Improvement of low-temperature NH3-SCR catalytic performance over nitrogen-doped MO x -Cr2O3-La2O3/TiO2-N (M = Cu, Fe, Ce) catalysts

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.

Probable surface NH₃-SCR reaction mechanism over CuCe/TiO₂-ZrO₂ catalyst is proposed to follow the E–R mechanism and the L–H mechanism, while the E–R mechanism dominates in the reaction and the oxidation of NO closes the catalytic cycle.

Tuning the catalytic properties of La–Mn perovskite catalyst via variation of A- and B-sites: effect of Ce and Cu substitution on selective catalytic reduction of NO with NH3

Perovskites with flexible structures and excellent redox properties have attracted considerable attention in industry, and their denitration activities can be further improved with metal substitution. In order to investigate the effect of Ce and Cu substitution on the physicochemical properties of perovskite in NH₃-SCR system, a series of La_1−xCe_xMn_1−yCu_yO₃ (x = 0, 0.1, y = 0, 0.05, 0.1, 0.2, 0.4) catalysts were prepared by citrate sol-gel method and employed for NO removal in the simulated flue gas, and the physical and chemical properties of the catalysts were studied using XRD, SEM, BET, XPS, DRIFT characterizations. The Ce substitution on A-site cation of LaMnO₃ can improve the denitration activity of the perovskite catalyst, and La_0.9Ce_0.1MnO₃ displays NO conversion of 86.7% at 350 °C. The characterization results indicate that the high denitration activity of La_0.9Ce_0.1MnO₃ is mainly attributed to the larger surface area, which contributes to the adsorption of NH₃ and NO. Besides, the appropriate Cu substitution on B-site cation of La_0.9Ce_0.1MnO₃ can further improve the denitration activity of perovskite catalyst, and La_0.9Ce_0.1Mn_0.8Cu_0.2O₃ displays the NO conversion of 91.8% at 350 °C. Although the specific surface area of La_0.9Ce_0.1Mn_0.8Cu_0.2O₃ is lower than La_0.9Ce_0.1MnO₃, the Cu active sites and the Ce³⁺ contents are more developed, making many reaction units formed on the catalyst surface and redox properties of catalyst improved. In addition, strong metal interaction (Ce⁴⁺ + Mn²⁺ + Cu²⁺ ↔ Ce³⁺ + Mn³⁺/Mn⁴⁺ + Cu⁺) and high concentrations of chemical adsorbed oxygen and lattice oxygen both strengthen the redox reaction on catalyst surface, thus contributing to the better denitration activity of La_0.9Ce_0.1Mn_0.8Cu_0.2O₃. Therefore, appropriate cerium and copper substitution will markedly improve the denitration activity of La–Mn perovskite catalyst. We also reasonably conclude a multiple reaction mechanism during NH₃-SCR denitration process basing on DRIFT results, which includes the Eley–Rideal mechanism and Langmuir–Hinshelwood mechanism.

Perovskites with flexible structures and excellent redox properties have attracted considerable attention in industry, and their denitration activities can be further improved with metal substitution.

Synthesis of W-modified CeO2/ZrO2 catalysts for selective catalytic reduction of NO with NH3

In this paper, a series of tungsten–zirconium mixed binary oxides (denoted as W_mZrO_x) were synthesized via co-precipitation as supports to prepare Ce_0.4/W_mZrO_x catalysts through an impregnation method. The promoting effect of W doping in ZrO₂ on selective catalytic reduction (SCR) performance of Ce_0.4/ZrO₂ catalysts was investigated. The results demonstrated that addition of W in ZrO₂ could remarkably enhance the catalytic performance of Ce_0.4/ZrO₂ catalysts in a broad temperature range. Especially when the W/Zr molar ratio was 0.1, the Ce_0.4/W_0.1ZrO_x catalyst exhibited the widest active temperature window of 226–446 °C (NO_x conversion rate > 80%) and its N₂ selectivity was almost 100% in the temperature of 150–450 °C. Moreover, the Ce_0.4/W_0.1ZrO_x catalyst also exhibited good SO₂ tolerance, which could maintain more than 94% of NO_x conversion efficiency after being exposed to a 100 ppm SO₂ atmosphere for 18 h. Various characterization results manifested that a proper amount of W doping in ZrO₂ was not only beneficial to enlarge the specific surface area of the catalyst, but also inhibited the growth of fluorite structure CeO₂, which were in favor of CeO₂ dispersion on the support. The presence of W was conducive to the growth of a stable tetragonal phase crystal of ZrO₂ support, and a part of W and Zr combined to form W–Zr–O_x solid super acid. Both of them resulted in abundant Lewis acid sites and Brønsted acid sites, enhancing the total surface acidity, thus significantly improving NH₃ species adsorption on the surface of the Ce_0.4/W_0.1ZrO_x catalyst. Furthermore, the promoting effect of adding W on SCR performance was also related to the improved redox capability, higher Ce³⁺/(Ce³⁺ + Ce⁴⁺) ratio and abundant surface chemisorbed oxygen species. The in situ DRIFTS results indicated that nitrate species adsorbed on the surface of the Ce_0.4/W_0.1ZrO_x catalyst could react with NH₃ due to the activation of W. Therefore, the reaction pathway over the Ce_0.4/W_0.1ZrO_x catalyst followed both Eley–Rideal (E–R) and Langmuir–Hinshelwood (L–H) mechanisms at 250 °C.

Interaction of W with Zr improved NH₃-SCR performance via enhancing redox and surface acidity.