Ciuparu Dragos, Haider Peter, Fernandez-García Marcos, Chen Yuan, Lim Sangyun, Haller Gary L, Pfefferle Lisa
Department of Chemical Engineering, Yale University, P.O. Box 208286, New Haven, Connecticut, USA.
J Phys Chem B. 2005 Sep 1;109(34):16332-9. doi: 10.1021/jp052243p.
Chemometric tools were employed to analyze the in-situ dynamic X-ray absorption spectroscopy data to probe the state of Co-MCM-41 catalysts during reduction in pure hydrogen and under single-wall carbon nanotube synthesis reaction conditions. The use of the progressive correlation analysis established the sequence in which changes in the spectral features near the Co K edge occurred, and the evolving factor analysis provided evidence for the formation of an intermediate Co(1+) ionic species during reduction of the Co-MCM-41 catalyst in pure hydrogen up to 720 degrees C. This intermediate species preserves the tetrahedral environment in the silica framework and is resistant to complete reduction to the metal in H(2). While the Co(2+) species is resistant to reduction in pure CO, the intermediate Co(1+) species is more reactive in CO most likely forming cobalt carbonyl-like compounds with high mobility in the MCM-41. These mobile species are the precursors of the metallic clusters growing carbon nanotubes. Controlling the rates of each step of this two-stage reduction process is key to controlling the size of the metallic Co clusters formed in Co-MCM-41 catalysts.
采用化学计量学工具分析原位动态X射线吸收光谱数据，以探究Co-MCM-41催化剂在纯氢还原过程及单壁碳纳米管合成反应条件下的状态。通过渐进相关分析确定了Co K边附近光谱特征变化的顺序，而演化因子分析为Co-MCM-41催化剂在高达720℃的纯氢中还原时形成中间Co(1+)离子物种提供了证据。这种中间物种保留了二氧化硅骨架中的四面体环境，并且在H₂中难以完全还原为金属。虽然Co(2+)物种在纯CO中难以还原，但中间Co(1+)物种在CO中更具反应性，很可能在MCM-41中形成具有高迁移率的类羰基钴化合物。这些可移动物种是生长碳纳米管的金属簇的前体。控制这一两阶段还原过程各步骤的速率是控制Co-MCM-41催化剂中形成的金属Co簇尺寸的关键。
