Preparation, Characterization And Catalytic Performance Of Cobalt-based Catalysts For Methane Combustion

Since world petroleum reserves are on the decline, a great deal of emphasis has been placed on developing alternatives for energy production. Large efforts have been undertaken to find effective processes and technology for the optimum utilization of the abundant natural gas (whose main constituent is methane) for energy production. Compared with the conventional flame combustion, a higher combustion efficiency and a utilization capacity were obtained in the catalytic combustion. Therefore, catalytic combustion has drawn increasing attentions extensively in the past few years. In our work, several series of cobalt-based catalysts were prepared for methane catalytic combustion, the effects of preparation method and conditions, plasma assisted treatment, promoter modification on those properties and catalytic performances were investigated in detail. These samples were characterized using X-ray diffraction, X-photoelectron spectroscopy, UV-vis and FT-IR spectral analysis, thermal gravity analysis, N₂ Adsorption- desorption (BET), transmission electron microscope, temperature programmed reduction and Hydrogen-oxygen titration technologies. The relation among structure and performance and the active sites for the redox reaction were analyzed and discussed in depth.For the catalysts prepared by the conventional impregnation, the suitable interaction between Si-containing support and active component was beneficial for the activation of C-H band in CH₄ molecule and the existence of active chemical adsorbed oxygen species, the higher activity at lower reaction temperature. T_50%, value on Co-Si catalyst was about 180℃lower than that of Co-Al. SiO₂ support with large surface area promoted the dispersion of the active component and the generation of cobalt oxide with higher valence state. The good catalytic performances were obtained on those catalysts supported by SiO₂. With the increasing calcination temperature from 300℃to 700℃, Co₃O₄ particles augmented and sintered on the samples surface which contained 10 wt. % cobalt, the crystallite size of Co₃O₄ was aggrandized from 7.2 nm to 20.3 nm by inchmeal. On the other hand, the activation energy for the combustion reaction was increased linearly with the augment of average crystallite size of Co₃O₄, the TOF value on prepared catalyst was decreased with it. Thus, the catalytic combustion was a typical structure-sensitive reaction. It could be concluded from the results of XRD and TPR measurements that, reduction process of Co-containing species went along through two steps: Co₃O₄→CoO→Co. After the reduction at 300℃for 30 min, Co₃O₄ was reduced to form CoO which would be further reduced and well dispersed as metallic Co under the higher reduction temperature. Otherwise, the increasing of calcination temperature enhanced the interaction between the active cobalt species and support, This was against the good redox cycle. Because of the existence of oxygen, the reduction temperature had a slight influence on the methane conversion and activation energy.These observations supported the fact that Si atom could band with one or several Co-O band to form different network during the gelation process. As a result, there were two kinds of cobalt species in the Co-Si catalysts prepared by sol-gel method in our work. One was very difficult to be reduced which entered into the framework of SiO₂ gel and formed the -Si-O-Co- band. The other was the conventional reducible cobalt oxide made from the residual part of Co atoms which was spilled over the gel network. With the increasing of cobalt content in the catalysts prepared, proportion of reducible Co₃O₄ within the whole Co-containing species enhanced to 30.8%. Active sites for the catalytic combustion, especially the proportion of OB₃ band in cobalt oxide, were augmented remarkably. As a result, the catalytic activity was promoted gradually. However, too much cobalt added to the gel mixture would increase the amount of Co₂SiO₄ which has no catalytic activity, the augment or sinter of crystal cobalt oxide was obtained. Then, the conversion of methane was decreased. Furthermore, indicated from the results of FT-IR and XRD characterization, the appropriate calcination temperature and H₂O: TEOS ratio could reduce the formation properties of Si-O-Co-O-Si bands and favor the good dispersion of Co₃O₄. The optimum cobalt content, calcination temperature and H₂O: TEOS value in our work were 22.5 %-26%, 600℃and 15.As to the catalyst prepared by sol-gel method, the results of thermal gravity analysis proved that decomposition of cobalt nitrate and the remaining organic compound removal was partially in progress during the plasma treatment, though the temperature was much lower than that of calcination. Owing to the high active species bombardment on the treated surface during the plasma treatment, the breakage of -Si-O-Co- bonds and the formation of Co₃O₄ was observed obviously. It could be concluded that the discharge treatment of samples prepared by sol-gel method induced a better generation of active cobalt species. The XPS characterizations suggested that plasma treatment was favorable for the enrichment of surface cobalt and active chemical adsorbed oxygen species, a value of surface cobalt was increased from 2.2% to 8.5% in mole. Meanwhile, a smaller diameter of crystallite particles (11.2 nm) and a better dispersion of Co₃O₄ on catalyst surface were obtained on Co-Plas-Solgel-2 compared with those of the conventional sample. The specific surface area of the glow plasma assised sample was increased from 305 m²/g to 320 m²/g. The ignition temperature (T_10% of plasma assisted catalyst was about 50℃lower than that of Co-Sol-gel-1, and its CH₄ conversion was two times higher than that of the conventional one during the whole range of catalytic combustion activity test(340-520℃). The plasma assisted sample exhibited significant enhancement in catalytic performances because of a better dispersion and more active sites, in addition, the sequence of plasma treatment and conventional calcinations had very little effect on its catalytic performance.The mesoporous SBA-15 was synthesized with hydrothermal crystallization method successfully, specific surface area, total pore volume and average pore diameter was 586.6 m²/g, 0.802 cc/g and 4.60 nm, respectively. 1μm rope-like block was observed in the investigation of the morphology by TEM, and a high ordered hexagonal structure was proved by low angle X-ray diffraction. As a catalyst support, mesoporous material with large specific surface area was effective on the restrain of the aggrandizement of supported active component. Hydrogen consumed in TPR experiment was enhanced obviously with the increase of cobalt content, and the crystallite size of Co₃O₄ was augmented from 8.3 nm to 15.4 nm step by step. Due to the structural and electronic effect of zirconia promoter, the reaction activity was neatly twice higher than that of conventional sample. The optimum zirconia content was 5%-10%. The characterization results indicated that the addition of palladium promoted the reduction and surface enrichment of cobalt oxide species obviously. Furthermore, higher dispersion and active surface area and more active sites were obtained on palladium promoted samples. Surface cobalt density was increased from 2.80 Co/nm² to 3.58 Co/nm² after the modification with Pd promoter. The existence of PdO promoted the translation from the surface lattice oxygen of Co₃O₄ into an active chemical adsorbed oxygen species. As a result, T_90% values of the catalysts containing more than 0.7 wt% Pd were lower than 350℃; their activities were 75% higher than that of the conventional sample. Among them, 2Pd-40Co/SBA-15 showed a good stability.