineralization. 3. RESULTS AND DISCUSSION 3.1 Research on the structure of the prepared catalyst Physical-chemical characteristics of TiO2-Mg catalysts are shown in Table 1. An Giang University Journal of Science – 2017, Vol. 5 (2), 59 – 65 61 Table 1. Physical-chemical features of TiO2-Mg catalysts. Samples The ratio of Mg (%) PZC SBET (m2/g) Average particle size from XRD (nm) TiO2 0 3.98 50.3 30.0 TiO2-0.5Mg 0.5 3.98 50.5 30.0 TiO2-1Mg 1 4.14 50.4 31.7 TiO2-3Mg 3 6.87 50.7 31.2 TiO2-5Mg 5 8.33 51.2 31.2 TiO2-10Mg 10 9.90 61.9 30.7 The analysis results of XRD patterns (Figure 1) of TiO2 catalysts containing Mg at concentration below 10% will not be seen appearance of typical pic of MgO at 2 = 42.8 và 62.2. On the other hand, apart from the typical pic of the TiO2 anatas phase (2 = 25.3; 37.8; 48.1) and the rutile phase (2 = 27.5; 36.1; 54.4), there has no new pic detected; suggestion in this case was Mg not going into the TiO2 crystal lattice to create a new phase, but only located on the surface of TiO2. This is more obvious evident when Mg content in the catalyst contained 25%, while XRD appeared more typical pic of MgO (Figure 1). In the SEM image (Figure 2), the catalyst containing Mg was almost identical to the original TiO2-P25 sample. The particle size distribution schema were similar to the results, the the particle size of the sample was about 30 nm. Thus the presence of of Mg at low levels in TiO2 catalyst does not alter the particle size and causes sintering phenomena when catalysts were calcined at 4500C. Figure 1. XRD patterns of TiO2-P25 catalyst samples: Anatase (A), Rutile (R); MgO (M) An Giang University Journal of Science – 2017, Vol. 5 (2), 59 – 65 62 a b Figure 2. The SEM image of the catalysts:(a) TiO2 Degussa P25; (b) TiO2-1Mg BET measured results performed in Table 2 shows the specific surface of catalysts containing Mg virtually unchanged when compared with the original TiO2 Degusa P25 catalyst and ranged from 50 to 60 m2/g, in which the specific surface increased by 18% when the Mg content reached to 10%. This may be due to the MgO particles positioning on the surface of TiO2, which possesses a porous structure that may contribute to the enhancement of a specific surface of the catalyst. The PZC values of the TiO2-Mg catalysts increase when the content of Mg in the sample increases, particularly TiO2-5Mg and TiO2-10Mg which possesses PZC at a very high value (8.3 and 9.9). This shows that at Mg content greater than 3% makes the TiO2 catalyst surface have base property and negatively charged. It is entirely appropriate because MgO is one strong base and this also prove that the MgO particles were located on the TiO2 surface not inside crystal structure. Thus, to obtain the TiO2 catalysts with positive or negative charged on surface, we can denature them with appropriate elements, in certain conditions. 3.2 Reactivity of photocatalysts The activity of the TiO2 catalysts containing Mg were studied in the degradation reaction of phenol in water with UV-VIS light. The results reflected in the conversion and mineralization of phenol after 180 minutes of reaction which is shown in Table 2 and Figure 3. Table 2. Comparison of the activity of TiO2-Mg catalysts Reaction conditions: phenol solution 50mg/l, TiO2=0.25 mg/l, room temperature, illumination source:UV- VIS (Halogen 150W light) Catalyst sample Phenol metabolism level (%) Mineralization levels (%) TiO2 44.70 38.97 TiO2-0.5Mg 52.50 48.67 TiO2-1Mg 76.20 73.70 TiO2-3Mg 46.68 43.81 TiO2-5Mg 20.31 18.16 TiO2-10Mg 14.00 13.02 An Giang University Journal of Science – 2017, Vol. 5 (2), 59 – 65 63 Figure 3. Comparison of the activity of TiO2-Mg catalysts according to the conversion and mineralization of phenol after 180 minutes The results above showed that the TiO2-1Mg catalyst is the most highly active one, the conversion and mineralization of phenol (76.2 and 73.7, respectively) being higher than TiO2 (44.7 and 39.0 respectively). TiO2 catalysts with Mg content greater than 1% of their activity decreased rapidly when increasing content of Mg, this indicated the presence of Mg at optimal content had ability to increase the activity of the catalyst. The UV-VIS spectrum of phenol is shown in Figure 4. The characteristic absorption area of phenol at wavelength of 270 nm was selected to signify the change in the concentration of phenol at different times. For reactions on the TiO2-1Mg catalyst after 180 minutes of reaction, absorption intensity was much lower than in case using the TiO2-P25 catalyst. This suggests that the phenol conversion in reaction to TiO2-1Mg catalyst is higher. 0 0,5 1 3 5 10 %Mg 0 10 20 30 40 50 60 70 80% Độ chuyển hóa Phenol Độ khoáng hóa An Giang University Journal of Science – 2017, Vol. 5 (2), 59 – 65 64 a b Figure 4. UV spectrum of the sample of the phenol decomposition reaction by time: (a) With TiO2-P25 catalyst , (b) With TiO2-1Mg catalyst In there : (1) at the beginning time, (2) After 30 minutes of reaction, (3) After 120 minutes of reaction, (4) After 180 minutes of reaction The main reason leading to the increasing activity of TiO2-1Mg catalyst certainly related to the presence of MgO covering surface of TiO2. According to Pachioni and his colleagues (Pachioni & Ferrari,1999) MgO on TiO2 surface was the center which trapped photo-generated electron on conductor band when TiO2 is excited by light. Thanks to this ability may has reduced recombination h+CB-e-VB and thus the formation of free radicals *OH from photo-generated holes was more advantageous. Besides being at the center at which captured electrons would occur reaction with oxygen to form O2*- radical, this radical also acts as a strong oxidizing agent. This process has been described by Bandara J. as follows: MgO/TiO2 + h  MgO/TiO2 (e-CB, h+VB) (1) MgO/TiO2 (e-CB, h+VB)  MgO(e-CB) /TiO2 (h+VB) (2) MgO(e-CB) /TiO2 (h+VB)  [Mg2+--O2-]-LC/ TiIVOH*+/OH* (3) TiIVOH*+/OH* + Organic Substance  Oxidation Products (4) MgO(e-CB) / [Mg2+--O2-]-LC + O2  MgO + O2*- (5) LC: only defect sides due to some of unsaturated coordination (Lower Coordination) According to author Hargreaves and his colleagues (Hargreaves, Hutchings, Joyner & Kiely, 1986), the increased photocatalytic activity of TiO2 may be due to the crystal network of MgO being an octahedral structure, in which the side (1 0 0) prevailed with oxygen vacacies. It thereby made the surface of MgO have anion gap and cation gap. In which, electron deficiency of anion gap played An Giang University Journal of Science – 2017, Vol. 5 (2), 59 – 65 65 a role as electron capturing centers. Thanks to this characteristic of MgO, it can capture e-CB tranforming to [Mg2+--O2-]-LC . However photocatalytic activity of TiO2 did not always increase with the enhancement of Mg content. The activity decreased when Mg content was higher than 1%. The reason is that while MgO has extremely high band gap energy (8-9 eV), this substance itself had no photoactivity, so the greater amount of this substance impeded the light absorption of TiO2 and prevented h+VB and e-CB diffusion towards the surface of TiO2, so lead to decrease of catalytic activity. On the other hand, the presence of MgO with high concentration (above 1%) on the surface of TiO2 significantly reduced capacity of phenol adsorption (reduce more than 18%), which contribute to reduction of the activity of the catalyst. 4. CONCLUSION A TiO2 Degussa P25 catalyst containing Mg with different content (0.5 -10%) was prepared by an impregnating method and showed: The presence of an MgO layer on the surface of the TiO2 catalyst almost did not change the particle size, the specific surface of the catalyst samples, and not create a new phase as well. The PZC values of TiO2-Mg samples were all higher than TiO2 Degussa P25 and gradually increased when the Mg content increased. The greater PZC values the catalyst has, the lower saturation adsorption of phenol on the surface was. This indicates that the surface of catalysts containing MgO has base property and negatively charged, this characteristic represents more obviously as increasing the content of Mg. Photocatalytic activity increased when Mg content was in 0.5-1% and peaked when Mg content reached 1%, then gradually decreased when Mg content was higher than 1% in the decomposition reaction of phenol with UV-VIS light. 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