The lean NOx traps behavior of (1–5%) BaO/CeO2 mixed with Pt/Al2O3 at low temperature (100–300 °C): The effect of barium dispersion

The present work investigates the lean NOx trapping performance of BaO (1, 3, 5 wt.%) modified CeO2 (BC) NOx storage catalysts, where 2 wt.% Pt/Al2O3 (PA) part was mechanically blended in. Before mixture, the storage catalysts (BaO/CeO2) were treated at 800 °C for 5 h in a steam and CO2-containing air. The storage catalysts were characterized in detail by X-ray diffraction (XRD), nitrogen adsorption–desorption, energy-filtered transmission electron microscopy (EFTEM), and X-ray photoelectron spectroscopy (XPS). Results show highest Ba dispersion on CeO2 for 3BC sample. 5BC sample forms large BaCO3 particle leading to lower Ba dispersion than 3BC sample. NOx storage capacity was investigated in steady-state and transient NO/O2 adsorption reactions in the temperature range from 100 °C to 300 °C. As BaO loading increases, the PA–BC catalysts exhibit nonlinear enhancement for NOx storage capacity (NSC) compared with PA–Ce sample. And PA–3BC sample possesses superior NOx storage capacity. The evolution and stability of NOx adsorbed species at 100 °C were explored by in situ DRITFS. DRIFTS results show that PA–3BC and PA–5BC samples present obviously different NOx adsorbed species from PA–Ce sample. Besides, nitrate formation is more facilitated for PA–3BC sample compared with the PA–5BC sample. These results can be contributed to high Ba dispersion on CeO2, which is advantageous to Ba–Ce contact and NOx trapping. And the Ba sites in intimate contact with CeO2 can effectively utilize the Ba sites and oxygen from CeO2 to form nitrates at low temperature, which possess higher stability than Ce nitrates.

TEM images for 1BC (a), 3BC (b) and 5BC (c) (storage materials) and EFTEM images of segistration of Ba and Ce mapping for 1BC (A), 3BC (B) and 5BC (C), and purple and green stand for Ce mapping and Ba mapping, respectively. For all Ba loaded CeO2 storage materials, Ba species at nm scale are located at the surface of CeO2 crystallite. Low Ba loading possesses good Ba dispersion and high NOx uptakes after hydrothermal treatment, while high Ba loading form larger Ba species particle.Figure optionsDownload as PowerPoint slideHighlights
► Ba modified CeO2 storage catalysts possess higher NOx uptakes than CeO2.
► Low Ba loadings modified CeO2 catalysts exhibit nonlinear NOx uptakes.
► NOx uptakes for Ba modified CeO2 depend on Ba dispersion on CeO2 and temperature.
► Nitrates formation at100 °C depends on the Ba dispersion on CeO2.