Electrocatalytic N2 Reduction Research Articles

Hydrophobic Microenvironment Modulation of Ru Nanoparticles in Metal-Organic Frameworks for Enhanced Electrocatalytic N2 Reduction.

The modulation of the chemical microenvironment surrounding metal nanoparticles (NPs) is an effective means to enhance the selectivity and activity of catalytic reactions. Herein, a post-synthetic modification strategy is developed to modulate the hydrophobic microenvironment of Ru nanoparticles encapsulated in a metal-organic framework (MOF), MIP-206, namely Ru@MIP-Fx (where x represents perfluoroalkyl chain lengths of 3, 5, 7, 11, and 15), in order to systematically explore the effect of the hydrophobic microenvironment on the electrocatalytic activity. The increase of perfluoroalkyl chain length can gradually enhance the hydrophobicity of the catalyst, which effectively suppresses the competitive hydrogen evolution reaction (HER). Moreover, the electrocatalytic production rate of ammonia and the corresponding Faraday efficiency display a volcano-like pattern with increasing hydrophobicity, with Ru@MIP-F7 showing the highest activity. Theoretical calculations and experiments jointly show that modification of perfluoroalkyl chains of different lengths on MIP-206 modulates the electronic state of Ru nanoparticles and reduces the rate-determining step for the formation of the key intermediate of N2H2 *, leading to superior electrocatalytic performance.

Screening for electrocatalytic reduction of N2 by borophene supported bimetallic atoms

Ammonia (NH3), as a high energy density chemical and hydrogen energy storage medium, has garnered significant interest in the realms of energy storage and catalysis. In contrast to the traditional Haber-Bosch process, the electrocatalysis nitrogen reduction reaction (eNRR) can synthesize green ammonia under mild reaction conditions. Based on density functional theory (DFT), the potential of 2D borophene supported transition metals as bimetallic atomic catalysts (BACs) in eNRR was investigated. Progressive screening through energy-oriented screening mechanism, Sc2B12, Ti2B12, V2B12 and Zr2B12 were identified as potential eNRR catalysts. Their complete reaction pathways and Gibbs free energy (ΔG) were further explored, and corresponding overpotentials were obtained. The results showed that overpotentials of the potential determining steps (PDS) of these catalysts were 0.34, 0.40, 0.39 and 0.13 V, respectively, demonstrating excellent eNRR catalytic activity and good selectivity. Cu2B12 shows remarkable hydrogen evolution reaction (HER) catalytic activity with an overpotential (η) of only 0.05 V. The study also revealed that the catalytic activity of TM2B12 is derived from its excellent conductivity, and the charge transmission enhances the catalyst activity of eNRR. Consequently, this study provides significant theoretical guidance and support for the synthesis of novel eNRR catalysts, and promotes further advancement in the field of catalysis.

B-doped MoS2/MoO2 heterostructure catalyst for the electrocatalytic reduction of N2 to NH3

B-doped MoS2/MoO2 heterostructure catalyst for the electrocatalytic reduction of N2 to NH3

Tuning oxygen vacancies on Bi2MoO6 surface for efficient electrocatalytic N2 reduction reaction

Electrocatalytic nitrogen reduction reaction (NRR) is a green and environmentally friendly ammonia production method, which is expected to replace the conventional Haber-Bosch process. In this paper, a Bi2MoO6 electrocatalyst with abundant oxygen vacancies was obtained by solvothermal method and sodium hydroxide etching treatment. The introduction of oxygen vacancies increased the active site of the catalyst and promoted the charge transfer and N2 adsorption, which finally significantly increased the activity of the catalyst. In 0.1 M Na2SO4 solution at a potential of -0.2 V (vs. RHE), the Bi2MoO6 with oxygen vacancies obtained an excellent Faradaic efficiency of 16.51% and an NH3 yield of 13.18 μg h−1 mg−1, which is about 4 times that of bare Bi2MoO6. This study provides a new idea to improve the activity of the catalyst for NRR.

Boosting Electrocatalytic N2 Reduction to NH3 by Enhancing N2 Activation via Interaction between Au Nanoparticles and MIL-101(Fe) in Neutral Electrolytes.

Electrocatalytic nitrogen reduction reaction (NRR) has attracted much attention as a sustainable ammonia production technology, but it needs further exploration due to its slow kinetics and the existence of competitive side reactions. In this research, xAu/MIL-101(Fe) catalysts were obtained by loading gold nanoparticles (Au NPs) onto MIL-101(Fe) using a one-step reduction strategy. Herein, MIL-101(Fe), with high specific surface area and strong N2 adsorption capacity, is used as a support to disperse Au NPs to increase the electrochemical active surface area. Au NPs, with a high NRR activity, is introduced as the active site to promote charge transfer and intermediate formation rates. More importantly, the strong interaction between Au NPs and MIL-101(Fe) enhances the electron transfer between Au NPs and MIL-101(Fe), thereby enhancing the activation of N2 and achieving efficient NRR. Among the prepared catalysts, 15 %Au/MIL-101(Fe) has the highest NH3 yield of 46.37 μg h-1 mg-1 cat and a Faraday efficiency of 39.38 % at -0.4 V (vs. RHE). In-situ FTIR reveals that the NRR mechanism of 15 %Au/MIL-101(Fe) follows the binding alternating pathway and also indicates that the interaction between Au NPs and MIL-101(Fe) strengthens the activation of the N≡N bond in the rate-limiting process, thereby accelerating the NRR process.

Tuning Surface Potential Polarization to Enhance N2 Affinity for Ammonia Electrosynthesis.

Electrocatalytic N2 reduction reaction (NRR) to synthesize ammonia is a sustainable reaction that is expected to replace Haber Bosch process. Laminated Bi2WO6 has great potential as an NRR electrocatalyst, however, the effective activity requires that the inert substrate is fully activated. Here, for the first time, success is achieved in activating the Bi2WO6 basal planes with NRR activity through Ti doping. The introduction of Ti successfully tunes the surface potential distribution and enhances the N2 adsorption. The subsequently strong hybrid coupling of d(Ti)-p(N) orbitals fills the electronic state of N2 antibonding molecular orbital, which greatly weakens the bonding strength of N≡N bonds. Further, in situ synchrotron radiation-based Fourier transform infrared(SR-FTIR) spectrum and theoretical calculations show that surface potential polarization enhances the adsorption of HNN* by Bi-Ti dual-metal sites, which is beneficial for the subsequent activation hydrogenation process. The Ti-Bi2WO6 nanosheets achieve 11.44% Faradaic efficiency (-0.2V vs. RHE), a NH3 yield rate of 23.14µgmg-1h-1 (15N calibration), and satisfactory stability in 0.1M HCl environment. The mutual assistance of theory and experiment can help understand and develop of excellent two-dimensional (2D) materials for the NRR.

Advances in zeolitic-imidazolate-framework-based catalysts for photo-/electrocatalytic water splitting, CO2 reduction and N2 reduction applications.

Harnessing electrical or solar energy for the renewable production of value-added fuels and chemicals through catalytic processes (such as photocatalysis and electrocatalysis) is promising to achieve the goal of carbon neutrality. Owing to the large number of highly accessible active sites, highly porous structure, and charge separation/transfer ability, as well as excellent stability against chemical and electrochemical corrosion, zeolite imidazolate framework (ZIF)-based catalysts have attracted significant attention. Strategic construction of heterojunctions, and alteration of the metal node and the organic ligand of the ZIFs effectively regulate the binding energy of intermediates and the reaction energy barriers that allow tunable catalytic activity and selectivity of a product during reaction. Focusing on the currently existing critical issues of insufficient kinetics for electron transport and selective generation of ideal products, this review starts from the characteristics and physiochemical advantages of ZIFs in catalytic applications, then introduces promising regulatory approaches for advancing the kinetic process in emerging CO2 reduction, water splitting and N2 reduction applications, before proposing perspective modification directions.

Mechanistic insights into the electrolyte effects on the electrochemical nitrogen reduction reaction using copper hexacyanoferrate/f-MWCNT nano-composites.

Developing an efficient, selective, and stable electrocatalysis system for the electrocatalytic N2 reduction reaction (ENRR) is a promising strategy for the green and sustainable production of ammonia. The activity, selectivity, and stability of various electrocatalysts in different electrolyte solvents, mainly acidic and alkaline electrolytes, are commonly compared in the literature. However, a mechanistic insight into the effect of these electrolytes on ENRR activity is lacking. Herein we demonstrate that the acidity or alkalinity of the electrolyte is a key factor in determining the rate-limiting step and, by extension, the ENRR performance of an electrochemical setup for the electroproduction of ammonia. Our results from ex situ X-ray photoelectron, Raman, and FTIR spectroscopy analysis of the fresh and spent Cu-hexacyanoferrate Prussian blue analogue-decorated functionalized carbon nanotube (CuFe PBA/f-CNT) catalyst reveal that NH4+-species are more strongly adsorbed on the catalyst surface during the ENRR in acidic than in alkaline electrolytes. The results of our detailed rotating ring-disc electrode voltammetry studies suggest that the ENRR over CuFe PBA/f-CNT is mostly controlled by surface adsorption in an acidic electrolyte and by mass transport in an alkaline electrolyte. In situ Raman spectroscopy confirms this finding and shows that the leaching of Fe(CN)6 species from the CuFe PBA/f-CNT composite in an alkaline electrolyte greatly affects the ENRR performance. We believe that the work presented herein offers a new insight into the mechanistic aspects of the ENRR in different electrolyte systems and hence can prove very valuable for the development of effective ENRR electrode/electrolyte systems for practical applications.

FeP-Fe3O4 nanospheres for electrocatalytic N2 reduction to NH3 under ambient conditions.

The electrocatalytic nitrogen reduction reaction (eNRR) under ambient conditions is deemed a promising alternative for NH3 synthesis. In this paper, an FeP-Fe3O4 nanocomposite electrocatalyst was prepared by phosphating annealing using Fe2O3 as a precursor, and the resulting FeP-Fe3O4 exhibited excellent N2-to-NH3-producing activity over a wide potential window. The highest faradaic efficiency of FeP-Fe3O4 is 11.02% at -0.1 V vs. reversible hydrogen electrode (RHE), and the maximum NH3 yield reaches 12.73 μg h-1 mgcat-1, comparable to or exceeding the reported values in this field. Furthermore, the FeP-Fe3O4 nanocomposite electrocatalyst presents high electrochemical stability, selectivity, and durability.

Recent Developments of Dual Single-Atom Catalysts for Nitrogen Reduction Reaction.

Ammonia is vital for fertilizer production, hydrogen storage, and alternative fuels. The conventional Haber-Bosch process for ammonia production is energy-intensive and environmentally harmful. Designing environmentally friendly and low-energy consumption strategies for electrocatalytic N2 reduction reaction (ENRR) in mild conditions is meaningful. Single-atom catalysts (SACs) have been studied extensively for NRR due to their high atomic utilization and unique electronic structure but are limited by their poor faradic efficiency and low ammonia formation yield. Dual single-atom catalysts (DSACs) have recently emerged as a promising solution for the effective activation of molecular N2 , providing diverse active sites and synergistic interactions between adjacent atoms. In this review, we summarize the latest advances in metal DSACs for electrochemical ENRR based on both theoretical calculations and experimental studies, including aspects such as their variety, coordination, support, N2 adsorption and activity mechanisms, the characterization of NRR and electrochemical cell Configuration. We also address challenges and prospects in this rapidly evolving field, providing a comprehensive overview of DSACs for ENRR.

Two-Dimensional Metal–Organic Framework TM Catalysts for Electrocatalytic N2 and CO2 Reduction: A Density Functional Theory Investigation

In this study, we screened novel two-dimensional metal–organic framework (MOF) materials, which can be used as efficient electrocatalysts in the N2 reduction reaction (NRR) and CO2 reduction reaction (CO2RR) through density functional theory (DFT) calculations. By systematically investigating the adsorption behaviors of N2 and CO2 in different MOF-TMs (TM = Fe, Co, Ni, Cu, Zn) and their electrocatalytic hydrogenation processes, we found that 2D MOF-Fe, MOF-Co, and MOF-Ni can be used as catalysts for electrocatalytic NRR. The free energy increase in the corresponding potential-limiting step is calculated to be 0.84 eV on MOF-Fe, 1.00 eV on MOF-Co, and 1.17 eV on MOF-Ni, all of which are less than or at least comparable to those reported values for the NRR. Moreover, only 2D MOF-Fe was identified as a suitable electrocatalyst for CO2RR. Instead of other hydrocarbons, the product CH3OH is selectively obtained in an electrocatalytic CO2 reduction reaction on a 2D MOF-Fe with a free energy increase of 0.84 eV in the potential-limiting step. Overall, the results of this study not only facilitate the potential application of 2D MOF-TMs as electrocatalysts but also provide new guidelines for rationally designing novel electrocatalysts for the NRR and CO2RR.

Impurity tuning for enhanced electrocatalytic reduction of N2 to NH3 over Fe/Co Co-doped MoS2 nanosheets

The electrochemical N2 reduction reaction (NRR) has attracted much attention for its use in the production of fertilizers and other chemicals. Herein, we demonstrate that the NRR can be enhanced when using Fe/Co co-doped MoS2 (FeCo–MoS2) nanosheets as the electrocatalyst. At the optimal doping concentration, the FeCo–MoS2 nanosheets provided a NH3 yield rate of 3.27 μg/h/cm2 at −0.55 V [vs reversible hydrogen electrode (RHE)], a value superior to those of undoped MoS2, Fe-doped MoS2, and Co-doped MoS2 samples by 191, 176, and 116%, respectively. Moreover, the Faradaic efficiency of 6.28% at −0.35 V (vs RHE) exceeded those of the undoped MoS2, Fe-doped MoS2, and Co-doped MoS2 samples by 217, 186, and 132%, respectively. Density functional theory calculations revealed that the preferred reaction mediated by the FeCo–MoS2 electrocatalyst followed a distal pathway, with its potential-determining step having an energy barrier (0.54 eV) lower than that of undoped MoS2 (1.13 eV), suggesting that the FeCo–MoS2 nanosheets possessed superior electrocatalytic activity. We conclude that Fe and Co are efficient dopants for boosting the NRR activity of MoS2 catalysts.

Transition metal doped and strain engineered VS−MoS2 as electrocatalysts for N2 electroreduction: A DFT study

S vacancy activated MoS2 (VS−MoS2) materials are greatly attractive as electrocatalysts in electrocatalytic N2 reduction reaction (NRR). Herein, we investigate the electrocatalytic NRR performance of transition metal (Cr, Mn, V, W) doped and strain engineered VS−MoS2 using first-principles calculations. For all the designed MoS2s, VS is easy to form due to the small defect formation energy (≤3.48 eV) and determined as active site for N2 adsorption and electrocatalytic NRR. Calculation results show that doping and strain regulate the electrocatalytic activity of VS−MoS2 by altering its electronic properties. When strain is absent, Cr-, Mn-, and V-VS−MoS2 exhibit better electrocatalytic NRR activity than undoped VS−MoS2, while W-VS−MoS2 shows slightly worse activity. However, the above situation changes with applying strain. Moreover, the catalytic activity of all the catalysts firstly decreases and then increases with increasing strain. Further comprehensive considering limiting potential (UL) and Faraday efficiency (FE), Cr-VS−MoS2 under 7 % strain is identified as the best NRR electrocatalyst with both excellent activity (UL = -0.18 V) and high selectivity (FE = 99.44 %). In addition, 5 % and 6 % strained Mn-VS−MoS2 are also good electrocatalysts for NRR with low UL and (-0.45 V and -0.23 V, respectively) and substantial FE (37.69 % and 19.29 %, respectively). Therefore, the electrocatalytic NRR performance of VS−MoS2 could be optimized through transition metal doping and strain engineering. Relevant experimental explores will be encouraged based on our theoretical findings.

Boosting protonation kinetics for ammonia electrosynthesis on Ru sites embedded in nanoporous ReSe2

Electrocatalytic N2 reduction reaction (NRR) is a promising approach for renewable NH3 production but is constrained by poor activity and selectivity, owing to the sluggish hydrogenation kinetics. Herein, we prepare a nanoporous Ru-doped ReSe2 (NP Ru-ReSe2) with controllable selenium vacancies by regulating the amount of Ru dopants. The catalyst exhibits excellent the NRR performance, which affords a high Faradaic efficiency of 22.7% and NH3 yield rate of 19.41 μg h−1 mg−1 at –0.4 V versus a reversible hydrogen electrode(RHE). In situ attenuated total reflectance–surface-enhanced infrared absorption spectroscopy studies combined with theoretical calculations unravel that a clear synergistic effect of Ru dopants and selenium vacancies onregulating the proton (H*) coverage on the ReSe2 surface to accelerate the proton-coupled processes as NN → NNH. while N2 dissociation becomes exothermic over the Ru active sites via Ru-to-N2 π back-donation, lowering energy barriers of the rate-determining step, ultimately enhancing the overall NRR catalytic performance.

Understanding the Mechanism of Li-Mediated Nitrogen Reduction Reaction

Green ammonia production is a key environmental goal. As a crucial step towards this goal, a direct electrocatalytic N2reduction (eNRR) by protons and electrons emerges as a highly desirable alternative. However, all pragmatic attempts to develop such an electrochemical route have so far been hindered by invariably low selectivities and large overpotentials. The most reliable method presently is the lithium-mediated eNRR (LiNR) in non-aqueous electrolytes.1 Despite significant progress made to achieve high FE and current density2, 3,the mechanism of LiNR is yet not fully understood due to so many intricate processes involved ranging from the atomic scale to the macroscopic scale. Moreover, most Li-NRR studies have the limitations of using a sacrificial solvent as proton donors and difficulties in scaling up production in batch reactors. The talk will, address the long-term stability and high activity of PtAu in the HOR process by suppressing the unwanted THF oxidation and improving the tolerance towards CO-poison. Moreover, we investigate the mechanism of the LiNR from the ab initio atomistic reaction process, mean-field microkinetics, to the mass-transport simulations, explicitly achieving an improved understanding the dynamic change of the solid-electrolyte interphase in different LiNR reaction conditions and the effect on the performance.Reference:1 J. Choi et al, Nat. Commun. 11, 1–10 (2020)2 K. Li, et al, Science 374, 1593-1597 (2021)3 S. Li, et al, Joule, 6, 2083-2101 (2022)

Oxygen vacancy modulation in interfacial engineering Fe3O4 over carbon nanofiber boosting ambient electrocatalytic N2 reduction

The oxygen vacancy modulation of interface-engineered Fe3O4 nanograins over carbon nanofiber (Fe@CNF) was achieved to improve electrocatalytic nitrogen reduction reaction (NRR) activity and stability via facile electrospinning and tuning thermal procedure. The optimal catalyst calcined at 800 ℃ (Fe@CNF-800) was endowed with abundant nanograin boundaries and optimized oxygen vacancy (Vo) concentration of iron oxides, thereby affording 37.1 μg h−1 mgcat.-1 (−0.2 V vs. reversible hydrogen electrode (RHE)) NH3 yield and rational Faraday efficiency (10.2%), with 13.6 times atomic activity enhancement compared to of that commercial Fe3O4. The interfacial effect of assembled nanograins in particles correlated with the formation of Vo and more intrinsic active sites, which is conducive to the trapping and activation of nitrogen (N2). The in-situ X-ray photoelectron spectroscopy (XPS) measurement revealed the real consumption of adsorbed oxygen when introducing N2 by the trapping effect of Vo. Density-Functional-Theory (DFT) calculation validates the promotive hydrogenation effect and elimination of hydrogen intermediate (H*) interacted with N2 transferring toward oxygen of the support. The optimal catalyst shows a lasting NRR activity at least 90 h, outperforming most reported Fe-based NRR catalysts.

Insights of active sites separation mechanism for highly efficient electrocatalytic N2 reduction to ammonia over glucose-induced metallic MoS2

Substituting the Haber-Bosch process with electrocatalytic N2 reduction reaction (NRR) is expected to be realized by optimizing the active sites to boost the sluggish N2 adsorption and the hydrogenation process without interference by the thermodynamically favored hydrogen evolution reaction (HER). Herein, a phase engineering strategy for MoS2 toward NRR is reported. The 1T phase are found to enhance the NRR activity with strong N2 adsorption. Then, the active site separation can weaken the competitive adsorption between H+ and N2 reactant on the Mo-edge, improving the NRR selectivity. More importantly, the HER selective basal plane can serve as an H-provider to further accelerate the hydrogenation process of NRR. The optimal MoS2/C-0.4 possesses a high ammonia yield rate of 80.38 μg h–1 mg–1cat. and Faradaic efficiency (FE) of 23.76%, outperforming almost all MoS2-based electrocatalysts and beyond. This work sheds light on synchronizing the catalytic activity and selectivity for catalytic reactions.

Operando analysis reveals potential-driven in situ formation of single-Fe-atom electrocatalysts for green production of ammonia

As a sustainable approach for N2 fixation, electrocatalytic N2 reduction reaction (N2RR) to produce ammonia (NH3) is highly desirable with a precise understanding to the structure-activity relationship of electrocatalysts. Here, firstly, we obtain a novel carbon-supported oxygen-coordinated single-Fe-atom catalyst for highly efficient production of ammonia from electrocatalytic N2RR. Based on such new type of N2RR electrocatalyst, by combining operando X-ray absorption spectra (XAS) with density function theory calculation, we reveal significantly that the as-prepared active coordination structure undergoes a potential-driven two-step restructuring, firstly from FeSAO4(OH)1a to FeSAO4(OH)1a'(OH)1b with the adsorption of another -OH on FeSA at open-circuit potential (OCP) of 0.58 VRHE, and subsequently restructuring from FeSAO4(OH)1a'(OH)1b to FeSAO3(OH)1a″ due to the breaking of one Fe-O bond and the dissociation of one -OH at working potentials for final electrocatalytic process of N2RR, thus revealing the first potential-induced insitu formation of the real electrocatalytic active sites to boost the conversion of N2 to NH3. Moreover, the key intermediate of Fe-NNHx was detected experimentally by both operando XAS and insitu attenuated total reflection-surface-enhanced infrared absorption spectra (ATR-SEIRAS), indicating the alternating mechanism followed by N2RR on such catalyst. The results indicate the necessity of considering the potential-induced restructuring of the active sites on all kinds of electrocatalysts for such as highly efficient ammonia production from N2RR. It also paves a new way for a precise understanding to the structure-activity relationship of a catalyst and helps the design of highly efficient catalysts.

Preparation of Cu atom doped LaFeO3/activated porous carbon composites and their electrocatalytic nitrogen reduction performance

The electrocatalytic N2 reduction reaction (NRR) under ambient conditions is a green and sustainable method for ammonia (NH3) synthesis, and the development of efficient electrocatalysts for NRR is a top priority. In recent years, LaFeO3 has been widely used in the field of catalysis because of its high stability, low cost, and green advantages. Through strategies such as heteroatom doping and carbon loading, we can effectively increase the content of oxygen vacancies and improve the electrical conductivity of the material to produce composites with unique electronic structures and excellent catalytic properties. In the present work, we prepared single-atom doped LaFeO3/activated porous carbon composites (LFC/AC) for electrocatalytic NRR. The NH3 yield and Faraday efficiency of LFC/AC were the highest at 23.876 μg h−1 mg−1 and 6.53% in 0.1 M Na2SO4 electrolyte solution, both of which were higher than those of LFC. A series of characterizations and tests have shown that LFC/AC has excellent stability, electrical conductivity, and electrocatalytic properties. The density flooding theory (DFT) simulations were performed to explore the main mechanisms to improve the NRR performance of the materials.

Electrocatalyst Microenvironment Engineering for Enhanced Product Selectivity in Carbon Dioxide and Nitrogen Reduction Reactions.

Carbon and nitrogen fixation strategies are regarded as alternative routes to produce valuable chemicals used as energy carriers and fertilizers that are traditionally obtained from unsustainable and energy-intensive coal gasification (CO and CH4), Fischer-Tropsch (C2H4), and Haber-Bosch (NH3) processes. Recently, the electrocatalytic CO2 reduction reaction (CO2RR) and N2 reduction reaction (NRR) have received tremendous attention, with the merits of being both efficient strategies to store renewable electricity while providing alternative preparation routes to fossil-fuel-driven reactions. To date, the development of the CO2RR and NRR processes is primarily hindered by the competitive hydrogen evolution reaction (HER); however, the corresponding strategies for inhibiting this undesired side reaction are still quite limited. Considering such complex reactions involve three gas-liquid-solid phases and successive proton-coupled electron transfers, it appears meaningful to review the current strategies for improving product selectivity in light of their respective reaction mechanisms, kinetics, and thermodynamics. By examining the developments and understanding in catalyst design, electrolyte engineering, and three-phase interface modulation, we discuss three key strategies for improving product selectivity for the CO2RR and NRR: (i) targeting molecularly defined active sites, (ii) increasing the local reactant concentration at the active sites, and (iii) stabilizing and confining product intermediates.