Comparative study of selected catalysts for methane partial oxidation
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Catalytic partial oxidation of methane (CPO) is an interesting alternative to the conventional steam methane reforming (SMR) for synthesis gas production. The obtained synthesis gas has the proper H2/CO ratio for methanol or Fischer-Tropsch synthesis. The conversion of methane and the selectivity to CO and H2 in the CPO process is favored at high temperatures. However, the exothermicity of the reaction, particularly, the combustion of methane, might provoke high local temperatures on the surface of the catalyst, leading to deactivation of the active phase by sintering. Although various transition metals are active for the CPO, Rh based catalysts have shown the most promising catalytic performance. On the other hand, due to the high cost of Rh, the loading in the catalyst must be reduced to an economically competing level. This can be achieved by increasing the dispersion of Rh on the surface of the support. However, the extreme reaction conditions working with in CPO makes it very challenging. Therefore, the main objective of the work reported in this thesis aims to develop a thermal stable catalytic system for the CPO process. In this thesis, we first focus on developing thermal stable Al2O3-based nanocomposites (ZrxCe1-xO2-Al2O3) by a modified Pechini route. These nanocomposites can be prepared by coating a Zr or Ce-containing polymeric resin, derived from an esterification between citric acid and polyethylene glycol, on the surface of Al2O3 via a simple evaporation-drying or a spray drying method, followed by calcination at certain temperatures. Especially, the thermal stability of the formed composite supports calcined between 1173 and 1473K is systemically studied by investigating the structure and phase evolution employing various characterization techniques. By comparing the UV-Raman spectra of the nanocomposite (surfaceregime sensitive) with the bulk phase information obtained by Vis-Raman and XRD, it is found that the ZrxCe1-xO2 phase is coated on the surface of the Al2O3. The formation of this protective layer of oxides on the Al2O3 surface can substantially improve the thermal stability of the nanocomposites. Besides, mixed oxides (i.e. Zr0.5Ce0.5 O2 or Zr0.25Ce0.75O2) are found to be more effective in improving the thermal stability of the alumina support than the single ceria or zirconia. Then, 5 wt % ZrxCe1-xO2-Al2O3 composites with high thermal stability are used as a candidate material for the CPO process. Additionally, the effect of two different commercial aluminas (Evonik Aeroxide AluC and Sasol Puralox SCCa) and the preparation method on the structure and thermal stability of the nanocomposites has been studied. Evonik Aeroxide AluC based nanocomposites prepared by spray drying give the highest thermal stability concerning sintering and alumina phase transformation. Rh is introduced in the alumina based nanocomposites by incipient wetness impregnation. Typically, loadings of 0.1 and 0.5 wt.% have been employed and the catalytic dispersion has been measured with volumetric H2 chemisorption. CPO has been investigated over these Rh-based catalysts by running experiments in a fixedbed reactor operating at 1 atm. Furnace temperatures ranging from ambient to 1173K have been employed to study the ignition-extinction behaviour. The feed gas typically consisted of a mixture of methane and air, with a CH4/O2 ratio of 2. Gas hourly space velocities ranging from 53 to 1500 LCH4/gcat h have been selected. Catalysts supported on CeO2-Al2O3 and Zr0.5Ce0.5O2-Al2O3 show the best behavior in CPO whereas those supported on ZrO2-Al2O3 present the worse performance. Ceria-containing materials present lower light off temperature, higher methane conversion and selectivity to synthesis gas. The better catalytic performance in the ceria containing catalyst might be attributed to the presence of oxygen vacancies, the capability of ceria for promoting the water gas shift reaction and the slightly higher dispersion of Ce-based materials. Additionally, supports with different c-CeO2 crystal sizes can be obtained on the Puralox and AluC-based nanocomposites. These materials are employed to deposit 0.5 wt. and 0.1 wt % Rh. It is found that the catalyst with smaller c-CeO2 crystal size is responsible of the lower ignition temperature probably due to enhanced reducibility. On the other hand, it is observed that a higher Rh dispersion in the catalyst will lead to a higher methane conversion and selectivity to synthesis gas. Additionally, the Rh dispersion is found to be closely related to the BET surface area of the support. The stability of the nanocomposites is also studied under reaction conditions. Puralox based catalysts are presenting higher conversions in the initial period due to the higher Rh dispersion, but the Ce-containing AluC-based nanocomposites prepared by spray drying shows a relatively stable conversion of methane, indicating that a highly thermal stable support can lead to a highly stable catalyst system, probably by stabilizing the Rh nanoparticles. It can be concluded that the composition and structure of the supports play an important effect on the partial oxidation of methane to produce synthesis gas over Rh-based catalysts. Thus, the synthesis of the proper support where the Rh nanoparticles can be stabilized is highly necessary to achieve a good performance, regarding both activity and stability under CPO conditions.