MoO_3在γ—Al_2O_3表面分散过程的动力学研究

KINETICS OF MoO_3 DISPERSION ON γ-A1_2O_3 SURFACE

采用原位X-光衍射仪对MoO_3在γ—Al_2O_3表面上的分散过程进行动力学研究。测定了不同温度下的MoO_3分散量随时间的变化。认为分散过程对于表面有效位置是一级反应,并得到了不同温度下的速率常数和分散过程的活化能。

The reaction kinetics of MoO3 dispersion on γ-A12O3 surface was investigated by in situ X-ray diffraction ( XRD ). The specific surface area of γ-A12O3 is 240 m2/g. γ-A12O3 and MoO3 were ground down into particles of size smaller than 250 mesh.Both kinds of particles, crystalline MoO3 (0.5000 g) and γ-A12O3 carrier (1.000g) were mixed thoroughly. Then the mixture was heated at certain temperature,while the residual amount of crystalline MoO3 was being measured in situ by the matrix - flushing method for quantitative X-ray diffraction analysis. From the experimental result, the process of MoO3 dispersion onto effective sites of γ-Al2O3 surface is a first -order reaction. The rate constants of MoO3 dispersion on γ-A12O3 at the various temperatures (400, 425, 450, 475 and 500 ℃ ) have been obtained. It is shown that the rate constants are not only as a function of size of particles ,homogeneity of mixing and dispersion temperature, but also as a function of ratios of MoO3/γ - A12O3. The apparent activation energy of MoO3 dispersion on γ-A12O3 surface is 62.5 kJ mol-1, and the preexponential factor A is 1.88×102 min-1. In this case, crystalline MoO3 dispersion on γ-Al2O3 surface is a chemical process, but this solid-solid dispersion is a difficult and slow one.