焙烧气氛对Ru/Al_2O_3催化剂上甲烷部分氧化制合成气反应性能的影响

Effect of Calcination Atmospheres on the Performance of Ru/Al_2O_3 Catalyst for Partial Oxidation of Methane to Syngas

采用气相色谱、质谱和原位时间分辨红外光谱等技术对空气和Ar气氛中焙烧的Ru/Al2O3催化剂样品上甲烷部分氧化(POM)制合成气反应进行了跟踪,并采用化学吸附、X射线衍射、拉曼光谱和H2-程序升温还原等技术对催化剂进行了表征.结果表明,在Ru/Al2O3-Air上POM反应出现振荡现象,而在Ru/Al2O3-Ar上则可较平稳地进行.经600°C还原后,Ru/Al2O3-Air上Ru的分散度仅为1%,而Ru/Al2O3-Ar上接近9%.这是导致两种样品上POM反应性能差异的主要因素.新鲜催化剂上存在两类Ru物种,分别是与载体相互作用较弱、较易还原的RuO2物种以及与载体相互作用较强、较难还原的Ru-O-Al物种.前者在POM反应过程中被还原为金属Ru0,后者则可随温度的升降发生周期性的还原和氧化,进而改变催化剂对CH4燃烧、重整或部分氧化等反应的相对活性,导致Ru/Al2O3-Air上POM反应尾气中各组分的浓度随时间而发生振荡.

The catalytic performance for partial oxidation of methane（POM） to syngas over the Ru/Al2O3 catalyst calcined in air（Ru/Al2O3-Air） and Ar（Ru/Al2O3-Ar） was studied.Steady-state reactivity was observed on the Ru/Al2O3-Ar sample,whereas oscillations in CH4,CO,H2,CO2,and H2O were observed on the Ru/Al2O3-Air sample.Comparative studies using CO and O2 chemisorption,X-ray diffraction,Raman,and temperature-programmed reduction（TPR） techniques were carried out in order to elucidate the property-structure relationship of the samples.The Ru dispersion on Ru/Al2O3-Air was only 1%,whereas that on Ru/Al2O3-Ar was about 9%.The significant difference in the Ru dispersion on the two samples is the major factor affecting the POM reaction.Two types of Ru species,RuO2 species weakly interacting with Al2O3 and the Ru-O-Al species strongly interacting with the support,were identified by H2-TPR experiments on the fresh catalyst.The former species could be easily reduced by H2（below 200 °C）,while the latter can only be reduced by H2 at above 700 °C.During the POM reaction at above 600 °C,the RuO2 species were in the metal state,whereas the Ru-O-Al species could undergo cyclic redox transformation.Such transformation is responsible for the oscillation on Ru/Al2O3-Air sample.