Effect of CaCO3 on catalytic activity of Fe–Ce/Ti catalysts for NH3-SCR reaction

Calcium poisoning mechanism on the selective catalytic reduction of NOx by ammonia over the γ-Fe2O3 (001) surface

γ-Fe₂O₃ has an excellent low-temperature selective catalytic reduction (SCR) deNO_x performance, but its resistance to alkaline earth metal calcium (Ca) is poor. In particular, the detailed mechanism of Ca poisoning on the γ-Fe₂O₃ catalyst at the atomic level is not clear. Hence, the density functional theory method was used in this research to investigate the influence mechanism of Ca poisoning on the NH₃-SCR over the γ-Fe₂O₃ catalyst surface. The findings reveal that NH₃, NO, and O₂ molecules can bind to the γ-Fe₂O₃ (001) surface to generate coordinated ammonia, monodentate nitroso, and adsorption oxygen species, respectively. The main active site is Fe₁-top. For the γ-Fe₂O₃ with Ca poisoning, the Ca atom has a high adsorption energy on the surface of γ-Fe₂O₃ (001), which covers the catalyst surface and reduces the active sites. The presence of Ca atom decreases the adsorption performance of NH₃, while slightly improving the NO and O₂ adsorption. In particular, the Ca atom restrains the NH₃ activation and NH₂ formation, which is detrimental to the NH₃-SCR process.

Promotional effect of Ca doping on Bi2Fe4O9 as peroxymonosulfate activator for gatifloxacin removal

A series of Ca-doped bismuth ferrite was prepared at various %w/w of Ca via a facile hydrothermal method to obtain Bi_2XCa_2(1-X)Fe₄O₉ (denoted as BFOCa-X, where X = 1, 0.95, 0.90, 0.80, 0.50). The BFOCa-X catalysts were characterized, and the results showed that they consist of pure phase BFO with nanosheet-like morphology. The as-prepared BFOCa-X catalysts were used as peroxymonosulfate (PMS) activator for gatifloxacin (GAT) removal. It was found that the catalytic activity decreased in the following order: BFOCa-0.8 (90.2% GAT removal efficiency in 45 min, k_app = 0.084 min^-1)>BFOCa-0.95 > BFOCa-0.9 > BFOCa-0.5 > BFO indicating that BFOCa-0.8 has the optimized active sites for catalysis. The Ca dopant contributed to the increased oxygen vacancies and surface hydroxyl groups, promoting the catalytic PMS activation process. The k_app value increased gradually with increasing catalyst loading and PMS dosage while pH 9 presented the highest GAT removal rate. The GAT degradation rate was inhibited by PO₄³-, humic acid and NH₄⁺ but promoted in the presence of Cl-, NO₃- and HCO₃-. It was also found that the GAT can undergo several degradation pathways in the catalytic PMS system, which eventually mineralized into innocuous compounds. The dominant reactive oxygen species (ROS) were identified using chemical scavengers, revealing that SO₄^•-, ¹O₂ and ^•OH contributed significantly to GAT degradation. Based on the XPS study, PMS was activated by the Fe²⁺/Fe³⁺ redox cycling and oxygen vacancies to produce SO₄^•-/^•OH and ¹O₂, respectively. Overall, the BFOCa-0.8 also showed excellent reusability up to at least 4 cycles with low Bi and Fe leaching (<7 and 62 μg L^-1, respectively), indicating that it has promising potential for application as PMS activator for antibiotics removal.

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.

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.

Selective catalytic reduction of NO over W–Zr-Ox/TiO2: performance study of hierarchical pore structure

A series of W–Zr-Ox/TiO2 catalysts with hierarchical pore structure were prepared and used for selective catalytic reduction of NO by NH3.