SPIN DOSIMETRY IN CATALYSIS RESEARCH

Various aspects of the problem of reliability of the standards for spi n dosimetry in solid samples containing transition metal ions are disc ussed. A method of preparing standards for d1 and d9 ions, based on va nadyl- and copper sulfate, is described. Reference samples with variou s spin concentration were prepared by uniform distribution of the para magnetic substance in a diamagnetic, chemically unreactive matrix. The testing of the quality of standards was performed by statistical meth ods considering the following factors: reproducibility of the average chemical composition between preparations, macro- and microhomogeneity within preparations and precision of EPR measurements. The statistica l analysis proved good quality of the standards produced by the elabor ated method except for microinhomogeneity. Several examples are given to illustrate application of spin dosimetry in catalysis research. E.g ., two different centers of reduced vanadium in vanadia-molybdena cata lysts, V(IV) stabilized by oxygen vacancies and V(IV) stabilized by Mo (VI) ions, respectively, were identified. The kinetic model of redox p rocesses occurring in V2O5-MoO3 catalysts upon interaction with oxygen and propylene was proposed. Quantitative determination of isolated Co (II) ions in CoO-MgO solid solutions revealed a strong correlation bet ween the number of these ions and the amount of adsorbed O2- species.