Exhaust Catalysts Research Articles

Pd/BEA hydrocarbon traps: Effect of hydrothermal aging on trapping properties and Pd speciation

Interest in Pd/zeolites as automobile exhaust catalysts has driven research focused on understanding the chemical environment of Pd active centers. Here, we studied the effects of hydrothermal deactivation on a Pd/BEA hydrocarbon trap. Hydrothermal aging (HTA) led to decreased ethylene storage capacity and oxidation activity. Characterization by x-ray absorption spectroscopy (XAS) and NOx adsorption indicated that for lower Si/Al ratio samples, the amount of ion exchanged Pd increased after HTA. Temperature programmed desorption of NOx after NO exposure revealed two desorption peaks, with a high temperature peak that increased in area with increased aging temperature. Based on infrared spectroscopy, XAS and reactor results, the high temperature desorption peak is consistent with NO stored on Pd+. The Pd+ formed during NO adsorption, existing originally as [Pd(OH)]+. This species was less active in ethylene storage and oxidation than the original Pd2+ species. [Pd(OH)]+ stored NOx to a higher temperature than Pd2+.

Professor Geoffrey Bond: ‘Mr Gold’ International Expert in Gold Catalysis 1927–2022

Professor Geoffrey C. Bond, PhD, DSc, FRSC, passed away after a short illness, peacefully in his sleep, on 9th January 2022 at Watford General Hospital, UK, close to his home in Rickmansworth. He was an international authority in the field of heterogeneous catalysis: the science that underpins almost all major industrial chemical processes. He specialised in catalysis by metals, which was the title of his first book published in 1962. Throughout his life he was in the vanguard of those extending the boundaries of his subject. Early work at the University of Hull, UK, featured the use of deuterium as an isotopic tracer to determine the detailed mechanisms of catalytic reactions. Next, at Johnson Matthey, UK, as Head of the Catalyst Research group he contributed to the development of vehicle exhaust catalysts. Then, at Brunel University, UK, he initiated a wide range of projects, culminating in his demonstration of the catalytic activity of gold: a metal which, until that time, ‘conventional wisdom’ had relegated to a class of inactive metals.

Quantitative detection of trace elemental palladium in automobile exhaust catalysts through electron probe microanalysis

Automobile exhaust catalysts containing only Pd have significant advantages over other types of catalysts, including exceptionally high activity at low temperatures, superior thermal stability at high temperatures, and resistance to sulfur toxicity. The present study employs the electron probe microanalysis (EPMA) technique to determine the trace elemental Pd in such catalysts. The particle size distribution and homogeneity of Pd in synthesised standard samples were studied after ball milling, which improved the sample uniformity. Furthermore, the impact of measurement parameters on the analysis precision was investigated. It was found that the optimal conditions are an acceleration voltage of 18 kV, a beam current of 140 nA, and a beam spot size of 40 μm. To avoid potential background interference when choosing background positions in the ZAF method, a calibration curve was established with a correlation coefficient higher than 0.99. This analysis method can be of substantial significance in the production process of automobile exhaust catalysts and in the recovery of precious metals from spent catalysts. Therefore, there is high potential to expand the applications of EPMA in this field.

Aqueous-Phase Glycerol Conversion over Ni-Based Catalysts Synthesized by Nanocasting

A morphological strategy consisting of nanocasting synthesis of nickel aluminate spinel precursor was addressed. Two nanocasted catalysts were synthesized involving different template-removal procedures (i.e., Teflon-assisted calcination vs. NaOH washing) for spinel recovery. As a reference, spinel NiAl2O4 supported by SBA-15 and bare nickel aluminate spinel were selected. The obtained solids were characterized in detail, examining their textural, acid–base, structural and compositional characteristics, either in the calcined or reduced forms. The as-obtained catalysts’ performance was evaluated in the aqueous-phase reforming of glycerol at 235 °C and 35 bar. Exhausted samples were also characterized to enlighten changes in catalyst properties during the aqueous-phase reaction. NiAl/SBA-15 and NiAl-NCF catalyst showed very poor catalytic performance for the glycerol transformation. NiAl-NCN catalyst presented improved activity with respect to NiAl, with a 20% higher hydrogen production rate but, as a drawback, higher methane formation for a whole range of glycerol conversions. Exhausted catalyst indicated nickel oxidized in liquid phase reaction.

ZrO2 and MxOy (M= La, Ce, and Nb) synergistically reinforced porous cordierite ceramics synthesized via a facile solid-state reaction

In an attempt to enhance the sintering and mechanical performance of porous cordierite ceramics (Mg2Al4Si5O18) as support materials for vehicle exhaust catalysts, ZrO2 and MxOy (M = La, Ce, and Nb) were simultaneously introduced into cordierite sintered at 1350 °C for 4 h. Then, the reinforced function of ZrO2 and MxOy on the properties of porous cordierite ceramics was systematically evaluated, especially for the sample co-doped with ZrO2 and La2O3. The results displayed a distinct enhancement in mechanical and thermal performance, and the cold compressive strength increased from 72.74 to 158.59 MPa as well as thermal conductivity ranged from 1.66 to 2.01 W m−1 K−1, respectively. It is found that ZrO2 facilitated activation sintering introduced via lattice distortion in cordierite and La2O3 accelerated the formation of cordierite and ZrSiO4 microcrystalline through the low-temperature liquid phase. The synergic effect between ZrO2 and La2O3, therefore, had a significant role in the reinforced mechanical and thermal performance of porous cordierite ceramics. This work not only offers a feasible strengthening strategy, but also expands the possibilities for building high-performance structural and functional ceramics.

Insights into the CeO2 facet-depended performance of propane oxidation over Pt-CeO2 catalysts

Propane (C3H8) is one of the gases hardest to convert in vehicles’ exhausts, which largely affects the emission control ability of after-treatment systems. In this paper, we take Pt-CeO2 catalysts as an example to investigate the supports’ facet-depended propane oxidation performance. Four types of CeO2 materials with different morphologies (nano-lamella, nano-cube, nano-polyhedron and nano-rod) were synthesized and were found to expose different dominating facets. Consequently, the nano-lamellar CeO2, mainly exposed (110) facet, delivered the best catalytic activity among various Pt-CeO2 catalysts. The ageing and reduction pretreatments tuned the amount of oxygen vacancies and Pt size/Pt charge which remarkably affected the performances of Pt-CeO2 catalysts. Based on the experiments and DFT calculations, it was concluded that oxygen on CeO2 (100) surface was easily removed in reduction atmosphere leading to electron-rich Pt formed on oxygen absent (100) surface, while on CeO2 (110) surface abundant oxygen vacancies were generated during reduction pretreatment and additional surface lattice oxygen contributed to form electron-deficient Pt kept in small cluster size. This research will help us to develop more efficient exhaust catalysts through a deep understanding of the electronic interactions between the metal and active surface of supports.

Algal bioaccumulation and toxicity of platinum are increased in the presence of humic acids

Environmental context The growth in demand for platinum has led to an increase in the presence of this metal in the environment but little is known about its toxicity to aquatic organisms. The presence of organic matter should contribute to decreasing metal bioavailability but the opposite was found for platinum. How ubiquitous natural organic matter can alter the accumulation and effects of platinum group elements remains to be fully elucidated. Rationale There is a growing interest for platinum in ecotoxicology, mainly because of its use in automobile exhaust catalysts. When it reaches aquatic ecosystems, platinum can interact with ligands such as natural organic matter. According to the Biotic Ligand Model, the formation of such complexes should reduce metal bioavailability. As a consequence, toxicity should decrease in the presence of organic matter. Methodology This study focused on the uptake of platinum by two microalgae species (Chlorella fusca and Chlamydomonas reinhardtii) and its subsequent inhibitory effects on growth (96 h). Cells were exposed to platinum (5–300 µg L−1) at three concentrations (0, 10 and 20 mg C L−1) of standard Suwannee River humic acid (SRHA). Platinum bound to humic acid was determined experimentally using partial ultrafiltration to relate metal uptake and toxicity to speciation. Results Unexpectedly, results show that platinum toxicity, expressed as ultrafiltrable Pt (not bound to humic acid) and total Pt concentrations, is enhanced in the presence of humic acid for both algae. For C. fusca, the half maximal effective concentration (EC50) values decreased from 93 to 37 and 35 µg L−1 of ultrafiltrable Pt in the presence of 10 and 20 mg C L−1 SRHA and from 89 to 36 and 0.31 µg L−1 for C. reinhardtii. Discussion In contradiction with the Biotic Ligand Model, the results show that the presence of SRHA can significantly and importantly increase platinum uptake and toxicity as determined in two unicellular green algae, C. reinhardtii and C. fusca. The present work raises the issue of the impact of platinum on microalgae under realistic environmental conditions (ubiquitous presence of organic matter), primary producers being of great ecological importance.

Selective Pd recovery from acidic leachates by 3-mercaptopropylphosphonic acid grafted TiO2: does surface coverage correlate to performance?

Modification of metal oxides with organophosphonic acids (PAs) provides the ability to control and tailor the surface properties. The metal oxide phosphonic acid bond (M-O-P) is known to be stable under harsh conditions, making PAs a promising candidate for the recovery of metals from complex acidic leachates. The thiol functional group is an excellent regenerable scavenging group for these applications. However, the research on organophosphonic acid grafting with thiol groups is very limited. In this study, four different metal sorbent materials were designed with different thiol surface coverages. An aqueous-based grafting of 3-mercaptopropylphosphonic acid (3MPPA) on mesoporous TiO2 was employed. Surface grafted thiol groups could be obtained in the range from 0.9 to 1.9 groups per nm2. The different obtained surface properties were studied and correlated to the Pd adsorption performance. High Pd/S adsorption efficiencies were achieved, indicating the presence of readily available sorption sites. A large difference in their selectivity towards Pd removal from a spend automotive catalyst leachate was observed due to the co-adsorption of Fe on the titania support. The highest surface coverage showed the highest selectivity (K d: 530 mL g-1) and adsorption capacity (Q max: 0.32 mmol g-1) towards Pd, while strongly reducing the co-adsorption of Fe on remaining TiO2 sites.

On-road mileage-based emission factors of gaseous pollutants from bi-fuel taxi fleets in China: The influence of fuel type, vehicle speed, and accumulated mileage

Urban taxis tend to be high mileage vehicles, and therefore potentially a significant contributor to local air pollution. To investigate the on-road emissions of gaseous air pollutants from urban taxi fleets, the mileage-based emission factors (EFs) for 20 gasoline/CNG bi-fuel taxis in China were measured using the in situ monitoring system. It was found that shifting the taxis' fuel from gasoline to CNG could cause significant emissions reduction in CO, slight drop in HC and small increase in NOx. With the fuel shift from China-VI gasoline to GB 18047-2017 CNG, approximately 82% reduction in CO, 14% reduction in HC and about 14% more NOx was observed. With fuel shift from China-III gasoline to GB 18047-2000 CNG, the reductions were 65%, 6% and 11%, respectively. When the China-VI gasoline-fueled taxis travel at higher speed (60 km/h or higher), the vehicles emit approxiamtly ten times more CO than that at lower speed (20–50 km/h). The mileage-based NOxEFs was strongly and positively related to the vehicles' speed for the taxis fueled by China-VI gasoline and GB 18047-2017 CNG. The ratio of NOx and HC emissions from gasoline-fueled taxis is lower than that from CNG-fueled taxis. Considerable reductions of the mileage-based EFs of CO, HC, and NOx from China-VI gasoline- and GB 18047-2017 CNG-fueled bi-fuel taxis were recorded. The taxi vehicles with higher accumulated mileage (greater than 200,000 km) were found with higher CO and HC emissions, due to the deterioration of vehicle engine performance and the exhaust catalyst system.

Highly enhanced dye adsorption of MoO3 nanoplates fabricated by hydrothermal-calcination approach in presence of chitosan and thiourea

Water pollution by organic dyes poses great challenge to the environment and living organism. Hence effective removal of organic dyes by cost effective methods have received significant attention in recent years. Herein, we report the complete removal of organic dyes (rhodamine B), methylene blue) and eosin yellow) from water via effective adsorption by MoO3 catalyst. Hydrothermally synthesised MoO2 (1) and amorphous MoSx (2) using ammonium molybdate without and with thiourea exhibited low dye adsorption. In contrast, crystalline micro/nanoplates of MoO3 (3 and 4) obtained from calcination of 1 and 2 showed highly enhanced dye adsorption. Particularly 4 showed higher dye adsorption compared to 3. UV–Visible absorption studies confirmed complete removal of organic dyes upon stirring with MoO3 catalyst. Dye removal studies further revealed that cationic dyes are adsorbed faster than anionic dye that could be attributed to the surface charge of MoO3. Interestingly, the adsorbed dyes were not released from MoO3 for more than 50 days. The exhausted MoO3 catalyst can be recovered by annealing at 400 °C. MoO3 catalyst has also been used as packing materials in dropper column and demonstrated effective removal of dyes by passing through dyes separately as well as mixture.

XANES Study of Tribofilm Formation With Low Phosphorus Additive Mixtures of Phosphonium Ionic Liquid and Borate Ester

The development of low phosphorus engine oils is important to minimize phosphorus-induced exhaust catalyst poisoning and resulting in harmful emissions. In this study, low phosphorus oil formulations were prepared by using an ashless additive mixture of borate ester (SB) with ionic liquid composed of a phosphonium cation and phosphate anion (P_DEHP) at 350 and 700 ppm phosphorus. Tribological properties of this binary additive system were evaluated using a reciprocating cylinder on a flat test configuration. Favorable interaction between P_DEHP and SB resulted in a significant reduction in friction coefficient and wear volume, in particular for P_DEHP(700P) + SB oil blend. Time-scale analysis of tribofilm formation was determined by running the tribological experiments for 5, 15, and 60 min duration. Electrical contact resistance (ECR) results revealed that the addition of P_DEHP at 350 ppm of phosphorus to SB at 500 ppm of boron can reduce the incubation time from 300 to 100 s for stable tribofilm formation. X-ray absorption near-edge spectroscopy (XANES) analysis of tribofilms indicates that the tribofilm mechanism for additive mixtures of P_DEHP and SB initially involves the formation of boron oxide-based films, which later interact with phosphorus to form boron phosphates in addition to iron phosphates. Incorporation of the high amount of boron phosphates in addition to boron oxide/acid and iron phosphates in the tribofilms contributed to the improved tribological performance of P_DEHP(700P) + SB oil. XANES results reveal that tribofilms formed due to the interaction of SB and P_DEHP evolve to a cross-linked structure, wherein the chain length of polyphosphates is increased with the increase in rubbing time.

An automatic on-line sorptive extraction platform for palladium determination in automobile exhaust catalysts based on a PTFE-turnings packed column, flow injection analysis and flame atomic absorption spectrometry

ABSTRACT A novel simple sensitive and effective on-line flow injection column preconcentration platform using poly-tetrafluoroethylene turnings as sorbent was developed prior to flame atomic absorption spectrometric (FAAS) determination of palladium in vehicle exhaust catalysts, hydrogenation catalysts and environmental water samples. The method is based on the sorption of palladium(II)-diethyldithiophosphate, which was found to be more effective than dithiocarbamate ones. Methyl isobutyl ketone (MIBK) was used as eluent prior atomisation and quantification. All parameters like pH, DDPA concentration, flow rates, preconcentration time that affect the sensitivity of the method as well as interferences of other metals were investigated in detail. The enhancement factor calculated as 114 for 60 s preconcentration time while the sampling frequency as 40 h−1. The detection limit was 0.9 μg L−1 and the relative standard deviation (RSD) was 2.8% at 20.0 μg L−1 concentration level. The accuracy of the proposed method was estimated by analysing certified reference material and spiked environmental samples. The analytical features of the method were compared with other on-line column preconcentration methods reported in the literature.

Recycling exhausted magnetic biochar with adsorbed Cu2+ as a cost-effective permonosulfate activator for norfloxacin degradation: Cu contribution and mechanism

Herein, we attempted to apply an exhausted magnetic biochar with adsorbed Cu2+ (Cu-Fe@BRC) directly as a PMS activator and explored the feasibility of this attempt. Density functional theory (DFT) and electrochemical analysis illuminated the adsorbed Cu2+ in Cu-Fe@BRC improved PMS activation and NOR degradation efficiency by elevating the adsorption capacity of PMS and performance of electron transfer. About 91.47% of norfloxacin (NOR) was rapidly degraded in Cu-Fe@BRC/PMS system with low Fe and Cu leaching. An in-depth mechanistic study was conducted with radical scavenging, radical capturing and solvent exchange, which demonstrated that the adsorbed Cu2+ could facilitate the formation of both different radicals and non-radical. Importantly, Cu-Fe@BRC can maintain a long-term stable operation and excellent catalytic performance in surface water treatment. The potential toxicity of by-product generated in the NOR degradation process was also predicated, and results suggested that most identified by-products were less toxic than NOR itself. Notably, the preparation cost of exhausted adsorbent-based catalysts could be negligible, so the expenditure of the corresponding oxidation process is reduced accordingly. Based on above, this work provides not only a low-cost exhausted biochar-based catalyst for water purification but also the insight into the PMS activation by adsorbed transition metal ions.

NH3 formation pathways from NO reduction by CO in the presence of water over Rh/Al2O3

Steady-state kinetic measurements with varying NO (0.05−0.2 kPa), CO (0.2−0.5 kPa), and water (1.5–9 kPa) pressures at 598 K rationalize that N2/N2O selectivity is determined by NO pressure while the NH3/(N2O + N2) selectivity is a single-valued function of NO/CO ratio. The existence of NCO* species on Al2O3 is confirmed by IR spectra. Monotonically increasing NH3/Rh ratios from H2O recirculation over the NCO* accrued in transient reaction studies on Rh/Al2O3 upon exposure to NO/CO mixtures suggests NH3 formation occurs by hydrolysis of isocyanate species bound on Al2O3. These findings demonstrate the critical role of H2O and NO/CO ratio in determining NH3 selectivity for operation of three-way automotive exhaust catalysts at rich air/fuel ratios.

Investigations of split injection properties on the spray characteristics using a solenoid high-pressure injector

An on-going challenge with Gasoline direct injection (GDI) engines is achieving rapid activation of the exhaust catalyst during cold starts, in order to reduce the Nitrogen Oxide (NOx) emissions. Injecting late in the compression stroke, in the efforts to form a stratified mixture, provides the fuel insufficient time to be entrained with the surrounding charge. This results in locally fuel rich diffusion combustion and the formation of high levels of particulate matter. Employing a split injection strategy can help tackle these issues. The current study examines the effects of a split injection strategy on the spray characteristics. Varying pulse width (PW) combinations, split ratios and dwell times are investigated using a Solenoid actuated high pressure injector. The injected quantity and the droplet characteristics of a target plume are investigated. The experiments were performed in a constant volume spray chamber. The droplet velocities and sizes were measured using Phase Doppler Particle Anemometry (PDA). Short and large PWs, in the range of 0.3–0.8 ms, were investigated. The results revealed that the highest injected quantity of fuel was measured with the shortest dwell time of 2 ms, owing to increased interactions between the injection events, which led to larger Sauter mean diameters (SMDs) measured. The SMDs for the shorter PW of 0.4 ms were generally larger than 0.8 ms PW. The droplets in this case were affected by the closely spaced opening and closing events of the Solenoid valve.

Electrical promotion-assisted automotive exhaust catalyst: highly active and selective NO reduction to N2 at low-temperatures

Low-temperature operation of TWC can be achieved even at 423 K by applying an electric field to the semiconductor catalyst.

Effect of idle time after starting the engine of a gasoline passenger car on the temperature rise of the exhaust catalyst and exhaust emissions

Effect of idle time after starting the engine of a gasoline passenger car on the temperature rise of the exhaust catalyst and exhaust emissions

Solvometallurgical Recovery of Platinum Group Metals from Spent Automotive Catalysts

Extraction and separation of platinum group metals (PGMs) from secondary raw materials are usually carried out via hydrometallurgical processes. These processes use strongly oxidizing acidic solutions, such as aqua regia or hydrochloric acid in the presence of chlorine gas, which may have negative environmental impacts and are dangerous. In this paper, a solvometallurgical approach was developed for the sustainable recovery of PGMs from spent automotive exhaust catalysts. The PGMs were leached using FeCl3 or CuCl2 as oxidizing agents in the organic solvents dimethyl sulfoxide (DMSO) or acetonitrile (CH3CN). Palladium was quantitatively dissolved in CuCl2/CH3CN, CuCl2/DMSO, and FeCl3/DMSO with only 10–20% codissolution of Pt and Rh. By adjusting the concentration of the oxidizing agent in CH3CN, Pd was selectively leached with 0.01 mol L–1 FeCl3, whereas Pt and Rh could be dissolved in a more concentrated FeCl3 solution (0.3 mol L–1). The solvoleaching of PGMs was investigated in depth by UV–vis spectra and electrochemical properties (i.e., cyclic voltammograms and formal reduction potentials of the Fe3+/Fe2+ and Cu2+/Cu+ couples in DMSO and CH3CN). After leaching, CH3CN was easily recovered by distillation. The Pd-containing residue was dissolved in water, from which Pd sponge was produced by reduction with formic acid. Meanwhile, the residue containing the solid chloride salts of Fe(III), Pt(IV), and Rh(III) was redissolved in ethylene glycol or DMSO for further purification by nonaqueous solvent extraction (NASX). The ionic liquid Aliquat 336 diluted in p-cymene showed selective extraction of Fe(III) and Pt(IV) while leaving Rh(III) in the raffinate. The loaded ionic liquid was recycled after selective stripping of Fe(III) with water and Pt(IV) with a thiourea solution. A flow sheet comprising solvoleaching and NASX is proposed. The closed-loop solvoleaching of PGMs with less-hazardous chemicals (FeCl3/CH3CN) avoids the emission of toxic or flammable gases (Cl2, H2, and NOx) while reducing the consumption of acids and bases and limiting the generation of waste water. In addition, NASX with an ionic liquid may be a more sustainable alternative for the conventional separation of PGMs.

β-Cyclodextrin-Silica Hybrid: A Spatially Controllable Anchoring Strategy for Cu(II)/Cu(I) Complex Immobilization

The development of new strategies for spatially controllable immobilization has encouraged the preparation of novel catalysts based on the organic-inorganic hybrid concept. In the present paper, a Cu-based multi-structured silica catalyst has been prepared and fully characterized. The inclusion of Cu(II) in β-cyclodextrins has been exploited with the double aim to stabilize the metal and to act as a source of Cu(I) catalytic sites. Multi-technique characterization by infrared, UV-visible, electron microscopy and X-ray absorption spectroscopies of the fresh and exhaust catalysts provided information on the local structure, redox properties and stability of the investigated hybrid systems. The catalytic system showed that copper nanospecies were dispersed on the support and hardly affected by the catalytic tests, confirming the stabilizing effect of β-CD, and likely of the N1-(3-Trimethoxysilylpropyl) diethylenetriamine spacer, as deduced by X-ray absorption spectroscopy analysis. Overall, we demonstrate a feasible approach to efficiently anchor Cu(II) species and to obtain a reusable single-site hybrid catalyst well suited for Cu(I)-catalyzed alkyne-azide cycloaddition.

Single‐atom Automobile Exhaust Catalysts

AbstractSingle‐atom catalysts (SACs) with isolated metal centers are attracting great research interest because of their maximized metal utilization rates and unique structures. In the last decade, significant efforts have been made to design highly efficient and cost‐effective SACs with applications in automobile exhaust aftertreatment. SACs have not only demonstrated their high potential for improving the catalytic performance of current automobile exhaust catalysts but also provided an ideal platform for understanding the origins of catalyst reactivity. In this review, we summarize the general strategies for promoting the reactivity of metal SACs while maintaining their thermal stability during exhaust‐abatement‐related reactions. Highlights are provided on well‐performed SACs for CO oxidation, unburnt hydrocarbon (HC) oxidation, and the selective catalytic reduction of NOx. The reaction mechanism and structure‐performance relationship of SAC during these reactions are also discussed. Finally, perspectives are given on the trending research fields for designing more efficient single‐atom automobile exhaust catalysts in the future.