The relationships among the macroscopic compositional parameters of a Cu-exchanged SSZ-13 zeolite catalyst, the types and numbers of Cu active sites, and activity for the selective catalytic reduction (SCR) of NOx with NH3 are established through experimental interrogation and computational analysis of materials across the catalyst composition space. Density functional theory, stochastic models, and experimental characterizations demonstrate that within the synthesis protocols applied here and across Si:Al ratios, the volumetric density of six-membered-rings (6MR) containing two Al (2Al sites) is consistent with a random Al siting in the SSZ-13 lattice subject to Lowenstein’s rule. Further, exchanged CuII ions first populate these 2Al sites before populating remaining unpaired, or 1Al, sites as CuIIOH. These sites are distinguished and enumerated ex situ through vibrational and X-ray absorption spectroscopies (XAS) and chemical titrations. In situ and operando XAS follow Cu oxidation state and coordinatio...

Catalysis in a Cage: Condition-Dependent Speciation and Dynamics of Exchanged Cu Cations in SSZ-13 Zeolites

This review addresses the most relevant aspects of vibrational spectroscopies (IR, Raman and INS) applied to zeolites and zeotype materials. Surface Bronsted and Lewis acidity and surface basicity are treated in detail. The role of probe molecules and the relevance of tuning both the proton affinity and the steric hindrance of the probe to fully understand and map the complex site population present inside microporous materials are critically discussed. A detailed description of the methods needed to precisely determine the IR absorption coefficients is given, making IR a quantitative technique. The thermodynamic parameters of the adsorption process that can be extracted from a variable-temperature IR study are described. Finally, cutting-edge space- and time-resolved experiments are reviewed. All aspects are discussed by reporting relevant examples. When available, the theoretical literature related to the reviewed experimental results is reported to support the interpretation of the vibrational spectra on an atomic level.

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Probing zeolites by vibrational spectroscopies

In situ diffuse reflectance infrared Fourier transform spectroscopy (In situ DRIFTS), temperature-programmed desorption (TPD), and temperature-programmed surface reactions (TPSR) were employed to investigate the adsorption and reactive properties of Cu-SSZ-13 and Cu-SAPO-34 zeolite catalysts; these fully formulated washcoat cordierite monoliths were hydrothermally treated at 750 °C in the simulated feed gases. The intrinsic mechanism and reasons for the differences in NH3-SCR activity were proposed based on the characterization results. The in situ DRIFTS and TPD results showed that ammonia could adsorb on both the Lewis and Bronsted acidic sites on these two catalysts; the ammonia on the Bronsted acidic sites might be active in the NH3-SCR reaction. For the different NOx adsorption processes, the total NOx desorption levels followed the following sequence: NO

In situ DRIFTS and temperature-programmed technology study on NH3-SCR of NOx over Cu-SSZ-13 and Cu-SAPO-34 catalysts

The development of Cu-chabazite (CHA) catalysts, i.e. Cu-SSZ-13 and Cu-SAPO-34, represents a significant technology breakthrough for the removal of NOx by selective catalytic reduction (SCR) with ammonia. Cu-CHA catalysts show an excellent hydrothermal stability towards high temperature aging and wide active temperature windows for the ammonia SCR reaction. This work summarizes the recent progress in the development of the Cu-CHA catalysts for the NH3-SCR reaction. The state of Cu in the reaction and the preparation methods on the catalytic performance are discussed. The advances in the understanding of the reaction mechanism are reviewed. The hydrothermal stability of the typical Cu-CHA catalysts are compared.

Recent advances in the selective catalytic reduction of NOx with NH3 on Cu-Chabazite catalysts

Structure–reactivity relationships for nanoparticle-based catalysts have been greatly influenced by the study of catalytic materials with either supported isolated metal atoms or metal clusters comprising a few atoms. The stability of these metal species is a key challenge because they can sinter into large nanoparticles under harsh reaction conditions. However, stability can be achieved by confining the nanoparticles in crystalline porous materials (such as zeolites and metal–organic frameworks). More importantly, the interaction between the metal species and the porous framework may modulate the geometric and electronic structures of the subnanometric metal species, especially for metal clusters. This confinement effect can induce shape-selective catalysis or different chemoselectivity from that of metal atoms supported on open-structure solid carriers. In this Review, we discuss the structural features, synthesis methodologies, characterization techniques and catalytic applications of subnanometric species confined in zeolites and metal–organic frameworks. We make a critical comparison between confined and non-confined isolated atoms and metal clusters, and provide future perspectives for the field. Supported isolated metal atoms and subnanometric metal clusters are emerging catalytic materials. This Review discusses the influence of confining subnanometric metal species in zeolites and metal–organic frameworks on their geometric and electronic structures and catalytic performance.

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Confining isolated atoms and clusters in crystalline porous materials for catalysis

Migration of copper species from the surface region upon activation of Cu–SAPO-34 leads to a several-fold increase in catalytic activity for the selective catalytic reduction of NOx with NH3. The temperatures required for the activation are higher than normal calcination temperatures and lead to a homogeneous transition metal distribution that renders Cu–SAPO-34 an industrially promising candidate for NOx removal.

Anna Katerinopoulou

Papers

Migration of Cu Ions in SAPO-34 and Its Impact on Selective Catalytic Reduction of NOx with NH3