Preparation method and application of titanium dioxide catalyst with controllable surface defects

Abstract

The invention discloses a preparation method of a titanium dioxide catalyst with controllable surface defects, and the titanium dioxide catalyst is applied to photocatalysis, so that low-concentration nitric oxide pollutants can be efficiently removed. According to the preparation method, distribution of bulk phase defects and surface defects of amorphous titanium dioxide is regulated and controlled through an inorganic acid permeation method, and precise construction of the titanium dioxide surface defects is achieved through gradient filling of the bulk phase defects. The prepared titanium dioxide catalyst rich in surface defects can efficiently and highly selectively remove low-concentration nitric oxide pollutants under the irradiation of visible light. The method is easy to operate, high in stability and good in photocatalytic activity, a reliable method is provided for green and low-carbon photocatalytic purification of indoor nitric oxide pollution gas, and the method is very worthy of popularization.

Description

Technical Field

[0001] The invention relates to a method for preparing a titanium dioxide catalyst with controllable surface defects and application of the catalyst in treating low-concentration nitrogen oxides, belonging to the technical field of air pollution treatment. Background Art

[0002] Nitrogen oxides (NO x , mainly NO) is a typical air pollutant, mainly from automobile exhaust and industrial waste gas. Low concentrations of NO in the atmosphere easily react with active free radicals to generate inorganic acids, ozone and PM2.5 particles, causing environmental problems such as acid rain, haze, photochemical smog, etc., which seriously threaten the health of residents.

[0003] Photocatalytic oxidation technology is considered a feasible method for atmospheric NO x Pollutant treatment technology. It is well known that the photocatalytic oxidation reaction of NO removal is dominated by the molecular oxygen activation pathway: the reactive oxygen species (ROS) generated by molecular oxygen activation can convert NO into nitrate and harmful byproduct NO2, affecting the activity and selectivity of NO oxidation removal. Therefore, the type and amount of ROS determine the removal efficiency of NO. It is generally believed that superoxide radicals (·O2 - ) is an effective ROS for NO oxidation, but the selectivity of NO oxidation products mediated by it is highly dependent on ·O2 - adsorption configuration. This means that ·O2 - Oxidation of NO carries the risk of generating toxic byproduct NO2. 1 O2) is also a common reactive oxygen species, but its oxidizing ability is weak and it can only convert NO into NO2. Among many ROS, the hydroxyl radical (·OH) with strong oxidizing ability can oxidize NO into nitrate through continuous oxidation reaction, effectively avoiding the formation of byproduct NO2. In particular, the selectivity of NO oxidation products mediated by ·OH is independent of the ROS adsorption configuration. At the same time, ·OH can react with 1 O2 synergistically oxidizes NO, thus compensating 1 Therefore, selective molecular oxygen activation to produce ·OH is expected to achieve deep oxidation of NO.

[0004] However, the slow molecular oxygen activation process caused by the easy recombination of catalyst carriers hinders the improvement of NO removal reaction activity and the regulation of product selectivity, thus limiting the practical application of this technology. Therefore, how to achieve high activity, high selectivity and safe purification of low-concentration NO is still a key issue that needs to be solved urgently by workers in the field of environmental pollution control. Summary of the invention

[0005] The primary technical problem to be solved by the present invention is to provide a method for preparing a titanium dioxide catalyst with controllable surface defects.

[0006] Another technical problem to be solved by the present invention is to provide an application of the catalyst for treating low-concentration nitrogen oxides.

[0007] In order to achieve the above object, the present invention adopts the following technical solutions:

[0008] According to a first aspect of an embodiment of the present invention, a method for preparing a titanium dioxide catalyst with controllable surface defects is provided, comprising the following steps:

[0009] (1) under continuous stirring, adding ammonia water in small amounts to a 0.25 mol / L titanyl sulfate solution several times until a white precipitate is formed, collecting the precipitate by centrifugation, adding hydrogen peroxide to the white precipitate and continuously stirring to obtain a suspension, reacting at 50 to 80° C. for 6 to 24 hours, cooling, centrifuging, washing, and drying to obtain an amorphous titanium dioxide catalyst;

[0010] (2) Add the prepared amorphous titanium dioxide to a boric acid solution, stir, heat to 100-120° C. and react for 10-12 hours. After cooling, centrifuge, wash and dry. The obtained substance is a yellow powder, i.e., a titanium dioxide catalyst with surface defects.

[0011] Furthermore, in the step (1), ammonia water is added in small amounts multiple times to adjust the pH value of the solution to 10, thereby forming a white precipitate.

[0012] Furthermore, in the step (1), 1 g of the white precipitate is mixed with 50 to 100 mL of hydrogen peroxide.

[0013] Furthermore, in step (1), the concentration of the hydrogen peroxide solution is: volume ratio H2O2:H2O=1:3-5.

[0014] Furthermore, in the step (1), the white precipitate is added to hydrogen peroxide and the resulting suspension is continuously stirred and reacted at 60° C. for 12 hours.

[0015] Furthermore, in step (2), the concentration of the boric acid solution is not less than 1 mol / L.

[0016] According to a second aspect of an embodiment of the present invention, a titanium dioxide catalyst having a surface rich in defects obtained by the above-mentioned preparation method is provided.

[0017] Furthermore, the surface defect-rich titanium dioxide catalyst is used in the photocatalytic reduction of nitric oxide.

[0018] Furthermore, the titanium dioxide catalyst with surface defects is used in the photocatalytic reduction of nitric oxide, wherein the photocatalytic conditions are visible light and the concentration of nitric oxide is between 100 and 1000 ppb.

[0019] H2O2 is the product of O2 reduction through a two-electron pathway, which can be further decomposed to generate ·OH, and is an important pathway for the formation of ·OH. Studies have shown that defect engineering can promote the two-electron molecular oxygen activation process by regulating the atomic arrangement and electronic structure of the catalyst. Inspired by this, amorphous photocatalysts with abundant defect sites are expected to become candidates for the activation of molecular oxygen to generate ·OH, among which the unsaturated metal defect sites on the surface have abundant localized electrons, which are conducive to the adsorption and two-electron activation of O2. However, everything has two sides. In the photocatalytic process, the bulk defects in the lattice of amorphous catalysts may become rapid recombination centers for photogenerated carriers, which usually reduces the photocatalytic activity of the catalyst. In order to improve the photocatalytic performance, the distribution of defects on the catalyst surface and in the bulk can be regulated by improving the electronic structure of defects on the catalyst surface and filling bulk defects.

[0020] The present invention uses inorganic acid molecules to regulate the distribution of surface defects and bulk defects of amorphous titanium dioxide catalysts. This method can adjust the electronic structure of surface defects and fill bulk defects, thereby optimizing the performance of the catalyst. Specifically, under the action of boric acid, a titanium dioxide catalyst rich in surface defects can be prepared, which has an optimized surface oxygen defect structure and can fill bulk defect sites.

[0021] The titanium dioxide catalyst prepared by this method can prolong the lifetime of photogenerated carriers and avoid the recombination of carriers in the bulk phase, thereby achieving efficient and highly selective photocatalytic oxidation to remove nitric oxide. This method provides a new green, low-carbon, efficient, safe and low-cost way to purify low-concentration nitric oxide pollutants in the atmosphere or indoor environment. In addition, the preparation method is simple to operate, low-cost, and suitable for large-scale industrial production.

[0022] On the other hand, boric acid molecules have proton hydrogen, which can react with the hydroxyl groups on the surface of titanium dioxide to form a hydrogen bond network of H3BO3-OH-TiO2, thereby promoting the formation of electron-rich titanium dioxide surface defects. At the same time, free boron atoms can penetrate into the titanium dioxide bulk phase, and the bulk phase Ti 3+ The defects are filled by boron atoms; the adjacent unsaturated titanium (Ti 3+ ) transfers electrons to boron atoms and transforms into saturated tetravalent titanium ions (Ti 4+ ). Boron atom filling destroys the local charge accumulation at the bulk defects, thereby inhibiting the recombination of electrons and holes at the bulk phase points.

[0023] Boric acid-modified titanium dioxide with rich surface defects effectively avoids bulk carrier recombination and accelerates electron transfer to the surface for molecular oxygen activation, achieving high-activity and high-selectivity photocatalytic removal of nitric oxide, providing a green, safe, efficient and pollution-free purification method for the removal of atmospheric nitrogen oxides. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 is a high resolution transmission electron microscopy (HRTEM) image in Example 1;

[0025] Figure 2 for Figure 1 Selected area electron diffraction (SAED) pattern of the sample;

[0026] Figure 3 XRD patterns of amorphous TiO2 (Am-TiO2) and boric acid-modified TiO2 rich in surface oxygen vacancies (HB-TiO2) prepared in Example 1;

[0027] Figure 4 EPR images of amorphous TiO2 (Am-TiO2) and boric acid-modified TiO2 rich in surface oxygen vacancies (HB-TiO2) prepared in Example 1;

[0028] Figure 5 The activity diagram of the photocatalytic removal of different concentrations of NO by the amorphous TiO2 (Am-TiO2) prepared in Example 1 and a series of boric acid-modified TiO2 rich in surface oxygen vacancies (HB-TiO2-x);

[0029] Figure 6 This is a diagram of the generation of byproduct NO2 during the photocatalytic removal of NO of different concentrations by the amorphous TiO2 (Am-TiO2) prepared in Example 1 and a series of boric acid-modified TiO2 rich in surface oxygen vacancies (HB-TiO2-x). DETAILED DESCRIPTION

[0030] In order to more clearly illustrate the embodiments of the present invention and / or the technical solutions in the prior art, the specific implementation methods of the present invention will be described below with reference to the accompanying drawings. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings and other implementation methods can be obtained based on these drawings without creative work. In addition, the reference to orientation only represents the relative position relationship between materials and components, rather than the absolute position relationship.

[0031] In order to better explain the present invention, the main contents of the present invention are further explained below in conjunction with specific embodiments, but the contents of the present invention are not limited to the following embodiments.

[0032] Example 1: Preparation of amorphous titanium dioxide catalyst

[0033] Dissolve titanyl sulfate octahydrate in deionized water to prepare a 0.25 mol / L titanyl sulfate solution. Under continuous stirring, add ammonia water in small amounts several times to adjust the pH value of the solution to 10. At this time, a white titanate precipitate is formed in the solution. Then continue stirring for 30 minutes to fully precipitate to obtain a white powdery substance. Collect the precipitate by centrifugation and wash it with deionized water several times until it is neutral. Take 1g of white precipitate and add it to 70mL of hydrogen peroxide solution (volume ratio H2O2:H2O=1:4), continue stirring for 90 minutes, transfer the obtained suspension to a Teflon-lined autoclave, and react at 50-80°C for 6-24 hours. After cooling, centrifuge, wash and dry. The product here is marked as Am-TiO2.

[0034] Example 2: Preparation of titanium dioxide catalyst with surface defects

[0035] Prepare 2 mol / L boric acid solution: dissolve H3BO3 (0.1 mol) in 50 mL hot water and stir well.

[0036] 0.5 g of the amorphous titanium dioxide prepared in Example 1 was added to the above solution, and after magnetic stirring for 30 minutes, the mixture was transferred into a Teflon-lined autoclave, reacted at 110° C. for 11 hours, cooled, centrifuged, washed, and dried. The obtained substance was a yellow powder, i.e., a titanium dioxide catalyst with defects on the surface of the product. The product here is marked as HB-TiO2-2.

[0037] Figure 1 The high-power transmission electron microscope (HRTEM) in Figure 2 observed that the surface defect-rich titanium dioxide catalyst (HB-TiO2-2) prepared in this example showed that it had a disordered crystal structure, while the corresponding Figure 2 The selected area electron diffraction (SAED) in the sample shows polycrystalline diffraction rings, corresponding to (110), (112) and (200) anatase crystal planes. This indicates that the surface-defective titanium dioxide catalyst (HB-TiO2-2) contains a large amount of nanocrystals. The XRD spectra of the amorphous titanium dioxide catalyst (Am-TiO2) prepared in Example 1 and the surface-defective titanium dioxide catalyst (HB-TiO2-2) are weaker in diffraction peaks. The diffraction peaks of HB-TiO2 can be compared with the characteristic diffraction peaks (101), (103), (200) and (105) of the anatase TiO2 standard card (JCPDS 01-0562), which indicates that the Am-TiO2 and HB-TiO2-2 prepared in Example 1 and Example 2, respectively, are anatase-type TiO2. Figure 3The low temperature electron paramagnetic resonance (EPR) spectrum shown shows that Am-TiO2 exhibits a stronger unsaturated Ti 3+ The signals (g1 = 2.002, g2 = 2.007, g3 = 2.008) and g4 = 2.022 correspond to the surface adsorption of oxides, which may be caused by the H2O2 treatment during the preparation process. 3+ The signal decreases, which may be due to the filling of bulk defect sites by boronic acid molecules and a small amount of boron atoms.

[0038] Example 3: Preparation of titanium dioxide catalyst with surface defects

[0039] The titanium dioxide catalyst with surface defects was prepared in the same manner as in Example 2, except that the concentration of the H3BO3 solution used was 1 mol / L, and the reaction was carried out in an autoclave at 120°C for 12 hours. The product here is marked as HB-TiO2-1.

[0040] Example 4: Preparation of Titanium Dioxide Catalyst with Rich Surface Defects

[0041] The titanium dioxide catalyst with surface defects was prepared in the same manner as in Example 2, except that the concentration of the H3BO3 solution used was 1.5 mol / L, and the reaction was carried out in an autoclave at 105°C for 10 hours. The product here is marked as HB-TiO2-1.5.

[0042] Example 5: Preparation of Titanium Dioxide Catalyst with Surface Defects

[0043] The titanium dioxide catalyst with surface defects was prepared in the same manner as in Example 2. The difference was that the concentration of the H3BO3 solution used was 2.5 mol / L, and the reaction was carried out in an autoclave at 100°C for 10 hours. The product here was marked as HB-TiO2-2.5.

[0044] Example 6: Photocatalytic NO performance test

[0045] A continuous flow reactor was used to evaluate the photocatalytic NO removal performance of each product.

[0046] The purification coating was prepared by the smearing method: first, 0.1 g of the sample was evenly ground with a quartz mortar, then added into 10 mL of deionized water, and after ultrasonic mixing, quickly poured into a culture dish with a diameter of 5 cm, and dried completely at 60 ° C. The catalyst powder can be considered to be evenly loaded.

[0047] The culture dish with uniform catalyst load is placed in a 1.2L or 10.8L stainless steel reactor (with a window made of quartz glass on the top). A 150W mercury lamp (with a filter, λ>420nm) or a 30W LED lamp (with a filter, 420nm≤λ≤750nm) is used as a visible light source, and the target pollutant is NO (1000ppb, 2000SCCM) obtained by diluting high-concentration compressed NO gas (117ppm, Ar) with air. After the gas path and light source are installed, NO gas is introduced for dark adsorption for 30 minutes to reach adsorption-desorption equilibrium. The light source is then turned on, and the concentration of NO and NO2 in the reaction process is monitored in real time using a Thermo Fisher nitrogen oxide analyzer (42i-DNMSDAA).

[0048] The calculation method of NO removal efficiency (η) and selectivity (s) is as follows:

[0049] NO removal rate (η%) = (C0-C) / C0×100%;

[0050] Selectivity of NO removal reaction (s%) = 1-D / (C0-C)×100%.

[0051] In the above formula, C0: adsorption equilibrium is the NO concentration in the reaction system (ppb); C: the instantaneous concentration of NO in the reaction system during the reaction (ppb); D: the instantaneous concentration of NO2 in the reaction system during the reaction (ppb).

[0052] The catalysts prepared in Example 1 and Examples 2 to 5 were tested for photocatalytic removal of NO under visible light irradiation, mainly to compare the photocatalytic activities, as shown in Table 1.

[0053] Table 1 Photocatalytic NO removal performance test results of each catalyst

[0054]

[0055] According to the results of photocatalytic removal of NO of different concentrations shown in Table 1, the titanium dioxide catalyst rich in surface defects prepared by the present invention can efficiently remove nitric oxide pollutants with a concentration of 100 to 600 ppb, with a removal efficiency of up to 92% and a selectivity of 99%. It can be seen that the titanium dioxide catalyst rich in surface defects prepared by the present invention is particularly suitable for photocatalytic removal of low-concentration nitric oxide pollutants under visible light conditions.

[0056] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions recorded in the aforementioned embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the present invention.

Claims

1. A method for preparing a titanium dioxide catalyst with controllable surface defects, characterized in that The following steps are involved: (1) under continuous stirring, adding ammonia water in small amounts to a 0.25 mol / L titanyl sulfate solution several times until a white precipitate is formed, collecting the precipitate by centrifugation, adding the white precipitate to hydrogen peroxide and continuously stirring the obtained suspension, reacting at 50 to 80° C. for 6 to 24 hours, cooling, centrifuging, washing, and drying to obtain an amorphous titanium dioxide catalyst; (2) Add the prepared amorphous titanium dioxide to a boric acid solution, stir, heat to 100-120° C. and react for 10-12 hours. After cooling, centrifuge, wash and dry. The obtained substance is a yellow powder, i.e., a titanium dioxide catalyst with surface defects.

2. The method for preparing a titanium dioxide catalyst with controllable surface defects according to claim 1, characterized in that: In the step (1), ammonia water is added in small amounts multiple times to adjust the pH value of the solution to 10, thereby forming a white precipitate.

3. The method for preparing a titanium dioxide catalyst with controllable surface defects according to claim 1, characterized in that: In the step (1), 1 g of white precipitate is mixed with 50-100 mL of hydrogen peroxide.

4. The method for preparing a titanium dioxide catalyst with controllable surface defects according to claim 1, characterized in that: In the step (1), the concentration of the hydrogen peroxide solution is a volume ratio of H2O2:H2O=1:3-5.

5. The method for preparing a titanium dioxide catalyst with controllable surface defects according to claim 1, characterized in that: In the step (1), the white precipitate is added to hydrogen peroxide to obtain a suspension which is continuously stirred and reacted at 60° C. for 12 hours.

6. The method for preparing a titanium dioxide catalyst with controllable surface defects according to claim 1, characterized in that: In the step (2), the concentration of the boric acid solution is not less than 1 mol / L.

7. A titanium dioxide catalyst rich in surface defects obtained by the method for preparing a titanium dioxide catalyst with controllable surface defects according to any one of claims 1 to 6.

8. Use of the titanium dioxide catalyst with surface defects as claimed in claim 7 in photocatalytic reduction of nitric oxide.

9. Use of the titanium dioxide catalyst with surface defects as claimed in claim 8 in photocatalytic reduction of nitrogen monoxide, characterized in that: The photocatalytic conditions are visible light and a nitric oxide concentration between 100 and 1000 ppb.

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