AP-XPS Study of the Reaction of O2 and CO2 with Zn–Au(111) Surface Alloys: Activation of O–O/C–O Bonds and the Formation of ZnO

Synchrotron-based ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) was used to study the dissociation of O2 and CO2 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 ZnOx species. Dissociative adsorption of O2 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: ZnOx and chemisorbed O atoms (Ochem) on Au or the Au–ZnO interface. Annealing from room temperature to 600 K induced an Ochem → ZnOx transformation that reflected the poor stability of the O atoms on Au(111). The Zn–Au(111) systems exhibited a reactivity toward CO2 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, CO2 underwent partial dissociation, depositing large amounts of Ochem on the surface with a minor formation of ZnOx. In addition, the deposited Ochem reacted with CO2 to form surface carbonate groups. Dosing of CO2 at 500–600 K mainly led to the formation of ZnOx, and the surface carbonate almost disappeared. In the presence of hydrogen, i.e., reaction feeds with a CO2 to H2 ratio of 1:3, the surface chemistry at 300 K was very similar to that seen for pure CO2 with the formation of ZnOx and carbonate groups. In contrast, at 500–600 K, the reaction with hydrogen induced the removal of ZnOx and CO3/HCOO species. Our AP-XPS results are consistent with the idea that CO2 hydrogenation on AuZn alloys involves a redox process where there is sequential oxidation by CO2 and reduction by H2 to yield methanol.