Aged Catalysts Research Articles

Hydrothermal and sulfur aging of CeTi/CeWTi catalysts for selective catalytic reduction of NOx with NH3

The CeO2-TiO2 (CeTi) and CeO2/WO3-TiO2 (CeWTi) catalysts were prepared by sol-gel method. The as-prepared catalysts were hydrothermally treated at 760 °C for 48 h in air containing 10 vol% H2O to obtain the hydrothermal aged catalysts. The sulfur aged catalysts were treated at 400 °C with 100 ppm SO2, 10% water vapor, air balance for 48 h and catalysts. The powder X-ray diffraction (XRD) and Raman results indicate that the crystallization of hydrothermal aged catalysts is more serious than sulfur aged catalysts. In addition, tungsten species can stabilize the CeTi catalyst from grain growth. According to the results of in situ diffuse reflectance infrared Fourier transform spectra (DRIFTS), temperature-programmed desorption of ammonia (NH3-TPD), H2 temperature-programmed reduction (H2-TPR) and ammonia oxidation, the aging process leads to loss of surface area, redox properties, surface acidities and surface ceria concentration, especially for the hydrothermal aging. The NH3-NO/NO2 SCR performances of sulfur aged catalysts are better than that of hydrothermal aged catalysts. Compared with CeTi catalyst, the addition of tungsten inhibits the crystallization of catalyst. So that more acid sites and active sites are retained. This is also the reason why tungsten addition improves the NH3-NO/NO2 SCR performance of CeTi catalyst.

A kinetic study of the NO to NO2 oxidation mechanism over Fe-FER: A combined analysis of operando surface and gas phase data

Abstract Diesel engines when compared with gasoline engines produce higher amount of NOx due the operating conditions. Among the NOx produced NO represents the great majority while, under air excess conditions, the removal of NOx is necessarily catalytic and still challenging. In the ammonia based SCR, it is often admitted that, when dealing with the original formulation developed for stationary sources (V/TiO2), the fast SCR proceeds in presence of an equimolar mixture of NO and NO2. Most of the catalytic formulations developed up to now (except Cu based zeolites) thus suffer the same rate determining step consisting in the NO to NO2 oxidation. Aiming at improving the catalytic efficiency through the catalyst composition, it is fundamental to understand the reaction mechanism at its elementary steps in order to properly design the active sites. The choice regarding the distinct possible hypothetical mechanisms was thus here guided by the nature of the active sites. It is indeed worth knowing, for a better tuning of the catalyst formulation, whether isolated, oligomeric or dual iron sites are involved in the catalytic loop. The approach in this work consists in a systematic study of the influence of the inlet NO concentration under O2 excess on both the initial reaction rate (NO to NO2 conversion level kept below 10%) and the NO coverage level onto iron measured by operando FTIR. Isothermal experiments were performed over an aged FeFER catalyst at three distinct temperatures. The whole set of data was then processed and compared to the expected evolution derived from five possible mechanisms. The best mechanism determined in the frame of this study involves isolated iron sites onto which NO and O2 co-adsorb and the corresponding rate determining step consists in the dissociation of the so-formed Fe_NOO2 intermediate species. The associated activation energy and reaction enthalpy values are then evaluated.

Cu-SAPO-18 for NH3-SCR Reaction: The Effect of Different Aging Temperatures on Cu2+ Active Sites and Catalytic Performances

Cu-based silicoaluminophosphate zeolite with an AEI structure (Cu-SAPO-18) was directly synthesized through hydrothermal crystallization. The fresh and aged catalysts were applied for the selective...

Thermal activation and aging of a V2O5/WO3-TiO2 catalyst for the selective catalytic reduction of NO with NH3

Real-world vanadium-based catalysts for the selective catalytic reduction (SCR) of NO with NH3 are occasionally exposed to high temperatures, which can induce catalyst aging. In this work, a 2.0 wt% V2O5/WO3-TiO2 catalyst based on commercial WO3-TiO2 was lab aged in dry and wet feed at different time lengths and temperatures. Aging carried out in static atmosphere or in flow only marginally influenced its performance, while e.g. temperature and water in the feed heavily affected the SCR activity. The low temperature NOx conversion (≤300 °C) increased after aging up to 600 °C for 16 h and was more pronounced after hydrothermal aging compared to thermal aging, which was associated with the increased surface coverage of the SCR-active vanadyl groups. Measurements of vanadium volatility in the temperature range selected for the aging temperatures revealed the high mobility of V species induced by the (hydro-)thermal treatments. The onset of catalyst deactivation, observed at lower aging temperature for the hydrothermally aged catalyst compared to the thermally treated one, is possibly due to a larger amount of mobile V and W species and the concurrent loss of specific surface area. The set of catalytic and characterization data showed that water is an essential component in aging protocols because it heavily affects the structure and the resulting catalytic activity of the SCR catalyst. Removal of residual sulfate groups, which are present on the commercial support, also contributed to catalyst activation at 550°C aging temperature as a result of structural changes evidenced by surface area measurements and by IR and Raman spectroscopy, including rearrangement of V species and apparent increase of Lewis acidity.

Selective catalytic reduction of NO with NH3 over the Cu/SAPO-34 catalysts derived from different Cu precursors

Abstract Hydrothermal stability of the Cu(A, N, Cl or S)/SAPO-34 samples derived from different Cu precursors (A: Cu-acetate; N: Cu-nitrate; Cl: Cu-chloride; and S: Cu-sulfate) was investigated. The physical and chemical properties of the samples were investigated by various analytical techniques, and their catalytic activities for NO conversion and NH3 oxidation were evaluated. The results of NH3-SCR activity show that the Cu(A)/SAPO-34 sample was better than that of the Cu(N, Cl or S)/SAPO-34 sample, which was due to the existence of a less amount of CuO species and a more amount of the isolated Cu2+ species. Moreover, catalytic activities of all aged catalysts decreased in the whole temperature range, which was attributed to the drop in the amount of isolated Cu2+ species in the hydrothermally treated Cu/SAPO-34 samples. After hydrothermal aging treatment, part of the isolated Cu2+ was transformed to CuO, leading to structural instability and deactivation of the catalysts.

Effects of Blending Ethanol with Gasoline on the Performance of Motorcycle Catalysts and Airborne Pollutant Emissions

ABSTRACT This study investigated the effects of blending ethanol with gasoline on the exhaust emissions of fuel-injected motorcycles. Regulated gasoline (RF), and 15 (E15) and 30 (E30) vol% ethanol fuel were used as test fuels. Measurements of several air pollutants (CO, HC, and NOx) and organic air pollutant groups were conducted for two new fuel-injected four-stroke motorcycles. In addition, various catalysts were inserted into the motorcycles’ tailpipes to determine the characteristics and performance of the catalysts in treating the exhaust. Compared to using RF, we found that using blended fuel potentially reduced the CO and HC emissions by 30–37% and 19–28%, respectively. New catalytic systems, in conjunction with using different fuels, reduced CO, HC, and NOx emissions in the tailpipe exhaust by 12–61%, 32–39%, and 81–85%, respectively. The CO and HC emissions were directly proportional in quantity to the running mileage of the catalyst, but the NOx emissions were unaffected by this mileage, although they increased as the catalyst aged. We also discovered that at identical running mileages for a catalyst, the fuel consumption increased by –1.7–6.5% and 4.1–15% when using E15 and E30 fuel instead of RF. Furthermore, the specific surface area and pore volume of the catalyst decreased with the aged catalyst the phosphorus and sulfur content in the catalyst increased with the catalyst’s running mileage; adding ethanol to the fuel decreased emissions of paraffins, olefins, and aromatics but increased those of carbonyls; and the ozone formation potential of volatile organic compounds (VOCs) in the tailpipe exhaust was 16.7–17.2% for paraffins, 22–33% for olefins, 26–45% for aromatics, and 4.9–25% for carbonyls.

In-situ DRIFTS study of CeO2 supported Rh catalysts for N2O decomposition

Ceria supported Rh catalysts exhibit superior activities for the decomposition of N2O in the presence of water and excess O2 comparing with other oxides supported Rh catalysts. In contrast to Rh/γ-Al2O, hydrothermal aging (750 °C for 20 h with 10% H2O in air) significantly enhances the catalytic activity of Rh/CeO2 for the decomposition of N2O. The presence of H2O, on the other hand, inhibits the reaction rate on all Rh catalysts, but this inhibition effect is less on the aged Rh/CeO2 than on its fresh state. The focus of this work is to explore the nature of this activity enhancement on the aged Rh/CeO2 catalyst and the effect of water. Ceria and alumina supported Rh catalysts were characterized by in-situ DRIFTS with CO and N2O as probe molecules to investigate the synergism between Rh species and ceria support and the interaction between water molecules and the Rh/CeO2 catalysts. The results suggest that Rh supported on ceria has more CO adsorption sites and better hydrothermal stability than that on alumina. On Rh/CeO2 catalysts, the surface Rh species was found to facilitate the reduction of Ce4+ ions to Ce3+ at the interfacial area and as a result increased the concentration of Ce3+ species relative to the CeO2 support after a reduction treatment. Ce3+ ions at the interface between Rh and CeO2 support can be easily oxidized to Ce4+ by N2O at room temperature and played an important role in the disassociation of N2O. After a hydrothermal aging treatment on Rh/CeO2, more surface Rh sites were found that can adsorb CO and N2O. In addition, on the aged Rh/CeO2 catalyst, a different type of Rh species was found to be situated in a more electron-rich environment. These combined factors may be responsible for its enhanced N2O decomposition activity on the aged Rh/CeO2 catalyst. DRITFS were also collected on fresh and aged Rh/CeO2 catalysts with different degrees of sample hydration. The intensity of the OH group at 3658 cm−1 over an aged catalyst was found to decrease with increasing temperature at a faster rate than that on the fresh catalyst. This finding may explain their difference in H2O sensitivity for the N2O decomposition reaction.

Kinetic Modeling of the Metal/Support Interaction for CH4 Reaction over Oxidized Pd/Al2O3

This paper deals with the kinetics of CH4 dissociation on model Pd/Al2O3 catalysts. The adsorption and conversion of CH4 over oxidized catalysts, fresh or aged, were studied with a temporal analysis of products (TAP) reactor. Single CH4 pulse TAP experiments were performed on a stabilized surface. The experiments are discussed in the light of a selected mechanism involving CH4 decomposition into CO2 taking into account an interaction between the metallic active sites and the support. TAP experiments over the oxidized catalysts confirm the involvement of the metal/support interface, with a spill-over effect. Optimized kinetics parameters validate this interaction of OH species adsorbed on Pd sites with alumina, showing a relatively good agreement between experimental and calculated outlet gas composition. Both fresh and aged catalysts follow the chosen mechanism with the same kinetic constants at 400 °C only an alteration of the catalytic surface areas explain the loss of activity.

Experimental determination and modelling of the diesel oxidation catalysts ageing effects

After-treatment systems are necessary to respect the pollutant emissions thresholds specified by regulations. However, due to system ageing, the efficiency of the after-treatment system may decrease and affect the vehicle emissions during real driving conditions. To address this issue, the model presented in this article is based on the on-engine tests performed to a set of diesel oxidation catalysts with different ageing levels, through which the ageing process is characterized. Then, the model is able to simulate the light-off temperature and slip increase due to ageing, and it is applied to a Worldwide Harmonized Light Vehicles Test Cycle for a new and a thermally aged catalysts.

Structure dependence of Nb2O5-X supported manganese oxide for catalytic oxidation of propane: Enhanced oxidation activity for MnOx on a low surface area Nb2O5-X

A series of Nb2O5-x with different structures were prepared as a carrier to manganese oxide catalysts for total oxidation of propane. The results demonstrated that a monoclinic structure of Nb2O5-x pre-calcined at 1025 °C, leads to significantly and surprisingly higher catalytic oxidation activity when MnOx is deposited at 400 °C even with extremely low specific area (around 3.94 m2/g) relative to the performance of MnOx/Nb2O5-x (∼>50 m2/g pre-calcined at 500 °C). Conversion vs temperature profiles for fresh and aged catalysts were generated and performance compared for different materials. Brunauer Emmett Teller (BET) and X-ray diffraction (XRD) were conducted to reveal the textural and structural features of niobium-based catalysts. Raman, X-ray photoelectron spectroscopy (XPS) and temperature programmed reduction (TPR) were performed to further understand the interaction between manganese and niobium oxides. Raman spectra indicated a new Nb-O-Mn species formed due to the strong interaction between the activated niobium oxide carrier at 1025 °C and manganese oxide. This study describes a synergistic catalytic oxidation effect between Mn oxides deposited on a specific phase structure of Nb2O5-x with very low specific surface area. The enhanced catalytic performance is directly related to the proper ratio of Mn3+/Mn4+ coupled with high ratio of Nb4+/Nb5+ on the surface and the oxygen vacancies generated between monoclinic Nb2O5-x and MnOx.

Neodymium versatate catalyst for the 1,3-butadiene polymerization – Effects of reaction parameters

Neodymium versatate was used as catalyst to perform solution 1,3-butadiene polymerizations. Catalysts were synthesized and aged at 5 °C for 0–60 days. Reactions were performed in the temperature range from 60 to 80 °C for 40–120 min. The obtained polymer products were characterized by Fourier-transform infrared spectroscopy (FTIR) and gel permeation chromatography (GPC). It is shown that the isomers content of the final resin is sensitive to ageing and to modification of the reaction temperature in the analyzed range. Besides, aged catalysts allowed for production of polybutadienes with higher molecular weights and higher polymer yields. Finally, it is shown that chain reincorporation reactions take place when reaction is allowed to continue after reaching monomer conversions that are close to 100%.

The effects of tungsten and hydrothermal aging in promoting NH3-SCR activity on V2O5/WO3-TiO2 catalysts

The fresh catalysts of V2O5-TiO2 (VTi) and V2O5/WO3-TiO2 (VWTi) were prepared via an impregnation method. The hydrothermal aged samples were obtained at 750 °C for 24 h under airflow with 10 vol% water vapor. Compared to the fresh catalysts, the catalytic performance of the aged catalysts (VTi-A and VWTi-A) has been significantly improved. Particularly, VWTi-A displayed the best activity over a wide temperature range (NOx conversion: >90% @ 250–550 °C). Structural characterizations indicated that the surface area of the aged catalysts was dramatically decreased, and the morphologies of catalysts also changed. The X-ray photoelectron spectroscopy (XPS), H2 temperature-programmed reduction (H2-TPR), and Raman results demonstrated that hydrothermal aging greatly influenced the migration and agglomeration of vanadium species. As a result, numerous active polymeric VOx species were emerged on the surface of catalysts. More importantly, XPS results revealed that more low valence vanadium species (V4+ and V3+) was produced on the VWTi-A catalysts than that of VTi-A after hydrothermal aging. These low valence vanadium species resulted in considerably higher SCR activity than the high valence vanadium species (V5+). Consequently, both polymeric surface VOx species and low valence vanadium species determined the enhanced low-temperature SCR behavior of the aged catalysts.

Development and Validation of a Two-Site Kinetic Model for NH3-SCR over Cu-SSZ-13. Part 2. Full-Scale Model Validation, ASC Model Development, and SCR-ASC Model Application

We present herein the final part in the development and validation of a two-site kinetic model for NH3-SCR over Cu-SSZ-13. To predict tailpipe emissions accurately, it was necessary to combine the kinetic model of the SCR catalyst developed in part I (Daya et al. 2018), with a reaction-diffusion model of the dual-layer ammonia slip catalyst (ASC). This dual-layer ASC model was developed following a three-step process, including development of the kinetic models of the individual layers, followed by parameterization of a parallel pore diffusion model of the dual-layer ASC. Reactor-scale validation of the dual-layer ASC model confirmed the kinetic model accuracy and highlighted the significance of intra-porous diffusion. Following this, the SCR model developed in part I of this paper was validated on an engine dynamometer through comprehensive steady-state experiments with inlet NH3 to NOx ratio (ANR) sweeps. The final SCR and ASC models were then evaluated on cold and hot heavy-duty transient (HDT) cycles, to examine the capability of predicting tailpipe NOx, NH3 slip as well as storage-based dynamics. Overall cycle-averaged NOx conversion was predicted within 3% using these models. Validated models have significant application in model-based control as well as improving catalyst design through improved functional understanding. The present Cu-SSZ-13 SCR model was simulated using the four-step SCR protocol (Kamasamudram et al. Catal. Today 151(3):212–222) to calculate the intra-catalyst dynamic capacity. These numerical experiments showed that the dynamic capacity decreases upon hydrothermal aging but leads to higher NOx conversion under standard and fast SCR conditions at 250 °C. This increase in NOx conversion is due to more uniform NH3 storage along the length of the catalyst, leading to higher NH3 utilization near the rear of the aged catalyst. Similar numerical experiments on the dual-layer ASC model demonstrated intra-layer washcoat distributions causing NO slip during transient drive cycles for both hydrothermal aging conditions.

A new understanding of CeO2-ZrO2 catalysts calcinated at different temperatures: Reduction property and soot-O2 reaction

CexZr(1-x)O2 (x = 0.27, 0.50 and 0.73) mixed oxides were prepared by co-precipitation method and then aged at 700–1000 °C. The reducibility of the aged catalysts was investigated by H2-TPR. It was found that the changes in reduction properties are related to the Ce/Zr ratio and Ce0.27Zr0.73O2 shows an increased H2 consumption with aging temperature increasing, even 1000 °C-aged samples show an increased reduction ability. Raman, XPS and HRTEM reveal that the promotion on reduction property of the aged samples should be ascribed to the increase of surface oxygen species, surface Ce/Zr ratio, O-vacancies and (110) and (100) planes. However, this promotion can not obviously improve soot oxidation activity. XRD, BET and O2-TPD results reveal that textural, structural and oxygen desorption properties are crucial to soot oxidation activity. The poor contact between soot and active sites would be an important factor that leads to a contradiction between H2-TPR and soot-O2 reaction activity.

Effect of hydrothermal aging on NOx reduction performance for Sb–V–CeO2/TiO2 catalyst

Herein, we investigated the NOx reduction performance of Sb–V–CeO2/TiO2 (SbVCT) catalyst subjected to hydrothermal aging, where 6 vol% of H2O was fed to the SbVCT for 16 h with variable temperatures of 550–750 °C. The structural, morphological, redox, and acid properties of fresh and aged catalysts were comprehensively characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), Raman, N2-physisorption, temperature programmed reduction/desorption (H2–TPR/NH3/NO–TPD) and in situ DRIFTS. The XRD, Raman and TEM results of the fresh and the one hydrothermally aged at 550, 600 °C catalysts indicated the presence of finely dispersed polymeric vanadia, antimony, and nano-crystalline ceria species on TiO2 support. The SbVCT catalysts hydrothermally aged at the temperatures ≤ 600 °C showed better NOx conversions than the others aged > 600 °C. This was because aging at the temperatures ≤ 600 °C could help to minimize the loss of surface redox/acid properties that were originally inherent to surface vanadyl and CeO2 species that went into CeVO4 and large CeO2 aggregates. This in turn led to the loss of surface redox and acidic characters indigenous to the SbVCT and reduced its NOx reduction performance. Moreover, the NOx reduction performance of hydrothermally aged V–WO3–TiO2 catalyst, when compared with that of SbVCT catalyst, showed relatively low NOx conversions at the temperatures below 300 °C.

Promoted Hydrothermal Stability of Pd/CeO2 Catalyst by Ba Doping

In this study, a group of Ba-doped Pd/CeO2 catalysts were prepared by impregnation method and hydrothermally aged at 750 °C for 20 h to investigate the effect of Ba addition on the hydrothermal stability of Pd/CeO2. The hydrothermal stability of Pd/CeO2 was promoted when doped with a moderate amount of Ba. Particularly, the aged catalyst with optimal 1.0 wt% Ba addition showed a significant improvement in CO oxidation activity, even when cofeeding water and carbon dioxide, presenting superior hydrothermal stability. The positive effects of Ba addition on inhibiting the sintering of CeO2 and aggregation of Pd during hydrothermal aging were proposed by combining with the structural properties, DRIFTS and XPS characterizations. More importantly, the Ba addition favored the preserving of oxidized Pd2+ and active oxygens, which played an important role in the enhanced CO oxidation activity of Ba-doped catalysts. Ba addition promoted the hydrothermal stability of Pd/CeO2 catalyst for CO oxidation by the steric effect on Pd.

On the hydrothermal stability of Cu/SSZ-13 SCR catalysts

Cu/SSZ-13 SCR catalysts have been extensively studied in the past decade or so. Hydrothermal stability of these catalysts has been identified as the most important criterion for application. In this perspective, we describe recent atomic-level understanding of their hydrothermal stability. In particular, electron paramagnetic resonance (EPR) is shown to rather accurately quantify isolated Cu(II) ions and CuO clusters in fresh and aged catalysts to demonstrate the remarkable hydrothermal stability for Cu2+ ions located in 6-membered ring windows, and the conversion of [Cu(OH)]+ ions in Chabazite cages to CuO clusters. The hydrothermal stability difference of the two isolated Cu(II) ions is confirmed with DFT simulations and the conversion of [Cu(OH)]+ to CuO is proposed to involve formation, migration and condensation of Cu(OH)2 intermediates. The structural destructive role of CuO clusters is attributed to mesopore formation from their migration, which more severely damages the catalysts than dealumination. Finally, perspectives are given on new strategies for low-temperature NOx removal, rational design and refinement of Cu/SSZ-13, and development of new Cu/zeolite SCR catalysts with even better performance than the state-of-the-art Cu/SSZ-13.

Induced effect of tungsten incorporation on the catalytic properties of CeVO4 systems for the selective reduction of NOx by ammonia

The SCR performances of aged W-doped CeVO4 catalysts were investigated in fast-, NO2- and standard conditions. Bulk CeV1-xWxO4 catalysts were prepared in soft conditions, according to a hydrothermal synthesis route at 180 °C leading to the ad hoc tetragonal structure. The aging procedure consisted in exposing the samples to 10 vol.% H2O diluted in air at 600 °C. The impact of this aging process was investigated as a function of tungsten loading, with x varying in the range 0.02–0.15. A clear beneficial effect is distinguishable at high temperature, i.e., above 300 °C, associated to a sharp selectivity enhancement irrespective of the operating conditions and tungsten loading. On the contrary, a slight detrimental effect on the reaction rate is clearly discernible in standard-conditions below 300 °C. It was found that the thermal aging induces more extensive diffusion of V5+ species assisted by the presence of substituted W6+. It was also demonstrated that tungsten diffuses more readily than V5+ leading ultimately to a significant surface enrichment and the formation of Ce4W9O33 mixed oxide evidenced from XRD and Raman spectroscopy. Hence, tungsten would rather act as a stabilizer than promoter avoiding the sintering of monomeric VOx species to more active polyvanadate species but also less selective. The gain observed in selectivity at high temperature can be equally explained by a greater stabilization of Brønsted acid sites and the preservation of the dispersion of monomeric vanadate species preventing the occurrence of ammonia oxidation.

Sol-gel one-pot synthesis of efficient and environmentally friendly iron-based catalysts for NH3-SCR

In current study, various pore-forming agents were employed for the synthesis of Fe-W-Ti catalysts via sol-gel one-pot approach, and the catalytic performance was investigated for selective catalytic reduction of NOx by ammonia (NH3-SCR). Among all the catalysts, Fe0.2W0.2Ti(U0.03, urea-modification) catalyst exhibited more than 95% NOx conversion in range of 240–420 °C, and excellent performance of resistance to SO2 and H2O was observed with a slight decline of NOx conversion to 92%. The characterization results revealed that urea-modified catalyst possessed low crystallinity of TiO2 with particle size of 3.8 nm and the dispersion of Fe-W species was increased significantly, which could be ascribed to the high specific surface area (144.92 m2/g) and superior porous structure. The sufficient strong acid, the remarkable reducibility and the increase in the ratio of divalent iron & adsorbed oxygen may be responsible for the excellent catalytic activity. Moreover, a dramatic drop in the specific surface area and surface acidity of the hydrothermal aged catalyst was observed, and the active species also became agglomerated with FeWO4 solid solution.

Effect of valence state and particle size on NO oxidation in fresh and aged Pt-based diesel oxidation catalysts

To stabilize Pt, Magnesium-modified SiO2-Al2O3 materials was used to impregnate with Pt, which could strengthen the bonding effect between Pt and Mg. Before and after aging, both showed a higher dispersion. High valence state of Pt in fresh modified catalyst was unfavorable of NO oxidation, indicating that the valence state of Pt was the leader factor in fresh catalytic performance. While for the aged Mg-modified sample, its reaction temperature of 30% NO conversion lowered by around 30 °C. The Pt stabilization via interacting with Mg derives a relation that the variation of Pt valence state and its exposed sites played a significant role in fresh and aged catalytic NO activity, respectively.