Sulfur Dioxide Capture by Heterogeneous Oxidation on Hydroxylated Manganese Dioxide

Oxygen vacancy-rich K-Mn3O4@CeO2 catalyst for efficient oxidation degradation of formaldehyde at near room temperature

Synthesis of catalysts with high catalytic degradation activity for formaldehyde (HCHO) at room temperature is highly desirable for indoor air quality control. Herein, a novel K-Mn3O4@CeO2 catalyst with excellent catalytic oxidation activity toward HCHO at near room temperature was reported. In particular, the K addition in K-Mn3O4@CeO2 considerably enhanced the oxidation activity, and importantly, 99.3 % conversion of 10 mL of a 40 mg/L HCHO solution at 30 °C for 14 h was achieved, with simultaneous strong cycling stability. Moreover, the addition of K species considerably influenced the chemical valence state of Mn from +4 (ε-MnO2) to +8/3 (Mn3O4) on the surface of CeO2, which obviously changed the tunnel structure and the number of oxygen vacancies. One part of K species is uniformly dispersed on K-Mn3O4@CeO2, and the other part exists in the tunnel structure of Mn3O4@CeO2, which is mainly used to balance the negative charge of the tunnel and prevent collapse of the structure, providing enough active sites for the catalytic oxidation of HCHO. We observed a phase transition from tunneled KMnO2 to Mn3O4 to tunneled MnO2 with the decreasing K+ content, in which K-Mn3O4@CeO2 exhibited higher HCHO oxidation activity. In addition, K-Mn3O4@CeO2 exhibited lower oxygen vacancy formation and HCHO adsorption energies in aqueous solution based on density functional theory calculations. This is because the K species provide more active oxygen species and richer oxygen vacancies on the surface of K-Mn3O4@CeO2, promote the mobility of lattice oxygen and the room-temperature reduction properties of oxygen species, and enhance the ability of the catalyst to replenish the consumed oxygen species. Finally, a possible HCHO catalytic oxidation pathway on the surface of K-Mn3O4@CeO2 catalyst is proposed.

Simultaneous Sulfite Electrolysis and Hydrogen Production Using Ni Foam-Based Three-Dimensional Electrodes

The electrochemical oxidation of sulfite ions offers encouraging advantages for large-scale hydrogen production, while sulfur dioxide emissions can be effectively used to obtain value-added byproducts. Herein, the performance and stability during sulfite electrolysis under alkaline conditions are evaluated. Nickel foam (NF) substrates were functionalized as the anode and cathode through electrochemical deposition of palladium and chemical oxidation to carry out the sulfite electro-oxidation and hydrogen evolution reactions, respectively. A combined analytical approach in which a robust electrochemical flow cell was coupled to different in situ and ex situ measurements was successfully implemented to monitor the activity and stability during electrolysis. Overall, satisfactory sulfite conversion and hydrogen production efficiencies (>90%) at 10 mA·cm^-2 were mainly attributed to the use of NF in three-dimensional electrodes with a large surface area and enhanced mass transfer. Furthermore, stabilization processes associated with electrochemical dissolution and sulfur crossover through the membrane induced specific changes in the chemical and physical properties of the electrodes after electrolysis. This study demonstrates that NF-based electrocatalysts can be incorporated in an efficient electrochemical flow cell system for sulfite electrolysis and hydrogen production, with potential applications at a large scale.

Adsorption and oxidation of SO2 on the surface of TiO2 nanoparticles: the role of terminal hydroxyl and oxygen vacancy–Ti3+ states

The absorption and oxidation reactions of SO₂ on TiO₂ nanoparticles were investigated by using a flow chamber, synchrotron X-ray absorption near-edge structure and high resolution synchrotron X-ray photoelectron spectroscopy techniques.

Mechanisms of bisulfite/MnO2-accelerated transformation of methyl parathion

Although bisulfite is able to activate manganese oxides for enhanced oxidation of organic contaminants with donor-electron functions, the removal mechanisms for some esters remain poorly understood. In this study, we investigated the bisulfite/MnO₂-accelerated transformation of methyl parathion (MP), a recalcitrant and toxic organophosphorus pesticide (OPP). The removal rate constants of MP depended on pH, oxygen conditions, and the ratio between [HSO₃^-] and [MnO₂]. MP transformation declined by 36% with the addition of pyrophosphate as a scavenging agent for Mn(III)aq. [Mn(OH)(SO₃H)]⁺, a reactive intermediate, may be involved in enhancing the transformation of MP. The overall reaction can be divided into three distinct processes. The first process comprises two steps: the dissolution of MnO₂ reduced by HSO₃^- and the formation of a Mn-sulfite complex by a relatively fast substitution-controlled process. The second process is much slower and forms a precursor organometallic complex between the MP and Mn(IV/III). The third process involves a series of redox/hydrolysis reactions via aqueous and surface reactions. The mechanisms of each process were interpreted using kinetic observation and product identification data. This study improved the fundamental understanding of the MnO₂/HSO₃^- reaction process, thereby increasing the feasibility for remediating OPP pollution of the soil-water environment.

Molecular Insights into NO-Promoted Sulfate Formation on Model TiO2 Nanoparticles with Different Exposed Facets

In this study, molecular insights into NO-promoted SO₂ oxidation on model TiO₂ with well-defined (001), (010), and (101) facets are investigated in the laboratory. The adsorbed SO₂ is significantly promoted to transform into sulfate by the coexisting NO on the three facets. The appearance of active oxygen species indicates an active oxygen species-initiated NO oxidation. The resulting NO₂ acts as an oxidant to convert adsorbed sulfite on hydroxyls to sulfate species. The (101) facet presents the best performance in the NO-promoted sulfate formation possibly owing to its desirable structure to accommodate SO₃^2-, NO₂, and molecular water. The uptake coefficient (γ_t) of SO₂ increases by the coexistence of NO on the (001) facet at 0% RH and is relative humidity (RH) dependent, which first decreases from 0% RH to 32% RH but then increases in the range of 32%-80% RH. The probable explanation is the combined contribution of the blocking effect of water and the hydration of SO₂. The finding in this study provides insight into the possibility of its occurrence on common mineral dusts and requires further investigation.

Re-evaluation of stability and toxicity of silver sulfide nanoparticle in environmental water: Oxidative dissolution by manganese oxide

Stability of silver sulfide nanoparticle (Ag₂S-NP) in the environment has recently drawn considerable attention since it is associated with environmental risk. Although the overestimated stability of Ag₂S-NP in aqueous solution has already been recognized, studies on transformation of Ag₂S-NP in environmental water are still very scarce. Here we reported that Ag₂S-NP could undergo dissolution by manganese(IV) oxide (MnO₂), an important naturally occurring oxidant in the environment, even in environmental water, although the dissolved silver would probably be adsorbed onto the particles (>0.45 μm) in environmental water, mitigating the measurable levels of dissolved silver. The extent and rate of Ag₂S-NP dissolution rose with the increasing concentration of MnO₂. In addition, environmental factors including natural organic matter, inorganic salts and organic acids could accelerate the Ag₂S-NP dissolution by MnO₂, wherein an increase in dissolution extent was also observed. We further documented that Ag₂S-NP dissolution by MnO₂ was highly dependent on O₂ and it was an oxidative dissolution, with the production of SO₄^2-. Finally, dissolution of Ag₂S-NP by MnO₂ affected zebra fish (Danio rerio) embryo viability, showing significant reduction in embryo survival and hatching rates, compared to embryos exposed to Ag₂S-NP, MnO₂ or dissolved manganese alone. These findings would further shed light on the stability of Ag₂S-NP in the natural environment - essential for comprehensive nano risk assessment.

DFT studies on the heterogeneous oxidation of SO2 by oxygen functional groups on graphene

Conversion of SO₂ to SO₃ on oxygen-functionalized graphene under ambient conditions.