Observation and rationalization of nitrogen oxidation enabled only by coupled plasma and catalyst

Abstract

Heterogeneous catalysts coupled with non-thermal plasmas (NTP) are known to achieve reaction yields that exceed the contributions of the individual components. Rationalization of the enhancing potential of catalysts, however, remains challenging because the background contributions from NTP or catalysts are often non-negligible. Here, we first demonstrate platinum (Pt)-catalyzed nitrogen (N2) oxidation in a radio frequency plasma afterglow at conditions at which neither catalyst nor plasma alone produces significant concentrations of nitric oxide (NO). We then develop reactor models based on reduced NTP- and surface-microkinetic mechanisms to identify the features of each that lead to the synergy between NTP and Pt. At experimental conditions, NTP and thermal catalytic NO production are suppressed by radical reactions and high N2 dissociation barrier, respectively. Pt catalyzes NTP-generated radicals and vibrationally excited molecules to produce NO. The model construction further illustrates that the optimization of productivity and energy efficiency involves tuning of plasma species, catalysts properties, and the reactor configurations to couple plasma and catalysts. These results provide unambiguous evidence of synergism between plasma and catalyst, the origins of that synergy for N2 oxidation, and a modeling approach to guide material selection and system optimization.

Reduced Energy Cost of Ammonia Synthesis Via RF Plasma Pulsing

“…The measurements reported here suggest that this mechanism alone cannot account for the ammonia produced in the reactor. Other activation pathways, such as nitrogen-vibrational excitation and enhanced dissociative adsorption, , must also play a role. We also point out that the nitrogen fixation efficiency increases with the amount of hydrogen introduced into the plasma volume.…”

Energy‐Efficient Small‐Scale Ammonia Synthesis Process with Plasma‐Enabled Nitrogen Oxidation and Catalytic Reduction of Adsorbed NOx

“…The reports available to date describe a combination of plasma with heterogeneous catalysts (both downstream and inside the active plasma compartment), which achieve a plasma-catalysis synergy, reducing the energy consumption of nitrogen fixation. [25,35,36] While the exact nature of such synergy remains an open question and likely depends on each specific case (e. g., plasma parameters), we can hypothesize the possibility of coupling our plasma process with catalysis to further decrease the EC of our plasma-based nitrogen oxidation process.…”

Plasma-Catalyst Reactivity Control of Surface Nitrogen Species through Plasma-Temperature-Programmed Hydrogenation to Ammonia

“…This simplification has negligible influence on microkinetic rates versus models parametrized with a harmonic oscillator model . The nonsteady state surface coverages in temperature-programmed reactions were solved with a mean-field microkinetic model . The coverage of surface species i is written as an ordinary differential equation (ODE) ∂ θ i ∂ t = ∂ θ i true( ∂ T β true) = ∑ j c i j r j and r j = k + j ∏ i θ i − c i j ∏ i P i − c i j − k − j ∏ i θ i c i j ∏ i P i c i j where β (15 K min –1 ) is the temperature ramping rate, r j is the net rate of reaction j , and c ij is the stoichiometric coefficient of i in reaction j .…”

“…11,12 Recent research has shown that nonthermal plasma activation of N 2 enables nitrogen adsorption on metal surfaces under conditions at which the materials are not thermally active for N 2 dissociation. 13−18 These nitrogen intermediates provide access to cyanide, 19,20 nitric oxide, 21 and ammonia 18 at mild conditions. Further, experiments indicate that optimal catalysts for plasma-promoted NH 3 synthesis are different from optimal thermal catalysts.…”