Isotopic and kinetic assessment of the mechanism of reactions of CH4 with CO2 or H2O to form synthesis gas and carbon on nickel catalysts
Isotopic and kinetic assessment of the mechanism of reactions of CH4 with CO2 or H2O to form synthesis gas and carbon on nickel catalysts
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DOI：
10.1016/j.jcat.2004.02.032
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发表时间：
2004-06-10
影响因子：
7.3
通讯作者：
Iglesia, E
中科院分区：
文献类型：
--
作者：
Wei, JM;Iglesia, E
催化剂的动力学和同位素测量以及严格排除输运和热力学工件的条件导致了CH4与CO2或H2O的反应及其在Ni/MgO催化剂上的化学计量分解的共同基本步骤序列。CH4/CO2和CH4/H2O混合物正反应的周转率与CH4压力（5-450 kPa）成正比，与CO2或H2O共反应物的分压（5-450 kPa）无关。这些周转率及其一级速率常数和活化能也与测量CH4分解的结果相似，表明这些反应在力学上是等效的，并且C-H键激活是所有三个反应中唯一与动力学相关的步骤。这些结论得到了重整和分解反应中相同的CH4/CD4动力学同位素效应(k（H)/k(D) = 1.62-1.71）和无法检测到的H2O/D2O同位素效应的证实。C-H键活化的动力学相关性与CH4/CD4/CO2混合物中化学转化和同位素混合的相对速率一致，也与从CH4/CO2/D-2和(CH4)-C-12/(CO2)-C-12/(CO)-C-13混合物反应中获得的助反应物活化和H-2和H2O脱附的准平衡性质的同位素证据一致。在CH4重整过程中，这些准平衡步骤导致平衡的水气转移反应，这一发现被流出物组成的测量所证实。这些基本步骤也为碳丝生长提供了一个预测模型，并确定了碳的热力学活性与基本步骤的各种动力学和热力学性质以及反应物和产物的普遍浓度的严格依赖，特别是由P-CH4 P-CO/P-CO2（或P-CH4 P-H2/P-H2O）比率给出。Ni表面的这些机理特征类似于先前为负载贵金属催化剂（Rh, Pt, Ir, Ru）建立的机理特征。在严格的动力学控制和相关的商业实践条件下，在广泛的成分和金属分散体范围内，这些C-H键激活周转率的直接测量使得Ni和贵金属催化剂在ch4重整反应中的反应活性首次得到直接和严格的比较。(C) 2004爱思唯尔公司版权所有。
Kinetic and isotopic measurements for catalysts and conditions that rigorously excluded transport and thermodynamic artifacts led to a common sequence of elementary steps for reactions of CH4 with CO2 or H2O and for its stoichiometric decomposition on Ni/MgO catalysts. Turnover rates for forward reactions of CH4/CO2 and CH4/H2O mixtures were proportional to CH4 pressure (5-450 kPa) and independent of the partial pressure of the CO2 Or H2O coreactants (5-450 kPa). These turnover rates and their first-order rate constants and activation energies are also similar to those measured for CH4 decomposition, indicating that these reactions are mechanistically equivalent and that C-H bond activation is the sole kinetically relevant step in all three reactions. These conclusions were confirmed by identical CH4/CD4 kinetic isotope effects (k(H)/k(D) = 1.62-1.71) for reforming and decomposition reactions and by undetectable H2O/D2O isotopic effects. The kinetic relevance of C-H bond activation is consistent with the relative rates of chemical conversion and isotopic mixing in a CH4/CD4/CO2 mixture and with the isotopic evidence for the quasi-equilibrated nature of coreactant activation and H-2 and H2O desorption obtained from reactions of CH4/CO2/D-2 and (CH4)-C-12/(CO2)-C-12/(CO)-C-13 mixtures. These quasi-equilibrated steps lead to equilibrated water-gas-shift reactions during CH4 reforming, a finding confirmed by measurements of the effluent composition. These elementary steps provide also a predictive model for carbon filament growth and identify a rigorous dependence of the carbon thermodynamic activity on various kinetic and thermodynamic properties of elementary steps and on the prevalent concentrations of reactants and products, specifically given by P-CH4 P-CO/P-CO2 (or P-CH4 P-H2/P-H2O) ratios. These mechanistic features on Ni surfaces resemble those previously established for supported noble metal catalysts (Rh, Pt, Ir, Ru). These direct measurements of C-H bond activation turnover rates allowed the first direct and rigorous comparison of the reactivity of Ni and noble metal catalysts for CH4-reforming reactions, under conditions of strict kinetic control and relevant commercial practice and over a wide range of compositions and metal dispersions. (C) 2004 Elsevier Inc. All rights reserved.