Frennet Alfred, Visart de Bocarmé Thierry, Bastin Jean-Marie, Kruse Norbert
Chimie Physique des Matériaux (Catalyse-Tribologie), Université Libre de Bruxelles, Campus Plaine, CP 243, 1050 Bruxelles, Belgique.
J Phys Chem B. 2005 Feb 17;109(6):2350-9. doi: 10.1021/jp048834h.
The aim of this paper is to demonstrate the importance of providing time-resolved information in catalysis research. Two truly in situ methods will be presented and compared for their merits and drawbacks: chemical transient kinetics (CTK) and pulsed field desorption mass spectrometry (PFDMS). The presentation will be given by way of example choosing the syngas (CO/H2) reaction over cobalt-based catalysts as a catalytic process. Despite numerous efforts in the past, the mechanism of this reaction is still under debate. In CTK the reaction is studied on a metal-supported catalyst under flow conditions in a pressure range extending from atmospheric pressure down to 100 Pa. Sudden changes in the partial pressures of the reactants then allow following the relaxation to either steady-state conditions ("transients") or cleanoff ("back transients"). In PFDMS short field pulses of several volts per nanometer are applied to a model catalyst which resembles a single metal particle grain (a "tip"). These pulses intervene during the ongoing reaction under flow conditions at pressures ranging from 10(-1) to 10(-5) Pa and cause field desorption of adsorbed species. This method is particularly suited to detect reaction intermediates in a time-dependent manner since the repetition frequency of the pulses can be systematically varied. It is shown that both methods lead to complementary results. While CTK allows conclusions on the mechanism of CO hydrogenation by following the time-dependent formation of hydrocarbon species, PFDMS provides insight into the initial steps leading to adsorbed CxHy species. A quantitative assessment of the CTK data allows the demonstration that the catalyst under working conditions is in an oxidized rather than metallic state. The initial steps to oxidation are also traced by PFDMS. Most importantly, however, CTK results allow formulation of a reaction mechanism that is common for both hydrocarbon and oxygenate formation and is based on the occurrence of a formate-type species as the most abundant surface intermediate.
本文旨在论证在催化研究中提供时间分辨信息的重要性。将介绍并比较两种真正的原位方法的优缺点：化学瞬态动力学（CTK）和脉冲场解吸质谱（PFDMS）。将通过选择钴基催化剂上的合成气（CO/H₂）反应作为催化过程为例进行阐述。尽管过去进行了大量研究，但该反应的机理仍存在争议。在CTK中，反应是在金属负载催化剂上，在流动条件下，压力范围从大气压降至100 Pa的情况下进行研究的。反应物分压的突然变化使得可以追踪到向稳态条件（“瞬态”）或清除（“反向瞬态”）的弛豫过程。在PFDMS中，将每纳米几伏的短场脉冲施加到类似于单个金属颗粒晶粒（“尖端”）的模型催化剂上。这些脉冲在压力范围为10⁻¹至10⁻⁵ Pa的流动条件下正在进行的反应过程中起作用，并导致吸附物种的场解吸。由于脉冲的重复频率可以系统地变化，因此该方法特别适合以时间依赖的方式检测反应中间体。结果表明，这两种方法得出的结果相互补充。虽然CTK通过追踪烃类物种随时间的形成来推断CO加氢的机理，但PFDMS则深入了解了导致吸附的CₓHᵧ物种的初始步骤。对CTK数据的定量评估表明，工作条件下的催化剂处于氧化态而非金属态。PFDMS也追踪了氧化的初始步骤。然而，最重要的是，CTK结果允许制定一种对于烃类和含氧化合物形成都通用的反应机理，该机理基于甲酸酯型物种作为最丰富的表面中间体的出现。
