Spin dosimetry in catalysis research

Various aspects of the problem of reliability of the standards for spin dosimetry in solid samples containing transition metal ions are discussed. A method of preparing standards for d¹ and d⁹ ions, based on vanadyl- and copper sulfate, is described. Reference samples with various spin concentration were prepared by uniform distribution of the paramagnetic substance in a diamagnetic, chemically unreactive matrix. The testing of the quality of standards was performed by statistical methods considering the following factors: reproducibility of the average chemical composition between preparations, macro- and microhomogeneity within preparations and precision of EPR measurements. The statistical analysis proved good quality of the standards produced by the elaborated 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 catalysts, V(IV) stabilized by oxygen vacancies and V(IV) stabilized by Mo(VI) ions, respectively, were identified. The kinetic model of redox processes 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 between the number of these ions and the amount of adsorbed O ₂ ^? species.