Hydrothermal Aging Process Research Articles

Interface Induced by Hydrothermal Aging Boosts the Low-Temperature Activity of Cu-SSZ-13 for Selective Catalytic Reduction of NOx.

Hitherto, sulfur poisoning and hydrothermal aging have still been the challenges faced in practical applications of the Cu-SSZ-13 catalyst for the selective catalytic reduction (SCR) of NOx from diesel engine exhaust. Here, we elaborately design and conduct an in-depth investigation of the synthetic effects of hydrothermal aging and SO2 poisoning on pristine Cu-SSZ-13 and Cu-SSZ-13@Ce0.75Zr0.25O2 core@shell structure catalysts (Cu@CZ). It has been discovered that Cu@CZ susceptible to 750 °C with 5 vol % H2O followed by 200 ppm SO2 with 5 vol % H2O (Cu@CZ-A-S) could still maintain nearly 100% NOx conversion across the significantly wider temperature region of 200-425 °C, which is remarkably broader than that of the Cu-SSZ-13-A-S (300-400 °C) counterpart. The experimental results show that the hydrothermal aging process results in the migration of highly active Cu species within the cage of Cu-SSZ-13 to the CZ surface, forming CuO/CZ with abundant interfaces, which significantly enhances the adsorption and subsequent activation of NO, leading to the generation of reactive N2O3 and HONO intermediates. Moreover, density functional theory (DFT) calculations reveal that the H of the HONO* species can function as Brønsted acid sites, effectively adsorbing NH3 to generate the active NH4NO2* intermediate, which readily decomposes into N2 and H2O. Furthermore, this pathway is the rate-determining step with an energy barrier of 0.93 eV, notably lower than that of the "standard SCR" pathway (1.42 eV). Therefore, the formation of the new CuO/CZ interface profoundly boosts the low-temperature NH3-SCR activity and improves the coresistance of the Cu@CZ catalyst to sulfur poisoning and hydrothermal aging.

Exploring the limit of Cu-based small-pore zeolite framework collapse during hydrothermal aging in NOx abatement from diesel vehicle exhaust

Cu-SSZ-13 small-pore zeolites have been commercialized for NOx abatement in diesel engines, while they still suffer from hydrothermal deactivation. To identify the changes occurring in catalysts during the whole hydrothermal aging (HTA) process, the selective catalytic reduction by ammonia (NH3-SCR) performance of hydrothermally aged catalysts was investigated until total deactivation occurred. Through precisely identifying the alterations in the zeolite framework and active Cu sites during HTA, three stages were distinguished under progressively more severe HTA conditions, which were dealumination/transformation of active Cu species, CuOx accumulation, and structural collapse. These three stages were found to cause a slight decrease, serious attenuation, and total deactivation of the deNOx efficiency, which stemmed from the loss of Bronsted acid sites, reduction in the number of active Cu ion sites and structural collapse, respectively. This work also depicted simulated deactivation curves of Cu-SSZ-13 with various compositions during the HTA process, which helps in understanding the hydrothermal aging limits of catalysts with various Si/Al ratios and Cu loadings.

Investigating the effect of chemical structures on water sorption and diffusion in amine-cured epoxy resins by molecular dynamics simulations

Mechanical degradation of polymer materials by moisture ingress is inevitable due to their chemical and physical properties. In this regard, it is necessary to better understand the mechanisms governing interactions between water and polymer. In this research, the sorption and diffusion of moisture in epoxy systems with different chemical structures of amines (DGEBA/DDS and DGEBA/DDM) has been investigated by molecular dynamics simulations. For the validation, the physical properties of the simulation models and the calculated values of the diffusion coefficient and equilibrium water content have been compared with the existing data and acceptable results have been obtained. The study results of polymer systems showed that both the polarity and the average radius of nanopores played an effective role in diffusivity and equilibrium water content values. It was also found that the interactions between water and polymer molecules at the water-polymer interface played an important role in the initiation of the moisture sorption process. The results of the simulation during the hydrothermal aging process showed that moisture first entered the pores in the epoxy network, then diffused deeper into the polymer, and finally, with a change in the polymer structure, caused water-polymer hydrogen bonds with stronger strength.

Enhanced Low-Temperature Activity and Hydrothermal Stability of Ce-Mn Oxide-Modified Cu-SSZ-39 Catalysts for NH3-SCR of NOx

Cu-SSZ-39 zeolite with an AEI structure exhibits excellent hydrothermal stability and can be a potential alternative to Cu-SSZ-13 zeolite SCR catalysts for NOx removal in diesel vehicles. However, the inferior low-temperature performance of Cu-SSZ-39 leads to substantial NOx emissions during the cold-start period, impeding its practical application. In this study, Ce-Mn oxide-modified Cu-SSZ-39 catalysts (CeMnOx/Cu-SSZ-39) and references (CeO2/Cu-SSZ-39 and MnOx/Cu-SSZ-39) were prepared by the ion-exchange of Cu ions followed by impregnation of the oxide precursors, with the aim of enhancing the NH3-SCR performance at low temperatures. The modified catalysts exhibited improved low-temperature activity and hydrothermal stability compared to the unmodified counterpart. In particular, CeMnOx/Cu-SSZ-39 showed the highest activity among the three catalysts and achieved NOx conversions above 90% within the temperature range of 180 °C to 600 °C, even after undergoing hydrothermal aging at 800 °C. Experimental results indicated that the synergistic effect between Ce and Mn in CeMnOx improves the redox properties and acidity of the catalyst due to the presence of Ce3+, Mn4+, and abundant adsorbed oxygen species, which facilitate low-temperature SCR reactions. Furthermore, the interaction of CeMnOx with Cu-SSZ-39 stabilizes the zeolite framework and hinders the agglomeration of Cu species during the hydrothermal aging process, contributing to its exceptional hydrothermal stability. The kinetics and NO oxidation experiments demonstrated that CeMnOx provides access to fast SCR reaction pathways by oxidizing NO to NO2, resulting in a significant increase in low-temperature activity. This study provides novel guidelines for the design and preparation of Cu-SSZ-39 zeolite with outstanding SCR performance over a wide temperature range.

Influence of hydrothermal conditions on the mechanical properties of hybrid composite pipes

Composite materials are becoming widespread in many sectors today, especially in the defence and space industry. Composite materials can be exposed to significant temperature changes and various environmental conditions depending on their usage areas. Therefore, composite materials with high resistance to high temperatures and harsh environmental conditions are being studied today. In this study, four types of 6-layer composite pipes with glass/glass/glass, basalt/basalt/basalt, glass/basalt/glass and basalt/glass/basalt stacking sequences were fabricated with filament wound method. In order to simulate the behavior of composite materials under environmental conditions, specimens were subjected to hydrothermal aging in pure water at 70°C for three different periods of 500, 1000 and 1500 hours. During the aging process, the mass changes of the pipes were measured regularly, and the water absorption behavior was determined with reference to ASTM D5229 standard. Hardness tests were carried out with Rockwell hardness tester based on ASTM D785 standard. As a result of the study, it was determined that the mechanical performance of hybrid specimens was higher than that of non-hybrid specimens. It was observed that the hydrothermal aging process caused a loss of mechanical strength in all specimens. It was concluded that the hybrid composite specimen with basalt/glass/basalt stacking order had the highest resistance to hydrothermal aging.

Effect of Hydrothermal Aging Behavior on Surface Treated Kevlar Fiber Laminated Composites

The water sorption characteristics of Kevlar fiber–reinforced epoxy composites were studied by immersion in water at 80 degrees Celsius. The hydrothermal aging process was conducted on treated and untreated Kevlar/epoxy composites; also, the composite was evaluated by the three-point bending test. The phosphoric acid (PA) pretreated with epichlorohydrin (ECH) was used for the surface modification of Kevlar. In the case of chemically modified fiber composites, water uptake was found to be dependent on the chemical treatment done on the fiber surface. The lowest water uptake was observed for composites treated with PA with ECH. The effect of thermal aging on the flexural strength of the treated Kevlar composite was 20.42% higher than the untreated composite. Consequently, the flexural modulus was 13.9% higher than the untreated Kevlar composite. Moreover, the water diffusion coefficient of treated composite 23.19% higher with untreated composite. It was concluded that fiber/matrix degradation time at the interface region was increased in the case of treated composite

The Regulation of Surface Copper Species Coupled with Ammonia-Evaporation and Hydrothermal Aging Process to Enhance Catalytic Hydrogenation Properties of Cu–SiO2 Catalysts

The Regulation of Surface Copper Species Coupled with Ammonia-Evaporation and Hydrothermal Aging Process to Enhance Catalytic Hydrogenation Properties of Cu–SiO2 Catalysts

Constructing a honeycomb‐inspired Ni‐Co LDHs@CFs enhanced polymer composites with high strength, aging resistance and excellent anti‐wear for tribological applications

AbstractThe development of carbon fibers/polymer composites (CFPCs) has made a profound impact on varied application prospects in the transmission and braking fields, where interface engineering between fibers/matrix holds the key to co‐enhanced mechanical and tribological properties. Herein, we designed a honeycomb‐inspired Ni‐Co layered double hydroxides (Ni‐Co LDHs) modified CFPCs, in which Ni‐Co LDHs interconnected nanosheets were grown in‐situ onto the carbon fibers (CFs) surface via the oxidation‐assisted hydrothermal method, and then polymer matrix fully impregnate and filled with both the CFs and LDHs porous containers by capillary forces. Benefiting from the special interface, the Ni‐Co LDHs grafted CFs reinforced polymer composites (Ni‐Co LDHs@CFs composites) possessed the excellent mechanical strength and outstanding tribological properties. The LDHs@CFs composites had 32% improvement in the tensile strength, and 84.9% strength retention ratio after the hydrothermal aging process. Furthermore, the friction coefficient was increased by 24% and wear rate was dramatically decreased by 42% from 6.148 × 10−14 to 3.557 × 10−14 m3 (N m)−1, indicating better torque transmitting capacity and anti‐wear properties. This work could provide a valuable reference for the construction of biomimetic interface to simultaneously improve the mechanical and tribological properties of the CFPCs.

Improved PVC/ZnO Nanocomposite Insulation for High Voltage and High Temperature Applications

Nanosized inorganic oxides have the trends to improve many characteristics of solid polymer insulation. In this work, the characteristics of improved poly (vinyl chloride) (PVC)/ZnO are evaluated using 0, 2, 4 and 6 phr of ZnO nanoparticles dispersed in polymer matrix using internal mixer and finally compressed into circular disk with 80 mm diameter using compression molding technique. Dispersion properties are studied by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), X-ray diffractometry (XRD), and optical microscopy (OM). The effect of filler on the electrical, optical, thermal, and dielectric properties of the PVC are also analyzed. Hydrophobicity of nano-composites is evaluated by measuring contact angle and recording hydrophobicity class using Swedish transmission research institute (STRI) classification method. Hydrophobic behavior decreases with the increase in filler content; contact angle increases up to 86°, and STRI class of HC3 for PZ4 is observed. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) are employed to evaluate the thermal properties of the samples. Also, continuous decrease of optical band gap energy from 4.04 eV for PZ0 to 2.57 eV for PZ6 is observed. In the meantime, an enhancement in the melting temperature, Tm, is observed from 172 to 215 °C. To check the stability of materials against hydrothermal stresses, all the fabricated materials are then subjected to a hydrothermal aging process for 1000 h and their structural stability is analyzed using optical microscopy and FTIR analyses.

Hydrothermally improved natural manganese-containing catalytic materials to degrade 4-chlorophenol

Natural manganese-containing mineral (NMM) was used as a catalyst in heterogeneous catalytic ozonation for 4-chlorophenol (4-CP) degradation. The surface and structural properties of NMM were modified by the hydrothermal aging process and called H-NMM. The catalytic activity of NMM and H-NMM were evaluated for the catalytic ozonation process (COP). The synergistic effect of NMM and H-NMM in ozonation processes for 4-CP degradation under optimal conditions (pH of 7, 1 g/L of NMM and H-NMM, 0.85 mg/min of O3, and 15 min of reaction time) was measured by 3.04 and 4.34, respectively. During the hydrothermal process, Mn4+ and Fe2+ were converted to Mn2+ and Fe3+, which caused better performance of the H-NMM than the NMM. During the catalytic ozonation process, Mn2+ is completely oxidized, which increases the production of Hydroxyl radical (•OH). The reactive oxygen species (ROS) generated in the system were identified using radical scavenging experiments. •OH, superoxide radical (•O2−), and singlet oxygen (1O2) represented the dominant reactive species for 4-CP degradation. The O3/H-NMM process indicated a powerful ability in the mineralization of 4-CP (66.31% of TOC degradation). H-NMM exhibited excellent stability and reusability in consecutive catalytic cycles, and the NMM exhibited desirable performance. This study offers NMM and H-NMM as effective, stable, and competitive catalysts for hastening and enhancing the ozonation process to mitigate environmentally related pollutants of high concern.

Silica modulated palladium catalyst with superior activity for the selective catalytic reduction of nitrogen oxides with hydrogen

A significant promotion effect of colloidal SiO2 on Pd/TiO2 catalyst for improving the activity and N2 selectivity in the selective catalytic reduction of NO with H2 (H2-SCR) was reported, both before and after hydrothermal aging. The SiO2 addition not only benefited the formation of more oxygen vacancies but also increased Pd dispersion and created rich Pd-SiO2 interfaces on Pd-SiO2/TiO2 catalyst. The H2-SCR of NO on both Pd/TiO2 and Pd-SiO2/TiO2 followed a Langmuir-Hinshelwood mechanism, in which the adsorbed bridging bidentate nitrates on TiO2 could react with dissociated H2 species on Pd sites at Pd-TiO2 interface. Particularly, on the promoted Pd-SiO2/TiO2, the Pd-TiO2 interface could facilitate NO adsorption and activation, and the created Pd-SiO2 interface could benefit H2 adsorption and dissociation, thus contributing to its significantly improved H2-SCR activity. Furthermore, the active Pd species (which should be Pd0 under reaction condition with higher surface concentration) within Pd-SiO2/TiO2 could be well stabilized during long-term hydrothermal aging process through the formation of rich Pd-SiO2 interfaces.

Degenerating effect of transformation and loss of active sites on NH3-SCR activity during the hydrothermal aging process for Cu-SSZ-13 molecular sieve catalyst

A series of Cu-SSZ-13 small-pore molecular sieve catalysts are prepared by the ion-exchange method, analyzed, and tested by changing the synthesis ratio, and performing hydrothermal aging treatment under various aging conditions to explain the degree of influence. The investigations are carried out to reveal that when the Si/Al ratio is 10, the catalyst has the best comprehensive performance and ZCuOH sites are more prone to transformation and loss than Z2Cu sites during the hydrothermal aging process. In-situ DRIFTS experiments and DFT molecular simulation calculations are used to explain the reasons for the degradation of catalytic performance caused by the hydrothermal aging, which completes the study of the hydrothermal aging mechanism for Cu-SSZ-13 molecular sieve catalysts. The result shows that Z2Cu active sites exhibit better hydrothermal aging resistance than ZCuOH, the catalyst is more sensitive to aging temperatures than durations, and its limit of the aging temperature is about 850 ℃.

Unraveling the nature of cerium on stabilizing Cu/SAPO-34 NH3-SCR catalysts under hydrothermal aging at low temperatures

To reveal how cerium stabilizes Cu/SAPO-34 at low-temperature hydrothermal aging, various amounts of cerium were introduced into Cu/SAPO-34 via impregnation method and treated at 70 °C with RH 80% for 96 h. Cerium as Ce3+ and CeO2 nanoparticle is located on the surface of Cu/SAPO-34, and Ce3+ plays a vital role on gradually decreasing surface acidity and blocking defect sites with an increase of Ce loading. After hydrothermal aging, Cu/SAPO-34 with high Ce loading shows the superior SCR activity comparable to fresh samples. It is proven that the surface acidity determines the stability of the structure during hydrothermal aging process, and lower surface acidity prevents the number of Cu(II) ions from decreasing significantly. Furthermore, the structure's stability helps the recovery of Cu(II) ions and renders an outstanding regeneration ability. Our finding paves the way for the design of new Cu/SAPO-34 catalysts with good SCR activity and long-term stability in real application.

The promotional role of Nd on the catalytic activity and hydrothermal stability of Cu-SAPO-18 catalyst for NH3-SCR reaction

The Cu-SAPO-18 was prepared by the one-pot method and modified through hydrothermal post-treatment, metal-doped Nd(x)/Cu-SAPO-18 (x = 0.4, 0.8, 1.2, 1.6) catalysts were obtained. The catalytic performance of fresh and aged Nd(x)/Cu-SAPO-18 was evaluated by ammonia selective catalytic reduction (NH3-SCR) reaction. The NOx conversion rate of Nd (1.2)/Cu-SAPO-18 was highest. More than 94.5% NOx conversion was maintained over the aged Nd(x)/Cu-SAPO-18 during 250–450°C, which was almost 10% higher than the conversion of aged Cu-SAPO-18. Various techniques including XRD, XRF, BET, SEM, 27Al MAS NMR, XPS, UV-vis, NH3-TPD, H2-TPR, were applied to explore the effect of Nd content on the physiochemistry property of Cu-SAPO-18. The addition of Nd effectively maintained the relatively large specific surface area and uniform pore size distribution. Moreover, the dealumination progress of Cu-SAPO-18 was mitigated and the aggregation of copper species during hydrothermal aging process was restrained. Thus, Nd(x)/Cu-SAPO-18 owed less CuOx and more amount of isolated Cu2+ ions, which stabilized catalyst structure by entering the double 6-membered ring (D6R). Meanwhile, the incorporation of Nd enhanced the redox property and stabilised surface acid sites of Cu-SAPO-18, which were responsible for facilitating the adsorption and activation of reactants.

Hydrothermal aging of polyimide film

AbstractThe hydrothermal aging of a commercial polyimide film, (poly[pyromellitic dianhydride‐co‐4,4′‐oxydianiline]) is investigated, providing further insight into the well‐known loss of properties that can occur for these materials in the presence of moisture. The study involved measuring ATR‐FTIR, ultimate tensile strength, and percent elongation at break, under accelerated hydrothermal aging conditions at three different temperatures (70, 80, and 90°C). ATR‐FTIR data was analyzed using chemometrics in order to identify significant trends that develop upon the accelerated aging conditions. The most dramatic changes were observed for the aging at 90°C. Changes in the ATR‐FTIR spectra for aging at all three temperatures can be attributed to hydrolysis of the imide groups. Ultimate tensile strength was also used to monitor the hydrothermal aging process. This data was used to construct an Arrhenius plot from which an activation energy of 71.8 KJ/mol was determined for the hydrothermal aging process. This value is comparable to that of textiles used in fire protective clothing, suggesting that polyimide is a viable candidate for modeling the degradation of these textiles. This paper also shows the large potential of chemometrics for polymer aging studies as it allows identifying degradation mechanisms from subtle chemical changes in the materials.

Degenerating Effect of Transformation and Loss of Active Sites on Nh3-Scr Activity During the Hydrothermal Aging Process for Cu-Ssz-13 Molecular Sieve Catalyst

Degenerating Effect of Transformation and Loss of Active Sites on Nh3-Scr Activity During the Hydrothermal Aging Process for Cu-Ssz-13 Molecular Sieve Catalyst

Degenerating Effect of Transformation and Loss of Active Sites on Nh3-Scr Activity During the Hydrothermal Aging Process for Cu-Ssz-13 Molecular Sieve Catalyst

Degenerating Effect of Transformation and Loss of Active Sites on Nh3-Scr Activity During the Hydrothermal Aging Process for Cu-Ssz-13 Molecular Sieve Catalyst

Insights into the impact of lanthanum on hydrothermal-induced migration and transformation of copper species in Cu/SAPO-34 catalyst for NH3-SCR

During the high-temperature hydrothermal aging process, the tolerance of the zeolite structure and the stability of the active Cu2+ ions directly determined the denitration (deNOx) performance of the copper-based zeolite catalysts in automobile exhaust treatment. Herein, we displayed the hydrothermal aging stability of La-free and La-doped Cu/SAPO-34 catalysts at variant temperatures (650, 750 and 850 °C). A series of characterization results signified that the addition of La into SAPO-34 framework by ion exchange method had obvious contribution to hold back the collapse of SAPO-34 structure and transformation of isolated Cu2+ ions. Different from Cu/SAPO-34 catalyst, Cu/La-SAPO-34 catalyst was more prone to maintain its superior catalytic activity after hydrothermal aging at 750 °C, which possessed great application potential. In addition, NH3-TPD and DRIFT spectra shed light on the variation of surface acidity over two kinds of catalysts inducing by hydrothermal treatment. It could be discovered that doping of La provided moderate surface acidity and excellent hydrothermal stability for Cu/SAPO-34 even up to 750 °C, which was in favor of the NH3-SCR reaction.

Density functional theory researches for atomic structure, properties prediction, and rational design of selective catalytic reduction catalysts: Current progresses and future perspectives

• Using DFT to explore the adsorption of gas molecules on catalysts surface. • Investigating the characteristics of catalytic active sites and reaction mechanisms through DFT. • Explaining the catalysts poisoning process through DFT. • Illustrating the catalysts hydrothermal aging process through DFT. • Catalysts modification and design based on DFT. In recent years, molecular simulation methods, especially density functional theory (DFT), have attracted much attention and been widely used to study the SCR reaction theoretically and microscopically. This review discusses the application of DFT in SCR research from the aspects of surface adsorption, active site characteristics, reaction mechanism, poisoning mechanism, and hydrothermal aging mechanism to illustrate the status of quantum chemical computing in this field. The gas adsorption includes the adsorption of nitrogen oxides, ammonia, and their co-adsorption. The site and mechanism include the properties of different active sites, intermediate species analysis, and reaction mechanism research. The poisoning includes related research on sulfur poisoning and alkali metal poisoning. The hydrothermal aging includes research on dealumination and loss of active sites. It is demonstrated that the catalysts can be improved and rationally designed based on quantum chemistry calculations. Finally, the application of DFT in this field is evaluated, hoping to provide good suggestions for DFT calculations to further understand the more complicated reaction mechanisms and optimally design the SCR catalysts.

Unexpected increase in low-temperature NH3-SCR catalytic activity over Cu-SSZ-39 after hydrothermal aging

The control of nitrogen oxides (NOx) from heavy-duty diesel vehicles is becoming increasingly urgent due to its detrimental effects on the environment. Cu-SSZ-39, with its exceptional NH3-SCR activity and hydrothermal stability, has attracted more and more attention in the field of diesel vehicle emission control. In this work, an unexpected phenomenon was observed for Cu-SSZ-39 after hydrothermal aging. Generally, the hydrothermal aging process causes deterioration of the catalytic performance of catalysts. However, a remarkable increase in low-temperature NH3-SCR catalytic activity was observed in Cu-SSZ-39 after it was exposed to 850 ℃ hydrothermal aging conditions for 16 h. EPR, DRIFTS, H2-TPR, UV–vis and XANES were utilized to detect the changes in Cu species. It was determined that a portion of the Cu2+ ions near double six-membered rings (d6r) transformed into CuxOy species after hydrothermal treatment. These CuxOy species facilitated the production of nitrate species which were critical intermediates in the NH3-SCR process, resulting in an increase in low-temperature catalytic activity.