Role of Vehicular Catalytic Converter Temperature in Emission of Pollutants: An Assessment Based on Isotopic Analysis of CO₂ and N₂O

Abstract

Vehicular catalytic converters are used to regulate, reduce, and convert toxic and environmentally unfriendly compounds in exhaust gases into relatively inert and less harmful chemical species. The efficiency, however, is largely affected by the operating temperature of the converter which is set by the hot exhaust gas released from the combustion chamber. A major gas released during combustion is CO 2 , and its multiply substituted isotopocule, namely, 13 C 16 O 18 O, provides a window of opportunity to probe directly the effective temperature of the converter in operation. Here, we report multiple isotopic measurements in exhaust CO 2 (δ 13 C, δ 17 O, δ 18 O, and Δ 47 ) of diesel (trucks and buses) and gasoline (sedans, trucks, and two-wheel motorcycles)-powered vehicles. For investigating the efficiency of a converter in reducing toxic compounds, we studied NO x processes through isotopic analysis of the exhaust N 2 O. We found that the degree of N 2 O reduction to N 2 in gasoline-powered vehicles is high when the temperature is above 200 °C (inferred by Δ 47 ). In contrast, diesel-powered vehicles produce N 2 O in abundance, probably a consequence of selective catalytic reduction of NO x , and the reduction efficiency depends on the converter temperature. In other words, the catalytic converters act as sinks and sources of N 2 O to the atmosphere in gasoline-and diesel-operated vehicles, respectively. We also report a new set of field data by measuring the isotopic compositions of CO 2 and N 2 O in the Hsuehshan tunnel, a ∼13 km long highway tunnel in Taiwan. Elevated N 2 O concentrations inside the tunnel indicate that the emission of N 2 O by heavy-duty diesel vehicles is much higher compared to the reduction by gasoline-operated passenger cars, making the vehicular exhausts a net source of N 2 O to the atmosphere. The combined study of clumped isotopes and N 2 O concentration in exhaust gases suggests that it is useful to probe the operational temperature of catalytic converters and monitor the pollution level in operation, thus providing an opportunity for manufacturers to optimize the catalytic efficiency to reduce the level of toxic pollutants to the environment.

Reduction of NOx on metal-free hydrogenated hexagonal boron nitride

“…21 However, platinum and palladium catalysts often exhibit peak activity at temperatures around 200 °C, 21 whereas catalytic converters in vehicles frequently surpass this threshold with passenger vehicles operating over a wide range of temperatures (280–650 °C). 26 Therefore, there is potential to find alternative catalysts that are active at higher temperatures. Perovskites have recently been of interest as a greener alternative for NO x removal but still rely (although quantitatively less) on metal atoms in their structure.…”