Spin–spin exchange in vanadium-containing catalysts studied by in situ-EPR: a sensitive monitor for disorder-related activity

Abstract

In unsupported V-containing catalysts, V4+ sites are frequently present in amorphous oxidic clusters and/or crystalline paramagnetic bulk phases in which they are coupled by effective spin–spin exchange interactions. This work presents two EPR procedures for evaluating these interactions and relating their strength to the degree of disorder and catalytic performance, namely (1) evaluating permanent structural disorder by calculating exchange energies and integrals and (2) monitoring transient electronic disorder caused by catalyst–reactant interaction in working catalysts using the ratio of the fourth and the square of the second moment of the EPR signal. Selected application examples comprise VPO catalysts such as (i) (VO)2P2O7, in which a certain degree of structural disorder revealed to be essential for high catalytic performance in the selective oxidation of n-butane to maleic anhydride, (ii) (NH4)2(VO)3(P2O7)2 in which transient perturbation of spin–spin exchange sensitively reflects the strength of reactant adsorption during ammoxidation of toluene and (iii) (NH4)2VOP2O7 in which an amorphous VO2+-containing phase could be identified as active component besides inactive crystalline phases. Based on operando-EPR measurements, reasons for activity differences in MoVTeNb oxide catalysts could be suggested.