Adsorption and bonding on platinum : influence of the surface structure and chemical composition

Sammanfattning: This thesis deals with the influence of the structure andchemical composition of platinum surfaces on the adsorption ofsome molecules. Three main lines can be distinguished : 1) thecharacterisation of clean/modified surfaces, 2) the adsorptionof some simple atoms and molecules on these surfaces andfinally 3) the adsorption of 2-butenal, a bi-functionalmolecule containing a C=C group conjugated with a C=O group.The main tools used in this work are scanning tunnellingmicroscopy, photoelectron spectroscopy and high-resolutionelectron energy loss spectroscopy, in combination with quantumchemical calculations.The platinum (111)(1× 1) and (110)(1× 2) surfacesare chosen as substrates. Pt(111) is a non-reconstructedclose-packed surface, while the Pt(110) surface is open, due toits missing row (MR) reconstruction, which results in thealternation of ridges, {111} microfacets and valleys. Titaniumgrows on Pt(111) in a Volmer-Weber mode. Pronounced reactionsbetween Pt and Ti are detected already at room temperature asthe Ti2p and Pt4f7/2core-level shifts are characteristic of the Pt3Ti alloy. Carbon segregated on the (110) surfaceappears as extended graphitic regions, which smoothen thesubstrate. Sn deposition at room temperature on Pt(110) resultsin the appearance of small islands, randomly spread over thesurface. The presence of mobile Sn ad-atoms and Pt-Sn-Ptalloyed chains in the valley of the MR reconstruction is alsoevidenced. Annealing the surface results in the rearrangementof the tin in the surface layer, together with a globalshortening of the terraces in the [110]direction and anincreased density of (1× n) (n>2) defects.The adsorption of atomic hydrogen and oxygen has beenperformed on the Pt(111)( √ 3x √3)R30º surfacealloy and on Pt(110)(1× 2) respectively. The adsorptionsite of these atoms is changed as compared to the onedetermined on the clean Pt(111) surface (i.e. fcc hollow site): H is adsorbed on-top site on the (√3× √3)surface alloy, and O sits on the ridge of the missing rowreconstruction in bridge site. Carbon monoxide adsorption wasperformed on Pt(111) and Pt(110)(1× 2) surfaces, modifiedor not by tin. On the unmodified (111) surface, CO adsorbsfirst in top site, thenin bridge. Changing the geometry of thesurface to the (110) results in the vanishing of the bridgesite population. Modifying these surfaces by Sn does not changethe CO adsorption site.Finally, 2-butenal (CH3-CH=CH-CH=O) has been adsorbed at 100 K on thePt(111) surface and the Sn/Pt(111) surface alloys. On theunmodified Pt surface, comparison with results obtained forpropene (CH2=CH-CH3) adsorption evidences the involvement of the C=Cgroup of the 2-butenal molecule in the bonding to the Ptsurface. The carbonyl group is also suggested to take part inthe bonding, through a σccη1(O) configuration. This carbonyl group bondingdisappears when Sn is alloyed to the surface, and the formationof a new physisorbed phase is detected.