Effects of Si/Al Ratio on Passive NOx Adsorption Performance over Pd/Beta Zeolites

In the current article, the effect of Si/Al ratio on the NO_x adsorption and storage capacity over Pd/Beta with 1 wt% Pd loading was investigated. The XRD, ²⁷Al NMR and ²⁹Si NMR measurements were used to determine the structure of Pd/Beta zeolites. XAFS, XPS, CO-DRIFT, TEM and H₂-TPR were used to identify the Pd species. The results showed that the NO_x adsorption and storage capacity on Pd/Beta zeolites gradually decreased with the increase of Si/Al ratio. Pd/Beta-Si (Si-rich, Si/Al~260) rarely has NO_x adsorption and storage capacity, while Pd/Beta-Al (Al-rich, Si/Al~6) and Pd/Beta-C (Common, Si/Al~25) exhibit excellent NO_x adsorption and storage capacity and suitable desorption temperature. Pd/Beta-C has slightly lower desorption temperature compared to Pd/Beta-Al. The NO_x adsorption and storage capacity increased for Pd/Beta-Al and Pd/Beta-C by hydrothermal aging treatment, while the NO_x adsorption and storage capacity on Pd/Beta-Si had no change.

Selective catalytic reduction of NOx with ammonia (NH3-SCR) over copper loaded LEV type zeolites synthesized with different templates

LEV type zeolites were synthesized with four different structure-directing agents and converted to copper loaded NH₃-SCR catalysts. The synthesis recipe was found to impact the respective Al population in the two topologically different framework sites in double and single 6-rings, resolvable by ²⁷Al MAS NMR spectroscopy. Hydrothermal stability was found to be related to the silanol concentration, Si/Al ratio, particle size, crystal morphology, crystal defects, external surface area, and microporosity. Catalytic activity in NH₃-SCR was dependent on preferential Al siting in the double 6-rings. Levinite synthesized using adamantylamine showed the strongest preference for Al atoms sitting in double 6-ring sites, and showed the highest catalytic turnover frequency. Unfortunately, because of the large crystal size, copper loading of this sample was limited to 0.6 wt% while other samples could be loaded with copper up to 3.3 wt%. An optimum combination of hydrothermal stability and catalytic activity was obtained with N,N'-bis-dimethylpentanediyldiammonium dibromide as structure-directing agent.

How changes in Cu-SSZ-13 catalytic oxidation activity via mild hydrothermal aging influence sulfur poisoning extents

Sulfur poisoning of Cu-SSZ-13 is a function of the catalyst's oxidation ability.

Mobility and Reactivity of Cu+ Species in Cu-CHA Catalysts under NH3-SCR-NOx Reaction Conditions: Insights from AIMD Simulations

The mobility of the copper cations acting as active sites for the selective catalytic reduction of nitrogen oxides with ammonia in Cu-CHA catalysts varies with temperature and feed composition. Herein, the migration of [Cu(NH₃)₂]⁺ complexes between two adjacent cavities of the chabazite structure, including other reactant molecules (NO, O₂, H₂O, and NH₃), in the initial and final cavities is investigated using ab initio molecular dynamics (AIMD) simulations combined with enhanced sampling techniques to describe hopping events from one cage to the other. We find that such diffusion is only significantly hindered by the presence of excess NH₃ or NO in the initial cavity, since both reactants form with [Cu(NH₃)₂]⁺ stable intermediates which are too bulky to cross the 8-ring windows connecting the cavities. The presence of O₂ modifies strongly the interaction of NO with Cu⁺. At low temperatures, we observe NO detachment from Cu⁺ and increased mobility of the [Cu(NH₃)₂]⁺ complex, while at high temperatures, NO reacts spontaneously with O₂ to form NO₂. The present simulations give evidence for recent experimental observations, namely, an NH₃ inhibition effect on the SCR reaction at low temperatures, and transport limitations of NO and NH₃ at high temperatures. Our first principle simulations mimicking operating conditions support the existence of two different reaction mechanisms operating at low and high temperatures, the former involving dimeric Cu(NH₃)₂-O₂-Cu(NH₃)₂ species and the latter occurring by direct NO oxidation to NO₂ in one single cavity.

Improved lifetime and stability of copper species in hierarchical, copper-incorporated CuSAPO-34 verified by catalytic model reactions

The first successful synthesis of hierarchical CuSAPO-34 (3.9 wt% Cu) is reported using the polymer Pluronic F127 as a mesoporous structure directing agent (SDA). X-Ray absorption spectroscopy (XAS) revealed single site Cu2+ with 4 nearest oxygen neighbours at 1.96 Å. A catalytic model reaction, the selective reduction of NO with different sized hydrocarbons as reductants, explained that Cu2+ is accessible and reactive in both micro- and mesopores of the hierarchical CuSAPO-34. The presence of mesopores resulted in superior lifetime of the hierarchical CuSAPO-34 in the catalytic model reaction, selective oxidation of propene.

Heavy metal poisoning resistance of a Co-modified 3Mn10Fe/Ni low-temperature SCR deNOx catalyst

Heavy metals have a great influence on the deNO_x efficiency of catalysts. The 3Mn10Fe/Ni catalyst that used nickel foam (Ni) as the carrier, Mn and Fe as the active components, and Co as a trace auxiliary was prepared using an impregnation method. The catalysts poisoned by Pb or Zn and Co-modified catalysts with Pb or Zn poisoning were studied. The addition of Pb or Zn significantly decreases the deNO_x activity of the 3Mn10Fe/Ni catalyst due to the decrease in the content of high-valence metal elements such as Fe³⁺ and Mn⁴⁺, lattice oxygen concentration, reduction performance, acidity, and the number of acid sites. However, after Co modification, the deNO_x activity of the poisoned catalysts can be improved effectively because the strong interaction between Pb or Zn and lattice oxygen is weakened, and the contents of lattice oxygen, high valence metal elements, reduction ability, and the number of acid sites increase.

Environmental benign synthesis of Nano-SSZ-13 via FAU trans-crystallization: Enhanced NH3-SCR performance on Cu-SSZ-13 with nano-size effect

Small pore zeolites with chabazite structure have been commercialized for selective catalytic reduction (SCR) of nitrogen oxides (NO_x) with ammonium (NH₃) from diesel exhaust. However, conventional zeolite synthesis processes detrimental effects on the environment due to the consumption of large amount of water, organic templates. Thus, this study proposed a green synthesis process with addition of minimal amount of water, structure directing agent and shortened steps to prepare nano-sized SSZ-13 (0.12 μm) using trans-crystallization strategy and exhibited enhanced performance for NO_x removal after copper ion-exchange. The operation temperature window (NO_x conversion >90 %) as well as the SO₂ and H₂O resistance over the green-route prepared nano-sized SSZ-13 (178-480 °C) outperformed the conventional SSZ-13 (29.8 μm, 211-438 °C) mainly due to the much shorter diffusion path. This clearly implied that the mass transportation was key for NH₃-SCR of NO_x on such small pore zeolite catalysts, which was further confirmed via an in-depth mass transportation calculation process. These results demonstrate that the Cu-nano-sized SSZ-13 prepared by the environmental benign route has great potential to act as a new generation of deNO_x catalyst for diesel exhaust and provided a guideline for researchers to develop new methods to synthesize nano-catalysts for air pollution control.

Understanding the Deactivation Phenomena of Small-Pore Mo/H-SSZ-13 during Methane Dehydroaromatisation

Small pore zeolites have shown great potential in a number of catalytic reactions. While Mo-containing medium pore zeolites have been widely studied for methane dehydroaromatisation (MDA), the use of small pore supports has drawn limited attention due to the fast deactivation of the catalyst. This work investigates the structure of the small pore Mo/H-SSZ-13 during catalyst preparation and reaction by operando X-ray absorption spectroscopy (XAS), in situ synchrotron powder diffraction (SPD), and electron microscopy; then, the results are compared with the medium pore Mo/H-ZSM-5. While SPD suggests that during catalyst preparation, part of the MoO_x anchors inside the pores, Mo dispersion and subsequent ion exchange was less effective in the small pore catalyst, resulting in the formation of mesopores and Al₂(M_OO₄)₃ particles. Unlike Mo/H-ZSM-5, part of the Mo species in Mo/H-SSZ-13 undergoes full reduction to Mo⁰ during MDA, whereas characterisation of the spent catalyst indicates that differences also exist in the nature of the formed carbon deposits. Hence, the different Mo speciation and the low performance on small pore zeolites can be attributed to mesopores formation during calcination and the ineffective ion exchange into well dispersed Mo-oxo sites. The results open the scope for the optimisation of synthetic routes to explore the potential of small pore topologies.

Combining Kinetics and Operando Spectroscopy to Interrogate the Mechanism and Active Site Requirements of NOx Selective Catalytic Reduction with NH3 on Cu-Zeolites

NO_x selective catalytic reduction (SCR) with NH₃ on Cu-zeolites is a commercial emissions control technology for diesel and lean-burn engines. Mitigating low-temperature emissions remains an outstanding challenge, motivating an improved understanding of the reaction mechanism, active site requirements, and rate-determining processes at low temperatures (<523 K). In this Perspective, we discuss how operando spectroscopy provides crucial information about how the structures, coordination environments, and oxidation states of Cu active sites depend on reaction conditions and sample composition; when combined with kinetic measurements, such operando data provide insights into the Cu site and spatial density requirements for reduction and oxidation steps relevant to the Cu(II)/Cu(I) SCR redox cycle. Isolated Cu ions coordinated to zeolite oxygen atoms ex situ become coordinated to NH₃ in situ and dynamically interconvert between mononuclear and binuclear NH₃-solvated Cu complexes to catalyze SCR turnovers. We conclude with future research directions that can benefit from combining quantitative kinetic measurements with operando spectroscopy.

Active sites and mechanisms in the direct conversion of methane to methanol using Cu in zeolitic hosts: a critical examination

In this critical review we examine the current state of our knowledge in respect of the nature of the active sites in copper containing zeolites for the selective conversion of methane to methanol.

Controllable positions of Cu2+ to enhance low-temperature SCR activity on novel Cu-Ce-La-SSZ-13 by a simple one-pot method

Regulation of Cu²⁺ ions to migrate to more active SCR reaction sites can efficiently improve low-temperature SCR activity.

Why does there have to be a residual Na ion as a co-cation on Cu/SSZ-13?

Only the residual Na⁺ ion as co-cation improves the hydrothermal stability and NH₃-SCR activity of Cu/SSZ-13.

Quantitative determination of the Cu species, acid sites and NH3-SCR mechanism on Cu-SSZ-13 and H-SSZ-13 at low temperatures

The NH₃-SCR mechanism and the number of acid sites and various Cu species on Cu-SSZ-13 and H-SSZ-13 were quantitatively determined.

Evaluation of Cu/SAPO-34 Catalysts Prepared by Solid-State and Liquid Ion-Exchange Methods for NO x Removal by NH3-SCR

Cu/SAPO-34 catalysts are prepared using solid-state ion exchange (SSIE) and liquid ion exchange (LIE). SSIE is conducted by calcining a physical mixture of H-SAPO-34 zeolite and CuO nanoparticles at elevated temperatures (500-800 °C). The conventional LIE method is conducted by exchanging Na-SAPO-34 with Cu(COOCH₃)₂ aqueous solution with a final calcination step at 500 °C. Catalysts were fully characterized, focusing on Cu species identification. The NH₃-SCR activity is evaluated for NO _x removal. Cu/SAPO-34 catalysts synthesized by SSIE at 700 °C achieved an optimal reaction rate, which was correlated with a higher proportion of Cu²⁺ ions. The activation energies of Cu/SAPO-34 catalysts prepared by SSIE and LIE with varying copper loadings are 32-38 and 42-47 kJ mol^-1, respectively. The SSIE catalysts achieve higher turnover frequency than LIE catalysts for a similar copper content, which decreases on increasing the copper loading. These results provide evidence that Cu ions exchanged into the Cu/SAPO-34 catalysts synthesized by SSIE present higher activity than those prepared by LIE for NO _x removal by NH₃-SCR.

The Role of SO3 Poisoning in CU/SSZ-13 NH3-SCR Catalysts

To reveal the role of SO3 poisoning in Cu/SSZ-13 NH3-SCR catalysts, fresh and sulfated Cu/SSZ-13 catalysts were prepared in the presence or absence of SO3 flux. The deactivation mechanism is probed by the changes of structural, copper species, and selective catalytic reduction (SCR) activity. The variations concentrate on the changes of copper species as the Chabazite (CHA) framework of Cu/SSZ-13 catalysts could keep intact at high ratios of SO3/SOx. The thermal gravimetric analyzer (TGA) results reveal that the copper sulfate formed during sulfation and the amounts of sulfate species increased with an increase in the SO3/SOx ratio. In contrast to the changing trend of copper sulfate, temperature program reduction (H2-TPR), and electron paramagnetic resonance (EPR) results manifest that, since the number of active copper ions declines with an increase of the SO3/SOx ratio, the active sites transform to these inactive species during sulfation. Due to the combination of NH3-SCR activity and the kinetic tests, it is shown that the decreased number of active sites is responsible for the declined SCR activity at low temperature. As Cu/SSZ-13 catalysts show excellent acid-resistance ability, our study reveals that the Cu/SSZ-13 catalyst is a good candidate for NOx elimination, especially when SO3 exists.

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.

New trends in tailoring active sites in zeolite-based catalysts

This review discusses approaches for tailoring active sites in extra-large pore, nanocrystalline, and hierarchical zeolites and their performance in emerging catalytic applications.

Small-Pore Zeolites: Synthesis and Catalysis

In the past decade or so, small-pore zeolites have received greater attention than large- and medium-pore molecular sieves that have historically dominated the literature. This is primarily due to the commercialization of two major catalytic processes, NOx exhaust removal and methanol conversion to light olefins, that take advantage of the properties of these materials with smaller apertures. Small-pore zeolites possess pores that are constructed of eight tetrahedral atoms (Si⁴⁺ and Al³⁺), each time linked by a shared oxygen These eight-member ring pores (8MR) provide small molecules access to the intracrystalline void space, e.g., to NOx during car exhaust cleaning (NOx removal) or to methanol en route to its conversion into light olefins, while restricting larger molecule entrance and departure that is critical to overall catalyst performance. In total, there are forty-four structurally different small-pore zeolites. Forty-one of these zeolites can be synthesized, and the first synthetic zeolite (KFI, 1948) was in fact a small-pore material. Although the field of 8MR zeolite chemistry has expanded in many directions, the progress in synthesis is framework-specific, leaving insights and generalizations difficult to realize. This review first focuses on the relevant synthesis details of all 8MR zeolites and provides some generalized findings and related insights. Next, catalytic applications where 8MR zeolites either have been commercialized or have dominated investigations are presented, with the aim of providing structure-activity relationships. The review ends with a summary that discusses (i) both synthetic and catalytic progress, (ii) a list of opportunities in the 8MR zeolite field, and (iii) a brief future outlook.

Methane-to-methanol conversion over zeolite Cu-SSZ-13, and its comparison with the selective catalytic reduction of NOx with NH3

Using in situ FT-IR and operando UV-vis-NIR DRS, performed on a series of different Cu–ion-exchanged SSZ-13 zeolites, both a mono-nuclear site or a dimeric copper active site are consistent for methane-to-methanol activation.

Operando Spectroscopic Studies of Cu-SSZ-13 for NH3-SCR deNOx Investigates the Role of NH3 in Observed Cu(II) Reduction at High NO Conversions

The small pore zeolite chabazite (SSZ-13) in the copper exchanged form is a very efficient material for the selective catalytic reduction by ammonia (NH₃) of nitrogen oxides (NOx) from the exhaust of lean burn engines, typically diesel powered vehicles. The full mechanism occurring during the NH₃–SCR process is currently debated with outstanding questions including the nature and role of the catalytically active sites. Time-resolved operando spectroscopic techniques have been used to provide new level of insights in to the mechanism of NH₃–SCR, to show that the origin of stable Cu(I) species under SCR conditions is potentially caused by an interaction between NH₃ and the Cu cations located in eight ring sites of the bulk of the zeolite and is independent of the NH₃–SCR of NOx occurring at Cu six ring sites within the zeolite.

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.

Redox-Driven Migration of Copper Ions in the Cu-CHA Zeolite as Shown by the In Situ PXRD/XANES Technique

Using quasi-simultaneous in situ PXRD and XANES, the direct correlation between the oxidation state of Cu ions in the commercially relevant deNO_x NH₃ -SCR zeolite catalyst Cu-CHA and the Cu ion migration in the zeolitic pores was revealed during catalytic activation experiments. A comparison with recent reports further reveals the high sensitivity of the redox-active centers concerning heating rates, temperature, and gas environment during catalytic activation. Previously, Cu⁺ was confirmed present only in the 6R. Results verify a novel 8R monovalent Cu site, an eventually large Cu⁺ presence upon heating to high temperatures in oxidative conditions, and demonstrate the unique potential in combining in situ PXRD and XANES techniques, with which both oxidation state and structural location of the redox-active centers in the zeolite framework could be tracked.

The dependence on ammonia pretreatment of N-O activation by Co(II) sites in zeolites: a DFT and ab initio molecular dynamics study

This work is focused on the donor properties of cobalt-exchanged cationic sites in zeolites. It is based on cluster and periodic density functional theory modeling for relevant {[Co(II)(NH3)n]-NO} adducts, where Co(II) means a cobalt cation embedded either in a periodic model of chabasite (CHA) zeolite or in model clusters. NO stretching frequencies were derived from MD trajectories and compared to harmonic values from cluster calculations. By relating calculated NO frequencies to experimental FTIR spectra, it was shown that the forms of {Co(II)-NO} adducts comprising three or four ammonia co-ligands dominate the spectrum taken in ammonia-saturation conditions while forms with two NH3 ligands prevail under intermediate ammonia saturation. Finally, this work confirms the critical dependence of Co(II) activation ability towards NO upon the center donor properties, reinforced by ligation of strong donor ammonia ligands. However, strongly bound ligands appear also to compete with interaction of the center with the electron-rich framework, and a balance must be observed to maintain optimal activation ability. Graphical abstract A snapshot from MD trajectory showing a fragment of periodic framework with twoCo(II)-NO centers, bound to one framework oxygen and strongly coordinating three ammonia ligands with four others forming the second coordination sphere.

Enhanced activity of desilicated Cu-SSZ-13 for the selective catalytic reduction of NOx and its comparison with steamed Cu-SSZ-13

Mesoporous Cu-SSZ-13 was created by first synthesizing zeolite H-SSZ-13 and subsequently desilicating the material by base leaching using NaOH in different concentrations.