Theses supervised by Doç. Dr. Emrah Özensoy
10 theses · İhsan Doğramacı Bilkent University
Kuantum Noktalar ile İyileştirilmiş Titanya Sistemleri ile Fotokatalitik Yükseltgemeli NOx Depolama
Increasing activities of industrial combustion systems, volcanic eruptions, agriculture activities and utilization of stationary and mobile fossil and biomass combustion systems are known to be the major causes of toxic nitrogen oxides (NOx) pollution. These pollutants are not only highly hazardous for the ecosystem but also can trigger the formation of secondary pollutants such as acid rain and tropospheric ozone. Abatement of toxic NOx gases can be achieved by thermal catalytic processes or physical/chemical adsorption systems. However, environmentally friendly, cost-efficient and sustainable alternative photocatalytic systems can also be designed which can exploit readily abundant solar radiation. One of the most well-known benchmarks for environmental photocatalysts is titanium dioxide with a wide band gap typically varying within 3.0-3.2 eV that can be activated via UV photons. This wide band gap prevents efficient absorption of visible light, which corresponds to around 5 times higher intensity compared to UV light. In order to increase the photocatalytic efficiency of the titanium dioxide, its visible-light exploitation capability should be enhanced. Although, this can be done by doping of TiO2 with nonmetal main group elements, recently the research focus has shifted towards utilization of semiconductor quantum dots (QDs) for this purpose. Visible response of the QDs can be modified by tuning their particle size. Furthermore, QDs provide additional advantages such as the generation of hot electrons or multiple charge carriers with a single high-energy photon. In the present work, CdTe QDs were employed as a direct band gap semiconductor (1.44 eV) compatible with the visible window of the solar spectrum to promote titania based photocatalysts. Due to its higher conduction band, CdTe can transfer its conduction band electrons to the conduction band of TiO2 and the hole that is created on the valence band of TiO2 can be transferred to the valence band of CdTe; leading to efficient electron-hole separation. Thus, visible light exploitation capacity of TiO2 can be enhanced along with its photocatalytic activity. Current photocatalytic activity results on QD functionalized titania systems exhibited much higher NOx storage in solid state and an enhancement of NO conversion values as compared to that of P25 titania benchmark photocatalyst. In addition, various reference materials were prepared and photocatalytically tested in order to shed light on the mechanism of this photocatalytic enhancement. These results provided insight regarding the functionality of the different structural components of the photocatalytic architecture in the photocatalytic NOx oxidative storage process. The influence of each structural component in this catalytic architecture was studied. Control experiments were conducted with the dispersant water and the capping agent thioglycolic acid. The results revealed that NO conversion and selectivity can be enhanced by the adsorbed water on titania surface. By adding thioglycolic acid on titania, the NO conversion is suppressed but the selectivity of the system increased. Finally, by replacing titania with a photocatalytically inactive material alumina, it was shown that only in the presence of CdTe quantum dots, there can be NO oxidation/conversion. In overall, utilizing CdTe quantum dots are advantageous for exploiting more of the solar irradiation; however, they suffer from low stability and short catalytic life time.
Bronsted ve lewis bazlarının Pd(111) tek kristal model katalizörünün formik asit dehidrejenasyon seçiciliğine olan etkileri
Formic acid (FA) is an environmentally friendly hydrogen-based energy vector that can be obtained from renewable biomass feedstocks. However, catalytic decomposition of FA involves two different competing chemical pathways called dehydration and dehydrogenation. Thus, molecular level studies focusing on the selective catalytic FA dehydrogenation are essential for establishing structure-reactivity relationships which can be used in order to increase the catalytic dehydrogenation selectivity. In the current work, effects of Bronsted and Lewis bases on catalytic FA dehydrogenation selectivity were studied under ultra-high vacuum (UHV) conditions on an atomically well-defined Pd(111) single crystal model catalyst surface by using temperature programmed desorption/temperature programmed reaction spectroscopy (TPD/TPRS), X-Ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED) techniques. Doubly-deuterated FA (DCOOD) was used as the FA source, while ammonia and manganese oxide were chosen as the model Bronsted and Lewis bases. Adsorption and subsequent surface decomposition reaction of DCOOD on Pd(111) showed that model catalyst was not totally selective towards dehydrogenation. Functionalizing the Pd(111) surface with ammonia suppressed the FA dehydration and boosted the dehydrogenation pathway, where positive influence of ammonia on FA dehydrogenation selectivity decayed when ammonia coverage was greater than 1 ML. A boost in hydrogen generation was observed in the catalytic FA dehydrogenation on manganese oxide-deposited Pd(111) surface (at sub-monolayer manganese oxide regime) as compared to that of a clean Pd(111) model catalyst. It was found out that manganese oxide can enhance FA dehydrogenation by acting as a promoter and/or catalytically contributing to the reaction depending on the oxidation state composition.
Fotokatalitik nox arıtımı için çekirdek-çeper yapılı kuantum-kuyu nanoplakaları ile zenginleştirilmiş TiO2 malzemeleri
Oleic acid capped core/crown CdSe/CdSeTe quantum-well nanoplatelets (NPL) were used in the surface functionalization of TiO2. Structural characterization of the synthesized photocatalytic architecture was carried out to shed light on its surface chemistry, electronic, and crystallographic structure. NPL/TiO2 composites were tested in NO photo-oxidation under ultraviolet-A (UVA) and visible (VIS) light, showing a remarkable activity in NOx abatement and high selectivity for nitrate storage as compared to standard benchmark TiO2 photocatalyst (i.e. P25). Improved photocatalytic behavior can be attributed to the decrease in the bandgap and enhanced photogenerated electron-hole pair separation as a result of the incorporation of CdSe/CdSeTe NPL onto TiO2. Stability of composites was also investigated in durability tests. Even though some decrease in photocatalytic activity and selectivity of NPL/TiO2 composites was observed, performance of the NPL/TiO2 composites was found to be significantly better than pure TiO¬2¬.
De-nox uygulamaları için yeni nesil hibrit perovskitler
Bu çalışmanın temel amacı, katalitik De-NOx uygulamalarına yönelik, LaCoxMn1-xO3 tipi hibrit perovskit malzemelerinin, yapısal ve işlevsel özelliklerinin incelenmesidir. Perovskitlerin karakteristik yapısı, termal dayanıklılıkları ve NOx/SOx adsrobsiyon/salınım özellikleri XRD, BET, XPS, in-situ FTIR, ex-situ FTIR, TEM, BET, TPD ve TPR teknikleri kullanılarak çalışılmıştır. Diğerlerine kıyasla, LaCo0.8Mn0.2veLaCo0.7Mn0.3O3 perovskitlerinin optimize edilmiş yüzeyleri sayesinde (yani özel B-noktası kompozisyonu nedeniyle),en yüksek NOx depolama kapasitesine sahip malzemeler oldukları belirlenmiştir. NOx ve SOx deneyleri, B noktalarına ait kompozisyonun, katalizör yüzeyine bağlanan türlerin adsorpsiyon geometrisi üzerinde önemli bir değişikliğe yol açmadığını göstermiştir. Diğer taraftan, NOx emiliminin H2 indirgenmesi sonrasında arttığı, IR deneyleriyle gösterilmiştir. Ayrıca, QMS deneylerinde ölçülen N2, özel koşullar altında, hibrit katalizörlerin N-O bağ aktivasyonunu başarıyla gerçekleştirebildiğini ve NOx türlerinin tamamen indirgenebildiğini göstermektedir. Mn'ce zengin perovskitler üzerindeki SOx türleri, H2 ile oldukça etkin bir şekilde indirgenerek, bertaraf edilebilmesine rağmen; Co'ca zengin perovskitlerde tersinmez sülfat türleri oluşumu nedeniyle kükürt giderim sürecinin daha verimsiz olduğu gözlenmiştir. SOx türleri ve NOx türlerinin aynı anda var olduğu durumda, katalizör yüzeyindeki aktif bağlanma noktalarına dair rekabeti, SOx türlerinin belirgin bir üstünlükle kazandığı saptanmıştır. Perovskitlerin kristal örgülerindeki oksijen miktarı, oksijen boşlukları ve indirgenme profilleri; B-bölgesindeki kompozisyona bağlı olarak farklılıklar göstermektedir. Perovskit kristal örgüsündeki, Co-O bağlarının, yapıdaki artan Mn yüklemesiyle birlikte, Mn-O bağlalarıya yer değiştirdiği, ex-situ FTIR deneyleriyle gösterilmiştir. Hibrit perovskitlerin spesifik yüzey alanları, yapıdaki Mn yüklemesinin artırılmasına bağlı olarak artmaktadır. Mevcut çalışmalar çerçevesinde elde edilen deneysel sonuçlar; hibrit perovskitlerin yüksek NOx depolama kapasitesi ve yüksek SOx toleranslarına sahip olduklarını ve yapılarının sentetik olarak kontrol edilmesiyle, yüzey kimyasal özelliklerinin, redox davranışlarının, NOx/SOx türleriyle etkileşimlerinin değiştirilerek, yüksek performansa sahip, özgün De-NOx katalizörlerinin tasarımına olanak verdiğini göstermektedir. Anahtar sözcükler: Perovskit, NOx, SOx, LaMnO3, LaCoO3, LaCoxMn1-xO3
SO2 ve NO2 moleküllerinin denox katalızörü olarak kullanılan üçlü ve dörtlü karmaşık oksit yapılar üzerindeki emilim ve salınım özelliklerinin incelenmesi
The main premise of the current study is the design, synthesis and functional characterization of novel catalytic materials with superior resistance against sulfur poisoning without compromising NOx storage capacity (NSC) in their NOx Storage Reduction (NSR) catalytic applications. BaO/TiO2-based materials are well known systems in deNOx catalysis, exhibiting promising performance towards sulfur poisoning. However, they suffer from limitations due to poor NSC and high affinity towards unwanted solid state interactionsbetweenTiO2 and BaO storage domains leading to the formation of BaTiOx.The main emphasis of the current work is the design of a novel catalytic system where ZrO2 and Al2O3 act as diffusion barriers between BaO and TiO2 domains while allowing good dispersion and preservation of the individual characteristicsof these active sites within a wide operational temperature window. Along these lines, binary and ternary mixed oxide materials, ZrO2/TiO2 (ZT) and Al2O3/ZrO2/TiO2 (AZT), and their Pt, BaO and/or K2O functionalized counterparts in the form of Pt/ZT, Pt/AZT, Pt/BaO/AZT, Pt/K2O/AZT and Pt/K2O-BaO/AZT with different mass loadings (i.e. 8 and 20 wt. % 20 BaO and 2.7, 5.4 and 10 wt. % K2O) were synthesized via sol-gel synthesis. Surface structure and catalytic properties of the synthesized materials were comprehensively investigated at the molecular level as a function of calcination temperature, catalyst composition, nature of the gas phase adsorbates (e.g. NO2, SO2, O2, H2, N2, N2O C5H5N etc.) interacting with the catalyst surface at various operational temperatures by means of XRD, Raman spectroscopy, BET analysis, in-situ FTIR and TPD. Current results indicate no evidence for the formation of undesired BaTiOx and/or KTiOx. NSC of fresh monolithic catalysts was also quantitatively measured under realistic operational conditions in a tubular flow reactor system. These flow reactor measurements indicated that Pt/8BaO/AZT and Pt/20BaO/AZT materials revealed promising NOx storage and sulfur regeneration performance at low (i.e. 473 K) and moderate (i.e. 573 K) temperatures in comparison to the conventional Pt/20Ba/Al2O3 benchmark catalyst. However, they were found to be surpassed by the conventional Pt/20BaO/Al2O3 benchmark catalyst at higher operational temperatures (i.e. 673 K). Therefore, activity loss at high temperatures was alleviated by incorporating a high-temperature storage functionality (i.e. K2O) to the catalyst structure. Upon this structural enhancement, Pt/5.4K2O/AZT catalyst was found to reveal much higher NSC at high temperatures (i.e. 673 K) as compared to BaO-based materials. An overall assessment of the results presented in the current study suggests that there exists a delicate trade-off between NOx Storage Capacity (NSC) and sulfur uptake/poisoning in NSR systems which is strongly governed by the BaO and K2O loading/dispersion as well as the surface structure of the support material.
Geniş sıcaklık aralığında çalışabilen yeni nesil denox katalizörlerinin tasarımı
The main objective of this study is to design novel DeNOx catalyst to widen the operational temperature range of exhaust emission control systems. For this purpose, single and multi NOx-storage domains (e.g. K2O, BaO) were loaded on an Al2O3/ZrO2/TiO2 (AZT) ternary mixed oxide support with various compositions and different catalytic systems were synthesized by utilizing sol-gel and wetness impregnation methods. These materials were characterized by means of XRD, N2 sorption, in-situ FTIR and TPD measurements in comparison to the Pt/20Ba/Al benchmark catalyst. K2O and BaO co-loading on AZT sample reveals better platinum dispersion than that of the single storage domain materials. Particularly, Pt/5.4K-8Ba/AZT system revealed promising NOx storage capacity (NSC) and high sulfur removal performance. NOx/SOx adsorption geometries and stabilities of the generated adsorbates were analyzed using in-situ FTIR and TPD. Although the Pt/20Ba/AZT and Pt/10K/AZT catalysts revealed high NSC, they showed poor sulfur regeneration characteristics. In conclusion, it was demonstrated that K2O and BaO co-impregnated samples can be utilized to design new catalytic architectures to widen the operational temperature window of exhaust emission control catalysts.
İndirgenebilir karışık metal oksitlerin kullanımı ile NDİ katalizörlerinin kükürt direncinin artırılması
Pt functionalized binary, ternary, and quaternary oxides (e.g. Pt/BaO/CeO2/ZrO2/Al2O3) were synthesized by wetness impregnation method and characterized by X-ray Diffraction (XRD), Brunauer–Emmett–Teller (BET) surface area analysis, in-situ Fourier Transform Infrared (FTIR), and temperature programmed desorption (TPD) techniques. Effect of the synthesis sequence on the NOx storage capacity was investigated by synthesizing subsequently impregnated and co-impregnated ternary oxides. Influence of BaO loading on NOx uptake of quaternary oxides was examined by utilizing two different BaO loadings namely; 8 wt% and 20 wt% on co-impregnated ternary oxide, Pt10-10CeZrAl. Co-presence of CeO2-ZrO2 oxide domains leads to an increase in NOx storage. As BaO loading increases in quaternary oxides, thermal stabilities of nitrates and nitrites increase due to the formation of bulk/ionic nitrates. Although BaO impregnation on co-impregnated ternary oxides leads to a decrease in specific surface area (SSA) values due to sintering, NOx adsorption on BaO-functionalized quaternary oxides was found to be higher than the BaO deficient ternary oxides. Upon sulfur poisoning, formation of strongly bound bulk/ionic sulfate/sulfite functional groups on BaO containing catalysts result in a need for higher temperatures for complete sulfur regeneration. Comparison of the CeO2-ZrO2 promoted systems with that of the Pt/ 20 wt% Ba/Al2O3 conventional NOx Storage Reduction (NSR) catalyst suggests that ceria-zirconia promotion enhances the sulfur tolerance. In conclusion, in this study a new NSR catalyst namely, Pt20Ba10-10CeZrAl, which is promoted with reducible mixed metal oxides, was synthesized and characterized. This novel NSR catalyst formulation revealed favorable sulfur resistance with minor sacrifice in NOx storage ability.
Enhanced photocatalytic nox oxidation‐storage over titania‐metal oxide physical mixtures under uv and visible light
Developing new technologies for the abatement of gaseous nitrogen oxides (NO, NO2, etc.) will still be one of the popular research fields; because fossil fuels (mainly coal and natural gas) will remain as the main energy sources for many decades to come. Although various technologies have been developed and implemented for DeNOx processes, alternative approaches are still open to discussion. Among these; Photocatalytic NOx Oxidation-Storage (PhoNOS) can offer promising opportunities to overcome this environmental challenge, as it can be utilized under ambient conditions with the help of UV and visible light irradiation. In this study; firstly, a new performance analysis method was developed other than the photonic efficiencies used in previous works. In this analysis method, a "DeNOx Index" was utilized. This index indicates the net change in total air pollution due to NOx species by comparing the relative contributions of NO and NO2 along with NO conversion and solid state NOx storage selectivity. This new method was first applied on previously studied TiO2-Al2O3 binary oxide samples (P2) synthesized by sol-gel co-precipitation method in comparison with commercially available Degussa P25 TiO2. Furthermore, TiO2-Al2O3 (P2) binary oxides were also physically/mechanically mixed with an alkaline earth oxide, CaO. Addition of CaO to P2 binary oxides decreased the NO conversion while enhancing the NOx storage. In order to alleviate the loss of NO conversion in CaO+P2 systems, physical mixtures of P25 TiO2 with two different commercial metal oxides (CaO and γ-Al2O3) were prepared and investigated. While CaO provides "higher alkalinity" (i.e. a desirable property for the solid state storage of acidic gaseous NOx species) than γ-Al2O3, mesoporous γ-Al2O3 can provide a higher porosity and specific surface area for the adsorption and storage of the oxidation products in the solid state. Considering these, binary or ternary mixtures with various compositions were prepared and catalytically tested under UV and Visible light irradiation. It was found out that the boosting effect of CaO on NOx storage is more significant than that of γ-Al2O3 for the binary oxides. On the other hand, it should be noted that ternary mixtures containing smaller amounts of titania with high performance can also be obtained by incorporating alumina into the mixture. In addition to these, performances of selected samples were studied under different humidity conditions and experimental durations. These experiments yielded interesting implications regarding NOx adsorption-oxidation phenomena on the investigated mixed oxide surfaces. Current findings indicate that further experiments are required to fully understand the fundamental mechanisms of photocatalytic NO oxidation and storage at the molecular level. Keywords: Photocatalytic NOx Oxidation-Storage, Titania, DeNOx Index, NO Conversion Activity
Formik asit dehidrojenlenme tepkimesi için yüksek karbon monoksit direncine sahip üç-metalli heterojen katalizörler
Hydrogen energy is considered to be a promising alternative for the sustainable and environmentally friendly solution of the global energy problem. One of the major obstacles of hydrogen energy applications is to maintain safe and efficient storage of hydrogen which can also be achieved chemically using suitable carrier materials. Formic acid (HCOOH, FA) can be utilized as a hydrogen carrier due to its low molecular weight (46 g/mol) and high hydrogen density (%4.4 weight). FA is a stable, non-flammable, and non-toxic biomass side-product rendering it a perfect candidate for an alternative hydrogen vector. Design of novel heterogeneous catalysts which can substitute the existing homogeneous catalytic systems may allow overcoming catalyst isolation and recovery costs and associated logistical problems hindering their applications in on-board operations. FA can be catalytically decomposed via dehydrogenation and dehydration reactions. Selective dehydrogenation of FA is crucial because, the production of CO from dehydration mechanism can suppress the activity of the catalyst by blocking/poisoning the precious metal sites. Consequently, development of CO-resistant, selective, catalytically active, and reusable heterogeneous catalysts has a great significance. In the current work, a new material that can produce H2(g) from FA under ambient conditions in the absence of additives with high CO-poisoning tolerance will be introduced, which is comprised of Pd-based trimetallic active centers functionalized with Ag and Cr in addition to amine-functionalized MnOx promoters dispersed on a SiO2 support surface. A novel trimetallic FA dehydrogenation catalyst was prepared and studied using analytical, ex-situ and in-situ spectroscopic techniques and compared to the results obtained for monometallic, bimetallic and active site-free counterparts. Trimetallic catalysts were found to reveal superior catalytic activity and stability compared to all of the currently investigated catalysts. Structural and catalytic properties of the trimetallic catalysts were investigated as a function of metal loadings. Structural characterization of the synthesized materials was carried out by Raman spectroscopy, Inductively-Coupled Plasma Optical Emission Spectroscopy (ICP-OES), X-ray Diffraction (XRD), Brunauer, Emmett and Teller (BET) Specific Surface Area Analysis, Transmission Electron Microscopy (TEM), High Resolution TEM (HRTEM), Scanning Transmission Electron Microscopy (STEM), and STEM/Energy Dispersive X-Ray (EDX), High-Angle Annular Dark Field (HAADF)/STEM. In addition, interaction of the catalyst surfaces with reactants and products were also monitored via in-situ FTIR spectroscopy for functional characterization. Detailed in-situ FTIR spectroscopic experiments were also performed using HCOOD, DCOOH and DCOOD in order to understand the nature of the adsorbed species, products and catalytic inhibitors.
Oksijen türlerinin Au(111) ve Ag(111) model katalizörleri üzerindeki doğası ve O-H, C-H, C-C, N-H bağ aktivasyonundaki rolleri
Metal-catalyzed heterogeneous oxidation reactions have high importance for the large-scale production of the commodity chemicals vastly used in the chemical industry. Controlling the selectivity in such processes to increase the product yield and minimize the production of undesired byproducts requires a molecular level understanding of the bond activation mechanisms. Thus, understanding the nature of oxygen species in various bond cleavage processes is critical. In the current work, nature of oxygen species was studied on the planar Au(111) and Ag(111) single crystal model catalyst surfaces via x-ray photoelectron spectroscopy (XPS), temperature programmed desorption/ temperature programmed reaction spectroscopy (TPD/TPRS), low energy electron diffraction (LEED) and infrared reflection absorption spectroscopy (IRAS) techniques under ultra-high vacuum (UHV) conditions. Ozone (O3) was utilized as the oxygen delivery agent providing atomic oxygen to the reacting surface. Various oxygen species were determined on both Au(111) and Ag(111) model catalysts and their role in O-H, C-H, C-C and N-H bond activation was investigated by using probe molecules such as methanol, acetaldehyde and ammonia. Three different oxygen species such as atomic oxygen (Oa), subsurface oxygen (Osub) and surface oxide (Oox) were determined on Au(111) single crystal. Oxygen accumulation on Au(111) surface at 140 K for ΘO < 1.0 MLE of oxygen coverage resulted in the surface atomic oxygen (Oa) formation while 2D surface oxide (Oox) started to grow for ΘO > 1.0 MLE of oxygen coverage at the same temperature. It was also shown that oxygen atoms dissolved (Osub) into the bulk of the Au(111) single crystal when oxygen was accumulated at 473 K. Atomic oxygen species (Oa) on Au(111) was found to be very active for the cleavage of O-H and C-H bonds in methanol; C-C bond in acetaldehyde; N-H bond in ammonia molecules. Surface oxide (Oox) overlayer was also active for methanol oxidation, however it showed very high selectivity towards CO2. Dissolved oxygen atoms (Osub) revealed almost no activity in methanol oxidation reactions on Au(111). In a similar manner, three different oxygen species were determined on the Ag(111) surface such as surface atomic oxygen (Oa), surface oxide (Oox) and bulk-like oxide (Obulk) species. Disordered atomic oxygen (Oa) and surface oxide (Oox) overlayers prepared at 140 K on Ag(111) for ΘO ≤ 0.2 MLE were found to be very active for O-H and CH bond cleavage producing formaldehyde as the dominant product. Increasing oxygen quantity for both oxygen species (0.7 MLE ≤ ΘO ≤ 1.3 MLE) resulted mostly CO2 formation. Oa (ΘO < 1.10 MLE) was also found to be highly active in N-H bond cleavage for ammonia and selective to N2 as the dominant product. On the other hand, ordered p(5×1) and c(4×8) surface oxide (Oox) overlayers on Ag(111) prepared 473 K were found to be almost entirely inactive for N-H cleavage. Extreme oxygen exposures on Ag(111) (ΘO > 1.93 MLE ) at 140 K led to bulk-like silver oxide (Obulk) species with poor N2 selectivity in ammonia oxidation and increasing extent of formation of toxic pollutants such as NO and N2O.