NITRIC-OXIDE REDUCTION AND CARBON-MONOXIDE OXIDATION OVER CARBON-SUPPORTED COPPER CHROMIUM CATALYSTS

Carbon supported copper-chromium catalysts are shown to be very active for both the reduction of nitric oxide with carbon monoxide and the o xidation of carbon monoxide with oxygen. Mixed copper chromium oxide a ctive phases have good activity in the simultaneous removal of nitric oxide and carbon monoxide from exhaust gases. The influence of several catalyst variables has been investigated. The activity per volume of catalyst increases with increasing loading, while the intrinsic activi ty shows a maximum around C/M = 100-50. An optimum catalyst for nitric oxide reduction and carbon monoxide oxidation has a copper/chromium r atio of 2/1. The apparent activation energy for the carbon monoxide ox idation over carbon supported copper-chromium catalysts is 77 kJ/mol, suggesting that the Cu-O bond rupture is the rate-limiting process. Th e reduction of nitric oxide takes place at higher temperatures. Since all catalysts have a low selectivity for molecular nitrogen formation at lower temperatures, the dissociation of nitric oxide is probably ra te determining, resulting in a slightly reduced catalyst system. In an excess of carbon monoxide the reaction is first-order in nitric oxide and zero-order in carbon monoxide. Moisture inhibits the reaction by reversible competitive adsorption, whereas carbon dioxide does not. Ox ygen completely inhibits the reduction of nitric oxide due to the more rapid reoxidation of the catalytic sites compared to nitric oxide. Th erefore, the reduction of nitric oxide takes place only when all oxyge n has been converted and, hence, is shifted to higher temperatures. As a possible consequence, the production of nitrous oxide is reduced. N itric oxide and molecular oxygen react preferentially with carbon mono xide, so, in an excess of oxidizing component, gasification of the car bon support occurs at higher temperatures after carbon monoxide has be en completely consumed.