Journal of Energy Chemistry 22(2013)459–467 Effect of supercritical water on the stability and activity of alkaline carbonate catalysts in coal gasification Jinli Zhang a , Xiaoxia Weng a , You Han a* , Wei Li a , Zhongxue Gan b , Junjie Gu b a. School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China; b. State Key Laboratory of Low Carbon Energy of Coal, ENN Group, Langfang 065001, Hebei, China [ Manuscript received November 30, 2012; revised January 17, 2013 ] Abstract The stability and activity of alkaline carbonate catalysts in supercritical water coal gasification has been investigated using density functional theory method. Our calculations present that the adsorption of Na 2 CO 3 on coal are more stable than that of K 2 CO 3 , but the stability of Na 2 CO 3 is strongly reduced as the cluster gets larger. In supercritical water system, the dispersion and stability of Na 2 CO 3 catalyst on coal support is strongly improved. During coal gasification process, Na 2 CO 3 transforms with supercritical water into NaOH and NaHCO 3 , which is beneficial for hydrogen production. The transformation process has been studied via thermodynamics and kinetics ways. The selectively catalytic mechanism of NaOH and the intermediate form of sodium-based catalyst in water-gas shift reaction for higher hydrogen production has also been investigated. Furthermore, NaOH can transform back to Na 2 CO 3 after catalyzing the water-gas shift reaction. Thus, the cooperative effects between supercritical water and Na 2 CO 3 catalyst form a benignant circle which greatly enhances the reaction rate of coal gasification and promotes the production of hydrogen. Key words supercritical water; alkaline carbonates; coal gasification; hydrogen production; density functional theory 1. Introduction Coal gasification has received much attention over decades [1,2] as an effective method for a clean conversion of coal into fuel gas, such as H 2 , CH 4 and other products. Cat- alysts in coal gasification have been proved to decrease the reaction temperature and improve the gas products via var- ious experiments [3-8]. Among diverse catalysts, alkaline carbonates are often used for its easy availability, relatively cheap cost and its adaption to wide gasification operational parameters [8-12]. The traditional gasifying agents are O 2 , CO 2 [13,14], H 2 and vapor steam [6,8,14-18]. Since supercritical wa- ter (SCW) is an excellent non-polar solvent with high solu- bility for organics and low solubility for inorganics, the cat- alytic coal gasification technology in SCW has been attracted great interest during the past two decades [5,7,19]. In SCW reaction system, product distribution could be controlled by controlling the solubility of SCW manipulated by tempera- ture and pressure [20]. And many experiments [21-27] have shown that the product distribution of coal gasification in SCW is different from that in vapor. Comparing with that in N 2 , Bi et al. [21–24] found that coal pyrolysis in SCW ob- tains higher carbon conversion and higher yield of light prod- ucts, suggesting that SCW enables coal to split into smaller molecules. Besides, the particle dispersion of alkaline carbon- ate catalysts could be affected by SCW. Our previous study showed that Na 2 CO 3 particles are more dispersed than K 2 CO 3 when they crystallize from SCW because of the low solubil- ity in SCW [28]. This is one of the reasons that Na 2 CO 3 has higher catalytic activity than K 2 CO 3 during the process of coal gasification in SCW, whereas their performance is con- trast in vapor steam [8]. However, the effect of SCW on the stability and activity of catalysts during the reaction is unclear yet. Therefore, there is a genuine need for quantitative stud- ies on the interaction mechanism of SCW and catalyst in coal gasification process. In this paper, we made an attempt to investigate the stabil- ities of Na 2 CO 3 and K 2 CO 3 catalysts on coal with and with- out SCW, and the catalytic mechanism of Na 2 CO 3 in SCW coal gasification process was also studied by applying density functional theory (DFT) method. Particularly, the cooperative effect of SCW and Na 2 CO 3 on H 2 production is focused on. This study will not only be helpful for the design of catalyst * Corresponding author. Tel: +86-22-27401476; Fax: +86-22-27890643; E-mail: yhan@tju.edu.cn This work was supported by the National High-Tech Research and Development Program of China (2011AA05A201), the National Natural Science Foundation of China (21106094) and Tianjin Science Foundation for Youths, China (12JCQNJC03100). Copyright©2013, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved.