Browsing by Author "Karaca, Kaan"

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Open Access
Geometry optimization with variationally consistent forces using higher-order finite element methods in Kohn-Sham density functional theory calculations
(2021-09) Karaca, Kaan
Variationally consistent atomic forces are computed for Kohn-Sham density func-tional theory (DFT) solved via a higher order ﬁnite element (FEM) framework. Force expressions are derived for pseudopotential and all-electron settings in a uniﬁed structure. Generalized gradient approximations are additionally ad-dressed together with nonlinear core correction in the same pseudopotential set-ting. Classical Lagrange basis functions are used as well as non-uniform rational B-spline (NURBS) basis in isogeometric analysis concept. Calculated forces have been shown to be variationally consistent with energies. Reference force values have been generated through Kohn-Sham DFT software packages and accuracy of forces is veriﬁed. Finally, geometry optimizations have been conducted. For this purpose, several optimization algorithms are tested for their robustness, compu-tational cost and ease of implementation. Fast inertial relaxation engine (FIRE) algorithm is eventually chosen as the optimization algorithm. Variationally con-sistent forces allow conducting geometry optimization even at coarse meshes, ﬁnding the energy minima of any particular setup. Optimized ground state ge-ometries have also been compared with those obtained from reference software packages, showing very close agreement with values reported in literature.
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Türkiye'nin ilk nükleer reaktörü: TR-1
(Bilkent University, 2020) Peker, Dilşad Simay; Kırmacı, Selin; Karaca, Kaan; Lomlu, Rana
Türkiye'nin ilk reaktörü olan TR-1 hakkında olan bu araştırma yazısı TR-1'in açılma kararından başlayarak kuruluş süresini, aktif yıllarındaki çalışmalarını ve kapanış sürecini sosyal, siyasi ve ekonomik olarak detaylı bir şekilde incelemektedir. Bu süreçlerde karşılaşılan problemlere, halkın bakış açısına, kapanış nedenlerine ve Türkiye'nin gelişimine katkısına da değinen bu yazı TR-1'den sonra nükleer alanında yapılan çalışmaları da anlamak için bir ışık tutmaktadır.
Embargo
Unraveling structure-functionality relationships of shape-defined Cu2O nanocrystal model catalysts for methanol decomposition
(2024-08) Karaca, Kaan
Methanol is one of the centerpieces of the chemical industry as a C1 building block and an intermediate producing high-value chemicals such as formaldehyde, methyl methacrylate, methyl tertiary-butyl ether/ tertiary-amylmethylether, and acetic acid. The global demand for methanol is expected to grow exponentially due to its applications in hydrogen production, direct methanol fuel cells, and olefin production via the Methanol to Olefins (MTO) processes. Cu-based catalysts have been widely studied both in academia and in the industry to transform methanol into value-added chemicals at the industrial scale. Some of these academic fundamental research studies have been performed either under ultra-high vacuum (UHV), cryogenic temperature conditions utilizing single-crystal nanocatalysts or under industrially relevant high temperature-pressure conditions utilizing complex mesoporous catalysts resulting in complex data which is challenging to analyze in a conclusive manner to obtain reliable mechanistic information due to the presence of the well-known limitations in heterogenous catalysis called “the materials gap” and “ the pressure gap”. Thus, uniquely defined model catalysts are required to bridge these gaps by offering well-ordered surfaces that can be studied under ambient conditions. This thesis focuses on the structure-functionality relationships of shape-defined Cu2O model catalysts for methanol decomposition. Cubic and octahedral Cu2O nanocrystal catalysts were synthesized and characterized by various ex-situ methods such as Scanning Electron Microscopy (SEM), X-Ray Diffraction (XRD), X-Ray Absorption Near Edge (XANES), Extended X-Ray Absorption Fine Structure (EXAFS), Attenuated Total Reflectance Infrared Spectroscopy (ATR-IR), X-Ray Photoelectron Spectroscopy (XPS) and H2-Temperature Programmed Reduction (H2-TPR). The nature of the surface-active sites were characterized by CO adsorption via in-situ Fourier Transform Infrared Spectroscopy (in-situ FTIR) and the morphology-dependent methanol and formaldehyde decomposition properties were studied via in-situ FTIR and Temperature Programmed Desorption (TPD). The results showed that c-Cu2O and o-Cu2O have distinct structure-functionality relationships for methanol decomposition. It is proposed that the labile surface oxygens that can be readily donated from the c-Cu2O surface can facilitate low-temperature (T ≤ 250 °C) methanol/methoxy oxidation to formates which in turn yield predominantly CO2 and H2O as the total oxidation products. In contrast, limited reducibility of the c-Cu2O surface only allows methanol/methoxy oxidation to first formaldehyde and then to dioxymethylene, eventually yielding predominantly CO and H2 as the thermal decomposition products, indicating the predominance of dehydrogenation catalytic pathways rather than total oxidation thus, unraveling the structure-functionality relationships of shape-defined Cu2O nanocrystal model catalysts for methanol decomposition.
Open Access
Variationally consistent Hellmann–Feynman forces in the finite element formulation of Kohn–Sham density functional theory
(Elsevier, 2023-01-01) Karaca, Kaan; Temizer, İlker
Hellmann–Feynman forces are derived within the numerical framework of the finite element method for density functional theory in the Kohn–Sham formalism. The variational consistency of the force expressions in all-electron and pseudopotential settings are carefully examined, with a particular focus on the implications arising from different representations for interaction terms that are associated with electrostatics. Numerical investigations in nonperiodic systems which range from diatomic molecules to carbon allotropes demonstrate the systematic convergence that is offered by the finite element framework, not only for energy and force but also for geometric configuration and molecular statics parameters. A range of higher-order discretizations employing fixed meshes are invoked within these examples based on classical finite elements as well as on isogeometric analysis. Overall, this work contributes to recent advances which demonstrate the viability of the finite element method for carrying out ab initio molecular dynamics.