The active chemical state of zinc (Zn) in a zinc-copper (Zn-Cu) catalyst during carbon dioxide/carbon monoxide (CO 2 /CO) hydrogenation has been debated to be Zn oxide (ZnO) nanoparticles, metallic Zn, or a Zn-Cu surface alloy. We used x-ray photoelectron spectroscopy at 180 to 500 millibar to probe the nature of Zn and reaction intermediates during CO 2 /CO hydrogenation over Zn/ZnO/Cu(211), where the temperature is sufficiently high for the reaction to rapidly turn over, thus creating an almost adsorbate-free surface. Tuning of the grazing incidence angle makes it possible to achieve either surface or bulk sensitivity. Hydrogenation of CO 2 gives preference to ZnO in the form of clusters or nanoparticles, whereas in pure CO a surface Zn-Cu alloy becomes more prominent. The results reveal a specific role of CO in the formation of the Zn-Cu surface alloy as an active phase that facilitates efficient CO 2 methanol synthesis.

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Zinc and zinc oxide films on Cu(111) have frequently been used as inverse model catalysts to study various chemical reactions. In the present work the growth and subsequent oxidation of zinc films are investigated by scanning tunneling microscopy (STM), low-energy ion scattering (LEIS), temperature-programmed desorption (TPD), and low-energy electron diffraction (LEED). Up to monolayer coverage zinc grows as a two-dimensional film with zinc atoms continuing the face-centered cubic lattice of the copper substrate. At higher coverages layer-by-layer growth is less strictly obeyed. Stacking faults are introduced as well, indicating a transition toward a hexagonal-close-packed structure of zinc. At 300 K intermixing is found to be slow but rises when the temperature is increased. Accordingly, the continuous downshift and broadening of the zinc desorption peak with increasing submonolayer coverage are attributed to different levels of intermixing formed during the TPD temperature ramp. Postoxidation of zinc films starts at steps of zinc islands and is most effective in the temperature range from 430 to 500 K, although at temperatures beyond ≈450 K partial desorption of zinc has to be taken into account, too. At low oxygen exposures zinc forms small ZnO clusters along Zn steps, while at higher exposures continuous “bands” of ZnO develop around metallic Zn islands. These ZnO bands decompose around 650 K into zinc atoms dissolving into copper bulk and oxygen atoms forming a surface oxide with copper. Finally, bulk dissolved zinc atoms desorb at temperatures of ≈850 K and above.

Metal halides are a class of layered materials with promising electronic and magnetic properties persisting down to the two-dimensional limit. While most recent studies focused on the trihalide components of this family, the rather unexplored metal dihalides are also van der Waals layered systems with distinctive magnetic properties. Here we show that the dihalide NiBr2 grows epitaxially on a Au(111) substrate and exhibits semiconducting and magnetic behavior starting from a single layer. Through a combination of a low-temperature scanning-tunneling microscopy, low-energy electron diffraction, X-ray photoelectron spectroscopy, and photoemission electron microscopy, we identify two competing layer structures of NiBr2 coexisting at the interface and a stoichiometrically pure layer-by-layer growth beyond. Interestingly, X-ray absorption spectroscopy measurements revealed a magnetically ordered state below 27 K with in-plane magnetic anisotropy and zero-remanence in the single layer of NiBr2/Au­(111), which we attribute to a noncollinear magnetic structure. The combination of such two-dimensional magnetic order with the semiconducting behavior down to the 2D limit offers the attractive perspective of using these films as ultrathin crystalline barriers in tunneling junctions and low-dimensional devices.