NOx storage and reduction over copper-based catalysts. Part 1: BaO + CeO₂ supports

The performance of noble metal-free copper-based NSR catalysts was studied by rapid-scan operando DRIFTS, using CeO₂, BaO and BaO + CeO₂ mixtures with different ratios as supports. The maximum temperature for CuO/CeO₂ utilization as NSR catalyst was 400 °C, as the stored NOx species decomposed above this temperature. At 400 °C, CuO/CeO₂ showed high NO oxidation capacity and NOx were stored on the catalyst mainly in the form of nitrates and lower populations of nitro groups. CuO/BaO was not suitable for NSR below 250 °C, because its oxidation activity was very low and its NOx storage capacity was negligible, but it did not exhibit upper temperature restrictions until 500 °C. The effect of temperature on NOx chemisorption on copper catalysts with BaO + CeO₂ mixed oxide supports was between those of CuO/CeO₂ and CuO/BaO. NSR experiments performed with high frequency H₂ or CO pulses (micropulses every 120, 60 or 30 s) at 400 °C showed that regeneration with H₂ was more effective than with CO, and this was attributed to the higher reactivity of H₂ rather than to the poisoning effect of the reaction products (H₂O and CO₂ respectively). Nitrates were the main form of chemisorbed nitrogen oxides on all catalysts in NSR experiments at 400 °C, and NOx chemisorption and desorption rates were faster for CuO/CeO₂ than for CuO/BaO. The general behavior of all catalysts tested for NSR at 400 °C was quite similar, and only certain differences were observed during the highest frequency pulses of CO, where CuO/CeO₂ showed the best resistance to deactivation. For all catalysts, N₂ was the only NOx reduction product detected during H₂ regeneration.