Catalyst TiO2 with N-doped, Mg-doped and N-Mg co-doped were created by impregnation method with Mg(NO3)2 (add Mg in) and mix method with urea (add N in). The XRD, SEM, DR, BET, FT-IR measurements were carried out for structural characterization of the TiO2 samples. The result performed, compared with the sample of initial TiO2-P25 particle size and specific surface area, samples of doped-catalyst changed insignificantly.
An Giang University Journal of Science – 2019, Vol 6, 60 – 69 ENHANCE THE ACTIVITY OF THE PHOTOCATALYST TiO2-P25 DEGUSSA BY DOPING IT WITH N AND Mg ON DEGRADATION OF PHENOL UNDER VISIBLE LIGHT Pham Phat Tan1 An Giang University Information: Received: 31/05/2018 Accepted: 29/09/2018 Published: 02/2019 Keywords: Photocatalyst, N-Mg co-doped TiO2, phenol, degradation ABSTRACT Catalyst TiO2 with N-doped, Mg-doped and N-Mg co-doped were created by impregnation method with Mg(NO3)2 (add Mg in) and mix method with urea (add N in) The XRD, SEM, DR, BET, FT-IR measurements were carried out for structural characterization of the TiO2 samples The result performed, compared with the sample of initial TiO2-P25 particle size and specific surface area, samples of doped-catalyst changed insignificantly The average crystalline size was 30-32 nm, specific surface area about 50 - 60 m2/g The crystalline phase components did not change, including only anatase and rutile Doped-catalyst samples narrowed the band gap of TiO2 (values of Eg decreased to under 3.0 eV) In there, N-doped sample possessed the lowest value of Eg (Eg = 2.53 eV), it meant this sample responded to the visible light area obviously In the degradation of phenol, N-Mg co-doped TiO2 had better catalytic performance than N-doped and Mg-doped alone The order of doping elements (N and Mg) shown to be an important factor on the catalytic activity of the doping catalysts The catalysts TiO2 were doped by N first then Mg demonstrated synergic effect and activity of the catalysts increased 1.5- 1.6 times compared with pure TiO2 By contrast, in the reverse order (first Mg then N) the inhibited effect was observed and the catalytic activity would decrease INTRODUCTION study with the aim of seeking for photocatalyst of TiO2, which is highly active when using solar energy in two directions, is indispensable, which contributes in applying the treatment of biodegradable organic pollutants with "green" technology TiO2-based photocatalysis is one of the advanced oxidation processes due to the high catalytic activity of TiO2 in the presence of UV light It still has some limitations, however, needed to be resolved For instance, (i) narrowing Eg (the band gap) will allow the use of extensible optical performance beyond the ultraviolet to the visible range (UV-VIS) of solar energy patterns; (ii) preventing the recombination of e-CB and h+VB of the TiO2 after the influence of the photon The If non-metallic elements are implanted such as N, S, F, I, C into TiO2 so that they replace partially the position of O in TiO2 crystal lattice, this will narrow the band gap of TiO2 to lower level, for example, Eg = 2.54 – 2.66 eV then TiO2 60 An Giang University Journal of Science – 2019, Vol 6, 60 – 69 photocatalyst activity will be promoted throughout the UV-VIS range of solar energy (Asahi, 2001; Ihara, 2003; Mozia, 2005) of given concentration in such a way as to Mg content in the catalyst reached 1.0% The mixture was stirred for two hours, then stabilized for 24 hours Drying a sample at 110 oC for three hours and heated at 450 oC for three hours (Denoted as TiO2-1Mg) The magnesium content above is optimal, and this was published before In that study, TiO2 Degussa P25 catalyst containing Mg with different content (0.5 -10%) was prepared by an impregnating method and showed 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 (P.P Tân, 2017) The second direction is to decelerate recombination of pairs of photo-generated electron on conductor band and photo-generated hole on the valence band of TiO2 to create many free radicals *OH – an extremely strong oxidizing agent plays a pivotal role in the oxidation of organic contaminants (Colmenares, 2006) This direction is conducted by doping TiO2 with metal elements: Cu, Ag, Fe, Ni, Pt, Pd, Zn, Zr, Cr, W, Ru (Bandara, 2004; Barakat, 2005; Iliev, 2006; Vaidya, 2004; Wang, 2004; Xu, 2004; Zhang, 2004) to create electron capturing centers on conductor band to prevent photo-generated electrons return valence band, then reduce recombination e-CB h+VB TiO2-P25 Degussa powder were mixed with urea with molar ratios 1:1, calcinated at 4000C, for two hours in the air with gradient 50C/minute (denoted as TiO2-1U) The ratio above is optimal and was published before (P.P Tân, 2008) Besides, the tendency of research metal-nonmetal co-doped TiO2 has been especially notified, and there were studies on metal-nonmetal co-doped TiO2 (Huang, 2007; Morikawa, 2006; Pan, 2006; Wei, 2004) The result increased the catalyst activity but some studies reflected reverse results The catalysts TiO2 contained both N and Mg (TiO2-N-Mg) were prepared in different order TiO2-1Mg powders were mixed with urea with molar ratios 1:1, calcinated at 4000C for two hours (denoted as TiO2-1Mg-1U) TiO2-1U powders were impregnated in Mg(NO3)2 solution with magnesium content at 1%, calcinated at 4500C for three hours (denoted as TiO2-1U-1Mg) Nowadays, very few studies in using magnesium and nitrogen concurrently to dop TiO2 are published and this considered as an interesting and promising study On the basis of science, doping TiO2 with a specific element with certain content to increase activity, however if co-doping with both elements happens, their activity may increase due to synergic effect among the roles of nitrogen and magnesium, but may decrease their activity due to inhibition among these factors This article, the photocatalysts N, Mg – co-doped TiO2 were prepared to degrade phenol under visible light The catalysts were finely pulverized before surveying physical-chemical characteristics and their activity Compared samples were carried out similarly but without the Mg(NO3)2 2.2 Reactor and light source The activity of doped TiO2 catalysts was surveyed on the reaction system discontinuously pyrex glass, with 150 ml volume, 160 mm high, 42 mm of diameter A 150W halogen light (OSRAM HLX) possesses wavelengths from 360 nm to 830 nm was placed in a cylinder of quartz and was cooled by the surrounding water EXPERIMENTS 2.1 TiO2-P25 modification with N, Mg TiO2-P25 Degussa powder (anatase ≈ 80%, rutile ≈ 20%, BET surface area ≈ 50 m2/g, particle size ≈ 30 nm) was impregnated by Mg(NO3)2 solution Reactant is phenol concentration of 50mg/l 61 An Giang University Journal of Science – 2019, Vol 6, 60 – 69 2.3 Process and analytical methods α phenol ( % ) = 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 CuKα anode electrode (1,5406 A0), 2θ scanning angle from 15o to 70o; method of scanning electron microscopy (SEM) was done on SEM instrument (JOEL-JSM5500-Japan) The BET surface area measurement was performed on the CHEMBET 3000 Co − Ct 100 Co In there, C0: initial concentration of phenol, Ct: concentration of phenol at time t, α: the conversion of phenol at time t Total Organic Carbon (TOC) measurements were performed using ANATOC II by direct injection of the samples Results of the total initial organic carbon and sample of corresponding reactants at time t were analyzed automatically The mineralization of the samples were calculated: The diffuse reflection spectra was measured on DRS (JASCO V-550-Japan) The Eg values were calculated by Eg = hC/λ = 1239,8/λ (eV) βTOC ( % ) = The activity of catalysts was assessed by the conversion and mineralization of phenol The concentration of phenol in the reaction time was determined in the characteristic absorption peaks: 211 and 270 nm (measured on UV-VIS Jasco V530 machine, Japan, mineralization was determined on ANATOC II, Australia) The calculation formula was as follows: TOCo − TOCt 100 TOCo In there, TOC0, TOCt: total initial organic carbon and sample of corresponding reactants at time t, β: the mineralization RESULTS AND DISCUSSION 3.1 Research on the structure of the prepared catalysts Physical-chemical characteristics of the catalysts are shown in Table Table Physical-chemical features of TiO2-P25 and doped-TiO2 catalysts Catalyst Sample SBET (m2/g) Average particle size (nm) Color TiO2-P25 50 30 White TiO2-1Mg 50 30 White TiO2-1U 60 30-32 Yellow TiO2-1Mg-1U 59 30-32 Light yellow TiO2-1U-1Mg 59 30-32 Ivory white The result from Table showed that with N, Mg alone or concurrently at TiO2-P25 did not change significantly the specific surface compared with TiO2-P25 pure, the specific surface fluctuates in the range of 50-60 m2/g This indicated, the doping TiO2-P25 pure with two elements above with 1% Mg content or catalyst and urea with molar ratios 1:1 or doping with both N and Mg did not change significantly their specific surface area though N was added first then Mg or in reverse order The calculated results from XRD patterns (Figure 1) performed the samples above had stable particle sizes within the range of 30 – 32 nm 62 An Giang University Journal of Science – 2019, Vol 6, 60 – 69 2 Figure XRD patterns of TiO2-P25 catalyst samples: (1) TiO2-P25, (2) TiO2-1Mg, (3) TiO2-1U, (4) TiO2-1U-1Mg (5) TiO2-1Mg-1U The XRD patterns showed that catalyst which were doped with N or Mg or two elements above had crystalline phase components and they were similar to the original TiO2-P25 All samples appeared specific peaks of anatase phase (2θ = 25.3; 37.8; 48.1) and rutile phase (2θ = 27.5; 36.1; 54.4), to doped samples, there was no any new peak TiO2- TiO2- TiO2-1U- TiO2-1U TiO2-1MgFigure 2: The SEM image of the catalysts: TiO2-P25 and doped-TiO2 63 An Giang University Journal of Science – 2019, Vol 6, 60 – 69 Figure The SEM image of the catalyst Mg doped TiO2-P25 TiO2-1U-1Mg TiO2-1U Figure The TEM image of the catalysts: TiO2-1U TiO2-1U-1Mg 450 0C In Figure 3, the elements were distributed equally on the surface of the catalyst TEM images (Figure 4) of the catalysts showed the particle size of doped-catalysts changed insignificantly (about 30-32 nm) The SEM images (Figure 2) indicated that the catalysts have the same shape and distribution This demonstrated that there was no clumped phenomenon of the crystal lattice in TiO2 catalyst samples despite doping with element alone or codoping with N and Mg when being calcinated at 64 An Giang University Journal of Science – 2019, Vol 6, 60 – 69 Therefore, doping TiO2-P25 with qualitative and quantitative components and doping condition as mentioned above, particularly, when co-doping with N first then Mg or in reverse order happened, physical properties and catalyst structure had same results This is an advantageous point to study their activity later 3.2 Order of adding N and Mg in TiO2-P25 photocatalyst Figure The DRS partterns of the catalysts: TiO2-P25 and doped-TiO2 The DRS patterns (Figure 5) proved that the TiO2doped catalysts had widening ability towards visible light absorbtion If N was added first then Mg (TiO2-1U-1Mg), the ability of visible light absorption will be lower than the opposite case (TiO2-1Mg-1U) However, both co-doped situations had weaker ability of visible light absorbtion than N-doped sample alone (TiO2-1U) DRS patterns reflected that N-doped sample (TiO2-1U) widened to visible light area most obviously, Eg decreased to just 2.53 eV The reason of increasing the ability of visible light absorption of doped sample was due to the role of N on TiO2 crystal lattice, this sample only doped with N, N went into TiO2 crystal lattice so that it performed as widening role in visible light absorption To co-doped sample, if N was added in TiO2 which was doped with Mg previously, MgO layer will prevent N from entering the lattice, thus reducing the role of N, led to reducing in visible light absorbtion (Bandara, Hadapangoda & Jayasekera, 2004) If the doping process was conducted in reverse order, added N first then Mg, the MgO layer will partly cover the TiO2 surface which led to decrease the ability of visible light absorbtion; as a result, this ability was not much preferable than the two cases above Ascending order of visible light absorption: TiO2-P25 < TiO2-1U-1Mg < TiO2-1Mg-1U < TiO2-1U The Eg values of the corresponding samples gradually decreased (Table 2) In terms of photosensitivity, the samples TiO2-P25 and TiO2-1Mg were white, while the N-containing samples were yellow However, if doped with N first then Mg, the catalyst color would be lighter, changed from light yellow to ivory white The N-doped sample alone was slightly lighter in color than the rest 65 An Giang University Journal of Science – 2019, Vol 6, 60 – 69 Table The Eg value of TiO2-P25 and doped-TiO2 catalysts Catalyst sample TiO2 P25 TiO2-1U-1Mg TiO2-1Mg-1U TiO2-1U White Ivory white Light yellow Bright yellow 400 431 436 450-540 Eg (eV) 3.10 2.88 2.84 2.76-2.30 Eg TB (eV) 3.10 2.88 2.84 2.53 Color Maximum absorption (nm) Depending on previous researches, the catalysts of N-doped alone and N-Cr-co-doped TiO2 simultaneously (Pan & Wu, 2006), N-Cu-codoped TiO2 (Morikawa, Irokawa & Ohwaki, 2006), N-Fe-co-doped TiO2 (Rane al et, 2006) indicated that co-doped samples had better ability of extending the light absorption compared with N-doped samples alone This is partly due to the color of oxides of crom, copper, and iron However, the result in Table showed that the ability of visible light absorption of Mg-N-codoped TiO2 was lower than that of N-doped alone This demonstrated that MgO might play a better role in preventing recombination e-CB-h+VB not because of its color (Bandara, Hadapangoda & Jayasekera, 2004) extremely important factor in choosing the suitable doping element, doping order to create a new highly photo-activity catalyst generation 3.3 Characteristic catalytic activity of TiO2-NMg catalyst sample The results above demonstrated the photocatalytic co-doped samples with the two elements above in order of N first then Mg had a narrowed surface and increased particle size compared to un-doped catalyst Their photosynthetic activity should be reduced, but the opposite was true The two codoped samples had crystalline compositions close to TiO2-P25, but their activity was still higher The above results proved that in co-doped photocatalyst sample with order of adding N first then Mg, synergy of N and MgO role, meant it had active ability in visible light, and prevented recombination e-CB-h+VB as well, has greatly improved the photocatalytic activity of the TiO2N-Mg catalyst samples Recently, there are not many studies in codoping two elements on TiO2, particularly the elements N and Mg The results of this study and the works of the authors mentioned above indicated that the ability of widening the light absorption to the visible light had a close-knit relationship to doping order and the color of doped metal or oxide of the metal This is an The TiO2 photocatalytic activity at TiO2-P25 surface was affected by order of N and Mg shown in Table and Figure Table The activity of TiO2-P25 and doped-TiO2 catalysts Catalytic sample Phenol degradation levels (%) Mineralization levels (%) TiO2-P25 44.7 39.0 TiO2-1U 54.2 48.1 TiO2-1Mg 66.3 61.3 TiO2-1Mg-1U 41.0 40.1 TiO2-1U-1Mg 71.3 65.8 66 An Giang University Journal of Science – 2019, Vol 6, 60 – 69 80 60 50 40 71.3 65.8 66.3 61.3 70 44.7 54.2 48.1 41 40.1 39 30 20 10 TiO2-P25 TiO2-1U TiO2-1Mg Phenol metabolism levels TiO2-1Mg-1U TiO2-1U-1Mg Mineralization levels Figure Comparison of the activity of TiO2-P25 and doped-TiO2 catalysts From the results above, if TiO2-P25 catalyst was doped with N or Mg alone, the phenol conversion after 180 minutes under UV-VIS light increased from 1.2 to 1.5 times compared with TiO2-P25 Degussa However, if co-doping TiO2-P25 with two elements in order of N first then Mg (TiO21U-1Mg), the activity rose 1.6 times compared with original sample and increased around 1.2 times compared with N-doped sample alone (TiO2-1U) be accounted for MgO possessed no photoactivity due to high band gap energy (Eg = eV) but when crystallines MgO located at TiO2 surface with certain contain, it played a role in capturing e-CB of semiconductors when receiving suitable light illuminated, then increased the photoactivity of catalyst Then when N was doped, maybe because N contacted to MgO layer directly, most of N could accessed inside crystal lattice TiO2 to occupy space of oxygen in lattice but perhaps it replaced partial oxygen in crystalline MgO, this is also the reason why this catalyst did not absorb visible light as good as TiO2-1U On the other side, because inside crystalline MgO the element N had valene III which was higher than the valence of oxygen element, it filled up partial defects on crystalline MgO Maybe this is one of the reasons for reducing the role of electron capture of MgO Optical response spectra of samples (Figure 4) cho thấy, TiO2-1U-1Mg sample did not absorb visible light as good as TiO2-1U, its band gap energy was higher (Eg=2.88 eV), meanwhile Eg of TiO2-1U was only 2.36 eV When the Mg was added to the sample TiO2-1U, the Eg value increased leading to the activity should have decreased but in turn its activity increased This illustrated that e-CB capturing ability on conductor band of MgO with narrowing the band gap of N made a synergy effect of two seperated role of N and MgO Therefore, this indicated two elements N and Mg were co-doped on TiO2 P25 Degussa catalyst confirmed synergic effect of N and MgO role Particularly, photoactivity was optimized when doping with N first then Mg However, if TiO2-P25 were co-doped with two elements in reverse order, Mg was added first then N (TiO2-1Mg-1U) then the result reflected quite opposition, photoactivity reduced about 10% compared with original sample This result may However, studies in co-doped TiO2 catalyst were published with different results: Huang L studied co-doped TiO2 with N and 1.6% Pt content, 67 An Giang University Journal of Science – 2019, Vol 6, 60 – 69 VIS light area when N is added in TiO2 first then Mg Compared with TiO2-P25, activity of photocatalyst that doped in this order increases about 1.6 times On the contrary, adding Mg first then N is noticed an inhibited effect and reduced photocatalytic activity in degradation of phenol In this situation, MgO may play a better role in preventing recombination e-CB-h+VB than N in the role of narrowing the band gap energy This is such an interesting content; the following research will investigate further the role in preventing recombination e-CB-h+VB and the role of narrowing the band gap energy by doping metal and nonmetal elements on the photocatalysts in processing of organic pollutant treatment in water with the solar energy source degradation of RB was the best under visible light (Huang, Sun, & Liu, 2007) Morikawa T doped TiO2 with N and Cu, the co-doped sample had higher acitivity than N-doped sample alone in the oxidation of acetaldehyde under visible light (Morikawa, Irokawa & Ohwaki, 2006) On the contrary, Pan C C claimed the photoactivity in the methylene green decomposition or co-doped with Cr first then N on TiO2 was lower than that of the seperated doped-samples (Pan & Wu, 2006) Therefore, depends on the condition of doping and characteristic of doping elements, the activity of co-doped catalyst has not performed consistent result yet This direction will be studied more in the future The above statements and research results in this study show that the co-doped TiO2 catalyst will open up a highly effective catalyst prospect in processing of organic pollutants treatment in water with solar energy source REFERENCES CONCLUSION Bandara J., Hadapangoda C C., & Jayasekera W composite G (2004) TiO2/MgO photocatalyst: the role of MgO in photoinduced charge carrier separation Applied Catalysis B: Environmental, 50, 8388 Asahi R., Morikawa T., Ohwaki T., Aoky K.,& Taga Y (2001) 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