Synergy of Homogeneous and Heterogeneous Chemistry Probed by In Situ Spatially Resolved Measurements of Temperature and Composition
Synergy of Homogeneous and Heterogeneous Chemistry Probed by In Situ Spatially Resolved Measurements of Temperature and Composition
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DOI：
10.1002/anie.201007346
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发表时间：
2011-01-01
影响因子：
16.6
通讯作者：
Forzatti, Pio
中科院分区：
文献类型：
--
作者：
Donazzi, Alessandro;Livio, Dario;Forzatti, Pio
多相和均相化学反应之间的相互作用是高温催化过程中的一个关键问题。特别是，对控制所需产品选择性的主要路线的评估对反应装置的设计和安全运行至关重要。这一评估尤其适用于短接触时间(SCT)反应堆中碳氢化合物的超快转化，这些反应堆在努力应对世界范围内对更有效地开发能源和材料资源的日益增长的需求方面发挥着关键作用。例如，超低排放燃气轮机的催化辅助燃烧，碳氢化合物催化部分氧化(CPO)为H2或CO/H2混合物(即合成气)，[1]以及低碳烷烃氧化脱氢(ODH)为烯烃。[2，3]作为一个共同特征，这些过程在自热和紧凑型反应器中运行，使用贵金属催化剂(钯、铑和铂)。碳氢化合物在贵金属上的SCT自热转化具有巨大的能量强度。强放热和吸热反应在催化剂表面以极高的速率进行。因此，在较小的反应堆体积内建立了温度(高达2008C mm？1)和浓度的急剧梯度。气温从250摄氏度到11008摄氏度通常在几毫米以内。就功率密度以及温度和浓度梯度的程度而言，这种严重程度通常是火焰和气相氧化过程的典型特征。然而，对于催化过程来说，这些条件代表了一个完全非常规的动力学体系。为了更好地掌握所涉及现象的强度和速度，我们需要将低碳烷烃的SCT转化视为火焰的催化当量。这种类比可以清楚地描绘出该过程的复杂性，并强调相关的科学问题：在这些非常高的温度下，催化过程在多大程度上会粘在催化剂表面上，因为在这种温度下，物种的吸附在热力学上是不利的？C±H键的气相活化(例如，自由基的形成和传播)能否与催化过程合作或竞争？在这方面，人们普遍认为，在常压下，在Rh上的CH4 CPO的气相路径可以忽略不计，而催化路径占主导地位。相反，铂上的短链烷烃的SCT-ODH主要在气相进行，从而导致烯烃和其他烃物种的产生。[7，8]在这些例子的基础上，我们可以得出结论，碳氢化合物的SCT转化是由催化或气相化学控制的。仅取决于气相中C？H键的稳定性。
The interaction between heterogeneous and homogeneous chemistries is a crucial issue for high-temperature catalytic processes. In particular, the assessment of the main routes that control the selectivity to the desired products is essential for the design and safe operation of reaction units. This assessment is particularly true for the ultrafast conversion of hydrocarbons in short-contact-time (SCT) reactors that play a pivotal role in the effort to cope with the worldwide growing demand for more efficient exploitation of energy and material resources. Examples are the catalytically assisted combustion for gas turbines with ultralow emissions, the catalytic partial oxidation (CPO) of hydrocarbons to H2 or CO/H2 mixtures (ie, syngas),[1] and the oxidative dehydrogenation (ODH) of light alkanes to olefins.[2, 3] As a common feature, these processes operate in autothermal and compact reactors, with noble-metal catalysts (palladium, rhodium, and platinum). An enormous energy intensity is peculiar to the SCT autothermal conversion of hydrocarbons over noble metals. Strongly exothermic and endothermic reactions proceed on the catalyst surface at extremely high rates. As a consequence, sharp gradients of temperature (up to 2008C mmÀ1) and concentration are established within the small reactor volumes. Temperatures ranging from 250 to 11008C are generally experienced within a few millimeters. Such a level of severity—in terms of power density and extent of temperature and concentration gradients—is typical of flames and gas-phase oxidation processes in general. For a catalytic process, however, these conditions represent a thoroughly unconventional kinetic regime. To best grasp the intensity and the speed of the involved phenomena, we need to think of SCT conversion of light alkanes as the catalytic equivalent of a flame. This analogy can clearly depict the complexity of the process and emphasize the related scientific issues: To what extent does a catalytic process “stick” to the catalyst surface at these very high temperatures, where the adsorption of species is thermodynamically unfavored? Can the gas-phase activation of CÀH bonds (eg, the formation and propagation of radicals) cooperate or compete with the catalytic process?In this respect, it is largely accepted that in the case of CH4 CPO over rhodium the gas-phase paths are negligible at atmospheric pressure and the catalytic route dominates.[4–6] Conversely, the SCT-ODH of short alkanes over platinum proceeds mainly in the gas phase, thus giving rise to the production of olefins and other hydrocarbon species.[7, 8] On the basis of these examples, we could conclude that either catalytic or gas-phase chemistry governs the SCT conversion of hydrocarbons, depending only on the stability of the CÀH bond in the gas phase.