Elucidation of the Active Phase and Deactivation Mechanisms of

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Elucidation of the Active Phase and Deactivation Mechanisms of Chromium Nitride in the Electrochemical Nitrogen Reduction Reaction Jared Nash, Xuan Yang, Jacob Anibal, Siyu Yao, Klaus Attenkofer, Jingguang G Chen, Yushan Yan, Bingjun Xu, and Marco Dunwell J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b05436 • Publication Date (Web): 03 Sep 2019 Downloaded from pubs.acs.org on September 3, 2019

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The Journal of Physical Chemistry

Elucidation of the Active Phase and Deactivation Mechanisms of Chromium Nitride in the Electrochemical Nitrogen Reduction Reaction Jared Nash,‡1 Xuan Yang,‡1 Jacob Anibal,1 Marco Dunwell,1 Siyu Yao,2 Klaus Attenkofer,3 Jingguang G. Chen,2,4 Yushan Yan,1* and Bingjun Xu1*

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Center for Catalytic Science and Technology, Department of Chemical and

Biomolecular Engineering, University of Delaware, 150 Academy Street, Newark, DE 19718, USA.

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Chemistry Division, Brookhaven National Laboratory, Upton, NY 11973, USA.

National Synchrotron Light Source II, Brookhaven National Laboratory, Upton, NY 11973, USA.

ACS Paragon Plus Environment

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Department of Chemical Engineering, Columbia University, New York, NY 10027, USA.


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ABSTRACT

Metal nitrides have been suggested to be effective catalysts for the electrochemical nitrogen reduction reaction (ENRR) based on computational investigations, however, experimental verification has been scarce. In this work, we demonstrate that chromium nitride is an active and selective ENRR catalyst in a Nafion-based membrane electrode assembly. The specific ENRR rate (1.4 × 10-11 mol∙cm-2∙s-1) and Faradaic efficiency (0.58%) on the chromium nitride catalyst are both approximately 20 times higher than those on Pt at -0.2 V vs. the reversible hydrogen electrode. Although the only bulk phase identified by X-ray diffraction of the chromium nitride catalyst is pure phase Cr2N, X-ray photoelectron spectroscopy (XPS) investigations reveal that CrN, CrNxOy and CrOx species, in addition to Cr2N, are present on the surface of the catalyst. In contrast, a synthesized chromium nitride sample with a bulk CrN phase shows a negligible ENRR rate. XPS shows that the synthesized sample does not possess any Cr2N species on the surface, which leads to the identification of Cr2N as the active phase in ENRR. Batch cell testing with 15N2 as the feed forms both 14NH3 and 15NH3, indicating the involvement of surface N in the activation of dinitrogen, i.e. a Mars-van Krevelen mechanism. Two mechanisms of catalyst deactivation are identified: 1) leaching of surface N at lower potentials (< -0.4 V), and 2) slow conversion of the active Cr2N phase to the inactive CrN phase at -0.2 V.


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INTRODUCTION

Industrial ammonia production (140 million tons in 20161) by fixating atmospheric N2 is among the top feats of catalytic science and engineering. The inception of the Haber-Bosch process resulted in the dramatic increase in the human population around the middle of the 20th century.2 N-fertilizers, with ammonia as the primary building block, were estimated to support almost half of the world population in 2008,2 and therefore, the impact of the Haber-Bosch process on humanity cannot be overstated. Further, the demand for ammonia will likely increase as the world population grows. The Haber-Bosch process operates at high pressure (150-300 atm) and relatively high temperature (400-500 °C) on an iron-based catalyst with nitrogen and hydrogen feedstocks.3-5 Despite more than a century of optimization, the Haber-Bosch process remains energy intensive, consuming ~1% of the world’s energy supply.6-9 Moreover, hydrogen used in the ammonia synthesis process is exclusively produced via reforming of fossil hydrocarbons, which leads to substantial CO2 emissions (>10,200,000 tons and ~3% of total global CO2 emissions in 201310). Transportation of N-fertilizers from centralized plants to end users further increases CO2 emissions and consumption of energy.1 Therefore, distributed ammonia production using renewable energy via the electrochemical nitrogen reduction reaction (ENRR) is an attractive alternative to reduce the overall carbon footprint of the ammonia synthesis process and, in turn, agricultural activities. The scalability of electrochemical ammonia synthesis is well-suited for distributed, on-site ammonia production and efficient utilization of intermittent renewable energy sources.11 With the prospect of wide adoption of carbon taxes, a renewable and distributed ammonia production network could become increasingly attractive. The ENRR could enable on-site, on-demand ammonia production powered by renewable energy sources such as solar or wind, and significantly reduce capital requirement as compared to centralized plants. However, the lack of selective and


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active ENRR catalysts hinders the commercial adoption of this process. Moreover, there is little fundamental understanding of ENRR, which makes rational catalyst design near impossible. The ENRR has been the subject of several recent experimental and computational studies. Experimentally, ENRR testing has been conducted with high temperature proton conductors, typically >500 °C,6, 12-16 which limits its overall system energy efficiency unless sophisticated heat integration is implemented. Lower temperature ENRR testing with polymer electrolyte membranes (under 100 °C) offers a promising alternative by reducing the need for process heat, as well as the possibility of ammonia decomposition.10, 17-21 The most mature polymer electrolyte membranes are proton exchange membranes (PEMs), among which, Nafion is most frequently used owing to its high proton conductivity and chemical stability. Due to the acidic nature of PEMs, platinum group metal (PGM) catalysts are the most commonly used catalysts because of their stability in acidic environments. Catalysts generally show low Faradaic efficiencies (FEs)17, 22-24

for the ENRR, which is consistent with computational predictions that metals and metal alloys

are unlikely to be selective for the ENRR.25-27 The low FE of metal catalysts are attributed to the correlated binding energies of various surface intermediates in the ENRR and the competing hydrogen evolution reaction (HER), known as the linear scaling relations. However, recent reports show higher FEs for gold sub-nanoclusters (8.11%28), tetrahexahedral gold nanorods (4.02%29), Pd in a phosphate buffer (8.2%30), ruthenium single-atom catalyst (29.6%31), and boron carbide nanosheet (15.95%32). Some recent work has looked at non-aqueous electrolytes which show higher FE, but lower production rates.33-34 One alternative to these metal catalysts is a family of catalysts, transition metal nitrides, which has been predicted to be active and selective for the ENRR, with the reaction going through a Mars-van Krevelen (MVK) mechanism based on computational studies.26, 35-36 Recently, VN has been reported to be an active ENRR catalyst,7, 37


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however, metal nitrides as a class of ENRR catalysts remain underexplored.38 It is unclear if the active site for VN, which is actually the oxynitride, is universal for all metals nitrides. In order to rationally develop catalyst synthesis strategies, the active site(s) on these metal nitride catalysts needs to be identified to determine the reaction descriptors. In this study, reactivity and extensive characterization of fresh and spent catalysts lead to the identification of the Cr2N species as the active phase in the ENRR. We employ a membrane electrode assembly (MEA) to evaluate the ENRR activity to mitigate the challenge of the low solubility of N2 in aqueous electrolytes. ENRR testing is performed at well-defined cathode potentials by using the anode side as a reference electrode. Identifying Cr2N as the active site shows that the active site can be different for different metal nitrides. Increasing the overpotential on the cathodes favors HER, and can actually remove nitrogen from the catalyst surface and deactivates the catalyst. Furthermore, we demonstrate that at low overpotential formation of the mononitride also deactivates the catalyst. These results show that the nitrogen coordination environment is crucial for peak performance.

EXPERIMENTAL

Catalyst and Characterization

CN-P was purchased from US Research Nanomaterials, Inc. Ball milling of CN-P was performed using a SPEX SamplePrep Miller/Mill 8000M ball miller for 30 min, 2 h, and 24 h in air. CN-S was synthesized using a urea-glass method as described in previous literature.39 Briefly, ethanol and chromium chloride were mixed together and stirred until the solids were dissolved. A 3:1 ratio of urea:chromium was used to create the final precursor. The solution was dried and placed in a tube furnace. The film was heated to 800 °C at a rate of ~3 °C/min in a nitrogen flow.


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The temperature was maintained at 800 °C for 6 h, then slowly cooled to room temperature. XRD measurements were performed on fresh and spent catalysts using a Bruker D8 diffractometer. XPS and ion etching were performed using a Thermo Fisher K-Alpha+ spectrometer. Nitrogen physisorption experiments were conducted on a Micromimetics ASAP 2020 machine. The amount of chromium leached from the nitride catalyst was evaluated with an Agilent Cary UV-Vis spectrometer.

Determination of Electrochemical Surface Areas Cyclic voltammetry (CV) measurements were performed in a glass cell (Pine Ins.) in a three-electrode configuration with a graphite rod as the counter electrode, a double junction saturated calomel electrode (SCE, Princeton Applied Research) as the reference electrode, and a glassy carbon electrode (5 mm diameter, Pine Ins.) with the a catalysts film as the working electrode. The glassy carbon electrodes were first polished with 0.05 µm alumina polishing solution (Buehler) and then rinsed and sonicated in DI water. The catalyst inks were prepared by mixing the catalyst with water and LIQUION (IonPower) to produce a 0.05% Nafion ink solution. The ink was sonicated for 60 min in an ultrasonicator (keeping the ink temperature

Elucidation of the Active Phase and Deactivation Mechanisms of

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