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•Mechanism of KBBC/PS system for mining water treatment under different pH conditions was studied.•Direct electron transfer process contributed under acidic and neutral ...conditions.•Various active species contributed under alkaline conditions.•Continuous flow experiments prove the potential applicability of KBBC/PS process.
The challenges of remediating polluted water in mine areas are exacerbated by low emission dispersion and long-lasting pollution, particularly the significant fluctuations in pH value. Notably, the treatment of flotation reagent polluted water with variable pH scenarios remains a challenge. In this study, metal-free biochar-based materials (KBBC)/persulfate system was used for flotation reagent contaminated wastewater treatment with a wide pH range. Regarding the intrinsic nature of the biochar-based materials, ball milling enhanced the degree of graphitization, while introducing a hard template of KCl increased the proportion of carbonyl functional groups on their surface, thereby facilitating persulfate activation. Experimental findings revealed that the KBBC/PS system displayed efficient degradation of aniline aerofloat (AAF) as a representative flotation reagent across a wide pH range (pH = 3.6–10.4 with buffer). Investigations on the mechanism indicated that, under acidic and neutral conditions, superoxide radicals and direct electron transfer processes constituted primary pathways for pollutant degradation. Meanwhile, various active species contributed under alkaline conditions with increased involvement of free radicals (•OH and SO4•−) and singlet oxygen. Furthermore, continuous flow experiments were set up to prove the potential applicability of the KBBC/PS process. This study not only deepens the understanding of persulfate activation by metal-free carbon-based materials across a wide pH range but also expands their potential applications in mine-polluted water purification.
Electrochemical conversion of CO(2) into hydrocarbons and oxygenates is envisioned as a promising path toward closing the carbon cycle in modern technology. To date, however, the reaction mechanisms ...toward the plethora of products are disputed, complicating the search for alternative catalyst materials. To conclusively identify the rate-limiting steps in CO reduction on Cu, we analyzed the mechanisms on the basis of constant-potential density functional theory (DFT) kinetics and experiments at a wide range of pH values (3–13). We find that *CO dimerization is energetically favored as the rate-limiting step toward multicarbon products. This finding is consistent with our experiments, where the reaction rate is nearly unchanged on a standard hydrogen electrode (SHE) potential scale, even under acidic conditions. For methane, both theory and experiments indicate a change in the rate-limiting step with electrolyte pH from the first protonation step under acidic/neutral conditions to a later one under alkaline conditions. We also show, through a detailed analysis of the microkinetics, that a surface combination of *CO and *H is inconsistent with the measured current densities and Tafel slopes. Finally, we discuss the implications of our understanding for future mechanistic studies and catalyst design.
Hydrogen generation from formic acid (FA), one of the most promising hydrogen storage materials, has attracted much attention due to the demand for the development of renewable energy carriers. ...Catalytic dehydrogenation of FA in an efficient and green manner remains challenging. Here, we report a series of bioinspired Ir complexes for highly robust and selective hydrogen production from FA in aqueous solutions without organic solvents or additives. One of these complexes bearing an imidazoline moiety (complex 6) achieved a turnover frequency (TOF) of 322 000 h–1 at 100 °C, which is higher than ever reported. The novel catalysts are very stable and applicable in highly concentrated FA. For instance, complex 3 (1 μmol) affords an unprecedented turnover number (TON) of 2 050 000 at 60 °C. Deuterium kinetic isotope effect experiments and density functional theory (DFT) calculations employing a “speciation” approach demonstrated a change in the rate-determining step with increasing solution pH. This study provides not only more insight into the mechanism of dehydrogenation of FA but also offers a new principle for the design of effective homogeneous organometallic catalysts for H2 generation from FA.
Driven by the persisting poor understanding of the sluggish kinetics of the hydrogen evolution reaction (HER) on Pt in alkaline media, a direct correlation of the interfacial water structure and ...activity is still yet to be established. Herein, using Pt and Pt–Ni nanoparticles we first demonstrate a strong dependence of the proton donor structure on the HER activity and pH. The structure of the first layer changes from the proton acceptors to the donors with increasing pH. In the base, the reactivity of the interfacial water varied its structure, and the activation energies of water dissociation increased in the sequence: the dangling O−H bonds < the trihedrally coordinated water < the tetrahedrally coordinated water. Moreover, optimizing the adsorption of H and OH intermediates can re‐orientate the interfacial water molecules with their H atoms pointing towards the electrode surface, thereby enhancing the kinetics of HER. Our results clarified the dynamic role of the water structure at the electrode–electrolyte interface during HER and the design of highly efficient HER catalysts.
On nickel–platinum alloy nanoparticles under alkaline conditions, the reactivity of interfacial water varies with its structure and the order of water dissociation. The inclusion of nickel re‐orientates interfacial water molecules with their hydrogen atoms pointing towards the electrode surface, thereby enhancing the kinetics of the hydrogen evolution reaction (HER).
Cu surfaces are known for their ability to electrochemically convert CO2 and CO into multicarbon (C2+) products. Although the impact of the electrolyte alkalinity on the electrochemical CO2 and CO ...reduction reactions (CO2RR and CORR, respectively) has been extensively investigated on Cu surfaces, systematic investigation of the pH dependence of the surface Cu speciation has been lacking. Herein, we demonstrate for the first time the pH dependence of Cu surface speciation and CO adsorption configuration in weakly to strongly alkaline electrolytes using in situ surface enhanced Raman spectroscopy. Oxygen-containing species, e.g., CuO x /(OH) y , are absent in near-neutral electrolyte of KHCO3 (pH = 8.9), and their onset potential becomes more positive with increasing the alkalinity of electrolytes. Concurrently, adsorbed CO transitions from atop-bonded (COatop) to bridge-bonded configuration (CObridge) as the pH of the electrolyte increases. CO is proposed to adsorb in the atop configuration only on Cu sites without neighboring oxygen-containing species, while the appearance of the CObridge band correlates with the presence of the oxygen-containing species. Surface speciation on Cu microparticles (Cu MPs) and oxide-derived Cu (OD-Cu) are similar in all the electrolytes investigated, while the CO adsorption bands show subtle differences on these two surfaces, reflecting variations in the microenvironment of CO adsorbed in the same configuration. Selectivity for C2+ products in the CORR on Cu MPs increases with the electrolyte alkalinity at −0.4 VRHE, which correlates with the presence of the oxygen-containing Cu species. However, the presence of the oxygen-containing Cu species alone is shown to be an unreliable predictor of the CORR performance on Cu MPs and OD-Cu, which could be attributed to the difference in the microenvironment of the surface Cu sites. Results reported in this work highlight that the assumption that the surface Cu speciation under reducing potentials is independent of the electrolyte pH must be treated with caution.
The successful deployment of advanced energy‐conversion systems depends critically on our understanding of the fundamental interactions of the key adsorbed intermediates (hydrogen *H and hydroxyl ...*OH) at electrified metal–aqueous electrolyte interfaces. The effect of alkali metal cations (Li+, Na+, K+, Cs+) on the non‐Nernstian pH shift of the step‐related voltammetric peak of the Pt(553) electrode is investigated over a wide pH window (1 to 13) by means of experimental and computational methods. The co‐adsorbed alkali cations along the step weaken the OH adsorption at the step sites, causing a positive shift of the potential of the step‐related peak on Pt(553). Density functional calculations explain the observations on the identity and concentration of alkali cations on the non‐Nernstian pH shift, and demonstrate that cation–hydroxyl co‐adsorption causes the apparent pH dependence of “hydrogen” adsorption in the step sites of platinum electrodes.
Keeping in step: A combination of experiment and computations shows that the apparent pH dependence of hydrogen adsorption in step and defect sites of platinum electrodes is due to the co‐adsorption of cations with hydroxyl. A model for this effect, which is of key importance for interpreting the electrocatalytic activity of platinum, is outlined.
The pH is an important parameter in the reaction mechanism of the electrochemical reduction of carbon dioxide and carbon monoxide to methane and ethylene on copper electrodes. We have investigated ...the influence of the pH on this reaction using Cu(111) and Cu(100) single crystal electrodes. The results support our recently proposed reaction mechanism, in which two different reaction pathways to ethylene can be distinguished: a first, pH-dependent pathway that has a common intermediate with the formation of methane that occurs mainly on Cu(111), and a second, pH-independent pathway via a carbon monoxide dimer. The latter pathway occurs on Cu(100) only.
Electron paramagnetic resonance (EPR) has been extensively used for the identification of free radicals that are generated from advanced oxidation processes (AOPs) so as to establish the reaction ...mechanism. However, some misinterpretations or controversies on the identity of detected EPR signals remain in the literature. This study, with Cu(II)-based AOPs as examples, comprehensively investigated the origin of 5,5-dimethyl-l-pyrroline N-oxide (DMPO) adducts in Cu(II) alone, Cu(II)/H2O2, Cu(II)/peroxymonosulfate (PMS), and Cu(II)/peroxydisulfate (PDS) systems. In most Cu(II) systems, DMPO-OH signals can be detected even without any peroxygens, indicating the presence of other origins of this adduct in addition to the genuine spin trapping of •OH by DMPO. According to the formed secondary radical adducts (DMPO-OCH3 from a nonradical process or DMPO-CH2OH from a radical oxidation) derived from methanol quenching, we propose that CuO+, instead of free radicals, is involved in the Cu(II)/PMS system, while •OH is indeed generated in the Cu(II)/H2O2 and Cu(II)/PDS systems under neutral conditions. Notably, 17O-incorporation experiments demonstrate that −OH in the detected DMPO-OH adduct originates 100% from water in the Cu(II) alone system but the amount of −OH is over 99.8% from the oxidant while peroxygens are added. In addition, DMPO-O2 – appears only in the Cu(II)/PDS system under highly alkaline conditions and H2O is not involved in superoxide formation.
Electrochemical properties of two new derivatives of phenothiazine, i.e., 3,7-bis(4-aminophenylamino)phenothiazin-5-ium chloride (PhTz-(NH2)2) and 3,7-bis(4-carboxyphenylamino)phenothiazin-5-ium ...chloride (PhTz-(COOH)2), has been investigated on glassy carbon electrode. The pH influence on their electrode reactions was specified. The compounds studied showed complex kinetics of electrode reactions. Heterogeneous constants of the electron transfer at pH = 7.0 calculated from the Tafel plot were equal to 8.3 × 10−4 and 7.4 × 10−4 cm s−1 and transfer coefficients to 0.52 and 0.36 for PhTz-(NH2)2 and PhTz-(COOH)2, respectively. For carboxylate derivative, significant influence of the pH of working solution on the transfer coefficient was mentioned. Based on cyclic voltammetry, quartz crystal microbalance and spectroscopy of electrochemical impedance, deposition of oxidation products was found for PhTz-(NH2)2 and PhTz-(COOH)2 in multiple potential cycling. The carboxylic groups critically influenced electropolymerization of appropriate compound. The formation of bonds typical for electropolymerization was also confirmed by diffusion reflectance IR spectra. The electropolymerization product exerted high electrochemical activity within a broad pH range. Heterogeneous rate constant of the electropolymerized product was found to be 4.5 × 10−1 s−1.
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