(94a) Compressed Natural Gas Engines Exhaust Gas Treatment

Compressed-natural-gas (CNG) is recognized as the cleanest hydrocarbon fuel for spark-ignition engines. It represents the best compromise towards availability and well-known technology, reliability and low cost. The low emission levels of particulate matter (PM) and CO₂ reduce the dangerousness of the exhaust stream [1]. The unburned hydrocarbon emissions (93% methane) are the main negative aspect of this type of engine: even if methane toxicity is not comparable with that of the unburned hydrocarbons emitted by gasoline spark-ignition and diesel engines, it is a strong green-house gas. The global heating power is 35 higher than that of CO₂. A research line of ours, carried out in cooperation with the FIAT Research Centre (Orbassano, Italy), is aimed at developing nanostructured Pd-spinel-type-oxide catalysts employing an overall noble metal load smaller than that used in conventional converters. Methane is indeed a rather hard-to-ignite hydrocarbon, which entails the use of three-fold higher amount of noble metals (about 200 g/ft³) compared to the three-way catalysts for gasoline-fired engines aftertreatment. In the present contribution, innovative Pd-spinel-type-oxide catalysts are introduced and deposited by ?in situ combustion synthesis? [2] over ceramic honeycombs to produce reliable catalytic converters, tested on a specific test rig and compared to conventional ones. After a large screening of methane combustion oxide catalysts obtained by combustion synthesis [2], in line with the other authors [3], the best catalyst was found to be Co_0.8Cr₂O₄(see table 1). The catalytic converters (Chauger cylindrical honeycombs; cell density: 200 cpsi; length: 25 mm; diameter: 34 mm) were prepared by firstly depositing a layer of g-alumina (10 wt% referred to the monolith weight) and then 15 wt% (referred to the galumina weight) of Co_0.8Cr₂O₄, the most active spinel. On some monoliths, Pd was then deposited by wet impregnation with an aqueous solution of Pd(NO₃)₂ so as to obtain 1wt% of Pd referred to the deposited catalyst (about 80 g/ft³ of catalytic converter). The monoliths were then kept at 500°C in order to promote the Pd stabilisation and the formation of finely dispersed Pd clusters. A calcination step at 700°C for 2h was finally performed. Adhesion tests and a full chemical-physical characterization (XRD, SEM, TEM, TPD/R/O, AAS) were carried out and will be detailed in the full presentation. Catalytic combustion experiments were performed in a stainless steel reactor heated in a horizontal split tube furnace with a heating length of 60 cm. The catalyzed monolith was sandwiched between two mullite foams for optimizing temperature control flow distribution. A thermocouple, inserted along one of the central monolith channel, was used to measure the inlet temperature. Lean inlet conditions (0.4% CH₄, 10% O₂, N₂ balance) were ensured by mass flow controllers. The reactor temperature gradient measured in the axial direction was not significant. The GHSV was set equal to 10,000 h^-10, whereas the composition of the off-gases was monitored by NDIR analysers (ABB). The methane conversion results show that all gAl₂O₃-supported catalysts shift the combustion temperature range towards values significantly lower (decrease of more than 350°C) than those typical of non-catalytic combustion. Starting from the pure gAl₂O₃ catalyzed monolith, only a slight improvement of the activity is noticeable compared to the blank monolith. Undoubtedly, the dispersion of active spinel-type-oxide phase (Co_0.8Cr₂O₄) on alumina is successful to enhance the activity towards methane oxidation. The half conversion temperature is lowered to 403°C, from the 760°C achieved for the monolith uncatalyzed converter. Further improvements (about 15°C) lowering are induced by the presence of Pd, thereby approaching the performance of the high-Pd containing catalysts. The role of Pd in methane catalytic combustion is well known and still addressed by a number of investigation [4, 5].

References

1. Kato T., Saeki K.,. Nishide H, Yamada T. , JSAE Review 22, 365 (2001).

2. Civera A., Pavese M., Saracco G., Specchia V., Catal. Today, 83, 199 (2003).

3. Cimino S., Pirone R., Lisi L., App. Catal. B: Environ., 35, 243 (2002).

4. Choudhary T.V., Banerjee S., Choudhary V.R., App. Catal., A: Gen. 234, 1 (2002).

5. Janbey A., Clark W., Noordally E., Grimes S., Thair S.,Chemosphere 52, 1041 (2003).