摘要
对P/V比在0.9-1.1间的七个VPO_x催化剂进行EPR谱和化学滴定测定。以VOSO_4为基准,求出单位EPR相对强度所代表的(VO)^(2+)(％)。和化学滴定数据结合,得到催化剂中V^(3+),V^(4+)和V^(5+)离子的相对含量。从EPR谱图特征,将催化剂分成两类:一类为空气中焙烧而成,新鲜催化剂的V^(4+)/V^(5+)在0.7以上,V^(3+)百分含量在9％以下。有宽而强,不对称的单—EPR峰轴讯号。反应后,△H_(pp)值增高,EPR强度下降。另一类为氮气中焙烧而成,新鲜催化剂的V^(4+)/V^(5+)在0.3以下,而V^(3+)在22％以上,存在类似于“双峰”,实质上是宽化的,低分辨的V^(4+)超精细结构(hfs).hfs随P/V比下降而减弱,至P/V=0.9完全消失。该类催化剂反应后△H_(pp)值下降,而EPR强度增高。除了顺酐生成速率最高的一个催化剂外,其它六个催化剂上的顺酐生成速率基本上和催化剂中V^(4+)/V^(5+)比呈线性关系。
EPR and chemical analysis were used to investigate the changes of oxidation numbers of Vanadium ions in seven VPO_x catalysts with P / V ratio of 0.9--1.1. VOSO_4 was applied as a standard of spin density. The percentage of (VO)^(2+) in VOSO_4 was determined by chemical analysis, and then the ratio of unit doubly integrated EPR signals of VOSO_4 and (VO)^(2+) (%) were obtained. The relative content of V^(3+), V^(4+), and V^(5+) ions in catalysts were also obtained by combination of EPR and chemical analysis. According to the characters of EPR signals, the catalysts were devided into two kinds. The V^(4+) / V^(5+) > 0.7, V^(3+) (%) < 9% were found for freshly calcined catalysts in air. They exhibited a broad but intense EPR signals. After reaction, the △H_(pp) increased, and EPR intensity decreased. But the other kind or freshly calcined catalysts in N_2 were quite different. The V^(4+) / V^(5+) < 0.3, V^(3+) (%) > 22% were found. A quasidouble peak EPR signal appeared. In fact, it was broadening, low resolution hyperfine structure due to the (VO)^(2+) (S= 1 / 2, I= 7 / 2) ions were apart from each other and dispersed both on the surface and in the bulk. The hfs was vanished, when P / V decreased to 0.9. In addition it should be noted that △H_(pp) decreased and EPR intensity increased after reac- tion. Besides, the linear relation was found between formation velocity of MA and ratio of V^(4+) / V^(5+) in catalysts apart from one best catalyst.
