Speaker
Description
CuNi nanoparticles have been successfully employed as catalysts in many chemical reactions. Depending on reaction conditions changes in their surface composition are observed, due to the adsorption of molecules. Here, we studied the Ni/Cu(100) single crystal surface as a model system for CO2 hydrogenation to explore the segregation trends under different reaction atmospheres. Exposure to an oxidizing atmosphere prior to hydrogenation results in the formation of NiO islands. LEEM, LEED, µNEXAFS, XPEEM and depth profile XPS results indicate that the NiO islands are encapsulated by a thin Cu shell (ca. 1.5-4 nm thick). Two reaction environments are explored, namely: CO2+H2 and CO2+CO+H2. In both cases the NiO islands progressively reduce to Ni. However, while in the first case Ni remains encapsulated, a thinning of the Cu shell is detected when CO was added. This suggests a clear effect on the stabilization energy when different intermediates are formed. DFT simulations found two families of intermediaries showing distinctive segregation trends: i) C-bound species that stabilize Ni on the surface and ii) O-bound species that stabilize Cu on the surface, thus identifying the nature of the metal-adsorbate bond as the driving force for segregation. A quite different behaviour is observed with the Ni/Cu(100) system, where no encapsulation is observed.
The dynamicity of the reduction process of the NiO islands could be corroborated by measurements with in situ conditions in a newly installed and state-of-the-art NAP XPEEM/LEEM instrument acquired in the framework of the CatLab project. The microscope, currently installed in the experimental hall of BESSY-II in Adlershof, offers a unique opportunity to observe surface phenomena in pressures up to 0.1 mbar and temperatures up to 800°C. A time dependence analysis of the reduction of NiO islands under hydrogenation conditions allows us to identify potential mechanisms for the phase redistribution.
