•Lower copper (<1.7%) loading ensured structure durability even after 950 °C aging.•Higher copper loading (>3.91%) accelerated structure collapse after 950 °C aging.•The optimal copper loadings for activity and stability didn’t overlap.•More vacant Brønsted acid site and less copper oxides benefited samples durability.Four Cu/SAPO-34 samples by one-pot method are utilized to examine their durability after 950 °C hydrothermal treatment and its relation with copper loading. The SCR results show fresh Cu/SAPO-34 with 3.91% copper loading (F-Cu-3.91) performs the superior NO conversion, wide temperature window and excellent nitrogen selectivity among fresh samples, and NO conversion is mainly determined by isolated Cu2+ contents at low temperature. After 950 °C aging treatment, Cu/SAPO-34 catalysts with copper loading under 1.70% present good stability, while the ones with copper loading above 3.91% show activity and crystallinity decline. Ex-situ DRIFTs, XRD and NH3-TPD results reveal the 950 °C aging process leads to Si-OH-Al bonds breakage and phase transition of chabazite support over Cu/SAPO-34 samples with high copper loading, meanwhile, the EPR and TPR outcomes prove the copper oxides’ further dispersion and coordination variation due to skeleton collapse. Finally, this work is trying to manifest the appropriate copper loading for a stable Cu/SAPO-34 catalyst and its deactivation mechanism during extreme working situation.Download high-res image (163KB)Download full-size image

•Pt greatly increases N2O and NO2 generation during SCR process.•Pt benefits copper species reduction but shows less impact on structure.•Hydrothermal treatment promotes rejuvenation of poisoned Cu/SAPO-34 by Pt.•Platinum-copper oxo-complexes weaken ammonia oxidation and improve SCR activity.•Pt enhances copper oxides further distribution during hydrothermal treatment.This study mainly focused on selective catalytic reduction of air pollutants NOx by ammonia (NH3-SCR), and a series of Pt impregnated Cu/SAPO-34 (homemade) samples were employed to elucidate its negative impact on DeNOx activity and hydrothermal treatment’s acceleration on NH3-SCR activity resurgence. Firstly, XRD, NH3-TPD and DRIFTs were performed to examine platinum interaction with zeolites structure and contribution to acidity. Then, Pt inhibition on NH3-SCR activity was evaluated and its impact on reaction network, including ammonia oxidation, NO oxidation and NO + NH3, was concluded based on gas switching tests. H2-TPR and EPR further reflected various Cu species coordination and redox capacity variation caused by platinum doping. It was intriguing to find out that the further hydrothermal treatment benefited rejuvenation of Pt-poisoned Cu/SAPO-34 and even activity enhancement from Pt presence under the condition of zeolites structure integrity. Hydrothermal treatment induced platinum sintering and STEM illustrated platinum species combined with copper oxides and generated oxo-complexes, weakening ammonia oxidation. And Pt presence promoted copper oxides further dispersion during hydrothermal treatment. Finally, platinum poisoning on Cu/SAPO-34 and its regeneration after hydrothermal treatment were concluded to indicate their application potential.Download high-res image (133KB)Download full-size image

•In rich period, N2O was originated from chemical adsorption of NO on Pt0 surface.•N2O formation was influenced by a complex reaction network in rich period.•Water-gas shift and C3H6 steam reforming produced H that decreased N2O formation.•CO2 reforming of C3H6 produced CO that led to an increase in N2O formation.•In lean period, N2O was formed via gaseous NO/O2 reacting with residual NCO.N2O formation over Pt-BaO/Al2O3 catalysts was found to be involved in a complex reaction network induced by C3H6 evolutions during NOx storage reduction process. C3H6 evolutions in the rich period travelled along cracking to CxHy(ad), partial oxidation to COad and followed by NCOad production. In the lean period, residual NCOad reacting with the introduced NO/O2 led to the formation of N2O. In the rich period, N2O was originated from the chemical adsorption of NO on Pt0 surface caused by C3H6 reduction. In addition, both H2O and CO2 in the feed had significant influences on N2O formation in rich period. In the presence of H2O, high performance of water-gas shift was found at T ≥ 300 °C, and steam reforming of C3H6 occurred at T ≥ 350 °C. These reactions produced a large number of Had that facilitated NH3 production and thereby decreased N2O formation. When CO2 was introduced to the feed, CO2 reforming of C3H6 appeared at T ≥ 300 °C as a result of COad production. These parts of COad were responsible for the increase in N2O formation.Download high-res image (96KB)Download full-size image

AbstractThe effect of synthesis methods on the activity of V/Ce/WTi catalysts was investigated for the selective catalytic reduction (SCR) of NOx by NH3. V/Ce/WTi-DP (deposition precipitation) catalyst showed excellent NH3-SCR performance, especially the better medium-temperature activity and the less N2O formation than V/Ce/WTi-IMP (impregnation). These catalysts were characterized by X-ray diffraction (XRD), Brumauer-Emmett-Teller (BET), X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction (H2-TPR), and in situ DRIFTS techniques. The XPS and H2-TPR results revealed that V/Ce/WTi-DP exhibited more surface Ce species, higher level of Oα and higher reducibility of Ce species. Reflected by in situ DRIFTS results, the deposition precipitation method (DP) contributed to a greater amount of weakly adsorbed NO2, monodentate nitrate and NH3 species with better reactive activity. Meanwhile, the cis-N2O22– species, an intermediate for N2O formation, was very limited. As a result, these advantages brought about the superior SCR activity and N2 selectivity for V/Ce/WTi-DP.The deposition precipitation method (DP) contributes to a great amount of active nitrates, NH3 species with better reactive activity and the limited cis-N2O22– species, which brings about the superior NH3-SCR performance for V/Ce/WTi-DP

Because of the potential crisis of Pt/Al2O3 catalysts being poisoned by S-containing impurities in diesel exhausts, it is important to better understand the deactivation of SO2 and its regeneration effects on Pt/Al2O3 catalysts. In this study, these effects induced by SO2 on Pt/Al2O3 were studied as a function of SO2 exposure time, and NO oxidation activity was investigated for different treated Pt/Al2O3 catalysts. The results show that the SO2 exposure period seriously affects the catalytic ability, especially after a long period of the sulfation process. TEM, CO-DRIFTS, TGA, and STEM-EDS elemental mapping results show that Pt physical and chemical states, and SOx species are considered to be essential parameters in the SOx effects on the catalysts. It is found that partial regeneration in the samples with different SO2 exposure times is primarily caused by an un-recoverable deactivation of edge and kinked Pt atoms during the sulfation process. Moreover, the mechanism of SOx loading and its elimination through the NO oxidation process, investigated by in situ DRIFTS, provides further cognizance on SOx species and their removal. The mechanism reveals a special intermediate SOx species, and thus a possible way in which gaseous SO2 and O2 transform to sulfate on catalysts is expected.

A series of CeO2–Al2O3 composites with different Ce content were prepared. The uniformity of the CeO2 dispersion was confirmed using H2-TPR and HR-TEM. Two aging treatments were conducted, and the CeO2–Al2O3 composites show superior hydrothermal stability. The sintering of CeO2 and Al2O3 are independent of each other based on XRD and HR-TEM results. On the other hand, the dynamic oxygen storage capacity (DOSC) is mostly activated after 20 h of aging at 750 °C, and deactivated after 10 h of aging at 1050 °C. Combining the results of structural and DOSC studies, the interaction between CeO2 and Al2O3 can be divided into two parts, (1) a chemical interaction which negatively impacts the DOSC, and (2) a spatial limitation which benefits the sample stability. The former interaction is eliminated after hydrothermal aging at 750 °C, while the later one exists even after hydrothermal aging at 1050 °C.

The promotion effect of Cu on the V/WTi catalyst for the selective catalytic reduction of NOx by NH3 was investigated in the temperature range of 150–400 °C. The Cu addition shows a superior NH3-SCR performance in comparison with the V/WTi sample. The catalysts were characterized by XRD, Raman, EPR, H2-TPR, XPS, and in situ DRIFTS techniques. The obtained results reveal that the Cu oxides in close proximity to V oxides on the surface facilitate the formation of double redox couples of V5+/V4+ and Cu2+/Cu+, which may play a critical role in the superior NH3-SCR performance. The electronic interactions caused by the redox cycle of Cu2+ + V4+ ↔ V5+ + Cu+ could significantly improve the redox properties of the vanadium species, which is beneficial for the activation of NH3 species bound to the vanadium species. Moreover, the redox cycle of Cu2+ + V4+ ↔ V5+ + Cu+ induces the formation of high-activity nitrate species adsorbed on Cu species. The kinetic analysis reveals that the Cu doping induces the decrease of the activation energy (Ea) of NH3-SCR.

Pd@Zr/CeO2 core-shell catalyst prepared by hydrothermal method was applied in CO oxidation reaction, exhibiting high CO oxidation activity at low temperature. XRD (X-ray diffraction) analysis demonstrated that the remarkable enhancement of catalytic performance was found to depend on the presence of more oxygen vacancies in the core-shell structure, which contributed higher content of and ready release of active oxygen species at low temperature, confirmed by H2-TPR (temperature programed reduction) results. Interestingly, introducing a small amount of zirconium (0.5 wt.%) exhibited a significant improvement of catalytic activity because the introduction of Zr further improved the amount of crystal defects and promoted the migration of oxygen species.Catalytic activity of different catalysts over CO oxidation reaction

A reversed microemulsion was first adopted to prepare the Ce−Zr−Sr ternary mixed oxide with different Sr content. The Pd-only three-way catalyst (TWC) was obtained by incipient wetness impregnation with 0.5 wt % Pd loading. The structural and oxygen handling properties have been analyzed by XRD, Raman, H2-TPR, and the dynamic oxygen storage capacity (DOSC). The Sr doping resulted in improved phase thermal stability and the low-temperature activity. The introduction Sr into ceria−zirconia lattice strongly modified the mobility of oxygen and enhanced the DOSC performance. Pd-only TWC based on the Ce0.67Zr0.33Sr0.03O2.03 support exhibited superior activity for C3H8 and NO conversion and amplified amplitude of the stoichiometric window.

The dynamic oxygen storage capacity (DOSC) and rate (DOSR) over Mn0.1Ce0.9Ox and Mn0.1Ce0.6Zr0.3Ox prepared by sol–gel method were investigated. The results show that Ce-based compounds containing Mn oxides with variable valences contribute to the majority of DOSC and DOSR performances at low temperature, and the DOSC and DOSR of Mn–Ce–O can be enhanced by the Zr addition at high temperature. Both materials were characterized by XRD, BET, Raman, TPR, EPR and XPS. XRD and Raman results indicate that Mn0.1Ce0.9Ox and Mn0.1Ce0.6Zr0.3Ox are characterized with the fluorite-type cubic structure similar to CeO2, and furthermore the thermal stability of M–Ce–O materials is improved by the introduction of some Zr atoms. EPR researches reveal that there are three types of Mn2+ species: isolated Mn2+ substituting for Ce4+ ions in the lattice with a cubic symmetry, ones in defect with a noncubic symmetry and at the surface of samples. From XPS, it can be concluded that Mn2+/Mn3+ redox couples exist on the surface of Mn0.1Ce0.9Ox and Mn0.1Ce0.6Zr0.3Ox samples.

Doping effect of CaO and MgO on the microstructure and dynamic oxygen storage capacity (OSC) of Ce0.67Zr0.33O2 was compared. XRD, Raman spectra and HRTEM observation confirmed that after MgO and CaO doping, more oxygen ion vacancies and lattice defect in the fluorite lattice were produced due to charge compensation and radius effect. On the other hand, improved phase thermal stability was obtained from doping. Results of dynamic OSC measurements showed that MgO and CaO doping enhance the dynamic OSC of Ce0.67Zr0.33O2. But after redox treatment difference was found between the two dopants. The improved OSC of CaO doping was due to the proper ionic radius and effective modification of microstructure. The decreased OSC of MgO doped Ce0.67Zr0.33O2 after redox treatment was due to the small solubility of Mg2+ in ceria lattice and collapse of the defective structure of Mg–Ce–Zr–O system.

P-modified alumina was prepared through gelification with phosphate compounds introduced during the formation of alumina gel. Common characterizations such as XRD, BET and IR were employed to investigate the effect of phosphate dopant on the structure and surface properties of alumina. The results showed that the addition of phosphate could effectively improve thermal stability and textural properties of alumina. For G5 sample, the α-phase formation temperature is enhanced up to 1573 K and the surface area retains 85 m2 g−1 as well as the pore volume 0.356 cm3 g−1 after calcination at 1473 K for 3 h. Moreover, the surface area and pore volume both have increasing trends with the increasing phosphorous contents in alumina after calcinations at the same temperature. There is also an increasing trend of lattice constants of P-modified aluminas with the increase of phosphorous content in alumina. Considering the defective spinel structure of γ-alumina, it can thus be deduced that some of the P5+ ions might occupy the vacancy of cationic site and interrupted the track of Al3+ ions transfer from tetrahedral to octahedral in the processes of sintering and transformation.

The oxygen storage capacity (OSC) of Ce0.67Zr0.33O2 catalysts were investigated with CO–He pulses at 500 °C and CO–O2 dynamic pulses at 300–700 °C after redox and hydrothermal treatments. BET, XRD, and EPR were used to study the effects of the pretreatment on the surface and bulk structures of the catalyst. The results show that during the CO–He pulses, migration of oxygen from the bulk to the surface becomes important. The rate of oxygen migration depends on the structure of the solid and the number of the oxygen vacancies in the lattice. The experiments with dynamic pulses of CO–O2 reveal that DOSC is closely related to the surface area of the samples of the same composition. However, at high temperatures, oxygen in the bulk also contributes to DOSC due to bulk-to-surface migration. A mechanism describing the reaction between the surface oxygen and CO and the oxygen migration process from the bulk to the surface is proposed.

Because of the potential crisis of Pt/Al2O3 catalysts being poisoned by S-containing impurities in diesel exhausts, it is important to better understand the deactivation of SO2 and its regeneration effects on Pt/Al2O3 catalysts. In this study, these effects induced by SO2 on Pt/Al2O3 were studied as a function of SO2 exposure time, and NO oxidation activity was investigated for different treated Pt/Al2O3 catalysts. The results show that the SO2 exposure period seriously affects the catalytic ability, especially after a long period of the sulfation process. TEM, CO-DRIFTS, TGA, and STEM-EDS elemental mapping results show that Pt physical and chemical states, and SOx species are considered to be essential parameters in the SOx effects on the catalysts. It is found that partial regeneration in the samples with different SO2 exposure times is primarily caused by an un-recoverable deactivation of edge and kinked Pt atoms during the sulfation process. Moreover, the mechanism of SOx loading and its elimination through the NO oxidation process, investigated by in situ DRIFTS, provides further cognizance on SOx species and their removal. The mechanism reveals a special intermediate SOx species, and thus a possible way in which gaseous SO2 and O2 transform to sulfate on catalysts is expected.