We have investigated to what extent Pt(211) is representative for Pt[n(111) × (100)] surfaces in adsorption/desorption behavior of water, hydrogen, and oxygen through temperature-programmed desorption. In contrast to surfaces with n > 3, H2O adsorbs to Pt(211) in a crystalline fashion far below the usual crystallization temperature of amorphous solid water. For D2, we find that desorption from (100) steps is independent of terrace length for n ≥ 3, but desorption from the neighboring (111) terraces varies. Larger terraces result in larger variations in binding energies as a consequence of decreasing proximity of adsorption sites to the step edge. For O2, we observe enhanced dissociation on Pt(211) resulting in a much larger maximum O coverage than surfaces with n > 3. The TPD characteristics suggest formation of 1D PtO2 structures, which are only formed for n = 3 with this (100) step type. Hence, Pt(211) can by no means be considered representative of Pt(111) terraces truncated by (100) steps. Our results stress that great caution is required when extrapolating results from theoretical studies based on this smallest unit cell containing the (100) step edge to catalysis by actual particles.
We have examined water desorption from Pt(111) terraces of varying width and its dependence on precoverage by deuterium (D) with temperature programmed desorption studies. We observe distinct water desorption from (100) steps and (111) terraces, with steps providing adsorption sites with a higher binding energy than terraces. Preadsorption of D at the steps causes annihilation of water stabilization at the steps, while it also causes an initial stabilization of water on the (111) terraces. When the (111) terraces also become precovered with D, this water stabilization trend reverses on all surfaces. Destabilization continues for stepped surfaces containing up to 8-atom wide (111) terraces with a (100) step type and these become hydrophobic, in contrast to surfaces with a (110) step type and with the infinite (111) terrace. Our results illustrate how surface defects and a delicate balance between intermolecular forces and the adsorption energy govern hydrophobic vs. hydrophilic behavior, and that the influence of steps on the adsorption of water on nano-structured platinum surfaces has a very long-ranged character.
The "hydrogen region" of platinum is a powerful tool to structurally characterize nanostructured platinum electrodes. In recent years, the understanding of this hydrogen region has improved considerably: on Pt(111) sites, there is indeed only hydrogen adsorption, while on step sites, the hydrogen region involves the replacement of adsorbed hydrogen by adsorbed hydroxyl which interacts with co-adsorbed cations. However, the hydrogen region features an enigmatic and less well-understood "third hydrogen peak", which develops on oxidatively roughened platinum electrodes as well as on platinum electrodes with a high (110) step density that have been subjected to a high concentration of hydrogen. In this paper, we present evidence that the peak involves surface-adsorbed hydrogen (instead of subsurface hydrogen) on a locally "reconstructed" (110)-type surface site. This site is unstable when the hydrogen is oxidatively removed. The cation sensitivity of the third hydrogen peak appears different from other step-related peaks, suggesting that the chemistry involved may still be subtly different from the other features in the hydrogen region.
We report on a combined TPD and STM study of O2 adsorption and dissociation on various Pt surfaces with varying (111) terrace widths and either (110) or (100) step geometries. Our quantitative TPD results show that (110) stepped surfaces adsorb considerably more oxygen at 100 K, regardless of terrace width, than either (100) stepped surfaces or planar Pt(111). These results suggest that O2 dissociates on the (110) stepped surfaces at 100 K, well lower than required for temperature-induced dissociation on (111) planes. The amount of oxygen desorbing from recombinative desorption of adsorbed oxygen atoms is also greater on (110) stepped surfaces. In addition, the partitioning of adsorbed oxygen between molecular and dissociative states depends on the step geometry; (110) stepped surfaces show an uptake plateau indicative of a threshold surface concentration for low-temperature dissociation, whereas (100) stepped surfaces do not. Scanning tunneling microscope (STM) images for various O coverages and surface deposition temperatures confirm low-temperature dissociation on a (110) stepped surface. The STM images also show that terrace width is not a factor in the lowered dissociation barriers for O2 on (110) stepped surfaces.
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