Electrical conductivity of a MoVTeNbO catalyst in propene oxidation measured in operando conditions

A complex ac electrical conductivity study was performed on a Te0.33MO3.33 (M = Mo + V + Nb) pure phase (M2-type phase) in operando conditions, using the differential step technique (DST). The aim is to obtain new information about the influence of the oxidation and moisture on the catalytic behaviour of this phase in propene oxidation to acrylic acid. Three successive series of experiments were performed on the same batch of sample: (i) cycles of heating (up to 300, 350 and 400 °C, respectively)–cooling (to room temperature) in inert and in the catalytic test mixture, without intermediate oxidation; (ii) the same, but intercalating heating–cooling oxidation cycles (up to 300, 350 and 400 °C, respectively) before the corresponding catalytic test runs; and (iii) catalytic testing runs performed by heating up to 400 °C in the presence of pre-absorbed or co-adsorbed humidity. Based on the electrical conductivity results, it appears that 350 °C is a critical temperature in terms of surface reduction/re-oxidation: below this temperature no major surface changes of the relative oxidation level occur on flushing in dry helium or in the catalytic test mixture, while above it, an important reduction occurs in the same atmospheres. Re-oxidation in oxygen is not efficient if performed below 350 °C, but if done above (up to 400 °C) the surface recovers almost the initial behaviour. These results suggest the reversibility of the oxidation state of the catalyst surface, which is important in the practical use of this material. The activity and selectivity of acrylic acid formation are discussed in relation with the relative levels of oxidation/reduction of the surface.