Study On Photoelectric And Adsorption Properties Of Ga, In And Rare Earth Ions Doped Aluminate

Zinc aluminate with good barrier resistance to UV, high thermal stability, low surface activity and chemical resistance, and it is widely used in various fields such as UV reflective coatings, catalyst carrier and catalyst and so on. At present, automobile exhaust has becomes an important environmental pollutant with a rapidly incrasing car ownership. Nitrogen oxides (NOx) and soot particles can not be removed efficiently by three-way catalyst under oxygen-rich oxidation, lean blow burning and low temperature condition, whereas spinel-type transition-metal oxides are distinguished in selective catalytic reduction of NOx. Among them, zinc aluminate doped by Ga can keep high catalytic activity for a long time in the removal of NOx with coexistence of H2O and SO2. Because adsorption properties are closely related to catalytic performances of catalysts, it has great practice meanings to study on the adsorption properties of spinel-oxide catalyst.This paper reviews the studies of the spinel for catalytic removal of NOx, and it generalizes adsorption properties of small molecule adsorbed on Ga doped-zinc aluminate as well as its research background, while we introduce quantum theory. Electronic and optical properties of zinc aluminate doped by Ga, In and rare earth elements are studied using first-principles calculations. Her research is now focusing on adsorption properties of NOx H2O, SO2, O2, alkali and alkaline oxides adsorption on the surface of Ga-doped zinc aluminate catalyst and effect of O2 on CH4-SCR reduction of NOx. Our main results are as follows:1. The influence of structural, electronic and optical properties of ZnAl2O4 doped by Ga, In and rare earth element were studied.Regarding the ZnGaxAl2-xO4 related studies, we discussed its structural, electronic and optical properties. We found that the volume of the crystal unit cell increase, while Zn-O, Ga-O and Al-O bond lengths increase with increasing of dopant amount x. ZnAl2O4 has a direct band gap of 3.895eV, whlie ZnGaxAl2-xO4 has a indirect band gap due to replacement of Al by Ga, and its bandgap is lowered. Ga ions substituting Al ions in ZnAl2O4 changes electron transition from the top of the valence to the bottom of the conduction, and it causes a red shift of absorbance shoulder and a increase of refractive index in infinite wavelength. In addition, the peak of energy loss function becomes flat.Structural, electronic and optical properties of ZnInxAl2-xO4 were investigated. It is found that the volume of the crystal unit cell increase, while Zn-O, In-0 and Al-O bond lengths increase with increasing of dopant amount x. A narrowing of badgap and the absorbance spectra shifts toward red, and refractive index in infinite wavelength increases after In ions substituting Al ions in ZnAl2O4. Strength of peaks of energy loss function increase for ZnInxAl2-xO4 after its strength of peak decreases.The results of the RE ions substituting Al ions in ZnAl2O4 showed that ZnAl2O4 doped by rare earth element (La, Ce, Pr and Nd) causes the deformation of octahedron, namely, [REO6], which is responsible for crystal lattice distortion. Due to Ce, Pr and Nd ions substituting Al ions in ZnAl2O4, Fermi levels of ZnCexAl2-xO4, ZnPrxAl2-xO4 and ZnNdxAl2-xO4 shif toward conduction band. A sharp peak occurs at the Fermi level for ZnAxAl2-xO4 (Ce and Pr), resulting in a notable decrease of stability of ZnAl2O4 matrix.2. Adsorption stability was evaluated and effect of O2 on reaction of NO2 with CH4 was analyzed on the basis of calculating NOX, H2O, SO2, alkali metal hydroxide and alkali metal oxides adsorption on the surface of Ga-doped zinc aluminate.DOS of ZnGaAlO4(100) and ZnGaAlO4 (110) was investigated. Alkali metal hydroxide and alkali metal oxides on bandgap were evaluated. It is found that bandgap of ZnGaAlO4(100) surface is wider than that of ZnGaAlO4(110), meaning reactivity of the former is higher than that of the latter. The bandgap of ZnGaAlO4(100) decreases after LiOH and NaOH adsorption on the surface but KOH, and LiOH-and NaOH-modified ZnGaAlO4(100) becomes more reactive. Similarly, the bandgap of ZnGaAlO4(100) decreases after after Li2O, Na2O and K2O adsorption on the surface, Li2O-, Na2O- and K2O-modified ZnGaAlO4(100) becomes more reactive in following sequence:Li2O> Na2O> K2O.Adsorption performances of NO2、SO2 and H2O was studied, it is found that adsorption energy of NO2 is higher than other, and H2O and SO2 adsorption at Zn is main adsorption configurations, and they do not inhibit the adsorption of H2O and SO2 on Ga as the active component, i.e., the catalyst exhibits excellent water and sulfur resistance. This is consistent with the relevant experimental results.Adsorpting energy of configurations (NO N, O-down adsorption at Zn) and binding energy of NO2 adsorption at Zn were studied. The mechanisms involved NO N, O-down adsorption at Zn based on DOS was investigated. Results showed that configuration of NO N-down adsorption at Zn is more stable than than that of NO O-down adsorption at Zn, and the density of NO-π decreases aftter NO N-down adsorption at Zn. Adsorption energy of NO2 N-down adsorption at Zn is about two times thigher than that of NO. The process of desorption of NO2 is energetically feasible after dioxygen oxidizes NO to NO2, namely, a spontaneous process on thermodynamics. Activation energy of NO2 adsorption on Ga is lower than that of structure of NO2 adsorption on Al. In addition, a systematic study of NO2 adsorption at high coverages ranging from 1/4 monolayer (ML),1/2 ML,3/4 ML,1ML were carried out and the results shown that N02-metal bond length increases, and average adsorption of NO2 decreases with increasing coverage from 1/4 to 1 ML. Besides, average net charge decreases with increasing coverage from 1/4 to 1 ML.Through studying co-adsorption, dissociation adsorption and multi-component adsorption, we found that adsorption strength of NO2 is stronger than that of other small molecules (CO2, N2, H2O and CH4), while NO2 and O2 co-dsorption is strongly repulsive on the catalyst surface. Additionally, O2 slightly increses dissociation energy and causes the formation of little stronger O-N and H-C bonds. O2 decreases reactant-; metal bond, instead, O2 increases product-metal bond.O2-reactant and 02-product adsorption on the catalyst surface, and effect of O2 on energy level of the reactants and products, were explored. Results showed that the reactants and products are raised to higher energy level after O2 adsorption on the surface, but their activation state were not changed. Furthermore, NO2 act as Lewis acid accepting electronic charge from the surface. Instead, charge transfer is observed from other molecules (CO2, N2, H2O and CH4) toward the surface, which acts as a weak acid. Moreover, the enthalpy of reaction of NO2 with CH4 in the absence of O2 is larger than that in the absence of O2, while activation energy in the presence of O2 is smaller than that in the absence of O2. Finally, physical nature of a lowering of SCR reaction temperature caused by O2 has been revealed theoretically.