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Sustainable and Near Ambient DeNOx Under Lean Burn Conditions: A Revisit to NO Reduction on Virgin and Modified Pd(111) Surfaces

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journal contribution
posted on 06.06.2014, 00:00 by Kanak Roy, Ruchi Jain, Chinnakonda S. Gopinath
Catalytic conversion of NO in the presence of H2 and O2 has been studied on Pd(111) surfaces, by using a molecular beam instrument with mass spectrometry detection, as a function of temperature and reactants composition. N2 and H2O are the major products observed, along with NH3 and N2O minor products under all conditions studied. Particular attention has been paid to the influence of O2 addition toward NO dissociation. Although O2-rich compositions were found to inhibit the deNOx activity of the Pd catalyst, some enhancement in NO reduction to N2 was also observed up to a certain O2 content. The reason for this behavior was determined to be the effective consumption of the H2 in the mixture by the added O2 and O atoms from NO dissociation. NO was proven to compete favorably against O2 for the consumption of H2, especially ≤550 K, to produce N2 and H2O. Compared with other elementary reaction steps, a slow decay observed with the 2H + O → H2O step under SS beam oscillation conditions demonstrates its contribution to the rate-limiting nature of the overall reaction. Pd(111) surfaces modified with O atoms in the subsurface (Md-Pd(111)) induces steady-state NO reduction at near-ambient temperatures (325 K) and opens up a possibility to achieve room temperature emission control. A 50% increase in the reaction rates was observed at the reaction maximum on Md-Pd(111), as compared with virgin surfaces. Oxygen adsorption is severely limited below 400 K, and effective NO + H2 reaction occurs on Md-Pd(111) surfaces. Valence band photoemission with a UV light source (He I) under different oxygen pressures with APPES clearly identified the characteristics of the Md-Pd(111) surfaces and PdO. The electron-deficient or cationic nature of Md-Pd(111) surfaces enhances the NO dissociation and inhibits oxygen chemisorption ≤400 K under lean-burn conditions.