摘要
应用XPS技术,对添加了3wt％第二组分金属氧化物M_xO_y(ZnO,La_2O_3,SnO_2,V_2O_5与Re_2O_7)的MoO_3/γ-Al_2O_3催化剂(MoO_3的浓度为12wt％)的氢还原行为进行了考察,样品的氢还原在电子能谱仪的样品预处理室进行,然后不暴露于大气直接进入能谱分析腔进行XPS测定。同时对第二组分金属氧化物的金属氧化价态也作了相应的XPS测定。结果表明,第二组分的引入,可显著地影响MoO_3/γ-Al_2O_3上MoO_3之氢还原行为,而且不同第二组分与MoO_3及担体的相互作用也有明显差别。
MxOy-MoO3/γ-Al2O3 (MxOy : ZnO, La2O3, SnO2, V2O5 and Re2O7) catalysts were prepared by the conventional two-step impregnation method. All samples consist of 12(wt)% MoO3, 3(wt)% MxOyand 85(wt)% γ-Al2O3.
In comparison with the XRD spectra of γ-Al2O3 and MoO3/γ-Al2O3, in the spectra of all samples of MxOy-MoO3/γ-Al2O3, there are no inherent lines of molybdena and MxOy present (see Fig. 1). It may indicate that MoO3and MxOy are dispersed approximately as monolayers on these MxOy-MoO3/γ-Al2O3 samples.
Specimens for XPS measurement were prepared by pressing ground catalysts into small pellet, the diameter and thickness of which are 12. 5 mm and 0. 7 mm respectively. Before reduction, XPS measurement of untreated specimen was carried out. The reduction of specimens was performed at 250, 400 and 550℃ subsequently in a preperation chamber as required before transfer to the spectrometer chamber without exposure to air. According to the area ratio of the XPS peaks concerned, relative atomic ratios among the component elements on the sample surface were obtained (see Table 2). The data from Table 2 show that the oxide MxOyis well dispersed on the molybdena and Mo/AI ratio is decreased with the increase of reduction temperature. The results are consist with the results obtained previously[6].
The XPS spectra of Mo3d from those specimens were treated by computer curve fitting program designed by Yu et al.[11] to determine the distribution of molybdenum species with various oxidation
state. The atomic ratio (%) Mo(i) / sum from i=4 to 6 Mo(i) from various specimens treated in different conditions are shown in Fig 2. The results show that adding metal oxide as the second component substantially promoted the reduction of supported MoO3 except the adding of SnO2. For instance, in Re2O7-MoO3/γ-Al2O3, after hydrogen reduction at 250℃. Mo (Ⅳ ) and Mo(Ⅴ) species were formed and after hydrogen reduction at 400℃, Mo(Ⅵ) was entirely reduced to Mo(Ⅳ) and Mo(Ⅴ) and the relative atomic ratio of Mo(Ⅳ) was near 80% on the surface. The extent of influence exerted by the metal oxide added as the second component on the reduction of MoO3, may be arranged in the following order: Re2O7 > V2O5 > La2O3 > ZnO > SnO2.
The XPS spectra of the metal oxide added as the second component indicate that, while MoO3 was reduced the XPS spectra of the metal in the second component will also change significantly except Sn3d5/2 of SnO2(see Fig. 3). A shift in the spectra of Sn3d5/2 can be found after SnO2-MoO3/ Al2O3was reduced. This shift could be attributed to the Fermi level shift by reduction due to the semiconductor property of SnO2. The XPS spectra of other metals in the second components are apparently changed even for the untreated specimens. The change in the spectra are enhanced with the increase of the reduction temperature. In Fig. 4 and Fig. 5, the XPS spectra of Zn2p3/2 and La3d5/ 2 are shown respectively. Shoulder peaks are seen to be present on the low binding energy side and it increases with the increase of the temperature of reduction. The shoulder peak may be attributed to the formation of new species and it probably indicates the existence of interaction between the metal oxide in the second component and molybdena. According to the experimental results presented, it is found that the extent of influence exerted by the metal oxide in the second component on reduction behaviour of MoO3 depends on whether or not the metal oxide can be reduced and/or possesses multiple oxidation states rather than a single one.
出处
《分子催化》
EI
CAS
CSCD
1992年第3期183-192,共10页
Journal of Molecular Catalysis(China)
基金
国家自然科学基金资助课题
关键词
氧化钼
金属氧化物
还原
催化剂
Molybdenum oxide, Metal oxide, Reduction behavior, XPS.
