【摘要】 用EXAFS方法研究了油脂加氢用Cu-Cr氧化物催化剂,该催化剂在加氢过程中铜由处于CuO-CuCr₂O₄中的氧化态向金属铜原子簇转化,铬由CuCr₂O₄状态向Cr₂O₃转化,验证并丰富了由EPR和XRD实验结果所提供的信息。更多还原

【Abstract】 It is well known that the Cu-Cr oxide catalysts are highly selective and active for the hydrogenation of edible oil. Here, a detailed EXAFS study on these catalysts is presented with the purpose to identify the local coordination and the valence states. The catalysts (oxidized sample ) were prepared by homogeneous co-precipitation of copper and chromium hydroxides at 100℃, followed by drying and calcination at 350 ℃ for 3 h, and then used in hydrogenation of edible oil at 230℃ ( reduced sample ) . The atomic ratio of Cu to Cr is 1: 1. EXAFS experiment was performed on the anode-rotating X-ray diffr-actometer ( Rigaku, 12 kW ) with Mo target and LiF ( 200 ) crystal as monochromator.Cu-K edge of the catalysts was studied with Cu and CuO as model compounds. It is found that the first coordination shell of Cu in oxidized catalyst is similar to Cu in CuO, the highest peak of Radical Structural Function ( RSF ) is due to Cu-O coordination, while coordination of Cu in the reduced sample is similar to that in Cu foil. The slight differences of RSF (shapes and peak positions) of the above samples from the CuO and Cu standards can be attributed to the more complex environment of Cu in catalysts. The EXAFS results suggest that Cu exist in the form of CuO as well as CuCr2O4 in oxidized state, whereas the CuO may be reduced to Cu and form small cluster bound to the oxide supports after catalytic reaction. The small coordination number of Cu-Cu in the reduced catalyst implies that the Cu clusters are very small and irregular ( Table 1). Cr-K edge was also studied with Cr2O3, CuCr2O4 and CrO3 as reference compounds. A remarkable shoulder peak present in Cr-K pre-edge of CrO3 is due to the tetrahedral coordination of Cr+6, the absence of this peak in our samples strongly indicates a different coordination structure and valence state of Cr. Fig. 2b further reveals the similar RSF of the oxidized sample to that of CuCr2O4, while RSF of the used catalyst is more like that of Cr2O3. It is therefore proposed that Cr in the oxidized state exists as CuCr2O4 (spinel type ) which decomposes and changes to Cu-Cr2O3 during reaction.The chemical shifts (Table 2 ) of Cr-K edge in both oxidized and reduced states are about 7.0 eV, being inbetween the values of Cr2O3 and CuCr2O4, and significantly different from that in CrO3. This suggests that the valence of Cr is near +3. The EPR of the catalysts after treatment in vacuum also shows that there is no high valence state of Cr.XRD experiment was done as well. Although the results of XRD are consistent with those of EXAFS, the diffraction lines are so weak and dispersed that it is hard to ascertain the components with confidence. Clearly, the more pertinent and reliable information offered by EXAFS in this work confirms once again the advantages of this technique in the study of imperfectly crystallized catalyst.更多还原