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, V 4+ 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) 2 P 2 O 7 , 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) (NH 4 ) 2 (VO) 3 (P 2 O 7 ) 2 in which transient perturbation of spin-spin exchange sensitively reflects the strength of reactant adsorption during ammoxidation of toluene and (iii) (NH 4 ) 2 VOP 2 O 7 in which an amorphous VO 2+ -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.

The coordinatively saturated vanadium MIL-47 as a low leaching heterogeneous catalyst in the oxidation of cyclohexene

“…Both spectra appear as broad lines and can be well-fitted by single Lorentzians. The position, which is practically the same in the two spectra, is compatible with a signal assignation to V +IV (g = 1.96) 32 . The absence of 51 V hyperfine structure is attributed to spin-spin interactions between the close-by V +IV ions in the MIL-47 structure.…”

Superior Activity of Isomorphously Substituted MOFs with MIL‐100(M=Al, Cr, Fe, In, Sc, V) Structure in the Prins Reaction: Impact of Metal Type

“…The superior catalytic performance of MIL‐100(V) has been attributed to the type of metal sites . Through the use of NO as a probe molecule and infrared spectroscopy, the presence of two bands attributed to the vibrations of V 3+ and V 4+ species were elucidated .…”

“…The authors proposed the vanadium sites to be mainly V 3+ cations, whereas V 4+ units could be situated within the vanadium trimers. This is in good agreement with the broad single signal at g =1.963 (Δ H =17.54 mT) observed in the EPR spectrum of MIL‐100(V), indicating the presence of vanadium with an oxidation state of 4+ (Figure ) . The presence of V 4+ and V 3+ plays a significant role in activating the reactant because the optimum proximity of dual active sites allows a favorable geometry of the reaction precursor, which may be the reason for the superior catalytic activity of MIL‐100(V).…”

Active Sites for Propane Oxidation: Some Generic Considerations

“…An adsorbate-type structure of the active site allowing dynamical response will be useful whereas a closely packed 3-D crystalline state will hinder such a process with high activation barriers. Such ''adsorbatelike'' active phases [77,78] can be identified with probe molecule reactions [79][80][81][82][83][84][85][86] and have been rationalized for covering some bulk catalysts [13,42,[87][88][89][90][91] as consequence of surface energy minimization.…”