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- Ammothum Kandy, Akshay Krishna
(författare)
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Linear models for multiscale materials simulations : Towards a seamless linking of electronic and atomistic models for complex metal oxides
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Multiscale modelling approaches, connecting data from electronic structure calculations all the way towards engineering continuum models, have become an important ingredient in modern materials science. Materials modelling in a broader sense is already amply used to address complex chemical problems in academic science, but also in many industrial sectors. As far as multiscale modelling is concerned, however, many challenges remain, in particular when it comes to coupling and linking the various levels along the multiscale ladder in a seamless and efficient fashion. This thesis focusses on the development of new and efficient linear models to improve the quality and parameterisation processes of the two-body potentials used in empirical and semi-empirical methods within a multiscale materials modelling framework. In this regard, a machinery called curvature constrained splines (CCS) based on cubic splines to approximate general two-body potentials has been developed. The method is linear, and parameters can be easily solved in a least-square sense using a quadratic programming approach. Moreover, the objective function is convex, implying that global minima can be readily found. This makes the optimisation process easy to handle and requires little to no human effort. Initial tests to validate the method were performed on molecular and bulk neon systems. Later, the method was extended to incorporate long-range interactions by including atomic charges. The capability of the method was demonstrated for ZnO polymorphs, and at the same time benchmarked towards the conventional Buckingham potentials applied to the same problem. The results indicate that the CCS+Q method performs on par with the Buckingham approach, but is much faster and easier to parameterise. The merits of the method is further demonstrated with an exploration of size and shape dependent stability of CeO2 nanoparticles.Having established the framework of the CCS methodology, the method was further used to develop repulsive potentials for the semi-empirical self-consistent charge density functional tight binding (SCC-DFTB) method. The generation of the repulsive potentials is normally a tedious and time-consuming task. The CCS methodology makes this process significantly more efficient, and further provides new opportunities to explore the limits of the SCC-DFTB method. The development of repulsive potentials for bulk Si polymorphs showed that it is possible to retrieve a good description of each individual polymorph, but impossible to obtain an acceptable joint description of all polymorphs. The results indicated that a transferable repulsive potential needs to have coordination dependence, and by the use of a many-body artificial neural network representation for the repulsive potential, it was indeed possible to obtain a global transferability. The CCS methodology was finally used to model a system of considerable chemical diversity and complexity, namely reduced CeO2 within the SCC-DFTB formalism. Here, the CCS framework facilitated the development of an efficient workflow that yielded a harmonized description of Ce ions in different oxidation states. In short, the introduced CCS-based workflow proved to extend the applicability of SCC-DFTB to complex oxide systems with correlated electronic states. To conclude, the CCS methodology is demonstrated to be a versatile tool for efficient linking between (and within) electronic and atomistic models.
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| 3. |
- Ammothum Kandy, Akshay Krishna, et al.
(författare)
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Curvature Constrained Splines for DFTB Repulsive Potential Parametrization
- 2021
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Ingår i: Journal of Chemical Theory and Computation. - : American Chemical Society (ACS). - 1549-9618 .- 1549-9626. ; 17:3, s. 1771-1781
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Tidskriftsartikel (refereegranskat)abstract
- The Curvature Constrained Splines (CCS) methodology has been used for fitting repulsive potentials to be used in SCC-DFTB calculations. The benefit of using CCS is that the actual fitting of the repulsive potential is performed through quadratic programming on a convex objective function. This guarantees a unique (for strictly convex) and optimum two-body repulsive potential in a single shot, thereby making the parametrization process robust, and with minimal human effort. Furthermore, the constraints in CCS give the user control to tune the shape of the repulsive potential based on prior knowledge about the system in question. Herein, we developed the method further with new constraints and the capability to handle sparse data. We used the method to generate accurate repulsive potentials for bulk Si polymorphs and demonstrate that for a given Slater-Koster table, which reproduces the experimental band structure for bulk Si in its ground state, we are unable to find one single two-body repulsive potential that can accurately describe the various bulk polymorphs of silicon in our training set. We further demonstrate that to increase transferability, the repulsive potential needs to be adjusted to account for changes in the chemical environment, here expressed in the form of a coordination number. By training a near-sighted Atomistic Neural Network potential, which includes many-body effects but still essentially within the first-neighbor shell, we can obtain full transferability for SCC-DFTB in terms of describing the energetics of different Si polymorphs.
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| 6. |
- Duchon, Tomas, et al.
(författare)
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Establishing structure-sensitivity of ceria reducibility : real-time observations of surface hydrogen interactions
- 2020
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Ingår i: Journal of Materials Chemistry A. - : ROYAL SOC CHEMISTRY. - 2050-7488 .- 2050-7496. ; 8:11, s. 5501-5507
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Tidskriftsartikel (refereegranskat)abstract
- The first Layer of atoms on an oxide cataLyst provides the first sites for adsorption of reactants and the Last sites before products or oxygen are desorbed. We employ a unique combination of morphological, structural, and chemical analyses of a model ceria cataLyst with different surface terminations under an H2 environment to unequivocally establish the effect of the Last Layer of atoms on surface reduction. (111) and (100) terminated epitaxiaL isLands of ceria are simultaneously studied in situ allowing for a direct investigation of the structure reducibility relationship under identical conditions. Kinetic rate constants of Ce4+ to Ce3+ transformation and equilibrium concentrations are extracted for both surface terminations. Unlike the kinetic rate constants, which are practically the same for both types of isLands, more pronounced oxygen release, and overall higher reducibility were observed for (100) isLands compared to (111) ones. The findings are in agreement with coordination -Limited oxygen vacancy formation energies calculated by density functional theory. The results point out the important aspect of surface terminations in redox processes, with particular impact on the catalytic reactions of a variety of catalysts.
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| 7. |
- Kullgren, Jolla, 1978-, et al.
(författare)
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SCC-DFTB simulations of ceria surfaces and nanoparticles
- 2017
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- First principles modelling, using e.g. the density functional theory (DFT), has become a valuable tool in materials research. However, today’s computer resources limit the size and time scales that can be studied with such techniques, thereby hindering the full utilization of computational chemistry for large-scale systems in practice. Thus, new developments of reliable approximate and/or parameterized methods are needed.One promising approximate method, conceptually similar to the DFT, is the self-consistent charge density functional based tight binding method (SCC-DFTB). SCC-DFTB calculations are parameterized against DFT data (see illustration in Figure 1) and are at least two orders of magnitude faster than a standard semi-local DFT calculation. However, to obtain an accuracy comparable to DFT for complex oxides is a task that has proven to be a challenge.In this talk, I will present our SCC-DFTB parameterization effort for the technologically important reducible oxide CeO2.1 I will discuss the strategy we have developed for the parameterization and the special complication that follows with reducible oxides. Furthermore, I will demonstrate the applicability of the generated parameters and show results from validation by comparing to data obtained from DFT calculations for CeO2. I will show results for oxygen vacancy formation in various ceria structures of different dimensionality, ranging from 0D (nano) to 3D (bulk) and for oxygen adsorption on ceria nanoparticles and preliminary results regarding ceria nanoparticle agglomeration.
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| 8. |
- Langhammer, David, 1991-, et al.
(författare)
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Adsorption and Catalytic Oxidation of NO2 on Anatase TiO2 : Concerted Nitrate Interaction and Photon-Stimulated Reaction
- 2022
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Ingår i: ACS Catalysis. - : American Chemical Society (ACS). - 2155-5435. ; 12:16, s. 10472-10481
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Tidskriftsartikel (refereegranskat)abstract
- The catalytic and photon-induced oxidation of NO2 on anatase TiO2 has been studied and compared with the surface nitrate species obtained after adsorption of HNO3. Using a combination of in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), density functional theory (DFT), and temperature-programmed desorption (TPD), it is shown that identical products are obtained in all reaction systems but that their formation rates differ significantly. The surface reaction products are identified as combinations of surface–NO3– species, where NO2 bonds to the lattice oxygen, and freely adsorbed NO3– ions. These products can be obtained either by dissociative adsorption of HNO3 or by catalytic/photocatalytic oxidation of NO2, which is facilitated by UV light. A concerted reaction mechanism is unraveled that involves reorientation of bidentate nitrate that pushes out a neighboring protonated lattice oxygen to form a surface–NO3– species and a terminal OH group. The thermal stability of these surface species has been studied by means of TPD and simultaneous in situ DRIFTS measurements that reveal a main desorption peak (m/z = 46) at around 430 °C, which is attributed to concerted nitrate desorption through pentoxide (N2O5) formation. A weaker and broader TPD peak is found at about 185 °C and is attributed to desorption of nitrate species bonded in a compressed configuration. The experimental results can be explained by the changing stability of the identified nitrate products, which depends strongly on the surface chemical environment and the surface coverage. The DFT results show that the stabilization of intermediate NO2 adsorbates and the final nitrate reaction products occurs through a bifunctional charge exchange mechanism that is mediated by the TiO2 crystal. In particular, a stable surface–NO3– and NO3– ion pair configuration has been identified. This mechanism explains both the thermal and photoinduced oxidation of NO2 and their thermal stability and different formation rates, yielding high photoinduced oxidation reaction rates. Our results provide insights into the structure and chemical stability of nitrate surface products on TiO2 particles and their formation mechanism, which is important for understanding their catalyzed transformation into the harmful compounds HONO and N2O during continued UV light illumination.
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| 9. |
- Lu, Zhansheng, et al.
(författare)
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Sulfidation of Ceria Surfaces from Sulfur and Sulfur Diffusion
- 2012
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Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 116:15, s. 8417-8425
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Tidskriftsartikel (refereegranskat)abstract
- Even very low levels of sulfur contaminants can degrade the catalytic performance of cerium oxide. Here, the interaction of atomic sulfur with the ceria (111) and (110) surfaces has been studied using first-principles methods. Two sulfoxy species are identified: oxido-sulfate(2-) species (SO2-) on both the CeO2(111) and (110) surfaces and hyposuffite (SO22-) on the (110) surface. Sulfide (S2-) is formed when a surface or a subsurface oxygen atoms is replaced by sulfur. These sulfide species are more stable at the surface. Furthermore, sulfite (SO32-) structures are found when sulfur is made to replace one Ce in the ceria (111) and (110) surfaces. The calculated sulfur diffusion barriers are larger than 1.4 eV for both surfaces, and thus sulfur is essentially immobile, providing a possible explanation for the sulfidation phenomena of the ceria-based catalyst. Thus, we find three different species from interaction of S with ceria which are all, due to their strong binding, capable of poisoning the surface, reduced or unreduced. Our results suggest that under reducing conditions sulfur is likely to be found in the (111) surface (replacing oxygen) but on the (110) surface (as SO22-).
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| 10. |
- Meier de Andrade, Ageo, 1990-
(författare)
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Transition metal and alloy catalysts in the light of computational materials modelling
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis is built around two pillars. One is heterogeneous catalysis in the broader context of green chemistry. The focus here is on identifying catalytically active materials suitable for the valorization of renewable feedstocks. The second pillar deals with materials modelling itself, both its role to help identify the features responsible for certain desired material properties and the assessment of model quality and how to overcome challenges when modelling complex systems.Density functional theory (DFT) has become a standard method in heterogeneous catalysis and materials science, as it generally combines good accuracy with an affordable computational cost. The choice of density functional (in relation to the system under study) strongly affect the accuracy of the DFT results. In this thesis, a subset of functionals have been tested and validated with respect to their ability to predict structural and energetic properties of single- and multi-component materials. It is shown that the inclusion of dispersion corrections by computing the nonlocal correlation self-consistently, as done in the vdW-DF-cx functional, increases the accuracy of computed results in relation to experimental data. In the evaluation of the functionals, the computed properties were rationalized in terms of (i) the reduced density gradient distribution, unique for each material, and (ii) the exchange enhancement factor, unique for each density functional and dependent on the reduced density gradient distribution. Moreover, a tool is presented that can guide researchers towards the most appropriate density functional for the problem in question. This involves a protocol that brings DFT results into better agreement with experiment. Heterogeneous catalysts are complex and catalyst research is often performed using model experiments and calculations. Here descriptors have important roles to play. Two descriptors for catalytic activity have been scrutinized in this thesis. The first is the work function of metal surfaces. Here, it is shown that the adsorption of selected ad-atoms on Ni surfaces provides a route to control the metal's work function over a wide energy range. The second descriptor is the difference in stability between the enol and keto tautomers of a model lignin molecule on a model metal catalyst surface in the context of lignin depolymerization. The aim is to explore the reasons underlying the relative stabilities and to enhance the preference for enol in the keto-to-enol tautomerization. The modelling results show that a mixed PdPt alloy surface stabilizes the enol tautomer, suggesting that this could be an active catalyst for lignin depolymerization.
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