Rh/TiO_2体系中金属-担体的相互作用

THE METAL-SUPPORT INTERACTION IN Rh/TiO_2 SYSTEM

本文用氢化学吸附、H_2-TPD、正戊烷氢解反应、原位FT-IR等方法研究了活化温度对Rh/TiO_2体系催化性能的影响,探讨了不同体系中金属-担体的相互作用.实验结果表明,Rh/TiO_2体系经高温氢还原后,金属-担体间强相互作用的产生与TiO_2的结构类型和纯度有关;氢从TiO_2表面扩散到晶格,与质子和晶格氧离子结合形成的OH^-基团直接相关.OH^-基团对诱发金属-担体间的强相互作用有重要意义.

The effect of the activation temperature on the catalytic properties of Rh supported on TiO_2 of various crystal forms and purities was studied. Hydrogen chemisorption, npentane hydrogenolysis, H_2-TPD, and in situ FT-IR were used to probe the extend and the origin of metal-support interactions in the systems under study. When either a 98% anatase or chemical pure anatase type TiO_2(88% anatase, 12% rutile) was used as a support for Rh, a 7-or 6-fold decrease in H_2 chemisorption capacity and a 5 order of magnitude suppression in the overall n-pentane hydrogenolysis activity (expressed asturnover frequency at 573K) after a high temperature reduction (773K, HTR) relative to a low temperature reduction (573K, LTR)were observed. However, little effect of activation temperatures was observed on hydrogen chemisorption capacity or n-pentane hydrogenolysis activity when a 99.99% or an optical grade rutile was used as the support for Rh. The desorption peaks in H_2-TPD diagram shifted to higher temperatures after HTR relative to those after LTR. In situ IR results showed that one could observe a band of about 3750cm typical of OH group for both Rh supported anatase catalysts afterLTR and HTR, but no band was observed in the same region after the same treatment for supported rutile catalysts. It is suggested that the strong metal-support interaction (SMSI) effect for metal/TiO_2 system is dependent on the crystal forms and purity of support and the presence of surface OH group which is formed by the combination of proton with the lattice O^(2-) ion, and it showed that the presence of hydrogen in the surface of Rh/TiO_2 catalyst plays an importment role in the initiation of SMSI effect.