| 1. |
- Beye, M., et al.
(författare)
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Selective Ultrafast Probing of Transient Hot Chemisorbed and Precursor States of CO on Ru(0001)
- 2013
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 110:18
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Tidskriftsartikel (refereegranskat)abstract
- We have studied the femtosecond dynamics following optical laser excitation of CO adsorbed on a Ru surface by monitoring changes in the occupied and unoccupied electronic structure using ultrafast soft x-ray absorption and emission. We recently reported [M. Dell'Angela et al. Science 339, 1302 (2013)] a phonon-mediated transition into a weakly adsorbed precursor state occurring on a time scale of >2 ps prior to desorption. Here we focus on processes within the first picosecond after laser excitation and show that the metal-adsorbate coordination is initially increased due to hot-electron-driven vibrational excitations. This process is faster than, but occurs in parallel with, the transition into the precursor state. With resonant x-ray emission spectroscopy, we probe each of these states selectively and determine the respective transient populations depending on optical laser fluence. Ab initio molecular dynamics simulations of CO adsorbed on Ru(0001) were performed at 1500 and 3000 K providing insight into the desorption process.
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| 2. |
- Dell'Angela, M., et al.
(författare)
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Real-Time Observation of Surface Bond Breaking with an X-ray Laser
- 2013
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Ingår i: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 339:6125, s. 1302-1305
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Tidskriftsartikel (refereegranskat)abstract
- We used the Linac Coherent Light Source free-electron x-ray laser to probe the electronic structure of CO molecules as their chemisorption state on Ru(0001) changes upon exciting the substrate by using a femtosecond optical laser pulse. We observed electronic structure changes that are consistent with a weakening of the CO interaction with the substrate but without notable desorption. A large fraction of the molecules (30%) was trapped in a transient precursor state that would precede desorption. We calculated the free energy of the molecule as a function of the desorption reaction coordinate using density functional theory, including van der Waals interactions. Two distinct adsorption wells-chemisorbed and precursor state separated by an entropy barrier-explain the anomalously high prefactors often observed in desorption of molecules from metals.
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| 3. |
- Gladh, Jörgen, et al.
(författare)
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Detection of adsorbate overlayer structural transitions using sum-frequency generation spectroscopy
- 2015
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Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 633, s. 77-81
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Tidskriftsartikel (refereegranskat)abstract
- We demonstrate that temperature-programmed vibrational sum-frequency generation (SFG) spectroscopy has a unique sensitivity to certain adsorbate overlayer structural transitions. In the CO stretching vibration of co-adsorbed CO/O(2x1)/Ru(0001) we observe pronounced dips in the spectral intensity as the adsorbate overlayer undergoes structural transitions with temperature. Combining with temperature-programmed desorption (TPD) a more complete picture of temperature-dependent structural transitions is obtained. We extract kinetic parameters from the SFG data and obtain good agreement with TPD when both techniques see the same transition. Infrared-infrared visible SFG is used to determine changes in inter-adsorbate coupling that allow us to experimentally assign the structural transitions. Furthermore, density functional theory calculations of the proposed structures and energetics are performed to verify the experimental assignments.
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| 4. |
- Gladh, Jörgen, et al.
(författare)
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X-ray emission spectroscopy and density functional study of CO/Fe(100)
- 2012
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 136:3
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Tidskriftsartikel (refereegranskat)abstract
- We report x-ray emission and absorption spectroscopy studies of the electronic structure of the pre-dissociative alpha(3) phase of CO bound at hollow sites of Fe(100) as well as of the on-top bound species in the high-coverage alpha(1) phase. The analysis is supported by density functional calculations of structures and spectra. The bonding of lying down CO in the hollow site is well described in terms of pi to pi* charge transfer made possible through bonding interaction also at the oxygen in the minority spin-channel. The on-top CO in the mixed, high-coverage alpha(1) phase is found to be tilted due to adsorbate-adsorbate interaction, but still with bonding mainly characteristic of vertical on-top adsorbed CO similar to other transition-metal surfaces.
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| 5. |
- Nilsson, Anders, et al.
(författare)
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Catalysis in real time using X-ray lasers
- 2017
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Ingår i: Chemical Physics Letters. - : Elsevier BV. - 0009-2614 .- 1873-4448. ; 675, s. 145-173
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Tidskriftsartikel (refereegranskat)abstract
- We describe how the unique temporal and spectral characteristics of X-ray free-electron lasers (XFEL) can be utilized to follow chemical transformations in heterogeneous catalysis in real time. We highlight the systematic study of CO oxidation on Ru(0001), which we initiate either using a femtosecond pulse from an optical laser or by activating only the oxygen atoms using a THz pulse. We find that CO is promoted into an entropy-controlled precursor state prior to desorbing when the surface is heated in the absence of oxygen, whereas in the presence of oxygen, CO desorbs directly into the gas phase. We monitor the activation of atomic oxygen explicitly by the reduced split between bonding and antibonding orbitals as the oxygen comes out of the strongly bound hollow position. Applying these novel XFEL techniques to the full oxidation reaction resulted in the surprising observation of a significant fraction of the reactants at the transition state through the electronic signature of the new bond formation.
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| 6. |
- Öberg, Henrik, et al.
(författare)
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Adsorption and Cyclotrimerization Kinetics of C2H2 at a Cu(110) Surface
- 2012
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Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 116:17, s. 9550-9560
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Tidskriftsartikel (refereegranskat)abstract
- The kinetics of acetylene adsorption and cyclotrimerization was studied by vibrational sum-frequency generation spectroscopy (SFG) and density functional theory (DFT) calculations. At low temperature, SFG shows two resonances corresponding to acetylene adsorbed in two different sites. Upon heating, two new vibrational resonances appear. We interpret these resonances as being due to C2H2 island formation and adsorbed C4H4, which is the intermediate in the subsequent cyclotrimerization reaction to form benzene. A kinetic model is applied, which allows determination of the relevant activation barriers. The barrier for C2H2 diffusion is determined to be 43 +/- 1 kJ/mol. The activation barrier for formation of the C4H4 intermediate is found to be 84 +/- 6 kJ/mol and the barrier for benzene formation 5 +/- 3 kJ/mol lower. Barriers to diffusion and formation of C4H4 and C6H6 obtained from DFT calculations are in quantitative agreement with the experiments once the locally high coverage in C2H2 islands is included.
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| 7. |
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| 8. |
- Öberg, Henrik, et al.
(författare)
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Optical laser-induced CO desorption from Ru(0001) monitored with a free-electron X-ray laser : DFT prediction and X-ray confirmation of a precursor state
- 2015
-
Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 640, s. 80-88
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Tidskriftsartikel (refereegranskat)abstract
- We present density functional theory modeling of time-resolved optical pump/X-ray spectroscopic probe data of CO desorption from Ru(0001). The BEEF van der Waals functional predicts a weakly bound state as a precursor to desorption. The optical pump leads to a near-instantaneous (<100 fs) increase of the electronic temperature to nearly 7000 K. The temperature evolution and energy transfer between electrons, substrate phonons and adsorbate is described by the two-temperature model and found to equilibrate on a timescale of a few picoseconds to an elevated local temperature of similar to 2000K. Estimating the free energy based on the computed potential of mean force along the desorption path, we find an entropic barrier to desorption (and by time-reversal also to adsorption). This entropic barrier separates the chemisorbed and precursor states, and becomes significant at the elevated temperature of the experiment (similar to 1.4 eV at 2000 K). Experimental pump-probe X-ray absorption/X-ray emission spectroscopy indicates population of a precursor state to desorption upon laser-excitation of the system (Dell'Angela et al., 2013). Computing spectra along the desorption path confirms the picture of a weakly bound transient state arising from ultrafast heating of the metal substrate.
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| 9. |
- Besharat, Zahra, et al.
(författare)
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Dehydrogenation of methanol on Cu2O(100) and (111)
- 2017
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 146:24
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Tidskriftsartikel (refereegranskat)abstract
- Adsorption and desorption of methanol on the (111) and (100) surfaces of Cu2O have been studied using high-resolution photoelectron spectroscopy in the temperature range 120-620 K, in combination with density functional theory calculations and sum frequency generation spectroscopy. The bare (100) surface exhibits a (3,0; 1,1) reconstruction but restructures during the adsorption process into a Cu-dimer geometry stabilized by methoxy and hydrogen binding in Cu-bridge sites. During the restructuring process, oxygen atoms from the bulk that can host hydrogen appear on the surface. Heating transforms methoxy to formaldehyde, but further dehydrogenation is limited by the stability of the surface and the limited access to surface oxygen. The (root 3 x root 3)R30 degrees-reconstructed (111) surface is based on ordered surface oxygen and copper ions and vacancies, which offers a palette of adsorption and reaction sites. Already at 140 K, a mixed layer of methoxy, formaldehyde, and CHxOy is formed. Heating to room temperature leaves OCH and CHx. Thus both CH-bond breaking and CO-scission are active on this surface at low temperature. The higher ability to dehydrogenate methanol on (111) compared to (100) is explained by the multitude of adsorption sites and, in particular, the availability of surface oxygen.
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| 10. |
- Boscolo Bibi, Sara, 1993-
(författare)
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Going for gold : A spectroelectrochemical and catalytic study of gold materials
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- With the increase in demand for renewable energy, understanding chemical processes is essential for improving the design of catalysts in order to achieve better performance. This thesis summarises the experimental investigation of three types of catalytic gold materials: gold oxide formed from gold films, oxide-derived gold (OD-Au) produced from gold films, and gold nanoparticles supported on metal oxides. Different spectroscopic techniques were employed, such as operando sum frequency generation (SFG) and in situ and ex situ X-ray spectroscopies. These methods allowed the probing of the electronic and chemical states of gold after oxidising electrochemical treatments. The results indicate the presence of subsurface gold oxide remnants after formation of OD-Au, which may help explain its improved catalytic properties with respect to pure gold. In addition, a mathematical model to couple the early stages of gold oxide formation with the nonlinear optical response of gold during this process is presented. This model suggests that the growth proceeds from small oxide islands to 3D oxide growth, while SFG oxidation variation is due to the suppression of the free electron density by negatively-charged adsorbing oxygen atoms. Gold oxide was also studied with both in situ and operando X-ray spectroscopies, showing the importance of a continuous electrochemical treatment during measurements to avoid beam induced effects. Furthermore, gold nanoparticles supported on metal oxides (TiO2 and γ-Fe2O3) were investigated mainly with mass spectrometry. The results indicate two different reaction pathways for oxidation of CO to CO2 depending on the type of metal oxide support. These findings could be used to help design future gold-based catalysts.
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| 11. |
- Dell'Angela, M., et al.
(författare)
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Vacuum space charge effects in sub-picosecond soft X-ray photoemission on a molecular adsorbate layer
- 2015
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Ingår i: Structural Dynamics. - : AIP Publishing. - 2329-7778. ; 2:2
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Tidskriftsartikel (refereegranskat)abstract
- Vacuum space charge induced kinetic energy shifts of O 1s and Ru 3d core levels in femtosecond soft X-ray photoemission spectra (PES) have been studied at a free electron laser (FEL) for an oxygen layer on Ru(0001). We fully reproduced the measurements by simulating the in-vacuum expansion of the photoelectrons and demonstrate the space charge contribution of the high-order harmonics in the FEL beam. Employing the same analysis for 400 nm pump-X-ray probe PES, we can disentangle the delay dependent Ru 3d energy shifts into effects induced by space charge and by lattice heating from the femtosecond pump pulse.
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| 12. |
- Ghadami Yazdi, Milad, et al.
(författare)
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Naphthalene on Ni(111) : experimental and theoretical insights into adsorption, dehydrogenation and carbon passivation
-
Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- An attractive solution to mitigate tars and also to decompose lighter hydrocarbons in biomass gasification is secondary catalytic reforming, converting hydrocarbons to useful permanent gases. Albeit in use for long time in fossil feedstock catalytic steam reforming, the understanding of the catalytic processes is still limited. Naphthalene is typically present in the biomass gasification gas and to further understand the elementary steps of naphthalene transformation, we investigated the temperature dependent naphthalene adsorption, dehydrogenation and passivation on Ni(111). TPD (temperature programmed desorption) and STM (scanning tunneling microscopy) in ultra-high vacuum environment from 110 K up to 780 K, combined with DFT (density functional theory) were used in the study. Room temperature adsorption results in a flat naphthalene monolayer. DFT favors the di-bridge[7] geometry but the potential energy surface is rather smooth. DFT also reveals a pronounced dearomatization and charge transfer from the adsorbed molecule into the nickel surface. Dehydrogenation occurs in two steps, with two desorption peaks at approximately 450 K and 600 K. The first step is due to partial dehydrogenation generating active hydrocarbon species that at higher temperatures migrates over the surface forming graphene. The graphene formation is accompanied by desorption of hydrogen in the high temperature TPD peak. The formation of graphene effectively passivates the surface both for hydrogen adsorption and naphthalene dissociation. In conclusion, the obtained results on the model naphthalene and Ni(111) system, provides insight into elementary steps of naphthalene adsorption, dehydrogenation and carbon passivation, which may serve as a good starting point for rational design, development and optimization of the Ni catalyst surface, as well as process conditions, for the aromatic hydrocarbon reforming process.
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| 13. |
- Giacomozzi, Linda, 1986-
(författare)
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The role of knockout driven fragmentation in collision with isolated complex molecular systems
- 2018
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This licentiate thesis covers results from collision induced dissociation (CID) experiments of Polycyclic Aromatic Hydrocarbons (PAHs), hydrogentated PAHs (HPAHs) and porphyrins. PAHs and HPAHs are believed to play key roles in the evolution of complex molecules in space, while porphyrins are essential for various functions in living organisms. The experiments were carried out in the Electrospray Ion Source Laboratory (EIS-LAB) at the Double ElectroStatic Ion-Ring ExpEriment (DESIREE) infrastructure, Stockholm University. In these studies, molecular ions collide with neutral targets at center-of-mass energies from a few tens to a few hundreds of eV. In this energy regime, fragmentation is mainly driven by nuclear scattering processes (nuclear stopping) rather than by interactions with the molecular electronic clouds (electronic stopping). Thus, knockout of single heavy atoms from the molecule in prompt billiard-ball type collision processes may occur in competition with statistical fragmentation processes in which the excitation energy is redistributed across all degrees of freedom before the decay. In this thesis, we have investigated the importance of knockout driven fragmentation processes in a series of experiments with markedly different molecular structures ranging from pure sp2-hybridized to pure sp3-hybridized systems. The experiments are interpreted in view of molecular structure and molecular dynamics simulations. These studies show that knockout processes are important for any molecular system, but they are in general difficult to isolate by means of mass spectrometric techniques. We have seen clear fingerprints of single carbon-atom loss in the collision experiments for PAHs and porphyrins. In contrast, in the case of the hydrogenated PAHs the fragmentation is dominated by the statistical loss of single carbon atoms due to their lower dissociation energy. We further report the threshold displacement energy for PAH, hydrogenated PAHs and porphyrin molecules--the key intrinsic property determining the cross section for prompt atom knockout.
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| 14. |
- Gladh, Jörgen, 1964-
(författare)
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Bonding and Desorption Mechanismsof CO on Metal Surfaces
- 2012
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- I have investigated two different systems CO/Fe(100) and CO/Ru(0001), toobtain new information on the binding and desorption processes. The twodifferent systems have served as a model system, one for a static examination,CO on iron, and for the dynamic case, CO on ruthenium. To perform theseinvestigations, several types of techniques have been used such as, X-rayabsorption spectroscopy, X-ray emission spectroscopy, and femtosecond laserinduced desorption techniques such as two-pulse correlation.For the CO/Fe(100) system, we found that the on-top CO “1 phase” canbe described by the Blyholder-Nilsson-Pettersson model. The pre-dissociativephase of CO bound at hollow sites, “3 phase”, can be described in a Dewar-Chatt-Duncanson like picture.For the CO/Ru(0001) system, it was found that all our data could be fitted from an empirical friction heat bath model. Moreover, it turned out, thatthere is a strong frictional coupling to the substrate electrons and phonons.
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| 15. |
- Gladh, Jörgen, et al.
(författare)
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Electron- and phonon-coupling in femtosecond laser-induced desorption of CO from Ru(0001)
- 2013
-
Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 615, s. 65-71
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Tidskriftsartikel (refereegranskat)abstract
- We studied femtosecond laser-induced desorption of CO from Ru(0001) using intense near-infrared and visible femtosecond laser pulses. We find a pronounced wavelength dependence with a factor 3-4 higher desorption yield at comparable fluence when desorption is induced via 400 nm light, compared to 800 nm and attribute this difference to the difference in penetration depth of the incident light. All our data can be described using empirical friction-modeling to determine the desorption mechanism with the same mechanism for both wavelengths. We find that both hot electrons and phonons contribute to the desorption process.
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| 16. |
- Gladh, Jörgen, 1964-
(författare)
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Ultrafast Probing of CO Reactions on Metal Surfaces : Changes in the molecular orbitals during the catalysis process
- 2017
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis presents experimental studies of three different chemical reaction steps relevant for heterogeneous catalysis: dissociation, desorption, and oxidation. CO on single-crystal metal surfaces was chosen as the model systems.X-ray absorption spectroscopy (XAS) and x-ray emission spectroscopy (XES) provide information about the electronic structure, and were performed on CO/Fe to measure both a non-dissociative, and a pre-dissociative state. The measurement on the pre-dissociative state showed a π → π* excitation, which implies a partly broken internal π bond in the molecule.Ultrafast laser-induced reactions were used to examine the dynamic properties of desorption and oxidation. Here CO/Ru and CO/O/Ru were used as model systems. Desorption of CO from a Ru surface involve both hot electrons and phonons. In the case of CO oxidation from CO/O/Ru a pronounced wavelength dependence of the branching ratio between desorption and oxidation was observed. Excitation with 400 nm showed a factor of 3-4 higher selectivity towards oxidation than 800 nm. This was attributed to coupling to transiently excited, non-thermalized electrons.Finally, by performing optical pump/x-ray probe XAS and XES changes in the electronic structure during the reaction could be followed, both for desorption and oxidation. In the CO/Ru experiment, two different transient excitation paths were observed, one leading to a precursor state, and one where CO moves into a more highly coordinated site. Using selective excitation in XES, these were shown to coexist on the surface. In the oxidation experiment, probing the reacting species located near the transition state region in an associative catalytic surface reaction was demonstrated for the very first time.
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| 17. |
- Hohmann, Lea, et al.
(författare)
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Effect of Coadsorbed Sulfur on the Dehydrogenation of Naphthalene on Ni(111)
- 2023
-
Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 128:1, s. 67-76
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Tidskriftsartikel (refereegranskat)abstract
- There are several difficulties when experimentally determined reaction mechanisms are applied from model systems to real catalysis. Besides the infamous pressure and material gaps, it is sometimes necessary to consider impurities in the real reactant feedstock that can act as promoters or catalyst poisons and alter the reaction path. In this study, the effect of sulfur on the dehydrogenation of naphthalene on Ni(111) is investigated by using X-ray photoelectron spectroscopy and scanning tunneling microscopy. Sulfur induces a (5 root 3 x 2) surface reconstruction, as previously reported in the literature. The sulfur does not have a strong effect on the dehydrogenation temperature of naphthalene. However, the presence of sulfur leads to a preferred formation of carbidic over graphitic carbon and a strong inhibition of carbon diffusion into the nickel bulk, which is one of the steps of destructive whisker carbon formation described in the catalysis literature.
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| 18. |
- LaRue, Jerry, et al.
(författare)
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Real-Time Elucidation of Catalytic Pathways in CO Hydrogenation on Ru
- 2017
-
Ingår i: The Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 8:16, s. 3820-3825
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Tidskriftsartikel (refereegranskat)abstract
- The direct elucidation of the reaction pathways in heterogeneous catalysis has been challenging due to the short-lived nature of reaction intermediates. Here, we directly measured on ultrafast time scales the initial hydrogenation steps of adsorbed CO on a Ru catalyst surface, which is known as the bottleneck reaction in syngas and CO2 reforming processes. We initiated the hydrogenation of CO with an ultrafast laser temperature jump and probed transient changes in the electronic structure using real-time X-ray spectroscopy. In combination with theoretical simulations, we verified the formation of CHO during CO hydrogenation.
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| 19. |
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| 20. |
- Marks, Kess, 1987-, et al.
(författare)
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Adsorption and decoposition of ethanol on Cu2O(111) and (100)
- 2019
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Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 123:33, s. 20384-20392
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Tidskriftsartikel (refereegranskat)abstract
- Ethanol dehydrogenation on metal oxides such as Cu2O is an important reaction for the production of renewable energy by fuel cells both via the production of H2 fuel and applied in direct alcohol fuel cells. To better understand this reaction we studied the adsorption, dissociation and desorption of ethanol on Cu2O(111) and (100) surfaces using high-resolution photoelectron spectroscopy (PES), vibrational sum frequency generation spectroscopy (SFG), and temperature programmed desorption (TPD) accompanied by density functional theory (DFT) calculations. On Cu2O(100) the first layer consists primarily of dissociatively adsorbed ethoxy. Second and third layers of ethanol physisorb at low temperature and desorb below 200 K. On the Cu2O(111) surface, adsorption is mixed as ethoxy, ethanol and the products following C-C cleavage, CHx and OCHx, are found in the first layer. Upon heating, products following both C-C and C-O bond breaking are observed on both surfaces and continued heating accentuates the molecular cracking. C-O cleavage occurs more on the (100) surface, whereas on the Cu2O(111) C-C cleavage dominates and occurs at lower temperatures than on the (100) surface. The increased ability of Cu2O(111) to crack ethanol is explained by the varied surface structure including both surface oxygen, electron rich O-vacancies and Cu.
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| 21. |
- Marks, Kess, 1987-
(författare)
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Experimental femtosecond-laser based investigations of model catalytic surface reactions
- 2018
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In order to be able to design novel catalytic processes more efficiently, detailed understanding of the catalyst-reactant interaction and the dynamics of the microscopic reaction steps is needed. The present thesis aims to contribute to the fundamental understanding of catalyst reactant systems by means of experiments using model systems in Ultra High Vacuum (UHV). The main body of work involves femtochemistry/mass spectrometry measurements as well as sum-frequency generation (SFG) measurements, which both make use of a femtosecond laser and a UHV sample environment. The results of two experimental investigations within the field of surface science are presented.The first paper concerns CO oxidation on ruthenium (0001) and in particular the energy transfer from substrate to adsorbates upon laser excitation. For these experiments laser-induced desorption was performed. We were able to control the branching ratios of competing mechanisms and understand the role of non-thermal electrons in the mechanisms.The second project aims to understand the adsorption and dehydrogenation of methanol on cuprous oxide (Cu2O) which is complicated by the fact that the cuprous oxide surface reconstructs differently under different conditions. The results presented in this part were acquired using mainly X-ray Photoelectron Spectroscopy and SFG. We were able to understand the restructuring of the Cu2O surface and to show that methanol adsorbs molecularly on Cu2O(111) instead of dissociatively as a methoxy and hydrogen species as it does on Cu2O(100).
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| 22. |
- Marks, Kess, 1987-
(författare)
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Experimental investigations of model catalytic surface reactions on metal and metal oxide surfaces
- 2019
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In the development of renewable energies catalysis plays an important role, for example in the production of H2 gas that drives fuel cells, or in the decomposition of annoying by-products of renewable energy production. Most catalysts and catalytic processes currently used in the industry have their roots in macroscopic empirical investigations and trial and error-based optimization. In order to be able to design novel catalytic processes more efficiently, detailed understanding of the catalyst-reactant interaction and the dynamics of the microscopic reaction steps is needed. The present thesis aims to contribute to the fundamental understanding of catalyst reactant systems by means of experiments using model systems in Ultra High Vacuum. For this purpose, several surface science techniques were employed such as vibrational sum-frequency generation (SFG), X-ray photoelectron spectroscopy (XPS), temperature programmed desorption (TPD) and femtochemistry.In the present thesis the results of three different projects are presented. The first concerns the adsorption and decomposition of naphthalene on Ni(111). Using scanning tunnelling microscopy (STM) and density functional theory (DFT) we identify the adsorption energy and geometry of the naphthalene molecule. Using SFG and TPD we investigate the temperature dependent breakdown of the naphthalene molecule and identify geometrical changes of the adsorbate as an intermediate step in the decomposition reaction. Additionally, we observe poisoning of the surface due to graphene growth using both STM and XPS and explore the possible effect of co-adsorption with oxygen on the reaction pathway and the poisoning of the catalyst.The second section concerns the adsorption and decomposition of ethanol and methanol on cuprous oxide (Cu2O). Using mainly XPS and SFG we show that ethanol adsorbs dissociatively on Cu2O(100) and (111) and that methanol adsorbs dissociatively on the (100) but molecularly on the (111) surface. Furthermore, we identify intermediate surface species and products of the temperature dependent dehydrogenation of both alcohols and show that the (111) surface is the more effective catalyst for decomposition.The third section explores the physics of non-thermal excitation methods and discusses CO oxidation on ruthenium (0001) induced by an optical laser and by X-rays from a free electron laser. Based on these femtochemistry experiments we discuss in particular the energy transfer both for direct excitation and for substrate mediated excitations. We show that we were able to control the branching ratios of competing mechanisms and understand the role of non-thermal electrons in the mechanisms of optical laser excitation. Furthermore, we show that it is possible to induce CO oxidation by direct X-ray core hole excitation and can rationalize the relaxation process that leads to CO oxidation.
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| 23. |
- Marks, Kess, et al.
(författare)
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Investigation of the surface species during temperature dependent dehydrogenation of naphthalene on Ni(111)
- 2019
-
Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 150:24
-
Tidskriftsartikel (refereegranskat)abstract
- The temperature dependent dehydrogenation of naphthalene on Ni(111) has been investigated using vibrational sum-frequency generation spectroscopy, X-ray photoelectron spectroscopy, scanning tunneling microscopy, and density functional theory with the aim of discerning the reaction mechanism and the intermediates on the surface. At 110 K, multiple layers of naphthalene adsorb on Ni(111); the first layer is a flat lying chemisorbed monolayer, whereas the next layer(s) consist of physisorbed naphthalene. The aromaticity of the carbon rings in the first layer is reduced due to bonding to the surface Ni-atoms. Heating at 200 K causes desorption of the multilayers. At 360 K, the chemisorbed naphthalene monolayer starts dehydrogenating and the geometry of the molecules changes as the dehydrogenated carbon atoms coordinate to the nickel surface; thus, the molecule tilts with respect to the surface, recovering some of its original aromaticity. This effect peaks at 400 K and coincides with hydrogen desorption. Increasing the temperature leads to further dehydrogenation and production of H-2 gas, as well as the formation of carbidic and graphitic surface carbon. Published under license by AIP Publishing.
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| 24. |
- Marks, Kess, 1987-, et al.
(författare)
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Naphthalene Dehydrogenation on Ni(111) in the Presence of Chemisorbed Oxygen and Nickel Oxide
- 2024
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Ingår i: Catalysts. - : MDPI AG. - 2073-4344. ; 14:2
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Tidskriftsartikel (refereegranskat)abstract
- Catalyst passivation through carbon poisoning is a common and costly problem as it reduces the lifetime and performance of the catalyst. Adding oxygen to the feed stream could reduce poisoning but may also affect the activity negatively. We have studied the dehydrogenation, decomposition, and desorption of naphthalene co-adsorbed with oxygen on Ni(111) by combining temperature-programmed desorption (TPD), sum frequency generation spectroscopy (SFG), photoelectron spectroscopy (PES), and density functional theory (DFT). Chemisorbed oxygen reduces the sticking of naphthalene and shifts H2 production and desorption to higher temperatures by blocking active Ni sites. Oxygen increases the production of CO and reduces carbon residues on the surface. Chemisorbed oxygen is readily removed when naphthalene is decomposed. Oxide passivates the surface and reduces the sticking coefficient. But it also increases the production of CO dramatically and reduces the carbon residues. Ni2O3 is more active than NiO.
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| 25. |
- Nilsson, Anders, et al.
(författare)
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X-ray absorption spectroscopy and X-ray Raman scattering of water and ice; an experimental view
- 2010
-
Ingår i: Journal of Electron Spectroscopy and Related Phenomena. - : Elsevier BV. - 0368-2048 .- 1873-2526. ; 177:03-feb, s. 99-129
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Tidskriftsartikel (refereegranskat)abstract
- Here we present a review of X-ray absorption spectroscopy and X-ray Raman scattering with the perspective to understand the spectra of water including changes with temperature, mass of the water molecule and presence of monovalent ions. The different detection schemes are discussed and it is concluded that transmission X-ray absorption measurements, using a small area where the thickness is uniform, and X-ray Raman scattering give the most reliable spectra. Different model systems are discussed such as the surface and bulk of ice and various adsorbed monolayer structures on metal surfaces.
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| 26. |
- Schiros, Theanne, et al.
(författare)
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Structure of water adsorbed on the open Cu(110) surface: H-up, H-down, or both?
- 2006
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Ingår i: Chemical Physics Letters. - : Elsevier BV. - 0009-2614 .- 1873-4448. ; 429:4-6, s. 415-419
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Tidskriftsartikel (refereegranskat)abstract
- We investigated the structure of the water monolayer on an open surface, Cu(1 1 0), at low temperature. We found that water adsorbs molecularly, adopting a 2:1 ratio of H-down and H-up configurations. This behavior of water on an open surface is quite different to the behavior on close-packed surfaces, such as Pt(1 1 1) and Ru(0 0 0 1), where water adsorbs primarily H-down, but can be understood on the basis of a range of different water adsorption sites across the observed (7 × 8) unit cell.
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| 27. |
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| 28. |
- Schiros, Theanne, et al.
(författare)
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The Role of Substrate Electrons in the Wetting of a Metal Surface
- 2010
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 132:9, s. 094701-
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Tidskriftsartikel (refereegranskat)abstract
- We address how the electronic and geometric structures of metal surfaces determine water-metal bonding by affecting the balance between Pauli repulsion and electrostatic attraction. We show how the rigid d-electrons and the softer s-electrons utilize different mechanisms for the redistribution of charge that enables surface wetting. On open d-shell Pt(111), the ligand field of water alters the distribution of metal d-electrons to reduce the repulsion. The closed-shell Cu d10 configuration of isostructural Cu(111), however, does not afford this mechanism, resulting in a hydrophobic surface and three-dimensional ice cluster formation. On the geometrically corrugated Cu(110) surface, however, charge depletion involving the mobile sp-electrons at atomic rows reduces the exchange repulsion sufficiently such that formation of a two-dimensional wetting layer is still favored in spite of the d10 electronic configuration
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| 29. |
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| 30. |
- Schreck, Simon, et al.
(författare)
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Atom-specific activation in CO oxidation
- 2018
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 149:23
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Tidskriftsartikel (refereegranskat)abstract
- We report on atom-specific activation of CO oxidation on Ru(0001) via resonant X-ray excitation. We show that resonant 1s core-level excitation of atomically adsorbed oxygen in the co-adsorbed phase of CO and oxygen directly drives CO oxidation. We separate this direct resonant channel from indirectly driven oxidation via X-ray induced substrate heating. Based on density functional theory calculations, we identify the valence-excited state created by the Auger decay as the driving electronic state for direct CO oxidation. We utilized the fresh-slice multi-pulse mode at the Linac Coherent Light Source that provided time-overlapped and 30 fs delayed pairs of soft X-ray pulses and discuss the prospects of femtosecond X-ray pump X-ray spectroscopy probe, as well as X-ray two-pulse correlation measurements for fundamental investigations of chemical reactions via selective X-ray excitation.
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| 31. |
- Öström, Henrik, et al.
(författare)
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Bonding of Saturated Hydrocarbons to Metal Surfaces
- 2003
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Ingår i: Physical Review Letters. - : The American Physical Society. - 0031-9007. ; 91:4, s. 046102-
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Tidskriftsartikel (refereegranskat)abstract
- The adsorption of octane on Cu(110) was studied by x-ray absorption and x-ray emission spectroscopy, in combination with spectrum calculations in the framework of density functional theory, as a model system for alkane adsorption on transition metals. Significant electron sharing between the adsorbate and metal surface and involvement of both bonding and antibonding C-H molecular orbitals in the molecule-metal bond was found. The calculations were extended to the case of octane adsorbed on Ni(110), and the position of the metal d band was found to be important for the bonding. The results were generalized to show that this is important for the efficiency as an alkane dehydrogenation catalyst.
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| 32. |
- Öström, Henrik, 1975-
(författare)
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Chemical Bonding of Hydrocarbons to Metal Surfaces
- 2004
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Using x-ray absorption spectroscopy (XAS), x-ray emission spectroscopy (XES) and x-ray photoelectron spectroscopy (XPS) in combination with density functional theory (DFT) the changes in electronic and geometric structure of hydrocarbons upon adsorption are determined. The chemical bonding is analyzed and the results provide new insights in the mechanisms responsible for dehydrogenation in heterogeneous catalysis.In the case of alkanes, n-octane and methane are studied. XAS and XES show significant changes in the electronic structure upon adsorption. XES shows new adsorption induced occupied states and XAS shows quenching of CH*/Rydberg states in n-octane. In methane the symmetry forbidden gas phase lowest unoccupied molecular orbital becomes allowed due to broken symmetry. New adsorption induced unoccupied features with mainly metal character appear just above the Fermi level in XA spectra of both adsorbed methane and n-octane. These changes are not observed in DFT total energy geometry optimizations. Comparison between experimental and computed spectra for different adsorbate geometries reveals that the molecular structures are significantly changed in both molecules. The C-C bonds in n-octane are shortened upon adsorption and the C-H bonds are elongated in both n-octane and methane.In addition ethylene and acetylene are studied as model systems for unsaturated hydrocarbons. The validity of both the Dewar-Chatt-Duncanson chemisorption model and the alternative spin-uncoupling picture is confirmed, as well as C-C bond elongation and upward bending of the C-H bonds.The bonding of ethylene to Cu(110) and Ni(110) are compared and the results show that the main difference is the amount of back-donation into the molecular π* orbital, which allows the molecule to desorb molecularly from the Cu(110) surface, whereas it is dehydrogenated upon heating on the Ni(110) surface. Acetylene is found to adsorb in two different adsorption sites on the Cu(110) surface at liquid nitrogen temperature. Upon heating the molecules move into one of these sites due to attractive adsorbate-adsorbate interaction and only one adsorbed species is present at room temperature, at which point the molecules start reacting to form benzene. The bonding of the two species is very similar in both sites and the carbon atoms are rehybridized essentially to sp2.
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| 33. |
- Öström, Henrik, et al.
(författare)
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Ethylene on Cu(110) and Ni(110) : Electronic structure and bonding derived from x-ray spectroscopy and theory
- 2004
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Ingår i: Surface Science. - : Elsevier. - 0039-6028. ; 559:2-3, s. 85-93
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Tidskriftsartikel (refereegranskat)abstract
- The bonding of ethylene to Cu(1 1 0) and Ni(1 1 0) is analyzed in detail using symmetry-resolved X-ray absorption (XAS) and emission (XES) spectroscopies in conjunction with density functional theory (DFT) calculations of geometric structure and spectra. XES, which probes the occupied valence states, reveals the formation of bonding and non-bonding orbitals of π-3d as well as π*-3d character. Additional mixing of σ and π states indicates rehybridization upon adsorption. The anti-bonding π-3d and π*-3d combinations are unoccupied and seen in XAS. A lower intensity of the π* transition for Ni is evidence of larger π* occupancy upon bonding. The position of the σ* shape-resonance indicates a 0.02 Å longer C–C bond on Ni than on Cu, in good agreement with the DFT structure optimizations. The XE spectra are well-reproduced both by specific spectrum calculations based on cluster models and by the carbon p-density of states calculated using periodic boundary conditions. The contribution of both π and π* levels to the new, surface-induced occupied states close to the Fermi-level lends support to the traditional Dewar–Chatt–Duncanson picture of the bonding. Theoretical charge-density difference plots support an alternative view of ethylene bonding in terms of the specific involvement of the excited molecular triplet state. Based on the variation in XE intensities the main difference between ethylene bonding to Cu and Ni is found to be an about two times larger occupancy of the π* orbital upon chemisorption on the transition metal, which comes along with C–C bond elongation and stronger σ–π rehybridization.
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| 34. |
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| 35. |
- Öström, Henrik, et al.
(författare)
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Geometric structure and chemical bonding of acetylene adsorbed on Cu(110)
- 2004
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Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 565:2-3, s. 206-222
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Tidskriftsartikel (refereegranskat)abstract
- The chemical bonding and geometric structure of acetylene adsorbed on Cu(I 10) is analyzed using X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS) and X-ray emission spectroscopy (XES) in combination with density functional theory (DFT) total energy geometry optimizations and spectral calculations. XPS reveals two different adsorbed species at liquid nitrogen temperature. The molecular alignment is deduced from angle-resolved XAS, revealing that in one site the molecules are aligned with the C-C axis along the [0 0 1] direction and in the other site with an average angle of 35degrees to the Cu rows. The position of the shape resonance is used to deduce a C-C bond length of 1.35 Angstrom, which is close to the values obtained from the DFT geometry optimizations. XES reveals strong sigma-pi mixing and new occupied states close to the Fermi level, originating from the out-of-plane pi* orbital, which becomes occupied upon adsorption in agreement with the Dewar-Chatt-Duncanson model of the bonding.
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| 36. |
- Öström, Henrik, et al.
(författare)
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Methanol decomposition on Ni(111) and O/Ni(111)
- 2022
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 156:2
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Tidskriftsartikel (refereegranskat)abstract
- Methanol decomposition on Ni(111) surfaces has been studied in the presence and absence of oxygen using temperature-programmed desorption and temperature-dependent sum frequency generation spectroscopy. Under both conditions the C–H and O–H bonds break, forming carbon monoxide and atomic hydrogen on the surface. No C–O bond scission was observed, limiting the number of reaction pathways. The O–H bonds break first (>150 K), forming surface methoxy, followed by C–H bond breakage (>250 K). All atomic hydrogen desorbs from the surface as H2 through H+H recombinative desorption. H2 desorbs at a higher temperature in the presence of oxygen (>300 K) than the absence of oxygen (>250 K) as the oxygen on the surface stabilizes the H atoms, forming surface hydroxide (OH). The surface oxygen also appears to stabilize the O–H and C–H bonds, leading to slightly higher dissociation temperatures. The CO molecules occupy both the bridge sites and the top sites of the Ni atoms as surface H appears to force the CO molecules to the top sites. There is a slight blueshift in the C–O bond vibration for both the O covered and O free surfaces due to CO being more mobile. On the O free surface, the C–O peak width broadens as low-frequency modes are activated. Finally, CO desorbs between 350 and 400 K.
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