CO2 reduction in reverse flow reactors

Aksu, Nevzat Can
Syngas, a mixture of CO and H2, is a very important industrial gas mixture, because it can be used for synthesizing products such as synthetic natural gas, ammonia and methanol. The main purpose of this work was to obtain CO by CO2 reduction. Reducible metal oxides are used as catalysts due to the fact that they can be reduced and re-oxidized upon changes in temperature, pressure and gas atmosphere. CeO2 was chosen for this cyclic reaction pathway based on the reports on similar studies in literature. It is preferred due to its not toxic nature and it high melting point. Pt was used to lower the oxygen desorption temperature and Al2O3 was also used to get higher surface area. Reversed flow reactors (RFR) work by changing the gas flow direction periodically as its name indicates. This type of reactor is used in various applications in industry. Oxidation-Reduction reactions are also very suitable for this reactor because materialin the reactor can be used as a heat sink for one exothermic and one endothermic reaction. In order to study the red-ox properties of the selected material, TPD analyses of Al2O3, Al2O3-CeO2, Pt/CeO2 and Pt/Al2O3-CeO2 were performed. Two different methods were used to prepare the samples. These are the incipient wetness method and the polyol method. Effects of sample preparation methods and Pt amount in sample were investigated. H2O and CO2 adsorption/desorption amounts for the samples that have different loadings of Pt were investigated. It was shown that H2O and CO2 adsorption/desorption amount increases with increasing Pt concentration in the catalyst. The H2O adsorption amount of the catalyst that has 1% Pt is 1.58 times the amount of catalysis that has 0.5% Pt. CO2 adsorption amount of catalysis that has 1% Pt is about 4.5 times the amount of catalysis that has 0.5% Pt. Desorption orders of H2O and CO2 from Pt/Al2O3-CeO2 were found to be first order and second order respectively. Thermodynamic analysis of equilibrium conversion of the red-ox reaction supports that CeO2 is turned to Ce2O3 after oxygen desorption. The results of thermodynamic calculations show that there is no oxygen desorption up to 1400 0C. This situation is also demonstrated by TPD experiments. When 1% Pt was added to the surface of CeO2, oxygen desorption peak was observed at about 900 0C. These experiments indicate that Pt can lower oxygen desorption temperature. One of the purposes of this thesis was to construct an automated flow reversal temperature programmed desorption system and making and analyzing TPD experiments. An automatic flow reversal system was designed and constructed to make cyclic reduction and oxidation reactions with metal oxides. The system was also tested in this work and it is seen that the system can work properly. During these tests, the effect of high surface area that was obtained by using Al2O3 on oxygen desorption is demonstrated. In addition to all of these, oxygen desorption experiments were made by the catalysts that were prepared by two different methods. Experiments show that the catalyst which was prepared by polyol method has 10 times more oxygen desorption amount than the catalyst which was made by incipient wetness method. By this way, effect of Pt dispersion on the oxygen evolution from the catalyst surface was demonstrated.


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Citation Formats
N. C. Aksu, “CO2 reduction in reverse flow reactors,” M.S. - Master of Science, Middle East Technical University, 2013.