Paper SS+NC-MoM10
From Near-surface to Surface CuPt Alloy: Cu Surface Segregation Induced by CO Adsorption

Monday, October 20, 2008, 11:20 am, Room 208

Bimetallic alloys offer a way of tuning electronic structure and hence also surface catalytic properties. For example, a CuPt near-surface alloy¹ has recently been suggested as a promising catalyst for the water-gas shift (WGS) reaction, CO + H₂O → H₂ + CO₂. Crucial to the catalytic performance of such alloys is the surface composition and structure under reaction conditions. Examples of segregation of one component to form a surface oxide in oxidizing environments are many. However, largely unexplored are the effects of molecular adsorbates. Using a combination of XPS, in-situ and ex-situ IR, LEED, ISS and TPD, we have studied reversible surface changes induced by elevated CO pressures and sample temperatures for surfaces with varying Cu concentrations (up to 3 ML) in the near-surface region of Pt(111). We show that at sample temperatures high enough to overcome Cu diffusion barriers (~470 K),² a CO pressure of 2 mbar is sufficient to induce segregation of Cu to the topmost surface layer, switching a CuPt near-surface alloy to a novel well-ordered CuPt surface alloy with very different properties. The thermodynamic driving force behind the surface changes is rationalized on the basis of the much greater bondstrength of CO to Pt surface atoms in the presence of Cu atoms in the topmost surface layer. This is observed experimentally as large, well-defined and high temperature CO desorption peaks (up to 580 K). The so-called “d-band model”³ explains our findings.

¹ J. Knudsen et al., J. Am. Chem. Soc. 129 (2007) 6485.
² N. Schumacher et al., Surf. Sci. 602 (2008) 702.
³ J. Greeley, J.K. Nørskov, M. Mavrikakis, Annu. Rev. Phys. Chem. 53 (2002) 319, and references therein.