Oxy-Reforming Of Methane With Carbon Dioxide And Oxygen On Supported Nickel Catalysts In A Fluidized Bed Reactor

The transformation of methane and carbon dioxide, the cheapest carbon-containing materials and the most problematic greenhouse gases, into more valuable compounds has long attracted the attention of researchers. The combination of CO₂ reforming and partial oxidation of methane, also called methane autothermal reforming [MATR] (Eq.(1)), has been substantial interest in recent years in alternative routes for the conversion of natural gas (methane) to syngas. This process has low-energy requirements due to the opposite contribution of the exothermic partial oxidation of methane (Eq. (2)) and the endothermic CO₂ reforming of methane (Eq. (3)). By controlling the feed composition, H₂/CO ratio in the product can be modified according to the need of the post processing. Oxygen in feed is helpful to avoid catalyst deactivation by carbon deposition.CH₄+xCO₂+(1-x)/2 O₂→(1+x) CO +2H₂,â–³H₂₉₈=(285x-38)kJ/mol(04+CO₂→2 CO+2 H₂,â–³H₂₉₈=247 kJ/mol (2)CH₄+1/2 O₂→CO+2 H₂,â–³H₂₉₈=-38 kJ/mol (3)Fluidization has a favorable effect on the inhibition of carbon deposition, which is probably because the catalyst particles are circulated between the oxidizing zone and reducing zone and carbon gasification proceeds readily in the oxidizing zone. Moreover, catalyst can maintain a suitable level of reducibility during fluidization that enhances the conversion of methane. The aim of this work is to investigate the promotion effect of rare earth oxide and the different Ni particle size on the catalytic activity and stability, and deactivation mechanism of catalyst in the reforming of methane with CO₂ and O₂.The Ni/SiO₂ catalyst was modified with several rare earth oxides and used for the MATR in a fluidized bed reactor. Gd₂O₃ is the most promising one among the tested rare earth oxides. Gd₂O₃-modified Ni/SiO₂ catalysts exhibited higher activity and stability for MATR in fluidized bed reactor. On the NiGd_0.45/SiO₂ catalyst, it was possible to convert methane into syngas with low H₂/CO ratio (12/CO<2) at above 75% methane conversion without clear deactivation in 100 hours. Characterizations found that Gd₂O₃-modified Ni/SiO₂ catalyst possessed higher CO₂ adsorption and activation ability due to the formation of surface carbonate species. H₂-TPR and XRD characterizations found that the strong interaction between nickel and SiO₂ took place, which improved the dispersion of Ni.The adsorption and activation of CH₄ and CO₂ on Ni/SiO₂ catalyst with different Ni particle size were investigated by diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. CH_x species (x=1³) were observed on the catalyst with different Ni particle size when CH₄ was adsorbed on its surface. CH₃-O species was also detected as the result of CH_x species interaction with surface hydroxyl. It was found that the dissociation of CH₄ depended intensively on the Ni particle size. CH₄ might dissociate more easily on Ni/SiO₂ catalyst with smaller Ni particle size. CO₂ was very difficult to dissociate on Ni/SiO₂ catalyst, while the adsorbed H from CH₄ dissociation could promote the dissociation of CO₂. The smaller Ni particle might be helpful to hydrogen overflow to the support for the activation of CO₂. In co-adsorption of CH₄ and CO₂, monodentate carbonate species was observed on the Ni/SiO₂ catalyst with smaller Ni particle size, which indicated that the smaller Ni particle could promote the reaction of CO₂ with surface oxygen species.The different-sized Ni catalysts (4.5 nm and 45.0 nm) were prepared and used for MATR in a fluidized-bed reactor. It was found that the activity and stability of Ni catalysts depend strongly on the particle size and the operating space velocity. Small sized Ni is more active and stable at space velocity (<54,000 h^-1). Characterizations disclosed that methane decomposition rate decreases with the enlarging Ni particle size. The results of pulse-MS illustrates that turnover frequency (TOF) of CH₄ decomposition was 9.8 s^-1 on the 45.0 nm Ni catalyst, comparable with previously reported values. On the 4.5 nm Ni catalyst, the calculated TOF of CH₄ increased to 12.6 s^-1.CO₂ and O₂ accelerated the conversion of methane, but the small particle Ni catalyst was more effective and stable. As the methane decomposition rate slowed on larger Ni particles and/or at higher space velocity, and the amount of formed surface carbon was insufficient to ensure complete conversion of the oxygen, surface Ni will be gradually oxidized by remaining O₂, leading to Ni deactivation.According to the above deactivation mechanism of Ni catalyst, we suggested that increasing the reducing atmosphere (such as H₂) would be a solution routine. In the followed section, we checked the performance of a small sized Ni catalyst (<5 nm) for the partial oxidation and CO₂ reforming of methane in a H₂-rich atmosphere. It was found that small sized Ni catalyst exhibited higher activity and stability even at higher space velocity (80,000 h^-1). The deactivation of Ni catalyst was depressed successfully by the H₂ in feed.Primary study on MTAR in the present of propane to produce syngas was practiced in a fluidized bed reactor. The results showed that the small sized Ni catalyst (<5 nm) exhibits higher activity and resistance to carbon deposition. The propane decomposition is a temperature sensitive process and the order of graphitization of the deposited carbon increased with the temperature. A large proportion of H-rich carbon species can be formed at lower temperature, which is possible helpful for the dissociation and activation of methane.