摘要
本文用穆斯堡尔港及表面微区元素分析,对几种不同组成We-Mo系样品进行了研究,结果表明:Fe-Mo样品中均有Fe_2(MoO_4)_3相生成.在表面上,铁、钽均不富集.当Fe/Mo>0.67时,物相中除有Fe_2(MoO_4)_3外,还有α-Fe_2O_3相.微分扫描量热法测定当Fe/Mo<0.67时随着MoO_3含量的增加,生成的Fe_2(MoO_4)_3由α-相转变为β-相的相变温度相应有所降低,表明铁用混合氧化物催化剂组分间存在有一定的相互作用.在Fe-Mo系催化剂中,当Fe(MoO_4)_3/MoO_3=1/4时,比表面较大,催化活性和生成苯甲醛的选择性也最高.此外,还提出在催化剂中可能存在的钼氧八面体对四面体的置换,以及在钼酸铁结构间隙中进入的Mo^(6+)所构成的表面氧化还原部位,有利于甲苯的选择氧化反应.
The Fe-Mo catalysts were prepared by the method similar to that reported by Forzatti [3] . The catalytic activity and selectivity were measured in a fixed-bed quartz reactor (i.d.-20mm)with air/toluene = 10 (mole ratio) at 450℃. Various techniques, such as XRD, surface micro-analysis ( SMA) , differential scanning calorimetry(DSC) , and MOssbauer spectroscopy(MBS), were used to study the physico-chemical properties of samples.The results are shown in Table 1.It can be seen that the catalytic selectivity for the conversion to benzaldehyde by MoO3is small, the sample Fe2O3 promotes the full oxidation of toluene. However, the sample S- 5 ( Fe/ Mo= 0. 29) has fairly good selectivity and yield of benzaldehyde.The measurement of SMA indicated within the limits of experimental error that Fe and Mo were not enriched on the surface of Fe-Mo catalysts.
Results of MBS measured at low velocity are shown in Fig. 1- (d) and 1- (e) . A double-line spectrum was obtained with a quadrupole splitting(Q.S.).It was reported in the literature[4,5] that the MBS of Fe2 (MoO4)3 at zero velocity appeared as a single peak and did not have a Q.S.This discrepancy is due to the fact that in their experiments the velocity of measurements was rather high, so no double-line spectrum could be obtained (see Fig. 1-(a) ) . It is well known that the crystal structure of ferric molybdate consists of (FeO6) octahedra sharing with (MoO4) tetrahedra in the corners.The (MoO4) tetrahedron is connected with an other ( FeO6) octahedron through the oxygen bridge. Thus a
Fe-O-Mo-O-Fe-chain structure is formed.Because the (FeO6) octahedron has only a
little distortion, the Q.S. value should be low and the measured value is 0.19 mm / s, and the I. S. with respect to SNP is 0.68mm / s, which is in accordance with the literature MBS parameter of Fe2(MoO4) 3. As regards to sample S-9, the iron content is higher than that needed for the formation of Fe2(MoO4) 3, and the inner magnetic field (Hi) indicates that the remaining iron exists as α-Fe2O3.Thus as shown in Fig. 1- (b), there is a single peak of Fe2 (MoO4)3 in the middle of the graph, and six peaks of α-Fe2O3 appear on each side of this sin-gle peak.Coupled with the results of XRD, it can be seen that while Fe / Mo = 0.67 the sample is Fe2(MoO4)3, while Fe/ Mo < 0.67 the smples are composed of two phases: MoO3 and Fe2 (MoO4)3, while Fe/ Mo> 0.67, besides Fe2(MoO4)3, α-Fe2O3 also exists, and neither solid solutions nor other compounds are formed in these samples.The DSC measurements showed that the phase transition temperature of molybdate from α-Fe2(MoO4)3 into β-Fe2(MoO4)3 in these samples is also decreased markably with the decreases of Fe / Mo atomic ratio of catalysts which clearly showed the existence of interaction between the components in these mixed iron-molybdenum oxide catalysts.
The MBS results reflect the microscopic structural character of substances and the chemical environment of the Mossbauer atom. In those samples, the I. S. and Q. S. have not changed, i.e.the environment of Fe and the movement of bonding electron within the ferric molybdate have not changed significantly in spite of the high variation in contents of MoO3.It has been reported in the literature[9] that the ferric molybdate was the active component in Fe-Mo catalyst for the oxidation reaction of methanol. It was also shown[10] that when MoO3 exists in surplus in the Fe/ Mo catalysts, it will promote the formation of stoichiometric ferric molybdate on the surface, and the catalyst has a high activity in the oxidation of methanol. However, the activity of selective oxidation of toluene by ferric molybdate (sample S-8) is rather low.From the above facts we can suggest a model for the surface structure of Fe-Mo catalyst as follows.A part of (MoO4)tetrahedra is replaced by the (MoO6) octahedra to form a chain structure, and some of the Mo6+enter into the structural interstices of ferric molybdate(see the figure on p 144).Thus,the catalyst contains Fe3+ and Mo6+ sites, which act as components for the Mars-Krevelen catalytic s
出处
《分子催化》
EI
CAS
CSCD
1989年第2期139-147,共9页
Journal of Molecular Catalysis(China)
基金
中国科学院科学基金
南京大学校内三项基金资助课题
