In-situ ETEM Observation of Competing Mechanisms for Filamentous Carbon Gasification
In-situ ETEM Observation of Competing Mechanisms for Filamentous Carbon Gasification
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丝状碳气化竞争机制的原位 ETEM 观察
DOI：
10.1093/micmic/ozad067.663
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发表时间：
2023
影响因子：
2.8
通讯作者：
Zhu, Yuanyuan
中科院分区：
文献类型：
--
作者：
Nielsen, Monia R;March, Seth;Sainju, Rajat;Zhu, Chunxiang;Gao, Pu-Xian;Suib, Steven L;Zhu, Yuanyuan
由碳沉积（也称为焦化）引起的金属催化剂失活是各种工业催化过程（例如将甲烷转化为合成气和氢气）中的一个重要问题。在典型的甲烷转化催化剂（例如负载型镍和负载型铁）上碳沉积的一个共同特征涉及丝状碳的形成。尽管控制碳沉积的方法不同，但催化活性损失通常是不可避免的，最终，需要通过气化去除碳来再生废（负载）金属催化剂。然而，目前的再生方法依赖于单步热碳气化，通常会因金属烧结和碳去除不完全而导致不可逆的失活。已经提出了各种机制来描述丝状碳气化。最常提到的是：1）碳本体扩散，即碳-催化剂界面处的碳溶解通过金属扩散以气化； 2）氧气/氢气溢出，气体物质在金属催化剂上吸附和解离，然后迁移到附近的碳上进行气化； 3）氧化还原机制，晶格氧氧化沉积在金属上的碳； 4) 在非氧化还原载体的情况下，氧化物颗粒向碳沉积迁移以气化。所有这些传统机制都依赖于碳与催化剂的相互作用。在这项工作中，我们在 750 oC、1 bar 1O2/4He 中首次进行了丝状碳的原位 ETEM 气化。如图 1 所示，废 Ni-W/CeO2 甲烷部分氧化 (POM) 催化剂的代表性 HRTEM 图像显示，一些丝状碳不含金属，一些丝状碳封装了金属催化剂。我们的 ETEM 实验是使用基于大气 MEMS 的气体池（DENSsolution，Climate G+ 系统）进行的，该气体池模拟了通常在大气压下进行的技术相关的工业催化剂再生。有趣的是，如图2所示，我们在气化过程中观察到固定和高度移动的催化剂颗粒，以及在有或没有碳催化剂相互作用的情况下气化的丝状碳。特别是，我们发现非催化丝状碳气化是可重复的。这直接与碳气化所需的通过金属-碳相互作用的传统机制相矛盾。最近对焦炭气化的原位光学显微镜研究得出了类似的结论，其中在没有碳碱接触的情况下实现了成功的气化[1]。这些新颖的原位真实空间观测表明，在通过热气化去除丝状碳的过程中可能有几种不同的机制在起作用，并且这些机制可能相互竞争并影响碳气化反应的速率。总之，原位 ETEM 是一种强大的技术，有助于直接评估不同碳气化条件下废催化剂再生的主要和/或再生机制的组合[2]。
Metal catalyst deactivation caused by carbon deposition, also known as coking, is a significant problem in various industrial catalytic processes, such as the conversion of methane to syngas and hydrogen. A common characteristic of carbon deposits over typical methane conversion catalysts such as supported Ni and supported Fe involves the formation of filamentous carbon. Despite different approaches to control carbon deposition, catalytic activity loss is often inevitable, and eventually, the removal of carbon via gasification is necessary to regenerate spent (supported) metal catalysts. However, the current regeneration approach relies on single-step thermal carbon gasification, often leading to irreversible deactivation due to metal sintering and incomplete carbon removal. Various mechanisms have been proposed to describe filamentous carbon gasification. The most frequently mentioned are: 1) carbon bulk diffusion, where carbon dissolution at the carbon-catalyst interface diffuses through the metal for gasification; 2) oxygen/hydrogen spillover, where gas species are adsorbed and dissociated on the metal catalyst, which can then migrate to the nearby carbon for gasification; 3) the redox mechanism, where lattice oxygen oxidizes the carbon deposited on metals; and 4) in the case of non-redox supports, the oxide particles migrate towards carbon deposition for gasification. All these conventional mechanisms rely on carbon-catalyst interaction.In this work, we performed the first in-situ ETEM gasification of filamentous carbon at 750 oC in 1 bar 1O2/4He. As shown in Figure 1, a representative HRTEM image of the spent Ni-W/CeO2 partial oxidation of methane (POM) catalyst shows that some filamentous carbon is metal-free, and some encapsulate the metal catalyst. Our ETEM experiments were conducted using an atmospheric MEMS-based gas cell (DENSsolution, Climate G+ system), which mimics the technically relevant industrial catalyst regeneration usually carried out under atmospheric pressure. Interestingly, as shown in Figure 2, we observed both immobile and highly mobile catalyst particles during the gasification and filamentous carbon that is gasified both with and without carboncatalyst interaction. In particular, we found that the non-catalyzed filamentous carbon gasification is reproducible. This directly contradicts the conventional mechanisms via metal-carbon interaction as required for carbon gasification. A similar conclusion was made in a recent in-situ optical microscopy study on char gasification, where successful gasification was achieved without carbon-alkali contact [1]. These novel in-situ real-space observations suggest that there are probably several different mechanisms at play during the process of removing filamentous carbon through thermal gasification and that these mechanisms may compete with each other and affect the rate of the carbon gasification reaction. In summary, in-situ ETEM is a powerful technique that facilitates direct assessment of the predominant and/or the combination of regeneration mechanism (s) for spent catalyst regeneration under different carbon gasification conditions [2].