| 1. |
- Gustafson, Johan, et al.
(author)
-
High-energy surface X-ray diffraction for fast surface structure determination
- 2014
-
In: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 343:6172, s. 758-761
-
Journal article (peer-reviewed)abstract
- Understanding the interaction between surfaces and their surroundings is crucial in many materials-science fields such as catalysis, corrosion, and thin-film electronics, but existing characterization methods have not been capable of fully determining the structure of surfaces during dynamic processes, such as catalytic reactions, in a reasonable time frame. We demonstrate an x-ray-diffraction–based characterization method that uses high-energy photons (85 kiloelectron volts) to provide unexpected gains in data acquisition speed by several orders of magnitude and enables structural determinations of surfaces on time scales suitable for in situ studies. We illustrate the potential of high-energy surface x-ray diffraction by determining the structure of a Pd surface in situ during catalytic CO oxidation and follow dynamic restructuring of the surface with subsecond time resolution.
|
|
| 2. |
- Martin, Natalia Mihaela, 1984, et al.
(author)
-
Structure-function relationship during CO2 methanation over Rh/Al2O3 and Rh/SiO2 catalysts at atmospheric pressure conditions
- 2018
-
In: Catalysis Science and Technology. - : Royal Society of Chemistry (RSC). - 2044-4753 .- 2044-4761. ; 8:10, s. 2686-2696
-
Journal article (peer-reviewed)abstract
- The effect of support material and chemical state of Rh for Rh/Al2O3 and Rh/SiO2 model catalysts during CO2 hydrogenation were studied by a combined array of in situ characterisation techniques including diffuse reflectance infrared Fourier transform spectroscopy, energy-dispersive X-ray absorption spectroscopy and high-energy X-ray diffraction at 250-350 °C and atmospheric pressure. The CO2 methanation proceeds via intermediate formation of adsorbed CO species on metallic Rh likely followed by their hydrogenation to methane. Linearly-bonded CO species is suggested to be a more active precursor in the hydrogenation compared to the bridge-bonded species, which seems to relate to particle size effects: for larger particles mainly the formation of inactive bridge-bonded CO species takes place. Further, analysis of the chemical state of Rh during reaction conditions reveal a minor formation of RhOx from dissociation of CO2 , which is a consequence of the increased activity observed over Rh/Al2O3 catalyst.
|
|
| 3. |
- Martin, Natalia Mihaela, 1984, et al.
(author)
-
Structure-function relationship for CO2 methanation over ceria supported Rh and Ni catalysts under atmospheric pressure conditions
- 2019
-
In: Catalysis Science and Technology. - 2044-4753 .- 2044-4761. ; 9:7, s. 1644-1653
-
Journal article (peer-reviewed)abstract
- In situ structural and chemical state characterization of Rh/CeO 2 and Ni/CeO 2 catalysts during atmospheric pressure CO 2 methanation has been performed by a combined array of time-resolved analytical techniques including ambient-pressure X-ray photoelectron spectroscopy, high-energy X-ray diffraction and diffuse reflectance infrared Fourier transform spectroscopy. The ceria phase is partially reduced during the CO 2 methanation and in particular Ce 3+ species seem to facilitate activation of CO 2 molecules. The activated CO 2 molecules then react with atomic hydrogen provided from H 2 dissociation on Rh and Ni sites to form formate species. For the most active catalyst (Rh/CeO 2 ), transmission electron microscopy measurements show that the Rh nanoparticles are small (average 4 nm, but with a long tail towards smaller particles) due to a strong interaction between Rh particles and the ceria phase. In contrast, larger nanoparticles were observed for the Ni/CeO 2 catalyst (average 6 nm, with no crystallites below 5 nm found), suggesting a weaker interaction with the ceria phase. The higher selectivity towards methane of Rh/CeO 2 is proposed to be due to the stronger metal-support interaction.
|
|
| 4. |
- Shipilin, Mikhail, et al.
(author)
-
Quantitative surface structure determination using in situ high-energy SXRD: Surface oxide formation on Pd(100) during catalytic CO oxidation
- 2014
-
In: Surface Science. - : Elsevier BV. - 0039-6028. ; 630, s. 229-235
-
Journal article (peer-reviewed)abstract
- We have performed a quantitative structure determination of the SQRT(5×5)R27° surface oxide, formed on Pd(100) under semi-realistic conditions for catalytic CO oxidation, using in situ high-energy surface X-ray diffraction. We describe the experiment and the extraction of quantitative data in detail. The structural results are in agreement with previous reports of a system consisting of a single layer of PdO(101) formed in pure O2 on top of Pd(100) and studied under ultra high vacuum conditions.
|
|
| 5. |
- Shipilin, Mikhail, et al.
(author)
-
Transient stuctures of PdO during CO oxidation over Pd(100)
- 2015
-
In: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 119:27, s. 15469-15476
-
Journal article (peer-reviewed)abstract
- In situ high-energy surface X-ray diffraction was employed to determine the surface structure dynamics of a Pd(100) single crystal surface acting as a model catalyst to promote CO oxidation. The measurements were performed under semi-realistic conditions, i.e. 100 mbar total gas pressure and 600 K sample temperature. The surface structure was studied in detail both in a steady gas ow and in a gradually changing gas composition with a time resolution of 0.5 sec. Our results show that \sqroot-PdO(101) surface oxide forms in a close to stoichiometric O2 and CO gas mixture as the mass-spectrometry indicates a transition to a highly active state with the reaction rate limited by the CO mass transfer to the Pd(100) surface. Using a low excess of O2 in the gas stoichiometry, islands of bulk oxide grow epitaxially in the same (101) crystallographic orientation of the bulk PdO unit cell according to a Stranski-Krastanov type of growth. The morphology of the islands is analyzed quantitatively. Upon further increase of the O2 partial pressure a polycrystalline Pd oxide forms on the surface.
|
|
| 6. |
- Blomberg, Sara, et al.
(author)
-
Combining synchrotron light with laser technology in catalysis research
- 2018
-
In: Journal of Synchrotron Radiation. - 1600-5775 .- 0909-0495. ; 25:5, s. 1389-1394
-
Journal article (peer-reviewed)abstract
- High-energy surface X-ray diffraction (HESXRD) provides surface structural information with high temporal resolution, facilitating the understanding of the surface dynamics and structure of the active phase of catalytic surfaces. The surface structure detected during the reaction is sensitive to the composition of the gas phase close to the catalyst surface, and the catalytic activity of the sample itself may affect the surface structure, which in turn may complicate the assignment of the active phase. For this reason, planar laser-induced fluorescence (PLIF) and HESXRD have been combined during the oxidation of CO over a Pd(100) crystal. PLIF complements the structural studies with an instantaneous two-dimensional image of the CO2 gas phase in the vicinity of the active model catalyst. Here the combined HESXRD and PLIF operandomeasurements of CO oxidation over Pd(100) are presented, allowing for an improved assignment of the correlation between sample structure and the CO2distribution above the sample surface with sub-second time resolution.
|
|
| 7. |
- Mehar, Vikram, et al.
(author)
-
Understanding the Intrinsic Surface Reactivity of Single-Layer and Multilayer PdO(101) on Pd(100)
- 2018
-
In: ACS Catalysis. - : American Chemical Society (ACS). - 2155-5435. ; 8:9, s. 8553-8567
-
Journal article (peer-reviewed)abstract
- We investigated the intrinsic reactivity of CO on single-layer and multilayer PdO(101) grown on Pd(100) using temperature-programmed reaction spectroscopy (TPRS) and reflection absorption infrared spectroscopy (RAIRS) experiments, as well as density functional theory (DFT) calculations. We find that CO binds more strongly on multilayer than single-layer PdO(101) (∼119 kJ/mol vs 43 kJ/mol), and that CO oxidizes negligibly on single-layer PdO(101), whereas nearly 90% of a saturated layer of CO oxidizes on multilayer PdO(101) during TPRS experiments. RAIRS further shows that CO molecules adsorb on both bridge-Pdcusand atop-Pdcussites (coordinatively unsaturated Pd sites) of single-layer PdO(101)/Pd(100), while CO binds exclusively on atop-Pdcussites of multilayer PdO(101). The DFT calculations reproduce the much stronger binding of CO on multilayer PdO(101), as well as the observed binding site preferences, and reveal that the stronger binding is entirely responsible for the higher CO oxidation activity of multilayer PdO(101)/Pd(100). We show that the O atom below the Pdcussite, present only on multilayer PdO(101), modifies the electronic states of the Pdcusatom in a way that enhances the CO-Pdcusbonding. Lastly, we show that a precursor-mediated kinetic model, with energetics determined from the present study, predicts that the intrinsic CO oxidation rates achieved on both single-layer and multilayer PdO(101)/Pd(100) can be expected to exceed the gaseous CO diffusion rate to the surface during steady-state CO oxidation at elevated pressures, even though the intrinsic reaction rates are 4-5 orders of magnitude lower on single-layer PdO(101)/Pd(100) than on multilayer PdO(101)/Pd(100).
|
|
| 8. |
- Schaefer, Andreas, 1981, et al.
(author)
-
Thermal reduction of ceria nanostructures on rhodium(111) and re-oxidation by CO2
- 2018
-
In: Physical Chemistry Chemical Physics. - : Royal Society of Chemistry (RSC). - 1463-9084 .- 1463-9076. ; 20:29, s. 19447-19457
-
Journal article (peer-reviewed)abstract
- The thermal reduction of cerium oxide nanostructures deposited on a rhodium(111) single crystal surface and the re-oxidation of the structures by exposure to CO2 were investigated. Two samples are compared: a rhodium surface covered to ≈60% by one to two O-Ce-O trilayer high islands and a surface covered to ≈65% by islands of four O-Ce-O trilayer thickness. Two main results stand out: (1) the thin islands reduce at a lower temperature (870-890 K) and very close to Ce2O3, while the thicker islands need higher temperature for reduction and only reduce to about CeO1.63 at a maximum temperature of 920 K. (2) Ceria is re-oxidized by CO2. The rhodium surface promotes the re-oxidation by splitting the CO2 and thus providing atomic oxygen. The process shows a clear temperature dependence. The maximum oxidation state of the oxide reached by re-oxidation with CO2 differs for the two samples, showing that the thinner structures require a higher temperature for re-oxidation with CO2. Adsorbed carbon species, potentially blocking reactive sites, desorb from both samples at the same temperature and cannot be the sole origin for the observed differences. Instead, an intrinsic property of the differently sized CeOx islands must be at the origin of the observed temperature dependence of the re-oxidation by CO2.
|
|
| 9. |
- Shipilin, Mikhail, et al.
(author)
-
The influence of incommensurability on the long-range periodicity of the Pd(100)-(root 5 x root 5)R27 degrees-PdO(101)
- 2017
-
In: Surface Science. - : Elsevier BV. - 0039-6028. ; 660, s. 1-8
-
Journal article (peer-reviewed)abstract
- The structural model of the (root 5 x root 5)R27 degrees PdO(101) surface oxide grown on Pd(100) has been proposed and refined by a number of authors over more than a decade. In the current contribution we discuss the long-range periodicity of this structure arising along one of the crystallographic directions due to its incommensurability with the substrate. Analyzing the results of surface sensitive diffraction studies, we determined a slight distortion of the previously reported perfect (root 5 x root 5)R27 degrees surface oxide unit cell. Considering it, we were able to achieve both qualitatively and quantitatively better fit to the experimental diffraction data than it was possible for the perfect structure. Further, taking into account the experimentally obtained scanning tunneling microscopy data and closely examining high-resolution patterns recorded by means of high-energy surface X-ray diffraction, we developed a qualitative structural model based on a larger non-orthogonal surface unit cell to shed more light on the long-range order of the PdO(101) surface oxide. The model comprises a shift of the atoms of the PdO perpendicularly to the direction of the incommensurability to correct for it. This structural model reproduces the fine details of the high-resolution diffraction patterns and qualitatively explains the periodic stripes of structural distortion observed in the images recorded by a scanning tunneling microscope.
|
|
