Studies Of Catalytic Decomposition Of H₂O₂ By NaOH Or Cu/Ni-doped OMS-2 Materials

Manganese oxide octahedral molecular sieve?OMS?is a type of molecular sieve with a rectangular pore section.According to the length and width of its pore section,it can be divided into OMS-1?OMS-7,OMS-2?manganese octahedral with 2*2 pore structure?.Faceted molecular sieve),as one of them,is a molecular sieve material with high conductivity(10^-2?^-1cm^-1),large specific surface area(10² m² g^-1),and simple synthesis method.OMS-2 is an excellent molecular sieve catalyst,which exhibits excellent catalytic performance in catalytic reactions such as the oxidation of CO,the oxidation of styrene,and the decomposition of hydrogen peroxide.This article contains two works,one is the preparation and characterization of Cu/Ni-doped OMS-2 and the study of catalytic hydrogen peroxide decomposition.The second is the study of alkali-catalyzed decomposition of hydrogen peroxide to uniformly release oxygen.The specific content is discussed in the following four chapters.In Chapter 1,we summarized the application of hydrogen peroxide and its catalytic decomposition,and reviewed the current status of hydrogen peroxide decomposition catalysts,as well as the structural characteristics and preparation methods of OMS-2molecular sieve catalysts.In Chapter 2,we have prepared seven Cu or Ni-OMS-2 molecular sieve catalysts with high Mn³⁺/Mn⁴⁺ratio and small grain size.The catalyst was characterized by X-ray powder diffraction?XRD?,nitrogen adsorption/desorption specific surface measurement,inductively coupled plasma emission spectroscopy?ICP?,and surface photoelectron spectroscopy?XPS?.In Chapter 3,we have studied the influence of three variables of preparation temperature,doping metal type and doping concentration on the structure and performance of the catalyst.It is found that the OMS-2 series catalysts are all Cryptomelane-Q,syn-KMn₈O₁₆ crystals with 2*2 pore structure.They are composed of nanorods with a diameter of 9?30 nm and a length of 69?360 nm.The volume ratios of micropores,mesopores and macropores of the seven catalysts are all in the range of1:8.0?14.2:7.0?10.0,and more than 75%of the specific surface area is provided by micropores and mesopores less than 34.48 nm.Among the seven molecular sieves,the volume of pores with a radius of curvature around 0.49 nm and a range of 27-59 nm is relatively large,corresponding to the 2*2 pores of the molecular sieve and the plywood stacked between the crystal grains.The catalyst surface O/Mn ratio is in the range of3.1?4.9,which is much higher than the crystal stoichiometric ratio and the bulk O/Mn ratio?2.09?2.15?.The Mn³⁺/Mn⁴⁺ratio is in the range of 0.6 to 2.0,which is higher than most of the reported OMS-2 series catalysts.The Mn⁴⁺/O ratio is in the range of0.06?0.19,and the Mn⁴⁺/?Mn+O?ratio is in the range of 0.05?0.15.The preparation temperature,the type and concentration of doped metal will not change the crystal structure,but will affect the crystal grain size,specific surface and pore size distribution,surface Mn³⁺/Mn⁴⁺ratio,Mn⁴⁺/O ratio and Mn⁴⁺/?Mn+O?ratio.With the increase of the preparation temperature,the grain sizes of Cu-OMS-2 or Ni-OMS-2 molecular sieves decrease,and the volume and specific surface area of the pores with a radius of curvature of 3.4-10 nm in the molecular sieve do not change much?<10%?,the volume of pores with a radius of curvature of 27?59 nm?grain stacking pores?is reduced?20.6%?,the specific surface area is approximately reduced?19.0%?,and the pores with a radius of curvature in the range of 59.6?273 nm The volume and specific surface area fluctuate randomly?volatility range-9%?26%?.With the increase of doped Ni or Cu concentration,the pore volume of the molecular sieve with a radius of curvature of0.31?1.94 nm and 0.5?34.48 nm first increases and then decreases.Since the surface area is mainly provided by micropores and mesopores less than 37 nm,The specific surface area also increases first and then decreases.The increase of the preparation temperature will increase the Mn³⁺/Mn⁴⁺ratio of the surface layer of the molecular sieve.The doping of Ni will cause the Mn³⁺/Mn⁴⁺ratio of the surface layer of the molecular sieve to decrease,and as the Ni doping concentration increases,the Mn³⁺/Mn⁴⁺ratio decreases.For Cu-doped Cu-OMS-2 molecular sieves,this change is just the opposite.In Chapter 4,we evaluated the relationship between the structure of the catalyst and its catalytic performance.Experiments on the catalytic decomposition of hydrogen peroxide show that the catalytic activity of the seven catalysts is higher than that of noble metals and the reported OMS-2 materials.The ratio of Mn³⁺/Mn⁴⁺on the surface of the catalyst is the biggest factor influencing the catalytic decomposition activity of hydrogen peroxide.The undoped OMS-2 has the highest conversion rate of hydrogen peroxide decomposition at 25?,and the corresponding Mn³⁺/Mn⁴⁺ratio is 0.9163.Doping with Cu or Ni will cause the ratio of Mn³⁺/Mn⁴⁺on the surface of the catalyst to deviate from this optimal value,resulting in a decrease in the conversion rate of catalytic hydrogen peroxide decomposition,and the greater the doping concentration,the more the Mn³⁺/Mn⁴⁺ratio deviates,and the catalytic activity decreases.Also more.In addition,as the surface layer Mn⁴⁺/O and Mn⁴⁺/?Mn+O?ratios increase,the catalytic decomposition activity of hydrogen peroxide first increases and then decreases.The undoped OMS-2 has the highest decomposition rate of hydrogen peroxide at 25°C,and the corresponding Mn⁴⁺/O and Mn⁴⁺/?Mn+O?ratios are 0.14 and 0.11,respectively.The doping of Ni will cause the surface layer Mn⁴⁺/O and Mn⁴⁺/?Mn+O?ratio to increase,and as the doping concentration increases,the Mn⁴⁺/O and Mn⁴⁺/?Mn+O?ratio increases.The doping of low concentration Cu will increase the surface Mn⁴⁺/O ratio and Mn⁴⁺/?Mn+O?ratio,and the doping of high concentration Cu will cause the surface Mn⁴⁺/O ratio and Mn⁴⁺/?Mn+O?ratio to decrease.As the preparation temperature increases,the AOS of Mn⁴⁺/O,Mn³⁺/O,Mn⁴⁺/?Mn+O?,Mn³⁺/?Mn+O?,Mn⁴⁺/O,and surface Mn decreases,The ratio of Mn/O,Oads/?Mn+O?,Mn³⁺/Mn⁴⁺increases,and the catalyst activation energy increases.In Chapter 5,we established a mathematical model of the decomposition of hydrogen peroxide and performed numerical calculations to establish a process for the uniform release of oxygen by the alkali-catalyzed decomposition of hydrogen peroxide.First,we determined the mathematical model parameters through the literature method and the experimental method,and then took the temperature of the water bath as a variable,and the average rate and stability of oxygen release as indicators,and performed numerical calculations,and established three temperature control techniques for stable oxygen release:water bath The temperature is always 30?,the initial temperature of the water bath is 30?,after 25 minutes it is set to 40?,the initial temperature of the water bath is 30?,and after 28 minutes it is set to 40?.According to the three processes to control the temperature of the water bath,0.32 g Na OH catalyzes the decomposition of 7.0 m L of 27.5%hydrogen peroxide solution to release oxygen uniformly,and the average oxygen release rate is 10.6?11.0 m L/min.