Identification of NOx species adsorbed on cobalt(II)-supported zirconia and sulfated zirconia and investigation of their reactivity toward methane
Kobalt(II) depolanmış zirkonyum ve zirkonyum sülfat üzerine adsorbe olmuş NOx moleküllerinin tanımlanması ve metan ile tepkimesinin araştırılması
- Tez No: 129226
- Danışmanlar: DOÇ. DR. MARGARİTA KANTCHEVA
- Tez Türü: Yüksek Lisans
- Konular: Kimya, Chemistry
- Anahtar Kelimeler: Adsorption of NO and NO/02, in situ FTIR, cobalt(II) supported on zirconia and sulfated zirconia, CH4-SCR, intermediates. Vll
- Yıl: 2002
- Dil: İngilizce
- Üniversite: İhsan Doğramacı Bilkent Üniversitesi
- Enstitü: Mühendislik ve Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Kimya Ana Bilim Dalı
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 84
Özet
ÖZET KOBALT(II) DEPOLANMIŞ ZİRKONYUM ve ZİRKONYUM SÜLFAT ÜZEİNE ADSORBE OLMUŞ NQ MOLEKÜLLERİNİN TANIMLANMASI ve METAN ile TEPKİMESİNİN ARAŞTIRILMASI AHMET SELİM VAKKASOĞLU Kimya Bölümü Yüksek Lisans Tez Yöneticisi: Assoc. Prof. Margarita Kantcheva Temmuz 2002 Sabit ve hareketli kaynaklardan yayılan azot oks it (NOx) moleküllerinin seçici katalitik indirgenmesinde kullanılan katalizörlerde yapılan geliştirme çalışmalarına olan ilgi son yıllarda artmıştır. Bu ilginin iki sebebinden biri, seçici katalitik indirgenme yönteminde kullanılan amonyağın yerine metan kullanımı, diğeri ise içten yanmalı motorlardan yayılan karbondioksit gazının azaltılmasıdır. Bununla birlikte, 1992'de, oksijenli ortamda metan ile NOx moleküllerinin seçici katalitik indirgenmesinde etkili katalizör görevi yapan kobalt(II) depolanmış ZS M5 zeolitinin keşfedilmesinden bu yana fazla gelişme sağlanamamıştır. Birçok katalitik sistemin su buharından ciddi şekilde etkilendiği görülmüştür. Ayrıca bu sistemler gerçek atık ortamlarında (
Özet (Çeviri)
ABSTRACT IDENTIFICATION of NOx SPECIES ADSORBED on COBALT(ll)-SUPPORTED ZIRCONIA and SULFATED ZIRCONIA and INVESTIGATION of THEIR REACTIVITY TOWARD METHANE AHMET SELİM VAKKASO?LU M.S. in Chemistry Supervisor: Assoc. Prof. Margarita Kantcheva July 2002 The development of new catalysts for the selective catalytic reduction (SCR) of nitrogen oxide (NOx) emissions from both stationary and mobile sources has gained an increasing interest in the past decade. This is due to the need for replacing the NH3-SCR process with possibly a CH4-SCR process (methane-selective catalytic reduction) and demands for improvement of fuel economy in internal combustion engines with low CO2 emissions. However, little progress had been made since the discovery in 1992 of cobalt(II)-exchanged ZSM5 and ferrierite as effective catalysts for the SCR of NOx with methane in presence of oxygen. Most of the catalytic systems studied are severely affected by the presence of water vapor and do not show practical activity under actual exhaust conditions, at least below 773 K. The search for highly active and hydro- thermally stable catalysts for reduction of NO with methane in presence of oxygen at low reaction temperatures continues. The present work is a fundamental study aimed at investigation the possibility of application of solid acids based on sulfated zirconia (active in the isomerization of light saturated hydrocarbons) as catalysts for CH4-SCR of NO. The activation of light alkanes in the isomerization process on catalysts such as sulfated zirconia starts with cleavage of the C-H bond. The same process is important in the activation of methane in the SCR of NO. Sulfated zirconia possesses moderately strong surface acidity, weaker than that ofZSM5. However, its structure is not sensitive to water and the surface sulfate groups are stable up to 850 K. This work involves FTIR spectroscopic studies on the interaction of methane with the surface of cobalt (II) supported on zirconia and sulfated zirconia and investigations of the role of preadsorbed NOx species in the process of hydrocarbon activation. The samples have been prepared by wet impregnation of zirconia and sulfated zirconia using cobalt(II) acetate as a precursor. The catalyst containing 5.3 wt % of cobalt and 4.5 wt% of sulfated ions (CoSZ sample) has been investigated in detail. In order to understand the effect of the sulfate ions on the performance of the cobalt(II)-zirconia system, the same amount of cobalt (5.0 wt %) was introduced into sulfate free- zirconia (CoZ sample). The catalysts have been characterized by XRD and in situ FTIR spectroscopy. The monoclinic structure of zirconia support converts to tetragonal after the deposition of cobalt(II). The sulfated zirconia and the CoZS catalyst have the same phase composition (67 % tetragonal). The application of sulfated zirconia as support causes decrease in the average crystallite size, increase in the surface area and better dispersion of the cobalt ions. The introduction of cobalt(II) to zirconia and sulfated zirconia leads to increase in the basicity of the surface hydroxyls. The adsorption of NO on both catalysts reveals that there are no exposed coordinatively unsaturated Zr4+ ions and leads to formation of cobalt(II) dinitrosyls. In addition, a process of NO disproportionation with the involvement of the surface OH groups takes place producing anionic nitrosyl (NO-), nitro (NO2") species and water molecules. The sulfate groups decrease the reducibility of the Co2+ sites and oxidize readily the anionic nitrosyls. On coadsorption of NO and 02 at room temperature on the samples studied, various kinds of surface nitro-nitrato species are observed differing in the modes of their coordination. The nitro-nitrato structures on the sulfated sample are characterized by a lower thermal stability than that on the CoZ sample. The difference in the thermal stability of the NOx~ (x = 2 and/or 3) parallels their activity toward methane. Most of the nitrate species on the sulfate-free catalyst do not interact with the hydrocarbon and they transform to N02~ (nitro) structures. In contrast, the nitro-nitrato species formed on the viCoSZ catalyst are able to oxidize the methane producing formic acid, formate, carbonate/carboxylate species and water. The presence of sulfate ions in the CoSZ catalyst affects the reactivity of the adsorbed NOx species and increases the acidity of the Lewis sites, which facilitates the activation of methane. The NOx-modified CoSZ catalyst displays higher activity than the NOx-free catalyst, which shows the importance of the surface nitro-nitrato complexes in the activation of methane as selective reducers of NO. Taking into account the higher hydrothermal stability of sulfated zirconia than that of ZSM5, the materials containing cobalt(II) supported on sulfated zirconia could be promising as CH4-SCR catalysts.
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