Promotional effects of cerium modification of Cu-USY catalysts on the low-temperature activity of NH3-SCR

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.

Effect of Cu loading content on the catalytic performance of Cu-USY catalysts for selective catalytic reduction of NO with NH3

Series of Cu-USY zeolite catalyst with different Cu loading content were synthesized through simple impregnation method. The obtained catalysts were subjected to selective catalytic reduction of NO_x with NH₃ (NH₃-SCR) performance evaluation, structural/chemical characterizations such as X-ray diffraction (XRD), N₂ adsorption/desorption, H₂ temperature-programmed reduction (H₂-TPR), NH₃ temperature-programmed desorption (NH₃-TPD) as well as detailed in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) experiments including CO adsorption, NH₃ adsorption and NO+O₂ in situ reactions. Results show that Cu-USY with proper Cu loading (in this work 5Cu-USY with 5 wt.% Cu) could be promising candidates with highly efficient NH₃-SCR catalytic performance, relatively low byproduct formation and excellent hydrothermal stability, although its SO₂ poisoning tolerability needs alleviation. Further characterizations reveal that such catalytic advantages can be attributed to both active cu species and surface acid centers evolution modulated by Cu loading. On one hand, Cu species in the super cages of zeolites increases with higher Cu content and being more conducive for NH₃-SCR reactivity. On the other hand, higher Cu loading leads to depletion of Brønsted acid centers and simultaneous formation of abundant Lewis acid centers, which facilitates NH₄NO₃ reduction via NH₃ adsorbed on Lewis acid centers, thus improving SCR reactivity. However, Cu over-introduction leads to formation of surface highly dispersed CuO_x, causing unfavorable NH₃ oxidation and inferior N₂ selectivity.

Promotional mechanism of enhanced denitration activity with Cu modification in a Ce/TiO2-ZrO2 catalyst for a low temperature NH3-SCR system

This study aims to investigate the enhanced low temperature denitration activity and promotional mechanism of a cerium-based catalyst through copper modification. In this paper, copper and cerium oxides were supported on TiO2-ZrO2 by an impregnation method, their catalytic activity tests of selective catalytic reduction (SCR) of NO with NH3 were carried out and their physicochemical properties were characterized. The CuCe/TiO2-ZrO2 catalyst shows obviously enhanced NH3-SCR activity at low temperature (<300 °C), which is associated with the well dispersed active ingredients and the synergistic effect between copper and cerium species (Cu2+ + Ce3+ ↔ Cu+ + Ce4+), and the increased ratios of surface chemisorbed oxygen and Cu+/Cu2+ lead to the enhanced low-temperature SCR activity. The denitration reaction mechanism over the CuCe/TiO2-ZrO2 catalyst was investigated by in situ DRIFTS and DFT studies. Results illustrate that the NH3 is inclined to adsorb on the Cu acidic sites (Lewis acid sites), and the NH2 and NH2NO species are the key intermediates in the low-temperature NH3-SCR process, which can explain the promotional effect of Cu modification on denitration activity of Ce/TiO2-ZrO2 at the molecular level. Finally, we have reasonably concluded a NH3-SCR catalytic cycle involving the Eley-Rideal mechanism and Langmuir-Hinshelwood mechanism, and the former mechanism dominates in the NH3-SCR reaction.

A review of Mn-based catalysts for low-temperature NH3-SCR: NOx removal and H2O/SO2 resistance

The development of high-efficiency catalysts is the key to the low-temperature NH₃-SCR technology. The introduction of SO₂ and H₂O will lead to poisoning and deactivation of the catalysts, which severely limits the development and application of NH₃-SCR technology. This review introduces the necessity of NO_x removal, explains the mechanisms of H₂O and SO₂ poisoning on NH₃-SCR catalysts, highlights the Mn-based catalysts of different active metals and supports and their resistance to H₂O and SO₂, and analyses the relationship between metal modification, selection of support and preparation method, morphology and structure design and SO₂/H₂O resistance. Given the current problems, this review points out the future research focus of Mn-based catalysts and also puts forward corresponding countermeasures to solve the existing problems.

Influence of different preparation methods on the activity of Ce and Mo co-doped ZSM-5 catalysts for the selective catalytic reduction of NOx by NH3

The Ce-doped different MoO₃ [conventional molybdenum oxide (con-MoO₃) or nano molybdenum oxide (nano-MoO₃) and synthetic molybdenum oxide (syn-MoO₃)] modification of ZSM-5 catalyst synthesized by different preparation methods (the combination of grinding and ion-exchange method and the combination of impregnation and ion-exchange method) was studied on selective catalytic reduction (SCR) of NO_x with NH₃. The results demonstrated that the SCR performance of the prepared Ce-doped syn-MoO₃ modification of ZSM-5 catalyst [Ce(0.9%)-syn-MoO₃(6%)/ZSM-5] by the combination of impregnation and ion-exchange method was better than Ce-doped con-MoO₃ modification of ZSM-5 [Ce(0.9%)-con-MoO₃(6%)/ZSM-5] and Ce-doped nano-MoO₃ modification of ZSM-5 [Ce(0.9%)-nano-MoO₃(6%)/ZSM-5] via the combination of grinding and ion-exchange method, especially when the temperature window is 200-350 °C. That is because it is easy to form Mo-O-Al by the smaller sized MoO₃ more easily interacting well with Brønsted acid under calcining temperature, which results in the decrease of Brønsted acid sites in the catalyst. Combing with the binding energy of Mo for all the catalysts, the combination of Mo and Al (Mo-O-Al) altered the chemical environment around the Mo species. Furthermore, Ce(0.9%)-syn-MoO₃(6%)/ZSM-5 exhibited excellent sulfur resistance.