H3PW12O40 Grafted on CeO2: A High-Performance Catalyst for the Selective Catalytic Reduction of NOx with NH3

Polyoxometalates-Mediated Selectivity in Pt Single-Atoms on Ceria for Environmental Catalysis

Optimizing the reactivity and selectivity of single-atom catalysts (SACs) remains a crucial yet challenging issue in heterogeneous catalysis. This study demonstrates selective catalysis facilitated by a polyoxometalates-mediated electronic interaction (PMEI) in a Pt single-atom catalyst supported on CeO2 modified with Keggin-type phosphotungstate acid (HPW), labeled as Pt1/CeO2-HPW. The PMEI effect originates from the unique arrangement of isolated Pt atoms and HPW clusters on the CeO2 support. Electrons are transferred from the ceria support to the electrophilic tungsten in HPW clusters, and subsequently, Pt atoms donate electrons to the now electron-deficient ceria. This phenomenon enhances the positive charge of Pt atoms, moderating O2 activation and limiting lattice oxygen mobility compared to the conventional Pt1/CeO2 catalyst. The resulting electronic structure of Pt combined with the strong and local acidic environment of HPW on Pt1/CeO2-HPW leads to improved efficiency and N2 selectivity in the degradation of NH3 and NO, as well as increased CO2 yield when inputting volatile organic compounds. This study sheds the light on the design of SACs with balanced reactivity and selectivity for environmental catalysis.

Recent Progress on Functionalized Nanoporous Heteropoly Acids: From Synthesis to Applications

Functionalized nanoporous heteropoly acids (HPAs) have garnered significant attention in recent years due to their enhanced surface area and porosity, as well as their potential for low-cost regeneration compared to bulk materials. This review aims to provide an overview of the recent advancements in the synthesis and applications of functionalized HPAs. We begin by introducing the fundamental properties of HPAs and their unique structure, followed by a comprehensive overview of the various approaches employed for the synthesis of functionalized HPAs, including salts, anchoring onto supports, and implementing mesoporous silica sieves. The potential applications of functionalized HPAs in various fields are also discussed, highlighting their boosted performance in a wide range of applications. Finally, we address the current challenges and present future prospects in the development of functionalized HPAs, particularly in the context of mesoporous HPAs. This review aims to provide a comprehensive summary of the recent progress in the field, highlighting the significant advancements made in the synthesis and applications of functionalized HPAs.

Effect of vanadium and tungsten loading order on the denitration performance of F-doped V2O5-WO3/TiO2 catalysts

F-doped V2O5-WO3/TiO2 catalyst has been confirmed to have excellent denitration activity at low temperatures. Since the V2O5-WO3/TiO2 catalyst is a structure-sensitive catalyst, the loading order of V2O5 and WO3 may affect its denitration performance. In this paper, a series of F-doped V2O5-WO3/TiO2 catalysts with different V2O5 and WO3 loading orders were synthesized to investigate the effect of denitration performance at low temperatures. It was found that the loading orders led to significant gaps in denitration performance in the range of 120-240 °C. The results indicated loading WO3 first better utilized the oxygen vacancies on the TiF carrier promoting the generation of reduced vanadium species. In addition, loading WO3 first facilitated the dispersion of V2O5 thus enhanced the NH3 adsorption capacity of VWTiF. In situ DRIFT verified the rapid reaction between NO2, nitrate, and nitrite species and adsorbed NH3 over the VWTiF, confirming that the NH3 selective catalytic reduction (NH3-SCR) reaction over VWTiF at 240 °C proceeded by the Langmuir-Hinshelwood (L-H) mechanism. This research established the constitutive relationship between the loading order of V2O5 and WO3 and the denitration performance of the F-doped VWTi catalyst providing insights into the catalyst design process.

Remarkable N2-selectivity enhancement of NH3-SCR over HPMo modified MnCo-BTC@SiO2 catalyst

In this work, the phosphomolybdate (HPMo) modification strategy was applied to improve the N2 selectivity of MnCo-BTC@SiO2 catalyst for the selective catalytic reduction of NOx, and further, the mechanism of HPMo modification on enhanced catalytic performance was explored. Among MnCo-BTC@SiO2-x catalysts with different HPMo concentrations, MnCo-BTC@SiO2-0.75 catalyst exhibited not only the highest NH3-SCR performance (∼95% at 200-300°C) but also the best N2 selectivity (exceed 80% at 100-300°C) due to the appropriate redox capacity, greater surface acidity. X-ray photoelectron spectrometer (XPS) and temperature programmed reduction of H2 (H2-TPR) results showed that the modification with HPMo reduced the oxidation-reduction performance of the catalyst due to electron transfer from Mo5+ to Mn4+/Mn3+ and prevent the excessive oxidation of ammonia adsorption species. NH3 temperature-programmed desorption of (NH3-TPD) results showed that the modification with HPMo could significantly improve the surface acidity and NH3 adsorption, which enhance the catalytic activity and N2 selectivity. In-situ diffused reflectance infrared Fourier transform spectroscopy (in-situ DRIFTS) revealed that modification with HPMo increased significantly the amount of adsorbed NH3 species on the Bronsted acid site and CB/CL, it suppressed the production of N2O by inhibiting the production of NH species, the deep dehydrogenation of ammonia adsorption species. This study provided a simple design strategy for the catalyst to improve the low-temperature catalytic performance and N2 selectivity.

Review on magnetic adsorbents for removal of elemental mercury from coal combustion flue gas

Under the influence of human activities, atmospheric mercury (Hg) concentrations have increased by 450% compared with natural levels. In the context of the Minamata Convention on Mercury, which came into effect in August 2017, it is imperative to strengthen Hg emission controls. Existing Air Pollution Control Devices (APCDs) combined with collaborative control technology can effectively remove Hg2+ and Hgp; however, Hg0 removal is substandard. Compared with the catalytic oxidation method, Hg0 removal through adsorbent injection carries the risk of secondary release and is uneconomical. Magnetic adsorbents exhibit excellent recycling and Hg0 recovery performance and have recently attracted the attention of researchers. This review summarizes the existing magnetic materials for Hg0 adsorption and discusses the removal performances and mechanisms of iron, carbon, mineral-based, and magnetosphere materials. The effects of temperature and different flue gas components, including O2, NO, SO2, H2O, and HCl, on the adsorption performance of Hg0 are also summarized. Finally, different regeneration methods are discussed in detail. Although the research and development of magnetic adsorbents has progressed, significant challenges remain regarding their application. This review provides theoretical guidance for the improvement of existing and development of new magnetic adsorbents.

Phosphotungstic Acid-Modified MnOx for Selective Catalytic Reduction of NOx with NH3

H3PW12O40-modified MnOx catalysts (denoted as Mn-HPW) were used for NOx elimination with co-fed NH3. The optimal Mn-HPW0.02 catalyst exhibited over 90% NOx conversion at 90–270 °C. The incorporation of HPW increased the amount of Lewis acid sites of the catalyst for adsorbing NH3, and accelerated the reaction between the adsorbed NH3 species and gas-phase NOx, thus, increasing the low-temperature catalytic activity. The oxidation ability of the Mn catalyst was decreased due to the addition of HPW, thus, mitigating the overoxidation of the adsorbed NH3 species and improving the de-NOx activity and N2 selectivity in the high-temperature region. DRIFT results revealed that the NH3 species on Lewis and Brønsted acid sites, bridged nitrate, and bidentate nitrate were important species/intermediates for the reaction. NH3-SCR over the Mn and Mn-HPW0.02 catalysts obeyed the Eley–Rideal and Langmuir–Hinshelwood mechanisms, simultaneously, at 120 °C.

Catalytic performance and reaction mechanisms of NO removal with NH3 at low and medium temperatures on Mn-W-Sb modified siderite catalysts

Iron-based catalysts have been explored for selective catalytic reduction (SCR) of NO due to environmentally benign characters and good SCR activity. Mn-W-Sb modified siderite catalysts were prepared by impregnation method based on siderite ore, and SCR performance of the catalysts was investigated. The catalysts were analyzed by X-ray diffraction, H₂-temperature-programmed reduction, Brunauer-Emmett-Teller, Thermogravimetry-derivative thermogravimetry and in-situ diffused reflectance infrared Fourier transform spectroscopy (DRIFTS). The modified siderite catalysts calcined at 450°C mainly consist of Fe₂O₃, and added Mn, W and Sb species are amorphous. 3Mn-5W-1.5Sb-siderite catalyst has a wide temperature window of 180-360°C and good N₂ selectivity at low temperatures. In-situ DRIFTS results show NH₄⁺, coordinated NH₃, NH₂, NO₃^- species (bidentate), NO₂^- species (nitro, nitro-nitrito, monodentate), and adsorbed NO₂ can be discovered on the surface of Mn-W-Sb modified siderite catalysts, and doping of Mn will enhance adsorbed NO₂ formation by synergistic catalysis with Fe³⁺. In addition, the addition of Sb can inhibit sulfates formation on the surface of the catalyst in the presence of SO₂ and H₂O. Time-dependent in-situ DRIFTS studies also indicate that both of Lewis and Brønsted acid sites play a role in SCR of NO by ammonia at low temperatures. The mechanism of NO removal on the 3Mn-5W-1.5Sb-siderite catalyst can be discovered as a combination of Eley-Rideal and Langmuir-Hinshelwood mechanisms with three reaction pathways. The mechanism of NO, oxidized by synergistic catalysis of Fe³⁺ and Mn^4+/3+ to form NO₂ among three pathways, reveals the reason of high NO_x conversion of the catalyst at medium and low temperatures.

A Universal and Reversible Wet Adhesive via Straightforward Aqueous Self-Assembly of Polyethylenimine and Polyoxometalate

The excellent adhesion of mussels under wet conditions has inspired the development of numerous catechol-based wet adhesives. Nevertheless, the performance of catechol-based wet adhesive suffers from the sensitivity toward temperature, pH, or oxidation stimuli. Therefore, it is of great significance to develop non-catechol-based wet adhesives to fully recapitulate nature's dynamic function. Herein, a novel type of non-catechol-based wet adhesive is reported, which is readily formed by self-assembly of commercially available branched polyethylenimine and phosphotungstic acid in aqueous solution through the combination of electrostatic interaction and hydrogen bonding. This wet adhesive shows reversible, tunable, and strong adhesion on diverse substrates and further exhibits high efficacy in promoting biological wound healing. During the healing of the wound, the as-prepared wet adhesive also possesses inherent antimicrobial properties, thus avoiding inflammations and infections due to microorganism accumulation.

NH3-Selective Catalytic Reduction of NOx to N2 over Ceria Supported WOx Based Catalysts: Influence of Tungsten Content

A series of HPW/CeO2 catalysts generated from 12-tungstophosphoric acid, H3PW12O40 (HPW), supported on ceria and presenting different tungsten loadings (2, 4.5, 9, 16, and 40 wt% W) were prepared and characterized by N2 physisorption, XRD, IR, Raman, and UV-Vis. The different characterization techniques suggested that low loading of tungsten resulted in mainly isolated sites, while high tungsten loading produced polymeric or tungsten clusters. Those materials exhibited high activity in NH3-SCR of NOx into N2. Moreover, the series of experiments indicated that low loading in tungsten (2% HPW/CeO2) displayed the highest activity with a remarkable N2 selectivity (99%) at medium-high temperature (300–515 °C), owing to the high amount of monomeric tungstate coverage on the catalyst surface.

Improved NOx Reduction over Phosphate-Modified Fe2O3/TiO2 Catalysts Via Tailoring Reaction Paths by In Situ Creating Alkali-Poisoning Sites

The deactivation issue arising from alkali poisoning over catalysts is still a challenge for the selective catalytic reduction of NO_x by NH₃. Herein, improved NO_x reduction in the presence of alkaline metals over phosphate-modified Fe₂O₃/TiO₂ catalysts has been originally demonstrated via tailoring the reaction paths by in situ creating alkali-poisoning sites. The introduction of phosphate results in the partial formation of iron phosphate species and makes the catalyst to mainly exhibit the characteristics of FePO₄, which is responsible for the widened temperature window and enhanced alkali resistance. The tetrahedral [FeO₄]/[PO₄] structures in iron phosphate act as the Brønsted acid sites to increase the catalyst surface acidity. In addition, the formation of an Fe-O-P structure enhances the redox ability and increases surface adsorbed oxygen. Furthermore, the created phosphate groups (PO₄^3-) serving as alkali-poisoning sites preferentially combine with potassium so that iron species on the active sites are protected. Therefore, the enhanced NH₃ species adsorption capacity, improved redox ability, and active nitrate species remaining in the phosphate-modified Fe₂O₃/TiO₂ catalyst ensure the de-NO_x activity after being poisoned by alkali metals through the Langmuir-Hinshelwood reaction pathway. Hopefully, this novel strategy could provide an inspiration to design novel catalysts to control NO_x emission with extraordinary resistance to alkaline metals.

Acid modification enhances selective catalytic reduction activity and sulfur dioxide resistance of manganese-cerium-cobalt catalysts: Insight into the role of phosphotungstic acid

Improving the SO₂ resistance of catalysts is crucial to driving commercial applications of Mn-based catalysts. In this work, the phosphotungstic acid (HPW) modification strategy was applied to improve the N₂ selectivity, SO₂ and H₂O resistance of the Mn-Ce-Co catalyst, and further, the mechanism of HWP modification on enhanced catalytic performance was explored. The results showed that HPW-Mn-Ce-Co catalyst exhibits higher NO_x conversion (~100% at 100-250 °C) and N₂ selectivity (exceed 80% at 50-350 °C) due to more oxygen vacancies, greater surface acidity, and lower redox capacity. In situ diffused reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) reveal that HPW changed the reaction path of Mn-Ce-Co catalysts, promoted the adsorption and activation of NH₃, and reduced the effect of SO₂ on the active bidentate nitrate species, and thereby exhibiting good SO₂ resistance. X-ray photoelectron spectrometer (XPS) and NH₃ temperature-programmed desorption of (NH₃-TPD) results show that HPW can inhibit the formation of metal sulfate, and SO₂ can be combined with Ce species more easily. The generated Ce₂(SO₃)₃ can not only protect Mn species but also increase the acid sites and weaken the poisoning effect of metal sulfate. This study provides a simple design strategy for the catalyst to improve the low-temperature catalytic performance and toxicity resistance.

Insight into the reaction mechanism over PMoA for low temperature NH3-SCR: A combined In-situ DRIFTs and DFT transition state calculations

Phosphomolybdic acid catalyst (PMoA/TiO₂) is a promising catalyst for selective catalytic reduction of NO_x with NH₃ (NH₃-SCR) due to its strong acidity and excellent redox property. This work presents the NH₃-SCR reaction mechanism by In-situ diffuse reflectance Infrared Fourier Transform Spectroscopy (In-situ DRIFTs) and density functional theory (DFT). In-situ DRIFTs results indicated that the NH₃-SCR performance over PMoA/TiO₂ followed both Eley-Rideal (E-R) and Langmuir-Hinshelwood (L-H) mechanisms. The reaction pathway, intermediate, transition state and energy barrier over PMoA to complete NH₃-SCR reaction were calculated by DFT. The results showed that the catalytic cycle includes foundational reaction (NH₃ + NO reaction) and regenerative reaction (NH₃ + NO₂ reaction). NH₂, NH₂NO, HNNOH and HO₂NNH species were the key intermediates. In the foundational reactions, NO₂ played an important role in the removal of remaining H atoms. The NH₃ dissociation on Lewis acid site, the internal hydrogen transfer on Brønsted acid site and the formation of HO₂NNH species were the rate-controlling steps. The catalytic cycle of NH₃-SCR over PMoA consists of standard SCR and fast SCR.

Recovering gaseous Hg0 using sulfureted phosphotungstic acid modified γ-Fe2O3 from power plants burning Hg-rich coal for centralized control

Conversion of gaseous elemental mercury (Hg⁰) to particulate-bound mercury (Hg^P) by the injection of disposable sorbents or to gaseous oxidized mercury (Hg²⁺) by catalysts is not suitable to control Hg⁰ emissions from power plants burning Hg-rich coal because removed Hg may be released as secondary pollution, particularly during the employment of fly ash and desulfurization gypsum. In this study, sulfureted phosphotungstic acid modified γ-Fe₂O₃ (HPW/γ-Fe₂O₃) was employed as a magnetic and reproducible sorbent to recover gaseous Hg⁰ in coal-fired flue gas for the centralized control. Sulfureted HPW/γ-Fe₂O₃ showed an excellent capacity to enrich gaseous Hg⁰ from low concentrations to ultra-high concentrations (>10 mg m^-3), which benefited to condensing it into liquid Hg. Moreover, sulfureted HPW/γ-Fe₂O₃ exhibited excellent para-magnetism, enabling it to be magnetically reclaimed after Hg⁰ capture; this magnetization did not disappear after multiple thermal desorption of Hg⁰ due to its excellent thermal stability. Furthermore, sulfureted HPW/γ-Fe₂O₃ was regenerated by re-sulfuration without decreasing the Hg⁰ capture performance. Therefore, gaseous Hg⁰ recovery using sulfureted HPW/γ-Fe₂O₃ is a promising, economical-effective, and eco-friendly technology for the centralized control of Hg pollution emitted from power plants that burn coal with a high Hg content.

Study on heteropolyacid-based catalysts with high activity and reusability for isoprene synthesis from formaldehyde and isobutene

SiW/S-1 catalysts exhibited excellent catalytic activity and reusability due to the highly dispersed SiW and MFI structure of S-1.

A promising Al-CeZrO4/HPW-incorporated SPEEK composite membrane with improved proton conductivity and chemical stability for PEM fuel cells

An improved sulfonated poly (ether ether ketone) (SPEEK) nanocomposite membrane was prepared by incorporating both phosphotungstic acid (HPW) and Al doped cerium-based oxides (Al-CeZrO4) in SPEEK matrix. The HPW was immobilized by Al-CeZrO4 so that firmly dispersed acid–base pairs were formed. The introduction of Al-CeZrO4 helped improve the chemical stability of the pristine (baseline) SPEEK membrane without compromising the conductivity, and the addition of HPW further enhanced the conduction of protons through acid–base interactions. Stability tests showed that when the SPEEK/Al-CeZrO4 nanocomposite membrane was immersed in a Fenton’s solution for 108 h at 80°C, a loss of 34.9% in proton conductivity was observed, which is 24.1% less than that of the pristine SPEEK membrane, indicating that the attenuation of membrane proton conductivity was inhibited. At the same time, the proton conductivity of the SPEEK/Al-CeZrO4/HPW nanocomposite membrane (that has already incorporated HPW) was increased by 15.5% compared to the SPEEK/Al-CeZrO4 nanocomposite membrane. Hence, Al-CeZrO4/HPW is considered as an effective inorganic nanofiller for improving both proton conductivity and chemical stability of SPEEK membranes, and the hybrid composite membrane is worth further studying.

Alkali and Phosphorus Resistant Zeolite-like Catalysts for NOx Reduction by NH3

At present, the deactivation of selective catalytic reduction (SCR) catalysts caused by the coexistence of alkali metal and phosphorus (P) remains an urgent problem and lacks corresponding strategies against catalyst poisoning. Herein, a novel zeolite-like Ce-Si₅Al₂O_x catalyst derived from an ultrasmall nanozeolite EMT precursor was synthesized without organic templates at ambient temperature. This catalyst was able to maintain above 95% NO_x conversion in the 270-540 °C temperature range. Moreover, 1 wt % potassium (K) and 5 wt % P loading had no influence on the SCR performance of the Ce-Si₅Al₂O_x catalyst at 300-480 °C. It was demonstrated that cerium (Ce) was highly dispersed in the amorphous aluminum (Al) silicate derived from EMT zeolites and expressed high catalytic performance. Besides, a large number of acid sites were reserved to absorb ammonia allowing effective participation in the SCR reaction and capturing alkali metals, thus improving the SCR performance and K resistance. Additionally, the strong interaction between Ce and aluminosilicate decreased cerium phosphate production, preventing deactivation of the catalysts. Thus, this novel low-cost zeolite-like Ce-Si₅Al₂O_x catalyst with a highly active ion-exchanged metal phase and abundant surface acid sites paves a way for designing new efficient and poisoning-resistant SCR catalysts for practical applications.

Significant Enhancement of Gaseous Elemental Mercury Recovery from Coal-Fired Flue Gas by Phosphomolybdic Acid Grafting on Sulfurated γ-Fe2O3: Performance and Mechanism

The existing technologies to control Hg emissions from coal-fired power plants can be improved to achieve the centralized control of Hg⁰ emissions, which continue to pose a risk of Hg exposure to human populations. In this work, MoS_x@γ-Fe₂O₃, formed by the sulfuration of phosphomolybdic acid (HPMo)-grafted γ-Fe₂O₃, was developed as a magnetic and regenerable sorbent to recover gaseous Hg⁰ from coal-fired flue gas as a cobenefit to the use of wet electrostatic precipitators. The thermal stability of γ-Fe₂O₃ was notably enhanced by HPMo grafting; thus, the magnetization of MoS_x@γ-Fe₂O₃ hardly decreased during the application. The kinetic analysis indicates that the chemical adsorption of gaseous Hg⁰ was mainly dependent on the amounts of surface S₂^2- and surface adsorption sites. Although the amount of S₂^2- on sulfurated γ-Fe₂O₃ decreased after HPMo grafting, the amount of surface adsorption sites significantly increased due to the formation of a layered MoS_x structure on the surface. Therefore, the ability of sulfurated γ-Fe₂O₃ to capture Hg⁰ was improved considerably after HPMo grafting. Furthermore, low concentrations of gaseous Hg⁰ in coal-fired flue gas can be gradually enriched by at least 1000 times by MoS_x@γ-Fe₂O₃, which facilitates the recovery and centralized control of gaseous Hg⁰ in flue gas.

Influence of the addition of vanadium to Pt/TiO2 catalyst on the selective catalytic oxidation of NH3 to N2

In this work, the effect of the addition of vanadium to the Pt/TiO₂ catalyst on the selective catalytic oxidation (SCO) of NH₃ to N₂ was investigated. It was found that the addition of vanadium significantly enhanced catalytic activity at all tested temperatures. The Pt/V/TiO₂ catalyst exhibited the highest NH₃ conversion (∼100%) and NH₃ to N₂ conversion (∼81%) at 250°C. The physicochemical characteristics of the catalysts were investigated via Brunauer-Emmett-Teller (BET) analysis, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), NH₃ temperature-programmed desorption (TPD), NH₃ temperature-programmed oxidation (TPO), and in situ Fourier-transform infrared (FTIR) spectroscopy. It was found that the addition of V to the catalyst enhanced the conversion of NH₃ as a result of the formation of new acid sites. The increase in the number of acid sites resulted in increased NH₃ to N₂ conversion via the internal selective catalytic reduction (i-SCR) mechanism. This mechanism involves the SCR of NO_x, which are formed by the oxidation of NH₃. Based on experimental results and analyses of the catalysts modified by the addition of V, it was shown that there was a close relation between reaction selectivity and the surface oxygen species of the catalyst and N₂ yield. Furthermore, the addition of V increased the durability of SO₂ by inhibiting the formation of ammonium bisulfate (ABS).

Surface acidity enhancement of CeO2 catalysts via modification with a heteropoly acid for the selective catalytic reduction of NO with ammonia

The surface acidity enhanced CeO₂ catalysts exhibited a SCR reaction path that both mechanisms existed but E-R dominated.