NICKEL-COBALT SOLID SOLUTION AND ITS APPLICATIONS AS CATALYSTS FOR CARBON DIOXIDE REFORMING OF METHANE

Global warming issue becomes more significant to human beings and other organisms on the earth. Among many greenhouse gases, carbon dioxide (CO2) has the largest contribution to global warming. To find an effective way to utilize the greenhouse gas is urgent. It is the best way to convert CO2 to useful compounds. CO2 reforming of methane is an attractive process to convert CO2 and methane into synthesis gas (CO/H2), which can be used as a feedstock for gasoline, methanol, and other hydrocarbons. Nickel and cobalt were found to have good activity for CO2 reforming. However, they have a poor stability due to carbon deposition. This research developed efficient Ni-Co solid solution catalysts with excellent activities and high stability for CO2 reforming of methane. First, the structure of binary oxide solid solution of nickel and cobalt was investigated. It was found that while the calcination of Ni(NO3)2 and Co(NO3)2 mixture with 1:1 molar ratio at a high temperature above 800 oC generated NiO-CoO solid solution, only Ni3O4-Co3O4 solid solution was observed after the calcination at a low temperature of 500 oC. Furthermore, if the calcination was carried out at a medium temperature arranged from 600 to 700 oC, both NiO-CoO and Ni3O4-Co3O4 solid solutions can be formed. This occurred because Co3O4 can induce the formation of Ni3O4, whereas NiO can stabilize CoO. In addition, the lattice parameter of Ni3O4, which was predicted by using Vegard’s Law, is 8.2054 Å. As a very important part of this dissertation, Ni-Co solid solution was evaluated as catalysts for CO2 reforming of methane. It was revealed that nickel-cobalt solid solution showed excellent catalytic performance and high stability for CO2 reforming of methane. However, the stability of Ni-Co solid solution catalysts is strongly dependent on their composition and preparation condition. The optimum composition is 50%Ni-50%Co. Furthermore, the structure of Ni-Co catalysts was characterized by XRD, Vvis, TPR, TPD, BET, AES, TEM, XANES and EXAFS. The relationship between the structure and the catalytic performance was established: (1) The reduced NiO-CoO solid solution possesses better catalytic performance and stability than the reduced Ni3O4-Co3O4 solid solution. (2) Ni is richer on surface in Ni-Co catalysts. And (3) the reduction of Ni-Co-O solid solution generated two types of particles, small and large particles. The small ones are dispersed on large ones as catalytic component.

ASTAXANTHIN PRODUCTION FROM HAEMATOCOCCUS PLUVIALIS UNDER VARIOUS LIGHT INTENSITIES AND CARBON DIOXIDE CONCENTRATIONS

The microalga Haematococcus pluvialis was cultivated in MES-volvox medium at various light intensities and CO2 concentrations. It was found that CO2 concentrations of 10 and 15%, in combination with high irradiance at initial pH =6.7, accelerate astaxanthin accumulation in H. pluvialis cells but obstruct cell growth. The purpose of this research study was to devise a one-stage process consisting of the simultaneous cultivation of H. pluvialis and astaxanthin production using high light intensity and high CO2 concentration. This could be achieved at 200 µE/m2s and 15% CO2 in growth medium at initial pH = 4.3. Compared to the traditional two-stage H. pluvialis cultivation system, this one-step process can save up to 8-9 days of astaxanthin production time. The astaxanthin content in H. pluvialis cells induced with high light intensity only or with a combination of high light intensity and high CO2 concentration had comparable astaxanthin content; 94 and 97 mg/g dry biomass, respectively. However, it was extremely low in nitrate-free medium at high irradiance alone or combined with high CO2 concentration, with an average value of 4 mg/g dry biomass. Cell density was 40% less in cultures under discontinuous illumination compared to continuous illumination. This process could serve as a microalgal CO2 mitigation system after further understanding of the CO2 fixation ability of H. pluvialis has been gained.