An overview is given of the selective catalytic reduction of NOx by ammonia (NH3‐SCR) over metal‐exchanged zeolites. The review gives a comprehensive overview of NH3‐SCR chemistry, including undesired side‐reactions and aspects of the reaction mechanism over zeolites and the active sites involved. The review attempts to correlate catalyst activity and stability with the preparation method, the exchange metal, the exchange degree, and the zeolite topology. A comparison of Fe‐ZSM‐5 catalysts prepared by different methods and research groups shows that the preparation method is not a decisive factor in determining catalytic activity. It seems that decreased turnover frequency (TOF) is an oft‐neglected effect of increasing Fe content, and this oversight may have led to the mistaken conclusion that certain production methods produce highly active catalysts. The available data indicate that both isolated and bridged iron species participate in the NH3‐SCR reaction over Fe‐ZSM‐5, with isolated species being the ...

https://www.tandfonline.com/doi/pdf/10.1080/01614940802480122

The State of the Art in Selective Catalytic Reduction of NOx by Ammonia Using Metal‐Exchanged Zeolite Catalysts

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Reactivity of Surface Species in Heterogeneous Catalysts Probed by In Situ X-ray Absorption Techniques

In this review we focus on the catalytic removal of a series of N-containing exhaust gases with various valences, including nitriles (HCN, CH3CN, and C2H3CN), ammonia (NH3), nitrous oxide (N2O), and nitric oxides (NOx), which can cause some serious environmental problems, such as acid rain, haze weather, global warming, and even death. The zeolite catalysts with high internal surface areas, uniform pore systems, considerable ion-exchange capabilities, and satisfactory thermal stabilities are herein addressed for the corresponding depollution processes. The sources and toxicities of these pollutants are introduced. The important physicochemical properties of zeolite catalysts, including shape selectivity, surface area, acidity, and redox ability, are described in detail. The catalytic combustion of nitriles and ammonia, the direct catalytic decomposition of N2O, and the selective catalytic reduction and direct catalytic decomposition of NO are systematically discussed, involving the catalytic behaviors as ...

Selective Transformation of Various Nitrogen-Containing Exhaust Gases toward N2 over Zeolite Catalysts.

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

Exploring materials with regulated local structures and understanding how the atomic motifs govern the reactivity and durability of catalysts are a critical challenge for designing advanced catalysts. Herein we report the tuning of the local atomic structure of nickel-iron layered double hydroxides (NiFe-LDHs) by partially substituting Ni2+ with Fe2+ to introduce Fe-O-Fe moieties. These Fe2+ -containing NiFe-LDHs exhibit enhanced oxygen evolution reaction (OER) activity with an ultralow overpotential of 195 mV at the current density of 10 mA cm-2 , which is among the best OER catalytic performance to date. In-situ X-ray absorption, Raman, and electrochemical analysis jointly reveal that the Fe-O-Fe motifs could stabilize high-valent metal sites at low overpotentials, thereby enhancing the OER activity. These results reveal the importance of tuning the local atomic structure for designing high efficiency electrocatalysts.

Introducing Fe 2+ into Nickel–Iron Layered Double Hydroxide: Local Structure Modulated Water Oxidation Activity

Active Sites in Fe/MFI Catalysts for NOx Reduction and Oscillating N2O Decomposition

Process for purifying alkane-containing feed streams by contacting them with one or more metal complexes with intra-crystalline channels according to the general formula: M a [M XbLc]d (OH)e wherein M represents at least one metal from Group 1b, 2a, 2b, 4a, 7b and/or the nm-noble metals of Group 8; M represents at least one metal from Group 6b, 7b and/or 8; X represents an (iso)cyano-, and/or (iso)cyanato- and/or (iso)thiocyano-moiety; L represents a nitroso-, an amino- and/or a halide-moiety, a, b, c, d and e (when OH is present) provide the electroneutrality; and c and e may equal O. Compounds such as magnesium hexacyano cobaltate and zinc hydroxy hexacyano cobaltate can be used advantageously in the purification of methane by selectively adsorbing carbon dioxide present therein.

van Ra Rutger Santen

Papers

Active Sites in Fe/MFI Catalysts for NOx Reduction and Oscillating N2O Decomposition

Gas purification process