Optimizing the distribution and proportion of various active sites for better NH3-SCR property over Cu/SSZ-13

The Cu/SSZ-13 catalyst with Si/Al ratio of 10 and Cu/Al ratio in the range of 0.005 to 0.5 was synthesized by ion exchange method. The effect of Cu/Al ratio on the performance, selectivity, and active center of the catalyst SCR was studied. The samples with too low Cu/Al ratio have low catalytic activity in the entire temperature range due to the small number of active sites. The samples with too high Cu/Al ratio have good performance in the low temperature range due to the large number of active sites. However, a large amount of copper that has not undergone ion exchange will generate CuO, which greatly enhances the oxidation of NH₃ at high temperatures, which in turn leads to a decrease in catalytic performance at high temperatures. XRD and BET experiments show that all catalyst samples have good CHA crystal structure and microporous structure, and the pore size is mostly about 2 nm. In situ DRIFTS analysis shows that when the Cu/Al ratio is low, the proportion of Z₂Cu in the active sites is higher, and the stability of the catalyst is enhanced. When the copper-aluminum ratio is high, the proportion of ZCuOH increases, and the low-temperature activity of the catalyst is higher.

Exploring the optimal ratio of elemental components of the Cu/SSZ-13 framework: the reformation of NH3-SCR properties

The effect of the ratio of skeleton elements on the performance of the Cu/SSZ-13 catalyst was studied.

Effect of hydrothermal aging temperature on a Cu-SSZ-13/H-SAPO-34 composite for the selective catalytic reduction of NO x by NH3

Cu-SSZ-13 suffers activity loss after hydrothermal treatment at high temperatures, particularly above 850 °C. The stability of Cu-SSZ-13 can be enhanced by compositing with H-SAPO-34. This work investigates the effect of aging temperature on the composites. For the structure, the extra-framework P in H-SAPO-34 migrates and interacts with the Al in Cu-SSZ-13, forming a new framework P-Al bond. This interaction is enhanced with the increment of the aging temperature. For the cupric sites, the aging at 750 °C results in the agglomeration of Cu²⁺ ions to CuO. However, the sample aged at 800 °C exhibits higher activities than that aged at 750 °C, which might be attributed to the increased formation of framework P-Al bonds promoting the redispersion of CuO to Cu²⁺ ions. The composite suffers severe deactivation due to the significant loss of Cu²⁺ ions after aging at 850 °C.

Atomic layer deposition of silica to improve the high-temperature hydrothermal stability of Cu-SSZ-13 for NH3 SCR of NOx

The improvement of stability is a crucial and challenging issue for industrial catalyst, which affects not only the service time but also the cost of catalyst. This is especially prominent for that applied in harsh environment atmospheres, such as the exhaust of diesel vehicles. Herein, we reported a new strategy to improve the high-temperature hydrothermal stability of Cu-SSZ-13, which is a promising catalyst for the treatment of exhaust emitted from diesel vehicles through the NH₃-SCR NO_x route. Different from that reported in literature, we managed to improve the high-temperature hydrothermal stability of Cu-SSZ-13 by coating the surface with a nanolayer of stable SiO₂ material using the atomic layer deposition (ALD) method. The coating of SiO₂ layers effectively suppressed the leaching of alumina from the SSZ-13 molecular sieve even after the hydrothermal aging at 800 °C for 16 h with 12.5% water in air. Meanwhile, the ultra-thin SiO₂ nanolayer does not block the pores of zeolites and affect the catalytic activity of Cu-SSZ-13 contribute to the superiority of the ALD technology.

High CO Adsorption Performance of CuCl-Modified Diatomites by Using the Novel Method “Atomic Implantation”

An atomic implantation method was used to modify diatomite with CuCl. The CuCl/diatomite samples were characterized by different techniques, including FTIR, XRD, BET, SEM-TEM, EDX, and CO-TPR. Characterization results revealed the formation of CuCl particles of 50–60 nm highly dispersed on diatomite surface. CO adsorption measurements showed that 2CuCl/diatomite exhibits the highest CO adsorption capacity among all CuCl-modified samples with diatomite. Its CO adsorption capacity of 2.96 mmol/g at 30°C is 10 times higher than that of unmodified diatomite (0.29 mmol/g). The CO adsorption on CuCl-modified diatomites was found to fit well with the Langmuir–Freundlich model.

An Aging Model of NH3 Storage Sites for Predicting Kinetics of NH3 Adsorption, Desorption and Oxidation over Hydrothermally Aged Cu-Chabazite

A unified transient kinetic model which can predict the adsorption, desorption and oxidation kinetics of NH3 over hydrothermally aged Cu-chabazite was developed. The model takes into account the variation of fractional coverages of NH3 storage sites due to hydrothermal aging. In order to determine the fractional coverage of these sites, the catalyst was aged for various times at a certain temperature followed by NH3 adsorption, desorption and temperature-programmed desorption (TPD) experiments. TPD profiles were deconvoluted mainly into three peaks with centres at 317, 456 and 526 °C, respectively. Hydrothermal aging resulted in the progressive increase in the intensity of the peak at 317 °C and decrease in the intensity of the peaks at 456 and 526 °C, along with decreased NH3 oxidation at high temperatures. A model for hydrothermal aging kinetics of the fractional coverage of storage sites was developed using three reactions with appropriate rate expressions with parameters regressed from experimental data. The model was then incorporated into a multi-site kinetic model for the degreened Cu-Chabazite by the addition of aging reactions on each storage site. The effects of both aging time and temperature on the kinetics NH3 adsorption, desorption and oxidation were successfully predicted in the 155-540 °C range. This study is the first step towards the development of a hydrothermal aging-unified kinetic model of NH3-Selective Catalytic Reduction over Cu-chabazite.

Deactivation of Cu/SSZ-13 NH3-SCR Catalyst by Exposure to CO, H2, and C3H6

Lean nitric oxide (NOx)-trap (LNT) and selective catalytic reduction (SCR) are efficient systems for the abatement of NOx. The combination of LNT and SCR catalysts improves overall NOx removal, but there is a risk that the SCR catalyst will be exposed to high temperatures and rich exhaust during the LNTs sulfur regeneration. Therefore, the effect of exposure to various rich conditions and temperatures on the subsequent SCR activity of a Cu-exchanged chabazite catalyst was studied. CO, H2, C3H6, and the combination of CO + H2 were used to simulate rich conditions. Aging was performed at 800 °C, 700 °C, and, in the case of CO, 600 °C, in a plug-flow reactor. Investigation of the nature of Cu sites was performed with NH3-temperature-programed desorption (TPD) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFT) of probe molecules (NH3 and NO). The combination of CO and H2 was especially detrimental to SCR activity and to NH3 oxidation. Rich aging with low reductant concentrations resulted in a significantly larger deactivation compared to lean conditions. Aging in CO at 800 °C caused SCR deactivation but promoted high-temperature NH3 oxidation. Rich conditions greatly enhanced the loss of Brønsted and Lewis acid sites at 800 °C, indicating dealumination and Cu migration. However, at 700 °C, mainly Brønsted sites disappeared during aging. DRIFT spectroscopy analysis revealed that CO aging modified the Cu2+/CuOH+ ratio in favor of the monovalent CuOH+ species, as opposed to lean aging. To summarize, we propose that the reason for the increased deactivation observed for mild rich conditions is the transformation of the Cu species from Z2Cu to ZCuOH, possibly in combination with the formation of Cu clusters.

A time- and cost-effective synthesis of CHA zeolite with small size using ultrasonic-assisted method

In this paper, the ultrasonic-assisted method has been used to the rapid hydrothermal synthesis of CHA zeolite in the absence of expensive organic template. The influence of different sonicated time on the crystallinity and micromorphology of CHA-type crystals has been studied in detail. The bath-type ultrasound processor produces acoustic waves at the frequency of 20 kHz and the power of ultrasound wave is 150 W. The synthesized products have been characterized by XRD, SEM, EDX, TG and N₂ adsorption analyses. Due to the acceleration on homogeneous nucleation caused by acoustic cavitation cracks during the ultrasonic vibrative process, the crystallization time can be shorten from 48 h to 10 h and the crystal size can be reduced from about 20 μm to 5 μm. The SEM observations of samples with different ultrasonic treatment time distinctly revealed the morphology evolution of the walnut-like CHA zeolites. From EDX, TG and N₂ adsorption comparisons, it is clear that untreated sample and ultrasonic sample both have similar element distribution, thermal stability, and pore size distribution. The research work in this paper has demonstrated the ultrasonic treatment can significantly improve crystallinity degree, reduce crystallization time and crystal size of CHA zeolite.

Effect of Copper Precursors on the Activity and Hydrothermal Stability of CuII−SSZ−13 NH3−SCR Catalysts

A series of CuII−SSZ−13 catalysts are prepared by in-situ hydrothermal method using different copper precursors (CuII(NO3)2, CuIISO4, CuIICl2) for selective catalytic reduction of NO by NH3 in a simulated diesel vehicle exhaust. The catalysts were characterized by X−ray diffraction (XRD), scanning electron microscope (SEM), X−ray photoelectron spectroscopy (XPS), N2 adsorption-desorption, hydrogen-temperature-programmed reduction (H2−TPR), ammonia temperature-programmed desorption (NH3−TPD), and 27Al and 29Si solid state Nuclear Magnetic Resonance (NMR). The CuII−SSZ−13 catalyst prepared by CuII(NO3)2 shows excellent catalytic activity and hydrothermal stability. The NO conversion of CuII−SSZ−13 catalyst prepared by CuII(NO3)2 reaches 90% at 180 °C and can remain above 90% at a wide temperature range of 180–700 °C. After aging treatment at 800 °C for 20 h, the CuII−SSZ−13 catalyst prepared by CuII(NO3)2 still exhibits above 90% NO conversion under a temperature range of 240–600 °C. The distribution of Cu species and the Si/Al ratios in the framework of the synthesized CuII−SSZ−13 catalysts, which determine the catalytic activity and the hydrothermal stability of the catalysts, are dependent on the adsorption capacity of anions to the cation during the crystallization process due to the so called Hofmeister anion effects, the NO3− ion has the strongest adsorption capacity among the three kinds of anions (NO3−, Cl−, and SO42−), followed by Cl– and SO42– ions. Therefore, the CuII−SSZ−13 catalyst prepared by CuII(NO3)2 possess the best catalytic ability and hydrothermal stability.

Inter-particle migration of Cu ions in physically mixed Cu-SSZ-13 and H-SSZ-13 treated by hydrothermal aging

Physically mixed Cu-SSZ-13 and H-SSZ-13 catalysts were prepared using two different methods to control the extent of contact. Migration of Cu ions between particles is proved by discrepancy of two physical mixed catalysts.

The Role of Impregnated Sodium Ions in Cu/SSZ-13 NH3-SCR Catalysts

To reveal the role of impregnated sodium (Na) ions in Cu/SSZ-13 catalysts, Cu/SSZ-13 catalysts with four Na-loading contents were prepared using an incipient wetness impregnation method and hydrothermally treated at 600 °C for 16 h. The physicochemical property and selective catalytic reduction (SCR) activity of these catalysts were studied to probe the deactivation mechanism. The impregnated Na exists as Na+ on catalysts and results in the loss of both Brönsted acid sites and Cu2+ ions. Moreover, the high loading of Na ions destroy the framework structure of Cu/SSZ-13 and forms new phases (SiO2/NaSiO3 and amorphous species) when Na loading was higher than 1.0 mmol/g. The decreased Cu2+ ions finally transformed into CuxO, CuO, and CuAlOx species. The inferior SCR activity of Na impregnated catalysts was mainly due to the reduced contents of Cu2+ ions at kinetic temperature region. The reduction in the amount of acid sites and Cu2+ ions, as well as copper oxide species (CuxO and CuO) formation, led to low SCR performance at high temperature. Our study also revealed that the existing problem of the Na ions’ effect should be well-considered, especially at high hydrothermal aging when diesel particulate filter (DPF) is applied in upstream of the SCR applications.

Nanoscale tomography reveals the deactivation of automotive copper-exchanged zeolite catalysts

Copper-exchanged zeolite chabazite (Cu-SSZ-13) was recently commercialized for the selective catalytic reduction of NO _X with ammonia in vehicle emissions as it exhibits superior reaction performance and stability compared to all other catalysts, notably Cu-ZSM-5. Herein, the 3D distributions of Cu as well as framework elements (Al, O, Si) in both fresh and aged Cu-SSZ-13 and Cu-ZSM-5 are determined with nanometer resolution using atom probe tomography (APT), and correlated with catalytic activity and other characterizations. Both fresh catalysts contain a heterogeneous Cu distribution, which is only identified due to the single atom sensitivity of APT. After the industry standard 135,000 mile simulation, Cu-SSZ-13 shows Cu and Al clustering, whereas Cu-ZSM-5 is characterized by severe Cu and Al aggregation into a copper aluminate phase (CuAl₂O₄ spinel). The application of APT as a sensitive and local characterization method provides identification of nanometer scale heterogeneities that lead to catalytic activity and material deactivation.

Structural and kinetic changes to small-pore Cu-zeolites after hydrothermal aging treatments and selective catalytic reduction of NOx with ammonia

Cu in aged zeolites changes structure during SCR.

Correlation of the changes in the framework and active Cu sites for typical Cu/CHA zeolites (SSZ-13 and SAPO-34) during hydrothermal aging

The relative framework stability of Cu/CHA zeolites (SAPO-34 and SSZ-13) was studied during hydrothermal aging at 800 °C, and the fundamental mechanism for the framework change was investigated. Additionally, the relationship between the variation in the framework and active SCR reaction sites was established. SAPO-34 showed stronger stability during hydrothermal aging than SSZ-13. The results showed that dealumination occurred in the SSZ-13 zeolite, leading to the loss of crystallinity and a severe decrease of the Brönsted acid sites. Simultaneously, the detached Al(OH)3 species deactivated the Cu species by the transformation of isolated Cu(2+) ions to CuAlOx species. While the vacancy in the SAPO-34 framework caused by desilication could be healed with the migration of extra-framework Al and P atoms to the defects. And the Cu species showed a certain degree of aggregation with the improved redox ability of the aged Cu/SAPO-34 zeolite and the acidic properties were well maintained.

Synthesis of nanosphere TiO2 with flower-like micro-composition and its application for the selective catalytic reduction of NO with NH3 at low temperature

TiO₂ nanospheres consisting of flower-like nanopowders were synthesized by a solvothermal method, and Cu/TiO₂(T) catalysts were prepared via an impregnation method.

Recent advances in automotive catalysis for NOx emission control by small-pore microporous materials

The ever increasing demand to develop highly fuel efficient engines coincides with the need to minimize air pollution originating from the exhaust gases of internal combustion engines. Dramatically improved fuel efficiency can be achieved at air-to-fuel ratios much higher than stoichiometric. In the presence of oxygen in large excess, however, traditional three-way catalysts are unable to reduce NOx. Among the number of lean-NOx reduction technologies, selective catalytic reduction (SCR) of NOx by NH3 over Cu- and Fe-ion exchanged zeolite catalysts has been extensively studied over the past 30+ years. Despite the significant advances in developing a viable practical zeolite-based catalyst for lean NOx reduction, the insufficient hydrothermal stabilities of the zeolite structures considered cast doubts about their real-world applicability. During the past decade renewed interest in zeolite-based lean NOx reduction was spurred by the discovery of the very high activity of Cu-SSZ-13 (and the isostructural Cu-SAPO-34) in the NH3-SCR of NOx. These new, small-pore zeolite-based catalysts not only exhibited very high NOx conversion and N2 selectivity, but also exhibited exceptionally high hydrothermal stability at high temperatures. In this review we summarize the key discoveries of the past ∼5 years that led to the introduction of these catalysts into practical applications. This review first briefly discusses the structure and preparation of the CHA structure-based zeolite catalysts, and then summarizes the key learnings of the rather extensive (but not complete) characterisation work. Then we summarize the key findings of reaction kinetic studies, and provide some mechanistic details emerging from these investigations. At the end of the review we highlight some of the issues that still need to be addressed in automotive exhaust control catalysis.

Improved high-temperature hydrothermal stability of Cu-SSZ-13 by an ammonium hexafluorosilicate treatment

Cu-SSZ-13 catalysts were treated with ammonium hexafluorosilicate (AHFS) to improve their hydrothermal stabilities.

The selective catalytic reduction of NOx over a Cu/ZSM-5/SAPO-34 composite catalyst

A series of Cu/ZSM-5/SAPO-34 composite catalysts with varying ZSM-5 mass fraction were synthesized using a pre-seed method, and their catalytic performances were tested for selective catalytic reduction (SCR) of NO with NH₃.

The role and activity of various adsorbed ammonia species on Cu/SAPO-34 catalyst during passive-SCR process

In this work, the adsorption and reaction performance of various adsorbed ammonia species during the passive-SCR process were investigated.