SELECTIVE CARBON MONOXIDE OXIDATION IN H2-RICH GAS STREAM OVER Co3O4/CeO2/ZrO2, Ag/CeO2/ZrO2, AND MnO2/CeO2/ZrO2 CATALYSTS

Activity and selectivity of selective CO oxidation in an H₂-rich gas stream over Co₃O₄/CeO₂/ZrO₂, Ag/CeO₂/ZrO₂, and MnO₂/CeO₂/ZrO₂ catalysts were studied. Effects of the metaloxide types and metaloxide molar ratios were investigated. XRD, SEM, and N₂ physisorption techniques were used to characterize the catalysts. All catalysts showed mesoporous structure. The best activity was obtained from 80/10/10 Co₃O₄/CeO₂/ZrO₂ catalyst, which resulted in 90% CO conversion at 200°C and selectivity greater than 80% at 125°C. Activity of the Co₃O₄/CeO₂/ZrO₂ catalyst increased with increase in Co₃O₄ molar ratio.     

Synthesis of Highly Effective CeO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>–MnO<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>–BaO Catalysts for Direct NO Decomposition

Abstract  

Ce–Mn mixed oxides with a Mn/(Ce + Mn) molar ratio of 0.25 were prepared by solvothermal (ST-1) and co-precipitation (CP) methods, and Ba was loaded on the Ce–Mn oxides. In addition, CeO₂–MnO _x –BaO catalysts with various compositions were directly prepared by the solvothermal (ST-2) method. The NO decomposition activities of these catalysts were examined. Among the catalysts examined, the ST-2 catalyst having a nominal composition of Ce_0.8Mn_0.15Ba_0.05O _x exhibited the highest activity; 77% NO conversion to N₂ was attained at 800 °C. These catalysts were characterized by X-ray diffraction, Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). The Raman and XPS results indicate that the CP catalyst had larger amounts of the BaMnO_3-δ and/or Mn₃O₄ phases. The ST-1 and ST-2 catalysts had highly dispersed Ba species on the surface. The ST-2 catalyst had Mn species with the lowest binding energy of Mn 2p and also had a high population of oxygen vacancies in the ceria lattice, suggesting that Mn species with a low oxidation state contributes to the formation of oxygen vacancies, which play an important role in this reaction.     

One-step synthesis of ternary MnO2–Fe2O3–CeO2–Ce2O3/CNT catalysts for use in low-temperature NO reduction with NH3

In this article, a facile one-step strategy for the synthesis of ternary MnO₂–Fe₂O₃–CeO₂–Ce₂O₃/carbon nanotubes (CNT) catalysts was discussed. The as-prepared catalysts exhibited 73.6–99.4% NO conversion at 120–180 °C at a weight hourly space velocity (WHSV) of 210 000 ml·g_cat^− 1·h^− 1, which benefited from the formation of amorphous MnO₂, Fe₂O₃, CeO₂, and Ce₂O₃, as well as high Ce^3 + and surface oxygen (O_ε) contents. The mechanism of formation of MnO₂–Fe₂O₃–CeO₂–Ce₂O₃/CNT catalysts was also proposed.     

The Effect of Acidic and Redox Properties of V2O5/CeO2–ZrO2 Catalysts in Selective Catalytic Reduction of NO by NH3

V₂O₅ supported ZrO₂ and CeO₂–ZrO₂ catalysts were prepared and characterized by N₂ physisorption, XRPD, TPR, and NH₃-TPD methods. The influence of calcination temperature from 400 to 600 °C on crystallinity, acidic and redox properties were studied and compared with the catalytic activity in the selective catalytic reduction (SCR) of NO with ammonia. The surface area of the catalysts decreased gradually with increasing calcination temperature. The SCR activity of V₂O₅/ZrO₂ catalysts was found to be related with the support crystallinity, whereas V₂O₅/CeO₂–ZrO₂ catalysts were also dependent on acidic and redox properties of the catalyst. The V₂O₅/CeO₂–ZrO₂ catalysts showed high activity and selectivity for reduction of NO with NH₃.     

Synthesis and catalytic properties of nanostructured Me/Mn<Subscript>0.5</Subscript>Ce<Subscript>0.5</Subscript>O<Subscript>2</Subscript> in the oxidation of carbon monoxide

The nanostructured solid solution Mn_0.5Ce_0.5O₂ is synthesized to develop effective noble metal free catalysts for the detoxification of technogenic contaminants. Its chemical and phase compositions and textural characteristics are studied by differential thermal analysis, X-ray diffraction analysis, laser mass spectrometry, and low-temperature nitrogen adsorption. The activity of the solid solution in the oxidation of carbon monoxide is determined by the flow method within a temperature range of 20–300°C at atmospheric pressure, a gas hourly space velocity of 1800 h⁻¹ for the following gas mixture composition, vol %: CO, 3.6; O₂, 8.0; N₂, balance. The activity of Mn_0.5Ce_0.5O₂ is shown to be appreciably higher than the activity of MnO_x and CeO₂, and the temperature of 100% conversion is 92, 120, and 210°C, respectively. Using the solid solution as a support and the technique of impregnation, we synthesize the nanostructured catalysts Cu/Mn_0.5Ce_0.5O₂ and Ag/Mn_0.5Ce_0.5O₂, which manifest high activity in the oxidation of carbon monoxide: the temperature of 100% conversion is 77 and 85°C, respectively. The new catalysts could be of interest for the purification of industrial and motor vehicle wastes.     

Effect of Mn content on physical properties of CeOx–MnOy support and BaO–CeOx–MnOy catalysts for direct NO decomposition

A series of manganese–cerium mixed oxides were prepared by a glycothermal method, and the NO decomposition activities of the Ba-loaded Ce–Mn oxides were examined. Among the catalysts examined, the highest NO conversion was obtained on the BaO/Ce–Mn oxide catalyst with a Mn/(Ce+Mn) ratio of 0.25. The X-ray diffraction and Raman analyses indicated the formation of Ce–Mn oxide solid solutions with a cubic fluorite structure. The electron spin resonance analysis indicated the presence of paramagnetic Mn²⁺ species in the composite catalysts. Incorporation of Mn²⁺ in the fluorite structure of CeO₂ causes an increase in the concentration of oxygen vacancies, which play an important role in the NO decomposition activity of the catalysts. The catalysts were also characterized by X-ray photoelectron spectroscopy and temperature-programmed reduction techniques. Based on the results obtained, the relationship between the physical properties of the catalysts and their NO decomposition activities was discussed.     

Selective catalytic reduction of NOx by NH3 at low temperature over manganese oxide catalysts supported on titanate nanotubes

Abstract

We report the NO conversion with NH₃ at low temperature over MnO_x-supported on titanate nanotubes. The effect of SO₂ and water on catalytic activity was also analyzed. For the catalytic activity tests, three catalysts with 1, 3, and 5?wt.% of MnO_x were synthesized. A high NO conversion (92%) at 180?°C is reported for the 1Mn/NTiO₂ catalyst. The further addition of MnO_x to the support improves the catalytic activity but NO conversion (95%) was shifted to a higher temperature (from 180 to 220?°C). However, the presence of SO₂ (50?ppm) and water (5 vol.%) diminishes the NO conversion up to 60%, which remains after 300?min. We found that addition of MnO_x increases the Lewis acid sites concentration of the nanotubes. As the Lewis acid sites increase (from 1.86 to 3.56 µmol m^?2), the Mn⁴⁺/Mn³⁺ ratio on the surface of the nanotubes decreases (from 4 to 1.6), which indicates that the surface of the catalysts is deficient in electrons. We concluded that a high Lewis acid sites concentration and a low Mn⁴⁺/Mn³⁺ ratio, hence a surface deficient in electrons, improves the catalytic activity to remove NO on the Mn/NTiO₂ catalysts.     

Nitric Oxide Reduction Using Hydrogen Over Perovskite Catalysts with Promotional Effect of Platinum on Catalytic Activity

The catalytic activities of strontium substituted La_0.7Sr_0.3MnO₃ type perovskite catalysts for NO reduction using H₂ as reducing agent has been studied, which is further improved by incorporation of Pt outside (0.1 wt.%Pt/La_0.7Sr_0.3MnO₃) and inside (La_0.7Sr_0.3Mn_0.97Pt_0.03O₃) the perovskite lattice structure. Pt shows excellent enhancement in catalytic selectivity towards N₂ when supported on the perovskite. The catalysts were characterized using thermo-gravimetric analysis (TGA), X-ray diffraction (XRD), scanning electron microscopy (SEM) and specific surface area. Catalysts evaluations were carried out using thermo-gravimetric analysis coupled with mass spectra (TG-MS).     

Novel promoting effect of acid modification on selective catalytic reduction of NO with ammonia over CeO2 catalyst

A series of acid modified CeO₂ catalysts were prepared and used for selective catalytic reduction (SCR) of NO with NH₃. The results showed that the SCR activity of pure CeO₂ was greatly enhanced by the modification of acid. The CeO₂ modified by 20% phosphotungstic acid exhibited the best NO conversion in a wide temperature range of 150–550 °C. The SCR activity was slightly influenced by SO₂ and H₂O, while such effect was reversible. The improvement of SCR activity and N₂ selectivity over CeO₂ catalyst modified by acid was attributed to the enhanced amount and intensity of Brønsted or Lewis acid sites.     

Promotion effect and mechanism of MnOx doped CeO2 nano-catalyst for NH3-SCR

MnO_x-CeO₂ (denoted as Mn–Ce) nanorod and MnO_x-CeO₂ nanooctahedra catalysts were synthesized by the hydrothermal method and were used for selective catalytic reduction of NO with NH₃. The catalytic performance tests showed that the NO removal efficiency of CeO₂ catalysts was obviously improved after loading MnO_x. The structure and properties of catalysts had been characterized by SEM、TEM、XRD、BET、XPS、H₂-TPR、NH₃-TPD and in situ DRIFTS. It was found that Mn–Ce catalyst were of uniform core-shell structure, higher concentrations of Mn⁴⁺ and Ce³⁺, better reducibility, the increase of weak acid sites. The results of in situ DRIFTS indicated that the NH₃-SCR reaction should obey the E–R mechanism. Moreover, the promotion effect and mechanism of MnO_x doped CeO₂ was demonstrated, which improved the catalytic activity of Mn–Ce catalysts.     

The effect of alkali promoters in oxidative coupling of methane with Mn-oxide catalysts

The catalytic oxidalive coupling of metnane to ethylene and ethane with manganese oxide catalysts promoted with alkali metal and alkali metallic-chloride has been studied at atmospheric pressure in a fixed bed flow reactor. The main studies of reaction were carried out over maganese oxide catalysts promoted with sodium chloride and the structure and surface morphology of these catalysts was characterized by an X-ray diffraction and a scanning electron microscope. The powdered MnO₂ was changed into Mn₂O₃, and MnO₂ containing alkali metallic-chlorides was not changed to new ternary oxides but changed into Mn₃O₄ and/or Mn₂O₃ at higher calcination temperature(above 780°C). The optimum content of NaCl promoted was 10–20wt%, an in over 10wt%, the conversion and the selectivity were kept constant. The main factor on deactivation of catalysts was the loss of thepromoter(NaCl). The addition of alkali metal salts to manganese oxide catalyst has enhanced C₂(C₂H₄ + C₂H₆) selectivity due to neutralizing acid sites more than the electronic factor. It was confirmed that chlorine in alkali metallicchloride has enhanced the formation of C₂H₄, resulting in a good C₂-yield (up to 25.7%).     

Oxidative coupling of methane and oxidative dehydrogenation of ethane over strontium-promoted rare earth oxide catalysts

Sr-promoted rare earth (viz. La, Ce, Pr, Nd, Sm, Eu, Gd, Dy, Er and Yb) oxide catalysts (Sr/rare earth ratio = 0·1) are compared for their performance in the oxidative coupling of methane (OCM) to C₂ hydrocarbons and oxidative dehydrogenation of ethane (ODE) to ethylene at different temperatures (700 and 800°C) and CH₄ (or C₂H₆)/O₂ ratios (4–8), at low contact time (space velocity = 102000 cm³ g⁻¹ h⁻¹). For the OCM process, the Sr–La₂O₃ catalyst shows the best performance. The Sr-promoted Nd₂O₃, Sm₂O₃, Eu₂O₃ and Er₂O₃ catalysts also show good methane conversion and selectivity for C₂ hydrocarbons but the Sr–CeO₂ and Sr–Dy₂O₃ catalysts show very poor performance. However, for the ODE process, the best performance is shown by the Sr–Nd₂O₃ catalyst. The other catalysts also show good ethane conversion and selectivity for ethylene; their performance is comparable at higher temperatures (≥800°C), but at lower temperature (700°C) the Sr–CeO₂ and Sr–Pr₆O₁₁ catalysts show poor selectivity. © 1998 SCI.     

Structure/activity correlation for unpromoted and CeO2‐promoted MnO2/SiO2 catalysts

The goal of this study was to understand the structure–activity relationship for unpromoted and ceria‐promoted MnO_x/SiO₂ catalysts used in CO oxidation. SiO₂ and CeO₂‐promoted SiO₂ (20% CeO₂) were used as supports to prepare MnO_x/SiO₂ catalysts with various manganese (Mn) loadings. X‐ray diffraction (XRD) and X‐ray photoelectron spectroscopy (XPS) data indicated a higher Mn dispersion on ceria‐promoted than on unpromoted MnO_x/SiO₂ catalysts. Analysis of the XRD patterns and Mn_2p XPS spectra indicated that Mn was present as MnO₂ on MnO_x/SiO₂ with low Mn loadings and ceria‐promoted MnO_x/SiO₂ catalysts and as mixed MnO₂/Mn₂O₃ on MnO_x/SiO₂ catalysts with high Mn loadings. Kinetic data obtained for CO oxidation on unpromoted and ceria‐promoted MnO_x/SiO₂ catalysts are presented and interpreted in correlation with the catalyst surface and bulk structure. A synergistic catalytic effect was observed in the case of the ceria‐promoted MnO_x/SiO₂ catalysts. Post‐reaction XRD and XPS analysis of catalysts indicated that the presence of ceria precludes formation of the less catalytically active Mn₃O₄ species from MnO₂ deposited initially on the SiO₂ support. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effects of WO3 doping on stability and N2O escape of MnOx–CeO2 mixed oxides as a low-temperature SCR catalyst

MnO_x–WO_x–CeO₂ catalysts synthesized using a sol–gel method were investigated for the low-temperature NH₃-SCR reaction. Among them, W_0.1Mn_0.4Ce_0.5 mixed oxides exhibited above 80% NO_x conversion from 140 to 300 °C. In addition, this catalyst exhibited high stability and CO₂ tolerance in a 50 h activity test at 150 °C. Substantially reduced N₂O production and enhanced N₂ selectivity were achieved by WO₃ doping, which was due to the weakened reducibility and increased number of acid sites. The decreased SO₂ oxidation activity as well as the reduced formation of ammonium and manganese sulfates resulted in a high SO₂ resistance of this catalyst.     

Optimum ratio of K2O to CeO2 in a wet-chemical method prepared catalysts for ethylbenzene dehydrogenation

Fe₂O₃–K₂O–CeO₂ catalysts with various ratios of K₂O to CeO₂ were prepared by the wet-chemical method. Their phase compositions, reducibility, valence states of elements and catalytic activities for ethylbenzene dehydrogenation were studied. The results demonstrated that when the weight ratio of K₂O: CeO₂ was 1.40, the catalyst had highest ethylbenzene conversion and styrene selectivity, which were attributed to the optimization of active phase content and electron transfer ability, etc. Further, higher CeO₂ content not only enhanced styrene selectivity, but also prolonged the life cycle of catalysts.     

Screening of doped MnOx–CeO2 catalysts for low-temperature NO-SCR

The effect of different dopants including niobium, iron, tungsten and zirconium oxide on the low-temperature activity of MnO_x–CeO₂ catalysts for the selective catalytic reduction (SCR) of NO_x with ammonia has been studied with coated cordierite monoliths in model gas experiments. A clearly higher activity and particularly superior nitrogen selectivity was obtained with the niobium-doped catalyst in comparison with the MnO_x–CeO₂ reference system. At 200 °C, the DeNO_x was 80% while the N₂ selectivity reached more than 96%. In contrast, a decrease of the SCR activity was observed when iron, zirconium or tungsten oxides were added to MnO_x–CeO₂. However, the addition of niobium oxide did not improve the resistance of the catalyst against SO₂ poisoning. A strong and irreversible deactivation occurred after exposure to SO₂.     

The effect of cobalt precursors on NO oxidation over supported cobalt oxide catalysts

The effect of cobalt precursors such as cobalt acetate and cobalt nitrate on NO oxidation was examined over cobalt oxides supported on various supports such as SiO₂, ZrO₂, and CeO₂. The N₂ physisorption, X-ray diffraction (XRD), transmission electron microscopy (TEM), temperature-programmed reduction with H₂ (H₂-TPR), NO chemisorptions, and temperature-programmed oxidation (TPO) with mass spectroscopy were conducted to characterize catalysts. The NO uptake as well as the catalytic activity for NO oxidation was dependent on the kinds of cobalt precursors and supports for supported cobalt oxides catalysts. Among tested catalysts, Co₃O₄/CeO₂ prepared from cobalt acetate showed the highest catalytic activity. The catalytic activity generally increased with the amount of chemisorbed NO. Reversible deactivation was observed over Co₃O₄/CeO₂ in the presence of H₂O. On the other hand, irreversible deactivation occurred over the same catalyst even in the presence of 5 ppm SO₂ in a feed. The strongly adsorbed SO₂ can prohibit NO from adsorbing on the active sites and also can prevent formed NO₂ from desorbing off the catalyst surface. The formation of SO₃ cannot be observed from the chemisorbed SO₂ on Co₃O₄/CeO₂ during TPO.     

Au/MnO2–TiO2 catalyst for preferential oxidation of carbon monoxide in hydrogen stream

The catalytic activity of nanosize gold catalysts supported on MnO₂–TiO₂ and prepared by deposition–precipitation method has been investigated for preferential oxidation of carbon monoxide in H₂ stream. The catalysts were characterized by inductively coupled plasma-atomic emission spectroscopy, X-ray diffraction, nitrogen sorption, transmission electron microscopy, and X-ray photoelectron spectroscopy. The influence of pH in the preparation process and the amount of MnO₂ loading on the catalytic properties of the Au/MnO₂–TiO₂ catalysts were also studied. Fine dispersion of gold nanoparticles on all the supports was obtained. Especially, Au/MnO₂–TiO₂ with MnO₂/TiO₂ mol ratio of 2:98, showed a mean Au particle size of 2.37 nm. The nanosized support constrained the size of gold. The addition of MnO₂ on Au/TiO₂ catalyst improved the selectivity of CO oxidation without sacrificing CO conversion in hydrogen stream between 50 and 100 °C. This could be attributed to the interactions of gold metal with MnO₂–TiO₂ support and the optimum combination of metallic and electron-deficient gold on the catalyst surface.     

Study of Mn-based Catalysts for Oxidative Dehydrogenation of Cyclohexane to Cyclohexene

The gas-phase oxidative dehydrogenation (ODH) of cyclohexane to cyclohexene in the presence of molecular oxygen has been studied over various Mn-based catalysts. It is found that LiCl/MnO_x/PC (Portland cement) catalyst exhibits the highest catalytic performance, and a 42.8% cyclohexane conversion, 58.8% cyclohexene selectivity and 25.2% cyclohexene yield can be achieved under 600 °C, 20,000 h⁻¹ and C₆H₁₂/O₂/N₂=14/7/79. There are good correlations between the selectivities to cyclohexene and the electrical conductivities of Li doped Mn-based catalysts, from which it is deduced that the non-fully reduced oxygen species (O₂⁻, O₂²⁻, O⁻) involved in a new phase of LiMn₂O₄ might be responsible for the high selectivity toward cyclohexene, whereas the Mn₂O₃ crystal phase results in the CO_x formation. The selectivity to cyclohexene increases with increasing molar ratio of Li to Mn in LiCl/MnO_x/PC.     

Catalytic reduction of nitrogen monoxide by propene in the presence of excess oxygen over gold based ceria catalyst

The catalytic reduction of nitrogen monoxide by propene in the presence of excess oxygen over gold based ceria catalyst was studied. Adsorption and temperature programmed desorption of NO/O₂ on Au/CeO₂ reveal that the catalyst adsorbs and desorbs NO over a large range of temperature. A maximum of 26% conversion of NO _x was obtained around 210 °C, with a selectivity of 50% to N₂.