SUPPLEMENTARY INFORMATION Articles https://doi.org/10.1038/s41929-018-0045-1 In the format provided by the authors and unedited. Overcoming ammonia synthesis scaling relations with plasma-enabled catalysis Prateek Mehta 1, Patrick Barboun 1, Francisco A. Herrera 2, Jongsik Kim 1, Paul Rumbach 2, David B. Go 1,2 *, Jason C. Hicks 1 * and William F. Schneider 1 * 1 Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, IN, USA. 2 Department of Aerospace and Mechanical Engineering, University of Notre Dame, Notre Dame, IN, USA. *e-mail: dgo@nd.edu; jhicks3@nd.edu; wschneider@nd.edu Nature Catalysis www.nature.com/natcatal 2018 Macmillan Publishers Limited, part of Springer Nature. All rights reserved.
Supplementary Information: Overcoming Ammonia Synthesis Scaling Relations with Plasma-enabled Catalysis Prateek Mehta, 1 Patrick Barboun, 1 Francisco A. Herrera, 2 Jongsik Kim, 1 Paul Rumbach, 2 David B. Go, 1, 2, Jason C. Hicks, 1, and William F. Schneider 1, 1 Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, Indiana 46556, United States 2 Department of Aerospace and Mechanical Engineering, University of Notre Dame, Notre Dame, Indiana 46556, United States (Dated: February 16, 2018) dgo@nd.edu jhicks3@nd.edu wschneider@nd.edu
SUPPLEMENTARY METHODS We provide below additional information about the computational and experimental methods used, along with supplementary figures and results. Scaling relations Supplementary Figures 1 and 2 show the scaling reactions used to calculate the reaction and activation energies of the elementary steps on stepped surfaces. Relevant adsorption energies and activation energies were obtained from Refs. [1, 2]. Supplementary Figures 3 and 4 show the scaling reactions used to calculate the reaction and activation energies of the elementary steps on terrace sites. Relevant energies were obtained from Refs. [2, 3]. We note that for terrace sites, we used a constant value of -0.32 ev for the H binding energy (the value for Pt(111) [2]). This choice ensures that the overall reaction energy is identical for steps and terraces, and is justified because the hydrogen adsorption energy varies very little for the metals studied here. A similar assumption was made in a previous study [1].
Supplementary Figure 1. Correlations between reaction energies of different elementary steps and the nitrogen adsorption energy on step sites
Supplementary Figure 2. BEP relationships between reaction and activation energies of different elementary steps on step sites Supplementary Figure 3. Correlations between reaction energies of different elementary steps and the nitrogen adsorption energy on terrace sites
Supplementary Figure 4. BEP relationships between reaction and activation energies of different elementary steps on terrace sites Plasma Characterization This section contains figures referenced in the plasma characterization section of the methods described in the text.
Supplementary Figure 5. Experimental set-up consisting of an L-shaped quartz tube DBD plasma reactor with 5 mm inner diameter tube and an optical fibre coupled to a spectrometer. The bottom part of the L-bend has a quartz window for easy optical access to the catalyst bed. Supplementary Figure 6. (a) Emission spectra of a 10 W DBD plasma with no catalyst for different N 2 /H 2 ratios at a total flow rate of 20 sccm. (b) Example of fitting experimental spectrum using Specair in the wavelength range of 300-385 nm (N 2 /H 2 = 5, 10 W DBD plasma with no catalyst).
Supplementary Figure 7. Vibrational temperature (T vib ) as a function of the gas composition N 2 /H 2 for three experimental conditions: a 10 W DBD plasma with no catalyst material (open circle), a 10 W DBD plasma with 100 mg Al 2 O 3 support, and a 10 W DBD plasma with 100 mg Al 2 O 3 support and 5% Ni catalyst. Error bars indicate the 95% confidence interval. Catalyst Characterization N 2 physisorption experiments were performed on all five catalysts at 77 K using a Quantachrome Nova 2200e physisorption system. These data are shown in Supplementary Table 1. All materials had surface areas in the range of 100-150 m 2 /g. The accessible metal sites were titrated through CO pulse chemisorption (Micromeritics Chemisorb 2750). Before measurement catalysts were pre-treated in 20 ml/min hydrogen at 773 K for 30 min to minimize any polycarbonyl formation during the CO pulse sequence. After 30 min the flow was changed to 20 ml/min helium for 30 min to remove any physisorbed species from the surface. The sample was then cooled to 308 K and injections of 30 % CO in He (Airgas) were pulsed into the system until the surface was saturated with CO. The amount of CO adsorbed was subsequently determined using an external calibration. CO uptake amounts are also reported in Supplementary Table 1. X-ray diffraction was performed on each powdered catalyst using a D8 Advance Davinci (Bruker). Diffraction patterns obtained on each material are shown in Supplementary Figure 8. A silicon standard was added to each sample to in order to correct for any shifts in peak location. Particle size distributions were determined for each material
based on TEM images taken by a FEI Titan Microscope. All particle size distributions are based on more than 300 particles. Images and distributions can be seen in Supplementary Figure 9. Average particle sizes are reported in Supplementary Table 1. Supplementary Table 1. Metal loading, surface area, CO uptake, and average particle size of the catalysts investigated in this study. Catalyst Metal loading (%) Surface Area (m 2 /g) CO uptake (µmol/g) Avg. Particle Size (nm) Fe 5 106.4 26.6 9.8 Ru 5 131.1 11.7 14.8 Co 5 140.6 5.5 7.3 Ni 5 133.5 16.5 15.0 Pt 5 152.7 16.8 2.5 Supplementary Figure 8. X-ray diffraction patterns for: a) 5 % Fe/Al 2 O 3, b) 5 % Ru/Al 2 O 3, c) 5 % Co/Al 2 O 3, d) 5 % Ni/Al 2 O 3, and e) 5 % Pt/Al 2 O 3. The vertical dashed lines correspond to the silicon standard. γ-alumina appears on all samples and is marked with a white diamond. The other metals are marked as follows: Fe (brown circles), Ru (red squares), Co (blue triangles), Ni (green diamonds), and Pt (black triangles).
Supplementary Figure 9. TEM images and particle size distributions for 5 % Fe/Al2 O3, 5 % Ru/Al2 O3, 5 % Co/Al2 O3, 5 % Ni/Al2 O3, and 5 % Pt/Al2 O3
Feed Composition Optimization N 2 rich feeds have been reported to produce higher ammonia yields than stoichiometric feeds in plasma-catalytic ammonia synthesis [4, 5]. To determine an optimal feed composition, we measured ammonia synthesis production rates with inlet N 2 :H 2 ratios ranging between 1:3 and 5:1 for two cases. In the first case, we used 100 mg of 5 % Ru/Al 2 O 3. In the second experiment, we used 100 mg of alumnia to measure background ammonia production from the plasma and/or the support. The measured ammonia production rates are shown as a function of feed ratio in Supplementary Figure 10. It is clear from the figure that N 2 :H 2 ratios between 1 and 3 result in the highest production rates. Accordingly, we used a N 2 :H 2 ratio of 2 for all rate determination experiments. Supplementary Figure 10. Observed rate as a function of the inlet feed composition in a reactor packed with 100 mg of 5 % Ru/Al 2 O 3 or 100 mg of Al 2 O 3 blank. Reaction conditions: 438 K, 10 W, flow rate = 20 ml/min. Error bars indicate the standard deviation of duplicate measurements.
Initial rate determination We determined initial rates through careful examination of plots of the production rate of ammonia as a function of the residence time, defined as mass of material divided by the mass flow rate of reactants (W/F ). All initial rate determinations were conducted at 438 K using a feed composition of N 2 /H 2 = 2 and a plasma power of 10 W. For these experiments, 100 mg of material was packed into the reactor, and the total flow rate of reactants was varied between 10 and 50 ml/min. Supplementary Figure 11 shows the ammonia production rates normalized per gram of material as a function of W/F (measured points are shown as filled circles). In all cases, production rates were lower at high W/F, and began to plateau in the limit of very low residence times, indicating that rates at these residence times were in the kinetically limited regime. A blank W/F curve was also measured using a reactor packed with 100 mg of alumina to evaluate the background reactions. Three different methods were used to extract initial rates from the collected data. In the first method, a line was fit through the data points in the reaction-limited regime. These fits were then extrapolated to W/F = 0 to calculate the initial rates. In the second method, a quadratic equation was fit to the data and the peak of the parabola was used to calculate site-time yields. Finally, because a kinetically limited regime was observed on each catalyst, the rate measured at the lowest residence time was assumed to be equal to the initial rate to compute site-time yields. The initial rates computed by the three methods varied only marginally. The reported site time yields are based on initial rates (shown as open circles in Supplementary Figure 11) linearly extrapolated to W/F = 0 (first method).
Supplementary Figure 11. Observed rates as a function of the residence time of reactants (W/F ) in the DBD reactor. Initial rates (plotted as open circles) were extracted by extrapolating the observed rates to W/F = 0 using uncertainty weighted linear regression (shown by dotted lines). Reaction conditions: 438 K, 10 W, inlet N 2 :H 2 = 2:1. Error bars indicate the standard deviation of the rates. SUPPLEMENTARY REFERENCES [1] Vojvodic, A. et al. Exploring the limits: A low-pressure, low-temperature haber-bosch process. Chem. Phys. Lett 598, 108 112 (2014). [2] Wang, S. et al. Universal transition state scaling relations for (de)hydrogenation over transition metals. Phys. Chem. Chem. Phys. 13, 20760 (2011). [3] Falsig, H. et al. On the structure sensitivity of direct no decomposition over low-index transition metal facets. Top. Catal. 57, 80 88 (2013). [4] Mizushima, T., Matsumoto, K., Ohkita, H. & Kakuta, N. Catalytic effects of metal-loaded membrane-like alumina tubes on ammonia synthesis in atmospheric pressure plasma by dielectric barrier discharge. Plasma Chem. Plasma Process. 27, 1 11 (2006). [5] Kim, H.-H., Teramoto, Y., Ogata, A., Takagi, H. & Nanba, T. Atmospheric-pressure nonthermal plasma synthesis of ammonia over ruthenium catalysts. Plasma Processes Polym. 14,
1600157 (2016).