Real biofuel and fossil-fuel soot combustion activities in active and passive regeneration of diesel/gasoline particulate filters under different O2/NOx concentrations.

In this work, we aim to investigate and compare the combustion reactivities of real biofuel soot and fossil-fuel soot in the active and passive regeneration conditions of DPF and GPF through temperature-programmed oxidation (TPO). Higher reactivity of biofuel soot is achieved even under GPF conditions with extremely low oxygen concentration (~ 1%), which provides a great potential for low-temperature regeneration of GPF. Such a result is mainly attributed to the low graphitization and less surface C = C groups of biofuel soot. Unfortunately, the presence of high-content ashes (~ 47%) and P impurity in real biofuel soot hinder its combustion reactivity. TPO evidences that the O2/NOX-lacking conditions in GPF are key factors to impact the combustion of soot, especially fossil-fuel soot. This work provides some useful information for understanding real biofuel and fossil-fuel soot combustion in GPF and DPF regeneration and further improvement in filter regeneration process.

Engineering a PtCu Alloy to Improve N2 Selectivity of NH3-SCO over the Pt/SSZ-13 Catalyst.

Improving the N2 selectivity is always a great challenge for the selective catalytic oxidation of ammonia (NH3-SCO) over noble-metal-based (especially Pt) catalysts. In this work, Cu as an efficient promoter was introduced into the Pt/SSZ-13 catalyst to significantly improve the N2 selectivity of the NH3-SCO reaction. A PtCu alloy was formed in the PtCu/SSZ-13 catalyst, as confirmed by X-ray diffraction, transmission electron microscopy, energy dispersive spectrometry mapping, and X-ray absorption spectroscopy results. As indicated by the X-ray photoelectron spectroscopy analysis, the Pt species in the alloyed PtCu nanoparticle was mainly present in the electron-rich state on PtCu/SSZ-13, while the electron-deficient Cu and isolated Cu2+ species were both present on the surface of PtCu/SSZ-13. Due to such a unique alloyed structure with an altered oxidation state, the N2 selectivity of NH3-SCO on the PtCu/SSZ-13 catalyst was remarkably improved, while the NH3-SCO activity was kept comparable to that on Pt/SSZ-13. The reaction path was changed from the NH mechanism on Pt/SSZ-13 to both NH and internal selective catalytic reduction mechanisms on the PtCu/SSZ-13 catalyst, which was considered the main reason for the enhanced N2 selectivity. This work provides a new route to synthesize efficient alloy catalysts for optimizing the N2 selectivity of NH3-SCO for NH3 slip control in diesel exhaust purification.

Unique κ-Ce2Zr2O8 Superstructure Promoting the NOx Adsorption-Selective Catalytic Reduction (AdSCR) Performance of the WO3/CeZrOx Catalyst.

Selective catalytic reduction of NOx by NH3 (NH3-SCR) for diesel emission control at low temperatures is still a great challenge due to the limit of the urea injection threshold and inferior SCR activity of state-of-the-art catalyst systems below 200 °C. Fabricating bifunctional catalysts with both low temperature NOx adsorption-storage capacity and medium-high temperature NOx reduction activity is an effective strategy to solve the issues mentioned above but is rarely investigated. Herein, the WO3/Ce0.68Zr0.32Ox (W/CZ) catalyst containing the κ-Ce2Zr2O8 pyrochlore structure was successfully developed by a simple H2 reduction method, not only showing superior NOx adsorption-storage ability below 180 °C but also exhibiting excellent NH3-SCR activity above 180 °C. The presence of the pyrochlore structure effectively increased the oxygen vacancies on the κ-Ce2Zr2O8-containing W/CZ catalyst with enhanced redox property, which significantly promoted the NOx adsorption-storage as active nitrate species below 180 °C. Upon NH3 introduction above 180 °C, the κ-Ce2Zr2O8-containing W/CZ catalyst showed greatly improved NOx reduction performance, suggesting that the pyrochlore structure played a vital role in improving the NOx adsorption-selective catalytic reduction (AdSCR) performance. This work provides a new perspective for designing bifunctional CeZrOx-based catalysts to efficiently control the NOx emissions from diesel engines during the cold-start process.

Boosting Methane Combustion over Pd/Y2O3-ZrO2 Catalyst by Inert Silicate Patches Tuning Both Palladium Chemistry and Support Hydrophobicity.

Supported palladium (Pd) catalysts are widely utilized to reduce the emission of exhaust CH4 from lean-burn engines by catalytic combustion. A large amount of water vapor in the exhaust makes hydroxyls accumulate on the catalyst surface at temperatures below 450 °C, leading to severe catalyst deactivation. Tuning palladium chemistry and inhibiting water adsorption are critical to developing active catalysts. Modifying the support surface with inert silicates would both change the palladium-support interaction and decrease water adsorption sites. This study reports an improved Pd/Y2O3-ZrO2 catalyst by constructing silicate patches on yttria-stabilized zirconia (Y2O3-ZrO2) support. The silicates hindered electron transfer from Y2O3-ZrO2 oxygen vacancies to palladium, which optimized palladium chemistry, especially the reducibility of active PdO species, and thereby boosted CH4 conversion under dry conditions. The temperature of 90% methane conversion (T90) over the catalyst decreased from 386 to 309 °C. Moreover, the inert silicates decreased surface oxygen vacancies of Y2O3-ZrO2 to improve support hydrophobicity, thereby inhibiting hydroxyl accumulation. The poisoning effect of water on the active sites located on the palladium-silicate interface was alleviated. When reaction gases contained 10 vol % water, the silicate-modified catalyst still showed higher activity with T90 of 404 °C, which is lower than T90 of 452 °C for unmodified catalyst. This work represents a step forward in preparing high-performance palladium catalysts for low-temperature wet methane combustion.

Catalysts for highly water-resistant catalytic decomposition of ozone: Hausmannite Mn3O4 on exposed (101) crystal surface.

Recently, ozone pollution has gradually replaced PM2.5 as the main pollutant affecting air pollution. In this study, we synthesized a series of Mn3O4 catalysts by hydrothermal method changing the precursors and tested their activities at different relative humidity, gas volume space velocity of 150,000 h-1 and 5 ppm ozone. Remarkably, Mn3O4-SO4 prepared with MnSO4 as precursor showed excellent catalytic ozone decomposition activity, almost completely converting 5 ppm of ozone at different relative humidity ranges. Finally, the most active Mn3O4-SO4 catalyst was tested for its usability limit at RH= 90%, after 28 h of testing under high humidity conditions, it had retained successfully the complete decomposition of low concentrations of ozone. The catalysts were characterized by XRD, Raman, HRTEM, XPS, BET, H2O-TPD and in situ IR NH3 adsorption. The characterization analysis revealed that the Mn3O4-SO4 surface could exposed a highly active (101) crystalline surface with high specific surface area, excellent hydrophobicity as well as ozone adsorption capacity, which were highly favorable for ozone decomposition under high humidity conditions. In this work, Mn3O4 exhibits good catalytic activity, which provides an additional option for future studies of manganese oxides.

Constructing a Pt/YMn2O5 Interface to Form Multiple Active Centers to Improve the Hydrothermal Stability of NO Oxidation.

The hydrothermal stability of NO oxidation is the key to the practical application of diesel oxidation catalysts in diesel engines, which in the laboratory requires that NO activity does not decrease after aging for 10 h with 10% H2O/air at 800 °C. On the one hand, the construction of a metal/oxide interface can lead to abundant oxygen vacancies (Ov), which compensate for the loss of activity caused by the aggregation of Pt particles after aging. On the other hand, YMn2O5 (YMO) has excellent thermal stability and NO oxidation capacity. Therefore, a Pt/YMn2O5-La-Al2O3 (Pt/YMO-LA) catalyst was prepared by the impregnation method. The support of the catalyst, YMn2O5-La-Al2O3 (YMO-LA), was obtained by mixing high specific surface LA and YMO ball-milling. Under laboratory-simulated diesel exhaust flow, the NO oxidation performance of Pt/YMO-LA did not decrease after hydrothermal aging. Combining high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), and oxygen temperature-programmed desorption (O2-TPD), the Pt/YMn2O5 interface was formed after hydrothermal aging, and the increased Ov can provide reactive oxygen to Pt and YMO. The cooperative catalysis of multiple active centers composed of Pt, YMO, and Ov is the crucial factor to maintain the NO oxidation performance. In addition, in situ diffuse reflectance infrared Fourier transform spectra (DRIFTs) show that an increase in Ov is beneficial to the adsorption and desorption of more nitrate and nitrite intermediates, thus achieving the hydrothermal stability of NO oxidation.

Efficient monolithic MnOx catalyst prepared by heat treatment for ozone decomposition.

The effects of calcined temperature on the properties and ozone decomposition activity of manganese oxide catalysts were investigated under high-humidity, low ozone conditions. An outstanding manganese-based catalyst (MnOx (260 ℃)) was prepared, which could decompose above 90% (RH = 0%) and 70% (RH = 90%) ozone after 6 h using. Specific characterization showed MnOx (260 ℃) had excellent properties. XRD results showed MnOx (260 ℃) was mainly Mn3O4 and partially MnO2. TEM indicated MnOx (260 ℃) exposed highly active crystal family plan MnO2 (110), and the lattice fringes of MnO2 (110) and Mn3O4 (103) overlapped. In situ DRIFT showed hydroxyl groups adsorbed on MnOx (260 ℃) were removed, which is beneficial to inhibiting the inactivation caused by surface water accumulation. O2-TPD results proven MnOx (260 ℃) had good oxygen migration ability. XPS results manifested that MnOx (260 ℃) had the most adsorbed oxygen. In short, when the calcination temperature is appropriate, MnOx (260 ℃) has coexisted multiple phases, exposed high active crystal family plan and removed surface hydroxyl, which is conducive to the exposure of oxygen vacancies and the inhibition of deactivation.

Soot combustion over CeO2 catalyst: the influence of biodiesel impurities (Na, K, Ca, P) on surface chemical properties.

This work assessed the impact of biodiesel impurities on CeO2 catalyst for soot combustion via soot-TPO experiments. The results showed that Na- and K-doped catalysts were assisted for soot combustion, while Ca- and P-doped catalysts had a negative effect. N2 adsorption-desorption and XRD results indicated that doping biodiesel impurities led to smaller surface area by blocking small pores. Surface chemical properties are suggested as major reasons for promoting soot combustion by means of XPS, H2-TPR, and O2-TPD. Na- and K-doped catalysts showed stronger redox ability and surface lattice oxygen mobility, poorly for Ca- and P-doped catalysts. Interestingly, a large number of surface oxygen species were observed on P-doped catalyst and it enhanced the ignition of bio soot. In the presence of NO, surface chemical properties including NOx storage/release capacity and NO oxidation ability were characterized by NO-adsorption DRIFTS, NO-TPO and NOx-desorption DRIFTS, alkali-doped catalyst with excellent NOx storage capacity that can release active oxygen species and gaseous NO2 accelerate heterogeneous soot combustion, and the poor NO conversion ability to NO2 that weakens the promotion effect of soot combustion. Particularly in the existence of P, the promotion effect of soot elimination in NO + O2 was further weakened by the reason of poor NOx storage capacity and NO oxidation ability.

Development of a thermally stable Pt catalyst by redispersion between CeO2 and Al2O3.

For catalytic systems consisting of Pt as the active component and CeO2-Al2O3 as the support material, the metal-support interaction between the Pt and CeO2 components is widely applied to inhibit aggregation of Pt species and thus enhance the thermal stability of the catalyst. In this work, a highly thermostable Pt catalyst was prepared by modifying the synthesis procedure for conventional Pt/CeO2/Al2O3 (Pt/Ce/Al) catalyst, that is, the CeO2 component was introduced after deposition of Pt on Al2O3. The obtained CeO2/Pt/Al2O3 (Ce/Pt/Al) catalyst exhibits significantly different aging behavior. During the hydrothermal aging process, redispersion of Pt species from the surface of Al2O3 to the surface of CeO2 occurs, resulting in a stronger metal-support interaction between Pt and CeO2. Thus, the formed Pt-O-Ce bond could act as an anchor to retard aggregation of Pt species and help Pt species stay at a more oxidative state. Consequently, excellent reduction capability and superior three-way catalytic performance are acquired by Ce/Pt/Al-a after hydrothermal aging treatment.

Entropy-stabilized single-atom Pd catalysts via high-entropy fluorite oxide supports.

Single-atom catalysts (SACs) have attracted considerable attention in the catalysis community. However, fabricating intrinsically stable SACs on traditional supports (N-doped carbon, metal oxides, etc.) remains a formidable challenge, especially under high-temperature conditions. Here, we report a novel entropy-driven strategy to stabilize Pd single-atom on the high-entropy fluorite oxides (CeZrHfTiLa)Ox (HEFO) as the support by a combination of mechanical milling with calcination at 900 °C. Characterization results reveal that single Pd atoms are incorporated into HEFO (Pd1@HEFO) sublattice by forming stable Pd-O-M bonds (M = Ce/Zr/La). Compared to the traditional support stabilized catalysts such as Pd@CeO2, Pd1@HEFO affords the improved reducibility of lattice oxygen and the existence of stable Pd-O-M species, thus exhibiting not only higher low-temperature CO oxidation activity but also outstanding resistance to thermal and hydrothermal degradation. This work therefore exemplifies the superiority of high-entropy materials for the preparation of SACs.

Catalytic performance promoted on Pt-based diesel oxidation catalyst assisted by polyvinyl alcohol.

Eliminating vehicle emission is of importance due to the severe limit value. The work reports a convenient strategy of improving dispersion of platinum-based catalyst with the assistance of polyvinyl alcohol in a varied addition amount. Following the "two-step" annealing techniques, the catalytic performance of the polymer-assisted catalysts in diesel was obviously enhanced because of the improved dispersion of the platinum. Based on experimental results, the long chains of polymer resulting in the steric effect are presumed to isolate platinum ion, inhibiting the aggregation of platinum particles and then improving its dispersion. And the hydroxyl bonding between the polymers could convey electron to platinum species, leading to the lower platinum valence state. Both effects are positive resulting in an excellent NO maximum conversion of around 65% at the optimal introduction of 5 mass% of polymer, as the diesel oxidation catalyst (DOC), which could be inclined to a good purification in the diesel aftertreatment. Hopefully, the convenient research method could initiate the exploration and application of polymer-assisted catalysts for well-dispersed noble metal nanoparticles in eliminating exhaust emission.

Key role of NO + C3H8 reaction for the elimination of NO in automobile exhaust by three-way catalyst.

Pd-only three-way catalysts with improved catalytic activity for NO elimination were prepared. In order to explore the catalytic reaction rules of NO reduction under a three-way catalytic system, a series of single reactions related to NO reduction were evaluated. It was found that the reaction temperatures of NO + H2 or NO + CO or NO + C3H6 reactions were below 250 °C, while that of NO + C3H8 was up to 350 °C. Thus, the reaction NO + C3H8 served as the key reaction in determining the purification efficiency of NO at the high-temperature stage. By in situ FTIR, we proposed that three possible steps were involved in NO + C3H8 reaction. The first step was the oxidation of C3H8 and NO to acetone and nitrate species by active oxygen species, respectively (C3H8 + O* → C3H6O, NO + O* → NO3-). XPS results revealed that the amount of active oxygen species in Pd/CeO2-ZrO2-Al2O3 (Pd/CZA, 73.7%) was much higher than that in Pd/CexZr1-xO2+Al2O3 (Pd/CZ+A, 64.1%). This was in line with the higher reaction efficiency of the first step over Pd/CZA. Then the NO + C3H8 reaction was accelerated by the first step, which consequently contributed to the higher NO elimination efficiency of Pd/CZA.

Different Reaction Mechanisms of Ammonia Oxidation Reaction on Pt/Al2O3 and Pt/CeZrO2 with Various Pt States.

NH3 emissions were limited strictly because of the threat for human health and sustainable development. Pt/Al2O3 and Pt/CeZrO2 were prepared by the impregnation method. Differences in surface chemical states, reduction ability, acid properties, morphological properties, reaction mechanisms, and ammonia oxidation activity were studied. It indicated that Pt species states were affected by different metal-support interactions. The homogeneously dispersed Pt species over Pt/Al2O3 exposed Pt(111) because of weak metal-support interactions; there even existed an obvious interface between Pt and Al2O3. While obscure even an overlapped interface was observed over Pt/CeZrO2, resulting in the formation of PtO because of the oxygen migration from CeZrO2 to Pt species (confirmed by CO-FTIR, the cycled H2-TPR and transmission electron microscopy results). It was noteworthy that different reaction mechanisms were induced by different states of Pt species; NH was the key intermediate species for ammonia oxidation reaction over Pt/Al2O3, but two kinds of intermediates, N2H4 and HNO, were observed for Pt/CeZrO2. It consequently resulted in the obvious distinction of the NH3-SCO catalytic performance; the light-off temperatures of NH3 over Pt/Al2O3 and Pt/CeZrO2 were 231 and 275 °C, respectively, while the maximum N2 selectivity (65%) was obtained over Pt/CeZrO2, it was obviously better than that over Pt/Al2O3.

Barium-promoted hydrothermal stability of monolithic Cu/BEA catalyst for NH3-SCR.

As a promising candidate for NOx elimination from the emission of diesel engines, the enhancement effect of Ba on the hydrothermal stability of cordierite supported CuBa/BEA at 600 °C for 48 h was investigated by XRD, NH3-TPD, H2-TPR, XPS, EPR, TEM and in situ DRIFTS. Different properties and amounts of active species are significant factors contributing to the enhanced hydrothermal stability of CuBa/BEA-HT. CuBa/BEA-HT has more Cu2+/Cu+ redox-couples and stronger interactions than Cu/BEA-HT, indicating an excellent redox property of the active species in CuBa/BEA-HT. The better redox property of CuBa/BEA-HT produces more nitrates that easily participate in the NH3-SCR reaction, which enhances the low-temperature activity. Furthermore, as observed from EPR and H2-TPR, the appearance of more isolated Cu2+ species and fewer CuO species also contribute to the higher hydrothermal stability of CuBa/BEA-HT.

Study on hydrothermal deactivation of Pt/MnO x -CeO2 for NO x -assisted soot oxidation: redox property, surface nitrates, and oxygen vacancies.

The study mainly focuses on surface properties to investigate the deactivation factors of Pt/MnO x -CeO2 by H2 temperature-programmed reduction, CO chemical adsorption, NO x -temperature-programmed desorption (TPD), O2-TPD, NO temperature-programmed oxidation, SEM, TEM, in situ diffuse reflectance infrared Fourier transform spectra, Raman, and thermogravimetric methods. The results show that there are three main factors to lead to hydrothermal deactivation of the catalyst: redox property, oxygen vacancy, and surface nitrates. The loss of oxygen vacancies decreases the generation and desorption of active oxygen and that of surface nitrates weakens the production of NO2 and surface peroxides (-O2-). These factors greatly result in the damage of the C-NO2-O2 cooperative reaction.

64-Slice spiral double-low CT to evaluate the degree of stenosis and plaque composition in diagnosing coronary artery disease.

This study examined the application of 64-slice spiral double-low computed tomography (CT) to evaluate the degree of coronary artery stenosis. We examined 45 patients with coronary heart disease by 64-slice spiral double-low CT and coronary angiography (CAG) to determine CT accuracy in evaluating coronary artery stenosis. Imaging analysis from 64-slice spiral double-low CT identified 199 segments with coronary stenosis from 45 patients, including 46 segments with mild stenosis, 38 with moderate stenosis and 115 with severe stenosis or artery occlusion. CT analysis agreed with CAG on the identification of the degree of stenosis in 122 segments, with an overall accuracy of 61.3%. The accuracy for serious stenosis or occlusion was the highest at 69.6%. We also found a strong correlation between coronary plaque compositions and the degree of stenosis. Correspondence analysis showed that the presence of soft plaques closely correlated with severe stenosis, whereas mixed plaques closely correlated with moderate stenosis. Overall, 64-slice spiral double-low CT imaging can effectively assess the degree of coronary artery stenosis in patients with coronary heart disease and accurately detect plaque composition. Thus, 64-slice spiral double-low CT imaging can predict the risk of coronary heart disease and the degree of coronary artery stenosis, which is helpful for early diagnosis and treatment of coronary heart disease.

Enhanced catalytic performance of a PdO catalyst prepared via a two-step method of in situ reduction-oxidation.

A supported PdO nanocatalyst prepared by a facile in situ reduction process followed by an oxidation step showed significantly enhanced activity for the removal of pollutants from natural gas vehicles. The excellent catalytic performance is mainly due to high dispersion of the uniform PdO nanoparticles generated during the novel two-step method.

Interactional effect of cerium and manganese on NO catalytic oxidation.

To preferably catalyze the oxidation of NO to NO2 in diesel after-treatment system, a series of CeO2-MnO x composite oxides was supported on silica-alumina material by the co-impregnation method. The maximum conversion of NO of the catalyst with a Ce/Mn weight ratio of 5:5 was improved by around 40%, compared to the supported manganese-only or cerium-only sample. And its maximum reaction rate was 0.056 µmol g-1 s-1 at 250 °C at the gas hourly space velocity of 30,000 h-1. The experimental results suggested that Ce-Mn solid solution was formed, which could modulate the valence state of cerium and manganese and exhibit great redox properties. Moreover, the strong interaction between ceria and manganese resulted in the largest desorption amount of strong chemical oxygen and oxygen vacancies, leading to the maximum O α area ratio of 62.26% from the O 1s result. These effective oxygen species could be continually transferred to the surface, leading to the best NO catalytic activity of 5Ce5Mn/SA catalyst. Graphical abstract.

Ce-Zr-La/Al2O3 prepared in a continuous stirred-tank reactor: a highly thermostable support for an efficient Rh-based three-way catalyst.

Two Ce-Zr-La/Al2O3 composite oxides, CZLA-C and CZLA-B, were synthesized using a co-precipitation method in a continuous stirred-tank reactor (CSTR) and a batch reactor (BR), respectively. Two Rh-based three-way catalysts (TWCs), Rh/CZLA-C and Rh/CZLA-B were obtained by a wet-impregnation method using the two composites as the supports. The physicochemical properties of the samples before and after thermal treatment at 1000 °C were characterized by N2 adsorption-desorption, X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), H2-temperature programmed reduction (H2-TPR) and CO chemisorption. The results indicated that CZLA-C shows higher thermal stability than CZLA-B due to a sparsely-agglomerated morphology. Compared with Rh/CZLA-B, Rh/CZLA-C displayed better reducibility and higher thermal stability and exhibited significantly higher activity in the catalytic removal of the simulated gasoline vehicle exhaust emission (NO, CO and C3H8). Our work can provide a facile and economical synthesis route to advanced support materials and catalysts for exhaust emission control.

Preparation of ceria-zirconia by modified coprecipitation method and its supported Pd-only three-way catalyst.

A CeO2-ZrO2 compound with mixed phase composition (CZ4) was prepared by modified co-precipitation method, and for comparison, single-phase Ce(0.2)Zr(0.8)O2, Ce(0.5)Zr(0.5)O2 and Ce(0.8)Zr(0.2)O2 were synthesized via simultaneous co-precipitation method. The textural, structural and redox properties, together with the catalytic performance of the supported Pd-only three-way catalysts were investigated systematically. The results revealed that the generation of numerous interface sites in Pd/CZ4 due to its mixed phase composition (as confirmed by TEM observation) had a positive influence on modifying its structural, redox properties and thermal stability. The XRD and Raman results revealed that the highest structural stability was obtained by Pd/CZ4 with negligible lattice variation and slightest grain growth after aging treatment. The XPS analysis demonstrated that the compositional heterogeneity of Pd/CZ4 could facilitate the formation of Ce(3+), and was beneficial to preserve high dispersion of Pd as well as maintain Pd at a more oxidized state. The H2-TPR and oxygen storage capacity measurements indicated that Pd/CZ4 possessed highest reduction ability as well as largest oxygen storage capacity regardless of thermal aging treatment. And consequently Pd/CZ4 exhibited improved three-way catalytic activity compared with the catalysts supported on single-phase Ce(x)Zr(1-x)O2 both before and after thermal aging treatment.