Lin Pin Ann, Gomez-Ballesteros Jose L, Burgos Juan C, Balbuena Perla B, Natarajan Bharath, Sharma Renu
Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, MD 20899-6203, USA.
University of Maryland - IREAP, College Park, MD 20742, USA.
J Catal. 2017 May;349:149-155. doi: 10.1016/j.jcat.2017.03.009. Epub 2017 Apr 3.
Rational catalyst design requires an atomic scale mechanistic understanding of the chemical pathways involved in the catalytic process. A heterogeneous catalyst typically works by adsorbing reactants onto its surface, where the energies for specific bonds to dissociate and/or combine with other species (to form desired intermediate or final products) are lower. Here, using the catalytic growth of single-walled carbon nanotubes (SWCNTs) as a prototype reaction, we show that the chemical pathway may in-fact involve the entire catalyst particle, and can proceed via the fluctuations in the formation and decomposition of metastable phases in the particle interior. We record in situ and at atomic resolution, the dynamic phase transformations occurring in a Cobalt catalyst nanoparticle during SWCNT growth, using a state-of-the-art environmental transmission electron microscope (ETEM). The fluctuations in catalyst carbon content are quantified by the automated, atomic-scale structural analysis of the time-resolved ETEM images and correlated with the SWCNT growth rate. We find the fluctuations in the carbon concentration in the catalyst nanoparticle and the fluctuations in nanotube growth rates to be of complementary character. These findings are successfully explained by reactive molecular dynamics (RMD) simulations that track the spatial and temporal evolution of the distribution of carbon atoms within and on the surface of the catalyst particle. We anticipate that our approach combining real-time, atomic-resolution image analysis and molecular dynamics simulations will facilitate catalyst design, improving reaction efficiencies and selectivity towards the growth of desired structure.
合理的催化剂设计需要对催化过程中涉及的化学途径有原子尺度的机理理解。非均相催化剂通常通过将反应物吸附到其表面来起作用，在该表面上特定键解离和/或与其他物种结合（以形成所需的中间体或最终产物）的能量较低。在这里，我们以单壁碳纳米管（SWCNT）的催化生长作为原型反应，表明化学途径实际上可能涉及整个催化剂颗粒，并且可以通过颗粒内部亚稳相形成和分解的波动来进行。我们使用最先进的环境透射电子显微镜（ETEM），在原子分辨率下原位记录了SWCNT生长过程中钴催化剂纳米颗粒中发生的动态相变。通过对时间分辨的ETEM图像进行自动原子尺度结构分析，对催化剂碳含量的波动进行了量化，并将其与SWCNT生长速率相关联。我们发现催化剂纳米颗粒中碳浓度的波动与纳米管生长速率的波动具有互补性。通过跟踪催化剂颗粒内部和表面碳原子分布的空间和时间演化的反应分子动力学（RMD）模拟，成功解释了这些发现。我们预计，我们结合实时原子分辨率图像分析和分子动力学模拟的方法将有助于催化剂设计，提高反应效率和对所需结构生长的选择性。
