Oxidation of Diesel-Generated Volatile Organic Compounds in the Selective Catalytic Reduction Process

Abstract

The main part of the VOCs (volatile organic compounds) contained in diesel exhaust (≈80%) is oxidized to CO and CO2 over an SCR (selective catalytic reduction) catalyst. CO is the major product of this oxidation, representing about 50−70% of the formed products (CO + CO2). This preferential formation of CO leads to a pronounced increase of CO emissions when an SCR process is added to a diesel engine. A small fraction of the VOCs is selectively oxidized to carboxylic acids over the SCR catalyst. This selectivity is due to the acidic properties of the catalyst causing the preferential desorption at the oxidation state of the acid. The main products of these oxidation reactions are the lower monocarboxylic acids and some dicarboxylic acids forming stable anhydrides, especially maleic and phthalic acid. The highest emissions of these acids are found at low temperatures; they decrease at higher temperatures. Formic acid is preferentially decomposed into carbon monoxide and water. It must therefore be assumed that the strong increase of CO mentioned above is due to a mechanism involving the thermal decomposition of formic acid formed from various primary VOCs.