Promotional Effects on NH3-SCR Performance of CeO2–SnO2 Catalysts Doped by TiO2: A Mechanism Study

Recent Progress on Low-Temperature Selective Catalytic Reduction of NOx with Ammonia

Selective catalytic reduction of nitrogen oxides (NOx) with ammonia (NH3-SCR) has been implemented in response to the regulation of NOx emissions from stationary and mobile sources above 300 °C. However, the development of NH3-SCR catalysts active at low temperatures below 200 °C is still needed to improve the energy efficiency and to cope with various fuels. In this review article, recent reports on low-temperature NH3-SCR catalysts are systematically summarized. The redox property as well as the surface acidity are two main factors that affect the catalytic activity. The strong redox property is beneficial for the low-temperature NH3-SCR activity but is responsible for N2O formation. The multiple electron transfer system is more plausible for controlling redox properties. H2O and SOx, which are often found with NOx in flue gas, have a detrimental effect on NH3-SCR activity, especially at low temperatures. The competitive adsorption of H2O can be minimized by enhancing the hydrophobic property of the catalyst. Various strategies to improve the resistance to SOx poisoning are also discussed.

Investigation on the Structural and Photocatalytic Performance of Oxygen-Vacancy-Enriched SnO2-CeO2 Heterostructures

In this study, pure CeO2 and oxygen-vacancy-enriched SnO2-CeO2 composite materials were prepared using the sol-gel method, and their microstructures and photocatalytic properties were investigated. The results indicate that SnO2 coupling promotes the separation and transfer of photogenerated electrons and holes and suppresses their recombination. The 50% SnO2-CeO2 composite material exhibited a decreased specific surface area compared to pure CeO2 but significantly increased oxygen vacancy content, demonstrating the highest photogenerated charge separation efficiency and the best photocatalytic performance. After 120 min of illumination, the degradation degree of MB by the 50% SnO2-CeO2 composite material increased from 28.8% for pure CeO2 to 90.8%, and the first-order reaction rate constant increased from 0.002 min-1 to 0.019 min-1.

A Study of CeSnOx and Pd/CeSnOx as Low-Temperature NOx Adsorbers with Excellent Hydrothermal Stability

In the present work, we report on two passive NOx adsorber (PNA) material candidates: the novel support CeSnOx with and without Pd loading. The NOx adsorption and storage capacities of fresh and hydrothermally aged CeSnOx and Pd/CeSnOx were investigated. The results show that CeSnOx exhibits a rather large NOx uptake and storage capacity (28.9 μmol/g), while the loading of Pd on CeSnOx can further increase the storage capacity to 37.6 μmol/g and affect the desorption temperature of NOx. It was found that the NOx desorption temperature of Pd/CeSnOx was compatible with the efficient operating window of selective catalytic reduction (SCR) catalysts. After a hydrothermal aging treatment at 800 °C for 12 h, the NOx adsorption and storage capacities of CeSnOx and Pd/CeSnOx increased, indicating excellent hydrothermal stability. The interaction of Pd with CeSnOx, the state of Pd species, and the structure of CeSnOx and Pd/CeSnOx are studied by combination of the characterization results.

An efficient strategy to screen an effective catalyst for NOx-SCR by deducing surface species using DRIFTS

HYPOTHESIS

Transition metal supported TiO₂ is one of the hottest catalysts in the field of selective catalytic reduction (SCR) of nitrogen oxides. Various formulas have been put forward for an enhanced activity. However, seldom work emphasizes on easy and fast screening of an effective catalyst.

EXPERIMENTS

In this work, Diffuse Reflection Fourier Transform Infrared (DRIFTS) screened catalyst by analyzing intermediates during SCR.

FINDINGS

TiO₂ provided main adsorption sites for NH₃ and the "Eley-Rideal" mechanism dominated the catalysis. The transition metals served as the bridge of electron transport. Moreover, the area reduction rate of adsorbed NH₃ and NH₄⁺ species in DRIFTS represented the electron-transfer rate as well as catalytic activity. In other words, a faster area reduction indicated a better SCR activity. Therefore, this work supplied a fast strategy to screen the most effective catalyst among different materials even without using a nitrogen oxides detector. At the same time, less ammonia and nitrogen oxides were used or discharged.