Observation and Rationalization of Nitrogen Oxidation Enabled Only by Coupled Plasma and Catalyst

Abstract

<div> <div> <div> <p>Heterogeneous catalysts coupled with non-thermal plasma (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 are often non-negligible. Here, we first demonstrate nitrogen (N2) oxidation by radio frequency plasma and platinum (Pt) combination at conditions in which nitric oxide (NO) yield from plasma or Pt is vanishingly small. 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, NO yields from NTP and thermal catalysis are suppressed by radical reactions and inhibited by 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 yield and energy efficiency involves tuning of plasma species, catalysts properties, and the reactor configurations to couple the two. These results provide unambiguous evidence of the benefits of combining plasma and catalysts and open approaches to design the coupled system. </p> </div> </div> </div>

Plasma-catalyst modeling for materials selection: challenges and opportunities in nitrogen oxidation

“…Disentangling the gain by NTP-catalyst combination is therefore challenging. Recently, an RF plasma was coupled with a Pt catalyst to generate NO at conditions at which neither component alone is active, results rationalizied through corresponding microkinetic models [14]. Here, we compare those model results with those predicted for a Au catalyst both as illustration of the modeling approach and an illustration of material dependence in this system.…”

“…We focus on (211) surfaces in the following analysis since they often show higher activities under thermal conditions [51]. NO is the major observed product at low O 2 mole fractions with similar plasma characteristics [14]. NO 2 or N 2 O production is therefore not included here for simplicity.…”

“…Here, we illustrate the coupling of a reduced plasma chemistry with DFT-parameterized surface reactions to predict the chemical product formation from plasma-catalyst combinations with N 2 oxidation to NO as an example (figure 1). We consider the mutual influences of Au and plasma on NO production rates and production and compare to previously reported Pt results [14]. N 2 oxidation is an alternative to ammonia synthesis for nitrogen fixation [15,16].…”

“…The concentrations of gaseous species in the reactor is modeled assuming a well-mixed flow throughout the reactor volume, where the concentrations of gaseous species in the reactor are the same as those at the outlet [14]. This is commonly termed a continuously stirred tank reactor and test calculations of plug flow reactor yield similar results:…”

“…N 2 oxidation is an alternative to ammonia synthesis for nitrogen fixation [15,16]. While NO is produced in non-thermal N 2 /O 2 plasmas, at low O 2 mole fractions in a low pressure, radio frequency plasma coupled with a Pt catalyst, NO production is observed at conditions at which neither catalyst nor plasma yields significant NO [14,17]. As highlighted in figure 1, NO formation pathways in non-thermal plasmas are wellestablished and can be described using a simple two-step Zeldovich model [18,19].…”