Three-way catalyst performance using natural gas with two different sulphur levels
Stoichiometric engines running on natural gas rely on three-way catalysts to meet limits e.g. Euro 6 regarding emissions of hydrocarbons (including methane), carbon monoxide and oxides of nitrogen. As is well known from decades of industry experience with three-way catalysts for petrol applications, sulphur naturally present in the fuel can, following combustion in the engine, cause poisoning of the aftertreatment system. Through complex mechanisms including steric effects, sulphur blocks active sites and prevents the metals in the washcoat from performing their task of facilitating the simultaneous oxidation and reduction of harmful components in the exhaust gas. Through related mechanisms, the presence of sulphur reduces the washcoat’s oxygen storage capacity, which severely limits the catalyst’s ability to oxidise hydrocarbons and carbon monoxide under rich conditions. However, desulphation processes can occur during normal driving, which might lead to partial (or even full) recovery of the catalyst’s performance. Little work has been done recently to understand the effect of sulphur and especially on the catalyst systems of modern natural gas vehicles.