Ni-ZrO₂ Catalyst Prepared By Citric-gel Method And Its Low-temperature Catalytic Methanation Performance In A Slurry-bed Reactor

Recent years, CO methanation technology, the key during the coal to synthetic natural gas（SNG）, has received wide attention. It is well known that methanation is a highly exothermic reaction and heat can be rapidly removed by low-temperature slurry bed reactor, which can avoid the catalyst sintering and carbon deposition caused by high temperatures in a catalysts bed. Since the reaction temperature is low in the slurry bed reactor, usually less than 350 °C and industrial methanation catalyst showed poor activity for CO methanation in lowtemperature slurry bed reactor. Therefore, it is the key to develop the catalyst suitable for the CO methanation reaction in slurry bed reactor. Ni/ZrO₂ catalyst has outstanding low-temperature CO methanation activity at a hydrogen-rich conditions, but the reseach on the condition of high content of CO for methanation is relatively less, especially CO methanation catalytic properties in lowtemperature slurry bed reactor.Ni-ZrO₂ catalysts with high Ni dispersion degree, small Ni particles and strong interaction between the support and Ni species were prepared by citric-gel method. Therefore, this article was studied the effect of different supports, different prepared methods and different calcined temperatures on the structure of Ni-based catalysts and low-temperature CO methanation properties in the slurry bed. And XRD, N₂ adsorption and desorption, H₂-TPR, TEM, H₂-chemisorption, CO-TPD and XPS were employed to investigate the relationship between the structure of Ni-based catalysts and CO methanation properties. The results are as follows.（1） Ni-Al₂O₃, Ni-SiO₂ and Ni-ZrO₂ catalysts were prepared by citric-gel method. Compared to Ni-Al₂O₃ and Ni-SiO₂ catalyst, Ni-ZrO₂ catalyst had high Ni metal dispersion, small Ni particle size. Ni-ZrO₂ catalyst exhibited the CO conversion of 61.6%, much higher than that of Ni-Al₂O₃（49.1%） and Ni-Si O₂（31.7%） catalysts under the hard condition of low temperature of 260 °C, pressure of 1.0 MPa, H₂/CO molar ratio of 3.0 and a high space velocity of 6000 mL·g?1cat·h?1 in a slurry-bed reactor.（2） Ni-ZrO₂ catalyst prepared by the citric-gel method exhibited larger specific surface area, higher Ni dispersion, stronger NiO-support interaction, moderate interaction with CO and larger desorption of CO molecular than those of catalysts prepared by the solution combustion and co-precipitation method. Accordingly, the Ni-ZrO₂ catalyst prepared by the citric-gel method exhibited the best low-temperature catalytic activity. In addition, the catalyst Ni-ZrO₂ prepared by three methods did not deactive during the investigated reaction time.（3） The structure of the catalyst was significantly affected by calcination temperature. As the calcination temperature rose, the BET specific surface area and pore volume decreased, the metal Ni dispersion firstly increased and then decreased, while the Ni particle size firstly decreased and then increased. Ni-ZrO₂ catalyst calcinated at 450 °C showed the highest Ni dispersion, smallest Ni crystallite size, strongest interaction between Ni species and support, and largest amount of CO desorption which can moderate the interaction with metallic Ni. Under the conditions of 260 °C, 1.0 MPa and H₂/CO molar ratiao of 3, Ni-ZrO₂ catalyst showed the highest CO conversion of 61.6% and no obvious deactivation during the investigated reaction time. The exorbitant calcination temperature resulted in transition of tetragonal ZrO₂ to monoclinic ZrO₂, and agglomeration of Ni particle size, which were not conducive to the methanation reaction. The Ni aggregation and Ni crystallite size growth of Ni-ZrO₂ catalyst calcined at 750 °C leads to the catalyst deactivation.