Rational Regulation of Reducibility and Acid Site on Mn-Fe-BTC to Achieve High Low-Temperature Catalytic Denitration Performance

Selective catalytic reduction with ammonia is the mainstream technology of flue gas denitration (de-NO_x). The reducibility and acid site are two important factors affecting the de-NO_x performance, and effective regulation between them is the key to obtain a highly efficient de-NO_x catalyst. Herein, a series of Mn-Fe-BTC with different ratios of Mn and Fe are synthesized, among which 2Mn-1Fe-BTC with 2:1 molar ratio of Mn and Fe has excellent low-temperature (LT) de-NO_x performance (above 90% NO conversion between 60 and 270 °C) and good tolerance to H₂O and SO₂ poisoning (88% NO conversion at 150 °C with 100 ppm of SO₂ and/or 6% H₂O). It is revealed that the reducibility properties and acid sites of Mn-Fe-BTC can be flexibly tuned by the ratio of Mn and Fe. The difference in electronegativity between Fe and Mn leads to the redistribution of valence electrons, which enables the controllable reducibility of Mn-Fe-BTC. Furthermore, different amounts of Mn and Fe lead to different electron transport, which determines the type and number of acid sites. The synergistic effect of Mn and Fe endows Mn-Fe-BTC with enhanced surface molecular adsorption capacity and enables the catalyst to selectively chemisorb NH₃ and NO at different active sites. This research provides guidance for the flexible regulation of reducibility and acid site of LT de-NO_x catalyst.

A Theoretical Investigation of Cu+, Ni2+ and Co2+-exchanged Zeolites for Hydrogen Storage

This study investigates the H 2 adsorption on Cu + , Ni 2+ and Co 2+ -exchanged SSZ-13 (CHA) and SSZ-39 (AEI) using periodic DFT computations. Most stable Cu + position was found to be the 6-membered-ring window for both zeolites. Similarly, for investigated Ni 2+ and Co 2+ loadings on 6-membered-ring windows, the third nearest neighbor Al positions, i.e., Al-O-Si-O-Si-O-Al coordination, was found to be the most stable position. H 2 adsorption was investigated for all the Cu + , Ni 2+ and Co 2+ centers. AEI and CHA resulted in similar H 2 -Cu interactions for the Al and B substituted structures. H 2 adsorption on Cu + located in the 8-membered-ring gave the highest adsorption energy for both frameworks. Replacing Al with B in the framework increased the electron back donation from Cu + (3d) orbitals to H 2 antibonding orbital (s H2 * ). The H 2 adsorption energies on the Ni 2+ and Co 2+ -exchanged zeolites were found to be between -15 and -44 kJ/mol. Higher energy values were observed on the AEI framework, especially when two Al atoms have the Al-O-Si-O-Al configuration. Lesser interaction of the d-orbitals in the case of the Co 2+ and Ni 2+ cations resulted in heat of H 2 adsorption close to optimum values required for H 2 storage on porous materials.

Theoretical Studies on the Mechanism of deNOx Process in Cu-Zn Bimetallic System-Comparison of FAU and MFI Zeolites

In the present study we propose a more promising catalyst for the deNOx process to eliminate harmful nitrogen oxides from the environment. The study was performed with a computer calculation using density functional theory (DFT) based on an ab initio method. Two zeolite catalysts, FAU and MFI, were selected with additional Cu-O-Zn bimetallic dimer adsorbed inside the pores of both zeolites. Based on the analysis of preliminary studies, the most probable way of co-adsorption of nitric oxide and ammonia was selected, which became the initial configuration for the reaction mechanism. Two types of mechanisms were proposed: with hydroxyl groups on a bridged position of the dimer or a hydroxyl group on one of the metal atoms of the dimer. Based on the results, it was determined that the FAU zeolite with a bimetallic dimer and an OH group on the zinc atom was the most efficient configuration with a relatively low energy barrier. The real advantage of the Cu-Zn system over FAU and MFI in hydrothermal conditions has been demonstrated in comparison to a conventional Cu-Cu catalyst.

Investigating the role of Cu-oxo species in Cu-nitrate formation over Cu-CHA catalysts

The speciation of framework-interacting CuII sites in Cu-chabazite zeolite catalysts active in the selective catalytic reduction of NOx with NH3 is studied, to investigate the influence of the Al content on the copper structure and their reactivity towards a NO/O2 mixture. To this aim, three samples with similar Cu densities and different Si/Al ratios (5, 15 and 29) were studied using in situ X-ray absorption spectroscopy (XAS), FTIR and diffuse reflectance UV-Vis during pretreatment in O2 followed by the reaction. XAS and UV-Vis data clearly show the main presence of Z2CuII sites (with Z representing a framework negative charge) at a low Si/Al ratio, as predicted. EXAFS wavelet transform analysis showed a non-negligible fraction of proximal Z2CuII monomers, possibly stabilized into two 6-membered rings within the same cage. These sites are not able to form Cu-nitrates by interaction with NO/O2. By contrast, framework-anchored Z[CuII(NO3)] complexes with a chelating bidentate structure are formed in samples with a higher Si/Al ratio, by reaction of NO/O2 with Z[CuII(OH)] sites or structurally similar mono- or multi-copper Zx[CuIIxOy] sites. Linear combination fit (LCF) analysis of the XAS data showed good agreement between the fraction of Z[CuII(OH)]/Zx[CuIIxOy] sites formed during activation in O2 and that of Z[CuII(NO3)] complexes formed by reaction with NO/O2, further confirming the chemical inertia of Z2CuII towards these reactants in the absence of solvating NH3 molecules.

Selective catalytic reduction of NO with NH3 over Cu-exchanged CHA, GME, and AFX zeolites: a density functional theory study

Density functional theory calculations have been applied to study the selectivity caused by the cage size during the selective catalytic reduction of NO by NH₃ over the Cu-exchanged zeolites with cha, gme, and aft cages.

Selective catalytic reduction of NO over Cu-AFX zeolites: mechanistic insights from in situ/operando spectroscopic and DFT studies

In situ/operando spectroscopic experiments and DFT calculations unravel the redox mechanism of NH₃-SCR over Cu-AFX zeolites.

Kinetic and spectroscopic insights into the behaviour of Cu active site for NH3-SCR over zeolites with several topologies

Zeolite topology has a great effect on the dependence of NH₃-SCR rates over Cu–zeolites at 473 K on Cu density. It is revealed by the time-resolved UV-vis measurements that zeolites mainly affect the oxidation property of Cu ion by O₂.

Recent Understanding of Low-Temperature Copper Dynamics in Cu-Chabazite NH3-SCR Catalysts

Dynamic motion of NH3-solvated Cu sites in Cu-chabazite (Cu-CHA) zeolites, which are the most promising and state-of-the-art catalysts for ammonia-assisted selective reduction of NOx (NH3-SCR) in the aftertreatment of diesel exhausts, represents a unique phenomenon linking heterogeneous and homogeneous catalysis. This review first summarizes recent advances in the theoretical understanding of such low-temperature Cu dynamics. Specifically, evidence of both intra-cage and inter-cage Cu motions, given by ab initio molecular dynamics (AIMD) or metadynamics simulations, will be highlighted. Then, we will show how, among others, synchrotron-based X-ray spectroscopy, vibrational and optical spectroscopy (diffuse reflection infrared Fourier transform spectroscopy (DRIFTS) and diffuse reflection ultraviolet-visible spectroscopy (DRUVS)), electron paramagnetic spectroscopy (EPR), and impedance spectroscopy (IS) can be combined and complement each other to follow the evolution of coordinative environment and the local structure of Cu centers during low-temperature NH3-SCR reactions. Furthermore, the essential role of Cu dynamics in the tuning of low-temperature Cu redox, in the preparation of highly dispersed Cu-CHA catalysts by solid-state ion exchange method, and in the direct monitoring of NH3 storage and conversion will be presented. Based on the achieved mechanistic insights, we will discuss briefly the new perspectives in manipulating Cu dynamics to improve low-temperature NH3-SCR efficiency as well as in the understanding of other important reactions, such as selective methane-to-methanol oxidation and ethene dimerization, catalyzed by metal ion-exchanged zeolites.

Solvation and Mobilization of Copper Active Sites in Zeolites by Ammonia: Consequences for the Catalytic Reduction of Nitrogen Oxides

ConspectusCopper-exchanged chabazite (Cu-CHA) zeolites are catalysts used in diesel emissions control for the abatement of nitrogen oxides (NO_x) via selective catalytic reduction (SCR) reactions with ammonia as the reductant. The discovery of these materials in the early 2010s enabled a step-change improvement in diesel emissions aftertreatment technology. Key advantages of Cu-CHA zeolites over prior materials include their effectiveness at the lower temperatures characteristic of diesel exhaust, their durability under high-temperature hydrothermal conditions, and their resistance to poisoning from residual hydrocarbons present in exhaust. Fundamental catalysis research has since uncovered mechanistic and kinetic features that underpin the ability of Cu-CHA to selectively reduce NO_x under strongly oxidizing conditions and to achieve improved NO_x conversion relative to other zeolite frameworks, particularly at low exhaust temperatures and with ammonia instead of other reductants.One critical mechanistic feature is the NH₃ solvation of exchanged Cu ions at low temperatures (<523 K) to create cationic Cu-amine coordination complexes that are ionically tethered to anionic Al framework sites. This ionic tethering confers regulated mobility that facilitates interconversion between mononuclear and binuclear Cu complexes, which is necessary to propagate SCR through a Cu²⁺/Cu⁺ redox cycle during catalytic turnover. This dynamic catalytic mechanism, wherein single and dual metal sites interconvert to mediate different half-reactions of the redox cycle, combines features canonically associated with homogeneous and heterogeneous reaction mechanisms.In this Account, we describe how a unified experimental and theoretical interrogation of Cu-CHA catalysts in operando provided quantitative evidence of regulated Cu ion mobility and its role in the SCR mechanism. This approach relied on new synthetic methods to prepare model Cu-CHA zeolites with varied active-site structures and spatial densities in order to verify that the kinetic and mechanistic models describe the catalytic behavior of a family of materials of diverse composition, and on new computational approaches to capture the active-site structure and dynamics under conditions representative of catalysis. Ex situ interrogation revealed that the Cu structure depends on the conditions for the zeolite synthesis, which influence the framework Al substitution patterns, and that statistical and electronic structure models can enumerate Cu site populations for a known Al distribution. This recognition unifies seemingly disparate spectroscopic observations and inferences regarding Cu ion structure and responses to different external conditions. SCR rates depend strongly on the Cu spatial density and zeolite composition in kinetic regimes where Cu⁺ oxidation with O₂ becomes rate-limiting, as occurs at lower temperatures and under fuel-rich conditions. Transient experiments, ab initio molecular dynamics simulations, and statistical models relate these sensitivities to the mobility constraints imposed by the CHA framework on NH₃-solvated Cu ions, which regulate the pore volume accessible to these ions and their ability to pair and complete the catalytic cycle. This highlights the key characteristics of the CHA framework that enable superior performance under low-temperature SCR reaction conditions.This work illustrates the power of precise control over a catalytic material, simultaneous kinetic and spectroscopic interrogation over a wide range of reaction conditions, and computational strategies tailored to capture those reaction conditions to reveal in microscopic detail the mechanistic features of a complex and widely practiced catalysis. In doing so, it highlights the key role of ion mobility in catalysis and thus potentially a more general phenomenon of reactant solvation and active site mobilization in reactions catalyzed by exchanged metal ions in zeolites.