Browsing by Subject "NOx"

Now showing 1 - 8 of 8

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).
Open Access
FT-IR spectroscopic investigation of the reactivity of NOx species adsorbed on Cu2+/ZrO2 and CuSO4/ZrO2 catalysts toward decane
(Elsevier, 2003-04-15) Kantcheva, M.
The nature of the NOx species produced on NO adsorption and its co-adsorption with O-2 at room temperature on zirconia-supported copper(II) catalysts has been studied by means of in situ FT-IR spectroscopy. The samples were prepared by impregnation of zirconia with aqueous solutions of copper(II) nitrate and sulfate. The structural identification of the surface NOx complexes exhibiting absorptions in the fundamental nitro-nitrato region was performed by analyzing the combination bands of the nitrate species. In order to understand which factors control the selectivity of the catalysts in the catalytic reduction of NO by longer chain hydrocarbons, the stability of surface nitro-nitrato species and their reactivity toward adsorbed decane at various temperatures was investigated. The nitrates on the CuSO4/ZrO2 catalyst are characterized by significantly lower thermal stability than the nitro-nitrato species on the Cu2+/ZrO2 sample. The difference in the thermal stability of the NOx- species (x is 2 and 3) parallels their reactivity toward the adsorbed decane. The sulfate-free catalyst contains bidentate nitro species that are inert toward the hydrocarbon. The bidentate nitro species start to decompose to NO at temperatures higher than 523 K. In contrast, the nitrate species formed on the CuSO4/ZrO2 catalyst are able to oxidize the adsorbed decane completely at 523 K producing acetates, formates, adsorbed CO and isocyanate species. It is proposed that the presence of stable nitro species on the sulfate-free copper(II)-zirconia catalyst is associated with its non-selective behavior above 573 K in the reduction of NO with decane in an excess of oxygen reported in the literature. (C) 2002 Elsevier Science B.V. All rights reserved.
Open Access
In-situ FT-IR spectroscopic investigation of NO(formula) + CH(formula) surface reactions on palladium promoted WO(formula)/TiO(formula)-ZrO(formula) mixed oxides
(2005) Ağıral, Anıl
The interaction of methane at various temperatures with NOx species formed by room temperature adsorption of NO/O2 mixture on tungstated zirconia-titania (25 wt % of WO3, denoted as WZT) and palladium(II)-promoted (1.5 wt % of Pd) zirconia-titania (1.5Pd/ZT) and tungstated zirconia-titania (1.5Pd/WZT) is investigated using in situ FTIR spectroscopy. The structure and surface properties of ZT, WZT, 1.5Pd/ZT and 1.5Pd/WZT are studied by XRD, DR-UV-Vis spectroscopy and FT-IR spectroscopy of adsorbed CO and NO. Zirconia-titania was prepared by a homogenous coprecipitation of urea at 70oC. Formation of crystalline ZrTiO4 compound at calcination temperature of 600oC is observed. Based on the data of XRD and DR-UV-Vis spectra, very good mixing of oxides has been achieved with high surface area (118 m2 /g) and small crystallite size (4.4 nm). The WZT sample has paratungstate type polytungstate species forming intermediate WOx surface domains which give rise to strong Brønsted acidity. Pd-containing samples were prepared impregnating the ZT and WZT samples with Pd(NO3)2.2H2O solution. The WZT support can stabilize isolated Pd2+ ions coordinated to surface oxygen atoms. The spectrum of CO adsorbed on the ZT sample reveals the presence of coordinatively unsaturated (cus) Zr4+ and Ti4+ sites. Their amount decreases considerably after the modification of the sample with WO3. The adsorption of CO and NO on the 1.5Pd/ZT and 1.5Pd/WZT samples indicates the presence of palladium ions in two different environments. The adsorption of NO at room temperature on the samples studied involves process of disproportionation of NO on surface oxide ions leading to formation of adsorbed anionic nitrosyl, NOí , and NO2. The addition of molecular oxygen to the NO causes its oxidation to NO2/N2O4. These gases adsorb molecularly over the surface and undergo self-ionization and disproportionation with the participation of surface hydroxyl groups. Introduction of WOx species and Pd2+ ions to the zirconia-titania mixed oxide hinders the processes of NO2/N2O4 self-ionization and disproportionation by elimination of the necessary active sites. NOx species formed at room temperature on the WZT and 1.5Pd/ZT samples suppress the oxidation of the methane, whereas in the case of the 1.5Pd/WZT catalyst the surface nitrates initiate the formation of nitromethane. Mechanism for the reduction of NO over the 1.5Pd/WZT catalyst is proposed, which involves a step of thermal decomposition of the nitromethane to adsorbed NO and partially oxidized hydrocarbons (methoxy and/or formate species) through the intermediacy of cis-methyl nitrite. The reduction of the adsorbed NO by the partially oxidized hydrocarbons leads to the products of the CH4-SCR, molecular nitrogen and carbon oxides. Under in situ conditions, nitromethane and cis-methyl nitrite are stabilized on the surface of the 1.5Pd/WZT catalyst, whereas the adsorption of the authentic reagent results in adsorbed nitromethane and its trans isomer. It is concluded that nitromethane formed in situ and authentic nitromethane follow different decomposition routes.
Open 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.
Open Access
N-O activation on precious metal-free metal oxide based NOx removal systems
(2022-01) Ercan, Kerem Emre
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.
Open 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.
Open Access
NOx storage and reduction pathways on zirconia and titania functionalized binary and ternary oxides as NOx storage and reduction (NSR) systems
(Elsevier, 2014-08-01) Say, Z.; Tohumeken, M.; Ozensoy, E.
Binary and ternary oxide materials, ZrO2/TiO2 (ZT) and Al2O3/ZrO2/TiO2 (AZT), as well as their Ptfunctionalized counterparts were synthesized and characterized via XRD, Raman spectroscopy, BET, in situ FTIR and TPD techniques. In the ZT system, a strong interaction between TiO2 and ZrO2 domains at high temperatures (>973K) resulted in the formation of a low specific surface area (i.e. 26 m2/g at 973K) ZT material containing a highly ordered crystalline ZrTiO4 phase. Incorporation of Al2O3 in the AZT structure renders the material highly resilient toward crystallization and ordering. Alumina acts as a diffusion barrier in the AZT structure, preventing the formation of ZrTiO4 and leading to a high specific surface area (i.e. 264 m2/g at 973K). NOx adsorption on the AZT system was found to be significantly greater than that of ZT, due to almost ten-fold greater SSA of the former surface. While Pt incorporation did not alter the type of the adsorbed nitrate species, it significantly boosted the NOx adsorption on both Pt/ZT and Pt/AZT systems. Thermal stability of nitrates was higher on the AZT compared to ZT, most likely due to the defective structure and the presence of coordinatively unsaturated sites on the former surface. Pt sites also facilitate the decomposition of nitrates in the absence of an external reducing agent by shifting the decomposition temperatures to lower values. Presence of Pt also enhances partial/complete NOx reduction in the absence of an external reducing agent and the formation of N2 and N2O. In the presence of H2(g), reduction of surface nitrates was completed at 623K on ZT, while this was achieved at 723K for AZT. Nitrate reduction over Pt/ZT and Pt/AZT via H2(g) under mild conditions initially leads to conversion of bridging nitrates into monodentate nitrates/nitrites and the formation of surface OH and NHx functionalities. N2O(g) was also continuously generated during the reduction process as an intermediate/byproduct.
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.