Supported Single-atom Catalysts Research Articles

Theoretical insight into electrochemical nitrate reduction on transition metal iron doped graphdiyne

AbstractProduction of ammonia (NH3) by electrocatalytic reduction of nitrate (NO3RR) not only eliminates harmful pollution, but also provides a way to reduce the energy consumption associated with predominated Haber‐Bosch process. However, realization of this process still faces many challenges because of the complexity of the reaction mechanism. Here we investigated the catalytic activity and selectivity of a series of graphdiyne supported single atom catalysts (SACs), namely TM/GDY, for the reduction of N to NH3 by first‐principles calculations. Among the 10 SACs studied, Fe/GDY was found to have good catalytic performance, consistent with the fact that the Fe‐doped GDY molecular layer was located near the top of the volcano plot, with a reaction limit potential of −0.44 V and showed excellent selectivity in inhibiting the competitive hydrogen evolution reaction (HER). The formation of the by‐products NO2, NO, N2O and N2 on Fe/GDY requires a considerable energy barrier, which ensures high selectivity. Furthermore, detailed electronic property analyses indicate that the GDY can work as an electron repository to effectively balance the charge transfers during the reaction process. This study not only offers an eligible NO3RR electrocatalyst but also provides an atomic understanding of the mechanisms of the NO3RR process behind.

Heterogeneous N-heterocyclic carbenes supported single-atom catalysts for nitrogen fixation: A combined density functional theory and machine learning study

Electrocatalytic nitrogen reduction reaction (NRR) has emerged as a sustainable and eco-friendly alternative for ammonia production at ambient conditions. Exploring highly efficient and selective electrocatalysts for NRR continues to gain significant attention, but remains a challenge. In this work, we conducted a series of single-atom catalysts (SACs) by embedding 29 kinds of transition metal (TM) atoms on the two-dimensional heterogeneous N-heterocyclic carbene, and systematically investigated their catalytic performance for NRR using density functional theory, high-throughput screening, and machine learning. Two promising candidates (TM = Mn and Ta) with high catalytic activity and selectivity were identified, with limiting potentials of - 0.51 and - 0.53 V, respectively. Moreover, considering solvation effects, the limiting potential for Mn was further reduced to -0.43 V. Machinelearning(ML)analysis revealedthat the adsorption energy of N2 emerged as an efficient descriptor for NRR activity, and transition metal atomic Mendeleev number (Nm), the molar volume of TM atoms (Vm) and the 1st ionization energy of TM atoms (Im) were intrinsic to the difference in NRR performance of these SACs. Our findings demonstrate a novel class of efficient SACs based on heterogeneous N-heterocyclic carbene, offering valuable insights for further design of NRR electrocatalysts with exceptional catalytic performance.

A First-Principles Study of Regulating Spin States of MoSi2N4 Supported Single-Atom Catalysts Via Doping Strategy for Enhancing Electrochemical Nitrogen Fixation Activity.

Regulating the spin states of catalysts to enhance activity is fascinating but challenging. Herein, by using first-principles calculations, single transition-metal (TM) atoms Mo, Re, and Os embedded in nitrogen vacancy of the MoSi2N4 monolayer (TM1/VN-MoSi2N4) were screened out as potential catalysts for electrochemical nitrogen reduction reaction to ammonia. Our findings suggest that the spin states of these active centers can be precisely and gradually tuned through a simple doping strategy. Additionally, doping one O atom into the Mo1/VN-MoSi2N4 system as an example significantly improves catalytic activity. The spin state of Mo1 transitions from high to intermediate while simultaneously breaking the C3v symmetry of the supported atom. These factors synergistically lead to better orbital overlap between the catalyst and intermediates, facilitating subsequent protonation processes and overall catalytic activity. This work provides novel insight into designing, precisely controlling, and revisiting the spin-related catalytic performance in heterogeneous catalysis.

CO Oxidation on Ir1/TiO2: Resolving Ligand Dynamics and Elementary Reaction Steps

Identifying the rate-controlling steps and the evolution of the ligand environment throughout the catalytic cycle on supported single-atom catalysts is crucial to bridge the gap between heterogeneous and homogeneous catalysis. Here we identified the rate-controlling elementary steps for CO oxidation on TiO2-supported Ir single atoms and isolated the corresponding intermediate Ir complexes. Kinetic measurements, operando spectroscopy, and quantum-chemical calculations indicate that the reaction mechanism has two kinetically relevant steps, CO adsorption/oxidation and O2 dissociation. By varying the reaction conditions, three Ir1 complexes (states) along the reaction cycle were isolated and identified using in situ spectroscopy. Furthermore, we show that all the intermediate Ir1 states share a common CO ligand that does not turn over. This study provides atomic level details on the active, intermediate complexes and reaction cycle of supported single-metal-atom catalysts, thereby offering future possibilities to control the ligand environment and reactivity.

Approaching Molecular Definition on Oxide-Supported Single-Atom Catalysts.

ConspectusSingle-atom catalysts (SACs) offer unique advantages such as high (noble) metal utilization through maximum possible dispersion, large metal-support contact areas, and oxidation states usually unattainable in classic nanoparticle catalysis. In addition, SACs can serve as models for determining active sites, a simultaneously desired as well as elusive target in the field of heterogeneous catalysis. Due to the complexity of heterogeneous catalysts bearing a variety of different sites on metal particles and the respective support as well as at their interface, studies of intrinsic activities and selectivities remain largely inconclusive. While SACs could close this gap, many supported SACs remain intrinsically ill-defined due to complexities arising from the variety of different adsorption sites for atomically dispersed metals, hampering the establishment of meaningful structure-activity correlations. In addition to overcoming this limitation, well-defined SACs could even be utilized to shed light on fundamental phenomena in catalysis that remain ambiguous when studies are obscured by the complexity of heterogeneous catalysts.In this Account, we describe approaches to break down the complexity of supported single-atom catalysts through the careful choice of oxide supports with specific binding motives as well as the adsorption of well-defined ligands such as ionic liquids on single metal sites. An example of molecularly defined oxide supports is polyoxometalates (POMs), which are metal oxo clusters with precisely known composition and structure. POMs exhibit a limited number of sites to anchor atomically dispersed metals such as Pt, Pd, and Rh. Polyoxometalate-supported single-atom catalysts (POM-SACs) thus represent ideal systems for the in situ spectroscopic study of single atom sites during reactions as, in principle, all sites are identical and thus equally active in catalytic reactions. We have utilized this benefit in studies of the mechanism of CO and alcohol oxidation reactions as well as the hydro(deoxy)genation of various biomass-derived compounds. More so, the redox properties of polyoxometalates can be finely tuned by changing the composition of the support while keeping the geometry of the single-atom active site largely constant. We further developed soluble analogues of heterogeneous POM-SACs, opening the door to advanced liquid-phase nuclear magnetic resonance (NMR) and UV-vis techniques but, in particular, to electrospray ionization mass spectrometry (ESI-MS) which proves powerful in determining catalytic intermediates as well as their gas-phase reactivity. Employing this technique, we were able to resolve some of the long-standing questions about hydrogen spillover, demonstrating the broad utility of studies on defined model catalysts.

High-throughput screening of B/N-doped graphene supported single-atom catalysts for nitrogen reduction reaction

Transitional metal single atom (TM1) doped graphene catalysts have been widely applied in electrochemical N2 reduction reaction (NRR). However, it remains a challenge for the rational design of highly active and selective electrocatalysts owing to limited knowledge of structure-activity correlations. Here, we adopted first-principle calculations to high-throughput screen the NRR performance of TM1 coordinated with two boron and two nitrogen atoms in graphene (TM1-B2N2/G). A “five-step” strategy was implemented by progressively considering different metrics such as stability, N2 adsorption, N2 activation, potential-determining step, and selectivity. As a result, a volcano plot of reactivity is established by using the valence electron number of TM1 as the descriptor. Among all catalysts, Cr1-B2N2/G exhibits superior performance with a limiting potential of -0.43 V with high selectivity of NRR interpreted by better spatial symmetry and excellent compatibility in terms of energy when N2 interacts with TM1. Our work reveals the general strategy of computational efforts to predict the next generation of advanced catalytic materials for NRR.

Computational study of the conversion of methane and carbon dioxide to acetic acid over NU-1000 metal–organic framework-supported single-atom metal catalysts

The conversion of methane (CH4) and carbon dioxide (CO2) to more valuable chemicals is attracting attention from an industrial and environmental perspective; however, this process is a great challenge in catalysis, because of the high stabilities of CH4 and CO2. Herein, we investigate the CH4 and CO2 conversion to acetic acid over NU-1000 metal–organic framework (MOF)-supported single-atom catalysts (SACs) (M-NU-1000, M = Mn, Fe, Co, Ni, and Cu) using density functional calculations. The reaction proceeds in three steps: first, a CH4 C–H bond dissociates to form a methyl intermediate. Then, an acetate intermediate is formed via C–C bond coupling between the methyl carbon atom and CO2. Finally, the acetate abstracts a proton to form the acetic acid product. We discover that Cu-NU-1000 exhibits the highest catalytic activity as compared to the other catalysts, based on its overall reaction barrier and rate constants. We also disclose two suitable descriptors to approximately predict the overall activation barrier of the reaction. One is the complexation energies difference between CO2 adsorbed on a methyl intermediate and CH4 adsorbed on the SACs used in this study. Another plausible descriptor is the difference between complexation energies of acetate intermediate and the CH4 adsorption. The supported NU-1000 MOFs are also found to accelerate the stability of all species formed along the reaction coordinates, especially during the transition states.

Theoretical prediction on stability, electronic and activity properties of single-atom catalysts anchored graphene and boron phosphide heterostructures

Introduction of graphene-like heterostructures supported single-atom catalysts (SACs) has been considered as a promising strategy for generating sustainable energy sources. By using first-principles calculation, different kinds of single transition metal (TM = Mn, Fe, Co and Ni) atoms anchored boron phosphide (BP-TM) and the effects of added graphene substrate on its electronic property and sensitivity toward the gas reactants are comparably investigated. The graphene support can provide the transferred electron to BP sheet and then regulate the adsorption strength and electronic structure of TM atoms anchored systems (BP-gra-TM). Besides, the adsorption of N2 molecule (parallel and end-on modes) on BP-gra-TM (including pristine and single vacancy (SV) within BP, i.e., BP-gra-SV-TM) are more stable than those on BP-TM and BP-SV-TM sheets, and then induce the change in electronic and magnetic properties of supported systems. Furthermore, the potential catalytic performances of four reactive substrates (BP-TM, BP-gra-TM, BP-SV-TM and BP-gra-SV-TM) are systematically explored for nitrogen reduction reaction (NRR). It is found that the NRR processes on BP-gra-Mn via enzymatic pathways and BP-gra-Fe via distal mechanism have lower limiting potential (0.52 V and 0.63 V) than other ones, indicating that the introduction of graphene can improve the surface activity of BP-TM sheets, which would provide useful guidance to design two-dimensional heterostructure-based SACs with high activity, efficiency and selectivity for the NRR.

Effect of ZrS2 load single/dual-atom catalysts on the hydrogen evolution reaction: A first-principles study.

The hydrogen evolution effect of ZrS2 carrier loaded with transition metal single-atom (SA) was explored by first-principles method. ZrS2 was constructed with transition metal single-atom and dual-atom. The structure-activity relationship of supported single-atom catalysts was described by electronic properties and hydrogen evolution kinetics. The results show that the ZrS2 carrier-loaded atomic-level catalysts are more likely to occur in acidic environments, where the Mo SA load has a higher hydrogen precipitation capacity than the Pt SA. In the case of dual-atom adsorption, most of the hydrogen reduction processes are higher than that of single atom loading, which indicates that the outer orbital hybridization is more likely to lead to the interfacial charge recombination of the catalyst. Thereinto, Ni/Pt @ZrS2 has the lowest Gibbs free energy (0.08 eV), and the synergistic effect of transition metals induces the deviation of the center of the d-band from the Fermi level and improves the dissociation ability of H ions. The design provides a new catalytic model for the HER and provides some ideas for understanding the two-site catalysis.

Unsymmetrically N, S-coordinated single-atom cobalt with electron redistribution for catalytic hydrogenation of quinolines

Engineering unsymmetrical coordination is an efficient strategy for improving the performance of carbon supported single-atom catalysts for diverse applications. However, the exploration of this strategy to prepare carbon supported single-atom catalysts for hydrogenation reaction is still in infancy, especially for hydrogenation of quinolines. Herein, we report a protein-metal-ion-network strategy for preparation of single-atom cobalt supported on hierarchically porous carbon (SA-Co/NSPC). SA-Co/NSPC features unsymmetrically N/S-coordinated metal center, i.e., Co1–N3S1. Moreover, the carbon matrices possess an ultra-thin two-dimensional morphology and hierarchically porous structures. SA-Co/NSPC catalyst with Co1–N3S1 active site exhibits excellent catalytic performance for hydrogenation of N-heterocycles. The DFT calculation results show that the site of Co1–N3S1 reveals a significant electron redistribution in comparison to the Co1–N4, leading to a lower H* diffusion barrier and quinoline hydrogenation reaction barrier. SA-Co/NSPC catalyst combined with the unique hierarchically porous structure and high active Co1–N3S1 owns extraordinary catalytic performance for hydrogenation of quinolines.

Sp-Hybridized Nitrogen as New Anchoring Sites of Iron Single Atoms to Boost the Oxygen Reduction Reaction.

Carbon supported single-atom catalysts with metal-Nx configuration are considered as one of the most efficient catalysts for the oxygen reduction reaction (ORR). However, most of the metal-Nx active sites are composed by pyridinic N at the defect locations of graphene-like supports. Here, we employ graphdiyne (GDY) as a new carbon substrate to synthesize an iron (Fe) single atom catalyst (Fe-N-GDY), showing excellent catalytic performance. Benefitting from the abundant acetylenic bonds in GDY, sp-N anchored metal atoms are created without forming defects. The sp-N and OH ligands regulate the electronic structure of Fe atoms and optimize the adsorption energy of ORR intermediates on the active sites by reducing the electron local density of Fe atoms, which accelerates the reaction kinetics and promotes the ORR activity of Fe-N-GDY. Furthermore, the practical application of Fe-N-GDY is corroborated by its high power density and long-term performance via assembling a zinc-air battery.

Catalytic applications of single-atom metal-anchored hydroxides: Recent advances and perspective

Developing isolated single atomic noble metal catalysts is one of the most effective methods to maximize noble metal atom utilization efficiency and enhance catalytic performances. Layered double hydroxides (LDHs) are two-dimensional nanoarchitectures in which M3+ and M2+ sites are atomically isolated due to static repulsions, providing special anchoring sites for single noble metal atoms and enabling the tuning of catalytic activity. Herein, a comprehensive review of the advances in LDHs supported single-atom catalysts (M/LDH SACs) is presented, focusing on the synthetic strategies, structure characterization, and application of M/LDH SACs in energy devices. Strong electronic coupling between single atomic noble metal atoms and corresponding anchoring sites of LDHs determines not only the catalytic activity of M/LDH SACs but also the stability during catalytic reactions. Furthermore, a perspective is proposed to highlight the challenges and opportunities for understanding the reaction mechanism and development of highly efficient M/LDH SACs.

Role of Polarons in Single-Atom Catalysts: Case Study of Me1 [Au1, Pt1, and Rh1] on TiO2(110)

The local environment of metal-oxide supported single-atom catalysts plays a decisive role in the surface reactivity and related catalytic properties. The study of such systems is complicated by the presence of point defects on the surface, which are often associated with the localization of excess charge in the form of polarons. This can affect the stability, the electronic configuration, and the local geometry of the adsorbed adatoms. In this work, through the use of density functional theory and surface-sensitive experiments, we study the adsorption of Rh1, Pt1, and Au1 metals on the reduced TiO2(110) surface, a prototypical polaronic material. A systematic analysis of the adsorption configurations and oxidation states of the adsorbed metals reveals different types of couplings between adsorbates and polarons. As confirmed by scanning tunneling microscopy measurements, the favored Pt1 and Au1 adsorption at oxygen vacancy sites is associated with a strong electronic charge transfer from polaronic states to adatom orbitals, which results in a reduction of the adsorbed metal. In contrast, the Rh1 adatoms interact weakly with the excess charge, which leaves the polarons largely unaffected. Our results show that an accurate understanding of the properties of single-atom catalysts on oxide surfaces requires a careful account of the interplay between adatoms, vacancy sites, and polarons.

Reversing the Catalytic Selectivity of Single-Atom Ru via Support Amorphization.

Supported single-atom catalysts (SACs), with the extremely homogenized active sites could achieve high hydrogenation selectivity toward one of the functional groups coexisting in the reactant molecule. However, as to the target group, the control of selective recognition and activation by SACs still remains a challenge. Herein, the phase engineering of the support is adopted to control the chemo-recognition behavior of SACs in selective hydrogenation. Single-atom Ru on amorphous porous ultrathin TiO2 nanosheets (Ru1/a-TiO2) is constructed, in which Ru is more positively charged than that in the crystalline counterpart (Ru1/c-TiO2). Moreover, in the nitro/vinyl selective hydrogenation process, Ru1/a-TiO2 shows superior nitro selectivity, opposite to the vinyl selectivity of Ru1/c-TiO2. Density functional theory calculations for single-atom Ru of different charge states show that the reactant adsorption configuration could be inverted in the amorphous TiO2, accounting for the chemo-recognition behavior controlled by the phase of support.

Mo2CS2-MXene supported single-atom catalysts for efficient and selective CO2 electrochemical reduction

Single-atom catalysts (SACs) recently attracted considerable attention in heterogeneous catalysis, owing to high atom-utilization and unique properties. In this paper, we investigated geometry, electronic structure, stabilities, catalytic activity, and selectivity of the various TM@Mo2CS2 (TM = Fe, Co, Ni, Cu, Ru. Rh, Pd, Ag, Os, Ir, Pt, and Au) anchored SACs for CO2 electrochemical reduction using periodic density functional theory and ab-initio molecular dynamics calculations. The single metal atoms tend to occupy the Mo-top site on the Mo2CS2 surface. Possible different reaction pathways to produce various C1 products such as CO, HCOOH, HCHO, CH3OH, and CH4 have been investigated for Fe, Co, Ni, and Ru supported SACs. Among the SACs investigated, Fe, Co, and Ru supported by Mo2CS2 catalysts selectively produce CH4, whereas Ru@Mo2CS2 has the lowest overpotential of 0.24 V. Ni primarily produces HCOOH with an overpotential is 0.37 V. Therefore, this research demonstrated the significant potential of Mo2CS2 surface for a single-atom catalyst for selective CO2 reduction and other electrochemical applications.

Theoretical insights into the CO/NO oxidation mechanisms on single-atom catalysts anchored H4,4,4-graphyne and H4,4,4-graphyne/graphene sheets

Novel two-dimensional (2D) carbon-based substrates supported single-atom catalysts (SACs) have gained enormous interests. Herein, the adsorption property of single transition metal (TM) catalysts anchored at H4,4,4-graphyne (TM-H4,4,4-GY) are investigated using the first-principle calculations. The graphene substrate can provide the electron transfer to H4,4,4-GY and then improve the stability of TM atoms on H4,4,4-GY sheet (TM-H4,4,4-GY/G). Different kinds of TM adatoms and gas reactants can effectively control the electronic and magnetic properties of Mn-H4,4,4-GY and Mn-H4,4,4-GY/G systems. Four catalytic mechanisms for NO and CO oxidations are comparably analyzed via Langmuir-Hinshelwood (LH), Eley-Rideal (ER), new ER (NER) and termolecular ER (TER) reactions. The calculated results reveal that the NO and CO oxidation processes exhibit various reaction rates on different substrates. Compared with NO oxidation on Mn-H4,4,4-GY surface, the ER and LH mechanisms as the starting state for CO oxidation are energetically more favorable than other ones. On the Mn-H4,4,4-GY/G sheet, the CO oxidation processes by ER and NER mechanisms have relatively smaller energy barriers and exhibit higher reaction rates than those of NO oxidation. Therefore, the oxidation reactions of CO acts as a leading role in the first stage and the NO oxidation may happen in the second stage. These results provide an effective descriptor about geometric structure, electronic, magnetic, gas sensoring and catalytic properties of novel 2D carbon-based catalysts.

Low‐dimensional material supported single‐atom catalysts for electrochemical CO2 reduction

AbstractConverting CO2 emissions to valuable carbonaceous chemicals/fuels under mild conditions provides a sustainable way to maintain carbon balance and alleviate the energy shortage. Low‐dimensional material (LDM) supported single‐atom catalysts (SACs) have been attracted significant attention for electrochemical CO2 reduction reaction (ECR) in recent years. This is mainly because integrating the single‐atoms and LDMs can inherit the advantages of themselves and the synergy effects between them are potential to enhance the ECR performance. In this review, we summarized the strategies for synthesizing LDM supported SACs for ECR, and different LDM supported SACs for ECR have been briefly introduced. Moreover, some optimization strategies for LDM supported SACs towards CO2 electroreduction are highlighted. At the end of this review, the perspectives and challenges of LDM supported SACs for ECR are provided.

N-Doped Graphene Supported Cu Single Atoms: Highly Efficient Recyclable Catalyst for Enhanced C-N Coupling Reactions.

Heterogenization of homogeneous catalysis through supported single-atom catalysts (SACs) provided a feasible solution to recycling catalysts while keeping its efficiency in chemical synthesis. In this work, Cu SACs anchored on N-doped graphene (Cu SACs/NG) were prepared and first used for C-N coupling reactions. During the preparation, Cu-N-C structures, including Cu-N4 moieties, were formed in a one-step pyrolysis method. As-prepared Cu SACs/NG exhibited excellent catalytic activity toward C-N coupling reactions with a broad scope of substrates and showed outstanding performance of recycling. Compared with Cu nanoparticles (Cu NPs/NG), the advantages of single-atom catalysts were validated via experimental and theoretical calculations. The enhanced performances were attributed to increasing the number of active sites and increasing the intrinsic activity of each active site. This work provides an alternative synthetic strategy for fabricating atomically dispersed SACs and represents a significant advance for coupling reactions.

Heterogeneous Single Atom Environmental Catalysis: Fundamentals, Applications, and Opportunities

AbstractThe development of efficient catalysts is pivotal for pollution abatement to guarantee a sustainable and green industrial production. Supported single atom catalysts (SACs) with the advantages of maximum atom utilization, unique electronic structure, and distinguished metal support interaction have recently become a hotspot in the catalysis community, and bring new opportunities for environmental catalysis. This review aims to provide a comprehensive overview of the latest progress on the environmental SACs from both the fundamental and practical points of view. It starts with highlighting the state‐of‐the‐art synthesis strategies of great values for the practical use to construct challenging SACs. The applications of SACs in the environmental remediation processes are critically summarized, including CO oxidation, NOx decomposition, volatile organic compounds incineration, CO2 conversion, and water purification. Meanwhile, the topic also sketches out the recent progress for the non‐mercury catalyzed acetylene hydrochlorination reaction to address the environmental‐benign synthesis. Emphasis is placed on the elucidation of the structure–activity relationships and the underlying catalytic mechanisms to shed light on the guidelines for catalyst optimization and upgradation. Finally, the remaining challenges and perspectives for this flourishing field are also discussed.

Temperature‐Dependent Diels‐Alder Cycloaddition on Polyoxometalate‐Supported Single‐Atom Catalysts M1/PTA (M=Mn, Fe, Co, Ru, Rh, Pd, Os, Ir and Pt; PTA=[PW1240]3−)

AbstractDiels‐Alder (D−A) reaction shaped both the art and science of total synthesis to some degree, while the effect of temperature on D−A activity over polyoxometalates supported single‐atom catalysts (SACs) has been infrequently studied and simulated using theoretical calculations. Herein, cycloaddition of 1,3‐butadiene (C4H6) and ethylene (C2H4) was employed as a model D−A reaction. The multitudes of SACs M1/PTA (M=Mn, Fe, Co, Ru, Rh, Pd, Os, Ir and Pt; PTA=[PW12O40]3−) were examined by DFT‐M06l computations to understand the reaction mechanism on a molecular level. The adsorption energies of reactant and product, and activation energy barriers for all the studied SAC systems have the same variation trends with the temperature variations. Considering that the adsorption for C4H6 is always stronger than that of C2H4 in all the studied systems, the initial adsorption configurations is the M1/PTA SACs adsorbed one C4H6 molecule. Three SACs, namely the Co1/PTA and Rh1/PTA at 100 K, Rh1/PTA at 300 K were identified, which show predominant catalytic activity and the corresponding activation energy barriers are 4.21, 8.51 and 5.11 kcal mol−1, respectively. The bonding interaction between adsorbate C4H6 and SACs arises from the occupied molecular orbitals (MOs) with a mixture of π orbitals of C4H6 and d atomic orbitals of the metal single atom. These theoretical calculations give new guidelines to develop high catalytic activity and cost‐effective SACs towards the D−A reaction.