Active Sites for Heteroatom Removal

探讨了催化剂脱除杂原子(硫或氮)的活性基础。模型表面是通过硫化钨或钼的单晶表面来制备的。波纹(211)和阶梯(532)表面被用来产生类似于二硫化钼边缘中心的硫化表面，一些人声称这是商业加氢脱硫催化剂中的活性中心。用低能电子衍射、俄歇电子能谱和扫描隧道显微镜对硫化表面的结构进行了表征。后者提供了一种直接的手段来想象在硫化过程中在这些表面上发生的复杂的重组。利用程序升温反应、反射/吸收红外光谱和光电子能谱对有机硫和有机氮化合物在硫磺表面的吸附和反应进行了研究。取代的噻吩类和吡啶类化合物被用来确定化学反应性改变和空间位阻对与表面相互作用的影响。实验结果与半经验分子轨道计算的有机化合物的修饰金属簇合物相补充；扩展的休克尔方法和INDO方法都被使用。脱除燃料中的硫和氮是减少光化学烟雾和酸雨的首选方法。随着煤炭和石油的低硫、低氮储量耗尽，这个问题变得更加尖锐。这项工作旨在解释已知的去除杂原子的催化剂的工作原理，并为开发从燃料或燃料前体中脱除氮和/或硫的改进系统提供基础。

The basis of catalytic activity for heteroatom (sulfur or nitrogen) removal is explored. Model surfaces are prepared by sulfiding single-crystal surfaces of tungsten or molybdenum. Corrugated (211) and stepped (532) surfaces are employed to create sulfided surfaces similar to the edge sites of molybdenum disulfide, claimed by some to be the active sites in commercial hydrodesulfurization catalysts. The structures of the sulfided surfaces are characterized with low-energy electron diffraction, Auger electron spectroscopy, and scanning tunnelling microscopy. The latter provides a direct means to image the complex restructuring that occurs on these surfaces during sulfiding. Adsorption and reaction of organosulfur and organonitrogen compounds on these sulfur- modified surfaces are followed with temperature-programmed reaction, reflection/absorption infrared spectroscopy, and photoelectron spectroscopy. Substituted thiophenes and pyridines are used to determine the effects of altered chemical reactivity and steric hindrances on interaction with the surface. Experimental findings are complemented with semiempircial molecular-orbital calculations of the organic compounds with modified metal clusters; both extended Huckel and INDO methods are used. Removal of sulfur and nitrogen from fuels is the preferred method for reduction of both photochemical smog and acid rain. As low-sulfur and low-nitrogen reserves of coal and petroleum are exhausted this problem becomes more acute. This work is designed to explain how catalysts known to remove heteroatoms work and to provide a basis for development of improved systems for nitrogen and/or sulfur removal from fuels or fuel precursors.