Karadaghi Lanja R, To Anh T, Habas Susan E, Baddour Frederick G, Ruddy Daniel A, Brutchey Richard L
Department of Chemistry, University of Southern California, Los Angeles, California 90089, United States.
Catalytic Carbon Transformation and Scale-Up Center, National Renewable Energy Laboratory, Golden, Colorado 80401, United States.
Chem Mater. 2022 Oct 11;34(19):8849-8857. doi: 10.1021/acs.chemmater.2c02148. Epub 2022 Sep 22.
Transition-metal carbides are promising low-cost materials for various catalytic transformations due to their multifunctionality and noble-metal-like behavior. Nanostructuring transition-metal carbides offers advantages resulting from the large surface-area-to-volume ratios inherent in colloidal nanoparticle catalysts; however, a barrier for their utilization is removal of the long-chain aliphatic ligands on their surface to access active sites. Annealing procedures to remove these ligands require temperatures greater than the catalyst synthesis and catalytic reaction temperatures and may further result in coking or particle sintering that can reduce catalytic performance. One way to circumvent this problem is by replacing the long-chain aliphatic ligands with smaller ligands that can be easily removed through low-temperature thermolytic decomposition. Here, we present the exchange of native oleylamine ligands on colloidal α-MoC nanoparticles for thermally labile -butylamine ligands. Analyses of the ligand exchange reaction by solution H NMR spectroscopy, FT-IR spectroscopy, and thermogravimetric analysis-mass spectrometry (TGA-MS) confirm the displacement of 60% of the native oleylamine ligands for the thermally labile -butylamine, which can be removed with a mild activation step at 250 °C. Catalytic site densities were determined by carbon monoxide (CO) chemisorption, demonstrating that the mild thermal treatment at 250 °C activates ca. 25% of the total binding sites, while the native oleylamine-terminated MoC nanoparticles showed no available surface binding sites after this low-temperature treatment. The mild pretreatment at 250 °C also shows distinctly different initial activities and postinduction period selectivities in the CO hydrogenation reaction for the ligand exchanged MoC nanoparticle catalysts and the as-prepared material.
过渡金属碳化物因其多功能性和类似贵金属的性能，是用于各种催化转化的有前景的低成本材料。纳米结构化过渡金属碳化物具有胶体纳米颗粒催化剂固有的大表面积与体积比所带来的优势；然而，其应用的一个障碍是去除其表面的长链脂肪族配体以暴露活性位点。去除这些配体的退火程序需要高于催化剂合成和催化反应温度的温度，并且可能进一步导致积炭或颗粒烧结，从而降低催化性能。解决这个问题的一种方法是用较小的配体取代长链脂肪族配体，这些较小的配体可以通过低温热解分解轻松去除。在此，我们展示了在胶体α-MoC纳米颗粒上，将天然油胺配体交换为热不稳定的丁胺配体。通过溶液¹H NMR光谱、FT-IR光谱和热重分析-质谱（TGA-MS）对配体交换反应的分析证实，60%的天然油胺配体被热不稳定的丁胺取代，后者可以在250°C的温和活化步骤中去除。通过一氧化碳（CO）化学吸附测定催化位点密度，结果表明在250°C的温和热处理激活了约25%的总结合位点，而天然油胺封端的MoC纳米颗粒在这种低温处理后没有可用的表面结合位点。在250°C的温和预处理在配体交换的MoC纳米颗粒催化剂和制备的材料的CO加氢反应中也显示出明显不同的初始活性和诱导期后选择性。
