摘要
运用光电子谱就地分析Ni/C样品表面在9D-575K温度范围的加氢反应.Ni/C样品的制备是通过分子束技术将CO在Ni表面化学吸附解离出C与Ni结合形成的.Ni/C样品形成后通H_2分子束观察表面加氢反应生成的中间产物随温度的变化. 实验表明。加氢反应生成CH_4诸过程中.由CH生成CH_2是最慢步骤,而生成CH则是最快步骤.C_2H_x及C_nH_x(n>2)等长链碳氢化合物也都是以CH为基础生成的.C在加氢过程中不仅作为反应物之一,而且也是很好的助催剂.有C存在的Ni/C样品能在远高于纯Ni表面的脱附温度下对氢进行化学吸附与转移.
Formation of hydrocarbon out of .CO and H_2 is one of the most frequently studied reaction systems in catalysis. But in our experiments, the dissociative chemisorption of CO was modelled prior to the reaction by preparing a deposit of reactive carbon on and in the Ni(poly) foil, where the Ni/C specimen was exposed to a molecular beam of H_2. A photoelectron spectroscopic in-situ analysis of kinetic surface intermediates of the hydrogenation of carbon on Ni(poly) foil was given by spectra taken during the running reaction in the temperature range from 90 to 575 K. Thermal desorption spectra of H_2 and CH_4were also observed in that temperature range. These experiments were proceeded in an UHV apparatus with molecular beam sources and various electron spectroscopies.
It was shown by experiments that of hydrogenation processes in forming CH_4, the forma tion of CH_2 out of CH is the slowest reaction while the generation of CH is the fastest reaction. Hence, formation of C_2H_x, and longer hydrocarbon chains as well starts by dimerization of CH. The barrier between CH and CH_2 is understood by consideration of the orbital hybridization ofthe adsorbed C_1 groups. The energi(?)s are dipicted by the electron binding energies of C Ls electron: the most significant difference is the one between CH and CH_2. It is also shown that the essential role of C as a co-catalyst which is able to chemisorb and to transfer H at temperature well above the desorption temperature of H from clean Ni is established.
