Research on modified silicone compound photocatalyst by doping N

In order to improve the visible light response and reaction efficiency of nitrogen-doped TiO2 set urea as nitrogen source, and silica gel as the load reagent adopt sol-gel method to prepare powder of nitrogen-doped TiO2 supported on SiO2 , use equal volume impregnation method to load MnO2 on catalyst, and then obtain the composite photocatalyst after drying, and roasting .The prepared composite catalysts were characterized by XPS, TEM, SEM, XRD and other methods. Moreover, the photo-catalytic activity of the composite catalysts under visible-light region was tested, and the influences of nitrogen content and calcinations temperature on the photo-catalytic activity were investigated. The results show that:(1) the range of light response of modified composite catalyst was expanded from ultraviolet region to visible-light region, which results in high visible-light catalytic activity in the degradation of methyl orange.(2) Through the mechanism of photo-catalytic reaction and the treatment effect analysis, MnO2 as catalytic resulting O2 can be served as a good electronic capture agent and improve the reaction efficiency.(3)With the reduction of nitrogen content,and the increase of calcination temperature, the visible-light catalytic activity was weakened.(4) The activity of catalyst was reused six times, without significant reduction and had excellent efficiency


Introduction
With the increasingly serious energy shortage and environmental pollution, fanding the suitable photocatalyst to realize the effective utilization of solar energy, has become a hot issue in the field. Therefore, TiO 2 with excellent catalytic performance attracts more attention in the environmental, [1] which has the advantages of low cost, good chemical stability, nontoxic,and no secondary pollution. But there are also shortcomings, e.g.,the forbidden band of TiO 2 is wide (Eg=3.2eV)and can only absorb UV-light; In addition, the hole and electron stimulated by light are easily compounded so as to reduce the photo-quantum efficiency [2][3] . Many domestic and international experts and scholars have tried to use the nonmetal ion doping method to extend the visible light absorption of TiO 2 , where nitrogen-doping can effectively hence the absorption of the catalyst in the visible-light region, as well as the efficiency of photo-catalytic oxidation [4] . SiO 2 as the load reagent can effectively control the growth of TiO 2 particles so as to get smaller size, and larger specific surface area, which can improve the separation performance, and make them inactivation to prolong the service life [5][6] . Researches show that quantum yield is the most important factor influencing treatment effect of photocatalyst, so improving the light quantum yield of photo-catalyst, promoting the separation of photongenerated carriers and reducing the recombination rate are key factors, in this paper, by preparing the nitrogen-doped TiO 2 /MnO 2 supported on SiO 2 composite photocatalyst the new ecological O 2 resolved from hydrogen peroxide which are produced in the process of catalytic hydrolysis can be used as electron capture agent to effectively promote the separation of photon-generated carriers and to reduce its compound probability [7] . This kind of photo-catalyst has advantages of visible light response, and smaller and equally distributed particle size, and can efficiently restrain the compound of electronic -hole pairs, which greatly improve the treatment effect of the pollutants.

preparation of Nitrogen-Doped TiO 2 /MnO 2 supported on SiO 2 composite photo-catalyst
The preparation was made as follow：After adjusting the PH value of Solution B (10mL of deionized water and 20mLof anhydrous ethanol ) to about 2, dropwise add Solution A(10mL of actic acid,10mL of dibutyl phthalate and 30mL anhydrous erhanol ) into it with strong stirring, and then continue to strongly stir for 30 minutes after dropping, add different volumes of urea whose concentration is 1mol/L to the mixed Solution with N/Ti molar ratio of 10%, 15%, 20% and 25% respectively, and continue to stir for 30 minutes, add a certain amount of SiO 2 as load reagent and strongly stir for 60 minutes, stand and air dry the mixture at room temperature until it become xerogel and then grind it in agate mortar, calcine it in muffle furnace for 3hours at different temperature of 350 ℃ ,400 ℃ ,450 ℃ and 500 ℃ respectively, grind it again and screen it by standard sieve with 300 meshes, adopt the equal volume impregnation method to load a certain concentration of manganese nitrate on the calcined sample of catalyst in ultrasonic instrument, after 1 hour's drying, and calcination a series of nitrogendoped TiO 2 /MnO 2 supported on SiO 2 composite photocatalyst with different calcination temperature is obtained.

performance of Nitrogen-Doped TiO 2 /MnO 2 supported on SiO 2 composite photo-catalyst
The adsorption ability and photo-catalytic performance of nitrogen-doped TiO 2 /MnO 2 supported on SiO 2 composite photo-catalyst was evaluated by the degradation of methyl orange, and the reaction was carried out in the self-made additional stirring temperature cycle optical instrument with 150w iodine tungsten lamp as light source. The reaction device keeps the temperature of the reaction system at 25 ℃, and the magnetic stir keeps uniform concentration and temperature of solution in the system ,Take a certain quality of nitrogen-doped TiO 2 /MnO 2 supported on SiO 2 composite photo-catalyst in reaction device, add 100mL of methyl orange solution with certain concentration, and make it adsorptiondesorption balance through 30 minutes' magnetic stirring under the condition of no light, and then turn on the iodine tungsten lamp for illumination. In the process of aphotic adsorption -desorption balance and illumination, take a sample every 20 minutes, use the principle of centrifugal separation to remove the photo-catalyst, adopt ultraviolet-visible spectrophotometer to measure the absorbance of supernatant fluid at the longest absorption wave, and then calculate the concentration according to the standard working curve based on the concentration(C t ) of the supernatant fluid at the degradation time(t) and the initial concentration(C 0 ) of degradation, calculat the degradation rate (D) of methyl orange to evaluate the photocatalytic activity of nitrogen-doped TiO 2 /MnO 2 supported on SiO 2 composite photo-catalyst: D=(c t -c 0 )/ c 0 ×100%

Figure1. The XRD spectrum of 20%N at different calcination temperatures
The X-ray diffraction (XRD) spectrum in Figure 1 shows that: all nitrogen doped TiO 2 /MnO 2 supported on SiO 2 composite photocatalysts with N/Ti molar ratio of 20% prepared at 300℃, 350℃, 400℃ and 450℃ respectively, are anatase phases, and the Ti-N diffraction peak does not appear, because high temperature made N volatilize, resulting in low N content, or larger Ti -N clusters fail to form in the complex catalyst; Meanwhile the movement of characteristic peak of anatase type TiO 2 does not appear, because of nitrogen-doping, it shows that nitrogen doping does not change its crystal tructure. Using Scherrer formula to calculate the average particle size, the results are 13.8nm, 14.6nm, 12.3nm, and 17.2nm.
With the increase of calcination temperature, the characteristic peak of anatase crystal phase becomes more keen-edged, which means the crystal shape is more and more regular and the size becomes larger and larger, because the increase of calcination temperature causes agglomeration of particles.  Figure 2 shows that: all nitrogen-doped TiO 2 /MnO 2 supported on SiO 2 composite photo-catalysts with different N/Ti molar ratio prepared at temperature of 400℃ are anatase phases. Using the Scherrer formula to calculate the average grain size, the results are 9.1nm, 8.4nm, 14.7nm, 12.4nm, 13. 2 nm, 12.3nm ,and 14.2nm. Nitrogen-doping enlarges the catalyst particles, and the size has a minimum value with the change of N/Ti molar ratio, which is corresponding to the catalyst with molar ratio of 20%. It is consistent with the verification results according to theoretical basis that more keen-edged diffraction peak and smaller full width at half maximum means smaller particle and better crystal.

Surface morphology analysis of Catalyst
Scanning electron microscope (SEM)  Figure 3 is the SEM images and energy spectrum diagrams of nitrogen-doped TiO 2 supported on SiO 2 composite photocatalyst, to which Mn 2+ is added. It can be seen from the figure: TiO 2 particle size ranges from few nanometers to dozens of nanometers which is successfully loaded on the surface of SiO 2 particle which relatively uniform distribution; the comparison of energy spectrum diagram shows that Mn 2+ has been successfully incorporated into and loaded on the surface of SiO 2 ,which slightly increases TiO 2 particle size. Thus it shows SiO 2 as loaded reagent can effectively improve the distribution of TiO 2 particles, and enhance the dispersing performance; and the Mn 2+ has been successfully incorporated and loaded on the surface of SiO 2 .   Figure 3 (a) and Figure 4 (b) it can be seen that the particle size increases with the increase of calcination temperature, which is due to the agglomeration of the grain caused by the increase of calcination temperature, verifying the analysis conclusion in XRD.   Figure 6 is the XPS energy spectrum diagram of composite catalysts at calcination temperature of 400 ℃,where A and B respectively represent the catalysts with and without nitrogen-doping. It can be seen from the figure that, for the nitrogen-doped catalyst samples, there is a new N 1s peak at 396.1eV, and a N 1s peak of the catalyst surface adsor bed N 2 at 401.7eV, thus the N atoms and TiO 2 molecules are combined by chemical bonds. Asahi [8] et al, attributed the peak appearing in the 396eV to N atoms mixed with the crystal lattice, and attributed peaks appearing in the 400eV and 402eV to the molecular adsorption of N 2 on the catalyst surface. They pointed out that the catalyst with visible light activity is associated with the N atoms, and this is because the hybridization of N 2p orbital and O 2p orbital happened, which can decrease the forbidden band width of TiO 2 , so as to be motivated by the visible light.   (1) shows test results of visible-light activity of catalyst. The comparison between d and a /b shows that the catalytic activity of visible light of nitrogen-doped TiO 2 /SiO 2 has been significantly improved. This is because the doped N atoms partly replaced O atoms enter into crystal lattice of TiO 2 , and formed an independent N 2p narrowband above the valence band, causing the response of composite catalysts to the visible light [8] . It can be seen from f, g, i and j in the figure, the catalytic activity of the composite photo-catalysts gradually increases with the increment of nitrogen-doping content, It will be optimal when the percentage of nitrogen doping content is 20%, at this moment the composite catalyst can completely get rid of methyl orange within 120 minutes, and that with different doping amount will not have the same effect under the same conditions. The reason may be that the newly created energy level after nitrogendoping and the bulk phase of TiO 2 form a compound semiconductor structure, hence the electric potential difference of both valence bands can make the hole of TiO 2 's valence band migrate to the newly formed energy levels, so as to effectively reduce the recombination probability of photo-generated electron hole, and improve the activity of catalyst [6] . can be seen from I and h in the figure that the removal rate decreases with the increase of temperature, and the reason may be that under high temperature, N element was oxidized and volatilizated in the heating process, leading to the decrease of N content in TiO 2 lattice, and the enlargement of size, and decrease of specific surface area, can also lower the activity of visible-light response. Figure 6(1) shows the catalyst activity test results of UV-light. It can be seen from the figure that the composite catalyst in ultraviolet region also has a good degradation effect. simultaneously, Figure 7 shows, Mn 2 + has obviously improved the processing efficiency of catalyst. This is because the Mn 2 + is oxidized into MnO 2 through calcination and loads on the surface of catalyst. It is used as catalyst to catalyze H 2 O 2 to produce O 2 which is served as electron capture agent to effectively promote the separation of photogenerated carrier and reduces its compound probability. Effect of run times on the photo-catalytic activity of TiO 2 Figure 9 shows the experimental results of repeated use of catalyst. Under the same cycle times, the catalytic activity of the composite catalyst adding SiO 2 as lode reagent is obviously higher. After reused six times, the degradation rate of composite catalyst is 75%, while that without SiO 2 is only 27% under the same condition. In conclusion, SiO 2 as load reagent can effectively inhibit the deactivation of catalyst, so as to improve its service life.

Photo-catalytic activity of Nitrogen-Doped TiO 2 /MnO 2 supported on SiO 2 composite photocatalyst
In conclusion, for nitrogen-doped TiO 2 /MnO 2 supported on SiO 2 composite photo-catalyst, nitrogendoping can expand the range of light response of TiO 2 and improve the activity in visible and ultraviolet region. SiO 2 as load reagent has smaller particle size, and larger specific surface area, which can improve the separation performance of catalyst, so as to prolong the service life of catalyst. The mechanism shows that holes migrating to the surface of the semiconductor TiO 2 react with H 2 O or OHabsorbing on the surface and the generated · OH, is free radical with strong oxidability, which can oxidize a variety of organic pollutants, and plays a main role in photo-catalytic system as oxidizing agent. The Mn 2 + is oxidized into MnO 2 by calcination, which loads on catalyst surface, and catalyzes · OH. The generated O 2 can react with electrons which migrate to the surface of the semiconductor TiO 2 to generate ·O 2 , and other activeoxygen free radicals. The reactive group will also be able to participate in oxidation-reduction reactions, and the generated O 2 is served as electron capture agent to promote the separation of photo-generated carrier and reduce the recombination probability, which significantly improve the photo-catalytic activity of the catalyst. At the same time, according to the reaction dynamics, this process can promote the generation of· OH which is served as the capture agent of hole. According to the simulated catalytic reaction mechanism of Fig.10 OOH, which can participate in many redox processes. Electronhole recombination is the most important reason for the serious decrease of reaction efficiency. Electron-hole recombination is very fast, but the rate of trapping carriers is relatively slow. Electrons are more active than holes. Therefore, O 2 produced by high temperature oxidation of Mn 2+ to MnO 2 is used as electron trapping agent in advance. Adsorption on the catalyst surface is very important, it can improve the surface electron migration rate to inhibit carrier recombination, thereby increasing the quantum efficiency of photocatalytic reaction, but also promote the formation of hydrogen peroxide molecules adsorbed on the catalyst surface. Therefore, the prepared composite catalyst can achieve visible light response and obtain high treatment effect.

Conclusions
Make full use of the good performance of visible-light response of nitrogen doped TiO 2 ,great adsorption of SiO 2 and excellent catalytic activity of MnO 2 , through the effective combination of sol-gel method [9] and incipientwetness impregnation method to prepare nitrogen-doped TiO 2 /MnO 2 supported on SiO 2 composite photo-catalyst.
The results show that the urea in the form of N atom replaces O atoms in the TiO 2 lattice, form Ti -O -N bong on the surface of TiO 2 , and constitute the new energy level structure, which made the absorption of catalyst cause red-shift and induce the visible light response, while the composite catalysts in both the ultraviolet and visible region show higher catalytic activity. SiO 2 as load reagent can not only control the growth of TiO 2 particles, to get smaller size and larger specific surface area, but also improve the separation performance of catalyst, so as to expend its service life. The Mn 2 + is oxidized to MnO 2 and loads on catalyst surface to make O 2 which migrated to the surface of the semiconductor TiO 2 as electron capture agent to promote the separation of photogenerated carrier and reduce the recombination probability. Then the generated active oxygen free radicals participate in oxidation-reduction reaction of dye pollutants to make them degrade, which significantly improves the photo-catalytic activity of the catalyst.