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Pyridyl-containing graphdiyne provides well-defined sites for stabilizing sub-2 nm copper nanoclusters, which show an optimum CH4 faradaic efficiency of 58% in the electrochemical CO2 reduction reaction.

Fine-tuning electronic structures of single-atom catalysts (SACs) plays a crucial role in harnessing their catalytic activities, yet challenges remain at a molecular scale in a controlled fashion. By tailoring the structure of graphdiyne (GDY) with electron-withdrawing/-donating groups, we show herein the electronic perturbation of Cu single-atom CO2 reduction catalysts in a molecular way. The elaborately introduced functional groups (-F, -H and -OMe) can regulate the valance state of Cuδ+, which is found to be directly scaled with the selectivity of the electrochemical CO2-to-CH4 conversion. An optimum CH4 Faradic efficiency of 72.3% was achieved over the Cu SAC on the F-substituted GDY. In situ spectroscopic studies and theoretical calculations revealed that the positive Cuδ+ centers adjusted by the electron-withdrawing group decrease the pKa of adsorbed H2O, promoting the hydrogenation of intermediates toward the CH4 production. Our strategy paves the way to precise electronic perturbation of SACs toward efficient electrocatalysis.

Understanding the seven coordination and O−O coupling pathway of the distinguished Ru-bda catalysts is essential for the development of next generation efficient water-oxidation catalysts based on earth-abundant metals. This work reports the synthesis, characterization and catalytic properties of a monomeric ruthenium catalyst Ru-bnda (H2bnda = 2,2'-bi(nicotinic acid)-6,6'-dicarboxylic acid) featuring steric hindrance and enhanced hydrophilicity on the backbone. Combining experimental evidence with systematic density functional theory calculations on the Ru-bnda and related catalysts Ru-bda (H2bda = 2,2ʹ-bipyridine-6,6ʹ-dicarboxylic acid), Ru-pda ((H2pda = 1,10-phenanthroline-2,9-dicarboxylic acid)), and Ru-biqa (H2biqa = (1,1ʹ-biisoquinoline)-3,3ʹ-dicarboxylic acid), we emphasized that seven coordination clearly determines presence of RuV=O with high spin density on the ORuV=O atom, i.e. oxo with radical properties, which is one of the necessary conditions for reacting through the O−O coupling pathway. However, an additional factor to make the condition sufficient is the favorable intermolecular face-to-face interaction for the generation of the pre-reactive [RuV=O···O=RuV], which may be significantly influenced by the secondary coordination environments. This work provides a new understanding of the structure-activity relationship of water-oxidation catalysts and their potential to adopt I2M pathway for O−O bond formation.

A molecular [Ru(bda)]-type (bda = 2,2’-bipyridine-6,6’-dicarboxylate) water oxidation catalyst with 4-vinylpyridine as the axial ligand (Complex 1) was immobilized or co-immobilized with 1-(trifluoromethyl)-4-vinylbenzene (3F) or styrene (St) blocking units on the surface of glassy carbon (GC) electrodes by electrochemical polymerization, in order to prepare the corresponding poly-1@GC, poly-1+P3F@GC, and poly-1+PSt@GC functional electrodes. Kinetic measurements of the electrode surface reaction revealed that [Ru(bda)] triggers the O–O bond formation via (1) the radical coupling interaction between the two metallo-oxyl radicals (I2M) in the homo-coupling polymer (poly-1), and (2) the water nucleophilic attack (WNA) pathway in poly-1+P3F and poly-1+PSt copolymers. The comparison of the three electrodes revealed that the second coordination sphere of the water oxidation catalysts plays vital roles in stabilizing their reaction intermediates, tuning the O–O bond formation pathways and improving the water oxidation reaction kinetics without changing the first coordination structures.

Langmuir adsorption model is a classic physical-chemical adsorption model and is widely used to describe the monolayer adsorption behavior at the material interface in environmental chemistry. Traditional adsorption dynamic modeling solely considered the surface physiochemical interaction between the adsorbent and adsorbate. The surface reaction dynamics resulting from the heterogeneous surface and intrinsic electronic structure of absorbents were rarely considered within the reported adsorption experiments. Herein, by employing the chlorine hybrid graphene oxide (

We report herein a series of Cp*Ir complexes containing a rigid 8-aminoquinolinesulfonamide moiety as highly efficient catalysts for the dehydrogenation of formic acid (FA). The complex [Cp*Ir(L)Cl] (HL = N-(quinolin-8-yl)benzenesulfonamide) displayed a high turnover frequency (TOF) of 2.97 × 104 h-1 and a good stability (>100 h) at 60 °C. Comparative studies of [Cp*Ir(L)Cl] with the rigid ligand and [Cp*Ir(L')Cl] (HL' = N-propylpypridine-2-sulfonamide) without the rigid aminoquinoline moiety demonstrated that the 8-aminoquinoline moiety could dramatically enhance the stability of the catalyst. The electron-donating ability of the N,N'-chelating ligand was tuned by functionalizing the phenyl group of the L ligand with OMe, Cl, and CF3 to have a systematical perturbation of the electronic structure of [Cp*Ir(L)Cl]. Experimental kinetic studies and density functional theory (DFT) calculations on this series of Cp*Ir complexes revealed that (i) the electron-donating groups enhance the hydrogen formation step while slowing down the β-hydride elimination and (ii) the electron-withdrawing groups display the opposite effect on these reaction steps, which in turn leads to lower optimum pH for catalytic activity compared to the electron-donating groups.

Electrocatalytic water splitting is an efficient means of producing energy carriers, such as H2. The hydrogen evolution reaction (HER) requires high-efficiency electrocatalysts. Understanding the active site structures of the HER electrocatalysts is essential for the rational design and development of water splitting devices. In this study, porous CoW sulfides were employed as model electrocatalysts for pH-universal HER. Multiple characterization studies, such as X-ray photoelectron spectroscopy, X-ray absorption spectroscopy and operando X-ray diffraction, were systematically used to investigate the reconstruction of the active species at the surface and in the bulk. The results show that during the HER, the structural transformation of the species CoW sulfides is strongly dependent on the pH of the electrolyte. Electrolytes of varying pH lead to varied reconstruction and influence the true catalytically active species responsible for the HER. The surface and the bulk of the electrocatalysts transform to different oxides/hydroxides when subjected to the HER. This is the first time that the pH-dependent bulk and surface structural evolution in the HER has been revealed. This study reveals the reconstruction and potential active site evolution of mixed-metal sulfides for the HER. We believe that the present study not only provides an idealized “pre-catalyst” for pH-universal highly-efficient HER, but also provides a thorough understanding about the identification of the real active sites and the mechanism of the structural evolution of the electrocatalysts during hydrogen evolution.

The [fac-Mn(bpy)(CO)3Br] complex is capable of catalyzing the electrochemical reduction of CO2 to CO with high selectivity, moderate activity and large overpotential. Several attempts have been made to lower the overpotential and to enhance the catalytic activity of this complex by manipulating the second-coordination sphere of manganese and using relatively stronger acids to promote the protonation-first pathway. We report herein that the complex [fac-Mn(bpy-CONHMe)(CO)3(MeCN)]+ ([1-MeCN]+; bpy-CONHMe = N-methyl-(2,2′-bipyridine)-6-carboxamide) as a pre-catalyst could catalyze the electrochemical reduction of CO2 to CO with low overpotential and high activity and selectivity. Combined experimental and computational studies reveal that the amide NH group not only decreases the overpotential of the Mn catalyst by promoting the dimer and protonation-first pathways in the presence of H2O but also enhances the CO2 electroreduction activity by facilitating C–OH bond cleavage, making [1-MeCN]+ an efficient CO2 reduction pre-catalyst at low overpotential.

Significance The present-day industrial ammonia synthesis is overreliance on the Haber–Bosch process, yet consumes more than 1% of the global energy supply along with gigatonne greenhouse-gas emission per year. Electrochemical nitrogen reduction reaction (ENRR) offers a sustainable path to produce ammonia under mild conditions, while its efficiency achieved by far is fairly low, which requires the development of both catalysts and electrolyzers. Here, by the cooperation of densely populated metal single atoms on graphdiyne substrate with the pressurized electrocatalytic system, the kinetics and the thermodynamic driving force of ENRR are effectively regulated, leading to a record-high ammonia yield rate. This work motivates the technological and material coevolution for the ENRR toward its envisioned application., Using renewable electricity to synthesize ammonia from nitrogen paves a sustainable route to making value-added chemicals but yet requires further advances in electrocatalyst development and device integration. By engineering both electrocatalyst and electrolyzer to simultaneously regulate chemical kinetics and thermodynamic driving forces of the electrocatalytic nitrogen reduction reaction (ENRR), we report herein stereoconfinement-induced densely populated metal single atoms (Rh, Ru, Co) on graphdiyne (GDY) matrix (formulated as M SA/GDY) and realized a boosted ENRR activity in a pressurized reaction system. Remarkably, under the pressurized environment, the hydrogen evolution reaction of M SA/GDY was effectively suppressed and the desired ENRR activity was strongly amplificated. As a result, the pressurized ENRR activity of Rh SA/GDY at 55 atm exhibited a record-high NH3 formation rate of 74.15 μg h−1⋅cm−2, a Faraday efficiency of 20.36%, and a NH3 partial current of 0.35 mA cm−2 at −0.20 V versus reversible hydrogen electrode, which, respectively, displayed 7.3-, 4.9-, and 9.2-fold enhancements compared with those obtained under ambient conditions. Furthermore, a time-independent ammonia yield rate using purified 15N2 confirmed the concrete ammonia electroproduction. Theoretical calculations reveal that the driving force for the formation of end-on N2* on Rh SA/GDY increased by 9.62 kJ/mol under the pressurized conditions, facilitating the ENRR process. We envisage that the cooperative regulations of catalysts and electrochemical devices open up the possibilities for industrially viable electrochemical ammonia production.

As a favorite descriptor, the size effect of Cu-based catalysts has been regularly utilized for activity and selectivity regulation toward CO2 /CO electroreduction reactions (CO2 /CORR). However, little progress has been made in regulating the size of Cu nanoclusters at the atomic level. Herein, the size-gradient Cu catalysts from single atoms (SAs) to subnanometric clusters (SCs, 0.5-1 nm) to nanoclusters (NCs, 1-1.5 nm) on graphdiyne matrix are readily prepared via an acetylenic-bond-directed site-trapping approach. Electrocatalytic measurements show a significant size effect in both the activity and selectivity toward CO2 /CORR. Increasing the size of Cu nanoclusters will improve catalytic activity and selectivity toward C2+ productions in CORR. A high C2+ conversion rate of 312 mA cm-2 with the Faradaic efficiency of 91.2 % are achieved at -1.0 V versus reversible hydrogen electrode (RHE) over Cu NCs. The activity/selectivity-size relations provide a clear understanding of mechanisms in the CO2 /CORR at the atomic level.

Heterostructure engineering plays a vital role in regulating the material interface, thus boosting the electron transportation pathway in advanced catalysis. Herein, a novel Bi

The purpose of this paper is to provide an effective method to evaluate the interface design of medical equipment to improve user experience. Ngo proposed the calculation method of fourteen aesthetic indexes and tested the method in the layout design of the website. However, he counts the proportion of each index into the same value, which is obviously inconsistent with people's cognitive habits. This paper extends Ego’s research to the application level. We creatively introduce the analytic hierarchy process and redistribute the values to make it consistent with human cognitive habits. The method can help designers quickly pick up the optimal design scheme.In order to construct an effective evaluation system, five indexes that balance symmetry economy regularity and rhythm were selected from ego's 14 aesthetic measures for interface elements displays based on five basic interface design principles, user research and experiences of professional designers. AHP (Analytic Hierarchy Process) that has five steps: 1. Build a hierarchical structure model 2. Construct a judgment matrix 3. Hierarchical single sort 4. Hierarchical total order 5. Consistency check. was used to calculate the weight of each index. An expert was invited to value the indexes and the valued data successfully passed the consistency test.In order to quickly get the calculation results, we programmed all the calculation methods and processed the data of interface elements into matrices. The evaluation method is constructed through MATLAB programming to provide an objective evaluation method for the improvement of the user experience of the interface design of medical equipment.We assume that the model could benefit designers in selecting the optimal design plan and improving the efficiency of using the interface. Then we started empirical research to validate the method. The operating interface of a certain brand of medical pathology product freezing microtome was selected as a sample. For experimentation, we chose five different design schemes of the interface, and processed the interface elements into rectangles. We got the position and area data of the rectangle on the surface from adobe xd.The result is that the evaluation method can well reflect the aesthetics of the medical equipment interface design, play a guiding role in the interface design of the medical equipment, and can improve and enhance the user experience of the medical equipment interface. Due to time reasons, this article does not include more elements that affect the usability of the interface, such as the visual weight of the elements, the purity of the hue, and so on. All the interfaces used have been decolorized because color will affect human evaluation during the research. We will consider including color in the evaluation system in the future research.

Because of the strong σ-donor and weak π-acceptor of the N-heterocyclic carbene (NHC), Mn-NHC complexes were found to be active for the reduction of CO2 to CO with high activity. However, some NHC-...

Iron-based nanosized ecomaterials for efficient Cr(VI) removal are of great interest to environmental chemists. Herein, inspired by the “mixed redox-couple” cations involved in the crystal structur...

Ultrathin two-dimensional (2D) nanosheets with efficient light-driven proton reduction activity were obtained through the exfoliation of novel metal-organic frameworks (MOF), which were synthesized by using a bis(4'-carboxy-2,2':6',2″-terpyridine) ruthenium complex as a linker and 3d transition-metal (Mn, Co, Ni, and Zn) anions as nodes. The nanosheet of the Ni2+ node exhibits a photocatalytic hydrogen evolution rate of 923 ± 40 μmol g-1 h-1 at pH = 4.0, without the presence of any cocatalyst or cosensitizer. A combined experimental and theoretical study suggests a reductive quenched pathway for the photocatalytic hydrogen evolution by the nanosheet. The transition-metal nodes at the edge of the nanosheets are proposed as the active sites. Density functional theory (DFT) calculations attributed the different catalytic activities of the nanosheets to the discrepancy of H adsorption free energy at various transition-metal nodes.

Electrochemical nitrogen fixation under ambient conditions is an attractive prospect to produce ammonia, yet the process still suffers from the sluggish kinetics and is extremely sensitive to the s...

Artificial nitrogen fixation through the nitrogen reduction reaction (NRR) under ambient conditions is a potentially promising alternative to the traditional energy-intensive Haber–Bosch process. F...

Herein, we converted spent LiFePO

Accurate control over the coordination circumstances of single-atom catalysts (SACs) is decisive to their intrinsic activity. Here, two single-site heterogeneous organometallic catalysts (SHOCs), Cp*Ir-L/GDY (L = OH

The [fac-Mn(bpy)(CO)3Br] complex is capable of catalyzing the electrochemical reduction of CO2 to CO with high selectivity, moderate activity and large overpotential. Several attempts have been made to lower the overpotential and to enhance the catalytic activity of this complex by manipulating the second-coordination sphere of manganese and using relatively stronger acids to promote the protonation-first pathway. We report herein that the complex [fac-Mn(bpy-CONHMe)(CO)3(MeCN)]+ ([1-MeCN]+; bpy-CONHMe = N-methyl-(2,2′-bipyridine)-6-carboxamide) as a pre-catalyst could catalyze the electrochemical reduction of CO2 to CO with low overpotential and high activity and selectivity. Combined experimental and computational studies reveal that the amide NH group not only decreases the overpotential of the Mn catalyst by promoting the dimer and protonation-first pathways in the presence of H2O but also enhances the CO2 electroreduction activity by facilitating C–OH bond cleavage, making [1-MeCN]+ an efficient CO2 reduction pre-catalyst at low overpotential., The amide NH group decreases the overpotential of Mn-based CO2 reduction catalysts by promoting the dimer and protonation-first pathways in the presence of H2O and enhances the CO2 electroreduction activity by facilitating C–OH bond cleavage.

Herein, we converted spent LiFePO4 battery to the sea urchin-like material (SULM) with a highly efficient and environment-friendly method, which can contribute to building a zero-waste city. With SULM as a Fenton-like catalyst, a highly-efficient degradation process was realized for organic pollutants with interface and solution synergistic effect. In our SULM+NH2OH+H2O2 Fenton-like system, NH2OH can effectively promote the interface iron (Fe(Ⅲ)/Fe(Ⅱ)) and solution iron (Fe(Ⅲ)/Fe(Ⅱ)) redox cycle, thus promoting the generation of reactive oxygen species (ROS). However, the ROS generation process and organic pollutants degradation pathway with the presence of NH2OH remains a puzzle. Here the detailed ROS generation mechanism and pollutants degradation pathway have been illustrated carefully based on experimental exploration and characterization. Therein, hydroxyl radicals (·OH) and singlet oxygen (1O2) are the main ROS for oxidizing and degrading organic pollutants. Notably, 1O2 can be converted from superoxide radicals (·O2) in SULM+NH2OH+H2O2 system. This study not only demonstrates the strategy of "trash-to-treasure" and "waste-to-control-waste" to simultaneously reduce the hazardous release from industrial solid waste and organic wastewater, it also provides new mechanistic insights for NH2OH mediated Fenton-like redox system.

Configuring reactive and stable catalytic interfaces is crucial to design efficient photocatalysts for Cr(VI) reduction. Herein, via the platinum decoration approach based on interfacial engineering, we developed an effective catalytic interface within novel semiconducting chalcopyrite quantum dots (Pt/CuFeS

Surface functionalization of carbon materials is of interest in many research fields, such as electrocatalysis, interfacial engineering, and supercapacitors. As an emerging carbon material, γ-graphyne has attracted broad attention. Herein, we report that the surface functionalization of a γ-graphyne-like carbon material (γ-G1) is achieved by immobilizing functional groups via the click chemistry. Texture analysis of aberration-corrected microscopy, X-ray photoelectron spectroscopy, and electrochemistry confirm the successful surface modification of γ-G1 through a strong covalent linkage 1,2,3-triazole. The direct linkage of functional groups on γ-G1 via the click chemistry represents a general method for preparing other functional materials by using γ-graphyne-like materials as a skeleton.

Mesoporous LaFeO3 was used as a p-type visible-light-absorbing semiconductor (VLAS) substrate for light-driven H2 generation. The successful modification of LaFeO3 with a molecular dye (P1*) and a molecular hydrogen production catalyst (NiP) paved a novel way to construct DS-PEC photocathodes for solar-driven H2 generation by using VLASs.

Significant advances during the past decades in the design and studies of Ru complexes with polypyridine ligands have led to the great development of molecular water oxidation catalysts and understanding on the O−O bond formation mechanisms. Here we report a Ru-based molecular water oxidation catalyst [Ru(bds)(pic)2] (Ru-bds; bds2− = 2,2′-bipyridine-6,6′-disulfonate) containing a tetradentate, dianionic sulfonate ligand at the equatorial position and two 4-picoline ligands at the axial positions. This Ru-bds catalyst electrochemically catalyzes water oxidation with turnover frequencies (TOF) of 160 and 12,900 s−1 under acidic and neutral conditions respectively, showing much better performance than the state-of-art Ru-bda catalyst. Density functional theory calculations reveal that (i) under acidic conditions, the high valent Ru intermediate RuV=O featuring the 7-coordination configuration is involved in the O−O bond formation step; (ii) under neutral conditions, the seven-coordinate RuIV=O triggers the O−O bond formation; (iii) in both cases, the I2M (interaction of two M−O units) pathway is dominant over the WNA (water nucleophilic attack) pathway., Developing efficient molecular water oxidation catalysts for artificial photosynthesis is a challenging task. Here the authors introduce a ruthenium based complex with negatively charged sulfonate groups to effectively drive water oxidation under both acidic and neutral conditions.

Configuring reactive and stable catalytic interfaces is crucial to design efficient photocatalysts for Cr(VI) reduction. Herein, via the platinum decoration approach based on interfacial engineering, we developed an effective catalytic interface within novel semiconducting chalcopyrite quantum dots (Pt/CuFeS2 QDs). Benefiting from the catalytic merits of the Pt modulated interfacial structure and electronic structure, Pt/CuFeS2 QDs show a broader light absorption capability extending to near-infrared radiation (NIR) range with superior carriers separation performance and faster charge transfer efficiency, which delivers a three-folder faster photocatalytic Cr(VI) reduction efficiency comparing to the original CuFeS2 QDs. Density functional theory (DFT) calculations unravel that Pt atoms prefer to be anchored with the surface S atoms to form a stable interfacial structure with faster electron transfer and Cr(VI) reduction dynamics. This work demonstrates that platinum decoration based on interfacial engineering is an effective strategy to simultaneously modulate the band structure and accelerate the interfacial reaction dynamics for semiconductor photocatalysts, which paves the way for designing highly efficient photocatalysts for light-driven environmental and energy engineering applications.

Iron-based nanosized ecomaterials for efficient Cr(VI) removal are of great interest to environmental chemists. Herein, inspired by the "mixed redox-couple" cations involved in the crystal structure and the quantum confinement effects resulting from the particle size, a novel type of iron-based ecomaterial, semiconducting chalcopyrite quantum dots (QDs), was developed and used for Cr(VI) removal. A high removal capacity up to 720 mg/g was achieved under optimal pH conditions, which is superior to those of the state-of-the-art nanomaterials for Cr(VI) removal. The mechanism of Cr(VI) removal was elucidated down to an atomic scale by combining comprehensive characterization techniques with adsorption kinetic experiments and DFT calculations. The experimental results revealed that the material was a good electron donor semiconductor attributed to the existence of "mixed redox couple of Cu(I)-S-Fe(III)" in the crystal structure. With the size-dependent quantum confinement effect and the high surface area, the semiconducting chalcopyrite QDs could effectively remove Cr(VI) from aqueous solution through a syngenetic photocatalytic reduction and adsorption mechanism. This study not only reports the design histogram of the iron-based CuFeS

Engineering novel Sn-based bimetallic materials could provide intriguing catalytic properties to boost the electrochemical CO

Previous research of molybdenum-based electrocatalysts for nitrogen reduction reaction (NRR) has been largely considered on either isolated Mo single atoms (MoSAs) or Mo carbide particles (e.g., Mo

Ru-Based complexes have advanced the study of molecular water oxidation catalysts (WOCs) both in catalysis and mechanism. The electronic effect has always been considered as an essential factor for the catalyst properties while less attention has been focused on the bite angle effect on water oxidation catalysis. The Ru-bda ([Ru(bda)(pic)2]; bda2− = 2,2′-bipyridine-6,6′-dicarboxylate; pic = 4-picoline) catalyst is one of the most active WOCs and it has a largely distorted octahedral configuration with an O–Ru–O bite angle of 123°. Herein, we replaced the carboxylate (–COO−) groups of bda2− with two methylenesulfonate (–CH2SO3−) groups and prepared a negatively charged ligand, bms2− (2,2′-bipyridine-6,6′-dimethanesulfonate), and the Ru-bms complex [Ru(bms)(pic)2]. The O–Ru–O bite angle changed from 123° in Ru-bda to 84° in Ru-bms, leading to a dramatic influence on the catalytic behavior. Systematic analysis of the reaction intermediates suggested that Ru-bms transformed all the way to Ru-bdavia oxidative decomposition under CeIV-driven water oxidation conditions.

To address the urgent issues of hazardous gas detection and the prevention of environmental pollution, various functional materials for gas sensing and catalytic reduction have been studied. Specifically, the p-type perovskite LaFeO

Mesoporous LaFeO3 was used as a p-type visible-light-absorbing semiconductor (VLAS) substrate for light-driven H2 generation. The successful modification of LaFeO3 with a molecular dye (P1*) and a molecular hydrogen production catalyst (NiP) paved a novel way to construct DS-PEC photocathodes for solar-driven H2 generation by using VLASs.

The mononuclear ruthenium complex [Ru(pdc)L3] (H2pdc = 2,6-pyridinedicarboxylic acid, L = N-heterocycles such as 4-picoline) has previously shown promising catalytic efficiency toward water oxidation, both in homogeneous solutions and anchored on electrode surfaces. However, the detailed water oxidation mechanism catalyzed by this type of complex has remained unclear. In order to deepen understanding of this type of catalyst, in the present study, [Ru(pdc)(py)3] (py = pyridine) has been synthesized, and the detailed catalytic mechanism has been studied by electrochemistry, UV–vis, NMR, MS, and X-ray crystallography. Interestingly, it was found that once having reached the RuIV state, this complex promptly formed a stable ruthenium dimer [RuIII(pdc)(py)2-O-RuIV(pdc)(py)2]+. Further investigations suggested that the present dimer, after one pyridine ligand exchange with water to form [RuIII(pdc)(py)2-O-RuIV(pdc)(py)(H2O)]+, was the true active species to catalyze water oxidation in homogeneous solutions.

The coordination of a substrate water molecule on a metal centered catalyst for water oxidation is a crucial step involving the reorganization of the ligand sphere. This process can occur by substi ...