Structure and properties of molybdenum oxide nitrides as model systems for selective oxidation catalysts

Abstract

Molybdenum oxide nitride (denoted as Mo(O,N)3) was obtained by ammonolysis of α-MoO3 with gaseous ammonia. Electronic and geometric structure, reducibility, and conductivity of Mo(O,N)3 were investigated by XRD, XAS, UV-Vis spectroscopy, and impedance measurements. Catalytic performance in selective propene oxidation was determined by online mass spectrometry und gas chromatography. Upon incorporation of nitrogen, Mo(O,N)3 maintained the characteristic layer structure of α-MoO3. XRD analysis showed an increased structural disorder in the layers while nitrogen is removed from the lattice of Mo(O,N)3 at temperatures above ~600 K. Compared to regular α-MoO3, Mo(O,N)3 exhibited a higher electronic and ionic conductivity and an onset of reduction in propene at lower temperatures. Surprisingly, α-MoO3 and Mo(O,N)3 exhibited no detectable differences in onset temperatures of propene oxidation and catalytic selectivity or activity. Apparently, the increased reducibility, oxygen mobility, and conductivity of Mo(O,N)3 compared to α-MoO3 had no effect on the catalytic behavior of the two catalysts. The results presented confirm the suitability of molybdenum oxide nitrides as model systems for studying bulk contributions to selective oxidation.

Systematic study on the influence of the morphology of α-MoO3 in the selective oxidation of propylene

“…This resembles the procedure described by Kühn et al [49]. Afterwards the resulting material was calcined at 550 °C for 4 h like all the other samples.…”

Role of Vanadium and Phosphorus in Substituted Keggin‐Type Heteropolyoxo Molybdates Supported on Silica SBA‐15 in Selective Propene Oxidation

“…Thep reedge peak heights of in situ Va nd Mo K-edge XANES spectra at 298 K( Figure 4) were attributed to the distorted MO 6 (M = V, Mo) building units of the Keggin ion with the metal centers in their highest oxidation states. [5,21] The XANESs pectra remained unchanged in the temperature range 298-473K.H ence, the Keggin structure appeared to be stable up to 473 K. Between 473 and 648 K, the pre-edge peak heighti ncreased with temperature. This indicated structural changes from octahedral to tetrahedral [MO x ]( M = V, Mo) units during thermal treatment under catalytic conditions.…”

Effects of Anion Substitution in (Mo,V)5O14 on Catalytic Performance in Selective Propene Oxidation to Acrolein

“…Resulting oxide nitrides represent suitable model system for studyingc orrelations between oxygen mobility and catalytic performance. [13] The corresponding oxide nitrides of a-MoO 3 exhibited an enhanced electronic conductivity. [14] In addition, mild ammonolysis resulted in am uch improvedr educibility,w hich can be attributed to increased oxygen mobility.C orrelations between catalytic performance and electrical properties could not be elucidated because of the low thermals tability of the corresponding oxide nitrides.…”

“…Recent studies have shown that nitrogen can be incorporated in the anion lattice of α‐MoO 3 without changing the crystallographic structure. Resulting oxide nitrides represent suitable model system for studying correlations between oxygen mobility and catalytic performance . The corresponding oxide nitrides of α‐MoO 3 exhibited an enhanced electronic conductivity .…”

“…[14] In addition, mild ammonolysis resulted in am uch improvedr educibility,w hich can be attributed to increased oxygen mobility.C orrelations between catalytic performance and electrical properties could not be elucidated because of the low thermals tability of the corresponding oxide nitrides. [13] However,M oO 3 is not the most relevant system for studies on selectiveo xidationo fp ropene. Consequently,t he range of oxide nitridess hould be extended to more relevant structures in propene oxidation.…”