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Distinguishing Plasma Contributions to Catalyst Performance in Plasma-Assisted Ammonia Synthesis Patrick M. Barboun, Prateek Mehta, Francisco Herrera, David B. Go, William F. Schneider, and Jason C. Hicks ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b00406 • Publication Date (Web): 15 Apr 2019 Downloaded from http://pubs.acs.org on April 18, 2019

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ACS Sustainable Chemistry & Engineering

Distinguishing Plasma Contributions to Catalyst Performance in Plasma-Assisted Ammonia Synthesis Patrick Barboun1, Prateek Mehta1, Francisco A. Herrera2, David B. Go1,2, William F. Schneider1, and Jason C. Hicks*1 1

Department of Chemical and Biomolecular Engineering, University of Notre Dame, 54417 Leahy Drive, Notre Dame, Indiana 46556, United States

2

Department of Mechanical Engineering, University of Notre Dame, 54417 Leahy Drive, Notre Dame, Indiana 46556, United States

KEYWORDS. N2 reduction, plasma catalysis, plasma-catalyst interactions, vibrationally excited species, N2 fixation *Email: [email protected]

ABSTRACT

Plasma-assisted catalysis is the process of electrically activating gases in the plasma phase at low temperatures and ambient pressure to drive reactions on catalyst surfaces. Plasma-assisted catalytic processes combine conventional heterogeneous surface reactions, homogeneous plasma-

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phase reactions, and coupling between plasma-generated species and the catalyst surface. Herein, we perform kinetically-controlled ammonia synthesis measurements in a dielectric barrier discharge (DBD) plasma-assisted catalytic reactor. We decouple contributions due to plasmaphase reactions from the overall plasma-assisted catalytic kinetics by performing plasma only experiments. By varying the gas composition, temperature, and discharge power, we probe how macroscopic reaction conditions affect plasma-assisted ammonia synthesis on three different γalumina-supported transition metal catalysts (Ru, Co, and Ni). Our experiments indicate that the overall reaction and plasma-phase reaction are first order in both N2 and H2. In contrast, the rate contributions due to plasma-catalyst interactions are first order in N2 but zeroth order in H2. Furthermore, we find that tuning the plasma discharge power is more effective in controlling catalytic performance than increasing bulk gas temperature in plasma-assisted ammonia synthesis. Finally, we show that adding a catalyst to the DBD reaction alters the way that productivity scales with the specific energy input (SEI).

INTRODUCTION Non-thermal, plasma-assisted catalysis is an emerging, synergistic process to activate and selectively transform a variety of stable molecules, such as methane (CH4), carbon dioxide (CO2), nitrogen (N2), and organic pollutants, to desired products.1-3 High-energy electrons generated in non-thermal (low-temperature, non-equilibrium) plasmas can excite ground-state gas-phase molecules, allowing them to react on catalytic materials at milder bulk temperatures and pressures than possible thermally. This type of synergy between solid catalysts and plasma species has been studied as a method to improve selectivity and low temperature and pressure activity for a variety


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of reactions, including dry reforming of hydrocarbons,4-9 steam reforming of methane,10 CO2 hydrogenation,11-15 pollutant decomposition,16-17 and ammonia (NH3) synthesis18-22, among others. Among these reactions, plasma-assisted ammonia synthesis has received significant attention due to the importance of ammonia as a chemical feedstock. Currently, nearly all of the globally manufactured ammonia is produced from N2 and H2 via the Haber-Bosch process,23-24 which is heavily reliant on fossil fuels. As such, there has been substantial interest in developing new sustainable methods of ammonia production that mitigate this issue.24 One potential production approach that is gaining attention involves delocalized ammonia synthesis plants that produce emissions-free ammonia on-site for industries or processes that rely heavily on ammonia products.25 For example, such a plant could be used for agriculture in areas with underutilized, “stranded” wind energy that is too far from population centers to be economically harvested.26-28 Additionally, hydrogen could be generated through electrochemical water splitting, as an alternative to methane reforming used in industrial ammonia synthesis processes. Because of the low pressures used for electrochemical hydrogen production and the high energy cost of using high pressure systems, the primary challenge for such distributed production methods is the need for reactions at near ambient pressures, an operating envelope where plasma-assisted catalysis presents a promising solution.29-30 Non-thermal plasmas in mixtures of N2 and H2 are known to produce ammonia without a catalyst,31 and adding a catalyst at low pressures has been shown to enhance productivity.19, 21-22, 32-33

In both of these processes, the ammonia synthesis performance is sensitive to macroscopic

reactor and plasma parameters, such as flow rate,34 gas composition,33, 35 bulk gas temperature,21 electrode material,19,

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and plasma power.21,

35-36

Plasma-assisted catalysis has also been

demonstrated on a wide range of metals, often with activity trends that diverge from traditional


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thermal synthesis.19,

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While the exact mechanism leading to the enhancement in ammonia

production is not fully understood, and many possibilities exist,1-2 computational and experimental evidence has suggested vibrationally-excited nitrogen species facilitate dissociative adsorption processes on metal surfaces.34, 37-38 In addition to affecting reactant adsorption, it has also been suggested that the plasma aids in desorption of strongly bound surface species for plasma-assisted ammonia decomposition, which could also play a significant role in ammonia synthesis.39-40 Despite these insights, many fundamental questions remain regarding how the N2 –H2 plasma interacts with metal catalysts. Further, the kinetics of plasma-assisted catalytic reactions are poorly understood, with catalyst performance typically reported as overall conversion and energy efficiency, doing little to distinguish between the contributions of the plasma phase reactions and the plasma-assisted surface catalyzed reactions. In this study, we report rigorous, kineticallycontrolled ammonia synthesis rate measurements of plasma-assisted catalysis on γ-aluminasupported Ru, Fe, and Ni catalysts (all 5 wt % loading) as well as appropriate plasma-only background reactions to quantify ammonia production via plasma processes, to distinguish between plasma and surface contributions to ammonia production. We then monitor these contributions while varying flow rate, gas composition, temperature, and input power to gain insight into the plasma-assisted catalytic consequences related to reactor operation.

METHODS AND MATERIALS Materials Three commercially-available catalysts from Riogen Inc. were used in this study: Ru, Ni, and Co, all supported on γ-Al2O3. Each catalyst had a 5 wt% loading on the support. For reactants, we used 99.999% N2 (Airgas NI UHP300) and 99.999% H2 (Airgas HY UHP300) with 99.997%


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He (Airgas HE HP300) occasionally used as a carrier gas or to flush the system. GC measurements were calibrated using a 10% NH3 in He gas mixture

Catalyst Characterization Crystallinity, surface area, particle size, and surface properties were evaluated for all materials. X-ray diffraction (XRD) patterns were collected using a D8 Advance diffractometer. Scans were conducted over the range of 30-70° 2θ with a scan speed of 2 s per step and a step size of 0.02°. For each XRD measurement, pure silicon was mixed with the sample to act as an internal standard with a 1:10 ratio of silicon to catalyst by mass. Surface area and pore diameter were estimated using a Quantachrome 2200e via N2 physisorption experiments performed at 77 K. Samples were degassed overnight under vacuum and a temperature of 150°C, and N2 adsorption and desorption isotherms were collected over a relative pressure range of 1.0x10-4 to 1.0 P/P0. Surface metal atoms were titrated via CO pulse chemisorption using a Micromeritics Chemisorb 2750 using a procedure outlined in previous papers.8-9, 34 Prior to chemisorption measurements, catalysts were pretreated in 20 SCCM (standard cubic centimeter per minute) H2 at 500°C for 30 min followed by 30 min in 20 SCCM He at 500°C to remove H2 and prevent potential polycarbonyl formation during CO adsorption. Particle sizes were determined from TEM images taken with a Jeol 2011 microscope. All average particle size determinations are based on more than 300 particles. XRD patterns and characterization data are shown in Figure S1 and Table S1 respectively.

Reactor Set-up and Conditions


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The packed-bed dielectric barrier discharge reactor used for rate measurements is depicted in Figure 1 and is similar to the reactor used in our previous studies.8-9, 34 The reactor consists of a quartz tube with an inner diameter of 5 mm and a wall thickness of 1 mm with a silica frit in the center to support packing materials. A 6 cm long 200-1400 steel mesh (McMaster-Carr) is wrapped around the outside of the quartz tube at the same location as the packed bed to act as the outer electrode. A tungsten rod with a diameter of 1.5 mm is inserted along the center of the tube to act as the inner electrode, providing a discharge gap of 1.75 mm and a discharge volume of 1.07 cm3. The DBD was generated in the reactor using a high voltage AC power source (PMV500) with a frequency in the range of 20-25 kHz. Voltage and charge were measured across a capacitor (10 nF) in series with the electrodes using an oscilloscope (Tetronix TDS3012B) connected to a voltage attenuator (1000:1, Tektronix P6015A). The power deposited into the plasma was measured by plotting charge-voltage (Q-V) Lissajous curves, an example of which is shown in Figure S2, and calculating the area inside the curve; a method commonly used for DBD systems.6, 41

He, N2 and H2 flow rates into the reactor were controlled using mass flow controllers (Aalborg

GFC 17). Total flow rates were varied from 30 – 50 SCCM. The reactor was heated by a furnace and maintained at 200C unless otherwise stated. The bulk gas temperature was monitored using a thermocouple located downstream from the discharge zone. Approximately, 100 mg of material was packed into the reactor discharge region zone as depicted in Figure S3. Prior to each experiment, catalysts were pretreated in situ in 20 SCCM of H2 at 500°C for 30 min.


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Figure 1 Diagram of the dielectric barrier discharge packed-bed reactor used in this study. RESULTS AND DISCUSSION In order to ensure that the rates measured in this study are kinetically limited reaction rates free from transport limitations, we performed a Koros-Nowak test on each of the materials used.42 In Figure 2, we report rate measurements of NH3 production on Ni, Co, and Ru catalysts as a function of residence time,9,

43

which was varied by changing reactant flow rate, for different

catalyst pellet sizes and dilution amounts. These metals were chosen because they were identified as the three most active materials in our previous study.34 Experiments were also performed using only γ-Al2O3 to quantify background contributions from reactions in the plasma-phase and the support. γ-Al2O3 is used rather than an empty reactor because it is the support material for all the metals studied, and it is commonly reported that enhancements in the local electric field arise from


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packing a dielectric material into a DBD, often resulting in reaction conversions greater than those observed in empty reactors.8, 31 Three residence time sweep experiments were performed on each catalyst: one using a small (45-90 μm) pellet size and no dilution, the next using a larger (90-150 μm) pellet size with no dilution, and finally an experiment using the same small particle size (45-90 μm) and a 50:50 dilution with γ-Al2O3. In all cases, the NH3 yield increased with residence time (Figure S4). A feed composition of 75% N2 and 25% H2 was used because this ratio resulted in the highest NH3 production rate from our previous study.34 The temperature and discharge power were also both held constant at 200°C and 10 W, respectively. Based on these experiments, it is clear that the pellet size and dilution have no effect on the observed rates, indicating that transport effects are not influencing the measured rates under these conditions. In addition to demonstrating the absence of transport limitations in the rate measurement, Figure 2 shows that over the range of flow rates studied, all catalysts show higher ammonia production rates than a reactor packed only with alumina. Under these same conditions, the measured thermal catalytic rates for each catalyst are zero, indicating that the observed rate enhancement is due to synergistic interactions between the plasma and the catalyst. However, in order to rigorously investigate this enhancement, the ammonia production rate of plasma-phase reactions must be separated from the plasma-assisted catalytic reaction rate in order to more accurately compare the intrinsic reactivity of the different catalysts.


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Figure 2 Rate measurements of plasma ammonia synthesis and plasma-assisted catalytic ammonia synthesis as a function of residence time in the catalyst bed. Temperature was maintained at 200°C with a constant discharge power of 10 W and a feed composition of 25% H2 and 75% N2. Measurements were performed on Ni, Ru, and Co as well as γ-Al2O3 where the residence time was changed by varying the flow rate from 50-30 SCCM. Experiments using 45-90 μm pellets, 90-150 μm pellets, and 45-90 μm pellets diluted in γ-Al2O3 were conducted on each material.

Plasma-assisted catalytic reactions are typically a convolution of homogeneous, plasma-phase reactions and heterogeneous, surface reactions, making evaluation of intrinsic catalyst activity challenging. Most published studies have focused on the overall conversion or selectivity achieved by different catalysts. While these metrics are good descriptors of overall reaction progression, they are not suitable for comparing the intrinsic activity or selectivity across different catalysts, nor can they differentiate between the contributions of homogeneous reactions and catalytic reactions. In this work, we aim to distinguish between the contributions of the homogenous plasma phase reaction, the thermal catalytic reaction, and the plasma-assisted catalytic reaction rates. To infer the contribution of the third component, we assume that the contributions are additive when measured under equivalent conditions at low conversions and product yields:


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(1)

𝑟NH3 = 𝑟plasma + 𝑟thermal catalysis + 𝑟plasma catalysis

Here, rNH3 is the observed ammonia production rate, rplasma is the homogeneous plasma-phase reaction rate, rthermal catalysis is the catalytic ammonia production rate in the absence of a plasma, and rplasma catalysis is the ammonia production rate due to synergies between the catalyst and the plasma. While these contributions are likely coupled, this expression is convenient for describing plasmaassisted catalytic systems, as both the thermal and plasma phase contribution can be quantified by performing background reactions through thermal and plasma only experiments. Thermal ammonia production rates are straightforward to measure through traditional thermal catalytic reactor tests. Conveniently, thermal ammonia synthesis rates are low at atmospheric pressure and low temperature such that rthermal catalysis is negligible, and Equation 1 can be simplified to Equation 2.

(2)

𝑟NH3 = 𝑟plasma + 𝑟plasma catalysis

With this model in mind, we can define useful metrics to evaluate the plasma-assisted catalytic component of the overall rate for different metals. Here, we consider the ratio of the plasma+catalyst ammonia production rate to the ammonia production rate of the background plasma reaction in order to quantify the relative enhancement provided by the catalyst.

𝑟NH3

𝑅𝑎𝑡𝑒 𝑅𝑎𝑡𝑖𝑜 = 𝑟plasma =

𝑟plasma + 𝑟plasma catalysis 𝑟plasma

=1+

𝑟plasma catalysis 𝑟plasma

(3)


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Figure 3a shows the ratio of the observed rate to the plasma-only rate for each catalyst as a function of the residence time W/F in the reactor. As evidenced from the experiments, the addition of a catalyst provides up to a two-fold improvement over the plasma-only reaction rate. Furthermore, the ratio of the overall rate to the plasma only rate does not appear to change significantly with the residence time, indicating that the forward ammonia production rates are being measured.

Figure 3 Residence time sweep experiments on 5% Ru/Al2O3 (red), 5% Ni/Al2O3 (green), 5% Co/Al2O3 (blue), and pure Al2O3 (black). Activity is expressed in terms of (a) the ratio of ammonia production rate with a catalyst and the production rate with pure Al2O3 via Equation 3 and (b) the site-time yield of ammonia on each catalyst via Equation 4. For each experiment, 100 mg of material was used, the flow rate was varied from 50-30 SCCM, the temperature was maintained at 200°C, and the power was maintained at 10 W.


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While mass normalized rates and rate ratios are effective at describing the productivity of the reaction and the synergistic effects of the plasma, neither metric can effectively describe the intrinsic activity of a metal for comparison between different materials. To get a clearer picture of the intrinsic activity of each of the catalysts in this study, background subtracted ammonia production rates were normalized on the number of CO-accessible surface metal sites, determined through CO pulse chemisorption. The resultant site-time yield (STY) is then defined as

𝑆𝑇𝑌 =

(𝑟NH3 ― 𝑟plasma) 𝐶𝑂 𝑠𝑖𝑡𝑒𝑠

=

𝑟plasma catalysis + 𝑟plasma ― 𝑟plasma 𝐶𝑂 𝑠𝑖𝑡𝑒𝑠

=

𝑟plasma catalysis 𝐶𝑂 𝑠𝑖𝑡𝑒𝑠

(4)

Subtracting out plasma contributions before normalizing on sites is necessary as the homogeneous reaction does not occur on metal sites; thus, without subtraction, catalytic rates would likely be significantly overestimated. It should be noted that this expression assumes the background plasma reaction rate measured with our blank experiments is not significantly affected by incorporating low metal loadings of different supported metal nanoparticles into the discharge zone, which is corroborated in studies that interrogated these possible interactions.44-45 Figure 3b shows the site-time yields for each catalyst computed using Equation 4. Upon subtraction and normalization, the differences between the metals are apparent. The activity trend, in terms of site-time yield, is Co > Ni > Ru, which is consistent with our previous work.34 We conclude that the differences in production rate observed here, as well as previously, are due to differences in the intrinsic activity of each catalyst.34

Gas Composition and Rate Law


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Figure 4 (a and d) Ammonia production rates, (b and e) site-time yields, and (c and f) ratio of plasma+catalyst rate to plasma background rate as a function of partial pressure of N2 (a,b, and c) or H2 (d, e, and f) are shown on 5% Ni/Al2O3 (green), 5% Ru/Al2O3 (red), 5% Co/Al2O3 (blue), and pure Al2O3 (white). All experiments were conducted at 1 atm, 200°C, 10W and 50 SCCM. When varying PN2, the ratio of (He+N2):H2 was maintained at 1:5. When varying PH2, the ratio of N2:(He+H2) was maintained at 3. To elucidate gas composition effects on the interactions between a catalyst and the plasma, experiments were performed to provide insights about the governing rate expression for the


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observed plasma-assisted catalysis. We assume power-law expressions for both the plasma-only rate (rplasma = k1PN2aPH2b) and the plasma-assisted catalytic rate (rplasma

catalysis

= k2PN2cPH2d).

Subsequently, reaction orders can be determined by varying the partial pressure of one reactant in excess of the other reactant. For determining a and b, we performed experiments at a feed composition of 16.7% N2 and 83.3% H2. From this point, the partial pressure of N2 supplied was reduced and replaced with He such that a constant total flow rate, H2 fraction, and pressure were maintained. We note that He could also participate in plasma phase reactions, however, due to the low partial pressures of reactant used in these studies, we expect its role to be negligible. Furthermore, Lissajous curves are largely unaffected by the presence of He (see Fig S5). This process was repeated for each catalyst, as well as for the plasma only background using only γAl2O3. The observed ammonia production rates on the catalysts as a function of PN2 are shown in Figure 4a. Linear curve fits (dotted lines) show that the observed rate increases linearly with PN2 in the background experiment as well as for each catalyst studied with an intercept of 0 at PN2 = 0, implying a first order dependence on PN2. Similarly, we performed an experiment varying the partial pressure of PH2 in excess N2 (shown in Figure 4b). The gas composition was 75% N2 and 25% H2, and the hydrogen mole fraction was decreased and replaced with He to maintain a constant total flow rate. As shown in Fig. 4b, linear dependence was again observed on all materials including the background experiments; however, the slope is constant on all materials. Since rNH3 for γ-Al2O3 is equivalent to rplasma, the first order dependence for each reactant (i.e. a = b = 1) indicates that both N2 and H2 play a role in the rate determining step for the homogeneous plasma-only reaction.


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Site-time yields with respect to the partial pressures of N2 and H2 are shown in Figures 4c and 4d respectively. Reaction orders for the plasma-assisted catalytic reaction become clear upon background subtraction. We find that for all catalysts, the site-time yields, and thus the plasmaassisted catalytic rate (rplasma catalysis), are first order in N2 partial pressure (Figure 4c). However, the site-time yields have no dependence on the partial pressure of H2 (Figures 4d), in contrast the first order-dependence seen for the observed rates without background subtraction (rNH3). These results indicate that H2 activation by the catalyst is fast and that the activation of N2 is the most critical step in driving the plasma-assisted catalytic reaction. The observed reaction rate orders are consistent with the hypothesis in our previous work that the generation of excited N2 species by the plasma increases N2 adsorption to the catalyst surface and promotes plasma-assisted catalytic reactions. 34 This same analysis is evident when looking at the data in terms of the ratios of the overall rates to the plasma-only rate. Based on Equation 3, the rate ratio is proportional to PN2c-a when varying PN2, or PH2d-b when varying PH2. Figure 4e shows that the rate ratio has no dependence on partial pressure of N2, confirming that both the plasma reaction and the plasma-assisted catalytic reaction have a first order dependence on N2. By contrast, the rate ratio with respect to H2, shown in Figure 4f, appears to have a dependence of -1 implying that the plasma-assisted catalytic reaction has no H2 dependence since we observe a first order dependence on H2 in the plasma reaction.

Temperature Dependence The effects of bulk gas temperature on the plasma-assisted catalytic performance were investigated. A constant discharge power of 10 W was used in these experiments, and steady state ammonia production rates were collected at various temperatures between 150°C and 325°C. The


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gas composition in these experiments was 75% N2 and 25% H2 with a constant flow rate of 50 SCCM. The initial value of 150°C was chosen as the lower extreme because the resistive heating caused by the plasma increased the temperature up to this value without external heating. Under these same conditions, no ammonia production was observed from thermal catalytic reactions (not shown for brevity).

Figure 5 (a) Ammonia production rates, (b) the ratios of the overall rates to the plasma-only rate, and (c) site-time yield are shown as a function of temperature on 5% Ru/Al2O3 (red), 5% Ni/Al2O3 (green), 5% Co/Al2O3 (blue), and pure Al2O3. For each experiment, 100mg of material was used,


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the total flow rate was 50 SCCM, the gas composition was 25% H2 and 75% N2 and the power deposited was maintained at 10 W. Figure 5a shows the mass normalized production rates of ammonia as a function of temperature. As with the residence time sweep experiments, a background reaction with only γAl2O3 was also performed to quantify the contribution of homogeneous plasma phase reactions to the overall ammonia productivity. From these experiments, background reaction rates increase with increasing temperature as shown in Figure 5a. In the presence of a catalyst, overall rate enhancement is observed at all temperatures, and rates improve with temperature similarly to the plasma background. The ratios of the overall reactions to the plasma-only reaction as a function of temperature are plotted in Figure 5b. While the overall ammonia production increases as a function of temperature on all materials, the relative enhancement provided by the catalyst decreases. This decrease in the ratio of the overall rate to the plasma rate with temperature indicates that contributions of the plasma-assisted catalytic surface reactions to the overall ammonia production rate decrease with increasing temperature. Similarly, the site-time yields for each material (Figure 5c) also decrease as a function of temperature demonstrating that the plasma-assisted reaction is non-Arrhenius, as has been observed in plasma-assisted catalytic dry reforming.9 The increase in plasma phase activity observed here contrasts with plasma dry reforming, where plasma-phase reaction rates were observed to drop off with increasing temperature.8, 46-47 For dry reforming, which is an endothermic reaction, Kim et. al. showed that below 400°C, plasma activity dominated overall reactivity, but as the temperature was increased, the plasma alone became nearly completely inactive, while plasma-assisted catalytic behavior became more noticeable. The absence of plasma-only reactivity was suggested to be due to the suppression of CH4 dissociation by the plasma at higher temperatures, while other species, such as vibrationally excited species,


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may enhance catalytic activity.8-9 Here, for this exothermic reaction, the opposite is observed; as the temperature is increased the activity of the plasma phase reactions increases, while the species driving surface reactions comprise a smaller fraction of the total number of active species.

Power and SEI While for thermal catalysis, temperature is the primary parameter describing energy input into a reactor; plasma-assisted catalysis introduces plasma power as another important parameter describing energy deposition into a reactor. Figure 6a shows the measured ammonia production rate for the plasma, as well as the plasma-assisted catalytic rates on all three catalysts as a function of power. Reaction rates increased linearly with power; a dependence that has also been observed in other studies.21 This rate increase can be explained by the generation of more and more intense discharge filaments that occurs with increasing power, which in turn drive plasma phase chemistry.41, 48


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Figure 6 (a) Ammonia production rates, (b) site-time yield, and (c) ratios of the overall rates to the plasma rate shown as a function of the power deposited into the reactor on on 5% Ru/Al2O3 (red), 5% Ni/Al2O3 (green), 5% Co/Al2O3 (blue), and only γ-Al2O3. For each experiment, 100 mg of material was loaded into the reactor, the temperature was maintained at 200°C, the gas composition was 25% H2 and 75% N2, and the flow rate was 50 SCCM.

Figure 6b shows the site-time yield of each material as a function of input power. Again, these results highlight the activity differences between the various metals and also show that the trends


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in activity do not deviate over the power range studied. Finally, from ratios of the overall rates to the plasma rate, shown in Figure 6c, the relative enhancement provided by the catalysts can be clearly observed at all powers investigated. Notably, the relative enhancement also increases with power, indicating that the catalysts are turning over more ammonia per site as more electrical energy is deposited into the reactor. Because this increase is associated with an increase in the plasma discharge area, we speculate that higher input power increases the concentration of active species that can react on the catalyst surface.

Figure 7 (a) Ammonia production rates, (b) ratios of the overall rates to the plasma-only rate, and (c) site-time yield as a function of SEI are shown at three different powers using 5% Ni/Al2O3 (closed symbols) and only γ-Al2O3 (open symbols). For all experiments, the reactant flow rate


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was varied from 50 – 30 SCCM, the temperature was maintained at 200°C and 100 mg of material was loaded into the reactor. We have demonstrated, as other studies have, that power is an important and relatively straightforward parameter to modify the plasma phase and plasma-assisted catalytic activity.7, 9, 21, 32

However, often a more useful way to describe energy deposition is the specific energy input

(SEI), which is defined as the power deposition divided by the flow rate of reactants into the reactor and can be interpreted as the average energy deposited per molecule of gas in the discharge. Figure 7a shows the observed ammonia production rates on 5% Ni/Al2O3 and the plasma only (with γAl2O3) background experiment as a function of the SEI at different powers and flow rates. These experiments were conducted on all materials, though Ru and Co are omitted for clarity and can be found in Figures S6 and S7. Plasma only data (Figure 7a; open squares, open circles, and open diamonds) at various flow rates and powers collapse to a single curve when plotted against SEI make. Plasma phase ammonia production peaks at an SEI of ~13 kJ/L, then begins to drop off at higher SEI values likely due to the higher conversion of reactants that occurs at these lower flow rates. In contrast, plotting the plasma-catalyst production rates (Figure 7a; closed squares, closed circles, and closed diamonds) versus SEI does not collapse Ni data, indicating that input power affects plasma-catalyst interactions in a way that it does not affect plasma-phase reactions. This result provides evidence that the SEI does not fully describe the catalytic contribution to the reaction. This is even more apparent when the ratios of the overall rates to the plasma-only rate are examined as a function of SEI (Figure 7b). Increasing the power improves the relative enhancement provided by the catalyst; however, no obvious trend with SEI can be observed. Likewise, as depicted in Figure 7c, the site-time yield on Ni at various powers plotted with respect


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to SEI shows plasma-assisted catalytic reactions are not solely controlled by SEI like the plasmaonly reaction. Hence, adding a catalyst to the plasma system appears to break the scaling between the plasma-phase reaction and the SEI, showing that there are plasma characteristics that change with power and not SEI that drive plasma-surface reactions.

Figure 8(a) Ammonia production rates, (b) ratios of the overall rates to the plasma-only rate, and (c) site-time yield as functions of residence time are shown at three different powers using 5% Ni/Al2O3 (closed symbols) and pure Al2O3 (open symbols). For all experiments, the reactant flow rate was varied from 50 – 30 SCCM, the temperature was maintained at 200°C and 100 mg of material was loaded into the reactor.


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To further interrogate these findings, the same rate data from Figure 7 are shown as a function of residence time at different powers (Figure 8). Figure 8a provides the measured rates, which show no clear trend with residence time. Depending on the power used, ammonia production rates can either increase or decrease with residence time, which is not normally observed in thermal catalysis, where rates are higher at shorter residence times. The increase in the plasma only measured ammonia production rate with residence time can be attributed to the increase in SEI. However, the effectiveness of residence time as a descriptor for catalytic activity becomes more apparent in the rate ratio and site-time yield plots. Figure 8b shows a plot of the ratios of the overall rates to the plasma-only rate versus residence time where the relative rate enhancement plateaus at short residence time (high flow rate), which indicates that measurements at this point are maximum values (i.e., approximated as initial rates at the W/F = 0 limit). This same effect is seen in Figure 8c, where the site-time yield provides a similar leveling effect at short residence times. Similar trends were observed on all three catalysts investigated as shown in Figure S6 and S7. Collectively, it is evident that SEI is a useful parameter for describing plasma phase activity; however, plasma-assisted catalytic reactions are influenced more significantly with power than SEI. Additionally, variation in parameters common to traditional kinetic evaluation of solid catalysts (i.e., residence time) provide further insight to the dynamical nature of the plasmacatalyst interaction and how the behavior in the presence of the metal catalyst deviates from plasma only processes.

Catalyst Stability Catalyst stability was investigated through a time-on-stream study with Co as the catalyst over a 7 hour period as shown in Figure 9a. The reaction was conducted at a constant discharge power


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of 10 W, a temperature of 200°C, a flow rate of 50 SCCM, and a feed composition of 75% N2 and 25% H2. The production rate of ammonia was stable over this time period, as has been seen in other studies of plasma-assisted catalytic ammonia synthesis.36 Post-reaction characterization of the supported Co catalyst was performed to determine if any significant changes to morphology or physical properties occurred during the reaction. Figures 9c and 9d show TEM images of the catalyst before and after the reaction. No obvious changes to the catalyst were observed from these images. Figure 9b shows the particle size distribution of the Co metal on the support before and after the reaction. No significant change in the particle size appeared during the 7 hours reaction time. Finally, N2 physisorption and CO titrations of the catalyst after reaction show minimal change in the surface area and a slight reduction in the number of measurable surface metal sites from 5.4 μmol/g to 4.8 μmol/g. Overall, these experiments provide evidence that the plasma has minimal effect on the morphology and physical properties of the catalyst under the conditions studied here. While it has been proposed that the DBD can affect the morphology of catalysts,1, 49 several studies have also shown that plasma treatment during catalyst synthesis can prevent catalyst sintering during calcination and reduction steps and is likely the reason for minimal changes in catalyst properties during this time-on-stream experiment.50-52


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Figure 9 a) A time-on-stream study of plasma-catalytic ammonia synthesis over 5% Co/Al2O3 for a 7 hrs reaction time at 200°C, 10W plasma discharge power, 50 SCCM total flow rate, and a feed composition of 75% N2 and 25% H2. b) Particle size distributions for the Co catalyst before and after reaction for 7 hrs. Histograms are based on more than 300 particles. TEM images of the Co catalyst before c) and after d) 7 hrs of reaction. CONCLUSION In this study, experiments were performed that decouple the plasma-phase and plasma-catalytic ammonia production rates to provide insights that are not easily obtained when only observing overall production rates. We show that, by applying experimental methods typically used for measuring catalytic productivity under low conversion, differential reactor conditions and by measuring appropriate background reactions, the contributions of plasma-catalytic reactions relative to the homogeneous plasma-phase reactions can be interrogated. By varying reactant gas


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composition, we observed significant differences with and without a catalyst in plasma-assisted experiments. On the catalysts examined in this study, N2 activation is the critical step in the plasma-assisted catalytic reaction pathway as shown by the first order dependence with respect to N2 partial pressure and the zero-order dependence with H2. By contrast, the first order dependence in both reactants observed in the plasma phase shows that the homogenous reaction is limited by steps involving both nitrogen and hydrogen. Together, these findings provide further evidence of pathway differences between plasma-phase reaction processes and plasma-assisted catalysis. To kinetically drive the reaction forward, bulk gas temperature and plasma input power were varied independently to determine if and/or how the plasma-assisted processes differ from the thermal reaction. Although both increasing bulk gas temperature and increasing power deposition improved the production of ammonia in the conditions studied, tuning discharge power is more effective at controlling plasma-assisted catalytic activity than bulk gas temperature. To gain insight into how the energy deposition into the system affected the production rates in either the plasma-phase only or plasma-assisted catalysis, the same specific energy input (SEI) was applied by varying input power and gas flow rate. The plasma-phase reactions correlate well with the SEI, where the same ammonia production rate can be obtained at different powers but the same SEI. However, in the presence of a catalyst, significant deviations from the plasma-phase reactivity are observed. When keeping the SEI constant, it is evident that higher ammonia production rates are obtained at a higher input power, which showcases the synergy between the plasma and the catalyst that enhance the plasma-surface reaction independent of SEI. Similar detailed investigations for other plasma-catalytic reactions will provide the basis for the rational development of more efficient plasma-assisted catalytic processes.


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AUTHOR INFORMATION Corresponding Author *Email: [email protected] SUPPORTTING INFORMATION Contains characterization for all catalysts studied, including XRD, particle size, chemisorption analysis and surface area analysis. Plasma characterization is also included as well as additional reactor tests that support the experiments shown in the main text. ACKNOWLEDGMENT The authors thank the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Sustainable Ammonia Synthesis Program under award number DE-SC-0016543, the U.S. Air Force Office of Scientific Research under Award No. FA9550-18-1-0157, and National Science Foundation under grant PHY-1254273 for financial assistance for this project. We thank the Notre Dame Materials Characterization Facility for use of the Bruker D8 Advanced Diffractometer, which was used to collect XRD patterns, and the Notre Dame Integrated Imaging Facility for access to the TEM. REFERENCES 1. Neyts, E. C.; Ostrikov, K.; Sunkara, M. K.; Bogaerts, A., Plasma Catalysis: Synergistic Effects at the Nanoscale. Chem. Rev.s 2015, 115 (24), 13408-13446. 2. Whitehead, J. C., Plasma–catalysis: the known knowns, the known unknowns and the unknown unknowns. J. Phys. D: Appl. Phys. 2016, 49 (24), 243001-243001. 3. Metha, P.; Barboun, P.; Go, D. B.; Hicks, J. C.; Schneider, W. F., Catalysis Enabled by Plasma Activation of Strong Chemical Bonds: A Review. ACS Energy Lett. 2019, 4 (5), 10.1021/acsenergylett.9b00263 (in press). 4. Allah, Z. A.; Whitehead, J. C., Plasma-catalytic dry reforming of methane in an atmospheric pressure AC gliding arc discharge. Catal. Today 2015, 256, 76-79. 5. Gallon, H. J.; Tu, X.; Whitehead, J. C., Effects of Reactor Packing Materials on H 2 Production by CO 2 Reforming of CH 4 in a Dielectric Barrier Discharge. Plas. Proc. Polym. 2012, 9, 90-97.


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TOC FIGURE:

SYNOPSIS Plasma-assisted catalysis offers an alternative method for low temperature and low-pressure ammonia synthesis and can be driven by renewable energy sources for distributed, small scale operation.


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Plasma-assisted catalysis offers an alternative method for low temperature and low pressure ammonia synthesis and can be driven by renewable energy sources for distributed, small scale operation.


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Distinguishing Plasma Contributions to Catalyst ... - ACS Publications

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