Machida Masato, Tokudome Yurika, Maeda Akihide, Koide Tomoyo, Hirakawa Taiki, Sato Tetsuya, Tsushida Masayuki, Yoshida Hiroshi, Ohyama Junya, Fujii Kenji, Ishikawa Naoya
Division of Materials Science and Chemistry, Faculty of Advanced Science and Technology, Kumamoto University, 2-39-1 Kurokami, Chuo, Kumamoto 860-8555, Japan.
Elements Strategy Initiative for Catalysts & Batteries (ESICB), Kyoto University, 1-30 Goryo-Ohara, Nishikyo, Kyoto 615-8245, Japan.
ACS Omega. 2020 Dec 11;5(50):32814-32822. doi: 10.1021/acsomega.0c05443. eCollection 2020 Dec 22.
In the present study, we prepared a 12 nm thick Ir overlayer via pulsed cathodic arc plasma deposition on a 50 μm thick Fe-Cr-Al metal (SUS) foil. Using this thin-film catalyst made NH-O reactions more environmentally benign due to a much lower selectivity for undesirable NO (<5%) than that of a Pt overlayer (∼70%) at 225 °C. Despite its small surface area, Ir/SUS exhibited promising activity as an ammonia slip catalyst according to a turnover frequency (TOF) >70-fold greater than that observed with conventional Ir nanoparticle catalysts supported on γ-AlO. We found that the high-TOF NH oxidation was associated with the stability of the metallic Ir surface against oxidation by excess O present in simulated diesel exhaust. Additionally, we found that the Ir overlayer structure was thermally unstable at reaction temperatures ≥400 °C and at which point the Ir surface coverage dropped significantly; however, thermal deterioration was substantially mitigated by inserting a 250 nm thick Zr buffer layer between the Ir overlayer and the SUS foil substrate (Ir/Zr/SUS). Although NO formation was suppressed by NH oxidation over Ir/Zr/SUS, other undesired byproducts (i.e., NO and NO) were readily converted to N by coupling with a VO-WO/TiO catalyst in a second reactor for selective catalytic reduction by NH. These results demonstrated that this tandem reactor configuration converted NH to N with nearly complete selectivity at a range of 200-600 °C in the presence of excess O (8%) and HO (10%).
在本研究中，我们通过脉冲阴极电弧等离子体沉积在50μm厚的铁铬铝金属（SUS）箔上制备了一层12nm厚的铱覆盖层。使用这种薄膜催化剂，在225℃时，由于对不良副产物一氧化氮（<5%）的选择性远低于铂覆盖层（约70%），使得氨氧化反应对环境更加友好。尽管其表面积较小，但根据周转频率（TOF），铱/ SUS作为氨逃逸催化剂表现出了良好的活性，比负载在γ - AlO上的传统铱纳米颗粒催化剂观察到的周转频率高70倍以上。我们发现，高TOF的氨氧化与金属铱表面对模拟柴油废气中过量氧气氧化的稳定性有关。此外，我们发现铱覆盖层结构在反应温度≥400℃时热不稳定，此时铱的表面覆盖率显著下降；然而，通过在铱覆盖层和SUS箔基板（Ir/Zr/SUS）之间插入250nm厚的锆缓冲层，热劣化得到了显著缓解。尽管在Ir/Zr/SUS上通过氨氧化抑制了一氧化氮的形成，但在第二个反应器中，其他不需要的副产物（即一氧化氮和二氧化氮）通过与VO-WO/TiO催化剂耦合，在氨选择性催化还原过程中很容易转化为氮气。这些结果表明，在存在过量氧气（8%）和水蒸气（10%）的情况下，这种串联反应器配置在200 - 600℃的范围内以几乎完全的选择性将氨转化为氮气。
