Synergetic Effect Between Plasma And Catalyst In Ammonia Decomposition For Hydrogen Production

Ammonia decomposition reaction has received an increasing attention due to the potential of using ammonia as a hydrogen storage medium in hydrogen economy. So far, only noble metal Ru exhibits good catalytic activity for ammonia decomposition in thermal catalysis mode, but metal Ru is too expensive for wide use. The problem of cheap metal catalysts such as supported Fe, Co, Ni lies in the very slow recombinative desorption of the surface adsorbed N atoms. In this study, the activity of the cheap metal catalysts was greatly enhanced by placing the catalyst bed into the plasma zone. Systematic studies were carried out to get insight into the activity enhancement. The following results and conclusions were obtained:1. A strong synergy was observed in ammonia decomposition when placing the cheap catalyst into the dielectric barrier discharge (DBD) plasma. For example, in the conditions of NH3flow rate being40ml/min, reaction temperature being410"C, input power being32.4W, when a bulk. Fe-based catalyst was used (10g), the ammonia conversion increased from7.8%to99.9%and the reaction temperature decreased by140℃compared to the thermal catalysis. The energy efficiency of H2production increased from0.43mol/kW-h to4.96mol/kW-h compared with the plasma only.2. The catalyst played a chief role in the synergy of plasma catalysis for ammonia decomposition. First, the catalyst increases the energy efficiency of the plasma by making use of the concomitant heat of the discharge. Second, the discharge was modified by the discharge of catalyst surface, changing from filamentary discharge to microdischarge which increased the uniformity and currents of discharge. Such a modification led to the increase of electron density and inelastic collision of NH3molecules with the electrons. Third, the catalyst can exploit the active species such as excited ammonia molecule (NH3*) to accelerate ammonia conversion.3. The vital role of plasma for the catalyst was to eliminate the rate-limiting step of the recombinative desorption of surface-adsorbed N atoms. This was confirmed by the self-established plasma desorption technique and other means such as OES, FTIR and15NH3isotope tracing. The acceleration of the recombinative desorption rate of surface-adsorbed N atoms was mainly implemented by heavy active species such as NH3*, NH2·and NH·instead of electrons.4. Metals and supports both had strong effects on the synergy of plasma and catalyst. Among Fe, Co, Ni and Cu, the supported Co catalyst exhibited the strongest synergy because the moderate strength of Co-N bond favored the formation and cleavage of M-N. More importantly, it was found that the dielectric constant of support had a close relationship with the ammonia conversion, which means that the dielectric constant is an important criterion to choose the support of catalyst in the plasma-catalysis mode.5. After improvement of catalyst preparation, the ammonia conversion and energy efficiency of Hi production increased by40%and more than two times, respectively. There are optimal values for catalyst dosage (10g for the bulk Fe-based catalyst) and feed flow of reactant (～200ml/min). In the presence of plasma, the activity of catalyst modified by promoter (KCl, KNO3, La(NO3)3or Ce(NO3)3) decreased for ammonia decomposition, which is dfferent from the condition of thermal catalysis.