# Ru/RScSi intermetallics for ammonia synthesis in mild conditions

Fabien Can, Charlotte Croisé, Khaled Alabd, Sophie Tencé, Etienne Gaudin, Antoine Villesuzanne, Xavier Courtois, Nicolas Bion

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Fabien Can, Charlotte Croisé, Khaled Alabd, Sophie Tencé, Etienne Gaudin, et al. Ru/RScSi intermetallics for ammonia synthesis in mild conditions. 18th ICC - International Congress on Catalysis, IFP Énergie Nouvelle; Ircelyon; Foundation de la Maison de la Chimie; Société Chimique de France, Jul 2024, LYON, France. hal-04891294

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# Ru/RScSi intermetallics for ammonia synthesis in mild conditions

Fabien Can¹*, Charlotte Croisé¹, Khaled Alabd², Sophie Tencé², Etienne Gaudin², Antoine Villesuzanne², Xavier Courtois¹, Nicolas Bion¹

¹Institut de Chimie des Milieux et Matériaux de Poitiers (IC2MP), Université de Poitiers, CNRS 86073 POITIERS Cedex 9, FRANCE

²Institut de Chimie de la Matière Condensée de Bordeaux (ICMCB), Université de Bordeaux, CNRS, 33600 PESSAC, FRANCE

*fabien.can@univ-poiteirs.fr

## Introduction

For a century now, ammonia synthesis is carried out using the Haber-Bosch (HB) process ($N_2 + 3H_2 \rightleftharpoons 2NH_3$), which requires severe operating conditions (T=400-600 °C; P=150-200 bars). It is currently responsible for 1.6% of the world $CO_2$ emissions, mainly due to fossil hydrogen sources[1]. Ammonia, mainly used for soil fertilization, is one of the most synthesized chemical compounds in the world (180 Mt/year) with a high growth projection due to its potential as a decarbonized alternative energy carrier. To support ammonia synthesis from alternative non-continuous green hydrogen production, the $NH_3$ synthesis should be performed in milder conditions, according to an increase in the performance of HB catalysts. With this aim, new families of solids are developed and electride and/or hydride supported materials received particular attention. In this study, ruthenium supported intermetallic-electrides, whose electron-donating effect is expected to stimulate $N_2$ conversion, were investigated and compared to a Ru/MgO reference catalyst. More precisely, the influence of rare earth in the Ru/RScSi intermetallic catalyst (R=rare earth: La, Ce, Pr, Gd, Nd, Sm) was studied for ammonia synthesis under mild temperature and pressure conditions (T=300-400 °C; P=1-5 bars). Transient experiments were also carried out on promising formulations to highlight the determining steps in the reaction mechanism.

## Materials and Methods

Catalysts were prepared according to [2]. The ammonia synthesis activity measurement (300-400 °C; 1-5 bars) was carried out in a home-made pressure reactor ($H_2:N_2$ (3:1), 60 mL min⁻¹, 0.1 $g_{cata}$). The compositions of the feed gas and effluent stream were monitored continuously (1 Hz) using online MKS 2030 Multigas infrared analyzer with high $NH_3$ sensitivity (2-3 ppm). Transient experiments were studied with pulses of $N_2$ or $H_2$ in the secondary gas atmosphere ($H_2/N_2$) or with alternating $H_2-N_2$ flows.

## Results and Discussion

First, the influence of rare-earth (R) element in Ru/RScSi electride-like intermetallic catalysts for ammonia synthesis under mild conditions (300-450 °C; 1-5 bar) was investigated. At 400 °C, the catalytic performances in ammonia synthesis (Table 1) follow the La to Gd lanthanides series ($Ru_{1.7}/LaScSi = Ru_{1.7}/CeScSi = Ru_{1.7}/PrScSi > Ru_{1.7}/NdScSi > Ru_{1.5}/SmScSi > Ru_{1.3}/GdScSi$) and appear correlated with the formation of the hydride phase. Hydride phase formation and stabilization was studied by means of XRD and $H_2$-TPD experiments.

$N_2$ isotopic exchange (Table 1) evidenced that the RScSi supports generally help for the dissociation of the $N_2$ molecule, and transient flow experiments (Figure 1B) indicated the

possible N- species storage and that hydrogen in the formed $NH_3$ molecule comes mainly from the gas-phase $H_2$ and not primarily from the hydride phase.

<table><caption>Table 1. $N_2$ isotopic exchange behaviors and $NH_3$ production rate of Ru/RScSi catalysts.</caption><thead><tr><th></th><th>Ru<sub>1.7</sub>/<br>LaScSi</th><th>Ru<sub>1.7</sub>/<br>CeScSi</th><th>Ru<sub>1.7</sub>/<br>PrScSi</th><th>Ru<sub>1.7</sub>/<br>NdScSi</th><th>Ru<sub>1.5</sub>/<br>SmScSi</th><th>Ru<sub>1.3</sub>/<br>GdScSi</th><th>Ru<sub>2.1</sub>/<br>MgO</th></tr></thead><tbody><tr><td>Initial</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td></td><td>55.4</td><td>50.1</td><td>13.9</td><td>1.5</td><td>0.4</td><td>4.4</td><td>0.0</td></tr><tr><td colspan="8">NH<sub>3</sub> production rate @ 1 bar (mmol/h/g)</td></tr><tr><td>NH<sub>3</sub>/400°C</td><td>2.56</td><td>2.40</td><td>2.33</td><td>2.05</td><td>1.30</td><td>0.63</td><td>0.66</td></tr><tr><td>NH<sub>3</sub>/300°C</td><td>0.14</td><td>0.58</td><td>0.04</td><td>0.02</td><td>0.13</td><td>0.01</td><td>0.01</td></tr></tbody></table>

In addition, $Ru_{1.7}/CeScSi$ catalyst showed remarkable catalytic activity at low temperature (300 °C) with 0.58 mmol$NH_3$/h/g at 1 bar (Table 1). Transient experiments (Figure 1A) combined with $H_2$-RTP hydride stability measurements suggest that $NH_3$ formation at 300 °C is impacted by (i) reversible hydrogen storage-release properties and (ii) the suspected formation/desorption of $NH_x$-like species.

Furthermore, the substitution of Sc (transition metal) and Si (p-block element) elements on Ru supported cerium-based intermetallics (R, rare earth) highlighted that ammonia synthesis at 300 °C could respect either rather associative or dissociative mechanism depending on the catalyst formulation.

<figure><img src="image_2.png" /><figcaption>Figure 1. $Ru_{1.7}/CeScSi$ catalytic performances in (A): $N_2/H_2$ pulses in $H_2/N_2$ atmosphere at 300 °C; (B): Alternative $H_2-N_2$ flows at 450 °C.</figcaption></figure>

## Significance

This work is of major industrial significance because of the expected growth in ammonia production, while fundamental aspects are developed about ammonia synthesis mechanisms at low temperature.

## References

1. Institute for Industrial Productivity. Industrial Efficiency Technology Database: Ammonia. http://ietd.iipnetwork.org/content/ammonia

2. C. Croisé et al., ChemCatChem, **2023**, 15, e202201172