Department of Chemistry, Center for Catalysis and Surface Science, Northwestern University, Evanston, Illinois 60208, USA.
J Am Chem Soc. 2011 Nov 9;133(44):17816-23. doi: 10.1021/ja2067274. Epub 2011 Oct 17.
This work characterizes the adsorption, structure, and binding mechanism of oxygenated organic species from cyclohexane solution at the liquid/solid interface of optically flat alumina-supported palladium nanoparticle surfaces prepared by atomic layer deposition (ALD). The surface-specific nonlinear optical vibrational spectroscopy, sum-frequency generation (SFG), was used as a probe for adsorption and interfacial molecular structure. 1-Hexanoic acid is an overoxidation product and possible catalyst poison for the aerobic heterogeneous oxidation of 1-hexanol at the liquid/solid interface of Pd/Al(2)O(3) catalysts. Single component and competitive adsorption experiments show that 1-hexanoic acid adsorbs to both ALD-prepared alumina surfaces and alumina surfaces with palladium nanoparticles, that were also prepared by ALD, more strongly than does 1-hexanol. Furthermore, 1-hexanoic acid adsorbs with conformational order on ALD-prepared alumina surfaces, but on surfaces with palladium particles the adsorbates exhibit relative disorder at low surface coverage and become more ordered, on average, at higher surface coverage. Although significant differences in binding constant were not observed between surfaces with and without palladium nanoparticles, the palladium particles play an apparent role in controlling adsorbate structures. The disordered adsorption of 1-hexanoic acid most likely occurs on the alumina support, and probably results from modification of binding sites on the alumina, adjacent to the particles. In addition to providing insight on the possibility of catalyst poisoning by the overoxidation product and characterizing changes in its structure that result in only small adsorption energy changes, this work represents a step toward using surface science techniques that bridge the complexity gap between fundamental studies and realistic catalyst models.
这项工作描述了在原子层沉积 (ALD) 制备的光学平整氧化铝负载钯纳米粒子表面上，从环己烷溶液中吸附、结构和含氧有机物种的结合机制。表面特定的非线性光学振动光谱，和频产生 (SFG)，被用作吸附和界面分子结构的探针。1-己酸是 1-己醇在 Pd/Al(2)O(3)催化剂的液/固界面上有氧异相氧化的过度氧化产物和可能的催化剂毒物。单组分和竞争吸附实验表明，1-己酸比 1-己醇更强烈地吸附在 ALD 制备的氧化铝表面和也通过 ALD 制备的带有钯纳米粒子的氧化铝表面上。此外，1-己酸在 ALD 制备的氧化铝表面上以构象有序吸附，但在带有钯颗粒的表面上，在低表面覆盖度下吸附物表现出相对无序，并且在较高的表面覆盖度下变得更加有序。尽管在具有和不具有钯纳米粒子的表面之间没有观察到结合常数的显著差异，但钯纳米粒子在控制吸附剂结构方面起着明显的作用。1-己酸的无序吸附很可能发生在氧化铝载体上，并且可能是由于氧化铝上结合位点的修饰造成的，这些结合位点靠近颗粒。除了提供关于过度氧化产物对催化剂中毒的可能性的见解，并描述导致吸附能变化很小的结构变化外，这项工作代表了朝着使用表面科学技术迈出的一步，这些技术弥合了基础研究和实际催化剂模型之间的复杂性差距。
