Fe-BEA Catalyst Research Articles

Catalytic hydrolysis of HCN on the coupled sites of Lewis acid and surface hydroxyl of Fe-BEA catalyst leading to the presence of HCHO positively affects NH3-SCR activity

Formaldehyde (HCHO) in the exhaust gas of diesel or natural gas engines is usually considered to be a serious hindrance to the NH3-SCR process. However, the promotion effects of HCHO on the NH3-SCR activity and its temperature dependence were discovered in this work for the first time. For the Fe-BEA catalysts, below 300 °C, a notable decrease in activity observed, up to 40 % at 300 °C, and HCN selectivity reached 35 %. However, with the increase in temperature, a significant promotion effect was produced. Not only for the increase of NOx conversion, but also the diminish of the by-product HCN. The disparity in the number and activity of Lewis acid sites (Fe3+) and surface hydroxyls was the primary reason why Fe-BEA catalysts with varying Fe contents exhibited different influence. The presence of HCHO resulted in a modification of the oxidation pathway of NH3 at high temperatures. Surface hydroxyls facilitated the hydrolysis reaction of the as-formed HCN to produce NH3, with NH3 on Lewis acid sites subsequently re-participating in the SCR process. This enhanced the NOx conversion and the selectivity of carbon-containing products. Furthermore, it was demonstrated that HCHO completely converted to hexamethylenetetramine (HMTA) by reacting with NH3 prior to passing through the SCR catalyst. The thermal decomposition of HMTA on the catalyst was identified as a crucial step for the subsequent generation of HCN, and the detailed mechanism of how to effectively inhibit the decrease of de-NOx activity and the formation of HCN in the HCHO-containing NH3-SCR conditions was elucidated. This study paves the way for the development of effective SCR catalysts for the synergistic control of HCN from mobile and stationary sources.

O3-assisted NH3-SCR over FeBEA catalyst at low reaction temperature

The injection of O3 upstream to the SCR catalyst, FeBEA zeolite, significantly improves its catalytic performance at 100–200 °C. This improvement is due to the oxidation of NO by O3, which proceeds even at ambient temperature, and to the further reduction of the NO + NO2 mixture as a result of fast SCR, NO2-SCR and accumulation of NH4NO3 on the catalyst surface.

Mechanism of catalytic sites participating in N2O formation over Fe-BEA and Cu-SSZ-13 NH3-SCR catalysts

N2O as a potent greenhouse gas, is increasingly emphasized in automotive exhausts. The mechanism of N2O formation over Fe-BEA and Cu-SSZ-13 catalysts was investigated by experiments and density functional theory (DFT) calculations to elucidate the effect of NO2/NOx ratios and water. An updated detailed experimental phenomenon was exhibited to DRIFTs, NH3-TPD and catalytic performance and the results showed that N2O formation increased with the rising NO2/NOx ratio, the NH3 absorbed on Lewis acid sites provided the N atom on N2O formation. The DFT calculation identified that the side reaction of NO2 with NH3 on active sites generated N2O. The NH2 and HONO bonded to Fe sites to form H2NNO and OH, and then the H atom of H2NNO migrated toward HNNO and H2O over Fe-BEA catalyst, while activated HONO reacted with NH2 to generate HNNOOH on Cu sites, then desorbed into N2O and H2O over Cu-SSZ-13 catalyst.

Development and validation of rare earth modified Fe-BEA SCR catalyst for mitigation of NOx from NH3 gas turbine

This study investigated NH3-SCR of NOx from the exhaust of 300 kW NH3 gas turbine using Fe-zeolite and rare earth modified Fe-zeolite catalyst. As part of greenhouse gas reduction program, a gas turbine for power generation using NH3 and/or natural gas as fuel is developed where the challenge was to develop a catalyst that reduces NOx with high efficiency with minimum NH3 consumption as well as small SCR unit size. From laboratory bench scale evaluations Fe-BEA catalyst was identified to show desired NOx conversion, however upon aging in presence of steam at 800 °C the performance degraded. The Gd modified Fe-BEA catalyst showed high NOx reduction >98% at 600 °C and remained durable even after aging at 800 °C. The characterization results of XRD, UV and NH3-TPD revealed that addition of rare earth cations to Fe-BEA decreased the agglomeration of active iron species and stabilized the zeolite structure leading to minimal decrease in NH3 storage sites all of which are responsible for increased NOx conversion. The Gd/Fe-BEA catalyst was scaled up and tested behind a 300 kW NH3 gas turbine and it was demonstrated that the reduction of NOx from 477 ppm to 4.3 ppm, which is below the set target of 70 ppm in the exhaust was achieved.

Evolution Mechanism of N2O for the Selective Catalytic Reduction of NOx by NH3 Over Cu-SSZ-13 Assisted Fe-BEA Catalysts

Evolution Mechanism of N2O for the Selective Catalytic Reduction of NOx by NH3 Over Cu-SSZ-13 Assisted Fe-BEA Catalysts

Effect of the NH4NO3 Addition on the Low-T NH3-SCR Performances of Individual and Combined Fe- and Cu-Zeolite Catalysts

We have measured NOx conversions and N2O productions over Fe-BEA and Cu-SAPO catalysts and over their sequential arrangements under Enhanced SCR conditions, resulting from the addition of an aqueous solution of ammonium nitrate (AN) to the typical Standard SCR feed stream, and we have compared them to those observed under Standard and Fast SCR conditions. The expected strong enhancement of the poor low temperature activity of the Fe-BEA catalyst was confirmed: both NH3 and NOx conversions and N2O formations similar to those of the Fast SCR reaction were achieved when cofeeding ammonium nitrate. On the other hand, the Cu-SAPO efficiency was drastically decreased by the addition of AN at low temperatures, possibly due to trapping of the ammonium nitrate salt within the SAPO zeolite, characterized by smaller pores than those of the BEA zeolite. The Cu-SAPO performances were recovered only at T > 250 °C with a huge release of N2O due to the thermal decomposition of AN. The combined system with the Fe-zeolite sample placed upstream of the Cu-zeolite also exhibited outstanding low temperature deNOx performances, with even lower N2O production than over the Fe-zeolite only at the same Enhanced SCR (E-SCR) conditions.

Ozone as an Enabler for Low Temperature Methane Control Over a Current Production Fe-BEA Catalyst

A process has been developed to oxidise methane at low temperatures in an excess oxygen environment using ozone as the oxidant. Ozone being more reactive than oxygen allows low temperature methane control. A range of catalysts have been screened and a commercially available current production Fe-BEA catalyst, from an automotive application has shown excellent potential for methane control with ozone in the feed, leading to a peak conversion efficiency of 81% at 168 °C in the absence of H2O and 60% conversion at 220 °C in the presence of H2O.

Water promotion mechanism on the NH3-SCR over Fe-BEA catalyst

The water promotion mechanism in the Fe-BEA catalyst on the NH3-SCR activity was investigated by in situ DRIFT studies. While a significant amount (mol/g) of Lewis acid sites was formed on the surface of the Fe-BEA without water addition, supplementary hydroxyl species caused Lewis acid sites to be transformed into Brønsted acid sites, thus resulting in more NO3−/NO2− and NH4+ species adsorbed on the surface which increased the SCR activity under the presence of water.

Effect of alumina and zirconia as binders on the activity of Fe-BEA for NH3-SCR of NO

Fe-BEA catalysts are active for the NH3-SCR of NO. For industrial application, a binder should be added to the Fe-BEA catalysts to make them tightly adhere to the monoliths. The addition of alumina and zirconia as binders to the Fe-BEA led to a different effect on NO conversion. The catalytic activity of the mixed samples was evaluated by the temperature programmed procedure in a flow-reactor system, and the mechanism was analyzed via SEM, BET, XRD and XPS. It was found that larger iron particles were formed by the migration of parent iron particles in the Fe-BEA catalyst with alumina. This led to the increase of Fe3+ magnitude and iron cluster, enhancing the abilities of NO oxidation and storage. Accordingly, the SCR activity increased slightly in low temperature but decreased sharply in high temperature. For the Fe-BEA with zirconia sample, NO oxidation and storage abilities decreased due to the less iron clusters. The increase of Fe3+ magnitude resulted in higher catalytic oxidation ability, which gave rise to little change in the SCR activity compared with the Fe-BEA.

Effects of steam introduction on deactivation of Fe-BEA catalyst in NH3-SCR of N2O and NO

The effects of water vapor on the catalytic activity of a Fe-BEA catalyst in the NH3-SCR reaction for N2O and NO were investigated using diffusive reflectance infrared Fourier transform spectroscopy (DRIFTS) tests and SCC-DFTB calculation. It was found that water vapor only decreased the N2O conversion rate and did not affect the conversion rates of NO. The DRIFTS analysis revealed that the presence of water increased the adsorption strength of nitrous oxide (N2O) on the catalyst surface. On the other hand, nitric oxide (NO) conversion route was only altered from NO2-SCR to standard selective catalytic reduction (SCR) upon injection of water vapor. It was confirmed that the water vapor not only inhibited the desorption of N2O during the reduction step but also induced stronger adsorption of N2O during the NH3-SCR reaction. A computational chemical model that includes hydroxyl zeolite, N2O, and NO supported the experimental result showing binding force between the hydroxyl zeolite and N2O is much higher than that between hydroxyl zeolite and NO. It is believed that the stronger adsorption induced by water vapor caused delayed dissociation of N2O thereby not allowing for catalytic conversion.

Effect of acid treatment of Fe-BEA zeolite on catalytic N2O conversion

The effect of acid treatment on the physical and chemical characteristics of BEA zeolite, as well as the catalytic activity of the Fe-BEA catalyst for N2O reduction under NH3-selective catalytic reduction (NH3-SCR) conditions, was examined. The acid treatment caused dealumination of BEA and enrichment of the silanol groups on vacant T-sites and the Bronsted acid sites. As the acid treatment time increased, the silanol groups and the weak acid sites in BEA also increased. Because the weak acid sites behave as anchoring sites for Fe ions, the catalytic activity also increased as the treatment time increased. However, extended exposure of BEA to acid decreased the catalytic activity of the Fe-BEA catalyst somewhat, and decreased the silanol groups and weak acid sites. The catalytic activity and the amount of weak acid sites were well correlated with the BEA acid treatment time.

Impact of Thermal and Chemical Ageing of Fe-BEA SCR Catalyst on NOx Conversion Performance

Emissions of nitrogen oxides (NOx) from heavy-duty diesel engines are subject to more stringent environmental legislation. Selective catalytic reduction (SCR) over metal ion-exchanged zeolites is in this connection an efficient method to reduce NOx. Understanding durability of the SCR catalyst is crucial for correct design of the aftertreatment system. In the present paper, thermal and chemical ageing of Fe-BEA as NH3-SCR catalyst is studied. Experimental results of hydrothermal ageing, and chemical ageing due to phosphorous and potassium exposure are presented. The catalyst is characterized by flow reactor experiments, nitrogen physisorption, DRIFTS, XRD, and XPS. Based on the experimental results, a multisite kinetic model is developed to describe the activity of the fresh Fe-BEA catalyst. Furthermore, the model can predict deactivation of the catalyst well by decreasing the number of active sites, representing loss of active iron sites due to migration or chemical blockage of the sites. By performing a systematic study of different deactivation mechanisms, a deactivation expression for the active sites can be formulated.

NH3-SCR Activity of H-BEA and Fe-BEA After Potassium Exposure

A previous developed multisite microkinetic model for hydrothermally deactivated H-BEA and Fe-BEA catalyst used for NH3-SCR applications has been applied and validated using potassium exposed H-BEA and Fe-BEA samples. The catalysts have been exposed to evaporated aqueous solution of potassium (10 ppm KNO3) in a flow-reactor for 14-48 h. The results for the H-BEA samples show an increase in NH3-SCR activity after 48 h of potassium exposure. This is correlated to an increase in NOX storage capacity, indicating that potassium forms new NOX storage sites participating in the SCR reaction. However, the Fe-BEA samples show different results. The SCR activity for the Fe-BEA catalyst decreases significantly after 14 h of exposure to potassium and after longer exposure time the activity slightly increases again compared to the sample exposed to potassium for 14 h. When applying the kinetic model to the flow-reactor measurements the model is able to predict the changes in SCR activity well when changing the site density of the active sites. In overall, combining the experimental results with the simulation indicates that exposure to potassium initially chemically blocks the active iron species which decrease with exposure time. Longer time of exposure to potassium indicates that the iron species are exchanged and lost due to formation of new potassium sites in the zeolite resulting in an overall lower SCR activity compared to a fresh Fe-BEA sample.

Oxidative Coupling and Hydroxylation of Phenol over Transition Metal and Acidic Zeolites: Insights into Catalyst Function

Reaction of phenol with hydrogen peroxide over H-MFI, Fe-MFI, H-BEA, Fe-BEA and TS-1 zeolite catalysts was investigated. Over H-BEA, biphenyl product was observed. It is suggested, that the larger pore size of H-BEA facilitates coupling of two phenol molecules. Two distinct reaction mechanisms are proposed for acid and redox catalysts.

Improved low-temperature SCR activity for Fe-BEA catalysts by H2-pretreatment

A series of iron-exchanged zeolite beta catalysts (0.5–4wt.% Fe) have been prepared by incipient wetness impregnation and tested for selective catalytic reduction (SCR) of NOX with ammonia as reductant. The catalysts were characterized using BET, NH3-TPD and XPS before and after H2-pretreatment at 650°C for 5h. The NH3-SCR activity tests show that the samples pretreated by hydrogen exhibit higher low-temperature SCR activity compared to the fresh samples, while the high-temperature activity remains almost constant. The results clearly show that the high-temperature H2-treatment has a significant influence on the extent of different iron species formed in the zeolite. Furthermore, H2-treatment of hydrothermally aged samples can recover some of the initial activity, although not completely due irreversible dealumination during the ageing. By H2-pretreatment SCR catalysts with high iron loading and high activity can be prepared.

Experimental evidence of the mechanism behind NH3 overconsumption during SCR over Fe-zeolites

Isotope labeled 15NO was used to investigate the mechanism of the unusual overconsumption of NH3 during standard SCR over Fe-zeolite, under conditions during which NH3 oxidation with O2 alone (without NO) was unfavorable. When the Fe-BEA catalyst was exposed to the 15NO+14NH3+O2 gas mixture at 250 and 300°C, the resulting products included 14NO, a product of 14NH3 oxidation under SCR conditions. However, 14NO was not detected when Fe-BEA was exposed to 14NH3+O2. Since the only source of 14N derives from 14NH3, with the labeled gas mixture used, the 14NO during SCR must have originated from oxidation of 14NH3. Furthermore, twice as much 14N14N was observed at 300°C under SCR conditions in comparison with NH3 oxidation using O2 alone. Under SCR conditions, the 14NO formed through the unusual oxidation route further reacted with 14NH3 to produce 14N2. Thus, for the first time, we have experimental evidence for the unusual overconsumption of ammonia during SCR over Fe-zeolites.

Effect of Thermal Ageing on the Nature of Iron Species in Fe-BEA

The adsorption of NH3 and NO over fresh and thermally treated Fe-BEA catalysts were studied using in situ DRIFT spectroscopy to follow the evolution of different iron species before and after thermal treatment. Fe-BEA samples were prepared and thermally treated in air at 700 °C for 12, 24 and 48 h, and at 800 and 900 °C for 48 h. Compared to the fresh sample, the NH3 adsorption experiments indicate dealumination of the zeolite and iron oxide particle growth for the aged samples. Furthermore, the NO adsorption experiments show distinct absorption peaks which are assigned to isolated iron species, iron clusters and larger iron oxide particles. The thermally treated samples show migration of isolated iron species from the zeolite pores forming iron oxide particles. Further ageing results in a continuous migration and formation of larger iron oxide particles located on the external surface of the zeolite.

Determination of the Redox Processes in FeBEA Catalysts in NH3−SCR Reaction by Mössbauer and X-ray Absorption Spectroscopy

The nature and oxidation state of iron species in Fe-exchanged BEA zeolites treated in synthetic air or nitrogen were determined by a combination of Mossbauer and X-ray absorption spectroscopy. The linear correlation between the edge energy of the X-ray absorption near edge structure (XANES) and the oxidation state determined by Mossbauer spectroscopy allowed determining the fraction of Fe2+ in situ. The distribution of Fe2+ and Fe3+ in the catalysts depends on the Fe concentration and the conditions of the thermal treatment. It is possible to stabilize isolated Fe2+ cations under ambient atmosphere in the zeolite pores, while FeBEA catalysts show a temperature-dependent oxidation and reduction of the active Fe species during the selective catalytic reduction of nitrogen oxides by NH3 (NH3-SCR), reflecting the equilibrium for NO oxidation.

Role of the Fe-zeolite structure and iron state in the N 2O decomposition: Comparison of Fe-FER, Fe-BEA, and Fe-MFI catalysts

The decomposition of nitrous oxide was compared over Fe-FER, Fe-MFI, and Fe-BEA with well established iron distribution in cationic positions and low amounts of less well-established oxide species. It was evidenced that, despite a comparable content of Fe(II) in the cationic positions, the catalytic activity of Fe-FER greatly exceeds that of Fe-BEA and Fe-MFI. While about one half of the iron sites in Fe-FER (Fe/Al < 0.15) participate in the decomposition of nitrous oxide after activation at 450 °C, the number of active sites in Fe-BEA or Fe-MFI was much lower, and, accordingly, without acceleration of the reaction by the addition of NO, these samples exhibit much lower catalytic activity than Fe-FER. This could be likely correlated with the concentration of Fe(II) in positions with a specific spatial iron arrangement at optimal Fe⋯Fe distances. For that role we propose a local structure with two adjacent β sites, where the Fe⋯Fe distance would be 7 to 7.5 Å, i.e. comparable to the length of the N 2O molecule, and provide potential for cooperation of the two iron cations on the N 2O molecule splitting. Such arrangement is absent in both the Fe-BEA and Fe-MFI structures.

Screening of Fe–BEA catalysts for wet hydrogen peroxide oxidation of crude olive mill wastewater under mild conditions

A series of Fe–BEA catalysts, differing in the amount of iron have been characterized by XRD, BET surface area, UV–vis spectroscopy and chemical analysis. The zeolite samples have been tested as heterogeneous catalysts for the wet hydrogen peroxide oxidation of crude olive mill wastewaters (OMW) under very mild conditions (at 28°C and atmospheric pressure). All experiments were performed on a laboratory scale set-up.BSE-1/3 catalyst with a moderate Fe content (Fe/Al=1.19) showed the best results in terms of catalytic activity and loss of active species into the aqueous solutions. The stability of Fe species has been shown to be strongly dependent on the Fe environment into the zeolite framework.Over the selected catalyst, application of catalytic procedure on diluted OMW solution permitted high removal efficiencies of pollutants. The process produces a removal capacity of 28% of total organic carbon (TOC), 40% of total phenols, 30% of chemical oxygen demand (COD) and 59% of colour, just after 12h. 5-Day biochemical oxygen demand (BOD5), and toxicity towards the bioluminescent bacteria Vibrio fischeri were selected to follow the performance of this process in terms of reducing the ecotoxicity of OMW. Results showed an increase in the biodegradability of the treated sample and a decrease of the microtoxicity from 100% to 70% load towards V. fischeri.Occurrence of small catalyst deactivation by carbonaeous during the oxidation reaction was observed through scanning electron microscopy (SEM) and elemental analysis.