Selective Catalytic Reduction Activity Research Articles

Hydrocarbon and Soot Oxidation over Cerium and Iron Doped Vanadium SCR Catalysts

AbstractV2O5−WO3/TiO2 (VWTi) catalysts are widely employed for selective catalytic reduction (SCR) of NOx. However, due to their poor thermal stability the application in diesel particulate filters (DPFs), i. e. 2‐way SCRonDPF is limited. In this study, the potential of Ce‐ and Fe‐doped VWTi systems for hydrocarbon and soot oxidation in addition to the SCR activity was systematically investigated for fresh and thermally aged samples. The formation of metal vanadates upon thermal aging, as identified by X‐ray diffraction, Raman and X‐ray adsorption spectroscopy, prevents drastic sintering of the support and maintains a high NOx−SCR and hydrocarbon oxidation activity. Additionally, the doped VWTi catalysts show a slight increase of the CO2 selectivity during hydrocarbon oxidation, which represents an important aspect for such multifunctional catalysts. Despite of the advantages, the formation of metal vanadates hinders the mobility of vanadium species and decreases the soot oxidation ability of the doped catalysts. Interestingly, a promising soot oxidation activity was identified for the VWTi−Fe sample after aging at 650 °C, which resulted in decomposition of the iron vanadate and generation of highly dispersed and mobile V2O5.

Modification of V2O5-WO3/TiO2 Catalyst by Loading of MnOx for Enhanced Low-Temperature NH3-SCR Performance.

V2O5-WO3/TiO2 as a commercial selective catalytic reduction (SCR) catalyst usually used at middle-high temperatures was modified by loading of MnOx for the purpose of enhancing its performance at lower temperatures. Manganese oxides were loaded onto V-W/Ti monolith by the methods of impregnation (I), precipitation (P), and in-situ growth (S), respectively. SCR activity of each modified catalyst was investigated at temperatures in the range of 100–340 °C. Catalysts were characterized by specific surface area and pore size determination (BET), X-ray diffraction (XRD), temperature programmed reduction (TPR), etc. Results show that the loading of MnOx remarkably enhanced the SCR activity at a temperature lower than 280 °C. The catalyst prepared by the in-situ growth method was found to be most active for SCR.

Submonolayer Vanadium and Manganese Binary Metal Oxides Supported on Three-Dimensionally Ordered Mesoporous CeO2 for Efficient Low-Temperature NH3–SCR

Submonolayer-supported catalysts exhibited excellent activity for ammonia selective catalytic reduction (NH3–SCR) and have attracted great interest. Here, a catalyst based on three-dimensional ordered mesoporous (3DOM) CeO2 supported by a submonolayer of vanadium and manganese bimetal oxides (3DOM VOx–MnOx/CeO2) was successfully synthesized. The optimal catalytic temperature was found to shift to lower temperature with increasing VOx–MnOx loading amount under the submonolayer coverage. The 5 wt% 3DOM VOx–MnOx/CeO2 catalyst with submonolayer coverage exhibited the best catalytic performance, achieving a NO conversion of more than 95% at 250 °C. It was found that three-dimensional CeO2 endowed the material with a high specific surface and highly ordered mesoporous channels, which could highly disperse VOx–MnOx below one monolayer coverage. Based on NH3–TPD, H2–TPR, Raman and XPS characterization, an increase in VOx–MnOx content under the submonolayer coverage resulted in an enhancement of the surface acidity, redox capacity, number of oxygen vacancy defects and charge transfer. In particular, the 5 wt% 3DOM VOx–MnOx/CeO2 catalyst showed the highest number of acidic sites that for NH3 activation and the fastest electron transfer required for oxygen to be regenerated. Therefore, the 5 wt% 3DOM VOx–MnOx/CeO2 catalyst with submonolayer coverage generated a significantly enhanced catalytic performance, which made it desirable for NO removal applications.

The poisoning effect of sintering dust on V2O5–WO3/TiO2 catalyst for NOx removal in iron ore sintering flue gas

ABSTRACT NOx from iron ore sintering flue gas can be removed by selective catalytic reduction (SCR) over V2O5–WO3/TiO2 (VWTi) catalyst. In this work, the poisoning effects of K2SO4 and CaSO4 on the VWTi catalyst were investigated. The SCR activities and physicochemical properties of the fresh and poisoned catalysts were characterized. The results confirmed the deactivation of the poisoned catalysts, and the SCR activity decreased with increasing the concentration of the doped poisoning precursors. This reduced SCR activity could be related to the decreased reducibility of vanadium species and lower content of surface chemisorbed oxygen. Characterization of the poisoned catalysts showed migration of V 2p3/2 towards lower binding energy, reduced amount of NH3 desorption, and elimination of V=O stretching vibration bond, which could be attributed to the extension of V=O bond, consequently leading to the formation of –V–O–Ca/K bonds. A possible poisoning mechanism of the VWTi catalyst was proposed and discussed.

Promotional effect of ion-exchanged K on the low-temperature hydrothermal stability of Cu/SAPO-34 and its synergic application with Fe/Beta catalysts

Kions were introduced onto Cu/SAPO-34 catalysts via the ion-exchange process in order to improve their stability under low-temperature hydrothermal aging. The changes in structure and copper-species contents of these catalysts upon hydrothermal aging were probed in order to investigate their effects on selective catalytic reduction (SCR) activity. For the fresh Cu/SAPO-34 catalysts, K ions had little influence on the chabazite framework but effected their acidities by exchanging with acid sites. After hydrothermal aging, the structural integrity and amount of active sites decreased on pure Cu/SAPO-34. While the K-loaded catalysts showed improved chabazite structure, acidity, and active site conservation with increasing K loading. However, although the 0.7 wt% K catalyst maintained the same crystallinity, active site abundance, and low-temperature SCR activity as the fresh catalyst upon aging, an apparent decrease in SCR activity at high temperature was observed because of the inevitable decrease in the number of Bronsted acid sites. To compensate for the activity disadvantage of K-loaded Cu/SAPO-34 at high temperature, Fe/Beta catalysts were co-employed with K-loaded Cu/SAPO-34, and a wide active temperature window of SCR activity was obtained. Thus, our study reveals that a combined system comprising Fe/Beta and K-loaded Cu/SAPO-34 catalysts shows promise for the elimination of NOx in real-world applications.

Preparation of metal-organic framework Cu+/Ni-MOF catalyst with enhanced catalytic activity for selective catalytic reduction of NOx

A series of Cu+/Ni-MOF materials with different Cu loading were prepared by a hydrothermal method and were applied on selective catalytic reduction with NH3-SCR. From characterization results of XRD, XPS and TPR, it could be inferred that Cu+ species have been successfully introduced into the Ni-MOF framework. Compared with monometallic Ni-MOF catalyst, all bimetallic Cu+/Ni-MOF sample showed much better NH3-SCR activity at low temperature and achieved about 30% increase in the maximum catalytic efficiency at 320 °C, which indicating a good synergistic effect between copper and nickel ions.

High performance of catalytic sheet filters of V2O5-WO3/TiO2 for NOx reduction.

Nitrogen oxide (NOx) emissions in fuel from the stationary as well as from mobile sources primarily from power stations, industrial heaters, cogeneration plants, and diesel engines represent a major worldwide environmental problem. The selective catalytic reduction (SCR) of NOx with NH3 over catalyst based on V2O5-WO3/TiO2 (VWT) is the most effective benchmark technique to efficiently reduce NOx emissions from stationary and mobile sources. Among the different transition metals (Mn, Nb, Co, Cr, Cu, Ce) used in current research work, the manganese could make up the loss of SCR activity caused by the decrease of V2O5 loading (50%) in prepared VWT powder catalyst. The optimal loading of Mn is 3 wt% in case of 3V9WT powder catalyst, which shows the best catalytic performance. 3Mn3V9WT powder catalyst exhibits enhanced NO conversion performance, i.e., ~ 95-98% with NH3 leakage < 20 ppm, for the temperature window of 260-320 °C in comparison with all other metal-doped 3V9WT catalyst powder. Manganese is the best substitute (50%) to vanadium in VWT catalyst without compromising the NO conversion performance even in presence of SO2 (1000 ppm), i.e., > 94% for the temperature of 300-320 °C. The recovery of SCR activity of the 3Mn3V9WT catalyst after SO2 effect was good, i.e., ~ 94% at 320 °C for long interval of time, where NH3 leakage was < 5 ppm.

Improvement of the activity and SO2 tolerance of Sb-modified Mn/PG catalysts for NH3-SCR at a low temperature

A series of MnM/palygorskite (PG) (M = La, W, Mo, Sb, Mg) catalysts was prepared by the wetness co-impregnation method for low-temperature selective catalytic reduction (SCR) of NO with NH3. Conversion efficiency followed the order Sb > Mo > La > W > Mg. A combination of various physico-chemical techniques was used to investigate the influence of Sb-modified Mn/PG catalysts. MnSb0.156/PG catalyst showed highest NO conversion at low temperatures in the presence of SO2 which reveals that addition of Sb oxides effectively enhances the SCR activity of catalysts. A SO2 step-wise study showed that MnSb0.156/PG catalyst displays higher durable resistance to SO2 than Mn/PG catalyst, where the sulfating of active phase is greatly inhibited after Sb doping. Scanning electron microscopy and X-ray diffraction results showed that Sb loading enhances the dispersion of Mn oxides on the carrier surface. According to the results of characterization analyses, it is suggested that the main reason for the deactivation of Mn/PG is the formation of manganese sulfates which cause the permanent deactivation of Mn-based catalysts. For Sb-doped Mn/PG catalyst, SOx ad-species formed were mainly combined with SbOx rather than MnOx. This preferential interaction between SbOx and SO2 effectively shields the MnOx as active species from being sulfated by SO2 resulting in the improvement of SO2 tolerance on Sb-added catalyst. Multiple information support that, owing to the addition of Sb, original formed MnOx crystallite has been completely transformed into highly dispersed amorphous phase accounting for higher SCR activity.

One-pot synthesis of Fe/Cu-SSZ-13 catalyst and its highly efficient performance for the selective catalytic reduction of nitrogen oxide with ammonia

Series of Fe/Cu-SSZ-13 catalysts with different Fe loading content were synthesized by simple one-pot strategy. The obtained catalysts were subjected to selective catalytic reduction (SCR) of NOx with NH3 and were characterized by various techniques. The results show that Fe0.63/Cu1.50-SSZ-13 catalyst with proper Fe content exhibits excellent catalytic activity with widest operation temperature window from 160 to 580°C, excellent hydrothermal stability as well as good resistance to sulfur poisoning when compared with Cu-SSZ-13, signifying its great potential for practical applications. Further characterizations reveal that the synthesized Fe/Cu-SSZ-13 catalysts present typical chabazite (CHA) structure with good crystallinity, while isolated Cu2+ and monomeric Fe3+ are revealed as the predominant copper and iron species. At low temperatures, isolated Cu2+ species act as primary active sites for SCR reaction, while monomeric Fe3+ species provide sufficient active sites for sustain the SCR activity at high temperature. Moreover, Fe over doping would lead to the damage of zeolite structure, destruction of isolated Cu2+ site, as well as the formation of highly oxidizing Fe2O3, thus causing deterioration of catalytic performances.

Poisoning Effect Comparison of ZnCl2 and ZnSO4 on Mn‐Ce/AC Catalyst for Low‐Temperature SCR of NO

AbstractIn this study, the effect of different zinc salts (ZnCl2 and ZnSO4) on the deactivation of activated carbon(AC)‐based Mn−Ce catalysts and the poisoning mechanism of zinc salts on Mn−Ce/AC are comparatively studied. SEM, BET, XRD, XPS, H2‐TPR, NH3‐TPD and In‐situ DRIFT are used to characterize the physical and chemical changes of Mn−Ce/AC catalyst. The Selective Catalytic Reduction (SCR) activities of Mn−Ce/AC has a noticeable decline after loading zinc salts, and the highest NO conversion of Zn poisoning catalysts do not exceed 70 %. And the acid site is occupied by Zn2+ on the catalyst, which affects the chemical adsorption of NH3 and hinders the formation of the intermediate ‐NH2. Moreover, the decrease of Mn4+, reducibility of manganese oxide and acid sites are more serious on ZnC−Mn‐Ce/AC. Besides, loading ZnSO4 can improve the content of Ce3+ and Oβ on catalyst surface, which can mitigate the poisoning effects of ZnSO4 on Mn−Ce/AC catalyst. Hence, ZnCl2 displays a more serious poisoning effect on Mn−Ce/AC catalyst compared with ZnSO4. Then the probable mechanism model of ZnCl2 and ZnSO4 on Mn−Ce /AC catalyst is proposed and compared.

Systematic effects of Fe doping on the activity of V2O5/TiO2-carbon nanotube catalyst for NH3-SCR of NOx

The present investigation concentrates on the reduction of NOx emissions, the indirect greenhouse gases, which generate ozone by reacting with the atmosphere. A series of composite catalytic support (TiO2-CNTs) with a constant TiO2-CNT ratio prepared by precipitation method was utilized to synthesize Fe-modified V2O5/TiO2-CNT catalyst for selective catalytic reduction (SCR) of NOx with ammonia (NH3). The structural and surface characteristics of the prepared catalyst were investigated by using Brunauer–Emmett–Teller surface area measurement, X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscope, and temperature-programmed desorption (TPD) techniques. In terms of catalytic activity, VFe3TiC10 catalyst showed better SCR performance having a NOx conversion efficiency of 95% and 98% at 250 °C and 300 °C, respectively. The improved performance could be attributed to high surface chemisorbed oxygen on the surface of the catalyst by the added Fe species which promotes the SCR activity. The catalytic activity results depicted that the addition of CNTs to TiO2 support enhanced the SCR activity of the catalyst at low reaction temperatures and it also provides better resistance to SO2. In addition, the effects of Fe modification on the NH3 removal efficiency of the catalyst were also investigated.

A new insight into the promotional effect of nitrogen-doping in activated carbon for selective catalytic reduction of NOX with NH3

A series of N-doped carbons were prepared to investigate the effect of different N-containing groups on selective catalytic reduction (SCR) of NOx with NH3. Combined the SCR activity with the results of porosity analysis and X-ray photoelectron spectroscopy, it's deduced that the pyridinic N (N-6) rather than the surface area or doped total N was mainly responsible for the promoted SCR activity. The electron paramagnetic resonance and O2-temperature programmed desorption (O2-TPD) experiments indicated that N-6 created numerous of oxygen vacancy. The NO+O2-TPD and transient response of NH3 further demonstrated that the increased oxygen vacancy enhanced the absorbability and reactivity of NOx, therefore the SCR reaction was elevated by accelerating the reaction in the Langmuir-Hinshelwood (L-H) mechanism. Furthermore, the NH3-TPD suggested that N-6 was conductive to the NH3 adsorption. In situ DRIFTs of NH3 adsorption and reaction illustrated that the increased NH3 mainly existed as NH2 species, which were quickly consumed by NO+O2, further elevated the reaction between gaseous NO and adsorbed NH3 in the Eley-Rideal (E-R) mechanism. The N-6 groups doped in the activated carbons facilitated the L-H and E-R reactions and thus promoted the SCR activity.

Effects of PbO poisoning on Ce–Mn/AC catalyst for low-temperature selective catalytic reduction of NO with NH3

As a common heavy metal in the sintering flue gas, Pb can exist in the form of oxide (PbO) and lead to the decrease in the denitration catalysts activity. Ce–Mn/AC (activated carbon) and PbO–Ce–Mn/AC catalysts were prepared by impregnation method and their selective catalytic reduction of NH3 with NO was studied. Results showed that selective catalytic reduction activity of Ce–Mn/AC decreased remarkably after doping PbO. And the NO conversion of Ce–Mn/AC reached 94.52% at 200 °C, while the value was reduced to 65.8% after doping PbO at the same temperature. The doping of PbO decreased the total pore volume and oxygen functional groups of activated carbon, increased crystallinity of Mn oxides on the catalyst, decreased Mn4+ and chemisorbed oxygen content and then inhibited the “fast selective catalytic reduction” denitration reaction for Ce–Mn/AC catalysts. On this basis, the poisoning effects of lead oxide on Ce–Mn/AC catalysts for low-temperature selective catalytic reduction were revealed.

Unraveling the Unexpected Offset Effects of Cd and SO2 Deactivation over CeO2-WO3/TiO2 Catalysts for NOx Reduction.

It is challenging for selective catalytic reduction (SCR) of NOx by NH3 due to the coexistence of heavy metal and SO2 in the flue gas. A thorough probe into deactivation mechanisms is imperative but still lacking. This study unravels unexpected offset effects of Cd and SO2 deactivation over CeO2-WO3/TiO2 catalysts, potential candidates for commercial SCR catalysts. Cd- and SO2-copoisoned catalysts demonstrated higher activity for NOx reduction than a Cd-poisoned catalyst but lower than that for an SO2-poisoned catalyst. In comparison to SO2, Cd had more severe effects on acidic and redox properties, distinctly decreasing the SCR activity. After sulfation of Cd-poisoned catalysts, SO42- preferentially bonded with the surface CdO and released CeO2 active sites poisoned by CdO, thus reserving the highly active CeO2-WO3 sites and maintaining a high activity. The sulfation of Cd-poisoned catalysts also provided more strong acidic sites, and the synergistic effects between the formed cerium sulfate and CeO2 contributed to the high-temperature SCR performance. This work sheds light on the deactivation mechanism of heavy metals and SO2 over CeO2-WO3/TiO2 catalysts and provides an innovative pathway for inventing high-performance SCR catalysts, which have great resistance to heavy metals and SO2 simultaneously. This will be favorable to academic and practical applications in the future.

Tailored Alkali Resistance of DeNOx Catalysts by Improving Redox Properties and Activating Adsorbed Reactive Species

SummaryIt is still challenging to develop strongly alkali-resistant catalysts for selective catalytic reduction of NOx with NH3. It is generally believed that the maintenance of acidity is the most important factor because of neutral effects of alkali. This work discovers that the redox properties rather than acidity play decisive roles in improving alkali resistance of some specific catalyst systems. K-poisoned Fe-decorated SO42−-modified CeZr oxide (Fe/SO42−/CeZr) catalysts show decreased acidity but reserve the high redox properties. The higher reactivity of NHx species induced by K poisoning compensates for the decreased amount of adsorbed NHx, leading to a desired reaction efficiency between adsorbed NHx and nitrate species. This study provides a unique perspective in designing an alkali-resistant deNOx catalyst via improving redox properties and activating the reactivities of NHx species rather than routinely increasing acidic sites for NHx adsorption, which is of significance for academic interests and practical applications.

Effect of oxide supports on palladium based catalysts for NO reduction by H2-SCR

The present work shows the synthesis of various mixed oxide catalysts prepared by co-precipitation followed by impregnation of palladium metal. The catalysts are denoted as Pd/MgO-CeO2, Pd/ZrO2-CeO2, Pd/Al2O3-CeO2, and Pd/TiO2-CeO2 and examined for selective catalytic reduction (SCR) of NO with H2 in the presence of excess oxygen (6% O2). The properties of catalysts were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), BET, transmission electron microscopy (TEM), H2-temperature-programmed reduction (H2-TPR), and Raman spectroscopy. The synthesized catalysts exhibit notable NO reduction potential at 175 ℃ and in a wide temperature range from 100 to 400 ℃. Compared to bare CeO2 support, the Pd/ZrO2-CeO2 catalyst greatly enhances the SCR activity with a wide temperature range (100–400 ℃). The experimental results showed that the Pd/ZrO2-CeO2 catalyst had the best catalytic activity with 92 % NO reduction and a wide temperature reduction window (150–400 ℃) at the gas hour space velocity (GHSV) of 20,000 h−1.

Effect of preparation and reaction conditions on the performance of In/H-Beta for selective catalytic reduction of NOx with CH4

In/H-Beta catalyst was prepared by optimizing the support, concentration of ion exchange liquid and calcination temperature to investigate the effects of synthesis conditions on catalytic activity of selective catalytic reduction of NOx with CH4. The results showed that the In/H-Beta catalyst exhibited the superior activity when concentration of exchange liquid was 0.033 M and calcination temperature was 500 °C, the NOx removal ratio could reach 97.6%. In addition, reaction conditions could affect the catalytic performance. When O2 concentration was 10%, CH4:NO ratio was no less than one, space velocity was lower than 23600 h−1 and NO initial concentration was no more than 700 ppm, In/H-Beta could exhibit superior catalytic activity. Moreover, the catalytic performances of In/H-Beta catalysts were discussed after enduring H2O or/and SO2. This novel strategy could open the door for selective catalytic reduction of NOx with CH4.

Nanosized V-Ce Oxides Supported on TiO2 as a Superior Catalyst for the Selective Catalytic Reduction of NO

Nanosized V-Ce oxides supported on TiO2 (VCT) were prepared and utilized in the low-temperature selective catalytic reduction (SCR) of NO with NH3. Compared with the other V-Ce oxides-based catalysts supported on Al2O3, ZrO2, and ZSM-5, VCT showed the best SCR activity in a low-temperature range. The NOx conversion of 90% could be achieved at 220 °C. Characterizations including X-ray diffraction (XRD), scanning election micrograph (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), temperature-programmed desorption with NH3 (NH3-TPD), and temperature-programmed reduction with H2 (H2-TPR) showed that V1.05Ce1/TiO2 exhibited a good dispersion of V2O5, enrichment of surface Ce3+ and chemical-absorbed oxygen, and excellent redox capacity and acidity, which resulted in the best SCR performance at low temperature.

Titania-Samarium-Manganese Composite Oxide for the Low-Temperature Selective Catalytic Reduction of NO with NH3.

A novel Ti-doped Sm-Mn mixed oxide (TiSmMnOx) was first designed for the selective catalytic reduction (SCR) of NOx with NH3 at a low temperature. The TiSmMnOx catalyst exhibited a superior catalytic performance, in which NOx conversion higher than 80% and N2 selectivity above 90% could be achieved in a wide-operating temperature window (60-225 °C). Specially, the catalyst also showed high durability against the large space velocity and excellent SO2/H2O resistance. Ti incorporation can efficiently inhibit MnOx crystallization and tune the MnOx phase during calcination at a high temperature. Subsequently, a high specific surface area as well as an increased amount of acid sites on the TiSmMnOx catalysts were produced. Further, the reducibility of the Sm-doped MnOx catalyst was modulated, facilitating NO oxidation and inhibiting NH3 nonselective oxidation. Consequently, a superior SCR activity was achieved at a low temperature and the operating temperature window of the TiSmMnOx catalyst was significantly widened. These findings may provide new insights into the reasonable design and development of the new non-vanadium catalysts with a high NH3-SCR activity for industrial application.

The poisoning mechanism of gaseous HCl on low-temperature SCR catalysts: MnOx−CeO2 as an example

The degradation of catalyst properties, rather than the deposition of NH4Cl, was the main reason for the deactivation of the MnCe low-temperature SCR (selective catalytic reduction) catalyst under HCl atmosphere. Cl ions prefer to be located at the oxygen vacancies around Mn cations on the MnCe surface. The adsorbed Cl ions could further promote the formation of oxygen vacancies. Therefore, the active Mn sites on the surface of MnCe would be quickly covered by Cl ions when HCl was present and thus promote the formation of Brønsted acid sites. However, the ionic NH4+ adsorbed on the Brønsted acid sites of MnCe and MnCe − Cl were not fully utilized in the SCR reaction. Meanwhile, the reactivity of the coordinated NH3 bonded with Lewis acid sites was significantly restrained by HCl due to the significant inhibition of the redox performance. Overall, HCl could obviously restrain the low-temperature SCR activity of MnCe.