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

Catalytic conversion of NO in the presence of H2 and O2 has been studied on Pd(111) surfaces, by using 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 towards 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 certain O2 content. The reason for this behaviour 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 favourably against O2 for the consumption of H2, especially 550 K, to produce N2 and H2O. Compared to other elementary reaction steps, a slow decay observed with 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 Oatoms 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. 50% increase in reaction rates was observed at reaction maximum on Md-Pd(111), compared to virgin surfaces. Oxygen adsorption is severely limited below 400 K and effectively NO + H2 reaction occurs on Md-Pd(111) surfaces. Valence band photoemission with UV light source (He I), under different oxygen pressures with APPES, clearly identified the characteristics of the MdPd(111) surfaces and PdO. Electron deficient or cationic nature of Md-Pd(111) surfaces enhance NO dissociation and inhibits oxygen chemisorption ≤ 400 K under lean-burn conditions.