新型反相微乳液制备纳米结构甲烷催化燃烧催化剂La0.95Ba0.05MnAl11O19-α的研究

提出一种由水, 异丙醇, 正丁醇组成的新型微乳液, 并以其为反应介质制备了甲烷高温燃烧La_0.95Ba_0.05MnAl₁₁O_19-a催化剂. 采用¹H NMR, FT-IR, 电导法及激光粒度散射法研究了新型微乳液中水的结构及相特性. 新型微乳液中水的体积分数小于一定值时, 电导率与水含量成非线性关系, k～φ曲线上存在一临界值(φ^P＝0.15). 水质子的化学位移随水含量的降低移向高场. 加入4% D₂O测定的O—D键的伸缩振动随水含量增加而向高波数方向移动. 异丙醇铝在新型微乳液中水解形成的Al(OH)₃ 胶体粒子的粒径范围为226～329 nm. 采用新型微乳液作为反应介质制备的Ba_0.05La_0.95MnAl₁₁O_19-α催化剂的粒径在30 nm, 明显小于纯水制备的样品(100 nm). BET 比表面积为65 m²/g, 比纯水制备样品高出约一倍. XRD结果显示, 1200 ℃焙烧10 h即可获得含单一β-Al₂O₃相的催化剂. Ba_0.05La_0.95MnAl₁₁O_19-α催化剂甲烷催化燃烧的T₁₀为420 ℃, 比纯水制备样品下降了90 ℃. 甲烷催化燃烧活性提高是由于含有较多Mnⁿ+纳米结构六铝酸盐的形成.

Novel Reverse Microemulsion and Its Application to Synthesis of Nanostructured La0.95Ba0.05MnAl11O19-α Catalyst for Methane Combustion

A novel type of reverse microemulsion system composed of water, i-propanol and n-butanol was developed as a reaction medium to prepare substituted-hexaaluminate La_0.95Ba_0.05MnAl₁₁O_19-α catalyst for methane combustion. The structure and properties of water in the novel reverse microemulsion system were investigated by means of ¹H NMR, FT-IR, electrical conductometry and laser light scattering apparatus. The electric conductivity of the novel reverse microemulsion system followed a nonlinear correlation when the volume fraction of water was low. There existed a critical point of φ^P (0.15) along the k～φ curve. ¹H NMR revealed that the chemical shift of the water proton signal moved to upfield with decreasing water content. FTIR showed that the ν_O-D band was shifted to higher frequency with increasing water concent. The size distribution of the Al(OH)₃ gel particles formed by hydrolysis of Al(i-OC₃H₇)₃ in the novel reverse microemulsion ranged from 226 to 329 nm with the average diameter of 267 nm. Substituted hexaaluminate La_0.95Ba_0.05MnAl₁₁O_19-α synthesized in the novel reverse microemulsion system possesses a surface area of 65 m²/g, comparable to 32 m²/g of similar samples prepared with pure water. XRD measurements revealed that β-Al₂O₃ was the only phase for this catalyst after calcination at 1200 ℃ for 10 h and its particle size measured by TEM is about 30 nm. This nanostructured hexaaluminate, when used as a catalyst for CH₄ combustion, allowed a light-off of a stream of 1% CH₄ in air at 420 ℃, as compared to 510 ℃ required for similar catalyst prepared with pure water. The excellent low-temperature catalytic activity of this catalyst arised from the formation of nanostrctured hexaaluminate that contained larger amount of manganese.