Vehicle-aged Catalysts Research Articles

Assessment of the Impact of Trace Elements in FAME Biodiesel on Diesel Oxidation Catalyst Activity after Full Lifetime of Operation in A Heavy-Duty Truck

Fatty acid methyl ester (FAME) biodiesel contains some trace amounts of Na, K, P, Ca, and Mg. Our objective was to investigate whether the presence of such elements can poison a diesel oxidation catalyst that has been used for an entire regulatory lifetime in a heavy-duty truck fueled by FAME biodiesel. The investigated vehicle-aged catalyst contained high loadings of S, P, and Na, as well as a visible layer of soot. Activity in the NO oxidation reaction was severely decreased compared to a fresh catalyst of the same type, while the CO and C3H6 oxidation reactions were less affected. Subsequent selective trace element removal procedures, followed by activity tests, were used to decouple the effect of different poisons. Sintering was observed to be the main cause of catalyst deactivation. Of the trace elements present on the catalyst, P had the greatest effect on catalyst activity, while the other trace elements had little effect.

Detailed Characterization Studies of Vehicle and Rapid Aged Commercial Lean NOx Trap Catalysts

Commercial lean NOx trap (LNT) catalysts were aged and characterized to elucidate the effect of aging on their performances and examine the deactivation of a rapid-aged catalyst toward an improved correlation with respect to a vehicle-aged catalyst. Detailed characterization studies were carried out on the flow reactor with small cylindrical cores extracted from the commercial LNT catalysts. Physicochemical characterization techniques were also implemented. The catalyst evaluation revealed that aging resulted in a significant deterioration of NOx storage and reduction functions as a consequence of precious metal sintering, loss of surface area of the NOx storage and support materials, phase transitions of the adsorber compounds, and large accumulation of poison species. Among the aged samples examined, the middle (lengthwise) vehicle-aged sample showed the highest NOx conversion, while the oven-aged catalyst was the most active of the aged samples for water gas shift reaction and oxygen storage.

Performance Studies and Correlation between Vehicle- and Rapid- Aged Commercial Lean NOx Trap Catalysts

Even though substantial improvements have been made for the lean NOx trap (LNT) catalyst in recent years, the durability still remains problematic because of the sulfur poisoning and sintering of the precious metals at high operating temperatures. Hence, commercial LNT catalysts were aged and tested in order to investigate their performance and activity degradation compared to the fresh catalyst, and establish a proper correlation between the aging methods used. The target of this study is to provide useful information for regeneration strategies and optimize the catalyst management for better performance and durability. With this goal in mind, two different aging procedures were implemented in this investigation. A catalyst was vehicle-aged in the vehicle chassis dynamometer for 100000 km, thus exposed to real conditions. Whereas, an accelerated aging method was used by subjecting a fresh LNT catalyst at 800 °C for 24 hours in an oven under controlled conditions. Engine dynamometer studies were performed with a Volvo mid-sized diesel engine with the purpose of testing the NOx storage and reduction performance, as well as the THC and CO conversion activity of the catalysts under controlled conditions. The aged catalysts activity was shown to be significantly degraded, mainly at low working temperatures compared to the fresh LNT, and one reason for this could be limited NO oxidation. In addition, the oven-aged sample was found to be well correlated to the vehicle-aged catalyst. On top of that, several vehicle emission cycles were carried out in the vehicle chassis dynamometer with a 2.0 l Volvo XC90 diesel vehicle in order to study the catalysts performance under real driving conditions and monitor the gradual deterioration of the vehicle-aged catalyst during the vehicle aging testing.

Multivariate analysis of the effect of biodiesel-derived contaminants on V2O5‑WO3/TiO2 SCR catalysts

This study investigates the effect of biodiesel-derived contaminants on vanadia-based NH3-SCR catalysts in heavy-duty exhaust aftertreatment. The aim was to study, not only the effect of single contaminants on the catalyst performance, but also of possible interaction effects between poisons.The effect of six potential catalyst poisons (Na, K, Mg, P, S and Zn) was evaluated using an experimental design and multivariate data analysis. Monolithic V2O5-WO3/TiO2 catalysts were subjected to accelerated laboratory-scale aging, where the six contaminants were fed simultaneously using a wet impregnation method. In addition to NOx conversion tests, the catalysts were characterized by means of ICP-OES, SEM-EDX, XPS, N2 physisorption and NH3-TPD. The lab-aged samples were compared to fresh and vehicle-aged catalysts.The accelerated aging method showed good reproducibility and gave rise to surface compounds similar to those found in vehicle-aged catalysts. Despite plausible differences regarding penetration depth of the contaminants into the walls of the catalyst, the aging method appears to be an efficient way to point out significant chemical poisons.The model obtained from the experimental design was found to correlate well with the experimental data and can therefore be used to predict effects of the various poisons and poison interactions. Significant effects on the NOx conversion were found for P, S, Na, Mg and K as well as for the interactions P×Na, P×K and S×Na. A poisoning effect was found for Mg, Na, K, P×K, and P×Na, where Na and K exhibited the strongest poisoning effect. The deactivating effect of alkali was lowered in the presence of phosphorus and sulfur, which is explained by the formation of phosphates and sulfates, preventing the interaction of the alkali metals with the vanadia active sites.

Structural Characteristics of Natural-Gas-Vehicle-Aged Oxidation Catalyst

Deactivation of the natural-gas-vehicle-aged Pt/Pd oxidation catalyst supported on γ-alumina-based washcoat was studied by electron microscopy, X-ray diffractometry, X-ray photoelectron spectroscopy, and catalytic activity measurements. Significant structural changes were detected in the used catalyst compared to the fresh one. Grain size of the washcoat had grown but its structure had remained the same, γ-alumina, as in the fresh catalyst. Sintering of the noble metal particles had occurred, particle sizes varied from ~5 up to ~100 nm. Decomposition of palladium oxide and platinum oxide to metallic Pd and Pt occurred followed by the formation of the bimetallic Pt/Pd crystals. Also reformation of palladium oxide was detected. In addition, the inlet part of the used catalyst was totally covered by a poisoning layer. Due to these structural changes and poisoning, the activity of the vehicle-aged catalyst had decreased significantly compared to the fresh one. All the changes were stronger in the inlet than in the outlet part of the used catalyst indicating higher operating temperature and more extensive thermal deactivation and poisoning in the inlet than in the outlet part of the converter.

Thermal durability of Cu-CHA NH3-SCR catalysts for diesel NOx reduction

Multiple catalytic functions (NOx conversion, NO and NH3 oxidation, NH3 storage) of a commercial Cu-chabazite (CHA) urea/NH3-SCR catalyst were assessed in a laboratory fixed-bed flow reactor system after differing degrees of hydrothermal aging. Catalysts were characterized by using X-ray diffraction (XRD), 27Al solid state nuclear magnetic resonance (NMR) and transmission electron microscopy (TEM)/energy dispersive X-ray (EDX) spectroscopy to develop an understanding of degradation mechanisms during catalyst aging. The catalytic reaction measurements of laboratory-aged catalysts were performed, which allows us to obtain a universal curve for predicting the degree of catalyst performance deterioration as a function of time at each aging temperature. Results show that as the aging temperature becomes higher, the zeolite structure collapses in a shorter period of time after an induction period. The decrease in SCR performance was explained by zeolite structure destruction and/or Cu agglomeration, as detected by XRD/27Al NMR and by TEM/EDX, respectively. Destruction of the zeolite structure and agglomeration of the active phase also results in a decrease in NO/NH3 oxidation activity and NH3 storage capacity of the catalyst. Laboratory aging conditions (16h at 800°C) compare well with a 135,000mile vehicle-aged catalyst, except for the chemically poisoned inlet section of the vehicle-aged catalyst.

Impact of thermal and vehicle aging on the structure and functionalities of a lean NOx-trap

Investigations of the aging behaviour of a commercial lean NOx trap (LNT) are reported in this paper. Two aging processes were tested and compared: a hydrothermal aging at 800 °C and a vehicle aging corresponding to a 80,000 km use. Samples were characterized with Operando FTIR, XRF, XRD, SEM-EDX, TEM, and BET analyses. LNT functionalities were evaluated using a flow reactor capable of performing NOx storage and conversion measurements, and correlated with the LNT structure evolution due to aging. The results highlighted that Pt sintering had an impact on the NO oxidation functionality and also deteriorated the Pt/Ba interface. It was hence partly responsible for the NSC decrease. Both aging processes had a similar impact on the LNT NOx storage capacity (NSC) although their structural evolutions were different: BaAl 2O 4 formation on the hydrothermal aged catalyst versus barium poisoned by sulphur on the vehicle aged catalyst.

Deactivation of diesel oxidation catalysts: Vehicle- and synthetic aging correlations

Deactivation characteristics of a diesel oxidation catalyst subjected to controlled lab aging were compared to field-aged catalysts after 0, 80,000, and 160,000km driving in order to establish the (dis)similarities between the two types of aging, and determine to what extent vehicle aging can be replaced by rapid synthetic aging. Emission tests confirmed the increasing deactivation of the field-aged catalysts and measurements of CO oxidation, Pt dispersion and hydrocarbon storage capacity were used together with XPS, TEM, and SEM–EDS to compare the performance with lab-aged catalysts. The synthetic aging was designed to account for a combination of thermal and chemical deactivation. A similar degree of Pt sintering was observed for both real and synthetic aging conditions, whereas differences arose from various contributions to the chemical deactivation. XPS showed that pre-oxidation and pre-reduction at 700°C was not sufficient to remove typical compounds of oil-derived catalyst poisons (P, Zn, Ca), which furthermore accumulated with mileage, on the vehicle-aged catalysts. The degree of sulfur poisoning of the lab-aged catalysts increased with the duration of low-temperature sulfur exposure and could not be recovered in atmospheres typical for normal driving conditions. The results show that synthetic aging can be used to simulate thermal aging and a general effect of irreversible chemical deactivation, and thereby accelerate durability test procedures of various catalysts where loss of catalytic activity by time of operation is of concern.

Carbon Filament Growth on Fully Formulated Pd/Rh Automotive Catalysts

Abstract Automotive exhaust catalysts are typically operated under stoichiometric conditions to minimize nitrogen oxide, hydrocarbon, and carbon monoxide pollutants. These catalysts do not form carbon filaments under normal operating conditions. In development of catalysts, however, a stabilization procedure is sometimes utilized on used catalysts (dynamometer or vehicle) to purge the catalyst of sulfur prior to measuring the catalytic activity in sweep and light-off testing. The stabilization procedure consists of running the catalyst under rich (excess fuel) conditions for 0.5hr. This study documents the existence of carbon filaments due to the stabilization procedure and discusses the effect of filaments on subsequent testing. Two separate catalysts were used in this study. The first was a 50,000 mile vehicle aged catalyst that had also been through the stabilization procedure and then sweep and light-off tested. The other was only dynamometer aged for 120hr at 850°C (1560°F).

Analytical electron microscopy of fresh and vehicle-aged catalysts

Automotive catalysts have a general loss of activity during aging, basically due to two principal deactivation mechanisms. One of them is thermally induced “sintering,” which results in catalytic surface area reduction. The other is chemically induced “poisoning,” which in part causes blockage of active metal sites. The conventional bulk techniques have indicated that various catalyst functions were affected differently by poisons and thermal damage; however, they generally did not provide detailed descriptions of the mechanisms of deactivation. Only analytical electron microscopy (AEM) can provide microchemical and microstructural information to gain a more thorough and fundamental understanding of catalytic deactivation.Fresh and vehicle-aged commercial automotive catalysts containing Pt, Pd, and Rh on alumina supports were prepared for AEM by a microtomy technique, which retains the spatial integrity of the catalyst pellet with uniform thickness. Then these AEM specimens were characterized in a transmission electron microscope (TEM) and in a dedicated scanning transmission electron microscope (STEM).