O2和CO在Ni(111)表面的吸附活化

采用高分辨电子能量损失谱(HREELS)、俄歇电子能谱(AES)和低能电子衍射(LEED)研究镍单晶表面氧物种及CO与O₂的共吸附。实验结果表明，Ni(111)表面氧化后存在两种氧物种，位于54 meV能量损失峰的表面化学吸附氧物种和位于69 meV能量损失峰的表面氧化镍。首先，随着暴露氧量的增加，表面化学吸附氧物种的能量损失峰蓝移至58 meV；其次，通过真空退火及与CO相互作用考察，发现表面化学吸附氧物种较不稳定。在室温条件下，表面预吸附形成的表面化学吸附氧物种与CO共吸附，导致端位吸附CO增多，表明氧优先吸附在穴位上，随着CO暴露量的增加化学吸附氧物种与CO反应脱去；而表面氧化镍需在较高温度和较高CO分压下才能被CO还原。预吸附CO可被氧逐渐移去。

Adsorption and Activation of O2 and CO on the Ni(111) Surface

Ni-based catalysts have been widely used in many important industrial heterogeneous processes such as hydrogenation and steam reforming owing to their sufficiently high activity yet significantly lower cost than that of alternative precious-metal-based catalysts. However, nickel catalysts are susceptible to deactivation. Understanding the adsorption and activation behavior of small molecules on the model catalyst surface is important to optimize the catalytic performance. Although many studies have been carried out in recent years, the initial oxidation process of nickel surface is still not fully understood, and the influence of the adsorption sequence of CO and O₂ and their co-adsorption is controversial. In this study, the surface oxygen species on Ni(111) and the co-adsorption of CO and O₂ were explored using high-resolution electron energy loss spectroscopy (HREELS), Auger electron spectroscopy (AES), and low energy electron diffraction (LEED). HREELS can provide useful information about the surface structure, surface-adsorbed species, adsorption sites, and interactions between surface oxygen species and CO on the surface. The results showed that there were two kinds of oxygen species after the oxidation of Ni(111), and the energy loss peaks at 54–58 meV were ascribed to surface chemisorbed oxygen species, and the peak at 69 meV to surface nickel oxide. The chemisorbed oxygen at low coverage displayed a LEED pattern of (2×2), revealing the formation of an ordered surface structure. As the amount of oxygen increased, the energy loss peak at 54 meV shifted to 58 meV. At an O₂ partial pressure of 1×10^-8 Torr (1 Torr = 133.32 Pa), the AES ratio of O/Ni remained almost unchanged after dosing 48 L, which indicated that the surface nickel oxide was relatively stable. The surface chemisorbed oxygen species was less stable, which could change to surface nickel oxide after annealing in vacuum. CO adsorbed on Ni(111) at room temperature with tri-hollow and a-top sites. Upon annealing in vacuum, a-top CO weakened first and then disappeared completely at 520 K, whereas tri-hollow CO was much more stable. The pre-adsorption of CO could suppress O₂ adsorption and oxidation of the Ni(111) surface. The presence of oxygen could then gradually remove and replace CO with O₂. The surface oxygen species preferred the tri-hollow sites, resulting in more a-top adsorbed CO during the co-adsorption of CO and oxygen. The surface chemisorbed oxygen species were more active and could react with CO at room temperature; however, the surface nickel oxide was less active, and could only be reduced at a higher temperature and higher partial pressure of CO.