Rivera Rocabado David S, Noguchi Tomohiro G, Hayashi Shio, Maeda Nobutaka, Yamauchi Miho, Ishimoto Takayoshi
Graduate School of Nanobioscience, Yokohama City University, Seto 22-2, Kanazawa-ku, Yokohama 236-0027, Japan.
Graduate School of Advanced Science and Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi-Hiroshima 739-8527, Japan.
ACS Nano. 2021 Dec 28;15(12):20079-20086. doi: 10.1021/acsnano.1c07825. Epub 2021 Dec 3.
The adsorption states of N and H on MgO-supported Ru nanoparticles under conditions close to those of ammonia synthesis (AS; 1 atm, 250 °C) were uncovered by modulation-excitation infrared spectroscopy and density functional theory calculations using a nanoscale Ru particle model. The two most intense N adsorption peaks corresponded to the vertical chemisorption of N on the nanoparticle's top and bridge sites, while the remaining peaks were assigned to horizontally adsorbed N in view of the site heterogeneity of Ru nanoparticles. Long-term observations showed that vertically adsorbed N molecules gradually migrated from the top sites to the bridge sites. Compared to those adsorbed vertically, N molecules adsorbed horizontally exhibited a lower dipole moment, an increased N─N bond distance, and a decreased N─N bond order (i.e., were activated), which was ascribed to enhanced Ru-to-N charge transfer. H molecules were preferentially adsorbed horizontally on top sites and then rapidly dissociated to afford strongly surface-bound H atoms and thus block the active sites of Ru nanoparticles. Our results clarify the controversial adsorption/desorption behavior of N and H on AS catalysts and facilitate their further development.
利用调制激发红外光谱和基于纳米级钌颗粒模型的密度泛函理论计算，揭示了在接近氨合成（AS；1个大气压，250℃）的条件下，氮和氢在氧化镁负载的钌纳米颗粒上的吸附状态。两个最强的氮吸附峰对应于氮在纳米颗粒顶部和桥位的垂直化学吸附，而其余峰则鉴于钌纳米颗粒的位点异质性归因于水平吸附的氮。长期观察表明，垂直吸附的氮分子逐渐从顶部位点迁移到桥位。与垂直吸附的相比，水平吸附的氮分子表现出较低的偶极矩、增加的N─N键距和降低的N─N键级（即被活化），这归因于增强的钌到氮的电荷转移。氢分子优先水平吸附在顶部位点，然后迅速解离，产生强烈表面结合的氢原子，从而阻塞钌纳米颗粒的活性位点。我们的结果阐明了氮和氢在氨合成催化剂上有争议的吸附/解吸行为，并促进了它们的进一步发展。
