Akademik Veri Yönetim Sistemi
Tezin Türü: Doktora
Tezin Yürütüldüğü Kurum: Orta Doğu Teknik Üniversitesi, Fen Bilimleri Enstitüsü, Fen Bilimleri Enstitüsü, Türkiye
Tezin Onay Tarihi: 2014
Öğrenci: SEVAL GÜNDÜZ
Danışman: TİMUR DOĞU
Özet:Due to fast depletion of fossil fuel resources and related environmental impact of CO2 emissions, the interest in hydrogen as a clean energy carrier has recently increased. Hydrogen production from bio-ethanol, which already contains large amount of water, by steam reforming process, has shown excellent potential with CO2 neutrality and renewability. Steam reforming of ethanol (SRE) process has a highly complex reaction network including numerous side reactions which decrease hydrogen yield and have a negative effect on process economy. In addition, highly endothermic nature of the steam reforming of ethanol reaction raises the doubts on the economic feasibility of the process. Therefore, the main objectives in the present study are; (i) designing and synthesizing novel mesoporous catalysts which are highly active for steam reforming of ethanol reaction and highly stable in the presence of steam at elevated temperature conditions and (ii) developing a new reaction system that uses an alternative heat source which is more efficient than conventional heating. In the scope of the present study, Co-Mg and Ni-Mg incorporated mesoporous alumina type materials were synthesized following two different techniques; direct synthesis (one-pot) and impregnation routes with Co/Al or Ni/Al molar ratio of 0.10. Catalytic activities of the synthesized materials were tested in both conventionally heated and focused-microwave reactor systems available in our laboratory. Afterwards, several characterization techniques were applied to both fresh and used catalysts in order to understand the reasons of catalytic performance differences obtained. Co-Mg incorporated catalysts prepared by direct addition of Mg (Co-Mg-MA and Co@Mg-MA) and by impregnation/lack of Mg (Co-Mg@MA and Co@MA) showed very different behavior towards steam reforming of ethanol process. While catalysts synthesized by direct addition of Mg exhibited superior activity towards steam reforming of ethanol reaction with an average hydrogen yield of 5.2 out of the maximum hydrogen yield value of 6.0, the ones synthesized by impregnation/lack of Mg showed no activity for hydrogen production. The main product obtained with these catalysts was ethylene indicating that ethanol dehydration reaction dominated the reaction pathway over these catalysts. According to XRD and XPS analysis, Co-Mg-MA and Co@Mg-MA catalysts synthesized by direct addition of Mg involved Coo and CoO phases which are the active structures for SRE reaction. However Co-Mg@MA and Co@MA catalysts were mainly composed of CoAl2O4 structure and no Coo and CoO phases were observed in their framework which resulted in low activity towards SRE reaction. Additionally, DRIFTS analysis of pyridine adsorbed samples showed that acidity of catalysts prepared by impregnation/lack of Mg (Co-Mg@MA and Co@MA) was very high which can be considered as the main reason of relatively high selectivity of ethanol dehydration reaction over these catalysts. The best catalyst towards SRE reaction, Co@Mg-MA, was also tested in focused-microwave reaction system and it was observed that microwave-assisted SRE reaction provided slightly higher and more stable hydrogen yield. The most surprising outcome of microwave-assisted SRE reaction was that coke elimination was achieved, most probably due to the uniform temperature distribution reached under microwave heating which eliminates Boudouard reaction that is the main route for coke deposition. Unlike Co-Mg incorporated catalysts, a drastic difference was not observed between the hydrogen yields of Ni-Mg incorporated catalysts. Hydrogen yields obtained by Ni-Mg-MA, Ni-Mg@MA and Ni@Mg-MA catalysts were 4.0, 4.5 and 5.0, respectively. This slight difference was attributed to different Ni particle sizes of materials synthesized by different routes. It is a known fact that large Ni particles show higher catalytic activity in the methanation of CO which result in higher CH4 selectivity and lower H2 yield. According to XRD analysis, particle sizes of Ni-Mg-MA and Ni@Mg-MA were 20 nm and 7 nm, respectively. Therefore it is obvious that lower hydrogen yield observed in Ni-Mg-MA catalyst was due to the higher Ni particles present in its structure. The activity tests carried out in focused-microwave reactor system provided more stable product distribution and coke elimination.