Development of carbon-based catalysts for small molecule activation

  • Entwicklung von kohlenstoffbasierten Katalysatoren für die Aktivierung kleiner Moleküle

Chen, Shiming; Leitner, Walter (Thesis advisor); Perathoner, Siglinda (Thesis advisor)

Aachen (2019)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2019. - Dissertation, Università degli studi di Messina, 2018


The present Ph.D. thesis was focused on the development of advanced technics for ammonia synthesis with sustainable methods, i.e. electrocatalytic processes using N2, H2O and renewable energy as input sources. Implementing this technology will thus result in a breakthrough change towards a sustainable, low-carbon chemical production based on the use of renewable energy sources. There is thus a rising interest in fossil-fuel-free direct ammonia synthesis. A flow electrochemical cell was developed for ammonia synthesis directly from water and N2 at room temperature and atmospheric pressure. Iron supported on carbon nanotubes (CNTs) was used as the electrocatalyst in this hemi-cell. An ammonia formation rate of 2.2×10-3 gNH3·m-2·h-1 was obtained at room temperature and atmospheric pressure in a flow of N2, under an applied potential of -2.0 V vs. Ag/AgCl. This value is higher than the ammonia formation rate obtained using noble metals (Ru/C) under comparable reaction conditions. Furthermore, hydrogen gas with total Faraday efficiency as high as 95.1% was obtained. Reaction condition was optimised with Fe2O3-CNT used as electrocatalyst. A 30% wt iron-oxide loading was found to be optimal. The performances greatly depend on the cell design, where the possibility of ammonia crossover through the membrane has to be inhibited. The reaction conditions also play a significant role. The effect of electrolyte (type, pH, concentration) was investigated in terms of current density, rate of ammonia formation and Faradaic efficiency in continuous tests up to 24h of time on stream. A complex effect of the applied voltage was observed. An excellent stability was found for an applied voltage of -1.0 V vs. Ag/AgCl. At higher negative applied voltages, the ammonia formation rate and Faradaic selectivity are higher, but with a change of the catalytic performances, although the current densities remain constant for at least 24h. This effect is interpreted in terms of reduction of the iron-oxide species above a negative voltage threshold, which enhances the side reaction of H+/e- recombination to generate H2 rather than their use to reduce activated N2 species, possibly located at the interface between iron-oxide and functionalized CNTs. Active sites for ammonia synthesis was also explored. We show here that, contrary to expectations, iron-oxide (Fe2O3) nanoparticles (supported over carbon nanotubes - CNTs) result more active in the direct electrocatalytic synthesis of ammonia from N2 and H2O than the corresponding samples after reduction to form Fe or Fe2N supported nanoparticles. A linear relationship is observed between the ammonia formation rate and the specific XPS (X-ray- photoelectron spectroscopy) oxygen signal related to O2- in Fe2O3 species, which is proofed by both chemically and electrochemically reduced samples. HRTEM (high-resolution transmission electron microscopy) data on the changes during the electrocatalytic tests confirmed that in-situ activated sites for ammonia synthesis were formed, due to the reconstruction of iron oxide particles. This opens new possibilities to understand the reaction mechanism under working conditions and design more efficient electrocatalyst for ammonia synthesis. Homogenous catalysts for ammonia synthesis was also explored. A series of ruthenium complexes were tested using the same conditions. Ru(PNP)Cl2 (PNP: 2,6-Bis[(di-tert-butylphosphanyl)methyl]pyridine) was found to be the best catalyst for ammonia synthesis among the series of analyzed complexes. This complex was also tested using different conditions, and it was found that suitable amounts of acetic acid can increase its catalytic performance. Comparing different compositions of nitrogen and hydrogen loadings, it was found that the ammonia formation rate increases with increasing nitrogen loading, from which we can deduce that activation of hydrogen was not the rate limitation step in these conditions.