Recent Progress on Establishing Structure–Activity Relationship of Catalysts for Selective Catalytic Reduction (SCR) of NOx with NH3

Insights into the Synergistic Promotion Effect of Cu and W Modification on the Catalytic Performance of α-Fe2O3 for NH3-SCR of NOx

A dual-element modification strategy was proposed to promote the catalytic performance of the α-Fe2O3 catalyst for the selective catalytic reduction of NOx by NH3 (NH3-SCR of NOx) at both low and high temperatures. By optimizing the loading amount of CuO (4 wt %) and WO3 (5 wt %), a wide operating temperature window (150-350 °C) was achieved on the modified α-Fe2O3 catalyst (W5/Cu4/Fe). Further characterizations revealed that the enhanced low-temperature activity could be attributed to the improved redox performance of α-Fe2O3 through CuO modification, while the superior high-temperature activity was primarily ascribed to the enhanced surface acidity induced by WO3 modification. The synergistic effect of CuO and WO3 modifications facilitated the NH3-SCR reaction on the modified α-Fe2O3 catalysts to efficiently proceed through the Eley-Rideal (E-R) mechanism pathway. This provided compelling evidence that the catalytic performance of NH3-SCR catalysts at different temperatures was dominantly governed by distinct factors (e.g., redox property and surface acidity), offering valuable insights for the rational design of robust catalysts for NOx abatement.

Unveiling the Promoting Effects of Zr and the Hydrothermal Treatment on the NH3-SCR Activity of the Cu-SAPO-18 Catalyst

Selective catalytic reduction of NOx by NH3(NH3-SCR) remains challenging for diesel vehicles due to the complex exhaust condition. Cu-SAPO-18 zeolite has emerged as an efficient catalyst for the NH3-SCR process, attributed to its unique small pore configuration and high NH3-SCR activity. Herein, Zr-modified Cu-SAPO-18 has been fabricated and evaluated for the reduction of NOx. The addition of Zr to Cu-SAPO-18 led to enhanced activity, with the 0.05 wt % Zr-doped Cu-SAPO-18 catalyst achieving more than 90% NOx conversion across a temperature span of 230 to 470 °C. More importantly, hydrothermal aging at 750 °C further enhanced the activity significantly, achieving nearly 100% NOx conversion at 175 °C. The presence of Zr led to an increased amount of Cu2+ species and acid sites, while the hydrothermal treatment promoted a significant increase in acidity and facilitated the conversion of some Cu2+ to CuxOy species, which is crucial for the enhanced activity. The present research provides insights into the development of high-efficiency small-pore zeolite-based NH3-SCR catalysts, offering valuable implications for both fundamental research and industrial catalysis.

Phase Transformation Behavior, Mechanical Properties Under Thermal Stress, and Slag-Induced Erosion of 2-4 mol% CeO2-Doped CaO-Stabilized Zirconia

We investigated the phase transitions, mechanical properties, and chemical durability of a composition of 9 mol% CaO-stabilized zirconia (9CSZ) doped with 2-4 mol% CeO2 under thermal stress against molten slag. The monoclinic phase fraction of 9CSZ was 7.14% at room temperature, and CSZ doped with 2-4 mol% CeO2 showed a slightly lower value of 5.55-3.72%, with only a minor difference between them. The microstructure of 9CSZ doped with 2-3 mol% CeO2 was similar to that of undoped 9CSZ, whereas the microstructure of 9CSZ doped with 4 mol% CeO2 exhibited noticeable grain refinement. The mechanical properties of CSZ at room temperature tended to improve as the CeO2 doping concentration increased. The Vickers hardness increased from 1088.4 HV to 1497.6 HV when the CeO2 doping amount was 4 mol%, and the specific wear amount decreased from 1.5941 to 1.1320 × 105 mm3/Nm. This tendency remained similar even after applying thermal stress. The monoclinic phase fraction of 9CSZ increased from 7.14% to 67.71% after the erosion experiment with the CaF₂-based slag. CeO2-doped CSZ had a lower monoclinic phase fraction than CSZ after the erosion experiment, but as CeO2 content increased from 2 to 4 mol%, the fraction rose to 4.07%, 30.85%, and 77.11%.

The Emergence of the Ubiquity of Cerium in Heterogeneous Oxidation Catalysis Science and Technology

Research into the incorporation of cerium into a diverse range of catalyst systems for a wide spectrum of process chemistries has expanded rapidly. This has been evidenced since about 1980 in the increasing number of both scientific research journals and patent publications that address the application of cerium as a component of a multi-metal oxide system and as a support material for metal catalysts. This review chronicles both the applied and fundamental research into cerium-containing oxide catalysts where cerium’s redox activity confers enhanced and new catalytic functionality. Application areas of cerium-containing catalysts include selective oxidation, combustion, NOx remediation, and the production of sustainable chemicals and materials via bio-based feedstocks, among others. The newfound interest in cerium-containing catalysts stems from the benefits achieved by cerium’s inclusion, which include selectivity, activity, and stability. These benefits arise because of cerium’s unique combination of chemical and thermal stability, its redox active properties, its ability to stabilize defect structures in multicomponent oxides, and its propensity to stabilize catalytically optimal oxidation states of other multivalent elements. This review surveys the origins and some of the current directions in the research and application of cerium oxide-based catalysts.

N2O Hydrogenation on Silver Doped Gold Catalysts, A DFT Study

In this study, the full reaction mechanism for N₂O hydrogenation on silver doped Au(210) surfaces was investigated in order to clarify the experimental observations. Density functional theory (DFT) calculations were used to state the most favorable reaction paths for individual steps involved in the N₂O hydrogenation. From the DFT results, the activation energy barriers, rate constants and reaction energies for the individual steps were determined, which made it possible to elucidate the most favorable reaction mechanism for the global catalytic process. It was found that the N₂O dissociation occurs in surface regions where silver atoms are present, while hydrogen dissociation occurs in pure gold regions of the catalyst or in regions with a low silver content. Likewise, N₂O dissociation is the rate determining step of the global process, while water formation from O adatoms double hydrogenation and N₂ and H₂O desorptions are reaction steps limited by low activation energy barriers, and therefore, the latter are easily carried out. Moreover, water formation occurs in the edges between the regions where hydrogen and N₂O are dissociated. Interestingly, a good dispersion of the silver atoms in the surface is necessary to avoid catalyst poison by O adatoms accumulation, which are strongly adsorbed on the surface.

Reaction Pathways of Standard and Fast Selective Catalytic Reduction over Cu-SSZ-39

Cu-SSZ-39 exhibits excellent hydrothermal stability and is expected to be used for NO_x purification in diesel vehicles. In this work, the selective catalytic reduction (SCR) activities in the presence or absence of NO₂ were tested over Cu-SSZ-39 catalysts with different Cu contents. The results showed that the NO_x conversion of Cu-SSZ-39 was improved by NO₂ when NO₂/NO_x = 0.5, especially for the catalysts with low Cu loadings. The kinetic studies showed two kinetic regimes for fast SCR from 150 to 220 °C due to a change in the rate-controlling mechanism. The activity test and diffuse reflectance infrared Fourier transform spectra demonstrated that the reduction of NO mainly occurred on the Cu species in the absence of feed NO₂, and when NO₂/NO = 1, NO could react with NH₄NO₃ on the Brønsted acid sites in addition to undergoing reduction on Cu species. Thus, NO₂ can promote the SCR reaction over Cu-SSZ-39 by facilitating the formation of surface nitrate species.

A Study on the Effect of Different Ball Milling Methods on the NH3-SCR Activity of Aluminum-Laden Bayan Obo Tailings

Rich in Fe, Ce, Mn, Si and other elements which have good catalytic activity, Bayan Obo rare-earth tailings are naturally advantaged as the carrier of denitrification catalysts. In this paper, pseudo boehmite (γ-Al2O3) was mixed with Bayan Obo tailings using different ball milling methods for modification to prepare NH3-SCR catalysts. The effect of different mixing methods on the SCR denitrification activity at a low temperature was investigated and the prepared catalysts were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), temperature programed desorption (NH3-TPD), temperature programed reduction (H2-TPR) and other means. The conversion rate of NOx at 250–350 °C was above 80% and the highest conversion rate of NOx of 90% was achieved at 300 °C. SEM and XRD revealed that the tailings modified by pseudo boehmite (γ-Al2O3) using the ordinary ball milling method have loose structure and good dispersion of active substances, and specific surface area (BET) analysis shows that the tailings have the maximum specific surface area and pore volume. However, over grinding and secondary spheronization were observed in the tailings modified by pseudo boehmite (γ-Al2O3) using high energy ball milling method, leading to the decrease of specific surface area and pore volume, poor dispersion of active substances, and ultimately low denitrification rate.

Authentication of emission monitoring data and optimization of desulfurization in the molybdenum roasting process based on BAT-OOPN and the response surface method

This paper presents a quantitative pollutant discharge model for a typical molybdenum roasting plant, which combines the best available technology and object-oriented Petri net concepts. The proposed model was used to verify whether the best available technology in a molybdenum roasting process meeting the current pollutant emission limits by comparing the results of multiple simulations with online monitoring data records. Theoretical SO2 emission values were obtained via multiple simulations and compared with the online monitoring data of a typical molybdenum roasting plant to verify the authenticity of the online monitoring data. The relationship between the different operating parameters and desulfurization efficiency is established through analyzing the historical operation parameters of the enterprise and response surface method. It was found that the optimal operating parameters for the flue gas desulfurization system of this plant could be characterized by a flue gas temperature of 90-93 °C, a pH range of 6.20-6.30, and a liquid-gas ratio of 23-25 L/m3.

Facile Fabrication of Ce/V-Modified Multi-Channel TiO2 Nanotubes and Their Enhanced Selective Catalytic Reduction Performance

To optimize one-dimensional (1D) TiO₂ nanofibers, tailor-made multi-channel TiO₂ nanotubes have been successfully fabricated by electrospinning technology. After loading with Ce and V, the CeVTi-tube catalyst exhibited a broad working temperature window and acceptable resistance to H₂ O and SO₂ for elimination of NO_x . The corresponding analysis revealed that the multi-channel structure provided more surface adsorbed oxygen species and that the wall of nanotubes anchored active components efficiently, which was beneficial to improve the stability as well as dispersion of the active components. Besides, a synergistic effect between Ce and V easily occurred at the CeVTi-tube catalyst, and its reducibility was significantly improved since the electron transformation between Ce and V was dramatically enhanced. Consequently, the tailor-made multi-channel CeVTi-tube catalyst exhibited satisfied de-NO_x efficiency at the temperature range of 220-460 °C. It seemed that the multi-channel TiO₂ nanotubes hold great potential as an excellent carrier for SCR catalysts.

The remarkable promotional effect of Sn on CeVO4 catalyst for wide temperature NH3-SCR process by citric acid-assisted solvothermal synthesis and post-hydrothermal treatment

A series of CeVO₄ catalysts were prepared by citric acid-assisted solvothermal synthesis and hydrothermal methods.