In situ bulk structural study on solid-state dynamics and catalytic activity correlations of a H4[PNbMo11O40] partial oxidation catalyst

Abstract

Abstract We report on the preparation and characterization of a niobium containing Keggin type heteropolyoxomolybdate (HPOM), H4[PNbMo11O40] · 13 H2O, with Nb substituting Mo in the Keggin ion. H4[PNbMo11O40] · 13 H2O exhibits a long-range order structure similar to that of H3[PMo12O40] · 13 H2O. The structural evolution of H4[PNbMo11O40] · 13 2O was studied under reducing (propene) and partial oxidation reaction conditions (propene and oxygen) by in situ X-ray diffraction and X-ray absorption spectroscopy combined with mass spectrometry and corroborated by thermogravimetric analysis. Following the loss of crystal water in the temperature range from 373 K to 573 K, a partial decomposition and reduction of the molybdenum and the formation of a characteristic cubic HPOM is observed at 573 K, similar to the structural evolution of H3[PMo12O40] · 13 H2O and H4[PVMo11O40] · 13 H2O during treatment in propene. However, in contrast to H3[PMo12O40] · 13 H2O and H4[PVMo11O40] · 13 H2O, the cubic HPOM decomposes rapidly and the MoO3 formed is subsequently reduced to MoO2. Nb centers in the Keggin ion considerably destabilize the HPOM and accelerate the decomposition towards MoO3. A similar structural evolution is observed under propene and oxygen. The formation of the cubic HPOM with Mo centers on extra-Keggin framework positions coincides with the onset of catalytic activity. However, at about 673 K the active phase further decomposes to a mixture of MoO3 and a more stable cubic HPOM resulting in a pronounced decrease of the catalytic activity. Evidently, preparation and structure-activity relationships for Nb containing mixed metal oxide catalysts need to take into account the particular structure directing properties of Nb during thermal activation.