Klas Andersson - Academia.edu (original) (raw)

Papers by Klas Andersson

The structure, bonding and chemistry of water and hydroxyl on certain well-defined metal single-c... more The structure, bonding and chemistry of water and hydroxyl on certain well-defined metal single-crystal surfaces are presented in this thesis. Synchrotron based core level spectroscopies (x-ray photoelectron (XP)-and x-ray absorption (XA) spectroscopy) in combination with scanning tunneling microscopy (STM), low energy electron diffraction (LEED) and density functional theory (DFT) calculations form the basis of the presented results. Taken together these techniques provide chemically quantitative, local electronic and geometric information. Conditions for the experimental investigations span the temperature range 35-520 K (-240 to 250 • C) and pressure range from ultra-high vacuum (UHV) [10 −11 Torr (∼10 −14 Atm)] to near ambient pressures [∼1 Torr (∼10 −3 Atm)]. With the sampled range of experimental conditions and techniques at hand we address the structure and bonding of water at metal surfaces along with activation barriers for water dissociation, structure and bonding in mixed water-hydroxyl phases and the fundamental importance of hydrogen (H-) bonding interactions on structure and kinetic barriers. Adsorption of water at the Pt(111), Ru(001) and Cu(110) surfaces at temperatures below 150 K under UHV conditions, i.e. below the temperature for significant ice sublimation rates, is found to proceed molecularly and no dissociation is observed. Complete 2-dimensional wetting layers can be formed on Pt(111), Ru(001) and Cu(110). At water adsorption temperatures above 150 K on Ru(001), it is found that previously reported isotope dependent features in thermal desorption spectra are due to qualitatively different surface chemistry for H 2 O and D 2 O. Whereas D 2 O desorbs molecularly intact, H 2 O dissociates in kinetic competition with the desorption channel above 150 K, the difference explained by the delicate change in energetics introduced by the approximately 0.1 eV lower zero point vibrational energy of the intramolecular O-H bond compared to O-D bond in the water isotopes. The molecularly intact water overlayer is found very sensitive to x-ray and electron induced damage and it is argued that this reconciles conflicting results in the literature over the, in essence, magnitude of the activation barrier for water dissociation on Ru(001). The structure of the mixed H 2 O:OH phases on the hexagonally close-packed Ru(001) and Pt(111) surfaces were studied and compared. On Ru(001) it consists of stripe-like structures 4 to 6 Ru lattice parameters wide where OH, in a non-donor configuration, decorates the edges of the stripes whereas the inner structure consists of intact water. The observed short-range order of the mixed H 2 O:OH stripes and the tendency of OH not to fully dissolve into the H 2 O-containing H-bond network on Ru(001) is radically different compared to the mixed H 2 O:OH phases observed on Pt(111). On Pt(111) two types of extended long-range order mixed H 2 O:OH H-bonding networks with 3×3 and (√ 3 × √ 3)R30 • symmetry were studied and found to be interrelated by geometric distortions originating from the asymmetric H-bond donor-acceptor properties of OH towards H 2 O. On the open Cu(110) surface the structure of the intact water monolayer is a mixed H-down and H-up structure in a 2:1 ratio. Similarly to the H 2 O/Ru(001)-system the molecularly intact water monolayer on Cu(110) start dissociating slightly above 150 K and is very sensitive to x-ray and electron induced damage. The studies on Cu(110) were extended to near ambient conditions utilizing in-situ XPS and compared to results on Cu(111). Whereas the Cu(111) surface remains adsorbate free, we find that the Cu(110) surface at room temperature up to about 430 K in the presence of only 1 Torr water holds significant amounts of water in a mixed H 2 O:OH layer. The differences are explained by the differing activation barriers for water dissociation, leading to the presence of OH groups on Cu(110) which lowers the desorption kinetics of water by orders of magnitude due to the formation of H 2 O-OH bonds of significant strength. By lowering the activation barrier for water dissociation on Cu(111) by pre-adsorbing atomic O, generating adsorbed OH, similar results to those on Cu(110) are obtained.

The structure, bonding and chemistry of water and hydroxyl on metal surfaces are presented. Synch... more The structure, bonding and chemistry of water and hydroxyl on metal surfaces are presented. Synchrotron based x-ray photoelectron- and x-ray absorption spectroscopy along with density functional theory calculations mainly form the basis of the results. Conditions span the temperature range 35 - 520 K and pressures from ultra-high vacuum (~10 fAtm) to near ambient pressures (~1 mAtm). The results provide, e.g, new insights on the importance of hydrogen bonding for surface chemical kinetics. Water adsorbs intact on the Pt(111), Ru(001) and Cu(110) surfaces at low temperatures forming 2-dimensional wetting layers where bonding to the metal (M) mainly occurs via H2O-M and M-HOH bonds. Observed isotope differences in structure and kinetics for H2O and D2O adsorption on Ru(001) are due to qualitatively different surface chemistries. D2O desorbs intact but H2O dissociates in kinetic competition with desorption similar to the D2O/Cu(110) system. The intact water layers are very sensitive to x-ray and electron induced damage. The mixed H2O:OH phase on Ru(001) consists of stripe-like structures 4 to 6 Ru lattice parameters wide where OH decorates the edges of the stripes. On Pt(111), two different long-range ordered mixed H2O:OH structures are found to be inter-related by geometric distortions originating from the asymmetric H-bond donor-acceptor properties of OH towards H2O. Water adsorption on Cu(110) was studied at near ambient conditions and compared to Cu(111). Whereas Cu(111) remains clean, Cu(110) holds significant amounts of water in a mixed H2O:OH layer. The difference is explained by the differing activation barriers for water dissociation, leading to the presence of OH groups on Cu(110) which lowers the desorption kinetics of water by orders of magnitude due to the formation of strong H2O-OH bonds. By lowering the activation barrier for water dissociation on Cu(111) by pre-adsorbing atomic O, generating adsorbed OH, similar results to those on Cu(110) are obtained.

The Journal of Physical Chemistry C, 2007

We report the first measurements on the quantitative partitioning of water between its molecular ... more We report the first measurements on the quantitative partitioning of water between its molecular and dissociated forms at a gas-metal interface under elevated water pressures and temperatures. By means of synchrotron-based in-situ photoelectron spectroscopy, mixed H 2 O and OH phases on Cu(110) at H 2 O pressures up to 1 Torr in the 275-520 K temperature range are studied. In increasing order of stability three phases with H 2 O:OH ratios of 2:1, 1:1 and 0:1 were observed. It was found that surprisingly large quantities of molecular water are present on the surface up to 428 K in 1 Torr H 2 O. A detailed comparison with previous ultra-high vacuum (UHV) studies shows that the observed species, phases and chemical kinetics under UHV compare very well with our results at elevated pressures and temperatures. The stability of the hydrogen-bonded H 2 O-OH complex at the surface, and its influence on the adsorption-desorption and dissociation kinetics, constitutes the essential link between our results and those obtained under UHV conditions.

Probing the coverage and chemical speciation of molecules at surfaces are of fundamental interest... more Probing the coverage and chemical speciation of molecules at surfaces are of fundamental interest in molecular environmental science. The concentration of water and its dissociation fragments at surfaces affect many highly important interfacial chemical processes and there exist no previous quantitative determinations of the coverage of water on clean metal surfaces at near ambient conditions. We have utilized Ambient Pressure Photoelectron Spectroscopy (AP-PES) to study the water/Cu(111) and Cu(110) systems at pressures up to 1 Torr in the temperature range 270-470 K.

Physical Review B, 2009

We demonstrate the sensitivity of x-ray absorption spectroscopy to hydrogen bonding using as expe... more We demonstrate the sensitivity of x-ray absorption spectroscopy to hydrogen bonding using as experimental model system water on Ru͑0001͒. We stepwise go from fully broken to complete H-bond network by varying the morphology from isolated monomers via two-dimensional clusters to a saturated monolayer as probed by scanning tunneling microscopy. The sensitivity of x-ray absorption to the symmetry of H bonding is further elucidated for the amino ͑-NH 2 ͒ group in glycine adsorbed on Cu͑110͒ where the E vector is parallel either to the NH donating an H bond or to the non-H-bonded NH. We show that the pre-edge in the x-ray absorption spectrum is associated with an asymmetric hydrogen-bonding situation while the postedge is directly associated with hydrogen bond formation. The results give further evidence for the much debated interpretation of the various spectral features of liquid water and demonstrate the general applicability of x-ray absorption spectroscopy to analyze H-bonded systems.

The Journal of Chemical Physics, 2010

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The Journal of Chemical Physics, 2008

Scanning tunneling microscopy (STM) and x-ray absorption spectroscopy (XAS) have been used to stu... more Scanning tunneling microscopy (STM) and x-ray absorption spectroscopy (XAS) have been used to study the structures produced by water on Ru(0001) at temperatures above 140 K. It was found that while undissociated water layers are metastable below 140 K, heating above this temperature produces drastic transformations whereby a fraction of the water molecules partially dissociate and form mixed H 2 O-OH structures. XPS and XAS revealed the presence of hydroxyl groups with their O-H bond essentially parallel to the surface. STM images show that the mixed H 2 O-OH structures consist of long narrow stripes aligned with the three crystallographic directions perpendicular to the closepacked atomic rows of the Ru(0001) substrate. The internal structure of the stripes is a honeycomb network of H-bonded water and hydroxyl species. We found that the metastable low temperature molecular phase can also be converted to a mixed H 2 O-OH phase through excitation by the tunneling electrons when their energy is 0.5 eV or higher above the Fermi level. Structural models based on the STM images were used for Density Functional Theory optimizations of the stripe geometry. The optimized geometry was then utilized to calculate STM images for comparison with the experiment.

We present an x-ray absorption spectroscopy results for fully broken to a complete Hbond network ... more We present an x-ray absorption spectroscopy results for fully broken to a complete Hbond network of water molecules on Ru(0001) by varying the morphology from isolated water molecules via two-dimensional clusters to a fully covered monolayer as probed by scanning tunneling microscopy. The sensitivity of x-ray absorption to the symmetry of H-bonding is further elucidated for the amino (-NH 2) group in glycine adsorbed on Cu(110) where the E-vector is parallel either to the NH donating an H-bond or to the non-H-bonded NH. The results give further evidence for the interpretation of the various spectral features of liquid water and for the general applicability of x-ray absorption spectroscopy to analyze H-bonded systems.

The Journal of Physical Chemistry C, 2017

The carbon formation causing deactivation during CO methanation was studied for a Ni/Al 2 O 3 cat... more The carbon formation causing deactivation during CO methanation was studied for a Ni/Al 2 O 3 catalyst. Sulfur-free methanation at low temperature (573 K) for various lengths of time was followed by temperature programmed hydrogenation (TPH) providing information on carbon types involved in the deactivation of the catalyst. Three main carbon hydrogenation peaks were evident from TPHs following methanation: ∼460 K, ∼650 K, and ∼775 K. It is suggested that the ∼460 K TPH peak was composed of two peaks: a surface carbide peak at 445-460 K, and a peak due to carbon dissolved into the nickel at 485 K based on CO and CH 4 adsorption measurements and XRD analysis. The 650 K and 775 K temperature peaks are assigned to polymerized carbon structures and the ∼775 K peak was found to be the primary cause of deactivation as judged by a linear correlation between its amount and the degree of catalyst deactivation. The longer the duration of the methanation test, the more carbon was built up on the Ni surfaces and the highest observed amount was quantified to be as much as eight carbon atoms per Ni surface atom (8 C/Ni surf), which would roughly correspond to an average coverage of four monolayers of graphene. From H 2 desorption measurements after reaction the 650 K TPH peak carbon structure is proposed to be partially hydrogenated, possibly resembling polycyclic aromatic-like carbon. The 775 K peak carbon species are likely more graphene-like. Results indicate that although carbon deposition nucleation may be initiated at the most active methanation sites, i.e. the Ni step sites, subsequent growth takes place over Ni terrace sites. A strongly inhomogeneous carbon growth distribution over the Ni nanoparticle surfaces could also account for our findings. Similar to suggestions regarding catalyst deactivation in Fischer-Tropsch synthesis, a surface CH* coupling mechanism is likely taking place and our results suggest these polymeric hydrocarbon species become more ordered, aromatic and eventually graphene-like over time. 2

Journal of the American Chemical Society, 2009

Counterintuitive to expectations and all prior observations of adsorbate-induced surface segregat... more Counterintuitive to expectations and all prior observations of adsorbate-induced surface segregation of the more reactive alloy component (the one forming the stronger bond with the adsorbate), we show that CO adsorption at elevated pressures and temperatures pulls the less reactive Cu to the surface of a CuPt near-surface alloy. The Cu surface segregation is driven by the formation of a stable self-organized CO/CuPt surface alloy structure and is rationalized in terms of the radically stronger Pt-CO bond when Cu is present in the first surface layer of Pt. The results, which are expected to apply to a range of coinage (Cu, Ag)/Pt-group bimetallic surface alloys, open up new possibilities in selective and dynamical engineering of alloy surfaces for catalysis.

Energy & Fuels, 2020

Biomass gasification is a sustainable way to convert biomass residues into valuable fuels and che... more Biomass gasification is a sustainable way to convert biomass residues into valuable fuels and chemicals via syngas production. However, several gas impurities need to be removed before the final synthesis. Understanding of the interactions and effects of biomass-derived producer gas contaminants (S and K) on the performance of reforming catalysts is of great importance when it comes to process reliability and development. In the present study, the steam reforming activity at 800°C of a sulfurequilibrated nickel catalyst during controlled exposure to alkali species (∼2 ppmv K) and in its absence was investigated using real producer gas from a 5 kW th O 2-blown fluidized-bed gasifier. Conversions of CH 4 , C 2 H 4 , and C 10 H 8 were used to evaluate the performance of the Ni/MgAl 2 O 4 catalyst and MgAl 2 O 4 support. A significant and positive effect on the catalyst activity is observed with addition of gas-phase KCl. This is assigned primarily to the observed K-induced reduction in sulfur coverage (θ S) on Nian effect which is reversible. The catalytic contribution of the K-modified pure MgAl 2 O 4 support was found to be significant in the conversion of naphthalene but not for light hydrocarbons. The product and catalyst analyses provided evidence to elucidate the preferential adsorption site for S and K on the catalyst as well as the role of the support. Whereas S, as expected, was found to preferentially adsorb on the surface of Ni particles, forming S-Ni sites, K was found to preferentially adsorb on the MgAl 2 O 4 support. A low but still significant K adsorption on S−Ni sites, or an effect on only the fraction of exposed Ni surface area near the metal− support interface, can, however, not be excluded. The result suggests that an improved Ni/MgAl 2 O 4 catalyst activity and an essentially carbon-free operation can be achieved in the presence of controlled amount of gas-phase potassium and high sulfur coverages on Ni. Based on the results, a mechanism of the possible K−S interactions is proposed.

At low coverage of water on Cu(110), substrate-mediated electrostatics lead to zigzagging chains ... more At low coverage of water on Cu(110), substrate-mediated electrostatics lead to zigzagging chains along [001] as observed with STM [T. Yamada, S. Tamamori, H. Okuyama, and T. Aruga, "Anisotropic water chain growth on Cu(110) observed with scanning tunneling microscopy" Phys. Rev. Lett. 96, 036105 ]. Using x-ray absorption spectroscopy we find an anomalous lowenergy resonance at ∼533.1 eV which, based on density functional theory spectrum simulations, we assign to an unexpected configuration of water units whose uncoordinated O-H bonds directly face those of their neighbors; this interaction repeats over trough sites with enhanced electron density and is analogous to the case of a hydrated electron. © 2013 AIP Publishing LLC.

The Journal of Physical Chemistry C, Jun 10, 2010

We combine low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), X-ray ... more We combine low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), and Auger electron spectroscopy (AES) with density functional theory (DFT) to reveal the structure and bonding of water-hydroxyl mixed layers adsorbed on Pt(111). We find that the stable water-hydroxyl adlayer forms a mixed phase of nearly coplanar hexamer structures resulting in ( 3 × 3)R30°and (3 × 3) unit cells, respectively. In the asymmetric (3 × 3) structure the lateral O-O distances alternate between long and short bond lengths similar to the chemical bonding network for OHions in solution. The chemical driving force behind this similarity is discussed in a molecular orbital picture.

Chem Phys Lett, 2006

We investigated the structure of the water monolayer on an open surface, Cu(1 1 0), at low temper... more 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.

Chemical Physics Letters, 2006

Surface Science, Jul 1, 2005

An X-ray photoelectron spectroscopy (XPS) study was undertaken of the water/Cu(1 1 0)-system find... more An X-ray photoelectron spectroscopy (XPS) study was undertaken of the water/Cu(1 1 0)-system finding non-dissociative adsorption on clean Cu(1 1 0) at temperatures below 150 K. Thermally induced dissociation of D 2 O is observed to occur above 150 K, similar to the H 2 O/Ru(0 0 1) system, with an experimentally derived activation barrier of 0.53-0.56 eV which is very close in magnitude to the derived activation barrier for desorption of 0.50-0.53 eV. X-ray and electron induced damage to the water overlayer was quantified and used to rationalize the results of a recent XPS study of the water/Cu(1 1 0)-system where partial dissociation was observed already at 90 K.