New and revisited insights into the promotion of methanol synthesis catalysts by CO2
New and revisited insights into the promotion of methanol synthesis catalysts by CO2
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DOI：
10.1039/c3cy00573a
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发表时间：
2013-11
影响因子：
5
通讯作者：
O. Martin;J. Pérez–Ramírez
中科院分区：
文献类型：
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作者：
O. Martin;J. Pérez–Ramírez
在513K，5.0 Mpa的甲醇合成条件下，考察了工业催化剂上的CO加氢、CO2加氢和水煤气变换(WGS)反应。为此，采用了一种新的方法：连续增加合成气进料中的二氧化碳浓度(R=CO2：(CO+CO2)=0-100)，从而形成火山型甲醇生成速率图，并在再次降低二氧化碳浓度时形成滞后环。H2O共进料实验表明，活性的提高可能与连接CO和CO2两条加氢途径的WGS活性有关。另一方面，过量的表面羟基似乎抑制了甲醇的产生，这解释了高二氧化碳浓度下活性下降的原因。通过X射线衍射、程序升温还原氢气、N2O脉冲化学吸附、X射线光电子能谱、俄歇电子能谱等手段对新旧催化材料进行了表征。结果表明，铜的比表面积对CO2加氢反应有一定的影响，但这一参数显然不是CO2催化合成甲醇的关键参数，这是氧化锌和铜协同作用的结果。Al_2O_3通过稳定表面进一步促进了这种结构特征。活性极大值的位置取决于表面比铜：锌。这种滞后行为是由于铜分散度的不断减小和铜物种以一价氧化态固定的结果，两者都不利于CO_2加氢。CO加氢反应受铜锌体积比的影响很大，从而影响催化剂的还原性能。这些事实可以通过使用浸渍的模型催化剂来证实。
CO hydrogenation, CO2 hydrogenation, and water–gas shift (WGS) reactions have been simultaneously investigated over industry-like catalysts based on Cu–ZnO–Al2O3, under methanol synthesis conditions (513 K, 5.0 MPa). For this, a novel methodology has been applied: the concentration of carbon dioxide in the syngas feed was consecutively increased (R = CO2:(CO + CO2) = 0–100) resulting in a volcano-type plot of the rate of methanol formation and forming a hysteresis loop when decreasing the CO2 concentration again. H2O co-feeding experiments revealed that the enhancement of activity can be correlated with the WGS activity linking both hydrogenation paths of CO and CO2. On the other hand, excessive amounts of surface hydroxyls seem to inhibit methanol production, explaining the drop in activity at high CO2 concentrations. An investigation of the catalytic performance was accompanied by an extensive characterisation of the fresh and used catalytic materials by X-ray diffraction, temperature-programmed reduction by H2, N2O pulse chemisorption, X-ray photoelectron spectroscopy, and Auger electron spectroscopy. It was shown that the copper surface area affects the CO2 hydrogenation; however, this parameter is unambiguously not the key descriptor for CO2-promoted methanol synthesis, which is a consequence of the synergistic interaction of zinc oxide and copper. This structural feature is further promoted by Al2O3 through stabilisation of the surface. The position of the activity maximum is determined by the surface ratio Cu : Zn. The hysteresis behaviour is a result of the continuous decrease of Cu dispersion and the fixation of copper species in its monovalent oxidation state, both detrimental for CO2 hydrogenation. CO hydrogenation is strongly affected by the Cu : Zn bulk ratio and thus the reducibility of the catalyst. These facts could be substantiated by the use of impregnated model catalysts.