Theses supervised by Prof. Dr. Emrah Özensoy
10 theses · İhsan Doğramacı Bilkent University
Metanol bozunması için şekil tanımlı Cu2O nanokristal model katalizörlerin yapı-fonksiyonellik ilişkilerinin aydınlatılması
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.
Katalitik metal hidroksit nanoyapıları: NixMn(1-x)(OH)2 ile oksijenli ortamlarda c-h aktivasyonu ve düşük sıcaklıkta karbon monoksit yükseltgenmesi
Metal hydroxides and mixed metal hydroxides have been frequently utilized in diverse applications such as battery technologies, electrocatalysis, electrosynthesis, photocatalysis, supercapacitors, electrochromic devices, and electrochemical sensors. Yet, precious metal-free hydroxides have not been utilized to their full potential in the field of catalytic aerobic C-H activation and catalytic low-temperature CO oxidation. In this work, we demonstrate that upon careful optimization of catalyst synthesis protocols, a novel catalytic architecture is achieved in the form of Ni0.6Mn0.4(OH)2 revealing remarkable catalytic performance in the aerobic oxidation of alkylarenes, particularly in the aerobic oxidation of xanthene to xanthone. This optimized catalyst also shows superior catalytic activity in low-temperature CO (g) oxidation. We also present an efficient catalytic regeneration protocol, which can redeem the full initial activity of the carbonpoisoned spent catalyst in xanthene oxidation. Catalytic functionality of this novel nanomaterial architecture is also examined in detail in light of comprehensive characterization experiments including ATR-IR, XRD, BET-SSA, TGA, TEM, EDX and XPS measurements.
Monometalik ve bimetalik aktif bölgelere sahip yeni gliserol kuru reform katalizörleri
Novel glycerol dry reforming catalysts with monometallic (Ru) and bimetallic (Ru and Ni or Ru and Co) active sites which are supported on a custom-design ternary mixed oxide support material (i.e., Al2O3-TiO2-ZrO2, AZT) with varying compositions were examined. Characterization of the synthesized catalytic system was carried out with XRD, Raman, BET, XPS, ICP-MS, SEM, and EDX techniques. Structure of the Ru active sites as a function of Ru loading was also investigated with in-situ FTIR spectroscopy via CO adsorption. Catalytic reactivity results revealed that 1 wt.% Ru/AZT70 catalyst can outperform the 1 wt.% Ru/La2O3-ZrO2 catalyst in GDR reaction, where the latter catalyst is known to be the best catalyst in the literature for GDR reaction. 0.5wt.% Ru/AZT70 catalyst showed close activity compared to 1wt.% Ru/AZT70 catalyst. Furthermore, catalytic promotional effect of Ni for low Ru loadings in GDR reaction was also demonstrated.
Karbon monoksit oksidasyonu için yüksek yüzey dağılımlıve nanometreden küçük çaplı iridyum katalizörler
Novel catalytic architectures composed of catalytic centers with sub-nanometer diameters for CO oxidation reaction were designed, synthesized, and characterized. Accordingly, well-dispersed iridium precious metal active sites were supported on various catalytic support materials. Namely, magnesium oxide (MgO), ceria (CeO2), lanthana-zirconia (La2O3–ZrO2) and titania-zirconia (TiO2–ZrO2) systems were chosen as different support systems. The favorable catalytic effect of highly-dispersed Ir active sites with sub-nanometer diameters were demonstrated in flow-mode catalytic performance tests, where the lower loadings of highly dispersed Ir sites showed comparable catalytic activity in CO oxidation to that of bigger Ir clusters with higher metal loading. Furthermore, influence of the catalyst pre-treatment conditions (e.g., reduction in H2, oxidation in O2, and calcination in air) on the catalyst structure and performance were also studied via XRD, Raman, BET, XPS, TEM, EDX, and in-situ FTIR spectroscopy techniques. Our results indicate that in all the catalytic systems, high-dispersion Ir sites can be generated on supports where Ir exists as small clusters with < 1 nm particle size. Moreover, catalyst pretreatment conditions revealed noticeable alterations in the catalyst structure in terms of average support particle size, reduction extent of the support, specific surface area, pore volume, pore size, and Ir oxidation state. Finally, catalytic performance results indicated that under reaction conditions yielding close to 100% CO conversion, 0.2 and 0.5 wt.% Ir catalysts led to comparable performance to that of 1 wt.% Ir catalyst demonstrating the advantage of catalytic systems with highly dispersed sub-nanometer diameter active sites with extremely low metal loading.
Değerli metal içermeyen metal oksit tabanlı nox giderim sistemleri üzerinde N-O aktivasyonu
Elevated operational costs of platinum group metal (PGM) based environmental catalytic systems shift the focus of catalysis research towards cost-effective materials. In search for PGM-free alternative catalytic materials for NOx removal, high catalytic performance and long catalyst lifetime emerge as two important technical challenges. Within the scope of this dissertation, novel B-site mixed perovskites LaCoxMn1-xO3 (x = 0.1-0.9) and Fe and/or Co based CeO2 catalysts were synthesized, investigated and optimized as high performance, PGM-free, and durable catalyst alternatives for NOx removal systems. The perovskite based catalytic architectures can be utilized as diesel oxidation catalyst (DOC) oxidizing NO/CO to NO2/CO2, which can be coupled with selective catalytic reduction (SCR) catalysts to reduce NOx species to N2. On the other hand, Fe/Co based CeO2 systems can be exploited as catalyst candidates in SCR of NOx. In both of these NOx aftertreatment systems, NO activation is required. A simple and reproducible synthetic protocol was utilized to obtain perovskite-based DOC catalysts whose comprehensive structural characterization was carried out via XRD, N2 adsorption-desorption isotherm, ICP-MS, TEM, H2-TPR, ex-situ and in-situ XANES, EXAFS, in-situ FTIR, XPS, and TPD techniques. The oxidative catalytic performance of the perovskites for CO and NO oxidation was determined in flow-mode catalytic activity tests. It was demonstrated that bulk-oxygen vacancies have a strong influence on the redox activity of the B-site mixed perovskites with the ABO3 structure (where A = La, B = Co, Mn) allowing them to efficiently switch between high and low oxidation states in a reversible fashion under relatively moderate redox conditions without requiring elevated temperatures for regeneration, unlike conventional LaMnO3 and LaCoO3-based simple perovskite systems. La1.01Co0.75Mn0.24O2.97 and La1.04Co0.65Mn0.31O2.97 were found to reveal the best NO and CO oxidation performances among the currently investigated perovskites (La1.01Co0.75Mn0.24O2.97, La1.04Co0.65Mn0.31O2.97, La0.97Co1.03O2.91, and La0.97Mn1.03O3.17), which were on par with a conventional precious-metal benchmark catalyst (i.e., 1 wt. % Pt/Al2O3). Influence of Fe and Co loading on monometallic (Fe or Co) or bimetallic (Fe-Co) catalysts with different CeO2 support materials were studied in SCR of NO to N2. The flow-mode NO reduction experiments point out that 4 wt. % Co/CeO2 is the best catalyst in the studied group of catalysts based on its high N2 selectivity at relatively low temperatures. Detailed structural characterization experiments conducted via XRD, N2 adsorption-desorption isotherm, ATR-FTIR, Raman, and in-situ FTIR techniques indicate correlations between catalyst structure and SCR functionality. Our experimental findings indicate that 4 wt. % Co/CeO2 has relatively higher catalytic performance under excess H2(g) concentrations. The NO activation performance of both La1.01Co0.75Mn0.24O2.97 and La1.04Co0.65Mn0.31O2.97 B-site mixed perovskites and 4 wt. % Co/CeO2 were tested under significantly harsh conditions indicating their strong potential to be used not only in mobile applications but also in stationary NOx removal systems. Keywords: Perovskite, Diesel Oxidation Catalyst, Selective Catalytic Reduction, NO activation, NOx, PGM-free, Heterogeneous Catalyst
Enerjık iyonik sıvıların iridyum bazlı katalizörler üzerinde ekzotermik bozunması
Hydrazine (N2H4) is the most commonly used propellant for in-orbit spacecraft propulsion. However, utilization of hydrazine in space missions has challenges associated with health, environment and safety risks. Energetic ionic liquids (EILs) such as ammonium dinitramide (ADN) present themselves as environmentally friendly alternative fuels to hydrazine. EILs can be decomposed efficiently and safely in the presence of a heterogenous catalyst. In this context, monometallic catalysts containing Ir and Al2O3 were synthesized using both wetness impregnation and incipient to wetness impregnation methods, and the structural properties of these catalysts were investigated. Furthermore, the effects of the Al2O3 support material on Ir dispersion and catalytic performance of anaerobic ADN decomposition were studied. In order to improve the Ir active site dispersion on the Al2O3 support material, promoters such as La and Ce were added to the catalyst systems and different pretreatment conditions were applied to the synthesized catalysts. Furthermore, LaMnO3 (perovskite) promoted alumina catalysts with Ir active sites were also studied. Catalysts with high performance, 5Ir/TH100 (5Ir/Al2O3), 5Ir/L3 (5Ir/La-Al2O3), and 5Ir/Sir10 (5Ir/Si-Al2O3) were investigated with in-situ X-ray Absorption Near Edge Spectroscopy (XANES), in-situ Extended X-ray Absorption Fine Structure (EXAFS), in-situ Fourier Transform Infrared Spectroscopy (in-situ FTIR), Temperature Programmed Desorption (TPD), X-ray Diffraction (XRD), Transmission Electron Microscopy (TEM), Energy Dispersive X-ray (EDX), Pyridine adsorption via FTIR, CO Chemisorption, X-ray Photoelectron Spectroscopy (XPS) and X-ray Fluorescence (XRF) analysis techniques. Our findings revealed that 5Ir/TH100 and 5Ir/L3 catalysts favorably lowered the onset temperature of the ADN decomposition reaction, whereas 5Ir/Sir10 boosted the pressure generation during the reaction. The formation of mostly metallic Ir nanoparticles on 5Ir/TH100 and 5Ir/L3 enables the lowering of the activation energy of the reaction. On the other hand, enhancement in the pressure generation for 5Ir/Sir10 catalyst is associated with the generation of small oxidic Irnx+ clusters which are strongly interacting with the SiOx-AlOx surface domains of the support material. The fundamental structure-functionality relationships unraveled in the current work may allow design of novel catalytic systems for aerospace monopropellant propulsion systems with higher performance by simultaneous exploitation of Ir active sites with different electronic properties.
Fotokatalik NOx yükseltgenmesi ve depolanmasının etanol ile zenginleştirilmiş TiO2 kullanılarak artırılması
Nitrogen oxides (NOx), especially nitric oxide (NO) and nitrogen dioxide (NO2) severely affect human health. In this regard, semiconductor photocatalysis present an appealing approach, since the only requirements for this procedure are sunlight, water and oxygen which are naturally abundant. Despite its favorable properties like chemical inertness, long-term stability and low cost, titania (TiO2) has a lower NOx abatement performance due to its low selectivity towards nitrites/nitrates as final product. In this work, we report a simple monohydric alcohol impregnation protocol at mild temperature range to synthesize colored TiO2 nanoparticles for efficient photocatalytic NOx oxidation and storage (PHONOS) application under UVA illumination. The ethanol induced coloration of commercial benchmark TiO2 (P25) and photocatalytic activity for NOx abatement were observed to be dependent on heat-treatment temperature; the highest activity was obtained at 150 °C. Comprehensive analyses of the optimized photocatalyst suggest the presence of surface functionalities of adsorbed formic acid and acetate. The doping of TiO2 with these in situ generated impurities results in the generation of Ti3+ and oxygen vacancies (Vos) (intrinsic defects) which are aimed to be observed using X-Ray Photoelectron Spectroscopy (XPS), Raman Spectroscopy and Diffuse Reflectance UV-Visible Spectroscopy (DR-UV-Vis). These fine-tuned materials demonstrated superior photocatalytic performance surpassing conventional P25 benchmark in short (1 h) and long term (15 h) evaluation studies. Special attention has been paid to the selectivity of the designed photocatalyst toward nitrate/nitrite formation and CaO was introduced as NOx storage domains to further improve the stability of best performing photocatalysts for extended time period.
MnOx /Pd(111) katalizörü üzerinde CO2 aktivasyonu
CO2 is an atmospheric pollutant (i.e., a greenhouse gas) and it can be converted into valuable chemicals such as methanol, methane, and formic acid. However, CO2 reduction is a challenging process due to the thermodynamic stability of CO2. In this work, we focus on the activation of CO2 by using an atomically well-defined MnOx/Pd(111) planar model catalyst. Pd(111) surface can dissociatively adsorb hydrogen molecules, but CO2 does not strongly bind to the Pd(111) surface. On the other hand, MnOx nano-structures can facilitate the activation of CO2 due to the presence of acid and base sites on the metal oxide surface. Therefore, MnOx/Pd(111) was chosen as a model catalyst to investigate catalytic CO2 activation. A multifunctional ultra-high vacuum system with quadrupole mass spectrometer (QMS), X-ray photoelectron spectrometer (XPS), and low energy electron diffraction (LEED) was used to perform the experiments. Manganese thin film growth mechanism on Pd(111) surface was determined by using XPS. Manganese was evaporated on Pd(111) substrate at two different temperatures (i.e., 85 K and 300 K). Formation of products after the dosing of the reactants on the MnOx/Pd(111) surface was examined via temperature programmed desorption (TPD). For both cases, formed manganese oxide thin film was investigated by using XPS to estimate the relative, Mn0, Mn2+ and Mn3+ surface concentrations. Prepared manganese film on Pd(111) at 300 K could activate CO2 to CO, which is a valuable chemical for the chemical industry. To prepare smaller clusters, manganese was evaporated on the Pd(111) single crystal surface at 85 K. At moderate manganese coverage, carbonate CO32- formation was detected on the MnOx/Pd(111) interfacial sites.
Pd(111) tek kristal model katalizörler üzerindeki poliol oksidasyonlarının katalitik bağ kırılma dizilerinin incelenmesi
Understanding the bond-breaking sequences of catalytic polyol oxidation on transition metal catalysts is critical for the chemical transformation of biomass derived chemical feedstock into value-added products which may also offer new alternatives to fossil fuel-based commodity chemicals. In the current work, oxidation of ethylene glycol on an atomically well-defined Pd(111) single crystal planar model catalysts was investigated via temperature programmed desorption (TPD) technique under ultra-high vacuum (UHV) conditions. Presence of surface oxygen atoms was found to promote the formation of formaldehyde (H2CO) and carbon dioxide as the most prominent catalytic oxidation products. Enhancement in formaldehyde generation was observed upon increasing the ethylene glycol-to-oxygen ratio. Our results indicate that the activation of C-C bonds was primarily facilitated by atomic oxygen, preceding the complete dehydrogenation of the C2HxOz surface species. The formation of H2CO was mainly attributed to the most unstable surface species in terms of C-C bond scission, namely -OCH2CO- and -OCH2CHO-. Other surface species such as -OCHCHO- and -OCHCO- led to additional decomposition products such as CO rather than formaldehyde.
Aerobik C-H aktivasyonu için çift metalli hidroksit katalizörleri
The increasing interest in the oxidation of sp3 C-H and O-H bonds has garnered tremendous attention due to its potential for facile production of oxygenated organics. Precious metal-free bimetallic hydroxide-based materials are commonly employed in various applications such as batteries and photocatalysts. However, their prospects in C-H activation reactions have been poorly explored. This research focuses on the development and evaluation of a bimetallic Fe-Mn hydroxide catalyst for aerobic C-H activation and O-H oxidation reactions without the need for an initiator. The Fe-Mn hydroxide catalyst was synthesized and carefully optimized to enhance its catalytic efficiency in the direct oxygenation of a wide scope of alkylarene compounds through C-H functionalization and oxidation of benzylic alcohols. A series of Fe-Mn bimetal hydroxides with different Fe/Mn ratios were synthesized using a customized chemical co-precipitation method. These catalysts were then tested for the catalytic oxidation of fluorene to fluorenone using molecular oxygen as the sole oxidant, with the Fe0.6Mn0.4(OH)y-12S catalyst demonstrating the best performance. Under mild reaction conditions, the catalyst exhibited remarkable performance in activating C-H bonds using molecular oxygen as the oxidant. Various substrates, including alkylarenes and alcohols, were investigated, consistently yielding high yields of oxygenated products with minimal catalyst loadings. XRD, XPS, XANES, ICP-MS, BET, and TGA were employed to gain insights into the structural features of the catalyst. Our findings indicate that the following structural properties of the optimized Fe0.6Mn0.4(OH)y-12S catalyst could be responsible for the currently observed enhanced catalytic reactivity: i) unique Mn oxidation state (ca. Mn2.6+), ii) Fe cationic sites containing a mixture of Fe2+ and Fe3+ species, where Fe3+ species are the dominating species, iii)realtively low specific surface area of 68 m2/g, iv) relatively disordered and defective crystal structure comprised of bimetallic hydroxides as well as additional oxide/oxyhydroxide phases, v) residual Na+ surface species enabling electronic promotion of the cationic active sites via electron donation. Keywords: Bimetallic hydroxide catalyst, aerobic oxidation, C-H activation, heterogeneous catalysis, alkylarenes.