Preparation Of Co-based Mixed Oxides With Spinel Structure And Their Performance For Catalytic Combustion Of Methane

Supported noble metal catalysts, although with outstanding catalytic activity for the combustion of methane, are not fully satisfactory based on high price and scarce resource. As a result there is a strong demand for the development of excellent catalysts both in catalytic performance and price. Co₃O₄ with spinel structure has been studied extensively due to high catalytic activity and low cost. However, the potential applications of Co₃O₄ as a catalyst are limited because of the poor resistance to sintering. While composite metal oxides are usually considered to have good thermal stability. Therefore, it is meaningful to synthesize Co-based composite oxides with spinel structure in order to enhance catalytic activity and thermal stability.In the paper, a series of Co_1-xM_xCo₂O₄ catalysts were prepared by various synthesis methods and used successfully for methane combustion. The selected methods included the co-precipitation method, sol-gel method using glucose as complexing agent and solution combustion method. The effects of the addition of other metal oxides, the doping amount of CeO₂, preparation methods, precipitants and calcination temperatures on crystal structure and properties of the catalysts were investigated by means of FI-IR, XRD, BET, H₂-TPR and DTA-TG techniques.1. Co_0.5M_0.5Co₂O₄ (M=Mg, Zn, Ce) mixed oxides with spinel structure were prepared by co-precipitation method, and the influence of different metal oxides on catalytic performance of CO₃O₄ was researched. The results indicated that: Compared with Co_0.5Mg_0.5Co₂O₄ and Co_0.5Mg_0.5Co₂O₄ catalysts, the best catalytic activity and structural stability were shown by Co_0.5Ce_0.5Co₂O₄ catalyst, which had higher lattice distortion and surface area, larger pore diameter and pore volume, smaller crystal size,stronger oxygen mobility,lower apparent activation. Furthermore, the complete conversion temperature of Co_0.5Ce_0.5Co₂O₄ catalyst decreased by 56℃in comparison with that of spinel-type composite oxide Cr-Mg-0 T₁₀₀=520℃) with good performance reported in the literature.2. The effects of different doping amounts of CeO₂ on properties of Co_1-xCe_xCo₂O₄ (x=0.0-0.5) catalysts were studied in the combustion of methane. Results showed that all catalysts synthesized using co-precipitation method formed spinel-type structure. Catalytic activity of Co_1-xCe_xCo₂O₄ composite oxides changed with cerium content, and the activity order was as follows: Co_0.7Ce_0.3Co₂O₄ > Co_0.8Ce_0.2Co₂O₄ > Co_0.9Ce_0.1Co₂O₄ > Co_0.6Ce_0.4Co₂O₄ > Co₃O₄ > Co_0.5Ce_0.5Co₂O₄. The best catalytic activity was shown by the Co_0.7Ce_0.3Co₂O₄ catalyst (Complete conversion temperature is 429℃), which could be explained in terms of larger lattice distortion and surface area, smaller crystal size, stronger reducing activity and oxygen mobility, lower apparent activation.3. A series of Co_0.7Ce_0.3CO₂O₄ catalysts were prepared by the co-precipitation, sol-gel and solution combustion methods, respectively. The experimental results indicated that the catalytic activity of Co_0.7Ce_0.3Co₂O₄ catalyst prepared by co-precipitation method were the best among all the samples, which could be attributed to its higher lattice distortion and surface area, larger pore diameter and pore volume, smaller crystal size, stronger oxygen mobility and lower reaction apparent activation energy of catalytic combustion of methane.4. Co_0.7Ce_0.3Co₂O₄ catalysts were prepared by co-precipitation method using K₂CO₃, (NH₄)₂CO₃ and C₂H₂O₄ as precipitants respectively. The research indicated that the precipitants mentioned above had important influence on catalytic activity, structure, surface area and reduction ability of the catalysts. As a result of larger lattice distortion, pore volume, surface area and oxygen mobility, smaller crystal size, lower apparent activation, the Co_0.7Ce_0.3Co₂O₄ catalyst prepared with K₂CO₃ precipitant was found to exhibit the highest catalytic activity and structural stability, and the ignition temperature (T₁₀) and complete conversion temperature (T₁₀₀) of methane over the catalyst are 262℃and 428℃.5. Co_0.7Ce_0.3Co₂O₄ mixed oxides were prepared by co-precipitation method and the calcination temperatures employed were 400, 500 and 600℃respectively. The influence of the calcination temperature of the mixed oxides on the performance of methane combustion was studied. Results displayed that all catalysts prepared under different calcination temperatures had spinel-type structure. And the higher the calcination temperature, the larger the average crystal size, the smaller the lattice distortion, pore volume and BET surface area, the higher the reaction apparent activation energy of catalytic combustion of methane, the lower the catalytic activity. Furthermore, the catalyst roasted under 400℃with larger BET surface area and smaller crystal size had the better catalytic activity for methane combustion.