Browsing by Subject "DeNOx"
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Item Open Access Core-crown quantum nanoplatelets with favorable type-II heterojunctions boost charge separation and photocatalytic NO oxidation on TiO2(Wiley, 2020-09) Ebrahimi, Elnaz; İrfan, Muhammad; Shabani, Farzan; Koçak, Yusuf; Karakurt, Bartu; Erdem, E.; Demir, Hilmi Volkan; Özensoy, EmrahFunctionalization of TiO2 (P25) with oleic acid‐capped CdSe(core)/CdSeTe(crown) quantum‐well nanoplatelets (NPL) yielded remarkable activity and selectivity toward nitrate formation in photocatalytic NOx oxidation and storage (PHONOS) under both ultraviolet (UV‐A) and visible (VIS) light irradiation. In the NPL/P25 photocatalytic system, photocatalytic active sites responsible for the NO(g) photo‐oxidation and NO2 formation reside mostly on titania, while the main function of the NPL is associated with the photocatalytic conversion of the generated NO2 into the adsorbed NO3− species, significantly boosting selectivity toward NOx storage. Photocatalytic improvement in NOx oxidation and storage upon NPL functionalization of titania can also be associated with enhanced electron‐hole separation due to a favorable Type‐II heterojunction formation and photo‐induced electron transfer from the CdSeTe crown to the CdSe core of the quantum well system, where the trapped electrons in the CdSe core can later be transferred to titania. Re‐usability of NPL/P25 system was also demonstrated upon prolonged use of the photocatalyst, where NPL/P25 catalyst surpassed P25 benchmark in all tests.Item Open Access Enhanced photocatalytic NOx oxidation and storage under visible-light irradiation by anchoring Fe3O4 nanoparticles on mesoporous graphitic carbon nitride (mpg-C3N4)(Elsevier, 2019) Irfan, Muhammad; Sevim, M.; Koçak, Yusuf; Balcı, Merve; Metin, Ö.; Özensoy, EmrahSeveral mesoporous graphitic carbon nitride (mpg-C3N4) photocatalysts were synthesized by using a hard-templating method comprising thermal polycondensation of guanidine hydrochloride over silica spheres at three different temperatures (450, 500 and 550 ℃). After structural characterization of these mpg-C3N4 photocatalysts, they were tested in NO(g) photo-oxidation under visible (VIS) light. The effects of polycondensation temperature on the structure and photocatalytic performance of mpg-C3N4 in NO photo-oxidation were studied. The results revealed that polycondensation temperature has a dramatic effect on the photocatalytic activity of mpg-C3N4 in NO photo-oxidation, where mpg-C3N4 synthesized at 500 ℃ (mpg-CN500) showed the best performance in NOx abatement as well as a high selectivity towards solid state NOx storage under VIS light illumination. Photocatalytic performance of the mpg-CN500 was further enhanced by the anchoring of 8.0 ± 0.5 wt.% Fe3O4 nanoparticles (NPs) on it. Fe3O4/mpg-CN500 photocatalyst showed both high activity and high selectivity along with extended reusability without a need for a regeneration step. Enhanced photocatalytic NOx oxidation and storage efficiency of Fe3O4/mpg-CN500 photocatalyst was attributed to their mesoporous structure, high surface area and slow electron-hole recombination kinetics, efficient electron-hole separation and facile electron transfer from mpg-CN500 to Fe3O4 domains enhancing photocatalytic O2 reduction, while simultaneously suppressing nitrate photo-reduction and decomposition to NO2(g).Item Open Access Enhanced sulfur tolerance of ceria-promoted NOx storage reduction (NSR) catalysts: sulfur uptake, thermal regeneration and reduction with H2(g)(Springer New York LLC, 2013) Say, Z.; Vovk, E. I.; Bukhtiyarov, V. I.; Ozensoy, E.SOx uptake, thermal regeneration and the reduction of SOx via H2(g) over ceria-promoted NSR catalysts were investigated. Sulfur poisoning and desulfation pathways of the complex BaO/Pt/CeO2/Al2O3 NSR system was investigated using a systematic approach where the functional sub-components such as Al2O3, CeO2/Al2O3, BaO/Al2O3, BaO/CeO2/Al2O3, and BaO/Pt/Al2O3 were studied in a comparative fashion. Incorporation of ceria significantly increases the S-uptake of Al2O3 and BaO/ Al2O3 under both moderate and extreme S-poisoning conditions. Under moderate S-poisoning conditions, Pt sites seem to be the critical species for SOx oxidation and SOx storage, where BaO/Pt/Al2O3 and BaO/Pt/CeO2/Al2O3 catalysts reveal a comparable extent of sulfation. After extreme S-poisoning due to the deactivation of most of the Pt sites, ceria domains are the main SOx storage sites on the BaO/Pt/CeO2/Al2O3 surface. Thus, under these conditions, BaO/Pt/CeO2/Al2O3 surface stores more sulfur than that of BaO/Pt/Al2O3. BaO/Pt/CeO2/Al2O3 reveals a significantly improved thermal regeneration behavior in vacuum with respect to the conventional BaO/Pt/Al2O3 catalyst. Ceria promotion remarkably enhances the SOx reduction with H2(g).Item Open Access Finding an optimum surface chemistry for [Formula] systems as NOx storage materials(Bilkent University, 2010) Şentürk, Göksu SedaTitania promoted NOx storage materials in the form of BaO/TiO2/Al2O3 were synthesized via two different sol-gel preparation techniques, with varying surface compositions and morphologies [1, 2]. The influence of the TiO2 units on the NOx storage component (8 - 20 wt. % BaO), the nature of the crystallographic phases, thermal stabilities and the dispersion of the surface oxide/nitrate domains were investigated. The structural characterization of the synthesized NOx storage materials were analyzed by means of BET surface area analysis, X-ray diffraction (XRD), ex-situ Raman spectroscopy, scanning electron microscopy (SEM), energy dispersive X- ray (EDX) and transmission electron microscopy (TEM). Comparative analysis of the results showed that the TiO2/Al2O3 support material derived by the co-precipitation of the corresponding hydroxides via the sol-gel technique, exhibited distinctively more homogenous distribution of TiO2 domains. The functionality/performance of these materials upon NOx and SOx adsorptions were monitored by temperature programmed desorption (TPD) and insitu Fourier transform infrared (FTIR) spectroscopy. An improved Ba surface dispersion was observed for the BaO/TiO2/Al2O3 materials synthesized via the coprecipitation of alkoxide precursors which was found to originate mostly from the increased fraction of accessible TiO2/TiOx sites on the surface. These TiO2/TiOx sites functioned as strong anchoring sites for surface BaO domains and were tailored to enhance surface dispersion of BaO. The relative stability of the NOx species adsorbed on the BaO/TiO2/Al2O3 system was found to increase in the following order: NO+ /N2O3 on alumina << nitrates on alumina < surface nitrates on BaO < bridged/bidentate nitrates on large/isolated TiO2 clusters < bulk nitrates on BaO on alumina surface and bridged/bidentate nitrates on TiO2 crystallites homogenously distributed on the surface < bulk nitrates on the BaO sites located on the TiO2 domains. The detailed study of the interaction of SOx with BaO/TiO2/Al2O3 ternary oxide materials showed that titania (TiO2) was a promising candidate for improving the sulfur tolerance on these type of surfaces. Adsorption of SOx on both pure Al2O3 and TiO2 showed that Al2O3 formed strongly bound SOx species, that were thermally stable up to T > 1073 K. SOx adsorption directly altered stability of the nitrate species on the Ti/Al (Protocol 1, Protocol 2) samples. SOx uptake properties of the BaO/TiO2/Al2O3 materials were found to be strongly influenced by the morphology of the TiO2/TiOx domains and the BaO loadings (8/20 wt% BaO). Consequently, the presence of titania domains was seen to decrease the SOx desorption temperatures and enhance the sulfur-tolerance of these materials by destabilizing the accumulated sulfate species. SOx exposure on the synthesized materials led to a significant decrease in the NOx adsorption capacities. The results obtained from FT-IR spectra showed that the sulfur deposition on the NOx storage materials promoted by TiItem Open Access Palladium doped perovskite-based NO oxidation catalysts: the role of Pd and B-sites for NOx adsorption behavior via in-situ spectroscopy(Elsevier, 2014) Say, Z.; Dogac, M.; Vovk, E. I.; Kalay, Y. E.; Kim, C. H.; Li, W.; Ozensoy, E.Perovskite-based materials (LaMnO3, Pd/LaMnO3, LaCoO3 and Pd/LaCoO3) were synthesized, characterized (via BET, XRD, Raman spectroscopy, XPS and TEM) and their NOx (x = 1,2) adsorption characteristics were investigated (via in-situ FTIR and TPD) as a function of the nature of the B-site cation (i.e. Mn vs Co), Pd/PdO incorporation and H2-pretreatment. NOx adsorption on of LaMnO3 was found to be significantly higher than LaCoO3, in line with the higher SSA of LaMnO3. Incorporation of PdO nanoparticles with an average diameter of ca. 4 nm did not have a significant effect on the amount of NO2 adsorbed on fresh LaMnO3 and LaCoO3. TPD experiments suggested that saturation of fresh LaMnO3, Pd/LaMnO3, LaCoO3 and Pd/LaCoO3 with NO2 at 323K resulted in the desorption of NO2, NO, N2O and N2 (without O2) below 700K, while above 700K, NOx desorption was predominantly in the form of NO + O2. Perovskite materials were found to be capable of activating N–O linkages typically at ca. 550K (even in the absence of an external reducing agent) forming N2 and N2O as direct NOx decomposition products. H2-pretreatment yielded a drastic boost in the NO oxidation and NOx adsorption of all samples, particularly for the Cobased systems. Presence of Pd further boosted the NOx uptake upon H2-pretreatment. Increase in the NOx adsorption of H2-pretreated LaCoO3 and Pd/LaCoO3 surfaces could be associated with the electronic changes (i.e. reduction of B-site cation), structural changes (surface reconstruction and SSA increase), reduction of the precious metal oxide (PdO) into metallic species (Pd), and the generation of oxygen defects on the perovskite. Mn-based systems were more resilient toward B-site reduction. Pd-addition suppressed the B-site reduction and preserved the ABO3 perovskite structure.Item Open Access Photocatalytic NOx oxidation and storage under ambient conditions for air purification(Bilkent University, 2012) Soylu, Aslı MelikeAir pollution is one of the most serious environmental problems in both urban and rural settings with a direct impact on human health. A variety of chemical compounds can be associated with air pollution and gaseous nitrogen oxides (NOx), such as NO and NO2, are especially among the most hazardous environmental pollutants. NOx abatement can be efficiently performed at elevated temperatures (i.e. T > 300oC), however, an important challenge in air purification is the abatement of gaseous NOx species under ambient conditions (i.e. at room temperature and under regular atmospheric conditions). Photocatalytic systems offer promising opportunities in order to tackle this important environmental challenge, as these systems can be tailored to efficiently clean/purify air under ambient conditions with the help of ultraviolet (UV) and/or visible (VIS) light. In the current work, a hybrid technology for the photocatalytic oxidation and storage of gas phase NOx species is proposed where titania based powders are investigated as candidate photocatalytic materials. With this aim, various components of a thermally activated conventional NOx Storage/Reduction (NSR) catalyst is combined with a photocatalytically activated NOx oxidation catalyst to obtain a photocatalytically activated NOx oxidation and storage material. In this regard, three different sets of samples were prepared and investigated. The first set of photocatalysts was prepared by employing Al2O3, a high surface area support material, in order to disperse the photocatalytically active titania in an effective manner. Using a ―sol-gel co-precipitation method‖, TiO2/Al2O3 binary oxides were synthesized (where TiO2:Al2O3 mole ratio was chosen to be 0.25, 0.5, 1.0) and characterized by X-ray diffraction, Raman Spectroscopy and BET. For these samples, the effects of specific surface area, calcination temperature and the crystallinity of TiO2 were investigated in relevance to the photocatalytic NOx oxidation/storage reaction. Next, an alkali/alkaline earth oxide storage component is added to the TiO2- Al2O3 mixture and the incorporation of the storage component is achieved via two different routes; (i) either through ―incipient wetness impregnation‖ of 5 or 10% (w/w) metal nitrate [M(NO3)x] salts on TiO2-Al2O3 and a subsequent calcination to obtain alkali/alkaline earth oxides [MyO] or (ii) by physically grinding 5 or 10% (w/w) BaO powder with TiO2-Al2O3 binary oxide to obtain a ternary mixture. For these samples, the route of metal oxide incorporation (impregnation vs. physical mixture), the type of metal oxide storage component (alkali vs. alkaline earth metal) and the percentage of metal oxide loading (5% vs. 10%, w/w) were examined in photocatlytic NOx oxidation/storage reaction. The photonic efficiencies of these samples were tested using a continuous flow system, composed of mass flow controllers, a custom-made UVA-illuminated reaction cell and an ambient chemiluminescence NOx analyzer. Photocatalytic performance of all samples were compared with that of a commercially available Degussa P25 TiO2 benchmark catalyst. Photocatalytic preformance tests revealed that the TiO2-Al2O3 binary oxides had much higher NOx storage capacities compared to Degussa P25 and the further addition of an alkaline earth oxide (BaO) storage component on TiO2-Al2O3 by physical mixing significantly enhanced the NOx capture in solid state and decreased unwanted gaseous NO2 emission to an almost negligible level. On the other hand, the ―incipient wetness impregantion‖ of metal nitrates resulted in metal titanate (MxTiyOz) formation on TiO2-Al2O3 binary oxide and diminished the photooxidation ability of the catalyst.Item Open Access Sulfur poisoning and regeneration behavior of perovskite-based NO oxidation catalysts(Springer New York LLC, 2017) Kurt M.; Say, Z.; Ercan, K. E.; Vovk, E. I.; Kim, C. H.; Ozensoy, E.SOxuptake and release properties of LaMnO3, Pd/LaMnO3, LaCoO3and Pd/LaCoO3perovskites were investigated via in situ Fourier transform infrared (FTIR) spectroscopy, temperature programmed desorption and X-ray photoelectron spectroscopy. Sulfation of the perovskite leads to the formation of surface sulfite/sulfate and bulk-like sulfate species. Pd addition to LaMnO3and LaCoO3significantly increases the sulfur adsorption capacity. Pd/LaMnO3sample accumulates significantly more sulfur than LaMnO3; however it can also release a larger fraction of the accumulated SOxspecies in a reversible fashion at elevated temperatures in vacuum. This is not the case for Co-based materials, where thermal regeneration of bulk sulfates on poisoned LaCoO3and Pd/LaCoO3is extremely ineffective under similar conditions. However, in the presence of an external reducing agent such as H2(g), Pd/LaMnO3requires much lower temperature (873�K) for complete sulfur regeneration as compared to that of Pd/LaCoO3(973�K). Sequential CO and SOxadsorption experiments performed via in situ FTIR indicate that in the presence of carbonyls and/or carbonates, Pd adsorption sites may have a stronger affinity for SOxas compared to that of the perovskite surface, particularly in the early stages of sulfur poisoning.Item Open Access TiO2-Al2O3 binary mixed oxide surfaces for photocatalytic NOx abatement(Elsevier, 2014) Soylu, A. M.; Polat, M.; Erdogan, D. A.; Say, Z.; Yıldırım, C.; Birer, Ö.; Ozensoy, E.TiO2-Al2O3 binary oxide surfaces were utilized in order to develop an alternative photocatalytic NOx abatement approach, where TiO2 sites were used for ambient photocatalytic oxidation of NO with O2 and alumina sites were exploited for NOx storage. Chemical, crystallographic and electronic structure of the TiO2-Al2O3 binary oxide surfaces were characterized (via BET surface area measurements, XRD, Raman spectroscopy and DR-UV-Vis Spectroscopy) as a function of the TiO2 loading in the mixture as well as the calcination temperature used in the synthesis protocol. 0.5 Ti/Al-900 photocatalyst showed remarkable photocatalytic NOx oxidation and storage performance, which was found to be much superior to that of a Degussa P25 industrial benchmark photocatalyst (i.e. 160% higher NOx storage and 55% lower NO2(g) release to the atmosphere). Our results indicate that the onset of the photocatalytic NOx abatement activity is concomitant to the switch between amorphous to a crystalline phase with an electronic band gap within 3.05-3.10 eV; where the most active photocatalyst revealed predominantly rutile phase together and anatase as the minority phase.Item Open Access Trade-off between NOx storage capacity and sulfur tolerance on Al2O3/ZrO2/TiO2–based DeNOx catalysts(Elsevier, 2019) Say, Zafer; Mihai, O.; Kurt, Merve; Olsson, L.; Özensoy, EmrahAl2O3/ZrO2/TiO2 (AZT) ternary mixed oxides functionalized with Pt and BaO were synthesized in powder and monolithic forms and were utilized in NOx Storage Reduction/Lean NOx Trap (NSR/LNT) catalysis as novel catalytic materials. Adsorption of NOx and SOx species and their interactions with the catalyst surfaces were systematically investigated via in-situ FTIR technique revealing different NOx coordination geometries governed by the presence and the loading of BaO in the powder catalyst formulation. While BaO-free Pt/AZT stored NOx as surface nitrates, BaO incorporation also led to the formation of bulk-like ionic nitrate species. NOx adsorption results obtained from the current Temperature Programmed Desorption (TPD) data indicated that NOx Storage Capacity (NSC) was enhanced due to BaO incorporation into the powder catalyst and NSC was found to increase in the following order: Pt/AZT < Pt/8BaO/AZT < Pt/20BaO/Al2O3 < Pt/20BaO/AZT. Increase in the NSC with increasing BaO loading was found to be at the expense of the formation of bulk-like sulfates after SOx exposures. These bulk-like sulfates were observed to require higher temperatures for complete regeneration with H2(g). Catalytic activity results at 473 K and 573 K obtained via flow reactor tests with monolithic catalysts suggested that Pt/AZT and Pt/8BaO/AZT catalysts with stronger surface acidity also revealed higher resistance against sulfur poisoning and superior SOx regeneration in spite of their relatively lower NSC. Monolithic Pt/ 20BaO/AZT catalyst revealed superior NSC with respect to the conventional Pt/20BaO/Al2O3 benchmark catalyst at 573 K after sulfur regeneration. On the other hand, this trend was reversed at high-temperatures (i.e. 673 K). Preliminary results were presented demonstrating the enhancement of the high-temperature NSC of AZTbased materials by exploiting multiple NOx-storage components where BaO functioned as the low/mid-temperature NOx-storage domain and K2O served as the high-temperature NOx storage domain. Enhancement in the high-temperature NOx-storage in the BaO-K2O multiple storage domain systems was attributed to the formation of additional thermally stable bulk-like nitrates upon K2O incorporation.Item Open Access Utilization of reducible mixed metal oxides as promoters for the enhancement of sulfur regeneration in nsr catalysts(Bilkent University, 2016-07) Samast, Zehra AybegümPt 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.