Investigation On The Mechanism Of The NO-CO Reaction Over NiO/CeO₂ Catalyst

In the previous work of our laboratory,we found that the ceria-supported nickel oxide catalyst has been reported to show prominent catalytic activity for NO-CO reaction.In the present study,the reaction mechanism of NO and CO over the NiO/CeO₂ catalyst has been examined following two kinds of reaction modes,"NO reaction with CO pre-treated catalyst" and the reversed "CO reaction with NO pre-treated catalyst",employing in situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy(DRIFTS) coupled with Mass Spectroscopy(MS) techniques.Some useful results are shown as follows:1.Two kinds of adsorption modes,first CO then NO and the opposite sequence of first NO then CO,give distinct results.NO interaction with CO pre-covered catalyst generates the NCO species and N₂(g),while during the process of CO interaction with NO pre-covered catalyst,there is neither NCO species nor N₂(g).Besides that,NO-CO co-adsorption on the catalyst also produces NCO complexes and N₂.The results indicate that the dissociation of NO on the CO pre-reduced surface should be the pathway for N₂ generation.2.In terms of thermal stability,the population of NCO surface species,formed during the interaction of NO-CO with the catalyst,depends on temperature.The DRIFTS results indicate that the intensities of this band increases as enhance the temperature,and goes through a maximum in the 150-170℃range then decreases to near zero at 210℃.The corresponding MS results also show that there is the formation of N₂ and CO₂ at this temperature.In terms of reactivity,isocyanates do not interact directly with CO but react with NO to yield N₂ and CO₂.Reaction between NCO species and NO giving an alternative route for N₂ generation.3.Base on the DRIFTS and MS results,the reaction steps of NO reduction by CO over NiO₂/CeO₂ are suggested to be:(â…°) removal of surface oxygen by CO to create vacancy sites; (â…±) dissociation of NO on the vacancy sites to produce ^*N and ^*O,and the coupling of two ^*N for N₂ generation,while ^*N reacts with ^*CO to produce NCO species;(â…²) NCO species reacts with NO to produce N₂ and CO₂(an alternative route for N₂ generation);and(â…³) the oxygen originated from NO dissociation is removed by CO.Therefore,the pre-reducing of NiO/CeO₂ catalyst by CO should be the critical step for this reaction.