A Rare-Earth Samarium Oxide Catalyst for Electrocatalytic Nitrogen Reduction to Ammonia [View all]

[NNadir]

The paper I'll discuss in this post is this one: A Rare-Earth Samarium Oxide Catalyst for Electrocatalytic Nitrogen Reduction to Ammonia (Yonghua Cheng, Haifeng Nan, Qingqing Li, Yaojing Luo, and Ke Chu
ACS Sustainable Chemistry & Engineering 2020 8 (37), 13908-13914).

I often reflect on Stuart Kaufmann's remark, which has stuck in my mind for nearly two decades, in his fabulous book The Origins of Order that life can be considered, "An Eddy in Thermodynamics." I once spent part of an afternoon with Freeman Dyson - one of the best afternoons of my life - and he approved of that description as well.

The nitrogen-nitrogen triple bond is one of the strongest, and thus one of the most thermodynamically stable, bonds there is, 9.79 ev/bond. This means that it is very, very, very difficult to break. Worse, the activation energy of breaking it is also enormous, 3.5 ev/bond. Yet, for life to exist, breaking this bond is essential because of the impossible to understate role of nitrogen in biochemistry, where it plays a huge role in proteomics and nucleic acids, as well as in amino sugars, the role of which is very critical in immunology, the science at the forefront of the world's current crisis.

Before the development of the Haber-Bosch process, which is brilliantly discussed by one of my favorite thinkers, Vaclav Smil, in his wonderful "down to Earth" popular science book, Enriching the Earth, almost all of the fixed nitrogen on Earth was formed via the agency of a molybdenum/iron metalloenzyme, nitrogenase.

Here is the structure of the metal center of nitrogenase:



The caption:

Fig. 2. (A) Schematic representation of the FeMo-cofactor model. Y represents the bridging ligand with relatively light electron density. (B) Stereoview of the FeMo-cofactor and surrounding protein environment

Kim and Rees Science Vol. 257, Issue 5077, pp. 1677-1682 (1992)

I once had the privilege of attending one of Emily Carter's lectures in connection with the publication of this paper:

Prediction of a low-temperature N2 dissociation catalyst exploiting near-IR–to–visible light nanoplasmonics (Martirez and Carter, Science Advances (2017) Vol. 3, no. 12, eaao4710).

I asked her kind of rhetorically, "What is it about molybdenum, anyway?" and she laughed and made a sort of noncommittal remark about orbitals, a subject about which she knows more than I have ever known or ever will know.

I am, by the way, convinced that the best way to replace the dependence of the world on dangerous natural gas - and in some places even coal - for the production of ammonia, which is said to consume about 2% of the world energy supply, does not involve photochemical bond activation, or, for that matter, the electrochemistry under discussion in this paper on Samarium. I think the Haber-Bosch Process is acceptable if the heat energy required comes from process intensification of the thermal downgrade of thermochemical hydrogen production using nuclear energy as the primary energy source.

As I often remark, electricity is a thermodynamically degraded form of energy, and it is only acceptable to use it for chemical processes in the case where it is waste electricity.

Dealing with the environmental - in particular atmospheric - consequences of the Haber-Bosch process, on which the world's food supply depends, is another issue entirely.

From the introduction:

Ammonia (NH3), as a pivotal nitrogen building block and a carbon-free hydrogen fuel carrier, is widely applied in the agricultural, clinical, environmental, and biomedical fields, along with many other fields.(1) Electrochemical dinitrogen reduction via nitrogen reduction reaction (NRR) provides a promising route for green NH3 synthesis.(2) Nonetheless, developing efficient electrocatalysts to boost the NRR and impede the hydrogen evolution reaction (HER) is highly imperative. To this end, extensive efforts, both theoretical and experimental, have been dedicated to exploring effective NRR catalysts, involving precious metals,(3−5) nonprecious compounds,(6−20) and metal-free materials.(21−23)

Lanthanide rare-earth compounds have gained a noticeable popularity in various applications of batteries, sensors, catalysis, and supercapacitors,(24) owing to their unique electron configurations, high surface chemical activity, and robust structure. Recent studies have identified CeO2,(25−27) DyF,(28) and La2O3(29) as promising rare-earth NRR catalysts. As a typical lanthanide oxide, Sm2O3 has recently attracted considerable interest in photocatalysis and electrocatalysis. For instance, Sm2O3 could act as an effective catalyst for the oxidative coupling of methane with high activity, selectivity, and durability.(30) Wang et al. prove that Sm2O3 can catalyze oxygen reduction actively and selectively and with high stability.(31)
Here, we first demonstrate Sm2O3 to be an effective and stable NRR electrocatalyst. Theoretical computations uncover that Sm2O3 can facilitate the NRR and hinder the HER. On the basis of the theoretical results, we synthesized Sm2O3 nanoparticles (NPs) which delivered an appealing NRR performance as well as robust stability.

The authors here did similar computational work to that reported by Carter, but went a step further into the experimental realm:

Density functional theory (DFT) computations are first performed to authenticate the NRR feasibility of Sm2O3. The dominant (222) facet is considered for building the Sm2O3 model. As shown in Figure 1a, the Sm2O3 (222) comprises abundant surface-exposed Sm atoms with a positive charge of +1.01 |e|, which provides the catalytic opportunity for polarizing and activating the negatively charged N2 molecules.(32) Initially, upon N2 adsorption (Figure 1b), the *N2 prefers an end-on adsorption configuration and gains −0.02 |e| from an active Sm atom, resulting in an N–N bond elongation of 1.12 Å. Remarkably, for *N2 → *NNH (Figure 1c), two N atoms in *NNH gain a large amount of charge (−0.57 |e|), and the N–N bond is dramatically elongated to 1.215 Å, indicating that Sm atoms enable the effective N2 protonation to catalyze the NRR.

Some pictures from the text:



The caption:

Figure 1. (a) Schematic of NRR on Sm2O3 (222). (b, c) Optimized models of (b) *N2 and (c) *NNH on active Sm atom: N1 and N2 represent the distal-N and nearest-N, respectively. (d) PDOS of *NNH on Sm atom. (e) Free energy profiles of reaction pathway on Sm2O3 (222) at zero and applied energy of −0.92 V (inset: free energies of various species on Sm2O3 (222).

The caption:

Figure 2. Characterizations of Sm2O3 NPs: (a) XRD, (b) unit cell of Sm2O3, (c) TEM, (d) HRTEM, (e) SAED, (f) lattice measurement, (g) atomic configuration of Sm2O3 (222) facet (side view), (h) XPS Sm 3d spectra, and (i) O 1s spectra.

The caption:

Figure 3. (a) LSV curves of Sm2O3 NPs. (b) Time-dependent current density curves of Sm2O3 NPs for 2 h at various potentials and (c) corresponding UV–vis absorption data of resultant electrolytes and (d) obtained NH3 yields and FEs. (e) UV–vis spectra of the electrolytes after 2 h of electrolysis on Sm2O3 NPs at various potentials. (f) NH3 yields of Sm2O3 NPs, Sm2O3/RGO, and bare RGO at −0.6 V.

The caption:

Figure 4. (a) UV–vis absorption spectra of the electrolytes after 2 h of electrolysis over Sm2O3 NPs at −0.6 V at different conditions. (b) 1H NMR measurement. (c) Cycling test. (d) Chronoamperometry test for 20 h.

From the succinct conclusion of the paper:

In summary, the combined computational and experimental results validate Sm2O3 to be a high-performance rare-earth electrocatalyst for the NRR. Theoretical calculations unveil that the active Sm centers could favorably boost the NRR and impede the HER. The prepared Sm2O3 NPs show an appealing NRR activity along with high stability. This work may provide a new pathway for the rational design of rare-earth catalysts for electroreduction of N2 to NH3.

Again, I basically am less than supportive, despite the fine science demonstrated in this paper, to electrochemical reduction of ammonia, except in the case where there is waste electricity.

However, it is environmentally and economically wise to avoid waste electricity, and the idea of storing it in batteries is about as environmentally odious as one can get in my opinion.

Continuous processes are always desirable, as opposed to batch or interrupted processes, in an environmental and economic senses - because these senses depend on thermodynamics.

This fact - facts matter - is why so called "renewable energy" will continue to fail to address environmental issues while simultaneously failing to address the ethical issues associated with human development goals.

I trust you will have a pleasant and safe weekend.