Self-Defense Effects of Ti-Modified Attapulgite for Alkali-Resistant NOx Catalytic Reduction

Highly efficient and selective removal of anionic dyes from aqueous solutions using polyacrylamide/peach gum polysaccharide/attapulgite composite hydrogels with positively charged hybrid network

To avoid the weakness (lower adsorption rate and selectivity) of peach gum polysaccharide (PGP) and improve the adsorption performance of polyacrylamide (PAAm) hydrogel (lower adsorption capacity), in the present work, the PGP was chemically tailored to afford ammoniated PGP (APGP) and quaternized PGP (QPGP), and attapulgite (ATP) was bi-functionalized with cation groups and carbon‑carbon double bond. Then, PAAm/APGP and PAAm/QPGP/ATP hydrogels were synthesized via redox polymerization. The synthesis procedure and properties of hydrogels were traced by FTIR, SEM, XPS, TGA, TEM, and BET methods, and the dye adsorption performance of the hydrogels was evaluated using the new coccine (NC) and tartrazine (TTZ) aqueous solutions as the model anionic dyes. Effects of initial dye concentration, pH, and ionic strength on the adsorption were investigated. Compared with PAAm/APGP hydrogel, PAAm/APGP/ATP hydrogel exhibits higher adsorption rate, superior adsorption capacity, stability, and selectivity towards anionic dye. The adsorption process of PAAm/QPGP/ATP hydrogel reached equilibrium in about 20 min and followed the pseudo-second-order kinetic model and Langmuir isotherm. The adsorption capacities towards NC and TTZ of PAAm/QPGP/ATP hydrogel were calculated as 873.235 and 731.432 mg/g. This hydrogel adsorbent originating from PAAm, PGP, and ATP shows great promise for application in practical water treatment.

Strong metal oxide-zeolite interactions during selective catalytic reduction of nitrogen oxides

In response to the stricter EU VII emission standards and the "150 ℃ challenge", selective catalytic reduction by ammonia (NH3-SCR) catalysts for motor vehicles are required to achieve high NO conversion below 200 °C. Compounding metal oxides with zeolites is an important strategy to design the low-temperature SCR catalysts. Here, we original prepared Cu-SSZ-13 @ MnGdOx (Cu-Z @ MGO), which achieved over 90% NO conversion and 95% N2 selectivity at 150 ℃. It has been demonstrated that a uniform mesoporous loaded layer of MGO grows on Cu-Z, and a recrystallization zone appears at the MGO-Cu-Z interface. We discover that the excellent low-temperature SCR activity derives from the strong metal oxide-zeolite interaction (SMZI) effects. The SMZI effects cause the anchor and high dispersion of MGO on the surface of Cu-Z. Driven by the SMZI effects, the Mn3+/Mn4+ redox cycle ensures the low and medium temperature-SCR activity and the Cu2+/Cu+ redox cycle guarantees the medium and high temperature-SCR activity. The introduction of MGO improves the reaction activity of -NH2 species adsorbed at Mn sites at 150 ℃, achieving a cycle of reduction and oxidation reactions at low temperatures. This strategy of inducing SMZI effects of metal oxides and zeolites paves a way for development of high-performance catalysts.

Ultrahighly Alkali-Tolerant NOx Reduction over Self-Adaptive CePO4/FePO4 Catalysts

Catalyst deactivation caused by alkali metal poisoning has long been a key bottleneck in the application of selective catalytic reduction of NOx with NH3 (NH3-SCR), limiting the service life of the catalyst and increasing the cost of environmental protection. Despite great efforts, continuous accumulation of alkali metal deposition makes the resistance capacity of 2 wt % K2O difficult to enhance via merely loading acid sites on the surface, resulting in rapid deactivation and frequent replacement of the NH3-SCR catalyst. To further improve the resistance of alkali metals, encapsulating alkali metals into the bulk phase could be a promising strategy. The bottleneck of 2 wt % K2O tolerance has been solved by virtue of ultrahigh potassium storage capacity in the amorphous FePO4 bulk phase. Amorphous FePO4 as a support of the NH3-SCR catalyst exhibited a self-adaptive alkali-tolerance mechanism, where potassium ions spontaneously migrated into the bulk phase of amorphous FePO4 and were anchored by PO43- with the generation of Fe2O3 at the NH3-SCR reaction temperature. This ingenious potassium storage mechanism could boost the K2O resistance capacity to 6 wt % while maintaining approximately 81% NOx conversion. Besides, amorphous FePO4 also exhibited excellent resistance to individual and coexistence of alkali (K2O and Na2O), alkali earth (CaO), and heavy metals (PbO and CdO), providing long durability for CePO4/FePO4 catalysts in flue gas with multipollutants. The cheap and accessible amorphous FePO4 paves the way for the development and implementation of poisoning-resistant NOx abatement.

Revealing the Dynamic Behavior of Active Sites on Acid-Functionalized CeO2 Catalysts for NOx Reduction

Unraveling the dynamics of the active sites upon CeO₂-based catalysts in selective catalytic reduction of nitrogen oxides by ammonia (NH₃-SCR) is challenging. In this work, we prepared tungsten-acidified and sulfated CeO₂ catalysts and used operando spectroscopy to reveal the dynamics of acid sites and redox sites on catalysts during NH₃-SCR reaction. We found that both Lewis and Brønsted acid sites are needed to participate in the catalytic reaction. Notably, Brønsted acid sites are the main active sites after a tungsten-acidified or sulfated treatment, and the change of Brønsted acid sites significantly affects the NO_x removal. Moreover, acid functionalization promotes the cerium species cycle between Ce⁴⁺ and Ce³⁺ for the NO_x reduction. This work is critical to deeply understanding the natural properties of active sites, and it also provides new insights into the mechanism for NH₃-SCR over CeO₂-based catalysts.

Improved N2 Selectivity of MnOx Catalysts for NOx Reduction by Engineering Bridged Mn3+ Sites

Mn-based catalysts are promising for selective catalytic reduction (SCR) of NO_x with NH₃ at low temperatures due to their excellent redox capacity. However, the N₂ selectivity of Mn-based catalysts is an urgent problem for practical application owing to excessive oxidizability. To solve this issue, we report a Mn-based catalyst using amorphous ZrTiO_x as the support (Mn/ZrTi-A) with both excellent low-temperature NO_x conversion and N₂ selectivity. It is found that the amorphous structure of ZrTiO_x modulates the metal-support interaction for anchoring the highly dispersed active MnO_x species and constructs a uniquely bridged Mn³⁺ bonded with the support through oxygen linked to Ti⁴⁺ and Zr⁴⁺, respectively, which regulates the optimal oxidizability of the MnO_x species. As a result, Mn/ZrTi-A is not conducive to the formation of ammonium nitrate that readily decomposes to N₂O, thus further increasing N₂ selectivity. This work investigates the role of an amorphous support in promoting the N₂ selectivity of a manganese-based catalyst and sheds light on the design of efficient low-temperature deNO_x catalysts.

Efficient NOx Reduction against Alkali Poisoning over a Self-Protection Armor by Fabricating Surface Ce2(SO4)3 Species: Comparison to Commercial Vanadia Catalysts

Resolving severe deactivation by alkali metals for selective catalytic reduction of NO_x with NH₃ (NH₃-SCR) is challenging. Herein, surface Ce₂(SO₄)₃ species as a self-protection armor originally exhibited antipoisoning of potassium over ceria-based catalysts. The self-protection armor was also effective for other alkali (Na), alkali-earth (Ca), and heavy (Pb) metals, considerably resolving the deactivation of ceria-based SCR catalysts in practical applications. The catalytic activity tests indicated that the presence of ∼0.8 wt % potassium did not deactivate sulfated CeO₂ catalysts, yet commercial V₂O₅-WO₃/TiO₂ catalysts almost lost the NO_x conversions. Potassium preferably bonded with surface sulfates to form K₂SO₄ accompanied with the majority of surface Ce₂(SO₄)₃ over sulfated CeO₂ catalysts, but preferably coupled with active vanadia to generate inactive KVO₃ species over V₂O₅-WO₃/TiO₂ catalysts. Such an active Ce₂(SO₄)₃ species facilitated the adsorption and reactivity of NH₃ and NO_x, enabling ceria catalysts to maintain high catalytic efficiency in the presence of potassium. Conversely, the introduction of potassium into V₂O₅-WO₃/TiO₂ catalysts caused a considerable loss of surface acidity, hindering catalyst reactivity during the SCR reaction. The self-protection armor of Ce₂(SO₄)₃ species may open a promising pathway to develop efficient ceria-based SCR catalysts with strong antipoisoning ability.

Nickel Nanoparticles Anchored on Activated Attapulgite Clay for Ammonia Decomposition to Hydrogen

Ammonia decomposition to hydrogen technique is an effectively way to solve the problems associated with the storage and transportation of hydrogen, but the development of a high-performance catalyst for ammonia decomposition is a great challenge. Ni species supported on activated attapulgite clay (AATP) is prepared by a homogeneous precipitation method for ammonia decomposition to COx-free H2. The structural properties of the Ni/AATP catalysts are characterized by thermogravimetric analysis, X-ray diffraction, scanning and transmission electron microscopy, H2 temperature-programmed reduction, and N2 sorption technique. It is revealed that the porous structure and high surface area of rod-like symmetric AATP results in highly dispersed NiO particles because the presence of a strong interaction between AATP and NiO particles. In particular, the Si-OH in AATP can react with Ni species, forming Si-O-Ni species at the interface between Ni and AATP. The Ni/AAPT catalysts are used for ammonia decomposition, the 20%-Ni/ATTP catalyst shows a 95.3% NH3 conversion with 31.9 mmol min−1 gcat−1 H2 formation rate at 650 °C. This study opens a new way to utilize natural minerals as an efficient support of catalysts towards ammonia decomposition reaction.

Compensation or Aggravation: Pb and SO2 Copoisoning Effects over Ceria-Based Catalysts for NOx Reduction

Severe catalyst deactivation caused by multiple poisons, including heavy metals and SO₂, remains an obstinate issue for the selective catalytic reduction (SCR) of NO_x by NH₃. The copoisoning effects of heavy metals and SO₂ are still unclear and irreconcilable. Herein, the unanticipated differential compensated or aggravated Pb and SO₂ copoisoning effects over ceria-based catalysts for NO_x reduction was originally unraveled. It was demonstrated that Pb and SO₂ exhibited a compensated copoisoning effect over the CeO₂/TiO₂ (CT) catalyst with sole active CeO₂ sites but an aggravated copoisoning effect over the CeO₂-WO₃/TiO₂ (CWT) catalyst with dual active CeO₂ sites and acidic WO₃ sites. Furthermore, it was uniquely revealed that Pb preferred bonding with CeO₂ among CT while further being combined with SO₂ to form PbSO₄ after copoisoning, which released the poisoned active CeO₂ sites and rendered the copoisoned CT catalyst a recovered reactivity. In comparison, Pb and SO₂ would poison acidic WO₃ sites and active CeO₂ sites, respectively, resulting in a seriously degraded reactivity of the copoisoned CWT catalyst. Therefore, this work thoroughly illustrates the internal mechanism of differential compensated or aggravated deactivation effects for Pb and SO₂ copoisoning over CT and CWT catalysts and provides effective solutions to design ceria-based SCR catalysts with remarkable copoisoning resistance for the coexistence of heavy metals and SO₂.

NOx Reduction over Smart Catalysts with Self-Created Targeted Antipoisoning Sites

Selective catalytic reduction of NO_x in the presence of alkali (earth) metals and heavy metals is still a challenge due to the easy deactivation of catalysts. Herein, NO_x reduction over smart catalysts with self-created targeted antipoisoning sites is originally demonstrated. The smart catalyst consisted of TiO₂ pillared montmorillonite with abundant cation exchange sites to anchor poisoning substances and active components to catalyze NO_x into N₂. It was not deactivated during the NO_x reduction process in the presence of alkali (earth) metals and heavy metals. The enhanced surface acidity, reducible active species, and active chemisorbed oxygen species of the smart catalyst accounted for the remarkable NO_x reduction efficiency. More importantly, the self-created targeted antipoisoning sites expressed specific anchoring effects on poisoning substances and protected the active components from poisoning. It was demonstrated that the tetrahedrally coordinated aluminum species of the smart catalyst mainly acted as self-created targeted antipoisoning sites to stabilize the poisoning substances into the interlayers of montmorillonite. This work paves a new way for efficient reduction of NO_x from the complex flue gas in practical applications.