Development of Nanoparticle Catalysts for Electrochemical Ammonia Production

Abstract

Ammonia is one the most common industrial chemicals produced in the United States. It has a strong foothold in the agricultural, plastic, and textile industries. A process termed Haber-Bosch, named for its inventors, is the current method used for ammonia based fertilizer production in the agricultural industry. The process requires high temperatures of 400-500°C and high pressures of 150 – 200 atm to convert nitrogen into ammonia [1]. These extreme conditions only result in a 15% conversion at equilibrium [1]. Another problem associated with the Haber-Bosch process is the reforming of hydrocarbons, like coal and natural gas, resulting in the consumption of 3 to 5% of the world’s natural gas production and the release of large amounts of CO2 [2]. The total energy consumption for an ammonia plant is about 7.9 MW h tNH3 -1, and 13.5 MW h tNH3 -1 for natural gas and coal, respectively [1], and the energy use for the entire ammonia industry is approximately 2% of the world’s energy consumption [2]. The extreme conditions, low efficiency, environmental pollution, and energy consumption of the Haber-Bosch process have helped to push research efforts towards a more economically and environmentally viable electrochemical catalyst approach. The focus of this project is to develop a suitable nanoparticle catalyst for a solid state electrolyte electrochemical process. Electrochemical reduction of nitrogen to ammonia has been performed over non-precious monometallic and multi-metallic combinations, such as iron (Fe) and nickel (Ni). These metals are chosen because they have theoretical nitrogen binding energies that are close to the theoretically-predicted optimum [3]. However, thus far, most electrocatalysts have resulted in low Faradaic efficiencies due to the affinity of the catalyst surface for preferential hydrogen binding [4]. A catalyst made up of a combination of the metals at the nano-scale could balance the need for low applied potential while keeping hydrogen binding to a minimum. In particular, recent work by Licht and co-authors demonstrated that an iron oxide nanoparticle catalyst can be used at elevated temperature (200 oC) to achieve Faradaic efficiencies of greater than 30% [2]. In our initial work, nano-sized catalysts of Fe, Ni, and Fe-Ni were screened for efficiencies over a three hour span. Monometallic Fe nanoparticles showed a drastic decrease in efficiencies over time, whereas the efficiency of the monometallic Ni nanoparticles increased with time. The bimetallic Fe-Ni nanoparticles resulted in a combination of both Fe and Ni properties, depending on the composition and morphology of the nanoparticles. A low surface area, larger diameter Fe-Ni nanoparticle material had decreasing efficiencies over time, whereas a high surface area, nanoparticle cluster Fe-Ni sample had increasing efficiencies over time. In this presentation, we will show recent results on our continued work to develop Fe-Ni bimetallic nanoparticles as an electrocatalyst for nitrogen reduction. In particular, we will discuss progress on controlling nanoparticle size, morphology, and composition to optimize efficiency benefits of Fe while being able to gain stability due to Ni. Experimentally, we will discuss our efforts to investigate the role of certain synthesis parameters on nanoparticle morphology and catalytic performance by changing the timing of synthesis steps to control changes in Fe oxidation state, nickel deposition, and core morphology. Also the effects of changing the molar ratio of Ni to Fe in synthesis and molar ratio of ligand to metal to influence particle size will be explored. References K. Kugler, B. Ohs, M. Scholz, and M. Wessling, Phys.Chem.Chem.Phys., 16, 6129 (2014)S. Licht, B. Cui, B. Wang, F. Li, J. Lau, and S. Liu, Science, 345, 637 (2014)E. Skulason, T. Bligaard, S. Gudmundsdottir, F. Studt, J. Rossmeisl, F. Abild-Pedersen, T. Vegge, H. Jonsson, and J. K. Norskov, Phys. Chem. Chem. Phys., 14, 1235-1245 (2012)Xu, G.C., R.Q. Liu, and J. Wan, Science in China Series B: Chemistry, 52, 8 (2009)