Effect of Porous Catalyst Support on Plasma-Assisted Catalysis for Ammonia Synthesis

Abstract

We report on the effect of catalyst support particle porosity on the conversion of NH3 synthesis from N2 and H2 in a coaxial dielectric barrier discharge (DBD) plasma reactor. The discharge was created using an AC applied voltage with the reactor at room temperature and near atmospheric pressure (550 Torr). Two different particles of almost equal diameter (∼1.5 mm)porous silica (SiO2) ceramic beads (average pore size: 8 nm) and smooth, nonporous soda lime glass beadswere compared in the DBD reactor. As the pore size in the SiO2 particles was smaller than the Debye length, penetration of the plasma into the pores of the particles was unlikely; however, reactive species generated in the plasma outside the particles could diffuse into the pores. The N2 conversion and energy yield of NH3 increased with applied voltage for both particle types, and these values were consistently higher when using the SiO2 beads. Discharge and plasma properties were estimated from Lissajous plots and using calculations with the BOLSIG+ software. The effect of these two different catalyst supports on the physical properties of the discharge was negligible. High resolution optical emission spectra revealed that the concentrations of N2 +, atomic N, and atomic H (Hα, Hβ) in the plasma discharge were lower with the porous SiO2 beads than with the glass beads at every applied voltage tested. This indicates that these active species participate in heterogeneous reactions at support particle surfaces and that the larger surface area presented by the porous particles led to higher rates of depletion of these intermediates and a higher rate of ammonia synthesis.

Kinetic Modeling Analysis of Ar Addition to Atmospheric Pressure N2–H2 Plasma for Plasma-Assisted Catalytic Synthesis of NH3

“…Ar 2 * is an electronically excited Ar 2 . Reactions for N and H species, including interactions between vibrationally and electronically excited species, radicals, ions, and surface species, were adapted from Hong et al as in our previous reports. , In our discussion of excited state chemistry, the electronically excited N 2 states N 2 ( C 3 Π u ), N 2 ( B 3 Π g ), and N 2 ( A 3 Σ u + ) are abbreviated as N 2 (C3), N 2 (B3) and N 2 (A3), respectively.…”

“…In the practical range of E/N that was explored (100–141 Td), E-R reactions always dominated over L-H reactions by similar orders of magnitude for the production of NH(s). The conditions (low reduced electric field) under which L-H reactions occur faster than E-R reactions and radical adsorption are challenging for plasma generation. , …”

“…We excluded Langmuir−Hinshelwood (L-H) reactions since the dissociative adsorption of vibrationally excited N 2 on quartz is unlikely. 26 When L-H and E-R reactions were investigated for the production of important surface intermediates on 5% Ru/Al 2 O 3 in a previous study from our group, 10 it was found that E-R reactions were 10 3 times faster for the production of NH(s) and 10 5 times faster for the production of NH 2 (s) than L-H reactions at 523 K, 550 Torr, 53 mL min −1 , feed ratio N 2 :H 2 = 1:3, E/N = 141 Td, and n e = 3.87 × 10 7 cm −3 . In the practical range of E/N that was explored (100−141 Td), E-R reactions always dominated over L-H reactions by similar orders of magnitude for the production of NH(s).…”

Dielectric barrier discharge plasma catalysis as an alternative approach for the synthesis of ammonia: a review

“…100 The results of optical emission spectroscopy to identify excited species in the N 2 -H 2 plasma reaction using the DBD plasma in the presence of various catalysts have been reported by several research groups, as shown in Table 2. 80,91,93,[100][101][102] Although the mechanisms of ammonia synthesis in a non-thermal plasma catalysis system have been reported and described by several groups, [103][104][105][106][107][108][109] Hong et al have presented for the rst time a detailed kinetic modelling of nonequilibrium N 2 -H 2 atmospheric pressure discharges for catalytic NH 3 synthesis. 110 As we all know, to form ammonia in the gas phase, the bonds of both molecular hydrogen and molecular nitrogen must be broken.…”

Ammonia Synthesis via Nitrogen-Coupled Methane Conversion at Ambient Temperature and Plasma Conditions

“…The nonthermal plasma is convenient to combine with the heterogeneous catalyst. In general, this hybrid system may synergistically improve the conversion rate and selectivity. − Despite a lot of efforts, due to the complex interaction between plasma and catalyst, , a deep and thorough understanding of the associated mechanism is still lacking. On one hand, the presence of a catalyst affects the discharge type and the performance of plasma.…”