Morphology Dependent Reactivity of CsO x Nanostructures on Au(111): Binding and Hydrogenation of CO 2 to HCOOH.

Cesium oxide (CsO _x ) nanostructures grown on Au(111) behave as active centers for the CO ₂ binding and hydrogenation reactions. The morphology and reactivity of these CsO _x systems were investigated as a function of alkali coverage using scanning tunneling microscopy (STM), ambient pressure X-ray photoelectron spectroscopy (AP-XPS), and density functional theory (DFT) calculations. STM results show that initially (0.05-0.10 ML) cesium oxide clusters (Cs ₂ O ₂ ) grow at the elbow sites of the herringbone of Au(111), subsequently transforming into two-dimensional islands with increasing cesium coverage (>0.15 ML). XPS measurements reveal the presence of suboxidic (Cs _y O; y ≥ 2) species for the island structures. The higher coverages of cesium oxide nanostructures contain a lower O/Cs ratio, resulting in a stronger binding of CO ₂ . Moreover, the O atoms in the Cs _y O structure undergo a rearrangement upon the adsorption of CO ₂ which is a reversible phenomenon. Under CO ₂ hydrogenation conditions, the small Cs ₂ O ₂ clusters are hydroxylated, thereby preventing the adsorption of CO ₂ . However, the hydroxylation of the higher coverages of Cs _y O did not prevent CO ₂ adsorption, and adsorbed CO ₂ transformed to HCOO species that eventually yield HCOOH. DFT calculations further confirm that the dissociated H ₂ attacks the C in the adsorbate to produce formate, which is both thermodynamically and kinetically favored during the CO ₂ reaction with hydroxylated Cs _y O. These results demonstrate that cesium oxide by itself is an excellent catalyst for CO ₂ hydrogenation that could produce formate, an important intermediate for the generation of value-added species. The role of the alkali oxide nanostructures as active centers, not merely as promoters, may have broad implications, wherein the alkali oxides can be considered in the design of materials tuned for specific applications in heterogeneous catalysis.