489 / 2018-09-09 10:32:16

Recent Advances of Syngas Methanation Catalysts: A Review

全体主题 > 生物质燃气

Natural gas is an important energy source in the world. The conversion of biomass-derived syngas, mainly H2 and CO to synthetic natural gas (SNG) via methanation is a feasible way to convert biomass energy into hydrocarbon energy carrier. Catalyst plays an important role in the methanation processes. An ideal catalyst should exhibit excellent catalytic activity and long-term stability with high carbon oxides conversion and methane selectivity which depends on its unique characteristics. In this paper, the research progresses of CO and/or CO2 methanation catalysts will be presented and the effects of catalyst active metals, supports, promoters and preparation methods will be discussed in detail. The conclusions are listed below:
① Noble metals (Ru, Rh, Pd, Pt, etc.) and Nickel exhibit excellent catalytic activity. However they can easily be deactivated by sintering and carbon deposition. The formation of Ru(CO)x and Ni(CO)4 results in the active sites losses and decreases the catalytic performance. The active metals particle size and dispersion directly affect the CH4 selectivity via different reaction mechanisms.
② Supports with high specific surface area and abundant porous structure can enhance the active metals dispersion and increase the catalytic performance. The ordered mesoporous structure is more conducive to improve the stability of methanation reactions due to its confinement effect. The CO2 adsorption capability can be improved by increasing the basic sites of support, while reducing the acid sites which can inhibit the carbon deposition on the catalyst surface and extend the service life. Strong metal-support interaction enhances the thermal stability of the catalyst but reduces the reducibility; while too weak metal-support interaction leads to the active component sintering.
③ Adding promoters improves the catalysts structure with more defect sites and/or oxygen vacancies, which provide more reaction sites to load active metals and suppress coke deposition respectively. The synergy between bimetallic catalysts promote the catalytic activity efficiently. Some alloys (e.g. Ni3Fe) or metal nitride (e.g. Co4N) show superior activity in CO and CO2 methanation. A series of promoters improve electron transfer ability of the catalysts, which increase the electron density of active metals, consequently enhance the catalytic activity.
④ The preparation method plays a vital role in the catalytic performance. More and more preparation methods are employed to manufacture catalysts with higher activity, selectivity and long-term stability. The calcination and reduction temperature are both critical parameters for the preparation processes. Good thermal conductivity can be obtained by innovative configuration, which is beneficial to heat transfer to prevent overheating. Core-shell structured catalysts significantly improve the catalytic stability.
Although syngas methanation has been well commercialized, the methanation of biomass-derived syngas still face various challenges. For instance, the competition between CO and CO2 methanation reactions, the lifetime of the catalysts, the impurities effects and so on. Therefore, a lot of problems should be solved in the future research, which are proposed as follows:
① The mechanisms should be further studied in depth. CO and CO2 methanation reaction mechanisms are still controversial. The mechanisms of synergy between active metals and supports or promoters are not yet clear. It will be easier to develop a high efficient catalyst through clear reaction mechanisms.
② CO and CO2 methanation are typical “gas-solid” multiphase reactions with adsorption, reaction, desorption and diffusion processes. Therefore, the catalyst structure plays a critical role in the reactions. It is necessary to explore new catalyst structure to improve the catalytic methanation reactions, such as hierarchical meso-macrostructured catalysts, etc.
③ In co-methanation of CO/CO2 mixture, CO2 methanation is totally suppressed until a very high conversion of CO is attained, due to the hydrogenation of surface carbon only produced by CO. On the other hand, CO conversion starts to decrease with the temperature over a certain value, owing to the RWGS reaction. Therefore, catalysts with both high CO and CO2 conversion rate and CH4 selectivity in a wide temperature window are expected.
④ It is worth noting that the impurities in bio-syngas such as ash, alkali metal, chlorine, olefins and tars, which produced from biomass gasification or pyrolysis, have a negative effect on catalytic methanation performance. Thus, the anti-carbon deposition, anti-poisoning properties of bio-syngas methanation catalysts should be investigated in depth.