Akademik Veri Yönetim Sistemi
Tezin Türü: Doktora
Tezin Yürütüldüğü Kurum: Orta Doğu Teknik Üniversitesi, Mühendislik Fakültesi, Kimya Mühendisliği Bölümü, Türkiye
Tezin Onay Tarihi: 2014
Tezin Dili: İngilizce
Öğrenci: Hale Ay
Danışman: DENİZ ÜNER
Özet:Carbon dioxide reforming of methane is a promising process for the utilization of two important greenhouse gases and the production of synthesis gas with a lower H2/CO ratio which is preferred in Fischer–Tropsch synthesis. Ni catalysts have taken great interest in dry reforming of methane due to their high catalytic activity, easy availability and low cost. However, the main restriction of Ni-based catalysts is the formation of carbon which causes catalyst deactivation. The objective of this thesis was to reduce carbon formation and improve the life time of Ni based catalysts. Al2O3 supported and CeO2 supported Ni, Co and Ni-Co catalysts were prepared via incipient wetness impregnation method. Al2O3 was used as support material due to its high thermal stability. CeO2 was used as another support material due to its redox Ce4+/Ce3+ sites and ability to exchange oxygen. Co was introduced to the Ni-based catalyst in order to understand the effect of bimetallic catalysts in dry reforming of methane. The catalysts prepared by incipient wetness impregnation method were calcined at two different temperatures (700°C and 900°C) to elucidate the effect of calcination temperature. As an alternative to conventional preparation methods, Ni/Al2O3 catalysts were also prepared by polyol method. Ni/Al2O3 and Ni-Co/Al2O3 catalysts exhibitied comparable activities in terms of CH4 and CO2 conversion; H2 and CO yield. While doing the calcination at 700°C or 900°C did not really affect the catalytic performances of Ni/Al2O3 and Ni-Co/Al2O3, it had significant influence on the performance of Co/Al2O3. Carrying out the calcination at higher temperatures was found to be more preferable in terms of carbon deposition. Higher amount of coke was deposited on Ni-Co/Al2O3 compared to Ni/Al2O3 when the calcination was done at 700°C. DRIFTS and microcalorimetry studies showed that CO2 activation took place on Al2O3 for both Ni/Al2O3 and Ni-Co/Al2O3 catalysts. 13C NMR characterization of deposited coke demonstrated that it not only originated from CH4, but CO2 had also significant role in coke formation. Polyol process was employed to obtain Ni/Al2O3 catalysts in the presence of PVP as stabilizer and ethylene glycol as both solvent and reducing agent. The effect of PVP/Ni ratio was studied. Ni/Al2O3-without-PVP and Ni/Al2O3-PVP/Ni=2.5 catalysts exhibited quite high performance at 600°C and 700°C without making a reduction step before the reaction. The activities of CeO2 supported catalysts decreased with increasing calcination temperature. While Ni/CeO2 and Ni-Co/CeO2 provided comparable high activities, Co/CeO2 was shown to be an inactive catalyst for dry reforming of methane reaction at the specified operating conditions. O2 and CO pulse experiments were done to get information about the redox properties of different kinds of ceria samples, including commercial ceria, coprecipitated ceria and ceria fibers. The ceria fibers having the lowest surface area was shown to have the highest oxygen uptake value. This phenomenon demonstrated that the oxygen storage capacity of the ceria samples depended strongly on the morphology.