AP-XPS Study of the Reaction of O<sub>2</sub> and CO<sub>2</sub> with Zn–Au(111) Surface Alloys: Activation of O–O/C–O Bonds and the Formation of ZnO
Vikram Mehar, Jeongjin Kim, Adrian Hunt, Iradwikanari Waluyo, José A. Rodríguez
Abstract
Synchrotron-based ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) was used to study the dissociation of O 2 and CO 2 on Zn–Au(111) alloys, which contained 0.2–0.3 monolayers of zinc. Although Au(111) is inert, the alloys displayed high activity for the cleavages of the O–O and C–O bonds at room temperature with the formation of ZnO x species. Dissociative adsorption of O 2 at 300 K destroyed the alloys, and the results of AP-XPS pointed to the coexistence of two types of oxygen species on the surface: ZnO x and chemisorbed O atoms (O chem ) on Au or the Au–ZnO interface. Annealing from room temperature to 600 K induced an O chem → ZnO x transformation that reflected the poor stability of the O atoms on Au(111). The Zn–Au(111) systems exhibited a reactivity toward CO 2 that was much larger than that seen for Au(111), Cu(111), or surfaces of late transition metals like Ni, Pd, or Pt. At 300 K, CO 2 underwent partial dissociation, depositing large amounts of O chem on the surface with a minor formation of ZnO x . In addition, the deposited O chem reacted with CO 2 to form surface carbonate groups. Dosing of CO 2 at 500–600 K mainly led to the formation of ZnO x, and the surface carbonate almost disappeared. In the presence of hydrogen, i.e., reaction feeds with a CO 2 to H 2 ratio of 1:3, the surface chemistry at 300 K was very similar to that seen for pure CO 2 with the formation of ZnO x and carbonate groups. In contrast, at 500–600 K, the reaction with hydrogen induced the removal of ZnO x and CO 3 /HCOO species. Our AP-XPS results are consistent with the idea that CO 2 hydrogenation on AuZn alloys involves a redox process where there is sequential oxidation by CO 2 and reduction by H 2 to yield methanol.