Low-temperature Hydrothermal Stability Research Articles

New insights into the effects of Nd on low-temperature hydrothermal stability of deNOx Cu/SAPO-34 catalysts: A comparative evaluation of the mechanism

Designing a highly efficient and cost-effective deNOx catalyst for curbing NOx emission in diesel vehicle exhaust is highly desired but challenging. Herin, a Nd-promoted Cu/SAPO-34 (Cu/Nd-S) catalyst with superior NOx reduction efficiency and low-temperature hydrothermal stability was proposed. The NOx conversion of fresh Cu/Nd-S-F at 200 °C was improved to 86.1 % compared to fresh Cu/S-F (78.1 %). XRD and H2-TPR results revealed that Nd3+ ions filled the defect sites of zeolites and combined with P species to form NdPO4, thus stabilizing the zeolite framework and maintaining the stability of active copper species. The interaction between Nd3+ ions and Cu2+ ions protected the active copper species (Cu2+-2Z and [Cu(OH)]+-Z) from the attack of low-temperature (70 °C) moisture. However, Cu/SAPO-34 suffered from the rapid and irreversible deterioration under 70 °C of water bath, leading to the loss of surface Brønsted acid sites (Al-(OH)-Si) and isolated Cu2+ ions, so aged Cu/S-70 exhibited inferior NOx conversion. Meanwhile, in situ DRIFTS transient experiments elaborated that the NH3-SCR reaction over Cu/SAPO-34 and Cu/Nd-SAPO-34 catalysts obeyed the E-R and L-H routes simultaneously. To be noted, the Nd-doping engineering afforded a promising strategy for boosting the low-temperature hydrothermal stability of Cu/SAPO-34 catalysts for the practical application of NOx removal.

Praseodymium effects on great improving low-temperature hydrothermal stability of Cu/SAPO-34 NH3-SCR catalysts

Based on the previous findings that the presence of hydroxyl groups on the outer surface is crucial for maintaining skeletal stability, we propose a strategy modified Cu/SAPO-34 using Pr ions in this study. Therefore, we conducted several measurements to investigate the effect of Pr ions on the low-temperature hydrothermal stability of Cu/SAPO-34. We find that Pr exists only on the surface of Cu/SAPO-34 as ions and oxides, with Pr3+ ions playing a protective role in occupying surface acidic sites. The addition of small amounts of Pr leads to the re-dispersion of Cu, resulting in improved low-temperature selective catalytic reduction (SCR) activity in the as-synthesized samples. Furthermore, it enhances the resistance to decomposition of the Si–(OH)–Al framework during low-temperature hydrothermal aging, thereby preserving the framework structure and allowing detached active Cu species to return to exchangeable positions, ultimately restoring SCR activity. However, as the Pr content increases, the enhanced acidity causes some structural damage, gradually weakening the protective effect. Our work demonstrates that Pr modification is a simple and effective solution to the issue of poor low-temperature hydrothermal stability in Cu/SAPO-34, providing a promising way for the application of light rare earth elements.

Advances in low-temperature hydrothermal stability of Cu/SAPO-34 zeolite

Advances in low-temperature hydrothermal stability of Cu/SAPO-34 zeolite

One-pot synthesis of bimetallic CeCu-SAPO-34 for high-efficiency selective catalytic reduction of nitrogen oxides with NH3 at low temperature

NH3 selective catalytic reduction (SCR) has been widely recognized as a promising technique for reducing nitrogen oxides from diesel vehicle exhausts. High-efficiency SCR catalysts that could perform at low temperatures are essential to denitration. In this work, a series of bimetallic CeCu-SAPO-34 molecular sieves were synthesized by one-step hydrothermal method. The CeCu-SAPO-34 maintained good crystallinity and a regular hexahedron appearance of Cu-SAPO-34 after introducing Ce species, while exhibiting a higher specific surface area and pore volume. The as-prepared CeCu-SAPO-34 with 0.02% (mass) Ce constituent exhibited the best catalytic activity below 300 oC and a maximum NOx conversion of 99% was attained; the NOx removal rates of more than 68% and 94% were achieved at 150 °C and 200 °C, respectively. And the introduction of cerium species in Cu-SAPO-34 improves the low-temperature hydrothermal stability of the catalyst towards NH3-SCR reaction. Additionally, the introduced Ce species could enhance the formation of abundant weak Brønsted acid centers and promote the synergistic effect between CuO grains and isolated Cu2+ to enhance the redox cycle, which benefit the NH3-SCR reaction. This work provides a facile synthesis method of high-efficiency SCR denitration catalysts towards diesel vehicles exhaust treatment under low temperature.

Promotional role of Co on Cu-SAPO-34 towards enhanced denitration performance and SO2 tolerance

It is well known that Cu-SAPO-34 catalyst has high activity for nitrogen oxides (NOx) abatement from diesel vehicle emission. However, its low sulfur resistance and hydrothermal stability still retard its reality application. Herein, a series of novel binary CuCo-SAPO-34 catalysts were designed and prepared through a simple one-pot method and applied for NOx selective catalytic reduction with ammonia. Compared with Cu-SAPO-34, the binary CuCo-SAPO-34 catalysts exhibited enhanced activity and SO2 tolerance. To investigate the effects of Co addition and the reaction mechanism, various characterizations including X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX) mapping, physical and chemical adsorption, the in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTs), etc., were adopted. The results reveal that the Co3O4 nanoclusters existing in CuCo-SAPO-34 could efficiently accelerate the oxidation of NO to NO2, and the addition of Co is conducive to improving its sulfur resistance. In addition, CuCo-SAPO-34 also displayed superior high- and low-temperature hydrothermal stability. The binary CuCo-SAPO-34 developed in this work may be a good candidate for Cu-SAPO-34 based catalysts in practical diesel vehicle emission control.

Destructive and Protective Effects of NH3 on the Low-Temperature Hydrothermal Stability of SAPO-34 and Cu-SAPO-34.

The influences of gaseous, weakly adsorbed, and strongly adsorbed NH3 on the low-temperature (<100 °C) hydrothermal stability of SAPO-34 and Cu-SAPO-34 were investigated. NH3 temperature-programmed desorption (NH3-TPD), 1H magic angle spinning nuclear magnetic resonance (MAS NMR), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) were adopted to characterize the adsorption states of NH3 and H2O in SAPO-34, and the destruction of the SAPO-34 framework was revealed by direct and cross-polarization 29Si, 27Al, and 31P MAS NMR. Gaseous NH3 coadsorbed with H2O inside SAPO-34 micropores and induced the hydrolysis of framework P-O-Al and Si-O(H)-Al bonds. Weakly adsorbed NH3 was released during aging and played a similar negative role to gaseous NH3. When being combined with hydrolyzed Al species from the framework, active Cu ions transformed to inactive CuAl2O4-like species, leading to deactivation in low-temperature SCR of Cu-SAPO-34. Strongly adsorbed NH4+ via 200 °C preadsorption protected the framework integrity of SAPO-34 and the SCR activity of Cu-SAPO-34.

The influence of sodium on the catalytic properties, low-temperature hydrothermal stability, and sulfur resistance of Cu-SAPO-34 for NOx reduction by NH3-SCR

In the actual diesel vehicle after-treatment systems, Cu-SAPO-34 selective catalytic reduction (SCR) of NOx catalyst simultaneously experiences the failure of sodium (Na) poisoning, low-temperature hydrothermal (LTH) deactivation and SO2 poisoning. This work investigated that the Na-loaded Cu-SAPO-34 catalysts were severally subjected to LTH treatment and sulfur poisoning for the first time. Our characterizations revealed that Na played entirely different roles in above two scenarios. In the LTH process, Na readily exchanges the protons of Si-OH-Al moieties on the zeolite framework, which inhibits the hydrolysis of the framework and further restrains the formation of SCR-inactive species. However, for SO2 poisoning, the presence of Na accelerates the deactivation of Cu-SAPO-34 because more stable sulfate species are formed on the catalyst, which severely block the pores and reduce the active sites. It is expected that these results could assist the design of SCR catalysts to meet ultra-low NOx emission.

The Influence of Sodium on the Catalytic Properties, Low-Temperature Hydrothermal Stability, and Sulfur Resistance of Cu-Sapo-34 for Nox Reduction by Nh3-Scr

The Influence of Sodium on the Catalytic Properties, Low-Temperature Hydrothermal Stability, and Sulfur Resistance of Cu-Sapo-34 for Nox Reduction by Nh3-Scr

Insights into novel mesoporous Cu-SAPO-34 with enhanced deNOx performance for diesel emission control

The formation of ammonium sulfates and existence of mass transportation limits during selective catalytic reduction of NO x with NH 3 (NH 3 -SCR) are severely affecting the de NO x performance of small pore zeolites. Herein, a novel hierarchical porous Cu-SAPO-34-Meso zeolite rich with mesopores was synthesized by one pot dual-template strategy. Cu-SAPO-34-Meso exhibited remarkable de NO x performance and broaden temperature window compared with Cu-SAPO-34-Con without mesopores, especially at low reaction temperature, which should be attributed to the mesopores in Cu-SAPO-34-Meso can accelerate the transportation of the reactants and this has been confirmed by the diffusion calculation. Even exposed on SO 2 feed gas, Cu-SAPO-34-Meso also displayed better SO 2 resistance compared with Cu-SAPO-34-Con. The in situ DRIFTS revealed that the enhanced SO 2 tolerance might be attributed to Cu-SAPO-34-Meso with mesopores could prevent the sulfur species blocking the pores and promote the decomposition of the sulfur species quickly. Therefore, the novel Cu-SAPO-34-Meso catalyst might be a good candidate for real diesel emission control application. • Mesoporous Cu-SAPO-34 with high crystallinity was synthesized. • Cu-SAPO-34-Meso exhibited enhanced NO x removal performance. • Cu-SAPO-34-Meso rich with mesopores can facilitate the mass transportation of the reactants. • Cu-SAPO-34-Meso rich with mesopores displays enhanced SO 2 resistance. • Cu-SAPO-34-Meso exhibit good high and low temperature hydrothermal stability.

Promotional effect and mechanism of the modification of Ce on the enhanced NH3-SCR efficiency and the low temperature hydrothermal stability over Cu/SAPO-34 catalysts

This study focuses on co-cations modified copper based molecular sieve catalyst CuCe/SAPO-34. The SCR performance and low temperature hydrothermal stability of Cu/SAPO-34 and CuCe/SAPO-34 before and after 70 ℃ hydrothermal aging were evaluated to conclude the impact of Ce on the structure stability and mechanism of NH3-SCR reaction. For fresh samples, CuCe/SAPO-34 showed higher catalytic activity at the whole temperature range, especially below 250 ℃ and above 400 ℃ in comparison to pure Cu/SAPO-34. For aged catalysts, CuCe/SAPO-34 exhibited higher efficiency at below 400 ℃ than Cu/SAPO-34, and showed more than 80 % NOx conversion at 175 ℃ to 450 ℃, whereas, that was only 225 ℃ to 450 ℃ for Cu/SAPO-34. The effect of co-cation Ce on the physicochemical structure, acidity, redox capacity, Cu species migration, and framework stability of the fresh and aged samples were comprehensively characterized by NH3-TPO, NO-TPO, H2-TPR, NH3-TPD, BET, XRD and XPS. For the fresh samples, the addition of co-cation Ce decreased the amount of CuxOy species, increased Cu2+ sites, and improve the redox ability of active sites. After low temperature hydrothermal aging, CuCe/SAPO-34 remained the structural integrity of CHA framework. Ce species protected copper ions and prevented the dealumination in the process of aging. Furthermore, in situ DRIFTS indicated that, with the Ce doping, more active copper sites and monodentate nitrates adsorbed sites formed, resulting in excellent SCR performance and stability. In conclusion, modification by Ce could improve the SCR performance and low temperature hydrothermal stability of Cu/SAPO-34 catalyst.

Promotional effect of ion-exchanged K on the low-temperature hydrothermal stability of Cu/SAPO-34 and its synergic application with Fe/Beta catalysts

Kions were introduced onto Cu/SAPO-34 catalysts via the ion-exchange process in order to improve their stability under low-temperature hydrothermal aging. The changes in structure and copper-species contents of these catalysts upon hydrothermal aging were probed in order to investigate their effects on selective catalytic reduction (SCR) activity. For the fresh Cu/SAPO-34 catalysts, K ions had little influence on the chabazite framework but effected their acidities by exchanging with acid sites. After hydrothermal aging, the structural integrity and amount of active sites decreased on pure Cu/SAPO-34. While the K-loaded catalysts showed improved chabazite structure, acidity, and active site conservation with increasing K loading. However, although the 0.7 wt% K catalyst maintained the same crystallinity, active site abundance, and low-temperature SCR activity as the fresh catalyst upon aging, an apparent decrease in SCR activity at high temperature was observed because of the inevitable decrease in the number of Bronsted acid sites. To compensate for the activity disadvantage of K-loaded Cu/SAPO-34 at high temperature, Fe/Beta catalysts were co-employed with K-loaded Cu/SAPO-34, and a wide active temperature window of SCR activity was obtained. Thus, our study reveals that a combined system comprising Fe/Beta and K-loaded Cu/SAPO-34 catalysts shows promise for the elimination of NOx in real-world applications.

Fabrication of Cu-CHA composites with enhanced NH3-SCR catalytic performances and hydrothermal stabilities

Cu-SSZ-13 and Cu-SAPO-34, both with CHA topology but different compositions, are highly active NH3-SCR catalysts for the abatement of harmful nitrogen oxides from the exhausts of lean-burn engines. However, the former suffers from high preparation cost and worse high-temperature hydrothermal stability, and the latter is limited by its poor low-temperature hydrothermal stability. Herein, a new Cu-CHA composite (named Cu-CHADNL) containing both microenvironments of SSZ-13 and SAPO-34 was synthesized by employing one-pot synthesized Cu-SSZ-13 as Cu source, seeds and part of Si/Al source. The resultant materials have cubic-like morphology, high Si contents, controllable Cu loadings and uniform Si/P/Al/Cu elemental distributions. Solid state MAS NMR, NH3-DRIFTS and H2-TPR demonstrate the coexistence of aluminosilicate and SAPO zones in the Cu-CHADNL crystals. Notably, Cu-CHADNL exhibits high NH3-SCR activity in wide temperature window and robust resistance against both steaming at 800 °C and soaking in 80 °C water. These advantages are owing to the unique composite microstructures of Cu-CHADNL and suggest the material has huge application potential in deNOx process.

Nature of cerium on improving low-temperature hydrothermal stability of SAPO-34

To probe the function of Ce on enhancing low-temperature stability of SAPO-34, Ce/SAPO-34 with different Ce contents was prepared via one-pot method and further treated at 70 °C in 80% relative humidity for 24 h. The structural results prove that the one-pot synthesis is an effective way to introduce Ce (Ce < 0.325 wt%) without influencing micro-structural and chemical composition. The low-temperature hydrothermal stability of Ce/SAPO-34 is enhanced by Ce loading. More than 92% CHA structure and total acidity are maintained over 0.188 wt% Ce/SAPO-34, while only about 20% of that left on pure H/SAPO-34. As Ce exists as Ce3+ at exchange sites on the surface of SAPO-34, the results of the linear relationship between protected surface acidity and structure/acidity conservation manifest that Ce3+ effectively hinders the structural collapse. This study provides new insight into surface acidity protection for solving the problem of low-temperature hydrothermal stability of SAPO-34.

The effect of crystallite size on low-temperature hydrothermal stability of Cu-SAPO-34

Two Cu-SAPO-34 catalysts with different crystallite sizes were obtained, and catalyst with smaller crystallite size had better low-temperature hydrothermal stability because of less Cu2+ species on the surface.

Improving the low-temperature hydrothermal stability of Cu-SAPO-34 by the addition of Ag for ammonia selective catalytic reduction of NOx

Ag promoted Cu-SAPO-34 has been developed to improve the low-temperature hydrothermal stability of Cu-SAPO-34 for the NH3-SCR reaction. Kinetic tests show that the addition of Ag into Cu-SAPO-34 doesn’t change the reaction mechanism. The ion-exchanged Ag species, which decrease the Brønsted acid density and provide more Lewis acid sites with moderate acidity, have a protective effect on the SAPO framework against low-temperature steaming treatment (LTST). Higher surface Cu content is observed for CuAg2.25-SAPO-34, although CuAg2.25-SAPO-34 and Cu-SAPO-34 have similar content of isolated Cu ions. In situ DRIFTS studies demonstrate, for the first time, the NH3 molecules adsorbed on the weak/moderate acid sites (independent of the acid types such as Brønsted acid or Lewis acid) are more active than on the strong acid sites for the low-temperature SCR reaction. Compared with the original catalyst Cu-SAPO-34, the enhanced SCR activity of CuAg2.25-SAPO-34 after LTST is ascribed to the better preservation of total acid sites, higher moderate acid density (Lewis acid sites) and larger amount of surface Cu species on the CuAg2.25-SAPO-34 catalyst.

Effect of Si islands on low-temperature hydrothermal stability of Cu/SAPO-34 catalyst for NH 3 -SCR

Abstract The effect of the Si islands on the stability of Cu/SAPO-34 as a selective catalytic reduction (SCR) catalyst during low-temperature hydrothermal treatment (LTH treatment) was experimentally investigated. Cu/TEAOH (high Si islands) and Cu/TEA catalyst (low Si islands) were synthesized and then aged at 70 °C with 30 vol.% H2O for 48 h. Importantly, structure defect of SAPO-34 caused by Si islands could reduce the steric hindrance for H2O attacking. The stability of the SAPO-34 framework and isolated Cu+/Cu2+ ions was negatively correlated with high content of Si islands. After LTH treatment, the characterization results showed that an obvious dealumination occurred in Cu/TEAOH-LTH catalyst, with a great loss of relative crystallinity and Bronsted acid sites. Furthermore, the isolate Cu+/Cu2+ aggregated into CuO species in Cu/TEAOH-LTH catalyst, which accounted for the serious decreasing NH3-SCR activities of Cu/TEAOH-LTH catalyst. The results showed that Cu/TEA catalyst with less Si islands showed a better stability of structure and NH3-SCR activity after LTH treatment.

Neodymium promotion on the low-temperature hydrothermal stability of a Cu/SAPO-34 NH 3 -SCR monolith catalyst

Abstract Nd modified Cu/SAPO-34 catalyst for selective catalytic reduction of NO x by NH 3 (NH 3 -SCR) is prepared by wet-impregnation method and the catalytic performance and low-temperature hydrothermal (LTH) stability are evaluated. The catalyst shows little effect on the fresh activity but presents excellent LTH resistance by the incorporation of Nd. After LTH treatment, the catalytic performance barely changed for CuNd/SAPO-34, while significantly degraded for Cu/SAPO-34 below 400 °C. Meanwhile, the NO x conversion at 200 °C for the aged catalyst is enhanced from 64% to 86% by adding Nd. In addition, the catalysts are characterized using N 2 sorption, XRD, SEM, DRIFTS, H 2 -TPR and XPS. The results reveal that the destruction of zeolite framework by the attack of vapor is evidently inhibited and the stability of copper species is also improved during LTH treatment by the introduction of Nd. Consequently, the LTH stability of CuNd/SAPO-34 is much better than that of Cu/SAPO-34.

Investigation of low-temperature hydrothermal stability of Cu-SAPO-34 for selective catalytic reduction of NOx with NH3

The low-temperature hydrothermal stabilities of Cu-SAPO-34 samples with various Si contents and Cu loadings were systematically investigated. The NH3 oxidation activities and NH3-selective catalytic reduction (SCR) activities (mainly the low-temperature activities) of all the Cu-SAPO-34 catalysts declined after low-temperature steam treatment (LTST). These results show that the texture and acid density of Cu-SAPO-34 can be better preserved by increasing the Cu loading, although the hydrolysis of Si–O–Al bonds is inevitable. The stability of Cu ions and the stability of the SAPO framework were positively correlated at relatively low Cu loadings. However, a high Cu loading (e.g., 3.67 wt%) resulted in a significant decrease in the number of isolated Cu ions. Aggregation of CuO particles also occurred during the LTST, which accounts for the decreasing NH3 oxidation activities of the catalysts. Among the catalysts, Cu-SAPO-34 with a high Si content and medium Cu content (1.37 wt%) showed the lowest decrease in NH3-SCR because its Cu2+ content was well retained and its acid density was well preserved.

Improvement of low-temperature hydrothermal stability of Cu/SAPO-34 catalysts by Cu2+ species

The destructive effect of H2O on SAPO-34 framework and Cu2+ species protection mechanism at low temperature were studied in this research. A series of Cu/SAPO-34 samples with varying Cu loadings (0–6.78wt%) were hydrothermally treated at 70°C with 80% humidity. Textural characterization results showed that this treatment led to the collapse of SAPO-34 framework at zero or low Cu loadings, which was caused by the breakage of Si–O–Al bonds proved by ex-situ DRIFTS and NMR results. The copper content increase enhanced SAPO-34 stabilization. Selective catalytic reduction (SCR) reaction rates were severely reduced after the treatment, while this decrease was gradually suppressed with increasing Cu loading up to 6.78wt%. NH3-TPD and EPR results revealed that both the number of Brønsted acid sites and Cu2+ species decreased after the treatment, which likely contributed to the reduced SCR reaction rates. Our study indicated that the isolated Cu2+ ions were both SCR active site and structure protective agent.