MoVTeNb mixed oxide is a highly active and selective catalyst for oxidative dehydrogenation of ethane to produce ethylene, which exhibits the so-called M1 and M2 crystalline phases. Thermal stability of MoVTeNb catalytic system was assessed under reactions conditions, for this end the catalyst was exposed to several reaction temperatures spanned from 440 to 550°C and the pristine and spent materials were thoroughly analyzed by several characterization techniques. The catalyst's stability limit to operate at reactions temperatures <500°C, since, when reaction temperature is ≥500°C brings about the removal of tellurium from the intercalated framework channels of M1 crystalline phase. Rietveld refinement of XRD patterns and microscopies results point out that the tellurium loss cause the progressive partial destruction of M1 phase, thus decreasing the active sites amount and forming a MoO 2 crystalline phase, which is inactive in the employed reactions conditions. Raman spectroscopy confirms the MoO 2 phase development in function of reaction temperature. From HRTEM and EDS analyses it was noticed that tellurium depart occurs preferentially from the end sides of the needle-like M1 crystals, across the [001] plane. A detailed analysis of the solid deposited at the reactor outlet displays that it consists mainly in metallic tellurium; hence, the tellurium detaching is carried out by means of a reduction process of Te 4+ to Te 0 due to a combination of reaction temperature and feed composition. In order to keep the catalytic performance exhibited by MoVTeNb mixed oxide, hot spots along the reactor bed should be avoided or controlled for maintaining the catalytic bed temperature <500°C.

No abstract

No abstract

No abstract

Both Fe(C) and Fe(O) nanoparticles have been successfully synthesized using a modified graphite arc-discharge method. X-ray diffraction (XRD), high-resolution transmission electron microscopy (HREM), and electron diffraction (SAED) analyses indicated that both of these Fe nanoparticles have an average grain size of 15−20 nm and r-Fe, γ-Fe, and Fe 3 C phases are clearly identified in those Fe(C) particles, while an r-Fe and oxide layer (Fe 3 O 4 ) are revealed in these Fe(O) particles. Mo 1ssbauer spectra and hyperfine magnetic fields at room temperature (RT) further confirm their distinct nanophases. At RT, the assemblies of Fe(O) nanoparticles exhibit ferromagnetic properties due to magnetocrystalline anisotropy effects. However, modified superparamagnetic relaxation is observed in the assemblies of Fe(C) nanoparticles.