This review describes the history and development of ammonia synthesis from its discovery to the early development of the Haber-Bosch process to the current use of plasma catalysis as an alternative ...to the possible process decentralization of this essential commodity. Proposed reaction mechanisms to-date for vacuum and atmospheric pressure discharges are summarized and discussed. The use of different discharges and materials employed as catalysts as well as the observed interactions between the plasma and the catalyst that can modify and activate the catalytic surface and the possible effects of the catalyst on plasma are herein summarized. Moreover, important differences between thermal catalysis and plasma catalysis are contrasted. It is intended that this review can also help to develop an understanding of the current knowledge and gaps for the ammonia synthesis through plasma catalysis with an emphasis on the challenges presently faced.
Herein, we demonstrate a synergistic approach with radiofrequency plasma to synthesize ammonia in the presence of Ni-MOF-74 as catalyst. The Ni-MOF displayed higher ammonia yields as compared to the ...pure Ni metal. Specifically, ammonia yields as high as 0.23 g-NH3 (g-catalyst·kWh)−1 and energy cost of 265 MJ mol–1 over Ni-MOF were observed. The enhanced catalytic activity of the Ni-MOF in the presence of plasma was attributed to the presence of pores, which improved mass transfer of guest and product molecules during reaction, the presence of open Ni metal sites, and lower surface hydrogen recombination. Furthermore, the ammonia energy yield of our plasma-Ni MOF catalyst is superior to those of the state-of-the-art RF plasma catalytic systems.
Herein, we demonstrate the synthesis of ammonia via atmospheric dielectric barrier discharge (DBD) plasma discharge over zeolite 5A. The presence of the zeolite in the DBD reactor promoted the ...formation of microdischarges and a change of the voltage–current characteristics of the reactor, leading to an enhanced catalytic performance. The perturbation of the zeolite surface electronic properties due to atmospheric plasma led to an enhanced reactive state at the zeolite surface which promoted the dissociation of nitrogen and the subsequent formation of ammonia. An energy yield of 15.5 g-NH3/kW h was observed at an equimolar N2 to H2 ratio in the presence of zeolite 5A, which is at least 50 times higher than that obtained in the absence of the zeolite.
Microporous crystals have emerged as highly appealing catalytic materials for the plasma catalytic synthesis of ammonia. Herein, we demonstrate that zeolitic imidazolate frameworks (ZIFs) can be ...employed as efficient catalysts for the cold plasma ammonia synthesis using an atmospheric dielectric barrier discharge reactor. We studied two prototypical ZIFs denoted as ZIF-8 and ZIF-67, with a uniform window pore aperture of 3.4 Å. The resultant ZIFs displayed ammonia synthesis rates as high as 42.16 μmol NH3/min gcat. ZIF-8 displayed remarkable stability upon recycling. The dipole–dipole interactions between the polar ammonia molecules and the polar walls of the studied ZIFs led to relatively low ammonia uptakes, low storage capacity, and high observed ammonia synthesis rates. Both ZIFs outperform other microporous crystals including zeolites and conventional oxides in terms of ammonia production. Furthermore, we demonstrate that the addition of argon to the reactor chamber can be an effective strategy to improve the plasma environment. Specifically, the presence of argon helped to improve the plasma uniformity, making the reaction system more energy efficient by operating at a low specific energy input range allowing abundant formation of nitrogen vibrational species.
Nonthermal plasma is a promising alternative for ammonia synthesis at gentle conditions. Metal meshes of Fe, Cu, Pd, Ag, and Au were employed as catalysts in radio frequency plasma for ammonia ...synthesis. The energy yield for all these transition metal catalysts ranged between 0.12 and 0.19 g-NH3/kWh at 300 W and, thus, needs further improvement. In addition, a semimetal, pure gallium, was used for the first time as catalyst for ammonia synthesis, with energy yield of 0.22 g-NH3/kWh and with a maximum yield of ∼10% at 150 W. The emission spectra, as well as computer simulations, revealed hydrogen recombination as a primary governing parameter, which depends on the concentration or flux of H atoms in the plasma and on the catalyst surface. The simulations helped to elucidate the underlying mechanism, implicating the dominance of surface reactions and surface adsorbed species. The rate limiting step appears to be NH2 formation on the surface of the reactor wall and on the catalyst surface, which is different from classical catalysis.
AlPO-18 membranes for CO2/CH4 separation Carreon, Maria L; Li, Shiguang; Carreon, Moises A
Chemical communications (Cambridge, England),
2012-Feb-25, Letnik:
48, Številka:
17
Journal Article
Recenzirano
The synthesis of reproducible and continuous AlPO-18 membranes is demonstrated. The separation performance of these membranes for equimolar CO(2)/CH(4) gas mixtures is presented. The AlPO-18 ...membranes displayed CO(2) permeances as high as ~6.6 × 10(-8) mol m(-2) s Pa with CO(2)/CH(4) separation selectivities in the ~52-60 range at 295 K and 138 kPa.
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 N
2
+
, 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.
The excess of mango peels is considered manufacturing waste in the sugar and juice industry. There is an increasing interest in looking for alternative ways to employ this waste to address this ...overload. Here, we show the efficient use of mango peels as a noncost carbon source for the synthesis of graphene. We demonstrate for the first time the synthesis of graphene on Cu substrates from mango peels, a biomass rich in pectin. It is observed that plasma presence is essential for the growth of graphene from mango peels. At 15 and 30 min of plasma exposure, we observed the presence of multilayered graphene, at longer plasma exposure, i.e., 60 min, there is the formation of monolayer graphene, attributed to the etching of multiple layers formed at short times due to long plasma exposure time. When employing this technique, precautions must be taken due to the etching effect of plasma, such as reducing either the plasma exposure time or the plasma power. Finally, we present a graphene growth pathway under plasma environment on the basis of our experimental observations.
The synergistic combination of solid catalysts and plasma for the synthesis of ammonia has recently attracted considerable scientific interest. Herein, we explore MgTiO
3
, CaTiO
3
, SrTiO
3
, and ...BaTiO
3
perovskites as effective catalysts for the synthesis and decomposition of ammonia
via
cold plasma. MgTiO
3
perovskite, which contains the most electronegative alkaline metal of all the studied perovskites, resulted in the highest ammonia synthesis rate with a value of 12.16 μmol min
−1
m
−2
, which is around 50 times the value of only plasma, 0.24 μmol min
−1
. The high electronegativity of Mg can be assisting the dissociation of the triple nitrogen covalent bond. This intrinsic property of Mg perovskite added to the homogeneity of the plasma arising from the dielectric constant value of this perovskite might be synergistically responsible for the high ammonia synthesis rate observed. Interestingly, ammonia production over MgTiO
3
perovskite is almost double the performance of traditional oxides and some microporous crystals. We also explored the ammonia decomposition reaction due to the possibility of the importance of the reversible reaction owing to the electron collision with the ammonia molecules formed. Ammonia decomposition increased as plasma power increased. This points out the benefit of running at low plasma power and the need to design plasma reactors where the newly formed ammonia molecules can be removed from the reaction system to avoid further electron collision. The highest ammonia decomposition yield was 44.37% at 20 W corresponding to an energy yield of 5.06 g-NH
3
kW h
−1
.
Plasma catalytic ammonia synthesis & decomposition on perovskites. The blend of intrinsic properties (Mg electronegativity) with plasma awakens properties (plasma homogeneity induced by the dielectric constant) leads to high ammonia synthesis rates.
Herein, we demonstrate that the performance of mesoporous silica SBA-15 and SBA-15-Ag during plasma ammonia synthesis depends on the plasma conditions. At high power, the mesoporous silica SBA-15 ...without Ag produces the largest amount of ammonia, but the addition of Ag provides a minor benefit at lower powers. Plasma conditions were analyzed through optical emission spectroscopy using N
2
, N
2
+
, and NH molecular bands and H
α
line. Stark broadening of H
α
line was used to find electron density, and N
2
molecular bands were used to assess N
2
vibrational excitation, important for plasma nitrogen decomposition. At similar input conditions, reactors with SBA-15 have higher electron density and higher N
2
vibrational temperature. Consistent with higher electron density, SBA-15 reactors have stronger N
2
+
emission intensity relative to the neutral N
2
. The addition of Ag results in higher N
2
rotational temperature, possibly due to localized heating. From the materials point of view, SBA-15 is a more robust catalyst with good surface area retention after plasma exposure due to the lack of local heating generated when a metal is in the structure. We identify two possible regimes during ammonia synthesis, a metal and a surface-plasma driven. At lower plasma densities, the addition of metal is beneficial, while at higher power and plasma density, the best performance is achieved without the aid of a metal catalyst.
Graphical Abstract
Mesoporous materials for Plasma Catalytic Ammonia Synthesis, at certain plasma conditions lead to different regimes, a plasma/surface and a metal dominated regimes of ammonia production.
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