Exploring the Impact of Oxygen Vacancies in Co/Pr-CeO2 Catalysts on H2 Production via the Water-Gas Shift Reaction

In this study, we utilized various Pr-doped CeO2 catalysts (Pr=5, 10, 20, and 30 wt.%) as a support medium for the dispersion of cobalt (Co) nanoparticles, aiming to investigate the impact of oxygen vacancies on the water-gas shift (WGS) reaction. Different characterization techniques were employed to understand the insights into the structure-activity relationship governing the performance of Pr doped ceria supported Co catalysts towards WGS reaction. Our findings reveal that Co/Pr-CeO2 catalysts at optimum Pr loading (10 wt.%) exhibit a superior CO conversion (88 %) facilitated by the presence of more oxygen vacancies induced by Pr doping into the CeO2 lattice, as opposed to the performance of the pure Co/CeO2 catalytic system. It was also found that the highest activity was obtained at increased intrinsic oxygen vacancies and strong synergy between Co and Pr/CeO2 support, fostering more favorable CO activation at the interfacial sites, thus accounting for the observed enhanced activity.

Efficient removal of Hg0 from cement kiln flue gas using CexFeyOz composite catalyst

In this study, a kind of CexFeyOz composite with oxygen vacancy structure and strong oxygen storage capacity was prepared by coprecipitation method. Under the condition of no HCl of flue gas, the Hg0 in the flue gas of cement kiln was efficiently and economically removed by using 6-8% oxygen. The results showed that the optimum preparation conditions of the catalyst were Ce-Fe molar ratio of 1-11 and calcination temperature of 550 °C. In addition, the reaction temperature, space velocity, the concentration of O2, SO2, and NO had significant effects on the removal efficiency of Hg0 at different rates. More precisely, at the reaction temperature of 350 °C, low airspeed, high concentration of O2, and low concentration of SO2 and NO, the efficiency reached the highest value. According to XPS results, the elemental valence of the CexFeyOz composite changed after the reaction. The redox pairs of Ce3+-Ce4+ and Fe3+-Fe2+ had the ability to transfer electrons, which enabled more oxygen adsorbed on the catalyst surface to be converted into O2-, leading to the improvement of the oxidation efficiency of Hg0.

Engineering of oxygen vacancy defect in CeO2 through Mn doping for toluene catalytic oxidation at low temperature

Catalytic oxidation is considered a highly effective method for the elimination of volatile organic compounds. Oxygen vacancy defect engineering in a catalyst is considered an effective approach for high-performance catalysts. Herein, a series of doped Mn_xCe_1-xO₂ catalysts (x = 0.05-0.2) with oxygen vacancy defects were synthesized by doping low-valent Mn in a CeO₂ lattice. Different characterization techniques were utilized to inspect the effect of doping on oxygen vacancy defect generation. The characterization results revealed that the Mn_0.15Ce_0.85O₂ catalyst has the maximum oxygen vacancy concentration, leading to increased active oxygen species and enhanced oxygen mobility. Thus, Mn_0.15Ce_0.85O₂ catalyst showed an excellent toluene oxidation activity with 90% toluene conversion temperature (T₉₀) of 197 °C at a weight hourly space velocity of 40,000 mL g^-1 h^-1 as compared to undoped CeO₂ (T₉₀ = 225 °C) and Ce based oxides in previous reports. In addition, the Mn_0.15Ce_0.85O₂ catalyst displayed strong recyclability, water resistant ability and long-time stability. The in situ DRIFT results showed that the Mn_0.15Ce_0.85O₂ catalyst has a robust oxidation capability as toluene is quickly adsorbed and actuated as compared to CeO₂. Thus, the present work lays the foundation for designing a highly active catalyst for toluene elimination from the environment.

Effecting pattern study of SO2 on Hg0 removal over α-MnO2 in-situ supported magnetic composite

α-MnO₂ was in-situ supported onto silica coated magnetite nanoparticles (MagS-Mn) to study the adsorption and oxidation of Hg⁰ as well as the effecting patterns of SO₂ and O₂ on Hg⁰ removal. MagS-Mn showed Hg⁰ removal capacity of 1122.6 μg/g at 150 °C with the presence of SO₂. Hg⁰ adsorption and oxidation efficiencies were 2.4% and 90.6%, respectively. Hg⁰ removal capability deteriorated at elevated temperatures. Surface oxygen and manganese chemistry analysis indicated that SO₂ inhibited the Hg⁰ removal through consumption of adsorbed oxygen and reduction of high valence manganese. This inhibiting effect was observed to be counteracted by O₂ at lower temperatures. O₂ tended to compete with SO₂ for active sites and further create additional adsorbed oxygen sites for Hg⁰ surface reaction via surface dissociative adsorption rather than replenish the active sites consumed by SO₂. The high valence manganese was also preserved by O₂ which was essential to Hg⁰ oxidation. The intervention of O₂ in the inhibition of SO₂ on Hg⁰ removal was weakened at temperatures higher than 250 °C. Aa a result, Hg⁰ tends to be catalytic oxidized in the condition of low reaction temperatures and with the presence of O₂ over α-MnO₂ oriented composites.

Preparation and Performance of Carbon-Based Ce-Mn Catalysts for Efficient Degradation of Acetone at Low Temperatures

Based on the porous carbon material from citric acid residue, catalysts of different Ce-Mn ratios were prepared with incipient-wetness impregnation (IWI) to delve into their acetone-degrading performance and relevant mechanisms. When the Ce-Mn molar ratio is 0.8, the prepared catalyst Ce_0.8-Mn/AC shows abundant and uniformly dispersed Mn and Ce particles on the surface. The content of Mn and Ce on the Ce_0.8-Mn/AC surface reaches 5.64% and 0.75%, respectively. At the acetone concentration of 238 mg/m³ (100 ppm), the laws of acetone degradation in different catalysts at different catalyzing temperatures and with various oxygen concentrations were studied, and we found that the rate of acetone degradation by Ce_0.8-Mn/AC can exceed 90% at 250 °C. Cerium oxide and manganese oxide are synergistic in the catalytic degradation of acetone. Adding cerium to manganese-based catalysts can increase the oxygen migration rate in the catalysts and thus raise the reduction rate of lattice oxygen in manganese oxide. The results offer new ideas and approaches for the efficient and comprehensive utilization of bio-fermentation by-products, and for the development of cheap and high degradation performance catalysts for acetone.

Ultra-high adsorption of Hg0 using impregnated activated carbon by selenium

Activated carbon was one of the main adsorptions utilized in elemental mercury (Hg⁰) removal from coal combustion flue gas. However, the high cost and low physical adsorption efficiency of activated carbon injection (ACI) limited its application. In this study, an ultra-high efficiency (nearly 100%) catalyst sorbent-Se_x/Activated carbon (Se_x/AC) was synthesized and applied to remove Hg⁰ in the simulated flue gas, which exhibited 120 times outstanding adsorption performance versus the conventional activated carbon. The Se_x/AC reached 17.98 mg/g Hg⁰ adsorption capacity at 160 °C under the pure nitrogen atmosphere. Moreover, it maintained an excellent mercury adsorption tolerance, reaching the efficiency of Hg⁰ removal above 85% at the NO and SO₂ conditions in a bench-scale fixed-bed reactor. Characterized by the multiple methods, including BET, XRD, XPS, kinetic and thermodynamic analysis, and the DFT calculation, we demonstrated that the ultrahigh mercury removal performance originated from the activated Se species in Se_x/AC. Chemical adsorption plays a dominant role in Hg⁰ removal: Selenium anchored on the surface of AC would capture Hg⁰ in the flue gas to form an extremely stable substance-HgSe, avoiding subsequent Hg⁰ released. Additionally, the oxygen-containing functional groups in AC and the higher BET areas promote the conversion of Hg⁰ to HgO. This work provided a novel and highly efficient carbon-based sorbent -Se_x/AC to capture the mercury in coal combustion flue gas. Graphical abstract Selenium-modified porous activated carbon and the interface functional group promotes the synergistic effect of physical adsorption and chemical adsorption to promote the adsorption capacity of Hg⁰.

A critical review on relationship of CeO2-based photocatalyst towards mechanistic degradation of organic pollutant

Nanostructured photocatalysts commonly offered opportunities to solve issues scrutinized with the environmental challenges caused by steep population growth and rapid urbanization. This photocatalyst is a controllable characteristic, which can provide humans with a clean and sustainable ecosystem. Over the last decades, one of the current thriving research focuses on visible-light-driven CeO₂-based photocatalysts due to their superior characteristics, including unique fluorite-type structure, rigid framework, and facile reducing oxidizing properties of cerium's tetravalent (Ce⁴⁺) and trivalent (Ce³⁺) valence states. Notwithstanding, owing to its inherent wide energy gap, the solar energy utilization efficiency is low, which limits its application in wastewater treatment. Numerous modifications of CeO₂ have been employed to enhance photodegradation performances, such as metals and non-metals doping, adding support materials, and coupling with another semiconductor. Besides, all these doping will form a different heterojunction and show a different way of electron-hole migration. Compared to conventional heterojunction, advanced heterojunction types such as p-n heterojunction, Z-scheme, Schottky junction, and surface plasmon resonance effect exhibit superior performance for degradation owing to their excellent charge carrier separation, and the reaction occurs at a relatively higher redox potential. This review attends to providing deep insights on heterojunction mechanisms and the latest progress on photodegradation of various contaminants in wastewater using CeO₂-based photocatalysts. Hence, making the CeO₂ photocatalyst more foresee and promising to further development and research.

Mercury-bearing wastes: Sources, policies and treatment technologies for mercury recovery and safe disposal

Due to the lenient environmental policies in developing economies, mercury-containing wastes are partly produced as a result of the employment of mercury in manufacturing and consumer products. Worldwide, the presence of mercury as an impurity in several industrial processes leads to significant amounts of contaminated waste. The Minamata Convention on Mercury dictates that mercury-containing wastes should be handled in an environmentally sound way according to the Basel Convention Technical Guidelines. Nevertheless, the management policies differ a great deal from one country to another because only a few deploy or can afford to deploy the required technology and facilities. In general, elemental mercury and mercury-bearing wastes should be stabilized and solidified before they are disposed of or permanently stored in specially engineered landfills and facilities, respectively. Prior to physicochemical treatment and depending on mercury's concentration, the contaminated waste may be thermally or chemically processed to reduce mercury's content to an acceptable level. The suitability of the treated waste for final disposal is then assessed by the application of standard leaching tests whose capacity to evaluate its long-term behavior is rather questionable. This review critically discusses the main methods employed for the recovery of mercury and the treatment of contaminated waste by analyzing representative examples from the industry. Furthermore, it gives a complete overview of all relevant issues by presenting the sources of mercury-bearing wastes, explaining the problems associated with the operation of conventional discharging facilities and providing an insight of the disposal policies adopted in selected geographical regions.

Mercury in natural gas streams: A review of materials and processes for abatement and remediation

The role of natural gas in mitigating greenhouse gas emissions and advancing renewable energy resource integration is undoubtedly critical. With the progress of hydrocarbons exploration and production, the target zones become deeper and the possibility of mercury contamination increases. This impacts on the industry from health and safety risks, due to corrosion and contamination of equipment, to catalyst poisoning and toxicity through emissions to the environment. Especially mercury embrittlement, being a significant problem in LNG plants using aluminum cryogenic heat exchangers, has led to catastrophic plant incidents worldwide. The aim of this review is to critically discuss the conventional and alternative materials as well as the processes employed for mercury removal during gas processing. Moreover, comments on studies examining the geological occurrence of mercury species are included, the latest developments regarding the detection, sampling and measurement are presented and updated information with respect to mercury speciation and solubility is displayed. Clean up and passivation techniques as well as disposal methods for mercury-containing waste are also explained. Most importantly, the environmental as well as the health and safety implications are addressed, and areas that require further research are pinpointed.

A novel catalytic oxidation process for removing elemental mercury by using diperiodatoargentate(III) in the catalysis of trace ruthenium(III)

A series of experiments were conducted at a bench scale reactor to investigate the effects of key influencing factors on the Hg⁰ removal from flue gas using the prepared diperiodatoargentate (III) (DPA) as an oxidant, trace ruthenium(III) as a catalyst, respectively. The experimental results showed that the average Hg⁰ removal efficiency reached to 87.5% under the optimal conditions in which the DPA concentration was 1.03 mmol/L, catalyst concentration was 2.0 μmol/L, reaction temperature was 40 °C and solution pH was 8.5. Meanwhile, it was found from the experiments that the high concentrations of SO₂ and NO could inhibit the Hg⁰ removal due to the competition between Hg⁰ and SO₂/NO, while the lower NO concentration exhibited a slight promotion for Hg⁰ removal. The evolutions of DPA(III) and Ru(III) before and after the reaction were characterized by an ultraviolet visible spectrophotometer (UV-vis), from which, the promotional mechanism of Ru(III) on Hg⁰ removal was analyzed. The spent solution was analyzed by a cold vapor atomic fluorescence spectrometer (CVAFS), which verified that Hg⁰ was oxidized into Hg²⁺ by the catalytic system of DPA(III)-Ru(III), and DPA was converted into Ag⁺.

Functionalized magnetic mesoporous silica/poly(m-aminothiophenol) nanocomposite for Hg(II) rapid uptake and high catalytic activity of spent Hg(II) adsorbent

Currently, magnetic mesoporous silica nanospheres have been employed widely as adsorbents due to their large surface area and easy recovery. Herein, the functionalized magnetic mesoporous silica/organic polymers nanocomposite (MMSP) was fabricated by the grafted poly(m-aminothiophenol) embedded the aminated magnetic mesoporous silica nanocomposite based on Fe₃O₄ magnetic core, which was shelled by mesoporous silica and further modified by (3-aminopropyl) triethoxysilane. The adsorption properties of as-developed MMSP were systematically explored by altering the experimental parameters. The results indicated that the adsorption capacity and removal percentage of the MMSP could reach 243.83 mg/g and 97.53% within only 10 min at pH 4.0, and the coexisting ions had no significant effect on the selective Hg(II) ions removal from aqueous solutions, meanwhile, the adsorbent recovered by a magnet still exhibited good adsorption performance after recycled 5 times. In addition, by analyzing experimental data, the adsorption process of Hg(II) ions belonged to spontaneous exothermic adsorption, and the possible adsorption mechanisms were proposed based on the pseudo-second-order model and Langmuir model. After adsorption study, the waste material adsorbed Hg(II) was developed as an efficient catalyst for transformation of phenylacetylene to acetophenone with yield of 97.06%. In this study, we designed an efficient and selective material for Hg(II) ions remove and provided a treatment of the post-adsorbed mercury adsorbent by converting the waste into an excellent catalyst, which reduced the economic and environmental impact from conventional adsorption techniques.

CeO2-Decorated ?-MnO2 Nanotubes: A Highly Efficient and Regenerable Sorbent for Elemental Mercury Removal from Natural Gas

CeO₂ nanoparticle-decorated ?-MnO₂ nanotubes (NTs) were prepared and tested for elemental mercury (Hg⁰) vapor removal in simulated natural gas mixtures at ambient conditions. The composition which had the largest surface area and a relative Ce/Mn atomic weight ratio of around 35% exhibited a maximum Hg⁰ uptake capacity exceeding 20 mg?g^?1 (2 wt %), as determined from measurements of mercury breakthrough which corresponded to 99.5% Hg⁰ removal efficiency over 96 h of exposure. This represents a significant improvement in the activity of pure metal oxides. Most importantly, the composite nanosorbent was repeatedly regenerated at 350 ?C and retained the 0.5% Hg⁰ breakthrough threshold. It was projected to be able to sustain 20 regeneration cycles, with the presence of acid gases, CO₂, and H₂S, not affecting its performance. This result is particularly important, considering that pure CeO₂ manifests rather poor activity for Hg⁰ removal at ambient conditions, and hence, a synergistic effect in the composite nanomaterial was observed. This possibly results from the addition of facile oxygen vacancy formation at ?-MnO₂ NTs and the increased amount of surface-adsorbed oxygen species.

Peroxymonosulfate catalyzed by rGO assisted CoFe2O4 catalyst for removing Hg0 from flue gas in heterogeneous system

The cobalt ferrite-reduced oxidized graphene (CoFe₂O₄/rGO) catalyst was synthesized by hydrothermal method and characterized by Powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Scanning electron microscope (SEM), Brunauere Emmette Teller (BET) and Hysteresis loop. For developing a new method of removing elemental mercury (Hg⁰) from flue gas, the effects of catalyst dosage, PMS concentration, solution pH and reaction temperature on the removal efficiency were investigated experimentally by using peroxymonosulfate (PMS) catalyzed by CoFe₂O₄/rGO at a self-made bubbling reactor. The average removal efficiency of Hg⁰ in a 30-min period reached 95.56%, when CoFe₂O₄/rGO dosage was 0.288 g/L, PMS concentration was 3.5 mmol/L, solution pH was 5.5 and reaction temperature was 55 °C. Meanwhile, based on the free radical quenching experiments, in which, ethyl alcohol and tert butyl alcohol were used as quenchers to prove indirectly the presence of ^•OH and SO₄^•-, the characterizations of catalysts and reaction products, and the existing results from other scholars. The reaction mechanism was proposed.

Selenium Functionalized Metal-Organic Framework MIL-101 for Efficient and Permanent Sequestration of Mercury

Abatement of mercury emission from coal-fired power plants remains a serious task for public health and environmental societies. Selenium functionalized metal-organic framework MIL-101 (Se/MIL-101) was prepared for mercury removal from power plants. The Se/MIL-101 exhibited a remarkable mercury adsorption capacity of 148.19 mg·g^-1, which was about 154 to 705 times larger than that of commercial activated carbons exclusively applied for mercury removal from power plants. The initial mercury adsorption rate for Se/MIL-101 reached up to 44.8 μg·g^-1·min^-1, which was 89- to 1659-fold higher than those of mercury sorbents reported in the literature. The Se/MIL-101 maintained an excellent mercury adsorption stability under simulated flue gas atmosphere containing SO₂, NO, and H₂O. Gaseous elemental mercury (Hg⁰) converted on the Se/MIL-101 to stable and water-insoluble mercury selenide (HgSe), which guaranteed a minimum re-emission even sequestration of mercury. Moreover, the mercury-laden Se/MIL-101 could also immobilize mercury in gypsum and efficiently capture mercury ions from desulfurization effluent to an undetectable level (<0.0035 μg·L^-1). With these advantages, Se/MIL-101 appears to be a promising material for efficient and permanent sequestration of mercury from power plants.

Density Functional Theory Study on Mechanism of Mercury Removal by CeO2 Modified Activated Carbon

Doping of CeO2 on activated carbon (AC) can promote its performance for mercury abatement in flue gas, while the Hg0 removal mechanism on the AC surface has been rarely reported. In this research, density functional theory (DFT) calculations were implemented to unveil the mechanism of mercury removal on plain AC and CeO2 modified AC (CeO2-AC) sorbents. Calculation results indicate that Hg0, HCl, HgCl and HgCl2 are all chemisorbed on the adsorbent. Strong interaction and charge transfer are shown by partial density of states (PDOS) analysis of the Hg0 adsorption configuration. HCl, HgCl and HgCl2 can be dissociatively adsorbed on the AC model and subsequently generate HgCl or HgCl2 released to the gas phase. The adsorption energies of HgCl and HgCl2 on the CeO2-AC model are relatively high, indicating a great capacity for removing HgCl and HgCl2 in flue gas. DFT calculations suggest that AC sorbents exhibit a certain catalytic effect on mercury oxidation, the doping of CeO2 enhances the catalytic ability of Hg0 oxidation on the AC surface and the reactions follow the Langmuir–Hinshelwood mechanism.

Application of Plasma Treatment in Preparation of Soybean Oil Factory Sludge Catalyst and Its Application in Selective Catalytic Oxidation (SCO) Denitration

At present, the most commonly used denitration process is the selective catalytic reduction (SCR) method. However, in the SCR method, the service life of the catalyst is short, and the industrial operation cost is high. The selective catalytic oxidation absorption (SCO) method can be used in a low temperature environment, which greatly reduces energy consumption and cost. The C/N ratio of the sludge produced in the wastewater treatment process of the soybean oil plant used in this paper is 9.64, while the C/N ratio of the sludge produced by an urban sewage treatment plant is 10⁻20. This study shows that the smaller the C/N ratio, the better the denitration efficiency of the catalyst. Therefore, dried oil sludge is used as a catalyst carrier. The influence of different activation times, and LiOH concentrations, on catalyst activity were investigated in this paper. The denitration performance of catalysts prepared by different activation sequences was compared. The catalyst was characterized by Fourier Transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and scanning electron microscope (SEM). The experimental results showed that: (1) When the concentration of the LiOH solution used for activation is 15%, and the activation time is four hours, the denitration effect of the catalyst is the best; (2) the catalyst prepared by activation before plasma roasting has the best catalytic activity.

Support effect of Mn-based catalysts for gaseous elemental mercury oxidation and adsorption

Mn-Based catalysts with a Mn loading of 4 wt% were prepared using an impregnation method.

Nanoscale Cobalt-Manganese Oxide Catalyst Supported on Shape-Controlled Cerium Oxide: Effect of Nanointerface Configuration on Structural, Redox, and Catalytic Properties

Understanding the role of nanointerface structures in supported bimetallic nanoparticles is vital for the rational design of novel high-performance catalysts. This study reports the synthesis, characterization, and the catalytic application of Co-Mn oxide nanoparticles supported on CeO₂ nanocubes with the specific aim of investigating the effect of nanointerfaces in tuning structure-activity properties. High-resolution transmission electron microscopy analysis reveals the formation of different types of Co-Mn nanoalloys with a range of 6 ± 0.5 to 14 ± 0.5 nm on the surface of CeO₂ nanocubes, which are in the range of 15 ± 1.5 to 25 ± 1.5 nm. High concentration of Ce³⁺ species are found in Co-Mn/CeO₂ (23.34%) compared with that in Mn/CeO₂ (21.41%), Co/CeO₂ (15.63%), and CeO₂ (11.06%), as evidenced by X-ray photoelectron spectroscopy (XPS) analysis. Nanoscale electron energy loss spectroscopy analysis in combination with XPS studies shows the transformation of Co²⁺ to Co³⁺ and simultaneously Mn^4+/3+ to Mn²⁺. The Co-Mn/CeO₂ catalyst exhibits the best performance in solvent-free oxidation of benzylamine (89.7% benzylamine conversion) compared with the Co/CeO₂ (29.2% benzylamine conversion) and Mn/CeO₂ (82.6% benzylamine conversion) catalysts for 3 h at 120 °C using air as the oxidant. Irrespective of the catalysts employed, a high selectivity toward the dibenzylimine product (97-98%) was found compared with the benzonitrile product (2-3%). The interplay of redox chemistry of Mn and Co at the nanointerface sites between Co-Mn nanoparticles and CeO₂ nanocubes as well as the abundant structural defects in cerium oxide plays a key role in the efficiency of the Co-Mn/CeO₂ catalyst for the aerobic oxidation of benzylamine.

Effect of the properties of MnOx/activated carbon and flue gas components on Hg0 removal at low temperature

Proposed removal mechanisms of Hg⁰ under a simulated multi-component gas over the Mn/AC sorbent.

Fe and Co modified vanadium–titanium steel slag as sorbents for elemental mercury adsorption

The cooperation of Fe and Co was beneficial to the Hg⁰ removal and the main active sites were Co.

Enhancement of Ce1−xSnxO2 support in LaMnO3 for the catalytic oxidation and adsorption of elemental mercury

Mn-based perovskite oxide was used as the active site for elemental mercury (Hg⁰) removal from coal-fired flue gas.

Ceria–zirconia modified MnOx catalysts for gaseous elemental mercury oxidation and adsorption

The developed ceria–zirconia modified MnO_x catalysts were found to exhibit enhanced Hg⁰ oxidation and removal performance.