Three-way catalyst emissions control technologies for spark-ignition engines—recent trends and future developments

Abstract

Exhaust gas emissions from passenger cars with traditional gasoline engines have been markedly reduced over the three decades since the introduction of catalysts. The first oxidation catalysts were superseded by two-catalyst three-way systems that were followed by single three-way catalysts (TWCs). The first Pt/Rh-based TWCs formulations were refined by incorporating increasingly thermally stable ceria-based components to keep the surface composition close to stochiometric during exhaust gas composition fluctuations. Substituting Pd for Pt gave further improvement in some applications, as can zoning catalyst type or loading on a single substrate. Optimisation of cell density, geometric area, and thermal capacity of the substrate provided additional benefits. Modern advanced TWCs are very thermally stable so they can be mounted directly on the exhaust manifold. Here, with a rapid engine start-up strategy catalyst operating temperatures are quickly achieved after the engine is started. As a result extremely low emissions can be achieved, and computer models are used to predict the performance of different catalyst loadings and layouts. This is a powerful tool in the design of TWC systems for new engines/vehicles to predict what will be optimal, and today high performance from relatively very low PGM loadings is possible, in spite of increasingly stringent tailpipe emissions limits and distances over which they must be maintained. Now the performance of TWCs is monitored during actual on road driving, and this is done via measurement of the catalyst oxygen storage capacity.

PdO x /Pd at Work in a Model Three-Way Catalyst for Methane Abatement Monitored by Operando XANES

“…1 CH 4 conversion profiles for Pd/ACZ and Pd/A under simulated stoichiometric conditions of natural gas engines during two heating-cooling cycles 850°C reveals that either CH 4 reforming or reverse water gas shift is possibly at work above 370°C. Both NO reduction by CH 4 and CH 4 (steam) reforming have been mentioned as possible concurrent reactions to CH 4 oxidation responsible for CH 4 removal from the exhaust [12]. The present data confirm that direct CH 4 oxidation is not the only reaction contributing to this process under dry stoichiometric conditions.…”

“…Figure 1 shows that such characteristics are maintained under stoichiometric conditions (CH 4 , CO, NO x and O 2 ) of CH 4 oxidation on the model Pd/ACZ three-way catalyst (TWC). The MS signals of the pollutants are provided in Figure S1.…”

“…Heating-cooling segments exert some influence on CO and NO abatement efficiencies. The conversion profile of NO is as temperature dependent as that of CH 4 . In the case of CO, though conversion is complete at 200°C the apparent increase of CO concentration between 370 and Fig.…”

“…Exhaust after treatment technologies of natural gas passenger cars equipped with engines working at stoichiometric air-to-fuel ratios require catalytic control of methane (the major component of natural gas) because of its large global warming potential. Present solutions are based on threeway catalysts (TWC), whose composition is typically very complex given the severe conditions of temperature and of fast transients experienced during operation [3,4]. Besides Platinum group metals (PGM) that are required to simultaneously reduce and oxidize the major engine exhaust pollutants (CO, NO x and hydrocarbons), Al 2 O 3 is a major component of TWC and offers the required surface area for PGM dispersion and thermal stability.…”

Templated growth of platinum nanowheels using the inhomogeneous reaction environment of bicelles

“…Although bicelles have been suggested as templating agents for the growth of nanomaterials, prior to our recent study 11 no successful examples had been reported, partially because of the difficulties in obtaining high yields of bicelles. 3,4 In an earlier study, 11 we investigated the synthesis of platinum materials to demonstrate the templating ability of bicelles, since platinum is technologically important for applications in sensing, 12,13 reduction of tailpipe emissions, [14][15][16] electrocatalytic energy conversion in proton exchange membrane (PEM) fuel cells, [17][18][19] and renewable energy production in artificial photosynthesis and water-splitting devices. 20,21 Our previous study primarily focused on the synthesis and structural characterizations of the platinum nanowheels obtained in the presence of bicelles.…”