Elsevier

Journal of Catalysis

Volume 359, March 2018, Pages 82-91
Journal of Catalysis

On the mechanism of the electrochemical conversion of ammonia to dinitrogen on Pt(1 0 0) in alkaline environment

https://doi.org/10.1016/j.jcat.2017.12.028Get rights and content

Highlights

  • Potential-dependent characterization of adsorbed/desorbed NH3 oxidation products.

  • Nsingle bondN bond formation takes place via NH dimerization.

  • The coverage with NH is decisive for the formation of N2.

  • Strongly adsorbed N and NO inhibit Nsingle bondN formation via NH coupling.

  • A new reaction scheme, which includes NO formation is proposed.

Abstract

The electrochemical oxidation of ammonia to dinitrogen is a model reaction for the electrocatalysis of the nitrogen cycle, as it can contribute to the understanding of the making/breaking of Nsingle bondN, Nsingle bondO, or Nsingle bondH bonds. Moreover, it can be used as the anode reaction in ammonia electrolyzers for H2 production or in ammonia fuel cells. We study here the reaction on the N2-forming Pt(1 0 0) electrode using a combination of electrochemical methods, product characterization and computational methods, and suggest a mechanism that is compatible with the experimental and theoretical findings. We propose that N2 is formed via an NH + NH coupling step, in accordance with the Gerischer-Mauerer mechanism. Other Nsingle bondN bond-forming steps are considered less likely based on either their unfavourable energetics or the low coverage of the necessary monomers. The Nsingle bondN coupling is inhibited by strongly adsorbed N and NO species, which are formed by further oxidation of NH.

Introduction

Although dinitrogen and ammonia are the most stable forms of nitrogen under standard conditions, implying that these should thermodynamically be the preferred end-products in electrochemical reactions within the nitrogen cycle, in fact there are a few electrodes that are able to reduce nitrate to nitrogen, to reduce nitrogen to ammonia, or to oxidize ammonia back to nitrogen [1], [2], [3]. The difficulty in making the electrocatalysis of the nitrogen cycle selective and efficient is related to kinetic limitations, emphasizing the importance of understanding how the fundamental structure/nature of catalysts and the double layer affect the activity and selectivity of nitrogen-based electrocatalytic processes. Therefore, advances in understanding, at atomic and molecular levels, complexities of processes involved in ammonia oxidation to nitrogen and, in the reverse reaction, nitrogen reduction to ammonia, will provide the necessary building blocks for improving the efficiency of the overall nitrogen cycle in aqueous systems [4].

The splitting of ammonia to N2 and H2, which comprises the reversal of the Haber-Bosch process, can have a great impact if it is utilized for hydrogen production in an electrolyser. In such an ammonia electrolysis process, NH3 is oxidized on the anode to N2, while H2O is reduced on the cathode to H2 [5], [6], [7], [8]. This carbon-free process can turn ammonia into a convenient hydrogen carrier, if the energy required to operate the electrolyser is provided by renewables. However, as most of the reactions between N-containing compounds, the selective conversion of NH3 to N2 is difficult, even though N2 is thermodynamically the only possible product in a relatively broad potential region from ca. +0.1 V to +0.5 VRHE (see the thermodynamics of possible reactions in the section S1 of the Supporting Information). In fact, the ammonia oxidation occurs at more positive potentials due to kinetic limitations, i.e. above +0.5 VRHE, where other products can be additionally formed.

Platinum is the most active polycrystalline surface for N2 formation at such potentials [9], [10], [11]. Studies with Pt single-crystal electrodes [12], [13], [14], [15] and with preferentially (1 0 0)-oriented Pt nanoparticles [3], [16], [17], [18] have shown that the N2 formation on Pt takes place exclusively on the (1 0 0) surface.

The two traditional mechanisms for the electrochemical ammonia oxidation reaction (AOR) include the progressive formation of NH2, NH and N, but they differ in the elementary step for Nsingle bondN bond formation, which can be either an N + N (Oswin-Salomon) or an NHx + NHy with x, y = 1–3 (Gerischer-Mauerer) bond-forming step [19], [20]. In the Gerischer-Mauerer mechanism, the N species is considered a poison. The dominant mechanism for Nsingle bondN coupling on Pt(1 0 0) or polycrystalline Pt remains unclear. Experimental studies support the Gerischer-Mauerer mechanism [10], [11], [15], [21], [22], [23], but computational findings indicate that the N dimerization is the most favourable Nsingle bondN coupling step on Pt(1 0 0) [24], [25], [26]. Beyond this disagreement, the two traditional mechanisms must be extended to include nitrogen oxides which form in parallel, such as NO [22], [23]. The formation of NO is not considered in the traditional mechanisms but it may influence the reaction rate and selectivity, e.g. by blocking active sites or by participating in bond-forming steps [27]. Moreover, from the strong pH dependence of the ammonia oxidation rate we recently concluded that a deprotonation step should take place before the Nsingle bondN bond formation, decoupled from the electron transfer [28].

The discussion above indicates that more efforts are required to understand the mechanism of the electrochemical AOR. Here, we study the reaction on Pt(1 0 0) with focus on the characterization of adsorbed and desorbed AOR products as a function of the potential. We utilize standard electrochemical methods in combination with online electrochemical mass spectrometry (OLEMS), ion chromatography (IC), and Fourier transform infrared (FTIR) spectroscopy. In addition, we study the ammonia dehydrogenation steps with density functional theory (DFT) calculations. Based on the new data, we propose a scheme for the ammonia oxidation reaction that consistently explains the experimental observations.

Section snippets

Electrochemical cells

For standard electrochemical measurements, a Teflon FEP cell was used with a Pt counter electrode and a saturated Ag/AgCl reference electrode, which was located in a separate vial to prevent contamination by chloride ions and was connected with the main compartment via a salt bridge. For OLEMS, IC and FTIR measurements, a glass cell with a Pt counter electrode and a reversible hydrogen reference electrode (RHE) was used.

Electrode preparation

The Pt(1 0 0) single-crystals were disks from Mateck GmbH, with 6 mm

Computational details

The DFT simulations were carried out with VASP [33] using the projector augmented-wave method [34] to describe ion-electron interactions and the PBE formalism to evaluate the exchange-correlation energy [35]. The 2 × 2 (1 0 0) slabs had five metal layers: the three topmost and the adsorbates were allowed to relax completely, while the other two were fixed at the bulk interatomic distances. The geometry optimizations were made with using the conjugate-gradient scheme and a plane-wave cut-off of

Voltammetric characterization of ammonia solutions

We start with describing the voltammograms acquired on Pt(1 0 0) in ammonia-free 0.1 M KOH and in a solution containing 10−3 M ammonia, as summarized in Fig. 1. The blank voltammogram in the ammonia-free solution (solid grey curve) exhibits four pairs of peaks, which are typical for Pt(1 0 0) in alkaline solution and are related to hydrogen and hydroxide adsorption/desorption from different types of sites [44], [45], [46].

In the ammonia-containing solution (solid black curve), the current

Discussion and conclusion

We propose below a reaction scheme that condenses our experimental and computational findings (Scheme 1). According to that scheme, adsorbed ammonia is progressively dehydrogenated to NH2 and NH. Afterwards, the dominant pathway for NH is its dimerization to N2H2, which is dehydrogenated to N2 gas. This fast process competes with the slow recombination of NH with OH to form N and H2O. The formed N is the precursor for the formation of NO via NOH. The coverage with N is not sufficient

Acknowledgements

This research was supported by a Marie Curie International Outgoing Fellowship within the seventh European Community Framework Programme to I.K. under Award IOF-327650, and by the U.S. Department of Energy, Office of Science, Materials Sciences and Engineering Division (contract DE-AC02-06CH11357). H. L. acknowledges support from the China Scholarship Council through a CSC scholarship. F.C.-V. acknowledges funding from The Netherlands Organization for Scientific Research (NWO), Veni project

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