Materials Science & Technology C h e m i c a l a g i n g o f c a t a l y t i c c o n v e r t e r s w i t h r e g a r d t o m e t h a n e c o n v e r s i o n P r o j e c t o v e r v i e w Motivation A durability study of a catalytic converter from a naturally aspirated 2.0 l bifuel natural gas vehicle (NGV) showed much greater deterioration of T.HC tailpipe emissions over mileage in natural gas operation than in gasoline operation (Fig. 1). The T.HC emissions in gasoline operation were almost stable over the mileage, while those in natural gas operation increased steadily even in the warmstarted EUDC cycle. 0.400 0.320 40 60 80 00 THC ECE THC EUDC 0.4 08 NGV Gasoline NGV Gasoline 0.3 06 1 2 1 2 04 02 00 1 2 1 2 Fig. 1 T.HC emissions of the bifuel vehicle in natural gas (green bars) and gasoline operaton (blue bars) in the ECE (left) and EUDC cycle (right) Surface analysis of the catalytic converter after 35,000 km field testing showed sintering effects and phosphate poisoning (Fig. 2). The phosphate contamination itself is expected to originate from the engine lubricating oil (oil additive component). Other oil components such as Mg, Ca and Zn have also been found on the catalyst surface. Fig. 2 XPS surface analysis of the new (lower line) and vehicle-aged (upper line) catalytic converters after a mileage of 35,000 km in real-world driving (P2s = phosphorus signal) Seite 1 von 5
A similar durability study on a dedicated OEM NGV, which also uses natural gas for engine starting, showed greatly improved emission durability behavior (Fig. 3). THC ECE 4 THC EUDC 7 4 0 7 3 3 2 1 0 500 km 8000 km 13000 km 20'000 km 36'000 km 0 500 km 8000 km 13000 km 20'000 km 36'000 km Fig. 3 T.HC emissions of a dedicated OEM natural gas vehicle in the ECE (left) and EUDC (right) cycles. One of the main differences between the two natural gas vehicle concepts is the cold start and warming-up period. During this phase, the engine is running with gasoline in the case of the bifuel NGV, whereas in the case of the OEM-NGV the engine is running on natural gas. In comparison with gasoline, natural gas has much lower fuel condensation behavior, especially at cold engine. As a consequence, the OEM-NGV concept should result in reduced contamination of the lubricating oil with fuel. Literature search Investigations by Beck et al. showed reduced T.HC conversion in catalytic converters after chemical aging with phosphorus and zinc, while reduced CO and NOx conversion was observed mainly after thermal aging (sintering) 1. Another study by the same research group showed that the chemical aging of catalytic converters with phosphorus and zinc only has a minor impact on oxygen storage capacity 2. An earlier study by Williamson et al. showed that phosphorus is primarly deposited on the washcoat surface of the catalytic converter, while zinc depositions were also found within the bulk of the washcoat 3. Depth profiling of the vehicle-aged catalytic converter mentioned above, performed at Empa, confirmed these results. Due to its molecular stability, catalytic methane conversion is more sensitive to chemical and thermal aging than other exhaust gas components. This is true for natural gas vehicles, but also for gasoline or ethanol vehicles (with minor concerns due to lower methane concentration). This chemical degradation of methane conversion for natural gas, gasoline and ethanol vehicles has not yet been investigated. Due to reduced exhaust gas temperature in more efficient engines, engines with start/stop functions or hybrid powertrains, methane conversion will gain in importance in the future. Project proposal 1 Beck D., Sommers J., DiMaggio C.: Axial characterization of catalytic activity in closed-coupled lightoff an underfloor catalytic converters; Applied Catalysis B: Environmental 11 (1997) 257-272 2 Beck D., Sommers J., DiMaggio C.: Axial characterization of oxygen storage capacity in closed-coupled light off and underfloor catalytic converters and impact of sulfur; Applied Catalysis B: Environmental 11 (1997) 273-290 3 Williamson W., Perry J., Gandhi H., Bomback J.: Effects of oil phosphorus on deactivation of monolithic three-way catalysts; Applied Catalysis, 15 (1985) 277-292 Page 2 of 5
The proposed project is intended to show whether engine oil-based chemical aging of catalytic converters leads to increased methane emissions in fossil fuel (gasoline, natural gas) and biofuel (ethanol, biogas) operation. Furthermore, it should be shown whether low-saps engine oils could reduce or prevent such an effect. The project will be divided into the following phases: Phase 1: Oil component transport from crankcase to catalytic converter Oil dilution with fuel (condensation and wall wetting) and the following fuel evaporation through the crankcase ventilation system is expected to act as a shuttle for the transport of catalyst poisons to the catalytic converter. The project therefore starts with the experimental investigation of the oil component transport with following vehicle concepts: 1 Gasoline/E85 vehicle: - EN288 standard gasoline (95 octane), standard engine oil (e.g. Saab 9 5 ) - EN288 standard gasoline (95 octane), low-saps engine oil - Ethanol E85, standard engine oil 1 Gasoline/NGV: - EN288 standard gasoline (95 octane), standard engine oil (e.g. VW Touran Ecofuel) - Natural gas from Swiss grid, standard engine oil - Kompogas (pre-processed biogas), standard engine oil The experimental setup is shown in Fig. 4. Messstelle 1: Ansaugluft Messstelle 2: Verbrennungsluft Messstelle 3: Engine out Messstelle 3: Nach Kat. Ansaugluft Katalysator Kurbelgehäuseentlüftung Blow by Ölverbrauch Fig. 4 Measuring points of engine oil components In a first step, the existing sampling and analysis methods at Empa will be adapted and validated for the investigation of elements, acting as catalyst poisons. The sampling will be performed in staggered wash-bottles with a downstream backup filter. This sampling system allows the simultaneous sampling of gaseous and solid components. Page 3 of 5
The detection limit for the relevant components is defined by the blank values of the sampling system and the material for the backup filter. The selection of a suitable adsorption medium is based on the highest separation efficiency. The following analysis will be done by plasmaspectroscopic methods. Because phosphorus and zinc are omnipresent elements originating from different sources, a tracer element, which is not used in the fuel or lubricating oil (e.g. In, Bi, Ge) will be added in known quantities to the investigated oil and be sampled and analysed in parallel with the oil additive components. Phase 2: Oil and catalyst aging in field study The following vehicle types are driven for two years (mileage target: 40,000 km) in normal field operation without engine oil replacement: 1 gasoline vehicle (e.g. Saab 9 5 ): operated with gasoline and standard engine oil 1 gasoline vehicle (e.g. Saab 9 5 ): operated with gasoline and low-saps engine oil 1 E85 vehicle (e.g. Saab 9 5 ): operated with E85 and standard engine oil 1 NGV (VW Touran Ecofuel): operated with natural gas and standard engine oil 1 NGV (VW Touran Ecofuel): operated with pre-processed biogas and standard engine oil A sample of the engine oil will be analysed every 16,000 km with regard to oil composition (GC analysis), total base number (TBN), water content, viscosity, abrasion elements and P und Zn. The oil component transport investigation (Phase 1) from crankcase to catalytic converter will be repeated with the same vehicles after the 40,000 km field testing. The catalytic converters will be analysed regarding their conversion efficiency, their specific surface areas (BET), chemical poisons on the surface and in the wash coat bulk (XPS, LA-ICP-MS) and thermal aging (HR-SEM/TEM) in new (degreened) condition and after the aging in the field testing. Phase 3: Investigation of mechanism for reduced methane conversion The underlying deactivation mechanism by which P and Zn poisoning reduces methane conversion will be investigated in the catalysis laboratory on specially prepared model catalysts. The activity of the methane and carbon monoxide oxidation as well as the reduction of nitrogen monoxide will be determined using different synthetic exhaust gas compositions and temperatures. The exhaust gas concentration will be measured by on-line electron impact mass spectrometry (EI-MS) and Tourier Transform Infrared Spectroscopy (FT-IR). Additionally, the adsorption and desorption behaviour as well as the capacity of reaction under reducing and oxidising conditions will be investigated for the vehicle-aged catalytic converters and the specially prepared model catalysts using thermogravimetry mass spectrometry (TG-MS) and differential scanning calorimetry (DSC). Page 4 of 5
Costs Personnel costs...... 630 kchf 36 months scientists (without overheads)...510 kchf 10 months technicians (without overheads)...120 kchf Material costs...... 200 kchf Vehicles (30% of 3 Saab 9 5 and 2 VW Touran Ecofuel)...70 kchf 10 catalytic converters...10 kchf Use of analytical tools (HR-SEM, TEM, XPS, LA-ICP-MS, )...110 kchf Oil analysis (Intertek)...10 kchf Total project costs......830 kchf Already approved financing Empa own contribution (50% of personnel costs)...315 kchf approved Association of Swiss lubrication industry BFF-VSS...100 kchf approved Duebendorf, 24th August 2007 Page 5 of 5