Trends of Pd₃Au(111) Alloy Surface Segregation in Oxygen, Carbon, and Nitrogen Environments

Catalytic properties of alloys are largely determined by the specific chemical composition at the surface. Differences in composition between surface and bulk regions depend intricately on both the parent metals and the surrounding environment. While a nonreactive environment favors surface segregation of the more noble alloy component, a reactive environment such as oxygen is expected to draw the more active component to the surface. Using ab initio thermodynamics, we explore here the structure and composition of the Pd3Au(111) alloy surface in oxygen, carbon, and nitrogen containing environments with reference to, e.g., gas phase O2, CH4, and N2 reservoirs, respectively. An extensive and systematic search of the available phase-space shows the segregation profile in an oxygen atmosphere following the anticipated picture described above, with O preferentially staying at the surface. In contrast, carbon at low coverages burrows deeper into the alloy substrate without a significant effect on the segregation profile. A nitrogen environment induces an intermediate behavior to oxygen and carbon, where the nitrogen atoms first favor either surface or subsurface sites depending on the detailed metallic composition profile. Our results overall demonstrate the complex response that has to be expected for an active alloy surface during catalysis while assessing the level of detail that is required to be accounted for in corresponding reaction models.