| 1. |
- Nilsson, Sara, 1990, et al.
(författare)
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Probing the role of grain boundaries in single Cu nanoparticle oxidation by in situ plasmonic scattering
- 2022
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Ingår i: Physical Review Materials. - 2475-9953. ; 6:4
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Tidskriftsartikel (refereegranskat)abstract
- Grain boundaries determine physical properties of bulk materials including ductility, diffusivity, and electrical conductivity. However, the role of grain boundaries in nanostructures and nanoparticles is much less understood, despite the wide application of nanoparticles in nanophotonics, nanoelectronics, and heterogeneous catalysis. Here, we investigate the role of high-angle grain boundaries in the oxidation of Cu nanoparticles, using a combination of in situ single particle plasmonic nanoimaging and postmortem transmission electron microscopy image analysis, together with ab initio and classical electromagnetic calculations. We find an initial growth of a 5-nm-thick Cu2O shell on all nanoparticles, irrespective of different grain morphologies. This insensitivity of the Cu2O shell on the grain morphology is rationalized by extraction of Cu atoms from the metal lattice being the rate limiting step, as proposed by density functional theory calculations. Furthermore, we find that the change in optical scattering intensity measured from the individual particles can be deconvoluted into one contribution from the oxide layer growth and one contribution that is directly proportional to the grain boundary density. The latter contribution signals accumulation of Cu vacancies at the grain boundaries, which, as corroborated by calculations of the optical scattering, leads to increased absorption losses and thus a decrease of the scattering, thereby manifesting the role of grain boundaries as vacancy sinks and nuclei for Kirkendall void formation at a later stage of the oxidation process.
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| 2. |
- Luque-Ceballos, Jonathan C., et al.
(författare)
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A theoretical study on the geometry and spectroscopic properties of ground-state and local minima isomers of (CuS)n=2-6 clusters
- 2018
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Ingår i: Physica E: Low-Dimensional Systems and Nanostructures. - : Elsevier BV. - 1386-9477. ; 97, s. 1-7
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Tidskriftsartikel (refereegranskat)abstract
- Spectroscopic properties of gas-phase copper sulfide clusters (CuS) n (n = 2–6) are calculated using Density Functional Theory (DFT) and time-dependent (TD) DFT approaches. The energy landscape of the potential energy surface is explored through a basin-hopping DFT methodology. Ground-state and low-lying isomer structures are obtained. The global search was performed at the B3PW91/SDD level of theory. Normal modes are calculated to validate the existence of optimal cluster structures. Energetic properties are obtained for the ground-state and isomer clusters and their relative energies are evaluated for probing isomerization. This is a few tenths of an eV, except for (CuS) 2 cluster, which presents energy differences of ∼1 eV. Notable differences in the infrared spectra exist between the ground-state and first isomer structures, even for the (CuS) 5 cluster, which has in both configurations a core copper pyramid. TDDFT provides the simulated absorption spectrum, presenting a theoretical description of optical absorption bands in terms of electronic excitations in the UV and visible regions. Results exhibit a significant dependence of the calculated UV/vis spectra on clusters size and shape regarding the ground state structures. Optical absorption is strong in the UV region, and weak or forbidden in the visible region of the spectrum.
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| 3. |
- Sanders-Gutierrez, Oscar Alan, et al.
(författare)
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Molecular dynamics and DFT study of 38-atom coinage metal clusters
- 2022
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Ingår i: Computational Materials Science. - : Elsevier BV. - 0927-0256. ; 201
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Tidskriftsartikel (refereegranskat)abstract
- The thermal behavior of 38-atom mono-, bi-, and trimetallic clusters consisting of Cu, Ag, and Au atoms, is analyzed employing molecular dynamics simulations and DFT calculations for selected cluster compositions. Low-energy structures were singled out to perform NVT molecular dynamics simulations at several temperatures, using the Andersen thermostat for temperature control. The caloric curve is used to estimate the melting temperature and the specific heat. The pair distribution function g(r) of the solid and liquid-phase clusters is examined at different temperatures. When comparing the estimated melting points (Tm) among the monatomic clusters, the order becomes TmCu38>TmAg38>TmAu38. For bimetallic clusters, an increase of Tm is observed for Cu-Au compared to their monatomic counterparts, while the opposite occurs for Cu-Ag clusters. For trimetallic clusters, two low-energy isomers of the Cu36Ag1Au1 cluster are investigated. In this case, Tm is estimated to be 475 K, for the two isomers with the lowest-energy and second-to-lowest energy, respectively. For all the clusters studied, the pair distribution function g(r) shows that the first peak position is not shifted as an effect of temperature and its maximum value varies with composition, while the second peak essentially vanishes upon melting. The common-neighbor analysis (CNA) technique is used to analyze the local structural changes for the trimetallic clusters, again demonstrating a clear structural change upon melting. The HOMO-LUMO energy gap indicates that the trimetallic isomers' behavior is metallic, while the average binding energy show these clusters' energetic stability to be similar.
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| 4. |
- Gericke, Sabrina M., et al.
(författare)
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Effect of Different In 2 O 3 (111) Surface Terminations on CO 2 Adsorption
- 2023
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Ingår i: ACS Applied Materials & Interfaces. - 1944-8252 .- 1944-8244. ; 15:38, s. 45367-45377
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Tidskriftsartikel (refereegranskat)abstract
- In2O3-based catalysts have shown high activity and selectivity for CO2 hydrogenation to methanol; however, the origin of the high performance of In2O3 is still unclear. To elucidate the initial steps of CO2 hydrogenation over In2O3, we have combined X-ray photoelectron spectroscopy and density functional theory calculations to study the adsorption of CO2 on the In2O3(111) crystalline surface with different terminations, namely, the stoichiometric, reduced, and hydroxylated surface. The combined approach confirms that the reduction of the surface results in the formation of In adatoms and that water dissociates on the surface at room temperature. A comparison of the experimental spectra and the computed core-level shifts (using methanol and formic acid as benchmark molecules) suggests that CO2 adsorbs as a carbonate on all three surface terminations. We find that the adsorption of CO2 is hindered by hydroxyl groups on the hydroxylated surface.
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| 5. |
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| 6. |
- Hagman, Benjamin, et al.
(författare)
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Steps Control the Dissociation of CO2 on Cu(100)
- 2018
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 1520-5126 .- 0002-7863. ; 140:40, s. 12974-12979
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Tidskriftsartikel (refereegranskat)abstract
- CO2 reduction reactions, which provide one route to limit the emission of this greenhouse gas, are commonly performed over Cu-based catalysts. Here, we use ambient pressure X-ray photoelectron spectroscopy together with density functional theory to obtain an atomistic understanding of the dissociative adsorption of CO2 on Cu(100). We find that the process is dominated by the presence of steps, which promote both a lowering of the dissociation barrier and an efficient separation between adsorbed O and CO, reducing the probability for recombination. The identification of steps as sites for efficient CO2 dissociation provides an understanding that can be used in the design of future CO2 reduction catalysts.
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| 7. |
- Kauppinen, Minttu Maria, 1991, et al.
(författare)
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Methanol Synthesis Over PdIn, In2O3, and CuZn From First-Principles Microkinetics: Similarities and Differences
- 2022
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 126:36, s. 15235-15246
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Tidskriftsartikel (refereegranskat)abstract
- Methanol synthesis via catalytic CO2 hydrogenation is an important reaction where a valuable fuel and chemical is produced from a greenhouse gas. In2O3- and Pd-promoted In2O3 have experimentally shown promising activity and selectivity, although the nature of the active sites remains under debate. In this study, the kinetic behavior of potential active sites in Pd-promoted In2O3 toward methanol synthesis and the competing reverse water-gas shift reaction is assessed by exploring pristine In2O3 and a PdIn intermetallic phase by using first-principles mean-field microkinetics. The PdIn intermetallic phase is modeled with PdIn(310) and In2O3 with In2O3(110). The results are compared to Zn-decorated Cu(211), representing the commercial Cu/ZnO-based catalyst. PdIn shows better performance than both the unpromoted In2O3 and Zn-decorated Cu at conditions relevant to the industrial process. For all three systems we find that stabilization of adsorbed hydrogen enhances activity toward methanol, which provides insights for further catalyst development.
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| 8. |
- Posada Borbon, Alvaro, 1990, et al.
(författare)
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A First-Principles-Based Microkinetic Study of CO 2 Reduction to CH 3 OH over In 2 O 3 (110)
- 2021
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Ingår i: ACS Catalysis. - : American Chemical Society (ACS). - 2155-5435. ; 11:15, s. 9996-10006
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Tidskriftsartikel (refereegranskat)abstract
- Methanol synthesis from catalytic recycling of CO2 is one viable route to produce fuel and a stock chemical from a greenhouse gas. Herein, hydrogenation of CO2 to CH3OH is investigated with density functional theory calculations combined with mean-field microkinetic modeling. The model explores the direct route for CO2 hydrogenation (HCOOH route) and the competing reverse water-gas shift (RWGS) reaction. The predicted temperature dependence of turnover frequencies, selectivities, and reaction orders are in good agreement with previous experimental results. The formation of methanol at relevant reaction temperatures (470-670 K) is found to be kinetically controlled by H2COOH dissociation to H2CO + OH, whereas the RWGS reaction is solely controlled by CO2 hydrogenation to COOH. An analysis of the kinetic behavior reveals that the stabilization of hydrogen adsorption should be one way to improve the catalyst performance.
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| 9. |
- Posada Borbon, Alvaro, 1990, et al.
(författare)
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Cluster Size Effects in Ethylene Hydrogenation over Palladium
- 2017
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 121:20, s. 10870-10875
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Tidskriftsartikel (refereegranskat)abstract
- Density functional theory calculations are used to investigate ethylene hydrogenation over palladium clusters in the range from Pd-13 to Pd-116. A comparison is made to single crystal surfaces, which exemplifies several novel aspects of clusters. We find that the adsorption energies are always higher for the clusters, even if the comparison is made to adsorption on stepped surfaces in the low-coverage regime. Clusters are found to accommodate higher adsorbate coverages than extended surfaces. The saturation coverage for Pd-13 is unity, whereas it is 0.33 on Pd(111). The activation energies for hydrogenation of C2H4 to C2H5 over Pd-38 are clearly different from Pd(111) and Pd(211), which stresses the limitation of extended surfaces as models for nanoparticles.
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| 10. |
- Posada Borbon, Alvaro, 1990
(författare)
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CO2 activation for methanol synthesis on copper and indium oxide surfaces
- 2019
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Catalytic recycling of CO2 to added-value chemicals, such as methanol (CH3OH), has been proposed as a possible way for sustainable production of fuel and chemicals, in addition to providing a route to mitigate climate change. Multiple systems are known to be active for the conversion of CO2 to methanol, and the state of the art catalyst is Cu/ZnO/Al2O3. This catalyst is, however, known to deactivate rapidly. Moreover, there is no scientific consensus on either the active phase or the reaction mechanism. In response to this, the search for a longer-lasting catalysts for methanol-synthesis has been intense. In recent years, an In2O3/ZrO2 catalyst has attracted much attention, thanks to its high selectivity, activity and durability. In this thesis, we investigate the surface active phase and its effect on CO2 adsorption on Cu(100) and In2O3(110) with the use of density functional theory (DFT) calculations and ab-initio thermodynamics. Our results are compared to ambient pressure X-ray photoelectron emission spectroscopy (XPS) experiments. CO2 adsorption is the initial step in the reduction process. Hence, understanding of the active catalyst phase, and its effect on the adsorption process, is the first step for the rationalization of the catalytic processes on these systems. Simultaneously, understanding the electronic structure that allows for the high activity, might aid the rational design of better catalysts for CO2 activation. Our results show that Cu(100) oxidizes from the pristine surface to a p(2×2) overlayer at 0.25 ML followed by a reconstruction to a (2√2×√2)R45 (MR) structure at 0.50 ML. Moreover, dissociative adsorption of CO2 on Cu(100) occurs predominantly at surface steps. In2O3(110) is found to heavily hydroxylate in presence of H2 and/or H2O. Hydroxylation with H2 causes the undercoordinated In-sites to change oxidation state (from In 3+ to In 2+), while H2O does not. We suggest that the redox capacity of the undercoordinated In-site are responsible for the adsorption of CO2 on indium oxide, whereas oxygen vacancies act as spectators. Our results are in qualitative agreement with the experimental observation of heavy hydroxylation and the suppression of the reverse water gas shift on indium oxide
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| 11. |
- Posada Borbon, Alvaro, 1990, et al.
(författare)
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CO2 adsorption on hydroxylated In2O3(110)
- 2019
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Ingår i: Physical Chemistry Chemical Physics. - : Royal Society of Chemistry (RSC). - 1463-9084 .- 1463-9076. ; 21:39, s. 21698-21708
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Tidskriftsartikel (refereegranskat)abstract
- Catalytic synthesis of methanol from CO2 is one route to produce added-value chemicals from a greenhouse gas. Here, density functional theory calculations and ab initio thermodynamics are used to study CO2 adsorption on In2O3(110) in the presence of H2 and H2O. We find that the surface is heavily hydroxylated by either H2 or H2O and that hydroxylation promotes H2-induced vacancy formation. Moreover, CO2 adsorbs rather in a CO2- configuration on hydroxylated In2O3(110) than on oxygen vacancy sites. The results suggest that hydroxylation-induced oxidation-state changes of In-ions play a significant role in CO2 adsorption and activation during methanol synthesis.
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| 12. |
- Posada Borbon, Alvaro, 1990
(författare)
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First principles studies of CO2 activation and reduction over indium oxide and copper surfaces
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Catalytic recycling of carbon dioxide (CO2) to added-value chemicals, such as methanol (CH3OH), have been proposed as a possible path for sustainable production of fuel and chemicals, in addition to providing a route to mitigate anthropogenic carbon emissions. Several catalytic systems are known to be active for conversion of CO2 to methanol, Cu/ZnO/Al2O3 being the main industrial catalyst for the process. This catalyst is, however, known to deactivate over time due to copper sintering. In recent years an alternative In2O3/ZrO2 catalyst has attracted attention, thanks to its reported high selectivity, activity and durability. In this thesis, the activation and reduction of CO2 over Cu(100) and In2O3(110) are investigated from first principles-based calculations and simulations. Reaction intermediates and thermodynamic calculation of surface energy, coupled with theoretical X-ray photoelectron spectroscopy and mean-field microkinetic modeling, are utilized to describe and rationalize surface conditions and reaction mechanisms for the dissociative adsorption of CO2 on Cu(100) and for its reduction to CH3OH on In2O3(110). The oxidation process of Cu(100) by dissociative CO2 adsorption is found to be controlled by step sites. The role of the step is found to be two-fold, lowering the dissociation energy and simultaneously providing physical separation of the products. Upon reaction, the surface is found to oxidize from the pristine to a disordered p(2×2) oxygen overlayer to a reconstructed (2√2×√2)R45-missing row structure. Dissociative adsorption of H2 is investigated on In2O3(110) and In2O3(111). The adsorption is found to be facile, and both surfaces are predicted to be hydroxylated at typical methanol synthesis reaction conditions. CO2 reduction to CH3OH on the hydrogen covered In2O3(110) is investigated along a formate (HCOO) mediated mechanism, where the rate controlling step is found to be formation of H2CO+OH. The role of the competing Reverse Water Gas Shift reaction is also evaluated. The presented findings exemplify the significance of describing catalytic systems under thermodynamically relevant reaction conditions. Additionally, the results provide some understanding and insight on the mechanistic aspects of CO2 activation and reduction to added-value chemicals.
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| 13. |
- Posada Borbon, Alvaro, 1990, et al.
(författare)
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Hydrogen adsorption on In2O3(111) and In2O3(110)
- 2020
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Ingår i: Physical Chemistry Chemical Physics. - : Royal Society of Chemistry (RSC). - 1463-9084 .- 1463-9076. ; 22:28, s. 16193-16202
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Tidskriftsartikel (refereegranskat)abstract
- In2O3-Based catalysts have been measured to have a high activity for CO2 hydrogenation to H3COH. Here, we use density functional theory calculations with and without Hubbard-U corrections in combination with ab initio thermodynamics to investigate the dissociative adsorption of H2 over In2O3(111) and In2O3(110). H2 is found to dissociate heterolytically with a moderate barrier on both facets. Diffusion of hydrogen leads to the preferred homolytic adsorption configuration. Vacancy formation by water formation is thermodynamically preferred at high hydrogen coverages. Both surfaces are found to be hydroxylated at typical reaction conditions with the highest coverage predicted for In2O3(110). O 1s core level shifts are calculated for different coverages. The hydroxylated surfaces show two distinct shifts corresponding to different types of OH-groups. The presence of surface oxygen vacancies is not visible in the O 1s signatures. The results show that hydroxylation of the surfaces results in changes of the oxidation state of In-ions, which suggests that the redox properties on In2O3 are important for catalytic reduction of CO2 to added value chemicals.
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| 14. |
- Posada Borbon, Alvaro, 1990, et al.
(författare)
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Initial oxidation of Cu(100) studied by X-ray photo-electron spectroscopy and density functional theory calculations
- 2018
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Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 675, s. 64-69
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Tidskriftsartikel (refereegranskat)abstract
- Density functional theory calculations and ambient pressure X-ray photoelectron spectroscopy are used to investigate initial oxidation of Cu(100). Surface stability with respect to oxygen coverage is calculated together with O 1s core level shifts. Oxidation of Cu(100) is found to occur via the formation of a p(2 × 2) overlayer (0.25 ML) followed by a reconstructed r2×r2 R45° -O missing-row (MR) structure (0.50 ML). A c(4 × 6) structure with a 0.3 ML coverage is close in stability for intermediate oxygen chemical potentials. The relative stability is found to be weakly dependent on the applied exchange-correlation functional. The calculated shifts in the O 1s binding energy are in good agreement with the measured evolution of the binding energy. The shift to higher O 1s binding energies with increasing oxygen coverage is found to correlate with the charge on neighboring copper atoms. The O 1s core-level shifts here obtained with CO2 as oxidant, are similar to previous measurements of Cu(100) oxidation with O2.
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| 15. |
- Posada Borbon, Alvaro, 1990, et al.
(författare)
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On the signatures of oxygen vacancies in O1s core level shifts
- 2021
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Ingår i: Surface Science. - : Elsevier BV. - 0039-6028. ; 705
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Tidskriftsartikel (refereegranskat)abstract
- Density functional theory calculations are used to investigate O1s surface core level shifts for MgO(100), ZnO(101¯0), In2O3(111) and CeO2(111). Shifts are calculated for the pristine surfaces together with surfaces containing oxygen vacancies and dissociated H2. Pristine surfaces show small negative shifts with respect to the bulk components and vacancies are found to have a minor effect on the O1s binding energies of neighboring oxygen atoms. OH-groups formed by H2 dissociation yield binding energies shifted to higher energies as compared to the oxygen atoms in the bulk. The results stress the difficulties in assigning core-level shifts and suggest that assignments of shifts in O1s binding energies to neighboring oxygen vacancies for the explored oxides should be reconsidered.
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| 16. |
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| 17. |
- Tal, Alexey A., et al.
(författare)
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Correlation between Ethylene Adsorption Energies and Core-Level Shifts for Pd Nanoclusters
- 2019
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 123:4, s. 2544-2548
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Tidskriftsartikel (refereegranskat)abstract
- Density functional theory calculations have been used to investigate the adsorption of ethylene on Pd nanoclusters together with shifts in core-level binding energies of Pd atoms bonded to the adsorbate. The adsorption energy is found to correlate with the core-level shifts (CLS), which is consistent with the notion that the core-level binding energy is a measure of differences in cohesion. The correlation between adsorption energies and core-level shifts is found to be stronger than the correlation between adsorption energies and generalized coordination numbers, indicating that descriptors preferably should account for electronic effects explicitly. The advantages of CLS as a descriptor for the screening of adsorption properties is discussed.
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