Magnetically Propelled Microrobots toward Photosynthesis of Green Ammonia from Nitrates

Abstract

Ammonia (NH₃) production is a critical industrial process, as ammonia is a key component in fertilizers, essential for global agriculture and food production. However, the current method of synthesizing ammonia, the Haber-Bosch process, is highly energy-intensive, and relies on fossil fuels, contributing substantially to greenhouse gas emissions. Moreover, the centralized nature of the Haber-Bosch process limits its accessibility in remote or resource-limited areas. Photochemical synthesis of ammonia, provides an alternate lower energy, carbon-free pathway compared to the prevailing industrial methods. The photoconversion of nitrate anions, often present in wastewater, offers a greener, more sustainable, and energy-efficient route for both ammonia-generation and wastewater treatment. Photochemical and chemical synthesis of ammonia requires intensive mass-transfer processes, which limits the efficiency of the method. To change the game, in this work, a key new technology of ammonia-generation, a catalytic ammonia generation (AmmoGen) microrobot, which converts nitrate to ammonia using renewable light energy is reported. The magnetic propulsion of the AmmoGen microrobots significantly enhances mass-transfer, and expedites the photosynthesis of ammonia. Overall, this "proof-of-concept" study demonstrates that microrobots can aid in catalytic small molecule activation and generation of value-added products; and are envisaged to pave the way toward new sustainable technologies for catalysis.

Keywords: ammonia; magnetically driven; microrobots; nitrate reduction; photosynthesis.

Figure 1

Schematic representation of magnetically propelled AmmoGen microrobots producing green ammonia from nitrates under visible light irradiation.

Figure 2

Fabrication of the AmmoGen microrobots. a) Schematic representation of the fabrication of the AmmoGen microrobots. Scanning Electron Microscopy (SEM) images of b) BiOI/PTA/Cu microparticles. c) Fe₃O₄@PEI particles. d) AmmoGen microrobots formed by electrostatic interactions between (b,c). e) Magnified image of the surface of (d) showing spherical Fe₃O₄@PEI nanoparticles deposited on the surface. f) Energy Dispersive X‐Ray (EDS) mapping of the elements present in the AmmoGen microrobot.

Figure 3

Characterization of the AmmoGen microrobots. a) UV–vis spectroscopic studies of the materials synthesized step‐by‐step: i) BiOI, ii) BiOI/PTA, iii) BiOI/PTA/Cu, and iv) AmmoGen microrobots; b) Zeta potentials of the materials synthesized step‐by‐step: i) BiOI, ii) BiOI/PTA, iii) BiOI/PTA/Cu, iv) Fe₃O₄@PEI, and v) AmmoGen microrobots. Error bars indicate standard deviations from three replicate measurements; c) Powder X‐ray diffraction (PXRD) studies of i) BiOI, ii) PTA, iii) BiOI/PTA, iv) Fe₃O₄@PEI, and v) AmmoGen microrobots; D) Fourier‐transform infrared (FT‐IR) spectroscopic studies of i) BiOI, ii) BiOI/PTA, iii) BiOI/PTA/Cu, iv) Fe₃O₄@PEI, and v) AmmoGen microrobots.

Figure 4

High‐resolution X‐ray photoelectron spectra (XPS) of the AmmoGen microrobots. Deconvoluted XPS of AmmoGen microrobots for core levels a) Bi 4f, b) I 3d, c) W 4f, d) Cu 2p, e) Fe 2p, and f) O 1s.

Figure 5

Magnetic propulsion of the AmmoGen microrobots. a) Propulsion trajectories of a single Ammogen microrobot represented by colored lines at different frequencies (f = 0, 1, 3, and 5 Hz) of the rotating magnetic field, Scale bar: 10 µm; b) Average instantaneous speed (mean ± standard deviation obtained for 20 individual particles) of the AmmoGen microrobots as a function of frequencies of the magnetic field; c) Propulsion trajectories of a single Ammogen microrobot represented by greenish lines by switching “on” and “off” the magnetic field; d) The speed of AmmoGen microrobots by switching “on” and “off” the rotating magnetic field. Gray and green points indicate the “off” and “on” conditions of the rotating magnetic field respectively; e) Coloured lines representing trajectories of the AmmoGen microrobots in automated and manual steering modes: i) Manual steering, ii) Zigzag mode, iii) Star‐shaped mode, iv) Random mode, and v) Collective motion. Scale bar: 10 µm.

Figure 6

Ammonia production by the AmmoGen microrobots. a) Schematic representation of the AmmoGen microrobots producing ammonia from nitrates; b) Comparison of the yields of ammonia generated by the different individual constituents and the “static” and “dynamic” AmmoGen particles; c) UV–vis spectroscopic measurements showing the production of ammonia by the indophenol blue method. The spectra in the brown and blue zones correspond to the ammonia produced by “static” and “dynamic” particles respectively for aliquots drawn at time intervals of 15, 30, 45, and 60 min; d) Comparison of the ammonia yields by the “static” and “dynamic” AmmoGen microrobots at different reaction time intervals of 15, 30, 45, and 60 min. Error bars correspond to the standard deviation of data obtained from three replicate experiments.