Commercial automotive catalysts: Chemical, structural and catalytic evaluation, before and after aging
Introduction
Among the proposed solutions to reduce environmental pollution, the automotive catalyst is one of the best-known technological applications to control vehicle emissions. Although well established, this technology is still in continuous development and has been adopted by the majority of the countries ruled by policies controlling emissions from vehicles.
Researchers and specialists in the field have been making stronger efforts to develop products and processes that meet stricter environmental regulations and attend the demand for high quality fuels (especially regarding the control of sulfur levels in gasoline), drastic reduction of vehicle emissions and longer durability of the automotive catalyst (100,000 km or 5 years). Not only does this development affect catalyst manufacturers but also automobile manufacturers and oil companies.
Thus, it is important to understand the phenomenon that happens on the catalyst when it is exposed to high temperatures and contaminants, especially from gasoline and lubricant oil, in order to achieve the required durability.
Sulfur compounds present in the gasoline forms SOx after combustion which may interact with most of the catalytic material, since promoters and stabilizing agents like La2O3, BaO and ZrO2, to major compounds like cerium oxide and alumina. SO2 may also interact with CeO2 forming cerium sulfates, spoiling the oxygen storage capacity (OSC) of the catalytic system. Besides, it interacts with noble metals’ (NM) poisoning part of the main active sites on the catalyst. It is known that this poisoning is more significant at low temperatures, it enhances from oxidizing to stoichiometric conditions and in the presence of hydrocarbons in the exhaust gas. It's partially reversible from 500 to 700 °C and much less pronounced in operations at higher temperatures [1]. During reducing cycles, SO2 stored as sulfate species decomposes producing H2S. The use of NiO in the automotive catalyst formulation significantly reduced this effect through “gathering” of the H2S formed, originating nickel sulfide, and later regenerating the oxide phase during the oxidizing cycle [2].
Another important aspect concerning automotive catalysis refers to thermal resistance that directly affects the catalyst durability. Thermal deactivation is a natural aging process but engine troubles like bad tune-up and incomplete burning may accelerate this process. As a consequence, the automotive catalyst operates at a higher temperature and, in some cases, self-ignition in its interior may even occur, damaging all its structure. These events promote sintering of the catalytic material leading to active area loss [3]. The sintering rate of a metal supported on an oxide of high surface area depends on the interaction between them and metal mobility, which is related to the partial pressures of the species on the surface. Thus, the sintering of palladium particles at high temperatures will be faster in reducing conditions because metallic palladium vapor pressure is higher than that of palladium oxide [4]. This kind of active area loss may occur in normal driving conditions including temperature peaks from 900 to 1000 °C. Thermal aging may also cause loss of OSC through sintering, leading to the loss of close contact between cerium particles and the metallic sites. This contact is necessary for oxygen transport [5].
Aged commercial catalysts were analyzed with the aim to study thermal and chemical deactivation effects. The catalysts were aged in an engine bench in diverse conditions, simulating severe catalyst operation. Several papers with the same emphasis were already published, but with focus on different characterization techniques [6], [7], [8]. In this paper, the main focus is the study of catalytic performance in relation to CO and propane oxidation and attempt to correlate these results with the data obtained from XRF, XRD, surface areas, and finally the emission tests performed before and after aging.
Section snippets
Aging procedure of the commercial catalysts
The catalysts were exposed to an aging process in chassis dynamometer during 150 or 300 h tests. The test temperature ranged from 500 °C (average), in the mildest operation, and above 1000 °C in the most drastic one. In order to analyze their performance, fresh and aged catalysts were evaluated based on the emission test results, which consisted of the determination of CO, NOx, hydrocarbon and aldehyde levels in the exhaust gas, according to Brazilian regulations ABNT NBR6601 and ABNT NBR 12026.
Results and discussion
Fig. 1 presents specific surface area, pore volume and average pore diameter of the catalysts. These values were obtained from the arithmetic mean of the measured values on the inlet, middle and outlet sections.
Catalysts H and I are fresh and show a surface area above 25 m2/g. Significant differences are noticed among the fresh catalysts, from the same fabrication year, in relation to surface area (27 and 35 m2/g, respectively) and pore volume (0.063 and 0.090 cm3/g, respectively). Pore diameter
Conclusions
The characterization results from a group of commercial automotive catalysts, engine bench aged, showed that the catalysts exposed to higher aging temperatures presented a decay in surface area and pore volume and an enhance in the pore diameter, besides the indication of the presence of δ-Al2O3, α-Al2O3 and PdO phases. In inlet regions, it seems that crystalline compounds containing contaminants like cerium phosphate are formed. CO and C3H8 conversion activities were consistent with the
Acknowledgements
The authors gratefully acknowledge the financial support from Petrobras S.A. and the master scholarship granted to Daniela Meyer Fernandes from CAPES.
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