Comparative Study of Different Doped Metal Cations on the Reduction, Acidity, and Activity of Fe9M1Ox (M = Ti4+, Ce4+/3+, Al3+) Catalysts for NH3-SCR Reaction

Lattice capacity-dependent activity for CO2 methanation: crafting Ni/CeO2 catalysts with outstanding performance at low temperatures

In the pursuit of understanding lattice capacity threshold effects of oxide solid solutions for their supported Ni catalysts, a series of Ca2+-doped CeO2 solid solutions with 10 wt% Ni loading (named Ni/CaxCe1-xOy) was prepared using a sol-gel method and used for CO2 methanation. The lattice capacity of Ca2+ in the lattice of CeO2 was firstly determined by the XRD extrapolation method, corresponding to a Ca/(Ca + Ce) molar ratio of 11%. When the amount of Ca2+ in the CaxCe1-xOy supports was close to the CeO2 lattice capacity for Ca2+ incorporation, the obtained Ni/Ca0.1Ce0.9Oy catalyst possessed the optimal intrinsic activity for CO2 methanation. XPS, Raman spectroscopy, EPR and CO2-TPD analyses revealed the largest amount of highly active moderate-strength alkaline centers generated by oxygen vacancies. The catalytic reaction mechanisms were revealed using in situ IR analysis. The results clearly demonstrated that the structure and reactivity of the Ni/CaxCe1-xOy catalyst exhibited the lattice capacity threshold effect. The findings offer a new venue for developing highly efficient oxide-supported Ni catalysts for low-temperature CO2 methanation reaction and enabling efficient catalyst screening.

Pt/Al2O3@Ce/ZrO2-S bifunctional catalysts prepared by mechanically milling for selective catalytic oxidation of high-concentration ammonia

The selective catalytic oxidation (SCO) is an effective method for removing slipped high-concentration ammonia from NH3-fueled engine exhaust gas. Herein a novel bifunctional catalyst was synthesized by mechanically mixing sulfated Ce/ZrO2 (Ce/ZrO2-S) with a small fraction of Pt/Al2O3 (Pt 0.1 wt.%) for SCO of NH3. As expected, the introduction of a small amount of Pt/Al2O3 significantly improved NH3 conversion ability of Ce/ZrO2-S catalysts toward low-temperature direction. When the mass ratio of Pt/Al2O3 to Ce/ZrO2-S was 7.5% (the corresponding mixed catalyst was denoted as P@CZS-7.5), T90 temperature was 312 °C. More importantly, P@CZS-7.5 catalyst exhibited a much better N2 selectivity (> 96%) in a wide temperature range (320 ~ 450 °C). H2-TPR results revealed that the addition of a trace amount of Pt/Al2O3 significantly led to a distinct shift of reduction temperature peak toward low-temperature direction, thereby greatly improved the low-temperature redox performance of mixed catalysts. Furthermore, NH3-TPD and BET results showed that P@CZS-7.5 catalyst exhibited a similar NH3 adsorption capacity to Ce/ZrO2-S catalyst, while the former had a relatively higher specific surface area than the latter. It was considered as a crucial factor for P@CZS-7.5 catalyst maintaining superior N2 selectivity in high-concentration NH3 (5000 ppm) removal processes. In situ DRIFTS results indicated that P@CZS-7.5 catalyst followed the internal selective catalytic reduction (i-SCR) mechanism in NH3-SCO reactions.

Mechanism of SO2/H2O enhanced rare earth tailings catalysts in NH3-SCR at medium and high temperature

Rare earth tailings (RET) NH3-SCR catalysts were prepared by mechanical and microwave activation of a large amount of rare earth tailings after beneficiation of Bayan Ebo rare earth ore. The effects of SO2/H2O on the denitrification performance of the RET catalysts were evaluated by conducting denitrification activity tests, SO2/H2O tolerance tests and in situ DRIFTs mechanistic analysis. The results showed that the denitrification activity was significantly increased in the presence of SO2/H2O. And in situ DRIFTs analysis showed that in the presence of SO2/H2O, SO2 could be adsorbed as SO32- groups by the hydroxyl groups on the catalyst surface and react with SO42- to form S2O72- species. And in the presence of NH3, S2O72- would decompose into unstable SO42- species and SO32- and continue to react cyclically to form S2O72- species, providing the RET catalyst provides more acid sites, facilitating the SCR reaction.

Overview of mechanisms of Fe-based catalysts for the selective catalytic reduction of NOx with NH3 at low temperature

With the increasingly stringent control of NOx emissions, NH3-SCR, one of the most effective de-NOx technologies for removing NOx, has been widely employed to eliminate NOx from automobile exhaust and industrial production. Researchers have favored iron-based catalysts for their low cost, high activity, and excellent de-NOx performance. This paper takes a new perspective to review the research progress of iron-based catalysts. The influence of the chemical form of single iron-based catalysts on their performance was investigated. In the section on composite iron-based catalysts, detailed reviews were conducted on the effects of synergistic interactions between iron and other elements on catalytic performance. Regarding loaded iron-based catalysts, the catalytic performance of iron-based catalysts on different carriers was systematically examined. In the section on iron-based catalysts with novel structures, the effects of the morphology and crystallinity of nanomaterials on catalytic performance were analyzed. Additionally, the reaction mechanism and poisoning mechanism of iron-based catalysts were elucidated. In conclusion, the paper delved into the prospects and future directions of iron-based catalysts, aiming to provide ideas for the development of iron-based catalysts with better application prospects. The comprehensive review underscores the significance of iron-based catalysts in the realm of de-NOx technologies, shedding light on their diverse forms and applications. The hope is that this paper will serve as a valuable resource, guiding future endeavors in the development of advanced iron-based catalysts.

Doping SnO2 with metal ions of varying valence states: discerning the importance of active surface oxygen species vs. acid sites for C3H8 and CO oxidation

To elucidate the valence state effect of doping cations, Li+, Mg2+, Cr3+, Zr4+ and Nb5+ with radii similar to Sn4+ (CN = 6) were chosen to dope tetragonal SnO2. Cr3+, Zr4+ and Nb5+ can enter the SnO2 lattice to produce solid solutions, thus creating more surface defects. However, Li+ and Mg2+ can only stay on the SnO2 surface as nitrates, thus suppressing the surface defects. The rich surface defects facilitate the generation of active O2-/Oδ- and acid sites on the solid solution catalysts, hence improving the reactivity. On the solid solution catalysts active for propane combustion, several reactive intermediates can be formed, but are negligible on those with low activity. It is confirmed that for propane combustion, surface acid sites play a more vital role than active oxygen sites. Nevertheless, for CO oxidation, the active oxygen sites play a more vital role than the acid sites.

Revealing the Excellent Low-Temperature Activity of the Fe1-xCexOδ-S Catalyst for NH3-SCR: Improvement of the Lattice Oxygen Mobility

The development of selective catalytic reduction catalysts by NH₃(NH₃-SCR) with excellent low-temperature activity and a wide temperature window is highly demanded but is still very challenging for the elimination of NO_x emission from vehicle exhaust. Herein, a series of sulfated modified iron-cerium composite oxide Fe_1-xCe_xO_δ-S catalysts were synthesized. Among them, the Fe_0.79Ce_0.21O_δ-S catalyst achieved the highest NO_x conversion of more than 80% at temperatures of 175-375 °C under a gas hourly space velocity of 100000 h^-1. Sulfation formed a large amount of sulfate on the surface of the catalyst and provided rich Brønsted acid sites, thus enhancing its NH₃ adsorption capacity and improving the overall NO_x conversion efficiency. The introduction of Ce is the main determining factor in regulating the low-temperature activity of the catalyst by modulating its redox ability. Further investigation found that there is a strong interaction between Fe and Ce, which changed the electron density around the Fe ions in the Fe_0.79Ce_0.21O_δ-S catalyst. This weakened the strength of the Fe-O bond and improved the lattice oxygen mobility of the catalyst. During the reaction, the iron-cerium composite oxide catalyst showed higher surface lattice oxygen activity and a faster replenishment rate of bulk lattice oxygen. This significantly improved the adsorption and activation of NO_x species and the activation of NH₃ species on the catalyst surface, thus leading to the superior low-temperature activity of the catalyst.

Effects of sulfation on hematite for selective catalytic reduction of nitrogen oxides with ammonia

Hematite (α-Fe₂O₃) is a promising candidate for NH₃ selective catalytic reduction (NH₃-SCR) of NO_x due to its good sulfur resistance. However, the activity of pure α-Fe₂O₃ is very low. In this work, α-Fe₂O₃ obtained excellent N₂ selectivity and medium-high temperature activity via a simple surface sulfation method. The α-Fe₂O₃-350 (sulfated at 350 °C) sample showed an NO conversion rate of ~ 100% in the range of 275-350 °C and exhibited excellent H₂O and SO₂ resistance ability at 300 °C. Furthermore, pure α-Fe₂O₃ was used as a model catalyst to fully uncover the effect of sulfation on FeO_x-based catalysts in NH₃-SCR reactions. Structural characterization indicated that the degree of surface sulfation of the catalyst would be deepened with increasing temperature, and the states of sulfate species on α-Fe₂O₃ changed from surface sulfates to bulk-like sulfates. Although sulfation treatment reduced the redox properties of α-Fe₂O₃, it significantly increased its surface acidity and thus the activity. Excessive bulk-like sulfates induced a decrease in activity. Sulfation inhibited the adsorption of NO_x on the α-Fe₂O₃ catalyst surface and reduced the thermal stability of nitrates at medium-high temperature. Thus, the Langmuir-Hinshelwood (L-H) mechanism was inhibited, and the reaction mainly followed the Eley-Rideal (E-R) mechanism.

Study on the mechanism of synthetic (Ce,La)CO3F sulfuric acid acidification and NH3-SCR loaded with Mn and Fe

A hydrothermal method was used to synthesise (Ce,La)CO3F grain simulated minerals, in accordance with the Ce-La ratio of bastnaesite in the mineralogy of the Bayan Ebo process. The NH3-SCR catalytic activity of the synthesised (Ce,La)CO3F was improved by loading transition metals Mn and Fe and sulphuric acid acidification treatments. The activity test results showed that the catalysts which were simultaneously acidified with sulphuric acid and loaded with transition metals Mn and Fe had a NO x conversion of 92% at 250 °C. XRD, SEM, XPS and in situ Fourier transform infrared spectroscopy (FTIR) were used to investigate the physical phase structure, surface morphology, reaction performance and mechanism of the catalysts, to provide theoretical guidance for the specific reaction path of cerium fluorocarbon ore in the NH3-SCR reaction. The results showed that the introduction of transition metals and sulphuric acid greatly increases the proportion of adsorbed oxygen (Oα) and facilitates the adsorption of NH3 and NO. The catalyst surface metal sulphate and metal oxide species act as the main active components on the catalyst surface to promoted the reaction, and cracks and pores appear on the surface to facilitate the adsorption of reactive gases. The reaction mechanism of the SO4 2--Mn-Fe/(Ce,La)CO3F catalyst, and characterisation of the adsorption and conversion behaviour of the reactive species on the catalyst surface, were investigated by Fourier transform infrared spectroscopy (FTIR). The results showed that the catalyst follows the E-R and L-H mechanisms throughout the reaction, with the E-R mechanism being the main reaction. The reaction species were NH4 +/NH3 species in the adsorbed state and NO. The NH3(ad) species on the Lewis acidic site is the main NH3(g) adsorbed species for the reaction, bonded to Ce4+ in the carrier (Ce,La)CO3F to participate in the acid cycle reaction, and undergo a redox reaction on the catalyst surface to produce N2 and H2O. The SO4 2- present on the catalyst surface can also act as an acidic site for the adsorption of NH3. The above results indicated the excellent performance of the SO4 2--Mn-Fe/(Ce,La)CO3F catalyst, which provided a theoretical basis for the high value utilization of bastnaesite.

Understanding the high performance of an iron-antimony binary metal oxide catalyst in selective catalytic reduction of nitric oxide with ammonia and its tolerance of water/sulfur dioxide

In recent years, Fe-based catalysts for the selective catalytic reduction of NO with NH₃ (NH₃-SCR) have been attracting more attention. In this work, a novel Fe-Sb binary metal oxide catalyst was synthesized using the ethylene glycol assisted co-precipitation method and was characterized using a series of techniques. It was found that the catalyst with a molar ratio of 7:3 (Fe:Sb) displayed the best NH₃-SCR activity with 100% conversion of NO_x (nitrogen oxides) over a wide temperature window and with good resistance to H₂O + SO₂ at 250 °C. The X-ray photoelectron spectroscopy (XPS) and in situ diffused reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) of NO_x adsorption results suggested that strong electron interactions between Fe and Sb in Fe-O-Sb species existed and electrons of Sb could be transferred to Fe through the 2Fe³⁺ + Sb³⁺ ↔ 2Fe²⁺ + Sb⁵⁺ redox cycle. The introduction of Sb significantly improved the adsorption behaviour of NO_x species on the Fe_0.7Sb_0.3O_x surface, which benefitted the adsorption/transformation of NO_x, thereby facilitating the NH₃-SCR reaction. In addition, the Fe_0.7Sb_0.3O_x catalyst demonstrated a good tolerance of H₂O and SO₂, since the decomposition of NH₄HSO₄ on the catalyst surface was promoted by the introduction of Sb.

Novel Methods for Assessing the SO2 Poisoning Effect and Thermal Regeneration Possibility of MOx-WO3/TiO2 (M = Fe, Mn, Cu, and V) Catalysts for NH3-SCR

In this study, the sulfur resistance and thermal regeneration of a series of MO_x-WO₃/TiO₂ (denoted as MW/Ti, M = Fe, Mn, Cu, V) catalysts were investigated. After in situ sulfur poisoning, the selective catalytic reduction (SCR) activity of the poisoned catalysts was inhibited at low temperatures but was promoted at high temperatures. After thermal regeneration, the FeW/Ti catalyst was more thoroughly regenerated among nonvanadium-based catalysts. To investigate the impacts of sulfur poisoning, characterizations including X-ray diffraction, thermogravimetric analysis, H₂ temperature-programmed reduction, and SO₂ temperature-programmed desorption were applied. It was discovered that different sulfur-containing species blocked the adsorption of NH₃/NO to a distinct extent over all of the catalysts, thus affecting the catalytic activity. The effect depends on which are dominant (NO or NH₃) during the reaction at different temperatures. The difference in regeneration depends on the formation of sulfate species. The ratio of M_x(SO₄)_y to NH₄HSO₄ generated on the catalysts was adopted to assess the possibility of regeneration. The ratios were 0.5, 1.4, 1.5, and 1.7 for VW/Ti, FeW/Ti, CuW/Ti, and MnW/Ti catalysts, respectively. The lower the ratio was, the easier the catalyst could be regenerated. Meanwhile, the sulfate species could be decomposed more easily on the poisoned FeW/Ti catalyst. FeW/Ti is an excellent candidate for low- and medium-temperature NH₃-SCR among nonvanadium-based catalysts.

Improved NO x Reduction in the Presence of SO2 by Using Fe2O3-Promoted Halloysite-Supported CeO2-WO3 Catalysts

Currently, selective catalytic reduction (SCR) of NO _x with NH₃ in the presence of SO₂ by using vanadium-free catalysts is still an important issue for the removal of NO _x for stationary sources. Developing high-performance catalysts for NO _x reduction in the presence of SO₂ is a significant challenge. In this work, a series of Fe₂O₃-promoted halloysite-supported CeO₂-WO₃ catalysts were synthesized by a molten salt treatment followed by the impregnation method and demonstrated improved NO _x reduction in the presence of SO₂. The obtained catalyst exhibits superior catalytic activity, high N₂ selectivity over a wide temperature range from 270 to 420 °C, and excellent sulfur-poisoning resistance. It has been demonstrated that the Fe₂O₃-promoted halloysite-supported CeO₂-WO₃ catalyst increased the ratio of Ce³⁺ and the amount of surface oxygen vacancies and enhanced the interaction between active components. Moreover, the SCR reaction mechanism of the obtained catalyst was studied using in situ diffuse reflectance infrared Fourier transform spectroscopy. It can be inferred that the number of Brønsted acid sites is significantly increased, and more active species could be produced by Fe₂O₃ promotion. Furthermore, in the presence of SO₂, the Fe₂O₃-promoted halloysite-supported CeO₂-WO₃ catalyst can effectively prevent the irreversible bonding of SO₂ with the active components, making the catalyst exhibit desirable sulfur resistance. The work paves the way for the development of high-performance SCR catalysts with improved NO _x reduction in the presence of SO₂.