Review on catalytic roles of rare earth elements in ammonia synthesis: Development and perspective

Boosting Electrochemical Nitrogen Fixation via Regulating Surface Electronic Structure by CeO2 Hybridization

Electrocatalytic nitrogen reduction reaction (NRR) paves a sustainable way to produce NH3 but suffering from the relatively low NH3 yield and poor selectivity. High-performance NRR catalysts and a deep insight into the structure-performance relationship are higher desired. Herein, a molten-salt approach is developed to synthesize tiny CeO2 nanoparticles anchored by ultra-thin MoN nanosheets as advanced catalysts for NRR. Specifically, a considerably high NH3 yield rate of 27.5 µg h-1 mg-1 with 17.2% Faradaic efficiency (FE) can be achieved at -0.3 V vs (RHE) under ambient conditions. Experimental and density functional theory (DFT) calculations further point out that the incorporation of MoN with CeO2 can promotes the enlargement of the electron deficient area of nitrogen vacancy site. The enlarged electron deficient area contributes to the accommodation of lone pair electrons of N2, which dramatically improves the N2 adsorption/activation and the key intermediates (*NNH and *NH3) generation, thus boosting the NRR performance.

Adsorption of rare earth elements on a magnetic geopolymer derived from rice husk: studies in batch, column, and application in real phosphogypsum leachate sample

There is a growing need to develop new strategies for rare earth element (REE) recovery from secondary resources. Herein, a novel approach to utilize biogenic silica (from rice husk) and metakaolin was employed to fabricate magnetic geopolymer (MGP) by incorporating metallic iron. The fabricated MGP adsorbent material was used to uptake Ce3+, La3+, and Nd3+ from synthetic solutions and real phosphogypsum leachate in batch and column modes. The MGP offers a negatively charged surface at pH above 2.7, and the uptake of REEs rises from pH 3 to 6. The kinetic study validated that the kinetics was much faster for Nd3+, followed by La3+ and Ce3+. A thermodynamic investigation validated the exothermic nature of the adsorption process for all selected REEs. The desorption experiment using 2 mol L-1 H2SO4 as the eluent demonstrated approximately 100% desorption of REEs from the adsorbent. After six adsorption-desorption cycles, the MGP maintained a high adsorption performance up to cycle five before suffering a significant decrease in performance in cycle six. The effectiveness of MGP was also assessed for its applicability in recovering numerous REEs (La3+, Ce3+, Pr3+, Sm3+, and Nd3+) from real leachate from phosphogypsum wastes, and the highest recovery was achieved for Nd3+ (95.03%) followed by Ce3+ (86.33%). The operation was also feasible in the column presenting suitable values of the length of the mass transfer zone. The findings of this investigation indicate that MGP adsorbent prepared via a simple route has the potential for the recovery of REEs from synthetic and real samples in both batch and continuous operations modes.

Analysis of enrichment, correlation, and leaching patterns of rare earth elements in coal fly ash assisted by statistical measures

Coal fly ash (CFA) is a typical industrial solid waste, which has recently been reported to contain rare earth elements (REEs). REEs are important materials in many industrial fields. Therefore, extracting REEs from CFA becomes a win-win strategy to both make full use of CFA and reclaim REEs. However, the stable crystalline structure of CFA is hard to break, which limits the extraction of REEs. The inter-correlation and the leaching patterns of the REEs in CFA also remain unclear. In this work, REEs were enriched by desilication, and the correlation and the influences of multiple acids of the leached REEs were investigated. It was found that desilication could increase the leachable amount of REEs from 137.37 ppm to 346.12 ppm. The light rare earth elements (LREEs) were less inter-correlated than heavy rare earth elements (HREEs) and desilication enhanced the leaching of LREEs more than that of HREEs. The ratio and type of the leaching acids both influenced the extraction of REEs from CFA: HCl and HF played important roles in the extraction from the untreated CFA while HNO3 and HF were more decisive for the desilicated CFA. In addition, we used statistical analysis to quantificationally confirm that desilication and acids both significantly influenced the extraction of REEs. This work provides evidence for the enrichment of REEs in CFA and acid choosing when leaching REEs from CFA.

A Conceptual Approach for the Design of New Catalysts for Ammonia Synthesis: A Metal-Support Interactions Review

The growing interest in green ammonia production has spurred the development of new catalysts with the potential to carry out the Haber-Bosch process under mild pressure and temperature conditions. While there is a wide experimental background on new catalysts involving transition metals, supports and additives, the fundamentals behind ammonia synthesis performance on these catalysts remained partially unsolved. Here, we review the most important works developed to date and analyze the traditional catalysts for ammonia synthesis, as well as the influence of the electron transfer properties of the so-called 3rd-generation catalysts. Finally, the importance of metal-support interactions is highlighted as an effective pathway for the design of new materials with potential to carry out ammonia synthesis at low temperatures and pressures.

Dinitrogen Reduction Coupled with Methanol Oxidation for Low Overpotential Electrochemical NH3 Synthesis Over Cobalt Pyrophosphate as Bifunctional Catalyst

Electrochemical dinitrogen (N₂ ) reduction to ammonia (NH₃ ) coupled with methanol electro-oxidation is presented in the current work. Here, methanol oxidation reaction (MOR) is proposed as an alternative anode reaction to oxygen evolution reaction (OER) to accomplish electrons-induced reduction of N₂ to NH₃ at cathode and oxidation of methanol at anode in alkaline media thereby reducing the overall cell voltage for ammonia production. Cobalt pyrophosphate micro-flowers assembled by nanosheets are synthesized via a surfactant-assisted sonochemical approach. By virtue of structural and morphological advantages, the maximum Faradaic efficiency of 43.37% and NH₃ yield rate of 159.6 µg h^-1 mg_ca ^-1 is achieved at a potential of -0.2 V versus RHE. The proposed catalyst is shown to also exhibit a very high activity (100 mA mg^-1 at 1.48 V), durability (2 h) and production of value-added formic acid at anode (2.78 µmol h^-1 mg_cat ^-1 and F.E. of 59.2%). The overall NH₃ synthesis is achieved at a reduced cell voltage of 1.6 V (200 mV less than NRR-OER coupled NH₃ synthesis) when OER at anode is replaced with MOR and a high NH₃ yield rate of 95.2 µg h^-1 mg_cat ^-1 and HCOOH formation rate of 2.53 µmol h^-1 mg^-1 are witnessed under full-cell conditions.

Applications of rare earth oxides in catalytic ammonia synthesis and decomposition

Due to their unique structural and electronic properties, rare earth oxides have been widely applied as supports and promoters in catalytic ammonia synthesis and decomposition.