Redox Properties Of Catalysts Research Articles

Surface properties enhanced MnxAlO oxide catalysts derived from MnxAl layered double hydroxides for acetone catalytic oxidation at low temperature

MnxAlO mixed oxide catalysts derived from MnxAl-LDHs were prepared and tested for acetone catalytic oxidation. Detailed results indicated that the surface intrinsic and formed oxygen vacancies can induce the Mn-O bond of structural unit [MnO6] weakened. Subsequently, it can improve the redox properties of catalysts, and enhance the capacity of gaseous oxygen species dissociation and adsorption. Among them, Mn3AlO catalyst displayed the best catalytic performance for acetone oxidation (T90 = 164 °C) with the production of low amount of byproduct (<5 ppm) and high CO2 yield (>99%) producted. Additionally, the Mn3AlO catalyst can proceed consecutively for 12 h reaction without notable deactivation. Furthermore, in situ DRIFT and theoretical calculations methods was adopted to explore the reaction mechanism. And η1(O)(ads) (adsorption mode of acetone), CH2=C(CH3)=O(ads), O*, CH3CHO*, CH2O* and COO(ads) were considered as the main intermediate species and/or transient state during the reaction process. It was revealed that the acetone and oxygen molecules were activated by the dehydrogenation (α-H abstraction) and dissociation process over Mn3AlO catalyst, respectively, and then the intermediate specie, CH2C(CH3)O and O*, were produced, which was followed by the breaking of CC bonds to produce the CH3CHO* and CH2O* species. Finally, these species were attacked by dissociated oxygen (O*) and therefore further dehydrogenation occurred, form H2O and CO2 via the COO adsorbed species. Particularly, CC bond breaking was the main rate determining step for acetone oxidation.

Catalytic Activity and Molecular Behavior of Lanthanum Modified CoSx/γ-Al2O3 Catalysts for the Reduction of SO2 to Sulfur in Smelter Off-Gas Using CO-H2 Mixture as Reductant

In this study, a series of three-component CoSx/γ-Al2O3 catalysts were developed for the reduction of high-content SO2 to sulfur in smelter off-gas. The introduction of a small amount of lanthanum additive greatly improved the catalytic activity and stability of Co-Cu/γ-Al2O3 catalysts using coal gas as reductant. The catalysts were characterized by XRD, XPS, BET, and SEM techniques. H2/CO-TPR was recorded to study redox properties of catalysts. With the use of a CO-H2 mixture with different proportions, a series of parallel experiments were performed to investigate the mutual impact and relation of CO and H2. Under optimum conditions, SO2 conversion of 99% and sulfur selectivity of 99% were obtained. The catalyst remained highly active after a long run of 200 h. Process simulation was used to investigate the molecular behavior of catalyst. Combined with in situ infrared characterization, the catalytic mechanism was put forward and verified.

Cucurbit[7]uril promoted Fenton oxidation by modulating the redox property of catalysts.

The redox property of a ferrocene derivative, the catalyst of Fenton oxidation, is modulated by the host-guest complexation of cucurbit[7]uril. The supramolecularly modulated catalyst significantly accelerates the catalytic oxidation of dye chromophores, thus providing a way to avoid product inhibition and broadening the range of substrates.

Fe/Beta@Meso-CeO2 Nanostructure Core–Shell Catalyst: Remarkable Enhancement of Potassium Poisoning Resistance

Fe/Beta@meso-CeO2 core–shell catalyst with abundant mesopores was designed and controllablly constructed by a template-assisted self-assembly method. This catalyst was fabricated by small-grain Beta molecular sieve supporting FeOx nanoparticles as the core and thin meso-CeO2 film (~ 2 nm) as the shell, and it exhibits remarkable resistance to potassium poisoning and superior SO2 tolerance for selective catalytic reduction (SCR) of NOx with NH3. Meso-CeO2 shells play a key role in influencing the acidity and redox properties of catalyst. It can not only serve as an effective protective “layer” to prevent K from exchanging the isolated iron ions to form oligonuclear FexOy clusters, but also increase chemisorbed oxygen species and promote the formation of active NO2 and cis-N2O2− species. Moreover, meso-CeO2 thin film can suppress the generation of sulfate species blocking the active sites over Fe/Beta@meso-CeO2 catalyst. Furthermore, the kinetics result further reveals that the coating of meso-CeO2 shell doesn’t change Ea for the SCR reaction, and the decreased kinetic rate for K-poisoning catalyst is caused by the decreased number of active sites for the reaction. Therefore, the present study provides a new path for synthesis and application of core–shell structural catalysts.

Iron oxide-based catalysts for low-temperature selective catalytic reduction of NOxwith NH3

AbstractSelective catalytic reduction (SCR) is now an established NOxremoval technology for industrial flue gas as well as for diesel engine exhaust gas. However, it is still a big challenge to develop a novel low-temperature catalyst for NH3-SCR of NOx, especially at a temperature below 200°C. In the past few years, many studies have demonstrated the potential of iron (Fe)-based catalysts as low-temperature catalysts for NH3-SCR of NOx. Herein, we summarize the recent progress and performance of Fe-based catalysts for low-temperature NH3-SCR of NOx. Catalysts are divided into three categories: single FexOy, Fe-based multimetal oxide, and Fe-based multimetal oxide with support catalysts. The catalytic activity and selectivity of Fe-based catalysts are systematically analyzed and summarized in light of some key factors such as activation energy, specific surface area, morphology, crystallinity, preparation method and precursor, acid sites, calcination temperature, other metal dopant/substitute, and redox property of catalysts. In addition, H2O/SO2tolerance and the NH3-SCR reaction mechanism over Fe-based catalysts, including Eley-Rideal and Langmuir-Hinshelwood mechanism, are emphasized. Lastly, the perspectives and future research directions of low-temperature NH3-SCR of NOxare also proposed.

Synthesis of Polyoxymethylene Dimethyl Ethers from Dimethyl Ether Direct Oxidation over Carbon‐Based Catalysts

AbstractThe synthesis of polyoxymethylene dimethyl ethers (DMMx) with a selectivity of 84.3 % by direct oxidation of dimethyl ether (DME) was realized over 30 %Ti(SO4)2/active carbon (AC) catalyst. This process also significantly promotes the growth of the C−O chain. The catalytic performances of Ti(SO4)2/AC and Ti(SO4)2/graphene (G) catalysts differ largely for DME oxidation reaction, although both AC and G are carbon materials. The carbonyl and hydroxyl groups on the surface of the carbon‐based catalysts play an important role in DME direct oxidation to DMMx. Owing to the differences of surface structure and chemical properties of AC and G materials, the different interaction between the Ti(SO4)2 and supports remarkably affects the sulfate structures on the supports surface and leads to the large differences in the acid and redox properties of catalysts.

Effect of Co addition on the performance and structure of V/ZrCe catalyst for simultaneous removal of NO and Hg0 in simulated flue gas

The effect of CoOx addition on the performance and structure of V2O5/ZrO2–CeO2 catalyst for simultaneous removal of NO and Hg0 in simulated flue gas was investigated by various methods including SEM, BET, XRD, XPS, H2–TPR and FT–IR. It was found that the introduction of CoOx not only greatly enhanced the redox properties of catalysts, but also increased the catalytic performance for simultaneous removal of NO and Hg0. The CoOx–modified V2O5/ZrO2–CeO2 catalyst displayed excellent catalytic activity for NO conversion (89.6%) and Hg0 oxidation (88.9%) at 250°C under SCR atmosphere. The synergistic effect among vanadium, cobalt, and the ZrCe support could induce oxygen vacancies formation and promote oxygen mobility via charge transfer. Besides, CoOx could assist vanadium species in rapidly changing the valence by the redox cycle of V5++Co2+↔V4++Co3+. All the above features contribute to the excellent catalytic performance through CoOx addition.

Base-free conversion of 5-hydroxymethylfurfural to 2,5-furandicarboxylic acid in ionic liquids

Base-free conversion of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) was carried out by a simple and green process based on a novel ionic liquid (IL)-promoted non-noble metal catalytic reaction system. Among various of ILs, 1-butyl-3-methylimidazolium chloride ([Bmim]Cl) was found to be the best IL without addition of base as it has stronger hydrogen bonds formation ability and good thermal stability, as well as free to form definite aggregates. In addition, a series of non-noble metal catalysts containing Ce, Fe, Zr were synthesized and the Ce0.5Fe0.15Zr0.35O2 catalyst was found to have excellent activity due to the existence of active iron and zirconium species and the formation of CeO2-based solid solution, which could create more oxygen vacancies and improve the redox properties of catalyst. Under the optimal reaction condition, almost complete HMF conversion and 44.2% FDCA yield were obtained after 24h at 160°C over Ce0.5Fe0.15Zr0.35O2 in [Bmim]Cl. More importantly, the catalyst could be reused without significant decrease of its catalytic activity. This research not only develops a more environmentally friendly catalytic system that promotes the base-free oxidation of HMF into FDCA, but also provides an economic route to produce FDCA.

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.

MnO x catalysts supported on γ-Al2O3, ZSM-5, and SAPO-34: Effect of support on the activity of Mn supported catalysts in NO abatement by NH3

The NH3–SCR of NO reaction has been studied using three different manganese containing catalysts supported on γ-Al2O3, ZSM-5, and SAPO-34. The nanocatalysts were prepared by homogeneous deposition precipitation (HDP) method and characterized via BET, TEM, H2-TPR, and NH3-TPD analysis. The activity of the catalysts varied significantly depending on the type of the support. The Mn-ZSM-5 and Mn/γ-Al2O3 catalysts were more active for NO conversion than Mn-SAPO-34 catalyst. The no correlation was found between surface area, strength and amount of acid sites, and NO conversion of the catalysts. The results of H2-TPR revealed lower onset reduction temperatures of MnOx species in Mn-ZSM-5 and Mn/γ-Al2O3 compared with Mn-SAPO-34, indicating easily reducibility of MnOx species in these catalysts. The redox properties of catalysts are responsible for the SCR activity. The Mn-SAPO-34 catalyst contains low quantitative of access reducible MnOx species, which is followed by considerable decrease in the activity. The NO conversion ability of Mn-supported catalysts has been correlated with support structure.

Rare earth metals modified Ni–S2O82 −/ZrO2–Al2O3 catalysts for n-pentane isomerization

The effects of adding rare earth (RE) metals, such as Ce, Yb and Pr to Ni–S2O82−/ZrO2–Al2O3 (Ni–SZA) on the structure of catalysts as well as their isomerization performance were studied. The prepared catalysts were characterized by XRD, BET, FT-IR, Py-IR, and H2-TPR. The results showed that the addition of RE metals can increase the strength and amounts of the acid sites, improve the redox properties of catalysts. The Yb–Ni-SZA catalyst showed the best redox properties, which could provide enough metallic sites. In addition, it provided the largest amounts of weak and moderately strong acid sites. Among RE metals modified Ni-SZA catalyst, Yb-Ni-SZA exhibited the highest isopentane yield of 61.7% at 160°C. The optimum isomerization catalytic performance of the catalysts decreased in the order of Yb-Ni-SZA>Pr-Ni-SZA>Ni-SZA>Ce-Ni-SZA.

Effect of nano-sized cerium–zirconium oxide solid solution on far-infrared emission properties of tourmaline powders

Far-infrared functional nanocomposites were prepared by the co-precipitation method using natural tourmaline [Formula: see text], where [Formula: see text] is [Formula: see text], [Formula: see text], [Formula: see text], or vacancy; [Formula: see text] is [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], or [Formula: see text]; [Formula: see text] is [Formula: see text], [Formula: see text], [Formula: see text], or [Formula: see text]; [Formula: see text] is [Formula: see text], [Formula: see text]; and [Formula: see text] is [Formula: see text], [Formula: see text], or [Formula: see text] powders, ammonium cerium(IV) nitrate and zirconium(IV) nitrate pentahydrate as raw materials. The reference sample, tourmaline modified with ammonium cerium(IV) nitrate alone was also prepared by a similar precipitation route. The results of Fourier transform infrared spectroscopy show that tourmaline modified with Ce and Zr has a better far-infrared emission property than tourmaline modified with Ce alone. Through characterization by transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS), the mechanism for oxygen evolution during the heat process in the two composite materials was systematically studied. The XPS spectra show that [Formula: see text] ratio inside tourmaline modified with Ce alone can be raised by doping Zr. Moreover, it is showed that there is a higher [Formula: see text] ratio inside the tourmaline modified with Ce and Zr than tourmaline modified with Ce alone. In addition, XRD results indicate the formation of [Formula: see text] and [Formula: see text] crystallites during the heat treatment and further TEM observations show they exist as nanoparticles on the surface of tourmaline powders. Based on these results, we attribute the improved far-infrared emission properties of Ce–Zr doped tourmaline to the enhanced unit cell shrinkage of the tourmaline arisen from much more oxidation of [Formula: see text] to [Formula: see text] inside the tourmaline caused by the change in the catalyst redox properties of [Formula: see text] brought about by doping with [Formula: see text]. In all samples, tourmaline modified with 7.14 wt.% Ce and 1.86 wt.% Zr calcined at 800[Formula: see text]C for 5 h has the best far-infrared emission property with the maximum emissivity value of 98%.

Catalytic activities and mechanism of formaldehyde oxidation over gold supported on MnO2 microsphere catalysts at room temperature

MnO2 microspheres with various surface structures were prepared using the hydrothermal method, and Au/MnO2 catalysts were synthesized using the sol-gel method. We obtained three MnO2 microspheres and Au/MnO2 samples: coherent solid spheres covered with wire-like nanostructures, solid spheres with nanosheets, and hierarchical hollow microspheres with nanoplatelets and nanorods. We investigated the properties and catalytic activities of formaldehyde oxidation at room temperature. Crystalline structures of MnO2 are the main factor affecting the catalytic activities of these samples, and γ-MnO2 shows high catalytic performance. The excellent redox properties are responsible for the catalytic ability of γ-MnO2. The gold-supported interaction can change the redox properties of catalysts and accelerate surface oxygen species transition, which can account for the catalytic activity enhancement of Au/MnO2. We also studied intermediate species. The dioxymethylene (DOM) and formate species formed on the catalyst surface were considered intermediates, and were ultimately transformed into hydrocarbonate and carbonate and then decomposed into CO2. A proposed mechanism of formaldehyde oxidation over Au/MnO2 catalysts was also obtained.

The promotional effect of MoO3 doped V2O5/TiO2 for chlorobenzene oxidation

A series of V2O5–MoO3/TiO2 catalysts prepared by an impregnation method was investigated on their catalytic performance of chlorobenzene oxidation. This study demonstrated that MoO3 is a dopant, which can improve catalyst redox properties and enhance chlorobenzene oxidation catalytic activity at low temperature. Compared to V2O5–TiO2 and V2O5–WO3/TiO2 catalysts, V2O5–MoO3/TiO2 catalysts showed outstanding activity on chlorobenzene oxidation, which is due to the promoting influence of MoO3. For excellent catalytic performance and good N2 selectivity with water, the V2O5–MoO3/TiO2 catalysts may have promising commercial potential for the synergetic control of dioxins and NOx emitted from stationary sources.

Bifunctional Mo3VOx/H4SiW12O40/Al2O3 catalysts for one-step conversion of glycerol to acrylic acid: Catalyst structural evolution and reaction pathways

A series of Mo3VOx/H4SiW12O40/Al2O3 catalysts were prepared and applied to one-step oxydehydration of glycerol to acrylic acid. The catalysts were thoroughly characterized by BET, XRD, TEM, TGA, NH3-TPD, and XAFS techniques. It was found that acetic acid was produced in parallel with acrylic acid with similar yield when adjusting Mo3VOx content in the catalysts. Increasing calcination temperature from 350 to 650°C led to structural evolution of supported active species and subsequent activity change. Crystallinity of Mo3VOx increased with calcination temperature and a large amount of (V,Mo)Ox hetero-polyoxo mixed oxides were formed above 550°C. The Keggin structure of H4SiW12O40 began to dissociate around 450°C causing the formation of various WOx species. Above 450°C, the yield of acrylic acid dropped while that of acrolein could reach its maximum value with increase of the catalyst calcination temperature. In contrast, the yield of acetic acid showed a minor change always. Based on the catalytic and characterization results, possible active centers and reaction pathways are thus proposed, and for the improvement of the bi-functional catalyst for the one-step conversion of glycerol, both the acidic and redox properties of catalysts should be finely tuned. This work provides new insight into the reaction mechanism and the catalyst design for the one-step oxydehydration of glycerol to acrylic acid.

CO and C3H8 total oxidation over Pd/La-Al2O3 catalysts: Effect of calcination temperature and hydrothermal treatment

A series of Pd/La-Al2O3 (PLA) catalysts with La-Al2O3 (LA) support calcined at different temperatures (500, 700, 900 and 1050 °C) were prepared using an incipient wetness impregnation method. The activity of the fresh and hydrothermally aged PLA catalysts were tested for total oxidation of CO and C3H8. The activity of the fresh PLA catalysts for CO and C3H8 oxidation increased with increasing calcination temperature of the support, while the activities of the aged catalysts declined and became essentially the same. CO chemisorption results revealed that the suppressed activities of the aged catalysts were mainly due to the decline of palladium dispersion. The turnover frequency (TOF) of CO oxidation increased with increasing reduction ability of the catalysts, with a fresh catalyst calcined at 1050 °C having the highest value (0.048 s−1). However, the TOF of C3H8 total oxidation was affected by not only the redox properties of catalysts but also the size of Pd particle, and large Pd particles possessed higher TOF value of C3H8 oxidation, with the highest value (0.125 s−1) being obtained on an aged catalyst calcined at 500 °C.

Promoting effect of ZrO2 carrier on activity and thermal stability of CeO2-based oxides catalysts for toluene combustion

ZrO2 was used as a support for the preparation of Cu–Mn–Ce mixed oxides catalysts (CMC) by impregnation method and calcination at different temperatures. A model reaction, toluene combustion was conducted to evaluate the performance of CMC/ZrO2 catalysts. High thermal stability and activity was obtained on these catalysts. The effects of CMC loading and calcination temperature on the structure and redox properties of catalysts were also investigated by XRD, H2-TPR, TEM and XPS. Results show that calcination leads to the interaction of ZrO2 and CMC via the formation of a new phase of Zr0.88Ce0.12O2 in the interface, which is more likely to be formed in the catalysts with low CMC loading levels. Zr0.88Ce0.12O2 seems to function as the actual carrier of active phase of CMC, which cannot only stabilize the surface active structure of CMC but also enhance the mobility of active oxygen and improve the catalytic performance of total oxidation as well.

Low-temperature catalytic conversion of lignite: 3. Tar reforming using the supported potassium carbonate

Alumina-supported K2CO3–LaMn0.8Cu0.2O3 was investigated for the catalytic conversion of tar, produced from lignite, into syngas under inert and steam-reforming conditions. A double-bubble fluidized bed reactor system, equipped with a micro gas chromatograph and a collecting system to analyze permanent gases and condensable species, was developed to screen the catalytic conversion of tar components below 700°C. The redox properties of catalysts, estimated by hydrogen temperature programmed reduction analyses, were correlated with their catalytic performance in tar conversion. The synthesized catalyst effectively converted tars into hydrogen-rich syngas and also improved tar reforming by inhibiting coke deposition.

Chemical deactivation of V2O5-WO3/TiO2 SCR catalyst by combined effect of potassium and chloride

V2O5-WO3/TiO2 catalyst was poisoned by impregnation with NH4Cl, KOH and KCl solution, respectively. The catalysts were characterized by X-ray diffraction (XRD), inductively coupled plasma (ICP), N2 physisorption, Raman, UV-vis, NH3 adsorption, temperature-programmed reduction of hydrogen (H2-TPR), temperature-programmed oxidation of ammonia (NH3-TPO) and selective catalytic reduction of NOx with ammonia (NH3-SCR). The deactivation effects of poisoning agents follow the sequence of KCl>KOH≫NH4Cl. The addition of ammonia chloride enlarges the pore size of the titania support, and promotes the formation of highly dispersed V = O vanadyl which improves the oxidation of ammonia and the high-temperature SCR activity. K+ ions are suggested to interact with vanadium and tungsten species chemically, resulting in a poor redox property of catalyst. More importantly, potassium can reduce the Bronsted acidity of catalysts and decrease the stability of Bronsted acid sites significantly. The more severe deactivation of the KCl-treated catalyst can be mainly ascribed to the higher amount of potassium resided on catalyst.

Effect of support on V2O5 catalytic activity in chlorobenzene oxidation

The present paper investigates the influence of the direct incorporation of ceria in TiO2 aerogels support (sulfated and unsulfated) prepared by a sol–gel route on the catalytic properties of vanadia based materials in the total oxidation of chlorobenzene. ICP-AES, N2 physisorption, XRD, DRIFTS, Raman spectroscopy, XPS, H2-TPR and NH3-TPD were employed for catalyst characterization. This study demonstrated that cerium oxide is present as CeO2 form at the catalyst surface, which can improve the catalyst redox properties and so enhance the catalytic activity at high temperature (400°C). Besides, sulfate containing vanadia–titania samples (doped or not with ceria) had their catalytic performances considerably improved (due to sulfates) at lower temperatures (range 200–300°C). This is due to an increased global acidity and higher reactivity of redox sites thanks to the superficial interaction between (i) vanadia and sulfate, and (ii) when ceria is present, between ceria and sulfate leading to higher efficiency of catalysts in the chlorobenzene oxidation.