Browsing by Subject "SOx"

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    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).
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    ItemOpen Access
    Finding an optimum surface chemistry for [Formula] systems as NOx storage materials
    (2010) Şentürk, Göksu Seda
    Titania 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 Ti
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    ItemOpen Access
    Investigation of NO2 and SO2 adsorption/desorption properties of advanced ternary and quaternary mixed oxides for DENOx catalysis
    (2015-11) Say, Zafer
    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.
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    ItemOpen Access
    Novel hybrid perovskite catalysts for DE-NOx applications
    (2015-09) Ercan, Kerem Emre
    The main purpose of this study is to identify the nature of hybrid perovskites in the form of LaCoxMn1-xO3 (x=0.0-1.0) for catalytic De-NOx applications. Characteristic structure, thermal stability and NOx/SOx adsorption/release properties of perovskites were studied by XRD, BET, XPS, in-situ FTIR, ex-situ FTIR, TEM, BET, TPD and TPR. LaCo0.8Mn0.2 and LaCo0.7Mn0.3O3 were found to yield the highest NOx storage Capacity (NSC) among other investigated perovskites due to their optimized B-site composition. NOx and SOx adsorption experiments pointed out that B-site substitution did not have a significant alteration on adsorption geometries of corresponding adsorbates. NOx uptakes of the investigated perovskites were observed to be enhanced via H2 reduction as verified by IR results. Furthermore, N2 (28 a.m.u) release monitored by QMS during NOx TPD revealed direct N-O bond activation and complete reduction of NOx species under certain conditions. SOx adsorption and reduction experiments suggested that SOx reduction via H2 is more effective for Mn-enrich perovskites, since Co-enriched materials formed irreversible sulfate species. It was observed that adsorbed NOx species can be readily replaced by SOx species in the co-presence of NOx and SOx. It was also demonstrated that the oxygen-defect density and the surface oxygen concentration of hybrid perovskites can be modified by fine-tuning the substitution at the B-site. Based on ex-situ FTIR results, it was established that Co-O linkages could be gradually replaced with Mn-O linkages by increasing the Mn loading in the perovskite composition. Furthermore, specific surface areas (SSA) of hybrid perovskites were found to be enhanced by increasing the Mn loading. Current results suggest that hybrid perovskites are promising novel catalytic architectures for De-NOx applications due to their high NSC and versatile chemical structure which can be fine-tuned to enhance SOx tolerance, redox properties and thermal stability.
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    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, Emrah
    Al2O3/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.