Adsorption and Activation of O2 and CO on the Ni(111) Surface
Yuan DUAN,Mingshu CHEN*(),Huilin WAN
State Key Laboratory of Physical Chemistry of Solid Surfaces, National Engineering Laboratory for Green Chemical Productions of Alcohols-Ethers-Esters, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, Fujian Province, P. R. China
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.
Received: 04 February 2018
Published: 07 March 2018
Fig 1 (a) HREELS spectra of Ni(111) after exposing to oxygen with various pressure at room temperature (RT); (b) HREELS spectra of Ni(111) after exposing to different amount of O2 with pO2 = 1×10-8 Torr at RT; (c) O/Ni AES ratio as a function of the oxygen pressure (Inset: LEED pattern of Ni(111) after exposing to 1×10-8 Torr O2 for 5 min at RT); (d) O/Ni AES ratio as a function of the oxygen exposing amount with pO2 = 1×10-8 Torr.
Fig 2 (a) HREELS spectra Ni(111) oxidized at room temperature then annealed at various temperatures in vacuum; (b) Auger ratio of O/Ni as a function of the annealing temperature (Inset: LEED pattern of the Ni(111) surface after exposing to 1×10-6 Torr O2 for 5 min at RT and following by annealing at 750 K in vacuum).
Fig 3 HREELS spectra for (a) CO absorption on Ni(111) at RT and (b) flashed to different temperatures. (b) a: surface saturated with CO.
Fig 4 (a), (b) HREELS spectra of O2 adsorbed on the Ni(111) surface with saturated CO; (c) AES ratio of O/Ni as a function of the O2 exposing amount.
Fig 5 HREELS spectra of (a) various amount of CO on Ni(111) surface after pre-adsorption of 1.2 L O2; (b) 6 L CO on the Ni(111) surface pre-adsorbed different amount of O2; (c) Plots of the ratio of the hollow CO/a-top CO and Auger ratio of C/O. (c): (a–e) are exposed to CO at room temperature with 0.6, 3, 15, 30 and 300 L, respectively; (f–g) are exposed to CO at 428, 520 K (pCO = 1×10-6 Torr, 10 min); (h–j) are exposed to CO at 520, 673, 793 K (pCO = 1×10-5 Torr, 10 min).
Fig 6 HREELS spectra of co-adsorption of CO and O2 on Ni(111). Inset: Plots of the ratio of the hollow CO/a-top CO and Auger ratio of C/O in this condition. (a) 6 L CO after 3 L O2; (b) +3 L O2; (c) +3 L O2; (d) +6 L O2; (e) +6 L CO.
Fig 7 HREELS spectra of the reaction of CO with surface nickel oxide in different experimental conditions. (a) 300 K 30 L O2; (b)–(e) exposed to CO at 300, 450, 550, 650 K (pCO = 1×10-7 Torr); (f) exposed to CO at 650 K (pCO = 1×10-6 Torr); (g) exposed to CO at 650 K (pCO = 1×10-5 Torr)
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