Plasma ammonia synthesis over mesoporous silica SBA-15

Abstract

Herein we demonstrate the synthesis of ammonia via atmospheric dielectric barrier discharge plasma discharge over silica. We evaluated the performance of three structural different silicas: non-porous silica, fumed-silica and mesoporous SBA-15 silica. The mesoporous SBA-15 sample resulted in an ammonia synthesis rate per gram of catalyst at least three times higher than that of the non-porous and fumed silica when employing a hydrogen rich gas feed. The hierarchical ordered pore size in the mesoporous range of SBA-15 allowed us to observe experimentally, the pore size effect in the plasma discharge. Specifically, the ammonia production is enhanced by the presence of mesopores that can effectively enable the discharge diffusion into the pore as predicted by theoretical calculations. The importance of the porous structure was confirmed by optical emission spectra which indicates a similarity on the relative intensity on the main plasma activated species in the gas phase such as N2 +, NH and H α for the three silicas employed in this work i.e. non-porous, fumed and mesoporous (SBA-15). However, the remarkable ammonia production when employing the SBA-15 shows that despite the similarities in the gas phase the main differences might be attributed to the diffusion of the plasma discharge into the pores.

Plasma Catalysis for Ammonia Synthesis: A Microkinetic Modeling Study on the Contributions of Eley–Rideal Reactions

“…The importance of ER reactions is expected to be great at the high radical densities typically present in DBD plasmas. ,, To what extent plasma-activated species can penetrate the pores of the catalyst and interact with the active metal sites and how plasma-activated species interact with support materials can be an important descriptor for efficient catalyst design. The mesoporous nature of zeolites and MOFs retains specific interest to enhance diffusion of plasma species and/or alter plasma characteristics that could help further optimization of the technology. − …”

“…19 In addition, microporous phases, like zeolites and metalorganic frameworks (MOFs), can also improve diffusion of the plasmaactivated species toward the active sites. 20,21 For a realistic representation of plasma-catalytic NH 3 synthesis, different plasma conditions must be accounted for. Therefore, in this paper we discuss the surface kinetics, (i) in the microdischarges of a filamentary plasma, (ii) in between the microdischarges of a filamentary plasma, and (iii) in a uniform plasma.…”

“…Microdischarges were also found to be promoted by zeolites in the DBD, changing the IV characteristics of the plasma and enhancing NH 3 synthesis . In addition, microporous phases, like zeolites and metalorganic frameworks (MOFs), can also improve diffusion of the plasma-activated species toward the active sites. , …”

Nonthermal Plasma Catalytic Ammonia Synthesis over a Ni Catalyst Supported on MgO/SBA-15

“…18 Compared with the conventional HB process, the NTP catalytic route can be operated under milder conditions and powered by intermittent electricity (generated from renewable solar and wind), allowing a small-scale and intermittent operation. 15,17 Recently, the rational screening and designing of proper packing materials and catalysts for NTP catalytic reactor are important to improving the catalytic performance in terms of reactant conversion, product yield, and energy efficiency. 20 Recent efforts have demonstrated that the use of porous materials (e.g., zeolite, 15,17 metal−organic frameworks 21,22 ) is highly appealing to greatly improve the performance of NTP catalytic ammonia synthesis.…”

“…15,17 Recently, the rational screening and designing of proper packing materials and catalysts for NTP catalytic reactor are important to improving the catalytic performance in terms of reactant conversion, product yield, and energy efficiency. 20 Recent efforts have demonstrated that the use of porous materials (e.g., zeolite, 15,17 metal−organic frameworks 21,22 ) is highly appealing to greatly improve the performance of NTP catalytic ammonia synthesis. For example, Gorky et al demonstrated the presence of mesoporous SBA-15 in the NTP reactor could promote the formation and possible diffusion of microdischarges.…”