New insights into MnCeO_(x)/TiO_(2) composite oxide catalyst:Barium additive accelerated ammonia conversion

MnCeO_(x)/TiO_(2)has been widely used in selective catalytic reduction(SCR)of NO_(x)at low temperature.However,it is often poisoned in the presence of water vapor and sulfur dioxide.In this work,the promotion mechanism of Ba modification was investigated.Results show that the doped BaO reacts with CeO_(2)and forms BaCeO_(3).This unique perovskite structure of BaCeO_(3)significantly enhances NO oxidation and NH_(3)activation of MnCeO_(x)/TiO_(2)catalyst so that the NO conversion and the resistances to SO_(2)improve.It is found that Ba species obviously promotes the NO adsorption ability and improve the redox properties of MnCeO_(x)/TiO_(2)catalyst.While the acid properties of the catalyst are inhibited by Ba modification and among which Lewis acid sites are dominant for both MnCeO_(x)/TiO_(2)and MnCe(Ba)O_(x)/TiO_(2)catalysts.Furthermore,in situ DRIFT experiments reveal that the NO reduction upon MnCeO_(x)/TiO_(2)and MnCe(Ba)O_(x)/TiO_(2)catalysts follows both E-R and L-H mechanisms,in which L-H is preferred.Ba species enhances the formation of active nitrate species,which accelerates the NO reduction through L-H mechanism.It is interesting that although Ba species weakens the NH_(3)adsorption,it induces the ammonia conversion to coordination ammonia,which in turn accelerates the catalytic reaction.

Sol-gel approach to low-temperature synthesis of single-phase metastable La_(2)Ga_(3)O_(7.5) melilite with enhanced grain-boundary oxide ionic conductivity via a kinetically favorable mechanism

Starting with the stoichiometric and highly homogeneous gel-precursor,single-phase metastable melilite La_(2)Ga_(3)O_(7.5),as the end-member of solid solution La_(1+x)Sr_(1−x)Ga_(3)O_(7+x/2)(0≤x≤1),has been synthesized by solid-state reaction at 700℃ for 2 h via a kinetically favorable mechanism and characterized by X-ray diffraction(XRD),Raman,X-ray photoelectron spectroscopy(XPS),field emission scanning electron microscopy(FESEM),transmission electron microscopy(TEM),AC impedance spectroscopy,etc.It has been revealed that the as-synthesized melilite La_(2)Ga_(3)O_(7.5) shows an orthorhombic symmetry with crystal cell parameters a=11.4690(1)A,b=11.2825(4)A,and c=10.3735(4)A,while has more Raman active modes than LaSrGa_(3)O_(7) with a tetragonal structure,which was also synthesized under the same conditions for comparison,but tends to slowly decompose into perovskite LaGaO_(3) and Ga2O_(3) when annealed at 700℃ for over 20 h driven by its meta-stability.Moreover,the metastable La2Ga3O7.5 shows a higher XPS binding energy for the excess oxide ions in the crystal structure than those at normal lattice sites.Its intrinsic grain oxide ion conductivity can reach as high as 0.04 and 0.51 mS·cm^(-1) at 550 and 700℃,respectively,characterized by a simple Arrhenius relationship ln(σT)-1/T with invariable activation energy,E_(a)=1.22 eV,over the temperature range from 300 to 700℃,along with an apparent grain boundary conductivity that is about double that from the grains thanks to the clean grain boundaries.This paper provides a new strategic approach to the synthesis of complex oxides that may be of high performance but are difficultly achieved by the conventional ceramic method at high temperatures.