| 1. |
|
|
| 2. |
|
|
| 3. |
- Göthelid, Mats, et al.
(författare)
-
Adsorption site, core level shifts and charge transfer on the Pd(111)-I(root 3 x root 3) surface
- 2006
-
Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 600:15, s. 3093-3098
-
Tidskriftsartikel (refereegranskat)abstract
- We use core level photoelectron spectroscopy and density functional theory (DFT) to investigate the iodine-induced Pd(1 1 1)-I(root 3 x root 3) structure formed at 1/3 NIL coverage. From the calculations we find that iodine adsorbs preferentially in the fcc hollow site. The calculated equilibrium distance is 2.06 angstrom and the adsorption energy is 68 kcal/mol, compared to 2.45 angstrom and 54 kcal/mol in the atop position. The adsorption energy difference between fcc and hcp hollows is 1.7 kcal/mol. Calculated Pd 3d surface core level shift on clean Pd(1 1 1) is 0.30 eV to lower binding energy, in excellent agreement with our experimental findings (0.28-0.29 eV). On the Pd(1 1 1)-I(root 3 x root 3) we find no Pd 3d surface core level shift, neither experimentally nor, theoretically. Calculated charge transfer for the fcc site, determined from the Hirshfeld partitioning method, suggests that the iodine atom remains almost neutral upon adsorption.
|
|
| 4. |
- Göthelid, Mats, et al.
(författare)
-
Surface concentration dependent structures of iodine on Pd(110)
- 2012
-
Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 137:20, s. 204703-
-
Tidskriftsartikel (refereegranskat)abstract
- We use photoelectron spectroscopy, low energy electron diffraction, scanning tunneling microscopy, and density functional theory to investigate coverage dependent iodine structures on Pd(110). At 0.5 ML (monolayer), a c(2 x 2) structure is formed with iodine occupying the four-fold hollow site. At increasing coverage, the iodine layer compresses into a quasi-hexagonal structure at 2/3 ML, with iodine occupying both hollow and long bridge positions. There is a substantial difference in electronic structure between these two iodine sites, with a higher electron density on the bridge bonded iodine. In addition, numerous positively charged iodine near vacancies are found along the domain walls. These different electronic structures will have an impact on the chemical properties of these iodine atoms within the layer.
|
|
| 5. |
- Sinner-Hettenbach, M., et al.
(författare)
-
Oxygen-deficient SnO2(110) : a STM, LEED and XPS study
- 2001
-
Ingår i: Surface Science. - 0039-6028 .- 1879-2758. ; 477:1, s. 50-58
-
Tidskriftsartikel (refereegranskat)abstract
- The reconstruction process of SnO2(110) was investigated by scanning tunneling microscopy (STM), low energy electron diffraction (LEED) and X-ray photoelectron spectroscopy (XPS). The high-temperature 1 x 1 reconstruction could be identified as an intermediate face, transferable either to the 1 x 2 surface by annealing to higher temperature or back to the 4 x 1 reconstruction by annealing again at lower temperature. In this way, a new method to prepare well-ordered 4 x 1 terminated surfaces is introduced and the O/Sn ratio as well as the core-level positions of Sn 3d and O 1s were traced by XPS over a temperature range of 700-1200 K on freshly sputtered and on low-temperature long-time annealed SnO2(110). STM images of the 1 x 1 surface show a new structure of in-plane oxygen, ordered locally in zigzag rows that contribute to the LEED pattern as a very diffuse 2 x 1 intensity.
|
|
| 6. |
|
|
| 7. |
- von Schenck, Henrik, 1972-
(författare)
-
Investigations of selectivity control in catalysis
- 2002
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Catalytic reactions are ever present in applied chemistry,from laboratory synthesis to industrial scale processes.Catalysts provide the means to influence both the rate ofreactions (activity) and the product outcome (selectivity),thereby having fundamental impact on the reaction efficiency.In the ideal case, catalysts allow for 100% conversion ofreactants to the desired products with little or no energyneeded to drive the reaction. In reality this is rarely thecase.In the work presented here, the underpinnings of chemicalselectivity in catalysis have been investigated on anatomic/molecular level. Three catalytic systems have beenstudied, representing different approaches to achieving productselectivity. Both experimental and theoretical methods havebeen used. The carbon-carbon bond forming reaction known as theHeck reaction has been studied using quantum chemical methodsbased on density functional theory. The migratory insertion ofpropene into the palladium-phenyl bond of selectedorganometallic complexes has been in focus. A cationicpalladium(II)-diimine model catalyst favored the production ofthe branched 2-phenylpropene product. This selectivity could besignificantly improved by introducing steric groups at thenitrogen positions of the ligand system. The use of neutralpalladium complexes for the same reaction shifted theselectivity towards the linear insertion product, i.e. 1-phenylpropene.Methylamine, adsorbed on supported rhodium clustercatalysts, undergoes selfhydrogenolysis when annealed,producing ammonia and methane. When adsorbed on supportedpalladium clusters, methylamine undergoes disproportionationwith dimethylamine as the primary product. These reactions wereinvestigated using solid state nuclear magnetic resonance. Theinitial steps of the reaction involved carbonnitrogen bondscission. It is suggested that carbon fragment stability on thetwo different metals is primarily responsible for the observedselectivity. On rhodium, carbon species readily dehydrogenate,while the nitrogen species react with surface hydrogen to formammonia. On palladium, carbon fragments undergo carbon-nitrogenbond formation rather than further carbon-hydrogen bondscission.Photoelectron spectroscopy in combination with periodicdensity functional methods were used to study the influence oftin on the selective hydrogenation of crotonaldehyde onplatinum. Catalyzed by platinum, the hydrogenation of theα,β- unsaturated aldehyde leads to the formation ofthe saturated aldehyde. With tinmodified catalysts, thereaction produces an increased amount of the unsaturatedalcohol. Crotonaldehyde was adsorbed on Pt(111) and the tinmodified surface alloys Pt(111)Sn-(2x2) andPt(111)Sn-(√3x√3)R30º. It was found that anincreased surface concentration of tin caused deactivation ofthe alkene functionality while the carbonyl bond interactedwith the surface through O-Sn coordination. Preliminary studiesof the catalytic properties of a tin modified Pt(110)-(1x2)catalyst were also performed. The surface alloy was firstcharacterized and later dosed carbon monoxide. An interestingpreference for adsorption along the Pt ridge-rows wasobserved.
|
|
| 8. |
- von Schenck, Henrik, et al.
(författare)
-
NMR study of the chemisorption and decomposition of methylamine on Rh/SiO2
- 2007
-
Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 111:21, s. 7783-7794
-
Tidskriftsartikel (refereegranskat)abstract
- Nuclear magnetic resonance (NMR) studies of the interaction of methylamine with the rhodium surface in a Rh/SiO2 catalyst at temperatures in the range of 253-298 K reveal two principal features in both the C-13 and N-15 spectra. One feature is a resonance occurring at a frequency very close to the resonance frequency for methylamine gas. This feature is attributed to methylamine molecules adsorbed without dissociation. Because the methylamine is not removed from the catalyst surface by evacuation to 10(-6) Torr, its interaction with the surface is reasonably strong. The silica support does not retain methylamine after such an evacuation, and therefore, the adsorbed methylamine is associated with the rhodium. The other feature in both the C-13 and N-15 spectra is a very broad resonance occurring downfield of the resonance for the nondissociatively adsorbed methylamine. It is attributed to partially dehydrogenated surface species designated by the generalized formula (CN)H-x. Incipient scission of carbon-nitrogen bonds is observed at room temperature. As the temperature is increased above room temperature, the extent of scission of the carbon-nitrogen bonds increases. Ammonia, methane, and higher-carbon-number hydrocarbons then appear in the gas phase. The nondissociatively adsorbed methylamine is continuously transformed into the species (CN)H-x, thereby releasing hydrogen on the surface. The hydrogen is consumed in the production of the ammonia and methane observed in the gas phase. Ammonia is formed much more readily than methane or the other hydrocarbons, with the result that the species remaining on the surface are rich in carbon relative to nitrogen.
|
|
| 9. |
- Von Schenck, Henrik, et al.
(författare)
-
NMR study of the chemisorption and surface chemistry of methylamine on Pd/SiO2
- 2008
-
Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 112:8, s. 3042-3048
-
Tidskriftsartikel (refereegranskat)abstract
- Results of an investigation of the interaction of methylamine with a Pd/SiO2 catalyst utilizing nuclear magnetic resonance (NMR) spectroscopy are presented. Two sets of experiments were conducted. In one set, a sample of catalyst was initially exposed to (CH3NH2)-C-13-N-15 at 77 K and then subjected to a lengthy equilibration period at 190 K. A C-13 spectrum at 77 K obtained on the sample at this point exhibited a single symmetric line at a frequency very close to the resonance frequency for methylamine in the absence of the catalyst. The line is attributed to methylamine adsorbed without dissociation. In C-13 spectra obtained at 77 K on the sample after each of a series of subsequent annealing periods at successively higher temperatures in the range 253-298 K, the line exhibited a gradual broadening on the downfield side. Deconvolution of the spectra resolved the original symmetric line from a very broad downfield line attributed to partially dehydrogenated surface species designated by the formula (CN)H-x. In the second set of experiments, (CH3NH2)-C-13-N-15 was adsorbed on a sample of catalyst at 298 K. The sample was then annealed at successively higher temperatures in the range 325-450 K. Deconvolution of the C-13 spectra obtained at 298 K after the various annealing periods revealed extensive formation of dimethylamine, reaching a maximum after the annealing period at 400 K. Ammonia and methane were first detected in the N-15 and C-13 spectra, respectively, after the annealing period at 375 K. During the annealing periods at 425 and 450 K, the dimethylamine decomposed almost completely, leaving only methane, ammonia, and residual surface species.
|
|
| 10. |
- von Schenck, Henrik, et al.
(författare)
-
Reactions of iodobenzene on Pd(111) and Pd(110)
- 2003
-
Ingår i: Applied Surface Science. - 0169-4332 .- 1873-5584. ; 212-213, s. 508-514
-
Tidskriftsartikel (refereegranskat)abstract
- We have investigated the formation and reaction of phenyl fragments on Pd(1 1 1) and Pd(1 1 0), using synchrotron radiationbased photoelectron spectroscopy. The phenyl fragments are formed by carbon-iodine bond scission of iodobenzene, taking place already at 80 K. Pd(1 1 0) is seen to be more reactive than Pd(1 1 1) in this reaction. With annealing, the phenyl fragments subsequently form both benzene and biphenyl products. Ring coupling is found to be somewhat more efficient on Pd(1 1 1).
|
|
| 11. |
- Weissenrieder, Jonas, et al.
(författare)
-
Oxygen structures on Fe(110)
- 2003
-
Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 527:03-jan, s. 163-172
-
Tidskriftsartikel (refereegranskat)abstract
- The adsorption of oxygen on a Fe(110) single crystal has been studied by means of high resolution photoelectron spectroscopy (HRPES) and scanning tunneling microscopy (STM). Core level spectra were analyzed in detail on both clean and adsorbate covered surfaces. A shoulder on the high binding energy side of the Fe 2p core level indicates a structure comprising multiple components interpreted as an exchange split of the final state due to interaction between the 2p and 3d electrons. After adsorption of oxygen, (2 x 5), (2 x 2) and (3 x 1) reconstructions were observed with atomically resolved STM. The iron surface was further exposed to gradually higher doses of oxygen. Deconvolution of the O 1s HRPES spectra revealed two components separated approximately by 0.4 eV. The component at lower binding energy dominates at low coverage, while the high binding energy component increases in intensity with increasing O coverage. The formation of oxides was observed in the Fe 2p spectrum in the region between 708 and 710 eV. Further, well ordered iron oxides were grown by exposure to oxygen at 250 degreesC. The O 1s core level contained a single component with a binding energy similar to that of the high energy component in the just discussed O 1s spectrum. Low energy electron diffraction and STM images of this structure showed a large moire pattern with a 22.1 Angstrom x 30.9 Angstrom unit cell.
|
|
