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Diesel soot is a significant contributor to air pollution. Soot particles present in diesel engine exhaust have a negative impact on the environment and human health. Diesel oxidation catalysts (DOCs) and diesel particulate filters (DPFs) currently use noble metal-based catalysts for soot oxidation. Due to the use of noble metals in the catalyst, the cost of diesel after-treatment systems is steadily rising. As a result, diesel vehicles have become commercially less viable than gasoline vehicles and electronic vehicles. The study focuses on an alternative diesel oxidation catalyst with efficiency similar to that of a noble metal catalyst but with a much lower cost. CeO2-Al2O3 catalysts are known for their oxygen storage capacity and high redox activity, making them suitable for soot oxidation. Adding Zr to these catalysts has been shown to influence their structural and chemical properties, significantly affecting their catalytic behavior. Therefore, the current study is focused on using Zr/CeO2-Al2O3 as a substitute for noble metal-based catalysts to enhance its performance for diesel soot oxidation in automotive exhaust. Evaporation-induced self-assembly (EISA) was used to prepare 1, 3, and 5 weight (wt) % Zr supported mesoporous CeO2-Al2O3 catalysts. Morphological, structural, and physicochemical properties of the synthesized catalysts were examined using Brunauer-Emmett-Teller (BET) absolute isotherm, Scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Temperature programmed reduction (TPR), and Temperature-programmed desorption of ammonia (NH3-TPD). XRD, BET, and SEM data confirmed that the catalysts were mesoporous and low-crystalline with a high surface area. The soot oxidation activity of the catalysts was evaluated using a thermogravimetric analysis (TGA) technique. The loose contacts soot oxidation activity test suggested that 50% oxidation of soot occurred at 390 °C in the absence of a catalyst. T50 of CeO2-Al2O3 catalyzed soot oxidation was 296 °C. Adding Zr to the catalyst significantly improved catalytic activity for diesel soot oxidation. We observed a further drastic change in T50 of soot over 1, 3, and 5% Zr/CeO2-Al2O3, which were 220 °C, 210 °C, and 193 °C, respectively. According to these results, incorporating Zr into the CeO2-Al2O3 catalyst significantly improved the oxidation process of soot.

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In this paper, a novel control solution, which combines a feedforward control law and an error-based version of the active disturbance rejection control (ADRC) scheme, is proposed for the exhaust gas temperature control during the thermal regeneration process of a diesel particulate filter in the exhaust line of a diesel engine. Attributed to the complexity of the controlled upstream diesel oxidation catalyst (DOC), its thermodynamics is firstly captured and characterized by a set of linear models through identification modeling. Then, a novel error-based ADRC controller, in which the separated components in conventional ADRC such as the extended state observer (ESO) and the feedback compensator are restructured into a single, modularized control function block, is designed by applying the identified nominal DOC model. In order to further unburden the error-based ESO for better achievements, a combined feedforward compensator is well designed on the basis of the principle of energy balance. Thus a hybrid, 2-degree-of-freedom (2-DOF) controller is developed for better dynamic performance of the controlled DOC system. Its stability performance is also analyzed in the work. The robustness and advantages of the presented hybrid control scheme are finally validated and compared with a well-tuned regular PID-based controller by means of extensive simulation and experimental tests. The results show that the proposed hybrid controller is capable of providing more accurate and faster temperature response and is less sensitive to the variation of system parameters and external disturbances. Moreover, as the error-based ADRC in the hybrid scheme takes the reference tracking error as its direct input and is compatible with the regular PID controller in terms of input and output interfaces, it herein provides an appealing control scheme for existing applications as a substitute for the conventional PID-based controllers to achieve improved performance.

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A diesel oxidation catalyst (DOC) is installed upstream of an exhaust after-treatment line to remove CO and hydrocarbons and generate NO2. The catalyst should possess both good oxidation ability and thermal stability because it sits after the engine. We present a novel high-performance DOC with high steam resistance and thermal stability. A selective dissolution method is adopted to modify the surface physicochemical environment of CeO2-SmMn2O5. The X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, Raman, electron paramagnetic resonance, hydrogen temperature-programmed reduction, and temperature-programmed desorption results reveal that surface Sm cations are partially removed with the exposure of more Mn4+ and Ce3+ cations and the presence of active surface oxygen species. This mechanism benefits the oxygen transformation from Ce to Mn and promotes the Ce3+ + Mn4+ ↔ Ce4+ + Mn3+ redox cycle according to the in situ near-ambient pressure X-ray photoelectron spectroscopy and in situ diffuse reflectance infrared Fourier transformation spectroscopy results. Under laboratory-simulated diesel combustion conditions, the catalyst demonstrates excellent low-temperature oxidation catalytic activity (CO and C3H6 conversion: T100 = 250 °C) compared to a Pt-based catalyst (CO and C3H6 conversion: T100 = 310 °C) with a WHSV of 120,000 mL g-1 h-1. Specifically, NO conversion reaches 68% when the temperature is approximately 300 °C.

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Diesel oxidation catalysts Pt-Pd-(y)ZrO2-(z)WO3/CeZrOx-Al2O3 with total Pt & Pd loading of only 0.68 wt.% were prepared and investigated for oxidation activity and stability of CO, C3H6, and NO. Introduction of ZrO2 greatly improved low-temperature activities and retained stability especially for CO and C3H6 oxidation after treated at 800 °C. With the optimal loading amount of 6 wt% ZrO2, 2 wt% WO3 was introduced to the system and showed higher activity. Reaction temperature for 50% CO and C3H6 conversion declined to 160 and 181 °C, and the maximal NO conversion increased to 50%. By using XRD, TEM, CO chemisorption, XPS, and H2-TPR analysis, it was found that ZrO2 could inhibit aggregation of Pt and Pd, improve metal dispersion, and increase surface-chemisorbed oxygen after high-temperature treatment, accounting for promoted performance. Also, there were more reducible oxide species in ZrO2-doped catalysts. ZrO2 could induce reduction of noble metal oxides and surface ceria by weakening Pt-O-Ce interaction, which increased the ability to dissociate H2 and spillover effect of dissociated hydrogen to ceria. Doping WO3 increased metal dispersion of fresh samples and brought more Pt0 species that were active sites for oxidation reactions. Thus, ZrO2 and WO3 could be effective additives for oxidation catalysts to synergistically improve their activities and thermal stability.

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Four commercial monolithic diesel oxidation catalysts (DOCs) with two different platinum group metal (PGM) loadings and Pt:Pd ratios of 1:0, 2:1, 3:1 (w/w) were investigated systematically for CO, C3H6, and NO oxidation, CO-C3H6 co-oxidation, and CO-C3H6-NO oxidation reactions via transient activity measurements in a simulated diesel engine exhaust environment. As PGM loading increased, light-off curves shifted to lower temperatures for individual and co-oxidation reactions of CO and C3H6. CO and C3H6 were observed to inhibit theoxidation of themselves and each other. Addition of Pd to Pt was found to enhance CO and C3H6 oxidation performance of the catalysts while the presence and amount of Pd was found to increase the extent of self-inhibition of NO oxidation. NO inhibited CO and C3H6 oxidation reactions while NO oxidation performance was enhanced in the presence of CO and C3H6 probably due to the occurrence of reduced Pt and Pd sites during CO and C3H6 oxidations. The optimum Pt:Pd ratio for individual and co-oxidations of CO, C3H6, and NO was found to be Pt:Pd = 3:1 (w/w) in the range of experimental conditions investigated in this study.

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To achieve high fuel efficiency and low emission in automobiles, it is necessary to develop highly active diesel oxidation catalysts (DOCs). Pd/CeO2 catalysts have been widely used as active catalysts for CO and C3H6 oxidation reactions. Additionally, Ag has been reported to enhance the oxygen storage capacity (OSC) of CeO2, which contributes to the oxidation ability of Pd/CeO2. In this study, Pd/Ag-CeO2 catalysts were used for CO and C3H6 oxidation reactions. When CeO2 was doped with appropriate amounts of Ag, reducibility and CO desorption rate were increased, which confirmed the high OSCs of Ag-doped catalysts. However, Ag particles were formed and the Ce3+/Ce4+ ratio decreased when CeO2 was doped with excess amounts of Ag. In addition, reduced Pd (Pd0), which is an active species for C3H6 oxidation, was formed and maintained even under oxidative reaction conditions. Since the removal of C3H6 is important for the oxidation of CO and C3H6, the catalyst with the highest Pd0 fraction (Pd/0.1Ag-CeO2 and Pd/0.3Ag-CeO2) presented improved catalytic activity. Consequently, the optimal amount of Ag enhanced the OSC of Pd/Ag-CeO2 catalysts and formed active Pd0 species under oxidative conditions, which resulted in the excellent catalytic activity of Pd/Ag-CeO2 for the CO and C3H6 oxidation reaction.

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The high NO2/NOX ratio in the after-treatment system is beneficial to its performance and achieved by NO catalytic conversion in diesel oxidation catalyst (DOC) located upstream (CRDPF), catalytic DPF (CDPF), or a combination of both (CCDPF). In order to effectively control the emission of particulates and nitrogen oxides, various types of diesel particulate filter models are established to compare NO2 catalytic formation, consumption, and efflux. The results show that the catalytic performance of NO conversion is limited by mass transfer in DOC catalytic coating, while it is almost non-existent in CDPF. At low temperature, the passive regeneration of CDPF is slower than that of CRDPF, but as the temperature increases, the passive regeneration speed of CDPF will exceed that of CRDPF. CCDPF is the most effective for the NO2 catalytic formation, consumption, and efflux in the hot-start and high-speed cycle and thereby is conducive to improve the performance of the diesel particulate filter and downstream selective catalytic reduction.

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Decomposition mass loss and pyrolysis products analyses of particles sampled at various locations along the tailpipe of a Euro-IV diesel engine were performed using a thermogravimetry in conjunction with Fourier transformation infrared spectrometry-mass spectrum. Diesel particles were collected at the same location with and without diesel oxidation catalyst (DOC) mounted on the test engine separately. The three poles in thermal gravity-differential thermal gravity images suggested that the decomposition process of diesel particles could be divided into three stages which correspond to the decompositions of lower boiling substances, higher boiling substances and soot respectively. It is noticed that no matter whether DOC was mounted or not, the further the particles were sampled away from the engine block, the lower the peak temperatures and the heavier the mass losses within the first two stages, which indicated that the soluble organic fraction in the particle samples increased and therefore lowering the activation energy of thermal decomposition. Hydroxyl, ammonia, CxHy fragments, benzene, toluene, and phenol were found to be the primary products of particle decomposition, which didn't change with the location of particle sample point. The employment of DOC increased the activation energy for particle oxidation and resulted in a higher peak temperature and lower mass loss within the first-stage. Moreover, the CO stretching bands of aldehyde and ketone at 1771 cm-1 was only detected without a DOC, while the NO2 peak at 1634 cm-1 was solely noticed with the presence of DOC. Compared to the first-stage pyrolysis products, more polycyclic aromatic hydrocarbons and less CxHy fragments were seen in the second-stage.

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Particle number is a key index for evaluating particulate emissions, and diesel oxidation catalysts (DOCs) are one of the most important technologies for controlling the particulate emissions of a diesel engine. In this paper, a novel phenomenological one-dimensional model was established to predict particle number and size distributions at a DOC outlet with the aim of investigating the effects of DOC on particle number emissions. The phenomenological model consisted of two submodels: submodel-1, a global kinetic model for calculating particle size in particle number and size distributions after particles had passed through the DOC, and submodel-2, an original global parametric model for calculating the particle number at the DOC outlet. The effects of the sampling process, fuel properties, and the engine operating condition were considered in submodel-2. An 8.8 L, direct-injection, heavy-duty diesel engine was tested. The particle number and size distributions at the DOC inlet and outlet were determined using an engine exhaust particle sizer. The test data, coupled with literature results, were used to calibrate and validate the phenomenological model. This model was then applied to investigate the influence of various factors on particle number and size distributions at the DOC outlet. It was found that dilution temperature, fuel sulfur content, exhaust gas temperature, and gas hourly space velocity (GHSV) played a key role in the particle number after DOC oxidation. The particle number concentration at the DOC outlet increased as fuel sulfur content and exhaust gas temperature increased and decreased as GHSV and dilution temperature increased. In general, results proved that this phenomenological model was accurate enough to predict particle number and size distributions at a DOC outlet under most operating conditions. It may serve as a useful tool for research and development focusing on PM reduction of diesel engines and air pollution control.

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The poor thermodynamic stability of gold nanoparticles (NPs) makes it very challenging to stabilize them in small sizes at elevated temperatures. Herein, we report the preparation of antisintering Au nanocatalyst by rationally selecting the sublattice matched MgGa2O4 spinel as support based on theoretical predictions. Au/MgGa2O4 retains Au NPs of 2-5 nm even after aging over the melting temperature of bulk gold (1064 °C)! By identifying the stable structure, the extraordinary stability is found to arise from the formation of a new phase structure, namely Au-MgGa2O4 metal-oxide "hetero-bicrystal" that remains as crystallite without melting even at 1100 °C. More than 80% of the loaded Au can be efficiently stabilized so that the catalysts can exhibit excellent low-temperature activities for diesel exhaust (CO and C3H6) oxidation after severely thermal and hydrothermal aging. These results may pave ways for constructing antisintering gold nanocatalysts for industrial applications.

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Based on heavy chassis dynamometers, an experimental study was conducted in a diesel bus with proton transfer reaction mass spectrometry (PTR-MS). It investigated the effects of volatile organic compound (VOC) emission characteristics with three different diesel oxidation catalyst (DOC)+catalyzed diesel particulate filter (CDPF) after-treatments for a typical Chinese city bus driving cycle (CCBC). The results reveal that the major compounds from the diesel bus are OVOCs, aromatic hydrocarbons, alkenes, alkanes, nitrogenous organic compounds, and polycyclic aromatic hydrocarbons (PAH), and that the OVOCs account for more than 50%of the total VOCs. With the same precious metal composition and ratio of the proportion in the CDPF catalyst, the emissions of VOCs decrease with an increase in precious metal load. The emission reduction rates of the VOCs are 36.2%, 40.1%, and 41.4%, respectively, when the precious metal loads are 15 g·ft-3 (type A after-treatment device), 25 g·ft-3 (type B), and 35 g·ft-3 (type C). The average emission rates of alkanes for the three kinds of DOC+CDPF after-treatments are all over 59% for the entire CCBC cycle. The type C after-treatment device can reduce the alkane emissions by 70.2%, with a slight advantage for the OVOC reduction compared with type A and type B devices. For unsaturated hydrocarbons, including aromatic hydrocarbons, alkenes, and PAHs, the after-treatment devices have a catalytic effect, but there is no significant difference between them. The emissions of nitrogenous organic compounds are greatly decreased, by 50.5%, with the type A after-treatment, but the reduction rate decreases with an increase in precious metal load. In addition, OVOCs, aromatic hydrocarbons, and alkenes are the most important contributors to ozone formation. The adoption of DOC+CDPF reduces the emissions of VOCs and, therefore, the ozone formation potential. Taking into account the emission reduction rates and costs of the three different after-treatments and for weighting coefficients of 0.8 and 0.2, respectively, the type B after-treatment is the optimal solution.

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Based on heavy chassis dynamometer, an experimental study was conducted in diesel bus with China Stage Ⅲ, which investigated the effects of gaseous emission characteristics under CCBC driving cycle, such as carbon monoxide (CO), total hydrocarbons (THC), nitric oxide (NO), nitrogen dioxide (NO2), nitrogen oxides (NOx) and carbon dioxide (CO2) with the fresh/aged oxidized catalyst (DOC) and oxidation catalysts coupled catalyzed particulate trap (DOC+CDPF, referred CCRT). The results showed that using fresh and aged DOC/CCRT, the diesel bus could reduce CO, THC and NO emissions, meanwhile increase NO2 emissions, but NOx and CO2 emissions remained basically unchanged. In idle speed, acceleration, deceleration and constant speed of working conditions, the diesel bus using the fresh DOC had better oxidation efficiency of the CO and THC emissions than the bus using the aged DOC. The diesel bus using the fresh CCRT had higher oxidation efficiency of THC emissions, but lower oxidation efficiency of CO emissions than the bus using the aged CCRT. The diesel bus using the fresh DOC/CCRT had a higher rate of NO emissions reductions and NO2 emissions increments than the bus using the aged DOC/CCRT, but it did not basically affect the NOx emissions.

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Pd is more prone to sulfation compared to Pt. Given the chemical similarity between Pt and Pd, the radical divide in their tendencies for sulfation remains a puzzle. We explain this intriguing difference using an extensive first-principles thermodynamics analysis and computed bulk and surface phase diagrams. In practically relevant temperatures and O2 and SO3 partial pressures, we find that Pt and Pd show significantly different tendencies for oxidation and sulfation. PdO formation is favored even at low oxygen chemical potential; however, PtO2 formation is not favorable in catalytically relevant conditions. Similarly, PdSO4, and adsorbed SO3 and oxygen species on clean and oxidized surfaces are highly favored, whereas PtSO4 formation does not occur at typical temperature and pressure conditions. Finally, several descriptors are identified that correlate to heightened sulfation tendencies, such as the critical O chemical potential for bulk oxide and surface oxide formation, chemical potentials O and SO3 for bulk sulfate formation, and SO3 binding strength on metal surface-oxide layers, which can be used to explore promising sulfur resistant catalysts.

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Planar laser-induced fluorescence (PLIF) enables noninvasive in situ investigations of catalytic flow reactors. The method is based on the selective detection of two-dimensional absolute concentration maps of conversion-relevant species in the surrounding gas phase inside a catalytic channel. Exemplarily, the catalytic reduction of NO with hydrogen (2 NO+5 H2 →2 H2 O+2 NH3 ) is investigated over a Pt/Al2 O3 coated diesel oxidation catalyst by NO PLIF inside an optically accessible channel reactor. Quenching-corrected 2D concentration maps of the NO fluorescence above the catalytic surface are obtained under both, nonreactive and reactive conditions. The impact of varying feed concentration, temperature, and flow velocities on NO concentration profiles are investigated in steady state. The technique presented has a high potential for a better understanding of interactions of mass transfer and surface kinetics in heterogeneously catalyzed gas-phase reactions.

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This study investigated the filtration and continuous regeneration of a particulate filter system on an engine test bench, consisting of a diesel oxidation catalyst (DOC) and a catalyzed diesel particulate filter (CDPF). Both the DOC and the CDPF led to a high conversion of NO to NO2 for continuous regeneration. The filtration efficiency on solid particle number (SPN) was close to 100%. The post-CDPF particles were mainly in accumulation mode. The downstream SPN was sensitively influenced by the variation of the soot loading. This phenomenon provides a method for determining the balance point temperature by measuring the trend of SPN concentration.

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A particulate oxidation catalyst (POC) was employed to perform experiments on the engine test bench to evaluate the effects on the nitrogen dioxide (NO2) and particulate matter (PM) emissions from diesel engine. The engine exhaust was sampled from both upstream and downstream of the POC. The results showed that the POC increased the ratios of NO2/NOx significantly in the middle and high loads, the ratio of NO2/nitrogen oxides (NOx) increased 4.5 times on average under all experiment modes with the POC. An engine exhaust particle sizer (EEPS) was used to study the particle number-weighted size distributions and the abnormal particle emissions with the POC. The results indicated that the average reduction rate of particle number (PN) was 61% in the operating range of the diesel engine. At the engine speed of 1,400 r/min, the reduction rates of PN tended to decrease with the larger particle size. In the long time run under the steady mode (520 Nm, 1,200 r/min), abnormal particle emissions after the POC happened seven times in the first hour, and the average PN concentration of these abnormal emission peaks was much higher than that in normal state. The particle emissions of peaks 1-5 equaled the particles emitted downstream of the POC in normal state for 1.9h in number concentration, and for 3.6h in mass concentration. The PN concentrations tended to increase over time in 5h under the steady engine mode and the increase of the PN in the size range of 6.04-14.3 nm was more evident.

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Pt is an active catalyst for diesel exhaust catalysis but is known to sinter and form large particles under oxidizing conditions. Pd is added to improve the performance of the Pt catalysts. To investigate the role of Pd, we introduced metallic Pt nanoparticles via physical vapor deposition to a sample containing PdO nanoparticles. When the catalyst was aged in air, the Pt particles disappeared, and the Pt was captured by the PdO, forming bimetallic Pt-Pd nanoparticles. The formation of metallic Pt-Pd alloys under oxidizing conditions is indeed remarkable but is consistent with bulk thermodynamics. The results show that mobile Pt species are effectively trapped by PdO, representing a novel mechanism by which Ostwald ripening is slowed down. The results have implications for the development of sinter-resistant catalysts and help explain the improved performance and durability of Pt-Pd in automotive exhaust catalytic converters.