Newton Mark A, Ferri Davide, Smolentsev Grigory, Marchionni Valentina, Nachtegaal Maarten
Department of Physics, University of Warwick , Gibbet Hill Road, Coventry CV4 7AL, United Kingdom.
Paul Scherrer Institut, CH-5232 Villigen, Switzerland.
J Am Chem Soc. 2016 Oct 26;138(42):13930-13940. doi: 10.1021/jacs.6b06819. Epub 2016 Oct 17.
The kinetics involved in a recently revealed ambient-temperature mechanism for the catalytic oxidation of carbon monoxide by oxygen over a 5 wt % Pt/AlO catalyst are evaluated within a periodic, plug flow, redox operation paradigm using combined mass spectrometry (MS), diffuse reflectance infrared spectroscopy (DRIFTS), and time-resolved Pt L-edge XAFS. The species that are the most active at room temperature are shown to be a high-wavenumber (ca. 1690 cm) carbonate that we associate directly with a room-temperature redox process occurring in a fraction of the Pt atoms present in the catalyst. Our results, however, do not exclude the participation of carbonate species native to the AlO support, though these species tend to store CO at ambient temperature and become significant participants in CO oxidation catalysis only at slightly higher temperatures (323-333 K). Pt carbonate formation (1690 cm) under CO and the reaction to yield CO is shown to be extremely rapid and subject to an average apparent activation energy (E), across the techniques applied, of 8.7 kJ mol, within the temperature range investigated (276-343 K). Reoxidation of Pt (XANES) and subsequent CO production mediated by Pt carbonates under O (MS/IR) displays a first-order dependence upon O partial pressure and a negative dependence upon the coverage of CO adsorbed on the Pt nanoparticles also present in this catalyst. This oxidative regeneration/CO production step is subject to an apparent activation energy (E) of 56.5 (±5) kJ mol, is kinetically limited by the desorption of molecular CO from Pt nanoparticles, and also is shown to be dependent upon the partial pressure of O present in the oxidizing half of the cycle that we associate with the direct interaction of O with molecular CO adsorbed on the nanoparticles that promotes their desorption. Finally, a minority reactive state producing CO in the oxidizing cycle that displays no dependence upon the CO coverage of the nanoparticles can be induced through simple thermal treatment of the catalyst. These results are discussed in terms of the number and types of species present within the reactive system and in terms of the wider possibilities for the development of effective low-temperature CO oxidation using Pt/AlO catalysts.
在周期性、活塞流、氧化还原操作范式下，使用质谱（MS）、漫反射红外光谱（DRIFTS）和时间分辨铂L边X射线吸收精细结构（XAFS）相结合的方法，评估了最近揭示的一种环境温度下氧气在5 wt% Pt/Al₂O₃催化剂上催化氧化一氧化碳的机制所涉及的动力学。结果表明，在室温下最具活性的物种是一种高波数（约1690 cm⁻¹）的碳酸盐，我们将其直接与催化剂中一部分铂原子发生的室温氧化还原过程联系起来。然而，我们的结果并不排除Al₂O₃载体原生碳酸盐物种的参与，尽管这些物种倾向于在环境温度下储存CO，并且仅在稍高温度（323 - 333 K）下才成为CO氧化催化的重要参与者。在CO存在下铂碳酸盐的形成（1690 cm⁻¹）以及生成CO的反应被证明极其迅速，在所研究的温度范围（276 - 343 K）内，应用的各种技术测得的平均表观活化能（Eₐ）为8.7 kJ mol⁻¹。在O₂存在下铂的再氧化（XANES）以及随后由铂碳酸盐介导的CO生成（MS/IR）对O₂分压呈现一级依赖性，对吸附在该催化剂中也存在的铂纳米颗粒上的CO覆盖度呈现负依赖性。这个氧化再生/CO生成步骤的表观活化能（Eₐ）为56.5（±5）kJ mol⁻¹，动力学上受分子CO从铂纳米颗粒上解吸的限制，并且还表明它取决于在循环氧化半程中存在的O₂分压，我们将其与O₂与吸附在纳米颗粒上的分子CO的直接相互作用联系起来，这种相互作用促进了它们的解吸。最后，通过对催化剂进行简单热处理，可以诱导出一种在氧化循环中产生CO的少数反应态，它对纳米颗粒上的CO覆盖度没有依赖性。我们根据反应体系中存在的物种数量和类型，以及使用Pt/Al₂O₃催化剂开发有效的低温CO氧化的更广泛可能性来讨论这些结果。
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