A method for regulating the crystal surface of manganese tetraoxide by phase transformation strategy

Abstract

The present invention discloses a method for controlling the exposed crystal faces of manganese dioxide using a phase transformation strategy. This method involves reacting manganese dioxide with a phase transformation agent, urea, to transform the manganese dioxide into a manganese dioxide with highly exposed crystal faces. By selecting different manganese dioxide single crystal nanowires as precursors and calcining them with the phase transformation agent, urea, under an inert atmosphere, the manganese dioxide can be transformed into manganese dioxide with different exposed crystal faces, such as {101}, {112}, and {103}. The manganese dioxide with highly exposed crystal faces prepared by the present invention can be used as a catalyst to decompose organic gaseous pollutants in the air, with the manganese dioxide with {103} exposed faces exhibiting the best catalytic decomposition performance.

Description

Technical Field

[0001] The invention relates to a method for regulating and controlling the crystal surface of manganese manganese tetraoxide, belonging to the technical field of crystal surface regulation. Background Art

[0002] A large number of studies have shown that the performance of inorganic nanomaterials is not only related to factors such as the crystal form, size, and morphology of the nanoparticles, but also has a close relationship with the exposed crystal faces of the nanomaterials. By controlling the directional growth of nanomaterials, controlling the synthesis of nanomaterials that expose specific highly reactive crystal faces, and ultimately achieving high catalytic activity of nanomaterials, it has important scientific significance and application value. According to the Wuff crystal growth law, crystal faces with low surface energy grow slowly, and crystal faces with high surface energy grow quickly. During the growth of crystals, high surface energy crystal faces usually decrease rapidly or even disappear during the growth process of the crystal. In practical applications, it is found that high surface energy crystal faces often have high reactivity. Therefore, how to expose specific highly active crystal faces on the crystal surface remains a challenging technical problem in the field of nanomaterials. Currently, there are two pathways for effectively exposing specific crystal faces: (1) the "bottom-up" pathway, which involves selecting a suitable capping agent to preferentially adsorb on certain specific crystal faces to reduce surface energy and inhibit crystal face growth, thereby exposing specific high-energy crystal faces; and (2) the "top-down" pathway, which involves selectively etching unwanted crystal faces of the prepared crystal while simultaneously selecting a suitable capping agent to adsorb on the specific crystal faces to protect them from etching by the corrosive agent, thereby preserving and exposing the specific crystal faces. Therefore, selecting a suitable capping agent or capping agent to preferentially adsorb on specific crystal faces and altering their surface properties (such as surface energy / surface corrosion resistance) is the key to achieving crystal face control and is also the difficulty of this technology.

[0003] Different metal oxides often require specific capping and covering agents to manipulate crystal facets, yielding crystals with highly exposed facets. The selection of these agents is crucial, but often challenging. Overcoming this limitation and achieving more controllable crystal facet manipulation is of great scientific research significance and practical application value. Summary of the Invention

[0004] In order to solve the problems of the singleness and limitations of the existing methods for regulating the crystal plane of metal oxides, the present invention provides a method for regulating the crystal plane of manganese manganese tetraoxide using a phase transformation strategy.

[0005] To achieve the above object, the technical solution adopted by the present invention is: a method for regulating the crystal surface of manganese tetraoxide, comprising the following steps:

[0006] (1) Synthesis of α-MnO2 single crystals with highly exposed crystal faces;

[0007] (2) mixing the manganese dioxide single crystal obtained in step (1) with urea as a phase transition controller in a certain proportion and grinding the mixture to obtain a mixed powder;

[0008] (3) calcining the mixed powder obtained in step (2) in an oxygen-free atmosphere.

[0009] Preferably, in step (1), ammonium sulfate, manganese sulfate or ammonium oxalate is used as a precursor and a hydrothermal method is used to sequentially prepare α-MnO2 single crystals with {110}, {100} or {310} highly exposed crystal faces.

[0010] Preferably, in step (1), the morphology of the α-MnO2 single crystal is nanowire.

[0011] Preferably, in step (2), the mass ratio of the phase change control agent to manganese dioxide is 0.3 to 3, preferably 0.5 to 2.

[0012] Preferably, in step (3), the oxygen-free atmosphere is one or more of nitrogen, argon, and helium.

[0013] Preferably, in step (3), the calcination temperature is 400-500°C; and the calcination time is 1-5 hours.

[0014] The application of the above-mentioned manganese tetraoxide catalyst with highly exposed crystal faces in purifying gaseous pollutants in the air.

[0015] In the above application, the gaseous pollutants are volatile organic pollutant gases such as formaldehyde and benzene series.

[0016] The beneficial effects of the present invention are:

[0017] 1. This invention solves the problem of crystal face control in traditional nanomaterials. It does not require the selection of specific capping agents and covering agents for different manganese dioxides. It is universally applicable to the preparation of manganese dioxide with highly exposed crystal faces from manganese dioxide precursors with different structures.

[0018] 2. The phase transition method for regulating crystal surface exposure provided by the present invention is significantly innovative compared to the traditional "bottom-up" and "top-down" methods that require the selection of specific capping agents and covering agents.

[0019] 3. The present invention uses urea as a phase transition agent, which has the advantages of being cheap, readily available, and environmentally friendly.

[0020] 4. The present invention controls the conversion of manganese dioxide to manganese tetraoxide by controlling the amount of urea added, the calcination temperature, and the time, and successfully synthesizes manganese tetraoxide with different highly exposed crystal faces. The process is simple and easy to prepare on a large scale.

[0021] 5. The present invention can transform a precursor manganese dioxide with different exposed crystal faces into trimanganese tetraoxide with different exposed crystal faces, such as {101}, {112}, and {103}. These trimanganese tetraoxides exhibit excellent catalytic decomposition performance for organic pollutants, with the trimanganese tetraoxide catalyst with the {103} exposed crystal face providing the highest catalytic activity. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 The XRD comparison diagrams of the manganese tetraoxide catalysts with different exposed crystal faces prepared in Example 1, Example 2 and Example 3 are shown.

[0023] Figure 2 ac are TEM images of the manganese tetraoxide catalyst with exposed {103} crystal planes prepared in Example 1; Figure 2 df is the TEM image of the manganese tetraoxide catalyst with exposed {101} crystal plane prepared in Example 2; Figure 2 gi is the TEM image of the manganese tetraoxide catalyst with exposed {112} crystal plane prepared in Example 3.

[0024] Figure 3 The XRD comparison diagrams of the catalysts prepared in Example 1, Comparative Example 1 and Comparative Example 2 are shown.

[0025] Figure 4 The XRD comparison diagrams of the catalysts prepared in Example 1, Example 4, Comparative Example 3 and Comparative Example 4 are shown.

[0026] Figure 5 The XRD comparison diagrams of the catalysts prepared in Example 1, Example 5 and Comparative Example 5 are shown.

[0027] Figure 6 The XRD comparison diagrams of the catalysts prepared in Example 1, Example 6, Example 7 and Comparative Example 6 are shown.

[0028] Figure 7 This is a comparison chart showing the change in formaldehyde conversion rate of trimanganese tetraoxide catalysts with different exposed crystal faces prepared in Examples 1, 2 and 3 as a function of reaction temperature.

[0029] Table 1 shows the formaldehyde conversion rates of the trimanganese tetraoxide catalysts prepared in Examples 1 to 7 at 80°C. DETAILED DESCRIPTION

[0030] The present invention is further described in detail below with reference to the accompanying drawings and embodiments, but the present invention is not limited to these embodiments. Any simple modifications and equivalent changes made to the following embodiments based on the technical essence of the present invention without departing from the content of the present invention are within the scope of protection of the present invention.

[0031] Example 1

[0032] Highly Exposed Crystal Facets of Manganese Tetroxide Catalyst Mn3O4-103

[0033] The preparation method is as follows:

[0034] (1) Dissolve 20 mmol KMnO4 and 10 mmol (NH4)2SO4 in 70 mL deionized water to obtain a uniform mixed solution;

[0035] (2) The solution obtained in step 1 was transferred to a 100 mL polytetrafluoroethylene-lined autoclave, and the mixture was hydrothermally treated at 180°C for 24 hours and then naturally cooled at room temperature to obtain a reaction product;

[0036] (3) The reaction product obtained in step 2 was filtered, rinsed with deionized water, and then placed in an oven and dried at 105°C to obtain {110}α-MnO2 single crystal nanowires;

[0037] (4) Grind and mix 0.4 g of manganese dioxide obtained in step 3 with 0.2 g of urea to obtain a mixed powder;

[0038] (5) The mixture obtained in step 4 was placed in a tube furnace and calcined at 400 °C for 3 h in a N2 atmosphere to obtain a manganese tetraoxide catalyst with highly exposed crystal faces {103}, which was labeled as Mn3O4-103.

[0039] Example 2

[0040] Highly Exposed Crystal Facets of Manganese Tetroxide Catalyst Mn3O4-101

[0041] The preparation method is as follows:

[0042] (1) Dissolve 1.25 g of KMnO4 and 0.525 g of MnSO4·H2O in 70 mL of deionized water to obtain a uniform mixed solution;

[0043] (2) The solution obtained in step 1 was transferred to a 100 mL polytetrafluoroethylene-lined autoclave, and the mixture was hydrothermally treated at 160°C for 12 hours and then naturally cooled at room temperature to obtain a reactant;

[0044] (3) The reaction product obtained in step 2 was filtered, rinsed with deionized water, and then placed in an oven and dried at 80°C to obtain a dry solid;

[0045] (4) The dried solid obtained in step 3 was placed in a muffle furnace and calcined at 300 °C for 2 h to obtain {100}α-MnO2 single crystal nanowires;

[0046] (5) Weigh 0.4 g of the manganese dioxide obtained in step 4 and grind and mix with 0.2 g of urea to obtain a mixed powder;

[0047] (6) The mixed powder obtained in step 5 was placed in a tube furnace and calcined at 400 °C for 3 h in a N2 atmosphere to obtain a manganese tetraoxide catalyst with highly exposed crystal faces {101}, which was labeled as Mn3O4-101.

[0048] Example 3

[0049] Highly Exposed Crystal Facets of Manganese Tetroxide Catalyst Mn3O4-112

[0050] The preparation method is as follows:

[0051] (1) Dissolve 20 mmol KMnO4 and 10 mmol (NH4)2C2O4·H2O in 70 mL deionized water to obtain a uniform mixed solution;

[0052] (2) The solution obtained in step 1 was transferred to a 100 mL polytetrafluoroethylene-lined autoclave, and the mixture was hydrothermally treated at 180°C for 24 hours and then naturally cooled at room temperature to obtain a reaction product;

[0053] (3) The reaction product obtained in step 2 was filtered, rinsed with deionized water, and then placed in an oven and dried at 105°C to obtain {310}α-MnO2 single crystal nanowires;

[0054] (4) Grind and mix 0.4 g of manganese dioxide obtained in step 3 with 0.2 g of urea to obtain a mixed powder;

[0055] (5) The mixture obtained in step 4 was placed in a tube furnace and calcined at 400 °C for 3 h in a N2 atmosphere to obtain a manganese tetraoxide catalyst with highly exposed crystal faces {112}, which was labeled as Mn3O4-112.

[0056] Example 4

[0057] The preparation method is the same as that of Example 1, except that the masses of manganese dioxide and urea in step (4) are 0.4 g and 0.8 g, respectively, to obtain a manganese tetraoxide catalyst with a highly exposed crystal face {103}.

[0058] Example 5

[0059] The preparation method is the same as that of Example 1, but the difference is that the calcination temperature in step (5) is set to 500°C to obtain a manganese tetraoxide catalyst with a high exposed crystal face {103}.

[0060] Example 6

[0061] The preparation method is the same as that in Example 1, but the difference is that the calcination time in step (5) is set to 1 h, and a manganese tetraoxide catalyst with a highly exposed crystal face {103} is obtained.

[0062] Example 7

[0063] The preparation method is the same as that in Example 1, but the difference is that the calcination time in step (5) is set to 5 h, and a manganese tetraoxide catalyst with a highly exposed crystal face {103} is obtained.

[0064] Comparative Example 1

[0065] The preparation method is the same as that of Example 1, but the difference is that the phase transition control agent urea in step (4) is replaced with sodium sulfide Na2S to obtain a mixed crystal of α-MnO2 and Mn3O4.

[0066] Comparative Example 2

[0067] The preparation method is the same as that of Example 1, but the difference is that the phase transition control agent urea in step (4) is replaced with thiourea CH4N2S to obtain a mixed crystal of α-MnO2 and Mn3O4.

[0068] Comparative Example 3

[0069] The preparation method is the same as that of Example 1, except that the masses of manganese dioxide and urea in step (4) are 0.4 g and 0.12 g, respectively, to obtain mixed crystals of α-MnO2 and Mn3O4.

[0070] Comparative Example 4

[0071] The preparation method is the same as that of Example 1, except that the masses of manganese dioxide and urea in step (4) are 0.4 g and 1.2 g, respectively, to obtain amorphous manganese oxide.

[0072] Comparative Example 5

[0073] The preparation method is the same as that of Example 1, but the difference is that the calcination temperature in step (5) is set to 350°C to obtain mixed crystals of α-MnO2 and Mn3O4.

[0074] Comparative Example 6

[0075] The preparation method is the same as that in Example 1, but the difference is that the calcination time in step (5) is changed to 0.5 h, and a mixed crystal of α-MnO2 and Mn3O4 is obtained.

[0076] Material characterization analysis

[0077] Figure 1 The XRD test diagram of the prepared manganese oxide catalyst with different exposed crystal faces is shown in Figure 2. Figure 1 It can be seen that the diffraction peaks of the three catalyst materials are consistent with the standard card of manganese tetraoxide (JCPDS PDF#24-0734). Figure 2It can be seen from the TEM images that the morphology of all samples is a nanowire structure; among them, the lattice spacing of the Mn3O4-103 catalyst is 0.28 nm, and its highly exposed crystal plane is {103}; the lattice spacing of the Mn3O4-101 catalyst is 0.49 nm, and its highly exposed crystal plane is {101}; the lattice spacing of the Mn3O4-112 catalyst is 0.30 nm, and its highly exposed crystal plane is {112}.

[0078] Figure 3 The XRD comparison diagrams of the catalysts prepared in Example 1, Comparative Example 1 and Comparative Example 2 are shown. Figure 3 It can be seen that when the phase change controller is urea, the catalyst materials obtained are all manganese tetraoxide; and when the phase change controller is reagents such as sodium sulfide and thiourea, the catalyst materials obtained are all mixed crystals of α-MnO2 and Mn3O4.

[0079] Figure 4 The XRD comparison diagrams of the catalysts prepared in Example 1, Example 4, Comparative Example 3 and Comparative Example 4 are shown. Figure 4 It can be seen that when the mass ratio of the phase change controller to manganese dioxide is 0.5:1 and 2:1, the obtained catalyst materials are all manganese tetraoxide; when the mass ratio of the phase change controller to manganese dioxide is 0.3:1, the obtained catalyst material is a mixed crystal of α-MnO2 and Mn3O4; when the mass ratio of the phase change controller to manganese dioxide is 3:1, the obtained catalyst material is manganese oxide with an amorphous structure.

[0080] Figure 5 The following is a comparison of the XRD patterns of the catalysts prepared in Example 1, Example 5 and Comparative Example 5. Figure 5 It can be seen that when the calcination temperature is 400℃ and 500℃ respectively, the catalyst materials obtained are all manganese manganese oxide; and when the calcination temperature is 350℃, the catalyst material obtained is a mixed crystal of α-MnO2 and Mn3O4.

[0081] Figure 6 The XRD comparison diagrams of the catalysts prepared in Example 1, Example 6, Example 7 and Comparative Example 6 are shown below. Figure 5 It can be seen that when the calcination time is 1 h, 4 h and 5 h respectively, the obtained catalyst materials are all manganese tetraoxide; when the calcination time is 0.5 h, the obtained catalyst material is a mixed crystal of α-MnO2 and Mn3O4.

[0082] Catalytic decomposition performance evaluation

[0083] The catalytic decomposition performance of manganese tetraoxide catalysts with different exposed crystal faces was evaluated, using formaldehyde as the target pollutant. The performance evaluation was conducted in a continuous flow fixed-bed reactor. The process was as follows: 100 mg of manganese tetraoxide catalyst (40-60 mesh) was placed in a quartz tube reactor. The gas flow rate through the reactor was 150 mL / min, the formaldehyde concentration was approximately 70 ppm, and the carrier gas was synthetic air with 21% O2 / 79% N2, corresponding to a space velocity (GHSV) of 90 L / g. cat h, the test temperature is 30℃-130℃, and the formaldehyde conversion rate is calculated based on the carbon dioxide produced, as shown below: HCHO conversion rate (%) = [CO2] out / [HCHO] in ×100%, of which [HCHO] in is the initial concentration of formaldehyde at the inlet, [CO2] out The concentration of HCHO and CO2 was measured by gas chromatography equipped with a methane reformer.

[0084] like Figure 7 As shown, the manganese oxide catalysts of the present invention with different exposed crystal faces exhibit different catalytic performance in decomposing formaldehyde. The reaction temperatures for complete catalytic decomposition of formaldehyde by Mn3O4-103, Mn3O4-101, and Mn3O4-112 are 80°C, 100°C, and 110°C, respectively. Among them, the Mn3O4-103 catalyst has the best performance, completely converting formaldehyde into carbon dioxide at 80°C.

[0085] As shown in Table 1, the formaldehyde conversion rates of the trimanganese tetraoxide catalysts prepared in Examples 1 to 7 at 80° C. It can be seen that the trimanganese tetraoxide catalysts with different exposed crystal faces prepared in Examples 1 to 7 all have excellent formaldehyde decomposition activity, with formaldehyde conversion efficiencies exceeding 70%.

[0086]

Claims

1. A method for regulating the crystal surface of manganese tetraoxide, characterized in that: The steps include: (1) Synthesis of α-MnO2 single crystals with highly exposed crystal faces; (2) mixing the manganese dioxide single crystal obtained in step (1) with urea as a phase transition controller in a certain proportion and grinding the mixture to obtain a mixed powder; (3) Calcine the mixed powder obtained in step (2) in an oxygen-free atmosphere.

2. The method according to claim 1, wherein In step (1), ammonium sulfate, manganese sulfate or ammonium oxalate is used as a precursor and a hydrothermal method is used to sequentially prepare α-MnO2 single crystals with {110}, {100} or {310} highly exposed crystal faces.

3. The method according to claim 1, wherein In step (1), the morphology of the α-MnO2 single crystal is nanowire.

4. The method according to claim 1, wherein In step (2), the mass ratio of the phase transition controller to manganese dioxide is 0.3 to 3.

5. The method according to claim 1, wherein In step (2), the mass ratio of the phase transition controller to manganese dioxide is 0.5-2.

6. The method according to claim 1, wherein In step (3), the oxygen-free atmosphere is one or more of nitrogen, argon, and helium.

7. The method according to claim 1, wherein In step (3), the calcination temperature is 400-500°C; and the calcination time is 1-5 hours.

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