In situ IR studies of selective catalytic reduction of NO with NH3 on Ce-Ti amorphous oxides

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.

Nanoreactor Based on Cyclodextrin for Direct Electrocatalyzed Ammonia Synthesis

The high-efficiency transition metal-free electrocatalytic nitrate reduction reaction (NO₃^-RR) for ammonia synthesis has received more attention because of its green and environmentally friendly characteristics. Here, we report an efficient electrochemical NH₃ synthesis directly from purely organic macrocyclic compounds α-, β-, and γ-cyclodextrins (CDs)-catalyzed transition metal-free electroreduction of nitrate under ambient conditions. In comparison with α-, and β-CDs, parent γ-CD presented uncommon catalytic performance with a relatively higher NH₃ yield that can reach up to 2.28 mg h^-1 cm^-2 with a Faradaic efficiency (FE) of 63.2% at -0.9 V versus a reversible hydrogen electrode in alkaline medium, and the potassium ion-coordinated γ-CD complex can achieve a maximum NH₃ production rate up to 4.66 mg h^-1 cm^-2 with an NH₃ FE of 79.3%. Further comparison with permethyl-γ-CD, d-glucose, and poly(vinyl alcohol) for the NO₃^-RR indicated that the typical torus-shaped cyclic conformation and edge hydroxyl groups of parent CDs play important roles in the electrocatalytic process. The K⁺-mediated 3D γ-CD-K⁺ frameworks containing six CDs as nanoreactors greatly strengthen the enrichment effect of nitrate through hydrogen-bonding interaction and electrostatic interaction and promote the mass transfer, thus leading to the efficient NO₃^-RR in an alkaline electrolyte. This work provides a convenient, green, and economic method for high-performance NO₃^-RR, which has potential applications in the fields of environment, energy, and industry.

Time-resolved in situ DRIFTS study on NH3-SCR of NO on a CeO2/TiO2 catalyst

Ce–Ti catalysts were considered as a promising replacement for V–Ti based catalysts for selective catalytic reduction (SCR) of nitrogen oxides (NO and NO2) with NH3.

Enhancing the K resistance of CeTiOx catalyst in NH3-SCR reaction by CuO modification

It is generally accepted that CeTiO_x catalyst owns outstanding catalytic activity for ammonia-selective catalytic reduction (NH₃-SCR), but the tolerance to alkali metals is still dissatisfactory. Thus, it is of great importance to further elevate the catalytic activity and resistance to alkali metals of CeTiO_x catalyst. In our work, a series of CeTiO_x, CuO/CeTiO_x, K-CeTiO_x and K-CuO/CeTiO_x catalysts were prepared to comprehensively analyze the influence of CuO modification on the physicochemical features, catalytic activity and anti-K ability of CeTiO_x catalyst. The results manifest that CuO modification effectively enhances low-temperature catalytic activity and anti-K poisoning ability of CeTiO_x catalyst by protecting the reduction ability and the surface acidity as well as weakening the adsorption strength of NO_x.

High-temperature vanadium-free catalyst for selective catalytic reduction of NO with NH3 and theoretical study of La2O3 over CeO2/TiO2

SCR catalysts of the La2O3–CeO2/TiO2 series for de-NOx with NH3 were prepared and optimized. The Ce10La2 catalyst has a great NO conversion efficiency, good N2 selectivity, good SO2 resistance, and good anti-aging properties at high temperature.

MOF-74-M (M = Mn, Co, Ni, Zn, MnCo, MnNi, and MnZn) for Low-Temperature NH3-SCR and In Situ DRIFTS Study Reaction Mechanism

Monometallic and bimetallic MOF-74-M (M = Mn, Co, Ni, Zn, MnCo, MnNi, and MnZn) catalysts were prepared by the solvothermal method for NH₃-SCR. XRD, BET, SEM, and EDS-mapping tests indicate the successful synthesis of the MOF-74-M catalyst with uniform distribution of metal elements and large specific surface area, and the morphology is almost hexagonal. Adding Mn element to a single-metal catalyst can enhance activity, which is mainly because of the existence of various valence states of Mn so that it has excellent redox properties; the catalytic activity of water and sulfur resistance tests showed that the catalytic activity of MOF-74-M increases after adding a proper amount of SO₂, mainly because of the increase in acidic sites. In situ DRIFTS results indicate that the low-temperature range of MOF-74-MnCo and MOF-74-Mn is dominated by the E-R mechanism and the high-temperature range is dominated by the L-H mechanism. The entire temperature range of MOF-74-Zn is dominated by the L-H mechanism.

Active Site of O2 and Its Improvement Mechanism over Ce-Ti Catalyst for NH3-SCR Reaction

The current study on Ce-Ti catalyst was mainly focused on the function of NH3 and NO adsorption sites. In our study, by comparing Ce-Ti (doped catalyst) to Ce/Ti (supported catalyst), the active site of O2 and its improvement mechanism over Ce-Ti catalyst for NH3-Selective catalytic reduction (SCR) reactions were investigated. For Ce-Ti catalyst, a cerium atom was confirmed entering a TiO2 crystal lattice by X-ray diffraction (XRD) and Raman; the structure of Ce-□-Ti (□ represents oxygen vacancy) in Ce-Ti catalyst was confirmed by X-ray photoelectron spectroscopy (XPS) and Photoluminescence spectra (PL spectra). The nature of this structure was characterized by electron paramagnetic resonance (EPR), Ammonia temperature-programmed desorption (NH3-TPD), hydrogen temperature-programmed reduction (H2-TPR), Nitric oxide temperature-programmed desorption (NO-TPD) and In situ DRIFT. The results indicated that oxygen vacancies had a promotive effect on the adsorption and activation of oxygen, and oxygen was converted to superoxide ions in large quantities. Also, because of adsorption and activation of NO and NH3, electrons were transferred to adsorbed oxygen via oxygen vacancies, which also promoted the formation of superoxide ions. We expected that our study could promote understanding of the active site of O2 and its improvement mechanism for doped catalyst.

The selective catalytic reduction of NO over Ce0.3TiOx-supported metal oxide catalysts

A Ce_0.3TiO_x oxide carrier was synthesized via a sol-gel process, and Ce_0.3TiO_x supported metal (M=Cd, Mn, Fe, W, Mo) oxide catalysts were prepared by the method of incipient-wetness impregnation. The catalysts were characterized by means of X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET) analysis, scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared (FT-IR) spectroscopy, UV-Vis diffuse reflectance spectroscopy (UV-Vis DRS), and Temperature-programmed reduction with H₂ (H₂-TPR). The catalytic activities for de-NO_x were evaluated by the NH₃-SCR reaction. Among all the catalysts tested, the 2wt.% Cd/Ce_0.3TiO_x catalyst exhibited the best NH₃-SCR performance, with a wide temperature window of 250-450°C for NO conversion above 90%. Moreover, the catalyst showed N₂ selectivity greater than 99% from 200 to 450°C.

The effects of calcination atmosphere on the catalytic performance of Ce-doped TiO2 catalysts for selective catalytic reduction of NO with NH3

A series of well-reported Ce_x–Ti catalysts with a low content of Ce species were synthesized by a sol–gel method.

Decomposition behavior of ammonium nitrate on ceria catalysts and its role in the NH3-SCR reaction

Ceria catalyst's redox properties promote NH₄NO₃ decomposition while acidity inhibits this process.

Selective catalytic oxidation of ammonia over MnOx–TiO2 mixed oxides

The formation of Mn–O–Ti and a high dispersal of Mn₂O₃ promoted oxygen activation and NH₃ adsorption. The formation of N₂O depends on temperature.