Sites For Selective Catalytic Reduction Research Articles

Insights into Synergy of Copper and Acid Sites for Selective Catalytic Reduction of NO with Ammonia over Zeolite Catalysts

Herein, we report the function of copper sites in Cu-SSZ-13, Cu-ZSM-5 and Cu-Beta catalysts with the same Si/Al ratio (14) and Cu/Al ratio (0.4) on selective catalytic reduction of NO with NH3 (NH3-SCR) and reveal the relationship between active sites (Cu sites, acid sites) and catalytic activity. The results show that the amount of isolated Cu2+ ions in the catalysts directly determines the formation of strong Lewis acid sites and reaction intermediate NO3− ions, thus affecting the low-temperature SCR performance, while the amount of highly stable Cu+ ions and Brønsted acid sites is related to the high-temperature SCR performance of the catalysts. Consequently, it contains enough isolated Cu2+ ions, highly stable Cu+ ions and Brønsted acid sites, which endows Cu-SSZ-13 with excellent NH3-SCR activity.

Design of robust Co-doped Mn3O4 spinel catalysts for selective catalytic reduction of NO with NH3 at low temperatures

Developing an efficient catalyst with high intrinsic active sites for selective catalytic reduction of NOx with ammonia (NH3-SCR) at ultra-low temperature flue gas is challenging but desirable. Herein, a Co-doped Mn3O4 catalyst (Co0.04Mn2.96O4) was created by a confinement-pyrolysis-oxidation strategy using bimetallic Co–Mn–MOF–74 as the precursor, which exhibited excellent SCR performance with 96% NO conversion at 130 °C under a GHSV of 100,000 h−1. Benefited from the uniform microstructure of catalyst and combined with characterization and theoretical calculations, an efficient Cotet–O–Mnoct active site is revealed for NH3-SCR, and the formation of oxygen vacancies on Cotet–O–Mnoct site plays a crucial role in the adsorption and activation of reactants, which facilitating the low–temperature NH3-SCR activity. This work provides new insights into the design and synthesis of highly active NH3-SCR catalysts with high intrinsic active sites used at low temperatures.

Evolution mechanism of active sites for selective catalytic reduction of NOx with NH3 over Fe-ZSM-5 catalysts doped by Ce/Cu

Evolution mechanism of active sites for selective catalytic reduction of NOx with NH3 over Fe-ZSM-5 catalysts doped by Ce/Cu

Theoretical and Spectroscopic Evidence of the Dynamic Nature of Copper Active Sites in Cu-CHA Catalysts under Selective Catalytic Reduction (NH3-SCR-NOx) Conditions.

The dynamic nature of the copper cations acting as active sites for selective catalytic reduction of nitrogen oxides with ammonia is investigated using a combined theoretical and spectroscopic approach. Ab initio molecular dynamics simulations of Cu-CHA catalysts in contact with reactants and intermediates at realistic operating conditions show that only ammonia is able to release Cu+ and Cu2+ cations from their positions coordinated to the zeolite framework, forming mobile Cu+(NH3)2 and Cu2+(NH3)4 complexes that migrate to the center of the cavity. Herein, we give evidence that such mobilization of copper cations modifies the vibrational fingerprint in the 800–1000 cm–1 region of the IR spectra. Bands associated with the lattice asymmetric T-O-T vibrations are perturbed by the presence of coordinated cations, and allow one to experimentally follow the dynamic reorganization of the active sites at operating conditions.

New insights into the role of vanadia species as active sites for selective catalytic reduction of NO with ammonia over VOx/CeO2 catalysts

A series of VOx/CeO2 catalysts were synthesized via vanadia supported on ceria with different BET surface areas. The catalysts were employed to investigate the active sites for the selective catalytic reduction of NO with NH3 (NH3-SCR). The kinetic results show that VOx/CeO2 catalysts exhibit nearly constant apparent activation energies (Ea), indicating the same SCR reaction mechanism. The V–O–Ce bridging modes and oligomeric VOx were identified and quantified by Raman, FT-IR and H2-TPR. The amounts of the V–O–Ce bridging modes calculated by H2-TPR are correlated to the NH3-SCR intrinsic reaction rates. The turnover frequencies (TOFs) show a constant value at the same temperature, which were calculated based on the number of V–O–Ce bridging modes of VOx/CeO2 catalysts. Therefore, it can be concluded that the V–O–Ce bridging modes are the active sites of VOx/CeO2 catalysts for the NH3-SCR reaction.

Investigation of Cu Species in CuBTC: Active Sites for Selective Catalytic Reduction of NO with NH<sub>3</sub>

The classical metal-organic Frameworks CuBTC showed remarkable low temperature activity in selective catalytic reduction of NO with NH3 (NH3-SCR). It was found the conversion of NO can reach as high as 100% in the range 210-300 °C on the activated sample. The nature of the active Cu species among CuBTC in NH3-SCR based on the activity data were characterized by TEM, XPS, XRD, EPR and IR. This high activity is not only due to the unsaturated metal centers of the frameworks, but also to the presence of high-dispersed Cu2O particles. Both Cu2+ and Cu+ species are the active sites for NH3-SCR.

Isolated Cu2+ions: active sites for selective catalytic reduction of NO

Cu chabazite catalysts show remarkable low temperature activity in selective catalytic reduction (SCR) of NO. This high activity is due to the unique character of the zeolite framework that allows only the presence of one type of isolated mononuclear Cu(2+) species. These Cu(2+) species are the active sites for SCR.

Lean selective catalytic reduction of NO2 by methane over Co‐zeolites

The reaction of NO2, CH4 and O2 was studied using low levels of methane compared to NO2 and O2 over protonic and cobalt‐exchanged ferrierite, ZSM‐5 and mordenite zeolites. Results suggest that two reaction pathways at low and high temperatures may be involved in the lean selective catalytic reduction (SCR) of NO2 by methane. At low temperatures, the reduction of NO2 to NO and N2 might be the initial reaction step. It is likely that NO2 or its adsorbed precursors initiate the reaction of methane at low temperatures. At high temperatures, the oxidation of NO and combustion of methane with oxygen might be involved. No appreciable differences were observed in the reduction of NO2 over Co‐zeolites as compared to known results of NO reduction over these materials. However, enhanced N2 formation rate was observed on H‐zeolites starting from NO2 instead of data reported for NO. Furthermore, it appears that the active sites for SCR are both acid and metal sites.

ChemInform Abstract: Chromia on Titania. Part 4. Nature of Active Sites for Selective Catalytic Reduction of NO by NH3.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Chromia on Titania: IV. Nature of Active Sites for Selective Catalytic Reduction of NO by NH3

The surface sites on titania-supported chromia (CrOx/TiO2), which are relevant for the selective catalytic reduction (SCR) of NO by NH3, and the undesired formation of N2O have been investigated using temperature-programmed desorption (TPD) combined with in situ diffuse reflectance FTIR spectroscopy (DRIFT). The combined TPD and DRIFT measurements carried out on the differently pretreated (reduced, oxidised, exposed to SCR conditions) CrOx/TiO2 indicate that Brønsted-bound ammonia is crucial for the SCR reaction. N2O is formed at temperatures above 450 K by direct oxidation of Lewis-bound NH3 with oxygen present in the SCR feed gas. TPD measurements carried out with CrOx/TiO2 previously loaded with NO and NH3, respectively, showed that neither oxidised nor reduced CrOx/TiO2 convert NO in the absence of NH3 at the temperatures used for SCR. Adsorbed NO has been identified in various forms, including nitrate (1613 and 1517 cm−1) and a Cr-ONO species (1458 cm1). When adsorbed as a single reactant, ammonia desorbs molecularly from the prereduced catalyst and undergoes direct oxidation on a preoxidised catalyst leading to N2O and H2O at temperatures higher than about 350 K. At temperatures above 450 K, the direct oxidation of ammonia yielding N2, becomes significant. In the absence of NO, direct oxidation of NH3 to N2 and N2O occurs via Lewis-bound ammonia. In the presence of oxygen, ammonia is oxidised to NO above about 500 K. TPD experiments with CrOx/TiO2, previously either reduced or oxidised and then exposed to the SCR feed gas, showed that the surface reduced with ammonia is inactive for the desired selective reaction between NO and NH3, indicating that a partially oxidised state of the surface must be maintained for SCR; this observation confirms the crucial role of the oxygen in the SCR feed gas. The studies indicate that Brønsted-bound ammonia, characterised by bands at 1662 and 1425 cm−1, enhances the adsorption of NO and is involved in the selective catalytic reduction of NO to N2.

Study of the NO 2-NH 3 Interaction on a Titania (ANATASE) Supported Vanadia Catalyst

The adsorption and coadsorption of nitrogen dioxide and ammonia on an anatase-supported vanadia catalyst prepared by ion exchange from an acidic aqueous solution of ammonium vanadate are investigated by IR spectroscopy and ESR. The sequence of formation of adsorbed species and the thermal stability of the surface complexes are studied. It is established that the active sites for selective catalytic reduction (SCR) of NO 2 by NH 3 on vanadiatitania catalysts are (i) the V 5+O groups, which interact with NO 2, and, as a result, vanadium is reduced to V 4+, while the nitrogen dioxide is oxidized to a nitrate coordinated to the vanadium (IV); and (ii) the V 5+-OH groups, which react with NO 2, thus giving rise to a strong Brønsted acidity. Ammonia is adsorbed with the participation of the induced Brønsted acid sites as NH + 4, whereas NH 3 coordinatively bound to the Lewis acid sites is displaced by NO 2. The presence of delocalized protons on the sample surface is registered. It is found that these protons play an essential role in the occurrence of the SCR process. Some conclusions are drawn concerning the mechanism of SCR of NO x by NH 3. A scheme is proposed according to which the reaction takes place between a NO − 3 ion, having a more pronounced symmetry, and a NH + 4 ion, both coadsorbed on the catalyst active phase, with NH + 4 NO − 2 appearing as an intermediate.

Role of WO 3 in mixed V 2O 5-WO 3/TiO 2catalysts for selective catalytic reduction of nitric oxide with ammonia

A series ofTiO 2-supported V 2O 5 and V 2O 5+WO 3 catalysts were prepared to study the role of WO 3 in the commercial selective catalytic reduction (SCR) catalysts. The activities of these catalysts were measured under both dynamic and steady-state conditions. The catalysts were characterized by temperature-programmed reaction (TPR), ammonia chemisorption and proton magic angle spinning nuclear magnetic resonance (MAS NMR) measurements. The following effects of WO 3 on V 2O 5/TiO 2 in the commercial WO 3-V 2O 5/TiO 2 (coimpregnated) SCR catalyst are observed; (1) It increases the activity and widens the temperature window for SCR. (2 ) It significantly increases the poison resistance to both alkali metal oxides and arsenious oxide. (3) It reduces ammonia oxidation (as well as SO 2 oxidation which is known from the published literature). These results have been correlated directly with ammonia chemisorption and proton MAS NMR measurements. These results suggest that the Brønsted acid sites are the active sites for SCR. Addition of WO 3 in V 2O 5/TiO 2 increases the Brønsted acidity (both Brønsted acid site density and strength of the acid, i.e., proton donicity), whereas the alkali poison decreases the Brønsted acidity.