Rare earth elements possess 4f orbitals without full electron occupancy and lanthanide contraction.This characteristic results in their unique catalytic performance when they are used as active components or as catalyst supports.Research into and the development of rare earth catalytic materials will significantly promote the high-efficiency utilization of abundant rare earth elements,such as lanthanum and cerium.Currently,rare earth catalytic materials play an important role in such areas as the petroleum chemical industry,the catalytic combustion of fossil fuels,automotive emissions control,the purification of industrial waste air,and solid solution fuel cells.In this paper,we review the application of and recent research progress that has been made on rare earth catalytic materials,including relative theoretical research.The effects of rare earth elements on the structure,activity,and stability of the catalysts of interest are described.
Wangcheng ZhanYun GuoXueqing GongYanglong GuoYanqing WangGuanzhong Lu
Mesoporous CeO2-MnOx binary oxides with different Mn/Ce molar ratios were prepared by hydrothermal synthesis and characterized by scanning electron microscopy （SEM）, N2 sorption, X-ray diffraction （XRD）, X-ray photoelectron spectroscopy （XPS） and H2 temperature-programmed reduction （H2-TPR）. The characterization results indicated that the CeO2-MnOx catalysts exhibited flower-like microspheres with high specific surface areas, and partial Mn cations could be incorporated into CeO2 lattice to form solid solution. The CeO2-MnOx catalysts showed better catalytic activity for CO oxidation than that prepared by the coprecipitation method. Furthermore, the CeO2-MnOx catalyst with Mn/Ce molar ratio of 1 in the synthesis gel （Ce-Mn-1） exhibited the best catalytic activity, over which the conversion of CO could achieve 90% at 135 ℃. This was ascribed to presence of more Mn species with higher oxida- tion state on the surface and the better reducibility over the Ce-Mn-I catalyst than other CeO2-MnOx catalysts.
The effects of calcination temperature on the physicochemical properties of manganese oxide catalysts prepared by a precipitation method were assessed by X-ray diffraction,N2 adsorption-desorption,X-ray photoelectron spectroscopy,H2 temperature-programmed reduction,O2 temperature-programmed desorption,and thermogravimetry-differential analysis.The catalytic performance of each of these materials during the selective oxidation of cyclohexane with oxygen in a solvent-free system was subsequently examined.It was found that the MnOx-500 catalyst,calcined at 500 °C,consisted of a Mn2O3 phase in addition to Mn5O8 and Mn3O4 phases and possessed a low surface area.Unlike MnOx-500,the MnOx-400 catalyst prepared at 400 °C was composed solely of Mn3O4 and Mn5O8 and had a higher surface area.The pronounced catalytic activity of this latter material for the oxidation of cyclohexene was determined to result from numerous factors,including a higher concentration of surface adsorbed oxygen,greater quantities of the surface Mn4＋ ions that promote oxygen mobility and the extent of O2 adsorption and reducibility on the catalyst.The effects of various reaction conditions on the activity of the MnOx-400 during the oxidation of cyclohexane were also evaluated,such as the reaction temperature,reaction time,and initial oxygen pressure.Following a 4 h reaction at an initial O2 pressure of 0.5 MPa and 140 °C,an 8.0% cyclohexane conversion and 5.0% yield of cyclohexanol and cyclohexanone were achieved over the MnOx-400 catalyst.In contrast,employing MnOx-500 resulted in a 6.1% conversion of cyclohexane and 75% selectivity for cyclohexanol and cyclohexanone.After being recycled through 10 replicate uses,the catalytic activity of the MnOx-400 catalyst was unchanged,demonstrating its good stability.
Silicoaluminophosphate（SAPO） molecular sieves doped with cobalt（Co-SAPO-5） were synthesized hydrothermally with different concentrations of Co.Each sample was characterized by X-ray diffraction,N2 adsorption-desorption,scanning electron microscopy,ultraviolet-visible spectroscopy,temperature-programmed desorption of NH3（NH3-TPD）,and infrared spectrascopy of adsorbed pyridine（Py-IR）.The results showed that Co was highly dispersed in the Co-SAPO-5 samples.In addition,a part of the Co content had been incorporated into the SAPO-5 framework,while the remainder existed on the surface as extra-framework Co.The surface areas of the Co-SAOP-5 samples were similar to the SAPO-5 sample.However,the pore volumes of the Co-SAOP-5 samples were lower than that of the SAOP-5 sample.As the concentration of Co increased,the pore volume gradually decreased because extra-framework cobalt oxide was present on the catalyst surface.NH3-TPD and Py-IR results revealed that the amount of Br（？）nsted acid and the total amount of acid for the Co-SAPO-5 samples were higher than that for the SAPO-5 sample.These values were also higher for samples with higher Co content.The catalytic activity of the Co-SAPO-5 samples was evaluated for the oxidation of cyclohexane with molecular oxygen.When Co was added to the SAPO-5 catalyst,the catalytic activity of the Co-SAPO-5 catalysts improved.In addition,the conversion of cyclohexane increased as the Co content in the Co-SAPO-5 catalysts increased.However,with a high conversion of cyclohexane（6.30%）,the total selectivity of cyclohexanone（K） and cyclohexanol（A） decreased sharply.The K/A ratio ranged from 1.15 to 2.47.The effects of reaction conditions（i.e.,reaction temperature,reaction time,initial oxygen pressure,and the catalyst amount） on the performance of the Co-SAPO-5 catalysts have also been measured.Furthermore,the stability of the Co-SAPO-5 catalyst was explored and found to be good for the selective oxidation of cyclohexane by molecular oxygen.
Selective oxidation of saturated hydrocarbons with molecular oxygen has been of great interest in catalysis, and the development of highly efficient catalysts for this process is a crucial challenge. A new kind of heterogeneous catalyst, cobalt-doped carbon nitride polymer(g-C_3N_4),was harnessed for the selective oxidation of cyclohexane. X-ray diffraction, Fourier transform infrared spectra and high resolution transmission electron microscope revealed that Co species were highly dispersed in g-C_3N_4 matrix and the characteristic structure of polymeric g-C_3N_4 can be retained after Co-doping, although Co-doping caused the incomplete polymerization to some extent. Ultraviolet-visible, Raman and X-ray photoelectron spectroscopy further proved the successful Co doping in g-C_3N_4 matrix as the form of Co(Ⅱ)-N bonds. For the selective oxidation of cyclohexane, Co-doping can markedly promote the catalytic performance of g-C_3N_4 catalyst due to the synergistic effect of Co species and gC_3N_4 hybrid. Furthermore, the content of Co largely affected the activity of Co-doped g-C_3N_4 catalysts, among which the catalyst with 9.0 wt%Co content exhibited the highest yield(9.0%) of cyclohexanone and cyclohexanol, as well as a high stability. Meanwhile, the reaction mechanism over Co-doped g-C_3N_4 catalysts was elaborated.
Yu FuWangcheng ZhanYanglong GuoYun GuoYunsong WangGuanzhong Lu
