X射线吸收谱研究碳纳米管内Rh-Mn纳米粒子结构的变化

Structural changes of Rh-Mn nanoparticles inside carbon nanotubes studied by X-ray absorption spectroscopy

利用X射线吸收谱技术研究了负载于多壁碳纳米管内的Rh-Mn纳米粒子在不同气氛和温度下的结构.结果表明,Rh-Mn粒子在空气中是由氧化铑团簇和混合锰氧化物组成.经过氢气在300°C下还原后,混合锰氧化物种转化成MnO.而氧化铑团簇在He气氛下当温度达到250°C时就会发生分解而形成金属铑团簇.对形成的铑团簇用H2或CO进行热处理,发现其分散性随温度升高而提高;同时,X射线吸收谱实验没有观察到Mn和Rh之间存在显著的相互作用,助剂Mn的主要作用是提高了Rh的分散性.

Supported Rh‐based catalysts such as Rh‐Mn nanoparticles（NPs） have potential use in the synthesis of ethanol from syngas. The structure of Rh‐Mn NPs in multi‐walled carbon nanotubes under different atmospheres and temperatures was studied by X‐ray absorption spectroscopy（XAS）. TEM images showed that the NPs dispersed in the carbon nanotubes had a uniform size of 2 nm. XAS data revealed that the Rh‐Mn NPs before reduction were composed of Rh2O3 clusters and mixed Mn oxide species. After reduction in a 10% H2‐90% He atmosphere, the mixed Mn oxides were conerted into nearly pure MnO. In contrast, the Rh2O3 clusters were easily decomposed to metallic Rh clusters even under a He atmosphere at 250 ℃. The Rh clusters remained in the metal state under the next reduction atmosphere, but their dispersion in the Rh‐Mn NPs increased with increasing temperature. No significant Mn‐Rh or Mn‐O‐Rh interaction in the reduced NPs was observed in the extended X‐ray absorption fine structure analysis. The results showed that there was no interaction between the MnO particles and Rh clusters and the role of the Mn promoter was mainly to improve Rh dispersion.