Site selectivity of specific reaction steps important for catalysis: CO dissociation on ruthenium surfaces

Kenneth Nielsen

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Abstract

This thesis presents the results of two dierent projects, both focused on catalysis. The main part is about the investigation of the structure sensitivity of the CO dissociation reaction with STM on a Ru(0 1 54) single crystal and the second part concerns the STM structure study of the Bi/Pt(111) overlayer system.
In the STM study of the structure sensitivity of the CO dissociation reaction on the Ru(0 1 54) sample, it was determined that after cooling the sample from 700K to 400K in 10-8Torr of CO or in the CO that was left after a TPD, the sample displayed periodic decorations on every other of the geometrically a ternating step. In some cases these decorations also extended into a hexagonal structure on the step and in both cases had a periodicity of 0.55±0.05 nm. They were identied as originating from atomic oxygen and the presence of these patches along with the presence of carbon in the amount of 0.014-0.020ML (determined by oxygen titration), which is close to the concentration of one kind of step (0.02 ML), lead to the conclusion that CO dissociation takes place on only one kind of step. Furthermore, from experiments with a 10 L CO dose at dierent constant temperatures and a 10 L post-saturation dose at 400 K, it was determined that only for the 500K dose was the oxygen structure observed. From this we conclude that a temperature between 450K and 500K is required to activate the CO dissociation on the steps and at higher temperatures recombination is activated.
From comparison of our results with simulated STM images and DFT based reaction energy level calculation, it was furthermore determined that it is the B-step conguration that is active for the CO dissociation and that a conguration with the oxygen atom occupying the hcp 3-fold hollow site on top of the step, right next to the dissociation site, is the most stable conguration after dissociation.
Preliminary results where the sample was exposed to high doses of CO, at a CO pressure of 10-5 Torr and a temperature of 550K (dissociation conditions) indicated that especially every other step had a very rough appearance after 7 min exposure. After the 14 min exposure they had straightened out again and after 60 min exposure something, most probably carbon species, was observed to grow from the bottom of one kind of step and from under-coordinated atoms on the other step.
In an collaborative ruthenium related extra-project it was determined that it was possible to shift the CO TPD behavior of dierent systems. It was showed that for the Ru(0 1 54) sample it was possible to shift the TPD shape from the single crystal type to the nano-particle type by sputtering the sample for 300 s. It was also showed that for a system consisting of a PVD nano-particle lm it was possible to shift its TPD behavior from the nano-particle type to the single crystal type by annealing it for 600 s at 900 K.
In the Bi/Pt(111) side project samples was prepared by PVD of bismuth on a Pt(111) single crystal, after which the sample was subjected to successively increasing heat treatments and characterized with XPS and STM. From the experiments it was determined that the 500C 3 min heat treatment was sucient to evaporate the bismuth in excess of 1ML from the sample. Furthermore, two small shifts was
observed in the XPS binding energy of the Bi 4f 7=2 peak. The first 0.08±0.02 eV shift up is likely due to the ordering of the overlayers after the first heat treatment at 300C 3 min. The second 0.10±0.02 eV shift down after the heat treatment to 500C for 30 min could be due to the evaporation of the excess bismuth layers or to alloying of the bismuth into the platinum surface. The STM images showed a rough surface after deposition, a crystalline island structure after the 300C 3 min heat treatment and a at surface with step heights comparable to the substrate after the 500C 3 min heat treatment. At higher heat treatments the surface layer is observed to break up, either due to evaporation or alloying, in a manner where holes are formed until these become so abundant that they become interconnected and the bismuth that is left appears as particles. On the 500C 3 min sample a hexagonal 1.05-1.07 X 1.07-1.08nm 62.2-65.8 structure was observed indicative of the p(4 X 4)
overlayer structure. Also in preliminary results from the 300 C 3 min sample and the high temperature particles, periodicities was observed but these need further confirmation.

Original languageEnglish
PublisherTechnical University of Denmark
Number of pages156
Publication statusPublished - 2011

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