CO adsorption on Au (110) and Pd70Au30 (110) : an in situ comparative study by environmental STM

Abstract

Surface structure and catalytic properties of metals are often intimately related. Well known surface structures (or reconstructions) in UHV conditions may evolve under gas pressure to yield new configurations. Scanning tunneling microscopy (STM) yields information on surface morphology and structure down to the atomic level. It is thus a well suited technique to follow in situ surface modifications due to gas environments at relatively high pressures [1]. In PdAu alloys, Au has a strong tendency to segregate to the surface at the thermodynamic equilibrium under UHV conditions. It is indeed the case for the Pd 70 Au 30 (110) surface for which the outmost layer is essentially formed by Au (85–90% as determined independently by LEIS, variable kinetic energy XPS and grazing incidence surface XRD). We can thus consider to have a Au surface‐layer on top of a bimetallic bulk. In such conditions it is interesting to make a comparative study on how CO adsorption affects the surface structure and morphology both of this Au‐terminated bimetallic surface and of a Au(110) surface. We have thus used an environmental STM that can be operated from UHV (<10 −9 Torr) to 10 3 Torr of controlled environments (in this case CO); the study was donne at room temperature (~ 300K). This STM is an Omicron MicroLH slightly modified (gold plating of copper elements and magnets; coating of the piezoelectric tube) so that it is compatible with elevated pressure and variable temperature operation [2]. Under UHV conditions Au(110) presents a (1x2)‐missing‐row reconstruction whereas Pd 70 Au 30 (110) is unrecontructed with a measured surface parameter closer to Au than to Pd. CO only adsorbs on Au(110) for pressures above 10 −3 Torr whereas CO adsorbs on Pd 70 Au 30 (110) at very low partial pressures (<10 −6 Torr) as it was shown by NAP‐XPS. In the range of pressures studied (10 −2 Torr – 5 10 2 Torr) the unreconstructued flat terraces on Pd 70 Au 30 (110) become rough at low pressure and a “rice grain” morphology is observed with typical domain sizes (oriented in the [1 −1 0] direction) around 4 nm and 0.05 nm corrugation that prevails with no specific variation up to 5 10 2 Torr (Figure 1). Complementary studies performed by NAP‐XPS clearly show the segregation of Pd the surface under CO pressure. So the roughening observed by STM (approximately one third of an atomic step in height) is essentiallydue to the diffusion of Pd atoms to the surface. In the case of Au(110), the evolution of the surface structure and morphology with increasing CO pressure shows different surface structures [3] (Figure 2): under vacuum conditions, the Au(110) surface exhibits a (1x2) reconstruction which yields aligned terraces in the [1 −1 0] direction at a larger scale. CO chemisorption at 0.01 Torr on this surface induces a slow deconstruction of the (1x2) surface leading to a (1x4) structure under 0.1 Torr of CO.At higher pressure (0.5 to 30 Torr) a dramatic restructuring is observed where the terraces aligned in the [1 −1 0] direction under vacuum evolve to yield monoatomic‐high islands. Their size subsequently increases with increasing CO pressure [4]. At 100 Torr of CO the surface exhibits a (1x1) structure prior to the new surface structure observed at 5 10 2 Torr of CO with ~0.05 nm deep holes arranged in a c(4x4) array. Intensity modulations around these holes were also observed. CO chemisorption induces a strong restructuring of both “Au” surfaces as it is evidenced by the high resolution in situ environmental STM images. However while the restructuring is limited to a roughning of the surface (due Pd segregation) for the bimetallic crystal, the structure and morphology of the Au‐pure crystal surface evolve (through different configurations) as CO pressure increases showing that we have to take into account the dynamics of the surface and thus the evolution of the active sites during reaction.