| The utilization of carbon-rich fossil fuels has allowed an unprecedented era of prosperity and advancement for human development. However, the concentration of carbon dioxide in the atmosphere has consequently risen from ～280 ppm before the industrial revolution to ～400 ppm in 2014, which reached the highest value in history. Recent studies have identified a near-linear relationship between global mean temperature and cumulative CO2 emissions. Serious greenhouse effect was caused by the increasing CO2 concentration in the atmosphere. Searching for new methods which can effectively reduce atmospheric CO2 concentration becomes imminent. Many methods including reduce the emissions of CO2, CO2 capture, storage and utilization were used in reducing the concentration of CO2. Among these techniques, CO2 methanation not only can effectively reduce the concentration of CO2 in the atmosphere but also can make up for the gap of natural gas.It is necessary for the catalyst to have higher low-temperature activity as lower temperature is favorable for CO2 methanation reaction. ZrO2 was considered as a promising support in methanation, as it was found to be the only metal oxide whose surface is acidic, basic, oxidizing, and reducing. Three different polymorphs were found of ZrO2 at ambient pressure:monoclinic, tetragonal, and cubic. Few investigations were made for Ni/ZrO2 supported on single-crystalline ZrO2 during CO2 methanation but Ni supported on mixture of different ZrO2 crystallines, and the main conclusion drawn on the effect of crystalline of support was that the oxygen vacancies on tetragonal were benefit to the methanation of CO2.In this work, a series of Ni-M/ZrO2 (M=Fe, Co, Cu) catalysts and Ni/ZrO2 catalysts supported on various ZrO2 phases were prepared by wetness impregnation method. Their activities in were determined in a fixed bed reactor. XRD, BET, H2-TPR, H2-TPD, H2 pulse chemisorption, NH3-TPD, CO2-TPD, XPS and in situ infrared spectroscopy measurements were taken to characterize the surface and bulk properties of the catalysts.Compared with Co and Cu, iron is a suitable second metal for Ni/ZrO2 catalyst for low-temperature CO2 methanation. The appropriate Fe content for the catalyst is 3 wt.%. Upon reduction at 400℃, most of the iron exists as Fe2+. Because its strong electron-donating ability, Fe2+ can promote the reduction of nickel and zirconia. Sufficient reduction of NiO produces more active sites on the catalyst surface, which enhance hydrogen adsorption. More oxygen vacancies produced by partial reduction of the ZrO2 support promotes the CO2 dissociation at relatively low temperature. Therefore, Fe3Ni30 exhibits high catalytic activity in low-temperature methanation. CO2 methanation over Ni-Fe/ZrO2 catalysts may proceed through CO2 dissociation into CO by the interaction with oxygen vacancies formed by ZrO2 reduction and subsequent reaction of CO with hydrogen to generate methane.Among the catalysts tested, Ni/t-ZrO2 performed the optimal methanation performance, over which the CO2 conversion reached 97.3% and CH4 selectivity achieved 93.8%. This first can be explained by relative higher specific surface area of the support and more acid sites and acid strength on the surface of the catalyst, which lead to the higher dispersion of active metals. Secondly, much more basic sites especially strong basic sites contributed to more absorption of CO2. And finally, higher reduction degree of NiO and more oxygen defects benefited to the methanation of CO2. |
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